Removal of 2,4-dimethylphenol pollutant in water by ...

Microporous and Mesoporous Materials 189 (2014) 200–209

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Removal of 2,4-dimethylphenol pollutant in water by ozonation catalyzed by SOD, LTA, FAU-X zeolites particles obtained by pseudomorphic transformation (binderless) Jullian Vittenet a,b, Jeremy Rodriguez a, Eddy Petit b, Didier Cot b, Julie Mendret b, Anne Galarneau a,⇑, Stephan Brosillon b,⇑ a b

Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France Université Montpellier 2, Institut Européen des Membranes, 2 place Eugène Bataillon, 34095 Montpellier Cedex 5, France

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Article history: Available online 9 October 2013 Dedicated to Dr. Michael Stöcker on the occasion of his retirement as Editor-in-Chief of Microporous and Mesoporous Materials. Keywords: Zeolite Ozone Hydroxyl radical 2,4-DMP AOP

a b s t r a c t Advanced oxidation processes (AOP) as ozonation coupled with inorganic materials have been recently demonstrated to be highly suitable for wastewater treatment. In petrochemical wastewaters, pollutants as alkylphenols are often detected. In this paper, we highlight the efficiency to couple ozone with zeolites as SOD, LTA and FAU-X for the degradation of 2,4-dimethylphenol (2,4-DMP). For process requirement the 3 zeolites were prepared as particles of 70 lm and 1 mm thanks to pseudomorphic transformation without binders. The zeolites were synthesized under their sodium (Na) form and potassium (K) ion-exchange was performed to enhance their basicity. By single ozonation (without zeolite) 100% of 2,4-DMP removal occurred in 25 min, but the pollutant is transformed into oxidized by-products corresponding to only 14% of the Total Organic Carbon (TOC) removal after 5 h. Adding zeolites to the ozonation process increased very slightly the kinetic of disappearance of the pollutant but increased the removal of its oxidation by-products, with 34% of TOC removal after 5 h. Among the zeolites, the best solid catalyst for ozonation was Na-LTA. Experiments with tert-butanol (t-BuOH) put in evidence that Na-LTA generates hydroxyl radicals from ozone, which increased the degradation of the by-products. No adsorption of 2,4-DMP and of the resulting oxidation by-products was detected by thermogravimetric analysis (TGA) on the materials after 5 h of ozonation. The TOC removal is consequently equivalent to the total mineralization of the organics into CO2. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Industrial petrochemical wastewaters contain aromatic compounds such as polycyclic aromatic hydrocarbons (PAHs), benzene–toluene–ethylbenzene–xylene (BTEX) and phenols. In petrochemical effluents, 2,4-dimethylphenol (2,4-DMP) is a typical pollutant, which derived from the cresol part of fractional distillation and extraction with aqueous alkaline solutions [1] and has been chosen as test molecule for our study. Nowadays, more attention has been paid in the removal of these pollutants with the evolution of water regulations given by the European water framework directive, which stated the improvement of all water bodies quality [2]. Furthermore, as reported by most of regulatory sources, 2,4-DMP and its oxidation by-products are known to have a high toxic character ⇑ Corresponding authors. Tel.: +33 4 67 16 34 68. E-mail addresses: [email protected] (A. Galarneau), [email protected] (S. Brosillon). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.09.042

[3]. Efforts have been made for the development of more efficient wastewater treatment processes as adsorption or degradation by oxidation. Adsorption on coat fly for thermal effluents treatment led to 87% of 2,4-DMP removal after 48 h [4]. Advanced oxidation processes (AOP) appeared to be an adequate way for the elimination of this kind of refractory compounds by producing highly oxidative species [5,6]. For 2,4-DMP removal, different AOP have been used in literature. For example, the Fe2+/H2O2/UV process has reached 90% of pollutant removal after 1 h 40 min [6]. Recent studies have shown the positive effect of combining ozone with zeolites or activated carbon to fasten xenobiotic pollutants removal in less than 1 h [7–9]. Zeolites are thermally and chemically more stable than activated carbon in presence of ozone [10] and therefore are more promising candidates for long term ozonation processes [9,11,12]. However, reaction mechanisms for pollutant degradation in zeolite/ozone process are still unclear and zeolite structure dependent. Some studies point out the adsorption capacity of zeolites for pollutants, which are confined into the pore. However pollutant adsorption capacity in

