Adoum Mahamat Ahmat

Thomas Thiebault

Régis Guégan
email: regis.guegan@aoni.waseda.jp

Keywords: Phenolic acids, Montmorillonite, Adsorption, Organic matter preservation

come

Phenolic acids interactions with clay minerals: A spotlight on the adsorption mechanisms of Gallic Acid onto montmorillonite

1

Phenolic acids interactions with clay minerals: a spotlight on the adsorption mechanisms of Gallic Acid onto Montmorillonite

Adoum Mahamat Ahmat (a,b)1 , Thomas Thiebault (c) , Régis Guégan (b,d) 2 (a) Institut Department of Civil Engineering and Environment. 764,Boulevard Lahure,59508 Douai,France. (b) Institut des Sciences de la Terre d'Orléans ISTO, UMR 7327, University of Orléans, 1A Rue de la Férollerie, 45500 Orléans La source, France.

(c) EPHE, PSL University, UMR 7619 METIS (SU, CNRS, EPHE), 4 place Jussieu, F-75005, Paris, France (d) Faculty of Science and Engineering, Global Center for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan.

Abstract

For a better understanding of the preservation of organic matter in clay minerals, the 1 adsorption of a model humic substance, the Gallic Acid (GA), onto a Na-montmorillonite 2 (Na-Mt) was performed in batch situation for various experimental conditions (pH=2, 5, 7) in 3 order to mimic the natural context. The adsorption efficiency and change in the clay mineral 4 were characterized via a set of complementary experimental techniques (Fourier transform 5 infrared spectroscopy, X-ray diffraction, elemental analyses). Adsorption isotherms at the 6 equilibrium were fitted with the models of Langmuir, Freundlich and Dubinin-Radushkevitch 7 allowing one to precisely quantify the adsorption through the derived fitting parameters. From 8 the adsorption data combined with complementary results of the modeled humic-clay 9 complexes, different types of interactional mechanisms were inferred as a function of 10 background acidity: (i) at pH=2 while protonated GA was the preponderant form, anionic GA 11 species can be adsorbed to the Na-Mt surface through electrostatic interaction, leading to the a

Introduction

Clay minerals were recognized to stabilize soluble organic compounds through the adsorption of dissolved OM in superficial horizons of soils [START_REF] Gonzalez | Role of clay minerals on soil organic matter and humification[END_REF][START_REF] Kögel-Knabner | Organo-mineral associations in temperate soils: Integrating biology, mineralogy, and organic matter chemistry[END_REF]Schmidt et al., 2011;[START_REF] Kaiser | Effects of mineral characteristics on content, composition, and stability of organic matter fractions separated from seven forest topsoils of different pedogenesis[END_REF]. The confinement of organic compounds within the interlayer space of clay minerals avoids any heterotrophic reactions and leads to their preservation [START_REF] Kaiser | Sorptive stabilization of organic matter by microporous goethite: Sorption into small pores vs. surface complexation[END_REF][START_REF] Wattel-Koekkoek | Mean residence time of kaolinite and smectitebound organic matter in Mozambiquan soils[END_REF]. It also favors the generation of polymeric macromolecules via the condensation of single monomers (Stevenson, 1982;[START_REF] Wang | Catalytic synthesis of humic substances by natural clays, silts, and soils[END_REF][START_REF] Yariv | Organo-clay complexes and interactions[END_REF]. From the numerous research works on the subject, and more particularly studies focusing on the understanding of the interaction between clay minerals and humic substances leading to the formation of humic-clay complexes, it appears that simple blocks of polymeric humic molecules or elementary monomolecular compounds play a major role in the interaction with the mineral surface and stabilization of the complexes [START_REF] Greenland | Interaction between humic and fulvic acids and clays[END_REF][START_REF] Feng | Chemical and mineralogical controls on humic acid sorption to clay mineral surfaces[END_REF][START_REF] Wang | Structural and sorption characteristics of adsorbed humic acid on clay minerals[END_REF][START_REF] Chotzen | Adsorption of soil-derived humic acid by seven clay minerals, a systematic study[END_REF][START_REF] Chen | Mechanisms of soil humic acid adsorption onto montmorillonite and kaolinite[END_REF].

The typology of physicochemical mechanisms allowing the establishment of perennial association between organic compounds and mineral surfaces were extensively studied in recent years. For example, ligand exchanges and cationic exchanges are reported as sustainable organo-mineral interaction mechanisms [START_REF] Keil | Mineral matrices and organic matter[END_REF][START_REF] Lambert | Organic pollutants adsorption on clay minerals[END_REF], while low energy bonding such as van der Waals effects and hydrogen bonding are acknowledged to establish easily reversible associations [START_REF] Plante | Changes in clay-associated organic matter quality in a C depletion sequence as measured by differential thermal analyses[END_REF][START_REF] Lutzow | Stabilization of organic matter in temperate soils: Mechanisms and their relevance under different soil conditions-A review[END_REF]. Aqueous media chemistry strongly constrains the preponderance of these mechanisms [START_REF] Arnarson | Mechanisms of pore water organic matter adsorption to montmorillonite[END_REF]. Background parameters as ionic strength and pH shape clays' state of charge as well as the degree of protonation of dissolved organic compounds. Hence, it impacts the type of adsorption driving process. Chen et al., 2017 stressed out higher adsorption performances of humic compounds onto 2: 1 clay minerals at low pH and suggested the increase of electrostatic phenomena to explain this observation.

