K González-Labrada

I Quesada-Peñate

F Velichkova

C Julcour-Lebigue

C Andriantsiferana

M-H Manéro

C Albasi

U J Jáuregui-Haza

DEGRADATION OF PARACETAMOL IN AQUEOUS SOLUTION: COMPARISON OF DIFFERENT UV INDUCED ADVANCED OXIDATION PROCESSES

Keywords: acetaminophen, photolysis, UV/H2O2, photo-Fenton, UV/TiO2

Environmental hazards are a consequence of the massive use of synthetic chemicals that are prevalent in nearly every aspect of modern life. The outburst of the so-(pharmaceuticals and pesticides among others) generates an additional problem due to the scarce available information on their impact on the environment or their interferences with the biological processes. Among them, paracetamol is one of the drugs the most widely found in hospital effluents, in the effluents of wastewater treatment plants, as well as in rivers. In this work, the feasibility of paracetamol degradation by different processes based on ultraviolet radiation is discussed: photolysis, photo-oxidation with hydrogen peroxide, photo-Fenton reaction, and heterogeneous photocatalysis with TiO2 are evaluated. The best results are obtained with the photo-Fenton process, for which more than 99% of the pharmaceutical is degraded within 5 minutes. At best experimental conditions the mineralization was superior to 75% after one hour of reaction.

I. INTRODUCTION

The rapid development of industry and society runs parallel with the increasing generation of wastewaters, containing highly toxic and poorly biodegradable compounds. This situation affects the efficiency of conventional biological treatments, the most commonly used processes in wastewater remediation. The so-called and pesticides among others) generate an additional problem due to the scarce available information on their impact in the environment or their interferences with biological processes [START_REF] Fent | gy of human pharmaceuticals[END_REF]. The presence of pharmaceutical compounds in water and wastewater has been repeatedly reported (Kümmerer, 2001;[START_REF] Ikehata | El -adation of aqueous pharmaceuticals by ozonation and advanced oxidation processes: A review Ozone[END_REF][START_REF] Al-Rifai | of pharmaceutically active and non-steroidal estrogenic compounds in three different wastewater recycling schemes in Australia[END_REF][START_REF] Gagné | pharmaceutical products in a municipal effluent and toxicity to rainbow trout (Oncorhynchus mykiss) hepatocytes, Ecotoxicol[END_REF], Quesada Peñate et al., 2009a). Many of these active substances are persistent as they are transparent to conventional wastewater treatments and they are becoming ubiquitous in the environment [START_REF] Carballa | rmones in a sewage treatment plant[END_REF][START_REF] Tauxe-Wuersch | drugs in sewage treatments plants in Switzerland and risk assessment[END_REF]. Some of these compounds -in particular antibiotics, anti-tumor agents and analgesics -are indeed neither biodegradable, nor adsorbable on sewage sludge. Therefore, there has been an increasing concern about the impact of pharmaceutical compounds on public health and on the environment, not only because of their acute toxicity, but also their genotoxicity, as well as the development of pathogen resistance and endocrine disruption [START_REF] Halling-Sørensen | aceutical substances in the environment a review Chemosphere[END_REF][START_REF] Graham | Werener and Gene Abundances Associated with Waste Discharges to the Almendares River near Havana, Cuba[END_REF].

To drastically reduce the load of pharmaceutical compounds in discharged effluents, additional treatments have to be applied. Advanced oxidation processes (AOPs) have already proven to be effective for this purpose [START_REF] Kolpin | wastewater contaminants in US streams, 1999 2000: a national reconnaissance[END_REF][START_REF] Ikehata | El -adation of aqueous pharmaceuticals by ozonation and advanced oxidation processes: A review Ozone[END_REF][START_REF] Al-Rifai | of pharmaceutically active and non-steroidal estrogenic compounds in three different wastewater recycling schemes in Australia[END_REF][START_REF] Mcardell | Input and Elimination of Pharmaceuticals and Disinfectants from Hospital Wastewater[END_REF]. AOPs can be performed either as a tertiary treatment after biological (secondary) treatment, or as a pre-treatment stage in order to enhance the biodegradability of trace organic contaminants.

AOPs are characterized by a variety of radical reactions that involve combinations of chemical agents (e.g. O3, H2O2), heterogeneous catalysts (transition metals and metal oxides) and auxiliary energy sources (e.g. UV-vis radiation, electronic current, and ultrasound). UV based AOPs include UV/H2O2, UV/O3, UV/H2O2/O3, UV/H2O2/Fe 2+ (or Fe 3+ ), and UV/TiO2. The efficiency of the various AOPs depends both on the rate of generation of free radicals and the extent of contact between the radicals and the organic compounds (Gogate and Pandit, 2004). In the Fenton reaction, hyecomposition of H2O2 using Fe 2+ in acidic medium (pH in the range 2 4) [START_REF] Oliveros | scale development of a light-enhanced Fenton reaction[END_REF]. Photo-Fenton oxidation is the photo-catalytically enhanced version of the Fenton process. In this process, UV light irradiation egeneration of the ferrous catalyst by reduction of Fe 3+ and destruction of ferric complexes, thus allowing the redox cycle to go on as long as H2O2 is available [START_REF] Pignatello | 3+ catalyzed degradation of chlorophenoxy herbicides by hydrogen peroxide[END_REF]Safarzadeh-Amiri et al., 1996, l997;[START_REF] Carneiro | Homogeneous photodegradation of C.I. Reactive Blue 4 using a photo-Fenton process under artificial and solar irradiation[END_REF]. In UV/TiO2 process, titanium dioxide semiconductor absorbs UV light and generates radicals. Specifically, UV illumination of TiO2 generates conduction band electrons and valence band holes. Band electrons interact with surface adsorbed molecular oxygen to yield superoxide radical anions, while band holes interact with water to produce hydroxyl radicals [START_REF] Crittenden | Water treatment: Principles and Design[END_REF][START_REF] Chen | adsorption and photocatalytic degradation of an organic contaminant in an aqueous suspension of TiO2 by inorganic ions[END_REF]Coronado and Soria, 2007).

Paracetamol (N-acetyl-4-aminophenol or acetaminophen, C8H9NO2) is a common analgesic and antipyretic drug, heavily used all over the world. It has been detected in hospital effluents, but also in wastewater treatment plants, rivers and muds with concentrations in the range 6-65 µg.L -1 (Bound and Voulvoulis, 2006;[START_REF] Gómez | rious therapeutic classes by solid-phase extraction and liquid Chromatography tandem mass spectrometry analysis in hospital effluent wastewaters[END_REF][START_REF] Kinney | ntaminants in biosolids destined for land application[END_REF][START_REF] Thomas | rmaceuticals from two Oslo city hospitals and a wastewater treatment works[END_REF][START_REF] Al-Rifai | of pharmaceutically active and non-steroidal estrogenic compounds in three different wastewater recycling schemes in Australia[END_REF]. Despite paracetamol is biodegradable [START_REF] Sang | endocrine disruptors in South Korean surface, drinking, and waste waters[END_REF], its degradation rate is slow and does not allow its complete elimination by conventional wastewater treatment plants. On the other hand, chlorine treatment used for disinfection purpose is known to transform paracetamol into highly toxic intermediates, such as 1,4 benzoquinone and N-acetyl-pbenzoquinone imine (Bedner and Maccrehan, 2006). It is known that the hydrolysis of paracetamol is proton and hydroxyl ion catalyzed, while this breakdown process is minimal at a pH close to 6.

Several previous publications have been devoted to the study of paracetamol degradation in water using different advanced oxidation processes like radiolysis, sonolysis, catalytic wet air oxidation (CWAO) on activated carbon, ozonation and photolysis [START_REF] Dalmázio | ppraisal on the degradation of paracetamol by TiO2/UV system in aqueous medium. Product identification by gas Chromatography-mass spectrometry (GC-MS)[END_REF][START_REF] Yang | Degradation of paracetamol in aqueous solutions by TiO2 photocatalisys[END_REF][START_REF] Andreozzi | means of ozonation and H2O2/UV system[END_REF]Quesada Peñate et al., 2009b[START_REF] Quesada Peñate | Jauregui ation of paracetamol by catalytic wet air oxidation and sequential adsorption catalytic wet air oxidation on activated carbons[END_REF][START_REF] Neamtu | [END_REF][START_REF] Aguinaco | Gimepharmaceuticals single ozonation: A comparative study of its application on Fenton like systems[END_REF]Torun et al., 2014).