J. Vittenet et al. / Microporous and Mesoporous Materials 189 (2014) 200–209

zeolites strongly depends on their Si/Al ratio. ZSM-5 or dealuminated FAU-Y with high Si/Al ratio featured hydrophobic character suitable for the adsorption of most of the studied pollutants [9,13,14]. But also zeolites have shown their ability to catalyze the decomposition of ozone and to accelerate the generation of OH radicals [9]. For example, it has been reported that the positive effect of zeolite on ozonation process was due to the generation of OH radicals by the Brönsted acid sites of an acid treated natural zeolite [15]. Other studies pointed out that the degradation mechanism of pollutants using ZSM-5 zeolites is not occurring via a radical mechanism but that zeolites act as an ozone reservoir, in which confined ozone reacted with the pollutants adsorbed at the surface of the zeolite [16,17]. LTA and FAU-X zeolites featuring no Brönsted sites and, in the case of LTA too small pore size to adsorb pollutant or ozone, have shown an increase on phenol removal in comparison to single ozonation with 100% and 87% of phenol removal after 30 min and 1 h, respectively [18]. These zeolites are known for their basic character and the generation of OH radicals was proposed with these zeolites, as single ozonation is promoted in basic medium [18]. However, these studies were conducted with commercial zeolite granulates formed by agglomeration of zeolite crystals and different kind of binders (alumina, clays), which can play a role in the catalytic activity of the solids. Also no studies were conducted on the rate of the 2,4-DMP oxidation by-products. This is an important issue as oxidized by-products can be as or more toxic than the initial pollutants. In the present study, SOD, LTA and FAU-X zeolites have been tested in ozonation processes for the degradation of 2,4-DMP. The removal of 2,4-DMP was followed within time as well as the oxidized by-products by the Total Organic Carbon (TOC) removal analysis. OH radicals formation was followed by adding t-BuOH scavengers. To understand the effect of the sole zeolites, large particles (70 microns or 1 mm diameter) of SOD, LTA and FAU-X needed for the ozonation process were prepared by pseudomorphic transformation without adding any binders [19–25]. Pseudomorphic transformation is a special synthesis pathway, which uses a silica source with the desire size and shape of particles and allows its transformation into zeolites keeping the morphology intact [19] by adjusting the rate of silica dissolution to the rate of zeolite crystallization. Ion-exchange of Na+ by K+ was performed to increase the basicity character of the zeolites [26,27]. Adsorption experiments of 2,4-DMP on the zeolites (without ozone) thermogravimetric analyses (TGA) of the zeolites after ozonation treatment were performed to assess the ability of the zeolites to adsorb pollutant and oxidized by-products, respectively. 2. Experimental 2.1. Chemicals Acetonitrile (ACN) and water (H2O) used for High Performance Liquid Chromatography (HPLC) analyses are HPLC grade (Sigma Aldrich). All other chemicals products are high grade commercially available: Sodium hydroxide (NaOH) and sodium aluminate (NaAlO2) from Carlo Erba, t-BuOH, potassium chloride (KCl), sodium chloride (NaCl) and 2,4-dimethylphenol (2,4-DMP) from Sigma Aldrich (Isle d’Abbeau – France). 2,4-DMP solutions were prepared with water purified by a Millipore Milli-Q UV Plus system. 2.2. Synthesis of SOD, LTA, FAU-X zeolites SOD, LTA and FAU-X zeolites particles were synthesized by pseudomorphic transformation (without binders) according to ref [19] to observe the impact of the zeolite framework on the 2,4DMP decomposition under ozonation. Zeolites pseudomorphs

