Claude Universite

Bernard -Lyon

M François-Noël

Gilly Composantes Sante

Yuting Tu

Shuanghong Tian

Lingjun Kong

Ya Xiong

Claude Descorme
email: claude.descorme@ircelyon.univ-lyon1.fr

Co-catalytic effect of sewage sludge-derived char as the support of Fenton-like catalyst

Keywords: Sewage sludge Inorganic component Co-catalytic effect Heterogeneous Fenton-like catalyst sewage sludge, photo-Fenton-like catalyst, visible light, Acid Orange I, discoloration, mineralization Catalytic wet air oxidation (CWAO) 2-Chlorophenol Sewage sludge based catalyst Iron oxide Stability

Sewage sludge-derived char was successfully employed as the support of heterogeneous Fenton-like catalyst for the first time. Physicochemical properties of the prepared sewage sludge-derived char supported iron oxide catalyst (FeSC) have been evaluated by XRD, SEM, EDS and N 2 -adsorption/desorption analysis. FeSC exhibited better performance in discoloration of AOII than wood sawdust-based carbon supported iron oxide catalyst (FeWC) both in batch experiment and continuous tests. The high inorganic components content of sewage sludge was concluded to have strong correlations with the high catalytic activity of FeSC. To further interpret the co-catalytic effect of inorganic components in sewage sludge, the inorganic components were removed from the sewage sludge and then a series of catalysts were prepared by the addition of iron, as well as silica and/or alumina to the sewage sludge free of the inorganic components. It was found that the removal of inorganic fraction remarkably decreased the catalytic activity of iron-containing catalyst. The insertion of SiO 2 favors the increase in catalytic activity due to the formation of both hydrogen bonds between H 2 O 2 and siloxane bridges and the acidic microenvironment near the surface of silica phase. While the addition of Al 2 O 3 in sewage sludge as basic sites can facilitate the degradation of H 2 O 2 , and the characteristic of Lewis acidity of alumina can accelerate the reduction of Fe 3+ to Fe 2+ by H 2 O 2 .

Introduction

A B

Fe-SC800

Fe-SC700

Fe-SC600 Intensity (a. u.)

(deg)

Analytical methods

Results and discussion

Characterization of the catalysts

The basic physical and chemical characteristics of the SC, FeSC and SC-A samples are summarized in The ash content values and the results of the ICP-OES analysis indicated a high content in inorganic components, such as iron, silicon and aluminium. After the acidic washing, the ash fraction was partial removed. The iron content in SC, FeSC and SC-A, determined by ICP-OES, was 4.5, 12.0 and 0.4 wt.%, respectively. SC and FeSC exhibited quite a low specific surface area, while SC-A showed a higher surface as a result of the acidic washing and the efficient ash removal. As indicated in Table 3-1, the pH PZC value decreased upon incorporation of iron in the SC support. This variation could be attributed to the higher amount of acidic oxygen surface functional groups induced by the presence of iron, which could be beneficial for the CWAO reaction [10].

The iron phase in the FeSC sample obtained by XRD measurement was the same as the result we got in the Chapter 2 (Fig. 3-2). Compared with the diffractogram of the pure SC and SC-A samples, the XRD pattern for FeSC exhibited several extra diffraction lines at 2 equal 35.5º, 62.6º and 30.1º.

These lines could respectively be assigned to the (311), ( 440) and (220) planes of Fe 3 O 4 (magnetite, JCPDS n°88-0866).

Fig. 3-2. XRD patterns of FeSC, SC and SC-A. Intensity (a.u.)

CWAO of 2-chlorophenol over the different catalysts

Heterogeneous vs. homogeneous reaction

Synthesis of the catalysts

Wavenumber (cm -1 )

Mn-g-C3N4 g-C3N4 Fig. 4567.

