Pierre Legout

Guillaume Lefebvre

Marie Bonnin

Jean-Christophe Gimel

Lazhar Benyahia

Olivier Colombani

Brice Calvignac

Synthesis of PDMS-b-POEGMA diblock copolymers and their application for the thermo-responsive stabilization of watersupercritical carbon dioxide emulsions

Keywords: supercritical carbon dioxide, emulsion, amphiphilic block copolymer, surfactant, poly(dimethyl siloxane), poly(ethylene glycol methacrylate), interfacial tension, stimuli-responsive

In this study, PDMS13-b-POEGMAx diblock copolymers consisting of a CO2-philic poly(dimethylsiloxane) (PDMS) block connected to a thermosensitive hydrophilic poly(oligoethylene glycol methacrylate) (POEGMA) block, were synthetized by reversible addition-fragmentation chain-transfer (RAFT) radical polymerization. Their ability to decrease the water-supercritical CO2 (scCO2) interfacial tension (g) and to stabilize water-scCO2 emulsions was investigated using an original home-made device developed in the laboratory.

This device is able to control the pressure from 1 to 250 bar and the temperature from 40 to 80°C. It was implemented with 2 visualization windows, a drop tensiometer and a remote optical head for dynamic light scattering (DLS) measurements. These experiments revealed that PDMS-b-POEGMA decreased g down to 1-2 mN/m and was the most efficient at high pressure (250 bar) and low temperature (40°C) where PDMS and POEGMA blocks exhibited the highest affinity for their respective phase. The diblock copolymers were shown to stabilize water-scCO2 emulsions.

Moreover, the thermosensitive behavior of the POEGMA block in water (with a lower critical solubility temperature around 65°C) resulted in the formation of temperature-responsive emulsions that could reversibly switch at 100 bar from stable at 40°C to unstable at 80°C. These results were rationalized based on the solubility of each individual block of the copolymers in water and scCO2 as a function of temperature and pressure.

Introduction

Supercritical carbon dioxide (scCO2), due to its unique properties (e.g., nontoxicity, nonflammability, moderate critical temperature and pressure) is increasingly considered for various industrial applications (e.g. extraction, foaming, chemical reaction, dry cleaning). [START_REF] Bertucco | High Pressure Process Technology Fundamentals and Applications[END_REF] Over the last 20 years, a special focus was set on the production of water-in-scCO2 (W/C) or scCO2-in-water (C/W) emulsions and microemulsions 2 as they can act as "universal solvents" by dissolving simultaneously polar, ionic and nonpolar molecules. These dispersed systems have numerous fields of application, including chemical processing, solubilization and separation, synthesis of polymer colloids or inorganic particles. [START_REF] Lee | Water-in-Carbon Dioxide Emulsions: Formation and Stability[END_REF][START_REF] Johnston | Emulsions and Latexes[END_REF][START_REF] Johnston | Colloids in Supercritical Fluids over the Last 20 Years and Future Directions[END_REF][START_REF] Beuvier | Synthesis of Hollow Vaterite CaCO3 Microspheres in Supercritical Carbon Dioxide Medium[END_REF] The stabilization of C/W or W/C emulsions can be achieved with molecular surfactants [START_REF] Harrison | Water-in-Carbon Dioxide Microemulsions with a Fluorocarbon-Hydrocarbon Hybrid Surfactant[END_REF][START_REF] Ryoo | Water-in-Carbon Dioxide Microemulsions with Methylated Branched Hydrocarbon Surfactants[END_REF][START_REF] Sagisaka | Hyperbranched Hydrocarbon Surfactants Give Fluorocarbon-like Low Surface Energies[END_REF][START_REF] Sagisaka | Water-in-CO2 Microemulsions Stabilized by Fluorinated Cation-Anion Surfactant Pairs[END_REF] or amphiphilic block [START_REF] Girard | Structure-Property Relationships in CO2-Philic (Co)Polymers: Phase Behavior, Self-Assembly, and Stabilization of Water/CO2 Emulsions[END_REF][START_REF] Da Rocha | Effect of Surfactants on the Interfacial Tension and Emulsion Formation between Water and Carbon Dioxide[END_REF][START_REF] Psathas | Water-in-Carbon Dioxide Emulsions with Poly(Dimethylsiloxane)-Based Block Copolymer Ionomers[END_REF] copolymers which lower the water-scCO2 interfacial tension (g) and sterically and/or electrostatically hinder the coalescence of water or scCO2 droplets. [START_REF] Ryoo | Long-Ranged Electrostatic Repulsion and Crystallization of Emulsion Droplets in an Ultralow Dielectric Medium Supercritical Carbon Dioxide[END_REF] Alternatively, solid particles can also lead to very stable water-scCO2 emulsions [START_REF] Johnston | Colloids in Supercritical Fluids over the Last 20 Years and Future Directions[END_REF][START_REF] Adkins | Water-in-Carbon Dioxide Emulsions Stabilized with Hydrophobic Silica Particles[END_REF] through the Pickering effect. [START_REF] Yang | An Overview of Pickering Emulsions: Solid-Particle Materials, Classification, Morphology, and Applications[END_REF][START_REF] Chevalier | Emulsions Stabilized with Solid Nanoparticles: Pickering Emulsions[END_REF] CO2-philic copolymers used to stabilize water-scCO2 emulsions were extensively reviewed by Girard et al., [START_REF] Girard | Structure-Property Relationships in CO2-Philic (Co)Polymers: Phase Behavior, Self-Assembly, and Stabilization of Water/CO2 Emulsions[END_REF] focusing on their interactions with CO2. Among the different polymers reviewed, those exhibiting the highest affinity for CO2 are poly(vinyl acetate) < poly(dimethyl siloxanes) (PDMS) < poly(fluoroalkyl acrylates), in this order. [START_REF] Shen | CO2-Solubility of Oligomers and Polymers That Contain the Carbonyl Group[END_REF] Moreover, shorter polymers have a higher solubility in scCO2 for entropic reasons. [START_REF] Lee | The Solubility of Low Molecular Weight Poly(Dimethyl Siloxane) in Dense CO2 and Its Use as a CO2-Philic Segment[END_REF] Only few studies were devoted to PDMS-based amphiphilic copolymers and their use as emulsifiers.

Indeed, the strong affinity of PDMS for CO2 suggests that amphiphilic PDMS-based copolymers should strongly reduce g and successfully stabilize W/C or C/W emulsions. Amphiphilic copolymers with a rather short PDMS block (from 1,000 to 10,000 g/mol) were connected to either a poly(2-hydroxymethacrylate) (PHEMA) block, [START_REF] Psathas | Mapping the Stability and Curvature of Emulsions of H 2 O and Supercritical CO 2 with Interfacial Tension Measurements[END_REF] poly(ethylene oxide) (PEO) grafts, [START_REF] Da Rocha | Effect of Surfactants on the Interfacial Tension and Emulsion Formation between Water and Carbon Dioxide[END_REF] or a pH-sensitive poly(acrylic acid) (PAA) [START_REF] Psathas | Water-in-Carbon Dioxide Emulsions with Poly(Dimethylsiloxane)-Based Block Copolymer Ionomers[END_REF] block. These three polymer families were shown to stabilize W-C emulsions. PEO-b-PDMS-b-PEO triblocks were used to prepare W-C miniemulsions. [START_REF] Psathas | Formation of Carbon Dioxide in Water Miniemulsions Using the Phase Inversion Temperature Method[END_REF] No attempts were made to correlate the polymer chemical structure with its ability to reduce g or to stabilize emulsions.

To characterize the affinity of molecular surfactants or block copolymers with both the water and CO2 phases, the hydrophilic CO2-philic balance (HCB) parameter has been proposed in the literature (Eq. 1). [START_REF] Da Rocha | Effect of Surfactants on the Interfacial Tension and Emulsion Formation between Water and Carbon Dioxide[END_REF]

𝐻𝐶𝐵 =

! !" "! !! "! ""

! #$ "! ## "! $$ Equation 1
With A the cohesive energies between the different chemical species denoted by subscripts: T for the CO2philic part of the amphiphilic molecule, H for its hydrophilic part, W for water and C for CO2.

The HCB originates from the hydrophilic lipophilic balance (HLB) which characterizes a surfactant used in a water-oil system. [START_REF] Johnston | Colloids in Supercritical Fluids over the Last 20 Years and Future Directions[END_REF][START_REF] Griffin | Calculation of HLB Values of Non-Ionic Surfactants[END_REF] However, unlike the HLB, which is only based on the surfactant chemical structure, the theoretical HCB calculation takes into account all cohesive energies involved and thus all interactions of the surfactant molecule with its environment under specific conditions (e.g. temperature, pressure, salinity).

With these considerations, the HCB calculation relates more closely to the extended version of the Winsor R ratio [START_REF] Becher | Microemulsions and Related Systems: Formulation, Solvency, and Physical Properties[END_REF] or to the hydrophilic lipophilic difference (HLD) calculation produced by Salager, [START_REF] Salager | Optimum Formulation of Surfactant/Water/Oil Systems for Minimum Interfacial Tension or Phase Behavior[END_REF][START_REF] Salager | Partitioning of Ethoxylated Octylphenol Surfactants in Microemulsion-Oil-Water Systems: Influence of Temperature and Relation between Partitioning Coefficient and Physicochemical Formulation[END_REF] both developed for water-oil systems. Due to the difficulty to determine the cohesive energy values in equation 1, the HCB is mainly used as a qualitative concept to assess the influence of all involved interactions in the presence of a surfactant at a water-CO2 interface (Figure 1) (we only found one exception where the HCB was calcula to Figure 3 1, when the HCB value is more than one (respectively less than one), the amphiphilic molecule or copolymer is considered as predominantly hydrophilic (respectively CO2-philic) and leads preferentially to the formation of C/W (respectively W/C) emulsions in accordance to the Bancroft's rule. [START_REF] Johnston | Colloids in Supercritical Fluids over the Last 20 Years and Future Directions[END_REF][START_REF] Bancroft | (Ethylene Glycol) Methyl Ether Methacrylate[END_REF] When the HCB reaches a value close to one, the amphiphilic molecule or copolymer shows a balanced state with equal affinities for water and CO2, leading to a minimum g. AIBN (Aldrich, 98%) was recrystallized from methanol and OEGMA 300 g/mol (Aldrich) was purified on a basic alumina column before use to remove inhibitor.

Characterization techniques

Nuclear magnetic resonance spectroscopy

Nuclear Magnetic Resonance (NMR) spectra were recorded using deuterated chloroform (CDCl

Size Exclusion Chromatography

Size exclusion chromatography (SEC) was performed using a system made of a guard column S2).