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were produced starting from silica particles of 70 lm and 1 mm diameter with Davicat 1404 (Davison Catalysts) and W432 (DAVISIL), respectively. In a typical synthesis, an alkaline solution was first prepared by mixing deionized water, NaOH and NaAlO2. After dissolution, amorphous silica particles were introduced softly and stirred using Archimedes screw to preserve the size and the shape of the particles. Then, the suspension was stirred for 1 h at ambient temperature and heated at 100 °C during 24 h in a Teflon-lined stainless steel autoclave. The resulting zeolites particles were recovered by filtration and washed until neutral pH. After an overnight drying at 80 °C, the materials were calcined in air for 8 h at 550 °C with a temperature ramp of 2 °C/min. Under stirring for ozonation processes FAU-X pseudomorphs were not stable and produced fine nanoparticles, which blocked the filters and therefore a classical synthesis of FAU-X was used instead. FAU-X zeolite was synthesized following a verified synthesis from the International Zeolite Association (IZA) with the following molar ratios 1 NaAlO2: 4 SiO2: 16 NaOH: 325 H2O using Aerosil 200 (Degussa) as source of silica [28,29]. Deionized water, NaOH and NaAlO2 were mixed and after dissolution the silica source was introduced and stirred for 30 min. Then, the mixture was heated at 80 °C for 21 h in a Teflon-lined stainless steel autoclave, filtrated and washed until neutral pH. Drying and calcination were performed under the same experimental conditions as for zeolite pseudomorphs particles. All the zeolites were synthetized in Na+ form. In order to increase the zeolite basicity, ion-exchange with K+ was performed by stirring 1 g of zeolite in 50 mL of KCl (0.5 M) solution for 1 h at 80 °C. Then, the resulting zeolites were washed with deionized water until neutral pH. The procedure was repeated three times and the sample was dried at 80 °C overnight [30]. Ca2+ exchange was performed on LTA zeolite in order to measure the nitrogen adsorption volume [19]. Indeed, Ca-LTA zeolite exhibits bigger pores diameter (0.50 nm) than the Na-LTA (0.42 nm) and allowed N2 adsorption. The resulting materials will be named: X-y where X indicates the counter ion of the materials and x represents the zeolite framework type. For example, LTA zeolite in a Na form will be called Na-LTA. 2.3. 2,4-DMP adsorption on zeolites Pollutant adsorption capacities of the zeolites were performed prior to ozonation process in order to quantify the 2,4-DMP adsorption capacity of the zeolites on the overall degradation mechanism. An amount of 0.2 g of zeolites was added in a stirred glass bottle (200 rpm) containing 100 mL of 2,4-DMP solutions at different concentrations. Adsorption of the pollutant was performed at 25 °C for all zeolites and followed within time until reaching the equilibrium. Isotherms of adsorption have been plotted for concentrations in pollutant between 1 to 200 mg/L. Aliquots of solutions were filtered with PTFE syringe filter 0.45 lm syringe prior measuring pollutant concentration by HPLC equipped with an UV detector. 2.4. Description of the ozonation procedure The ozonation degradation of the 2,4-DMP pollutant was performed in a glass stirred batch reactor (2 L) under thermostatic control (Fig. 1). O3 was produced from synthetic air using a laboratory O3 generator (Ozonia LAB2B ozone generator). The O3 gas flow was 40 L/h and the O3 concentration was 2 g/m3. This amount of ozone produced at the lab scale is well below industrial ozonation processes, which is around 180 g/m3. This low amount of ozone allows to observe the degradation kinetic by slowing down the process. In a typical ozonation procedure, 3 g of materials were added in 1.5 L of 2,4-DMP solution (50 mg/L) and the suspension was stirred at 400 rpm at 25 °C. Then, O3 was immediately injected in the

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Fig. 1. Schematic diagram of the ozonation reactor.

glass reactor. To follow the kinetic of the pollutant and the oxidized by-products removal aliquots of the solution were taken within time and filtered using syringe filter. A stirring rate of 400 rpm was preferred over the stirring rate at 200 rpm used in adsorption experiments, because at 200 rpm the reaction was showing some ozone transfer limitation for the removal of the pollutant. The total removal of 2,4-DMP was achieved in 32 min instead of 25 min for 200 and 400 rpm, respectively (Fig. S1). No differences between stirring rates of 375 and 400 rpm was noticed, revealing that at 400 rpm the reaction is not limited by ozone transfer. Furthermore the stirring rate does not influence the rate of Total Organic Carbon (TOC) removal, same curves are obtained at 200 and 400 rpm (Fig. S1). The pH of the solution was followed within time and before each ozonation experiment, the pHmeter was calibrated with buffers solutions. To evidence the presence of radicals, a t-BuOH radicals scavenger (25 mg/L) was added to the pollutant solution before to start ozonation process. At the end of each ozonation treatment (after 5 h) the zeolites were collected by filtration, dried at 50 °C and analyzed by different techniques described below.

2.6. Zeolites characterization X-ray diffraction (XRD) patterns of the zeolites were performed using a Bruker D8 Advance diffractometer with a Bragg–Brentano geometry and equipped with a Bruker Lynx Eye detector. XRD patterns were recorded in the range 4–50° (2h) with an angular step size of 0.0197° and a counting time of 0.2 s per step. Scanning electron microscopy (SEM) pictures of the zeolites were obtained using a Hitachi S4800 microscope. EDX spectroscopy analyses were performed with a Thermofisher Nanotrace. Thermogravimetric analyses (TGA) of zeolites before and after ozonation treatment were determined with a TA Instruments SDT 2960 simultaneous DSCTGA thermogravimeter. The materials were heated in air flow at 600 °C with a temperature ramp of 10 °C/min. Textural properties of the zeolites were determined by N2 adsorption/desorption isotherm at 196 °C on a Micromeritics Tristar 3000 apparatus. Samples were previously outgassed in vacuum at 250 °C for 12 h. Surface areas were determined according to the BET equation using the Rouquerol criteria, necessary to well compare surface area of microporous materials [31,32]. Microporous and total pore volumes were calculated using the t-plot method.

2.5. Pollutant removal analysis

3. Results and discussion

The remaining 2,4-DMP concentration in the aqueous solution was measured by HPLC equipped with an UV detector (Waters 600E pump-717 plus Autosampler and PDA2996, at k = 279 nm). HPLC analysis was carried out on a C18 grafted silica column (Interchim, UptisphereÒ C18-ODB) with the following characteristics: 150 mm length, 4.6 mm and 5 lm average silica particles. Aliquot injections were conducted with a mobile phase of 45:55 (ACN/H2O) at 0.8 mL/min. The volume injected was 20 lL. The corresponding retention time of 2,4-DMP was 5 min in these analytical conditions. The TOC of the solutions was determined with a Shimadzu TOC-V meter with 130 mL/min of gas flow rate (air) and 190 kPa of gas pressure.