Table 4-7. Fenton oxidation, in which hydrogen peroxide is used as oxidant and Fe(II) ions as catalyst, has been proved to be an effective technology for the treatment of industrial wastewater containing non-biodegradable organic pollutants [1]. During recent decades, heterogeneous Fenton and Fenton-like catalysts have attracted a great deal of interest as they exhibit wider applicable pH range and can avoid the continuous loss of catalyst compared with the homogeneous Fenton process [2]. Many organic and inorganic materials have been used as catalyst supports by immobilizing iron ions or iron oxides on them. Among those supports, Nafion membrane [3,4], activated carbon [5,6], silica [7,8], Al 2 O 3 [9,10], zeolite [11], clay [12] are commonly used. And in recent years, some research has been carried out to explore the applications of solid waste as support of heterogeneous Fenton-like catalysts, such as coffee used grounds [13], rice husk ash [14], fly ash [15], red mud [16] and so on.

Evaluation of the catalytic ozonation activity of

The disposal of municipal sewage sludge has become one of the most important environmental problems nowadays [17]. The traditional methods of sewage sludge disposal, such as ocean dumping, landfill and incineration, may cause secondary pollutions to soil, ground water and air [18][19][20]. Therefore, the resource utilization of sewage sludge has attracted more and more attention and some new processes are being developed, such as sewage sludge composting, pyrolysis and gasification [21][22][23]. Among them, pyrolysis is considered as an interesting route since it is effective in reducing the volume of sludge and producing useful end products [24].

The solid residue formed during pyrolysis of sewage sludge is known as 'sludge-derived char (SC)' in the literatures [25,26]. Most of them were used as adsorbents of dyes, phenol and metals in wastewater treatment [27][28][29]. It was found that the BET surface areas of SC obtained from the carbonization alone have never been reported higher than 400 m 2 /g, which is much less than that of usual commercial activated carbon [30], thus SC is not a favorable adsorbent. Recently, SC was employed as an emerging catalyst support in aqueous and gaseous reactions. For example, Zhang et al. [31] used sewage sludge carbon as the support of photocatalyst TiO 2 for the removal and recovery of ionic mercury [Hg (II)] from water. Ansari and Bandosz [32] prepared H 2 S adsorbents/catalysts by incorporating calcium oxide into the dried sewage sludge and waste paper mixture-based carbonization material. However, there is no report about using SC as the support of Fenton-like catalysts. Furthermore, since SC is an inorganic-organic alloyed mixture, it should have some special behaviors compared with other commonly used supports. This situation encouraged us to investigate the potential of SC as the support of heterogeneous Fenton-like catalyst.

The aim of this study is to investigate the potential of SC as the support of heterogeneous Fenton-like catalyst. The catalytic activity of the prepared complex catalyst FeSC was evaluated by degradation efficiency of a representative azo dye, Acid Orange II (AOII). Considering the high ash content of SC obtained after pyrolyzed at 800 • C, which reaches around 70% as reported [33], co-catalytic effect of these inorganic components was investigated in detail.

Experimental

Materials

Dewatered sewage sludge was obtained in June from Lie De municipal wastewater treatment plant in Guangzhou, China. Pine wood sawdust was washed thoroughly with deionized water before use. Commercial activated carbon prepared from coconut husk was purchased from Tianjin Chemical Reagent Co. Ltd., China. These three kinds of precursors were dried at 105 • C to constant weight, subsequently ground and sifted through a 100-mesh sieve, and stored in a desiccator at room temperature.

Analytical grade of ferrous sulfate (FeSO 4 •7H 2 O), tetraethyl orthosilicate (TEOS), anhydrous ethanol, hydrogen peroxide solution (30%, w/v) and Al 2 (SO 4 ) 3 were obtained from Guangzhou Chemical Reagent Company, China. The AOII was a commercial dye (Hengrun Dyestuff Chemical Co., Ltd., Guangzhou) and used without further purification.