Besides background properties, organic compounds intrinsic characteristics also determine the extent of the adsorption and constrain the nature of preponderant bonding mechanisms [START_REF] Bu | Effects of complexation between organic matter (OM) and clay mineral on OM pyrolysis[END_REF]. In the presence of compounds with different spatial structures and molecular functions, a competition to the occupation of adsorption sites can occur. For example, the adsorption of amino acids onto montmorillonites is more effective than the uptake of phenolic acids contained in the same aqueous mixture [START_REF] Gao | Competition and surface conditioning alter the adsorption of phenolic and amino acids on soil minerals[END_REF].

Understanding these mechanisms is mandatory to assess the potential use of clay minerals in the adsorption of contaminants of various natures. Clays and clay minerals are investigated raw, or after chemical modification, as an economically viable removal pathway of petroleum by-products [START_REF] Meleshyn | Adsorption of phenanthrene on Na-Montmorillonite: A model study[END_REF]Lamishane et al., 2016) and emerging pharmaceutical molecules [START_REF] Li | Removal of diphenhydramine from water by swelling clay minerals[END_REF]Thiebault et al., 2015;[START_REF] De Oliveira | Adsorption of Diclofenac onto organoclays: effect of surfactant and environmental (pH and temperature) conditions[END_REF] commonly encountered in hydrographic networks. The uptake performance may be boosted via inorganic pillaring [START_REF] Liu | Adsorption of levofloxacin onto an ironpillared montmorillonite (clay mineral): Kinetics, equilibrium and mechanism[END_REF] or surfactant intercalation (De Oliveira and Guégan

2016).

Thus, the study of the fundamental mechanisms of organo-mineral aggregation has applications in various fields. In this paper, the emphasis is placed on the organo-clay association, and its further protective role. This role has been assessed in sedimentary environments through different approaches [START_REF] Kennedy | Clay mineral continental amplifier for marine carbon sequestration in a greenhouse ocean[END_REF][START_REF] Arndt | Quantifying the degradation of organic matter in marine sediments: A review and synthesis[END_REF][START_REF] Mahamat-Ahmat | Organic matter-clay interaction along a seawater column of the Eastern Pacific upwelling system (Antofagasta bay, Chile): implications for source rock organic matter preservation[END_REF], 2017). It appeared that organo-clay associations involving ionic exchanges and those resulting from interlayer intercalation allowed efficient isolation against microbial stresses. In pedological context, the mechanism has mainly been studied from the perspective of complex polymeric humic acids [START_REF] Chen | Mechanisms of soil humic acid adsorption onto montmorillonite and kaolinite[END_REF][START_REF] Derakhshani | Optimization of humic acid removal by adsorption onto bentonite and montmorillonite nanoparticles[END_REF].

While the main interaction mechanisms ensuring the stability of the organo-mineral complexes are rather difficult to determine in the case of humic substances (Tombácz et al., 2004;[START_REF] Chotzen | Adsorption of soil-derived humic acid by seven clay minerals, a systematic study[END_REF][START_REF] Chen | Mechanisms of soil humic acid adsorption onto montmorillonite and kaolinite[END_REF], this research work aims at characterizing precisely the main driving force leading to the aggregation of OM. For this purpose, we focus on a simple carboxylic acid (gallic acid) as simple blocks or monomeric components in polymeric humic acids. Gallic Acid (GA) is a phenolic acid found in vascularized plants, inputted in soil horizons as single molecules or apart of large polyphenolic macromolecules such as tannins and various ligno-cellulosic by-products. Here, in this research work, we focus on the sorption mechanisms of GA onto montmorillonite under different experimental conditions.

Materials and methods

Interaction of the carboxylic acids with a clay mineral

A natural Wyoming Na-montmorillonite (Na-Mt) was supplied by the Source Clay Minerals Repository of the Clay Minerals Society. Its structural formula can be expressed as: (Ca 0.12 Na 0.32 K 0.05 ) [Al 3.01 Fe(III) 0.41 Mn 0.01 Mg 0.54 Ti 0.02 ] [Si 7,98 Al 0.02 ] O 20 (OH) 4 . Gallic Acid (GA) was provided from Sigma Aldrich at 97.5 % purity grade and was used without further treatment. This weak acid can be seen as a phenolic compound owing a carboxylic function with 2 alcohol groups (Fig. 1). Its physico-chemical parameters (topological surface, pKa) are summarized in Table 1.

The interactions between GA and Na-Mt were conducted in different experimental conditions under various pH conditions: 2, 5 and 7. Typically, the adsorbent mass was 150 mg in 50 mL solution spiked with a GA concentration between 10 mg. L -1 and 2 g. L -1 . The pH value was adjusted with both NaOH and HCl solutions (0.1 M). The solutions were stirred at 200 rpm with a magnetic stirrer during 24 hours in order to reach the equilibrium (kinetic data not shown) at room temperature. The solid fraction (i.e. organo-clay complexes) was separated from the liquid one through a centrifugation step (5000 rpm; 10 min) and then lyophilized before flash pyrolysis analyses.

Experimental Techniques

Adsorbed organic carbon was measured using flash pyrolysis (Thermo Scientific Flash 2000 organic analyzer) performed on organo-clay complexes in powder form.