Synthesis

Synthesis of PDMS13-CTA 4-(dimethylamino)pyridine (0.12 g, 0.1 mmol, 0.15 eq), PDMS-OH (7.5 g, 6.8 mmol, 1 eq), CH2Cl2

dry (30 g) and 4-(cyano-dithiobenzoate) pentanoic acid (2.9 g, 10 mmol, 1.5 eq) were added in a 100 mL roundbottom flask equipped with a stir bar and an addition funnel. The solution was stirred and degassed with argon for 10 minutes. DCC (2.1 g, 10 mmol, 1.5 eq) was dissolved in dry CH2Cl2 (8 g) and added dropwise to the reaction flask at 0°C. Then, the reaction medium was stirred at room temperature overnight. The solid was removed by filtration, and the filtrate was washed once with an aqueous solution of sodium bicarbonate (20 mL, 10 % w/w) and twice with pure water (2⨯20 mL). The filtrate was then dried with anhydrous MgSO4 and concentrated on a rotary evaporator before precipitation of the polymer in a water/methanol mixture (20/80 v/v).

A liquid pinkish polymer was finally obtained (6.2 g, yield = 65 %).

Synthesis of PDMS13-b-POEGMAx

In a typical run corresponding to PDMS13-b-POEGMA46, OEGMA (20 g, 66.7 mmol, 40 eq), AIBN initiator (0.05 g, 3.3 mmol, 0.2 eq), PDMS-CTA (2 g,

POEGMA -(CH2-CH2-O)4-5-CH3; 3.45, t, 2H, -CH2-O- CH2-CH2-O-(C=O)-); 3.6, m, 16H×DPn(POEGMA) - (C=O)-CH2-CH2-O-(CH2-CH2-O)3-4-CH3; 3.66, t, 2H, - CH2-O-CH2-CH2-O-(C=O)-)); 4.1 (92 H, m, 2H×DPn(POEGMA) corresponding to DPn(POEGMA) = 46, -(C=O)-CH2-CH2-O-(CH2-CH2-O)3-4-CH3); 4.3 (m, 2H, -CH2-O-CH2-CH2-O-(C=O)-); 7.42 (m, 2H, m- ArH); 7.56 (m, 1H, p-ArH); 7.9 (dt, 2H, 3 J(H,H) = 8.4
Hz, 4 J(H,H) = 1.2 Hz, 2H, o-ArH).

Size exclusion chromatography for PDMS13-b-

POEGMA46: Mn = 16,000 g/mol, Ɖ = 1.1.

See Table 1 for the characteristics of the other polymers.

Interfacial tension measurements

The interfacial tension measurements were performed on a high pressure, high temperature homemade device (Figure 2) developed in partnership with the stability of the emulsions in the variable volume view cell (on the left). In the latter, two windows allowed the macroscopic observation of the water-scCO2 mixture. Moreover, a deported DLS apparatus was installed on the bottom observation window for further measurements (see section 6 of the experimental section).

Formation of water-carbon dioxide emulsions and stability

Emulsions of water and scCO2 were prepared in the variable volume view cell of the home-made device (Figure 2). First, the studied copolymer was solubilized in deionized water and placed in the view cell. Then, scCO2 was injected to the desired volume fraction and the pressure was controlled with the syringe pump. The temperature of the apparatus was controlled with the heated bath. The copolymer/water/scCO2 mixture was emulsified with a 28 mm diameter four bladed propeller at 900 rpm for 15 min, resulting in the formation of a white and turbid emulsion (see Figure 8b). Then, the stirring was stopped, and the emulsions were characterized visually. DLS measurements were conducted on the aqueous bottom phase which appeared after creaming of the emulsion with a Cordouan® particle size analyzer (VASCO Kin) operating at a wavelength of 638 nm and a fixed scattering angle of 170°. The remote head of the apparatus was positioned at the bottom of the first window as depicted on Figure 2. Traces of the scattered light intensity and intensitytime correlation functions, g2(Dt), with Dt the delay time, were recorded in the bottom phase before (aqueous polymer solution) and after creaming of the emulsion. As measurements were not performed on diluted systems in the limit of low angles, only apparent sizes could be obtained.

Cloud point measurements

The cloud points of the polymers were measured both for the diblock copolymers and for their constituting homopolymeric blocks (PDMS and POEGMA).

Cloud point pressure of the homopolymers and of the diblock copolymers in scCO2

The cloud point pressure in scCO2 was measured both for the diblock copolymers and for their constituting homopolymeric blocks according to Wang and al. The linear evolution of the number average molecular weights (Mn) of the diblock copolymers and the decrease of their dispersity (Ɖ) with conversion indicate a good control of the polymerization of the POEGMA blocks (Figure 4). Additionally, Figure 5 shows that the chromatograms of the macro-RAFT agents shifted to higher molecular weights upon polymerization, confirming the formation of diblock copolymers.

However, UV detection revealed that part of the macro-CTA was not converted into a diblock copolymer (Figure 5a), resulting in higher molecular weights of the POEGMA blocks than initially targeted (Table 1). Pure diblock copolymers could be obtained after removing unreacted PDMS-CTA by selective precipitation in pentane/diethyl ether (80/20 v/v), see Figure 5b.

Considering the conversion and percentage of PDMS-CTA that effectively reacted (see Table 1 and Figure 4a), the experimental molecular weights were consistent within experimental incertitude with the initial First, the effect of the polymer concentration was investigated at 40°C and 250 bar. The γ value of 24 mN/m obtained without polymer (Table 2) was in accordance with the values in the literature under the same conditions of T and P after an equilibration time of few minutes. [START_REF] Georgiadis | Interfacial Tension Measurements of the (H2O + CO2) System at Elevated Pressures and Temperatures[END_REF] Moreover, the results presented in Table 2 show that the PDMS13-b-POEGMAx decrease γ Note: Three measurements on three different droplets were conducted for each condition with a maximum variation of 10% around the average given in this table (all values can be found in Table S2).

This very effective reduction of γ could be attributed to the selective affinity of each block for their respective phase. Furthermore, it is worth mentioning that 0.1 % w/v polymer is sufficient to considerably reduce the surface tension whatever the degree of polymerization of the POEGMA block (x). Increasing the concentration of the polymer, even by a factor of 10, contributed only to a modest additional reduction of the interfacial tension. This indicates that with a low amount of polymer, the water-scCO2 interface is already almost saturated. Therefore, the critical micellar concentration (CMC) of the polymer is probably below 0.1 % w/v under these conditions.

We have considered the highest concentration (1 % w/v) in the following experiments where the effect of P and T was investigated (Table 3). Note: Three measurements on three different droplets were conducted for each condition with a maximum variation of 10% around the average given in this table (all values can be found in Table S3).

The γ values measured at different pressures without polymer (Table 3) are in accordance with the literature.

In fact, γ decreases almost linearly but slightly with pressure (Figure 6); which is explained by the decrease of the density difference between water and scCO2. [START_REF] Da Rocha | Effect of Surfactants on the Interfacial Tension and Emulsion Formation between Water and Carbon Dioxide[END_REF][START_REF] Georgiadis | Interfacial Tension Measurements of the (H2O + CO2) System at Elevated Pressures and Temperatures[END_REF] The results show that an increase of P induces a slight decrease of γ both with and without polymer (Table 3).

This will be described hereafter considering the effect of temperature as well. The effect of P on g is stronger for higher x values. A significant decrease of g is observed with the decrease of x at 100 bar, whereas this effect progressively vanishes by increasing the pressure. The behavior of PDMS13-b-POEGMA46 at 80°C can be explained in view of the HCB concept. Indeed, with a nonionic copolymer such as the PDMS13-b-POEGMA46, the increase of the CO2 density caused by the increase of pressure induced an increase of the interaction energy between CO2 and the CO2-philic PDMS block (noted ATC in Eq. 1). Considering the large weight fraction of the POEGMA block (Table 1), we assume that PDMS13-b-POEGMA46 has a hydrophilic tendency (HCB > 1). As a consequence, an increase of ATC with the pressure can be correlated with a better partition of the polymer at the water-scCO2 interface and thus a decrease of γ (right part of the "v-shape" on Figure 1), which is in agreement with our results. 40°C and 100 bar, the POEGMA block is still highly soluble in the water phase, but the PDMS block is not soluble in the scCO2 phase. As the interfacial tension is still efficiently decreased under these conditions (albeit not as well as at higher pressures), the polymer still goes to the interface; probably in the form of unimers with a partly collapsed PDMS block. We stress that for the interfacial tension measurements, the diblock copolymer is initially dispersed in the water phase where it is macroscopically soluble (see section 4 of the SI). Moreover, for each interfacial measurement, a pendant drop is created only after T and P have reached an equilibrium. The fact that g is still reduced in these conditions (40°C, 100 bar) in the presence of polymer implies that the latter does move to the interface from the water phase in spite of the low affinity of the PDMS block for the scCO2 phase. Similarly, at 80°C and 250 bar, the PDMS block is still highly soluble in the scCO2 phase, but the POEGMA block is no more soluble in the water phase. In these conditions, the diblock copolymer is not soluble in the water phase (see section 4 of the SI). It follows that when the droplet is formed starting with the polymer inside the aqueous phase, the polymer is phase separated. It may therefore be argued that the reduction of g in these conditions results from the presence of ill-defined aggregates of PDMS13-b-POEGMA46 at the interface. However, a control experiment conducted by initially dissolving the polymer in the scCO2 phase where it is macroscopically soluble (see section 4) led to the same reduction of g. As a conclusion, the observed reduction of g at 80°C and 250 bar is probably due to the presence of unimers with a partly collapsed POEGMA block at the interface (Figure 7). does not go to the interface.

Stabilization of emulsions at 40°C

In view of their efficiency to reduce g, the diblock copolymers were tested as stabilizers for water-scCO2 emulsions. 50/50 water/scCO2 volume ratios were mechanically emulsified in a pressurized batch reactor in the presence of 0.5 % w/v of polymer with respect to the total volume. At 40°C and for pressures ranging from 100 to 250 bar, white opaque emulsions were obtained right after mixing in all cases (Figure 8b). After less than 5 min, creaming was observed leading to the appearance of a white opaque emulsion above and a slightly turbid (for PDMS13-b-POEGMA46 at P = 100-200 bar, Figure 8d) or nearly transparent (for all other conditions, Figure 8c) aqueous phase below. In spite of creaming, the upper phase remained white and opaque for at least 15 min (some samples were monitored for more than 30 min and a white opaque upper phase was still there); revealing that the copolymers limited the coalescence of droplets to some extent, while the coalescence occurred within seconds without polymer.