The oxidation of paracetamol promoted by UV/TiO2 in aqueous medium was investigated by [START_REF] Dalmázio | ppraisal on the degradation of paracetamol by TiO2/UV system in aqueous medium. Product identification by gas Chromatography-mass spectrometry (GC-MS)[END_REF], who obtained 90% of drug degradation and 35% of mineralization in two hours. [START_REF] Yang | Degradation of paracetamol in aqueous solutions by TiO2 photocatalisys[END_REF] studied photolysis and photocatalytic oxidation of acetaminophen to determine the optimal operating conditions for its degradation. UVA (365 nm) irradiation alone converted a negligible amount of paracetamol, whereas paracetamol concentration decreased substantially under UVC (254 nm) irradiation with marginal changes in total organic carbon (TOC). Paracetamol oxidation from aqueous solutions was also studied by means of ozonation and H2O2 photo-oxidation [START_REF] Andreozzi | means of ozonation and H2O2/UV system[END_REF]. Both oxidative systems were able to destroy the aromatic ring of the substrate with a partial conversion of the initial carbon content into carbon dioxide. For the adopted experimental conditions, mineralization yields up to 30% and 40% were observed with ozonation and H2O2 photo-oxidation, respectively. Main reaction intermediates and products were identified for both systems by HPLC MS and GC MS analyses [START_REF] Andreozzi | means of ozonation and H2O2/UV system[END_REF]. [START_REF] Neamtu | [END_REF] studied the degradation of paracetamol in a bench-scale setup by means of simple ozonation (O3) and ozonation enhanced by UV light (UV/O3). Both oxidative systems were capable of removing the substrate with mineralization yields up to 51% for ozonation and 53% for UV/O3. The main advantage of photo-ozonation compared to simple ozonation was a more rapid conversion (92% vs. 79% after 90 min). The paracetamol decay followed a pseudo firstorder reaction with a superior rate constant (higher by 54%) for the UV promoted system in comparison with the direct ozonation. Mineralization was also slightly accelerated (+4%) in the UV/O3 system, due to the additional production of hydroxyl radicals induced by the UV light [START_REF] Neamtu | [END_REF]. The oxidation of a mixture of paracetamol and sulfamethoxazole was carried out by means of single ozonation as initial process for in-situ production of hydrogen peroxide and in a subsequent step this hydrogen peroxide was used for a sequential treatment in combination with Fenton or photo-Fenton systems [START_REF] Aguinaco | Gimepharmaceuticals single ozonation: A comparative study of its application on Fenton like systems[END_REF]. From single ozonation, fast and complete elimination of both pharmaceuticals was achieved, but at the end of the ozonation step only 10% of total organic carbon was removed. The addition of ferrous ions to the reaction media after single ozonation did not improve the extent of mineralization. Conversely, the application of photo-Fenton process resulted in a markedly increase in the mineralization efficiency, nevertheless total mineralization was not attained. A simple kinetic model of the processes allowed the calculation of pseudo-first order kinetic constants. Higher rate constants for the photo-Fenton system were due to the photochemical regeneration of ferrous ions [START_REF] Aguinaco | Gimepharmaceuticals single ozonation: A comparative study of its application on Fenton like systems[END_REF].

In this work, the effect of several operating conditions on paracetamol removal efficiency, namely pH and initial concentrations of H2O2, Fenton reagent and TiO2, was studied. In addition, the mineralization of the substrate was also evaluated.

II. MATERIALS AND METHODS

A. Materials C8H9NO2 (99 %), FeSO4

2O as the source of Fe 2+ , H2SO4 (95-97 %) and NaOH (99 %) were all provided by Sigma-Aldrich. TiO2 PC 500 catalyst was supplied by Millennium. It was characterized by its anatase structure (> 99 %), with crystallite diameter between 5 and 10 nm. H2O2 (30 % w/w) and H3PO4 (85 %) were purchased from Fluka. HPLC eluent acetonitrile (C2H3N) was provided by Scharlau-Chemie. Ultrapure water supplied by Purelab Option system (Elga) was used throughout the study. For all experiments, the initial concentration of paracetamol was 100 mg.L -1 .

B. Photolysis, H2O2 photo-oxidation and photo-Fenton experiments

The corresponding experimental set-up is displayed in Fig. 1. It is composed of a 1L Pyrex reactor coupled to a 0.5 L glass photochemical reactor. The latter includes a medium-pressure mercury vapor lamp (450W Hanovia PC451.050 lamp, arc length 4.8 cm) placed in a jacketed borosilicate glass immersion well. Of total energy radi-ated, approximately 40-48 % is in the ultraviolet portion of the spectrum (with a maximum at 366 nm), 40-43 % in the visible, and the balance in the infrared. The glass lamp holder completely cuts off wavelengths shorter than 280 nm, about 50% of the emission at 310 nm and results in 94% transmittance above 355 nm.

Both reactors are connected to a thermostat for temperature control (30±1°C). The solution circulates between the reactors by means of a peristaltic pump at a flow rate of 80 mL.min -1 and it is continuously stirred in both reactors.

The direct photolysis was carried out at three pH values (2.6, 5.5 and 8.0) adjusted with H2SO4 or NaOH. In the UV/H2O2 experiments, oxidant concentration was varied between 13.9 and 41.7 mmol.L -1 , corresponding to 1 to 3 times the stoichiometric amount for complete mineralization.

For the photo-Fenton oxidation, different combinations of Fe 2+ (FeSO4 2O) and H2O2 concentrations were applied following a 2 2 factorial design plan (Fe 2+ : 0.30 and 0.90 mmol.L -1 ; H2O2: 13.9 and 41.7 mmol.L -1 ).

C. TiO2 photocatalysis experiments

Photocatalysis experiments were carried out in a 1 L cylindrical Pyrex reactor with air injection to provide oxygen and magnetic stirring. Reaction temperature was set to 30 ± 1°C. A low-pressure mercury lamp (Philips PL-L 24W/10/4P model with a maximum emission at 365 nm and a measured light intensity of 47.5 W.m -2 ) was used and placed in a thermo-regulated cylinder. TiO2 concentration was varied from 0 to 3 g.L -1 . This experimental set-up is shown in Fig. 2.

D. Chemical analysis

Oxidation experiments were carried out during 5 hours, during which samples were taken at regular time intervals to follow the evolution of paracetamol conversion and mineralization. Photo-Fenton samples were preliminary treated with a quenching solution (NaOH 0.1 mol.L -1 ) that precipitated iron. This precipitate was removed by 0.2 µm nylon filters. Photocatalysis samples were passed through the same filters to eliminate powdered TiO2.

For all the processes, liquid samples were characterized using a liquid chromatograph equipped with a UV/Vis detector (Varian ProStar 310). Paracetamol was separated from oxidation intermediates on a Prontosil C18 reversed phase column, using a 90:10 mixture of acidified water (pH 2.2) and acetonitrile as isocratic mobile phase (flow rate of 0.25 mL.min -1 ). The detection of the compound was achieved at a wavelength of 254 nm.

To monitor the mineralization level, the residual TOC was measured on a SHIMADZU TOC-V CSN analyzer equipped with a non-dispersive infrared detector. The TOC values were obtained from the difference between the Total Carbon (TC) and Inorganic Carbon (IC) contents of the samples For TC, the combustion reaction was achieved at 680°C on a platinum catalyst, using oxygen flow at 150 mL.min -1 . IC was measured by addition of HCl 2N and CO2 degassing through nitrogen sparging.

III. RESULTS AND DISCUSSION

Table 1 shows the results of paracetamol photodegradation for non-catalytic processes. The initial pH value (pH0) was varied to study its effect on the considered processes. During the oxidation, the pH of the reaction medium usually decreased due to the formation of acidic intermediates (final pH ranging between 3.0 and 5.0).

Irrespective of the pH value, the degradation rate of paracetamol by photolysis at > 280 nm was almost negligible, a maximum conversion of 4.2% being achieved after 5 hours with pH0 = 2.6. It is known that the hydrolysis of paracetamol is proton and hydroxyl ion catalyzed, while this breakdown process is minimal at a pH close to 6 (Koshy and Lach, 1961). Previous works confirmed that at pH between 3.7 and 5.5 UVAvis radiation only results in a poor removal of the molecule, as a consequence of its weak absorption within the range 300 to 800 nm [START_REF] Yanxiang | onite KSF suspension[END_REF][START_REF] Aguilar | Mocteophen[END_REF].

