Ingegneria Geotecnica

E Chimica

Dei Materiali

PH.D Candidate

Maria Grazia Madaffari

Prof. Eng Nicola

Prof Arézou Eng

Modaressi

Eng Paolo

S Calabrò

Prof. Eng Reggio Nicola Moraci

Calabria

Thèse Présentée

Grade De Docteur

Mme Modaressi

Arézou Co-Directrice De Thèse

M Hicher

Pierre-Yves Rapporteur

M Simonini

Paolo Rapporteur

Mme Pantet

Anne Examinatrice

M Aversa

Stefano Examinateur

M Lanzo

Giuseppe Examinateur

UNIVERSITÀ DEGLI STUDI MEDITERRANEA DI REGGIO CALABRIA DOTTORATO DI RICERCA IN

Acknowledgments

It is no easy task to acknowledge all of the people I have been fortunate to meet and know in the three year course of my Ph.D. studies. I am truly thankful to my Supervisors, Prof. Arézou Modaressi and Prof. Nicola Moraci.

I would also like to express my profound thanks to Prof. Moraci for having had confidence in me and for having guided me in this research work both with his masterful intellect and with his openness to innovative ideas. I am really thankful to Prof. Arézou Modaressi for having offered me an exceptional opportunity to develop and for having welcomed and hosted me at the École Centrale Paris. The time that she has dedicated, her suggestions and her style will always remain as indelible lessons to me.

I have also really appreciated the precious support of Eng. Paolo S. Calabrò.

I would also like to acknowledge Prof. Fernando Lopez Caballero, Prof. Elsa Vennat and Prof. Barbara Malinowska for having always been available and helpful.

I would like to thank the members of the thesis committee: Prof. Pierre-Yves Hicher, Prof. Paolo Simonini, Prof. Anne Pantet, Prof. Stefano Aversa and Prof Giuseppe Lanzo, I am indebted to Prof. Pierre-Yves Hicher and Prof. Paolo Simonini for accepting my request to review the manuscript. Special thanks also go to my friends and colleagues at the École Centrale Paris and the Mediterranean University in Reggio Calabria for having rendered this period of research and study both pleasant and fruitful.

Finally, I am deeply grateful to my parents and to my brother for having been an everlasting source of support and encouragement. [START_REF] Weber | Simulation and design models for adsorption processes[END_REF]Smith, 1987, Appelo, 2005)

List of Figures

List of Tables

Introduction

In this chapter a background representing the basis and the theme of the research activity will be described. Particular attention will be paid to the global challenge of sites pollution, focusing on the contaminants that most affect groundwater matrix.

The state of the art of the innovative and well-accepted Permeable Reactive Barrier Groundwater Remediation Technology will be introduced, considering advantages and main issues concerning its long-term performance of its implementation at different scales. Finally, the choice of research subject, objectives and method will be presented. Regarding the parameters on the number of sites, ESDAC has detailed the definition for each parameter. In a "contaminated site" (CS) the presence of the contamination must be confirmed and management measures should be decided on the basis of the potential risk, in a "potentially contaminated site" (PCS), an unacceptable contamination is suspected and investigations need to verify the risk, while "management of contaminated sites" is aimed at remediation. A distinction is made between "estimated" and "identified" sites.

Contaminated sites in Europe

The Figures 1-1 The EIONET-CSI data collection can be integrated with heavy metal data provided by other projects such as the European Pollutant Release and Transfer Register (E-PRTR) and data about water and soil chemical characteristics available after the Land Use/Cover Area frame statistical Survey (LUCAS) soil survey, aimed at digital soil mapping to overcome the disadvantages due to the privacy issue in contaminated soil identification.

Regarding the activities that mostly contribute to soil contamination, waste disposal and treatment account for 38% of sources of local soil contamination over the total number of sources identified, industrial and commercial activities account for 34%, while the agricultural sites and waste water treatment facilities account for 8%. In particular, concerning the industrial and commercial activities, the production sector, including metal, chemical and oil industries and energy production, accounts for 60% of contamination, while the service sector, mainly gasoline stations, accounts for 33% [START_REF] Panagos | Contaminated Sites in Europe: Review of the Current Situation Based on Data Collected through a European Network[END_REF]. In France, the basol database (http://basol.developpement-durable.gouv.fr) on contaminated soils and sites gathers their localization, the nature of pollutants and their impact on the environment as well as the technical situation of the site.

Groundwater remediation solutions

The most widely used technology to remediate contaminated groundwater has been the "Pump-and-Treat" (P&T) system, which consists in extraction of water from the ground and its treatment in a water remediation plant. This kind of technology requires removal of a large amount of groundwater for a long time, leading to a depletion of resources and high treatment costs. Furthermore its remediation efficiency depends on the extraction of contaminants, some of which are highly linked to the aquifer solid matrix. For this reason, one of the main issues in research on groundwater remediation has been focused on the development of sustainable groundwater remediation technologies, during recent decades.

PRB definition and history

Permeable reactive barrier (PRB) is an in-situ technology for remediation of contaminated groundwater [START_REF] Tratnyek | Keeping up with all that literature: the ironrefs database turns 500[END_REF]USEPA, 2002). It consists of a engineered zone of reactive material placed in an aquifer in order to intercept the contaminant plume perpendicularly and to remove the contamination from groundwater flowing through it [START_REF] Rumar | Assessment of Barrier Containment Technologies[END_REF] (Figure 1-3). This technology is based on the natural aquifer gradient to move the contaminated groundwater through the reactive zone. As the contaminants pass through the barrier, they react with the reactive media that either transform them into less harmful compounds or block them into the reactive zone [START_REF] Powell | Permeable Reactive Barrier Technologies for Contaminant Remediation[END_REF][START_REF] Carey | Guidance on the Use of Permeable Reactive Barriers for Remediating Contaminated Groundwater[END_REF][START_REF] Skinner | The feasibility of a permeable reactive barrier to treat acidic sulphate-and nitrate-contaminated groundwater[END_REF]. Different tested reactive materials have the ability to remove organic and inorganic contaminants from groundwater. Moreover, PRB reduces the exposure of workers to contaminants and allows for use of the land surface, developing the treatment underground. This technology has been extensively investigated, obtaining results which make it a sustainable alternative to conventional P&T [START_REF] Korte | Zero-Valent Iron Permeable Reactive Barriers: A Review of Performance[END_REF][START_REF] Carey | Guidance on the Use of Permeable Reactive Barriers for Remediating Contaminated Groundwater[END_REF][START_REF] Wilkin | Capstone Report on the Application, Monitoring, and Performance of Permeable Reactive Barriers for Ground-Water Remediation: Volume 1 Performance Evaluations at Two Sites[END_REF][START_REF] Puls | Long-term performance of permeable reactive barriers: lessons learned on design, contaminant treatment, longevity, performance monitoring and cost -an overview[END_REF][START_REF] Skinner | The feasibility of a permeable reactive barrier to treat acidic sulphate-and nitrate-contaminated groundwater[END_REF][START_REF] Henderson | Long-term performance of zero-valent iron permeable reactive barriers: a critical review[END_REF][START_REF] Chen | Interactions between BTEX, TPH, and TCE during their bio-removal from the artificially contaminated water[END_REF]. However, the application in real scale of most of the successful laboratory-based investigations on PRB long-term performance and improvements in treating a broad spectrum of contaminants need to be validated by empirical evidence [START_REF] Warner | Chlorinated solvent and DNAPL remediation[END_REF].

Since the 1970s, limestone-based installations have been used for passive remediation of surface water or shallow groundwater contaminated by acid mine drainage (Pearson and McDonnell, 1975;[START_REF] Hedin | Passive treatment of acid mine drainage with limestone[END_REF].

At the University of Waterloo, Zero Valent Iron (ZVI) was discovered to be able to be used to remediate groundwater contaminated by halogenated organic solvent (Gillham and O'Hannesin, 1992). The first ZVI-PRB was built at the Canadian Forces Base at Borden, in Ontario in 1991. Since this time, the implementation of ZVI-PRB has been quickly developed and accepted.

Configurations

On the basis of a well-done contaminated site characterization, the appropriate configuration can be chosen among the different conventional and innovative solutions to optimize the PRB design.

Continuous PRB is the most common configuration that has minimal impact on the groundwater flow. It is completely filled with reactive material and it can be chosen when groundwater flow and plume geometry are well understood (Figure 1234). In Funnel-and-Gate configuration, the impermeable Funnels drive the contaminant plume to the reactive zone (Gate) (Figure 12345). In the case of very large contamination or high aquifer heterogeneity , a multiple Funnel-and-Gate can be installed, while multiple reactive media layers can be designed to treat a number of contaminants. Reactive Vessels configuration is very similar to the Funnel-and-Gate. The difference is that the vessel can be extracted in order to replace the reactive material when it is exhausted and it enables investigation and fixing performance problems. An example is the panel-drain ® process developed by Soletanche-Bachy Company (Figure 123456). Caissons PRB is similar to Funnel-and-Gate, consisting in low permeability sections that direct the flow to the reactive material contained in the caisson. The main characteristic is the upward flow of groundwater through the reactive zone.

GeoSiphon™ is a configuration patented by the Westinghouse Savannah River Company and it is based on a siphon effect to induce the upward flow of contaminant plume though the reactive zone to a discharge point of groundwater table (Figure 1234567).

PRB construction techniques

The choice of the PRB installation method constitutes a critical issue for the success of PRB performance because of the influence of design and construction steps on barrier operation. Different aspects such as geotechnical and civil design, method of construction, waste material management, service and infrastructures availability and safety measures should be considered.

The groundwater table depth is one of the first site-characteristics to evaluate in construction method selection. For a shallow depth, excavation is the suitable technique, while for a deeper barrier, injection is more appropriate. This choice is a function of the contaminant present and disposal methods of generated soil, too. The heterogeneous nature of aquifer and aquitard as well as the contaminated site soil quality influence the type of excavation.

Excavation techniques, that are easier and that generate less spoil than injection, include five main construction methods.

Biopolymer trenching can be used to construct a barrier with a depth of up to 21 m.

Using a standard backhoe 9 m depth can be reached, while using a modified one it is possible to excavate down to 25 m. (Gavaskar, 1999). This technique consists of pumping biopolymer slurry, e.g. guar gum, into the trench to make the trench wall stable [START_REF] Day | Directive 2006/118/EC of the European Parliament and of the Council on the protection of groundwater against pollution and deterioration[END_REF] (Figure 12345678). During excavation work and trench wall stabilization biopolymer remains in the trench and other kinds of support may not be necessary. After reactive material placement, enzymes can be used to degrade the biopolymer. Continuous trenching is appropriate for treatment zones 11 m deep and 0.3-0.9 m wide. A thick wall can be obtained through an arrangement of parallel trenches [START_REF] Naidu | Permeable Reactive Barrier. Sustainable Groundwater Remediation[END_REF]. This method consist in filling with reactive material and backfilling in not to leave the trench open at any time. A standard backhoe can be used for PRB construction also without side wall support, if the depth of the trench is less than 6 m and can remain about 4 h without caving in.

Otherwise, for a trench more than 6 m deep, the side walls can be supported by trench boxes and hydraulic shores.

Sheet piling can be used to construct a continuous barrier as well as a reactive zone and an impermeable zone in Funnel-and-Gate configuration. Usually, sheet piling is placed around the perimeter of the PRB during the trench excavation and is removed after the reactive material filling and the backfilling, allowing natural groundwater flow. To build the impermeable zone of Funnel-and-Gate, a crane with a vibrating hammer is normally used to install the sheet piles interconnected by interlocking edges (Figure 123456789). This is a suitable method where there is horizontally limited space to work and it does not generate much soil. It is appropriate for depths of about 15 m. Slurry wall does not have the same limit in depth as the sheet pile. It is usually constructed for the impermeable zone of a Funnel-and-Gate barrier. The trench is excavated using a backhoe, a modified backhoe or a clamshell digger, in function of the design depth. The most common slurries is made of soil/bentonite, cement/bentonite or other composites [START_REF] Meggyes | Construction methods of permeable reactive barriers[END_REF]. During the excavation, appropriate slurry, used to maintain trench stability, permeates the sides of excavation, forming a hydrated filter cake along the sides. A soil/bentonite backfill is put in place to fill the excavated space.

Injection techniques include hydraulic fracturing, jetting and soil mixing.

Hydraulic fracturing consists in installing a series of wells along the perimeter of the barrier under construction and creating a vertical fracture in them. A mixture made of iron/gel or slurry of soil/bentonite, soil/cement or composite slurry is injected into the well to create reactive zone or impermeable zone respectively. Enzyme can be used to degrade the gel. Attention should be paid to choose high quality and purity gel useful for the suspension and transport of iron, in order not to have impact on iron reactivity and permeability.

The practice of inject grouting to improve mechanical characteristics of soil has been recently proposed to be used for construction of the funnel.

Soil mixing techniques allow the attainment of more uniform PRB and a higher hydraulic conductivity than the jetting. This consists in creating a row of columns, using a hollow-stem auger with mixing paddles that rotate mixing the soil at the desired depth and injecting the reactive slurry.

EU-projects about PRB research

PRBs have not been fully and readily accepted and incentivized as new general remediation technologies in Europe, probably because (1) there is lack of reliable information on long-term performance, longevity and long-term effect, because of still missing long-lasting projects [START_REF] Puls | Long-term performance of permeable reactive barriers: an update on a U.S. multi-agency initiative[END_REF][START_REF] Yoon | Innovative construction and performance monitoring of a permeable reactive barrier at Dover Air Force Base. Chemical Oxidation and Reactive Barriers: Remediation of Chlorinated and Recalcitrant Compounds[END_REF][START_REF] Rochmes | Erste Erfahrungen mit Reaktiven Wänden und Adsorberwänden in Deutschland[END_REF][START_REF] Sarr | Zero-Valent-Iron Permeable Reactive Barriers -How Long will they Last?[END_REF][START_REF] Simon | Long-Term Behaviour of Permeable Reactive Barriers Used for the Remediation of Contaminated Groundwater[END_REF][START_REF] Vidic | Permeable reactive barriers: case study review[END_REF], (2) all degradation pathways are not identified and precise mass balances cannot be determined, while the toxicity of intermediary or final products is also under discussion [START_REF] Wienberg | Vollständige, stoffspezifische Bilanzen des Schadstoffumsatzes beim Einsatz reaktiver Wände. Sanierung von Altlasten mittels durchströmter Reinigungswände, Vorträge und Diskussionsbeiträge des Fachgespäches am 27[END_REF], (3) at present, insufficient information is available on the costs of PRBs, considering also the possibility of performance decreases over time and (4) knowledge regarding the applicability and longevity of combined contamination solutions, especially regarding heterogeneous and complex scenarios, is at an early stage [START_REF] Rochmes | Erste Erfahrungen mit Reaktiven Wänden und Adsorberwänden in Deutschland[END_REF][START_REF] Scherer | Chemistry and microbiology of reactive barriers for in situ groundwater cleanup[END_REF]. For these reasons, some projects have been promoted and proposed in order to ease the research and application of groundwater remediation technologies, such as the Permeable Reactive Barrier. (Birke, V. et al., 2007, http://www.rubinonline.de/) SAFIRA, "Sanierungsforschung in regional kontaminierten Aquiferen", in English meaning "remedial research applied to regionally contaminated aquifers", an R&D network using specifically designed in-situ reactors in a semi-technical scale for testing different reactive materials, was the first initiative to study the potentials of PRBs in a broader scope [START_REF] Weiß | SAFIRA 2. Statusbericht; Modellstandort, Mobile Testeinheit, Pilotanlage[END_REF]

ZVI-PRB: main issues

Some of main issues concerning PRB regard long-term removal efficiency, the hydraulic behavior and the release of contaminants. In fact, usually, when a PRB is designed, a sufficient reactive material is quantified to be placed in the barrier that is previewed as having a certain longevity.

When the barrier is built, it comes into contact with groundwater constituents and a number of reactions can take place, forming mineral precipitation and other products that might fill the pores of porous material and reduce hydraulic conductivity.

Concerning ZVI, the possible causes of hydraulic efficiency decrease are indicated in the expansive nature of iron corrosion products [START_REF] Caré | Mechanical properties of the rust layer induced by impressed current method in reinforced mortar[END_REF][START_REF] Zhao | Composition and expansion coefficient of rust based on X-ray diffraction and thermal analysis[END_REF], the gas formation [START_REF] Henderson | Impact of solids formation and gas production on the permeability of ZVI PRBs[END_REF][START_REF] Reardon | Anaerobic corrosion of granular iron: measurement and interpretation of hydrogen evolution rates[END_REF], 2005[START_REF] Reardon | Capture and storage of hydrogen gas by zero-valent iron[END_REF], the biofilm formation [START_REF] Gu | Biogeochemical dynamics in zero-valent iron columns: implications for permeable reactive barriers[END_REF], the secondary mineral precipitates accumulation [START_REF] O'hannesin | Long-Term Performance of an in Situ ''Iron Wall'' for Remediation of VOCs[END_REF][START_REF] Liang | Geochemical and microbial reactions affecting the long-term performance of in situ ''iron barriers[END_REF][START_REF] Kamolpornwijit | Preferential flow path development and its influence on long-term PRB performance: column study[END_REF][START_REF] Jeen | Performance evaluation of granular iron for removing hexavalent chromium under different geochemical conditions[END_REF] and the retention of fine particles derived from upstream soil in the pores [START_REF] Moraci | Critical aspects related to Fe0 and Fe0/pumice PRB design[END_REF].

The iron corrosion is developed through the reactions between ZVI and groundwater, including its constituents, involving Eh decrease and pH increase [START_REF] Blowes | Treatment of inorganic contaminants using permeable reactive barriers[END_REF][START_REF] Geranio | Review of Zero Valent Iron and Apatite as reactive materials for Permeable Reactive Barrier[END_REF]. This means that ZVI can react with water oxygen, magnesium, carbonates, calcium and sulfates dissolved inorganic constituents (Puls et al., 1999;[START_REF] Puls | Long-term performance of permeable reactive barriers: lessons learned on design, contaminant treatment, longevity, performance monitoring and cost -an overview[END_REF][START_REF] Jeen | Performance evaluation of granular iron for removing hexavalent chromium under different geochemical conditions[END_REF].

Under anaerobic conditions, water is reduced and at high pH, ferrous hydroxide or green-rust minerals can be formed (Zolla et al., 2007[START_REF] Wilkin | Long-term Performance of Permeable Reactive Barriers Using Zero-valent Iron: An Evaluation at Two Sites[END_REF], whereas under aerobic conditions, dissolved oxygen reacts as the oxidant and ferric oxides and oxy-hydroxides can precipitate (Wilkinet al., 2000).

Notwithstanding the fact that on the basis of a number of laboratory research studies, iron corrosion products can lead to hydraulic conductivity and removal efficiency decrease, in some field studies PRB performance has not declined. This means that the rate of decline can be different and that many PRBs are too new for gathering this kind of information. [START_REF] Kouznetsova | Modelling the long-term performance of zero-valent iron using a spatio-temporal approach for iron aging[END_REF].

ZVI-PRB for heavy metals

The proposal of the research activity, of which results will be presented in the following chapters, deals with the remediation of heavy-metal contaminated groundwater using PRB made of granular ZVI mixed with other granular materials.

The choice to treat heavy-metals contaminants was driven by the knowledge of high presence of this kind of pollutants in water matrix, e.g. as previously mentioned for EU countries (paragraph 1.2.2).

Nickel, Copper and Zinc removal using ZVI/granular material mixtures

Among the different heavy metal elements, the choice of the defined contaminants to study was on two main considerations.

The first concerns reactive materials. The main reactive material chosen to be used in the research has been ZVI, which is able to remove organic and inorganic contaminants. It is the most widely used in in-situ and pilot installations as well as in a number of laboratory studies, focused on its behavior improvement, and it can be considered the most versatile and the most suitable reactive material for treating a complex contamination. It is, however, true that the choice of reactive material and its characteristics depends on different considerations on contamination and site configuration, so that it is not possible to take a decision a priori and evaluation is necessary, case by case.

Given ZVI/granular material mixtures as reactive material to be used, the possible heavy metals removal mechanisms involved using ZVI have been considered so as to optimize their representation and the possibility of studying them. ZVI can remove heavy metals by reduction, adsorption onto corrosion products or co-precipitation [START_REF] Noubactep | A critical review on the mechanism of contaminant removal in Fe 0 eH2O systems[END_REF][START_REF] Noubactep | An analysis of the evolution of reactive species in Fe 0 /H2O systems[END_REF][START_REF] Noubactep | A Fe0-based alloys for environmental remediation: thinking outside the box[END_REF][START_REF] Cundy | Use of iron-based technologies in contaminated land and groundwater remediation: a review[END_REF][START_REF] Rangsivek | Removal of dissolved metals by zero-valent iron (ZVI): kinetics, equilibria, processes and implications for stormwater runoff treatment[END_REF][START_REF] Moraci | Heavy metals removal and hydraulic performance in zero-valent iron/pumice permeable reactive barriers[END_REF]. Three heavy metals have been chosen for study in function of their different affinity to iron oxides and the different redox behaviors of the couples, each one made of one of the chosen heavy metals and ZVI. The proposed heavy metals are Nickel (Ni), Copper (Cu) and Zinc (Zn).

The difference of electrode potential (E 0 ) between the elements involved gives information about the different redox behaviors. Zinc, whose E 0 Zn(II)/Zn(0) value is -0.763 V, cannot be reduced by ZVI, whose E 0 Fe(II)/Fe(0) value is -0.44 V. Copper can be readily reduced by ZVI because its E 0 Cu(II)/Cu(0) is +0.337 V. Nickel has a standard electrode potential close to that of ZVI. The value of E 0 Ni(II)/Ni(0) is -0.25 V. This means that redox process is not sure and that the main Nickel removal process using ZVI is not redox. The metallic ions can be removed by four main mechanisms: coprecipitation with iron hydroxides, adsorption onto the (hydr)oxide surfaces, isomorphic substitution for Fe in the iron oxide structure, or adsorptive sizeexclusion [START_REF] Herbert | Metal retention by iron oxide precipitation from acidic ground water in Dalarna[END_REF][START_REF] Wang | Relationships between heavy metals and iron oxides, fulvic acids, particle size fractions in urban roadside soils[END_REF][START_REF] Vodyanitskii | The role of iron in the fixation of heavy metals and metalloids in soils: a review of publications[END_REF], Bilardi, et al., 2013). Reduction can be developed through different reaction paths: direct reduction on ZVI, reduction by aqueous Fe(II), reduction by adsorbed or structural Fe(II), reduction by molecular (H2) or atomic (H) hydrogen [START_REF] Noubactep | A Fe0-based alloys for environmental remediation: thinking outside the box[END_REF][START_REF] Bilardi | Simultaneous removal of CuII, NiII and ZnII by a granular mixture of zero-valent iron and pumice in column systems[END_REF]. Furthermore, the removal of the three heavy metals has been tested in a number of batch and column tests using ZVI/Pumice mixtures [START_REF] Moraci | Heavy metals removal and hydraulic performance in zero-valent iron/pumice permeable reactive barriers[END_REF][START_REF] Moraci | Long-Term Efficiency of Zero-Valent Iron -Pumice Granular Mixtures for the Removal of Copper or nickel From Groundwater[END_REF][START_REF] Calabrò | Estimate of the optimum weight ratio in Zero-Valent Iron/Pumice granular mixtures used in permeable reactive barriers for the remediation of nickel contaminated groundwater[END_REF]Bilardi et al., 2013a and2013b;[START_REF] Moraci | Critical aspects related to Fe0 and Fe0/pumice PRB design[END_REF][START_REF] Bilardi | Simultaneous removal of CuII, NiII and ZnII by a granular mixture of zero-valent iron and pumice in column systems[END_REF] In France, Nickel constitutes 5.85% of the in-situ contaminants and is positioned at the 9 th position according to basol database. Copper is the 5 th (8.25%) and Zinc is the 8 th (5.93%).

Hydraulic issue: mixtures

Different strategies have been evaluated to overcome the problem related to the permeability loss of ZVI-PRB. [START_REF] Moraci | Heavy metals removal and hydraulic performance in zero-valent iron/pumice permeable reactive barriers[END_REF] proposed mixing ZVI with Pumice at a different weight ratios in order to avoid the fast permeability reduction observed using ZVI under the same boundary conditions. The proposal applied to heavy-metal removal has led to good results in terms of long-term removal efficiency and hydraulic behaviors. Different mixtures made of ZVI/sand, ZVI/Pumice, ZVI/gravel, ZVI/anthracite have been tested for TCE contaminated groundwater removal by O'Hannesin and Gillham (1998[START_REF] Ruhl | Evaluation of Two-Component Fe(0) Fixed Bed Filters with Porous Materials for Reductive Dechlorination[END_REF] and [START_REF] Bi | Effects of Mixing Granular Iron with Sand on the Kinetics of Trichloroethylene Reduction[END_REF] but these systems have been unefficient for this kind of contaminant.

Other suggestions to resolve the hydraulic issue were to increase the thickness of the barrier, distributing better the materials, to equalize the zones up-and downgradient the barrier through addition of pea gravel, to place a pre-treatment zone up gradient, to adjust the pH, to utilize larger ZVI grains and mechanical mixing [START_REF] Blowes | Treatment of inorganic contaminants using permeable reactive barriers[END_REF][START_REF] Li | Evaluation of five strategies to limit the impact of fouling in permeable reactive barriers[END_REF].

During the research activity, the choice in trying overcome the permeability issue has been to mix ZVI with a new granular material. The mixtures have been tested to study their long-term heavy-metals removal efficiency and hydraulic behaviors. ZVI has been mixed with granular Lapillus, which is a volcanic material characterized by a chemical composition and structure a little different from Pumice. Its characteristics will be further described in Chapter 2.

PRB design

The design of PRB requires well-done contaminated site characterization. On this basis, the suitable reactive material and its characteristics, PRB dimensions and configuration, and method of construction can be chosen. One of the first steps is to select the reactive material that should be reactive with respect to the contaminant to be removed, environmentally compatible, stable, easily available and cheap. The selection regarding the removal efficiency of the potential reactive material is based on the results of batch and column tests. The latter give information not only about the possible efficacy of contaminant removal, but also about the variables influencing long-term removal and hydraulic behavior, e.g. the available reactive surface, the residence time, the velocity rate and the concentration of contaminant.

However, different authors are working on the model development, to produce more complete or easier design tools. During the research activity, a model of the experimental tests carried out has been developed and this will be introduced in Chapters 6.

As mentioned above, one of the possible causes of hydraulic behavior decrease that can be avoided theoretically a priori regards the choice of grain size distribution, considering PRB as a geotechnical filter.

Filter criteria

The choice of the geotechnical characteristics of the granular reactive medium, namely grain size distribution, initial values and evolution ranges of porosity and permeability depend strictly on the proprieties of the aquifer. In fact the geotechnical characteristics of base soil, that are grain size distribution, porosity, internal stability and permeability, must be known and taken into account in PRB filter design. [START_REF] Moraci | Critical aspects related to Fe0 and Fe0/pumice PRB design[END_REF] pointed out the important geotechnical issue that has to be considered in the choice of reactive medium grain size distribution and this consists of satisfying filter design criteria towards the surrounding soil. There are the main design criteria for granular filter design in mono-dimensional flow conditions: internal stability, retention and permeability (Moraci et al., 2012a) The internal stability criteria regards the ability of granular filter to prevent the loss of its own small particles due to disturbing forces [START_REF] Kenney | Internal stability of granular filters[END_REF].

Considering the PRB granular filter, it is important that it does not undergo significant variations in permeability and particle size distribution due to the dragging force of fluid. This stability depends on the grain size distribution, the soil relative density and the applied hydraulic gradient. To evaluate the influence of the first of these it is sufficient, in a first approach, to observe the grain size distribution.

If it presents a concave upward curve, a gap-graded soil or a broadly graded soil can be considered to be internally unstable (Moraci et al., 2012b). The criteria proposed by Kezdi (1969) and [START_REF] Sherard | Sinkholes in dams of coarse, broadly graded soils[END_REF] based on classical retention criteria for granular soils and that introduced by [START_REF] Kenney | Internal stability of granular filters[END_REF] on the basis of experimental and theoretical results, are commonly used to evaluate the internal instability of a granular soil that is subjected to seepage. The retention and permeability criteria deal properly with the function of the filter.

When water flows through the soil, erosion can occur because of dragging of fine particles. The function of the filter is in fact to retain the soil particles and to avoid the increase of internal pore pressure at soil-filter interface at the same time. This means that the granular filter should be fine enough to retain loose soil particles and coarse enough to avoid the development of high internal pore pressure, without modifying the natural seepage flow [START_REF] Giroud | Development of criteria for geotextile and granular filters[END_REF]. A PRB filter can respect the permeability criteria, verifying the high internal pore pressure and flow rate requirements. In fact, on the one hand the hydraulic conductivity of the filter has to be higher than the surrounding soil to avoid the development of high internal pore pressure at soil-filter interface, and on the other hand the difference between the flow rate in the aquifer and that in the aquifer where PRB is placed should not be more than 10% [START_REF] Moraci | Geotextile filter: design, characterization and factors affecting clogging and blinding limit states. Theme lecture[END_REF], to have an acceptable filter.

Geometric, physical, hydraulic, chemical and biological factors can affect the PRB hydraulic performance and the seepage and its evolution between the base soil and the granular filter [START_REF] Caré | Modeling the Permeability Loss of Metallic Iron Water Filtration Systems[END_REF]. Among these factors, there are the shape of particles and the particle size distribution, the structure of the filter, including the pore constriction size and distribution, the particle surface roughness, the filter density, the particle specific density, the applied total head, the hydraulic gradient, the mass flow rate and changes in porosity of the filter in function of the water and soil chemistry and bacterial activity.

Conclusions

Considering the most used reactive material and its removal efficiency on the contaminants that most affect the groundwater matrix, at least in Europe, the research activity carried out has focused on the improvement of ZVI-PRB to remediate heavy metals contaminated groundwater.

In particular, ZVI has been tested mixed with Lapillus, considering that tested ZVI/Pumice and ZVI/sand mixtures have demonstrated a better hydraulic and heavy metal removal efficiency than pure ZVI. Filter criteria have been taken into account to prepare the mixtures.

Experimental results and model development will be introduced in the following chapters.

In

Materials Characterization

Introduction

As introduced in Chapter 2, in this work an internally porous material, Lapillus, is mixed with ZVI to investigate the long-term removal efficiency and hydraulic behavior of ZVI:Lapillus mixtures. Furthermore, a model to simulate experimental data obtained by previous batch and column tests using ZVI:Pumice mixtures is developed. For this reason, Lapillus, Pumice and ZVI were characterized.

In this chapter, the materials used in the experimental activity will be described.

These are reactive materials whose heavy metals removal efficiency and hydraulic behavior will be investigated, and the heavy metal contaminants used to prepare the synthetic contaminated solutions. The reactive materials used are Zero Valent Iron, Pumice and Lapillus. They have been characterized to know their physic-chemical and geotechnical properties and they have been observed using a scanning electronic microscope (SEM) to observe their shape and surface in order to obtain useful information for understanding the removal mechanisms and the processes leading to the reduction of hydraulic conductivity.

ZVI

The Zero Valent Iron used is of the FERBLAST RI 850/3.5 type, distributed by Pometon S.p.A., Mestre, Italy. The material is mainly made of iron (>99.74%) and the impurities include Mn (0.26%), O, S and C. (Bilardi et al., 2013). ZVI has been characterized by grain size analysis. Its grain size distribution is shown in Figure 2345.

The mean grain size (d50) is about 0.5 mm and the coefficient of uniformity (U) is 2.

The used ZVI microstructure was observed using Scanning Electron Microscopy (SEM). ZVI samples were prepared for SEM observation placing them on an appropriate support and fixing them on using silver varnish. Afterwards, they were conserved under vacuum conditions. Only before placing them into the instrument a plasma using a source of Gold-Palladium (Au-Pd) was used to put some atoms on the surface of the sample in order to create on the surface a layer capable of conducting the electrons for a clear observation.

SEM

Pumice

Pumice is a product of explosive volcanic eruption and it is the main component of pyroclastic deposits. Fischer and Schmincke (1984) defined pumice as highly vesicular silicic to mafic glass foam, which will commonly float on water. [START_REF] Whitham | Pumice[END_REF] and Esposito and Guadagno (1997) have investigated the characteristics of pumice, presenting new measurements and analysis about physical geotechnical properties, internal surface area, connectedness of vesicles and their size distribution. They studied the behavior of pumice in contact with water, highlighting how the penetration of water into particle pores modify the weightvolume relationship. The experiments demonstrated a similar pattern in water adsorption of pumice samples characterized by different sizes, internal porosity and density. In fact, an initial rapid absorption, resulting in a significant part of pore space occupied by water, is followed by a slow steady absorption of water lasting weeks or months, until the sinking of pumice.