It was technically not possible to determine which phase was dispersed in the other. However, the fact that the amphiphilic copolymers had a predominantly hydrophilic character due to the weight fraction of the POEGMA block (see Table 1) suggests that scCO2 in water emulsions were formed according to Bancroft's rule. [START_REF] Johnston | Colloids in Supercritical Fluids over the Last 20 Years and Future Directions[END_REF][START_REF] Bancroft | (Ethylene Glycol) Methyl Ether Methacrylate[END_REF] This hypothesis is somewhat supported by the fact that the emulsions creamed (water in scCO2 emulsions would have sedimented). 4). For PDMS13-b-POEGMA16, DLS revealed a strong increase of the relaxation time after emulsification and creaming, suggesting the presence of very large scatterers which should have caused a strong increase of scattered light intensity and turbidity if their amount had been sufficient (see SI, Figure S9). For this polymer, the decrease of the scattered intensity in the aqueous phase after creaming could therefore only be attributed to a much lower polymer concentration in this phase than before emulsification. In other words, a significant quantity of the diblock copolymer was present in the upper emulsion and stabilized it. Moreover, both the decrease of scattered light intensity and the absence of significant turbidity (Figure 8c) of the creamed aqueous phase implied that the quantity of very large scatterers was actually negligible. We speculate that these large scatterers corresponded to a very small amount of scCO2 droplets which had not yet creamed.

The same conclusion could be drawn for PDMS13-b-POEGMA31 at 100 and 250 bar (see Table 4 and Figure S9). For PDMS13-b-POEGMA31 at 150 and 200 bar and for PDMS13-b-POEGMA46 at 250 bar, DLS revealed a small decrease of the size of the scatterers (see Figure S9) which could also account for the lower scattered intensity. This means that the relative polymer concentration in these conditions is probably higher than the relative scattered intensity indicated in Table 4.

Still, we hypothesize that the polymer concentration was also lower in this phase after emulsification and creaming considering 1) the trend observed in the other conditions and 2) the fact that the emulsion is stabilized only in the presence of polymer. We note that the relative decrease of scattered intensity in the aqueous phase after emulsification and creaming was much less pronounced for increasing x. This result seems consistent with the fact that the PDMS13-b-POEGMAx have a HCB > 1 and therefore a stronger affinity for the water than for the scCO2 phase: decreasing x leads to a more balanced affinity of the diblock copolymer for both phases and therefore to its migration to the emulsion rather than to the excess water phase. * For these samples, DLS revealed that the size of the scatterers was slightly smaller than before emulsification and creaming so that the decrease of scattered light intensity could be attributed either to a decrease of polymer concentration, or to a decrease of the size of the scatterers or to a combination of both.

This means that the relative polymer concentration in these conditions is probably higher than the value of the relative scattered intensity.

Thermo-responsive emulsions

Finally, the impact of the thermo-sensitive behavior of the POEGMA block on the stabilization of emulsions was tested by emulsifying a 50/50 water/scCO2 mixture in the presence of 0.5 % w/v of PDMS13-b-POEGMA46.

At 100 bar, a thermo-reversible emulsification was observed. Indeed, scCO2 and water formed a stable emulsion at 40°C which destabilized within few seconds into two transparent phases (Figure 8a) by increasing the temperature to 80°C. Attempts to re-emulsify the two phases at 100 bar and 80°C were not successful, whereas decreasing again the temperature to 40°C and stirring for a few minutes afforded again an emulsion.

This result was in perfect agreement with the understanding of the system depicted on figure 7: the polymer desorbed and adsorbed reversibly from the interface at 100 bar by increasing or decreasing the temperature.

The same experiment was also conducted at 250 bar.

At this pressure, it was not possible to emulsify water and scCO2 at 80°C. However, if the emulsion was first formed at 250 bar and 40°C and then heated up to 80°C, destabilization did not occur. This could probably be attributed to the fact that the diblock copolymer was not soluble in water at 80°C and 250 bar so that long times or strong shearing would have been necessary to bring enough polymer at the interface to emulsify the system.

On the contrary, by starting at 40°C and 250 bar, the polymer was already at the interface. Raising the temperature to 80°C probably caused a collapse of the POEGMA block but did not remove the polymer from the interface (indeed, it still exhibited some surface activity at 80°C and 250 bar as shown above, see Figure 6) and did not cause destabilization of the emulsion. It is even possible that the collapsed POEGMA blocks developed attractive interactions at the interface, strengthening the efficiency of the polymer protection at the interface due to the sticking of unimers at the interface.

Conclusion

To develop new potential applications for water-scCO2 emulsification processes, a family of well-defined oil/water Pickering emulsions. [START_REF] Tang | CO2-Switchable Pickering Emulsions: Efficient and Tunable Interfacial Catalysis for Alcohol Oxidation in Biphasic Systems[END_REF][START_REF] Tang | Stimuli-Responsive Pickering Emulsions: Recent Advances and Potential Applications[END_REF] Moreover, in our laboratory, we aim to develop sustainable water-scCO2 emulsification processes [START_REF] Beuvier | Synthesis of Hollow Vaterite CaCO3 Microspheres in Supercritical Carbon Dioxide Medium[END_REF] for which the droplets of the emulsions act as micro-or nanoreactors in which the CaCO3 crystallization is carefully controlled to produce nanostructured and porous particles 37 used as drug carriers. [START_REF] Tran | Lysozyme Encapsulation within PLGA and CaCO3 Microparticles Using Supercritical CO2 Medium[END_REF][START_REF] Hassani | Lysozyme Encapsulation into Nanostructured CaCO3 Microparticles Using a Supercritical CO2 Process and Comparison with the Normal Route[END_REF][START_REF] Zeynep | Double Hydrophilic Polyphosphoester Containing Copolymers as Efficient Templating Agents for Calcium Carbonate Microparticles[END_REF] These copolymers could be relevant in this context too.

Figure 1 .

 1 Figure 1. Schematic representation of the effect of various formulation parameters on ln(g) for a nonionic surfactant at the water-CO2 interface.

( 5

 5 μm beads, 50 × 7.5 mm) connected to a PLgel Mixed-D column (5 μm beads with a distribution of pore sizes from 50 Å to a few µm, 300 × 7.5 mm) and a PLgel column (5 μm beads with individual pore sizes of 50 Å, 300 × 7.5 mm

1. 6

 6 mmol, 1 eq) and 20 g of toluene were added in a Schlenk flask. Five freeze-pump-thaw-argon cycles were carried out to remove oxygen. Then, the reaction medium was heated in an oil bath at 70°C. To monitor polymerization kinetics, aliquots were regularly collected under argon and analyzed by 1 H NMR (monomer conversion) and SEC (Mn, Ɖ). The overall monomer conversion was determined by 1 H NMR spectroscopy in DMSO-d6 by the relative integration of the protons of the polymer at 4.1 ppm and the vinylic protons of the monomer at 5.6 and 6.1 ppm. The kinetic plots allowed determining the time required to reach the desired conversion. At this time, the reaction was stopped by cooling the reaction medium in an ice bath and opening it to air. The final conversion was confirmed by 1 H NMR. To purify the copolymer, it was precipitated in a mixture of 80 % pentane and 20 % diethyl ether (by volume). A viscous pinkish copolymer was finally obtained (7.5 g, yield = 63 %).

1 H

 1 NMR (CDCl3) ppm for PDMS13-b-POEGMA46: 0.1 (m, 82 H = 6×DPn(PDMS) + 6 corresponding to DPn(PDMS)= 13, CH3-Si-CH3); 0.5 (m, 4H, -CH2-Si); 0.6-2.8 (~286 H corresponding to the following protons: ~ 0.8 ppm, t, 3H, CH3-CH2-CH2-CH2-Si-; ~ 1 ppm, s, 3H×DPn(POEGMA), -CH2-C(CH3)-C=O; ~ 1.3 ppm, m, 4H, -CH2-CH2-Si-; ~ 1.7 ppm, m, 2H, CH3-CH2-CH2-CH2-Si-; ~ 1.8 ppm, m, 2H×DPn(POEGMA), -CH2-C(CH3)-C=O; 2.3 pm, s, 3H, CH3-C(CN)-); 2.4-2.8, m, 4H, -(C=O)-CH2-CH2-C-)); 3.0-3.8 (~892 H corresponding to the following protons: 3.3, s, 3H×DPn

Top 29 ∆𝑃

 29 Industrie and controlled by the Windrop® software (Teclis). This device was composed of a motorized and variable volume view cell (Top Industrie) for the scCO2 injection and a syringe pump (260D P2 ISCO, USA) for the injection of the aqueous solution containing the amphiphilic copolymer. This solution was injected in a 50 mL stirred measurement cell (Top Industrie) equipped with a light source. A video camera was mounted on an optical rail for proper alignment and connected to a computer for video acquisitions. The measurement cell initially contained a small amount of water (approx. 3 mL), stirred to ensure the CO2 phase saturation. The CO2 was first injected to the desired pressure and the cell was thermalized between 40 and 80°C with a heated bath (Fisherbrand Isotemp 6200H7).Then, the aqueous polymer solution was introduced in the measurement cell with the syringe pump to obtain a 5 µL pendant drop by setting a desired pressure (typically 3 to 5 bar above the pressure of the measurement cell to avoid backflow). Finally, the interfacial tension measurement between the aqueous solution and the water-saturated CO2 was started and maintained, keeping the volume of the droplet constant, until 𝛾 remained stable for at least 2 minutes. The interfacial tension measurements were carried out by fitting the Laplace equation (Eq. 2) to the profile of the recorded pendant drop images, where ∆P is the pressure difference across the water-CO2 interface, R0 is the radius of curvature at the apex of the drop, H0 and H1 are the principal radii of curvature and z is the vertical distance. measurements at different pressures and different temperatures were made with the same aqueous solution for each polymer and each concentration. However, for each measurement, a new droplet was formed after equilibration of T and P in the apparatus. After each measurement, a sufficient amount of aqueous solution was pushed into the droplet to detach it from the needle.

Figure 2 .

 2 Figure 2. Schematic view of the home-made device used to measure both g in the measurement cell (on the right) or

  Commercial PDMS-OH was used for the experiments with this block. The synthesis of the POEGMA homopolymer was achieved in conditions similar to those described for the PDMS13-b-POEGMAx diblock copolymers except that CTA-COOH was used as chain transfer agent instead of PDMS-CTA with a [OEGMA]0:[CTA-COOH]0:[AIBN]0 molar ratio of 40:1:0.2 and with a final conversion of 56 %. In scCO2, the cloud point was measured at 40°C as a function of P, whereas in water it was measured as a function of T for fixed P values.

30

 30

1 . 1 .