Preparation of the catalysts

Sewage sludge-derived char supported iron oxide catalyst (named as FeSC) was prepared by one-step method combing synthesis with loading. Firstly, 20 g dried sludge was added into 150 mL of Fe solution (0.5 mol/L FeSO 4 , pH = 2.5). After the suspension was stirred for 24 h, it was evaporated in a rotary evaporator at 60 • C, and then dried at 105 • C for 12 h. Subsequently, the dried solid was pyrolyzed in a horizontal furnace at 800 • C for 2 h with heating rate of 20 • C/min and nitrogen flow of 40 mL/min. The pyrolyzed sample was washed with deionized water several times until the conductivity became constant, then dried at 105 • C overnight and ground to less than 200-mesh. The same procedure was used to prepare wood sawdust-based carbon supported iron oxide catalyst (labeled as FeWC), except that 20 g wood sawdust was mixed with 35 mL of FeSO 4 solution (0.5 mol/L, pH = 2.5). Commercial activated carbon supported iron oxide catalyst (FeAC) was prepared by classical incipient wetness impregnation [34] using 20 g AC and 0.1244 g FeSO 4 •7H 2 O, and also treated in N 2 flow at 800 • C for 2 h. The different amount of materials was used to guarantee all the catalysts have similar iron content.

To remove the inorganic components in sewage sludge, some activated sludge mixed liquor was obtained directly from the aeration tank of Lie De, and then was incubated for about two weeks in laboratory till the ratio of volatile suspended solids (VSS) and total suspended solids (TSS) was over 85%. Then some suspension was filtrated and dried. The obtained bacteria powder was suspended in 0.5 mol/L HCl solution for 12 h, subsequently filtrated and washed with deionized water several times until the pH of the rinse water was about 6. The ash content of the resulted dry sludge (referred to as SS-A) was 4.31 wt.%. Then the SS-A was used as precursor for catalysts preparation. Various amounts of SiO 2 and Al 2 O 3 were added to the precursor. The name of the catalysts and their preparation conditions are listed in Table 1. To synthesize SiO 2 -containing samples, fixed amounts of ethanol, TEOS and SS-A were stirred and heated at 80 • C for 2 h, then the mixture was dried at 105 • C for 12 h, and ground to fine powders (labeled as SiSS-A). After that, given amounts of FeSO 4 or/and Al 2 (SO 4 ) 3 solution were mixed with SiSS-A for 24 h, then dried and pyrolyzed as described above.

To evaluate the role of carbon phase, which generated from the carbonization of sewage sludge, in the heterogeneous Fenton-like catalytic reaction, FeAlSi catalyst was also prepared without the addition of any sewage sludge.

Characterization of catalysts

The average particle size was obtained on MasterSizer 2000 Particle Size Analyzer. The ash contents of catalysts were determined using a standard method according to ASTM D2866-94. The pH values of isoelectric point (pH IEP ) were determined from electrophoretic mobility measurements using a zeta potential analyzer (Zetasizer Nano ZS, Marlven). The specific surface area (S BET ) was determined with an Autosorb-iQ-MP gas sorption analyzer (Quantachrome Instruments, USA) by nitrogen adsorption at 77 K. The chemical state of iron on catalysts were followed by X-ray diffraction (XRD) using D/max 2200 vpc Diffratometer (Rigaku Corporation, Japan) with a Cu K␣ radiation at 40 kV and 30 mA. Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-6330F-mode field emission scanning electron microscope equipped with an energy-dispersive X-ray (EDX) detector (JED-2300). X-ray photoelectron spectroscopy (XPS) measurements were performed using an ESCALAB 250, Thermo-VG Scientific (UK) system, with Avantage software for data acquisition and analysis. An Al K␣ monochromatized radiation operating at 15 kV (150 W) was used, and the spectrometer, calibrated with reference to Au 4f7/2 (84.0 eV), Ag 3d5/2 (368.3 eV) and Cu 2p3/2 (932.6 eV), was operated in CAE mode with 20.0 eV pass energy. Data acquisition was performed with a pressure lower then 10 -7 Pa. Spectra analysis was performed using peak fitting with XPS peak 4.1 and Shirley type background subtraction. Bulk iron, alumina and silica content of the synthesized samples were measured by inductively coupled plasma optical emission spectrometry (ICP-OES, optima 5300DV, Perkin-Elmer).