The addition of H2O2 improved the pollutant conversion, and the best results were obtained for medium pH0 (5.5) and oxidant dose (27.8 mmol.L -1 ). Under these conditions, 30% of paracetamol was eliminated after 60 minutes of reaction. Still, the TOC conversion reached less than 10% after 5 hours of reaction for all the conditions. Indeed, H2O2 does not absorb significantly beyond 300 nm and the production of emain low under the applied light irradiation.

Photocatalysis with T iO2 irradiated at 365 nm yielded a paracetamol conversion of more than 60% at nearneutral pH (Fig. 3), but no effect of the catalyst concentration was found in the investigated range (0.3-3 g.L -1 ). Such a plateau was already observed and attributed to the opposite effects between the increase in active surface and decrease of light penetration when increasing TiO2 concentration [START_REF] Aguilar | Mocteophen[END_REF]. While greater catalyst concentrations increase available active sites, light penetration in the reaction system decreases, activating a reduced amount of TiO2 in the suspension. On the other hand, at a higher catalyst loading, the originally activated TiO2 may be deactivated through collision with ground-state catalyst according to the equation: TiO2* + TiO2 TiO2 # + TiO2 where TiO2* has active species adsorbed on its surface and TiO2 # is the deactivated form of TiO2 [START_REF] Yang | Degradation of paracetamol in aqueous solutions by TiO2 photocatalisys[END_REF]. Large catalyst concentrations would also cause agglomeration and sedimentation of TiO2, reducing the available catalyst surface for photon absorption. All these reasons might explain the negligible increase in paracetamol degradation beyond an optimal TiO2 dosage, 0.5 g.L -1 in this study. The present results are also in line with the work of [START_REF] Yang | Degradation of paracetamol in aqueous solutions by TiO2 photocatalisys[END_REF], which observed that more than 95% of 2.0 mM paracetamol was degraded within 80 min under UVC in the presence of TiO2 0.4 g.L -1 , with no improvement over 0.8 g.L -1 of catalyst. In our conditions, the TOC conversion after 4 hours was about 30% with 0.3 g.L -1 of TiO2.

On the other hand, photo-Fenton oxidation achieved over 99% conversion of paracetamol within only five minutes. Table 2 shows the levels of pollutant mineralization for this process. For the highest quantity of catalyst and oxidant the mineralization yield was superior to 75% after one hour of reaction, outperforming the results of the other photo-processes under UVA(/B)-vis light irradiation. In other words, photo-Fenton oxidation was found the best process for paracetamol destruction and the contribution of concomitant non-catalytic photo-processes (direct photolysis and UV/H2O2) remained very low. A similar performance was reported by Santos-Juanes [START_REF] Jordá | Mineralization level and reaction time: The keys for increasing plant efficiency[END_REF] at a pilot plant scale: a solution of 157.5 mg.L paracetamol was satisfactorily decontaminated by 15 minutes of photo-Fenton treatment with 20 mg.L of Fe 2+ and 200 mg.L of H2O2, yielding more than 20% TOC conversion. Indeed, from bioassays performed with P. putida the authors concluded that the biodegradability efficiency increased from 30% for untreated paracetamol solution to about 70% for a mineralization yield of ca. 20% (complete biodegradability being achieved over 72% of mineralization).

Finally, the efficiency of UV-induced AOPs can be compared to that of other oxidation processes carried out in our previous works: catalytic wet air oxidation (CWAO) over activated carbon [START_REF] Quesada Peñate | Jauregui ation of paracetamol by catalytic wet air oxidation and sequential adsorption catalytic wet air oxidation on activated carbons[END_REF] and high frequency ultrasound (Quesada Peñate et al., 2009b). None of these techniques could compete with the results of photo-Fenton oxidation (at optimal conditions), as they did not exceed 60% mineralization yield after 2 hours.

IV. CONCLUSIONS

Efficient degradation of paracetamol in aqueous solution could be achieved by a combination of UV-vis irradiation ( > 280 nm) with Fenton reagent, yielding mineralization level as high as 75% in one hour. Conversely, direct photolysis, UV/H2O2 and photocatalysis with TiO2 resulted in only a partial conversion of the pollutant after 4-5 hours, with a mineralization yield lower or only slightly higher than the required value of 20% reported by Santos-Juanes [START_REF] Jordá | Mineralization level and reaction time: The keys for increasing plant efficiency[END_REF] for an efficient detoxification.

To confirm this conclusion, next study will focus on the investigation of the biodegradability improvement with the perspective of an integrated process together with biological treatment.

Fig. 1 .

 1 Fig. 1. Experimental set-up for photolysis, UV/H2O2 and photo-Fenton oxidation.

Fig. 2 .

 2 Fig. 2. Experimental set-up for heterogeneous photocatalytic degradation.Table 1. Conversion of paracetamol (%) by non-catalytic photo-processes Time, min

Fig. 3 .

 3 Fig. 3. Photocatalytic degradation of paracetamol at different TiO2 concentrations.Table2. Mineralization of paracetamol (%) by photo-Fenton process (pH=2.6; n.a: non available) Time, min H2O2, mmol.L -1

Table 1 .

 1 Conversion of paracetamol (%) by non-catalytic pho-

			to-processes
					Time, min
	Process	pH0	H2O2, mmol.L -1	10	60	180	300
		2.6	-	0.7	0.9	2.5	4.2
	UV	5.5	-	0.5	1.3	1.2	1.2
		8.0	-	0.7	0.9	1.0	3.0
	UV/H2O2 2.6	13.9	1.1	7.8	19.9 26.7
		2.6	41.7	2.8	8.2	27.1 29.2
		5.5	13.9	0.7	5.2	28.4 41.3
		5.5	27.8	3.6 30.0 57.7 69.0
		5.5	41.7	2.5 21.6 58.6 68.8

Table 2 .

 2 Mineralization of paracetamol (%) by photo-Fenton process (pH=2.6; n.a: non available)

	Time, min


Aurelien Chassery
email: aurelien.chassery@cea.fr

Helene Lorcet

Joel Godlewski

Karine Liger

Christian Latge

Xavier Joulia

EXPERIMENTAL STUDY OF THE TRITIUM DISTRIBUTION IN THE EFFLUENTS RESULTING FROM THE SODIUM HYDROLYSIS

Within the framework of the dismantling of fast breeder reactors in France (PHENIX, SUPERPHENIX, RAPSODIE), several processes are under investigation regarding sodium disposal. One of them, called ELA (radioactive sodium waste treatment process), is based on the implementation of the sodium-water reaction, in a controlled and progressive way, to remove residual sodium mainly from the sodium purification systems called cold traps. This sodium contains impurities such as sodium hydride, sodium oxide and tritiated sodium hydride. The hydrolysis of these various chemical species leads to the production of a liquid effluent, mainly composed of an aqueous solution of sodium hydroxide, and a gaseous effluent, mainly composed of nitrogen (inert gas), hydrogen and steam. The tritium is distributed between these effluents, and, within the gaseous effluent, according to its forms HT and HTO. HTO being 10,000 times more radiotoxic than HT, a precise knowledge of the mechanisms governing the phase distribution of tritium is necessary. Indeed, it will help to design the process needed to optimize the treatment of the off-gas before its release into the environment.

This paper presents the first experimental results from a parametric study on the tritium distribution between the various effluents generated during hydrolysis operations. This parametric study has been performed in a laboratory scale hydrolysis process designed at the CEA Cadarache.

I. INTRODUCTION

Liquid sodium, used as a coolant fluid in Sodium Fast Breeder Reactors (SFBR), can contain several impurities such as oxygen or hydrogen, present from the start or introduced during certain servicing or handling operations on components or fuel assemblies. Another contaminant that can be found in the sodium is the tritium, a radioactive hydrogen isotope, mainly produced by fission reactions. In order to operate in safe conditions, the sodium is purified in devices called cold traps, in which the crystallization of the sodium oxide (Na 2 O), sodium hydride (NaH) and co-crystallization of tritiated sodium hydride (NaT) is induced by cooling. The crystals thus formed are trapped on a woven steel wool support. When these devices need to be dismantled, the residual sodium and impurities must be removed.

Various methods have been proposed or developed in the past to remove residual sodium from cold traps. [START_REF] Latge | A new process for the removal of impurities in the cold traps of Liquid Metal Fast Reactors[END_REF] One of them rests on the implementation of the sodium-water reaction (SWR) to remove films or clusters of residual sodium. As described in previous paper (Ref. 2), this reaction is almost instantaneous, highly exothermic a and is characterized by the generation of large volumes of hydrogen gas and the production of soda.