 11 using a high-pressure phase equilibrium apparatus (Separex, France). This high-pressure unit was composed of a variable volume view cell equipped with three sapphire windows and a mechanical stirrer with a pitched blade turbine. The temperature of the cell was maintained at 40°C using a water bath. Phase transitions were recorded with a USB camera. In a typical experiment, 100 mg of the tested polymer was transferred into the high-pressure visualization cell. The cell was then sealed and filled with 20 mL of CO2, leading to 0.5% (weight/volume, w/v) of polymer in the cell. Finally, the cell was pressurized at 300 bar using a membrane pump (Separex, France). The complete dissolution of the polymer in scCO2 could be visually observed after 10 min of mechanical stirring. Then, a video was recorded while the pressure inside the cell was slowly reduced at 20 bar/min using a syringe pump (260D P2 ISCO, USA) connected to the observation cell. The video was then post-treated by ImageJ and the grey intensity was calculated in order to determine the cloud point corresponding to the phase separation of the polymer from CO2. The image grey intensity was calculated from the red (R), green (G) and blue (B) components as (R + B + G)/3 (see SI section 4, figureS5). The measurements were repeated three times to test their reproducibility.Cloud point temperature of the polymers in waterThe cloud point temperatures of the POEGMA and PDMS homopolymers were studied in water saturated with CO2 at 1 bar and 250 bar. The high-pressure phase equilibrium apparatus (Separex, France), described above, was filled with 20 mL of aqueous polymer solution (1.0 % w/v). Saturation of CO2 in water was ensured by first raising the temperature to 80°C where CO2 was the most soluble in water, stirring for 5 min and then returning to 40°C. Then the temperature of the cell was raised by 5°C steps and the cloud point was determined based on the macroscopic turbidity observed with a USB camera.PDMS13-b-POEGMAx copolymers were soluble in water at room temperature for different concentrations (0.1; 0.5 and 1 % w/v). Their cloud point temperatures (TCP) were determined in water at 1 bar from turbidity measurements (see SI, section 4).Results and discussion1. Synthesis of PDMS13-b-POEGMAx diblock copolymers Synthesis of PDMS13-CTASynthesis of the chain transfer agent-functionalized PDMS (PDMS-CTA) was done by esterification of a commercially available PDMS-OH (Figure3, DPn = 13, Mn = 1,180 g/mol) with a carboxy-functional CTA in the presence of DCC and DMAP, see Figure3.[START_REF] Salager | Partitioning of Ethoxylated Octylphenol Surfactants in Microemulsion-Oil-Water Systems: Influence of Temperature and Relation between Partitioning Coefficient and Physicochemical Formulation[END_REF] After optimization (see SI for details, section 1 and tableS1), the optimal selected conditions corresponded to a PDMS-OH:CTA-COOH:DCC:DMAP molar ratio of 1:1.5:1.5:0.15. The reaction was conducted overnight in dichloromethane at room temperature. After purification, the polymer was characterized by 1 H NMR in DMSO-d6, revealing the quantitative transformation of the terminal hydroxy functions of PDMS-OH into CTA functions (Figure3). Indeed, proton Hc shifted from 3.75 ppm in PDMS-OH to 4.3 ppm in PDMS-CTA (Figure 3b), whereas proton Hb shifted from 3.57 to 3.67 ppm. No proton characteristic of PDMS-OH could be found in either the raw (data not shown) or purified PDMS-CTA, confirming full conversion.

Figure 3 .

 3 Figure 3. a) Synthetic scheme for the preparation of the PDMS-CTA macro-RAFT agent and of the PDMS-b-POEGMA diblock copolymers; b) 1 H NMR of the PDMS-CTA and the reference PDMS-OH in the 3.4-4.4 ppm region.

Figure 4 .

 4 Figure 4. a) Evolution of Mn (full symbols) and Ɖ (empty symbols) with conversion during the synthesis of PDMS13b-POEGMAx diblock copolymers (x = 16 p and r, x = 31 l and x = 46 n and o). The straight lines correspond to the theoretical evolution of Mn = MPDMS-CTA + [OEGMA]0×conversion×MOEGMA/(f×[PDMS-CTA]0) (see Table 1 for conditions, with MPDMS-CTA = 1460 g/mol and MOEGMA = 300 g/mol). b) Evolution of ln([M]0/[M]t) as a function of time.

[

  OEGMA]/[CTA] ratio and conversion. In the end, three PDMS13-b-POEGMAx with x = 16, 31 and 46 were obtained. The synthesis of a fourth polymer with a very short POEGMA block (x ~ 5) was also attempted. However, the same problem of efficiency of the PDMS-CTA block was observed and the POEGMA block was too short for further selective purification (see SI, section 3 for details).

Figure 5 .

 5 Figure 5. Size exclusion chromatograms of the macroCTA PDMS13-CTA, (⎯) and of the PDMS13-b-POEGMAx diblocks, DPn = 16 ( ... ), DPn = 31 (--), DPn = 46 ( _ _ ). a) Before and b) after purification by selective precipitation in pentane/diethyl ether (80/20 v/v). UV detection was used here to detect residual PDMS-CTA.

below 5

 5 mN/m with polymer concentrations ranging from 0.1 to 1 % w/v in water. Table 2. g in mN/m as a function of the concentrations of PDMS13-b-POEGMAx (C) at 40°C, 250 bar and for different x values.

Figure 6

 6 Figure6compares the effect of pressure on g for PDMS13-b-POEGMA46 at either 40 or 80°C. In the absence of polymer, the pressure dependence of g is similar for the two temperatures although g increases slightly when the temperature increases, as already observed in the litterature.[START_REF] Georgiadis | Interfacial Tension Measurements of the (H2O + CO2) System at Elevated Pressures and Temperatures[END_REF] On the contrary, there was a drastic difference between these two temperatures in the presence of PDMS13-b-POEGMA46. At 40°C, the pressure dependences of g with and without polymer are similar. However, the interfacial tension is strongly reduced in the presence of polymer; suggesting a significant but invariant surface activity of the polymer with the pressure at 40°C. At 80°C, PDMS13-b-POEGMA46 is not able to reduce g at all at 100 bar, leading to a value close to that of the neat water-scCO2 interface. Remarkably, g decreases strongly (compared to the case without polymer) with increasing pressure at 80°C, suggesting an increase of the surface activity of the polymer with the pressure. The polymer is however far less surface active at 80°C than at 40°C, even at 250 bar. These observations are confirmed by representing the evolution of the surface pressure π (N/m 2 ) (which represents the difference between γ without or with polymer respectively) as a function of the CO2 density (see SI, FigureS7for details) as already done for other systems in the literature.[START_REF] Adkins | Effect of Branching on the Interfacial Properties of Nonionic Hydrocarbon Surfactants at the Air-Water and Carbon Dioxide-Water Interfaces[END_REF]

Figure 6 .

 6 Figure 6. Average interfacial tension as a function of the pressure at different temperatures, 40°C ( ) and 80°C ( •, ) with 1 % w/v PDMS13-b-POEGMA46 (full symbols) and without polymer (empty symbols). The three measurements performed for each condition are represented on the figure to give an idea of the reproducibility.

Figure 7 .

 7 Figure 7. Schematic illustration of the organization of PDMS13-b-POEGMA46 at the scCO2 (C)-Water (W) interface for different pressures and temperatures indicated in the figure, explaining the impact of these parameters on the variation of g, see text: a) unimers with their two blocks well-swollen in their respective phase; b) unimers with a partly collapsed POEGMA block; c) unimers with a partly collapsed PDMS block; d) precipitated polymer which

Figure 8 .

 8 Figure 8. Typical pictures of the water/scCO2/PDMS13b-POEGMAx mixtures in different states: a) unmixed polymer aqueous solution (bottom phase) and scCO2 phase (top phase), b) completely emulsified system just

Table 4 .

 4 Intensity of the scattered light in the aqueous phase after emulsification and creaming expressed in percent relative to the initial scattered intensity of the PDMS13-b-POEGMAx diblock copolymer aqueous solution at 1 % w/v (100 bar, 40°C) before any emulsification. . Not applicable because these samples were significantly turbid (see Figure 8d), implying a strong influence of multiple scattering on the results. The turbidity was attributed to the presence of a significant amount of relatively large scCO2 droplets which had not yet creamed.

  PDMS13-b-POEGMAx copolymers, with three different POEGMA block sizes, were synthesized by RAFT. This work aimed at characterizing the relationship between the affinity of each block for their respective phase, their ability to decrease g and the consequences on the stability of emulsions in different conditions of P and T. Hence, the development of a home-made experimental device (with implemented characterization techniques: direct visualization, DLS and droplet tensiometry) led to the following significant findings. The PDMS13-b-POEGMAx diblock copolymers were able to reduce g by up to a factor of 20. Their high-performance (g was reduced down to 1.2 mN/m) was similar to that of other Pluronics® and Fluorinated copolymers. 2 These polymers were able to form emulsions for which coalescence was significantly retarded, although creaming could not be suppressed. The thermosensitivity of the copolymers due to the POEGMA block resulted in a strong variation of g with T (in the 40-80°C range), which allowed the thermo-reversible stabilization/destabilization of water-scCO2 emulsions (at 100 bar). This stimuli-responsivness could be useful for various applications such as emulsion polymerization, catalyst recovery, and cosmetics, as recently reviewed by Tang et al. for stimuli-responsive

ASSOCIATED CONTENT 1 .

 1 Optimization of the conditions for the synthesis of PDMS13-CTA; 2. Efficiency of the PDMS13-CTA during the RAFT polymerization; 3. Synthesis and characterization of PDMS13-b-POEGMA5; 4. Determination of the cloud point pressure in CO2 and of the cloud point temperature in water; 5. g measurements; 6. Investigation of the aqueous phase before and after emulsification.

Table 1 .

 1 Experimental Determined by 1 H NMR on the purified polymer; b Molar ratio of the reagents; c Determined from the relative areas of the peaks in SEC, see SI section 2; d Theoretical Mn = MPDMS-CTA + [OEGMA]0×conversion×MOEGMA/(f×[PDMS-CTA]0); e Mn and Ɖ determined by SEC in THF using light scattering and refractive index detections.

	x a	[OEGMA]0: [PDMS-CTA]0: [AIBN]0 b	OEGMA conversion (%)	CTA efficiency (%) c	M n theo (g/mol) d	M n exp (g/mol) e	Ɖ e	wt% POEGMA
	16 20 : 1 : 0.2	52	65	5000	7 500	1.1	84
	31 40 : 1 : 0.2	30	48	8 000	12 000	1.1	89
	46 40 : 1 : 0.2	60	45	16 000	17 000	1.1	93

conditions for the synthesis of the PDMS13-b-POEGMAx diblock copolymers and characteristics of the final polymers

a

Table 3 .

 3 Values

	Without
	polymer

of g in mN/m as a function of P, at 40°C, without polymer or with 1 % w/v of PDMS13-b-POEGMAx in the aqueous phase for various x. x = 16 x = 31 x = 46

to P. Legout) and by the French ANR in the frame of the CarboMIC project (ANR-19-CE09-0012-01). The authors thank Boris Jacquette (IMMM) for characterizations of the copolymers by SEC, Frédérick Niepceron for the cryoTEM experiment, Domitille Lefebvre (MINT, IMMM) for her implication on the g measurements, Teclis-scientific (France) and Top Industrie (Vaux Le Pénil, France) for their implication in the design of the experimental bench and software, Cordouan Technologies for the technical support regarding the particle size analyzer.

