Nadia Morin-Crini

Eric Lichtfouse
email: eric.lichtfouse@gmail.com

Marc Fourmentin
email: marc.fourmentin@univ-littoral.fr

Ana Rita
email: ritalado@fe.up.pt

Lado Ribeiro

Constantinos Noutsopoulos
email: noutsopouloscnoutso@central.ntua.gr

Francesca Mapelli
email: francesca.mapelli@unimi.it

Éva Fenyvesi
email: fenyvesi.e@cyclolab.hu

Melissa Gurgel
email: melissagav@feq.unicamp.br

Adeodato Vieira

Lorenzo A Picos-Corrales

Juan Carlos

Marc Fourmen

Constantinos Noutsopoul

Éva Fenyv

Melissa Gur

• Juan

Carlos Moreno-Piraján

Liliana Giraldo
email: lgiraldogu@unal.edu.co

Tamás Sohajda
email: sohajda@cyclolab.hu

Mohammad Mahmudul Huq

Jafar Soltan
email: j.soltan@usask.ca

Giangiacomo Torri
email: torri@ronzoni.it

Monica Magureanu

Corina Bradu
email: corina.bradu@g.unibuc.ro

Grégorio Crini
email: gregorio.crini@univ-fcomte.fr

Marc Fourment

Juan Carlos Moreno

Removal of emerging contaminants from wastewater using advanced treatments. A review

Keywords: Emerging contaminants, Wastewater, Advanced treatments, Adsorption, Biological technology, Oxidation processes

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Emerging contaminants are a group of natural and synthetic chemicals, and biological agents that are not or poorly regulated. These substances are currently of concern because they have known or suspected adverse impacts on the environment and the human health. Indeed, emerging contaminants are not necessarily of new use, but newly identified and for which data on their presence and fate in the environment, and their effects on organisms is not completely understood. The list of these substances is particularly long and includes pharmaceuticals such as antibiotics, anti-inflammatory drugs, anti-diabetics, analgesics, antidepressants, antisiolitics, lipid regulators, natural and synthetic hormones, illicit drugs and endocrine disruptors; cosmetics and personal care products including fragrances, fats, oils, detergents, disinfectants, sunscreens and insect repellants; pesticides; industrial products such as surfactants, additives, solvents, flame retardants and nanomaterials; microplastics and pathogens. The number of chemical substances concerned is constantly changing both in terms of parent substances and their degradation products, for example resulting from biological treatment. Due to advantages offered by these products in everyday life, many of them are employed and released continuously into the environment, even at very low concentrations, and some can cause chronic toxicity, endocrine disruption in humans and aquatic organisms, and the development of antibiotic resistant bacteria. It is therefore necessary to mobilize efforts to protect human health and biodiversity (Patle et al. 2020;[START_REF] Hube | Mitigation of emerging pollutants and pathogens in decentralized wastewater treatment processes: a review[END_REF][START_REF] Karpińska | New aspects of occurrence and removal of emergin pollutants[END_REF].

Nowadays, it is recognized that the presence of many emerging contaminants can be correlated with the discharge of wastewater effluents, mainly from wastewater treatment plants located in highly industrialized and urban areas (Fig. 1) [START_REF] Priac | Alkylphenol and alkylphenol polyethoxylates in water and wastewater: a review of options for their elimination[END_REF][START_REF] Priac | Alkylphenol and alkylphenol polyethoxylates in water and wastewater: a review of options for their elimination[END_REF][START_REF] Gilabert-Alarcón | Regulatory challenges for the use of reclaimed water in Mexico: a case study in Baja California[END_REF][START_REF] Mert | Efficient removal approach of micropollutants in wastewater using membrane bioreactor. In: Yonar T (ed) Wastewater and water quality[END_REF][START_REF] Picos-Corrales | Environmentfriendly approach toward the treatment of raw agricultural wastewater and river water via flocculation using chitosan and bean straw flour as bioflocculants[END_REF]. Indeed, conventional domestic wastewater treatment plants, often stabilization ponds and activated sludge systems, have not been designed to treat recalcitrant organic pollutants including emerging substances [START_REF] Botero-Coy | An investigation into the occurrence and removal of pharmaceuticals in wastewater Colombian[END_REF][START_REF] Gogoi | Occurrence and fate of emerging contaminants in water environment: a review[END_REF]Crini and Lichtfouse 2019;[START_REF] Tolboom | Algal-based removal strategies for hazardous contaminants from the environment -a review[END_REF][START_REF] Patel | Pharmaceuticals of emerging concern in aquatic systems: chemistry, occurrence, effects, and removal methods[END_REF]). As a result, the discharged effluents always contain a heterogenous mixture of substances. There are complementary solutions to existing treatments to deal with this residual contamination, such as the use of activated carbon coupled with oxidation steps and membrane filtration. Other advanced treatments have been studied in recent years as well [START_REF] Rodriguez-Narvaez | Treatment technologies for emerging contaminants in water: a review[END_REF][START_REF] Bourgin | Evaluation of a full-scale wastewater treatment plant upgraded with ozonation and biological post-treatments: abatement of micropollutants, formation of transformation products and oxidation by-products[END_REF][START_REF] Collivignarelli | Overview of the main disinfection processes for wastewater and drinking water treatment plants[END_REF][START_REF] Miklos | Evaluation of advanced oxidation processes for water and wastewater treatment -a critical review[END_REF][START_REF] Liu | Emerging pollutants -Part II: treatment[END_REF][START_REF] Khan | Recent trends in disposal and treatment technologies of emerging-pollutants -a critical review[END_REF].

This review presents advanced treatment methods such as adsorption-oriented processes using biosorbents, biological-based technologies, and advanced oxidation processes. Biosorbents such as cyclodextrin bead polymers have great potential in environmental applications although they are still at laboratory stage. Selected biological approaches include constructed wetlands, biomembrane reactors, strategies based on the use of algae, fungi or bacteria, and enzymatic degradation. Advanced oxidation processes such as electrochemical technologies, catalytic ozonation and plasma also represent the most promising approaches. This article is an abridged version of the chapter published by [START_REF] Morin-Crini | Remediation of emerging contaminants[END_REF] in the series Environmental Chemistry for a Sustainable World.

Adsorption-oriented processes for the removal of emerging contaminants

Liquid-solid adsorption using commercial activated carbons is a key method for removing contaminants from wastewaters or drinking water sources. Indeed, this industrial technique of contaminant removal can produce high quality water, while also being a process that is both technologically simple and economically feasible [START_REF] Crini | Wastewater treatment by sorption. In: sorption processes and pollution[END_REF]. It is known that commercial activated carbons in granular or powder form are effective adsorbents for treating a wide range of contaminants, including pesticides and pharmaceuticals, for adsorbing organic matter to reduce chemical oxygen demand and biochemical oxygen demand, for decoloring water, or for treating substances that cause a specific taste or odor (Crini andBadot 2007, 2010;[START_REF] Couto | Effect of solution pH and influence of water hardness on caffeine adsorption onto activated carbons[END_REF][START_REF] Ferreira | Adsorption of paracetamol using activated carbon of Dende and Babassu coconut mesocarp[END_REF]Crini et al. 2019;[START_REF] Khan | Recent trends in disposal and treatment technologies of emerging-pollutants -a critical review[END_REF]). However, even though the high adsorption capacity of active carbons is widely recognized, there are some drawbacks, namely their rapid saturation and regeneration. This regeneration step of saturated carbon is costly, not straightforward, and leads to a loss of the adsorbent. However, even though the high adsorption capacity of active carbons is widely recognized, active carbon technology has several drawbacks. Active carbons are quite expensive, the higher the quality, the greater the cost. The different qualities of carbon are strongly related to the raw material used, as well as dependent on the carbonization conditions and the way in which activation is carried out (physical or chemical). There are also two shortcomings, the rapid saturation and the need of regeneration of saturated carbon, which is also expensive, not straightforward, and results in adsorbent loss. It is therefore interesting to find new materials capable of having the same performance as carbon without its disadvantages. For more than 30 years, numerous studies have been published to meet this challenge. The research is divided into two directions: the use of natural, cheap, and abundant non-conventional materials in raw or modified form and the development of new synthetic materials. The materials proposed include cyclodextrin beads, metal-organic frameworks, molecularly imprinted polymers, chitosan-based materials, and nanocellulose.

Removal of pharmaceuticals using cyclodextrin bead polymers

Over the past two decades, much work has been done on the use of cyclodextrin polymers for the manufacture of complex emerging substances for environmental applications [START_REF] Morin-Crini | Water-insoluble β-cyclodextrin-epichlorohydrin polymers for removal of pollutants from aqueous solutions by sorption processes using batch studies: a review of inclusion mechanisms[END_REF][START_REF] Fenyvesi | Applications of steroid drugs entrapped in cyclodextrins[END_REF][START_REF] Cova | Cyclodextrin polymers and cyclodextrin-containing polysaccharides for water remediation[END_REF][START_REF] Yadav | A review on remediation technologies using functionalized cyclodextrin[END_REF]. These commercial polymers are becoming very popular for the tracking and removal of trace emerging substances. Conventional water treatment technologies are not very effective in reducing the concentration of these pollutants to a desirable level, prompting researchers to innovate and to propose complementary methods to conventional treatments, such as the use of cyclodextrin bead polymers.

Cyclodextrins are carbohydrates produced from starch and characterized by cyclic structure with hydrophilic surface and moderately hydrophobic inner cavity; cyclodextrins are able to encapsulate compounds in their cavity [START_REF] Szejtli | Introduction and general overview of cyclodextrin chemistry[END_REF]. Since their discovery at the end of the nineteenth century in France, a great body of knowledge has been collected on the effect and mechanism of binding various organics, with numerous pharmaceuticals among them [START_REF] Crini | Review: a history of cyclodextrins[END_REF]. Within the cavity of cyclodextrins, the water molecules are readily replaced by less hydrophilic guest compounds such as low water-soluble drugs (Fig. 2). In this way, inclusion complexes or clathrates are formed without creating or disrupting covalent bonds. The host cyclodextrin and the guest are reversibly connected by hydrophobic interactions and van der Walls forces; therefore, the complex can dissociate when the conditions change, e.g., upon dilution, heating or in the presence of other competing guests (Szejtli 1998; Crini et al. 2018). The stability of the inclusion complexes can be enhanced by hydrogen bonds between the hydroxyl groups on outer surface of cyclodextrin and the hydrophilic part of the guest protruding from the cavity. The most important consequences of the inclusion complex formation, such as enhancing the solubility of poorly soluble substances, protecting the included guest molecules against the environmental effects such as oxidation, hydrolysis, decomposition on heat and light, enzymatic degradation, and improving the taste, are utilized broadly by the pharmaceutical industry [START_REF] Frömming | Cyclodextrins in Pharmacy[END_REF][START_REF] Loftsson | Cyclodextrins in drug delivery (review)[END_REF][START_REF] Szejtli | Elimination of bitter, disgusting tastes of drugs and foods by cyclodextrins[END_REF][START_REF] Uekama | Recent aspect of cyclodextrin-based drug delivery system[END_REF]). The number of active ingredients marketed in the form of cyclodextrin complexes is above 100 at the end of 2021 and is continuously increasing (CycloLab archive). Among the three natural cyclodextrins, α-cyclodextrin, β-cyclodextrin and γ-cyclodextrin consisting of 6, 7 and 8 glucopyranose units, respectively, β-cyclodextrin possesses a cavity size with an internal diameter of 0.60-0.65 nm and a depth of 0.78 nm especially suitable for inclusion of a wide range of drug molecules [START_REF] Szejtli | Introduction and general overview of cyclodextrin chemistry[END_REF]. While the unmodified β-cyclodextrin is used mostly as excipient in pharmaceutical formulations for oral administration, its highly soluble hydroxypropylated and sulfobutylated derivatives have also been approved for parenteral applications (EMA 2017).

While the pharmaceutical industry aims to improve the solubility and consequently the bioavailability of drugs via complexation, the objective in wastewater purification is the opposite: to remove the dissolved non-biodegradable pharmaceuticals by adsorption/sorption/biosorption, both terms being used in the literature. As natural cyclodextrins are water-solubles, they must be transformed into water-insoluble sorbents/biosorbents, using three main synthesis routes:

(1) by cross-linking with proper bi-or polyfunctional reagents, such as diepoxy compounds, diisocyanates, dior polycarboxylic acids, and fluoroterphthalonitriles [START_REF] Wiedenhof | Properties of cyclodextrins. III cyclodextrin-epichlorhydrin resins[END_REF][START_REF] Crini | Macroporous polyamines containing cyclodextrin: synthesis, characterization, and sorption properties[END_REF][START_REF] Yamasaki | Preparation of crosslinked β-cyclodextrin polymer beads and their application as a sorbent for removal of phenol from wastewater[END_REF][START_REF] Zhao | Water-insoluble beta-cyclodextrin polymer crosslinked by citric acid: synthesis and adsorption properties toward phenol and methylene blue[END_REF][START_REF] Alsbaiee | Rapid removal of organic micropollutants from water by a porous β-cyclodextrin polymer[END_REF][START_REF] Xu | Simple synthesis of a swellable porous β-cyclodextrin-based polymer in the aqueous phase for the rapid removal of organic micro-pollutants from water[END_REF][START_REF] Yang | Removal of various pollutants from wastewaters using an efficient and degradable hypercrosslinked polymer[END_REF]; Fig. 3 shows a cyclodextrin polymer obtained by a cross-linking reaction between cyclodextrin molecules and a cross-linking agent;

(2) by copolymerization of polymerizable cyclodextrin derivatives with acrylic or vinyl monomers [START_REF] Wimmer | Synthesis of novel insoluble cyclodextrin polymers[END_REF][START_REF] Janus | New sorbents containing beta-cyclodextrin. synthesis, characterization, and sorption properties[END_REF][START_REF] He | Oil-absorbent beads containing beta-cyclodextrin moieties: preparation via suspension polymerization and high oil absorbency[END_REF]); (3) or by grafting on a macromolecular support such as chitosan, cellulose, alginate, or silica [START_REF] Crini | Separation of structural isomers using cyclodextrin-polymers coated on silica beads[END_REF][START_REF] Sakairi | Cyclodextrin-linked chitosan: synthesis and inclusion complexation abilities[END_REF][START_REF] Aoki | Adsorption of 4-nonylphenol ethoxylates onto insoluble chitosan beads bearing cyclodextrin moieties[END_REF][START_REF] Chung | Competitive adsorption of a phthalate esters mixture by chitosan bead and α-cyclodextrin-linked chitosan bead[END_REF][START_REF] Kono | Characterization and bisphenol A adsorption capacity of beta-cyclodextrin-carboxymethylcellulose-based hydrogels[END_REF][START_REF] Omtvedt | Efficient grafting of cyclodextrin to alginate and performance of the hydrogel for release of model drug[END_REF][START_REF] Yamasaki | Preparation of new photo-crosslinked β-cyclodextrin polymer beads[END_REF].

All these gels are preferably prepared in the form of tiny beads by emulsion/suspension polymerization. The spherical Fig. 2 Formation of an inclusion complex of cyclodextrin with a drug. Source Éva Fenyvesi, Budapest, Hungary; Marc Fourmentin, Dunkerque, France shape of the particles has the advantage of high surface area and good accessibility of the cyclodextrin cavities in addition to the technological advantages in scaled up production. Owing to the several hydroxyl groups on cyclodextrins, these gels are highly hydrophilic and swell in water. The degree of swelling depends on the conditions of preparation, e.g., type, concentration and ratio of reactants, temperature, and reaction time. [START_REF] Wiedenhof | Properties of cyclodextrins. III cyclodextrin-epichlorhydrin resins[END_REF][START_REF] Fenyvesi | Preparation of cyclodextrin polymers in bead form and control of their characteristics by the parameters of preparation[END_REF][START_REF] Romo | Application of factorial experimental design to the study of the suspension polymerization of beta-cyclodextrin and epichlorohydrin[END_REF]). The higher the degree of swelling the higher is the accessibility of the inner cyclodextrin cavities, but the smaller the mechanical stability against compression. The binding capacity of epichlorohydrin-cross-linked β-cyclodextrin bead polymer is higher in case of lower degree of cross-linking and reduced temperature [START_REF] Zhu | Binding of bile acids by polymerized cyclodextrin[END_REF][START_REF] Baille | Functionalized β-cyclodextrin polymers for the sorption of bile salts[END_REF]. For ionic or ionizable pharmaceuticals, cyclodextrin polymers modified by ionic groups can be more efficient biosorbents due to the contribution of electrostatic forces to hydrophobic interactions. Depending on the conditions, the binding mechanism can be complicated including inclusion and association complex formation, electrostatic interactions, ion exchange, and chelation. [START_REF] Morin-Crini | Water-insoluble β-cyclodextrin-epichlorohydrin polymers for removal of pollutants from aqueous solutions by sorption processes using batch studies: a review of inclusion mechanisms[END_REF]). Both the π-π interactions with the network and host-guest interactions with the cyclodextrin cavities may contribute to the uptake [START_REF] Huang | A phenyl-rich β-cyclodextrin porous crosslinked polymer for efficient removal of aromatic pollutants: Insight into adsorption performance and mechanism[END_REF].

Cyclodextrins and their polymers have been found useful for various environmental applications including wastewater treatment [START_REF] Crini | Synthesis and applications of adsorbents containing cyclodextrins[END_REF][START_REF] Gruiz | Cyclodextrins in innovative engineering tools for risk-based environmental management[END_REF][START_REF] Landy | Remediation technologies using cyclodextrins: an overview[END_REF][START_REF] Morin-Crini | Environmental applications of waterinsoluble β-cyclodextrin-epichlorohydrin polymers[END_REF][START_REF] Fenyvesi | Applications of steroid drugs entrapped in cyclodextrins[END_REF][START_REF] Cova | Cyclodextrin polymers and cyclodextrin-containing polysaccharides for water remediation[END_REF][START_REF] Yadav | A review on remediation technologies using functionalized cyclodextrin[END_REF]). The early sorption/biosorption experiments with cyclodextrin bead polymers aimed at the removal of various organic contaminants such as phenols and dyes. Although these beads usually outperformed activated charcoal, it became clear that cyclodextrin polymers are too expensive for the removal of Fig. 3 Cyclodextrin polymer obtained by a cross-linking reaction between cyclodextrin molecules and a cross-linking agent followed by the diclofenac sorption onto the cyclodextrin polymer. Source compounds present at high concentration. Due to their specific affinity toward drugs [START_REF] Fenyvesi | Controlled release of drugs from CD polymers substituted with ionic groups[END_REF][START_REF] Vyas | Cyclodextrin based novel drug delivery systems[END_REF][START_REF] Mocanu | Cyclodextrins anionic polysaccharide hydrogels: synthesis, characterization, and interaction with some organic molecules (water pollutants, drugs, proteins)[END_REF], they found their application for the sorption of non-biodegradable emerging contaminants, such as pharmaceuticals, which persits the traditional wastewater treatment and are usually detected in treated wastewater at very low concentrations.

The epichlorohydrin-cross-linked cyclodextrin polymers successfully removed hormones and other endocrine disrupting chemicals at nanomolar concentrations [START_REF] Oishi | Removal of dissolved estrogen in sewage efuents by beta-cyclodextrin polymer[END_REF]. High removal rate (70-> 90%) was achieved for 17-β-estradiol, a contraceptive of emerging concern, in the range of 1 × 10 -11 to 1 × 10 -8 mol/L concentration by using β-cyclodextrin polymer. Similar performance was observed for polymers prepared from γ-cyclodextrin and α-cyclodextrin. The removal ratio was hardly reduced when in addition to 17-β-estradiol, the model solution was spiked also with cholesterol in 100-fold molar excess related to the hormone because cholesterol, another steroid typical in wastewaters but with no endocrine disrupting properties, has lower binding affinity to the cyclodextrins cavity. The small-scale laboratory batch experiments with real municipal wastewater validated these results. It was recently published that carbonyldiimidazol-cross-linked β-cyclodextrin and γ-cyclodextrin polymer nanosponges efficiently removed the antipsychotic drug primavanserin from single solutions (93% and 80% removal rates, respectively) and from postreaction raffinates in small-scale batch experiments (0.1 g sorbent in 5 mL solution) [START_REF] Hemine | Cyclodextrin polymers as efficient adsorbents for removing toxic non-biodegradable pimavanserin from pharmaceutical wastewaters[END_REF].

The low mechanical strength of the gel beads does not allow the use of column technique in large scale. The higher swelling, e.g., lower degree of cross-linking, results in lower permeability, and thus, lower elution rate of water through the gel bed is attained. Therefore, fluidization technique was used in up-scaled demonstration of the technology. Another option is applying inorganic core such as silica or graphene oxide to get beads of high mechanical stability, or the introduction of rigid structures such as phenyl moieties into the polymer to form pores. For instance, silica coated with β-cyclodextrin using hexamethyl diisocyanate as cross-linking agent was used for the removal of emerging contaminants, including estrogen hormones, from water [START_REF] Bhattarai | Development of novel cyclodextrin adsorbents for the removal of emerging contaminants from water[END_REF]. More than 95% of 17-β-estradiol was removed from a single component solution and more than 90% of most estrogens from a multicomponent system, even after 4 regeneration cycles. Also magnetic graphene oxide modified with β-cyclodextrins/poly(l-glutamic acid) showed excellent binding capacity for 17-β-estradiol [START_REF] Jiang | Fabrication of β-cyclodextrin/poly(l-glutamic acid) supported magnetic graphene oxide and its adsorption behavior for 17β-estradiol[END_REF]): the sorption capacity was 85 mg/g sorbent at 0.05 g/L sorbent to solution ratio and 25 °C using a model solution of ~ 7 × 10 -7 mol/L of 17-β-estradiol. Single component solutions were eluted through a thin layer of β-cyclodextrin polymer with permanent porosity (cross-linked with tetrafluoroterephthalonitrile) to remove around 80% of ethinylestradiol and propranolol as model drugs at 1 × 10 -4 mol/L (Aisbaiee et al. 2016). In another study, a solution (8 mL) of 90 pollutants was eluted through the same porous β-cyclodextrin polymer (1 or 5 mg) to achieve nearly complete (over 95%) removal of several drugs including abacavir, amphetamine, cimetidine, codeine, diclofenac, estrone, famotidine, fluoxetine, gemfibrozil, ibuprofen, testosterone, triclosan to mention only a few and several pesticides [START_REF] Ling | Benchmarking micropollutant removal by activated carbon and porous β-cyclodextrin polymers under environmentally relevant scenarios[END_REF]. Figure 3 illustrates the diclofenac biosorption onto a cyclodextrin polymer. More recently, using the same cross-linking agent, the polymer (100 mg) obtained removed over 95% of both ethinylestradiol and estriol (~ 1 × 10 -7 mol/L, 1 L) by filtration from a multicomponent model solution containing other emerging contaminants [START_REF] Xu | Simple synthesis of a swellable porous β-cyclodextrin-based polymer in the aqueous phase for the rapid removal of organic micro-pollutants from water[END_REF]. A further possibility for preparing filters of good mechanical stability is sintering cyclodextrin bead polymers with a thermoplastic polymer such as polyethylene [START_REF] Jurecska | Characterization of cyclodextrin containing nanofilters for removal of pharmaceutical residues[END_REF]. This process, however, results in reduced binding capacity.