Chemical composition

The Pumice used in this research is classified as "2B" by the producer Pumex Spa. It originates from the Aeolian Island of Lipari, Sicily, Italy. In Table 2-1, its mineralogical composition of is shown. In addition it contains about 4% of structural water and traces of CaO, SO3, MgO, TiO2, FeO, MnO, P2O5 (Bilardi et al., 2013a).

Grain size distribution

In Figure 2-5, the used Pumice grain size distribution is shown. The mean grain size (d50) is about 0.3 mm and the coefficient of uniformity (U) is 1.4.

Mercury Intrusion Porosimetry (MIP)

The Mercury Intrusion Porosimetry analysis is based on the method described by

Washburn in 1921 to determine the effective pore diameters in a coarsely granular sample using mercury as a non-wetting liquid to be forced into the pores at known pressures. The sample is placed into a sample cup that is first evacuated to remove all the gases adsorbed from the sample. Afterwards, pure mercury is pumped into the sample through a series of increasing pressures steps and the intruded volume is measured. Based on the equation of [START_REF] Young | An essay on the cohesion of fluids[END_REF] and Laplace (1806) that describes the equilibrium of the internal and external pressures for spherical surfaces,

Washburn derived an equation directly applicable with mercury. This latter does not wet most of the substances and has to be forced to enter into the pores because it does not penetrate into them by capillary action. The method is based on the equation

2-1. 𝐷 = -4𝛾𝑐𝑜𝑠𝜃 𝑃 (2 -1)
where D is the pore diameter, P the applied pressure, γ the surface tension and θ the contact angle. In fact, there are three necessary physical parameters to be able to describe the intrusion of a liquid into a capillary. These are the surface tension, the contact angle and the geometry of the boundary line of contact among solid, liquid and vapor. The Washburn procedure is based on the hypothesis of cylindrical pores and the circular cross-section of the opening. AutoPore IV of Micromeretrics (Model 9505/9500) was used to characterize Pumice. The Pumice samples were first dried using a lyophilizer (Figures 234567). The used Pumice can be classified as a porous powder for this kind of application, therefore a penetrometer for powder was used (Figures 2-8 a and b).

The specifications of instrument characteristics are reported in

Pumice quantity used was about 0.11 g for each tests to be able to use the available mercury volume of the penetrometer. Penetrometer type 14-0138 was used. Its characteristics are reported in In Table 2-4, the maximum diameter identified by analyzing the results and the cumulative volume of pores per unit of Pumice mass are reported. In Figures 23456789101112, 2-15 the log differential intrusion, cumulative intrusion and incremental intrusion as function of Pore Diameter are reported for A, B, C and D tests respectively. Through a general overview, it seems that pumice does not have well-defined families of pores and a large range of pore sizes can be observed. The numbers reported on each graph concern the pore sizes corresponding to the picks.

The D test was the first one carried out and it was used in order to evaluate the steps of pressure to be used for the subsequent tests. For this reason, precise data about the first steps of pressure are not available. The maximum diameter and the different pore size families as found by Mercury Porosimetry data analysis can be validated by observation of the SEM images. The volume of Mercury filling the penetrometer at the beginning of High Pressure chamber analysis 𝑉 𝑚 , after the vacuum is reached, is equal to the weight of mercury filling the penetrometer at the end of the Low Pressure step (0.2MPa) divided by the 𝑌 𝑚 density of mercury. This can be expressed by the equation 2-2.

𝑉 𝑚 = 𝑊 𝑝𝑠𝑚 -𝑊 𝑠 -𝑊 𝑝 𝑌 𝑚 (2 -2)
where the weight of mercury is equal to the 𝑊 𝑝𝑠𝑚 assembly weight minus the 𝑊 𝑠 sample weight minus the 𝑊 𝑝 empty penetrometer.

Knowing the volume of penetrometer 𝑉 𝑝 thanks to the previous calibrations and taking into account that part of the used stem volume has filled the inter-particles sample volume, after Low Pressure step, it is possible to calculate the bulk volume of Pumice particles 𝑉 𝑏 through equation 2-3.

𝑉 𝑏 = 𝑉 𝑝 -𝑉 𝑚 -𝑉 𝑐𝑢𝑚(>𝐷𝑚𝑎𝑥𝑝𝑜𝑟𝑒𝑠) (2 -3)
The solid volume 𝑉 𝑠 can be calculated through equation 2-4.

𝑉 𝑠 = 𝑉 𝑏 -𝑉 𝑚,𝑖𝑛𝑡 (2 -4)
Where 𝑉 𝑚,𝑖𝑛𝑡 is the total internally intruded volume, the 𝑉 𝑏 the bulk volume. The 𝑉 𝑚,𝑖𝑛𝑡 total internally intruded volume is equal to the difference between 𝑉 𝑐𝑢𝑚(𝑚𝑎𝑥𝐻𝑃)

cumulative intrusion volume at the end of the measurement test and the inter-

particle volume 𝑉 𝑐𝑢𝑚(>𝐷𝑚𝑎𝑥𝑝𝑜𝑟𝑒𝑠) (equation 2-5) 𝑉 𝑚,𝑖𝑛𝑡 = 𝑉 𝑐𝑢𝑚(𝑚𝑎𝑥𝐻𝑃) -𝑉 𝑐𝑢𝑚(>𝐷𝑚𝑎𝑥𝑝𝑜𝑟𝑒𝑠) (2 -5)
Dividing the used pumice weight by the 𝑉 𝑏 bulk volume and the 𝑉 𝑠 solid volume, pumice apparent density 𝜌 𝑎𝑝𝑝 and real density 𝜌 𝑠 can be obtained respectively. The average value for 𝜌 𝑎𝑝𝑝 is 1.2 gr/cm 3 , while for 𝜌 𝑠 is 2.48 gr/cm 3 . The 𝜑 𝑖 internal porosity can be calculated by the equation 2-6:

𝜑 𝑖 = 1 - 𝜌 𝑎𝑝𝑝 𝜌 𝑠 (2 -6)
The calculated internal porosity was about 50%.

A quantity of information to characterize porous media can be deduced by analyzing Mercury Intrusion Porosimeter data, for example the number of pores and the material compressibility.

Pycnometer

The pumice density 𝝆 measured using Pycnometer is about 2 gr/cm 3 .

There are some differences among the density deduced by Pycnometer, the apparent density 𝜌 𝑎𝑝𝑝 and the real density 𝜌 𝑠 deduced by analyzing the Hg-porosimeter data.

The sequence is 𝜌 𝑎𝑝𝑝 < 𝝆 < 𝜌 𝑠 . The difference can be searched on the wettability of pumice using different fluids (i.e. Hg and water) and in the capillarity phenomena that can be developed in the micro pores of pumice when it is put in contact with water. In fact, fast water sorption on pumice when it is put in the water, sinking experiments and long-time pore occupancy by water are reported in a number of studies (Whitham and Spark, 1986).

Lapillus

Lapillus is a sedimentary pyroclastic material. Sedimentary rocks result from consolidation of loose materials which were created from erosion and alteration of pre-existing rocks. Pyroclastic rocks are a particular group of sedimentary rocks and are formed after volcanic products deposition in conjunction with explosive eruptions [START_REF] Scesi | Il rilevamento geologico-tecnico[END_REF]. The used lapillus is a natural material distributed by

Grain size distribution

The grain size distribution of Lapillus as received is reported in figure 2-21. The uniformity coefficient U (d60/d10) is equal to 10.4.

Mercury Intrusion Porosimetry (MIP)

As previously done for pumice, four tests (A, B, C and D) using lapillus were carried out using MIP. Log differential intrusion, cumulative intrusion and incremental intrusion variations are shown as function of the pore diameter for each test in the Considering the same procedure used for pumice, the average apparent density 𝜌 𝑎𝑝𝑝 and the average real density 𝜌 𝑠 of lapillus were equal to 2.2 gr/cm 3 and 3 gr/cm 3 respectively. The calculated internal porosity 𝜑 𝑖 was about 40%.

Pycnometer

The Lapillus density 𝝆 measured using Pycnometer resulted about 2.8 gr/cm 3 . As can be expected, the sequence 𝝆 𝒂𝒑𝒑 < 𝝆 < 𝝆 𝒔 is respected.

Heavy-metal contaminated solutions

The chemical composition of the synthetic solution used in both tests is reported by Bilardi et al. (2012b). The Nickel contaminated solution was prepared diluting Nickel Nitrate (Nickel(II) nitrate hexahydrate, purity > 99 %; Sigma-Aldrich) in distilled water in order to obtain the preset Nickel concentrations (10, 50 and 100 mg/L).

The Zinc contaminated solution was prepared diluting Zinc Nitrate (Zinc(II) nitrate hexahydrate, purity 99; Sigma-Aldrich) in distilled water in order to obtain the preset Zinc concentration (50 mg/L). The Copper contaminated solution was prepared diluting Copper Nitrate (Copper(II) nitrate hydrate, purity > 99%; Sigma-Aldrich) in distilled water in order to obtain the preset Zinc concentration (500 mg/L).

Conclusions

The used Zero Valent Iron is of the type FERBLAST RI 850/3.5, distributed by Pometon S.p.A., Mestre in Italy. The material is mainly made of iron. The mean grain size (d50) is about 0.5 mm and the coefficient of uniformity (U) is 2. By SEM analysis, ZVI particles seem to have an almost smooth surface, while some small particles, that have been observed on the surface, seem to be iron oxides. These letters seem to have different shapes and to be highly porous internally, with a surface smooth or highly rough. The Pumice used in this research is "Pumice 2B" provided by Pumex Spa. It originates from the Aeolian Island of Lipari, Sicily in Italy and it is mainly made of SiO2 (71.75%) , Al2O3 (12.33%), Fe2O3 (1.98%), Na2O (3.6%) and K2O (4.5%). The mean grain size (d50) is about 0.3 mm and the coefficient of uniformity (U) is 1.4.

Pumice was characterized by MIP. Its maximum average diameter is about 140-110 μm. The calculated internal porosity was about 50%. The Pumice 𝜌 density measured using Pycnometer is about 2 gr/cm 3 , the apparent density 𝜌 𝑎𝑝𝑝 is 1.2 gr/cm 3 and real density 𝜌 𝑠 is 2.48 gr/cm 3 .

Lapillus is a sedimentary pyroclastic material. The Lapillus used is a natural material and it is distributed by SEM "Società Estrattiva Monterosi"s.r.l., Viterbo, Italy. It has granular form and a red-marron colour (Figure 2-19) and it originated from explosive volcanic activity of the Sabatini Mountains. It is made of SiO2 (47%), Al2O3 (15%), Fe2O3 and FeO (7-8%), MgO (5.5%)

The coefficient of uniformity U is about 3.2 and the mean grain size (d50) is approximately 0.4 mm. Observing Lapillus using SEM, it seems to be less porous than Pumice and to have a more irregular and rougher surface than Pumice. The average apparent density 𝜌 𝑎𝑝𝑝 and the average real density 𝜌 𝑠 of Lapillus is equal to 2.2 gr/cm 3 and 3.86 gr/cm 3 respectively. The calculated internal porosity 𝜑 𝑖 is about 40%. The Lapillus density 𝜌 measured using Pycnometer resulted about 2.8 gr/cm 3 .

In Chapter 3 the characterization results of Pumice samples are used to develop a model for the simulation of nickel removal efficiency observed in batch tests performed using pure Pumice by previous Ph.D. studens.

Modelling of Nickel removal efficiency in batch tests using pure double Porous Materials and ZVI

Introduction

In this chapter the experimental data of batch tests carried out using pumice, ZVI and

Experimental results analysis using Pumice

Pumice removal efficiency has been tested through batch and column tests during the previous Ph.D. thesis at Mediterranea University of Reggio Calabria by Ph.D. students [START_REF] Rigano | Studio dell'efficienza di barriere permeabili reattive per la bonifica di acquiferi contaminati da metalli pesanti[END_REF][START_REF] Suraci | Studio sperimentale dell'efficienza di barriere permeabili reattive per la bonifica di acque di falda contaminate da metalli pesanti[END_REF][START_REF] Bilardi | Short and long term behaviour of Fe0 and Fe0/Pumice granular mixtures to be used in PRB for groundwater remediation[END_REF]. Batch tests were carried out setting two different liquid-solid ratios (10mL:1g here called Low Mass (LM) and 4mL:1g here called High Mass(HM)) and three different initial concentrations of Nickel (5mg/L , 50 mg/L , 500 mg/l). In LM and HM tests 5.4 g and 8 g of reactive material were used respectively. First, Nickel contaminant removal efficiency observed in batch tests using pure pumice will be considered. Table 3-1 shows a summary of batch tests carried out with pure pumice and Nickel-contaminated synthetic solutions. It should be considered that Pumice has been tested in column tests as pure reactive material too [START_REF] Moraci | Heavy metals removal and hydraulic performance in zero-valent iron/pumice permeable reactive barriers[END_REF]. The results collected by this test were considered and analyzed together with those of batch tests to have more information and in order to be able to hypothesize the possible Nickel removal mechanisms developed by using pumice only. observable that the main nickel removal by pumice is immediate and it reaches a certain percentage of initial concentration, after which pumice is almost not able to remove further quantities anymore, if not really slowly. In addition, the nickel mass quantity removed by pumice and the removal rate in pure Pumice tests depend on the solid : liquid ratio and the initial concentration. Looking at the same concentration, it seems that removed nickel mass and removal rate increase with the solid : liquid ratio. Considering the same solid : liquid ratio, the values of contaminant concentration are proportional to the nickel initial. In each test, a minimum concentration, called Cmin , is reached and its value depends on the initial concentration C0. Table 3-

Ni removal efficiency using pure Pumice

Heavy metals removal by pumice can depend on the surface functional groups and on high internal porosity. Iron and aluminium oxides are very effective for heavy metal removal due to their high specific surface areas and reactive surface functional groups (Potter, 1999). The charge on these hydroxyl surface functional groups depends on pH value; at low pH, protonation favors the formation of positively charged groups while at high pH de-protonation leads to the formation of negative groups [START_REF] Hohl | Characterisation of surface chemical properties of oxides in natural waters[END_REF]. The pH at which the net surface charge is zero is termed the zero point of charge or pHzpc [START_REF] Parks | The zero point of charge of oxides[END_REF]. Oxide minerals containing silica have a pHzPc of about pH 2, Fe oxides about pH 7-8 and oxides around pH 9 (Kinniburgh et al., 1976;[START_REF] Schulthess | Adsorption of heavy metals by silicon and aluminium oxide surfaces on clay minenls[END_REF]. At pH value greater than pHzpc, heavy metal cations will be tight to the reactive material surface through electrostatic attraction. Thanks to the chemical affinity of same surface groups for certain heavy metals, surface complexes formation is possible. The adsorption proceeds through the diffusion consisting of transfer of adsorbate from solution to adsorbent surface, the migration of adsorbate into pores and interaction with available sites on the interior surface of pores. The factors influencing the adsorption are surface area of adsorbent, contact time or residence time, particle size of adsorbent, solubility of substances, affinity of the solute for the adsorbent, size of the molecule with respect to the size of the pores, degree of ionization of the adsorbate molecule, pH and initial concentration.

Batch test model

To develop a model to simulate the nickel removal in batch tests carried out with pure Pumice, two main approaches can be followed: one is an approach to representative elementary volume, while the second is a particle approach. In this chapter, both are developed, analyzing the differences.

A conceptual model representing the real test conditions is considered. In a certain volume, a fixed ratio of granular porous reactive mass and contaminant solution are put in contact. At the beginning, all of the mass of nickel is in solution at a certain initial concentration, defined as C0, and all the reactive sites on the surface of the reactive material are available. Overtime, the concentration of nickel in solution, called CNi, decreases, while the nickel molecules are sorbed on the reactive material surface sites. Sorption terms include different phenomena: adsorption, chemisorption, absorption and ion exchange. Adsorption consists of solute clinging to the solid surface. Solute incorporation on a sediment, soil, rock surface by chemical reactions is defined as chemisorption. Absorption is a phenomenon that happens in porous materials, when the solute diffuses into the particle and is adsorbed onto the interior surface. Ion exchange occurs when a cation is exchanged with another of the same sign present on the solid surface [START_REF] Fetter | Contaminant Hydrogeology[END_REF][START_REF] Molfetta | Ingegneria degli acquiferi[END_REF].

Reversible linear kinetic sorption model

It can be hypothesized that the nickel removal processes on Pumice particles are sorption-based. This hypothesis can be validated by considering the chemical composition and the structure of Pumice. Hypothesizing that the rate of solute sorption is related to the amount that has already been sorbed, it is possible to consider a reversible sorption model. The model of batch tests is finalized to model batch tests and can be used for column tests. So it is important to consider the relation between the considered removal mechanism rate related to others possible and its rate in comparison to the flow rate in the column.

Of course, the term kinetic is considered by [START_REF] Fetter | Contaminant Hydrogeology[END_REF] in view of the insertion of the representing equation in the advection-dispersion mono-dimensional equation.

So it is necessary to compare the removal mechanism rate with the flow rate to choose the best model able to represent the relative rate.

A mechanism is defined as kinetic in that context as the opposite the of equilibrium process. The equilibrium models are based on the assumption that the rate of change in concentration due to sorption mechanisms is much greater than the change due to other processes and that the flow rate is low enough to make equilibrium achievement possible.

The phenomenon can be hypothesized as kinetic and reversible, considering that the sorption rate can be dependent on the already sorbed nickel, as can be observed in the experimental results, and the reaction time is comparable to the residence time in the column test, function of the flow rate. The first approach to model pure pumice batch tests is based on the equations 3-1 and 3-2:

𝜕𝐶 𝑁𝑖 𝜕𝑡 + 𝜕𝐶 𝑁𝑖 * 𝜕𝑡 = 0 (3 -1) 𝜕𝐶 𝑁𝑖 * 𝜕𝑡 = 𝑘 2 𝐶 𝑁𝑖 -𝑘 3 𝐶 𝑁𝑖 * (3 -2)
where CNi and CNi * are the present nickel concentration and the removed nickel mass divided by the solution volume respectively. The coefficients k2 and k3 can be easily found through batch tests experimental data. This approach can probably work also for batch tests with Lapillus, having a higher sorption capacity than Pumice.

Sorption and diffusion model

The second approach can be developed considering the pumice as an assembly of solid particles with a multi-scale structure. Therefore, the pumice is described as a spherical or cylindrical material of diameter Dp with cylindrical micro-pores of diameter dp and length Dp (Figure 3-3).

When the material is placed in the solution, a diffusion mechanism due to the concentration gradient inside the micro pores takes place and the transport of the dissolved substance ceases when the concentration in the micro-pores is equalized. In this approach, in addition to the sorption mechanism related to the external surface of the particles, an internal sorption can be considered or not.

𝑚 𝑁𝑖 = (𝑚 𝑁𝑖 ) 𝑒 + (𝑚 𝑁𝑖 ) 𝑒 𝑠 +(𝑚 𝑁𝑖 ) 𝑖 (3 -3)
Considering that identical phenomena occur in all inter-pores, the definition of concentration is introduced in each scale by:

𝐶 𝑒 = (𝑚 𝑁𝑖 ) 𝑒 𝑉𝑜𝑙 𝑠𝑜𝑙 (3 -4) 𝑐 𝑖 = (𝑚 𝑁𝑖 ) 𝑖 𝑛 • 𝑉 1𝑝𝑜𝑟𝑒 (3 -5)
where 𝐶 𝑒 is the concentration of nickel in external solution, 𝑐 𝑖 is the concentration of nickel in solution contained in the internal pores volume, 𝑉𝑜𝑙 𝑠𝑜𝑙 is the volume of solution and 𝑉 1𝑝𝑜𝑟𝑒 is the volume of one internal pore of pumice particle, corresponding to the one pore internal solution volume under hypothesis of totally saturation. 𝑛 is the total number of internal pores defined as the number of pumice particles 𝜂 times the average number of internal pores of each particle 𝜃 (equation 3-6).

𝑛 = 𝜂 • 𝜃 (3 -6)

As the experimental results have shown, the mechanisms involved depend not only on the concentration of the species but also on the mass of the reactive material; for example mechanisms where the surface interactions take place. Therefore, it will be useful to introduce normalized quantities with respect to the mass of pumice and unit surface for example. This will facilitate the generalization of the model to mixtures too.

Let us introduce a model parameter defined by the following equation 3-7:

𝑀 𝑝 = 𝑚 𝑝 𝑉𝑜𝑙 𝑠𝑜𝑙 (3 -7)
𝑀 𝑝 is a parameter equal to the ratio between the pumice mass 𝑚 𝑝 and the volume of solution 𝑉𝑜𝑙 𝑠𝑜𝑙 . The pumice mass is equal to the sum of the mass of each particle or to the average mass of particles 𝑚 𝑝 𝑖 multiplied by the number of pumice particles 𝜂 (equation 3-8).

𝑚 𝑝 = ∑ 𝑚 𝑝 𝑖 = 𝜂 • 𝑚 𝑝 𝑖 (3 -8)
Considering that the 𝑛 number of particles of pumice can be deduced using an average diameter 𝐷 𝑝 equal to 𝑑 50 , it can be calculated hypothesizing a spherical geometry (equation 3-9) or a cylindrical geometry (equation 3-10):

Spherical geometry:

𝑛 = 𝑚 𝑝 𝜌 𝑎𝑝𝑝 4 3 𝜋 ( 𝑑 50 2 ) 3 (3 -9)
Cylindrical geometry:

𝑛 = 𝑚 𝑝 𝜌 𝑎𝑝𝑝 𝜋 4 (𝑑 50 ) 3 (3 -10)
These relations show that there is a difference of 30% in the number of pores simply due to the assumption on the form of the grains.

Sorption model

Considering the adsorbed nickel mass, it is possible to introduce the definition of 𝑆 * that is equal to the (𝑚 𝑁𝑖 ) 𝑠 mass of nickel adsorbed divided by the 𝑚 𝑝 pumice mass (equation 3-11).

𝑆 * = (𝑚 𝑁𝑖 ) 𝑠 𝑚 𝑝 (3 -11)
To quantify 𝑆 * Langmuir (1915, 1918) equilibrium sorption isotherm can be used as given in equation 3-12:

𝑆 * = 𝛼𝛽 0 𝐶 1 + 𝛼𝐶 (3 -12)
Where (𝐶 𝑠 ) 𝑒 = 𝛼𝛽 0 𝐶 0 1 + 𝛼𝐶 0 𝑀 𝑝 (3 -14) where:

(𝐶 𝑠 ) 𝑒 = (𝑚 𝑁𝑖 ) 𝑠 𝑉𝑜𝑙 𝑠𝑜𝑙 (3 -15)
The nickel concentration in the solution after this instantaneous sorption on the external surfaces becomes:

𝐶 𝑒 = (𝐶 𝑒 ) 0 -(𝐶 𝑠 ) 𝑒 = 𝐶 0 -(𝐶 𝑠 ) 𝑒 (3 -16)
To develop a model that can take into account the sorption on internal surface it is necessary to introduce the model parameters as function of unit surface. The Langmuir equilibrium sorption isotherm is therefore represented by the following equation:

𝑆 * = 𝛼𝛽 𝑆 0 𝑆 𝑠𝑢𝑟 𝐶 0 1 + 𝛼𝐶 0 (3 -17)
where 𝑆 𝑠𝑢𝑟 is the specific surface per unit mass of absorbent and 𝛽 𝑆 0 is defined by the equation 3-18:

𝛽 𝑆 0 = 𝛽 0 (𝑆 𝑠𝑢𝑟 ) 𝑒 (3 -18)
If the sorption isotherm for the external surface is aimed, it designates the external surface 𝐴 𝑒𝑥𝑡 per unit mass, referred to (𝑆 𝑠𝑢𝑟 ) 𝑒 , given by equation 3-19.

(𝑆 𝑠𝑢𝑟 ) 𝑒 = 𝐴 𝑒𝑥𝑡 𝜌 𝑎𝑝𝑝 𝑉 𝑒𝑥𝑡 (3 -19)
Hypothesizing a spherical geometry, the external surface 𝐴 𝑒𝑥𝑡 , the external volume 𝑉 𝑒𝑥𝑡 and the specific external surface per unit mass (𝑆 𝑠𝑢𝑟 ) 𝑒 of pumice are given by equations 3-20, 3-21 and 3-22 respectively.

𝐴 𝑒𝑥𝑡 = 𝜋(𝐷 𝑝 ) 2 (3 -20) 𝑉 𝑒𝑥𝑡 = 𝜋 6 (𝐷 𝑝 ) 3 (3 -21) (𝑆 𝑠𝑢𝑟 ) 𝑒 = 6 𝜌 𝑎𝑝𝑝 𝐷 𝑝 (3 -22)
For cylindrical geometry, they can be calculated using equations 3-23, 3-24 and 3-25.

𝐴 𝑒𝑥𝑡 = 3𝜋 2 (𝐷 𝑝 ) 2 (3 -23) 𝑉 𝑒𝑥𝑡 = 𝜋 4 (𝐷 𝑝 ) 3 (3 -24) (𝑆 𝑠𝑢𝑟 ) 𝑒 = 6 𝜌 𝑎𝑝𝑝 𝐷 𝑝 (3 -25)
It should be noted that the specific surface is independent of the mass or the number of pumice grains in the solution.

To convert the (𝑆 * ) 𝑒 quantity in the correspondent concentration, meaning the mass of nickel removed from the external solution volume by external surface of pumice per unit of external solution volume (𝐶 𝑠 ) 𝑒 , it is sufficient to multiply it by 𝑀 𝑝 (equation 3-26).

(𝐶 𝑠 ) 𝑒 = (𝑆 * ) 𝑒 𝑀 𝑝 = 𝛼 𝛽 𝑆 0 (𝑆 𝑠𝑢𝑟 ) 𝑒 𝐶 0 1 + 𝛼𝐶 0 𝑀 𝑝 (3 -26)
To evaluate the sorption on the surfaces of internal pores, the surface of those pores for a unit mass of pumice designated by (𝑠 𝑠𝑢𝑟 ) 𝑖 should be considered. Therefore:

(𝑠 𝑠𝑢𝑟 ) 𝑖 = 𝐴 1𝑝𝑜𝑟𝑒 𝜌 𝑎𝑝𝑝 𝑉 𝑒𝑥𝑡 (3 -27)
If 𝐴 1𝑝𝑜𝑟𝑒 designates the internal surface of one pore, it can be calculated by equation 3-28:

𝐴 1𝑝𝑜𝑟𝑒 = 𝜋𝑑 𝑝 𝐷 𝑝 (3 -28)
Hypothesizing a spherical geometry, the internal specific surface of one pore per unit mass of one pumice particle is defined by equation 3-29.

(𝑆 𝑠𝑢𝑟 ) 𝑖 = 6 𝑑 𝑝 𝜌 𝑎𝑝𝑝 (𝐷 𝑝 ) 2 (3 -29)
For cylindrical geometry, equation 3-30 can be used.

(𝑆 𝑠𝑢𝑟 ) 𝑖 = 4𝑑 𝑝 𝜌 𝑎𝑝𝑝 (𝐷 𝑝 ) 2 (3 -30)
Therefore, the mass of nickel absorbed by internal pores surface per unit mass of pumice (𝑐 * ) 𝑖 is defined by equation 3-31:

(𝑐 * ) 𝑖 = 𝛼 𝛽 𝑆 0 (𝑆 𝑠𝑢𝑟 ) 𝑖 𝑐 𝑖 1 + 𝛼𝑐 𝑖 = 𝛼 𝛽 0 𝐴 1𝑝𝑜𝑟𝑒 𝐴 𝑒𝑥𝑡 𝑐 𝑖 1 + 𝛼𝑐 𝑖 (3 -31)
The corresponding concentration (𝑐 𝑠 ) 𝑖 , meaning the mass of nickel absorbed by internal pore surface of one pore per unit volume of one pore, is given by equation 3-32.

(𝑐 𝑠 ) 𝑖 = 𝛼 𝛽 𝑆 0 (𝑆 𝑠𝑢𝑟 ) 𝑖 𝑐 𝑖 1 + 𝛼𝑐 𝑖 𝑚 𝑝 𝑖 𝑉 1𝑝𝑜𝑟𝑒 = 𝛼 𝛽 𝑆 0 𝐴 1𝑝𝑜𝑟𝑒 𝑉 1𝑝𝑜𝑟𝑒 𝑐 𝑖 1 + 𝛼𝑐 𝑖 (3 -32)
where the volume of one pore 𝑉 1𝑝𝑜𝑟𝑒 is defined by equation 3-33.

𝑉 1𝑝𝑜𝑟𝑒 = 𝜋 4 (𝑑 𝑝 ) 2 𝐷 𝑝 (3 -33)
The internal surface and the volume of one pore ratio is given by equation 3-34.

𝐴 1𝑝𝑜𝑟𝑒 𝑉 1𝑝𝑜𝑟𝑒 = 4 𝑑 𝑝 (3 -34)
Although the sorption term on one internal pore seems to be small, it could become non negligible according to the number of pores per grain.

Internal diffusion model and external sorption

The variation of nickel concentration in space and time in the internal pores volume of pumice neglecting the sorption at internal pore scale is described by the Fick's lwas on molecular diffusion [START_REF] Fick | On liquid diffusion[END_REF] 3-35:

𝜕𝑐 𝑖 𝜕𝑡 -𝐷 𝜕 2 𝑐 𝑖 𝜕𝑥 2 = 0 (3 -35)
Where 𝐷 is the diffusion coefficient in one internal pore and 𝑐 𝑖 is the concentration of Ni at the pore scale where the diffusion takes place [START_REF] Crank | The Mathematics of Diffusion[END_REF].

The boundary condition of this 1D problem is imposed by the concentration in the external solution. Therefore:

𝑐 𝑖 (𝑥 = 0, 𝑡) = 𝑐 𝑖 (𝑥 = 𝐷 𝑝 , 𝑡) = 𝐶 𝑒 (𝑡) (3 -36)
Where

𝐶 𝑒 = 𝐶 0 -(𝐶 𝑠 ) 𝑒 - (𝑚 𝑁𝑖 ) 𝑖 𝑉𝑜𝑙 𝑠𝑜𝑙 (3 -37)
(𝐶 𝑠 ) 𝑒 is nickel mass adsorbed by pumice on its external surfaces per unit of external solution volume. The mass of nickel internally diffused (𝑚 𝑁𝑖 ) 𝑖 is equal to:

(𝑚 𝑁𝑖 ) 𝑖 = 𝑉 1𝑝𝑜𝑟𝑒 ∫ 𝑐 𝑖 𝑑𝑥 𝐷 𝑝 𝑛 (3 -38)
Where 𝑐 𝑖 is the solution of equation (3-35).

Therefore at each instant the concentration in the solution can be evaluated using equation 3-39.

𝐶 𝑒 = 𝐶 0 -(𝐶 𝑠 ) 𝑒 -𝐶 𝑑 (3 -39)
Where the concentration 𝐶 𝑑 is defined by equation 3-40.

𝐶 𝑑 = (𝑚 𝑁𝑖 ) 𝑖 𝑉𝑜𝑙 𝑠𝑜𝑙 = 𝑉 1𝑝𝑜𝑟𝑒 . 𝑛. ∫ 𝑐 𝑖 𝑑𝑥 𝑉𝑜𝑙 𝑠𝑜𝑙 . 𝐷 𝑝 (3 -40)
On the basis of analysis data of Mercury Intrusion Porosimetry (MIP) tests carried out on pumice samples, the number of pores contained in unit mass of pumice 𝑛 𝑝𝑜𝑟𝑒 can be deduced:

𝑛 𝑝𝑜𝑟𝑒 = 𝑛 𝑚 𝑝 (3 -41)
Multiplying it for the mass of pumice per unit volume of external solution 𝑀 𝑝 , the number of pores per one solution volume 𝑁 is obtained:

𝑁 = 𝑛 𝑉𝑜𝑙 𝑠𝑜𝑙 = 𝑛 𝑝𝑜𝑟𝑒 𝑀 𝑝 (3 -42)
Considering that the 𝑉 1𝑝𝑜𝑟𝑒 volume of one pore can be considered equal to the 𝐷 𝑝 length of the grain times the transversal section 𝐴 𝑚 , the concentration of nickel diffused into the pores can be calculated by the following equation:

𝐶 𝑑 = 𝑁𝐴 𝑚 ∫ 𝑐 𝑖 𝑑𝑥 (3 -43)
The number of pores per unit mass of pumice 𝑛 𝑝𝑜𝑟𝑒 can be calculated using the

Multiscale sorption model with internal diffusion

In the case where sorption can take place on the internal surfaces, the mass conservation equation in one pore becomes:

𝜕𝑐 𝑖 𝑠 𝜕𝑡 -𝐷 𝜕 2 𝑐 𝑖 𝑠 𝜕𝑥 2 + 𝜕(𝑆) 𝑖 𝜕𝑡 = 0 (3 -47)
Where 𝑐 𝑖 𝑠 designates the concentration in the pore in presence of sorption. It varies with time due to diffusion mechanism into internal pores of pumice particles and the sorption on internal surfaces. Introducing equation (3-32) into the above equation, we obtain:

(1 + 𝛼 𝛽 𝑆 0 𝐴 1𝑝𝑜𝑟𝑒 𝑉 1𝑝𝑜𝑟𝑒 (1 + 𝛼𝑐 𝑖 𝑠 ) 2 ) 𝜕𝑐 𝑖 𝑠 𝜕𝑡 -𝐷 𝜕 2 𝑐 𝑖 𝑠 𝜕𝑥 2 = 0 (3 -48)
The boundary conditions remain unchanged with respect to the case without internal sorption, though the value of 𝐶 𝑒 has implicitly changed now as the diffusion process is retarded:

𝑐 𝑖 𝑠 (𝑥 = 0, 𝑡) = 𝑐 𝑖 𝑠 (𝑥 = 𝐷 𝑝 , 𝑡) = 𝐶 𝑒 (𝑡) (3 -49)
where the nickel concentration in external solution volume 𝐶 𝑒 depends on the nickel mass adsorbed by external surface per unit of external solution volume (𝐶 𝑠 ) 𝑒 and the mass diffused in the internal pores and absorbed by internal pores surface to pores per unit of external solution volume 𝐶 𝑑 𝑠 (equation 3-50).