Funding Sources

This work was supported by the Region Pays de Loire (France) in the frame of the SPEED program (PhD grant to P. Legout) and by the French ANR in the frame of the CarboMIC project (ANR-19-CE09-0012-01). ACKNOWLEDGMENT This work was supported by the Region Pays de Loire (France) in the frame of the SPEED program (PhD grant

AUTHOR INFORMATION

Corresponding Authors * olivier.colombani@univ-lemans.fr; brice.calvignac@univ-angers.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


Thibaut Triquet

Claire Tendero

Laure Latapie

Marie-Hélène Manero

Romain Richard

Caroline Andriantsiferana
email: caroline.andriantsiferana@iut-tlse3.fr

Ouverte TiO 2 MOCVD coating for photocatalytic degradation of ciprofloxacin using 365 nm UV LEDs -kinetics and mechanisms

This work presents a solution for the photocatalytic degradation of the antibiotic ciprofloxacin (CIP) in water, without using P25 TiO 2 powder and thus getting rid of expensive separation steps. It consists in using a TiO 2 coating that is directly deposited on the optical window of a photocatalytic micro-reactor and 365 nm UV LEDs as radiation source. P25 TiO 2 powder was also studied as reference. HPLC-MS was used to determine the transformation products and the pathways reactions. CIP was slowly degraded by the photolysis reaction at 365 nm: (75 % removal after 8 h of UV irradiation). However, no significant decrease of the total organic carbon (TOC) was noticed, thus showing the presence of transformation products not degraded by the action of UV-light alone. For a low catalyst amount (i.e 0.12 g of TiO 2 , whatever the form, powder or coating, per liter of contaminated water,), excellent CIP degradation by photocatalysis was observed. Complete CIP degradation after 1 h of irradiation was required using P25 and 8 h using TiO 2 coating. Different preferential reaction pathways were identified for both TiO 2 catalysts. The Langmuir-Hinshelwood model showed a very good representation of the kinetics, unlike its simplified pseudo-first order model. Photocatalysis experiments did not show a complete mineralization (60-70 % of TOC removal), but most of the aromatic transformation products were degraded. The last transformation products were identified as small aliphatic acids. There is therefore a real interest in using MOCVD coating of TiO 2 for sustainable wastewater treatment to avoid expensive catalyst separation. A study with a spiked real effluent from a wastewater treatment plant was performed and a satisfactory degradation was obtained. Slower kinetics were found due to the presence of additional organic products and scavenger compounds such as HCO 3 -.

Introduction

Although water is an important resource on earth, its purity is essential for human consumption. In the past decades, many toxic molecules such as micropollutants (i.e. pharmaceuticals products: antibiotics, anti-inflammatories, analgesics) are present in waterways and underground leading to serious consequences for the environment. Even if the concentration of these micropollutants are very low (ranging from ng.L -1 to μg.L -1 ), anthropogenic activity induces regular increase of their occurrence in natural waters. Due to this increase, global and European environmental standards have been lowered to reduce environmental concerns [1]. Among all micropollutants, the most persistent are fluoroquinolones such as ciprofloxacin (CIP). These molecules are of great danger for the ecosystems due to bioaccumulation, which leads to high toxicity levels [START_REF] Moreira Meireles | Ciprofloxacin degradation by first-, second-, and third-generation manganese porphyrins[END_REF]. Classical wastewater treatment (i.e. filtration, decantation, biological processes…) cannot totally eliminate these molecules and additional treatments are therefore required. Separation techniques such as adsorption and stripping processes can be used but also degradation techniques such as Advanced Oxidation Processes (AOP) (chemical oxidation/reduction) [START_REF] Sivagami | Advanced oxidation processes for the treatment of tannery wastewater[END_REF]. AOP degrade the target molecules [START_REF] González-Labrada | Enhancement of ciprofloxacin degradation in aqueous system by heterogeneous catalytic ozonation[END_REF][START_REF] Merle | Influence of activated carbons on the kinetics and mechanisms of aromatic molecules ozonation[END_REF] into inorganic compounds (production of water and carbon dioxide) or harmless transformation products [START_REF] Andreozzi | Advanced oxidation processes (AOP) for water purification and recovery[END_REF]. AOP are furthermore very efficient due to the production of strong chemical oxidants, mainly hydroxyl radicals (OH • ), which are strongly reactive due to their high oxidation potential [START_REF] Abha | Application of advanced oxidation processes and toxicity assessment of transformation products[END_REF][START_REF] Jiménez | Produced water treatment by advanced oxidation processes[END_REF][START_REF] Aboussaoud | Combined ozonation using alumino-silica materials for the removal of 2,4-Dimethylphenol from water[END_REF]. Several AOP treatments have been studied and shown to be effective for the removal of fluoroquinolones. Individual processes (ozonation, photocatalysis, Fenton reaction…) and combined processes (O 3 /UV, O 3 /H 2 O 2 /UV and UV/H 2 O 2 ) also provide interesting performances [START_REF] Agustina | A review of synergistic effect of photocatalysis nd ozaonation on wastewater treatment[END_REF][START_REF] González-Labrada | Optimization of ciprofloxacin degradation in wastewater by homogeneous sono-Fenton process at high frequency[END_REF][START_REF] Ajo | Hospital wastewater treatment with pilot-scale pulsed corona discharge for removal of pharmaceutical residues[END_REF]. Amongst these AOP, photocatalysis is known to be an efficient technique to degrade organic compounds such as fluoroquinolones under both UV [START_REF] Paul | Photolytic and photocatalytic decomposition of aqueous ciprofloxacin: transformation products and residual antibacterial activity[END_REF][START_REF] Li | Transformation products formation of ciprofloxacin in UVA/LED and UVA/LED/TiO2 systems: Impact of natural organic matter characteristics[END_REF][START_REF] Gomes | Detoxification of parabens using UV-A enhanced by noble metals-TiO 2 supported catalysts[END_REF] and visible light [START_REF] Suwannaruang | Visible light-induced degradation of antibiotic ciprofloxacin over Fe-N-TiO 2 mesoporous photocatalyst with anatase/rutile/ brookite nanocrystal mixture[END_REF][START_REF] Karuppaiah | Efficient photocatalytic degradation of ciprofloxacin and bisphenol A under visible light using Gd2WO6 loaded ZnO/ bentonite nanocomposite[END_REF][START_REF] Paul | Visible-light-mediated TiO 2 photocatalysis of fluoroquinolone antibacterial agents[END_REF]. This photochemical reaction occurs in presence of a metal oxide semiconductor catalyst. Titanium dioxide (TiO 2 ) is the most commonly used catalyst. Indeed, TiO 2 can absorb UV-light (λ < 385 nm) to create free electrons (e-) and free holes (h + ) [START_REF] Matthews | Photo-oxidation of organic material in aquaous suspensions of titanium dioxide[END_REF]. TiO 2 is mostly used due to its low cost, photo-chemical stability. Furthermore, both its specific area and crystalline structures drive its photocatalytic efficiency [START_REF] Dewil | New perspectives for advanced oxidation processes[END_REF]. The main drawback of using TiO 2 is the particle size: it is usually a powder composed of nanoparticles, which are suspected to increase the risk of chronic intestinal inflammation and carcinogenesis [START_REF] Bettini | Food-grade TiO 2 impairs intestinal and systemic immune homeostasis, initiates preneoplastic lesions and promotes aberrant crypt development in the rat colon[END_REF]. Due to the very small size of the catalyst, an expensive filtration step is therefore required after treatment in order to separate the catalyst from the water. To avoid this separation step, an alternative technique consists of using supported catalysts [START_REF] Ahmad | Photocatalytic systems as an advanced environmental remediation: recent developments, limitations and new avenues for applications[END_REF] on classical supports such as glass [START_REF] Dulian | Photocatalytic methylene blue degradation on multilayer transparent TiO 2 coatings[END_REF], metal [START_REF] Schiemann | Synthesis of high surface area TiO 2 coatings on stainless steel by electrophoretic deposition[END_REF], activated carbon [START_REF] Andriantsiferana | Photocatalytic degradation of an azo-dye on TiO 2 /activated carbon composite material[END_REF][START_REF] Alalm | Enhancement of photocatalytic activity of TiO 2 by immobilization on activated carbon for degradation of pharmaceuticals[END_REF] or directly at the surface of a reactor [START_REF] Suhadolnik | Continuous photocatalytic, electrocatalytic and photo-electrocatalytic degradation of a reactive textile dye for wastewater-treatment processes: batch, microreactor and scaled-up operation[END_REF]. To obtain a TiO 2 coating on the surface of a support, several techniques are available including sol-gel deposition [START_REF] Hakki | The role of thermal annealing in controlling morphology, crystal structure and adherence of dip coated TiO 2 film on glass and its photocatalytic activity[END_REF], Metal Organic Chemical Vapor Deposition (MOCVD) [START_REF] Bento | Photocatalytic activity of undoped and sulfurdoped TiO 2 films grown by MOCVD for water treatment under visible light[END_REF], impregnation and oxidation processes [START_REF] Jiang | Low temperature synthesis of few-layer titanate nanobelts on Ti mesh and the hot-water induced transformations to highly photocatalytic active titania nanorods[END_REF] such as anodization technology [START_REF] Carvalho Cardoso | Bubble annular photoeletrocatalytic reactor with TiO 2 nanotubes arrays applied in the textile wastewater[END_REF] or plasma electrolytic oxidation [START_REF] Thukkaram | Antibacterial activity of a porous silver doped TiO 2 coating on titanium substrates synthesized by plasma electrolytic oxidation[END_REF]. MOCVD is of great interest since it allows the control of the physical characteristics of the coating, namely the crystalline structure and the morphology. Moreover, this deposition technique is known to ensure adherent and conformal coverage of high aspect ratio surfaces, regardless of the nature of the substrate (metal, glass or silicon). Therefore, this technique has been selected for this work along with glass as the support.