In laboratory experiments modeling post-purification of wastewater by fluidization technology, epichlorohydrin-cross-linked β-cyclodextrin polymer beads (20 g) were used to remove hormones, such as β-estradiol, ethinylestradiol and estriol (87-99%) from the spiked test solution (800 mL) containing some other emerging contaminants. The removal rate for the non-steroidal antiinflammatory drugs was in the range of 15-70% [START_REF] Nagy | Removal of emerging micropollutants from water using cyclodextrin[END_REF]. Based on the positive results of this experiment, a scaled-up fluidization procedure was planned and performed in a pilot-scale wastewater purification plant (Fig. 4). β-Cyclodextrin polymer beads (1 kg) were applied to polish 300 L purified municipal wastewater spiked with 9 emerging pollutants at approximately 2 × 10 -8 mol/L (among other drugs, e.g., diclofenac, 17-α-ethinylestradiol and 17-β-estradiol included in the watch list of substances for Union-wide monitoring in the field of water policy (European Water Framework Directive 2015/495/EU) [START_REF] Fenyvesi | Removal of hazardous micropollutants from treated wastewater using cyclodextrin bead polymer -A pilot demonstration case[END_REF]. The hormones such as estradiol (> 99.9%), 17-α-ethinylestradiol (99.9%) and estriol (96.1%) were effectively removed. The removal rate for the non-steroidal anti-inflammatory drugs diclofenac, ibuprofen, ketoprofen and naproxen were 85.5, 86.9, 18.1 and 13.5%, respectively, following roughly the cyclodextrin-drug association constants and showing that the main interaction was likely the inclusion complex formation. It should be noted that similarly low removal rate (18%) was obtained by [START_REF] Orprecio | Polymer-immobilized cyclodextrin trapping of model organic pollutants in flowing water streams[END_REF] for naproxen applied at 5 × 10 -6 mol/L in single component solution (5 L) using similar epichlorohydrin-cross-linked β-cyclodextrin polymer (10 g) as column packing. The pilot-scale demonstration of the fluidization technology with the cyclodextrin polymer was successful, with most of the pharmaceuticals as emerging contaminants, especially those on the European watch list, being efficiently removed in a short time. The beads are easily regenerated by extracting the sorbed components with methanol or ethanol [START_REF] Orprecio | Polymer-immobilized cyclodextrin trapping of model organic pollutants in flowing water streams[END_REF][START_REF] Jurecska | Characterization of cyclodextrin containing nanofilters for removal of pharmaceutical residues[END_REF][START_REF] Fenyvesi | Removal of hazardous micropollutants from treated wastewater using cyclodextrin bead polymer -A pilot demonstration case[END_REF][START_REF] Hemine | Cyclodextrin polymers as efficient adsorbents for removing toxic non-biodegradable pimavanserin from pharmaceutical wastewaters[END_REF]. The recovery of valuable drugs from industrial effluents is also conceivable after regeneration of the cyclodextrin polymer sorbent [START_REF] Hemine | Cyclodextrin polymers as efficient adsorbents for removing toxic non-biodegradable pimavanserin from pharmaceutical wastewaters[END_REF].

For the full-scale application of the cyclodextrin-based biosorbents in wastewater treatment, the first step is the scaled-up production of these polymers. All the examples overviewed in this section used sorbents/biosorbents prepared in laboratory except the epichlorohydrin-cross-linked β-cyclodextrin bead polymer produced on pilot-plant scale. Several research groups are working on development of economic and environmental-friendly technologies for scaling up. The tetrafluoroterephthalonitrile-cross-linked Dexsorb® adsorbents of Cyclopure Company, which can quickly and safely remove from water hundreds of pollutants including drugs, pesticides, and short-and long-chain polyfluorinated alkyl substances, are probably close to the industrial production (Cyclopure 2020).

All these studies show that cyclodextrin polymers are a promising alternative in the treatment of wastewater containing a cocktail of emerging trace substances. Industrialists will now have to be convinced to use these materials in their municipal wastewater treatment plants.

Metal-organic frameworks for the removal of emerging contaminants

In recent years, metal-organic frameworks have attracted attention as promising materials for the removal of emerging contaminants in effluents by adsorption-oriented, catalytic degradation and/or membrane processes [START_REF] Dias | Towards the use of metal-organic frameworks for water reuse: a review of the recent advances in the field of organic pollutants removal and degradation and the next steps in the field[END_REF][START_REF] De Andrade | Adsorption of pharmaceuticals from water and wastewater using nonconventional low-cost materials: a review[END_REF][START_REF] Mon | Metal-organic framework technologies for water remediation: towards a sustainable ecosystem[END_REF][START_REF] Bedia | A review on the synthesis and characterization of metal organic frameworks for photocatalytic water purification[END_REF][START_REF] Li | Water contaminant elimination based on metal-organic frameworks and perspective on their industrial applications[END_REF]Li et al. , 2020b;;[START_REF] Dhaka | Metal-organic frameworks (MOFs) for the removal of emerging contaminants from aquatic environments[END_REF][START_REF] Rojas | Metal-organic frameworks for the removal of emerging organic contaminants in water[END_REF][START_REF] Russo | Applications of metal organic frameworks in wastewater treatment: a review on adsorption and photodegradation[END_REF][START_REF] Zango | A critical review on metal-organic frameworks and their composites as advanced materials for adsorption and photocatalytic degradation of emerging organic pollutants from wastewaters[END_REF]. Metal-organic frameworks are a kind of crystalline materials with permanent porosity, more than 50% of their crystal volume, formed by the self-assembly of metal ions or their aggregates/clusters linked together by multifunctional organic ligands. They consist of small organic molecules, usually C6-rings, which are connected by metal clusters as a three-dimensional structure. Like cyclodextrin molecules, metal-organic frameworks have cavities where specific host-guest interactions can take place. These active sites can adsorb and/or degrade pollutants. In addition to superhigh pore volume, metal-organic frameworks also have values of surface area, between 1000 and 10,000 m 2 /g, superior to those of conventional materials such as carbons. They have also physicochemical properties easily tuned. Reviews by [START_REF] Dias | Towards the use of metal-organic frameworks for water reuse: a review of the recent advances in the field of organic pollutants removal and degradation and the next steps in the field[END_REF] and [START_REF] Bedia | A review on the synthesis and characterization of metal organic frameworks for photocatalytic water purification[END_REF] describe the different methodologies that can be used for the synthesis of metal-organic frameworks, paying attention to the purification and activation Fig. 4 Wastewater purification by applying cyclodextrin polymer beads for sorption of emerging pollutants and micropollutants such as residual pharmaceuticals. Source Éva Fenyvesi, Budapest, Hungary steps. A typical example of a metal-organic framework is chromium terephthalate . It is a polymer built from trimeric chromium (III) triangular cluster complexes, bridged by linear terephthalate linkers. The material has a highly porous threedimensional structure with large pores, higher that > 30 Å, high surface area, higher that 3,000 m 2 /g, and a huge cell volume, e.g., 702,000 Å 3 .

Because of their distinctive characteristics, metal-organic frameworks are very useful in chemistry and supramolecular chemistry including gas separation, adsorbent for carbon capture, and hydrogen for energy storage, nanotechnology for electrocatalysis, photocatalysis, and biocatalysis, materials science, drug delivery systems, biomedical imaging, sensing, fuel cells, and water and wastewater treatment (adsorption, semiconductor photocatalysis). They have many properties for the removal of pollutants that render them interesting for water treatment. Indeed, metallo-organic frames have not only a high pore volume, large surface area and adjustable pore size, but also multiple topologies, hierarchical structure, adjustable surface chemistry (easily functionalized cavities), and high adsorption capacity [START_REF] De Andrade | Adsorption of pharmaceuticals from water and wastewater using nonconventional low-cost materials: a review[END_REF][START_REF] Rojas | Metal-organic frameworks for the removal of emerging organic contaminants in water[END_REF]. Their recyclability also gives metal-organic frameworks an advantage over conventional adsorbents. Metallo-organic frames can be synthesized on a large scale and are versatile materials as they can be shaped into monoliths, pellets, membranes or beads for columns, suitable for decontamination devices.

Metal-organic frameworks can be used as adsorbent of pollutants or as platform for pollutant degradation through catalytic processes. This research is recent as the first work on metal-organic framework for dye removal and the first metal-organic framework used in the catalytic degradation of phenol were reported in 2010 by [START_REF] Haque | Adsorptive Removal of methyl orange from aqueous solution with metal-organic frameworks, porous chromium-benzenedicarboxylates[END_REF] and in 2007 by [START_REF] Alvaro | Semiconductor behavior of a metal-organic framework (MOF)[END_REF], respectively. In water and wastewater treatment, these organic-inorganic hybrid crystalline porous materials have proven to be excellent adsorbents for the removal of harmful species such as pharmaceuticals and personal care products, artificial sweeteners and feed additives, agricultural products, organic dyes, benzene, toluene, ethylbenzene and xylene compounds, pesticides, and industrial products such as alkylphenols, products of the photographic industry and plasticizers [START_REF] Rojas | Metal-organic frameworks for the removal of emerging organic contaminants in water[END_REF]. Their performance is better than that of other adsorbents, especially activated carbons. However, their performance and selectivity with respect to the pollutants targeted for disposal must be regulated by judicious selection of the metal ion and organic linker. Numerous examples of applications in the field of water and wastewater treatment are presented and discussed in the following references: Dias and Petit (2015), de [START_REF] De Andrade | Adsorption of pharmaceuticals from water and wastewater using nonconventional low-cost materials: a review[END_REF], [START_REF] Mon | Metal-organic framework technologies for water remediation: towards a sustainable ecosystem[END_REF], [START_REF] Bedia | A review on the synthesis and characterization of metal organic frameworks for photocatalytic water purification[END_REF], [START_REF] Li | Water contaminant elimination based on metal-organic frameworks and perspective on their industrial applications[END_REF], [START_REF] Dhaka | Metal-organic frameworks (MOFs) for the removal of emerging contaminants from aquatic environments[END_REF], and [START_REF] Rojas | Metal-organic frameworks for the removal of emerging organic contaminants in water[END_REF].

The extensive data in the literature show that metal-organic frameworks, as a new class of porous materials, have a great potential for water purification by (selective) adsorption and/or catalytic degradation such as photocatalysis and could also find applications in other advanced oxidation processes such as photo-Fenton and electro-Fenton, in soil remediation or in membrane filtration. This is a research topic in full development, particularly in the context of nanotechnologies, but is it is still in its early stages and requires a thorough assessment of parameters such as safety, lifetime, reusability, and industrial conditions. In addition, the low stability of metal-organic frameworks in water is still a major challenge for their environmental application in industry [START_REF] Rojas | Metal-organic frameworks for the removal of emerging organic contaminants in water[END_REF]. For a real application and compared to traditional semiconductors such as TiO 2 , two other aspects need to be improved. Firstly, the synthesis conditions must allow larger quantities of material to be obtained in continuous operation. Current processes, such as solvothermal methods, are limited by the slowness of the reactions and have a relatively high cost [START_REF] Li | Water contaminant elimination based on metal-organic frameworks and perspective on their industrial applications[END_REF]. Second, research is also needed on their potential toxicity and possible health effects. Indeed, many metal-organic frameworks are constructed from toxic metals, e.g., Cd, Cr, Ag and Co, and some molecules such as the dyes used are also considered as emerging contaminants.

Application of molecularly imprinted polymers for the removal of emerging contaminants

Over the past decade, several studies have reported innovative results on molecularly imprinted polymers and non-imprinted polymers for the removal of personal care products, pharmaceuticals, and endocrine disrupting compounds, in industrial effluents, surface waters, and drinking water [START_REF] Murray | Application of molecularly imprinted and non-imprinted polymers for removal of emerging contaminants in water and wastewater treatment: a review[END_REF][START_REF] Shen | Molecularly imprinted polymers for clean water: analysis and purification[END_REF][START_REF] Huang | Application of molecularly imprinted polymers in wastewater treatment: a review[END_REF]Chen et al. 2016a). The molecular imprinting technique is an emerging technology in which the synthesis of a material is performed in the presence of a template molecule. Subsequent removal of the template provides a material with "memory" sites capable of selectively recognizing and re-binding to the original template of a mixture [START_REF] Alexander | Molecular imprinting science and technology: a survey of the literature for the years up to and including 2003[END_REF]Chen et al. 2016a;[START_REF] Cantarella | Functional nanomaterials for water purification[END_REF][START_REF] Belbruno | Molecularly imprinted polymers[END_REF].

Molecularly imprinted polymers are prepared from crosslinked polymers containing cavities specific to a target analyte. These cavities are created by the copolymerization of cross-linking monomers and functional monomers with an imprinting molecule or template. After polymerization, the template is removed, leaving a cavity specific to the analyte. The molecularly imprinted polymer then selectively rebinds to the compound to be treated [START_REF] Alexander | Molecular imprinting science and technology: a survey of the literature for the years up to and including 2003[END_REF][START_REF] Belbruno | Molecularly imprinted polymers[END_REF]. The main advantages of these synthetic polymers are the ease of preparation and the ability to create "tailor-made" binding sites simply by adapting the synthesis procedure for the desired target molecule used as a template during polymerization [START_REF] Huang | Application of molecularly imprinted polymers in wastewater treatment: a review[END_REF][START_REF] Cantarella | Molecularly imprinted polymer for selective adsorption of diclofenac from contaminated water[END_REF]. Molecularly imprinted polymers have traditionally been used as solid-phase extraction media for analytical chemistry, e.g., for pre-concentration and selective removal of substances (Chen et al. 2016a;[START_REF] Belbruno | Molecularly imprinted polymers[END_REF].

Non-imprinted polymers are cross-linked polymeric materials that have macropores containing adsorption sites for organic molecules. They are synthesized using the same procedure as molecularly imprinted polymers, but in the absence of a template. Non-imprinted polymers therefore have the same chemical properties as molecularly imprinted polymers but do not contain specific cavities [START_REF] Murray | Application of molecularly imprinted and non-imprinted polymers for removal of emerging contaminants in water and wastewater treatment: a review[END_REF]. Non-imprinted polymers exhibit strong hydrophobic interactions (non-specific binding) between organic pollutants and polymers. The main difference between molecularly imprinted polymers and non-imprinted polymers is their specificity. Molecularly imprinted polymers can selectively remove a target substance such as 17-β-estradiol from biological wastewater, whereas nonimprinted polymers can remove several substances simultaneously. Molecularly imprinted polymers are also generally more porous, having a greater surface area, which may also explain the higher binding efficiency. However, while their specificity is an advantage for solid-phase extraction application, the same may not be true for complex wastewater (BelBruno 2019).

Molecularly imprinted polymers offer promising applications for water and wastewater treatment. They are advantageous for treatment of trace contaminants as they can be specifically designed to remove one or a group of target compounds [START_REF] Alexander | Molecular imprinting science and technology: a survey of the literature for the years up to and including 2003[END_REF]. This is an advantage over nonspecific technologies such as activated carbon [START_REF] Pichon | Role of molecularly imprinted polymers for selective determination of environmental pollutants -a review[END_REF][START_REF] Murray | Application of molecularly imprinted and non-imprinted polymers for removal of emerging contaminants in water and wastewater treatment: a review[END_REF]. Molecularly imprinted polymers have been studied for their ability to remove, degrade and/or destroy, in combination with other technologies, such as advanced oxidation processes, emerging substances such as hormones [START_REF] Noir | Selective removal of 17beta-estradiol at trace concentration using a molecularly imprinted polymer[END_REF][START_REF] Zhongbo | Selective removal of estrogenic compounds by molecular imprinted polymer (MIP)[END_REF][START_REF] Chen | Molecularly imprinted hollow spheres for the solid phase extraction of estrogens[END_REF], pharmaceuticals [START_REF] Pichon | Role of molecularly imprinted polymers for selective determination of environmental pollutants -a review[END_REF][START_REF] Madikizela | Applications of molecularly imprinted polymers for solid-phase extraction of non-steroidal anti-inflammatory drugs and analgesics from environmental waters and biological samples[END_REF][START_REF] Belbruno | Molecularly imprinted polymers[END_REF][START_REF] Cantarella | Molecularly imprinted polymer for selective adsorption of diclofenac from contaminated water[END_REF], endocrine disruptors [START_REF] Fernández-Alvarez | Removal and destruction of endocrine disrupting contaminants by adsorption with molecularly imprinted polymers followed by simultaneous extraction and phototreatment[END_REF], personal care products [START_REF] Alsudir | Competitive CE-UV binding tests for selective recognition of bisphenol A by molecularly imprinted polymer particles[END_REF]) and pesticides [START_REF] Murray | Application of molecularly imprinted and non-imprinted polymers for removal of emerging contaminants in water and wastewater treatment: a review[END_REF].

Le [START_REF] Noir | Selective removal of 17beta-estradiol at trace concentration using a molecularly imprinted polymer[END_REF], investigating the percolation of wastewater containing 17-β-estradiol through solid-phase extraction columns prepared with molecularly imprinted polymers (flow rate of 50 mL/min), demonstrated that these materials were capable to eliminate the equivalent of 22 ± 4 ng of 17-β-estradiol per L of effluent. The authors also showed that the use of molecularly imprinted polymers allows easy regeneration for subsequent reuse, yielding the same efficiency and reducing overall treatment costs. In another work, the same authors developed a method for regenerating molecularly imprinted polymer with a template of 17-β-estradiol using solvent extraction under UV light [START_REF] Fernández-Alvarez | Removal and destruction of endocrine disrupting contaminants by adsorption with molecularly imprinted polymers followed by simultaneous extraction and phototreatment[END_REF]. This method was able to regenerate and reuse both the polymeric material and the solvent, while simultaneously degrading 17-β-estradiol. For that, acetone was used as a solvent under both UV-C and UV-visible light. After a 10-h cycle, the molecularly imprinted polymer was completely regenerated, and no residual 17-β-estradiol was found in acetone. [START_REF] Cantarella | Molecularly imprinted polymer for selective adsorption of diclofenac from contaminated water[END_REF] studied the adsorption of diclofenac to molecularly imprinted polymers synthetized by a simple and inexpensive bulk polymerization reaction. The authors also reported that materials can be regenerated and reused by simply washing the material with a 3 mL methanol/acetic acid solution for 2 min. The performance remained the same after at least four adsorption/regeneration cycles. The proposed polymers showed exceptional adsorption capacity for the adsorption of diclofenac and the process was highly selective. In 10 min, 5 mg of molecularly imprinted polymer removed ∼90% of diclofenac from an aqueous solution at a concentration of 1 × 10 -4 mol/L, while only 8% being removed by the corresponding non-imprinted polymer. The adsorption capacity was 110 and 13 µmol/g for the molecularly imprinted polymer and the non-imprinted polymer, respectively. Due to their easy and inexpensive synthesis, high efficiency and selectivity, easy regeneration and reusability, the authors concluded that the technology could be used on a large scale for water treatment.

The use of molecularly imprinted polymers and nonimprinted polymers for the removal of emerging contaminants using adsorption-based or solid-phase extraction processes is a novel approach that has potential in the field of water engineering (BelBruno 2019). Polymers, particularly non-imprinted polymers, are inexpensive to manufacture and can be produced in large quantities [START_REF] Alexander | Molecular imprinting science and technology: a survey of the literature for the years up to and including 2003[END_REF]. After use, the materials can be easily regenerated and reused several times. However, further research is needed to determine how to best integrate this technology into treatment plants. In addition, their potential toxicity and possible health effects are subject to discussion [START_REF] Murray | Application of molecularly imprinted and non-imprinted polymers for removal of emerging contaminants in water and wastewater treatment: a review[END_REF].

Adsorption of emerging contaminants onto chitosan-based materials

Like cyclodextrin polymers, chitosan-based materials have great potential in environmental applications, mainly for metal chelation and dye removal [START_REF] Crini | Green adsorbents for pollutant removal. Springer Nature, Cham Crini G, Lichtfouse E[END_REF][START_REF] Pakdel | Review on recent progress in chitosan-based hydrogels for wastewater treatment application[END_REF][START_REF] Morin-Crini | Applications of chitosan in food, pharmaceuticals, medicine, cosmetics, agriculture, textiles, pulp and paper, biotechnology, and environmental chemistry[END_REF][START_REF] Vidal | Removal of organic pollutants from wastewater using chitosan: a literature review[END_REF]Brião et al., 2020). Chitosan is a biopolymer derived from chitin, the second most abundant polysaccharide (after cellulose) on Earth. This aminopolysaccharide can interact and adsorb a wide range of pollutants such as metals, dyes, organics, including emerging substances. In water and wastewater treatment, it represents an alternative as an ecological complexing agent due to its low cost (chitin comes from shellfish wastes), its intrinsic characteristics (renewable, non-toxic and biodegradable resource, hydrophilicity) and its chemical properties (polyelectrolyte at acidic pH, high reactivity, coagulation, flocculation and biosorption properties) resulting from the presence of reactive hydroxyl and especially amine groups in the macromolecular chains. Its use in water treatment is justified by two other important advantages: exceptional pollutant binding capacities, excellent selectivity, and versatility [START_REF] Crini | Green adsorbents for pollutant removal. Springer Nature, Cham Crini G, Lichtfouse E[END_REF][START_REF] Morin-Crini | Applications of chitosan in food, pharmaceuticals, medicine, cosmetics, agriculture, textiles, pulp and paper, biotechnology, and environmental chemistry[END_REF]. Chitosan-based materials can be used in solid form for the removal of pollutants from water and wastewater by filtration or adsorption processes or in liquid state, i.e., dissolved in acidic media, for applications in coagulation, flocculation, and membrane filtration technologies such as polymer assisted ultrafiltration. Among the proposed materials, cross-linked chitosan hydrogels deserve particular attention (Morin- Crini et al. 2019).

Chitosan-based materials have been investigated for the removal of emerging substances such as perfluorooctane sulfonate [START_REF] Zhang | Removal of perfluorooctane sulfonate from aqueous solution by crosslinked chitosan beads: sorption kinetics and uptake mechanism[END_REF], bisphenol A [START_REF] Dehghani | Adsorptive removal of endocrine disrupting bisphenol A from aqueous solution using chitosan[END_REF][START_REF] Zhou | Comparative adsorption of emerging contaminants in water by functional designed magnetic poly(N-isopropylacrylamide)/chitosan hydrogels[END_REF], amoxicillin [START_REF] Adriano | Adsorption of amoxicillin on chitosan beads: kinetics, equilibrium and validation of finite bath models[END_REF], sulfamethoxazole [START_REF] Qin | Rapid and selective extraction of multiple sulfonamides from aqueous samples based on Fe 3 O 4 -chitosan molecularly imprinted polymers[END_REF][START_REF] Zhou | Comparative adsorption of emerging contaminants in water by functional designed magnetic poly(N-isopropylacrylamide)/chitosan hydrogels[END_REF], ciprofloxacin [START_REF] Afzal | Enhancement of ciprofloxacin sorption on chitosan/biochar hydrogel beads[END_REF], diclofenac [START_REF] Rizzi | Chitosan film as eco-friendly and recyclable bio-adsorbent to remove/recover diclofenac, ketoprofen, and their mixture from wastewater[END_REF], ketoprofen [START_REF] Rizzi | Chitosan film as eco-friendly and recyclable bio-adsorbent to remove/recover diclofenac, ketoprofen, and their mixture from wastewater[END_REF], and caffeine [START_REF] Sanford | Study of natural adsorbent chitosan and derivatives for the removal of caffeine from water[END_REF]. For example, [START_REF] Adriano | Adsorption of amoxicillin on chitosan beads: kinetics, equilibrium and validation of finite bath models[END_REF] studied the adsorption of the beta-lactamic antibiotic amoxicillin on chitosan beads. This highly excreted antibiotic is difficult to eliminate in conventional wastewater treatment systems due to its structure and amphoteric properties resulting from the presence of carboxylic (pKa 2.68), amine (pKa 7.49) and hydroxyl (pKa 9.63) groups [START_REF] Anastopoulos | Removal of caffeine, nicotine and amoxicillin from (waste)waters by various adsorbents a review[END_REF]. Its charge changes gradually with pH, due to the different functional groups. The pH of wastewater affects not only the ionization of the molecules, but also the surface charge of the materials used as adsorbents for their removal. Adsorption results described by [START_REF] Adriano | Adsorption of amoxicillin on chitosan beads: kinetics, equilibrium and validation of finite bath models[END_REF] showed that 0.5 g of cross-linked chitosan were capable of removing amoxicillin from a 2 mL solution at concentrations ranging from 0.2 to 3 mg/L at pH = 6.5, with performances being concentration-dependent. The authors reported a maximum adsorption capacity of 8.71 ± 0.6 mg/g with an equilibrium time of 2 h. The adsorption mechanism was due to strong interactions between the carboxylic, amine and hydroxyl of the amoxicillin molecule and those of chitosan.