The samples were also characterized via Fourier Transform Infrared Spectroscopy (FT/IR) in the range 650-4000 cm -1 . Measurements were realized using a Thermo Nicolet 6700 FT spectrometer equipped with a Deuterated Triglycine Sulfate (DTGS) detector at room temperature and for different temperatures controlled by a Linkam thermal device allowing us to characterize the thermal behavior on a wide range of temperature: 50-550°C.

The analyses were performed in transmission mode and each spectrum corresponded to the average of 256 scans collected at 2 cm -1 of resolution.

The d 001 spacing's of the organo-mineral complexes was determined by the first 00l reflection from the X-rays patterns which were recorded on a conventional θ-θ Bragg-Brentano configuration by using a Thermo Electron ARL'XTRA diffractometer equipped with a Cu anode (CuK α = 1.5418 Å) coupled with a Si(Li) solid detector. The diffractograms on dry samples (100°C for 24 h) were performed between 1 and 24° (2θ) with an angular and time steps of 0.04° and 10 s, respectively.

Thermal gravimetric analyses were carried out under atmospheric conditions at the heating rate of 10 °C min -1 from room temperature (25°C) to 800 °C using the thermal gravimetric analyzer (Model: STA PT 1600, manufactured by Linseis Company, Germany).

Sorption Modeling

The fitting of the resulting adsorption isotherms by using Langmuir, Freundlich and Dubinin-Radushkevich (DR) equation models drive to numerous thermodynamic parameters allowing to precisely quantify the affinity of GA with the Na-Mt. Briefly, Langmuir model assumes that the whole organic molecules are adsorbed on singularized sites on the accessible surface of the adsorbent, and each site hosts a unique molecule. This Langmuir model is expressed by the following equation [START_REF] Levan | Binary Langmuir and Freundlich isotherms for ideal adsorbed solutions[END_REF]:

q e = q max K L /[1 + (K L C e )]
(1)

where q e is adsorbed amount when equilibrium is reached (mol g -1 ); C e is the remaining concentration in the solution at equilibrium (mol L -1 ); q max is the maximum sorption capacity of the Na-Mt, and K L is the Langmuir constant (L mol -1 ) which is related to Gibbs free energy ΔG° (kJ mol -1 ) through the thermodynamic equation ( 2):

ΔG° = -RTln K L (2)
where R represents the universal gas constant (8.314 J mol -1 K -1 ) and T the temperature

(K).
Freundlich and D-R equations takes into account surface heterogeneities on the adsorption process and deal with the variabilities in the interaction mechanisms leading to the adsorption of organic compounds that can form or be organized in multi-layers. Freundlich adsorption model is a linear relation [START_REF] Levan | Binary Langmuir and Freundlich isotherms for ideal adsorbed solutions[END_REF][START_REF] Özcan | Adsorption of acid blue 193 from aqueous solutions onto BTMA-bentonite[END_REF] expressed through the following equation:

ln q e = ln K F + 1/n (ln C e ) (3)
Where K F (g L -1 ) and n are constants and indicate respectively the extent of the adsorption and the degree of non-linearity between GA and the smectite. Indeed, when the term 1/n ranges between 0.1 and 1, it suggests that the adsorption mechanism is favorable [START_REF] Liu | Adsorption of methyl violet from aqueous solution by halloysite nanotubes[END_REF]. D-R isotherms allow one to acquire complementary thermodynamic parameters. Its equation is written as:

ln q e = ln q m + β*ε 2 (4)
Where ε is the Polanyi potential, computed through the relation ( 5)

ε = RT ln (1+1/Ce) (5)
q m is the theoretical potential saturation capacity of the sorbent and β is the constant related to the activity (mol 2 J -2 ) connected to the mean free energy E of adsorption (kJ mol -1 ) via the equation ( 6):

E = 1 / √2β (6)
This later parameter gives information whether the adsorption mechanism involves a cation exchange or physical adsorption. Indeed, if the magnitude of E is below 8 kJ mol -1 , physisorption is envisaged, while for E > 8 kJ mol -1 the adsorption process follows an ion exchange or a chemisorption mechanism.

Moreover, we used an error function (F error ) in order to evaluate which equation model was best suited to describe these processes. A lower result from the error function indicated a smaller difference between adsorption capacity calculated by the model (q i cal ) and the experimental (q i exp ). F error can be expressed according to the following Eq. ( 7)

F error = ∑ (q i cal -q i exp / q i exp ) 2 (7)
Where q i cal is a value of q predicted by the fitted model; q i exp is a value of q measured experimentally; i indicates the values of the initial GA concentration of the experiments; and P is the number of experiments performed.

Results and discussions

pH dependence on the adsorption of GA onto Na-Mt

The adsorption isotherms at the equilibrium stress out the actual affinity of GA with Na-Mt (Fig. 2). Indeed, adsorption isotherms display a gradual growth of the adsorbed amount when the equilibrium concentration increases. The slope of this growth attenuates at high starting concentrations emphasizing a saturation state, excepted for the isotherm realized at pH=5.