3.1. Zeolite pseudomorphs synthesis and characterization Different sources of silica have been used for the synthesis of the SOD, LTA and FAU-X zeolites. Preshaped silica as Davicat 1404 and W432 consisting of irregular particles with sharp edges and average size of 70 lm and 1 mm (Fig. S2), respectively, were used for the pseudomorphic transformation of silica particles into zeolites as already shown in previous study [19]. In the following, only the more stable particles under stirring were choosen for the ozonation processes: SOD from Davicat 1404 (70 lm) and LTA from W432 (1 mm) (Figs. 3 and 4). Unfortunately, under stirring for ozonation processes FAU-X pseudomorphs were not stable


Fig. 2. XRD pattern of (a) Na-SOD, (b) K-SOD, (c) K-SOD after ozonation, (d) Na-LTA, (e) K-LTA, (f) K-LTA after ozonation, (g) Na-FAU-X, (h) K-FAU-X and (i) K-FAU-X after ozonation.

and produced fine nanoparticles, which blocked the filters and therefore a more classical synthesis of FAU-X was used using Aerosil silica nanoparticles (ca. 50 nm) as silica source, which gave rise

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to particles of inhomogeneous size from few lm to 500 lm formed by an agglomeration of 1 lm FAU-X particles (Fig. 5). Only the fraction above 250 lm was retained to perform ozonation process. SEM pictures showed that Na-SOD pseudomorphs particles of 70 lm (Fig. 3) were constituted by an agglomeration of 5 lm yarn balls, which is a typical form of SOD crystals [33,34]. Na-LTA pseudomorphs particles of 1 mm (Fig. 4) were built by an aggregation of small typical cubic crystals of 1–2 lm. XRD patterns of the native silicas reported the amorphous nature of the materials as evidenced by the typical broad peak around 20° in 2h (Fig. S3) and after pseudomorphic transformation all the XRD peaks characteristic of the zeolite frameworks were obtained (Fig. 2). N2 adsorption/desorption analysis of the native silica materials stated isotherms of type IV (according to an IUPAC classification) characteristic of mesoporous materials for Davicat 1404 and W432 and an isotherm of II for Aerosil characteristic of non porous or macroporous materials (Fig. S4). The textural properties of the native silica and their zeolite pseudomorphs as well as large particles FAU-X in terms of pore diameter, surface area and pore volume are reported in Table 1. Textural properties of the SOD zeolite were not measured by N2 adsorption due to its too small pores diameter (0.28 nm) to adsorb N2. For LTA zeolite, Ca ion-exchange was performed prior N2 adsorption as Na-LTA features too small pores to adsorb nitrogen in opposite to Ca-LTA. Indeed, the exchange of 2 Na+ cations by 1 Ca2+ cation in the cavity of the zeolite allows nitrogen to penetrate into LTA porosity. Ca-LTA and Na-FAU-X featured isotherms of type I characteristic of microporous materials (Fig. 6). Microporous volumes for Ca-LTA and Na-FAU-X large particles are 0.21 and 0.24 mL/g, respectively. As pure LTA and FAU crystals feature a microporous volume of 0.26 and 0.36 mL/g, respectively, this suggests that the large particles of zeolites contain also some amorphous silica and/or silico-alumina parts, which will correspond to 81% and 67% of zeolite LTA and FAU-X, respectively, in the large particles based on microporous volumes analysis. Pseudomorphic zeolites Na-SOD and Na-LTA were obtained with Si/Al molar ratios of 0.9 and Na/Al ratio of 0.97 (Table 2), revealing the presence of some remaining amorphous sodium aluminate phase. For the large particles of Na-FAU-X, a Si/Al molar ratio of 1.23 and Na/Al ratio of 0.94 were obtained suggesting the presence of amorphous silica phase. In order to increase the basicity of the zeolites, a cation-exchange with K+ was performed. The Si/Al ratio for SOD and LTA does not change and 80% of Na+ was exchanged by K+. For FAU-X a slight decrease in Si/Al ratio from 1.23 to 1.16 was noticed suggesting the removal of some of the amorphous silica phase. LTA features the highest content of K+. XRD patterns of the K-exchanged zeolites are similar to the native Na-zeolites (Fig. 2) and the shape and size of the large particles of zeolites remain unchanged (Fig. S6).

Fig. 3. SEM pictures of Na-SOD particles zeolites with an average size of 70 lm.

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Fig. 4. SEM pictures of Na-LTA particles zeolites with an average size of 1 mm.

Fig. 5. SEM picture of Na-FAU-X particles zeolites of 250–500 lm.