) ( , 2 ) ( ) ( 2 ) ( 2 1 g s l s H NaOH O H Na    (1) ) ,( 2 ) ( ) ( 2 ) ( g s l s H NaOH O H NaH    (2) ) ( ) ( 2 ) ( 2 2 s l s NaOH O H O Na   (3) ) ( ) ( ) ( aq aq water s OH Na NaOH     (4)
In order to use these reactions in a controlled and progressive way, a process called ELA (Enceinte de Lavage en Actif), is under development mainly for the cleaning of cold traps from the prototype fast breeder reactor PHENIX. This process is continuously controlled by limiting the water flow rate in the vessel to avoid over pressure, hydrogen and temperature excursions.

In order to design the process, a precise knowledge of the composition of the effluents is needed. Indeed, the a The heat generated by a mole of sodium is, respectively for each reaction, Δ r H 0 1 = -141 kJ•mol -1 Na , Δ r H 0 2 = -82 kJ•mol -1 Na , Δ r H 0 3 = -76 kJ•mol -1 Na and Δ r H 0 4 = -45 kJ•mol -1 Na . Δ r H 0 4 is the heat of solution of NaOH at infinite dilution. tritium distribution between the gaseous and liquid effluents, and within the gaseous effluent, according to its chemical form HT(g) or HTO(g), is critical for designing the elements needed to treat the off-gas before its release into the environment.

The sodium wastes expected to be hydrolyzed in ELA should contain a large majority of tritium as tritiated sodium hydride NaT. By analogy with the reaction (2), the chemical phenomena involved in the hydrolysis of NaT should lead to the production of gaseous tritium in its HT(g) form only. However, HTO(g) has been identified in the gaseous effluent produced by hydrolysis operations performed at the CEA Cadarache, and sometimes in an important amount (up to 90 % of the tritiated gaseous products).

In the phenomenological model developed in this study, any conversion of HT(g) into HTO(g), by oxidation or by isotopic exchange reaction, has been dismissed 2 . It is assumed that HTO is produced by the chemical reactions in liquid state and that part of this HTO(l) changes into HTO(g) either by evaporation or by vaporization. A more precise description of the phenomena involved is provided in Ref. 2.

In order to validate some of the hypotheses proposed in this scenario, a laboratory scale process has been designed at the CEA Cadarache. Experimental results of a parametric study on the tritium distribution in the effluents are presented and discussed in this paper. This study is carried out in order to provide a numerical model of the phenomena involved in such an operation to predict the composition of the effluents produced, especially regarding tritium contamination.

II. LABORATORY SCALE HYDROLYSIS PROCESS

Due to technical limitations, specific activities of the samples and experimental conditions are not representatives of ELA. Another difference between the experimental device used for this study and ELA rests on the means used to perform the hydrolysis operations. Water is not spread under the sodium but above it, drop by drop. Nevertheless, the results of this parametric study should validate some of the hypotheses proposed to explain the tritium distribution.

The different parts of the process are described in the following paragraphs and presented in Fig. 1.

II.A. Hydrolysis Reactor

The main part of the experimental device is a 2-liter glass reactor into which small amounts of sodium (up to 15 g) are hydrolyzed. The sodium is maintained on a perforated metallic plate that allows the liquid effluent produced to flow toward the bottom of the reactor for future collection. In order to dilute the hydrogen produced, the vessel is swept by a flow rate of Argon. Water is added drop-by-drop over the sodium by means of a syringe pump. Flowmeters, pressure sensors, thermocouples and a moisture sensor are used to perform mass and energy balances.

II.B. Gaseous Tritium Trapping System

At the outlet of the reactor, tritiated water vapor HTO(g) is trapped into a first series of bubblers. The remaining gaseous tritium, in its HT(g) form, is then converted into HTO(g) by an oxidation reaction with CuO at a temperature of 873 K, before being trapped into a second series of bubblers. For each experiment, tritium collected in the liquid effluents in the reactor and in the bubblers is quantified by liquid scintillation counting.

III. PARAMETRIC STUDY ON THE TRITIUM DISTRIBUTION IN THE EFFLUENTS

III.A. Description of the Experiments

A series of experiments have been performed in order to study the influence of water flow rate, argon flow rate, initial mass and specific activity of the sample.

The sodium hydrolyzed in this study is from the secondary loop of the reactor PHENIX. Small cylindrical samples with an initial mass of approximately 5 or 10 g are used.

Argon flow rate is varying from 1.00 to 4.50 l•min -1 and water flow rate from 5.0 to 17.5 ml•h -1 .

III.B. Results and Discussion

Experimental results are gathered in Table I. Argon flow rates, water flow rates, initial mass of the samples, total specific activities and tritium distributions are presented for a series of thirteen experiments.

TABLE I. Results of the Parametric Study on the Tritium Distribution in the Effluents Resulting from a Sodium Hydrolysis

III.B.1. Influence of the Total Specific Activity on the Tritium Distribution

An influence of the total activity on the tritium distribution, according to its chemical form (HT or HTO), seems to be observed in the Fig. 2a and is confirmed in the Fig. 2b, where experimental results of the tritium distribution gathered from former experiments also performed at the CEA have been added to the study. The concentration of HTO seems to rise until the achievement of a limit for high total activities, whereas the concentration of HT continues to rise.

III.B.2 Influence of the Water and Argon Flow Rates on the Distribution of HTO between the Effluents

According to the experimental results plotted on Fig. 3, water flow rate seems to have an influence on the distribution of HTO. As expected in the phenomenological study, the higher the water flow rate is, the higher the overall hydrolysis reaction rate should be which leads to a higher quantity of energy released to vaporize the tritiated water.

In the studied range of parameters, argon flow rate, related to the evaporation of HTO, seems to have no influence on the phase change of tritiated water according to the results plotted on Fig. 4.

IV. CONCLUSIONS AND PERSPECTIVES

The first experimental results of a parametric study on the tritium distribution between the effluents resulting from a sodium hydrolysis have been gathered in this paper. This experimental study was carried out at the CEA Cadarache. An important influence of the total tritium concentration in the sample hydrolyzed has been highlighted. As for the phenomena suspected to be responsible for the phase change of tritiated water, in the studied range of parameters, vaporization induced by the heat of reactions seems to be dominant over the evaporation induced by the inert gas flow rate.

To complete this primary experimental study, other experiments will be performed with samples with higher specific activities. Additional tests to study more precisely the influence of the inert gas flow rate are also planned.

These experimental results will be used to validate a numerical model for the tritium distribution currently under development at the CEA.

Fig. 1 .

 1 Fig. 1. Laboratory scale hydrolysis process.

Fig. 2a .

 2a Fig. 2a. Influence of the total specific activity on the tritium distribution.

Fig. 2b .

 2b Fig. 2b. Influence of the total specific activity on the tritium distribution (extended with additional experimental results).

Fig. 3 .

 3 Fig. 3. Influence of the water flow rate on the distribution of HTO.

Fig. 4 .

 4 Fig. 4. Influence of the argon flow rate on the distribution of HTO.


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

Elham Farouk

Henri Delmas

Sequential adsorption -photocatalytic oxidation process for wastewater treatment using a composite material TiO 2 /activated carbon

Keywords: Activated carbon, Adsorption, Advanced oxidation process, Hydrogen peroxide, Photocatalysis, Titanium dioxide

A composite material was tested to eliminate phenol in aqueous solution combining adsorption on activated carbon and photocatalysis with TiO2 in two different ways. A first implementation involved a sequential process with a loop reactor. The aim was to reuse this material as adsorbent several times with in situ photocatalytic regeneration. This process alternated a step of adsorption in the dark and a step of photocatalytic oxidation under UV irradiation with or without H2O2. Without H2O2, the composite material was poorly regenerated due to the accumulation of phenol and intermediates in the solution and on TiO2 particles. In presence of H2O2, the regeneration of the composite material was clearly enhanced. After five consecutive adsorption runs, the amount of eliminated phenol was twice the maximum adsorption capacity. The phenol degradation could be described by a pseudo first-order kinetic model where constants were much higher with H2O2 (about tenfold) due to additional •OH radicals. The second implementation was in a continuous process as with a fixed bed reactor where adsorption and photocatalysis occurred simultaneously. The results were promising as a steady state was reached indicating stabilized behavior for both adsorption and photocatalysis.