From literature data, there is no doubt that chitosan-based hydrogels have high adsorption capacities [START_REF] Morin-Crini | Applications of chitosan in food, pharmaceuticals, medicine, cosmetics, agriculture, textiles, pulp and paper, biotechnology, and environmental chemistry[END_REF]). However, these materials are at the laboratory stage and pilot studies need to be conducted. Although various laboratories and a few companies can synthesize chitosanmaterials, it is very difficult to find commercial sources of cross-linked hydrogels with guaranteed reproducible properties. However, the performance can vary depending on the conditions and the method used to prepare the hydrogels. Despite an abundance of literature reports on industrial wastewaters, there is still little information available detailing comprehensive comparing various conventional commercial adsorbents under similar conditions. Like other non-conventional materials, further research is needed to determine how to best integrate this technology into treatment plants.

Nanocellulose as a novel adsorbent for environmental remediation

Nanocelluloses are innovative materials with at least one dimension at the nanoscale that are attracting increasing interest in the field of nanotechnology and materials science. These materials are highly ordered β-(1 → 4) glucan chains that are produced naturally or by chemical processes. Nanocelluloses are mainly obtained from naturally occurring cellulose sources. Indeed, they can be obtained from biomass, plants or bacteria, using fairly simple, scalable and effective isolation techniques. Most nanocellulose is produced from lignocellulose. The term "nanocellulose" encompasses several cellulose-based materials, whose chemical and physical properties generally vary depending on their source and method of extraction: cellulose nanocrystals, nanofibrillated celluloses, and rigid bacterial nanocellulose [START_REF] Moon | Cellulose nanomaterials review: structure, properties and nanocomposites[END_REF][START_REF] Lam | Applications of functionalized and nanoparticle-modified nanocrystalline cellulose[END_REF].

Pure nanocellulose is non-toxic, biodegradable, and biocompatible. In addition, it has other advantages such as low cost, abundance, practically renewable, intrinsic properties (large surface area) and high reactivity (easy to modify), making it sustainable material for several applications including electronics, optoelectronics and engineering, paper industry, composites, antibacterial coatings, food packaging, cosmetics and personal hygiene products, medical applications (tissue scaffolds, drug delivery), bio-imaging, biosensors, enzyme immobilization, catalysis and energy storage and production [START_REF] Putro | Nanocellulose based biosorbents for wastewater treatment: Study of isotherm, kinetic, thermodynamic and reusability[END_REF][START_REF] Thomas | Nanocellulose, a versatile green platform: from biosources to materials ant their applications[END_REF].

Nanocellulose in its various forms, including cellulose nanocrystals, cellulose nanofibrils, and bacterial cellulose, is also promising material for environmental applications such as water treatment [START_REF] Lam | Applications of functionalized and nanoparticle-modified nanocrystalline cellulose[END_REF][START_REF] Herrera-Morales | Examining the use of nanocellulose composites for the sorption of contaminants of emerging concern: an experimental and computational study[END_REF], 2019;Mahfoudhi and Boufi 2017;[START_REF] Putro | Nanocellulose based biosorbents for wastewater treatment: Study of isotherm, kinetic, thermodynamic and reusability[END_REF][START_REF] Voisin | Nanocellulose-Based Materials for Water Purification Nanomaterials[END_REF][START_REF] Abouzeid | Current state and new trends in the use of cellulose nanomaterials for wastewater treatment[END_REF][START_REF] Abujaber | Magnetic cellulose nanoparticles coated with ionic liquid as a new material for the simple and fast monitoring of emerging pollutants in waters by magnetic solid phase extraction[END_REF][START_REF] Mohammed | Cellulose nanomaterials: promising sustainable nanomaterials for application in water/ wastewater treatment processes[END_REF][START_REF] Shak | Nanocellulose: recent advances and its prospects in environmental remediation[END_REF][START_REF] Wang | A critical review of cellulose-based nanomaterials for water purification in industrial processes[END_REF][START_REF] Ibrahim | A short review on recent utilization of nanocellulose for wastewater remediation and gas separation[END_REF][START_REF] Sayyed | Cellulose-based nanomaterials for water and wastewater treatments: a review[END_REF][START_REF] Thakur | Recent advances in nanocellulose processing functionalization and applications: a review[END_REF], air filtration [START_REF] Nemoto | Simple freeze-drying procedure for producing nanocellulose aerogel-containing, high performance air filters[END_REF], membrane filtration [START_REF] Cruz-Tato | Metalized nanocellulose composites as a feasible material for membrane supports: design and applications for water treatment[END_REF][START_REF] Abouzeid | Current state and new trends in the use of cellulose nanomaterials for wastewater treatment[END_REF][START_REF] Shak | Nanocellulose: recent advances and its prospects in environmental remediation[END_REF][START_REF] Mautner | Nanocellulose water treatment membranes and filters: a review[END_REF], flocculation [START_REF] Shak | Nanocellulose: recent advances and its prospects in environmental remediation[END_REF], gas separation [START_REF] Ibrahim | A short review on recent utilization of nanocellulose for wastewater remediation and gas separation[END_REF]catalytic degradation (Mahfoudhi andBoufi 2017;[START_REF] Thomas | Nanocellulose, a versatile green platform: from biosources to materials ant their applications[END_REF][START_REF] Shak | Nanocellulose: recent advances and its prospects in environmental remediation[END_REF]. Nanocelluloses have been used for the removal of metals, dyes, nitrates, organics, and microbes from aqueous solution by filtration-, adsorption-or membrane-oriented processes. However, for industrial applications, native nanocelluloses have a low adsorption capacity due to their hydrophilic crystal structure. Therefore, it is necessary to incorporate functionalities by chemical modification of the cellulose primary hydroxyl groups to obtain materials and composites suitable for hydrophobic contaminants. This functionalization is also necessary to increase the selectivity and affinity of the substances. [START_REF] Hokkanen | Removal of heavy metals from aqueous solutions by succinic anhydride modified mercerized nanocellulose[END_REF] and [START_REF] Yu | Adsorption of heavy metal ions from aqueous solution by carboxylated cellulose nanocrystals[END_REF] reported that modification using succinic anhydrate and sodium bicarbonate can increase adsorption to nanocellulose up to tenfold. [START_REF] Shak | Nanocellulose: recent advances and its prospects in environmental remediation[END_REF] and Abou-Zeid et al. (2019) reviewed the different strategies for the preparation of nanocellulose for applications in wastewater treatment. An important feature for an application in the water domain is the fact that nanocellulose materials are easily regenerated and reusable.

Numerous studies have shown that nanocellulose-based materials are among the most promising green adsorbents, and comprehensive reviews have been published [START_REF] Herrera-Morales | Examining the use of nanocellulose composites for the sorption of contaminants of emerging concern: an experimental and computational study[END_REF], 2019;Mahfoudhi and Boufi 2017;[START_REF] Putro | Nanocellulose based biosorbents for wastewater treatment: Study of isotherm, kinetic, thermodynamic and reusability[END_REF][START_REF] Voisin | Nanocellulose-Based Materials for Water Purification Nanomaterials[END_REF][START_REF] Abouzeid | Current state and new trends in the use of cellulose nanomaterials for wastewater treatment[END_REF][START_REF] Mohammed | Cellulose nanomaterials: promising sustainable nanomaterials for application in water/ wastewater treatment processes[END_REF][START_REF] Shak | Nanocellulose: recent advances and its prospects in environmental remediation[END_REF][START_REF] Wang | A critical review of cellulose-based nanomaterials for water purification in industrial processes[END_REF][START_REF] Mautner | Nanocellulose water treatment membranes and filters: a review[END_REF][START_REF] Ibrahim | A short review on recent utilization of nanocellulose for wastewater remediation and gas separation[END_REF][START_REF] Sayyed | Cellulose-based nanomaterials for water and wastewater treatments: a review[END_REF][START_REF] Thakur | Recent advances in nanocellulose processing functionalization and applications: a review[END_REF]. [START_REF] Herrera-Morales | Examining the use of nanocellulose composites for the sorption of contaminants of emerging concern: an experimental and computational study[END_REF], 2019) demonstrated that modified nanocelluloses were capable of effectively adsorbing pharmaceuticals such as sulfamethoxazole and acetaminophen (paracetamol). [START_REF] Abujaber | Magnetic cellulose nanoparticles coated with ionic liquid as a new material for the simple and fast monitoring of emerging pollutants in waters by magnetic solid phase extraction[END_REF] proposed magnetic cellulose nanoparticles electrostatically modified with ionic liquids to adsorb pharmaceuticals (paracetamol, ibuprofen, naproxen, and diclofenac) in less than 30 min with extraction recoveries of 86%-16%. Although many studies have been published with promising results, nanocelluloses, and more generally nanomaterials, are still at the laboratory study stage [START_REF] Shak | Nanocellulose: recent advances and its prospects in environmental remediation[END_REF][START_REF] Thakur | Recent advances in nanocellulose processing functionalization and applications: a review[END_REF].

Biological-based technologies for the degradation and elimination of emerging contaminants

The biological treatment approach, specifically the activated sludge process, is the most used wastewater treatment technology. While it was originally designed to remove organic carbon, it was subsequently extended to remove nitrogen and phosphorous. However, conventional biological-based technologies are not capable of degrading the wide range of organic pollutants present in complex wastewater. These systems have been adapted and different strategies have been studied, including water pre-treatment, the implementation of complementary methods based on potabilization techniques such as ozonation and carbon adsorption, or pilot studies such as membrane filtration. These latter techniques, reserved for very specific water treatment, are already used in industry, but their cost is a constraint for their further implementation. Other techniques such as constructed wetlands, algal-based technologies and enzymatic degradation, and bioreactors are being developed [START_REF] Mohsenpour | Integrating micro-algae into wastewater treatment: a review[END_REF][START_REF] Parde | A review of constructed wetland on type, treatment and technology of wastewater[END_REF][START_REF] Plöhn | Wastewater treatment by microalgae[END_REF][START_REF] Vymazal | Recent challenges in constructed wetlands for wastewater treatment: a review[END_REF].

Constructed wetlands for the removal of emerging contaminants

Constructed wetlands are artificial ecosystems that rely on plants activity for water remediation [START_REF] Kaur | Constructed wetlands for the removal of organic micro-pollutants. In: current developments in biotechnology and bioengineering. Emerging organic micro-pollutants[END_REF] and can be categorized according to different operational parameters, such as the type of flow, e.g., horizontal vs vertical and free water vs sub-surface, the hydraulic regime, and the plant species used for phytodepuration. These systems, likewise natural wetlands, take advantage of the numerous physical, chemical and biological processes that occur at the interface between the root system and the wastewater under treatment, including, for instance, adsorption on soil/sediment, volatilization, uptake, and degradation [START_REF] Gorito | A review on the application of constructed wetlands for the removal of priority substances and contaminants of emerging concern listed in recently launched EU legislation[END_REF]. Constructed wetlands are a suitable technology to remove total suspended solids, ammonia, phosphorous, and to reduce the chemical oxygen demand and biochemical oxygen demand. They offer several advantages in terms of environmental and economic sustainability. Representing a low-cost and easy to maintain technology, constructed wetlands are considered an interesting opportunity for wastewater treatment in developing countries [START_REF] Mahmood | Natural treatment systems as sustainable ecotechnologies for the developing countries[END_REF]) and for decentralized systems treating sewage produced by small communities [START_REF] Zraunig | Long term decentralized greywater treatment for water reuse purposes in a tourist facility by vertical ecosystem[END_REF]. Several studies highlighted the potentiality of constructed wetlands for the removal of emerging organic contaminants, including antibiotics and antibiotic resistance genes (Chen et al. 2016b;[START_REF] Huang | Effect of plant physiological characteristics on the removal of conventional and emerging pollutants from aquaculture wastewater by constructed wetlands[END_REF], other pharmaceuticals and endocrine disruptors such as bisphenol A [START_REF] Syranidou | Bisphenol-A removal by the halophyte Juncus acutus in a phytoremediation pilot, characterization and potential role of the endophytic community[END_REF][START_REF] Campos | Removal of the endocrine disruptors ethinyl estradiol, bisphenol A, and levonorgestrel by subsurface constructed wetlands[END_REF], and synthetic dyes [START_REF] Riva | Root bacteria recruited by Phragmites australis in constructed wetlands have the potential to enhance azo-dye phytodepuration[END_REF]). Indeed, phytodepuration has been also proposed as a tertiary treatment downstream of conventional wastewater treatment plants which are not designed to remove these types of contaminants, often present in their effluents [START_REF] Castiglioni | Removal of pharmaceuticals in sewage treatment plants in Italy[END_REF]. Within this framework, antibiotic resistance is a hot topic in relation to water reuse, as its determinants (including genes and bacteria) are recognized as contaminants of emerging concern and are currently being addressed by the first EU regulation (2020/741) on wastewater reuse to be implemented in the coming years.

The contaminant removal in constructed wetlands results from the synergistic effect played by plant roots and their associated microbiome. The plant root system is an environmental niche that provides favorable conditions for microorganisms, which are specifically recruited by the release of root exudates and can help the plants cope with adverse conditions, including pollution [START_REF] Rolli | Cryfor-help in contaminated soil: a dialogue among plants and soil microbiome to survive in hostile conditions[END_REF]). Noteworthy, microorganisms can play a direct role in organic contaminant degradation. Constructed wetlands can be designed using single or mixed plant species, which belong mainly to the genera Typha, Phragmites, Iris and Juncus. A constructed wetland system realized co-cultivating the halophytes species Tamarix parviflora, Juncus acutus, Sarcocornia perrenis, and Limoniastrum monopetalum was able to efficiently decrease organic matter and pathogen concentration in the effluent [START_REF] Fountoulakis | Use of halophytes in pilot-scale horizontal flow constructed wetland treating domestic wastewater[END_REF]. This is a promising result in the context of bioremediation of wastewater produced by aquaculture, an antropic activity whose environmental impact is increasing, especially due to the intensive use of antibiotics. Constructed wetlands containing Iris pseudacorus and Phragmites australis, planted as single or mixed, were able to remove antibiotics and antibiotic resistance genes, and the removal rate varied according to the planting pattern and the target molecule. The tested plants showed the best removal performances in single species. Iris pseudacorus removed up to 77.64%, 68.70%, and 58.21% of enrofloxacin, sulfamethoxazole, and total antibiotic resistance genes, while Phragmites australis showed removal efficiencies of 81.11%, 64.94%, and 56.26% for the same molecules and genes [START_REF] Huang | Effect of plant physiological characteristics on the removal of conventional and emerging pollutants from aquaculture wastewater by constructed wetlands[END_REF]. Indeed, several authors showed that the performance of constructed wetland systems strictly depends on both operational parameters and the emerging contaminant to be removed. A microcosm scale study using Phragmites australis in a vertical subsurface flow constructed wetland, showed high removal efficiencies for all considered micropollutants, except for 2-ethylhexyl-4-methoxycinnamate, in both spiked experiments with 36 multi-class pollutants and non-spiked freshwater aquaculture effluents containing atrazine, isoproturon, perfluorooctanesulfonic acid, clarithromycin, erythromycin, fluoxetine, norfluoxetine, and 2-ethylhexyl-4-methoxycinnamate [START_REF] Gorito | Constructed wetland microcosms for the removal of organic micropollutants from freshwater aquaculture effluents[END_REF]. The high removal capacity of Phragmites australis was also demonstrated on the antiepileptic carbamazepine in a study that highlighted how several bacterial species isolated from Phragmites australis endosphere (i.e., the internal root tissue) were able to remove and degrade this recalcitrant molecule (Sauvêtre and Schröeder 2015). On the opposite, subsurface horizontal flow constructed wetlands planted with Heliconea zingiberales and Cyperus haspan showed a carbamazepine removal efficiency lower than 10% although the system successfully reduced the effluent concentrations of the pharmaceutical sildenafil and the personal care product methylparaben [START_REF] Delgado | Occurrence and removal of pharmaceutical and personal care products using subsurface horizontal flow constructed wetlands[END_REF]. A strategy to improve emerging contaminant degradation in constructed wetlands could be the use of substrates endowed with adsorptive ability, such as manganese oxides which were indicated as enhancer of triclosan removal during microcosm experiments [START_REF] Xie | Enhanced triclosan and nutrient removal performance in vertical up-flow constructed wetlands with manganese oxides[END_REF]).

In the last decade, the root microbiome of constructed wetland plants has gained increasing attention, given the importance of microbial metabolism for the degradation of pharmaceuticals and other organic molecules. For example, several bacterial endophytes isolated from Juncus acutus plants grown in a pilot constructed wetland were able to tolerate and/or use bisphenol A, ciprofloxacin and sulfamethoxazole as sole carbon source [START_REF] Syranidou | Bisphenol-A removal by the halophyte Juncus acutus in a phytoremediation pilot, characterization and potential role of the endophytic community[END_REF]. Therefore, selected bacterial strains could be used in the so-called microbial assisted phytodepuration given their ability to promote plant growth and, on the other hand, to tolerate and/or degrade emerging contaminants. A study aimed at removing the model azo-dye Reactive Black 5 in constructed wetland microcosms planted using Juncus acutus showed the beneficial effect of plant growth promoting bacteria inoculation. Plants were supplemented by two different single inocula and a mixed consortium of previously selected root-associated bacteria and showed a higher removal of Reactive Black 5 in the effluent compared to the non-inoculated control ones, suggesting that 'microbial assisted phytodepuration' could be successfully exploited for the remediation of textile industry wastewater, an issue in several developing countries [START_REF] Riva | Root bacteria recruited by Phragmites australis in constructed wetlands have the potential to enhance azo-dye phytodepuration[END_REF]. Finally, phytodepuration represents a wastewater treatment technology of interest also for water reclamation [START_REF] Petroselli | Integrated system of phytodepuration and water reclamation: A comparative evaluation of four municipal wastewater treatment plants[END_REF]), a current world-wide priority in the light of global warming and water crisis, although its benefits and risks should be carefully evaluated according to the final purposes of water reuse especially in terms of antibiotic resistance spread into the environment [START_REF] Riva | Microbial assisted phytodepuration for water reclamation: environmental benefits and threats[END_REF].

Algal-based removal strategies for emerging contaminants

One solution could be the use of biological materials such as algae and fungi [START_REF] Parlade | Effect of cultivation conditions on β-estradiol removal in laboratory and pilot-plant photobioreactors by an algal-bacterial consortium treating urban wastewater[END_REF][START_REF] Silva | The use of algae and fungi for removal of pharmaceuticals by bioremediation and biosorption processes: a review[END_REF][START_REF] Tolboom | Algal-based removal strategies for hazardous contaminants from the environment -a review[END_REF][START_REF] Tomasini | Fungal bioremediation. Fundamentals and applications[END_REF][START_REF] Plöhn | Wastewater treatment by microalgae[END_REF]. Algae are known to be effective in the treatment of water contaminated with organic pollutants through biological processes such as bioremediation (phytoremediation) and biosorption. The use of algae for the removal of emerging contaminants has many advantages such as the use of low-cost materials, low capital investment, simple operation, reduced maintenance, and the absence of formation of degradation by-products.

Algae are also highly adaptive microorganisms and can grow autotrophically, heterotrophically or mixotrophically. They can grow in very harsh environmental conditions such as low nutrient levels, and extreme pH and temperature. Algae can acclimatize not only to change depending on the temperature and nutrient availability, but also salinity and light. In general, the characteristics of municipal wastewaters and industrial effluents, e.g., textile and pulp and paper industries, are suitable for algae cultivation, since these waters are a source of nutrients. Over the past two decades, extensive research has shown the potential application of advanced algal-based technologies for the removal of pollutants [START_REF] Silva | The use of algae and fungi for removal of pharmaceuticals by bioremediation and biosorption processes: a review[END_REF][START_REF] Tolboom | Algal-based removal strategies for hazardous contaminants from the environment -a review[END_REF]. [START_REF] Tolboom | Algal-based removal strategies for hazardous contaminants from the environment -a review[END_REF] reviewed algal-based removal strategies for the removal of emerging contaminants through efficient biological degradation. In laboratory-scale studies, algae-based bioreactors such as open ponds and bubble column photobioreactors can remove pharmaceuticals and endocrine disruptors. Open ponds are artificial ponds of limited depth (around 0.03-0.07 m) used for the cultivation of microalgae without agitation. Their advantages are low investment cost, ease of use and low operating cost. However, a lot of space is required for algae growth. Other problems include poor use of light by the cells, variations in pH and dissolved oxygen, water loss due to evaporation, diffusion of carbon dioxide into the environment, temperature fluctuations, and inefficient agitation. The operational factors that influence algae growth in these open systems are mixing, dilution rate and depth. Open ponds operate with a long hydraulic retention time to consume carbon dioxide during the day (photosynthesis) and provide oxygen for aerobic biodegradation. Closed systems have been designed to overcome most of the problems associated with open systems. Photobioreactors are closed tubular systems, mainly designed in vertical, horizontal, and helical form (coil tube, flat plate, bubble column, air column, or agitation tank). Each system has its own advantages and drawbacks [START_REF] Silva | The use of algae and fungi for removal of pharmaceuticals by bioremediation and biosorption processes: a review[END_REF]. Closed bioreactors provide a tightly controlled environment for the isolation of the microalgal strain, ensuring increased productivity, biomass quality and the ability to explore a wider range of strains. The benefits are an easier pH and temperature control, higher volumetric efficiency, better use of the growing area, higher capture of radiant energy and less water loss. Closed systems also reduce the contamination risk and the loss of carbon dioxide to the atmosphere. This concept is promising, but the costs of such reactors are higher (expensive to install and to maintain) and are an obstacle to its development. Their design must be carefully optimized for each individual strain. A detailed discussion on these systems and their physicochemical characteristics can be found in the following references: de Godos et al. ( 2012), [START_REF] Singh | Development of suitable photobioreactor for algae production -a review[END_REF], [START_REF] Slade | Micro-algae cultivation for biofuels: cost, energy balance, environmental impacts and future prospects[END_REF], [START_REF] Meneses-Jácome | Sustainable energy from agro-industrial wastewaters in Latin America[END_REF], [START_REF] Matamoros | Capability of microalgae-based wastewater treatment systems to remove emerging organic contaminants: a pilot-scale study[END_REF], Ghosh et al. (2016), [START_REF] Gouveia | Microalgae biomass production using wastewater: treatment and costs scale-up considerations[END_REF], [START_REF] Norvill | Photodegradation and sorption govern tetracycline removal during wastewater treatment in algal ponds[END_REF][START_REF] Silva | The use of algae and fungi for removal of pharmaceuticals by bioremediation and biosorption processes: a review[END_REF]. [START_REF] Tolboom | Algal-based removal strategies for hazardous contaminants from the environment -a review[END_REF] reported high removal percentages (> 90%) for metoprolol, triclosan, and salicylic acid, moderate (50-90%) for carbamazepine and tramadol and very low (< 10%) for trimethoprim and ciprofloxacin by inoculation of different microalgae. Similar results were also obtained for metoprolol [START_REF] Bai | Algae-mediated removal of selected pharmaceutical and personal care products (PPCPs) from Lake Mead water[END_REF][START_REF] Gentili | Algal cultivation in urban wastewater: an efficient way to reduce pharmaceutical pollutants[END_REF], tramadol [START_REF] Ali | Removal of pharmaceutical pollutants from synthetic wastewater using chemically modified biomass of green alga Scenedesmus obliquus[END_REF], salicylic acid (Escapa et al. 2017a(Escapa et al. , 2017b)), and triclosan [START_REF] Matamoros | Capability of microalgae-based wastewater treatment systems to remove emerging organic contaminants: a pilot-scale study[END_REF][START_REF] Bai | Algae-mediated removal of selected pharmaceutical and personal care products (PPCPs) from Lake Mead water[END_REF]. However, for ciprofloxacin, a high elimination of > 90% was reported by [START_REF] Bai | Algae-mediated removal of selected pharmaceutical and personal care products (PPCPs) from Lake Mead water[END_REF]. Carbamazepine [START_REF] Matamoros | Capability of microalgae-based wastewater treatment systems to remove emerging organic contaminants: a pilot-scale study[END_REF]) and trimethoprim [START_REF] De Wilt | Micropollutant removal in an algal treatment system fed with source separated wastewater streams[END_REF] showed less promising results with elimination rates of 62% and 60%, respectively. In fact, the removal efficiency depended mainly on the algae species used, such as Chlorella vulgaris, Chlorella sorokiniana and Nannochloris sp. For example, Escapa et al. (2017aEscapa et al. ( , 2017b)), who evaluated the removal capacity of Chlorella vulgaris, Tetradesmus obliquus and Chlorella sorokiniana from wastewater containing paracetamol and salicylic acid using algae-based bioreactors, demonstrated that Tetradesmus obliquus removed both pharmaceutical contaminants better than Chlorella vulgaris in batch culture. The removal efficiency of salicylic acid by Tetradesmus obliquus was greater than 93% and that of Chlorella vulgaris was higher than 25%. Parameters that play an important role in pollutant degradation processes are not only the consortium of microorganisms present and its efficiency, but also the conditions used during degradation such as temperature, seasons, retention time, environmental pH, dissolved oxygen, and light periods. Performance also depends on chemical factors related to the pollutants, such as their structure and stereochemistry, concentration, toxicity, and the presence of several pollutants.