Table 1 Textural features of the SOD, LTA and FAU-X zeolite particles and native silicas. Materials

Pore diameter (nm)

SBET (m2/g)

Microporous volume (mL/g)

Total pore volume (mL/g)

Average particle size (lm)

Davicat 1404 W432 Aerosil K-SOD Ca-LTA Na-FAU-X K-FAU-X

10.1 12.6 >50 0.21 0.42 0.78 0.78

319 356 187 – 644 735 509

0 0 0 – 0.21 0.24 0.23

0.70 1.14 0.44 – 0.25 0.25 0.25

70 1000 0.05 70 1000 250–500 250–500

Fig. 6. N2 adsorption/desorption isotherms at LTA zeolites.

196 °C of (a) K-FAU X and (b) Ca-

(50 mg/L) adsorption was observed for Na-LTA, K-LTA, and K-SOD (2 g/L). These results are explained by the larger size of the pollutant (7.4 Å) in comparison to the pore size of the zeolites: SOD (2.1 Å) and LTA (4.2 Å). Similar result was obtained with the adsorption of phenol (5.6 Å) over LTA zeolite [18]. Concerning KFAU-X (with 7.8 Å pore opening), only a slight adsorption corresponding to 0.6 mg2,4-DMP/g was noticed and the equilibrium time was reached in 60 min. The initial weight ratio of pollutant to zeolite was 25 mg2,4-DMP/gFAU. The adsorption isotherm of 2,4-DMP was built for K-FAU-X by varying the pollutant concentration from 1 to 200 mg/L (Fig. S7). No relevant adsorption of 2,4-DMP was observed with only 1.6 mg 2,4-DMP adsorbed per g FAU for the highest 2,4-DMP initial concentration of 200 mg/L (corresponding to an initial amount of 100 mg2,4-DMP/gFAU). The contribution of the pollutant adsorption over zeolites SOD, LTA and FAU-X is therefore negligeable in the ozonation process. 3.3. 2,4-DMP removal by ozonation with and without zeolites

3.2. Adsorption of 2,4-DMP in the large particle zeolites Prior O3 experiments, the adsorption of 2,4-DMP was performed to characterize the adsorption capacity of the zeolites. No 2,4-DMP

Ozonation experiments were performed with 2,4-DMP (50 mg/ L) at 25 °C with an initial pH of 4.5. The pollutant removal by ozonation was followed within time as well as the total organic carbon (TOC) removal with and without zeolites (Fig. 7). TOC allows to

J. Vittenet et al. / Microporous and Mesoporous Materials 189 (2014) 200–209 Table 2 Chemical analysis in molar ratio (±0.05) determined by EDX of large particle zeolites SOD, LTA, FAU-X before and after ozonation.

⁄

Zeolites

Si

Al

Na

K

Si/Al

(Na + K)/Td

Na-SOD (Da1404) K-SOD (Da1404) K-SOD after O3 Na-LTA (W432) Na-LTA after O3 K-LTA (W432) K-LTA after O3 Na-FAU-X (Aerosil) K-FAU-X (Aerosil) K-FAU-X after O3

1 1 1 1 1 1 1 1 1 1

1.11 1.09 1.18 1.12 1.10 1.12 1.11 0.81 0.86 0.87

1.05 0.21 0.15 1.09 1.07 0.21 0.17 0.76 0.08 0.05

– 0.79 0.75 – – 0.87 0.86 – 0.61 0.62

0.90 0.92 0.86 0.90 0.91 0.90 0.90 1.23 1.16 1.14

1 0.96 0.82 1.02 1.01 1.01 0.97 0.84 0.74 0.72

Td = (Al + Si)/2.

Fig. 7. Kinetics of (a) 2,4-DMP removal and of (b) TOC removal for single ozonation () and for the ozone/zeolites process with: K-SOD(d), Na-LTA (h), K-LTA (j) and K-FAU X zeolite (). Conditions of ozonation procedure: 25 °C, flow O3 = 40 L/h and [O3]entering = 2 g/m3, [2,4-DMP]0 = 50 mg/L, 2 g/L zeolites, volume of solution 1.5 L.

monitor not only the removal of 2,4-DMP but also the removal of all the oxidized by-products resulting from the oxidation of 2,4DMP. The total degree of mineralization (total transformation into CO2) would correspond to 100% TOC removal. Concerning the 2,4DMP removal, single ozonation (without zeolite) lead to no detectable amount of the pollutant after 25 min of reaction (Fig. 7). The fast removal of 2,4-DMP (with the same initial concentration) by ozone was previously reported [6] with 90% of the 2,4-DMP removed in 10 min at pH 2.5 and in 3 min at pH 9.5. This highlights the high reactivity of ozone with 2,4-DMP. Adding zeolites (K-SOD, Na-LTA, K-LTA, K-FAU-X) in the ozonation process featured a slightly faster degradation of the 2,4-DMP (Fig. 7a). All the synthetized zeolites except K-FAU-X allowed to achieve complete disappearance of the pollutant in 22 min in presence of O3 (Fig. 7a). K-