Introduction

Advanced oxidation processes (AOPs) are widely used in wastewater treatment for the removal of organic and inorganic contaminants or to improve biodegradability of industrial wastewater. Heterogeneous photocatalytic process using TiO2 as catalyst is one of the most promising AOPs. The radicals generated (OH•, O2 -•, HOO•) can non-selectively mineralize a large range of organic pollutants which are difficult to eliminate [START_REF] Lagunas-Allue | Photocatalytic degradation of boscalid in aqueous titanium dioxide suspension: Identification of intermediates and degradation pathways[END_REF]: pharmaceutical compounds [START_REF] Elmolla | Photocatalytic degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution using UV/TiO2 and UV/H2O2/TiO2 photocatalysis[END_REF][START_REF] Hhh Lin | Photocatalytic oxidation of 5-fluorouracil and cyclophosphamide via UV/TiO2 in an aqueous environment[END_REF], dyes [START_REF] Konstantinou | TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations. A review[END_REF], pesticides [START_REF] Rizzo | Bioassays as a tool for evaluating advanced oxidation processes in water and wastewater treatment[END_REF], phenols [START_REF] Benoit-Marquié | Photocatalytic degradation of 2,4-dihydroxybenzoïc acid in water : efficiency optimization and mechanistic investigation[END_REF], etc. Titanium dioxide (TiO2) photocatalyst was broadly used because of its photochemical stability and comparatively low cost [START_REF] Arana | TiO2-photocatalysis as a tertiary treatment of naturally treated wastewater[END_REF][START_REF] Areerachakul | Granular activated carbon (GAC) adsorption-photocatalysis hybrid system in the removal of herbicide from water[END_REF][START_REF] Zhang | Development of TiO2 photocatalyst reaction for water purification[END_REF][START_REF] Shon | Chemical coupling of photocatalysis with flocculation and adsorption in the removal of organic matter[END_REF]. However, the main drawback of the practical use of the TiO2 is the difficulty to separate the TiO2 powder from water. The need of an uneasy and costly final filtration limits the industrial-scale development of these photocatalytic processes. Therefore, to solve this problem, techniques have been developed for immobilizing TiO2 catalyst onto the surface of a solid: sol-gel method [START_REF] Lee | Photocatalytic oxidation of microcystin-LR in a fluidized bed reactor having TiO2-coated activated carbon[END_REF], hydrothermal method [START_REF] Toyoda | Preparation and performance of anatase-loaded porous carbons for water purification[END_REF], impregnation [START_REF] Tao | Development of a TiO2/AC composite photocatalyst by dry impregnation for the treatment of methanol in humid airstreams[END_REF], atomic layer deposition [START_REF] Kim | Photocatalytic decomposition of toluene by nano-diamond supported TiO2 prepared using atomic layer deposition[END_REF], dip coating [START_REF] Sun | Photocatalytic degradation and kinetics of Orange G using nano-sized Sn(IV)/TiO2/AC photocatalyst[END_REF], hydrolysis [START_REF] El-Sheikh | Deposition of anatase on the surface of activated carbon[END_REF], metal organic chemical vapour deposition [START_REF] Andriantsiferana | Photocatalytic degradation of an azo-dye on TiO2/activated carbon composite material[END_REF][START_REF] Mills | Novel TiO2 CVD films for semiconductor photocatalysis[END_REF][START_REF] Zhang | Preparation of photocatalytic TiO2 coatings of nanosized particles on activated carbon by AP-MOCVD[END_REF], carbonization method [START_REF] Teng | The role of carbon in the photocatalytic reaction of carbon/TiO2 photocatalysts[END_REF], microwave irradiation [START_REF] Horikoshi | Formation and efficacy of TiO2/AC composites prepared under microwave irradiation in the photoinduced transformation of the 2-propanol VOC pollutant in air[END_REF]. The supporting materials generally suggested for degradation of organic compounds in water are materials such as silica, alumina, zeolites or clays [START_REF] Tanguay | Dichloromethane photodegradation using titanium catalysts[END_REF], glass, quartz or steel [START_REF] Fernandez | Preparation and characterization of TiO2 photocatalysts supported on various rigid supports (glass, quartz and stainless steel), Comparative studies of photocatalytic activity in water purification[END_REF][START_REF] Zazueta | Radiation field modeling and optimization of a compact and modular multi-plate photocatalytic reactor (MPPR) for air/water purification by Monte Carlo method[END_REF], activated carbon [START_REF] Andriantsiferana | Photocatalytic degradation of an azo-dye on TiO2/activated carbon composite material[END_REF][START_REF] Matos | Effect of the type of activated carbons on the photocatalytic degradation of aqueous organic pollutants by UV-irradiated titania[END_REF] or membrane [START_REF] Goei | Asymmetric TiO2 hybrid photocatalytic ceramic membrane with porosity gradient: Effect of structure directing agent on the resulting membranes architecture and performances[END_REF]. Improvement of photo-efficiency has not been systematically observed. Among these particle supports, activated carbon (AC) is very promising. Activated carbon adsorbs the pollutants and then releases them onto the surface of TiO2. Consequently, the pollutants are more concentrated around the TiO2 than in the bulk solution leading to an increase in the degradation rate of the pollutants [START_REF] Tryba | Application of TiO2-mounted activated carbon to the removal of phenol from water[END_REF][START_REF] Tsumura | Composites between photoactive anatase-type TiO2 and adsorptive carbon[END_REF]. The intermediates produced during degradation could also be adsorbed onto activated carbon and then be oxidized. Other authors [START_REF] Herrmann | Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants[END_REF][START_REF] Matos | Influence of activated carbon upon titania on aqueous photocatalytic consecutive runs of phenol photodegradation[END_REF][START_REF] García-Munoz | Photocatalytic degradation of phenol and isoproturon: Effect of adding an activated carbon to titania catalyst[END_REF] have reported a synergistic effect for simple mixtures of AC and TiO2, pollutants being more rapidly photodegraded in the mixed system which contains activated carbon. This so-called synergetic effect has been explained by the formation of a common contact interface between the different solid phases, in which AC acts as an efficient adsorption trap to the organic compounds. The pollutant is then more efficiently transferred to the TiO2 surface [START_REF] Ao | Enhancement effect of TiO2 immobilized on activated carbon fiber for the photodegradation of pollutants at typical indoor air level[END_REF][START_REF] Chiang | Effects of pore structure and temperature on VOC adsorption on activated carbon[END_REF][START_REF] Wang | Removal of 2,4 -dichlorophenol from aqueous solution by static-air-activated carbon fibers[END_REF].

The main fundamental problem in using TiO2 as photocatalyst is the energy loss in the electron-hole recombination which results in a lower efficiency of the degradation. Hence the prevention of electron-hole recombination becomes very important, the addition of electron acceptors into the reaction media is recommended: molecular oxygen present in air [START_REF] Augugliaro | Photocatalytic degradation of nitrophenols in aqueous titanium dioxide dispersion[END_REF][START_REF] Gupta | Photocatalytic mineralisation of perchlororthylene using titanium dioxide[END_REF] or H2O2 have been employed as effective electron acceptor in most photocatalysis applications. H2O2 could serve as electron scavenger to prevent the recombination and, in addition, generate more hydroxyl radicals [START_REF] Elmolla | Photocatalytic degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution using UV/TiO2 and UV/H2O2/TiO2 photocatalysis[END_REF]. In most of cases, the presence of H2O2 improved the photocatalytic oxidation performances [START_REF] Elmolla | Photocatalytic degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution using UV/TiO2 and UV/H2O2/TiO2 photocatalysis[END_REF][START_REF] Tryba | Application of TiO2-mounted activated carbon to the removal of phenol from water[END_REF][START_REF] Herrmann | Heterogeneous photocatalysis: an emerging technology for water treatment[END_REF]] some authors reported a negative effect of H2O2 [START_REF] Chu | Modeling the quantum yields of herbicide 2,4-D decay in UV/H2O2 process[END_REF][START_REF] Dionysiou | Effect of hydrogen peroxide on the destruction of organic contaminants-synergism and inhibition in a continuous-mode photocatalytic reactor[END_REF][START_REF] Chen | Study on the photocatalytic degradation of glyphosate by TiO2 photocatalyst[END_REF][START_REF] Achilleos | Factors affecting diclofenac decomposition in water by UV-A/TiO2 photocatalysis[END_REF]. Ilisz et al. [START_REF] Ilisz | Investigation of the photodecomposition of phenol in near-UV-irradiated aqueous TiO2 suspension. I: Effect of charge-trapping species on the degradation kinetic[END_REF] mainly observed the formation of primary oxidation products (hydroquinone, catechol and presumed aliphatics) at low concentration of H2O2 (< 0.01 mol/L) while hydroquinone and catechol concentration strongly decreased and ring-opening products were predominant at high H2O2 concentration (> 0.05 mol/L), more interesting in a complete degradation objective. Adan et al. [START_REF] Adan | Phenol photodegradation with oxygen and hydrogen peroxide over TiO2 and Fe-doped TiO2[END_REF] proposed the optimal molar ratio between concentrations of hydrogen peroxide and pollutant to be between 10 and 100.