The mechanisms involved in algae photobioreactors are complex and are being elucidated [START_REF] De Godos | Tetracycline removal during wastewater treatment in high-rate algal ponds[END_REF][START_REF] Matamoros | Capability of microalgae-based wastewater treatment systems to remove emerging organic contaminants: a pilot-scale study[END_REF][START_REF] Xiong | Biodegradation of carbamazepine using freshwater microalgae Chlamydomonas mexicana and Scenedesmus obliquus and the determination of its metabolic fate[END_REF][START_REF] Silva | The use of algae and fungi for removal of pharmaceuticals by bioremediation and biosorption processes: a review[END_REF][START_REF] Tolboom | Algal-based removal strategies for hazardous contaminants from the environment -a review[END_REF]). Nevertheless, the elimination processes include biodegradation, photodegradation and biosorption including cell sorption and/or bioaccumulation to algae. Other mechanisms and interactions are also cited, such as volatilization (stripping), biotransformation, bioprecipitation (biomineralization), and oxidation/reduction reactions. Bioaccumulation of pollutants in algae cells can also induce the generation of reactive oxygen species, free radicals, and non-radical forms such as hydrogen peroxide and single oxygen. For example, de [START_REF] De Godos | Tetracycline removal during wastewater treatment in high-rate algal ponds[END_REF], studying the mechanism of elimination of tetracycline by Chlorella vulgaris, reported that photodegradation and biosorption were the main interactions to explain biodegradation. [START_REF] Matamoros | Capability of microalgae-based wastewater treatment systems to remove emerging organic contaminants: a pilot-scale study[END_REF] showed that biodegradation and photodegradation were the most relevant removal pathways for 26 contaminants in municipal wastewater in a study conducted in two pilot ponds of "high quality algae." Volatilization was considered negligible for pharmaceuticals due to their low Henry's constant values. However, for recalcitrant hydrophobic compounds, volatilization and sorption pathways were predominant. [START_REF] Xiong | Biodegradation of carbamazepine using freshwater microalgae Chlamydomonas mexicana and Scenedesmus obliquus and the determination of its metabolic fate[END_REF], studying the removal of carbamazepine by the microalgae Chlamydomonas mexicana and Scenedesmus obliquus, demonstrated that both species simultaneously promote the biodegradation, adsorption and bioaccumulation of carbamazepine.

All the studies concluded that this technique could open new opportunities for wastewater treatment and to reduce environmental pollution. There are still areas to be explored or improved, such as the testing of other algal species, often the same species are used alone or in combination, the selection of different microorganisms with specific metabolism for different pollutants (currently there is a lot of research on the use of genetically modified microorganisms), the improvement of knowledge on the often slow kinetic and/or biodegradation mechanisms, and the optimization and better control of the operating parameters such as temperature, pH, and dissolved oxygen, as they are difficult to control on a large scale, which is a great challenge for this application. It is also necessary to acquire knowledge on other problems such as the incomplete transformation of certain recalcitrant pollutants.

Fungi for the removal of emerging contaminants

Fungi have also been recognized for their ability to transform a wide range of recalcitrant compounds using nonspecific intracellular and extracellular oxidizing enzymes. The use of fungi for the removal of emerging contaminants through biological processes such as mycoremediation and biosorption has the same advantages as those cited for the use of algae [START_REF] Zhang | Microbial community functional structure in response to antibiotics in pharmaceutical wastewater treatment systems[END_REF][START_REF] Badia-Fabregat | Identification of some factors affecting pharmaceutical active compounds (PhACs) removal in real wastewater. case study of fungal treatment of reverse osmosis concentrate[END_REF][START_REF] Asif | Degradation of pharmaceuticals and personal care products by white-rot-fungi -a critical review[END_REF][START_REF] Tomasini | Fungal bioremediation. Fundamentals and applications[END_REF].

Fungi are indeed microorganisms known for their effectiveness in treating water contaminated by pollutants such as pharmaceuticals. Fungal reactors or mycoreactors can be suspended or immobilized growth systems. They can operate under aerobic or anaerobic conditions. This technology requires specific and controlled conditions to maintain a sustainable and efficient process. For example, the oxidative metabolism of fungi can be strongly affected by the presence of nutrients, pH, immobilization on different supports, and agitation or static growth conditions. Recent comprehensive reviews on fungal technologies for the degradation of pharmaceuticals and personal care products have been published [START_REF] Asif | Degradation of pharmaceuticals and personal care products by white-rot-fungi -a critical review[END_REF][START_REF] Rodríguez-Rodríguez | fungal bioremediation: fundamentals and applications[END_REF][START_REF] Silva | The use of algae and fungi for removal of pharmaceuticals by bioremediation and biosorption processes: a review[END_REF][START_REF] Tomasini | Fungal bioremediation. Fundamentals and applications[END_REF]. [START_REF] Cruz-Morató | Degradation of pharmaceuticals in non-sterile urban wastewater by Trametes versicolor in a fluidized bed bioreactor[END_REF] studied the degradation of pharmaceuticals in non-sterile urban wastewater at the Universitat Autónoma de Barcelona (Spain) by Trametes versicolor in a fluidized bed reactor. Of the 80 pharmaceuticals analyzed, 13 were detected in the effluent of sterile urban wastewater, the most abundant belonging to the group of analgesic/anti-inflammatory compounds, especially naproxen (35.58 ± 4.8 μg/L) and ibuprofen (12.61 ± 1.79 μg/L). Complete elimination of both analgesics, ibuprofen and naproxen, occurred within 24 h after fungal treatment. A similar conclusion was reported by Marco-Urrea et al. (2013). Other analgesics as acetaminophen and codeine were initially detected at concentrations of 3.87 ± 0.41 μg/L and 0.02 ± 0.001 μg/L and were completely removed after 8 h and 2 days, respectively [START_REF] Cruz-Morató | Degradation of pharmaceuticals in non-sterile urban wastewater by Trametes versicolor in a fluidized bed bioreactor[END_REF]. The analgesics ketoprofen and salicylic acid were also initially detected in wastewater at concentrations of 0.48 ± 0.07 μg/L and 0.85 ± 0.11 μg/L, respectively. Changes in concentrations during treatment showed unexpected behavior with increases and decreases, but after 8 days, the concentrations were 0.31 ± 0.04 μg/L (ketoprofen) and 1.24 ± 0.07 μg/L (salicylic acid), corresponding to a 35% removal of ketoprofen and a 46% increase in salicylic acid concentration. The possible release of these compounds may be explained by the deconjugation of glucuronides during biological treatment. Complete removal of 7 of the 10 initially detected pharmaceuticals was achieved in non-sterile conditions after 24 h of fungal treatment, while only 2 were partially removed and 1 of the pharmaceuticals tested increased its concentration. Antibiotics such as erythromycin (detected at 0.3 µg/L) and metronidazole (detected at 0.05 µg/L) were successfully removed: erythromycin was completely eliminated within 15 min while the elimination of metronidazole was achieved after 2 days. Carbamazepine, a well-known recalcitrant psychiatric drug in activated sludge treatment was also detected in the wastewater at 0.7 µg/L. [START_REF] Cruz-Morató | Degradation of pharmaceuticals in non-sterile urban wastewater by Trametes versicolor in a fluidized bed bioreactor[END_REF] showed that carbamazepine and its metabolites were also strongly eliminated by Trametes versicolor during batch treatment in a fluidized bed bioreactor under sterile conditions, in agreement with the results published by Zhang and Geiβen (2012). In addition, the Vibrio fischeri luminescence test (Microtox® test) showed a significant reduction in wastewater toxicity after treatment. [START_REF] Cruz-Morató | Degradation of pharmaceuticals in non-sterile urban wastewater by Trametes versicolor in a fluidized bed bioreactor[END_REF] concluded that it was possible to use a fluidized bed bioreactor to remove pharmaceuticals at environmentally relevant concentrations under non-sterile conditions by Trametes versicolor.

In another work, the same authors treated hospital wastewaters (collected in the main sewer of the University Hospital of Girona, Dr. Josep Trueta, Girona, Spain) in a fungal bioreactor with Trametes versicolor, under sterile and nonsterile conditions [START_REF] Cruz-Morató | Hospital wastewater treatment by fungal bioreactor: removal efficiency for pharmaceuticals and endocrine disruptor compounds[END_REF]. Preliminary analytical monitoring of hospital wastewater showed that the most frequently detected families of substances were analgesics, antibiotics, psychiatric drugs, endocrine disruptors and X-ray contrast media, at concentrations ranging from ng/L to mg/L. Results showed that 46 of the 51 detected pharmaceuticals were partially degraded and/or completely eliminated in non-sterile experiments after 8 days. The initial total amount of pharmaceuticals in the bioreactor was 8185 μg in sterile treatment and 8426 μg in non-sterile treatment, and the overall load removal was 83.2% and 53.3% in their respective treatments. In particular, diclofenac, a recalcitrant compound in municipal wastewater treatment plants, and human metabolites of carbamazepine were efficiently removed. The lower removal detected in the nonsterile treatment is explained by the higher concentrations of caffeine (149 μg/L) and iopromide (419.7 μg/L), which are some of the most difficult compounds to degraded by the fungus (< 40% removal). Excluding caffeine and iopromide concentrations, the overall elimination of pharmaceuticals and endocrine disruptors was above 94% in both treatments. The treatment time required to achieve complete removal of the targeted pollutants highly depended on the type of pharmaceutical. For instance, almost complete elimination of analgesics was observed within 24 h, whereas similar results were observed for antibiotics within 5 days. Experiments also showed a significant reduction in wastewater toxicity after treatment, confirming the relevance of fungal treatment. The fact that the overall percentage of pollutant removal was similar in sterile and non-sterile treatments indicated that Trametes versicolor played a major role in this process and that the synergistic interaction between native bacteria and fungi did not improve removal efficiency. This result was also relevant as it indicated that sterility was not a mandatory condition for applying Trametes versicolor for this purpose. However, one of the main disadvantages of this process was that after treatment, the pH had to be neutralized since it reached acidic conditions due to the secretion of organic acids by the fungus. Cruz-Morató et al. (2014a) concluded that fungi-based technologies were a potential alternative to pretreat complex hospital wastewater. [START_REF] Vasiliadou | Biological removal of pharmaceutical compounds using white-rot fungi with concomitant FAME production of the residual biomass[END_REF] studied the biological removal of 13 pharmaceutical compounds using white rot fungi, Trametes versicolor and Ganoderma lucidum, with concomitant production of fatty acid methyl esters from the residual biomass. Both stains were used individually or simultaneously for the oxidative removal of pharmaceuticals. Trametes versicolor and Ganoderma lucidum allowed 100% removal of diclofenac, gemfibrozil, ibuprofen, progesterone, and ranitidine by individual or simultaneous strains after 7 days of incubation. Lower removals, ranging from 15 to 41%, were obtained for other less biodegradable substances such as 4-acetamidoantipyrin, clofibric acid, atenolol, caffeine, carbamazepine, hydrochlorothiazide, sulfamethoxazole, and sulpiride, although the combination of the two strains improved the efficiency of the system. [START_REF] Vasiliadou | Biological removal of pharmaceutical compounds using white-rot fungi with concomitant FAME production of the residual biomass[END_REF] also demonstrated efficient production of biodiesel from the residual fungal mass. The white rot fungus Trametes versicolor also reduced the estrogenic activity of a mixture of emerging contaminants in wastewater treatment plant effluent [START_REF] Shreve | The white-rot fungus Trametes versicolor reduces the estrogenic activity of a mixture of emerging contaminants in wastewater treatment plant effluent[END_REF][START_REF] Singhal | Degrading organic micropollutants: the next challenge in the evolution of biological wastewater treatment processes[END_REF]. Cruz del Álamo et al. ( 2018) also reported on the performance of an advanced bio-oxidation process based on fungi Trametes versicolor immobilized in a continuous rotating bioreactor to degrade pharmaceutical compounds in municipal wastewater. Further examples can be found in the review by [START_REF] Asif | Degradation of pharmaceuticals and personal care products by white-rot-fungi -a critical review[END_REF] and in two books recently edited by [START_REF] Tomasini | Fungal bioremediation. Fundamentals and applications[END_REF], and by [START_REF] Yadav | Recent advancement in white biotechnology through fungi[END_REF].

Numerous studies have demonstrated that white rot fungi can degrade a wide range of pharmaceuticals and personal care products, including even xenobiotics and recalcitrant compounds [START_REF] Yadav | Recent advancement in white biotechnology through fungi[END_REF]. Although the technical feasibility and effectiveness of this technology has been demonstrated, further research is needed before this method can be applied on a large scale.

Other biological strategies for the biodegradation of emerging contaminants

Other biological approaches for the degradation of emerging contaminants and the reduction of their negative impact include enzymatic degradation. Bioremediation using enzymes, such as transferases, hydrolases, and oxidoreductases, is a cost-effective and environmentally friendly biotechnology. Enzymes are biological catalysts that facilitate the conversion of substrates into products by providing favorable conditions that reduce the activation energy of the reaction. Each enzyme has its own mode of action: transferases catalyze the transfer of a functional group from a donor to an acceptor; hydrolases facilitate the cleavage of C-C, C-O, C-N and other bonds by water; oxidoreductases catalyze the transfer electrons and protons from a donor to an acceptor [START_REF] Karigar | Role of microbial enzymes in the bioremediation of pollutants: a review[END_REF]. The microbial enzyme plays an important role in bioremediation. Laccases are multinuclear copper-containing oxidoreductases and can perform electron oxidation of a broad spectrum of environmental contaminants. They are known for their potential to oxidize a broad spectrum of phenol-based compounds, dyes, inorganic substances, and pesticides. This group of versatile enzymes is also known as a green catalyst with significant potential to tackle emerging substances. These enzymes are an environmentally friendly substitute for conventional chemical reactions. Recent comprehensive reviews published by Iqbal's group are available on these topics (Ahmed et al. 2017a;Bilal et al. 2019aBilal et al. , 2019b). Iqbal's group concluded that laccases are one the most promising enzyme groups with potential uses in bioremediation and treatment of recalcitrant pollutants and xenobiotics. However, knowledge about this enzyme capable of degrading these substances is still limited, partly due to the lack of rapid screening methods [START_REF] Stadlmair | Monitoring enzymatic degradation of emerging contaminants using a chip-based robotic nano-ESI-MS tool[END_REF]. Nevertheless, this is an area of research in full development and significant advances are expected in the future.

Membrane bioreactors

Membrane bioreactor technology has been known since the 1970s, but its development took off in the 2000s. Its market value is currently continuing to increase as environmental regulations are becoming increasingly strict. The technology is becoming more and more profitable as the costs of membranes and the related membrane processes continue to fall. Indeed, among industrial water treatment techniques, membrane water treatment is now considered to be competitive with other techniques both from a chemical efficiency and an economic point of view. However, it has not yet overwhelmed the market due to some serious drawbacks such as membrane fouling.

Full-scale membrane bioreactor technology is a coupling of two wastewater treatment methods, biological treatment and membrane separation [START_REF] Domańska | Efficiency of municipal wastewater treatment with membrane bioreactor[END_REF][START_REF] Judd | The status of membrane bioreactor technology[END_REF][START_REF] Huang | Recent advances in membrane bioreactor technology for wastewater treatment in China[END_REF]. Conventional treatment of municipal wastewater generally involves three-stages: sedimentation of solids in the feed water (primary treatment), coupled with a coagulation stage when it rains for example, followed by treatment of organic matter by aerobic degradation, e.g., activated sludge and secondary treatment, and finally a second sedimentation process to remove biomass. Tertiary treatment is also possible, either to remove residual pollution and/or to disinfect the treated wastewater.

Membrane bioreactor technology can replace both physical separation and biodegradation processes by filtering the biomass through a membrane. As a result, the quality of water produced is much higher than that generated by conventional treatment, thus avoiding the need of another tertiary process (Ma et al. 2018;[START_REF] Jalilnejad | Advances in membrane bioreactor technology[END_REF]. Membrane bioreactors are slightly more efficient than activated sludge for the removal of organic matter because they combine biodegradation and membrane separation. Biodegradation converts minerals, organic matter and organic compounds into less toxic and refractory substances or, ideally, mineralizes them, while the final step using membranes reduces the chemical oxygen demand, promotes the retention of suspended solids and pathogens and salts, depending on the type of membrane used (microfiltration, ultrafiltration, nanofiltration or reverse osmosis). Compared to other conventional treatment processes, membrane bioreactor technology is also a smaller and simpler integrated process (compact installation), with less sludge production, complete biomass retention, high performance of nutrient removal, low energy density requirements, and much smaller footprint [START_REF] Jalilnejad | Advances in membrane bioreactor technology[END_REF]. The main shortcomings of membrane filtration are the generation of large volumes of concentrate that must be treated separately and the fouling of the membranes (which can considerably reduce their performance and lifetime), leading to a significant increase in maintenance and operating costs [START_REF] Huang | Recent advances in membrane bioreactor technology for wastewater treatment in China[END_REF][START_REF] Iorhemen | Membrane bioreactor (MBR) technology for wastewater treatment and reclamation: membrane fouling[END_REF]. A challenge of interest to the scientific community is the search for sustainable strategies to reduce membrane fouling. Other problems often mentioned are the generally longer retention times of the sludge, the higher biomass concentration of the reactors and the high variance of the separation step between the liquid phase and the sludge. It is also important to note that membrane bioreactor technology, like other techniques such as adsorption or membrane filtration, is a simple transfer process rather than a transformation/degradation process (as in the case of advanced oxidation processes). Attention should therefore be paid to the treatment of wastewater sludge (Ma et al. 2018).

Membrane bioreactors were originally designed and used for the treatment of domestic wastewater to remove conventional pollutants such as organic matter, minerals, and metals, and pollution including suspended solids, chemical oxygen demand, and biological oxygen demand [START_REF] Radjenovic | Analysis of pharmaceuticals in wastewater and removal using a membrane bioreactor[END_REF][START_REF] González | Evaluation of two pilot scale membrane bioreactors for the elimination of selected surfactants from municipal wastewaters[END_REF][START_REF] Huang | Recent advances in membrane bioreactor technology for wastewater treatment in China[END_REF]). Membrane bioreactors have subsequently been extended to a wide range of wastewaters, including urban runoff, mine wastewater and industrial effluents [START_REF] González | Evaluation of two pilot scale membrane bioreactors for the elimination of selected surfactants from municipal wastewaters[END_REF]. Research interests in membrane reactor technique have grown rapidly in recent years. Indeed, studies also showed that membrane bioreactors are effective in removing emerging substances, mostly pharmaceuticals [START_REF] Maeng | Influences of solid retention time, nitrification and microbial activity on the attenuation of pharmaceuticals and estrogens in membrane bioreactors[END_REF][START_REF] Ojajuni | Removal of organic micropollutants using membrane-assisted processes: a review of recent progress[END_REF][START_REF] Phan | Impact of hazardous events on the removal of nutrients and trace organic contaminants by an anoxic-aerobic membrane bioreactor receiving real wastewater[END_REF][START_REF] Sanguanpak | Effects of mixed liquor pH on membrane fouling and micro-pollutant removals in membrane bioreactors for municipal landfill leachate treatment[END_REF][START_REF] Alvarino | Assessing the feasibility of two hybrid MBR systems using PAC for removing macro and micropollutants[END_REF][START_REF] Besha | Removal of emerging micropollutants by activated sludge process and membrane bioreactors and the effects of micropollutants on membrane fouling: a review[END_REF][START_REF] Gurung | Assessing membrane fouling and the performance of pilot-scale membrane bioreactor (MBR) to treat real municipal wastewater during winter season in Nordic regions[END_REF][START_REF] Abargues | Fate of endocrine disruptor compounds in an anaerobic membrane bioreactor (AnMBR) coupled to an activated sludge reactor[END_REF][START_REF] Jiang | Effect of hydraulic retention time on the performance of a hybrid moving bed biofilm reactor-membrane bioreactor system for micropollutants removal from municipal wastewater[END_REF]Ma et al. 2018;[START_REF] Mert | Efficient removal approach of micropollutants in wastewater using membrane bioreactor. In: Yonar T (ed) Wastewater and water quality[END_REF]Bodzek et al. 2019;[START_REF] Borea | Are pharmaceuticals removal and membrane fouling in electromembrane bioreactor affected by current density?[END_REF][START_REF] Monteoliva-García | Removal of a pharmaceutical mix from urban wastewater coupling membrane bioreactor with advanced oxidation processes[END_REF][START_REF] Lim | Removal of organic micropollutants in anaerobic membrane bioreactors in wastewater treatment: critical review[END_REF][START_REF] Pathak | Membrane reactors for the removal of organic micro-pollutants[END_REF][START_REF] Racar | Challenges of municipal wastewater reclamation for irrigation by MBR and NF/RO: physicochemical and microbiological parameters, and emerging contaminants[END_REF][START_REF] Vieira | Removal of endocrine disruptors in waters by adsorption, membrane filtration and biodegradation[END_REF]. [START_REF] Maeng | Influences of solid retention time, nitrification and microbial activity on the attenuation of pharmaceuticals and estrogens in membrane bioreactors[END_REF] demonstrated that membrane bioreactor technology is an excellent approach for removing trace pharmaceuticals from complex wastewater. They reported high removal efficiency (> 80-90%) for acetaminophen, ibuprofen, bezafibrate, and estrogens including estrone, estradiol, and estriol. [START_REF] Phan | Impact of hazardous events on the removal of nutrients and trace organic contaminants by an anoxic-aerobic membrane bioreactor receiving real wastewater[END_REF] reported that membrane bioreactor technology can generally achieve high pollutant removal efficiency with the effluent quality largely complying with the Australian guidelines for water recycling, except for caffeine, estrone, and triclosan. Performance is a function of temperature [START_REF] Suárez | Mass balance of pharmaceutical and personal care products in a pilot-scale single-sludge system: influence of T SRT and recirculation ratio[END_REF][START_REF] Gurung | Assessing membrane fouling and the performance of pilot-scale membrane bioreactor (MBR) to treat real municipal wastewater during winter season in Nordic regions[END_REF]) and pH-dependent [START_REF] Sanguanpak | Effects of mixed liquor pH on membrane fouling and micro-pollutant removals in membrane bioreactors for municipal landfill leachate treatment[END_REF]. [START_REF] Suárez | Mass balance of pharmaceutical and personal care products in a pilot-scale single-sludge system: influence of T SRT and recirculation ratio[END_REF] evaluated the removal efficiency of sulfamethoxazole and erythromycin as a function of temperature and the results indicated that an increase in temperature resulted in 30% higher removal than in cold weathers. However, an increase in temperature to a high level, e.g., 45 °C, may inhibit metabolic activity [START_REF] Besha | Removal of emerging micropollutants by activated sludge process and membrane bioreactors and the effects of micropollutants on membrane fouling: a review[END_REF]. Similarly, low temperature impairs treatment efficiency. [START_REF] Gurung | Assessing membrane fouling and the performance of pilot-scale membrane bioreactor (MBR) to treat real municipal wastewater during winter season in Nordic regions[END_REF] also indicated that the removal efficiency of bisoprolol, diclofenac and bisphenol A was highly dependent on temperature, with maximum removal of 65, 38 and > 97%, respectively. Sanguanpak et al (2015) reported that membrane bioreactors performed better for analgesics and anti-inflammatory drugs, such as ibuprofen, diclofenac, and ketoprofen, under low pH conditions (the optimal pH was about 6), mainly due to the increased lipophilicity and the ionizability of pH-dependent of the molecules.