Catalytic activity

All the batch experiments were carried out in a glass conical beaker (250 mL) with a stopper shaken in a thermostatic bath at an equivalent stirring velocity around 200 rpm at 30 • C. In each run, a given amount of powder catalyst was added into 100 mL AOII solution. The solution pH was adjusted to 4.0 ± 0.1 with NaOH and H 2 SO 4 during the experiments. The samples for determination of adsorption kinetics were taken at predetermined time intervals and were analyzed immediately after filtration through 0.22 m Millipore membrane filters to remove suspended particles. After the adsorption/desorption equilibrium was established in 60 min, 60 mL suspension was used for Fenton-like reaction and initiated by adding 15 mM H 2 O 2 to the solution. Samples were withdrawn at a given time interval during the reaction and immediately analyzed after filtration. Each experiment was repeated thrice, the data points represent the average value and error bars indicate the standard deviation.

To investigate the lifetime of catalysts, a continuous catalytic oxidation was conducted in a novel integrated membraneheterogeneous Fenton-like catalytic continuous reactor (MFR) designed by our laboratory. As shown in Fig. 1, the MFR consists of a reaction tank, a submerged membrane module, a liquid feed system, an effluent collecting system, and an air bubbling system.

Analytical methods

The pH of AOII solution was measured with a PHS-3C pH meter (Rex Instrument Factory, Shanghai, China). The concentration of AOII was analyzed on UV-vis spectrophotometer (UV-3150, Shimadzu) with maximum absorbance at 484 nm. H 2 O 2 was analyzed spectrophotometrically by the titanium sulfate method. The leaching concentration of Fe in the solution was measured by inductively coupled plasma optical emission spectrometry (ICP-OES, optima 5300DV, Perkin-Elmer). The total organic carbon (TOC) of the reaction solution was measured by a TOC analyzer (Shimadzu, Japan).

Results and discussion

Characterization of catalysts

The basic physical and chemical parameters of FeSC are presented in Table 2. It can be seen that ash content of FeSC is 93.01%, indicating the catalyst mainly consists of inorganic components, and only a small amount of carbon. The main inorganic elements in ash detected by ICP-OES are Fe, Si, Al, Ca, K and Na, and their weight contents are 13.91%, 6.78%, 5.48%, 1.22%, 1.08% and 0.30%, respectively. As shown in Table 2, the ash content of SC is 74.63% and the Fe content is only 3.91%. That means the Fe in FeSC is mainly the added element during the preparation process of FeSC, while Si and Al are the native element in original sewage sludge.

The iron chemical state in FeSC was studied by XRD (Fig. 2). New broad bands at 2Â angles of 35.5 • , 62.6 • and 30.1 • are distinctly observed by comparing the XRD pattern of FeSC catalyst with that of SC. These diffraction peaks are assigned to (3 1 1), (4 4 0) and (2 2 0) planes of Fe 3 O 4 crystallite (JCPDS 88-0866), respectively [35,36]. The oxidation state of Fe on the surface of catalysts was also characterized by XPS (Supplementary data, Fig. S1). The binding energy of Fe 3p was determined to be 56.4 eV, which was ascribable to Fe 3 O 4 [37]. According to the results of XRD, other elements, Si, Al, Ca and Na in the forms of SiO 2 , Al 2 O 3 , CaSO 4 and NaPO 3 are also detected in FeSC. It is noteworthy that crystallographic structure of SiO 2 and Al 2 O 3 are particularly more recognizable compared with other oxides due to their higher content in the bulk powder sample [38]. The SEM micrograph of FeSC is shown in Fig. 3, the morphology of FeSC presents roughness surface with "coral type" structure. In addition, the Fe content on the surface of FeSC measured by EDS analysis is 15.26%, in agreement with the result obtained by ICP-OES, therefore, it can be deduced that the Fe 3 O 4 particles are almost uniformly distributed in FeSC. The average size of the crystalline particles for Fe 3 O 4 is calculated to be 12.1 nm based on the Debye-Scherrer formula from XRD study For comparison, main physicochemical parameters of another biomass carbon-based catalyst (FeWC) and its corresponding support (WC) are also determined and listed in Table 2. The Fe content of FeWC is similar to that of FeSC, however, FeWC is almost free of Si and Al elements. The BET surface area of FeWC is a little smaller than that of FeSC. As shown in Fig. 3, the surface morphology of FeWC presents in the form of plates which come from the carbonization of the cellulosic materials with small particle size iron oxide dispersed on them, while FeSC presents a more heterogeneous appearance.