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FAU-X achieved the total removal of 2,4-DMP in 25 min similarly as single ozonation. The 2,4-DMP pollutant removal was monitored by HPLC, and TOC removal study showed that 2,4-DMP was transformed into oxidized by-products (Fig. 7b). Oxidized by-products of phenolic compounds are known to be highly toxic as quinone and catechol [35,36] and the oxidation level of the 2,4-DMP is therefore very important to estimate for wastewater treatment and not only the removal of a given pollutant as usually presented in literature. For single ozonation only 14% of the TOC removal was reached after 5 h. Remarkable increase of the TOC removal was observed by adding zeolites with more than 26% of TOC eliminated in 5 h. Na-LTA featured the highest result with 34% of TOC removal followed by K-LTA, K-SOD and K-FAU-X. The different particles size of the zeolites did not affect the degradation of the pollutant as TOC removal was nearly similar for K-SOD and KLTA featuring particles size of 70 lm and 1 mm, respectively. In all ozonation experiments a plateau in TOC removal was reached after 1h30 of reaction, which indicates that oxidized by-products of 2,4-DMP formed at this stage are probably more stable towards oxidation than the first range of oxidized intermediates. In addition, this slowdown in the mineralization process can be explained by the use of a low amount of O3 used in our process, which might not be sufficient to degrade resistant organic compounds issued from 2,4-DMP oxidation. To explain the increase of TOC removal with zeolites, the evolution of the pH of the solution was followed. Indeed it is well known that at high pH (>7) the formation of radical species is favored during ozonation due to the interaction of O3 with OH-, which produces OH radicals. These radicals are one of the main reactant able to mineralize pollutants in aqueous media. This increase of pollutant degradation in high pH solution was already observed for single ozonation of phenols compounds [7,37–39]. In our case, the starting pH of the solution is around 4.5 and during single ozonation the pH of the solution decreases from 4.2 to 3.2 after 5 h of reaction (Fig. 8). This decrease of pH is attributed to the formation of carboxylic acids as oxidation byproducts of 2,4-DMP. A decrease of the pH of the solution was already observed in previous ozonation studies of 2,4-DMP and phenol for initial solutions with pH below 7 [6,40]. In opposite, when zeolites are added in the ozonation process, the pH immediately increases around 6.5–7.5 (Fig. 8). The increase of pH was in the following order: K-SOD > K-FAU-X > Na-LTA > K-LTA. For natural zeolites this increase of pH was noticed from pH = 5.5 to pH = 10 and attributed to a partial hydrolysis of the zeolite. The cations (Na+, K+) in the zeolites are partially exchanged by hydronium ions (H+) of the solution, and the free hydroxide ions formed are responsible for the pH increase in the mixture [41]. The increase of pH of an aqueous solution (without ozone and pollutant) was checked with the zeolites used in this study (Fig. 8): the fastest and highest increase of pH was observed with K-SOD (pH 9.6 in 7 min, pH 10.4 in 1 h), and the slowest and lowest with Na or K-LTA (pH 7.8 in 7 min, pH 9.5 in 38 min). In the case of ozonation experiments, the increase of pH due to cation-exchange of the zeolites in water was compensated by the formation of carboxylic acids by-products, and the pH of the solution was buffered around pH 6.5–7.5. In order to estimate the influence of the pH in the TOC removal single ozonation experiments were conducted in buffer solutions at pH 6.5 and 7.4. An increase of the TOC removal was observed from 14 to 21% with both buffer solutions after 5 h (Fig. S8), which remains below of the TOC removal obtained with zeolites and especially with Na-LTA with a TOC removal of 34% at pH 7. Therefore, the increase of the pH of the solution by adding zeolites in ozonation process is not a sufficient explanation for the higher TOC removal obtained with zeolites, except maybe for K-FAU-X (with 26% TOC removal at pH 7). In other words, the

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K-FAU-X, a slight decrease in Si/Al ratio after ozonation revealing a slight desilication could result from the rapid increase of pH in the first minutes of the reaction for these two zeolites, especially for K-SOD. Some slight decrease in cation to tetrahedra (Si, Al) ratio (Table 2) was observed with a loss of 1%, 4%, 3% and 14% of cations for Na-LTA, K-LTA, K-FAU-X, K-SOD, respectively. This evidences some partial cation removal from the zeolite due to the exchange with the protons of the solution and the concomitant production of hydroxyls ions. But except for K-SOD, which shows the highest level of cation removal and can explain the abrupt increase of pH in the first minutes of ozonation, the level of cation removal does not follow the increase of pH (Fig. 8). The Na-LTA zeolite is the chemically most stable zeolite under ozonation conditions (with only 1% of sodium exchange) and evidences the best activity in TOC removal. The increase of pH of Na-LTA is lower than the one obtained for K-FAU-X giving the lowest increase in TOC removal. Therefore some additional phenomenon as production of OH- from cation-exchange is responsible for the better activity of Na-LTA. 3.4. Evidence of OH formation using tert-butanol, a radical scavenger