Racar et al. ( 2020) investigated the removal of several emerging substances, mainly pharmaceuticals, from wastewater from a treatment plant located in Čakovec (Croatia) by a membrane bioreactor. A 6-month analytical monitoring of wastewater showed 12 substances were systematically detected with a high variation in concentration. The highest values (up to 500 μg/L) were found in the winter period. Azithromycin (92.54 ± 113.90 μg/L), clarithromycin (50.49 ± 80.95 μg/L), and diclofenac (71.57 ± 57.41 μg/L) were the most prevalent, while the other measured pharmaceuticals showed wide variations in concentration, with a high concentration in November for acetamiprid, clothianidin, imidacloprid and thiamethoxam, which were found at had significantly lower concentrations in the other months. The authors demonstrated that membrane bioreactor technology achieves high removal rates (to levels below the limit of quantification) for all substances, regardless of the season. [START_REF] Vieira | Removal of endocrine disruptors in waters by adsorption, membrane filtration and biodegradation[END_REF] recently compared the efficiency of adsorption, membrane separation and biodegradation to remove endocrine disruptors including parabens, bisphenols, phthalates, estrogens, and nonylphenols, and pesticides. The authors comprehensively discussed the advantages and disadvantages of each process.

Literature data show that the three main removal mechanisms for substances that occur in membrane bioreactors are adsorption/sorption/biosorption onto sludge flocs and bound microbial products, biological degradation including aerobic degradation, anaerobic degradation, metabolism and co-metabolism, and ion trapping mechanisms, and membrane separation. For adsorption process, the efficiency largely depends on the physicochemical characteristics of emerging substances, e.g., hydrophobicity, hydrogen bond, and electrostatic interactions. For biological degradation, performance depends also on the biodegradability and bioavailability of the substances, and the conditions used (oxidation-reduction potential plays an important role in the microbial diversity, enzymatic functions and activities). [START_REF] Suárez | Mass balance of pharmaceutical and personal care products in a pilot-scale single-sludge system: influence of T SRT and recirculation ratio[END_REF] reported that musks (galaxolide, tonalide and celestolide) and estrogens such as estrone and estradiol can be well degraded under aerobic and anoxic conditions, whereas the transformation of ibuprofen, roxithromycin, erythromycin, citalopram, and naproxen occur only in the aerobic process. In contrast, the degradation of diclofenac, sulfamethoxazole, diazepam, trimethoprim, and carbamazepine is much less effective in the presence of oxygen species. Membrane separation is another mechanism (size exclusion, charge repulsion) contributing to pollutant removal and it depends on the type of membrane used. [START_REF] Alvarino | Assessing the feasibility of two hybrid MBR systems using PAC for removing macro and micropollutants[END_REF] demonstrated that the removal efficiency of diclofenac and roxithromycin in an ultrafiltration membrane bioreactor was higher than that in a microfiltration membrane bioreactor due to the retention by the cake layer. However, ultrafiltration does not eliminate all pollutants. The solution would be to generalize the use of nanofiltration and osmosis membranes, but then there is the problem of energy consumption and maintenance costs of the membranes, which can clog up more quickly.

Most of the studies published in the literature on the behavior of membrane bioreactors in relation to emerging substances (mainly pharmaceuticals) have been conducted on laboratory studies. Pilot projects at industrial scale have yet to be conducted on a much wider range of substances including different families of pharmaceutical compounds, personal products and cosmetics, pesticides, and industrial substances. Another important challenge is the presence of nanoparticles and nanoproducts in wastewater that can create clogging problems.

The sustainable application of membrane bioreactor technology requires a better understanding of the fate of pollutants, research on biotransformation mechanisms and the combination of bioreactors with emerging technologies such as advanced oxidation processes [START_REF] Borea | Are pharmaceuticals removal and membrane fouling in electromembrane bioreactor affected by current density?[END_REF][START_REF] Monteoliva-García | Removal of a pharmaceutical mix from urban wastewater coupling membrane bioreactor with advanced oxidation processes[END_REF]). The idea developed in this recent research theme is to use the redox reactions that occur on the surface of a conductive membrane, under anodic or cathodic polarization. These reactions can facilitate the transformation of refractory pollutants, which is a clear advantage over conventional processes, and/or reduce membrane fouling, while having negligible effects on microbial activities in the effluent.

There are two other aspects that are becoming increasingly important in the field of water treatment. The first is the recycling of water, especially for irrigation of agricultural soils or golf fields. Recycling or reusing wastewater could also be a way of supplementing available water supplies. Effluents from membrane bioreactor technology are capable of meeting or exceeding current drinking water regulations. However, there are several barriers to water reuse. For example, the public perception of recycled water for drinking water production is less than favorable, while the reuse of water for irrigation is generally accepted. The second is to consider wastewater as a resource and not as a waste. Conventional municipal wastewater treatment plants such as those applying activated sludge are energy-intensive, produce large quantities of residues (sludge) and fail to recover the potential resources available in wastewater. Furthermore, in this respect, wastewater is often considered as a waste. However, wastewater should be considered a source of organic matter, phosphorus, nitrogen, metals, and energy. Membrane techniques could make it possible to recover compounds with high added value. Of course, there is the question of the presence of emerging substances. In this context, biological membrane technologies coupled with other methods could be useful.

Advanced oxidation technologies to degrade emerging contaminants

Removal of emerging contaminants by wastewater disinfection

The main objective of disinfection is to either eliminate pathogens from water to produce potable water or to reduce the pathogen content of treated wastewater in wastewater treatment plants. Disinfection is indeed the final treatment step to produce potable water. It is an important step because the use of water disinfection as a public health measure reduces the spread of diseases.

Pathogenic microorganisms are destroyed or inactivated using chemical or physical disinfectants such as chlorine, chlorine dioxide, hypochlorite, ozone, peracetic acid, bromide, iodine, non-ionizing radiation including UV radiation and ultrasonic radiation, and ionizing radiation (gamma ray). Among chemicals, chlorine and its compounds, chlorine dioxide and ozone, are the most common disinfectants used in the water industry. The main mechanisms of germicidal action of disinfectants are related to the direct oxidation of the cell of microorganisms by the disinfectant or to the alteration of the permeability of the cell wall, or even to the photochemical deterioration of their DNA or RNA (UV radiation) [START_REF] Asano | Water reuse: issues, technologies, and applications[END_REF]. Disinfectants are also known to remove organic contaminants from water, which act as nutrients or shelter for microorganisms. They must also have a residual effect to prevent microorganisms from growing in the pipe after treatment, which would result in recontamination of the water [START_REF] Collivignarelli | Overview of the main disinfection processes for wastewater and drinking water treatment plants[END_REF]. The main problem with disinfection is that the processes can lead to the formation of organic and inorganic disinfection by-products such as trihalomethanes, chlorite and chlorate, and aldehydes (von Sonntag and von Gunten 2012). Advanced technologies include the combination of ozone and hydrogen peroxide, ozone and UV radiation, hydrogen peroxide and UV radiation, UV radiation with titanium dioxide, alone or combined with other processes, such as land filtration, membrane technologies, nanotechnology, photovoltaic method, solar photocatalytic, and sonodisinfection.

Disinfection processes using disinfectants, alone or in combination with additional physical/chemical agents, have also been proposed to eliminate emerging substances [START_REF] Ikehata | Degradation of aqueous pharmaceuticals by ozonation and advanced oxidation processes: a review[END_REF][START_REF] Ikehata | Ozonation and advanced oxidation treatment of emerging organic pollutants in water and wastewater[END_REF][START_REF] Snyder | Ozone oxidation of endocrine disruptors and pharmaceuticals in surface water and wastewater[END_REF][START_REF] Gagnon | Degradation of selected acidic and neutral pharmaceutical products in a primary-treated wastewater by disinfection processes[END_REF]Kim and Tanaka 2009;Hey et al. 2012aHey et al. , 2012b;;Noutsopoulos et al. 2013a;[START_REF] Yang | Treatment of organic micropollutants in water and wastewater by UV-based processes: a literature review[END_REF]. In municipal wastewater treatment, disinfection is usually the last step (tertiary or final treatment) before the treated wastewater is released to the aquatic environment. However, the treatment can also take place after primary and/or secondary wastewater treatment. A review of the abundant literature published over the past 20 years highlights that the three main technologies studied for both disinfecting water from pathogens and removing substances are chlorination, ozonation and UV irradiation. These are the technologies used at industrial scale, either alone or in combination with other chemicals.

The main chlorination compounds used in the wastewater industry are chlorine (Cl 2 ), sodium hypochlorite (NaOCl) and calcium hypochlorite (Ca(OCl) 2 ). Of these, chlorine gas is generally used in large wastewater treatment plants, although a switch to sodium hypochlorite has been noted in many cases over the last decade due to safety concerns, while calcium hypochlorite is allowed in smaller plants. The effect of chlorination on the removal of emerging contaminants has been widely documented [START_REF] Hu | Products of aqueous chlorination of bisphenol A and their estrogenic activity[END_REF][START_REF] Petrovic | Occurrence and removal of estrogenic short-chain ethoxy nonylphenolic compounds and their halogenated derivatives during drinking water production[END_REF]Boyd et al. 2005;[START_REF] Westerhoff | Fate of endocrinedisruptor, pharmaceutical, and personal care product chemicals during simulated drinking water treatment processes[END_REF][START_REF] Zhang | Naproxen removal from water by chlorination and biofilm processes[END_REF][START_REF] Greyshock | Triclosan reactivity in chloraminated waters[END_REF][START_REF] Thurman | Accurate-mass identification of chlorinated and brominated products of 4-nonylphenol, nonylphenol dimers, and other endocrine disrupters[END_REF][START_REF] Korshin | Comparative study of reactions of endocrine disruptors bisphenol A and diethylstilbestrol in electrochemical treatment and chlorination[END_REF][START_REF] Stackelberg | Efficiency of conventional drinking-water-treatment processes in removal of pharmaceuticals and other organic compounds[END_REF][START_REF] Simazaki | Removal of selected pharmaceuticals by chlorination, coagulation-sedimentation and powdered activated carbon treatment[END_REF]Benotti et al. 2009a, b;[START_REF] Acero | Kinetics of aqueous chlorination of pharmaceuticals and their elimination from water matrices[END_REF][START_REF] Quintana | Investigating the chlorination of acidic pharmaceuticals and by-product formation aided by an experimental design methodology[END_REF][START_REF] Chen | Occurrence and assessment of treatment efficiency of nonyphenol, octylphenol and bisphenol A on drinking water in Taiwan[END_REF]Ga et al. 2014;de Jesus Gaffney et al. 2016). The degree of removal varies for different types of emerging contaminants. Nevertheless, all these studies on chlorination experiments are focused in pure water or drinking water rather than in wastewater matrix. On the other hand, studies on the ability of chlorination to remove emerging contaminants from wastewater are rather rare [START_REF] Renew | Simultaneous determination of flurorquinolone, sulfonamide, and trimethoprim antibiotics in wastewater using tandem solid phase extraction and liquid chromatographyelectrospray mass spectrometry[END_REF][START_REF] Belgiorno | Review on endocrine disruptingemerging compounds in urban wastewater: occurrence and removal by photocatalysis and ultrasonic irradiation for wastewater reuse[END_REF][START_REF] Nakamura | Formation of chlorinated estrones via hypochlorous disinfection of wastewater effluent containing estrone[END_REF][START_REF] Ying | Occurrence and removal of pharmaceutically active compounds in sewage treatment plants with different technologies[END_REF][START_REF] Li | Mass flows and removal of antibiotics in two municipal wastewater treatment plants[END_REF]Noutsopoulos et al. 2013aNoutsopoulos et al. , 2013bNoutsopoulos et al. , 2013c[START_REF] Noutsopoulos | Removal of endocrine disruptors and nonsteroidal anti-inflammatory drugs through wastewater chlorination: the effect of pH, total suspended solids and humic acids and identification of degradation by-products[END_REF][START_REF] Nika | Chlorination of benzothiazoles and benzotriazoles and transformation products identification by LC-HR-MS/MS[END_REF].

Based on the extensive data in the literature, chlorination appears to be effective in the removal of several nonsteroidal anti-inflammatory drugs, antibiotics, estrogens and antidepressants, e.g., diclofenac, naproxen, sulfamethoxazole, amitriptyline hydrochloride, and methyl salicylate, endocrine disrupters, e.g., nonylphenol, bisphenol, triclosan, and 17-β-estradiol, benzotriazoles and benzothiazoles, while other chemicals in these categories have significantly lower degradability, e.g., ibuprofen, ketoprofen, 17-β-estradiol and tolytriazole. The performance of the chlorination process in removing emerging contaminants is related both to the characteristics of the matrix, such as organic matter content of the wastewater, presence of total suspended solids and pH, and to the physicochemical characteristics of the chemicals. As suggested by [START_REF] Noutsopoulos | Removal of endocrine disruptors and nonsteroidal anti-inflammatory drugs through wastewater chlorination: the effect of pH, total suspended solids and humic acids and identification of degradation by-products[END_REF], the performance of the chlorination process for the removal of targeted endocrine disruptors and non-steroidal antiinflammatory drugs was not affected by the pH for typical wastewater pH values around 7-8. On the other hand, some studies reported a significant impact of pH on the removal of new contaminants [START_REF] Acero | Kinetics of aqueous chlorination of pharmaceuticals and their elimination from water matrices[END_REF][START_REF] Gallard | Chlorination of bisphenol A: kinetics and by-products formation[END_REF][START_REF] Pinkston | Transformation of aromatic ether-and amine-containing pharmaceuticals during chlorine disinfection[END_REF][START_REF] Deborde | Reactions of chlorine with inorganic and organic compounds during water treatment -kinetics and mechanism: a critical review[END_REF] for pH values significantly different from those prevailing in wastewater treated by secondary and/or tertiary treatment. In general, pH can effectively affect process performance at low values (lower than those prevailing in treated wastewater) that favor the prevalence of the strongest oxidizing species (i.e., hypochlorous acid). In addition, it is expected that not only the available free chlorine species (HOCl and OCl -), but also the chemical characteristics (pKa, chemical structure) of the compounds under different pH conditions may affect chlorination performance. With respect to other wastewater characteristics, [START_REF] Noutsopoulos | Removal of endocrine disruptors and nonsteroidal anti-inflammatory drugs through wastewater chlorination: the effect of pH, total suspended solids and humic acids and identification of degradation by-products[END_REF] reported that the effect of total suspended solids content of wastewater, and thus organic matter content, on the degradation of emerging contaminants during chlorination is more intense for chemicals with high K ow values, e.g., nonylphenol and its ethoxylates, and triclosan, and thus a high affinity to be distributed to the particulate phase. In the same study, the effect of humic acids on the removal of emerging contaminants and non-steroidal anti-inflammatory drugs by wastewater chlorination was rather minimal. A significant number of studies on endocrine disruptors and chlorination by-products of pharmaceuticals provide an analytical discussion of the relevant mechanisms [START_REF] Hu | Products of aqueous chlorination of bisphenol A and their estrogenic activity[END_REF][START_REF] Hu | Products of aqueous chlorination of 17a-estradiol and their estrogenic activities[END_REF][START_REF] Petrovic | Occurrence and removal of estrogenic short-chain ethoxy nonylphenolic compounds and their halogenated derivatives during drinking water production[END_REF][START_REF] Gallard | Chlorination of bisphenol A: kinetics and by-products formation[END_REF][START_REF] Bedner | Transformation of acetaminophen by chlorination produces the toxicants 1,4-benzoquinone and n-acetyl-pbenzoquinone imine[END_REF][START_REF] Korshin | Comparative study of reactions of endocrine disruptors bisphenol A and diethylstilbestrol in electrochemical treatment and chlorination[END_REF][START_REF] Thurman | Accurate-mass identification of chlorinated and brominated products of 4-nonylphenol, nonylphenol dimers, and other endocrine disrupters[END_REF][START_REF] Lei | 3D QSPR models for the removal of trace organic contaminants by ozone and free chlorine[END_REF][START_REF] Sharma | Oxidative transformations of environmental pharmaceuticals by Cl 2 , ClO 2 , O 3 and Fe(VI): Kinetics assessment[END_REF][START_REF] Quintana | Reaction of β-blockers and β-agonist pharmaceuticals with aqueous chlorine. Investigation of kinetics and by-products by liquid chromatography quadrupole time-of-flight mass spectrometry[END_REF][START_REF] Bulloch | Analytical and biological characterization of halogenated gemfibrozil produced through chlorination of wastewater[END_REF][START_REF] Soufan | Aqueous chlorination of diclofenac: kinetic study and transformation products identification[END_REF][START_REF] Noutsopoulos | Removal of endocrine disruptors and nonsteroidal anti-inflammatory drugs through wastewater chlorination: the effect of pH, total suspended solids and humic acids and identification of degradation by-products[END_REF][START_REF] Nika | Chlorination of benzothiazoles and benzotriazoles and transformation products identification by LC-HR-MS/MS[END_REF]. In many cases, toxicity measurements show that some of the by-products of chlorination are more toxic than the original compounds.

Chlorine dioxide (ClO 2 ) has a stronger disinfectant activity than chlorine. Considering its higher cost compared to conventional chlorination, disinfection with ClO 2 is generally adopted in cases where minimizing the production of chlorine-based disinfection by-products is desirable. Because of its instability, ClO 2 is produced on-site by mixing a chlorine solution with a sodium chlorite solution. The literature on the effectiveness of ClO 2 in removing emerging contaminants during wastewater disinfection is rather limited. In their comprehensive review, Hey and colleagues (2012a) investigated the effect of ClO 2 on the removal of a wide range of 56 pharmaceuticals. According to this study, it was shown that, in addition to the effectiveness of ClO 2 in removing many emerging contaminants, approximately onethird of the target compounds studied were virtually unaffected by the oxidant, even at doses as high as 20 mg/L. In their follow-up study, Hey et al (2012b) reported the effective elimination of three non-steroidal anti-inflammatory drugs, namely naproxen, diclofenac, and mefenamic acid, and a lipid-regulating agent, gemfibrozil, while no elimination was reported for ibuprofen and clofibric acid. The inability of ClO 2 to remove ibuprofen and carbamazepine has also been reported previously by Lee and van Gunten (2010), while satisfactory removal has been reported for sulfamethoxazole and 17α-ethinyl estradiol. In addition, Huber et al. (2005aHuber et al. ( , 2005b) ) showed an effective elimination of estrogenic hormones at very low doses of ClO 2 and a short contact time (5 min). Its ability to remove emerging contaminants has also been demonstrated for a range of antibiotics [START_REF] Sharma | Oxidative transformations of environmental pharmaceuticals by Cl 2 , ClO 2 , O 3 and Fe(VI): Kinetics assessment[END_REF][START_REF] Navalon | Reaction of chlorine dioxide with emergent water pollutants: product study of the reaction of three β-lactam antibiotics with ClO 2[END_REF]Wang et al. 2019).