𝐶 𝑒 = 𝐶 0 -(𝐶 𝑠 ) 𝑒 -𝐶 𝑑 𝑠 (3 -50)
𝐶 𝑑 𝑠 can be calculated by the following equation:

𝐶 𝑑 𝑠 = 𝑁𝐴 𝑚 ∫ 𝑐 𝑖 𝑠 𝑑𝑥 (3 -51)
Same considerations can be done to apply that model to nickel removal efficiency using pure Lapillus. Of course having different structure and chemical composition the relative weight of the two main removal mechanisms, sorption and diffusion, will be different.

Model implementation

The model has been implemented using the values reported in The variables used were:

𝐶 𝑠 = 𝛼𝛽 0 𝐶 0 1 + 𝛼𝐶 0 𝑀 𝑝 (3 -52) 𝐶 𝑒 = 𝐶 0 -𝐶 𝑠 -𝑁 • 𝑆 𝑚 ∫ 𝑐 𝑖 𝑑𝑥 (3 -53) 𝐷 𝑝 0
The geometry was defined as a one dimension domain having three points (x=0, x=d/2, x=d), introducing d equal to the d50 of studied pumice.

The initial condition was set as follows:

𝑐 𝑖 = 0[ 𝑚𝑔 𝐿 ] (3 -54)
The Dirichlet Boundary Condition were set as follows:

𝑐 𝑖 (𝑥 = 0) = 𝑐 𝑖 (𝑥 = 𝑑) = 𝐶 𝑒 (3 -55)

Coefficients evaluation

The used values for number of pores, average pores sections and Langmuir coefficients were evaluated as will be explained.

Number of pores and average pores sections

The numbers of pores and average pore section obtained as defined in the model pumice description are calculated using MIP data and considering that the maximum pore diameter is about 113μm, the second is relative to the maximum pore diameter equal to 138 μm.

Langmuir coefficients evaluation

The evaluation of Langmuir coefficients is based on the analysis of batch tests results and experimental conditions. The latter are summarized in Observing the experimental data, it is possible to make some hypotheses about removal mechanisms. considered to be able to model nickel sorption behavior on pumice is the Langmuir Sorption Isotherm. That relationship is described in the following equation:

𝐶 * = 𝛼𝛽𝐶 1 + 𝛼𝐶 (3 -56)
where C* is the mass of solute sorbed per dry unit weight of solid, expressed in (mg/Kg), α is an absorption constant related to the binding energy, expressed in (L/mg), β is the maximum amount of solute that can be absorbed by the solid, expressed in (mg/Kg) and C is the concentration of solute in solution.

To define the appropriate values of α and β coefficients, it is possible to consider and analyze different hypotheses, in order to establish the best one that allows the better representation of experimental results.

Since sorption is an equilibrium and instantaneous process, it is necessary to find an isotherm relationship using the residual concentration values measured at the first sampling time. To evaluate α and β coefficients for Langmuir relationship, nickel removed mass over reactive mass ratio for each solid to liquid ratio and initial concentration should be calculated and reported in a graph. The objective is in fact to find one α coefficient and one β coefficient valid for sorption mechanism between solution of nickel and pumice. The above-described model provides for the variation and influence of solid to liquid ratio, introducing a representative parameter, called 𝑀 𝑝 , as follows:

𝐶 𝑠 = 𝛼𝛽 0 𝐶 0 1 + 𝛼𝐶 0 𝑀 𝑝 (3 -57)
𝑀 𝑝 is equal to the solid to liquid ratio and useful to convert the 𝐶 * quantity in the correspondent concentration, meaning the nickel removed mass from the solution volume by external surface of pumice, called 𝐶 𝑠 .

Three hypothesis are the most immediate for α and β coefficients evaluation. The first is to find them graphically using the first time sampling C* values for all the tests (or without 500 mg/L initial concentration test) and in that way it is necessary to consider the error due to the different first time sampling: for LM, it is 2 hours for 5 mg/L of initial concentration and 120 hours for 50 mg/L of initial concentration; for HM, it is half an hour for all tests. The second one is to set the maximum amount of solute that can be absorbed by the solid to define β first. The third one is to find the best values of α and β to be able to better interpolate all the experimental tests (considering or not the value of 500 mg/L initial concentration test).

To evaluate the coefficients 𝛼 and 𝛽 0 , batch test results analysis was carried out. A maximum value of nickel mass removed to pumice mass ratio is hypothesized considering and comparing the results of batch tests with different solid to liquid ratio equal to 1mg:10 ml and 1mg:4ml. Let us consider the Langmuir coefficients as introduced in the model. The definition regarding them needs to be introduced. In particular, the α value used is defined as α0 value, equal for all the tests carried out with solution of nickel and pumice and expressed in (L/mg) multiplied by 𝑀 𝑝 solid : liquid ratio (g/mL). β introduced in the model is 𝛽 0 that as defined above represent the maximum amount of solute that can be absorbed by the solid and this is expressed in (mg/Kg).

The calculated values to be used for definition by graphic methods are reported in the following table: The Figures 345678) and b) and 3-9 report the data as it should be plotted to find the Langmuir coefficients for the LM tests and for HM tests considering two or three initial concentrations.

Sensitivity analysis

A global sensitivity analysis of model output was carried out to study the global importance of each input factor of the model to the output of the model. The method utilized was based on the Hybrid Random Balance Design -Fourier Amplitude Sensitivity Test (Hybrid RDB-FAST or HRF) [START_REF] Saltelli | A quantitative, model independent method for global sensitivity analysis of model output[END_REF][START_REF] Tarantola | Random balance designs for the estimation of first order global sensitivity indices[END_REF].

The value of coefficient of variance given as input for the analysis was for each input factor equal to 0.5. For Dm, d=Dp, β0, α0, Npores_g, Sm (Am) the average value used is the same as the model input.

In Equation 3-58 represents the 1D mass balance equation [START_REF] Bear | Hydraulics of ground water[END_REF]. for Ni in its general form applied at the external pore scale: It is possible to identify a zone where the residual concentration rate decreases towards a steady-state zone which implies:

𝜕𝐶 𝜕𝑡 + ∑ 𝑅 ̇𝑘 𝑘 = 0 (3 -59)
Considering that only one retention mechanism is active at this stage, it is possible to introduce the equation 3-60:

𝑅 ̇2 = -𝛼(𝐶 -𝐶 𝑒 ) (3 -60)
where Ce is the equilibrium concentration which may change during the process. For Ce evaluation it is possible to refer to the observation about batch tests results where Ce=0.7C0 relation has been found. Of course, this variable is function of the water occupying the internal pore and of the external concentration, so it increases progressively.

Modelling of nickel removal efficiency using pure ZVI

To model the Nickel removal efficiency of pure ZVI observed in batch tests, a representative elementary volume approach was used. On the basis of the literature studies about heavy-metal removal using ZVI and the experimental results of batch tests and column tests performed with pure ZVI, a reversible kinetic sorption model was tested to simulate the experimental results.

Batch tests model

The batch conceptual model is the same used for batch tests based on pure pumice use, considering the variation of solid to liquid ratio and the nickel initial concentration. This approach to model the nickel removal efficiency observed in pure ZVI batch tests is based on the equations 3-1 and 3-2

The coefficients are easily deduced by experimental batch tests results analysis.

The results of simulation and the experimental data are reported in the figures 3-18, 3-19, 3-20 and 3-21. For the column tests realized with ZVI, the permeability variation is a really important issue. However, the introduction of that model consists of a particularization of the more general ZVI : Pumice mixtures column model.

Modelling of batch tests using ZVI : Pumice (or Lapillus) mixtures

The model of nickel removal efficiency of ZVI : Pumice (or Lapillus) mixtures is based on the combination of the two models simulating the nickel removal efficiency using pure ZVI and pure Pumice, introducing each effect as a proportion of the considered weight ratio. In this case, two different possibilities can be chosen. It is possible to simulate that kind of batch test using for Pumice the reversible kinetic sorption model or the sorption-internal diffusion model. To introduce the ZVI influence on nickel removal efficiency the model based on reversible kinetic sorption was used, as for pure ZVI tests.

Conclusions

In this chapter, nickel contaminant removal efficiency observed in batch tests using pure Pumice, pure ZVI and ZVI:Pumice were considered and modelled.

To simulate the nickel removal in batch tests carried out with pure Pumice, two main approaches were followed: one is an approach to representative elementary volume, while the second is a particle approach.

For the first one, a conceptual model consisted on considering in a certain volume, a fixed ratio of granular porous reactive mass to contaminant solution put in contact.

The nickel concentration at the beginning of test C0 decreased with time, while the nickel molecules were sorbed on the reactive material surface sites. A reversible linear kinetic sorption model was used, considering the sorption-based removal process as studied in the literature. The second approach was developed considering the pumice as an assembly of solid particles with a multi-scale structure. Therefore, the pumice is described as a spherical or cylindrical material of diameter Dp with cylindrical micro-pores of diameter dp and length Dp.

When the material is placed in the solution, a diffusion mechanism due to the concentration gradient inside the micro pores takes place and the transport of the dissolved substance ceases when the concentration in the micro-pores is equalized. In this approach, in addition to the sorption mechanism related to the external surface of the particles, an internal sorption can be considered or not.

To model the nickel removal efficiency of pure ZVI observed in batch tests, a representative elementary volume approach was used. Considering the literature studies on heavy-metal removal using ZVI and the experimental results of batch tests and column tests realized with pure ZVI, a reversible kinetic sorption model was tested to simulate the experimental results.

The model based on reversible linear kinetic sorption seemed to be able to represent more appropriately the experimental results obtained from tests on pure ZVI than those on pure pumice. These letters are better modelled considering external sorption and internal diffusion.

A first step of modelling the mixtures was performed and it should be study in depth.

However, it seemed that a simple combination or addition of the models developed for each pure reactive material is not appropriate to simulate the experimental results obtained from tests on mixtures. If it is true, it could mean that the effects of removal processes developed by each reactive material of the mixture are not overlapping.

It was In Chapter 4, the results of some column tests performed using ZVI:Pumice 4 Long-term removal efficiency and hydraulic behaviour of ZVI:Pumice granular mixtures: general review and modelling

Introduction

In this chapter, an analysis of the results of column tests using ZVI and ZVI:Pumice mixtures to remediate heavy-metal contaminated groundwater, carried out by the previous Ph.D. students [START_REF] Rigano | Studio dell'efficienza di barriere permeabili reattive per la bonifica di acquiferi contaminati da metalli pesanti[END_REF]Suraci, 2010;Bilardi, 2011), is proposed in order to collect useful information for the development of a coupled removalhydraulic model. In fact, as studied in Chapter 3, a non-equilibrium sorption model can be used to simulate batch tests performed using pure ZVI and pure Pumice.

The dissertation is focused on the tests carried out using synthetic solutions contaminated by nickel, because for this element the availability of column tests and, consequently, of studies on the influence control variables is greater.

𝑀 𝑟 𝑗-𝑗 +1 = 𝑀 𝑖𝑛 𝑗-𝑗 +1 -𝑀 𝑜𝑢𝑡 𝑗-𝑗 +1 (4-1)
where the 𝑀 𝑟 is the retained mass between times j and j+1, 𝑀 𝑖𝑛 is the mass input through 𝑥 𝑖 section between times j and j+1 and 𝑀 𝑜𝑢𝑡 is the mass in output from the 𝑥 𝑖+1 section between times j and j+1. The equation (4-1) is more appropriate as the considered time is longer, because the possible contaminant mass in solution becomes negligible compared to that retained or removed by the reactive material.

The nickel mass flowing through the transversal section 𝑥 𝑖 during a ∆𝑡 𝑗 interval can be calculated by the following equation (4-2):

𝑀 𝑥 𝑖 ,∆𝑡 𝑗 = (𝐶 𝑥 𝑖, 𝑡 𝑗 +𝐶 𝑥 𝑖, 𝑡 𝑗-1 ) 2 • (𝑡 𝑗 -𝑡 𝑗-1 ) • 𝑣 • 𝑆 𝑥 𝑖 (4-2)
where Cxi, tj is the concentration measured at xi, the i -th point along the filter length and tj, the j -th time, v is the flow velocity and Sxi the transversal section corresponding to the i -th point along the filter length. The nickel mass retained by the reactive material contained in ∆𝑥 𝑖 column sector during a ∆𝑡 𝑗 interval can be calculated through the following equation (4-3):

𝑀 𝑟,∆𝑥 𝑖 ,∆𝑡 𝑗 = 𝑀 𝑥 𝑖-1 ,∆𝑡 𝑗 -𝑀 𝑥 𝑖 ,∆𝑡 𝑗 (4-3)

The cumulative nickel mass retained by the ∆𝑥 𝑖 column sector until 𝑡 𝑗 time can be calculated as the following equation (4-4):

𝑀 𝑟,∆𝑥 𝑖 ,𝑡 𝑗 = ∑ 𝑀 𝑟,∆𝑥 𝑖 ,∆𝑡 𝑗 𝑗 (4-4)

The total mass retained by the entire filter length during time is given by equation (4-5):

𝑀 𝑟,𝑡 𝑗 = ∑ ∑ 𝑀 𝑟,∆𝑥 𝑖 ,∆𝑡 𝑗 𝑗 𝑛 𝑖 (4-5)
The mass of nickel retained by unit of reactive material mass is calculated by dividing it by the mass of reactive material contained in the correspondent filter length.

Pure ZVI column tests

Column tests using pure ZVI and carried out by the previous Ph.D. students (Rigano, 2008;Suraci, 2010;Bilardi, 2011)

𝜑 = 1 - 𝑊 𝑍𝑉𝐼 𝜌 𝑍𝑉𝐼 𝑉 𝑖𝑐 (4-6)
where the 𝑊 𝑍𝑉𝐼 is the mass of Zero Valent Iron, 𝜌 𝑍𝑉𝐼 is the ZVI density and 𝑉 𝑖𝑐 is the internal volume of the column. The initial value of residence time was calculated by using the following equation (4-7):

T res = PV v•S (4-7)
where v is the flow velocity, S is the transversal section and PV is the initial value of Pore Volume calculated through equation (4-8): (1) Suraci ( 2010), ( 2 In fact, the evolution of the nickel removed mass as function of nickel input mass in the entire length of the column test performed with the highest velocity, and that removed in 3 cm of the slowest are comparable, as can be observed in Figure 4-4 a)

𝑃𝑉 = 𝜑 • 𝑉 𝑖𝑐 (4-8)
(3 cm is the shortest filter length for which data are available for the slowest test).

Furthermore the greater part of the removal capacity is performed by the first 3 centimetres in the slowest test, consequently if the total removed mass is calculated, the trend of removed mass at outlet is overlapped with that of three centimetres.

In term of hydraulic behaviour, a difference can be observed. Permeability evolution is reported as function of the nickel removed mass in Figure 4-4 b). A light permeability loss can be observed for the fastest, while no significant changes are observed for the slowest test until around to 1000 mg of nickel mass removed. After this, the permeability evolution slope is higher for the slowest test than that of the fastest one. As can be deduced from observation of Figure 4-1, the contact time for nickel removal seems to be an important parameter, considering that in the slowest test almost all the input mass is removed within 3 centimetres, while for the fastest test the total filter length is not enough to remove entirely the input mass, after a certain time.

ZVI-Pumice column tests

In Table 4-2 the characteristics of column tests performed using ZVI:Pumice mixtures to investigate their long-term nickel removal efficiency and hydraulic behaviour are reported. ZVI:Pumice weight ratio, flow velocity and nickel(Ni) initial concentration ere varied to study their influence. They were carried out by the previous Ph.D.

students [START_REF] Rigano | Studio dell'efficienza di barriere permeabili reattive per la bonifica di acquiferi contaminati da metalli pesanti[END_REF]Suraci,2011;Bilardi,2012).

Consideration about weight ratio influence

Three column tests were carried out by the previous Ph.D. student, Paolo Suraci, to investigate the influence of the weight ratio on the nickel removal efficiency of ZVI-Pumice mixtures [START_REF] Calabrò | Estimate of the optimum weight ratio in Zero-Valent Iron/Pumice granular mixtures used in permeable reactive barriers for the remediation of nickel contaminated groundwater[END_REF]. Results are here re-proposed and reviewed to finalize the observations and analysis to develop the results modelling step. In [START_REF] Moraci | Heavy metals removal and hydraulic performance in zero-valent iron/pumice permeable reactive barriers[END_REF] studied the removal efficiency of the three considered weight ratios. As can be observed, the 50:50 weight ratio seems to have the best behaviour in terms of removal capacity and this is due to the different ZVI content: 153 gr in 1 metre of filter made of 10:90 w.r., 244 gr and 530 gr in 50 cm of 30:70 w.r. and 50:50 w.r. respectively. Considering the evolution of the relative concentration, two main consideratios can be made. Firstly, using 10:90 and 30:70 weight ratios, the considered column lengths, 58 cm for 10:90 w.r. and 50 cm for 30:70 w.r. , are almost sufficient efficiently to reduce the nickel concentration until 120 hours after the interaction test start. In particular, 0.04 and 0.02 are the relative concentrations measured in that point. Using 50:50 weight ratio, 28 cm are enough to reduce the relative concentration to arount to the same above-mentioned value after 120 h.

The second consideration regards the general form of distribution of the experimental data. In all three tests, a different removal efficiency can be observed between the reactive materials placed in the first centimetres from the bottom of the column and that placed afterwards. This is more evident for the test containing a greater ZVI quantity. In fact, at the first sampling time, or rather 120 h, the relative concentration is 0.73, 0.67 and 0.56 in 10:90 w.r., 30:70 w.r. and 50:50 w.r. tests respectively. This value tends to increase more rapidly as lesser is the ZVI content.

However, after 1656 hours of interaction, the relative concentration at 1.5 centimetres is about 0.93, 0.88, 0.8 for 10:90 w.r., 30:70 w.r. and 50:50 w. respectively, while at the end of the tests, that is, after 3816 hours, the relative concentration at 1.5 centimetres is about 0.99, 0.97, 0.89 for 10:90 w.r., 30:70 w.r. The observed difference in terms of removal efficiency between the first 1.5 or 3 centimetres and the rest of the column can be researched for different reasons, e.g.

the deactivation of ZVI because of the high content in silice of Pumice [START_REF] Moraci | Heavy metals removal and hydraulic performance in zero-valent iron/pumice permeable reactive barriers[END_REF], the competition of nickel removal mechanisms with others that need more contact time than that ensured in the first centimetres in the experimental conditions used, the ZVI corrosion that can be more favoured in the second part of the column than the first and that implies a decrease in removal efficiency, a nonhomogeneous distribution of the two components of the mixtures due to the large difference in specific weight.

The nickel retained mass in each sector divided by the length of the sector will be introduced in Chapter 7.

Considerations about flow velocity and nickel initial concentration influences

In this section, some column tests performed varying the flow velocity and the nickel initial concentration are re-presented. In Here the data about concentratio are re-proposed in view of the modelling development, that is the objective of Chapter 6. In Figure 4 Different ZVI/Lapillus weight ratio mixtures were tested to study the long-term heavy-metal removal efficiency and the hydraulic performance. Four parameters were varied to study their influence on nickel removal and the hydraulic behaviour:

weight ratio, flow velocity, contaminant initial concentration and filter thickness.

Column tests at the same boundary conditions were carried out using zinc contaminated solutions. As for Copper contaminant the easily remediation observed by previous studies using pure ZVI and ZVI/Pumice mixtures was taken into account in the program drafting [START_REF] Moraci | Long-Term Efficiency of Zero-Valent Iron -Pumice Granular Mixtures for the Removal of Copper or nickel From Groundwater[END_REF]. Pluri-contaminated by nickel, zinc and copper was used to study the effect of simultaneous removal of the three contaminants on the long-term removal efficiency and hydraulic behaviour of ZVI/Lapillus mixtures.

Methodology

Different sets of column tests were planned in order to study the influence of some boundary and experimental conditions. These were performed following the same procedure with regards to column test assembling, performance monitoring, exhausted reactive media extraction, release tests and dismantling. The entire procedure, including the description of the experimental apparatus and the instruments used for performance monitoring will here be introduced.

Once the experimental data on contaminant concentration evolution with time and filter thickness and on hydraulic conductivity evolution with time were known, an attempt was made to observe, interpret and explain the long-term removal performance and hydraulic behaviour, looking also at physical and chemical parameters (pH, Eh and temperature). The scheme shown in Figure 5-1 summarizes the procedure followed for the interpretation of available experimental data for the i- distribution along the filter, the water flowed (f.w.)). This allows identification of the most efficient filter sector, that can vary with time, and the hydraulic conductivity evolution as function of the contaminant mass retained in the most efficient filter sector. Finally, the calculation of removal kinetics permits observation of the evolution of removal kinetics as a function of time in different filter thickness sectors.

Contribution of laboratory tests to PRB design

The contribution of laboratory tests to PRB design consists of simulating the flow of contaminant plume through the barrier. The column transversal section represents the elemental transversal section of the PRB, while its length corresponds to the PRB thickness. The inlet of the column test corresponds to the up-gradient side of the PRB, while the output represents the down-gradient side. This conceptual model of the column would be representative of the real removal process scheme if the same input contaminant mass to unit transversal area ratio were taken into account under the same boundary (C0 initial concentration) and experimental conditions (v flow velocity). Thus, a criterion proposed to reproduce the in-situ hydraulic conditions consists of choosing the experimental flow velocity and Peclet number as similar as possible to the real values [START_REF] Crittenden | Predicting GAC performance with rapid small-scale column tests[END_REF]. The values used in this research were chosen on the basis of the range of the flow velocity registered for a number of PRBs in situ varying in 0.015-5.7 m/d range [START_REF] Henderson | Long-term performance of zero-valent iron permeable reactive barriers: a critical review[END_REF].

On the geotechnical side, it would be an appropriate model if the geotechnical characteristics of the base soil were taken into account and the granular filter design criteria respected (Moraci et al, 2012a,b and[START_REF] Reardon | Capture and storage of hydrogen gas by zero-valent iron[END_REF].

The limits of a similar model regard the differences between the laboratory and real scales and physical-chemical initial condition and its evolution.

Experimental apparatus and procedure

Column tests were carried out filling a 50 cm (or 100 cm) high and 5 cm large column with reactive material, pure Lapillus or ZVI/Lapillus mixtures at different weight assess the permeability of the systems. To determine the hydraulic conductivity, the column test was interrupted and a tank or a burette, filled with the same contaminated solution used during the test, was connected to the column in order to perform the procedure. This latter lasted a limited time in order to re-establish the operational flow in the column, without causing an unacceptable disturbance to the system.

Experimental program and research objectives

The experimental program aimed at studying the long-term removal efficiency and hydraulic behaviour of new reactive media to be used in permeable reactive barrier Technology, is shown in Table 5- behaviour, and to find similar conditions, if they exist, and the best compromise to optimize ZVI use.

One column test was performed using pure Lapillus put in contact with a solution of nickel at an initial concentration of 50 mg/L using a 0.276 mm/min average constant flow velocity. The objective of the experiment was to investigate whether this material has a certain removal capacity, how much contaminant mass it is able to retain in the experimental conditions and the possible removal mechanisms involved.

It was also useful to represent a benchmark for column tests carried out using ZVI/Lapillus mixtures.

Three tests were realized varying the weight ratio of each of the mixtures. In particular, 10:90, 30:70, 50:50 ZVI/Lapillus weight ratios were tested to observe the influence of ZVI quantity and the role of Lapillus in terms of removal efficiency and hydraulic behaviour, in order to establish the weight ratio corresponding to the best compromise between removal and preservation of hydraulic behaviour.

Tests varying the initial concentration of Nickel and the flow velocity were carried out.

Three column tests using solution of nickel at 10, 50 and 100 mg/L of initial concentration and three tests using constant flow velocity equal to 0.055, 0.276 and 1.382 mm/min were carried out.

All the column tests were carried out using 50 cm columns. One test only was performed in a 100 cm long column in order to investigate the influence of filter thickness. In particular, 30:70 weight ratio ZVI/Lapillus granular mixture was tested in this study with a solution contaminated by nickel at 50 mg/L initial concentration flowing at a constant mean flow velocity equal to 0.276 mm/min. The Zinc efficiency removal was investigated using 50:50 ZVI/Pumice and ZVI/Lapillus weight ratios mixtures, using Zinc contaminant solution at 50 mg/L initial concentration flowing at 0.276 mm/min constant flow velocity.

Two column tests using tri-contaminant solution containing nickel, copper and zinc at 50, 500 and 50 mg/L initial concentrations respectively flowing at 0.276 mm/min constant flow velocity were carried out. During the first time when nickel concentration is most reduced within 40 cm of the filter length, in the first half length of the filter, pH is about 6.5 and it seems to decrease with time, while at the outlet its value is around 7. The Eh in the first ports is positive and it seems to increase with time, while at the outlet it is negative or less than that corresponding to the first ports, at least. During the second time (starting after 500 hours), an increase of pH over time is observed for the first ports, while its values are lower than those of the first time to the outlet. A specular behaviour of Eh can be observed. Therefore, pH decrease and Eh increase seem to be related to a first reduction in removal efficiency as it has been developed at each port.

Results of column tests

Considering a removal efficiency defined as η 𝑟𝑒𝑚 by the equation 5-1.

η 𝑟𝑒𝑚 = 𝐶 0 -𝐶 𝑚 𝐶 0 (5 -1)
where 𝐶 0 is the initial concentration and 𝐶 𝑚 is the measured concentration, it is possible to evaluate the removal efficiency of different double porous filter lengths as function of the input mass, as shown in Figure 5-9 a). shown for each column sector. Specific removal rates represent the quantity of nickel mass retained by unit of lapillus mass in the unit of time. They were calculated for each sector of filter length, considering the sampling time intervals. Their evolution in time and their difference among the sectors can let the conclusion be drawn that the highest removal specific removal rates have been developed in the first centimetres.

Weight ratio influence

In this paragraph an analysis of results of column tests carried out with different ZVI/Lapillus weight ratios will be proposed. The 10:90 ZVI/Lapillus column test has been performed using 138 gr of ZVI and 1242 gr of lapillus, the one with 30:70 ZVI/Lapillus weight ratio was prepared using 480 gr of ZVI and 1120 gr of Lapillus while for the 50:50 ZVI/Lapillus weight ratio test 970 gr of both reactive materials needed.

The three columns tests have been carried out under the same experimental conditions of flow velocity (0.276 mm/min) and nickel initial concentration (C0=50 mg/L).

Removal efficiency

In Figures 5-11a) and b) and 5-12 a), the nickel normalized concentration is shown as function of time for different sampling ports for 10:90 , 30:70 and 50:50 ZVI/Lapillus mixtures respectively. As can be observed, the less efficient mixture is the 10:90 w.r., while the most efficient is the 50:50 w.r. In fact, after 1500 hours of interaction, the 10:90 filter is no longer able to reduce the nickel concentration efficiently and nickel breakthrough occurs . The same phenomena can be observed after 2300 hours for the 30:70 w.r. column test, while after 4000 hours the 50:50 w.r. column test is still highly efficient.

Furthermore, some considerations concerning how the reactive material exhaustion occurs can be made. As ZVI weight content increases, breakthrough occurs at increasingly high times at the different ports, especially up to 38 cm. Then, for the subsequent ports, breakthrough is more rapid. In fact ZVI reacts not only with heavy metals but also with nitrates and water. Thus, its removal capacity is not intact when the reactive zone for heavy metals removal moves upward the column. [START_REF] Noubactep | Processes of Contaminant Removal in ''Fe0-H2O''Systems Revisited. The Importance of Co-Precipitation[END_REF][START_REF] Noubactep | A critical review on the mechanism of contaminant removal in Fe 0 eH2O systems[END_REF]Noubactep ( , 2010aNoubactep ( , 2010b[START_REF] Noubactep | Aqueous Contaminant Removal by Metallic Iron: Is the Paradigm Shifting?[END_REF] showed as metal ions can be removed from the aqueous phase in packed ZVI beds by adsorption, co-precipitation, and adsorptive size-exclusion when the pH >4.5 (Bilardi et al., 2013a).

Looking hours (1880 mg nickel mass input). At 18 cm, pH decreases from 8.8 to around 7.4 and Eh increases remaining in the negative values range, while at 50 cm pH decreases from 9.9 to 9 and Eh increases from -144 mV to -115 mV, during the first considered interval.

On the other hand, an 18 cm long filter , corresponding to 4 hours residence time and 350 gr of ZVI, is enough to remediate nickel contaminated water until 2280 hours (3420 mg nickel mass input).

The next interval, the second one, goes by until 5328 h. During this time, at 1.5 cm., pH is about 6.8 and Eh is about 10 mV, at 18 cm., pH decreases from 7.5 to 6.9 and Eh increases from negative to around 10 mV, while at the outlet pH decreases to 6.6 value and Eh becomes positive and equal to 56 mV, limited nickel concentration is detected at the outlet. However, 28 cm of 50:50 ZVI/Lapillus reactive filter, containing 543 gr of ZVI and corresponding to 6 hours of residence time, are enough to remediate nickel-contaminated water until 3312 hours (4980 mg nickel mass input). Subsequently, a 38 cm long filter is necessary, assuring a residence time of 8.6 hours and containing 1474 gr of ZVI, to reduce significantly the nickel concentration, until 3984 hours (6000 mg nickel mass input).

To better understand how nickel removal efficiency is developed and if and how it can influence the permeability, different histograms will be proposed to analyze in which sector the mass of nickel is mainly retained and how this changes with time.

In Figures 5-17 a) and b) and 5-18, the cumulative mass of nickel retained in each sector divided by the length of the sector is reported for different times, namely 432, 1440 and 2088 hours for 10:90 w.r. ZVI/Lapillus. It can be observed that the nickel mass input to the column is mainly retained by the first sector (from 0 to 3 cm from column inlet). The nickel retained in the first sector increases with time like that retained on the following sectors. However, the cumulative contaminant mass retained by each sector decreases as the distance from the inlet increases.

Hydraulic behavior

The hydraulic conductivity evolution with time for the three mixtures ZVI/Lapillus is shown in Figure 5-24 b). The highest reduction of hydraulic conductivity was observed in the mixture 50:50, while the best hydraulic behavior was observed for the mixture 10:90 w.r. This can be due mainly to higher quantity of ZVI corrosion products and gas formation [START_REF] Caré | Modeling the Permeability Loss of Metallic Iron Water Filtration Systems[END_REF]. Furthermore the nickel retained and mineral precipitation can play a role. The effects of these factors can be localized in different filter zones. For example, the nickel retained mass would be more concentrated in the shorter filter zone close to the inlet, as the ZVI quantity is higher.

Regarding the corrosion effects, although it is not easy to individuate where the most quantity of the most expansive products are localized, some information can be gathered by considering the pH and Eh values.

Comparing the evolution of pH and Eh values with the hydraulic behavior it is possible to observe some correspondences between the data measured at the outlet (but also at 18 cm form inlet) and the time when the most permeability loss occurs. In fact, hydraulic conductivity reduction is slower as the ZVI content is smaller and this occurs after a certain interaction time. Considering the pH and Eh values evolution in at outlet are mainly taken into account. In fact, the most hydraulic conductivity loss seems to occur when the pH is decreasing at the outlet. Because the initial pH is higher in 50:50, the difference to be covered to have stable conditions is also higher.

In particular, when the pH decreases from 9 at 1500 hours, permeability loss seems to star too.

In Figures 5-24a), hydraulic conductivity evolution is reported as function of the nickel mass retained by section in the most efficient sector as individuated for each weight ratios tested (Figure 5-25a) and as function of nickel mass retained in the most efficient sector divided by ZVI mass contained in the considered sector (Figure 5-25b).

Looking at the permeability evolution with time, the more rapid and higher loss of permeability is observed for higher ZVI content. There being more ZVI particles number in a section as ZVI content increases in weight ratio, after a certain time void volume is more reduced because of ZVI particle expansion, precipitates formation or other factors due to ZVI presence.