For this study, the selected fluoroquinolone target molecule is the antibiotic CIP, which is representative of a persistent micropollutant after conventional wastewater treatment [START_REF] Margot | Treatment of micropollutants in municipal wastewater: ozone or powdered activated carbon?[END_REF][START_REF] Al-Maadheed | Antibiotics in hospital effluent and domestic wastewater treatment plants in Doha, Qatar[END_REF]. To degrade this molecule, photocatalysis was investigated using a TiO 2 MOCVD coating on the upper glass window of a channel reactor and irradiated by a monochromatic LED lamp at 365 nm. The objective of this work is to study the performance of this catalyst under specific operating conditions where the quantity of catalyst used is 5-10 times lower than most of the work reported in the literature. The performance is then compared to the classical powder catalyst P25. In addition, a precise determination of the transformation products formed has been developed through the use of high-performance analytical techniques. Photocatalysis and photolysis experiments are compared with an objective of identifying the reaction pathways. This study also investigates the choice of an accurate model for the photocatalytic CIP degradation. The use of kinetics models can be an advantage for describing performances and to compare different catalysts. Two categories of models can be used: theoretical and empirical kinetics models [START_REF] Ateia | Modeling the degradation and disinfection of water pollutants by photocatalysts and composites: a critical review[END_REF]. In this study, theoretical kinetics model will be investigated. Most of the authors modelled the photocatalytic degradation with a first-order equation corresponding to a simplified Langmuir-Hinshelwood (L -H) model or with a second-order model [36]. However, these models are not always adequate due to simplifying assumptions which can be discussed [START_REF] Asenjo | Correct use of the Langmuir-Hinshelwood equation for proving the absence of a synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon[END_REF]. Therefore, the L-H model, studied in this study, appears to be a more suitable and very effective modeling method [START_REF] Ateia | Modeling the degradation and disinfection of water pollutants by photocatalysts and composites: a critical review[END_REF]. Some authors also propose modelling by coupling mass-balance equations (considering time, convection and diffusion terms), reaction mechanisms of each species (reaction rate, adsorption coefficient and kinetics constants) [START_REF] Suhadolnik | Mechanism and kinetics of phenol photocatalytic, electrocatalytic and photoelectrocatalytic degradation in a TiO 2nanotube fixed-bed microreactor[END_REF]. Finally, the influence of the liquid matrix is investigated by replacing distilled water with a real effluent from a wastewater treatment plant.

Material and method

Chemicals

Analytical grade ciprofloxacin, C 17 H 18 FN 3 O 3 (CIP, 98 % purity) with physico-chemical properties given in Table 1, TiO 2 powder (DEGUSSA-P25), formic acid, CH 2 O 2 (formic acid, >99 % purity) and Titanium tetraisopropoxide (TTIP, 99.999 %) were purchased from Sigma Aldrich.

A water sample collected from the outlet of a wastewater treatment plant (Castanet-Tolosan, France) was analysed by the Departmental Laboratory 31: Water -Veterinary -Air in Launaguet (France). The following measurements were carried out: COD (Chemical Oxygen Demand), BOD (Biochemical Oxygen Demand), nitrite (NO 3 -), nitrate (NO 2 -), ammonium (NH 4

+

), carbonates (CO 3 2-), bicarbonates (HCO 3 -), AT (Alkalimetric title) and CAT (Complete Alkalimetric title). The results are presented in Table 2.

MOCVD deposition

TiO 2 coating was grown in a tubular, horizontal hot-wall reactor that was described in [START_REF] Sarantopoulos | Photocatalyseurs à Base De TiO2 Préparés Par Infiltration Chimique En Phase Vapeur[END_REF], on Pyrex® glass and silicon substrates. Titanium tetraisopropoxide (TTIP, 99.999 %, Sigma-Aldrich) was thermally regulated in a bubbler and carried to the deposition zone, with 99.9992 % pure nitrogen as carrier gas. Table 3 summarizes the process conditions. Substrates were weighed before and after treatment to estimate the mass coated on the glass.

Table 1

Physico-chemical properties of CIP.

Characterization of the coating

Scanning Electron Microscopy (SEM, LEO-435 VP-PGT) was used to determine both the morphology and evaluate the thickness of the coating, which was deduced from cross-section measurements.

Atomic Force Microscopy (AFM, Agilent 5500) enabled the evaluation of the coating roughness.

The crystalline structure was deduced from X-Ray diffraction measurements under 2 • grazing incidence with a Cu Kα anode in theta-theta mode (GI-XRD, Bruker D8).

UV-vis spectrometry (Perkin-Elmer LAMBDA 19) was finally used to characterize the optical properties of the samples such as transmittance and band-gap.

Experimental set-up

Fig. 1 represents the experimental set-up used for all photolysis and photocatalysis experiments. It consists of a 100 mL storage tank filled with the initial solution. A peristaltic pump at a constant flow rate (200 mL.min -1 ) ensures the circulation between the tank and the photoreactor equipped with a glass window (130 × 10 × 12 mm). A monochromatic LED panel (365 nm) is placed 15 cm above the reactor and irradiates the liquid inside the reactor. An air aeration system maintains the temperature at 25 • C (±1 • C).

Photolysis and photocatalysis experiments

Photolysis and photocatalysis reactions were carried out with the same experimental protocol. A first step of adsorption was performed for 1.5 h in the dark (UV off). An initial solution of 100 mL at 20 mg.L -1 CIP was used. For each experiment, uniform irradiation was maintained for 8 h (Irradiance =10 mW.cm-2 , λ = 365 nm). For the photolysis experiments, the photo-reactor was equipped with a clean glass window, whereas for the photocatalysis experiments, a clean glass window with TiO 2 powder (12 ± 1 mg) or a glass window with the TiO 2 coating on its surface was used. The liquid samples were collected in the storage tank and filtered through a 0.45 μm Nylon membrane before being analyzed.

Every experiment was carried out three times in order to determine the standard deviation.

The concentration of CIP used for this study was higher than classical concentrations measured inside water after wastewater treatment (20 mg.L -1 vs C < 1 μg.L -1 ). This choice was motivated by the operating conditions of the study (100 mL of treated solution). To analyze a solution with very low concentration, every sample has to be concentrated before the HPLC analysis. Therefore, the volume to be concentrated has to be between 500 mL and 1 L. In this study, a concentration step is then not possible due to the experimental device, which strongly limits the volume of the treated solution (only 100 mL). Under these conditions, the concentration of the initial solution has to be higher than the real effluent in order to analyze the solution and detect all transformation products formed. Moreover, the experimental device includes many elements (reactor, pump, tube, storage tank) and part of the CIP and transformation products could be absorbed inside the system, which would modify the results.

Analytical methods

To follow the concentration of ciprofloxacin during the process, each sample was analyzed by HPLC-UV. The HPLC used is a HPLC-PDA Thermo Accela (Shimadzu, Japan), equipped with a UV detector at 280 nm and a C18 column (2.6 μm, 100 mm x 3 mm). The method used for HPLC-UV is a multi-ramp. The mobile phase is fed by two channels two phases: (A) as acetonitrile and (B) as acidic water (0.1 % of formic acid). The mobile phase is firstly 90 % of B and 10 % of A. The percentage of A is then increased from 10 % to 90 % over 7 min and then held for 5 min. A ramp of 7 min is then used to return to 10 % of A and 90 % of B and this proportion is maintained for another 5 min. During the whole analysis, the flowrate was maintained at 0.3 mL.min -1 at 40 • C.

For the determination of reactive intermediates, a HPLC-MS a Thermofisher UltiMate 3000 HPLC was used, coupled with an Orbitrap High Resolution Mass Spectrometer (HRMS -Exactive) equipped with an Electrospray ionisation source (ESI). The UV detector was set at 280 nm, a Gemini C18 column was used (3 μm, 100 mm × 2 mm). The flow rate was maintained at 0.2 mL.min -1 at 40 • C. The mobile phase is similar to the classic HPLC-UV used for experiment (A/B). During the first 2 min of the run, the mobile phase was maintained at 10/90 (v/v), then a ramp was performed for 7 min to reach 90/10 (v/v). This composition was maintained for 8 min before another ramp to reach the initial composition of 10/90 (v/v). This composition was then maintained for 6 min until the next injection. For the determination of intermediates, positive and negative ionization modes of the MS were used.

TOC measurements were carried out with a TOC-L (Total Organic Carbon Analyzer) from Shimadzu (Japan). Each sample was firstly analysed to determine the organic carbon by a 20 μL injection then a second analysis of 50 μL with hydrochloric acid (0.1 mol.L -1 ) was conducted to determine the inorganic carbon.

Lastly, reactive intermediates were analysed by Ionic Chromatography (IC) using a Thermo Scientific ICS 5000+ ion chromatography with a conductimetric detector. The anion column used is an AS19 (4 μm, 2 × 250 mm) with a flow rate of 0.250 mL.min -1 at 25 • C. The mobile phase is a solution of KOH. For 10 min, a concentration of 5 mmol.L -1 of KOH is maintained. A ramp is then set for 15 min to achieve a concentration of 45 mmol.L -1 of KOH, which is maintained for 10 min. Finally, another ramp of 2 min enables the concentration to be decreased to 5 mmol.L -1 of KOH. The crystalline structure was a pure anatase, as confirmed by the XRD analysis: no peaks from the rutile structure were detected according to the JCPDS cards depicted for comparison as shown in Fig. 3. Previous work on TiO 2 structures [START_REF] Al-Mamun | Photocatalytic activity improvement and application of UV-TiO2 photocatalysis in textile wastewater treatment[END_REF] has shown that anatase phase has a better photocatalytic activity than other phases such as rutile or brookite.

Results

Characterization of TiO 2 coatings

UV-vis transmission measurements performed on the TiO 2 coated Pyrex at 200-800 nm highlight near total absorption of the UV LED radiation, as shown in Fig. 4. The interference fringes in the visible range were in agreement with the thickness of the coating estimated by SEM. Low transmittance can be seen at 365 nm, which is the wavelength used for the photocatalysis reaction. This coating will therefore have photocatalytic properties. Moreover, in order to determine the optical band-gap Eg of the coatings, the Tauc plot [START_REF] Viezbicke | Evaluation of the Tauc method for optical absorption edge determination ZnO thin films as a model system[END_REF] of the absorption coefficient α for indirect semi-conductors, deduced from transmission measurements as shown in Fig. 5 was used. A value of 3.1 eV is found, which is in agreement with a previous work for similar coatings [START_REF] Miquelot | TiO 2 nanotree films for the production of green H 2 by solar water splitting: From microstructural and optical characteristics to the photocatalytic properties[END_REF].

Kinetics of photocatalysis

Fig. 6 shows a weak adsorption of CIP (3%) after 8 h in the glass and hydraulic system, a significant degradation of CIP by photolysis (75 % after 8 h) and a total degradation of CIP by photocatalysis (with powder of TiO 2 -P25 and coating after respectively 1 h and 8 h). Indeed, the photolysis of CIP was expected from its absorption UVspectrum given in Fig. 7. The LED panel emitted UV at 365 nm and even though CIP absorbance is weak (0.071) for this wavelength, it appeared sufficient to eliminate ciprofloxacin by UV alone (photolysis reaction) [START_REF] Van Doorslaer | UV-A and UV-C induced photolytic and photocatalytic degradation of aqueousciprofloxacin and moxifloxacin: reaction kinetics and role of adsorption[END_REF].