Fig. 8. (a) Evolution of the pH of the solution within time for single ozonation () and for the ozone/zeolites process with: K-SOD (d), Na-LTA (h), K-LTA (j) and KFAU-X (). Conditions of ozonation procedure: 25 °C, flow O3 = 40 L/h and [O3]entering = 2 g/m3, [2,4-DMP]0 = 50 mg/L, 2 g/L zeolites, volume of solution 1.5 L. (b) Evolution of the pH of suspensions of zeolites (2 g/L) in water: K-SOD (d), NaLTA (h), K-LTA (j) and K-FAU-X ().

increase of the amount of OH radicals generated by the increase of pH is not the only mechanism responsible for the high increase in TOC removal with zeolites. Another explanation for the increase of the TOC removal, corresponding to a decrease of organic matter in the solution, could be the adsorption of the oxidized by-products of the 2,4-DMP on the zeolites, instead of their expected transformation into CO2. To verify this hypothesis TGA analyses were performed on zeolites after ozonation. TGA analyses do not show any loss of organics as TGA curves of the zeolites before and after ozonation are similar with only a weight loss attributed to water (Fig. 9). Only K-SOD showed a slight higher weight lost (4%) in comparison to the native zeolite probably due to higher water adsorption in the pores. Based on TGA analysis, it could be stated that the increase in TOC removal cannot be attributed to the adsorption of by-products on the solids. Zeolites do not behave as adsorbents in the removal of the 2,4-DMP by ozonation. Generation of OH radicals seemed to be the most suitable mechanism. As zeolite framework type did not show significant different effect on the pollutant degradation, it can be also supposed that it is the external surface of the zeolites, which is involved in the reaction of OH radicals generation with O3. Zeolites stability after ozonation has been checked by XRD (Fig. 2 and S9) and chemical analysis (Tables 2 and S1). No significant differences before and after ozonation were found by XRD. Concerning chemical analysis no change in Si/Al ratio (no increase due to dealumination in the initial acidic media and no decrease due to intermediate basic media) due to framework dissolution or degradation by hydrolysis was noticed for Na-LTA. For K-SOD and

As Na-LTA zeolite revealed the highest level of mineralization of 2,4-DMP and appeared as a promising material for toxic compounds removal by ozonation, further studies on OH identification were conducted with this material. In typical ozonation of organic compounds in water, it has been identified that O3 can react by either a direct route or by a second pathway implying the generation of OH radicals. This second pathway called indirect reaction is influenced by the pH of the solution, the solution composition and the temperature [42,43]. The generation of these highly oxidative species provided many advantages and is more efficient for the degradation of refractory compounds [44]. To evidence the presence of OH radicals in the ozonation of 2,4-DMP in the presence of zeolites the addition of a stable OH radical scavenger as t-BuOH in the solution was performed. Prior this experiment t-BuOH was added to the solution for a single ozonation without zeolites to verify if simple ozonation could also produce OH radicals. No significant difference in pollutant removal was noticed, only a slight decrease in TOC removal from 14% to 12% was observed (Fig. 10). Therefore, 2,4-DMP degradation by single ozonation process is principally governed by the direct reaction of 2,4-DMP with O3. Adding t-BuOH to ozonation process using Na-LTA zeolite decreased drastically the level of mineralization from 34% to 15%, which clearly highlighted the generation of OH radicals by the zeolite. Na-LTA allowed a considerable enhancement of the 2,4-DMP mineralization by OH radicals successive attacks. This was not the case with high silica zeolites as ZSM-5 where no radical pathway was identified and where the zeolites acted more as O3 reservoir [5,10,12,16]. As this generation of OH radicals is only partially due to a pH effect of the solution, it may be hypothesized that OH radicals are generated by the interaction of O3 due to its dipole nature with hydroxyl groups at the surface of the material as reported earlier for other oxides (Al2O3, FeOOH) [17,45,46]. Further studies will be conducted to identify the nature of these hydroxyls groups coming either from defects on zeolite crystals or from amorphous sodium alumina phase remaining in the large Na-LTA particles obtained by pseudomorphic transformation. 3.5. Reuse of Na-LTA in ozonation of 2,4-DMP Different batches of Na-LTA have been employed and a good reproduction of the results has been obtained. However, one batch of Na-LTA has been used in a second run of ozonation after filtering and drying (at 50 °C) and shows a decrease of activity in TOC removal from 34% (first run) to 22% (second run) (Fig. S10). In paral-


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Fig. 9. TGA of K-SOD (a) before and (b) after ozonation, of K-LTA (c) before and (d) after ozonation, of Na-LTA (f) before and (e) after ozonation, of K-FAU-X (h) before and (g) after ozonation.