As was observed in the previous paragraph, not all the sections are equally interested by contaminant retention. In fact in the 10:90 w.r. column test, the sections comprised between 0 and 3 cm are those which contain the most cumulative quantity of nickel retained per section. The same thing is evident for the sector comprised between 3 and 8 cm of 50:50 ZVI/Lapillus w.r. column test. An intermediate phenomena, not allowing one to establish in the same evident way what column sector is more efficient, happens for 30:70 ZVI/Lapillus w.r. column test.

However for this test, the sector between 3 and 8 cm can be considered the most efficient during the time. On the basis of these considerations, permeability evolution has been represented as a function of nickel mass retained by section located in the most efficient sector for each column test, in order to investigate whether the average mass of contaminant retained by the most efficient sections can be considered as cause of permeability loss. Except for the same hydraulic behaviour in the three tests until a certain time (200 h), no correspondence between nickel retained in the most efficient section and permeability loss can be found.

However if the permeability evolution is represented in function of contaminant mass retained in the most efficient section divided by ZVI quantity contained in the section, a certain correspondence can be observed between permeability loss in 30:70 and 50:50 w.r.. The different hydraulic behaviour of 10:90 w.r. filter in this case can be due to the low ZVI content, that does not entail important permeability loss, and to the high Lapillus quantity that can be an important factor of nickel retention and that avoids high permeability loss.

Specific removal rate

The Looking at the graphs the influence is evident of advection phenomenon, representing an important factor of contaminant transport through the porous reactive material under the experimental conditions of the tests carried out. As can be observed in the semi-logarithmic graphs, the specific removal rate for each sector tend to a certain value. This is more evident for 10:90 w.r. where a faster removal efficiency exhaustion occurs, due to the lower ZVI content. In the tests where ZVI content is higher, the dispersion of specific removal rate is higher. It should be related to the faster removal efficiency exhaustion in tests with less ZVI content.

Looking at the graphs in natural scale, different information can be drawn up. In particular, the variation of nickel removal capacity of each sector is more evident and the presence of sectors more efficient than others. In correspondence to the reactive media placed in the first sector (0-3cm) the highest specific removal rate was developed. It happens at the beginning of the tests. Afterwards, in the 10:90 w.r. test the specific removal rate decreases for the first sector, while it increases for the next sectors, never exceeding the value relative to the first sector at the considered time (except for the 8-18 cm sector). This does not happen in the other w.r. tests. In fact, in 30:70 w.r. when the coefficient relative to the first sector decreases with time, that obtained for 3-8 cm and 8-18 cm sectors increases going beyond the first. For a certain time the specific removal rate of the 8-18 cm sector is higher than that of the 3-8 cm sector. The same thing happens for 50:50 w.r. tests where after about 500 hours the specific removal rate related to the 3-8 cm sector is always the highest.

The specific removal rate obtained for the column sectors included from 18 to 50 cm have generally lower values than those relative to 0 -18 cm of column. Some factors of this behaviour can be found in the lower contaminant concentration that arrives at the higher filter length, if a retention capacity function of the contaminant concentration is assumed, or in the lower removal efficiency observed at the higher filter length. The other three terms that can influence the interaction between contaminant flow and reactive material are dispersion, diffusion and reaction terms. Probably, diffusion plays a minor role with respect to the effects of the other phenomena, considering the number of Peclet and the relative higher importance of advection and dispersion terms. However, diffusion effect should be almost the same along the column and in both tests, considering that the grain size distribution is almost the same, as tortuosity. Dispersion effects on contaminant concentration evolution can be more evident as the considered point is longer from the input in the main flow direction, corresponding to different times from test beginning. To investigate whether diffusion and dispersion have influence on contaminant concentration evolution with time and along the column, it is possible to compare the 3 cm and 8 cm of column length of the pure Lapillus test. This comparison shows a light influence of these two phenomena on the contaminant concentration evolution.

To investigate the relative influence of each phenomenon on contaminant concentration evolution with time, a comparison can be made between the results related to 8 cm long pure Lapillus filter and those of 8 cm long 10:90 w.r. filter. The difference between the two considered cases during the first 600 hours of interaction suggests the influence of the presence of ZVI on Nickel removal represented by the reaction term. In fact, the higher removal efficiency of 10:90 w.r. 8 cm long filter compared to that of the pure Lapillus filter of the same length that can be observed during the first 600 hours of interaction can be due to the ZVI removal efficiency.

Furthermore, the superposed experimental data of both tests during the subsequent time can suggest that contaminant retention on Lapillus can be the main removal phenomena after 600 hours for the mixture test and that a second time characterized by a lower Nickel removal efficiency of ZVI has been developed. However, the role of ZVI and especially its distribution along the column seem to be really important for Nickel removal, related to the residence time too. Nickel normalized concentration evolution with time is reported for 3 cm length of 10:90 and 30:70 ZVI:Lapillus weight ratios filters and for 8 cm of 10:90 w.r. columns in Figure 5-32 a). 8 cm of the first and 3 cm of the latter filter correspond to the same ZVI content. In the same Figure 5-32 a) normalized concentration for 3 cm of pure ZVI column length, containing 240 g of ZVI, and that for 3 cm and 8 cm of pure Lapillus column lengths are reported too.

The same length of pure Lapillus filter does not have the same removal efficiency but its behaviour is similar to that of 3 cm pure ZVI filter and 3 cm of 50:50 w.r. filter. In 5-32 b), Nickel relative concentration is reported for 8 cm of 30:70 ZVI/Lapillus w.r.

filter and for 28 cm of 10:90 ZVI/Lapillus w.r. each one containing 77 g of ZVI. The benchmark is represented using a dashed line and it is referred to 28 cm of pure Lapillus filter thickness. It is possible to observe that the Nickel removal behaviour is similar using a thickness of 28 cm and 8 cm of Lapillus and 30:70 ZVI/Lapillus w.r. filters respectively. Furthermore, it is possible to hypothesize that the high difference in terms of removal between the 28 cm of pure Lapillus filter and the same length of 10:90 ZVI/Lapillus w.r. filter should be due only to the presence of the ZVI. The difference observed between the efficiency of 28 cm long 10:90 w.r. and 8 cm long 30:70 w.r. filters can be due to the higher residence time In Figure 5 This can probably be due to Lapillus residual removal efficiency, considering its higher quantity, and to the higher residence time ensured by the most efficient test .

Another comparison can be made to better understand the parameters mostly influencing the Nickel removal efficiency using ZVI/Lapillus mixtures. The previous couples of filters lengths at different weight ratio selected in order to have the same ZVI content can be compared to see how much ZVI mass and column length, as the residence time, influence Nickel removal.

In Figure 5-34 b) the Nickel mass retained is shown as function of the Nickel mass input for 8 cm long 10:90 w.r. filter and 3 cm long 30:70 w.r. filter containing each one about 22 g of ZVI. In the same graph, data related to 28 cm long 10:90 w.r. filter and to 8 cm long 30:70 w.r. filter are reported too. Each one of these two tests contains about 77 g of ZVI. The less efficient is the considered 3 cm long filter test corresponding to 0.66 hours of residence time, the most efficient is that which is 28 cm long that needs 6 hours to be passed through. Furthermore, both tests, where a thickness of 8 cm is considered, have the same removal efficiency, although one contains a mass of ZVI almost four times greater than the other. This is surely true until 3000 mg of Nickel mass input. The residence time corresponding to 8 cm filter length in this case about 1.8 hours. The residence time corresponding to 8 cm long filter is about 1.8 hours, while that relative to 3 cm is about 0.68 hours.

In Figure 5 efficiency equal to 95.8% and 98.8%, if 28 and 38 cm are considered respectively, after 8000 mg of Nickel mass in input.

Flow velocity influence

Similarly to what has been already done with ZVI/Pumice mixtures (Bilardi et al. Thick.

(cm)

React.

area (cm)

React.

Vol.

(cm 3 ) Fe 0

(gr)

Lapil.

( In Table 5-7, pH and Eh values measured at 1.5, 18 and 50 cm from inlet for different sampling times are reported for tests using v1.

In Table 5-8, the values shown for pH and Eh are related to the column test carried out using v3.

The duration of the test performed at v1 velocity is 15000 hours (it is still in progress), corresponding to 4500 mg of Nickel in input, the test using v2 was for 6000 hours with 9000 mg of Nickel in input, while the duration of the test using v3 was 859 hours and the Nickel mass in input was 6442 mg.

Influence of flow velocity on the removal efficiency

To analyse the Nickel removal efficiency of the test performed at v1 the interpretation of contaminant concentration and pH and Eh evolutions together can offer some useful information.

Firstly, regarding this test it is important to highlight that in Figure 5-39 b) only the sampling ports where Nickel concentration was above detection level have been taken into account. As can be observed in Figure 5-39 a), Nickel concentration was efficiently reduced by 28 cm long filter considering a mass in input equal to 4500 mg .

3 cm long filter is able to significantly remove Nickel concentration until about 300 hours (900 mg of Nickel mass input). At about 170 hours, in fact, the pH value at 1.5 cm from the inlet is about 7.11 and the Eh is positive. As can be observed for the above-analysed column tests, in this chemical condition a breakthrough occurred or is ready to occur. In the other sampling ports corresponding to higher distances from inlet, pH is about 9 or more, while the Eh is negative and equal to 100 mV in absolute value. For about 10000 hours (3000 mg of Nickel mass input), 8 cm long filters are able to reduce the Nickel concentration under detection limit. At this time, at 18 cm pH has decreased to 8.4 (from 9.2 at 1600 h) and Eh has increased to -70 mV (from -126 mV at 1600 h), while at the outlet pH has decreased to 8.9 (from 9.6 at 528 h) and Eh has increased to -115 mV (from -154 mV minimum measured value at 528 h).

Simultaneously, at the sampling port placed at 8 cm from the inlet, Nickel measured concentration is about 20 mg/L, while at the sampling port at 18 cm from the inlet Nickel concentration starts to increase (up to 1-2 mg/L, maximum valued measured until the end of the test). With regard to the pH and Eh evolution, information about the processes that have developed in the three columns can be gathered. Two things can be brought to light:

one is related to the ranges of the values, the second regards their variation over time. In fact, pH keeps about 6-6.5 and Eh is about 40-5 mV for all the test duration at each sampling port in column tests carried out at v3. In tests performed at v1 and v2 they vary in different ranges: pH range is 9.7 -6 in v1 and 8.8 -6 in v2 tests, while Eh range is -154 -30 mV in v1 and -113 -60 mV in v2 tests.

Moreover, in the column test performed with v2 the maximum value of pH and the minimum value of Eh were measured mainly at the outlet in the first sampling time, while in the test performed at v1 the pH increases more slowly and decreases after a certain time, reaching the maximum value after a long interaction time. This behaviour is observed at sampling ports at 18 cm and 50 cm from the inlet in v1 test.

The results suggest that in v3 column tests there is not enough contact time for iron corrosion and nickel removal. In the other two tests, a certain contact time is assured for their development, higher in the v1 test than in the v2 one. This parameter together with the chemical condition that the Lapillus support allows can influence the formation process of products, their structure, density and order. Probably in opportune hydraulic conditions for corrosion, the higher contact time may assure products more well-done, with an ordered structure and higher density than in faster development processes.

It should be highlighted that in v2 after about 1300 hours the pH is already about 6 and Eh positive and the same phenomenon is observed at the outlet after about 3000 hours. pH is always higher than 8.6 and Eh smaller than -110 mV at the outlet of v1 test for all the test duration. Almost the same thing happens at the sampling port at 18 cm from the inlet. If the products that cause pore clogging can be formed in chemical conditions of 6 pH and positive Eh reached after previous corrosion processes, the v2 is already subject to hydraulic issue, while v1 is not yet so.

In Figure 5 As can be expected, taking into account the previous consideration about the importance of residence time for removal efficiency, the greater Nickel mass is retained in the first 8 cm of the filter tested using v1. It is equally distributed in the first 18 cm of filter thickness in column test carried out at v2. Whereas the entire filter is involved in Nickel removal efficiency in the v3 test and the Nickel retained mass is concentrated in the filter length included between the sampling port at 18 cm from the inlet and the outlet.

Influence of flow velocity on hydraulic behavior

In order to investigate the hydraulic behaviour, the evolution of the hydraulic conductivity for the column tests carried out at different flow velocities has been analysed as function of different parameters. This study aims to investigate the conditions that mainly influence the hydraulic behaviour of the reactive mixtures .

In Figures 5-43 does. Furthermore, the main value calculated for the most efficient sectors is higher and the flow velocity is also higher.

For the v1 test, the highest specific removal rate is related to the first 8 cm of the filter and its value seems to be stabilized after about 2000 hours of interaction around 0.001 mg/(gr*h) average value. This situation changes after 14000 hours. The evolution of specific removal rate coefficients, as calculated for the v2 test, shows a variation of removal efficiency of each sector with time probably more affected by advection term than the previous test. Each highly efficient sector is characterized by a similar evolution of specific removal rate coefficients, decreasing after the maximum value has been reached. Looking at the graph of the v3 test, it can be observed that the highest values are related to the second half length of the filter.

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1 Figure 5-47 specific removal rate evolution with time for each filter sector for v3 test.

Initial concentration influence

Influence of initial concentration on removal efficiency

In Figure 5-48 a) and b) the Nickel normalized concentration evolution with filter thickness at different sampling times and with time at each sampling port for the column test performed using solution contaminated by Nickel at 10 mg/L of initial concentration is shown respectively. In table 5-10 the values of pH and Eh measured at different times at the sampling port placed at 1.5, 18 and 50 cm from the inlet of the considered column are reported. As can be observed, a 3 cm long filter is able to remove efficiently the Nickel mass input in the column until about 1000 hours of interaction, corresponding to 300 mg of Nickel input, subsequently the 8 cm thick filter needs to remediate the contaminated solution until 4656 hours at least, corresponding to 1400 mg of Nickel input mass. Looking at the pH and Eh evolution, some consideration can be made comparing them to those of column tests carried out using solution contaminated by Nickel at 50 mg/L of initial concentration. The pH is about 6 at the sampling port placed at 1.5 cm from the inlet, when the corresponding filter thickness is still highly efficient.

Moreover, while for the previously analysed column tests, negative Eh values were measured when pH was more than 8, using Nickel concentration equal to 10 mg/L, chemical conditions are reducing for pH equal to 6.7. Another difference is related to the reducing chemical condition developed during the first interaction time (until Furthermore, more reducing conditions and higher pH value were developed at half of the total filter length than at the end of the filter during the first interaction time. The same phenomenon was observed for the test carried out at v1 only. Usually the observed chemical conditions and measured pH values were more reducing and higher respectively as filter length increases.

In Figures 5-49 a) and b) the Nickel normalized concentration evolution with filter thickness at different sampling times and with time at each sampling port is shown respectively, for the column test carried out using solution contaminated by Nickel at 100 mg/L of initial concentration. In table 5-11, the value of pH and Eh measured at different times at the sampling port placed at 1.5, 18 and 50 cm from the column inlet are reported.

The 50 cm long filter of 30:70 ZVI/Lapillus mixture is able efficiently to remediate the contaminated solution until about 100 hours after the test beginning, corresponding to 320 mg of Nickel input mass.

Looking at the measured pH and Eh values, the first one was about 6 for all the filter length and the test duration while the latter ranged from 30 to 60 mV. As the contaminant mass in input increases, the removal efficiency of the test performed with solution containing Nickel at 100 mg/L initial concentration decreases up to 50% after 8000 mg of Nickel have been flowed through it. The test carried out using a solution of Nickel at 50 mg/L of initial concentration keeps a high performance (99%) until 4000 mg of Nickel input mass. Afterwards, the removal efficiency of the filter decreases up to 88% after 9000 mg of Nickel input mass. The data related to the test performed with 10 mg/L Nickel initial concentration of Nickel show an efficiency equal to 99.9% for the whole duration of the test, corresponding to 2000 mg of Nickel.

In Figure 5

Influence of initial concentration on hydraulic behavior

In Figures 5-52 and 5-53, the hydraulic conductivity evolution is shown as function of time, of Nickel mass input, of water input, of cumulative Nickel retained mass for the tests performed using solutions at different Nickel initial concentrations.

The filter that has better performed under the hydraulic point of view was that with the highest Nickel initial concentration, that subject to the highest hydraulic conductivity loss was the one carried out with the lowest Nickel initial concentration.

Looking at the evolution of hydraulic conductivity as function of the above-mentioned parameters, it can be gathered that the Nickel mass input as the Nickel retained mass does not play an important role in this study. On the other hand, time and input water seem to be the parameters that mostly can influence hydraulic performance.

Comparing them to the most affecting parameters (among those considered) of hydraulic behaviour in flow velocity influence study, it seems that the water input into the column could be the parameter that can better explain the hydraulic behaviour of the column tests carried out, among the considered parameters. The first value of specific removal rate for the first sector is higher the higher is the Nickel initial concentration, although the values calculated for different sectors of the tests carried out using solution of Nickel at 50 and 100 mg/L initial concentration for the different times are in the same range. In addition, the evolution of specific removal rates with time in the test carried out with 10 mg/L Nickel initial concentration solution is different from that observed in the other. In fact, in the first test, the value related to each sector stays constant and that of the first 3 cm long filter is about 0.003 mg/(gr*h) while that of the others sectors is very low. Looking at the graphs corresponding to the other two tests, a certain variation of specific removal rate values with time and with filter thickness is observed. In particular, a clear behaviour of specific removal rate evolution with time can be distinguished among the different filter thickness sectors for the test carried out using solution of Nickel at 50 mg/L initial concentration, whereas the specific removal rate calculated

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1 for the first 28 cm of the filter vary around the same average value, equal about 0.002 mg/(gr*h) in the case of the test performed using solution of Nickel at 100 mg/L initial concentration.

Influence of filter thickness on hydraulic behavior

In Figure 5-58 and 5-59 a) and b) the hydraulic behaviour of the LC will be compared to that of the filter tested in the study on the influence of the initial Nickel concentration, while in Figure 5-60 and 5-61 a) and b) it will be compared to that of the hydraulic performance of the filter tested varying the flow velocity. In fact, the hydraulic conductivity of LC stays constant for the test duration, while that of SC decreases of one order of magnitude in a shorter time. A hydraulic behaviour similar to that performed by the LC can be observed for the filter tested at v1 and v3 flow velocity. The difference on the hydraulic behaviour between the two filters tested using the solution at the same contaminant initial concentration and the same flow velocity can deal with different chemical conditions development. [START_REF] Bilardi | Short and long term behaviour of Fe0 and Fe0/Pumice granular mixtures to be used in PRB for groundwater remediation[END_REF] 1.E-07

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1

Removal efficiency

In Figure 5-62 a) and b) the Zinc normalized concentration is shown as function of time at each sampling port and as function of filter thickness for each sampling time.

The 8 cm thick filter is able to efficiently remediate until 400 hours from the test beginning. Afterwards, the 18 cm thick filters are enough to remediate until 1600 hours. Moreover, at sampling port placed at 28 cm from the inlet, highly reduced Zinc concentration was revealed. mixture at different filter thickness, corresponding to ZVI content. The removal efficiency seems not be dependent on the residence time as well as the Nickel removal is, while it is rather influenced by the ZVI content. In fact, a higher removal efficiency can be observed for the pure ZVI test, that is 3 cm long and contains 240 g of reactive medium, than a 8 cm long filter of 50:50 ZVI/Lapillus w.r. mixture, containing 152 g. The results obtained performing the pure ZVI test can be comparable with that of the 18 cm long mixture filter.

Hydraulic behaviour and specific removal rate coefficients

In Figure 5-64 a), the evolution of hydraulic conductivity as function of Zinc retained mass is shown. The pure ZVI test is subject to a high hydraulic conductivity loss corresponding to about four orders of magnitude, while no important hydraulic changes were observed for the 50:50 ZVI:Lapillus w.r. mixture.

In Figure 5-63 b), the evolution of specific removal rate for each filter sector as function of time is shown. For the entire test duration, the highest values can be found for the first 3 centimetres, while the second highest values can be observed for the 3-8 cm filter sector.

Results of column tests carried out with pluri-contaminated solution

The study proposed in this section is focused on the investigation of long-term removal efficiency performance and hydraulic behaviour of 50:50 ZVI:Lapillus weight ratio mixture to be used in pluri-contaminated solution remediation. A solution containing Nickel, Copper and Zinc at 50, 500 and 50 mg/L initial concentration respectively was used in the test. The obtained results and their comparison with that of column tests performed with the same weight ratio mixture and monocontaminant solutions are introduced. The benchmarks of (pure ZVI filter) tested with the same mono-contaminant and three-contaminant solutions in the same experimental conditions [START_REF] Suraci | Studio sperimentale dell'efficienza di barriere permeabili reattive per la bonifica di acque di falda contaminate da metalli pesanti[END_REF][START_REF] Bilardi | Short and long term behaviour of Fe0 and Fe0/Pumice granular mixtures to be used in PRB for groundwater remediation[END_REF] were taken into account in the analysis. The characteristics of the considered column tests are reported in

Removal efficiency

The investigation about removal efficiency is developed in three steps. The first consists of comparing the contaminant concentration evolution in monocontaminant and three-contaminant tests. The second is focused on studying whether a priority in removal exists and whether a difference between monocontaminant and three-contaminant cases can be observable, as well as in the similar study performed using ZVI/Pumice mixtures. [START_REF] Bilardi | Simultaneous removal of CuII, NiII and ZnII by a granular mixture of zero-valent iron and pumice in column systems[END_REF]. Finally, the results obtained using 50:50 ZVI:Lapillus weight ratio will be compared with those of pure ZVI filters tested under the same experimental conditions with mono and threecontaminant solutions.

In Figure 5 Nickel removal, the 3 cm thick pure ZVI filter is not able to reduce efficiently the contaminant concentration in any case, neither in mono-contaminant nor in threecontaminant tests. On the other hand, when Zinc contaminant is used, the 3 cm of pure ZVI filter are able to remove efficiently the contaminant in mono-contaminant solution, while it does not have the same efficiency when three-contaminant solution is used. Probably, this difference is due to the presence in three-contaminant solution of Copper, that can be rapidly removed by redox processes [START_REF] Bilardi | Simultaneous removal of CuII, NiII and ZnII by a granular mixture of zero-valent iron and pumice in column systems[END_REF]. It mostly influences Zinc removal, while it is not the most affecting parameter of Nickel removal.

Hydraulic behaviour

The investigation of the long-term hydraulic behaviour will be focused on comparing the calculated hydraulic conductivity evolution for the column tests carried out using mono-contaminant solutions of Nickel and Zinc and the value obtained for the threecontaminant solution tests.

In Figure 5-69 a) the evolution of hydraulic conductivity with time for column testes performed using 50:50 weight ratio ZVI:Lapillus and solutions contaminated respectively by Nickel (50 mg/L), Zinc (50 mg/L) and Nickel, Copper and Zinc (50, 500 and 50 mg/L respectively).

The three filters have not developed the same long-term hydraulic behaviour.

Considering that the composition of the used mixtures, the used ZVI mass and the experimental conditions (e.g. water flowed and residence time) were the same, the observed difference in hydraulic performance probably deals with the mass of retained contaminants or their compounds and reactions involving Iron, contaminants. To investigate whether the same order in hydraulic conductivity loss is observed using pure ZVI, the values measured in the 3 cm long pure ZVI filters performed with Nickel-contaminated (C0=50 mg/L), Zinc-contaminated (C0=50 mg/L), Coppercontaminated (C0=500 mg/L) and three-contaminated (C0=50 mg/L for Nickel and Zinc, C0=500 mg/L for Copper) solutions are reported in Figure 5-69 b). As can be expected, the ZVI filters rapidly lost their initial hydraulic conductivity and its final value was about 5E-07 cm/sec for all the tests, except for that performed using Nickel-contaminated solution. This difference can be sought in chemical conditions development or in the filter length being not high enough to develop hydraulic issue, taking into account the importance of residence time to remove Nickel.

The hydraulic conductivity loss in pure ZVI filters takes more time to develop in experiments using Zinc-contaminated than those with Copper-contaminated and three-contaminated solutions. Moreover, the loss of hydraulic conductivity has the same rate for all the tests. Also for the tested 50:50 ZVI:Lapillus weight ratio mixtures, that with the three-contaminant solution is subject to the earliest and fastest loss in hydraulic conductivity. That performed using Zinc-contaminated solution is not subject to hydraulic issue in the experimental test time.

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1 In this section, the development of a model to be used to simulate the long-term removal efficiency and the hydraulic behavior is outlined. Moreover, the model is useful to understand the mechanisms that occur during pollutants removal.

Firstly, the methodology used to develop the model will be described as well as the approach adopted and the fundamentals of equations that constitute the basis of the model.

A brief discussion on the mechanisms of heavy metal removal using ZVI, Pumice and Lapillus will be proposed to be able to choose the more appropriate model concerning the removal term. Once the most adequate has been hypothesized, the methodology followed to choose the values of coefficients and simulate the experimental results will be described. Finally the simulations and sensitivity analysis results will be shown.

Methodology

The model of column, considered as filter of doubly porous material, has been conceived in order to develop a tool to simulate the long-term removal efficiency and hydraulic behavior, as coupled, and to understand the removal processes and the mechanisms leading to hydraulic conductivity loss as well as how they evolve with time and along the filter thickness into the reactive packed bed.

The general steps for model development are described in Figure 6 Both removal efficiency and hydraulic behavior can be influenced by microstructure evolution and this should be introduced through a coupled model. Furthermore, this path has been constantly based on observation of experimental data, geometrical hypothesis and back-analysis.

Continuum approach

To simulate the column tests, the models of flow and solute transport through the subsurface porous media have been considered. They consist of partial differential equations and are based on considering the porous medium as a continuum domain that is occupied by different phases, solid matrix and void space and for which a representative elementary volume (REV) can be found. [START_REF] Bear | Theory and applications of transport in porous media[END_REF]. The void space can be occupied by liquid and gaseous phases in different proportions, as function of saturation. A phase can be defined as a portion of space that is separated from others by an interface or that is occupied by a material characterized by a single set of constitutive relations describing its behavior. The gaseous phase can be only one, gas being miscible, while the liquid phases can be more than one. A domain occupied by a phase can be considered a continuum if state variables and properties can be assigned to every point within it.

Considering a porous medium domain made of solid matrix and at least one fluid phase, subdomains occupied by the first and the latter can be identified. The subdomain occupied by a solid is characterized by a behavior as a continuum with respect to a solid, it is so for that occupied by water. The continuum approach applied to one phase is based on averaging its behavior at molecular level, in order to obtain a microscopic level description. To overcome the difficulties concerning the solving of problems at this level, a macroscopic level can be introduced, in which properties can be assigned for every point in the porous macroscopic level domain.

Thus, the real porous medium domain, made of at least two phases, each one considered as a continuum at microscopic level and together completely occupying the porous medium domain, is modeled as a continuum in which each phase behaves as a continuum filling up the porous medium domain as a whole (overlapping continua).

Average values of variables describing the behavior of a phase can be assigned at each point of porous medium domain taking them over an elementary volume centered into the point. Moving it into the porous medium domain, a field of macroscopic variables, as differentiable functions of spatial coordinates can be obtained.

Coefficients result from the averaging process, e.g. porosity, permeability and dispersivity. In fact, the REV should be sufficiently large that, taken at different locations of the entire domain, it will contain a solid phase and a void space. [START_REF] Bear | Theory and applications of transport in porous media[END_REF].

Fundamentals of equations

The development and use of a model allows us to predict the behavior of an aquifer in response to excitations due to the implementation of management decisions, to better understand a system from different points of view, to provide information for field experiments design, for monitoring networks and for complying with regulations. [START_REF] Bear | Theory and applications of transport in porous media[END_REF]. The aim of this work is to develop a numerical model to simulate dynamic laboratory tests carried out to study the long-term heavy metal removal efficiency and hydraulic behavior of zero valent iron and pumice or lapillus mixtures to be used in Permeable Reactive Barrier application.

Because of the complexity of different practical problems it is often not possible to solve the mathematical models analytically. Thanks to the ability to model more complex conditions than analytical solutions, numerical models are used to solve the mathematical ones using computer programs. [START_REF] Estabragh | Comparison Between Analytical and Numerical Methods in Evaluating the Pollution Transport in Porous Media[END_REF] describe the differences in terms of results between the two kinds of solution approaches.

The model used to search the best fitting of carried out column tests results is based on the one-dimensional advection-dispersion-sorption equation derived by the equation for three dimensional solute transport in saturated media under uniform steady flow presented by [START_REF] Bear | Hydraulics of ground water[END_REF].

The hypothesis of the well-known above-mentioned equation consists of homogeneous, isotropic and saturated porous media with flow conditions valid for Darcy's law. The model is based on the conservation of a mass of solute in a small representative elementary volume. (Fetter, 1992).

The results that will be shown at a later stage are obtained from some simulations of a numerical model based on a system of five or six equations with five or six unknowns respectively.

Comparing the batch tests and column tests results, it was possible to deduce and hypothesize non-equilibrium sorption conditions. a is the depletation rate, η represents the coefficient of volumetric expansion (Caré et al, 2013, Bilardi et al., 2013a). The 𝜑 𝑚𝑎𝑐𝑟𝑜 macro-porosity term used in equation (6-2) is defined by equation (6-6) and it is equal to the ratio between the void volume Vv and the total volume Vol of the column physical model. Where 𝜑 𝑚𝑖𝑐𝑟𝑜 is the pumice intra-particles porosity equal to the ratio between the pumice particles internal void volume Vvp and total volume Vol. Before discussing and defining the reaction terms, a review of available models will be introduced to explain the choice of the model used .

General review of available models

A number of numerical models are available to represent the transport through unsaturated and saturated soil. Vleach, CHEMFLO 2000, Sesoil, Sutra, Mofat, VS2DTI, Pestan, Bioplume III, HELP are used for unsaturated conditions, while Bioscreen, Modflow, Phast, WhAEM2000, AT123D and FEFLOW 5.4 are useful for saturated conditions.

In Figure 6-2 a scheme regarding modelling approaches available for the description of transformation processes of contaminants in transport equations is shown.

Some approaches for the simulation of contaminants transport are based on multicomponent transport codes that incorporate different chemical models to describe the interaction between dissolved species and immobile solid surfaces. [START_REF] Kantar | Heterogeneous processes affecting metal ion transport in the presence of organic ligands: Reactive transport modeling[END_REF]. For each kind of chemical process considered, there are different developed models. One example is the MIN3P model developed by [START_REF] Mayer | A numerical model for multicomponent reactive transport in variably saturated porous media[END_REF] and [START_REF] Mayer | Multicomponent reactive transport modeling in variably saturated porous media using a generalized formulation for kinetically controlled reactions[END_REF] consisting of a multicomponent reactive transport model for variably saturated porous media (Bilardi et al., 2012 b).

The single component models are not chemically explicit and are not based on considering the various sorption processes separately. They are focused on the overall result of processes resulting in solute removal from solution. This kind of model has a more limited number of parameters to be defined than the multicomponent ones and therefore are easier to calibrate.

To choose the more appropriate model, removal mechanisms of heavy metal (and Nickel in particular) onto Pumice (or Lapillus) and ZVI will be summarized.

Nickel removal on Pumice, Lapillus and ZVI

The heavy metal removal in zero valent iron can take place by means of reductive precipitation and co-precipitation, adsorption onto iron surface and onto iron corrosion products [START_REF] Suponik | Removal of heavy metals from groundwater affected by acid mine drainage[END_REF]. The kind of removal mechanism depends on the characteristics of the metal to remove, on the presence of other substances and physicochemical conditions of the ZVI and surrounding zones. On the other hand, in groundwater the corrosion of zero valent iron, oxidized to ferrous and ferric iron, causes an increase in pH, an decrease in ORP, the consumption of dissolved oxygen and the generation of hydrogen (Puls et al., 1999). The reactions involved during the oxidation of iron in the water are described by equations 6-7 to 6-11. As described in the paper written by [START_REF] Li | Sequestration of Metal Cations with Zerovalent Iron Nanoparticles: A Study with High Resolution X-Ray Photoelectron Spectroscopy (HRXPS)[END_REF], the removal mechanism of metal ions with a standard potential very close to that of iron are sorption/surface complex formation and/or reductive precipitation (Figure 6-3). The small difference of standard electrode potential of Fe 0 and Ni 0 reduces at minimum the possibility of reductive precipitation process in metallic form. It is possible to say that the main processes for Nickel (II) removal are adsorption on the iron corrosion products surface and less likely directly onto zero valent iron surface. There is the possibility that the co-precipitation with iron, forming oxides and hydroxides, takes place, too.

As has been described in Chapter 3, heavy metals removal by pumice as well as lapillus can depend on the high internal porosity and the content of iron and aluminium oxides that are very effective in removing heavy metal contaminants due to their high specific surface areas and reactive surface functional groups.