GA adsorption isotherms are properly fitted by the three equation models used as r 2 values display values between 0.95 and 0.99 and F error values are between 0.0010 and 0.130 for GA (Table 2). Based on r 2 values, experimental data seem to be better adjusted to the Langmuir model, however its function errors are higher than 0.1 and to those for both Freundlich and DR equations, which spread out from 0.001 to 0.004. This is a side effect of the logarithmic scale adopted in Freundlich and DR representations. Although Langmuir equation properly fitted experimental data, the two latter equations appear to be more suitable for modeling the adsorption of the phenolic acid onto the clay mineral surface. Indeed, clay mineral shows a heterogeneous surface leading to a distribution of several adsorption sites that are taken into account in both Freundlich and DR equation models.

Under low pH conditions (i.e. below the pKa of GA), the adsorption is particularly enhanced, with the predominance of the protonated form of GA (i.e. acidic form) in solution, due to the decrease of the repulsion between the neutral GA and Na-Mt. However, the situation may be rather twisted with antagonist effects. Indeed, under such pH conditions (pH=2), the pH value is lower than the pH of zero net proton charge (pH ZNPC ) of Na-Mt, estimated about 4.5 (Tombàcz and Szekeres, 2006). For pH < pH ZNPC , the electric charge of the edge-sites of clay mineral surface changes from anionic to cationic, favoring the adsorption of phenolic acids in their anionic form, (i.e. base) allowing a possible ion exchange process which enhances the amount of adsorbed organic acids. In contrast, at pH=5 and 7, since both the clay mineral edge-sites and the equilibrium ratio between the acid/base forms change (Fig. 3), the adsorption involves additional interactional mechanisms, affecting the lineshape of the adsorption isotherms while reducing the adsorbed amounts. This lowering is due to the repulsion between anionic charges of both GA and Na-Mt. with the angular position of the 00l reflection, increases from 9.6 Å for a dehydrated clay mineral to about 13.3 Å at pH=2. While an intercalation occurs at low pH, this phenomenon is not observed for the other two pH, where a slight increase of the interlayer space to a value of 11 Å can be noticed (Fig. 5). This is probably due to both the clay mineral charge density and the preponderance of the neutral (RCO 2 H) form which play an important role in the adsorption as well its associated interaction mechanism (e.g. coordinative bonding ).

The contribution of FT/IR spectroscopy gives important information in the characterization of GA-clay complexes derived from GA and Mt and confirms its actual adsorption. Absorption bands observed at 1350, 1384 and 1470 cm -1 are assigned to stretching modes of C -O. and bending modes of C-H of O-H respectively (Fig. 6). The strong vibration at 1700 cm -1 was assigned to C=O stretching of GA's carboxylic function. Besides the existence of a drift or a temperature gradient in temperature during the FT/IR experiments, the complete degradation of GA is accomplished at relative high temperature in contrast to bulk GA which is decomposed at about 300°C estimated through thermal gravimetric measurements (Fig. 8). This last observation about the preservation of GA at high temperature is related to its confinement within the interlayer space of Mt obtained for GA at pH=2.

TG analyses allow assessing the loss of weight of the adsorbent during a gradual heating. Organo-clay minerals display usually three main weight losses during the heating.

The first one is associated to the evaporation of free and adsorbed water, ranging between the initial temperature and 150°C. The second one is related to the thermal oxidation of adsorbed organic compounds between 150 and 600°C with a maximal decomposition temperature related to the characteristics of organic moieties. Finally, for temperatures higher than 600°C, only the dehydroxylation of clay minerals is expected [START_REF] Xie | Thermal characterization of organically modified montmorillonite[END_REF]. temperature is noticed at 247°C and corresponds to the decomposition of acidic group of GA [START_REF] Rao | Derivatographic studies of gallic acid[END_REF][START_REF] Hussein | Synthesis and characterization of (zinc-layered-gallate) nanohybrid using structural memory effect[END_REF]. In the organic matter combustion range, GA-Na-Mt complexes aggregated at pH = 5 display one loss of weight (i.e. 340°C), while those formed at pH=2 exhibit two noticeable losses at 310 and 410°C (Fig. 8). This splitting after interaction at pH=2 might be explained by the distinction between GA adsorbed onto the external surface of the clay mineral (i.e. lower temperature) and intercalated GA (i.e. higher temperature) [START_REF] Zhu | Novel intercalation mechanism of zwitterionic surfactant modified montmorillonites[END_REF]. Hence, this result appears to be consistent with XRD results, in which an intercalation of GA only occurs after interaction at pH=2.

Adsorption mechanisms and geochemical model of GA-Mt complexes

The observation that GA adsorption is higher at pH < pKa is in agreement with the results of Rabiei et al., 2016 which stressed out higher GA adsorptions at pH=3 (33%) compared to their experiments performed at pH=7 (20%). This pH-dependency is related to the protonation capacity of GA's polar appendage (COOH / COO -). It must be noted however, that the pH control of GA adsorption cannot be generalized to the interaction of other phenolbased molecules. The study by Dolaksis et al., 2018 for instance, suggested that the adsorption of phenolic cores devoid of polarizing COOH appendix is pH-independent. They showed that low energy mechanisms (hydrogen bridges) drive the adsorption of chlorophenols and nitrophenols onto silicate surfaces.