In order to compare the TiO 2 powder and TiO 2 coating experiments, both experiments were carried out with a similar amount of TiO 2 (around 12 ± 1 mg). With the TiO 2 powder, less than 30 min of irradiation were required to remove 75 % of CIP, whereas 8 h were necessary with direct photolysis. Moreover, it was observed that CIP was totally degraded by photocatalysis with the P25 powder after 1 h of irradiation. Other studies have reported the same trend with TiO 2 powder: CIP could be degraded by both techniques with faster kinetics obtained with photocatalysis [START_REF] Van Doorslaer | UV-A and UV-C induced photolytic and photocatalytic degradation of aqueousciprofloxacin and moxifloxacin: reaction kinetics and role of adsorption[END_REF][START_REF] Salma | Dependence of transformation product formation on pH duringphotolytic and photocatalytic degradation of ciprofloxacin[END_REF][START_REF] Li | Transformation products formation of ciprofloxacin in UVA/LED and UVA/LED/TiO2 systems: Impact of natural organic matter characteristics[END_REF]. For photocatalysis with TiO 2 coating, 5 h of irradiation were necessary to eliminate 75 % of CIP and 8 h for a total degradation. This phenomenon has been already reported with methylene blue and carbamazepine [START_REF] Murgolo | Degradation of emerging organic pollutants in wastewater effluents by electrochemical photocatalysis on nanostructured TiO 2 meshes[END_REF][START_REF] Dulian | Photocatalytic methylene blue degradation on multilayer transparent TiO 2 coatings[END_REF]. It clearly appeared that a faster kinetic degradation of CIP was achieved with TiO 2 powder than with the coating. In the case of TiO 2 coating, even if the mass of the coating was equal to the amount of powder, part of the coating could not react with the liquid phase. This is due to the fact that the coating was fixed on the glass and therefore not directly available. Moreover, when the TiO 2 powder was used, the photocatalyst was homogeneously distributed in the liquid and directly in contact with CIP molecules which implied a faster reaction. The hydrodynamics inside the reactor was assessed, and the flow was laminar (Reynolds number of 303). In laminar flow, there is not enough turbulence to allow efficient contact between the liquid and the coating. To improve the kinetics, a higher flowrate should be used. However, this configuration has been chosen to have a longer irradiation time (i.e. long residence time for a small reactor). With a higher flowrate, the CIP would not have enough time to react inside the reactor since the residence time would be too short.

Experimental data were used to determine various kinetic parameters. First, the kinetic parameters of photolysis reaction were determined using a first-order kinetic model (1) [START_REF] Cox | Photolysis of metamitron in water in the presence of soils and soil components[END_REF][START_REF] Zhang | Influence of chemical speciation on photochemical transformation of three fluoroquinolones (FQs) in water: kinetics, mechanism, and toxicity of photolysis products[END_REF]:

- dC dt = k photolysis C (1)
A combination of the photolysis model with the complete Langmuir-Hinshelwood model was used to predict the degradation of the CIP during photocatalysis experiments:

- dC dt = kK ad C 1 + K ad C + k photolysis C (2)
where C represents the concentration of CIP, k is the kinetic constant of photocatalysis reaction, K ad is the adsorption constant on the surface of catalyst and k photolysis is the kinetic constant of photolysis reaction.

In the literature [36], most authors have simplified the Langmuir-Hinshelwood model by considering the product K ad C << 1. In this case, the model can be simplified and takes the form of a pseudo-first order kinetic model where kK ad = k app .

-

dC dt = kK ad C + k photolysis C = k app C + k photolysis C (3)
Both photocatalysis models have been investigated due to new contradictory results [START_REF] Asenjo | Correct use of the Langmuir-Hinshelwood equation for proving the absence of a synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon[END_REF], where the pseudo-first order seems not suitable for photocatalysis modelling. The aim of this modelling is to show which model can be used for this system.

A classical method of determining non-linear equations has been used for the Langmuir-Hinshelwood coefficients. This method was applied via a MatLab code using the simulannealbnd function [START_REF] Schälte | Evaluation of derivative-free optimizers for parameter estimation in systems biology[END_REF][START_REF] Kirkpatrick | Optimization by simulated annealing[END_REF] and using the minimisation of the summation of the square of the difference between calculated and experimental concentrations as convergence criteria.

Kinetic degradation by photolysis reaction was studied and the classic first order kinetic model gave good results. As shown in Fig. 6, this model represents rather correctly the CIP elimination (r 2 = 0.959) and the kinetic constant obtained k photolysis was estimated at 0.00295 min -1 .

For photocatalysis experiments, the Langmuir-Hinshelwood model (Eq. 2) was compared to a pseudo-first order model (Eq. 3). For both models, the value of k photolysis used was the value estimated previously. The parameters obtained with the TiO 2 coating and P25 TiO 2 for both models are presented in Table 4. The pseudo-first order kinetic model showed a good representation of the CIP degradation (r 2 > 0.98), however its use will be discussed. An excellent agreement was obtained between the Langmuir-Hinshelwood kinetic models and the experimental values as shown in Fig. 6. Value for pseudo-first order kinetic calculated in Table 4 for P25 is comparable to the literature value for photocatalysis of diclofenac with P25: 0.06 min -1 (which is also an antibiotic with similar formula as ciprofloxacin) [START_REF] Arlos | Influence of methanol when used as a water-miscible carrier of pharmaceuticals in TiO 2 photocatalytic degradation experiments[END_REF].

An significant difference between values of k app and the product kK ad was noticed: 0.0648/0.105 min -1 for P25 and 0.0085/0.0097 min -1 for Fig. 6. Kinetics of degradation of ciprofloxacin by photolysis and photocatalysis. the TiO 2 coating. These deviations show that the hypothesis of K ad C <<< 1 cannot be verified. Indeed, the values of K ad C are equal to 1.07 for P25 experiments and 0.57 for coating experiments respectively. The pseudo-first order model is not adapted to these kinetic representations: the Langmuir-Hinshelwood model cannot be simplified as systematically proposed in most publications.

To follow the mineralisation rate of the CIP in solution, a TOC (Total Organic Carbon) measurement was done before and after treatment (8 h). Fig. 8 represents the evolution of TOC for the different experiments. With the photolysis treatment of the solution, 75 % of CIP was degraded but the TOC was decreased by only 9.7 %. This means that photolysis completely degraded the CIP but not all the transformation products. Photolysis is a selective reaction; it can only degrade organic compounds which absorbed UV at the same wavelength as the lamp.

Concerning the photocatalysis experiments, a significant degradation of TOC was obtained: 61 % with P25 and 68 % with the coating. Due to the photocatalyst, CIP and an important part of the transformation products can be degraded. There was no significant difference between the powder and the coating. However, although there was a significant decrease of the TOC, there was no total mineralisation of the CIP. Other molecules were present in the solution and have to be identified.

Transformation products

Despite the fact that CIP was totally degraded by photocatalysis, some of the transformation products still remained in the solution. By following the production and degradation of these transformation products, it is possible to explain why the total mineralisation was not achieved. All aromatic transformation products were identified by HPLC-MS [START_REF] Li | Transformation products formation of ciprofloxacin in UVA/LED and UVA/LED/TiO2 systems: Impact of natural organic matter characteristics[END_REF] and all of the suspected aromatic transformation products are listed in Table 5. These transformation products are representative of three different reaction pathways: oxidation/defluorination, dealkylation and oxidation. Through this analysis, it is possible to track their production and consumption during each experiment. Each retention time for HPLC analysis at 290 nm was assigned and the transformation products were followed by the evolution of the HPLC peak area. The evolution is shown in Fig. 9.

As shown in Fig. 9(a), during photolysis, ten different aromatic transformation products were found. While these transformation products were quickly produced, their degradation was weak and slow. Some of them were not degraded; they showed stability to the degradation reaction under UV light (for 8 h of UV treatment). The slow TOC decrease can be explained by the non-degradation of these aromatic transformation products. Photolysis is a selective phenomenon as it can only degrade organic compounds that can absorb at the wavelength of the light used [START_REF] Speight | Redox transformations[END_REF]. In this study a monochromatic light at 365 nm was used; the transformation products may not absorb this specific wavelength and hence were not degraded by photolysis. With this specific wavelength, only weak bonds (C-F) or non-aromatic cyclic compounds (C 4 N 2 H 9 ) seem to be broken.

In contrast, Fig. 9(b) and Fig. 9(c) show that all photocatalysis transformation products were formed and degraded during irradiation. While photolysis is a selective phenomenon, photocatalysis is a nonselective process. Through the production of super reactive radicals, many organic compounds can be degraded by the attack of these powerful oxidants. In this study, all transformation products were monitored by HPLC-UV at 290 nm, which is the representative wavelength of aromatic compounds. Nevertheless, at the end of the process, every aromatic compound was mostly degraded as shown in the last chromatogram in Fig. 9 regardless of the form of the catalyst (TiO 2 coating or P25). The main difference between using P25 and TiO 2 coating is the quantity of transformation product produced. A higher amount of transformation products was formed with P25 due to the fast degradation of CIP. For example, the maximum HPLC peak area for transformation product A was 44.7 mUA.min for P25 while it was only 8.1 mUA.min for TiO 2 coating. When TiO 2 coatings was used, fewer transformation products were created than with P25 or pure UV: 8 products with TiO 2 coating compared with 9 products with P25 TiO 2 and 10 products by photolysis. Two reaction pathways seem to be preferential with TiO 2 coating: the oxidation and dealkylation pathways. However, the kinetics of the oxidation and dealkylation reactions could be faster than defluorination kinetics.

For both photocatalytic experiments, the TOC values remained high (32 % for the coating and 39 % for P25) indicating that other organic compounds were present. Indeed, the presence of smaller organic compounds that were not degraded was expected. The last sample of each experiment was analysed by ionic chromatography. Table 6 shows the identification and quantification of compounds present at the end of treatment. At least three different aliphatic acids were found: formic acid, oxalic acid, as well as acetic and/or propionic acids, which have the same retention time. As expected from the oxidation, defluorination and dealkylation pathways, the other compounds detected were as follows: F -, NO 2 -, NO 3 2-.

The last sample from each experiment presented a weak presence of small aliphatic acids: formic and oxalic acids. Under these experimental conditions (pH = 6.7 ± 0.1), formic acid was present in form of formate anion (HCOO -) and oxalic acid in form of oxalate dianion (C 2 O 4 2-).