The adsorption proceeds through the diffusion consisting of transfer of adsorbate from solution to adsorbent surface, the migration of adsorbate into pores and interaction with available sites on the interior surface of pores (Figure 6-4). The factors influencing the adsorption are surface area of adsorbent, contact time or residence time, particle size of adsorbent, solubility of substances, affinity of the solute for the adsorbent, size of the molecule with respect to size of the pores, degree of ionization of the adsorbate molecule, pH and initial concentration.

Sorption models

The sorption processes are adsorption, by which a solute clings to a solid-surface, ion exchange, during which a charged ion is exchanged with another of the same sign present on the solid surface, chemisorption, when the solute is incorporated in solid phase by chemical reactions and absorption, that occurs when the solute can diffuse into the internal pores of a porous particle and be sorbed onto interior surfaces. (Fetter, 1992).

The aim of this kind of model is focused on the practical interest of considering the removal of the solute from solution and its partitioning between solution and solid phase.

Considering the sets of chemical reactions and the ground-water flow rates, [START_REF] Rubin | Transport of reacting solutes in porous media: Relationship between mathematical nature of problem formation and chemical nature reactions[END_REF] classified all reactions into two kinds consisting in equilibrium and nonequilibrium or kinetic reactions. The first are sufficiently fast reactions related to the groundwater flow rate so that it is possible to assume a local equilibrium between solute and surrounding. The non-equilibrium reactions are slow sorption processes compared to the rate of fluid flow in the porous media, so that it is not possible to hypothesize the development of equilibrium conditions between solute and sorbed phase.

The resulting model for linear equilibrium adsorption is easy but has shown some inadequacies comparing with experimental data. For this reason, more complex models were introduced taking into account the different rates of sorption processes in different parts of soil medium. [START_REF] Van Genuchten | Non equilibrium transport parameters from miscible displacement experiments[END_REF].

Considering the experimental data of the reactive materials tested in this research activity, the reaction time, is considered as the time that the reactive material needs to remediate efficiently contaminated water in batch tests, and the residence time, that the column test can ensure. In

Column model

A mono-dimensional advection-dispersion-diffusion equation is applied to the representative elemental volume (REV) of the column and the equation (6-1) can be rewritten as : where the 𝜑 𝑒 is the effective porosity, smaller than the total porosity 𝜑 (equal to voids volume to total volume ratio), is the porosity through which flow can occur, considering that sometimes, part of void space is unavailable for fluid flow or contains immobile fluid. 𝐶 is the solute concentration, 𝐷 𝐿 is the hydrodynamic dispersion coefficient parallel to the principal direction of flow that can be written also as:

𝐷 𝐿 = 𝛼 𝐿 𝑣 𝑥 + 𝐷 * (6 -13)
where 𝛼 𝐿 represents the longitudinal dynamic dispersivity, 𝑣 𝑥 is the average linear velocity in the longitudinal direction and 𝐷 * is the effective diffusion coefficient that is equal to:

𝐷 * = 𝜔𝐷 𝑑 (6 -14)
where 𝜔 is a coefficient that is related to the tortuosity (Bear 1972) and 𝐷 𝑑 is the diffusion coefficient in water. Tortuosity can be defined in different ways. It consists of an index of the shape of the flow path followed by water molecules flowing through a porous medium [START_REF] Fetter | Contaminant Hydrogeology[END_REF]. 𝑣 𝑥 represents the average velocity of the fluid flowing through a porous medium, referred to seepage velocity too. It is given by the Darcy velocity 𝑣 divided by the effective porosity 𝜑 𝑒 :

𝑣 𝑥 = 𝐾𝑖 𝜑 𝑒 (6 -15)
where 𝐾 is the hydraulic conductivity and 𝑖 represents the hydraulic gradient.

The term 𝐶 𝑖 * represents the removed solute mass by ith reactive material divided by the solution volume. This term is replaced by the model hypothesized as to be able to represent the experimental results and the remediation mechanisms developed by the used reactive materials as described in the literature.

Choice of model parameters coefficients

The hydrodynamic dispersion is given by the sum of diffusion and dispersion. This means that it can be divided into the two components. Practically, the diffusion is often neglected in groundwater flow conditions. To evaluate the relative contribution of mechanical dispersion and diffusion to solute transport in column tests, the number of Peclet, described by equation 6-16 for column studies, can be calculated.

𝑃𝑒 = 𝑣 𝑥 𝑑 𝐷 𝑑 (6 -16)
where 𝑣 𝑥 is the advection velocity, 𝑑 is the average grain diameter and 𝐷 𝑑 the molecular diffusion coefficient. The Peclet number, as applied to the column studied, represents the ratio of mass transport by advection to mass transport by molecular diffusion [START_REF] Fetter | Contaminant Hydrogeology[END_REF]. Considering an average diameter (d50) varying among 0.3, 0.4 and 0.5 mm, the 𝐷 𝑑 coefficient varying between the maximum (1.5×10 -9 m 2 sec -1 ) and minimum (0.7×10 -9 m 2 sec -1 ) values [START_REF] Gui | Reduction of chromate by granular iron in the presence of dissolved CaCO3[END_REF][START_REF] Jeen | Reactive transport modeling of trichloroethene treatment with declining reactivity of iron[END_REF][START_REF] Mayer | Multicomponent reactive transport modeling in variably saturated porous media using a generalized formulation for kinetically controlled reactions[END_REF][START_REF] Jeen | Effect of initial corrosion rate on longterm performance of iron reactive barriers: column experiments and numerical simulation[END_REF]O et al., , Moraci et al., 2014) ) Figure 6-5 shows the results of experimental measurements using uniform sand columns and tracers [START_REF] Perkins | A review of diffusion and dispersion in porous media[END_REF]. Although only pure ZVI can be classified as uniform sand, while Pumice and Lapillus are sand with a low percentage classified as silt, the observation of the graph considering the number of Peclet calculated for performed column tests, can provide important information.

Looking at the graph, for very low velocities, the 𝐷 𝐿 to 𝐷 𝑑 ratio is constant and about 0.7. For the Peclet number lower than 0.4, diffusion is the key-phenomenon for solute transport and dispersion can be neglected, for its values higher than 6 dispersion is the most effective mechanism of mass transport, while for value between 0.4 and 6

there is a transition zone, where the effectiveness of dispersion and diffusion is the same.

Considering the Peclet number as calculated for performed column tests, for all the tests performed using v2 and v3 hypothesizing both values of 𝐷 𝑑 the advective dispersion is the main mass transport phenomenon. The experiments performed using v1 correspond to the transition zone, where an influence of both dispersion and diffusion is valid. A study of the ratio of dispersion coefficient 𝐷 𝐿 over the diffusion coefficient 𝐷 𝑑 in pure water as function of Peclet number 𝑃𝑒 was proposed by Bear (1972) for a column study for given packed columns, using tracers. Two main domains have been discerned: one corresponding to 𝑃𝑒 < 0.5 where diffusion is the dominant mechanism and the other characterized by 𝑃𝑒 > 0.5 where dispersion is the main process (Bear,1972;Plumb and Whitaker, 1988;[START_REF] Appelo | Geochemistry, Groundwater And Pollution[END_REF].

The dispersivity coefficient 𝛼 𝐿 in column tests is equal to the representative grain diameter used in packing the column [START_REF] Appelo | Geochemistry, Groundwater And Pollution[END_REF]. The dispersivity coefficient 𝛼 𝐿 has been used equal to 𝑑 50 .

6.9.1 Methodology for model development

Batch tests and column tests

In this paragraph simulations using removal coefficients obtained by analysis of batch tests introduced in Chapter 3 will be described, showing the results.

In the following application, the effective porosity 𝜑 𝑒 will be considered equal to the total inter-particles porosity 𝜑 𝑚𝑎𝑐𝑟𝑜 and referred to 𝑛.

𝜕𝐶 𝑖 * 𝜕𝑡 = 𝜌 𝑏 𝑛 𝜕𝑆 𝑖 * 𝜕𝑡 (6 -17)
where 𝜌 𝑏 is the bulk density, defined by solid mass to total volume ratio, 𝑆 𝑖 * is the solute mass retained by unit mass of ith reactive material.

In equation 6-18 a non-equilibrium sorption model is described as more general than possible, considering a reactive media made of more than one material. Different reactive materials can be characterized by their solute removal efficiency, if taken as pure material but their efficiency can be affected by the presence of other reactive material, when they are used as mixed. Coefficients for batch test performed using 30:70 ZVI:Pumice w.r. mixture were calculated using the same procedure. In Secondly, a certain difference in simulation results can be observed between the test simulated using equation and coefficients related to each pure reactive material and that performed using one equation and coefficient related to test carried out with the mixture. This can be an evidence of not superposing effects, probably due to each influencing the other in removal efficiency.

Conclusions

A mono-dimensional advection-dispersion-diffusion equation was applied to the representative elemental volume (REV) of the column. To evaluate the influence of dispersion and diffusion term on the contaminant mass transport through the porous media, the Peclet number was calculated for all the column tests to be simulated. This Concerning the reaction term, a non-equilibrium sorption model was used.

The sorption onto the mixtures was modeled considering the mixture as one material.

This means that one term of sorption was used in the dispersion-advection equation.

To simulate the results and experimental data of some ZVI:Pumice and ZVI:Lapillus Afterwards, the results concerning the tested ZVI:Pumice and ZVI:Lapillus mixtures using Zinc-contaminated solutions and three-contaminated solutions will be compared.

Comparison about Nickel-contaminated tests

The column tests performed using ZVI:Pumice and ZVI:Lapillus mixtures placed in contact with Nickel-contaminated solutions will be analyzed by comparing them.

Firstly, pure Pumice and pure Lapillus column tests results will be considered. The comparison regarding performance of mixtures at the same weight ratio will be based on equal ZVI content, equal residence time and equal removal efficiency.

Results obtained by ZVI:Pumice and ZVI:Lapillus mixtures column tests performed with solutions at the same Nickel initial concentration will be compared, as well as those get from testes performed at the same flow velocity.

Benchmark columns

An analysis of long-term Nickel removal efficiency using pure volcanic materials is possible basing it on the experimental results of pure Pumice and pure Lapillus column tests carried out with Nickel-contaminated solution at an initial concentration equal to 50 mg/L flowing into the column at 0.276 mm/min constant flow velocity.

Porosity, Pore Volume and residence time have been calculated as described in equations (4-9), (4-7) and (4-8) respectively.

In [START_REF] Moraci | Heavy metals removal and hydraulic performance in zero-valent iron/pumice permeable reactive barriers[END_REF]. Pumice was placed as first layer at the bottom of the column, while ZVI occupied 3 cm as upper layer [START_REF] Moraci | Heavy metals removal and hydraulic performance in zero-valent iron/pumice permeable reactive barriers[END_REF]. The characteristics and the experimental data which will be reported are relative only to the Pumice layer. The evolution of Nickel normalized concentration with time is shown in Figure 7-1 a) and b) comparing the experimental data obtained at two different lengths of pure Pumice and pure Lapillus filters, specifically at 3 cm and 38 cm respectively. 3 cm is the filter length corresponding to the first sampling port, 38 cm is the highest length for which experimental data of pure Pumice filter are available.

A certain Nickel removal efficiency is already performed in 3 cm reactive granular filter length, although it is limited, for both tests. Looking at the data concerning this length, a light Nickel retention capacity is observable for tested Pumice; in fact after 120 hours from the beginning of the test , the relative Nickel concentration is about half of the initial value. After 245 hours, a constant value of residual concentration is reached and it is constantly equal about 0.9 until 1500 hours. In the case of the pure Lapillus test, 3 cm filter thickness can be passed through in a lower residence time than the same length Pumice filter, performing a Nickel removal efficiency higher than Pumice. In fact, the contaminant normalized concentration is about 0.6 after 600 hours of test duration. This value of relative concentration increases with time to about 0.8 after 2300 hours. In Figure 7-2 a) Nickel normalized concentration evolution is reported for 3 cm of ZVI:Lapillus and 8 cm of ZVI:Pumice mixtures filters. Both filter lengths contain about 10 gr of ZVI (Table 7-3). Although Nickel initial concentration is reduced by 30% at 120 hours, the 8 cm of ZVI:Pumice filter seems to be more efficient than the 3 cm of ZVI:Lapillus, which can be due to the higher residence and contact time or to the higher dispersion effect of the longer ZVI:Pumice filter than that of the ZVI:Lapillus.

However, removal efficiency decreases for both considered filters. Furthermore, it should be considered that the residence time in the ZVI:Lapillus filters is lower than the ZVI:Pumice filters in the above comparisons and especially for 10:90 mixtures. Using about 70 gr of ZVI in the 30:70 w.r. filters, the ZVI/Pumice mixtures tested in the column has a length of 13 cm, while the ZVI/Lapillus mixture is 8 cm long. As represented in Figure7-4 b, the Nickel removal efficiency is higher in the ZVI:Lapillus filter, even if the behavior of the two mixtures using the considered lengths is almost the same.

Using 180 gr and 250 gr of ZVI mixed with Pumice or Lapillus to obtain 30:70 w.r. mixtures, data related to the 38 cm of ZVI/Pumice mixture and the 18 cm of ZVI/Lapillus mixture filter lengths should be considered for the first ZVI content, while that concerning the 50 cm long ZVI/Pumice mixture filter and the 28 cm long ZVI/Lapillus mixture filter can be compared on the basis of the latter's ZVI content (Table 7-3). In these two cases, as reported in Figure 7-5 Considering the data as reported in Figure 7-6 and 7-7 a) and b), it is possible to observe the influence of the residence time of the contaminant solution during its passage through the porous reactive media on removal efficiency, as well as for the previously analyzed weight ratio filters. In fact, using about 55 gr of ZVI filling the column mixed with the same quantity of Lapillus or Pumice, the 3 cm of ZVI/Lapillus mixture filter and the 5 cm of ZVI/Pumice mixture filter do not assure the necessary residence time in order to reduce the initial Nickel concentration significantly.

However, the same ZVI quantity used in a 10:90 ZVI/Lapillus w.r. column test corresponds to 18 cm filter length, that can reduce to 0.004 the Nickel normalized concentration until 400 hours.

Equal efficiency

The third analysis to study the difference of weight ratio influence on Nickel removal efficiency between ZVI/Pumice and ZVI/Lapillus mixtures can be conducted searching the filter lengths that can have the same Nickel removal efficiency evolution, preset the weight ratio.

After having compared the different evolution of normalized Nickel concentration with time at different sampling ports of each column test carried out at a certain weight ratio, regarding those that demonstrate greater similarity, the corresponding ZVI content and the residence time have been calculated. ZVI:Lapillus tests, the distribution is more homogeneous along the column as the ZVI content decreases. for the ZVI:Lapillus tests, while the specific removal rate concerning the filter thickness of the ZVI:Pumice mixtures higher than 3 cm seem to remain constant.

Comparison of flow velocity influence

The third step in the study concerning the difference on Nickel removal efficiency between the ZVI:Pumice and the ZVI:Lapillus mixtures regards the influence of flow velocity on long-term Nickel removal efficiency and hydraulic behavior. In Table 7- The v1 test seems to be highly efficient for all the tests' duration in both filters.

Concerning the v2 tests, the ZVI:Lapillus filter maintains a high removal efficiency for all of the test duration. In fact, its Nickel removal efficiency is about 88% after 9000 mg of Nickel input , while using the ZVI:Pumice mixture, it is about 58%, corresponding to 6000 mg.

Long-term hydraulic behavior

Comparison of initial contaminant concentration

In this section, a comparison based on the influence of contaminant initial concentration between the long-term Nickel removal efficiency and hydraulic behavior of the ZVI:Pumice mixtures and that of the ZVI:Lapillus will be proposed.

The weight ratio used was 30:70 for both mixtures and the constant flow velocity was 0.276 mm/min. In Table 7 mg/L are subject to hydraulic conductivity decrease of two and one order of magnitude respectively.

Regarding the pH and Eh evolutions, a difference can be observed comparing the data related to the ZVI:Pumice and ZVI:Lapillus mixtures. In fact, considering the tests where pH and Eh do not stay constant ( those carried out at v3 and Nickel solution at 100 mg/L initial concentration), in the ZVI:Pumice mixture filters the pH increases with the filter thickness, while the Eh decreases with it. Furthermore, they tend to reach the same value along the column with time. In ZVI:Lapillus, this general behaviour is not followed during the first interaction time, during which the pH at half filter length is higher and Eh is lower than at outlet (for 170 hours for tests carried out with v1 and 1400 hours for tests carried out using solution with initial Nickel concentration equal to 10 mg/L, at least). conductivity reduction during the time, while a reduction of four orders of magnitude was observed for the pure ZVI test.

Conclusions

The comparative analysis between results of column tests performed using In , the Nickel concentration evolution along the column length from the beginning of the test until about 2500 h and from 4200 to 7300 h is shown respectively, for the test performed using v2. In  the Iron concentration evolution is shown for the same test and sampling times. Firstly, the volcanic Lapillus and ZVI:Lapillus mixtures seem to be more efficient in terms of long-term heavy-metal contaminated groundwater remediation and to have a better long-term hydraulic behavior than Pumice and ZVI:Pumice mixtures.

The higher removal efficiency of Lapillus can deal with the higher content in iron oxides, the lower content in silica and the greater roughness of the surface, that increases the specific external surface, important parameter in removal processes, as well as the larger pores, that makes more easily accessible the internal surface particle by contaminated water, than Pumice.

Concerning the mixtures, the higher specific weight of Lapillus than that of Pumice allows for a higher homogeneity of the mixture composition along the column (or PRB) height depending on the filling procedure used during the installation. It allows also for a higher ZVI content per unit volume or filter unit thickness in ZVI:Lapillus than in ZVI:Pumice, which means a higher number of reactive sites, considering the ZVI as the most efficient material. Furthermore, comparing the efficiency of mixtures filters at the same weight ratio and the same ZVI content, ZVI:Lapillus mixtures allow us to obtain a higher efficiency with less thick filters. Considering the importance of residence time for some heavy metal removal (e.g. Nickel), this highlights the higher Since gas production can be an important phenomenon leading to hydraulic conductivity reduction, a column study performed using a gamma-densimetre, that gives information about the volumetric mass of the solid phase during time, can be carried out and their results can be compared with the other two above-mentioned investigations to identify the influence of each factor in the hydraulic performance of column tests. Thus, the hydraulic model applied for the tested mixtures can be better developed and calibrated.

A microstructural study on the exhausted reactive material will be useful to better understand the mechanisms developed during the removal processes.

Finally, tests with real contaminated groundwater should be carried out to investigate whether and how different chemical composition and condition of contaminated water affect the contaminant removal efficiency and hydraulic behavior of the used mixtures.

Figure 1 - 8 Figure 1 - 8

 1818 Figure 1-1 a) identified PCS, CS and RS; b) estimated PCS and CS (Panagos et al., 2013) ............. 2 Figure 1-2 Distribution of contaminants affecting soil and groundwater in Europe (Panagos et al., 2013) .......................................................................................................................................................................... 3 Figure 1-3 Example of permeable reactive barrier (USEPA, 1998) .......................................................... 5 Figure 1-4 continuous PRB trench system (USEPA, 1998) ........................................................................ 6 Figure 1-5 funnel-and-gate system (USEPA, 1998) ....................................................................................... 7 Figure 1-6 Panel-drain principle (Soletanche-Bachy's patent) (Courcelles et al., 2008)............... 7 Figure 1-7 GeoSiphon™ configuration (Di Molfetta et al., 2005) ............................................................. 8 Figure 1-8 Biopolymer trenching (Naidu and Birke, 2015) ....................................................................... 9 Figure 1-9 Sheet piles installation (Naidu and Birke, 2015) ..................................................................... 9 Figure 2-1 SEM pictures of ZVI samples using an enlargement of a) 31X and b) 100X .............. 26 Figure 2-2 SEM pictures of ZVI samples using an enlargement of a) 100X and b) 150X ............ 27 Figure 2-3 SEM pictures of ZVI samples using an enlargement of a) 500 X and b) 2k X ............ 27 Figure 2-4 SEM pictures of ZVI samples using an enlargement of a) 4k X and b) 7.5k X ............ 27 Figure 2-5 Pumice and ZVI grain size distributions ................................................................................... 29 Figure 2-6 SEM pictures of Pumice samples using an enlargement of a) 300 X and b) 150 X . 30 Figure 2-7 a) lyophilizer instrument and b) plate with sample in the lyophilizer ........................ 31 Figure 2-8 a) used penetrometer and Pumice sample; b) sample chamber .................................... 32 Figure 2-9 a) used penetrometer filled with Pumice sample; b) greasing step .............................. 33 Figure 2-10 a) penetrometer placed into Low pressure chamber; b) Low Pressure Chamber on ..................................................................................................................................................................................... 33 Figure 2-11 a) Low pressure analysis before beginning; b) AutoPore IV of Micromeretrics (Model 9505/9500) Low Pressure and High Pressure Chambers ....................................................... 33 Figure 2-12 log differential intrusion, cumulative intrusion and incremental intrusion as function of pore diameter obtained for test A .............................................................................................. 34 Figure 2-13 log differential intrusion, cumulative intrusion and incremental intrusion as function of pore diameter obtained for test B .............................................................................................. 35 Figure 2-14 log differential intrusion, cumulative intrusion and incremental intrusion as function of pore diameter obtained for test C ............................................................................................... 35 Figure 2-15 log differential intrusion, cumulative intrusion and incremental intrusion as function of pore diameter obtained for test D .............................................................................................. 36 Figure 2-16 SEM pictures of Pumice samples using an enlargement of a) 30 X and b) 200 X . 37 Figure 2-17 SEM pictures of Pumice samples using an enlargement of a) 250 X and b) 250 X .......................................................................................................................................................................................... 37

Figure 3

 3 Figure 3-9 C0/C* as function of C0 for HM tests considering tests with nickel at 5 and 50 mg/L initial concentration ................................................................................................................................................

Figure 3 -

 3 Figure 3-10 Cs as function of C0 using Langmuir model............................................................................

Figure 3 -

 3 Figure 3-11 sensitivity analysis output for LM batch test model using solution of nickel at 5 mg/L C0 .........................................................................................................................................................................

Figure 3 -

 3 Figure 3-12 sensitivity analysis output for LM batch test model using solution of nickel at mg/L C0 .........................................................................................................................................................................

Figure 3 -

 3 Figure 3-13 sensitivity analysis output for LM batch test model using solution of nickel at 500 mg/L C0 .........................................................................................................................................................................

Figure 3 -

 3 Figure 3-14 sensitivity analysis output for HM batch test model using solution of nickel at 5 mg/L C0 .........................................................................................................................................................................

Figure 3 -

 3 Figure 3-15 sensitivity analysis output for HM batch test model using solution of nickel at mg/L C0 .........................................................................................................................................................................

Figure 3 -

 3 Figure 3-16 sensitivity analysis output for HM batch test model using solution of nickel at 500 mg/L C0 .........................................................................................................................................................................

Figure 3 -

 3 Figure 3-17 Experimental data and simulation output for LM batch test using pure ZVI and solution of nickel at 5 mg/L initial concentration .......................................................................................

Figure 3 -

 3 Figure 3-18 Experimental data and simulation output for LM batch test using pure ZVI and solution of nickel at 50 mg/L initial concentration ....................................................................................

Figure 3 -

 3 Figure 3-19 Experimental data and simulation output for HM batch test using pure ZVI and solution of nickel at 5 mg/L initial concentration .......................................................................................

Figure 3 -

 3 Figure 3-20 Experimental data and simulation output for HM batch test using pure ZVI and solution of nickel at 50 mg/L initial concentration ....................................................................................

Figure 4 - 1

 41 Figure 4-1 Nickel removed mass by ZVI a) in 0.055 mm/min test after 5000 h and b) in 1.382 mm/min test after 200 h .......................................................................................................................................

Figure 4 - 2

 42 Figure 4-2 Nickel removed mass by ZVI a) in 0.055 mm/min test after 8216 h and b) in 1.382 mm/min test after 328 h .......................................................................................................................................

Figure 4 - 3

 43 Figure 4-3 Nickel removed mass by ZVI a) in 0.055 mm/min test after 9704 h and b) in 1.382 mm/min test after 472 h .......................................................................................................................................

Figure 4 -

 4 Figure 4-4 a) Nickel mass removed in function of nickel mass input; b) permeability in function of nickel mass removed .......................................................................................................................

Figure 4 -

 4 Figure 4-5 normalized concentration of nickel in 10:90 w.r. ZVI:Pumice, C0=50ppm, f.r= ......

Figure 4 -

 4 Figure 4-6 normalized concentration of nickel in 30:70 w.r. ZVI:Pumice, C0=50ppm, f.r.= column test ..................................................................................................................................................................

Figure 6 -

 6 Figure6-4 reaction processes in a porous medium[START_REF] Weber | Simulation and design models for adsorption processes[END_REF] Smith, 1987, Appelo, 2005).

  Figure6-4 reaction processes in a porous medium[START_REF] Weber | Simulation and design models for adsorption processes[END_REF] Smith, 1987, Appelo, 2005). .......................................................................................................................................................................................

Figure 6 - 5

 65 Figure 6-5 Graph of dimensionless dispersion coefficient versus Peclet number (Perkins and Johnson, 1963). .......................................................................................................................................................

Figure 6 -

 6 Figure 6-6 first-order rate coefficients for batch tests performed using solution of Nickel at 50 mg/L initial concentration and pure ZVI at a) LM and b) HM solid to liquid ratios ..................

Figure 6 -

 6 Figure 6-7 first-order rate coefficients for batch tests performed using solution of Nickel at 50 mg/L initial concentration and a) pure Pumice at HM solid to liquid ratio and b) 30:70 w.r. mixture at LM and HM solid to liquid ratios ...............................................................................................

Figure 6 -

 6 Figure 6-8 simulation results based on the use of coefficients related to batch tests performed using pure reactive materials ...........................................................................................................................

Figure 6 -

 6 Figure 6-9 simulation results based on the use of coefficients related to batch tests performed using 30:70 ZVI:Pumice w.r. mixture ............................................................................................................

Figure 6 -

 6 Figure 6-10 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 50 mg/L initial concentration and v1 flow velocity. ......................................................................................................................................................................

Figure 6 -

 6 Figure 6-11 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 50 mg/L initial concentration and v3 flow velocity. ......................................................................................................................................................................

Figure 6 -

 6 Figure 6-12 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 8 mg/L initial concentration and v2 flow velocity. ......................................................................................................................................................................

Figure 6 -

 6 Figure 6-13 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 8 mg/L initial concentration and v3 flow velocity. ......................................................................................................................................................................

Figure 6 -

 6 Figure 6-14 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 8 mg/L initial concentration and v1 flow velocity. ......................................................................................................................................................................

Figure 6 -

 6 Figure 6-15 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 95 mg/L initial concentration and v3 flow velocity. ......................................................................................................................................................................

Figure 6 -Figure 7 - 8 Figure 8 - 2 Figure 8 - 3 Figure 8 - 4 Figure 8 - 5 Figure 8 - 6 Figure 8 - 7 Figure 8 - 8 Figure 8 - 9 Figure 8 -

 67882838485868788898 Figure 6-16 simulation and experimental results of column test performed with 30:70 ZVI:Lapillus w.r. mixture, solution of Nickel at 50 mg/L initial concentration and v2 flow velocity. ......................................................................................................................................................................

  ........................................................................................... 67 Table 3-5 Concentration measured during batch tests ............................................................................. 68 Table 3-6 Nickel mass removed for each bath test at the first sampling time (with time) ........ 72 Table 3-7Nickel removed mass and Pumice mass ratio for each batch test at the first sampling time ................................................................................................................................................................................. 72 Table 3-8 values used to evaluate Langmuir coefficient .......................................................................... 72 Table 3-9 Langmuir coefficients used for the simulation ........................................................................ 74 Table 4-1 pure ZVI column tests carried out by the previous Ph.D. students ................................. 88 Table 4-2Characteristics of column tests performed using ZVI:Pumice mixtures ........................ 92 Table 4-3 Characteristics of column tests performed using ZVI:Pumice mixtures and Zinccontaminated solutions .......................................................................................................................................... 97 Table 5-1Column tests program to study long-term removal efficiency and hydraulic behavior of ZVI:Lapillus mixtures ...................................................................................................................................... 109 Table 5-2 pH and Eh(mv) measured values for 10:90 ZVI/Lapillus weight ratio column test ....................................................................................................................................................................................... 117 Table 5-3 pH and Eh(mv) values for 30:70 w.r. ZVI/Lapillus ............................................................. 119 Table 5-4 pH and Eh(mv) values for 50:50 w.r. ZVI/Lapillus ............................................................. 120 Table 5-5 characteristics of column tests considered in comparison based on the same ZVI content........................................................................................................................................................................ 132 XXII

  ZVI:Lapillus column test carried out under v3 .......................................................................................... 145 Table 5-9 Characteristics of column tests carried out in order to study the initial concentration influence ....................................................................................................................................... 152 Table 5-10 pH and Eh values at different filter length and different sampling times for 30:70 ZVI/Lapillus column test carried out using a solution of Nickel at 10 mg/L initial concentration ........................................................................................................................................................... 153 Table 5-11 pH and Eh values at different filter length and different sampling times for 30:70 ZVI:Lapillus column test carried out using a solution of Nickel at 100 mg/L initial concentration ........................................................................................................................................................... 155 Table 5-12 Characteristics of column tests carried out in order to study the configuration influence .................................................................................................................................................................... 160 Table 5-13 Characteristics of carried out column tests to study the zinc removal .................... 164 Table 5-14 Characteristics of carried out column tests to study the tri-contaminant removal ....................................................................................................................................................................................... 168 Table 6-1 reaction time as observed by batch tests results ................................................................. 181 Table 6-2 column tests characteristics ......................................................................................................... 181 Table 6-3 Peclet number for carried out column tests varying d50, Dd and v ............................... 183 Table 6-4 k2 (1/h) and k3 (1/h) coefficients as found analyzing batch tests .............................. 188 Table 6-5 Pi coefficients evaluated considering k2 coefficient and the solid to liquid ratio in batch test ................................................................................................................................................................... 189 Table 6-6 solid to liquid ratio and reactive mass to contaminant mass ratio for LM and HM batch test and for 30:70 ZVI:Pumice w.r. mixture performed with 0.276 mm/min constant flow velocity (and solution of Nickel at 50 mg/L initial concentration) ......................................... 191 Table 6-7 coefficients of non-equilibrium sorption model as found by optimization............... 193 Table 7-1 Characteristics of pure Pumice and pure Lapillus carried out column tests ............ 200 Table 7-2 Characteristics of column tests considered in comparison of weight ratio influence between ZVI:Pumice and ZVI:Lapillus mixtures ....................................................................................... 203 Table 7-3 Filter lengths and correspondent ZVI mass for mixtures of ZVI:Pumice and ZVI:Lapillus at different weight ratios........................................................................................................... 204 XXIII Table 7-4 Characteristics of column tests considered in comparison of flow velocity influence between ZVI:Pumice and ZVI:Lapillus mixtures ....................................................................................... 215 Table 7-5 Characteristics of column tests considered in comparison of contaminant initial concentration influence between ZVI:Pumice and ZVI:Lapillus mixtures ..................................... 221 Table 7-6 Characteristics of column tests considered in comparison of long-term Zinc removal efficiency between ZVI:Pumice and ZVI:Lapillus mixtures ................................................ 224 1 Groundwater contamination and PRB remediation technology

  Since 2000 the European Environmental Agency has drawn up six reports about the state of the art of contaminated sites in Europe. Similar data have been obtained in the data collections reported in the different versions. The Joint Research Centre of the European Commission published in May 2014 the last work of data collection exercises completed regarding the indicator CSI 015 "Progress in the management of Contaminated Sites in Europe" (Van Liedekerke et al., 2014). The European Soil Data Centre (ESDAC) of the European Commission has conducted a project to collect and manage the contaminated sites data collected through the European Environmental Information and Observation Network (EIONET). Precisely, EIONET includes 28 Member States of the European Union together with Iceland, Liechtenstein, Norway, Switzerland, Turkey, and the West Balkan cooperating countries: Albania, Bosnia, Herzegovina, the former Yugoslav Republic of Macedonia, Montenegro, and Serbia as well as Kosovo. The data collection regarded management of contaminated sites, remediation targets and technologies, contribution of polluting activities to local soil contamination, environmental impacts and expenditure (http://www.eea.europa.eu/data-andmaps/indicators/progress-in-management-of-contaminated-sites-3/assessment).

  a) and b) provide data on identified numbers of PCS and CS and on estimated numbers of PCS and CS, represented respectively. A key aspect for research and development of remediation, analyzed by Van Liedekerke et al. (2014) consists of the percentage of contaminants affecting soil and fluid matrices. A similar distribution of contaminants affecting soil and groundwater can be deduced from the analysis results reported in Figure 1-2. In particular, heavy metals represent the dominant contaminant category. The second category mainly found is that of mineral oil.