At a pH < pKa, GA is mainly protonated (neutral) and this form represents about 99.6 % of the chemical species at pH=2 (Fig. 3). However, in acidic condition, the charge density of the edge-sites of Na-Mt changes and switches below the pH of zero net proton charge estimated at 4.5 (Table 1). Here, despite the possibility of an alteration of the structure and probably the chemical composition of the layered material, the adsorption of GA appears to be enhanced in acidic conditions. The parameters derived from the fitting procedure, and more particularly those of the D-R model giving a free energy of adsorption E slightly above 8 kJ mol -1 , underlining the possibility of chemical process for adsorption or at least strong electrostatic interactions. Although being less known and implied in the adsorption of anionic compounds onto Mt, electrostatic interaction was recognized as the main driving force leading to the intercalation of various kind of anionic compounds: tannic and benzoic acids, anionic surfactants [START_REF] Yan | Adsorption of benzoic acid by CTAB exchanged montmorillonite[END_REF][START_REF] Zhang | Integrated investigations on the adsorption mechanisms of fulvic and humic acids on three clay minerals[END_REF][START_REF] An | Adsorption of tannic acid on chitosan-montmorillonite as a function of pH and surface charge properties[END_REF], that may occur nevertheless, in particular experimental conditions (low pH range) as it is the case here.

Moreover, the increase of the basal spacing of Na-Mt after the interaction with GA at pH=2 (i.e. + 3.7 Å) is consistent with the molecular size of GA (i.e. z=3.7 Å, Figure 1), emphasizing the possible intercalation of GA through a horizontal monolayer within Na-Mt layers.

The adsorption of anionic species, even weak, acts as a coating that may favor further adsorption of organic molecules as organoclay materials do, nevertheless cannot only explain the totality of the adsorbed amount reaching about 1.5 x 10 -4 mol g -1 . Indeed, GA is in anionic form at a low concentration. The amount of anionic species may increase during the sorption mechanism since the uptake involves the displacement of acid basic equilibrium. This may allow in fine the adsorption of higher amount of anionic species. However, the effect of this acid basic shift is limited. Protonated species of GA remain preponderant and are likely to be adsorbed through other bonding mechanisms. Since GA is mainly neutral at pH < pKa as explained before, its adsorption should be driven through physisorption mechanisms such as molecular interaction (ππ interaction, van der Waals forces) with the prior adsorbed molecules and by coordinative bonding through inorganic exchangeable cations located within the interlayer space and with the carboxylic moieties as both FT / IR and XRD data highlighted. A recent work stressed out the importance of ion-dipole interaction (a coordinative bonding mechanism) as the main driving force for the adsorption of nonionic surfactants [START_REF] Guégan | Structure and dynamics of nonionic surfactant aggregates in layered materials[END_REF] onto a Mt surface and can be according to previous studies in the literature (Sonon and Thompson, 2005;[START_REF] Deng | Bonding mechanisms and conformation of poly(ethylene oxide)-based surfactants in interlayer of smectite[END_REF] extended to nonionic compounds and here GA in its neutral form (Fig. 7).

The preponderance and the role of this interaction is confirmed at a pH value of 5 where the maximum adsorbed amounts at the equilibrium reaches 1.3 x 10 -4 mol g -1 . Based on the diagram of preponderance of GA species in regards to pH, its neutral form represents about 20% at a pH=5 (Fig. 3). The experiment with the highest starting concentration of 2 g L -1 leads to 5.88 x 10 -4 mol g -1 (if one does the hypothesis that such amount is adsorbednumber of GA moles normalized to the mass of Mt introduced) where 20% are in the RCO 2 H form (neutral one), thus driving to a value of 1.176 x 10 -4 mol g -1 , close to the experimental one estimated at 1.3 x 10 -4 mol g -1 . At this pH value, the clay mineral displays a cation exchange capacity as well as an anionic charge surface increasing the repulsion between adsorbent and organic anions. Hence, the protonated form of GA is favored for adsorption, and may interact to the inorganic exchangeable cations through coordinative bonding forces. However, it is important to mention that 20% of the inorganic exchangeable cations are located onto the external surface of the clay mineral (Swartzen-Allen and Matijević, 1975;[START_REF] Shainberg | Sodium / Calcium Exchange in Montmorillonite and Illite Suspensions 1[END_REF][START_REF] Patzko | Ion exchange and molecular adsorption of a cationic surfactant on clay minerals[END_REF]Logdson and Laird, 2004), where they can be easily mobilized for a cation-exchange or other interactions such as coordinative bonding or other mechanism such as cationic bridges involving the anionic species and divalent compensating cations. Here, the adsorption of GA does not lead to any intercalation as the XRD data displayed and exclusively remains on the external surface. The presence of the inorganic exchangeable cations at about 20% of the CEC on the external surface can be easily mobilized through coordinative bonding or via complexation reactions (Fig. 7) with the phenolic acid of which adsorbed amount match a value lesser than the 20% of the CEC (1.6 x 10 -4 mol g -1 ).