These two ions were present in low concentrations: 0.810 mg.L -1 (formate) and 0.360 mg.L -1 (oxalate) for the coating and 1.04 mg.L -1 (formate) and 0.210 mg.L -1 (oxalate) for the P25 TiO 2 . From these concentrations, the carbon concentration from these ions could be determined which was found to represent 0.31 mg C .L -1 and 0.33 mg C . L -1 , which was only 8.1 % and 6.9 % of the final TOC for the coating and P25 TiO 2 respectively. The CIP molecule has only one fluorine atom, therefore the maximum quantity of residual fluorine in the liquid phase has to be equal to the molar number of initial CIP (6.04 × 10 --6 mol), which represents 1.15 mg F .L -1 . The analysis of the last sample showed a lower concentration of fluorine than expected (0.96 mg F .L -1 for the coating and 1.06 mg F .L -1 for P25 TiO 2 ). Due to this difference, remaining fluorine atoms were still attached to carbon aromatic rings. As shown in Fig. 9(b) and (c), some aromatic transformation products could still be present in the water. Due to their large number of carbon atoms, even a small amount of them can have a significant impact on the value of the TOC explaining a non-zero TOC at the end of the treatment. It is therefore possible that a large part of the remaining TOC is due to the presence of these aromatics in small quantities. Even if the performances of photocatalysis are not entirely satisfactory at the high concentration of CIP tested (20 mg.L -1 ), it is reasonable to expect that this photocalysis process to eliminate CIP may lead to better performances at environmental concentrations (< 1 μg.L -1 ).

The use of TiO 2 coating is part of sustainable processes. A glass window with a TiO 2 coating can be used several times in a row. The same glass window was used to treat five different initial solutions of ciprofloxacin at 20 mg/L of 100 mL. For these reuses, the same kinetic and mineralization performances were obtained, which shows the suitability of this process.

Analysis of a real effluent

The same experiments were carried out with a spiked real effluent for which the composition is given in Table 2. Fig. 10 compares the previous results obtained using ultra-pure water with the photolysis and photocatalysis (with coating) reaction performance using the real effluent. Only photocatalysis with coating was investigated because of the good efficiency obtained in clean water. With the real effluent, slower kinetic degradations were observed. For photolysis, 35 % of CIP still remains after 8 h of irradiation (compared with 20 % when using clean water) and for photocatalysis, 6 % of CIP remains after 8 h of irradiation (compared with less than 1 % when using clean water).

These differences could be explained by the complex nature of the real effluent which can strongly affect the performance of photocatalysis. A high TOC value of the real effluent was found (TOC = 40 mg C .L -1 ) in comparison with ultra-pure water (13 mg C .L -1 ). This higher value was due to the presence of organic compounds in this effluent and some of them are probably hardly oxidizable. That is why the organic compounds already present in the solution compete with CIP. The same interpretation explains the lower performance of photolysis with a TOC decrease of only 1 mg C .L -1 (TOC f = 39 mg C .L -1 ).

Compounds such as inorganic ions were also present (HCO 3 -for example), having a well-known scavenger effect. The same trend was observed during the photocatalytic degradation of carbamazepine [START_REF] Gao | Effects of inorganic ions on the photocatalytic degradation of carbamazepine[END_REF] and cylindrospermopsin [START_REF] Zhang | The effect of basic pH and carbonate ion on the mechanism of photocatalytic destruction of cylindrospermopsin[END_REF]. Indeed, HCO 3 -ions can directly react with hydroxyl radicals in order to create other radicals (CO 3 2-• ) that are more highly selective and less reactive. Thus, reactive site of TiO 2 were less readily available for the degradation of CIP or transformation products. After photocatalytic degradation, 20 mg C .L -1 still remained in the solution corresponding to 50 % of TOC decrease. As in ultra-pure water experiment, CIP was totally removed after 8 h (10 h in real effluent) but a total mineralisation was not reached even after 8 h of treatment. Fig. 11 shows the same trend for both reactions; many transformation products still remained in solution after 8 h of UVtreatment. The photocatalysis reaction results in better performance than photolysis although all aromatic transformation products were still detected in the solution. As for CIP, the degradation kinetics of the transformation products were slower in this complex matrix due to the presence of other organic compounds. Nevertheless, even if the degradation was incomplete after 8 h of irradiation, the concentration of these molecules decreased thereby showing the non-selectivity of photocatalysis. On the other hand, some transformation products did not seem to be degraded by photolysis showing its selectivity and confirming the efficiency of the photocatalytic process.

Conclusion

The TiO 2 coating presented interesting photocatalytic properties even with smaller amount of TiO 2 deposited compared to most studies.

Table 6

Identification and quantification of the final compounds present in water after treatment.

This study shows that antibiotics such as CIP can be degraded by a photocatalysis reaction using a wavelength present in solar radiation (365 nm) without doping the catalyst. Photocatalysis is confirmed as a non-selective process whereas photolysis is confirmed as a selective one. As expected, CIP can be degraded by photolysis, however most of the transformation products remain stable. The TiO 2 coating showed a slower degradation with CIP than with P25 but less transformation products were formed. The degradation by photocatalysis with both P25 and TiO 2 coating showed that most of the transformation products were degraded with 61 % and 68 % of TOC degradation respectively. The final transformation products were essentially aromatics molecules and small aliphatic acids (oxalic, formic, propionic and acetic acids). Concerning the chemical mechanisms, three different pathways, were identifiednamely oxidation, alkylation and defluorinationby the presence of transformation products detected. Concerning the representation of kinetics degradation of CIP, the pseudo-first order model cannot be used even if the concentration of CIP is small (K ad C values close to 1), however excellent representation by the Langmuir-Hinshelwood has been noticed with a value of 0.0284 L.mg -1 for K ad and 0.343 mg.L -1 . min -1 for k (for coating experiment). Moreover, the set-up used for photocatalysis experiments with TiO 2 coating proved its suitability: a same TiO 2 /window glass can be used several times without being saturated. Experiments with a real effluent show satisfactory efficiency for the photocatalytic degradation of antibiotics using MOCVD coating, despite a high TOC value and the presence of scavengers such as inorganic ions.

Credit author statement

Caroline Andriantsiferana and Claire conceived, supervised and designed the experiments. Thibaut TRIQUET performed the experiments, HPLC and TOC analysis and did the calculation for each kinetics models. Laure Latapie did every HPLC-MS analysis. Claire Tendero did the TiO 2 coating on glass by MOCVD with all characterizations of it (SEM, AFM, XRD). Thibaut TRIQUET, Claire Tendero, Caroline Andriantsiferana, Laure Latapie, Romain Richard and Marie-Hélène Manero wrote the paper.

Fig. 1 .

 1 Fig. 1. Experimental setup representation (a); experimental setup (b); photo-reactor (c).

  TiO 2 coatings were columnar as shown in the SEM photos given in Fig. 2. Fig. 2(a) shows a top view of the coating indicating full coverage of the substrate and Fig. 2(b) shows a cross section of the coating used to estimate its thickness at 1.6 ± 0.2 μm. The quadratic roughness (31 ± 1 nm) was deduced from AFM measurements.

Fig. 2 .

 2 Fig. 2. SEM photos of TiO 2 coating on silicon substrate: (a) top view and (b) cross section of the coating.

Fig. 3 .Fig. 4 .

 34 Fig. 3. XDR analysis of TiO 2 coating.

Fig. 5 .

 5 Fig. 5. Determination of the band gap energy of coating.

Fig. 7 .

 7 Fig. 7. Absorbance spectrum of ciprofloxacin.

Fig. 8 .

 8 Fig. 8. Evolution of TOC in photolysis and photocatalysis reaction before and after treatment.

Fig. 9 .

 9 Fig. 9. Evolution of transformation products by photolysis (a), photocatalysis with P25 (b) and coating (c) reactions and last chromatogram for photocatalysis reaction.

Fig. 10 .

 10 Fig. 10. Comparison of CIP degradation in ultra-pure water and a real effluent.

Table 2

 2 Analysis of the real effluent.

	Analysis	Method	Results	Units
	NH 4 + NO 3 -NO 2 -CO 3 2-	ISO 11,732 ISO 13,395 ISO 13,395 ISO	<0.4 <0.5 <0.1 <1	mg N .L -1 mg N .L -1 mg N .L -1 m CO3 .L -1
	HCO 3 -Chemical Oxygen Demand (COD) Biochemical Oxygen Demand	9963-1 ISO 9963-1 ISO 15,705 1899-1	212 <0.5 2.0	mg HCO3 .L -1 mg O2 .L -1 mg O2 .L -1
	(BOD)
	Alkalimetric Title	ISO	<5	ppm
		9963-1
	Complete Alkalimetric Title	ISO	174	ppm
		9963-1
	pH	ISO 10,523	7.4 (at 19 • C)	-

Table 3

 3 Operating conditions for MOCVD process.

	Precursor temperature in the bubbler	50 • C
	Deposition temperature Carrier gas (N 2 ) flow rate Dilution gas (N 2 ) flow rate	500 • C 25 cm 3 . min -1 500 cm 3 . min -1
	Deposition pressure	5 Torr

Table 4

 4 Kinetics coefficients of CIP degradation.

		Pseudo-first order	Langmuir-Hinshelwood model
		Simplified model
		(K ad C << 1)
	Study	k app (min -1 )	r 2	K ad (L. mg -1 )	k (mg. L -1 . min -1 )	kK ad (min -1 )	∑ (C mod -C exp ) 2
	P25	0.0648	0.986	0.0533	1.961	0.105	0.4146
	Coating	0.0085	0.993	0.0284	0.343	0.0097	0.0018

Table 5

 5 Transformation products: molecule structure, reaction pathway and HPLC retention time.

	Molecule	Molecule structure [14]	Retention time (min)	Reaction pathway	Detection Photolysis	Photocatalysis P25	Photocatalysis coating
	A		3.0	Oxidation Defluorination	Yes	Yes	Yes
	B		3.1	Oxidation Defluorination	Yes	Yes	Yes
	C		3.3	Dealkylation	Yes	Yes	Yes
	D		4.06	Oxidation	Yes	Yes	Yes
	E		4.37	Dealkylation Defluorination	Yes	Yes	Yes
	F		4.6	Oxidation	Yes	Yes	Yes
	G		4.72	Oxidation	Yes	Yes	Yes
	H		4.98	Dealkylation	Yes	Yes	Yes
	I		3.9	Oxidation Defluorination	Yes	Yes	No
	J		4.3	Oxidation Defluorination	Yes	No	No

Acknowledgements

We would like to express kind regards to Pierre Albrand from Laboratoire de Génie Chimique (Université de Toulouse, CNRS, INPT, UPS, Toulouse, France) for his help to create the MATLAB code for kinetics coefficient determination. We would also like to express kind regards to the "Pole LCMS" from Laboratoire de Génie Chimique (Université de Toulouse, CNRS, INPT, UPS, Toulouse, France) for their help for the identification of transformation products by LCMS. Fig. 11. Evolution of transformation products during (a) photolysis and (b) photocatalysis (coating) experiments with the spiked real effluent.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.