Catalytic activity of FeSC catalysts

Heterogeneous Fenton-like catalytic activity of FeSC catalyst was assessed in the term of degradation efficiency of AOII, as shown in Fig. 4. In adsorption phase, 26.9% of AOII is removed by SC, while only 22.8% of AOII discoloration is observed in FeSC system. This indicates that the impregnation of iron oxides can reduce the amount of AOII adsorbed on catalyst. During a two-hour oxidation process, the AOII degradation in the presence of only H 2 O 2 is almost negligible, and only 21.9% of AOII is removed by SC, indicating the catalytic activity of SC is very low. The removal of AOII could mainly be dependent on 3.91 wt.% of Fe in SC. The little amount of Fe originated from the original sewage sludge exists in the form of Fe 3 O 4 and acts as the role of Fenton-like catalyst. As can be seen in Fig. 4, the degradation efficiency of AOII catalyzed by FeSC locates at 94.6% after oxidation for 2 h. Therefore, FeSC shows obviously better catalytic activity than the SC support although it presents smaller adsorption capacity of AOII.

As is known, both the heterogeneous and homogeneous processes make contribution to the oxidation of AOII in the systems with catalysts. So the iron concentration in the liquid phase at the end of the operation was measured by ICP-OES. The result shows that, after 3 h of reaction the concentration of Fe in solution is 0.373 mg/L for FeSC system, and the amount (wt.%) of Fe lost from the solid never raised above 0.14% as referred to the total Fe initially incorporated. Meanwhile little amount of Ca, Cu, Mg and Zn were also detected in the filtrate, their concentrations were 0.813 mg/L, 0.029 mg/L, 0.271 mg/L and 0.027 mg/L, respectively. And the concentrations of Cr, Mn, Ni and Pb were below the detection limit of ICP-OES. In these conditions some, although relatively low, contribution of aqueous catalytic reaction may take place. To determine the influence of homogeneous phase reaction, we also carried out two homogeneous experiments using Fe(II) and Fe(III) at the concentration that was found in the filtrate after the reaction, and the discoloration efficiency of AOII were only 9.7% and 6.2%, respectively. This result suggests that degradation of AOII for the FeSC-based Fenton-like system could mainly be attributed to heterogeneous process, not homogeneous phase reaction.

In order to understand the catalytic potential of FeSC more clearly, a comparative Fenton-like experiment was performed with FeWC. As shown in curve C of Fig. 4, removal efficiency of AOII in adsorption phase is 17.8%, a little lower than that of FeSC, this can be explained by the lower BET surface area of FeWC. And the discoloration of AOII at the end of oxidation phase for FeWC is only 66.2%. The apparent rate constants (k ap ) of AOII degradation were obtained by fitting the results from 60 min to 180 min shown in Fig. 4 with pseudo first-order kinetic equation, which has been widely applied by researches for the description of the heterogeneous Fenton-like oxidation of organic pollutants [START_REF] Xue | Adsorption and oxidation of PCP on the surface of magnetite: kinetic experiments and spectroscopic investigations[END_REF][START_REF] Üstün | Advanced oxidation and mineralization of 3-indole butyric acid (IBA) by Fenton and Fenton-like processes[END_REF]. As shown in Table 3, very high and good correlation coefficients (R 2 ) were gotten, meaning there is good linear correlation between ln(C 0 /C t ) and time. The k ap for FeSC system was calculated to be 22.45 × 10 -3 min -1 , while that for FeWC system was only 7.31 × 10 -3 min -1 , the former was about 2 times higher than the later. Because the contents and existence forms of Fe in FeSC and FeWC are similar, the difference of their catalytic activities is possibly dependent on non-iron inorganic components co-existing in them.