The UV radiation process is a well-known disinfection step used to effectively remove bacteria, viruses and protozoa. The main types of lamps are low-intensity low-pressure lamps, high-intensity low-pressure lamps and high-intensity medium-pressure lamps, the former being the most used for disinfection purposes [START_REF] Asano | Water reuse: issues, technologies, and applications[END_REF]). The main germicidal mechanism of UV irradiation is associated with direct DNA damage, while the removal of organic pollutants is based on their direct photolysis during absorption of UV-C protons at the wavelength of 254 nm. The effectiveness of UV irradiation on the removal of pathogens and organic micropollutants is highly dependent on the applied dose (in mJ/cm 2 or mWs/cm 2 ), calculated as the product of average UV intensity and contact time. The effectiveness of UV irradiation for the removal of emerging contaminants has been well documented, primarily for pharmaceuticals and endocrine disruptors [START_REF] Andreozzi | Pharmaceuticals in STP effluents and their solar photodegradation in aquatic environment[END_REF][START_REF] Doll | Fate of pharmaceuticals -photodegradation by simulated solar UV-light[END_REF][START_REF] Lopez | Kinetic investigation on UV and UV/H 2 O 2 degradations of pharmaceutical intermediates in aqueous solution[END_REF][START_REF] Rosenfeldt | Degradation of endocrine disrupting chemicals bisphenol A, ethinyl estradiol, and estradiol during UV photolysis and advanced oxidation[END_REF][START_REF] Vogna | Kinetic and chemical assessment of the UV/H 2 O 2 treatment of antiepileptic drug carbamazepine[END_REF][START_REF] Chen | Biological assessment of bisphenol A degradation in water following direct photolysis and UV advanced oxidation[END_REF][START_REF] Neamtu | Photodegradation of endocrine disrupting chemical nonylphenol by simulated solar UV-irradiation[END_REF]Pereira et al. 2007a, b;[START_REF] Canonica | Phototransformation of selected pharmaceuticals during UV treatment of drinking water[END_REF]Benotti et al. 2009a, b;Kim et al. 2009a;Kim and Tanaka 2009;[START_REF] Yuan | Degradation of selected pharmaceuticals in aqueous solution with UV and UV/H 2 O 2[END_REF][START_REF] Wert | Evaluation of UV/H 2 O 2 treatment for the oxidation of pharmaceuticals in wastewater[END_REF][START_REF] Zhang | Kinetic degradation model and estrogenicity changes of EE2 (17α-ethinylestradiol) in aqueous solution by UV and UV/H 2 O 2 technology[END_REF][START_REF] Baeza | Transformation kinetics of biochemically active compounds in low-pressure UV photolysis and UV/H 2 O 2 advanced oxidation processes[END_REF][START_REF] Salgado | Assessing the removal of pharmaceuticals and personal care products in a full-scale activated sludge plant[END_REF][START_REF] Salgado | Assessing the removal of pharmaceuticals and personal care products in a full-scale activated sludge plant[END_REF][START_REF] Salgado | Photodegradation kinetics and transformation products of ketoprofen, diclofenac and atenolol in pure water and treated wastewater[END_REF][START_REF] Hansen | Energy effectiveness of direct UV and UV/H 2 O 2 Treatment of estrogenic chemicals in biologically treated sewage[END_REF][START_REF] Pablos | Emerging micropollutant oxidation during disinfection processes using UV-C, UV-C/H 2 O 2 , UV-A/TiO 2 and UV-A/TiO 2 /H 2 O 2[END_REF][START_REF] Bennett | Advanced oxidation processes for treatment of 17[beta]-estradiol and its metabolites in aquaculture wastewater[END_REF][START_REF] Mole | Correlating effluent concentrations and bench-scale experiments to assess the transformation of endocrine active compounds in wastewater by UV or chlorination disinfection[END_REF]. Kim et al. (2009aKim et al. ( , 2009b) ) reported that of the 42 pharmaceuticals studied under real wastewater conditions, only a few showed high removal, e.g., ketoprofen, diclofenac, and antipyrine, while several others and particularly antibiotics such as clarithromycin, erythromycin, and azithromycin, showed very low degradation due to UV irradiation, even at doses above 2700 mJ/cm 2 . Consequently, Noutsopoulos et al. (2013c) concluded that with the application of the low-pressure UV doses generally adopted for pathogen removal (10-80 mJ/cm 2 ), no significant removal should be expected for many emerging contaminants. Based on this study, the endocrine disruptors bisphenol A and nonylphenol and the non-steroidal anti-inflammatory drugs ibuprofen and naproxen showed a low degradability even at doses as high as 1000 mJ/cm 2 , confirming the results of previous studies [START_REF] Rosenfeldt | Degradation of endocrine disrupting chemicals bisphenol A, ethinyl estradiol, and estradiol during UV photolysis and advanced oxidation[END_REF][START_REF] Vogna | Kinetic and chemical assessment of the UV/H 2 O 2 treatment of antiepileptic drug carbamazepine[END_REF][START_REF] Chen | Biological assessment of bisphenol A degradation in water following direct photolysis and UV advanced oxidation[END_REF], Pereira et al. 2007a, b;[START_REF] Canonica | Phototransformation of selected pharmaceuticals during UV treatment of drinking water[END_REF][START_REF] Yuan | Degradation of selected pharmaceuticals in aqueous solution with UV and UV/H 2 O 2[END_REF][START_REF] Wert | Evaluation of UV/H 2 O 2 treatment for the oxidation of pharmaceuticals in wastewater[END_REF][START_REF] Baeza | Transformation kinetics of biochemically active compounds in low-pressure UV photolysis and UV/H 2 O 2 advanced oxidation processes[END_REF][START_REF] Salgado | Assessing the removal of pharmaceuticals and personal care products in a full-scale activated sludge plant[END_REF][START_REF] Pablos | Emerging micropollutant oxidation during disinfection processes using UV-C, UV-C/H 2 O 2 , UV-A/TiO 2 and UV-A/TiO 2 /H 2 O 2[END_REF]. The moderate effect of UV has also been confirmed by [START_REF] Bennett | Advanced oxidation processes for treatment of 17[beta]-estradiol and its metabolites in aquaculture wastewater[END_REF] for estrogens. The authors suggested that the complete degradation of estrogens could only be achieved at UV doses (500-100 mJ/cm 2 ) much higher than those used for pathogen removal. As concluded by many researchers (Kim et al. 2009a;[START_REF] Yuan | Degradation of selected pharmaceuticals in aqueous solution with UV and UV/H 2 O 2[END_REF]Pablos et al. 2012;Noutsopoulos et al. 2013a), the structure of each chemical as well as its physical characteristics, i.e., decadal molar absorption coefficient at 254 nm, largely govern its sensitivity to degradation. Studies of UV radiation transformation by-products on emerging contaminants are rather limited [START_REF] Salgado | Photodegradation kinetics and transformation products of ketoprofen, diclofenac and atenolol in pure water and treated wastewater[END_REF][START_REF] Bennett | Advanced oxidation processes for treatment of 17[beta]-estradiol and its metabolites in aquaculture wastewater[END_REF]) and the hypothesis that photodegradation intermediates may be more recalcitrant or toxic than parent compounds has therefore yet to be fully demonstrated. Several modifications have been suggested to improve the efficiency of UV irradiation such as the use of mediumpressure lamps (Kim et al. 2009a;Pereira et al. 2007) or the UV-based advanced oxidation processes such as UV/H 2 O 2 , UV/Cl 2 and VUV/O 3 [START_REF] Kruithof | UV/H 2 O 2 treatment: a practical solution for organic contaminant control and primary disinfection[END_REF][START_REF] Yuan | Degradation of selected pharmaceuticals in aqueous solution with UV and UV/H 2 O 2[END_REF][START_REF] Xiang | Kinetics and pathways of ibuprofen degradation by the UV/chlorine advanced oxidation process[END_REF][START_REF] Lian | Kinetic study of hydroxyl and sulfate radical-mediated oxidation of pharmaceuticals in wastewater effluents[END_REF]. For example, full-scale application of UV combined with hydrogen peroxide in an existing water treatment plant in North Holland for a running time of more than 2 years has proven that this practice was effective and reliable to control organic micropollutants [START_REF] Kruithof | UV/H 2 O 2 treatment: a practical solution for organic contaminant control and primary disinfection[END_REF]). The UV/H 2 O 2 process was installed between the sand filtration and granular activated carbon filtration processes. Substances such as mecoprop and diclofenac were removed by 98%, while the removal of the other compounds (pesticides) varied from 60 to 91%. Carbon filters could effectively remove residual hydrogen peroxide and at least conceptually, also any by-products formed in the oxidation process, as well as assimilable organic carbon that can feed microbes in biofilm formed within the water distribution lines.

Ozone is a very active oxidant and therefore a very effective germicide. It has been widely demonstrated that ozone has a remarkable ability to eliminate not only bacteria and viruses, but also pathogenic protozoa, compared to chlorine and ClO 2 . Ozone is produced on site and an ozonation unit consists of the air compressor, including cooling, drying and filtration accessories, the ozone generator, the contact tank for ozonation and the waste gas destruction device. Due to the high cost of the method and the high doses required in the wastewater, compared to natural water treatment, ozonation was not considered to be an attractive method for wastewater disinfection and its use was limited to water disinfection. However, after having corroborated its ability to remove emerging contaminants as well as its well-known disinfection performance, ozonation has received much attention over the last decade, especially in cases where specific provisions for the removal of emerging contaminants through wastewater treatment have been regulated, e.g., the Swiss Water Protection Act) Ozone reacts either through its molecular form (O 3 ) or through the activity of hydroxyl radicals that are formed during its decomposition reactions in water. The ozone molecule reacts effectively with many compounds, especially those containing aromatic rings, unsaturations or heteroatoms, while hydroxyl radicals are powerful non-selective oxidants. Ozone tends to react preferentially with the hydrophobic fractions of organic compounds such as hydrophobic acid and neutral species. The effectiveness of ozonation is affected by several parameters such as temperature, pH, presence of organic compounds (e.g., chemical oxygen demand), natural organic matter, total suspended solids, nitrates, and nitrites. For example, it has been proposed that nitrites act as a radical scavenger, inhibiting the effectiveness of ozonation [START_REF] Lee | Oxidative transformation of micropollutants during municipal wastewater treatment: comparison of kinetic aspects of selective (chlorine, chlorine dioxide, ferrate VI, and ozone) and non-selective oxidants (hydroxyl radical)[END_REF]. Several studies have been reported on the effectiveness of ozonation in removing emerging contaminants, although most of these studies use pure water rather than a wastewater matrix (Ahmed et al. 2017b;[START_REF] Sun | Comparison of different advanced treatment processes in removing endocrine disruption effects from municipal wastewater secondary effluent[END_REF][START_REF] Bourgin | Evaluation of a full-scale wastewater treatment plant upgraded with ozonation and biological post-treatments: abatement of micropollutants, formation of transformation products and oxidation by-products[END_REF][START_REF] Lacson | Fluidized-bed Fenton treatment of imidacloprid: optimization and degradation pathway[END_REF][START_REF] Paucar | Ozone treatment process for the removal of pharmaceuticals and personal care products in wastewater[END_REF][START_REF] Thanekar | Improvement in biological oxidation process for the removal of dichlorvos from aqueous solutions using pretreatment based on hydrodynamic cavitation[END_REF]Wang et al. 2018a).

According to extensive data in the literature, ozone has a high reactivity with pesticides, estrogens, endocrine disruptors, beta-blockers and many non-steroidal anti-inflammatory drugs and parabens, while a lower elimination capacity has been recorded for some antidepressants, antiepileptics, benzotriazoles, perfluorooctanesulfonic acids and perfluorooctanonic acids.

In view of the above, it is anticipated that through wastewater disinfection, an appreciable removal of emerging contaminants can be achieved, therefore adding on their total abatement in wastewater treatment plants, when the removal through primary and secondary treatment is take into consideration as well. Among different disinfection methods, chlorination and ozonation seem to provide better results, regarding emerging contaminants removal, while UV as stand-alone disinfection method, without being upgraded to UV assisted advanced oxidation processes, exhibit rather moderate performance. To guarantee satisfactory removal capacities for a wide range of compounds, higher doses than those typically used for the removal of pathogens should be applied. These higher doses are important to prevent the regrowth of bacteria after treatment. Further research is needed in order to conclude about the possible toxic characteristics of alternative disinfection methods by-products compared to those of their parent compounds. Conclusively, when disposal of treated wastewater in low dilution water streams, including streams, rivers, lakes, and shallow marine waters, is practiced or wastewater reuse is desirable (e.g., for agricultural use), the use of wastewater disinfection is mandatory to ensure a microbiologically acceptable water content. Optimizing disinfection methods to provide the removal of pathogens and the reduction of the emerging contaminant load that is released to the aquatic environment appears to be technologically feasible.

Electrochemical, photochemical and ultrasonic technologies for the removal of emerging contaminants in industrial wastewaters

Advanced oxidation processes represent one of the most promising strategies for the removal of emerging contaminants present in wastewater treatment effluents. Although these processes use different reagent systems, all techniques are based on the generation of reactive radicals. Hydroxyl radical-mediated advanced oxidation processes are base on the hydroxyl radical, which is a non-selective and very powerful oxidizing agent, used not only to degrade organic and inorganic substances but also to inactivate biological agents such as pathogenic microorganisms. Other radicals can be involved such as SO 4

•-in sulfate radical-mediated advanced oxidation processes that have gained attention due to its high redox potential comparable with that of HO • and chlorine radical-mediated advanced oxidation processes (e.g., UV/ chlorine) that are based on Cl • , Cl 2

•-, Cl O • . Several types of advanced oxidation processes are presented in Fig. 5. The technology can improve biodegradability, enhance color removal, degrade and mineralize recalcitrant molecules, and reduce toxicity. Advanced oxidation processes are potential techniques for industrial wastewater treatment, e.g., removal of pollutants from pulp and paper and textile industry, and for drinking water production (to remove pathogens and/ or organic compounds in combination with an adsorption step). Conventional municipal wastewater treatment plants have serious shortcomings that can also be addressed by advanced oxidation processes. However, these systems are not yet widely used in industry, mainly because of their high cost to treat large volume of effluents. The principles, performances, advantages, drawbacks, and applications of advanced oxidation processes are detailed in numerous reviews [START_REF] Ribeiro | An overview on the advanced oxidation processes applied for the treatment of water pollutants defined in the recently launched Directive 2013/39/EU[END_REF][START_REF] De Araújo | Advanced oxidation processes: a review regarding the fundamentals and applications in wastewater treatment and industrial wastewater[END_REF][START_REF] Mishra | A review on advanced oxidation processes for effective water treatment[END_REF][START_REF] Moreira | Electrochemical advanced oxidation processes: a review on their application to synthetic and real wastewaters[END_REF][START_REF] Miklos | Evaluation of advanced oxidation processes for water and wastewater treatment -a critical review[END_REF][START_REF] Babu | Detoxification of water and wastewater by advanced oxidation processes[END_REF][START_REF] Wang | Degradation of antibiotics by advanced oxidation processes: an overview[END_REF]. Moreover, their performance can be affected by a wide range of wastewater constituents, including the natural organic matter such as humic and fulvic acids, carbohydrates, and proteins, and inorganic species like carbonate, bicarbonate, nitrite, sulfate, chloride. Both organic and inorganic species can react with radicals, thus competing with emerging contaminants for oxidation or generating other radicals with lower oxidation potential, but promoting effects can also be observed, as recently reviewed by Lado [START_REF] Ribeiro | Impact of water matrix on the removal of micropollutants by advanced oxidation technologies[END_REF].

Among the many advanced oxidation processes studied for the elimination of non-biodegradable compounds (the so-called refractory compounds), electrochemical, photochemical, photocatalytic, and ultrasonic technologies are subject to particular attention. These technologies use single or combined methods, such as electro-Fenton, photoelectro-Fenton, and sono-electrolysis, in homogeneous (e.g., Organic compounds with lower weight and higher hydrophilicity;

CO 2 + H 2 O + etc. Water emerging contaminants O 3 in alkaline medium CatalyƟc ozonaƟon H 2 O 2 /catalyst O 3 /H 2 O 2 /UV O 3 /H 2 O 2 /ultrasound S 2 O 8 2-/catalyst S 2 O 8 2-/UV Photo-Fenton Heterogeneous photocatalysis Electrochemical oxidaƟon Electro-Fenton Anodic oxidaƟon O 3 /H 2 O 2 Non-thermal plasma Etc.
Energy:

-UV radiaƟon -ionizing radiaƟon -electrical -ultrasound, etc. The general aspects of Fenton and photo-Fenton processes, electrochemical oxidation processes and sonochemistry were published by [START_REF] Ameta | Fenton and photo-Fenton processes. In: advanced oxidation processes for waste water treatment[END_REF], by [START_REF] Radha | Electrochemical oxidation processes. In: advanced oxidation processes for waste water treatment[END_REF] and by Torres-Palma and Serna-Galvis (2018), respectively. Briefly, the Fenton process, based on the Fenton reagent, uses H 2 O 2 and an iron soluble salt, generating hydroxyl radicals at atmospheric pressure and room temperature. High efficiency, relatively cheap reagents, no need of energy to activate H 2 O 2 and the consequent easy implementation and operation are the advantages of such treatment. Some disadvantages are the generation of a secondary waste (sludge) and the narrow range of optimal pH (2.5-3). As alternatives, the photo-assisted Fenton process can be more efficient than Fenton alone, mainly due to the faster regeneration of Fe 2+ and electro-Fenton is another option, where Fe 2+ is produced from sacrificial cast iron anodes, or even photo-electro-Fenton. Heterogeneous photocatalysis is another process that has been extensively investigated for water/wastewater treatment and is based on the use of wide band-gap semiconductors which generate electrons and holes (and subsequent chain reactions involving hydroxyl radicals) when irradiated with photons of energy higher than the semiconductor band-gap. The most widely used photocatalyst is TiO 2 , due to its outstanding activity, photochemical stability, good band gap energy, low cost, and relatively low toxicity. Nevertheless, a drawback of the photo-assisted processes is the limited thickness of the water layer that is effectively penetrated by the UV radiation. Therefore, to increase the treatment efficiency, shallow bed reactors should be used. Fenton, photo-Fenton, photocatalysis, and ozonation-based processes have been commonly studied, while electrochemical technologies and sonolysis are less applied but deserve special attention in the literature.

Advanced oxidation processes methods constitute a potential additional (secondary or tertiary) treatment for the elimination of pharmaceuticals. They may ideally produce the complete mineralization of organic pollutant, generating H 2 O, CO 2 and other inorganic substances, or at least their transformation into more innocuous byproducts. For example, the partial degradation of nonbiodegradable organic substances can lead to biodegradable intermediates. For this reason, advanced oxidation processes can be used as pre-treatments before biological processes in a wastewater treatment plant. The electrochemical advanced oxidation processes have also been proposed for the degradation of pesticides and dyes, for the degradation of organic pollutants from wastewater and for water disinfection. Advantages often cited include easiness operation, high efficiency with possibility to mineralize compounds, no sludge production, possible coupling with other process, low temperature required for its operation, and possibility to use solar panel to decrease the energy consumption. Electrochemical technologies emerge also as a good alternative to carry out the on-site generation of disinfectant agents from the species naturally contained in wastewater. These technologies can be applied as a pre-treatment to transform recalcitrant compounds in biological wastewaters or in post-treatment before their discharge. However, until now most studies are conducted at laboratory scale and under controlled conditions. Further research and operational and investment costs assessment are necessary for scale-up new electrochemical technologies.

Promising results using electrochemical, photochemical/photocatalytic and sonochemical processes have been published for example by Torres-Palma and collaborators [START_REF] Giraldo | Degradation of the antibiotic oxacillin in water by anodic oxidation with Ti/IrO 2 anodes: evaluation of degradation routes, organic by-products and effects of water matrix components[END_REF][START_REF] Serna-Galvis | High frequency ultrasound as a selective advanced oxidation process to remove penicillinic antibiotics and eliminate its antimicrobial activity from water[END_REF], 2019;[START_REF] Jojoa-Sierra | Elimination of the antibiotic norfloxacin in municipal wastewater, urine and seawater by electrochemical oxidation on IrO 2 anodes[END_REF][START_REF] Valero | Electrochemical advanced oxidation processes for Staphylococcus aureus disinfection in municipal WWTP effluents[END_REF][START_REF] Villegas-Guzman | Selecting the best AOP for isoxazolyl penicillins degradation as a function of water characteristics: effects of pH, chemical nature of additives and pollutant concentration[END_REF][START_REF] Tomasini | Fungal bioremediation. Fundamentals and applications[END_REF]. The authors studied the degradation of the antibiotic oxacillin in water by anodic oxidation with Ti/IrO 2 anodes using an undivided stirred tank reactor [START_REF] Giraldo | Degradation of the antibiotic oxacillin in water by anodic oxidation with Ti/IrO 2 anodes: evaluation of degradation routes, organic by-products and effects of water matrix components[END_REF]. A decrease of 70% of the initial chemical oxygen demand was obtained and the level of biodegradability increased from 0.03 to 0.84, indicating that the system was able to transform the pollutant into highly oxidized and more biodegradable products with less antimicrobial activity. The more relevant initial aromatic by-products were identified and a degradation pathway of the electrochemical oxidation of the oxacillin antibiotic was proposed. [START_REF] Giraldo | Degradation of the antibiotic oxacillin in water by anodic oxidation with Ti/IrO 2 anodes: evaluation of degradation routes, organic by-products and effects of water matrix components[END_REF] concluded that electrochemical oxidation had a high potential to eliminate antibiotics. In another work, Serna-Galvis and co-workers demonstrated that high frequency ultrasound in the presence of additives was a selective and efficient advanced oxidation process to remove penicillinic antibiotics and to eliminate its antimicrobial activity from water [START_REF] Serna-Galvis | High frequency ultrasound as a selective advanced oxidation process to remove penicillinic antibiotics and eliminate its antimicrobial activity from water[END_REF]. Torres-Palma's group also studied the degradation of isoxazolyl penicillins by photo-Fenton, photocatalysis and ultrasound. The three processes achieved total removal of the antibiotic and antimicrobial activity and increased the biodegradability of the solutions (Villegas-Guzman et al. 2017). However, significant differences concerning the mineralization extent was observed depending on solution pH, chemical nature of additives, e.g., water matrix characteristics, and contaminant concentration. The authors also demonstrated that electrochemical advanced oxidation was a pertinent approach for Staphylococcus aureus disinfection in wastewater treatment plants [START_REF] Valero | Electrochemical advanced oxidation processes for Staphylococcus aureus disinfection in municipal WWTP effluents[END_REF].

Martínez-Pachón et al. ( 2019) studied the advanced oxidation of antihypertensives losartan and valsartan by photoelectro-Fenton at near-neutral pH using natural organic acids and a dimensional stable anode-gas diffusion electrode system under light emission diode lighting. Organic acids as citric, tartaric and oxalic acids were used as complexing agents of iron ions in order to maintain the performance of the Fenton reaction at near-neutral pH value. The authors showed that after 90 min of electro-Fenton treatment using the optimized conditions, a degradation of 70% of valsartan and 100% of losartan were achieved. The total degradation of the two antihypertensives was achieved with a photo-electro-Fenton for the same time period. The degradation performance was attributed to the increase of the initial dissolved iron in the system in the presence of the organic ligands, facilitating the Fe 3+ /Fe 2+ turnover in the catalytic photo-Fenton reaction and, consequently, hydroxyl radical production. The increased photo-activity of the complexes was also associated with their high capability to complex Fe 3+ and to promote ligand-to-metal charge transfer, which was of key importance to feed Fe 2+ to the Fenton process. The results showed that the system evaluated more efficient to eliminate sartan family compounds using light emission diode lighting in comparison with traditional UV-A lamps used in this type of works. Moreover, three transformation products of valsartan and two of losartan were identified by high-resolution mass spectrometry using hybrid quadrupoletime-of-flight mass spectrometry. The several organic compounds remaining after the photo-electro-Fenton treatment were effectively treated in a subsequent aerobic biological system [START_REF] Martínez-Pachón | Advanced oxidation of antihypertensives losartan and valsartan by photo-electro-Fenton at near-neutral pH using natural organic acids and a dimensional stable anode-gas diffusion electrode (DSA-GDE) system under light emission diode (LED) lighting[END_REF].

The review of the literature related to wastewater treatment issue revealed that the electrochemical-, photochemical-, and ultrasonic-based technologies are powerful to degrade emerging pollutants such as antibiotics, although some processes are not capable to completely mineralize the organic pollutants. The two main advantages of these advanced oxidation processes are: (i) the oxidizing species are generated in situ (no need of chemicals storage and handling) and (ii) most of processes (except the Fenton type) do not require a rigorous control of solution pH. Other advantages include operation control simplicity, reactor design compactness, adaptability of the technology to various organic loads of wastewater, and effectiveness in disinfection. For the electrochemical treatment, one of the main challenges to its successful implementation for industrial application is to reduce energy consumption and cost (including the cost of electrodes). Another important challenge is to improve the understanding of the reaction mechanisms and to identify potential toxic intermediates. It is also important to improve the long-term stability and electrolytic efficiency of the materials. In this respect, progress is expected from the application of nanotechnologies.

Carbon nanomaterials for the catalytic ozonation of emerging contaminants

An effective technology to degrade organic emerging contaminants is catalytic ozonation. This treatment uses a catalyst to decompose ozone into a number of strong oxidant radicals such as . OH (Wang et al. 2016c). These radicals quickly oxidize organic compounds (Fig. 6).

Homogeneous catalysts are water soluble metal salts which are not recoverable after an ozonation process, thus their application involves chemical costs as well as further pollution. On the contrary, heterogeneous catalysts are solid particles that can be recovered, regenerated, and reused. Numerous works have reported different kinds of materials as catalysts in water treatment processes, namely metal oxides, activated carbon and carbon nanomaterials [START_REF] Restivo | Catalytic ozonation of metolachlor under continuous operation using nanocarbon materials grown on a ceramic monolith[END_REF][START_REF] Restivo | Catalytic ozonation of organic micropollutants using carbon nanofibers supported on monoliths[END_REF][START_REF] Restivo | Carbon nanofibers doped with nitrogen for the continuous catalytic ozonation of organic pollutants[END_REF][START_REF] Rocha | Nitrogen-doped graphene-based materials for advanced oxidation processes[END_REF]Wang et al. 2016bWang et al. , 2016c)). Many reports have shown superior performance of metal oxide-based catalysts. However, these catalysts can be subject to metal leaching into effluent water [START_REF] Rocha | Nitrogen-doped graphene-based materials for advanced oxidation processes[END_REF]. Consequently, research efforts have been done for the development of metal-free catalyst. Among metal-free catalysts, carbon materials such as activated carbon, multi-walled carbon nanotubes, graphene oxide and reduced-graphene oxide are the most studied ones. These materials have been studied both as sole catalysts and as support materials. This section focuses on the studies that concern application of carbon-based nanomaterials, i.e., multi-walled carbon nanotubes, graphene oxide, and reduced-graphene oxides, both as catalysts and catalyst support materials in catalytic ozonation processes.