Similar observations are noticed at pH=7, where the adsorbed amount of neutral GA is not enough to lead to any intercalation as both XRD (Fig. 5) and FT/ IR data showed and restrict at this pH range principally anionic species in the adsorption. Indeed, in contrast to the previous pH, where the proportion of neutral species of GA represents about 96 and 20 % at pH=2 and 5 respectively, at a pH=7, it is insignificant with only 0.25 % (Fig. 3). While, the anionic form of GA is preponderant, an adsorption is surprisingly observed without however any intercalation as it was the case for pH=5, leading to an adsorbed amount of about 1 x 10 -4 mol g -1 (Fig. 2). Here, the adsorption may involve interaction mechanisms such as cationic bridges between anionic GA and divalent inorganic cations. Hence, even mostly compensated with Na + , around 20% of the compensating inorganic cations of Na-Mt are Ca 2+ , that are able to sorb anionic species through cationic bridges [START_REF] Errais | Anionic RR120 dye adsorption onto raw clay: Surface properties and adsorption mechanism[END_REF]Thiebault et al., 2016;[START_REF] Rabiei | Gallic acid-loaded montmorillonite nanostructure as a new controlled release system[END_REF]. It should be mentioned that this mechanism could also play a key-role at pH = 5, although the distinction between these mechanisms is not possible based on these experiments (Fig. 7).

Putting into perspective the behavior of GA with other phenolic-based compounds and clay minerals

The adsorption of GA is enhanced at low pH, as polyphenolic humic molecules behave [START_REF] Feng | Chemical and mineralogical controls on humic acid sorption to clay mineral surfaces[END_REF][START_REF] Zhang | Integrated investigations on the adsorption mechanisms of fulvic and humic acids on three clay minerals[END_REF][START_REF] Chotzen | Adsorption of soil-derived humic acid by seven clay minerals, a systematic study[END_REF]. Polyphenolic molecules as fulvic and humic acids, exhibit indeed greater adsorption rates with growing acidity [START_REF] Gouré-Douby | Adsorption of fulvic and humic like acids on surfaces of clays: Relation with SUVA index and acidity[END_REF]. For instance, the study of [START_REF] Chen | Mechanisms of soil humic acid adsorption onto montmorillonite and kaolinite[END_REF] focusing on the adsorption of soil macromolecular humic acids onto both montmorillonite and kaolinite, pointed out the enhancement of the adsorption at low pH. GA adopts a similar behavior when background pH ranges below its pKa and allows a preeminence of its protonated species.

In the case of natural polyphenolic molecules (humic substances), it has been repeatedly observed that in addition to background acidity, the nature of the adsorbent has a predominant role. Clay minerals of the 1:1 group such as kaolinite appear more conducive to the adsorption of humic -type of polyphenols. This is promoted by edges electrostatic phenomena. The extent of the adsorption through edge electrostatic interaction principally depends on the physico-chemical properties and mineralogy of a clay mineral, and more particularly its pH of zero net proton charge (pH ZNPC ) which is estimated to 4.5 for Na-Mt (Tombàcz and Szekeres, 2006), lower than other soil clayey components such as kaolinite (Wiliams and Wiliams, 1978;[START_REF] Gupta | Surface force measurements at the basal planes of ordered kaolinite particles[END_REF]. With its particular properties: a pH ZNPC and background protonation leading to a proper dispersion of clay mineral particles, kaolinite favors the adsorption of macromolecular humic acids at large content in contrast to Na-Mt besides its large CEC and specific surface area values [START_REF] Chotzen | Adsorption of soil-derived humic acid by seven clay minerals, a systematic study[END_REF]. With similar background chemistry, GA is likely to mimic the behavior of such soil poly-phenolic compounds. Since sorbent's charge distribution varies according to clayey groups; mineralogical typology must be added to the key criteria ruling GA's adsorption However, care should be taken in comparing the uptake of these phenolic-based compounds as differences in molecular masses, spatial arrangements and number of charges may induce differences in sorptive behaviors.

Although being poorer from quantitative perspective, the interaction of GA with Na-Mt remains interesting from a protective point of view since its neutral form may intercalate (Fig. 2; Fig. 7) under specific acidic conditions (pH < pKa). Interlayers isolations are reported to reduce bioavailability (Theng et al., 2001) and help to prevent biotic redox transformations.

In our case, the pH dependence of the adsorption appears to be consistent with the trends shown by other phenolic and polyphenolic molecules. However, the nature of the interactional mechanisms varies according to the molecules and does not display any systematic pattern. Although our results suggest intercalation and surface complexations via weak mechanisms (coordinative bonding), ligand exchanges are often involved in the interaction of long phenolic chains.

This encourages to complexify our view of the aggregation modalities of clays and phenol-like molecules and the possibility of organic matter stabilization process that it inducts. Na-Mt interactional mechanisms seems to differ whether the involved organic molecule are heavy macromolecular acids or singularized phenolic compounds. Thus, in a pedological environment where micro-fauna regime induces high rates of polymeric lyses and solubilizes a high quantity of phenolic acids, the stabilization process follows a different pathway from the configuration where polymeric humic acids are weakly degraded. In the first case, our data suggest that intercalation should prevail during the adsorption of polyphenolic acids by-products in acidic pH conditions, while several studies point out adsorption on the edge and ligand exchanges in the ecological context where polymeric forms are preserved [START_REF] Feng | Chemical and mineralogical controls on humic acid sorption to clay mineral surfaces[END_REF]Zang et al., 2012;[START_REF] Chotzen | Adsorption of soil-derived humic acid by seven clay minerals, a systematic study[END_REF][START_REF] Chen | Mechanisms of soil humic acid adsorption onto montmorillonite and kaolinite[END_REF].