It is known that reaction intermediates can form during the oxidation of organic dyes, and thus total organic carbon (TOC) removal was measured at the end of adsorption and Fenton-like oxidation phases. The results show that TOC removal efficiencies are 25.4% and 39.1% at 60 min and 180 min for SC, 15.4% and 50.6% for FeWC, while they are 18.4% and 69.7% for FeSC. Therefore, the conclusions of TOC removal are similar as for AOII degradation, indicating that FeSC as a heterogeneous Fenton-like catalyst can lead to a fast discoloration of AOII as well as a remarkable mineralization of the solution.

Co-catalytic effect of inorganic components in SC

To further investigate the effect of inorganic components in FeSC on the catalytic performance in Fenton-like reaction, the inorganic components of sewage sludge was firstly removed and then the remainder was used as precursor (SS-A) for SC-A and FeSC-A preparation. The physicochemical data of the prepared SC-A and FeSC-A are presented in Table 4. The ash content of SC-A and FeSC-A are only 10.02% and 39.97%, much lower than that of FeSC, and the Al and Si contents are at micro level. Then the catalytic behavior of SC-A and FeSC-A was also evaluated in the presence of H 2 O 2 at pH 4.0 (see curves B and C in Fig. 5). After 1 h adsorption and 2 h oxidation, the discoloration efficiencies of AOII are 11.2% and 51.3% for SC-A and FeSC-A, much lower than the 94.6% removal efficiency obtained with the FeSC catalyst. The pseudo first-order degradation kinetic constant for FeSC-A system is calculated to be 4.21 × 10 -3 min -1 (as shown in Table 3), which is also less than that for FeSC. This experimental result further indicates that the inorganic components in the SC have co-catalytic effect on the degradation of AOII in heterogeneous Fenton-like process.

Co-catalytic mechanism of inorganic components in SC

As mentioned previously, except for Fe the other inorganic elements in SC are mainly Si and Al, which exist in the forms of SiO 2 and Al 2 O 3 confirmed by XRD (Fig. 2). Their contents in SC are calculated to be 22.14% and 16.09%, respectively, being 73.46% of total noniron inorganic components. In other words, the co-catalytic effect of SC compared with WC is possibly mainly originated from the SiO 2 and Al 2 O 3 . In order to understand the enhancement mechanism, a series of Fe-loaded sewage sludge carbon with various SiO 2 and Al 2 O 3 content as Fenton-like catalysts were prepared by employing the SS-A as precursor, their physicochemical properties are listed in Table 4, and their catalytic activities were also determined, respectively. As shown in Table 4, all the modified SS-A based catalysts present low BET surface area (less than 25.0 m 2 /g), and the values are very close to each other. Before comparing the activity of silica and/or alumina incorporated iron-containing catalysts, two blank runs were performed to evaluate the ability of SiSC-A and AlSC-A to oxidize AOII in aqueous solutions in the presence of H 2 O 2 . Only 8.6% and 14.8% AOII removal was observed by these two kinds of Fe free catalysts through adsorption and oxidation processes under the same conditions as mentioned above. That means neither SiSC-A nor AlSC-A exhibits any considerable catalytic activity towards the degradation of AOII by H 2 O 2 .