Figure 1 -

 1 Figure 1-1 a) identified PCS, CS and RS; b) estimated PCS and CS (Panagos et al., 2013)

Figure 1 - 2

 12 Figure 1-2 Distribution of contaminants affecting soil and groundwater in Europe (Panagos et al., 2013)

Figure 1 - 3

 13 Figure 1-3 Example of permeable reactive barrier (USEPA, 1998)

Figure

  Figure 1-4 continuous PRB trench system (USEPA, 1998)

Figure

  Figure 1-5 funnel-and-gate system (USEPA, 1998)

Figure 1 - 6

 16 Figure 1-6 Panel-drain principle (Soletanche-Bachy's patent) (Courcelles et al., 2008)

Figure 1 - 7

 17 Figure 1-7 GeoSiphon™ configuration (Di Molfetta et al., 2005)

Figure 1 - 8

 18 Figure 1-8 Biopolymer trenching (Naidu and Birke, 2015)

Figure

  Figure 1-9 Sheet piles installation (Naidu and Birke, 2015)

  PEREBAR is a European Project regarding "Long-term Performance of Permeable Reactive Barriers used for the Remediation of Contaminated Groundwater". It was a Research Project within the 5 th Framework Programme of the European Union. The Universitaet Karlsruhe (Technische Hochschule) in Germany worked as coordinator while the Federal Institute For Material Research And Testing in Germany, G.U.T. Gruppe Umwelt & Technik Gmbh in Austria, the Ingenieursbuero Fader Umweltanalytik in Germany, Mecsekerc Koernyezetvedelmi Rt in Hungary, the Miskolc University in Hungary, the National Technical University of Athens in Greece and the University of Leeds in United Kingdom have taken part as participants.The objective of the PEREBAR project was to assess and improve the long-term performance of permeable reactive barrier systems, focusing on systems based on sorption and precipitation of heavy metals and sorption and decomposition of organic compounds. The primary PEREBAR model site is an area in southern Hungary contaminated by uranium mining. (http://www.perebar.bam.de/).

  Moraci et al. (2012b) have developed a new theoretical method based on simulation of seepage through a granular soil represented by a sequence of parallel layers containing constrictions and spherical particles with different packing. The particle packing and the distance between two layers is a function of soil relative density. The soil internal stability is evaluated by comparing particles contained in a layer to the constrictions of the next layer in the hydraulic flow direction.

  Chapter 2 the reactive materials used in the research are introduced. Zero Valent Iron, Pumice and Lapillus were characterized by grain size analysis, pycnometer measurements, Scanning Electron Microscopic (SEM) observation and Mercury Intrusion Porosimeter analysis. In Chapter 3, the results of batch tests performed using Pumice, ZVI and ZVI:Pumice mixture put in contact with solutions of Nickel at different initial concentration at different solid to liquid ratio are considered to develop a Nickel removal model. In Chapter 4 the experimental data obtained from column tests performed using ZVI and ZVI:Pumice mixtures to remediate heavy-metal contaminated groundwater are reviewed in the light of the development of the column test model. In Chapter 5, the experimental results of column tests performed using Lapillus and ZVI:Lapillus mixtures are shown and discussed. In Chapter 6, a model to simulate the long-term removal efficiency of the previously tested mixtures is proposed. In Chapter 7, a comparison among ZVI:Pumice and ZVI:Lapillus mixtures tested under the same experimental conditions is described. In Chapter 8, release tests results and the evolution of Iron concentration compared to that of Nickel along the column length and during the time are reported.

  Some pictures taken during the observation are shown in Figures2-1, 2-2, 2-3 and 2-4. In the first (Figure2-1 a), an overview of the sample can be seen. A small number of grains can be observed and one can be clearly seen. The others can be less easily distinguished from each other probably because of the plasma layer not being sufficient or of the varnish (Figure2-1 b). Thus the observation focused on the grain clearly distinguishable whose size is representative of d50 of the grain size distribution (Figure3-1 a). On the almost smooth surface of this ZVI particle, the are some small particles, that according to the analysis carried out by SEM-EDX seem to be iron oxides. These little particles that are compounds of Iron were observed using an enlargement of 500 X (Figure2-3 a), 2000 X (Figure 2-3 b), 4000 X (Figure 2-4 a) and 7500 X (Figure 2-4 b). They increase the roughness and the specific surface of the ZVI particle. They seem to have different shapes (Figures 2-3) and can be either highly porous or not (Figures 2-4). Their surface can be smooth or highly rough (Figures 2-4 a), while for porous small particles, pores of 1 μm or less can be observed (Figures 2-4 b).

Figure

  Figure 2-1 SEM pictures of ZVI samples using an enlargement of a) 31X and b) 100X

FigureA

  Figure 2-5 Pumice and ZVI grain size distributions

Figure 2 -

 2 Figure 2-6 SEM pictures of Pumice samples using an enlargement of a) 300 X and b) 150 X

  Figure 2-7 a) lyophilizer instrument and b) plate with sample in the lyophilizer

  Figure 2-9 a) used penetrometer filled with Pumice sample; b) greasing step

FigureFigure 2 -

 2 Figure 2-12 log differential intrusion, cumulative intrusion and incremental intrusion as function of pore diameter obtained for test A

  SEM "Società Estrattiva Monterosi s.r.l.", Viterbo, Italy. It has granular form and a redmaroon colour (Figure2-19) and originated from explosive volcanic activity of the Sabatini Mountains.

FigureFigure 2 -

 2 Figure 2-19 Lapillus sample

Figure 2 -

 2 Figure 2-21 Lapillus grain size distribution (sample from quarry)

Figure 2 -

 2 Figure 2-22 Lapillus and ZVI grain size distributions (as used in column tests)

Figure 2 -

 2 Figure 2-23 SEM pictures of Lapillus samples using an enlargement of a) 34 X and b) 40 k X

  and 2-25). Furthermore, the particle surface presents roughness at small scale also (Figure2-26 and 2-27).

Figure 2 -

 2 Figure 2-25 SEM pictures of Lapillus samples using an enlargement of a) 75 X and b) 75 k X

Figure 2 -

 2 Figure 2-28 to 2-31.

Figure 2 -Figure 2 -

 22 Figure 2-28 log differential intrusion, cumulative intrusion and incremental intrusion as function of pore diameter obtained for test A on Lapillus samples.

FigureFigure 2 -

 2 Figure 2-30 log differential intrusion, cumulative intrusion and incremental intrusion as function of pore diameter obtained for test C on Lapillus samples.

  their mixtures for nickel removal and performed during previous Ph.D. cycles will be re-analyzed to propose a general overview and interpretation. The aim of this work is to observe the Nickel removal efficiency of the reactive materials as they are able to develop in this kind of interaction test, to hypothesize the removal mechanisms involved and to develop a model to simulate the results. The results of MIP analysis, SEM observation, grain size distribution and pycnometric analysis of Pumice samples, as obtained in Chapter 2, are taken into account for analysis of experimental results of batch tests and the modelling development for their simulation.

  Batch tests were performed preparing vials containing the reactive materials and the contaminated solution, putting them in rotation through an end over end rotating stirrer (Stuart Scientific Drive STR/4.1) and sampling the contaminant solution at different times. Reactive material and contaminant solution were placed in septumcapped vials with no head-space with a prefixed liquid to solid ratio. At each sampling time, the vial from which liquid samples was drawn was sacrificed.The Figures3-1a) and b) show the results of batch tests carried out with pure Pumice and with low solid : liquid ratio (Low Mass -LM) and with high solid : liquid ratio (High Mass -HM) with different Nickel initial concentration. The star symbol refers to 5 mg/L of initial Nickel concentration, the circle to 50 mg/L and the rhombus to 500 mg/L.

Figure 3 - 1

 31 Figure 3-1 Batch tests results: variation of nickel normalized concentration over time for a) LM and b) HM tests Pumice immediately reduces the initial nickel concentration of a certain percentage increasing with the solid : liquid ratio. It is possible to make some hypothesis. It is

Figure 3

 3 Figure 3-2 a) relationship between the Cmin and C0 in batch tests; b) Nickel concentration evolution as function of filter thickness for different sampling times in column test

Figure 3 - 3

 33 Figure 3-3 Schematic double scale structure of a pumice particle a) Cylindrical form assumption b) spherical form assumption c) geometry of an internal pore

  𝛽 0[-] defines the capacity of the absorbent and 𝛼 [L 3 M -1 ] reflects the affinity between the absorbent and adsorbate. This model was chosen on the basis of observations of batch and column tests experimental results. 𝑆 * , contaminant adsorbed mass on reactive mass increasing with initial concentration 𝐶 0 , in correspondence of an initial values range, and not depending on 𝐶 0 after the maximum 𝑆 * value is reached can be hypothesized. A certain removal limit, which is function of the initial concentration 𝐶 0 , is in fact observed in each test. In particular, the maximum value of 𝑆 * , function of 𝐶 0 as defined inLangmuir model, is found analyzing the batch tests experimental results and considering the almost instantaneous removal effect of nickel onto the pumice external surface, developed during the first sampling time at half an hour from the start of the batch test. Considering an instantaneous sorption on the external surface of pumice depending on the initial concentration of the solution can be taken into account using equation 3To convert the (𝑆 * ) 𝑒 quantity, representing the mass of nickel instantaneously sorbed on the external surface, into the correspondent concentration (𝐶 𝑠 ) 𝑒 , meaning the mass of nickel removed from the external solution volume by external surface of pumice, it is enough to multiply it by 𝑀 𝑝 (equation 3-14).

Figures 3 -

 3 Figures 3-4 a) show the variation of nickel normalized concentration with time for all tests during the entire experimental period. Figures 3-4 b) and 3-5 and 3-6 a) and b)show the variation of concentration for each test during the first fifty hours.

Figure 3 -Figure 3 - 5

 335 Figure 3-4 a) Nickel normalized concentration evolution for all tests and the entire duration time b) nickel concentration for batch test at LM liquid:solid ratio and 5 mg/L nickel initial concentration until 50 h

𝛽 0

 0 in the Langmuir equation has the meaning of maximum contaminant mass to reactive media mass ratio. The Figure 3-7 shows the results of the batch tests carried out in terms of ratio between nickel mass removed and pumice mass as function of time. Results of four tests are reported. It is possible to hypothesize an immediate removal efficiency due to sorption and diffusion mechanisms effects time-depending for each tests.

Figure

  Figure 3-7 Nickel mass removed to pumice mass ratio as function of time

Figure 3 - 8

 38 Figure 3-8 C0/C* as function of C0 for a) LM tests and b) HM tests

  Figures 3-11 to 3-16 the output of sensitivity analysis of the model are shown for each condition of batch tests carried out. As can be observed, the Sm parameter seems to influence the most the results if LM conditions with solution of nickel at 5 mg/L of initial concentration are used. For the other two initial concentrations (50 and 500 mg/L) of nickel, Npores_g, d and Sm have the highest influence in both LM and HM conditions.

Figure 3 -

 3 Figure 3-11 sensitivity analysis output for LM batch test model using solution of nickel at 5 mg/L C0

  DL and v are the longitudinal dispersion coefficient and the velocity respectively. Rk represents the rate of mass change due to mechanism k. On the basis of the consideration about batch and column tests experimental results, it is possible to hypothesize what Ni removal mechanisms take place. During the first interaction time, the immediate removal mechanism is probably attributable mostly to sorption process onto the external Pumice surface (k=1).After the flow of several pore volumes have flowed, the pumice reactive sites onto surface seem completely occupied and the input constant concentration C0 cannot be reduced anymore. It is possible to observe effects of a non-equilibrium removal mechanism, too, leading to a further decrease of the residual concentration. This second process could be attributable to the Ni concentration gradient between the inter-grains flowing water and internal pore stagnant water. It can take place after water has occupied at least part of the internal pores. The concentration gradient becomes the driving force of the molecular diffusion of nickel between the water outside and inside the pumice pores. It is possible to observe that at each of the considered column lengths, the concentration values tend to an equilibrium concentration value Ceq, that may vary slowly with time.

Figure 3 -

 3 Figure 3-17 Experimental data and simulation output for LM batch test using pure ZVI and solution of nickel at 5 mg/L initial concentration

  mixtures and pure ZVI, performed by previous Ph.D. student are taken into account and re-analyzed, to introduce the experimental results in view of the development of column test modelling.

  For each column test, nickel concentration, pH, Eh, temperature and permeability values are available for different sampling times and ports. Furthermore, one column test was performed during this Ph.D. course to test the Zinc removal efficiency of 50:50 ZVI:Pumice w.r. mixture and to compare it to the previously tested 30:70 ZVI:Pumice w.r. mixture performance.4.2 Pure Pumice, Pure ZVI and ZVI:Pumice granular mixtures column tests general reviewConsidering that nickel concentration values are known for different distances of column length and for different times, it is possible to calculate the nickel mass retained by the reactive material placed between two sections of the column length.To calculate the nickel mass that has been retained or removed by the reactive medium, a mass balance equation was used. It can be defined as the point placed 𝑥 𝑖 along the column, where index 𝑖 varies between zero and the 𝑛 total number of the sampling ports, and 𝑡 𝑗 time, where index 𝑗 is for the progressive number of sampling times. nickel mass retained in a column sector placed between 𝑥 𝑖 and 𝑥 𝑖+1 , during the interval between 𝑡 𝑗 and 𝑡 𝑗+1 , under the hypothesis of homogeneous distribution of nickel mass retained in the transversal section 𝑆, can be calculated by the following equation (4-1) :

  )Bilardi (2011) *test interrupted because of column clogging Two column tests using pure ZVI and a contaminant solution of nickel at initial contaminant concentration of 40 mg/L were carried out using two flow velocities: one of 0.055 mm/min and one of 1.382 mm/min. The distribution of nickel removed along the filter with time, pH, Eh values and permeability evolution were considered.In Figure4-1 -4-3 a) and b), the nickel removed mass by ZVI unit mass are reported for different sectors of filter lengths at different times corresponding to the same input mass for both flow rate tests.

Figure 4 - 1

 41 Figure 4-1 Nickel removed mass by ZVI a) in 0.055 mm/min test after 5000 h and b) in 1.382 mm/min test after 200 h

Figure 4 - 2 Figure 4 - 3

 4243 Figure 4-2 Nickel removed mass by ZVI a) in 0.055 mm/min test after 8216 h and b) in 1.382 mm/min test after 328 h

Figures

  Figures 4-5 -4-7, results of 10:90, 30:70 and 50;50 weight ratios are reported respectively. In the a) figures represent the results relative to the interaction time from the beginning to about 1440 hours after, the b) figures represent the time afterwards until the end of the tests. The three tests were realized under a constant

Figure 4 -

 4 Figure 4-5 normalized concentration of nickel in 10:90 w.r. ZVI:Pumice, C0=50ppm, f.r=

  and 50:50 w.r. tests respectively. In the 10:90 test, the value of 0.99 is already reached at 2448 hours. Looking at the end of the less efficient part of filter length, corresponding to the outlet of the 30:70 and 50:50 w.r. and to 58 cm of 10:90 w.r. filters, the final relative concentration is around 0.8, 0.63 and 0.47 for 10:90, 30:70 and 50:50 w.r column tests respectively. This can suggests that the reaction between ZVI and nickel is immediate, considering the contact time ensured by the constant flow rate under which these tests have been realized. Under almost the same contact time, the higher the ZVI content the higher the removal efficiency. The calculated residence times are 21 h for 10:90 w.r. column test (considering 1 meter length) and 13 h for 30:70 and 50:50 column tests, considering the entire column length used. On the other hand, the contaminant flow needs about 20 minutes to pass through 1.5 centimenters of reactive filter, that are the most reactive.

  Figures 4-10, the nickel normalized concentration evolution is shown for tests performed using 30:70 ZVI:Pumice mixture with a solution of nickel at 50 mg/L of initial concentration and a flow velocity equal to 1.382 mm/min.

Figure 4 - 8

 48 Figure 4-8 normalized concentration of nickel in 30:70 w.r. ZVI:Pumice test using solution of nickel at 50 mg/l of initial concentration and v3 flow velocity a) until 132 h and b) 784 h.

Figure 4 -

 4 Figure 4-9 normalized concentration of nickel in 30:70 w.r. ZVI:Pumice tests a) using solution of nickel at 40 mg/l of initial concentration and v1 flow velocity and b) using solution of nickel at 8 mg/l of initial concentration and v2 flow velocity

  -14 a) the evolution of Zinc concentration normalized by the initial value at outlet is shown for the two tests. It seems that the breakthrough is reached at the same time, altough the subsequent Zinc concentration increase is slower in the 50:50 ZVI:Pumice w.r. mixture test. In Figure 4-14 b) the hydraulic performance evolution as function of time is shown for both tests. The hydraulic conductivity stays constant for both tests.

Figure 4 -

 4 Figure 4-10 a) Evolution of Zinc normalized concentration with time and b) hydraulic conductivity evolution for 30:70 and 50:50 ZVI:Pumice mixtures tested with solution of Zinc at 50 mg/L initial concentration and 0.276 mm/min constant flow velocity

  th column test. Comparing the output of this procedure for each column test involved in a set of experiments performed varying one parameter, it was possible to study its influence. In the red and green squares the output and the intermediate processing steps are reported respectively.

Figure 5 -

 5 Figure 5-1interpretation procedure of experimental data available for the i -th column test

  ratio (Figure 5-2 a), to study their removal efficiency and hydraulic behavior, under a constant velocity rate of contaminant solution. Columns experiments were carried out using polymethyl methacrylate (PMMA-Plexiglas™) columns, equipped with sampling ports located at different distances from the inlet (Figure 5-3 a).

Figure 5 -

 5 Figure 5-2 a) ZVI:Lapillus mixture; b) bottom of column with o-ring and inox grid; c) and with geotextile filter

Figure 5 -

 5 Figure 5-4 a) set of column; b) peristaltic pump; c) multi-parameter

Figure

  Figure 5-5 Figure 0 5 a) AAS and b) ICP/OS

Figure 5 -

 5 Figure 5-6 a) reactive material sampling, b) and c) two steps of column tests disassembling .

  1. The column tests, using solution contaminated by nickel, were carried out in order to evaluate the influence of the were carried out using zinc-contaminated solution and pluricontaminated solution of nickel, zinc and copper.The investigation is focused on identifying also the most important parameters influencing the removal mechanisms of contaminants and the long term hydraulic a

  test using pure lapillus demonstrated a certain capacity of the volcanic material to reduce the nickel concentration in the above mentioned experimental conditions. 1289 gr of lapillus were used in the column test which lasted 2800 hours (116 days). The experimental results are represented in Figures 5-7 a), b) and c) as function of time. In Figures 5-7 a), b) and c) the evolution of the nickel normalized residual concentration (C/C0) of nickel, of pH and Eh values related to the three sampling ports located at a distance of 1.5, 18 and 50 cm from the column inlet are reported respectively. A correspondence among nickel residual concentration and pH and Eh values can be observed. Concerning the sampling ports where a high nickel concentration is observed, the pH is similar to input solution (6.5) and the Eh is positive and equal to about 30 or 40 mV. Two different time intervals can be identified in the test duration. The first one starts at the beginning of the test and goes on until 500 hours. During the first interval, an increase in nickel residualconcentration, an increase in Eh value, from 15 to 40mV and a decrease in pH value, from almost 6.5 to 6 are observed for the sampling ports located at 3 and 18 cm from the column inlet. During the same time, low values of nickel concentration are measured at the outlet, while pH stays constant (around to 7) and Eh seems to decrease, (however its values are less than that of the first sampling port). After 500 hours of interaction testing, a new decrease of nickel concentration can be observed at the first ports, a pH increase up to 7 and Eh decrease down to 0 or negative values.At the outlet, an increase of nickel concentration is observed and in the same time pH decreases to around 6 and Eh increases to 40.

Figure 5 - 7

 57 Figure 5-7 Variation of a) nickel normalized concentration ,b) pH and c) Eh (mV) over time for the pure Lapillus column test

  Figure 5-8 Pure Lapillus column test a) nickel normalized concentration ,b) pH and c) Eh (mV) variations with time

Figure 5 -

 5 Figure 5-9 a) Nickel removal efficiency as function of the input mass for different lengths of pure Lapillus filter; b) hydraulic conductivity profile over time

Figure 5 -

 5 Figure 5-10 a) cumulative nickel mass retained per unit Lapillus mass (mg/gr) as function of nickel mass input; b) nickel mass retained by unit mass of Lapillus per unit time for each column sector (mg/(gr*h))

Figure 5 -Figure 5 -

 55 Figure 5-11 Nickel normalized concentration evolution for different sampling ports with time for a) 10:90 and b) 30:70 weight ratios ZVI/Lapillus column tests

Figure 5 -

 5 Figure 5-14 Nickel normalized concentration for 30:70 w.r. ZVI:Lapillus column tests until 2448 h

  a) and b) and 5 -15 a), the nickel normalized concentration evolution along the 30:70 w.r. column is represented. pH and Eh (mV) values are reported in

  a) and b), the evolution of nickel normalized concentration for 50:50 w.r. column is shown. During a first interval (from the test beginning to about 2280 hours), the remediation occurs within 18 cm (Figure 5-16 a), in a second interval a filter thickness of 28 cm and afterwards 38cm (Figure 5-16 b) are enough for reducing of nickel concentration. In

Figure 5 -

 5 Figure 5-17 Nickel mass retained in each sector divided by the length of the sector in 10:90 w.r. ZVI/Lapillus column test at a) 432 hours and b) 1440 hours

Figure 5 -Figure 5 -

 55 Figure 5-19 Nickel mass retained in each sector divided by the length of the sector in 30:70 w.r. ZVI/Lapillus column test at a) 432 hours and b) 1440 hours

Figure 5 -Figure 5 -Figure 5 -

 555 Figure 5-22 Nickel mass retained in each sector divided by the length of the sector in 50:50 w.r ZVI/Lapillus column test at a) 432 hours and b) 1440 hours

  ratio tests, pH and Eh values ranges are larger in 50:50 w.r., than in the other tests, while the final pH and Eh values are almost the same. It means that 50:50 w.r. column passes through more different chemical conditions, but the values to which pH and Eh tends are similar for the three test. This is more evident if values

  three tested ZVI/Lapillus mixtures with different weight ratios have been characterized by different rates of nickel retention and different maximum quantity of nickel that can be retained in each transversal section. This is mainly due to the different composition of the mixtures and different proportion between ZVI and Lapillus particles in the transversal section and along the filter among the three tests at different weight ratios. To investigate how the specific removal rate can vary along the column, with time and for each ZVI/Lapillus tested mixture, the partial mass of nickel retained by each column sector at different intervals of time was calculated and divided by the mass of ZVI or by the total mass of reactive materials contained in each sector. These values are shown in normal and semi-logarithmic scale for 10:90 w.r. test, in figures 5-26 and 5-27, for 30:70 w.r. test in figure 5-28 and 5-29 and for 50:50 w.r. test in figure 5-30 and 5-31 respectively.

Figure 5 -Figure 5 -Figure 5 -

 555 Figure 5-26 a) Partial Nickel mass retained in each column sector during each interval divided by the ZVI mass contained in the sector per unit time (semilog scale); b) Partial Nickel mass retained in each column sector during each interval divided by the reactive material mass contained in the sector per unit time (semilog scale) in 10:90 w.r. column test

Figure 5 -Figure 5 -Figure 5 -

 555 Figure 5-29 a) Partial Nickel mass retained in each column sector during each interval divided by the ZVI mass contained in the sector per unit time; b) Partial Nickel mass retained in each column sector during each interval divided by the reactive material mass contained in the sector per unit time in 30:70 w.r. column test

Figure 5 -

 5 Figure 5-33 a) Nickel normalized concentration evolution determined over time at a) 18 cm of 10:90 w.r. and 3 cm of 50:50 w.r. column tests, 18 cm of lapillus and 3 cm of ZVI and at b) 28 cm of 10:90 w.r., 8 cm of 30:70 w.r. and 28 cm of lapillus.

  -34a) Nickel normalized concentration evolution at 38 cm of 30:70 ZVI:Lapillus w.r. column test and at 18 cm of 50:50 ZVI/Lapillus w.r. column test is shown (Table5.5). The first considered length corresponds to 365 gr of ZVI and 8.5 hours of residence time. The second one length contains 349 g of ZVI and the corresponding residence time is 4 hours. Both considered thicknesses are highly 2000 hours. Afterwards the 18 cm long filter of 50:50 w.r. starts to reduce Nickel removal efficiency before the 38 cm long filter of 30:70 w.r. mixture.

Figure 5 -

 5 Figure 5-34 a) Nickel normalized concentration evolution determined along time at 38 cm of 30:70 w.r. and 18 cm of 5 0:50 w.r. column tests and b) Nickel retained mass as function of Nickel mass input at 8 cm of 10:90 w.r., 3 cm of 30:70 w.r., 28 cm of 10:90 w.r. and 8 cm of 30:70 w.r. column tests.

Figure 5 -(Figure 5 -

 55 Figure 5-35 a) Nickel mass retained mass as function of Nickel input mass at 8 cm of 10:90 w.r., 3 cm of 30:70 w.r., 18 cm of 10:90 w.r. and 8 cm of 50:50 w.r. column tests and b) at 38 cm of 30:70 w.r., 18 cm of 50:50 w.r., 18 cm of 10:90 w.r. and 3 cm of 50:50 w.r. column tests.

Figure

  Figure 5-37 a) Nickel mass retained as function of Nickel mass input in 8 cm filter length and b) 18 cm filter length of the three tested weight ratios

  2012c), three column tests varying the flow velocity were carried out in order to study the influence of flow velocity on long-term Nickel removal efficiency and hydraulic behaviour. The characteristics of the columns are summarized in Table5-6.The used weight ratio ZVI:Lapillus was 30:70, while the Nickel initial concentration was 50 mg/L. This means that only the residence time varies significantly among the three tests. For the test performed using a constant average flow velocity v1 equal to 0.055 mm/min (9.2e-07 m/s and 0.079 m/d), the initial residence time was about 56 hours, for that using v2 equal to 0.276 mm/min (4.6e-06 m/s and 0.4 m/d), it was about 11 hours, while for the test carried out with v3 equal to 1.382 mm/min (2.3 e-05 m/s and 1.9 m/d) flow velocity, the initial residence time was about 2 hours.In Figure5-39 a) and b) the Nickel normalized concentration evolution at each sampling time as function of filter thickness and at each sampling port as function of time is shown respectively for column test carried out at v1 constant average flow velocity. In Figure 5-40 a) and b) the Nickel normalized concentration evolution is represented for column tests carried out using v3 constant average flow velocity.

Figure 5 -

 5 Figure 5-39 Evolution of Nickel normalized concentration a) along the column at different sampling time and b) at each sampling port during the time for 30:70 ZVI/Lapillus column test carried out under 0.055 mm/min constant flow velocity

  40 a) the Nickel normalized concentration evolution with filter thickness for each sampling time for v3 test is shown. In Figure5-39 b) its evolution at each sampling port with time is depicted.

Figure 5 -

 5 Figure 5-40 Evolution of Nickel normalized concentration a) along the column at different sampling time and b) at each sampling port during the time for 30:70 ZVI/Lapillus column test carried out under 1.382 mm/min constant flow rate

  -41 a) the Nickel cumulative retained mass by the three tests carried out at three different constant flow velocities is shown. Until 1500 mg of Nickel in input the removal efficiency is high for all the tests. As this quantity increases, the removal efficiency of v3 test decreases until 60% corresponding to 4000 mg of Nickel in input, while that relative to the other two tests stays at high values (about 99%) until 6000 mg of Nickel input mass.

Figure 5 -

 5 Figure 5-41 a) cumulative nickel mass retained as function of Nickel mass Input for v1, v2 and v3 tests; b) nickel mass retained per unit of sector length for each filter sector for v1 test at 12384 hours.

Figure 5 -

 5 Figure 5-42 a) Nickel mass retained per unit of sector length for each filter sector for v2 test at 2448 hours and b) for v3 test at 499 hours.

Figure 5 -FigureFigure 5 -

 55 Figure 5-43 Hydraulic conductivity evolution a) as function of time and b) as function of input contaminant mass

Figure 5 -

 5 Figure 5-46 specific removal rate evolution with time for each filter sector a) for v1 test and b) for v2 test.

  Three column tests were performed to study the influence of the initial contaminant concentration on long-term Nickel removal efficiency and hydraulic behaviour. 30:70 ZVI:Lapillus mixtures filters have been used with solutions contaminated by Nickel at 10, 50 and 100 mg/L of initial concentration. The characteristics of column tests and their benchmarks (pure ZVI and pure Lapillus) are reported inTable 5-9.

Figure 5 -

 5 Figure 5-48 Evolution of Nickel normalized concentration a) along the column at different sampling time and b) at each sampling point with time for 30:70 ZVI/Lapillus column test carried out using a solution of Nickel at 10 mg/L initial concentration

  in the entire filter length, also in the first centimetres where this has not been observed for the previous tests.

  The removal efficiency of the tests performed varying the Nickel initial concentration is shown in Figure 5-50 a) as Nickel cumulative mass retained as function of Nickel mass input. It can be observed that all the three filters are able to retain 2000 mg of Nickel in input.

Figure 5 -

 5 Figure 5-49 Evolution of Nickel normalized concentration a) along the column at different sampling time and b) at each sampling port with time for 30:70 ZVI:Lapillus column test carried out using a solution of Nickel at 100 mg/L initial concentration

  -50 b) and 5-51 a) and b), the distribution of Nickel mass retained by different sectors is shown for the tests performed using solutions of Nickel at 10, 50 and 100 mg/L initial concentration respectively, for a preset Nickel input mass. As , in the first considered test, the Nickel retained mass is concentrated in the first 3 cm of the filter, in the second one it is distributed in the first 18 cm of the filter, in the last in the first 28 cm.

Figure 5 -Figure 5 -

 55 Figure 5-50 a) cumulative Nickel mass retained as function of Nickel mass input for column tests performed using solution of Nickel at 10, 50 and 100 mg/L initial concentration; b) retained Nickel mass per unit of sector length for each filter sector for test with Ni at 10 mg/L initial concentration at 5568 hours.

FigureFigure 5 -

 5 Figure 5-52 Hydraulic conductivity evolution a) as function of time and b) as function of input contaminant mass

Figure 5 -FigureFigure 5 -

 55 Figure 5-54 specific removal rate evolution with time for each filter sector a) for column test performed using solution of Nickel at 10 mg/L of initial concentration and b) for test performed using solution of Nickel at 50 mg/L of initial concentration

FigureFigure 5 -FigureFigure 5 -

 55 Figure 5-58 Hydraulic conductivity evolution a) as function of time and b) as function of input contaminant mass for tests at different contaminant initial concentration

Figure 5 -

 5 Figure 5-62 Zinc normalized concentration a) as function of time at each sampling port and b) as function of filter thickness for each sampling time.

  63 b) the Zinc mass retained mass per unit of sector length for different filter sectors at 2328 hours is shown. It can confirm the high Zinc removal of the first centimetres, also after a long time of interaction.

Figure 5 -

 5 Figure 5-63 a) cumulative zinc mass retained mass as function of zinc mass input b) Zinc mass retained per unit of sector length for different filter sectors.

Figure 5 -

 5 Figure 5-64 a) Hydraulic conductivity evolution as function of Zinc retained mass; b) removal specific rate evolution with time for each filter sector for column test performed using solution of Zinc at 50 mg/L of initial concentration

  -65 a) and b) the results concerning the 50:50 weight ratio ZVI:Lapillus mixture and the pure ZVI filters tested using mono-contaminant and threecontaminant solutions for Nickel and Zinc contaminants respectively are shown. The 18 cm thick filter of 50:50 ZVI:Lapillus mixture corresponds to about 340 gr of ZVI, while in the 3 cm of pure ZVI filter the mass of reactive material is 240 gr. Concerning

Figure 5 -

 5 Figure 5-65 a) Zinc and b) Nickel normalized concentration evolution at 18 cm long 50:50 ZVI:Lapillus w.r. mixtures and at outlet of pure ZVI filters carried out using mono and three-contaminant solutions

Figure 5 -

 5 Figure 5-66 a) Zinc and b) Nickel cumulative retained mass as function of contaminant input mass for 18 cm long 50:50 ZVI:Lapillus w.r. mixtures and pure ZVI filters carried out using mono and threecontaminant solutions

Figure 5 -

 5 Figure 5-67 Hydraulic evolution with time a) for 50:50 ZVI/Lapillus w.r. mixtures and b) for pure ZVI tests.

  Chapter 5, the removal efficiency and hydraulic behavior of mixtures based on ZVI and internally porous materials can be influenced significantly by different factors.

  -1. The mass balance represents the first step and it has been based on mechanisms of diffusion, advection and reaction. Afterwards, the observation of microstructure evolution represents the basis of interpretation and understanding of permeability evolution.