Conclusions

The series of organo-clay interactions performed here under evolutionary batch equilibrium conditions allowed to characterize the adsorption mechanisms of GA, a common phenolic acid in natural pedological media, onto Na-Mt. The parameters derived from fitting of GA data with a reasonable agreement (high r 2 values) to the adsorption models used:

Langmuir, Freundlich and D-R equations, gave pertinent insights for a proper description of the phenomena under different approaches which pointed out the good affinity of the phenolic acid to Na-Mt. Additional experimental results obtained by FT/IR, TGA and XRD corroborated the actual adsorption of GA onto Na-Mt.

From the set of data, it appears that this monomolecular acid is mainly adsorbed through coordinative bonding interaction in contrast to macromolecular humic acids at low pH range, where ligand exchanges or complexation reaction occur according to the literature.

Here, GA, a singularized element of large pedological compounds, interacts with the clayey mineral mainly via both surface and interlayer processes at low pH, leading to its confinement within the interlayer space which may allow a sustainable preservation of the organic matter in that way.

Acknowledgement

This study was supported by the project MONITOPOL funded by the French region Centre Val de Loire (grant number 00117247). The authors are grateful to Marielle Hatton for her analytical contribution.

Figures Captions

Fig. and Na-Mt composite samples as a function of the starting GA concentrations in solution for pH=2. Only the results going up to 0.1 g L -1 are presented here, since no particular evolution of (001) planes was observed beyond this concentration. Fig. 8: TG (solid lines) and DTG (dashed lines) curves of GA/Na-Mt (C GA = 0.1 g L -1 ) composite samples after interaction at pH=2 (dark gray lines); pH=5 (dark lines) and pH=7

(light gray lines).

Tables Captions

Table 1: Physico-chemical properties of implemented clay mineral (Na-Mt) and GA.

Fig. 4

 4 Fig.4shows the X-ray diffraction patterns evolution of the GA-clay complexes (dried

  Fig. 8 gathers TG and DTG curves of GA-Na-Mt composites. Weight losses associated to the dehydration remains independent from pH values. Corresponding DTG peaks occur between 98 and 105°C. With growing temperature, the distinction between each sample is more pronounced. Hence, between 150 and 600°C, no significant weight loss is displayed at pH=7, whereas noteworthy weight losses are observed at the lower pH values (2 and 5). The first decomposition

  Fig. 1: Chemical structure of Gallic Acid (GA).

Fig. 2 :

 2 Fig. 2: Adsorption isotherms of Gallic Acid. Beige squares represent the data obtained at pH

Fig. 4 :

 4 Fig. 4: Graph of the 3D evolution of XRD diffraction patterns of dehydrated Gallic Acid (GA)

Fig. 5 :

 5 Fig. 5: Evolution of the d 001 basal spacing determined by the 00l reflection of Na-Mt layers

Fig. 6 :

 6 Fig. 6: FT/IR spectra of GA/Na-Mt composite samples (pH=2): thermic evolution.

Fig. 7 :

 7 Fig.7: Schematic representation of the possible adsorption mechanisms leading to the

  Figure 1 Click here to download Figure: Fig1.pdf

  Figure 4 Click here to download Figure: Fig 4.PDF

  Figure 7 Click here to download Figure: Fig 7.pdf

  Figure 8 Click here to download Figure: Fig8.pdf

Table 2 :

 2 Adsorption isotherm constants determined with Langmuir, Freundlich, and Dubinin-Radushkevich model fit for the adsorption of GA onto Na-Mt for different pH.

	Supplementary data

Table 2 Click here to download Table: table 2.docx

 2

	y=5.87 Å
	x=7.64 Å
	z=3.72 Å

Swartzen-Allen, S.L., Matijević, E., 1975. Colloid and surface properties of clay suspensions: II. Electrophoresis and cation adsorption of montmorillonite. Journal of Colloid Interface Science 50, 143-153. Sonon, L.S., Thompson, M.L., 2005. Sorption of a nonionic polyoxyethylene lauryl ether surfactant by 2:1 layer silicates. Clays and Clay minerals 53, 45-54. Schmidt, M.W.I., Torn, M.S., Abiven, S., Dittmar, T., Guggenberger, G., Janssens, I.A., Kleber, M., Kögel-Knabner, I., Lehmann, J., Manning, D.A.C., Nannipieri, P., Rasse, D.P., Weiner, S., Trumbore, S.E., 2011. Persistence of soil organic matter as an ecosystem property. Nature 478, 49-56. Stevenson, F.J. 1982. Humus Chemistry. John Wiley & Sons, New York. Theng, B.K.G., Aslabie, J., Fraser, R., 2001. Bioavailability of phenanthrene intercalated into an alkylamonium-montmorillonite clay. Soil Biology and Biogeochemistry 33, 845-848. Thiebault, T., Guégan, R., Boussafir, M., 2015. Adsorption mechanisms of emerging micropollutants with a clay mineral: case of tramadol and doxepine pharmaceutical products. Journal of Colloid and Interface Science 453, 1-8. Thiebault, T., Boussafir, M., Guégan, R., Le Milbeau, C., Le Forestier, L., 2016. Clayey-sand filter for the removal of pharmaceuticals from wastewater effluent: percolation experiments. Environmental Science Water Research & Technology 2, 529-538. Tombácz, E., Libor, Z., Illés, E., Majzik, A., Klumpp, E., 2004. The role of reactive surface sites and complexation by humic acids in the interaction of clay mineral and iron oxide particles. Organic Geochemistry 35, 257-267. Tombácz, E., Szekeres, M., 2006. Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite. Applied Clay Science 34, 105-124.