In this work, phenol has been chosen as a model molecule representing hazardous phenol compounds in the industry. Indeed, phenolic compounds are identified as highly toxic compounds and non-biodegradable molecules. To degrade these compounds, a photocatalytic process has been implemented using commercial composite multilayer material TiO2/AC. The photocatalytic activity of this material was evaluated in two different reactors and two different reaction modes. First, a sequential method of water treatment was carried out involving adsorption then photocatalytic oxidation in a loop reactor, with or without H2O2 to examine the improvement of photocatalysis by H2O2 addition. In this configuration, the water treatment was not achieved by oxidation but by adsorption, the oxidative step being needed only for pollutant degradation and in situ activated carbon regeneration. Then, the performance of the composite material was studied in a continuous annular reactor with simultaneous adsorption and photocatalysis.

Materials and Methods

Materials

Phenol (Bioxtra 99.5%) and hydrogene peroxide (30% w/w) were supplied by Sigma Aldrich.

Ahlstrom directly supplied the composite material TiO2/AC, and separately the TiO2 catalyst (PC 500, Millenium, anatase > 99%) and the AC. The composite material was a tissue composed with a mixture of synthetic fibers, acrylic polymer and granular activated carbon, a mixture of TiO2 is coated on one side (EP1069950B1 European patent). The weight composition of this tissue is 4.5% TiO2, 63.5% AC, 27.5% fibers, 4.5% SiO2. SiO2 was used as an inorganic binder for titanium deposited on the paper fibers. It is transparent with UV light and photo-stable. The granular activated carbon made from coconut with particle diameter in the range 250-600 μm, and its specific surface was 1,065 m 2 /g. This activated carbon was mainly microporous (microporous and mesoporous volumes of 0.46 and 0.046 cm 3 /g respectively). The diameter of the TiO2 crystals was between 5 and 10 nm and the specific surface area SBET about 320 m 2 /g.

Analytical Methods

Solid analysis

The textural characterization was deduced from nitrogen adsorption at 77 K (ASAP 2010M; Micromeritics, Norcross, GA, USA). The specific surface area SBET was determined from the BET method for relative pressure range (p/p0) of 0.01-0.20 [START_REF] Brunauer | Adsorption of gases in multimolecular layers[END_REF]. Methods from Horvath and Kawazoe [START_REF] Adan | Phenol photodegradation with oxygen and hydrogen peroxide over TiO2 and Fe-doped TiO2[END_REF] and Barrett et al. [START_REF] Brunauer | Adsorption of gases in multimolecular layers[END_REF], referred as HK and BJH, were employed to assess the micropore and mesopore volumes respectively.

Liquid analysis

Phenol concentration measurements were performed by a high performance liquid chromatography equipment with UV detection at 254 nm (UV2000 dual wavelength; Thermo Finnigan, Les Ullis, France) using a C18 column (Prontosil, 4 mm × 250 mm i.d., 5 μm particles, I.C.S., Lapeyrouse-Fossat, France), with a mobile phase composed of acidified deionized water (W) (water acidified by H3PO4; pH = 2.2) and Acetonitrile (A) fed at 1 mL/min. The detector wavelength was set to 254 nm and the temperature of the column maintained at 30°C. For the adsorption step, an isocratic method was utilised (volumic composition Aqueous/Organic is 60/40). For the oxidation step, the separation of phenol from the oxidation intermediates was achieved with a mobile phase of variable volume composition W/A programmed at 1 mL/min (0-3 min: W only; 3-16 min: gradient W/A from 100/0 to 60/40; 16-25 min: 60/40).

The chemical oxygen demand (COD) was measured using colorimetric or digestion method with Hach equipment: a digital reactor block (DRB 200 HACH LANGE; Thermo Fisher Scientific, Illkirch, France), a spectrophotometer (DR/2500 HACH LANGE; Thermo Fisher Scientific, Illkirch, France) using 0-1,500 mgO2/L range Hach tubes (potassium dichromate, salt of silver and mercury, sulfuric acid). The precision of the method was assessed with standard solutions and showed a standard deviation of less than 5%.

The total organic carbon (TOC) was also measured during photocatalytic oxidation step. First, the inorganic carbon present in the solution was eliminated with concentrated phosphoric acid (84%) and the solution was degassed by a current of nitrogen. The sample was then injected in a TOC-meter (TC Multi Analyser 2100 N/C; Analytic Jena, France) where the organic molecules were totally oxidized at 850˚C, over a platinum catalyst. The quantity of CO2 released by the reaction was then measured by infrared spectrometry (IR). Total organic carbon analysis was made when H2O2 was added in the oxidation step. Indeed, COD is no longer possible due to H2O2 interference in the dichromate method.

Experimental Set-ups and Procedures

Batch adsorption

Isotherms of the original granular AC and TiO2/AC material were performed. In brown flasks, 0.5 g of adsorbent (AC or TiO2/AC) was added to 100 mL of phenol solutions (0.5-5.0 g/L concentration range). The suspensions were left under stirring in a thermo-regu-lated bath at 25˚C for 8 days to ensure equilibrium. The solutions were not buffered, the final pH was in the range of 4 to 5. Then, the solutions were filtered on 0.25 μm nylon filter membranes before analysis [START_REF] Matos | Effect of the type of activated carbons on the photocatalytic degradation of aqueous organic pollutants by UV-irradiated titania[END_REF]. The amount of adsorbed phenol was deduced from initial C0 and final HPLC measurements of concentrations in the liquid phase.

The loop reactor

The 2 L batch loop reactor (Fig. 1) composed of a reactor section (0.65 L) and a recycling tank (1.35 L) was designed to operate with a sheet of composite TiO2/AC (dimension: 360 mm × 120 mm × 3 mm; weight: 17 g with 10.9 gAC and 0.8 gTiO 2 ). The sheet was maintained barely immersed at the top of the flowing aqueous solution of phenol (C0 = 0.88 g/L) which was circulated by a peristatic pump (MasterflexL/S; Thermo Fisher Scientific, Illkirch, France) at a constant flow rate (1,000 mL/min). The recycling tank was cooled by thermostatic water bath at 25˚C (Julabo F12-ED; SELI, Toulouse, France). The equipment did not include the air supply as enough gas-liquid surface was available under both the composite sheet and the recycling tank to keep dissolved oxygen in the liquid. The range of emission of the 2 lamps (PL-L 24W; Philips, Bossee, France) was 340-400 nm with a maximum at 365 nm. The mean value of the light intensity measured with a radiometer (model RS232 Lutron; Thermo Fisher Scientific, Illkirch, France) was 55 W/m 2 at the surface of the composite material.

During the sequential process, two successive steps were achieved: adsorption then photo-oxidation corresponding to one cycle. The adsorption step was carried out in the dark during 4 days to approach the maximum loading corresponding to the adsorption equilibrium. Then, the oxidation step was started by switching on the UV lamps for 3 days. During the two steps samples were taken at regular time intervals and analysed by HPLC for determining the phenol concentration. A part of the samples was also used for measuring COD or TOC. TOC measurements have been chosen in presence of H2O2 in the oxidation step. After each oxidation step, the reactor was emptied from the oxidized solution of phenol and then filled by 2 L of a new one at same initial concentration (C0 = 0.88 g/L) for starting a next cycle. High initial concentration of phenol has been chosen to quickly saturate the composite material and so show its regeneration.