Figure 6 - 1

 61 Figure 6-1 Scheme of methodology

  the terms of variation of solute concentration C with time, the diffusion-dispersion DL term expressed by Fick's law, the advection term depending on the vx flow rate and the reaction terms Ri. The D coefficient is expressed by the equation (6-5) and this includes the D* diffusion coefficient and the hydrodynamic dispersion coefficient parallel to the principal direction of flow, defined as αL longitudinal dynamic dispersion times the seepage velocity vx. Each term of equation (6-1) is multiplied by the 𝜑 𝑒 effective porosity value to refer all the numerical models and then the results to a unit volume. The equations (6-2) and (6-3) express the total porosity variation with time. 𝜃 𝑧 represents the volumetric fraction occupied by ZVI, α

Figure 6 - 3

 63 Figure 6-3 Nickel removal using ZVI (Suponik et al., 2014)

Figure 6 -

 6 Figure 6-4 reaction processes in a porous medium (Weber and Smith, 1987, Appelo, 2005).

Figure 6 - 5

 65 Figure 6-5 Graph of dimensionless dispersion coefficient versus Peclet number (Perkins and Johnson, 1963).

Figure 6

 6 Figure 6-6 first-order rate coefficients for batch tests performed using solution of Nickel at 50 mg/L initial concentration and pure ZVI at a) LM and b) HM solid to liquid ratios

Figure 6 - 8

 68 Figure 6-8 simulation results based on the use of coefficients related to batch tests performed using pure reactive materials

Figure 6 -

 6 Figure 6-11 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 50 mg/L initial concentration and v3 flow velocity.

Figure 6 -

 6 Figure 6-12 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 8 mg/L initial concentration and v2 flow velocity.

Figure 6 -

 6 Figure 6-13 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 8 mg/L initial concentration and v3 flow velocity.

Figure 6 -

 6 Figure 6-14 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 8 mg/L initial concentration and v1 flow velocity.

Figure 6 -

 6 Figure 6-15 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 95 mg/L initial concentration and v3 flow velocity.

Figure 6 -

 6 Figure 6-16 simulation and experimental results of column test performed with 30:70 ZVI:Lapillus w.r. mixture, solution of Nickel at 50 mg/L initial concentration and v2 flow velocity.

  parameter, as applied to the column studied, represents the ratio of mass transport by advection to mass transport by molecular diffusion. Considering the evaluation of Peclet number, advection-dispersion term seems to be more influent for the performed column tests and v3 (1.382 mm/min), while for the other tests the terms of diffusion and the advection-dispersion can have equal influence on contaminant transport.

  mixtures tested, the values of different parameters as well as the initial and boundary conditions were changed to be equal to experimental conditions.The values found for sorption term are lower than the correspondent found by batch tests analysis. In particular the values used for column simulation related to the ZVI:Pumice mixture are 5, 15 or 50 times higher than the corresponding value found experimentally from batch tests. Furthermore, it can be observed that the column tests with 30:70 ZVI:Pumice mixture using a solution of Nickel at 40 and v1 and v3 flow velocity can be simulated using the same values of sorption term parameters.In Chapter 7, a comparison of experimental results of ZVI:Pumice and ZVI:Lapillus mixtures tested in column is performed to understand the factors that influence their long-term removal efficiency and hydraulic behavior and what is their different influence on the performance of the two mixtures. Chapter 6, a deeper interpretation of column tests results obtained using ZVI:Pumice and ZVI:Lapillus mixtures needs, to understand the factors of their long-term removal efficiency and hydraulic behavior and how they influence the sorption processes. In fact, a non-equilibrium sorption model seems to be able to simulate the column tests results.In this chapter, a comparative analysis of column tests results obtained testing ZVI:Pumice and ZVI:Lapillus mixtures is carried out. Firstly, results concerning the remediation of Nickel-contaminated solutions will be considered. The comparison regarding long-term removal efficiency and hydraulic behavior will concern the benchmark tests, the column tests carried out using mixtures at different weight ratio, those performed using Nickel-solution at different initial concentrations and those carried out varying the flow velocity.

Figure 7 - 1

 71 Figure 7-1 Nickel normalized concentration evolution with time using a) 3 cm filter length and b) 38 cm filter length of pure Pumice and pure Lapillus reactive materials

Figure 7 - 3 Figure 7 - 4

 7374 Figure 7-3 Nickel normalized concentration evolution in 10:90 w.r. column tests at a) 28 cm long ZVI:Pumice and 18 cm long ZVI:Lapillus filters and at b) 58 cm long ZVI:Pumice and 28 cm long ZVI:Lapillus filters.

Figure 7 - 5

 75 Figure 7-5 Nickel normalized concentration evolution in 30:70 w.r. column tests at a) 38 cm long ZVI:Pumice and 18 cm long ZVI:Lapillus filters and at b) 50 cm long ZVI:Pumice and 28 cm long ZVI:Lapillus filters.

Figure 7 - 6

 76 Figure 7-6 Nickel normalized concentration evolution in 50:50 w.r. column tests at a) 5 cm long ZVI:Pumice and 3 cm long ZVI:Lapillus filters and at b) 13 cm long ZVI:Pumice and 8 cm long ZVI:Lapillus filters.

Figure 7 - 7

 77 Figure 7-7 Nickel normalized concentration evolution in 50:50 w.r. column tests at a) 33 cm long ZVI:Pumice and 18 cm long ZVI:Lapillus filters and at b) 50 cm long ZVI:Pumice and 28 cm long ZVI:Lapillus filters.

Figure 7 -Figure 7 -

 77 Figure 7-10 Nickel mass retained in each sectors divided by the length of the sector in 10:90 w.r. of a) ZVI:Pumice and b) ZVI:Lapillus mixtures column tests at 2448 hours from the test beginning

Figure 7 -Figure 7 -

 77 Figure 7-13 Partial Nickel mass retained in each filter sectors during each time interval divided by the reactive media mass contained in the sector per unit time for 10:90 a) ZVI:Pumice w.r. and b) ZVI:Lapillus w.r.

Figure 7 -

 7 Figure 7-15 Partial Nickel mass retained in each filter sectors during each time interval divided by the reactive media mass contained in the sector per unit time for 50:50 a) ZVI:Pumice w.r. and b) ZVI:Lapillus w.r.

  6, the characteristics of the considered column tests are reported. The fixed weight ratio is 30:70 for both the ZVI:Pumice and ZVI:Lapillus mixtures tests. The initial Nickel concentration is about 50 mg/L for the ZVI:Lapillus tests and 40 mg/L for the ZVI:Pumice tests. The constant flow velocities used were 0.055 mm/min (v1), 0.276 mm/min (v2) and 1.382 mm/min (v3).

  tested with a v1 flow velocity, after about 2200 and 10000 hours after test beginning respectively. For the first considered time, around 2200 hours, it can be observed that both mixtures are highly efficient and that the Nickel retention is mostly concentrated in the first 3 cm of the ZVI:Lapillus filter (Figures 7-16 a), while it is better distributed along the filter in the ZVI:Pumice column test (Figures 7-16 b). This difference in Nickel retention distribution along the column is highlighted as time increases. In fact, at about 10000 hours after the test beginning, the input Nickel mass is almost totally removed by the first 8 cm of the ZVI:Lapillus filter length, mainly by the first 3 cm, while the input Nickel mass in the ZVI:Pumice is removed in a more homogeneous way by the filter and with a lower efficiency than the ZVI:Lapillus filter does.

Figure 7 -Figure 7 -

 77 Figure 7-16 Nickel retained mass in each sector divided by the length of the sector in v1 tests with a) ZVI:Pumice and b) ZVI:Lapillus mixtures column tests at about 2200 hours from the test beginning

  the ZVI:Lapillus (Figures b) mixtures tested with a v3 flow velocity, after about 60 and 400 hours after test beginning respectively. At about 60 hours, the Nickel input mass has been retained with high efficiency by both filters. Concerning the Nickel retention distribution along the filter, it seems to be more homogeneous in the ZVI:Lapillus filter than in the ZVI:Pumice one. This is more evident if a higher interaction time is considered, as shown in Figures 7.19. However, the Nickel removal efficiency seems not to vary much between the two mixtures when v3 flow velocity tests are considered. The Nickel retained mass as function of Nickel input mass is shown for the three tested flow velocities in Figures 7-20 a) and b) for the ZVI:Pumice and the ZVI:Lapillus mixtures respectively. Comparing them, the higher removal efficiency of the ZVI:Lapillus than ZVI:Pumice mixtures is confirmed for each flow velocity used.

Figure 7 -

 7 Figure 7-18 Nickel mass retained in each sector divided by the length of the sector in v3 tests with a) ZVI:Pumice and b) ZVI:Lapillus mixtures column tests at about 63 hours from the test beginning

Figure 7 -Figure 7 -

 77 Figure 7-19 Nickel retained mass in each sector divided by the length of the sector in v3 tests with a) ZVI:Pumice and b) ZVI:Lapillus mixtures column tests at about 400 hours from the test beginning

Figure 7 -

 7 Figure 7-21 permeability evolution as function of the Nickel mass input for v1,v2 and v3 tests performed with 30:70 w.r. a) ZVI:Pumice and b) ZVI:Lapillus mixtures.

Figure 7 -Figure 7 -

 77 Figure 7-22 permeability evolution as function of the input water for v1,v2 and v3 tests performed with 30:70 w.r. a) ZVI:Pumice and b) ZVI:Lapillus mixtures.

  24 a) and b), the Nickel retained mass is shown as function of the Nickel input mass for the 30:70 ZVI:Pumice w.r. mixture and for the 30:70 ZVI:Lapillus w.r mixture respectively. Regarding the Nickel mass input in column tests performed with the lowest Nickel initial concentration, the removal efficiency is high for both filters. Looking at the tests carried out with the intermediate Nickel initial concentration the removal efficiency rapidly decreased in the ZVI:Pumice mixture filter, while it remained high for the ZVI:Lapillus mixture, as previously observed.

Figure 7 -

 7 Figure 7-24 Nickel mass removed as function of Nickel mass input for tests performed with 30:70 w.r. a) ZVI:Pumice using solutions of Nickel at 10, 50 and 100 mg/L initial concentrations and b) ZVI:Lapillus mixtures using Nickel solution at 8 and 50 mg/L initial concentrations

Figure

  Figure 7-25 hydraulic conductivity evolution with time for tests performed with 30:70 w.r. a) ZVI:Pumice using solutions of Nickel at 10, 50 and 100 mg/L initial concentrations and b) ZVI:Lapillus mixtures using Nickel solution at 8 and 50 mg/L initial concentrations

Figure 7 -

 7 Figure 7-26 Evolution of Zinc normalized concentration with filter thickness at different sampling times for a) 50:50 ZVI:Pumice w.r. mixture test and b) 50:50 ZVI:Lapillus w.r. mixture test

Figure 7 -

 7 Figure 7-27 a) Zinc normalized concentration evolution as function of time and b) Zinc retained mass as function of input Zinc mass for 28 cm long ZVI:Lapillus mixture filter and 50 cm long ZVI:Pumice mixture filter.

Figure 7 -Figure 7 -

 77 Figure 7-28 hydraulic conductivity evolution with time for 50:50 ZVI:Pumice and ZVI:Lapillus w.r. mixtures and pure ZVI filters performed using solution of Zinc at 50 mg/L initial concentration and v2 constant flow velocity.

ZVI:

  Pumice mixture and those obtained using ZVI:Lapillus mixtures was based on Nickel and Zinc removal. The first step of analysis of column tests based on the Nickel-contaminated groundwater remediation regarded the comparison of longterm removal efficiency of tested pure volcanic materials, pure Pumice or pure Lapillus, carried out with Nickel-contaminated solution at an initial contaminant concentration equal to 50 mg/L put into the column at 0.276 mm/min constant flow velocity. A certain Nickel removal efficiency is already performed in 3 cm reactive granular filter length, although it is limited, for both tests and higher in pure Lapillus test. This difference is more evident considering the experimental data at 38 cm. Therefore, a comparison among the different weight ratio mixtures tested was made. It was based on analysis of results of column tests performed using 10:90, 30:70 and 50:50 ZVI:Pumice w.r. and ZVI:Lapillus w.r mixtures under the same experimental conditions, considering the same ZVI content, same residence time and same removal efficiency. According to the first two criteria, the ZVI:Lapillus w.r mixtures are always more efficient. In fact, considering the third one for 10:90 w.r. mixtures, 8 cm of ZVI:Lapillus mixture have the same efficiency of 58 cm of ZVI:Pumice filter, for 30:70

Figure 8 - 1 Figure 8 - 2 Figure 8 - 3 Figure 8 - 5 Figure 8 - 7 Figure 8 - 8 Figure 8 - 9 Figure 8

 818283858788898 Figure 8-1 Nickel concentration evolution as function of filter thickness for different sampling times for test with 30:70 ZVI:Lapillus w.r. mixture, Ni solution at 50 mg/L initial concentration and v3 flow velocity (until 63 h)

  removal efficiency of Lapillus compared to Pumice.Of course, the hypothesized causes of removal efficiency of Lapillus should be confirmed by further investigations. Physical-chemical analysis of exhausted material should be performed to understand what the main removal processes are and the most important characteristics that affect them, as well as the possible interactions between ZVI and Lapillus . Measurement of specific surface and characterization of roughness of Pumice and Lapillus using BET can be important for understanding how they influence the removal efficiency. As was observed through the SEM pictures, Pumice seems to have a high internal porosity and smooth external surface, while Lapillus seems to have a lower internal porosity with larger pores and a much rougher external surface than Pumice.Concerning the coupled model of the column test, the sorption model seems to be good for representing long-term Nickel removal efficiency of the studied mixtures.More investigations should be carried out, to develop the removal efficiency model applied to other heavy metals removal, considering mono-and three-contaminant solutions. In effect, the introduction of other terms or equations representing other contaminant removal processes, the removal efficiency of each material constituting the mixtures as well as their influence on each other can be considered. Regarding the model of hydraulic conductivity evolution, further study and investigation is needed. Considering the different possible causes of its evolution, e.g. iron corrosion products, secondary mineral precipitates, colloids and gas formation, measurements of differential pore water pressure along the filter thickness can provide important information about the position of the layer characterized by the highest porosity reduction. A further tool to use to make a comparison study consists in the application of a chemical-hydraulic modeling to the column tests, to identity where the accumulation of the products with the most expansive nature is, and how it varies with time. Of course, this should be investigated for each contaminant and for pluricontaminant solutions, because the evolution of removal efficiency and hydraulic behavior can vary with the considered filter thickness, the contaminant to be removed and the chemical conditions as function of time.

Table 1 -

 1 1 Main reactive materials used in PRB for heavy metals and metalloids contamination[START_REF] Epri ; Ladwig | Groundwater Remediation of Inorganic Constituents at Coal Combustion Product Management Sites -Overview of Technologies, Focusing on Permeable Reactive Barriers[END_REF] ................................................................................................................................................................ 14 Table 1-2 List (not exhaustive) of PRBs installed to treat heavy metal contaminated groundwater (adapted from RECORD, 2010) ............................................................................................... 16 Table 2-1 Pumice mineralogic composition .................................................................................................. 28 Table 2-2 Specificatio of AutoPore IV of Micromeretrics (Model 9505/9500) .............................. 31 Table 2-3 used penetrometer characteristics ............................................................................................... 32 Table 2-4 Pumice characteristics obtained by four MIP tests ................................................................ 32 Table 2-5 Lapillus characteristics obtained by four MIP tests ............................................................... 46 Table 3-1 Batch tests carried out using pumice ........................................................................................... 50 Table 3-2 Cmin reached as function of C0 .......................................................................................................... 51 Table 3-3 values of parameters used for model implementation ......................................................... 66 Table 3-4 Batch tests experimental conditions .

Table 5 -

 5 6 Characteristics of carried out column tests to study the flow rate influence ........... 142 Table 5-7 pH and Eh values at different filter length and different sampling times for 30:70 ZVI:Lapillus column test carried out under v1 .......................................................................................... 144 Table 5-8 pH and Eh values at different filter length and different sampling times for 30:70

1.4 PRB for heavy metals

  . The pilot plant treats groundwater contaminated by a complex mixture of CHCs, i.e., mainly chlorobenzenes, and other pollutants at Bitterfeld, Federal State of Saxony-Anhalt.

			alumina
			Hybrid ion
			exchange
	resin AQUAREHAB was an EU financed large scale project which started 1st May 2009 and Rare earth lasted for 56 months, until 2013. The project, coordinated by VITO involving a elements
	Kanchan™ consortium of 19 partners, was aimed at developing innovative rehabilitation arsenic filter
	technologies for soil, groundwater and surface water contaminated by a number of Granulat
	Ferric contaminants. These technologies include activated riparian zone/wetland, open Hydroxide™
	trench with smart biomass containing carriers, capping of sediment & stimulated Clays
	Precipitation ZVI biobarriers, multifunctional permeable reactive barriers and reactive zone with Obtained by chemical reduction or as solid Ferrous solution with Fe
	injectable Fe-based particles. Methods and tools to improve their design , to evaluate sulfate (Cr)
	Sodium their effects on river basin as well as the development of a framework to evaluate and dithionite
	(Cr) disseminate the research output in other river basins have been objectives of the Organic research project too. (https://aquarehab.vito.be/home/Pages/home.aspx) carbon
	Cations	Adsorption	ZVI	Neutral-to-alkaline pH is optimal
	(e.g. Fe, Mn,		Humasorb™
	Cd, Pb, Ni, Be,		Ferrous
	Ba, Tl)		sulfate (HFO)
			Zeolites
			Clays
	Different reactive materials have been tested for heavy metals contaminated Precipitation Phosphates May include sulfides, sulfates, carbonates, oxides
	ZVI groundwater remediation using PRB technology. In table 1-1, the reactive materials Limestone and hydroxides
	Organic are reported in function of the kind of metal or metalloid and of the main mechanism carbon
	employed to remove it.	Neutralized
			red mud
			Oxygen
	Constituents Mechanism Media sparging	Comments
	Oxyanions	Adsorption	ZVI	Neutral-to-acid pH is optimal
			Surfactant
	(e.g., As, Se, V,		modified
	Cr, Sb, Mo,		zeolites	Rare-earth or Fe-doping improves adsorption
	SO4)		Basic oxygen	capacity
			furnace slag
			Amorphous	High levels of sulfates may depress adsorption
			ferric
			hydroxide
			Neutralized
			red mud
			Diatomaceous
			earth
			Ferrous
			sulfate (HFO)
			Activated

Table 1 -1 Main reactive materials used in PRB for heavy metals and metalloids contamination (EPRI, 2006)

 1

	In table 1-2 the characteristics of PRBs installed to treat heavy metal contaminated
	groundwater are reported.

Table 1 -2 List (not exhaustive) of PRBs installed to treat heavy metal contaminated groundwater (adapted from RECORD, 2010)

 1

Table 2 -1 Pumice mineralogic composition

 2

	Pumice mineralogical composition
	(gr/100 gr)
	SIO2	71.75
	Al2O3	12.33
	Fe2O3	1.98
	Na2O	3.59
	K2O	4.47

  Table 2-2.

	Characteristics	Specifications
	Low Pressure
	Measurement	0 to 50 psia (345 kPa)
	Resolution	0.01 psi (69 Pa)
	Pore Diameter	360 to 3.6 μm
	Transducer Accuracy	±1 of full scale
	High Pressure
	Measurement	From atmospheric pressure to 33,000 psia (228 MPa)
	Resolution	0.2 psi (1400 Pa) from 3,000 psia (21 MPa) to 33,000 psia (228 MPa)

Table 2

 2

	Stem Volume	0.412 mL
	Penetrometer Volume	3.2934 mL
	Penetrometer Weight	56.9905 g
	Maximum Head Pressure	0.030682 MPa
	Assembly Weight	99.539 g

-3. a) b) Figure 2-8 a) used penetrometer and Pumice sample; b) sample chamber Penetrometer Constant 11.117 μL/pF

Table 2 -3 used penetrometer characteristics

 2

	Average	Average	Average
	sample quantity	Max Diameter	Internal Volume
	(g)	(μm)	(mL/g)
	0.11	143-113	1.25-1.02

Four tests (A, B, C and D) were carried out to characterize the porosity of Pumice.

The results are summarized in table 2-4.

Table 2 -4 Pumice characteristics obtained by four MIP tests

 2

	In Figures 2-9

Table 2 -5 Lapillus characteristics obtained by four MIP tests

 2

  The pore transversal section 𝐴 can be calculated as average value considering the total intrusion volume, as sum of 𝑉 𝑝𝑜𝑟𝑒,𝑖 volume of each pore and dividing it for the product of 𝑛 number of pores and 𝐷 𝑝 length of grain (equation 3-46).

	𝐴 𝑚 =	∑	𝑖	𝑉 𝑖𝑛𝑐.𝑖𝑛𝑡𝑟𝑢𝑠.,𝑖 𝐷 𝑝 𝑛 𝑝𝑜𝑟𝑒	=	∑ 𝑆 𝑖 * 𝑛 𝑖 𝑖 𝑛 𝑝𝑜𝑟𝑒	(3 -45)
		𝐴 =	∑ 𝑉 𝑝𝑜𝑟𝑒,𝑖 𝑛 𝑛 𝐷 𝑝	(3 -46)
	equation 3-44.
				𝑛 𝑝𝑜𝑟𝑒 =	1 𝐷 𝑝	∑	𝑉 𝑖𝑛𝑐.𝑖𝑛𝑡𝑟𝑢𝑠.,𝑖 𝐴 𝑖	(3 -44)

𝑖

where 𝑉 𝑖𝑛𝑐.𝑖𝑛𝑡𝑟𝑢𝑠.,𝑖 is the value of the incremental intrusion volume per unit mass of pumice, a data derived by MIP analysis, and 𝐴 𝑖 is the transversal area of the pore corresponding to the mean value of diameter of the range i. The transversal section 𝐴 𝑚 can be calculated by equation 3-45

Table 3

 3

	-3 for the

Table 3 -3 values of parameters used for model implementation

 3

Table 3

 3 -6.

	Tests	Reactive medium (mg)	Solution Volume (L)	initial contaminant mass (mg) C0 5 C0 50 C0 500 mg/L mg/L mg/L
	LM	5400	0.054	0.27	2.7
	HM	8000	0.032	0.16	1.6	16

Table 3 -4 Batch tests experimental conditions

 3

	In Table 3-4, data from batch tests carried out with Low Mass and High Mass and
	three different concentrations (5 mg/L, 50 mg/L, 500 mg/L) are reported.

Table 3 -5 Concentration measured during batch tests

 3

  In Table3-6 the values of nickel mass removed per unit of solution volume for all the tests at the first sampling time (reported) are summarized.

			Cs (mg/L)
	C0 (mg/L)	5	50	500
	LM	2.5 (2h)	15.2 (120h)
	HM	2.9 (0.5h)	11.4 (0.5h)	88.9 (0.5h)

Table 3 -6 Nickel mass removed for each bath test at the first sampling time (with time)

 3 To evaluate the Langmuir relationship coefficients, the C* values have to be considered for all the tests. Table3-7 reports the values of nickel removed mass to pumice mass ratio for each batch test at the defined times.

			C*(mg/Kg)
	C0 (mg/L)	5	50	500
	LM	24.9 (2h)	151.8 (120h)
	HM	11.6 (0.5h)	45.6 (0.5h)	355.6 (0.5h)

Table 3 -7Nickel removed mass and Pumice mass ratio for each batch test at the first sampling time

 3

Table 3 -8 values used to evaluate Langmuir coefficient

 3

Table 3 -

 3 9 reports the values of α and β coefficients as calculated under the three above-mentioned hypotheses.

The combination of Langmuir coefficient values that better represent the experimental data of each test, concerning the first sampling time, is reported in figure 3-10 and corresponds to that reported in table 3-9 as second (α0=5E+02 and β =6.67E-4 mg/Kg) Figure 3-

10 Cs as function of C0 using Langmuir model

	Hypoth. α0	β (mg/kg) comments
		1E+03	3.45E-04	based on LM tests
		5E+02	6.67E-04	based on HM test (5 and 50 ppm init. Conc.)
		1E+04	6.76E-05	based on HM test (all init. Conc. tests)
	1 st	1E+03	5.06E-04	average related to tests with 5 and 50 ppm init.conc. LM and HM both)
		4E+04	3.60E-04	average related to all the tests (5, 50 and 500 ppm init.conc, LM and HM both)
	2 nd	3E+04 6E+04	1.16E-05	β defined on the base of C* related to 5 HM test
	3 th	5.9E+03 2.06E-04	optimization

Table 3 -9 Langmuir coefficients used for the simulation

 3

  are reported in table 4-1. Mass of reactive material, filter thickness, initial contaminant concentration, flow velocity, porosity, residence time and test duration are defined for each test. The 𝜑 porosity was calculated

	through equation (4-6):

Table 4 -1 pure ZVI column tests carried out by the previous Ph.D. students

 4

		Mass (gr)	Initial			Residence
	media Reactive	(cm) thickness and filter	concentrati t contaminan	(mm/min) velocity Flow	φ(%) Porosity	time
			on (mg/L)
	Fe 0 (1)	240 (-3)	Ni 50	0.276	44	0.8
	Fe 0 (2)	1680 -19.25	Ni 40	0.055	48	32
	Fe 0 (2)	1680 -22.3	Ni 40	1.382	48	1.3
	Fe 0 (2)	1680 -22.3	Ni 8	1.382*	48	1.3
	Fe 0 (2)	1680 -22.3	Ni 8	0.055	48	32
	Fe 0 (2)	1680-22.25	Ni 95	1.382	48	1.3
	Fe 0 (2)	240 -3	distilled water	0.276	44	0.8

Table 4 -2Characteristics of column tests performed using ZVI:Pumice mixtures

 4

	Reactive media	Fe 0 and Pumice mass(gr)	Contaminan t and initial concentratio n (mg/L)	Flow (mm/min) velocity	Initial (%) porosity n	Residence time
	Fe 0 -Pum.
	10:90 w.r.	153 -1374	Ni 50	0.276	36	21.5
	(1)
	Fe 0 -Pum.
	30:70 w.r.	244 -570	Ni 50	0.276	44	13.3
	(1)
	Fe 0 -Pum.
	50:50 w.r.	530 -530	Ni 50	0.276	44	13.1
	(1)
	Fe 0 -Pum.
	30:70 w.r.	240-560	Ni 40	0.055	44	71.4
	(2)
	Fe 0 -Pum.
	30:70 w.r.	240-560	Ni 8	1.382	44	2.9
	(2)
	Fe 0 -Pum.
	30:70 w.r.	240-560	Ni 8	0.276	46	14.8
	(2)
	Fe 0 -Pum.
	30:70 w.r.	240-560	Ni 8	0.055	45	71.4
	(2)
	Fe 0 -Pum.
	30:70 w.r.	240-560	Ni 40	1.382	45	2.9
	(2)
	Fe 0 -Pum.
	30:70 w.r.	240-560	Ni 95	1.382	45	3
	(2)
	Fe 0 -Pum.
	30:70 w.r.	240-560	Ni 95	0.055	46	73
	(2)

(1) Suraci (

2010

),

(2)

Bilardi (2011)

7 normalized concentration of nickel in 50:50 w.r. ZVI:Pumice, C0=50ppm, f.r. = column test

		t 120h t 288 h t 432h	1.2 1	10:90 ZVI:Pumice
		t 600h
		t 744h	0.8
	Figure 4-			0.4 0.6 C/C 0	t 1584h t 1656h
					t 2280h
				0.2	t 2448h
					t 3240h
				0
				0	20 Column length (cm) 40	60
	1 1.2	30:70 ZVI:Pumice	t 120h t 288 h t 432h t 600h	1.2 1	30:70 ZVI:Pumice
			t 744h
	0.8		t 912h	0.8
	0.6 C/C 0		t 1104h t 1248h t 1440h	0.6 C/C 0	t 1584h t 1656h t 2280h t 2448h
	0.4			0.4	t 2616h
					t 2784h
	0.2			0.2	t 2928h t 3096h
					t 3240h
	0			0	t 3816h
	0	20 Column length (cm) 40	60	0	20 Column length (cm) 40	60

Table 4 -3 Characteristics of column tests performed using ZVI:Pumice mixtures and Zinc-contaminated solutions

 4

	Tres
	(h)

Table 5 -1Column tests program to study long-term removal efficiency and hydraulic behavior of ZVI:Lapillus mixtures

 5

	Reac Mat	W.r.	Init. cont. conc (mg/ L)	Reach eig. (cm)	Reac area (cm)	Reac Vol. (cm 3 )	Fe 0 (gr)	Lap. (gr) or Pum.	n (%)	Flow min) vel (mm/	PV (cm 3 )	Tres (h)
	Lap.		Ni 50	50	18.09 904.3		1289	35	0.276 318.5	10.6
	Fe 0 / Lap.	10:90 Ni 50	50	18.09 904.3	138	1242	36	0.276 322.2	10.7
	Fe 0 / Lap.	30:70 Ni 50	50	18.09 904.3	480	1120	37	0.276 334.3	11.1
	Fe 0 / Lap.	50:50 Ni 50	50	18.09 904.3	970	970	38	0.276 340.1	11.3
	Fe 0		Ni 50	3	18.09 54.26	240		44	0.276 23.76	0.8
	Fe 0 / Lap.	30:70 Ni 50	100	18.09 1808 1098 2562	34	0.276	607	20.2
	Fe 0 / Lap.	30:70 Ni 50	50	18.09 904.3	473	1104	37	0.055 336.7	56.1
	Fe 0 / Lap.	30:70 Ni 50	50	18.09 904.3	468	1093	38	1.382 348.1	2.3
	Fe 0 / Lap.	30:70	Ni 100	50	18.09 904.3	477	1113	37	0.276 337.5	11.3
	Fe 0 / Lap.	30:70 Ni 10	50	18.09 904.3	480	1120	38	0.276 341.3	11.4
	Fe 0 / Pum.	50:50 Zn 50	50	18.09 904.3	481	481	49	0.276 442.2	14.7
	Fe 0 / Lap.	50:50 Zn 50	50	18.09 904.3	970	970	38	0.276 340.1	11.3
			Ni 50,
	Fe 0 / Lap.	50:50	Cu 500,	50	18.09 904.3	963	963	38	0.276 344.5	11.5
			Zn 50
			Ni 50,
	Fe 0 / Lap.	30:70	Cu 500,	50	18.09 904.3	480	1120	37	0.276 334.2	11.1
			Zn 50

Figure 5-13 Nickel normalized concentration for 10:90 w.r. ZVI/Lapillus column test

	t(h)\ dist (cm)	pH	1.5	Eh (mV)	pH	18	Eh (mV)	pH	50 (mV) Eh	t(h)\ dist (cm)	pH	1.5 (mV) Eh	pH	18	Eh (mV )	pH	50 (mV) Eh
	151	6.18 54.2 8.11		-52	7.3	-3.5	1296	6.2	37.6	6.99		21	7.6 -20.6
	440	6.52 30.2 7.96		-56.2 7.92 -54.6	1680	6.2	35.3	6.26		38	7.1	7.3
	672	6.47 29.4 7.29		-9.8	7.31	-4.1	2424	6.1	42.6	6.5		29	7.1	8.4
	960	6.53 26.9	6.8		21.6 7.04	14	3024	6.5	33	6.68		18	6.3	37.7

Table 5 -2 pH and Eh(mv) measured values for 10:90 ZVI/Lapillus weight ratio column test

 5

  at the evolution of nickel normalized concentration with time and the An 18 cm filter length is enough to reduce significantly the nickel concentration as input until 432 hours (650 mg nickel mass input). 18 cm of 10:90 w.r. ZVI/Lapillus mixtures in the carried out column test correspond to about 4 hours of residence time of solution in reactive filter, 50 gr of ZVI and 450 gr of lapillus. After this time, 28 cm

	1 1.2 a)	10:90 ZVI/Lapillus	t 120h	1 1.2 b)		t 1752h t 1440h t 1104h	t 1920h t 1585h t 1248h
			t 240h			t 2280h	t 2448h
	0.8		t 432h	0.8		t 2952h	t 3144h
	0.6 C/C 0 of filter length are necessary to remediate efficiently until around to 1248 hours t 600h t 744h 0.6 C/C 0 (1880 mg nickel mass input). This length corresponds to 6 hours residence time, 77
	0.4 gr of ZVI and 695 gr of lapillus.	t 912h	0.4
	0.2			0.2		10:90
	0			0	ZVI/Lapillus
	0	20 Column length (cm) 40	60		0	20 Column length (cm) 40	60
	corresponding pH and Eh values for 10:90 w.r. ZVI/Lapillus mixture tested, it is
	possible to observe that for pH values more than 7 and Eh negative values, very low
	values of nickel concentration are measured. At 1.5 cm a certain nickel concentration
	is always measured, as pH is about 6.5 and Eh is around 30 mV. At 18 cm, low values
	of nickel concentration are measured until the pH remains around 8 and Eh is about -
	50 mV. As pH decreases and Eh increases, nickel measured concentration increases
	and nickel breakthrough occurs.