The change of the AOII discoloration rate with the various contents of SiO 2 in catalyst support is presented in Fig. 5 A. The comparison was based on the identical amount of iron. It is found that increase of degradation rate is followed by the order of FeSiSC-A 3 > FeSiSC-A 2 > FeSiSC-A 1 > FeSC-A, which is consistent with that of SiO 2 content. The changes of H 2 O 2 concentration in the liquid phase during the experiments (Fig. 5B) follow a parallel trend to the AOII oxidation: a better performance is reached when the content of SiO 2 in the FeAlSiSC-A catalysts increases. The results further confirm that co-catalytic effect of SC partly originates from SiO 2 -like inorganic component in it. There are several possible explanations for the co-catalytic effect of SiO 2 . Firstly, H 2 O 2 can adsorb on SiO 2 by hydrogen bonds with the oxygen in siloxane bridges, Si O Si [START_REF] Żegli Ński | Pi ękoś, A study of interaction between hydrogen peroxide and silica gel by FTIR spectroscopy and quantum chemistry[END_REF] (Fig. 6A), leading to a higher concentration of H 2 O 2 on surface of FeSC then that in the bulk solution and hence facilitating Fentonlike reaction. Except for siloxane, there is another kind of functional group on the silica surface, silanol (Si OH) [START_REF] Clark | Solid acids for green chemistry[END_REF], which undergoes protonation when the pH values are lower than the pH IEP of SiO 2 and deprotonation at pH > pH IEP [START_REF] Kang | Effects of inorganic substances on water splitting in ion-exchange membranes: II. Optimal contents of inorganic substances in preparing bipolar membranes[END_REF][START_REF] Wang | Structure characteristics and mechanical properties of kaolinite soils. I. Surface charges and structural characterizations[END_REF]. The pH IEP value of SiO 2 is widely reported at 2.0-3.5 [START_REF] Wang | Structure characteristics and mechanical properties of kaolinite soils. I. Surface charges and structural characterizations[END_REF][START_REF] Mullet | Comparison of two electrokinetic methods -electroosmosis and streaming potential -to determine the zetapotential of plane ceramic membranes[END_REF], lower than the solution pH in our experiments. Therefore, H + has an obvious tendency to release from Si O H, and results in the formation of negative charge SiO 2 particles and an acidic microenvironment near the catalyst surface [START_REF] Tian | Degradation of Acid Orange II at neutral pH using Fe2(MoO4)3 as a heterogeneous Fenton-like catalyst[END_REF], as shown in Fig. 6B. This explanation can be partly evidenced by the different zeta potential at pH 4.0 of SiO 2 -containting catalysts that are presented in Table 4, the surface charge became more negative when the content of SiO 2 increases. It is known that low pH favors • OH generation and • OH has higher oxidation potential at pH around 3.0 than that in less acidic solution [START_REF] Chen | Decolorization of KN-R catalyzed by Fe-containing Y and ZSM-5 zeolites[END_REF]. Hence it is possible that the insertion of SiO 2 can facilitate the adsorption of H 2 O 2 on the catalyst surface and increase the activity to generate • OH.

The role of alumina phase in iron-contenting samples as heterogeneous Fenton-like catalyst were also investigated. As shown in Fig. 7, the removal efficiency of AOII after the preliminary adsorption process is 21.1% for FeAlSC-A 1 , 39.7% for FeAlSC-A 2 and 18.2% for FeSC-A. The different adsorption capacities can be correlated with the different BET surface area values and different zeta potential of these three catalysts as shown in Table 4. The pH IEP values of Al 2 O 3 and SC-A are located at 8.5-10.4 [START_REF] Wang | Structure characteristics and mechanical properties of kaolinite soils. I. Surface charges and structural characterizations[END_REF] and 3.13, thus the insertion of Al 2 O 3 will lead to an increase in pH IEP of the particle, and presents less negative charge, which can facilitate the adsorption of sulfonic dye numerator by electrostatic phenomena. In consideration of AOII discoloration, the increase in the content of Al 2 O 3 also resulted in a higher AOII degradation efficiency. The pseudo first-order kinetic constant of AOII oxidation are determined to be 6.71 × 10 -3 min -1 for FeAlSC-A 1 and 8.17 × 10 -3 min -1 for FeAlSC-A 2 , respectively. As mentioned before, the surfaces of FeAlSC-A catalysts are much less negatively charge than that of FeSiSC-A catalysts. Therefore, the pH in microenvironment around the Al 2 O 3 particles is almost neutral and not as acidic as that of FeSiSC-A catalysts. It indicates that the co-catalytic mechanism of alumina phase is different from that of silica phase. For the enhancement mechanism of Al 2 O 3 in the catalyst, Timofeeva et al. [START_REF] Timofeeva | gunov, FeAl12-Keggin type cation as an active site source for Fe, Al-silica mesoporous[END_REF] demonstrated that insertion of alumina phase can increase the strength of basic sites of Fe-containing samples detected with CDCl 3 adsorption, leading to the acceleration of H 2 O 2 degradation and the increase in activity of catalysts. This explanation is in agreement with the result of our work that insertion of Al 2 O 3 can increase the pH IEP of catalyst. Furthermore, alumina as a Lewis acid can attract electron density from ferric ion, and destabilizing the Fe 3+ state, then facilitate the reduction of Fe 3+ to Fe 2+ by H 2 O 2 , which usually is the rate-limiting step in the Fenton's reagent chain reaction [START_REF] Lim | Highly active heterogeneous Fenton catalyst using iron oxide nanoparticles immobilized in alumina coated mesoporous silica[END_REF].