Appendix 1: Spectroscopic observations (FT/IR) of GA-Na-Mt complexes aggregated at different levels of GA starting concentrations.

Highlights

-Gallic acid (GA) adsorption onto Na-Mt was performed in batch conditions; -Data were fitted to Dubinin-Radushkevitch, Langmuir and Freundlich equations; -Following the acidity, both edge and interlayer interactions modes were observed;

-GA intercalates under its neutral form;


Alexandre Devaux

Mathieu Brédif

Charlotte Hoarau

Sidonie Christophe

UNDERGROUND VISUALIZATION: VIRTUAL REALITY, EX SITU AND IN SITU AUGMENTED REALITY

, the 3D model of sewers has been made available for web visualization and interaction. In this paper, we show that it is possible to address various use contexts, when adding the ability to visualize and interact with the sewer 3D model in virtual reality (VR) and in ex situ / in situ augmented reality (AR). These experiments will be further experimented to validate our hypotheses to favor collaboration, immersion for simulation and training, and more secured urban development.

Augmented Reality (AR) has been identified as a relevant medium for 3D urban geovisualization, potentially improving the user experience while offering urban data immersion and interaction [START_REF] Döllner | Virtual 3 d city models as foundation of complex urban information spaces[END_REF][START_REF] Drettakis | Design and evaluation of a real-world virtual environment for architecture and urban planning[END_REF][START_REF] Hugues | GIS and Augmented Reality : State of the Art and Issues[END_REF][START_REF] Normand | A new typology of augmented reality applications[END_REF]. In particular, AR has been extensively used indoor to visualize 3D models, such as building models [START_REF] Devaux | 3d urban geovisualization: in situ augmented and mixed reality experiments[END_REF], potentially enabling participation between practitioners.

In this work-in-progress paper, we present an ongoing experiment of underground visualization, investigating virtual and augmented reality using a pre-existing digitized network of sewers [START_REF] Nguyen | 3d immersive underground navigation[END_REF].

A 3D model of sewers should be easy to visualize and manipulate for final users: 3D geovisualization is clearly relevant to fit these needs, but the capacity of a 3D geovisualization system to handle various possible use contexts is less addressed by now. According to the use contexts, the users may have preferences or needs to handle a more or less realistic model. Moreover, they may need to visualize the 3D model ex situ (e.g. at the office for mission preparation) or in situ (during the mission on the ground).

The main issue here is to be able to improve the user experience, according to the use context: we aim et designing a flexible system that could manage the possible transitions between visual devices, 3D models varying in levels of detail, in order to fit the users' needs. * Corresponding author

EXPERIMENTATIONS

Based on the open-source 3D geovisualization web framework iTowns offering 3D visualization and precise interactive measurements [START_REF] Nguyen | A 3d heterogeneous interactive web mapping application[END_REF][START_REF] Devaux | Realtime projective multitexturing of pointclouds and meshes for a realistic street-view web navigation[END_REF], iTowns Development Team, 2018), we propose methods to co-visualize 3D city models and 3D underground models, with various visualization and interaction modalities.

Virtual Reality (VR). On a desktop PC or Virtual Reality (VR) device, the sewer model could be visualized in 3D with texture (Fig. 1-a) or with wireframe rendering superimposed on streetview imagery and building models (Fig. 1-b).

Augmented Reality (AR).

In AR with a HoloLens, we offer ex situ visualizations at reduced scale (Fig. 2-a) or at real scale (Fig. 2-b), as well as in situ visualizations at real scale (Fig. 2-c-d).

In this last in situ experiment, the users can then walk and "see through" the ground: it may be especially useful and intuitive for sanitation workers in order to prepare an intervention or for practitioners requiring to dig the sidewalks.

PERSPECTIVES

From a geographic information sciences perspective, the geovisualization of complex urban spaces and phenomena is essential to favor visuospatial thinking and to support the understanding and the decision-making for operational purposes. Scientific and technical challenges, as well as further experiments regarding in situ augmented and mixed reality are addressed in [START_REF] Devaux | 3d urban geovisualization: in situ augmented and mixed reality experiments[END_REF].

Those preliminary experiments should now be evaluated regarding their capacity to fit the user needs, their potential to increase interaction modalities and to reduce the cognitive effort to understand the user environment.

Figure 1 .

 1 Figure 1. Ex situ Virtual Reality (VR) using a WebGL application: a) 3D view of the textured sewer model with interactive measurements. b) 3D view from the street level of the textured 3D city model and the wireframe sewer model.

Figure 2 .

 2 Figure 2. Augmented Reality (AR) of the wireframe model (HoloLens): a) reduced scale ex situ, b) real scale ex situ, c-d) real scale in situ. The technical contribution of this 3D geovisualization web application is to take advantage from a shared implementation, allowing to switch seamlessly from one device to another. A demo of the capacities of the tool will be presented during the workshop/theme session on Virtual & Augmented Reality: Technology, Design & Human Factors, organized by ISPRS Working Group IV/9.