Among carbon nanomaterials, multi-walled carbon nanotubes are the most widely studied catalyst for ozonation processes due to their strong catalytic activity and re-usability. Multi-walled carbon nanotubes were first reported as catalyst in ozonation process by Liu et al. (2009a, b). These materials achieved 80% conversion of oxalic acid in 40 min as compared to around 2% conversion by non-catalytic ozonation. The authors also studied the catalytic performance of oxidized multi-walled carbon nanotubes. For this purpose, nanotubes were oxidized by pre-ozonation treatment at various degrees. This pre-ozonation treatment implanted different acidic groups such as -COOH and -OH on catalyst surface leading to inferior performance of multi-walled carbon nanotubes. The catalytic activity decrease could be attributed to the negative charge generated by the acidic groups which causes a drop of pH pzc value nearly to the pH of oxalic acid aqueous solution. This leads to less adsorption of anion species of oxalic acid, eventually to less degradation of oxalic acid during catalytic ozonation. In a similar study, [START_REF] Gonçalves | Influence of the surface chemistry of multi-walled carbon nanotubes on their activity as ozonation catalysts[END_REF] investigated the role of surface chemistry of multi-walled carbon nanotubes on catalytic ozonation of oxalic acid. This study gave a very similar picture as the previous one in which the surface acidic sites hinder multi-walled carbon nanotubes' catalytic ability. The authors also compared the performances of multiwalled carbon nanotubes with those of commercial activated carbon, concluding that multi-walled carbon nanotubes are more effective catalysts since they impose less internal mass transfer limitations on the reactants during the catalytic ozonation reaction. A catalytic reaction is mass transfer limited when the participating reactants experience mass transfer resistance inside micro-pores before reaching the active sites. Activated carbons possess a large quantity of micropores, which is likely to impose significant mass transfer resistance. On the other hand, multi-walled carbon nanotubes barely possess any micro-pores. The same research group compared the catalytic performance of commercial multi-walled carbon nanotubes with that of activated carbon in catalytic oxidative degradation of sulfamethoxazole [START_REF] Gonçalves | Ozonation of sulfamethoxazole promoted by MWCNT[END_REF]. In terms degradation of this antibiotic, catalytic ozonation with multi-walled carbon nanotubes was comparable to non-catalytic ozonation. Nevertheless, catalytic ozonation led to higher degree of mineralization expressed as total organic carbon removal. Surprisingly, activated carbon showed superior catalytic activity compared to multi-walled carbon nanotubes in terms of total organic carbon removal. This result was ascribed to the fact that sulfamethoxazole is more readily adsorbed in micro-pores and not to its oxidative degradation. The authors also studied the toxicity effects of its oxidation by-products by Microtox® bioassays. It was found that the lowest toxicity was achieved when the ozonation was carried on in the presence of multiwalled carbon nanotubes.

One of the main research directions in this field is to improve the catalytic activity of carbon nanotubes by modifying their surface chemistry and specific surface area by different methods including heteroatom-doping, oxidation, and grinding. [START_REF] Qu | Ozonation of indigo enhanced by carboxylated carbon nanotubes: performance optimization, degradation products, reaction mechanism and toxicity evaluation[END_REF] studied carboxylated carbon nanotubes in catalytic ozonation of indigo. The modified nanotubes showed higher indigo removal both in terms of indigo concentration, total organic carbon and toxicity removal. Yet, this study is inconclusive as it did not compare the results with those obtained with pristine carbon nanotubes. [START_REF] Soares | Modification of carbon nanotubes by ball-milling to be used as ozonation catalysts[END_REF] improved multi-walled carbon nanotubes' catalytic performance by increasing their specific surface area by shortening their tube size. This size reduction was achieved by ball-milling. The ball-milled multi-walled carbon nanotubes showed fairly improved performance in catalytic ozonation of oxalic acid. [START_REF] Rocha | Nitrogen-doped graphene-based materials for advanced oxidation processes[END_REF] showed that reduced graphene oxide works as a catalyst in ozonation of oxalic acid. Catalytic ozonation with reduced-graphene oxide was 50% more efficient in the pollutant removal than non-catalytic ozonation (for 120 min reaction time). Wang et al. (2016b) used reduced-graphene oxide as catalyst for ozonation of p-hydroxybenzoic acid when nearly full mineralization of p-hydroxybenzoic acid was achieved in 60 min. This study identified carbonyl groups as the active sites and suggested that superoxide radicals ( . O 2 -) and singlet oxygen ( 1 O 2 ) are the dominant species responsible for p-hydroxybenzoic acid degradation. Graphene oxide, the parent material of reduced-graphene oxide, can also be catalytically active in many reactions as its surface is rich in oxygen functional groups. For instance, the ozonation of N,N-diethyl-m-toluamide, a widely used pesticide, in the presence of graphene oxide was investigated by [START_REF] Liu | Ozone/graphene oxide catalytic oxidation: a novel method to degrade emerging organic contaminant N, N-diethyl-m-toluamide (DEET)[END_REF]. This process showed increased removal of the target pollutant as compared to non-catalytic ozonation. But lack of mineralization data raises doubts on its applicability. [START_REF] Ahn | Effect of graphene oxidation degree on the catalytic activity of graphene for ozone catalysis[END_REF] showed that over-oxidized graphene oxide produces significantly higher amounts of . OH than graphene oxide or reduced-graphene oxide, which is likely to lead to higher removal of recalcitrant organics. Song et al. (2019b) studied both graphene oxide and reduced-graphene oxide as catalysts for catalytic ozonation of p-chlorobenzoic acid and benzotriazoles. They found graphene oxide more efficient in degrading p-chlorobenzoic acid while reduced-graphene oxide showed higher activity in the case of benzotriazole. However, the authors noted that graphene oxide suffers from gradual degradation during the catalytic ozonation process. This deterioration of graphene oxide material is most likely caused by corrosive attack of ozone or . OH [START_REF] Radich | Is graphene a stable platform for photocatalysis? Mineralization of reduced graphene oxide with UV-irradiated TiO 2 nanoparticles[END_REF]. In an extensive study, Wang et al. (2018c) synthesized reduced-graphene oxide starting from graphite from used lithium ion battery. The obtained material showed higher removal of oxalic acid than commercially reduced-graphene oxide. A strong correlation between the amount of defective sites on reduced-graphene oxide and its catalytic activity was found. This correlation was again justified by density functional theory calculations.

Heteroatom doping of graphene oxide and of reducedgraphene oxide can significantly improve their catalytic activity. N, B, P and S have been studied as dopants for reduced-graphene oxides. [START_REF] Rocha | Nitrogen-doped graphene-based materials for advanced oxidation processes[END_REF] doped reduced-graphene oxide with N from different nitrogen precursors such as melamine and urea. This doping process implanted three N-containing functionalities into the reduced-graphene oxide namely pyridinic, pyrrolic and quaternary-N. The improved performance of the N-doped reduced-graphene oxide in mineralizing oxalic acid and phenol was attributed to these functional groups which work as active centers. Moreover, these N-functional groups increase the pH PZC , making the material more positively charged at the pH of oxalic acid solution (3.0). This enables increased adsorption of oxalate anions, which is the dissociated form of oxalic acid at pH of 3.0, favoring its catalytic surface reaction. This could also explain the lack of improvement in the phenol degradation, which is found in molecular form at the aqueous solution pH (pK a around 10). [START_REF] Bao | Promoting catalytic ozonation of phenol over graphene through nitrogenation and Co 3 O 4 compositing[END_REF] also confirmed improved catalytic ozonation performance of N-doped reduced-graphene oxide. [START_REF] Yin | Heteroatoms doped graphene for catalytic ozonation of sulfamethoxazole by metal-free catalysis: performances and mechanisms[END_REF] showed catalytic ozonation using N-and P-doped reduced-graphene oxides is far more efficient than non-catalytic ozonation in the degradation of sulfamethoxazole. Wang et al. (2019aWang et al. ( , 2019b) ) used a novel microwave method to dope N into reduced-graphene oxide. This method generates higher amounts of N-doped that eventually leads to higher removal of 4-nitrophenol and oxalic acid. The improved performance of this microwave method was attributed to the higher degree of graphene oxide reduction and generation of more defects and carbon dangling bonds.

In a very detailed study, Song et al. (2019a) synthesized and tested N-, P-, B-and S-doped reduced-graphene oxide for catalytic ozonation degradation of p-chlorobenzoic acid and benzotriazoles and for elimination of bromate (BrO 3 -). Bromates are carcinogenic by-products of ozone-based processes. Although P-doped reduced-graphene oxides showed the fastest elimination of both p-chlorobenzoic acid and benzotriazole, the authors concluded that in terms of normalized pseudo-first-order reaction constant (k obs ), N-doped reduced-graphene oxide shows the fastest removal.

The normalization was done by dividing the k obs by atomic percentage of the corresponding heteroatoms in each of the as prepared reduced-graphene oxide. On the other hand, S-doped reduced-graphene oxide was found to be unstable given the increase on the total organic carbon content of the aqueous solution during the catalytic ozonation process. Wang et al. (2018b) doped multi-walled carbon nanotubes with F using HF as F precursor. The materials showed significantly increased catalytic activity in ozonation of oxalic acid as compared to N-doped multi-walled carbon nanotubes. Highly electronegative active sites like N and O are inferred to decompose ozone by nucleophilic attack. Interestingly, this study showed that an excessive doping of electronegative atoms is counterproductive. Nevertheless, given the difficulty in handling highly toxic HF, F doping may not be feasible at commercial level.

Usually, catalytically active metals and metal oxides are deposited on porous materials such as alumina, zeolite, activated carbon, silica, and various metal oxides [START_REF] Ghuge | Catalytic ozonation for the treatment of synthetic and industrial effluents -Application of mesoporous materials: a review[END_REF]. Carbon nanomaterials such as multi-walled carbon nanotubes and graphene oxide/reduced-graphene oxides have also been used as support materials because of their excellent compatibility with metals and metal oxides, resistance to adverse environment, mechanical strength and excellent electron transfer ability [START_REF] Lin | Instantaneous formation of metal and metal oxide nanoparticles on carbon nanotubes and graphene via solvent-free microwave heating[END_REF][START_REF] Khan | Graphene based metal and metal oxide nanocomposites: synthesis, properties and their applications[END_REF]. Studies have also shown synergy between active catalyst materials and these carbon materials [START_REF] Sampaio | Synergistic effect between carbon nanomaterials and ZnO for photocatalytic water decontamination[END_REF].

The greatest number of works deals with the synthesis of supported manganese and iron oxides and their use in the oxidative degradation of emerging pollutants such as pesticides and pharmaceuticals [START_REF] Sui | Heterogeneous catalytic ozonation of ciprofloxacin in water with carbon nanotube supported manganese oxides as catalyst[END_REF][START_REF] Li | Efficient catalytic ozonation of bisphenol-A over reduced graphene oxide modified sea urchin-like α-MnO 2 architectures[END_REF][START_REF] Bai | Algae-mediated removal of selected pharmaceutical and personal care products (PPCPs) from Lake Mead water[END_REF]Wang et al. 2016a). [START_REF] Sui | Heterogeneous catalytic ozonation of ciprofloxacin in water with carbon nanotube supported manganese oxides as catalyst[END_REF] synthesized a MnO x /multi-walled carbon nanotube composite for catalytic ozonation of ciprofloxacin, a persistent antibacterial agent. MnO x /multi-walled carbon nanotube led to a removal of ciprofloxacin of 87.5% in 15 min as opposed to 40.2% elimination with unsupported MnO x and 26.7% removal with non-catalytic ozonation. [START_REF] Li | Efficient catalytic ozonation of bisphenol-A over reduced graphene oxide modified sea urchin-like α-MnO 2 architectures[END_REF] reported the synthesis of a sea urchin-like α-MnO 2 /reduced-graphene oxide composite for catalytic ozonation of bisphenol A. The process showed significantly higher bisphenol A removal efficiency compared to that with pristine α-MnO 2 , pristine reduced-graphene oxide and non-catalytic ozonation. The authors concluded that reduced-graphene oxide is inactive in catalytic ozonation of bisphenol A. Wang et al. (2016a) synthesized γ-MnO 2 /reduced-graphene oxide composite for catalytic ozonation of 4-nitrophenol, showing improved degradation and mineralization compared to non-catalytic ozonation. Prepared γ-MnO 2 /reduced-graphene oxide showed improved performance over commercial MnO 2 . Wang et al. (2019a) prepared a CeO 2 /oxidized-carbon nanotube composite for catalytic ozonation of phenol. The composite catalyst achieved nearly 100% total organic carbon removal in 60 min with virtually no activity loss up to 5 cycles. Depositing reduced-graphene oxide on metal oxide can also be fruitful as shown by [START_REF] Ren | Catalytic ozonation of di-n-butyl phthalate degradation using manganese ferrite/reduced graphene oxide nanofiber as catalyst in the water[END_REF]. In this report, MnFe 2 O 4 nano-fiber catalyst was improved by reduced-graphene oxide doping.

Overall, catalytic ozonation processes with carbon nanomaterial-based catalysts offer the following advantages: (i) higher degree of organic pollutant degradation as compared to activated carbon catalysts; (ii) minimal metal leaching issue; and (iii) compatibility under a wide range of conditions. However, the separation method of catalyst from treated water is an important issue to be addressed before technology scale-up for industrial application.

Non-thermal plasma for the removal of emerging contaminants

Growing interest in finding effective solutions for the removal of emerging contaminants from water led to the investigation of unconventional water treatment methods. Among them, non-thermal plasma is a promising approach and is now considered the youngest member of the so-called advanced oxidation processes family. Recently, significant research efforts were devoted to enhancing the efficiency of plasma treatment of water contaminated with harmful organic pollutants such as pharmaceuticals and pesticides [START_REF] Magureanu | Decomposition of methylene blue in water by corona discharges[END_REF]. The efforts are directed toward elaborating technically and economically feasible solution, toward bringing new insight the reaction mechanism and final characteristics and quality of the treated water.

Non-thermal plasma can be generated directly in the liquid or in the gas phase. Plasmas in contact with water are very complex systems, which produce a diversity of molecular, ionic and radical reactive species responsible for the degradation of organic pollutants. In these systems, the generation of reactive species is initiated by collision of the high energy electrons (formed by the electrical discharge) with the gas constituents and water molecules (in the gas phase, or at the gas-liquid interface). The nature of active species and their availability in the liquid phases depend on the plasma reactor configuration, e.g., corona, dielectric barrier discharges, gliding arc, and plasma jet, on the discharge characteristics, solution properties and gas composition (Brissetet al. 2008;[START_REF] Park | Reactive nitrogen species produced in water by non-equilibrium plasma increase plant growth rate and nutritional yield[END_REF]. A number of reactive oxygen species and reactive nitrogen species have been detected in the gas and/or liquid phase of the cold plasma discharge systems, such as:

•OH, HO 2 •, H 2 O 2 , O•, •O 2 -, O 3 , •NO, •NO 2 , NO 2
-, NO 3 -, and ONO 2 -. [START_REF] Lukes | Applications of membrane bioreactors for water reclamation: micropollutant removal, mechanisms and perspectives[END_REF][START_REF] Locke | Elementary chemical and physical phenomena in electrical discharge plasma in gas-liquid environments and in liquids[END_REF][START_REF] Bruggeman | Non-thermal plasmas in and in contact with liquids[END_REF].

Ozone, hydrogen peroxide and hydroxyl radical are the most investigated reactive oxygen species in such plasmas. Numerous studies deal with the identification and quantification of these three species by different spectrophotometric and chromatographic methods (Ono and Oda 2002;[START_REF] Park | Study on optical emission analysis of AC air-water discharges under He, Ar and N 2 environments[END_REF][START_REF] Xiong | Absolute OH density measurements in an atmospheric pressure dc glow discharge in air with water electrode by broadband UV absorption spectroscopy[END_REF]Guo et al. 2019a;[START_REF] Kanazawa | Application of chemical dosimetry to hydroxyl radical measurement during underwater discharge[END_REF][START_REF] Marotta | Advanced oxidation process for degradation of aqueous phenol in a dielectric barrier discharge reactor[END_REF][START_REF] Lukes | Hydrogen peroxide and ozone formation in hybrid gas-liquid electrical discharge reactors[END_REF][START_REF] Bilea | Characterization of the chemical activity of a pulsed corona discharge above water[END_REF].

The hydroxyl radical is a key reactive oxygen species, considered to be the main actor in the oxidative degradation of organic pollutants. Its formation in electrical discharges actually includes plasma processes among the advanced oxidation process. Due to its short lifetime, •OH interacts with organic pollutants very near the gas-water interface [START_REF] Kanazawa | Application of chemical dosimetry to hydroxyl radical measurement during underwater discharge[END_REF][START_REF] Marotta | Advanced oxidation process for degradation of aqueous phenol in a dielectric barrier discharge reactor[END_REF][START_REF] Ajo | Pulsed corona discharge in water treatment: the effect of hydrodynamic conditions on oxidation energy efficiency[END_REF]. In the bulk liquid, hydrogen peroxide, a stable molecular reactive oxygen species, is formed mainly by recombination of hydroxyl radicals [START_REF] Locke | Review of the methods to form hydrogen peroxide in electrical discharge plasma with liquid water[END_REF]. The accumulation of H 2 O 2 in water during the plasma treatment was highlighted by numerous works [START_REF] Locke | Review of the methods to form hydrogen peroxide in electrical discharge plasma with liquid water[END_REF]Magureanu et al. 2016;[START_REF] Bradu | Degradation of the chlorophenoxyacetic herbicide 2,4-D byplasma-ozonation system[END_REF][START_REF] Bilea | Characterization of the chemical activity of a pulsed corona discharge above water[END_REF]. The radical recombination reaction leading to stable molecular species may limit the process effectiveness [START_REF] Locke | Elementary chemical and physical phenomena in electrical discharge plasma in gas-liquid environments and in liquids[END_REF]. However, H 2 O 2 is a potential source of hydroxyl radicals, which may be used judiciously. For instance, H 2 O 2 decomposition with the regeneration of •OH radicals could be promoted by an adequate catalyst [START_REF] Parvulescu | Plasma chemistry and catalysis in gases and liquids[END_REF][START_REF] Jović | Effect of different catalysts on mesotrione degradation in water falling film DBD reactor[END_REF][START_REF] He | Aqueous tetracycline degradation by non-thermal plasma combined with nano-TiO 2[END_REF][START_REF] Aziz | Comparative study on 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol removal from aqueous solutions via ozonation, photocatalysis and nonthermal plasma using a planar falling film reactor[END_REF]Guo et al. 2019a). The proposed catalysts are either soluble transitional metal salts such as Fe 2+ , Fe 3+ , Mn 2+ , and Co 2+ [START_REF] Dojčinović | Decolorization of reactive textile dyes using water falling film dielectric barrier discharge[END_REF][START_REF] Jović | Effect of different catalysts on mesotrione degradation in water falling film DBD reactor[END_REF] or heterogeneous catalysts such as TiO 2 , activated carbon, and based-graphene materials [START_REF] He | Decomposition of tetracycline in aqueous solution by corona discharge plasma combined with a Bi 2 MoO 6 nanocatalyst[END_REF][START_REF] Aziz | Comparative study on 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol removal from aqueous solutions via ozonation, photocatalysis and nonthermal plasma using a planar falling film reactor[END_REF][START_REF] Vanraes | Removal of atrazine in water by combination of activated carbon and dielectric barrier discharge[END_REF][START_REF] Vanraes | Removal of several pesticides in a falling water film DBD reactor with activated carbon textile: energy efficiency[END_REF]Guo et al. 2019aGuo et al. , 2019bGuo et al. , 2019c)).

Another long-lived species generated in non-thermal plasma in oxygen-containing gaseous atmosphere is ozone. Appreciable O 3 concentrations were detected in the gas phase of electrical discharges in contact with water [START_REF] Lukes | Generation of ozone by pulsed corona discharge over water surface in hybrid gase liquid electrical discharge reactor[END_REF][START_REF] Marotta | Advanced oxidation process for degradation of aqueous phenol in a dielectric barrier discharge reactor[END_REF]Magureanu et al. 2016). However, ozone diffusion across the gas-liquid interface is limited and its concentration in water is often under the detection limit of employed analytical methods [START_REF] Marotta | Advanced oxidation process for degradation of aqueous phenol in a dielectric barrier discharge reactor[END_REF][START_REF] Dobrin | Degradation of diclofenac in water using a pulsed corona discharge[END_REF]Magureanu et al. 2016). Nevertheless, the transfer of plasma generated O 3 from the gas to the liquid could be improved to facilitate its reaction with the target organic pollutants or their degradation by-products. In this respect, plasma-ozonation systems were proposed (Magureanu et al. 2016;[START_REF] Bradu | Degradation of the chlorophenoxyacetic herbicide 2,4-D byplasma-ozonation system[END_REF]. In this case, the water to be treated is continuously circulated between a plasma reactor and an ozonation reactor in which the effluent gas from the plasma reactor was bubbled through the aqueous solution (Fig. 7). With this combined plasma-ozonation system, faster removal of the target compound and higher degree of mineralization was obtained compared to the single plasma or ozonation processes. The improvement was attributed to the enhanced transfer of O 3 in the water and its further interaction with the H 2 O 2 accumulated in solution, leading to increased generation of •OH (peroxone reaction).

When non-thermal plasma is generated in air, beside reactive oxygen species, reactive nitrogen species can be formed in significant amounts. Nitric oxide (•NO) is an important secondary species, produced through the interaction of the parent (N 2 ) and/or primary species (N•, O•, •OH) in gas phase. In further reactions, •NO can effectively fix oxygen atoms through the interaction with O• or with O-donor species produced in the discharge (e.g., O 3 and HO 2 •) to form nitrogen dioxide (•NO 2 ) [START_REF] Brisset | Chemical effects of air plasma species on aqueous solutes in direct and delayed exposure modes: discharge, post-discharge and plasma activated water[END_REF][START_REF] Aritoshi | Production and removal mechanisms of discharge NOx treatment in N 2 /O 2 gas mixture[END_REF]. In humid air and in the aqueous phase, the formation of nitrous and nitric acids (HNO 2 and HNO 3 ) takes place. The presence of peroxynitrous acid (ONOOH) in water was also reported. It was suggested that this peroxyacid is produced in the reaction between nitrous acid and hydrogen peroxide (reaction favored in acidic pH), or via the reaction between dissolved nitric and nitrous oxides and different radical reactive oxygen species (•OH, HO 2 • and •O 2 -) [START_REF] Goldstein | Chemistry of peroxynitrites as compared to peroxynitrates[END_REF]Mousa et al. 2007;[START_REF] Lukes | Applications of membrane bioreactors for water reclamation: micropollutant removal, mechanisms and perspectives[END_REF][START_REF] Tian | Atmospheric pressure dielectric barrier discharges interacting with liquid covered tissue[END_REF]. The generation of peroxynitric acid (O 2 NOOH) through the reaction between peroxynitrous acid and hydrogen peroxide has also been proposed [START_REF] Boehm | Hydrogen peroxide and beyond-the potential of high-voltage plasma activated liquids against cancerous cells[END_REF][START_REF] Ikawa | Physicochemical properties of bactericidal plasma-treated water[END_REF][START_REF] Nakashima | Ion-exchange chromatographic analysis of peroxynitric acid[END_REF].Thus, a variety of nitrogen-containing species is present in the aqueous phase as well. The reactive nitrogen species accumulation in water depends on their solubility and their lifetime. The dominant species for electrical discharges in contact with water are considered to be the nitrogen oxy-and peroxyacids and their conjugate ions: HNO x /NO x -(Tian and Kushner 2014). Detailed analysis of the generation, transport and interactions of reactive nitrogen species in plasma in contact with water can be found in [START_REF] Locke | Elementary chemical and physical phenomena in electrical discharge plasma in gas-liquid environments and in liquids[END_REF][START_REF] Bruggeman | Non-thermal plasmas in and in contact with liquids[END_REF][START_REF] Bradu | Reactive nitrogen species in plasma-activated water: generation, chemistry and application in agriculture[END_REF].