Annular continuous reactor

Experiments were carried out in an annular reactor of 120 mL shown in Fig. 2. On the central axis, the UV lamp was placed in a jacketed thermo-regulated cylinder (25˚C). The mean value of the light intensity measured with the radiometer was 72 W/m (at the inside surface of the jacket cylinder). A peristaltic pump with a flow rate of 2 mL/min was used circulate the solution from the inlet tank to the annular reactor. The size of the sheet of composite TiO2/AC was chosen to entirely fill the annular space (mTiO2/AC = 12.7 g, mTiO2 = 0.6 g). The experimental protocol was as follows. The composite material TiO2/AC was pre-loaded in a beaker for 3 days using a solution of phenol (C0 = 0.209 g/L) until the equilibrium was reached (Cfinal = 0.018 g/L). After this dark adsorption period, the oxidation process was then started using this loaded composite material as a pseudo-fixed bed in the annular reactor. As soon as the lamp was switched on, this reactor was continuously fed with a phenol solution, Cinlet = 0.018 g/L. The samples were taken each hour at the exit of the reactor via a sampling port. The experiment was conducted at 25˚C. All samples were analysed by HPLC to determine the phenol concentrations. [START_REF] Lagunas-Allue | Photocatalytic degradation of boscalid in aqueous titanium dioxide suspension: Identification of intermediates and degradation pathways[END_REF] aerated and stirred reactor, [START_REF] Elmolla | Photocatalytic degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution using UV/TiO2 and UV/H2O2/TiO2 photocatalysis[END_REF] pump, [START_REF] Hhh Lin | Photocatalytic oxidation of 5-fluorouracil and cyclophosphamide via UV/TiO2 in an aqueous environment[END_REF] UV lamp, [START_REF] Konstantinou | TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations. A review[END_REF] annular reactor.

Fig. 2. Continuous reactor

Results and Discussion

Adsorption Isotherms

The adsorption isotherms of the original granular AC and TiO2/AC material are presented in Fig. 3. Both isotherms exhibited the same shape, corresponding to type I of international union of pure applied chemistry classification (1985) as expected regarding the high AC microporosity (monolayer adsorption). Calculation of isotherm parameters has been performed using linearization of the Langmuir model equation. The Langmuir parameters given in Table 1 showed a higher value of the maximum adsorption capacity qmax (0.45 g/g) for the activated carbon alone. This saturation capacity was more important than the reported values in the literature for phenol adsorption. Usual values of qmax are between 0.20 and 0.35 g/gAC under ambient temperature and unbuffered conditions showing the excellent performances of this activated carbon [START_REF] Laszlo | pH-dependent adsorption and desorption of phenol and aniline on basic activated carbon[END_REF][START_REF] Kumar | Adsorption of phenol and 4-nitrophenol on granular activated carbon in basal salt medium: Equilibrium and kinetics[END_REF][START_REF] Liu | Characterization of mesoporous activated carbons prepared by pyrolysis of sewage sludge with pyrolusite[END_REF][START_REF] Andriantsiferana | Competitive Adsorption of p-Hydroxybenzoic Acid and Phenol on Activated Carbon: Experimental Study and Modeling[END_REF][START_REF] Mohamed | Competitive adsorption of phenolic compounds from aqueous solution using sludge based activated carbon[END_REF]. The composite material adsorption capacity is of course lower -as AC corresponds to only 63.5% of this material weight-but still very convenient. Note that in addition, when incorporated in the tissue, AC seems to lose about 12% of its specific adsorption capacity which could be due to partial pore clogging.

Sequential Process in Loop Reactor

Sequential process -5 cycles without H2O2

Preliminary experiment with phenol and UV light was performed. Direct photolysis of phenol during 3,500 min did not change the concentration as expected from the absorbance spectrum of phenol (no absorbance above 310 nm). Five cycles of adsorption-photocatalytic regeneration have been carried out with the composite material (Fig. 4(a) and (b)). In comparison with classic adsorption kinetics with granular carbon, the kinetics were very slow probably due to the diffusion limitation of the phenol inside the layer of natural and synthetic fibers containing AC. Table 2 showed a gradually decrease of the quantities adsorbed and the regeneration efficiency Re calculated from Eq. (1). The total quantity of adsorbed phenol (0.218 g/g) was 40% higher than the theoretical maximum adsorbed amount for a concentration equal to 0.88 g/L (0.17 g/g, see isotherm on Fig. 3) proving the effective regeneration of the composite material. The final degraded fraction decreased from 95% at cycle 1 to 1% at cycle 5 due to an incomplete regeneration of the material. The first-order kinetic model (Eq. ( 2)) was used to describe the kinetics of phenol degradation (Fig. 4 The model fitted well until about one hour, then the presence of an important quantity of intermediates (hydroquinone, catechol, benzoquinone, aliphatic compounds, etc.) and the competitive adsorption between phenol and these intermediates might change kinetics [START_REF] Cordero | Influence of L-type activated carbons on photocatalytic activity of TiO2 in 4-chlorophenol photodegradation[END_REF]. Consequently, the calculation of kapp was done using the first part of these kinetics. Decrease of the successive apparent contants kapp confirmed the continuous degradation of the regeneration efficiency and the pseudo-first order of the kinetic (depending on total concentration). This phenomenon was classically observed in photocatalysis when high quantity of organic molecules had to be degraded [START_REF] Ahmed | Heterogeneous photocatalytic degradation of phenols in waste-water: a review on current status and developments[END_REF]. Above a certain level of phenol concentration, the catalyst surface becomes saturated, and this may even lead to a decrease in the observed rates [START_REF] Grabowska | Mechanism of phenol photodegradation in the presence of pure and modified-TiO2: A review[END_REF]. The COD evolution was represented in Fig. 5. The degradation did not exceed 5% for the 4th cycle. For the first cycle, even if 95% of the phenol was removed in the liquid phase, 40% of the initial COD remained in solution due to the reaction intermediates. At the end of each oxidation step, the contribution of phenol to the COD was calculated using phenol concentration (HPLC): 15%, 42%, 80% and 84% for the 1st, 2nd, 3rd and 4th cycles respectively (Table 2). These results confirmed that the conditions of oxidation were degraded from the second cycle. At the end of the 5th cycle, the activated carbon inside the composite material was analyzed. The surface area SBET was reduced by 80% (from 1,065 m 2 /g to 208 m 2 /g) and the microporous volume by 84% (from 0.460 cm 3 /g to 0.076 cm 3 /g). An important quantity of phenol and oxidation intermediates were accumulated in the micropores without being further oxidized. These molecules deeply adsorbed in these pores did not migrate from the activated carbon to the catalyst to be degraded. The expected mechanism (adsorption of phenol → phenol diffusion from AC to TiO2 → total phenol mineralization under UV → total regeneration of the composite material) was no longer taking place. At the photo-catalyst surface, the phenol and intermediates remained concentrated and cycle after cycle more organic compounds get adsorbed. The photocatalytic activity decreased due to the accumulation of reaction intermediates formed at the surface as a result of partial oxidation of phenol. The presence of these molecules reduced number of reaction sites and reactivity with time. The catalyst used presented a low selectivity toward total oxidation with respect to partial oxidations [START_REF] Kim | Photocatalytic decomposition of toluene by nano-diamond supported TiO2 prepared using atomic layer deposition[END_REF]. This loss of activity might suggest a poisoning of the surface of the catalyst. Another explanation could be proposed: at the surface of the catalyst, these intermediates were probably in competition with H2O and O2 on the adsorption sites. Then less molecules of water and oxygen would be adsorbed at the surface of the catalyst, therefore, an increase of the recombination of the electron/hole pairs could occur. This phenomenon could explain a lower production of oxidative species (OH• and O2 -•) and an additional loss of photo-activity.

Sequential process -5 cycles with H2O2

In order to improve the oxidation process the effect of H2O2 addition on the rate of phenol degradation was investigated. At the beginning of each oxidation step, 50 mL of H2O2 (30% weight) were added to the solution. For the first oxidation, the corresponding initial H2O2/phenol molar ratio = 24 was almost twice the stoichiometric amount of hydrogen peroxide for complete mineralization, in agreement with the values proposed by Adan et al. [START_REF] Adan | Phenol photodegradation with oxygen and hydrogen peroxide over TiO2 and Fe-doped TiO2[END_REF] in the range 10 to 100). In presence of H2O2, Fig. 6(a) showed an effective adsorption for all of the five cycles. The amount of adsorbed phenol decreased, but much more slowly than without H2O2 and the regeneration efficiency remained superior to 60%: the regeneration was still partly achieved even after 5 cycles (Table 2). The total quantity of phenol adsorbed during 5 cycles (0.389 g/g) was 140% superior to the theoretical maximum amount adsorbed (see Fig. 3) and almost twice the quantity eliminates without H2O2. From these results, the coupling of H2O2 and TiO2 significantly improved the performance of the AC regeneration. As shown in Fig. 6(b), the phenol degradation was only slightly decreased from 100% to 96%. The pseudo first-order kinetic model (Eq. ( 2)) fitted well, the apparent constant kapp were between 4 and 12 times higher than without H2O2 (Table 2). The intermediates degradation has gone further as shown by the evolution of the TOC in Fig. 7. Indeed, the TOC conversion rate was between 60% and 70%. The comparison between total TOC and phenol contribution TOC phenol showed an important presence of intermediates (Table 2) showing the partial oxidation of the phenol. This is in agreement with Zhang et al. [START_REF] Zhang | Adsorption of dyes and phenol from water on resin adsorbents: effect of adsorbate size and pore size distribution[END_REF] who reported rather high values of TOC even at total degradation of the aromatic molecules. These intermediates product are thought to be aliphalic acids known to be difficult to oxidize [START_REF] Santos | Route of the catalytic oxidation of phenol in aqueous phase[END_REF].