from the first run is in fact H,Na-LTA (even if the amount of proton is very small; 1% of cation) and further sodium cannot be exchanged. Only the second mechanism responsible for the highest activity of the zeolites in comparison to single ozonation due presumably to hydroxyls groups present as defects in zeolite crystals or in the additional alumina phase between zeolite crystals occurs reversibly. 3.6. Effect of NaCl addition on ozonation

Fig. 10. Kinetic of TOC removal for single ozonation (e) with and () without tBuOH radical scavenger and for ozone/Na-LTA process (+) with and (h) without tBuOH. Conditions of ozonation procedure: 25 °C, flow O3 = 40 L/h and [O3]enter3 ing = 2 g/m , [2,4-DMP]0 = 50 mg/L, 2 g/L zeolites, volume of solution 1.5 L, [tBuOH] = 25 mg/L.

lel, the pH of the solution was followed during the ozonation and no increase of pH was noticed as in the first run; the pH remaining at pH 4.5 (Fig. S10). This indicates that the exchange of Na+ by protons in the solution does not occur anymore. The Na-LTA resulting

An excess of NaCl was added to a suspension of Na-LTA in water to limit the exchange of the sodium of Na-LTA with the proton of the solution, which is irreversible. The pH of the mixture increases only from pH 5.5 to pH 7.2, showing that NaCl allows to reversibly exchange protons and sodium cations in the solution. The effect of NaCl in ozonation processes is very important as some wastewater contains some salts as NaCl, which can be present in high concentration level in tannery and dye manufacturing wastewaters, for instance [47]. Previous studies have reported that NaCl or more precisely Cl- increased the rate of ozone decomposition in aqueous media [48,49]. But in another hand, it was also demonstrated that Cl- could act as scavenger for OH radicals [47]. NaCl (50 g/L) was added to the ozonation process using Na-LTA and a drastic decrease of TOC removal from 34% to 12% was observed (Fig. 11) as previously obtained with the addition of t-BuOH. For ozonation without zeolite no significant difference in TOC removal was no-

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Mansouri (IEM) and Bertrand Rebiere (UM2) for N2 adsorption/ desorption and EDX analyses respectively. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2013. 09.042. References

Fig. 11. Kinetic of TOC removal for single ozonation (⁄) with and () without NaCl and for ozone/Na-LTA process (s) with and (h) without NaCl. Conditions of ozonation procedure: 25 °C, flow O3 = 40 L/h and [O3]entering = 2 g/m3, [2,4DMP]0 = 50 mg/L, 2 g/L zeolites, volume of solution 1.5 L, [NaCl] = 50 g/L.

ticed after NaCl addition with only a slight decrease from 14% to 10%. The interaction between Cl and O3 leading to the formation of chloro complexes or to the formation of Cl2. radicals, which are less active [50] does not seem to influence seriously the degradation of the polluant. However, the decrease of mineralization level using Na-LTA and NaCl confirms the probable role of Cl as OH radicals scavenger. Consequently, Na-LTA cannot be used to treat saline wastewater.

4. Conclusions The synthesis of large particles of several zeolites (SOD, LTA and FAU-X) has been successfully achieved by pseudomorphic transformation without using inorganic blinders for a better understanding of phenols pollutant degradation during heterogeneous ozonation. Addition of zeolites to ozonation processes increases drastically the mineralization level of the pollutant. Na-LTA was the most active zeolite. Furthermore, its pseudomorphic synthesis as 1 mm particles can allow to avoid clogging problems in the ozonation processes and facilitate the separation from the solution by simple filtration. No adsorption of either 2,4-DMP or its oxidized by-products was observed on the zeolites. The increase of pH of the solution by adding zeolite due to the partial exchange of sodium by protons (1% of cation in the case of Na-LTA) producing hydroxyl ions is not the only cause of the better activity. The use of radical scavengers (t-BuOH, Cl-) during ozonation with Na-LTA has highlighted the OH radicals generation, which comes from the interaction of ozone with the produced hydroxyl ions and also most probably from an interaction with the hydroxyl groups at the surface of the zeolite particles due to defects in the crystals or from the additional sodium alumina phase present in the Na-LTA particles. Further studies will be conducted to identify the nature of these hydroxyls groups in the large Na-LTA particles obtained by pseudomorphic transformation. Zeolites and particularly Na-LTA appeared as a promising material for the removal of refractory compounds in ozonation process because of their stability with O3 and their ability of OH radical generation. Acknowledgments The authors acknowledge the French National Agency for Research (ANR) for supporting this study through the convention ANR ECOTECH 2010 project PETZECO (1081C0230/ANR-10-ECOT011-03), Nathalie Masquelez (IEM) for the TGA analyses, Bruno Navarra (IEM) for the development of the pilot, Abdeslam El

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