Table 5 -

 5

3. The experimental data on nickel relative concentration evolution are divided into three different intervals. In the first figure, data related to the interval between the test beginning and 912 hours are represented. During this period, 18 cm of filter length are enough for remediation. pH is about 8 and Eh is negative at 18 and 50 cm, while pH is about 6.5 and Eh is positive at 1.5 cm; 18 cm filter length correspond in this test to 4 hours of residence time, 173 gr of ZVI and 403 gr of Lapillus. After this period, 28 cm filter length, corresponding to around 6 hours of residence time and 269 gr of ZVI and 627 gr of Lapillus, are needed to reduce the nickel input concentration until 2448 hours (3680 mg nickel mass input). After this time, the filter length from 28 to 50 cm seems to not have the same removal efficiency of the previous part of the column. A filter length greater than about 28 cm from column inlet has not the same removal efficiency observed in the filter zone closer to the inlet. Probably, this is due to the involvement of the ZVI in the farther filter zones from the inlet in other kinds of reactions, e.g. corrosion, during the time when nickel does not get to, leading to a decrease of its own nickel removal efficiency. This phenomenon is more evident in the 10:90 ZVI/Lapillus column. Figure 5-

16 Nickel normalized concentration for 50:50 w.r. ZVI/Lapillus column test

	t(h)\ dist (cm)	pH	1.5	Eh (mV)	pH	18	Eh (mV)	pH	50 (mV) Eh	t(h)\ dist (cm)	pH	1.5 (mV) Eh	pH	18 (mV) Eh	pH	50	Eh (mV )
	272	6.88		23.4	8.8		-69.9	9.9	-144.9 2856 6.64 26.7	7.4	-1	7.6		-32
	504	6.68		13.8 8.73		-103 9.26 -130.6 3456 6.82	5.3	7.2	-28.3	8.2		-74
	1128 6.53		23.5 8.54		-90	9.14 -122.4 4176	6.8	6	6.1	59.7 6.6		56
	1512	6.3		36	7.23		18.7 9.02 -115.5 4632 6.79	8.4	6.9	8.8	6.2		69

Table 5 -4 pH and Eh(mv) values for 50:50 w.r. ZVI/Lapillus In

 5

Table 5

 5

	-4, the pH and Eh measured values for

This is probably due to higher ZVI content. However, an 8 cm long reactive filter of ZVI/Lapillus mixture with 50:50 weight ratio, corresponding to 2 hours of residence time and 155 gr of ZVI, is enough to remediate nickel contaminated water until 1248

Table 5

 5

	ZVI
	content	22	28	77	77	50	58	365	349
	(gr)
	w.r. filter length (cm)	10:90 8 cm	30:70 3 cm	10:90 28 cm	30:70 8 cm	10:90 18 cm	50:50 3 cm	30:70 38 cm	50:50 18 cm
	T res (h)	1.7	0.66	6	1.8	3.86	0.68	8.5	4
	ZVI:
	Lapillus particles	1:63	1:16	1:63	1:16	1:63	1:7	1:16	1:7
	number

-5 different filter thickness with the corresponding ZVI mass and residence time is reported. The residence time is calculated using the initial porosity. The ratio between ZVI and Lapillus particles numbers was calculated considering the d50 of each component of mixtures.

Table 5 -5 characteristics of column tests considered in comparison based on the same ZVI content

 5

Table 5 -6 Characteristics of carried out column tests to study the flow rate influence

 5

			Flow
	gr)	n (%)	veloci ty (mm/	PV (cm 3 )	Tres (h)
			min)

test carried out under 0.055 mm/min constant flow velocity

	t(h)\	1.5		18		50
	dist (cm)	pH	Eh (mV)	pH	Eh (mV)	pH	Eh (mV)
	171	7.11	16	8.95	-107.2	7.44	-38.4
	528	6.48	32.8	9.47	-143	9.64	-154.5
	840	6.5	34.3	8.66	-103.3	9.71	-145.6
	1608	6.85	11.6	9.21	-126	9.42	-134.6
	2184	6.45	29.3	9.12	-127.6	9.1	-124
	9576	6.83	16.2	8.4	-73.2	8.9	-115.4
	14856	5.9	22.3	6.38	5.2	8.6	-111.6

Table 5 -7 pH and Eh values at different filter length and different sampling times for 30:70 ZVI:Lapillus column test carried out under v1

 5

Table 5 -8 pH and Eh values at different filter length and different sampling times for 30:70 ZVI:Lapillus column test carried out under v3

 5

	t(h)\	1.5		18		50
	dist (cm)	pH	Eh (mV)	pH	Eh (mV)	pH	Eh (mV)
	4	6.2	46.1	6.15	46.8	6.5	17.7
	78	6.23	46.6	6.12	47.2	6.55	17
	379	6.24	44.9	6.2	47.2	5.98	62.4
	595	6.33	39.3	6.27	41.5	6.11	59.2
	739	6.25	43.4	6.19	40.1	6.46	45.7

Table 5 -9 Characteristics of column tests carried out in order to study the initial concentration influence

 5

	Tres
	(h)

Table 5 -10 pH and Eh values at different filter length and different sampling times for 30:70 ZVI/Lapillus column test carried out using a solution of Nickel at 10 mg/L initial concentration

 5

Table 5 -11 pH and Eh values at different filter length and different sampling times for 30:70 ZVI:Lapillus column test carried out using a solution of Nickel at 100 mg/L initial concentration

 5

		5		18		50
		pH	Eh (mV)	pH	Eh (mV)	pH	Eh (mV)
	168	6.63	35.7	6.83	37.7	6.58	26.8
	672	6.13	51.7	6.01	61.7	6.22	43.3
	1109	6.15	47.5	6.05	53.6	5.93	61.4
	1445	6.22	42.7	6.06	53.3	5.91	63.3
	2237	6.34	34.2	6.3	40.9	6.21	46.4

Table 5 -13 Characteristics of carried out column tests to study the zinc removal

 5

	Tres
	(h)

(*)

Table 5

 5

	-

Table 5 -14 Characteristics of carried out column tests to study the tri-contaminant removal

 5

	*(Suraci, 2011);**(Bilardi 2012)

  table 6-1 the reaction times observed for the batch tests performed using different weight ratios of ZVI:Pumice mixtures and solution of Nickel at 5 and 50 mg/L initial concentrations are reported. In table 6-2 the characteristics of column tests performed using different weight ratios of ZVI:Pumice mixtures and solution of Nickel at 50 mg/L initial concentrations are reported. It can be observed that the residence time is always lower than the time necessary for the

	total development of removal reaction, as observed in batch tests. Of course, this
	should be considered as a preliminary comparison, taking into account the different
	experimental conditions. Moreover, this observation can justify the choice of the tipe
	of sorption model (i.e. non-equilibrium (kinetic) sorption model). Different
	simulations and the results of batch tests modelling (Chapter 3) can validate this
	choice.

Table 6 -1 reaction time as observed by batch tests results

 6

			Reaction time (h)
	Ni Co	0:100	10:90	30:70	50:50	100:0
	(mg/l)	w.r.	w.r.	w.r.	w.r.	w.r.
	5	>48	24	24	8	8
	50	>120	120	48	48	24
	w.r	Mtot(g) Vc(cm 3 ) n(%) PV(cm 3 )	TR(h)
	0:100	560	902	45	411	13.7
	10:90	650	981	46	459	15.3
	30:70	800	981	46	459	15.3
	50:50	1060	981	46	448	15
	100:0	1680	435	51	222	7.4
		Table 6-2 column tests characteristics

Table 6 -3 Peclet number for carried out column tests varying d50, Dd and v

 6

	and the seepage velocity used to perform

  The equation 6-18 represents the used model to simulate experimental column tests results using coefficients obtained by batch tests. It assumes that the first-order rate coefficient evolution as function of solid to liquid ratio can be described by a linear relation. 𝑃 𝑖 (m 3 /(kg*s)) is a first-order rate coefficient divided by the reactive mass and multiplied by the solution volume. 𝑃 𝑖 value is derived by the batch tests and takes into account the solid to liquid ratio, in batch tests performed using the ith reactive material. 𝑀 𝑖 represents the reactive mass of the ith reactive material contained in the column. 𝑉𝑜𝑙 is the total internal volume of the column. 𝑘 3𝑖 is the backward rate coefficient meaning that the rate of solute sorption depends on the amount that has already been sorbed. The terms 𝑓 𝑖𝑗 represent the influence on solute removal efficiency of the ith reactive material due to the presence of the jth reactive material.Considering that by observing and analyzing batch test results, 𝑃 𝑖 and 𝑘 3𝑖 coefficients can be obtained easily, one step of simulation development has been focused on use of the two parameters (𝑃 𝑖 and 𝑘 3𝑖 ) as found for reactive materials used as pure. The term of mutual influence has been neglected. With this assumption the equations

	𝜌 𝑏 𝑛	𝜕𝑆 𝑖 * 𝜕𝑡	=	𝜕𝐶 𝑖 * 𝜕𝑡	= 𝑃 𝑖	𝑀 𝑖 𝑉𝑜𝑙 • 𝑛	𝐶 -𝑘 3𝑖 𝐶 𝑖 * -∑ 𝑓 𝑖𝑗	(6 -18)
	used in the simulation to describe sorption term are represented by (6-19) and (6-
	20).
				𝜕𝐶 𝑝 * 𝜕𝑡	= 𝑃 𝑝	𝑀 𝑝 𝑉𝑜𝑙 • 𝑛	𝐶 -𝑘 3𝑝 𝐶 𝑝 *	(6 -19)
				𝜕𝐶 𝑧 * 𝜕𝑡	= 𝑃 𝑧	𝑀 𝑧 𝑉𝑜𝑙 • 𝑛	𝐶 -𝑘 3𝑧 𝐶 𝑧 *	(6 -20)
	where 𝐶 𝑝 * and 𝐶 𝑧 *

𝑗

where

Table 6

 6 -4 the 𝑘 2 and 𝑘 3 values as calculated for each reactive material are reported. In Table6-5 the 𝑃 𝑖 coefficients equal to 𝑘 2 obtained by a done batch tests, divided by the reactive mass and multiplied by the solution volume used in the same batch test are shown.

	reactive		k2 (1/h)	k3 (1/h)
	material	LM	HM	LM	HM
	ZVI	2.57E-01	1.24E+00	3.83E-01	7.56E-01
	Pumice	3.00E-03	2.04E-01	9.67E-05	6.91E-01
	30:70	3.39E-01	9.66E-01	3.49E-01	9.56E-01

Table 6 -4 k2 (1/h) and k3 (1/h) coefficients as found analyzing batch tests

 6

		1.2					1.2
		1	a)		Pumice	1	b)	30:70 ZVI:Pumice w.r.
		0.8					0.8
	C / Co	0.6		y = e -0.204x		C / 0.6 Co				y = e -0.339x
		0.4					0.4
					Ni 50 HM			y = e -0.966x	Ni50 LM
		0.2			Espo. (Ni 50 HM)	0.2				Ni50 HM
											Espo. (Ni50 LM)
		0					0				Espo. (Ni50 HM)
		0	0.5	1	1.5	2	2.5	0	0.5	1	1.5	2	2.5
				t (h)						t (h)

Table 6 -5 Pi coefficients evaluated considering k2 coefficient and the solid to liquid ratio in batch test

 6

	In Figures 6-8 and 6-9, the results of simulations of 30:70 ZVI:Pumice w.r. mixture
	column test using a solution of Nickel at 50 mg/L initial concentration and a constant
	flow velocity equal to v2 (0.276 mm/min) and coefficients obtained by batch tests
	analysis are shown. In Figure 6-8 the results of simulation based on the two equations
	(6-19) and (6-20) concerning one the efficiency of Pumice and one that of ZVI are
	shown. In Figure 6-9 the results of simulation were obtained using one equation to
	describe the sorption onto mixtures particles:
	𝜕𝐶 𝑚𝑖𝑥 * 𝜕𝑡	= 𝑃 𝑚𝑖𝑥	𝑀 𝑚𝑖𝑥 𝑉𝑜𝑙 • 𝑛	𝐶 -𝑘 3𝑚𝑖𝑥 𝐶 𝑚𝑖𝑥 *	(6 -25)
	where the 𝑃 𝑚𝑖𝑥 has been calculated by the 𝑘 2𝑚𝑖𝑥 coefficient reported in table 6-4 and
	𝑘 3𝑚𝑖𝑥 were obtained using the following equation:
	𝑘 2𝑚𝑖𝑥 𝐶 -𝑘 3𝑚𝑖𝑥 𝐶 𝑝 * = 0	(6 -26)
	𝑘 2𝑚𝑖𝑥 and 𝑘 3𝑚𝑖𝑥 coefficients were obtained by batch tests performed using 30:70
	ZVI:Pumice w.r. mixtures.

Table 6 -6 solid to liquid ratio and reactive mass to contaminant mass ratio for LM and HM batch test and for 30:70 ZVI:Pumice w.r. mixture performed with 0.276 mm/min constant flow velocity (and solution of Nickel at 50 mg/L initial concentration) 6.9.3 Model application to mixture filters

 6 Considering the values of 𝑃 𝑚𝑖𝑥 and 𝑘 3𝑚𝑖𝑥 obtained from batch tests, they have been changed in order to search the best values for this parameters, referred to 𝑃 𝑚𝑖𝑥,𝑜 and 𝑘 3𝑚𝑖𝑥,𝑜 , where the subscripted o means optimization. Pumice and ZVI:Lapillus mixtures. Varying the mixtures all the values referred to the material were changed, e.g. apparent and real density for porous materials, initial porosity. Varying the flow velocity and the contaminant initial concentration the related parameter values and boundary conditions were changed to be equal to experimental conditions.The values of 𝑃 𝑚𝑖𝑥,𝑜 and 𝑘 3𝑚𝑖𝑥,𝑜 found for the different column tests simulated are reported in table 6-7.As can be observed, the values of both coefficients are lower than the correspondent found by batch tests analysis. In particular 𝑃 𝑚𝑖𝑥 values, related to batch tests, seem to be 5, 15 or 50 times higher than the corresponding 𝑃 𝑚𝑖𝑥,𝑜 . This can give some indications to performed batch tests with conditions more similar to those of column test. The 𝑘 3𝑚𝑖𝑥 values as found for batch tests are two or three orders of magnitude higher than the corresponding 𝑘 3𝑚𝑖𝑥,𝑜 . Further investigations can be effected to understand what are the main factors affecting the term 𝑘 3 representing the reversibility of sorption or the influence of already sorbed mass on sorption rate. Furthermore, it can be observed that the column tests with 30:70 ZVI:Pumice mixture using a solution of Nickel at 40 and v1 and v3 flow velocity can be simulated using the same values of 𝑃 𝑚𝑖𝑥,𝑜 and 𝑘 3𝑚𝑖𝑥,𝑜 parameters. These values can be used to simulate

	Considering what has been observed through the previous simulation results the
	sorption onto the mixtures has been modeled as developed by only one material that
	has the characteristics of the mixture.
	This means that one term of sorption has been used in the dispersion-advection
	equation.

This can suggest that the sorption in packed column is mainly dependent on the contaminant concentration. For low values of the latter, the sorption is highly affected by flow velocity.

Table 6 -7 coefficients of non-equilibrium sorption model as found by optimization Figure 6-10 simulation and experimental results of column test performed with 30:70 ZVI:Pumice w.r. mixture, solution of Nickel at 50 mg/L initial concentration and v1 flow velocity.

 6 It is possible to observe that the first-order rate coefficient 𝑃 𝑚𝑖𝑥,𝑜 depends on the initial concentration value. In fact, for values of 40-50 mg/L, it does not vary with the flow velocity, while this happens for lower initial concentration values (8 mg/L) and 𝑃 𝑚𝑖𝑥,𝑜 value increases as the flow velocity increases. The 𝑘 3𝑚𝑖𝑥,𝑜 backward rate, that

		1
		0.9
		0.8
		0.7
		0.6
	C/C 0	0.4 0.5
		0.3
		0.2
		0.1
		0
		0	10	20	30	40	50	60

takes into account the mass already absorbed onto the reactive material, is of the same order of magnitude for all the tests, unless for that performed at the highest initial concentration value (95 mg/L). This makes sense because in this latter case the

column length (cm) 30:70 ZVI:Pumice v1 Ni 40

	mass retained after a certain time is higher than in tests performed using lower initial
	concentration under the same conditions.
	t 152h
	t 400 h
	t 820 h
	t 2260 h
	t 3036 h
	152 h sim
	t 400 h sim
	t 820 h sim
	t 2260 h sim
	t 3000 h sim

30:70 ZVI:Pumice v2 Ni 8

						29 h
						264 h
						432 h
						600 h
						768 h
						936 h
						1104 h
						1272 h
						1447 h
						1673 h
						29 h sim
						264 h sim
						432 h sim
						600 h sim
						768 h sim
						936 h sim
						1104 h sim
						1272 h sim
						1447 h sim
						1673 h sim
	1
	0.9			30:70 ZVI:Pumcie v1 Ni 8		140 h 336 h
	0.8					600 h
	0.7					816 h 1080 h
	0.6					1512 h
	0.5 C/C 0 0.4					1841 h 140 h sim 336 h sim
	0.3					600 h sim 816 h sim
	0.2					1080 h sim
	0.1					1512 h sim
						1841 h sim
	0
	0	10	20	30	40	50	60
				column length (cm)

30:70 ZVI:Pumice v3 Ni 95

							4 h
							12 h
							34 h
							48 h
							78 h
							h
							h
							h
							h
							h
							h
							h
							4 h sim
							12 h sim
							34 h sim
							48 h sim
							78 h sim
							h sim
							h sim
							h sim
							h sim
							h sim
							h sim
							h sim
		1
		0.9			30:70 ZVI:Lapillus v2 Ni50		t 120h t 240h
		0.8					t 432h t 600h
		0.7					t 744h t 912h
	C/C 0	0.6 0.5					t 1104h t 1248h t 1440h 120 h sim
		0.4					240 h sim 432 h sim
		0.3					600 h sim 744 h sim
		0.2					912 h sim 1104 h sim
							1248 h sim
		0.1					1440 h sim
		0
		0	10	20	30	40	50	60
					column length (cm)

  Table 7-1 the characteristics of the two benchmark column tests are reported. The experimental data and characteristics of pure Pumice column test concern the 30:70 weight ratio ZVI:Pumice column test, where the two reactive materials are placed in two layers in serie, not mixed

Table 7 -1 Characteristics of pure Pumice and pure Lapillus carried out column tests

 7

	React. mat.	Initial cont. conc. (mg/L)	React. Thick. (cm)	React. area (cm 2 )	React. Vol. (cm 3 )	Lapill. (gr) or Pum.	n (%)	Flow min) veloc. (mm/	PV (cm 3 )	Tres (h)
	Lapill.	Ni 50	50	18.09	904.3	1289	35	0.276	318.5	10.6
	Pum.	Ni 50	47	18.09	850	560	45	0.276	383.4	12.8

Table 7 -2 Characteristics of column tests considered in comparison of weight ratio influence between ZVI:Pumice and ZVI:Lapillus mixtures

 7

	Tres
	(h)

  In Figures7-3 a) and b) data concerning 45 and 85 gr of ZVI are shown respectively. It can be observed that the ZVI:Lapillus filter is always more efficient than the ZVI:Pumice filter length containing approximately the same ZVI quantity. Even though the 18 cm and 28 cm long ZVI:Lapillus filters are compared to the 28 cm and 58 cm long ZVI:Pumice filters respectively, the first can reduce Nickel initial concentration to 0.004 value until 432 hours and 900 hours, while the second can reduce it by 50-60% at maximum already at the first sampling time. This can demonstrate the higher Nickel removal efficiency of Lapillus compared to Pumice.

Table 7 -4 Characteristics of column tests considered in comparison of flow velocity influence between ZVI:Pumice and ZVI:Lapillus mixtures 7.2.3.1 Long-term removal efficiency

 7 In Figures 7-16 and 7-17, the Nickel retained per unit reactive mass in each filter sector is shown for the ZVI:Pumice (Figures a) and ZVI:Lapillus (Figures b) mixtures

		50	50	18.09	904.3	473	1104	37	0.055	336.7	56.1
	Fe 0 / Lap	Ni 50	50	18.09	904.3	480	1120	37	0.276	334.3	11.1
	Fe 0 / Lap	Ni 50	50	18.09	904.3	468	1093	38	1.382	348.1	2.3
	Fe 0 / Pum	Ni 50	50	18.09	904.3 244	570	47	0.055	428.6	71.4
	Fe 0 / Pum	Ni 50	40	18.09	904.3	240	560	44	0.276	398.5	13.3
	Fe 0 / Pum	Ni 50	40	18.09	904.3	240	560	48	1.382	432.6	2.9

Table 7 -5 Characteristics of column tests considered in comparison of contaminant initial concentration influence between ZVI:Pumice and ZVI:Lapillus mixtures

 7 -9, the characteristics of the column tests carried out and useful for this study are reported.

	React. mat.	Initial cont. conc. (mg/L)	React. Thick. (cm)	React. area (cm 2 )	React. Vol. (cm 3 )	Fe 0 (gr)	Lapill. (gr) or Pum.	n (%)	Flow min) veloc. (mm/	PV (cm 3 )	Tres (h)
	Fe 0 / Lap	Ni 50	50	18.09	904.32	480	1120	37	0.276	334.34	11.1
	Fe 0 / Lap	Ni 100	50	18.09	904.32	477	1113	37	0.276	337.58	11.3
	Fe 0 / Lap	Ni 10	50	18.09	904.32	480	1120	38	0.276	341.36	11.4
	Fe 0 / Pum	Ni 8	50	18.09	904.32	240	560	46	0.276	444	14.8
	Fe 0 / Pum	Ni 50	50	18.09	904.32	244	570	44	0.276	644.6	21.5

cm (29 grZVI) 8 cm (77 gr ZVI) 18 cm (173 gr ZVI) 28 cm (269 gr ZVI) 38 cm (365 gr ZVI) 50 cm (4880 gr ZVI) b)

cm 10:90 w.r. 3 cm 30:70 w.r. 28 cm 10:90 w.r. 8 cm 30:70 w.r. b)

The first comparison can be made considering the 8 cm of 10:90 w.r. test (22 gr of ZVI) and 3 cm of 30:70 w.r. test (28 gr of ZVI). In Figure 5-32 a) the Nickel normalized concentration evolution measured at 3 cm of 30:70 w.r. ZVI/Lapillus column test and that at 8 cm of 10:90 w.r. ZVI/Lapillus column test are shown using continuous lines. As it is possible to observe, 8 cm of 10:90 w.r. filter seems to be more efficient than 3 cm of 30:70 ZVI/Lapillus w.r. mixture. Its Nickel removal efficiency loss proceeds more slowly and the residual efficiency remains higher during the time than that of the 3 cm long filter of 30:70 w.r. ZVI/Lapillus that contains the same ZVI weight and less Lapillus quantity.

At 120 hours the relative concentration is equal to 0.7 for 30:70 w.r and 0.006 for 10:90 w.r. After 600 hours, it is about 0.9 for 30:70 w.r. and 0.7 for 10:90w.r. After 2000 hours of interaction, it is around 0.99 for 30:70 w.r. column tests and about 0.8 for 10:90 w.r. There are different variables that can play a role in this behaviour: advection, diffusion, dispersion and reaction term. Both columns were carried out under the same flow velocity. This means that the advection term is the same and that the contaminant front should take the same time to reach a certain filter length, under the same experimental conditions. a)

The evolution of Nickel concentration at 3 cm of 30:70 w.r. column length is almost the same as that at 3 cm of 10:90 w.r. filter length. This can suggest that the optimized ZVI content in 3 cm of column is that of the 10:90 w.r, because it maximizes the Nickel removal efficiency. This means also that in the first 3 cm of 30:70 w.r. column, the ZVI content can be in excess in relation to the Nickel removal efficiency that can be developed in 3 cm of column length, in the considered interaction interval. This is more evident compering the results above-considered to that measured at 3 cm of pure ZVI column. The experimental data relative to 3 cm length of the three considered tests are overlapped until 600 hours of interaction, suggesting that the optimal ZVI content for certain filter length and time exists.

Considering the three tests carried out, the best ZVI content for the 3 cm column length and the assured residence time is that of the 10:90 w.r. test. This is true until 600 hours, after this time pure ZVI 3 cm long filter has a higher residual removal efficiency than the other tests. 600 hours correspond to 900 mg of input Nickel mass.

3 cm and 8 cm of the carried out column tests lengths mean about 0.66 and 1.7 hours of residence time respectively.

The evolution of the relative concentration corresponding to 8 cm of 10:90 w.r.

column and 3 cm of 30:70 w.r. column, as with that at 3 cm of pure ZVI column, shows the ability of lapillus to optimize the removal efficiency of the ZVI and the influence of residence time compared to the ZVI content, for certain contaminant initial concentration and constant velocity. In fact, Lapillus plays an important role, because it is able to reduce the Nickel normalized concentration to 0.7 value if it is used as pure reactive material. The importance of residence time for Ni removal was highlighted also for ZVI/Pumice mixtures [START_REF] Moraci | Heavy metals removal and hydraulic performance in zero-valent iron/pumice permeable reactive barriers[END_REF][START_REF] Calabrò | Estimate of the optimum weight ratio in Zero-Valent Iron/Pumice granular mixtures used in permeable reactive barriers for the remediation of nickel contaminated groundwater[END_REF]Bilardi et al. 2013a;[START_REF] Moraci | Critical aspects related to Fe0 and Fe0/pumice PRB design[END_REF] In

Influence of filter thickness

Influence of filter thickness on removal efficiency

In Figure 5-56 a) and b) the Nickel normalized concentration evolution with the filter thickness at different sampling times and with time at each sampling port is shown respectively, for the LC test. An 80 cm long filter can reduce efficiently (with a removal efficiency of 99%) the Nickel concentration until 6500 hours of interaction test, corresponding to 9000 mg of Nickel input mass.

In Figure 5-57 the evolution of Nickel concentration with time for the same sampling ports of the LC and SC tests is reported. A similar removal efficiency can be observed.

Whereas the long-term hydraulic behaviour has been not similar, as will be discussed in the next sub-paragraph.

Conclusion

In this chapter it is proposed the use of natural volcanic lapillus as new admixing agent for ZVI. The granular mixture composed by ZVI and lapillus was tested toward nickel removal through column tests and the effect of the mixture weight ratio, flow velocity and contaminant initial concentration on the long-term removal efficiency and hydraulic behavior was investigated.

It was observed as pure Lapillus tested in column with a solution of nickel at 50 mg/L of initial concentration and a 0.276 mm/min constant flow velocity performs a not negligible nickel removal efficiency. Considering the same ZVI content (240 g), the removal efficiency after the input of 2.5 g of nickel mass was 99.3%, 98.9%, 95.1%

and 34% for the granular mixtures at weight ratios of 10:90 (for which the maximum ZVI content available was 138g), 30:70 and 50:50 and pure ZVI filter respectively.

Hydraulic conductivity determinations have shown the highest permeability reduction for the granular mixture at weight ratio equal to 50:50. For the 10:90 and 30:70 w.r. mixtures, the nickel mass retained was almost homogenously distributed along the first 18 cm from the column inlet, while for 50:50 w.r. mixture the nickel mass is retained mainly in the first 8 cm leading to a higher reduction of the hydraulic conductivity. The flow velocity plays an important role in nickel removal, in fact the higher the flow velocity and the lower the contact time and the removal efficiency.

Furthermore, the removed nickel mass was retained in the first 8 cm of the filter in the test carried out at 0.079 m/d, it is equally distributed in the first 18 cm of the filter for the test carried out at 0.4 m/d, whereas the entire filter is involved in nickel removal processes in the test carried out at 1.9 m/d. The removal efficiency of the reactive medium decreases as contaminant concentration in input increases. In particular, the test performed with 10 mg/L of initial nickel concentration shows an efficiency equal to 99.9%. and the nickel retained mass was concentrated in the first 3 cm of the filter, whereas it was distributed in the first 28 cm of the filter in the test carried out with the higher value of initial nickel concentration (i.e. 100 mg/L).

Repeatability of tests should be studied to understand more precisely the influence of ZVI and lapillus in long-term removal efficiency and hydraulic behaviour.

In Chapter 6, a model to simulate the experimental results observed in column tests performed with mixtures is developed.

Comparison of weight ratio influence

Let us analyze and compare the weight ratio influence on Nickel removal efficiency of ZVI:Pumice and ZVI:Lapillus mixtures. The three tested weight ratios of ZVI:Pumice and ZVI:Lapillus and the benchmark tests will be considered. They consist of 10:90, 30:70 and 50:50 for both ZVI:Pumice w.r. and ZVI:Lapillus w.r., pure Pumice, pure

Lapillus and pure ZVI column tests carried out under the same experimental boundary conditions.

Table 7-2 summarizes the characteristics of the considered column tests.

The comparative analysis among these tests to study the weight ratio influence of the two different mixtures on Nickel removal can be based mainly on two parameters: ZVI content and removal efficiency.

Equal ZVI content

The first analysis proposed is about experimental data interpretation obtained considering couples of filter lengths corresponding to equal ZVI content between ZVI:Lapillus and ZVI:Pumice columns at the same weight ratio. Under the hypothesis of complete homogenous mixing of the two reactive materials constituting each mixture filling the column, filter lengths containing the same ZVI quantity in both kinds of mixtures were identified in order to compare their performance, at fixed weight ratio. In the following

Long-term Nickel removal efficiency

In Figures 7-26 a) and b) the evolution of Zinc normalized concentration as function of filter thickness for different sampling times is shown for the ZVI:Pumice and ZVI:Lapillus mixtures respectively. As can be observed, both filters are able initially to reduce Zinc concentration efficiently in the available filter thickness (50 cm). This is true for the ZVI:Pumice mixture filter until about 500 hours of interaction, while the ZVI:Lapillus mixture filter is highly efficient after 2328 hours also. Furthermore, the latter maintains a constant high removal efficiency until about 500 hours using a 8 cm long filter.

In this comparison, the different ZVI content between the two filters should be considered. In fact, the ZVI:Pumice and ZVI:Lapillus mixtures contain 481 gr and 970 gr of ZVI respectively, one almost the double of the other (Table 7-10). Taking into account this aspect, a comparison between the available data at the sampling ports placed at 28 cm from the inlet and at the outlet of the ZVI:Lapillus and ZVI:Pumice mixtures filters will be proposed. In Figure 7-27 a) the Zinc normalized concentration evolution at sampling ports placed at 28 cm from inlet and at outlet of the ZVI:Lapillus and ZVI:Pumice mixtures filters respectively is shown as function of time.

As can be observed, the ZVI:Pumice mixture filter is able to reduce the Zinc w.r. mixtures, 8 cm of ZVI:Lapillus mixture have the same efficiency of 38 cm of ZVI:Pumice filter and for 50:50 w.r. mixtures, 8 cm of ZVI:Lapillus mixture have the same efficiency of 33 cm of ZVI:Pumice filter.

Regarding the comparison among column tests with 30:70 ZVI:Pumice w.r. and ZVI:Lapillus w.r. mixtures performed using different flow velocities (0.055, 0.276 and 1.382 mm/min), the higher removal efficiency of the ZVI:Lapillus than ZVI:Pumice mixtures is confirmed for each flow velocity used. The v1 test seems to be highly efficient for all the tests duration (14000 h for ZVI:Pumice and 19000 for ZVI:Lapillus) in both filters. Concerning the v2 tests, the ZVI:Lapillus filter maintains a high removal efficiency for all the test duration. In fact its Nickel removal efficiency is about 88% after 9000 mg of Nickel input , while using ZVI:Pumice mixture it is about 58% corresponding to 6000 mg. Regarding the mixtures tested using v3 flow velocity, it can be observed that the Nickel removal efficiency evolution as function of Nickel input mass is almost the same and it is about 60% after 400 mg of Nickel input. In Chapter 8 the results of release tests performed for some column tests and the iron and nickel concentration evolution is shown and discussed.

8 Further information about removal efficiency

Introduction

As observed in Chapter 7, some differences in long-term removal efficiency and hydraulic behavior were found between ZVI:Pumice and ZVI:Lapillus mixtures.

In this chapter, some further analysis is performed to evaluate whether ZVI:Lapillus mixtures release nickel after the test end and how the iron concentration varies as function of time and filter thickness. The evolution of iron concentration along the column and with time is shown and discussed, comparing it with the evolution of Nickel for the same considered column tests.

Furthermore, results of release tests, performed to evaluate the possibility of release of contaminant by exhausted reactive materials when fresh water flows through it, are shown and described for two column tests.

Iron and Nickel concentration evolution

The Iron concentration was measured in samples withdrawn from the solution flowing into the column at different sampling times and ports for some column tests.

Knowing the evolution of Iron along the column with time can be an important information to understand whether Iron is released in the water during the removal processes and to hypothesize what processes lead to its release.

In this paragraph, two column tests are considered through which the mixture 30:70 ZVI:Lapillus w.r. was tested using a solution of Nickel at 50 mg/L initial concentration: one was performed using a 50 cm long filter under a 1.382 mm/min constant flow velocity (v3), the second using a 100 cm long filter under a 0.276 mm/min constant flow velocity (v2). In

Nickel release tests

The results of release tests performed in two column tests are shown in Figures 8-9

Conclusions

Observing the data of nickel and iron concentration evolution along the column and during time, it can be observed that the evolution of iron is strictly related to that of nickel, because its evolution starts and ends where it is observed as the same for the