Even if the reactive nitrogen species involvement in the organic compounds' degradation was less studied, there is solid evidence that these species participate in the degradative oxidation pathway of water pollutants. As an example, nitro-substituted by-products and N,N-dimethyl-nitroaniline have been detected in the degradation of methyl orange in corona discharge in contact with water by [START_REF] Cadorin | Treatment of methyl orange by nitrogen non-thermal plasma in a corona reactor: the role of reactive nitrogen species[END_REF]. It was assumed that the organic molecules interact with peroxynitrous acid either directly or indirectly via dissociation into NO 2 and •OH [START_REF] Moussa | Postdischarge long life reactive intermediates involved in the plasma chemical degradation of an azoic dye[END_REF][START_REF] Cadorin | Treatment of methyl orange by nitrogen non-thermal plasma in a corona reactor: the role of reactive nitrogen species[END_REF].

A large variety of discharge configurations has been used to produce plasma either directly in liquid (i.e., with both electrodes submerged in liquid) or in the gas phase, in contact with liquid [START_REF] Bruggeman | Non-thermal plasmas in and in contact with liquids[END_REF][START_REF] Jiang | Review on electrical discharge plasma technology for wastewater remediation[END_REF]Magureanu and Parvulescu 2016;[START_REF] Locke | Elementary chemical and physical phenomena in electrical discharge plasma in gas-liquid environments and in liquids[END_REF]). The early studies on plasma removal of aqueous pollutants addressed mainly organic dyes, due to facile observation of the solution decolorization [START_REF] Malik | Synergistic effect of pulsed corona discharges and ozonation on decolourization of methylene blue in water[END_REF][START_REF] Sugiarto | Oxidative decoloration of dyes by pulsed discharge plasma in water[END_REF][START_REF] Burlica | Organic dye removal from aqueous solution by glidarc discharges[END_REF][START_REF] Grabowski | Breakdown of methylene blue and methyl orange by pulsed corona discharge[END_REF][START_REF] Magureanu | Degradation of organic dyes in water by electrical discharges[END_REF][START_REF] Magureanu | Decomposition of methylene blue in water by corona discharges[END_REF]Stara et al. 2009). An analysis of reported results revealed that the energy efficiency of the process is significantly higher for the plasma in gas phase as compared to liquid-phase discharges, especially for large surface to volume ratio of the solution to be treated, i.e., in case of thin liquid films or liquid spray [START_REF] Malik | Water purification by plasmas: which reactors are most energy efficient?[END_REF].

Therefore, most of the recent studies on plasma degradation of water contaminants have been carried out using electrical discharges generated in gas phase in contact with liquid. Very simple geometries, such as corona, dielectric barrier discharges or gliding arc above liquid, have often been reported for the removal of antibiotics [START_REF] Shaer | Antibiotics degradation and bacteria inactivation in water by cold atmospheric plasma discharges above and below water surface[END_REF][START_REF] Smith | Mechanism of Ampicillin Degradation by Non-Thermal Plasma Treatment with FE-DBD[END_REF][START_REF] Sarangapani | Degradation kinetics of cold plasmatreated antibiotics and their antimicrobial activity[END_REF][START_REF] Xu | Degradation effect and mechanism of gas-liquid phase dielectric barrier discharge on norfloxacin combined with H 2 O 2 or Fe 2+[END_REF][START_REF] Zhang | Degradation of norfloxacin in aqueous solution by atmospheric pressure non-thermal plasma: mechanism and degradation pathways[END_REF][START_REF] Acayanka | Evaluation of energy balance in a batch and circulating non thermal plasma reactors during organic pollutant oxidation in aqueous solution P[END_REF]) and pesticides [START_REF] Hijosa-Valsero | Removal of priority pollutants from water by means of dielectric barrier discharge atmospheric plasma[END_REF][START_REF] Hu | Degradation of selected organophosphate pesticides in wastewater by dielectric barrier discharge plasma[END_REF][START_REF] Li | Degradation of nitenpyram pesticide in aqueous solution by low-temperature plasma[END_REF][START_REF] Singh | Rapid removal of carbofuran from aqueous solution by pulsed corona discharge treatment: kinetic study, oxidative, reductive degradation pathway, and toxicity assay[END_REF][START_REF] Singh | Removal of 2,4-dichlorophenoxyacetic acid in aqueous solution by pulsed corona discharge treatment: effect of different water constituents, degradation pathway and toxicity assay[END_REF]. Since the penetration depth of the plasma-generated reactive species into the solution is very small, in such configurations, the volume of plasma-treated solution is generally very small, in the milliliter range [START_REF] Smith | Mechanism of Ampicillin Degradation by Non-Thermal Plasma Treatment with FE-DBD[END_REF][START_REF] Zhang | Degradation of norfloxacin in aqueous solution by atmospheric pressure non-thermal plasma: mechanism and degradation pathways[END_REF][START_REF] Xu | Degradation effect and mechanism of gas-liquid phase dielectric barrier discharge on norfloxacin combined with H 2 O 2 or Fe 2+[END_REF]. [START_REF] Smith | Mechanism of Ampicillin Degradation by Non-Thermal Plasma Treatment with FE-DBD[END_REF] reported the complete removal of ampicillin in 1 mL solution of high concentration (20 mM) after only 3 min of treatment with a dielectric barrier discharge above liquid. When treatment of larger solution volumes is attempted, the time required for pollutant removal becomes much longer. Using a pin-to-water corona 2020) obtained almost complete removal of doxycycline with concentration of 50 mg/L in 50 mL water after 90 min treatment, while the degradation of oxytetracycline was even slower. [START_REF] Acayanka | Evaluation of energy balance in a batch and circulating non thermal plasma reactors during organic pollutant oxidation in aqueous solution P[END_REF] needed 120 min to remove approximately 80% of the initial amoxicillin in 500 mL 0.1 mM aqueous solution using a gliding arc above liquid. Obviously, the treatment time is not an accurate measure of the efficiency of the plasma process, since the degradation depends on a number of factors, such as the molecular structure of the target compound, its concentration, the solution volume and properties, the gaseous atmosphere, the input power and the discharge characteristics, to name only a few. An example to illustrate the large extent of this influence is the comparison between the abovementioned results of El [START_REF] Shaer | Antibiotics degradation and bacteria inactivation in water by cold atmospheric plasma discharges above and below water surface[END_REF] and the data reported by [START_REF] Singh | Rapid removal of carbofuran from aqueous solution by pulsed corona discharge treatment: kinetic study, oxidative, reductive degradation pathway, and toxicity assay[END_REF][START_REF] Singh | Removal of 2,4-dichlorophenoxyacetic acid in aqueous solution by pulsed corona discharge treatment: effect of different water constituents, degradation pathway and toxicity assay[END_REF], who also used a pin-to-water corona to degrade the pesticides carbofuran and 2,4-D and achieved complete removal within less than 10 min treatment. Several authors provided information on the energy efficiency of the removal process. For instance, [START_REF] Xu | Degradation effect and mechanism of gas-liquid phase dielectric barrier discharge on norfloxacin combined with H 2 O 2 or Fe 2+[END_REF] investigated the degradation of norfloxacin (initial concentration 10 mg/L in 10 mL water) by a dielectric barrier discharge. Although fast removal of the antibiotic has been achieved (4 min), the reported efficiency (defined as the amount of pollutant removed per unit of energy consumed in the process) was rather low, i.e., in the range of tens of mg/kWh. The addition of H 2 O 2 and Fe 2+ significantly improved the results, optimum catalyst concentration leading to the reduction of treatment time to 0.5 min. The positive effect of iron catalyst was attributed to the Fenton reaction in the presence of plasma-generated H 2 O 2 [START_REF] Li | Degradation of nitenpyram pesticide in aqueous solution by low-temperature plasma[END_REF][START_REF] Jović | Effect of different catalysts on mesotrione degradation in water falling film DBD reactor[END_REF][START_REF] Aziz | Comparative study on 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol removal from aqueous solutions via ozonation, photocatalysis and nonthermal plasma using a planar falling film reactor[END_REF][START_REF] Xu | Degradation effect and mechanism of gas-liquid phase dielectric barrier discharge on norfloxacin combined with H 2 O 2 or Fe 2+[END_REF].

A comparison between the corona discharge above liquid and a corona generated in gas bubbles inside the solution demonstrated much faster degradation of the target antibiotics in the second case [START_REF] Shaer | Antibiotics degradation and bacteria inactivation in water by cold atmospheric plasma discharges above and below water surface[END_REF]. This discharge geometry, with one or several hollow needles submerged in liquid as high voltage electrode and plasma produced in gas bubbles at the tip of the needles, has been employed by several research groups for the removal of antibiotics, mostly in combination with catalysts (Wang et al. 2018d;Guo et al. 2019aGuo et al. , 2019bGuo et al. , 2019c)). A slightly different configuration, with the gas blown through a tube containing the high voltage needle electrode, has also been used [START_REF] He | Aqueous tetracycline degradation by non-thermal plasma combined with nano-TiO 2[END_REF][START_REF] He | Decomposition of tetracycline in aqueous solution by corona discharge plasma combined with a Bi 2 MoO 6 nanocatalyst[END_REF][START_REF] Hu | Degradation and mineralization of ciprofloxacin by gas-liquid discharge nonthermal plasma[END_REF]. Several authors produce plasma in the gas and bubble the effluent gas through the solution to be treated [START_REF] Kim | Degradation of veterinary antibiotics by dielectric barrier discharge plasma[END_REF][START_REF] Kim | Elucidation of the degradation pathways of sulfonamide antibiotics in a dielectric barrier discharge plasma system[END_REF][START_REF] Lee | Degradation of sulfonamide antibiotics and their intermediates toxicity in an aeration-assisted non-thermal plasma while treating strong wastewater[END_REF]Tang et al. 2018aTang et al. , 2018b[START_REF] Tang | Percarbonate promoted antibiotic decomposition in dielectric barrier discharge plasma[END_REF]Wang et al. 2019c;Li et al. 2020a). It is unlikely that highly reactive species with short lifetime would reach the liquid, so in this case plasma is simply used as a source of ozone.

Besides the generation of large amounts of reactive species in the plasma, their efficient transfer to the treated liquid is essential for the enhancement of pollutants removal efficiency. This has been demonstrated for instance in the experiments of [START_REF] Acayanka | Evaluation of energy balance in a batch and circulating non thermal plasma reactors during organic pollutant oxidation in aqueous solution P[END_REF], where the authors compared the degradation of amoxicillin by a gliding arc plasma above the target solution with the results obtained when the solution is sprayed through the discharge region. Faster degradation of the antibiotic has been reported for the spray configuration than in the batch mode, i.e., three times larger rate constant, and 2.5 times higher energy yield. [START_REF] Hijosa-Valsero | Removal of priority pollutants from water by means of dielectric barrier discharge atmospheric plasma[END_REF] have also confirmed the importance of large surface-to-volume area by comparing the removal of several pesticides in water using either a batch dielectric barrier discharge geometry or a dielectric barrier discharge with falling liquid film. The energy efficiency for the removal of atrazine is 10 times higher in the configuration with liquid circulation, while for the insecticides lindane and chlorfenvinfos, it exceeds one order of magnitude. Such more elaborated reactor design has been extensively investigated for the degradation of various pesticides [START_REF] Hijosa-Valsero | Removal of priority pollutants from water by means of dielectric barrier discharge atmospheric plasma[END_REF][START_REF] Jović | Degradation of triketone herbicides, mesotrione and sulcotrione, using advanced oxidation processes[END_REF][START_REF] Jović | Effect of different catalysts on mesotrione degradation in water falling film DBD reactor[END_REF][START_REF] Vanraes | Removal of atrazine in water by combination of activated carbon and dielectric barrier discharge[END_REF][START_REF] Vanraes | Removal of several pesticides in a falling water film DBD reactor with activated carbon textile: energy efficiency[END_REF][START_REF] Bradu | Degradation of the chlorophenoxyacetic herbicide 2,4-D byplasma-ozonation system[END_REF][START_REF] Yu | Degradation of DEET in aqueous solution by water falling film dielectric barrier discharge: effect of three operating modes and analysis of the mechanism and degradation pathway[END_REF][START_REF] Aziz | Comparative study on 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol removal from aqueous solutions via ozonation, photocatalysis and nonthermal plasma using a planar falling film reactor[END_REF]) and antibiotics [START_REF] Magureanu | Degradation of antibiotics in water by non-thermal plasma treatment[END_REF]Rong and Sun 2014;Rong et al. 2014;[START_REF] Xin | Degradation of triclosan in aqueous solution by dielectric barrier discharge plasma combined with activated carbon fibers[END_REF][START_REF] Iervolino | Enhanced removal of water pollutants by dielectric barrier discharge nonthermal plasma reactor[END_REF]. Most authors used coaxial geometry, with the liquid pumped upward through a cylindrical inner electrode and then flowing as a thin film on the outer surface of this tube, thus being in direct contact with plasma. However, planar geometries have also been employed, with the liquid film flowing either horizontally [START_REF] Xin | Degradation of triclosan in aqueous solution by dielectric barrier discharge plasma combined with activated carbon fibers[END_REF] or vertically [START_REF] Aziz | Comparative study on 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol removal from aqueous solutions via ozonation, photocatalysis and nonthermal plasma using a planar falling film reactor[END_REF]) between the electrodes. One of the challenges in the dielectric barrier discharge with falling liquid film is to produce a stable thin film of liquid. Hama [START_REF] Aziz | Comparative study on 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol removal from aqueous solutions via ozonation, photocatalysis and nonthermal plasma using a planar falling film reactor[END_REF] obtained a homogeneous and stable solution layer of thickness estimated to 150 µm, flowing along large area (68 × 29 cm) glass sheets and used this planar dielectric barrier discharge configuration to degrade the herbicide 2,4-D. A comparison between plasma treatment and other advanced oxidation processes from the point of view of the energy yield revealed that the dielectric barrier discharge in combination with Fenton oxidation is the most efficient treatment process, followed in this order by ozonation, plasma alone, photocatalytic ozonation and, at last, photocatalysis. Although the good removal efficiency of ozonation is confirmed, the authors mention its major drawback related to low mineralization. For improved degradation of the target compound and its intermediate oxidation products, either photocatalytic ozonation, or plasma combined with Fenton oxidation are recommended [START_REF] Aziz | Comparative study on 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol removal from aqueous solutions via ozonation, photocatalysis and nonthermal plasma using a planar falling film reactor[END_REF].

It is now generally accepted that the addition of Fe 2+ to the plasma treatment significantly improves the removal of aqueous contaminants, another example being the herbicides mesotrione and sulcotrione [START_REF] Jović | Degradation of triketone herbicides, mesotrione and sulcotrione, using advanced oxidation processes[END_REF][START_REF] Jović | Effect of different catalysts on mesotrione degradation in water falling film DBD reactor[END_REF]. In this case, it has been found that the effect of Fe 2+ exceeds that of Mn 2+ and Co 2+ . The combination of plasma with TiO 2 catalysts also appears successful for pollutants removal. [START_REF] He | Aqueous tetracycline degradation by non-thermal plasma combined with nano-TiO 2[END_REF] reports a considerable rise in the removal rate of the antibiotic tetracycline, from 61.9 to 85.1%, accompanied by the enhancement of total organic carbon removal, from 25.3% with the plasma alone to 53.4% in the presence of TiO 2 .

Another approach to increase the efficiency of pollutants removal by plasma adopted by [START_REF] Vanraes | Removal of atrazine in water by combination of activated carbon and dielectric barrier discharge[END_REF][START_REF] Vanraes | Removal of several pesticides in a falling water film DBD reactor with activated carbon textile: energy efficiency[END_REF] is to locally enhance the pollutant concentration in the plasma region. They used a coaxial dielectric barrier discharge with falling film and added an activated carbon textile mesh with extremely large surface area over the inner electrode. This highly adsorptive material was found to significantly contribute to the removal of target compounds in plasma.

A method to improve the mass transfer of the plasmagenerated ozone into the treated liquid, already mentioned previously, is to bubble the effluent gas from the plasma through the solution [START_REF] Gerrity | An evaluation of a pilot-scale nonthermal plasma advanced oxidation process for trace organic compound degradation[END_REF][START_REF] Magureanu | Degradation of antibiotics in water by non-thermal plasma treatment[END_REF][START_REF] Bradu | Degradation of the chlorophenoxyacetic herbicide 2,4-D byplasma-ozonation system[END_REF]. Very high energy efficiency has been reported in suchdual plasma-ozonation systems, either employing dielectric barrier discharge with falling liquid film for the removal of β-lactam antibiotics (reaching 105 g/ kWh for amoxicillin) [START_REF] Magureanu | Degradation of antibiotics in water by non-thermal plasma treatment[END_REF], or with a corona discharge above liquid to degrade the herbicide 2,4-D (5 g/kWh) [START_REF] Bradu | Degradation of the chlorophenoxyacetic herbicide 2,4-D byplasma-ozonation system[END_REF].

One of the most efficient plasma systems reported up to now is based on a pulsed corona reactor similar to an electrostatic precipitator, with the liquid introduced as droplets or jets through the plasma zone and short high voltage pulses [START_REF] Panorel | Oxidation of aqueous pharmaceuticals by pulsed corona discharge[END_REF][START_REF] Preis | Pulsed corona discharge: the role of ozone and hydroxyl radical in aqueous pollutants oxidation[END_REF]. Energy yields of tens of g/kWh have been achieved in this system for the removal of various pharmaceuticals and these high values have been attributed to the large contact area between the plasma and the liquid [START_REF] Ajo | Pulsed corona discharge in water treatment: the effect of hydrodynamic conditions on oxidation energy efficiency[END_REF]. This configuration has been adapted for the treatment of hospital wastewater at pilot scale (50 L) [START_REF] Ajo | Hospital wastewater treatment with pilot-scale pulsed corona discharge for removal of pharmaceutical residues[END_REF] and tests have been run with promising results for both untreated sewage of a public hospital and for biologically treated wastewater effluent of a health care institute.

Another report of a pilot-scale plasma system has been done by [START_REF] Gerrity | An evaluation of a pilot-scale nonthermal plasma advanced oxidation process for trace organic compound degradation[END_REF] using the reactor developed by Aquapure Technologies Ltd. for the degradation of several pharmaceuticals in trace concentrations. The pilot unit contains a plasma reactor, based on a pulsed corona above water, and an ozone contactor which uses the ozone-rich gas from the plasma reactor. The tests have been done on tertiary-treated wastewater and spiked surface water with contaminants concentrations of tens to hundreds of ng/L. Rapid degradation of the target compounds has been demonstrated and the authors concluded that plasma treatment may be a possible alternative to more common advanced oxidation processes, since the energy requirements for pollutants degradation are comparable and no additional feed chemicals are needed.

To increase the performance of non-thermal plasma processes for the degradation of harmful water pollutants, significant research efforts have been focused on the optimization of the electrical discharge, aimed at improving the energy yield. Numerous studies have been dedicated to the understanding of the process chemistry through investigation of the plasma-generated reactive species, as well as the identification and quantification of the intermediate degradation products.

An issue that requires more attention is the characterization of the plasma treated water from both chemical and (eco)toxicological points of view. There are only few studies dealing with this complex characterization and more efforts are needed in order to correctly evaluate the plasma treatment performances. Nevertheless, it is worth mentioning that progress has been made in developing new combined processes such as plasma-ozonation or plasma-catalysis and performing tests on real wastewater effluents (like those from hospitals) at pilot-scale. In this context, non-thermal plasma appears to be a new potential candidate for water treatment used for emerging contaminants removal.

Conclusion

We have described advanced treatment methods for treating emerging contaminants. Extensive research on non-conventional adsorbents highlights the growing interest of scientists in developing systems that are increasingly effective in removing mixtures of trace pollutants, simple to implement from a technological point of view, economically viable and ecofriendly, with little or no impact on the environment. Materials such as cyclodextrin polymers, metal-organic frameworks, molecularly imprinted polymers, chitosanbased materials and nanocelluloses have great potential in environmental applications. However, they are still at the laboratory study stage. Further research is needed to determine the means of integration of these adsorbents into full scale treatment plants. Among the various disinfection methods, chlorination and ozonation appear to give better results in terms of removing both pathogens and emerging contaminants. However, additional investigations are needed to determine the possible toxic characteristics of disinfection by-products compared to their parent compounds, as well as the possible synergic adverse effects of cocktails of by-products even at trace concentrations. The approach consisting in the use of ozonation and adsorption on activated carbon has been used for about ten years in countries such as Switzerland and Germany, due to its efficiency, simplicity and technical feasibility at industrial scale, and for economic reasons. Another feature of these technologies is their facile integration into existing treatment facilities. Biological approaches such as constructed wetlands, biomembrane reactors, strategies based on the use of algae, fungi and bacteria, and enzymatic degradation are also a field of research in full development and significant advances are expected in the future. Finally, advanced oxidation processes represent one of the most promising strategies because of their efficiency and simplicity; they can also be integrated into existing treatment facilities as primary, secondary and/or tertiary methods. Significant advances are expected in the next few years, although here again, investment, operation and maintenance costs must be considered. Industrialists will now have to be persuaded to use these technologies in their municipal wastewater treatment plants.

Fig. 1

 1 Fig. 1 Over-consumption of pharmaceuticals and personal care products leads to contamination of rivers via wastewater from sewage treatment plants. Sinaloa, Mexico. source: Lorenzo A. Picos-Corrales, Sinaloa, Mexico

Fig. 5

 5 Fig. 5 Advanced oxidation processes for wastewater treatment. Source Corina Bradu, Bucharest, Romania

Fig. 6

 6 Fig. 6 Radical pathway of catalytic ozonation process source Mohammad Mahmudul Huq, Saskatoon, Canada

Fig. 7

 7 Fig. 7 Experimental set-up for plasma-ozonation system. Source: Monica Magureanu, Magurele, Romania

  photo-Fenton: Fe 2+ /H 2 O 2 /UV) or heterogeneous systems (heterogeneous photo-catalysis: TiO 2 /H 2 O 2 /UV; anodic oxidation or photo-electrocatalysis). Different energy sources and/or catalysts are employed. The heterogeneous catalysis is often preferred to the homogeneous processes, due to the easier recovery of the catalyst. According to the energy source, there are three sub-types of processes, those using:

	(i) UV radiation (O 3 /UV, H 2 O 2 /UV, O 3 /H 2 O 2 /UV, or photo-Fenton Fe 2+ /H 2 O 2 /UV); (ii) ultrasound energy (O 3 /ultra-sound, H 2 O 2 /ultrasound) and (iii) electrical energy (elec-
	trochemical oxidation, anodic oxidation and electro-Fenton).

Acknowledgements NM-C and GC (Besançon, France) thanks the FEDER (Fonds Européen de Développment Régional) for its financial support (NIRHOFEX Program: "Innovative materials for wastewater treatment") and the Université de Franche-Comté (France) for the research grant awarded to Guest Professor CB. ARLRo (Porto, Portugal) would like to thank FCT funding under the DL57/2016 Transitory Norm Programme and the scientific collaboration under projects: PTDC/QUI-QAN/30521/2017-POCI-01-0145-FEDER-030521funded by FEDER funds through COMPETE2020-Programa Operacional Competitividade e Internacionalização (POCI) and by national funds (PIDDAC) through FCT/MCTES and Base-UIDB/50020/2020 and Programmatic-UIDP/50020/2020 funding of LSRE-LCM of the Associate Laboratory LSRE-LCM, funded by national funds through FCT/MCTES (PIDDAC). MM and CB (Romania) acknowledges financial support from Ministry of Research, Innovation and Digitization CNCS/CCCDI-UEFISCDI-projects no. PN-IV-PCE143/2021 and PN-III-P2-2.1-26PTE/2020, respectively. Francesca Mapelli (Milan, Italy) acknowledges the Cariplo Foundation for its financial support (GA n° 2018-0995-WARFARE "Novel wastewater disinfection treatments to mitigate the spread of antibiotic resistance in agriculture").

Declarations

Conflict of interest The authors declares no conflict of interest.