For the first cycle, the synergetic factor S, calculated with Eq. ( 3), was equal to 7.5 confirming the better photocatalytic performance with H2O2.

Where: kapp (with H2O2) is the value of kapp with the composite material, with H2O2 and with UV kapp (without H2O2) is the value of kapp with the composite material and with UV kapp (alone) is the value of kapp without the composite material and with UV (reference). This better performance was due to two additional pathways of •OH production in presence of H2O2 [START_REF] Achilleos | Factors affecting diclofenac decomposition in water by UV-A/TiO2 photocatalysis[END_REF][START_REF] Muruganandham | Photocatalytic decolourisation and degradation of Reactive Orange 4 by TiO2-UV process[END_REF]:

Pathway 1: H2O2 + electron → •OH + OH - (4) Pathway 2: O2 + electron → •O2 - (5) H2O2 + •O2 -→ •OH + OH -+ O2 (6)
The quantity of added H2O2 was chosen in agreement with literature to avoid the negative effect of H2O2. Indeed, introduced in excess, H2O2 could react with hydroxyl radicals and the holes (reactions ( 7) and ( 8)): a weaker oxidizing radical •HO2 being produced, inducing a decrease of the photocatalytic performances [START_REF] Elmolla | Photocatalytic degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution using UV/TiO2 and UV/H2O2/TiO2 photocatalysis[END_REF].

H2O2 + •OH → H2O + •HO2 (7) H2O2 + hole → H + + •HO2 (8)
After the 5th cycle, the AC present in the composite material was analyzed and compared to the virgin one: the surface area SBET was reduced by 47% (from 1,065 m 2 /g to 562 m 2 /g) and the microporous volume by 75% (from 0.460 cm 3 /g to 0.113 cm 3 /g). In comparison with experiments without H2O2, this better preservation of surface area and porous volume proved phenol and intermediates to be less accumulated in the pores leading to more efficient regeneration.

Continuous Annular Reactor

In continuous mode, adsorption and photocatalytic oxidation operated simultaneously. Under UV light, Fig. 8 shows a transient behaviour with initial decreasing phenol concentration due to the added photocatalytic activity. Then the steady state was obtained corresponding to equilibrium between adsorption and oxidation and proving the composite to get a stable behaviour without noticeable deactivation. In these conditions, 55% of the phenol was removed corresponding to an overall reaction rate of 5 10 -6 (gphenol/gTiO 2 ) s -1 .

Conclusions

This study was conducted to put on the map an innovative process to degrade aromatic molecules. To offer an economically interesting process, the conditions were: (1) to use a commercial material and cheap UV lamps; (2) to prolong the adsorption activity of the material with in situ regeneration.

A first implementation was involved using a loop reactor. This sequential process alternated 5 cycles with a step of adsorption in dark and a step of photocatalytic oxidation under UV irradiation. Without H2O2, the regeneration of the composite multilayer material became ineffective from the third cycle on. Cycle after cycle, more pollutants were present at the surface of the composite material and in the solution. The apparent constants decreased drastically, reaction intermediates accumulated on the surface of the photocatalyst, reducing number of reaction sites and reactivity with time. After several uses, this accumulation induced a poisoning of the catalyst. The presence of H2O2 improved the conditions of regeneration. For the 5 cycles, the phenol degradation was between 100% and 96% and the TOC conversion rate was between 60% and 70%. At industrial scale, the process could be implemented as follow: (1) a step of continuous adsorption by controlling the outlet concentration; (2) a step of regeneration in batch mode using a small volume of water and in presence of UV and H2O2 controlling and optimizing the quantity of UV light, the catalyst loading and the duration of irradiation.

Fig. 1 .

 1 Fig. 1. Loop reactor; [1] 2 UV lamps, [2] Composite material TiO2/AC, [3] loop reactor, [4] Intermediated thermo regulated reactor, [5] pump.

Fig. 3 .

 3 Fig. 3. Adsorption isotherms (Experimental and Langmuir model).

  (b)) and to obtain the values of the apparent constant of degradation at each cycle.

Fig. 4 .

 4 Fig. 4. Evolution of the phenol concentration -Sequential process without H2O2; (a) Adsorptions, C0 = 0.88 g/L; (b) Oxidations under UV irradiation, [TiO2] = 0.4 g/L.

  a: regeneration efficiency calculated with Eq. (1) b: phenol concentration at the beginning of each oxidation step c: first-order apparent constant Kapp: COD or TOC: COD or TOC at the beginning of each oxidation step f: final COD or TOC: COD or TOC at the end of each oxidation step g: Calculated from HPLC phenol molar concentration (C6H6O+7O2 → 6CO2 + 3H2O); h: Calculated from HPLC phenol molar concentration (TOC = 6 × 12 × [phenol]).

Fig. 5 .

 5 Fig. 5. Carbon oxygen demand (COD) removal during 4 cycles, [TiO2] = 0.4 g/L.

Fig. 6 .

 6 Fig. 6. Evolution of the phenol concentration -Sequential process during 5 cycles in presence of H2O2; (a) Adsorptions (C0 = 0.88 g/L), (b) Oxidations under UV irradiation, [TiO2] = 0.4 g/L, [H2O2] = 7.6 g/L.

Fig. 7 .

 7 Fig. 7. Total organic carbon (TOC) removal with H2O2; [TiO2] = 0.4 g/L, [H2O2] = 7.6 g/L.

Fig. 8 .

 8 Fig. 8. Evolution of the phenol concentration using continuous reactor during UV irradiation; Cinlet = 0.017 g/L, mTiO 2 = 12.7 g.

Table 1 .

 1 Parameter Constants of Langmuir Model

		Parameters of Langmuir model
	Adsorbent	[qe = qmax(CeKL)/(1+CeKL)]
		qmax (g/g)	KL (L/g)	R 2
	Granular AC	0.45	11.66	0.99
	TiO2/AC material	0.25	3.39	0.98

Table 2 .

 2 Sequential Process with the Loop Reactor. Influence of the Presence of H2O2 on Comparative Parameters

		C y c l e	1	2	3	4	5	T o t a l
		Adsorbed quantity (g/g)	0.093	0.074	0.024	0.013	0.014	0.218
		Regeneration efficiency Re a (%)	100	80	26	13	14	-
	Without H2O2	Initial phenol concentration b (g/L) First order apparent constant Kapp c (min -1 ) Final degradated fraction d ( % )	0.062 0.0046 9 5	0.228 0.0042 8 0	0.469 0.0017 3 7	0.768 0.0017 2 0	--1	---
		Initial COD e (mg/L)	150	545	1,120	1,830	-	-
		Final COD f (mg/L)	60	260	900	1,721	-
		Final CODphenol g (%)	9	110	713	1,448	-	-
		Adsorbed quantity (g/g)	0.0945	0.0903	0.0827	0.0630	0.059	0.389
		Regeneration efficiency Re a (%)	100	95	87	66	62	-
		Initial phenol concentration b (g/L)	0.054	0.093	0.159	0.332	0.365	-
	With	First order apparent constant Kapp c (min -1 )	0.057	0.017	0.011	0.010	0.008	-
	H2O2	Final degradated fraction d (%)	100	100	97	97	96	-
		Initial TOC e (mg/L)	54	345	475	443	627	-
		Final TOC f (mg/L)	17	152	240	161	177
		Final TOCphenol h ( m g / L )	0	0	0	7	9

Acknowledgements

The authors wish to acknowledge Cedric Vallet from Ahlstrom who supplied all material.