SiO 2 presents lower pH IEP value compared with that of Al 2 O 3 , the surface charge of silica-contenting catalyst FeAlSiSC-A at pH 4.0 is lower than that of FeAlSC-A 2 (Table 4), and resulting in a slightly decrease of AOII adsorption. While as can be seen in Fig. 7, FeAlSiSC-A shows higher catalytic activity than FeAlSC-A 2 . The pseudo first-order degradation kinetic constant is calculated to be 1.511 × 10 -2 min -1 for FeAlSiSC-A (Table 3), which is about 1.8 times as much as that of FeAlSC-A 2 and FeSiSC-A 2 . This experimental result can further indicates the co-catalytic effect of SiO 2 and Al 2 O 3 in SC. On the other hand, maybe there is a synergistically catalytic effect of silica and alumina in FeAlSiSC-A, further research will be carried out to confirm this hypothesis.

To evaluate the function of sewage sludge-based carbon as support in heterogeneous Fenton-like reaction, the degradation of AOII with bare FeAlSi was carried out. The catalyst concentration is 0.8682 g/L for FeAlSi and 1.6256 g/L for FeAlSiSC-A to get the similar amount of Fe, Al and Si in these two systems. It can be observed that both the adsorption and oxidation of AOII in FeAlSi system are less significant than that of FeAlSiSC-A system (Fig. 7). The rate constant is calculated to be only 3.39 × 10 -3 min -1 for FeAlSi using the pseudo-first-order equation, much lower than that of FeAlSiSC-A. This comparison suggests that the carbon in FeSC also plays a very important role in the Fenton-like process. As is known, carbon materials are good adsorbents for aromatic chemicals and the preconcentration of AOII to be oxidized in the vicinity of reactive centers is considered to be beneficial in many literatures [35,[START_REF] Rodríguez-Reinoso | The role of carbon materials in heterogeneous catalysis[END_REF]. Furthermore, the dispersion of the inorganic oxide within the carbon matrix could prevent them from aggregating into clusters during thermal treatment, and increase the number and properties of the reaction surface sites.

Stability of the catalysts

The preliminary evaluation of the stability of the FeSC catalyst was evaluated by AOII degradation in a novel integrated membrane-heterogeneous Fenton-like catalytic continuous reactor (MFR). For comparison, commercial activated carbon which has been extensively used as catalyst support in catalytic wet peroxide oxidation was employed to prepare FeAC catalyst. The Fe content in FeAC is determined to be 10.35% by ICP-OES, and the BET surface area is 541.3 m 2 /g. As shown in Fig. 8, for FeSC and FeWC, the degradation efficiency of AOII reach around 36% and 97% at 120 min, respectively, which are then kept constant until at least 600 min. On the contrary, that for FeAC reaches 93.7% rapidly in the initial 60 min and then decreases with the operation time, which is only 46.6% at 720 min. It indicates that FeSC presents both a high catalytic activity and a long-term stability.

Conclusions

A novel heterogeneous Fenton-like catalyst was first successfully prepared by employing the sewage sludge-derived char (SC) as the catalyst support. It was evidenced that the prepared FeSC catalyst showed better catalytic activity in Fenton-like oxidation of AOII than that of wood sawdust-based carbon supported iron oxide catalyst (FeWC). After compared the physicochemical properties of these two iron-containing catalysts, the presence of inorganic components in sewage sludge, mainly SiO 2 and Al 2 O 3 , was speculated to be the main reason for the good catalytic performance of FeSC. To further confirm this interpretation, the inorganic components in sewage sludge were firstly removed, and various amounts of iron, silica and alumina were added into the treated precursor to prepare heterogeneous Fenton-like catalysts. It was found that the removal of inorganic fraction yielded significant deterioration of catalytic activity. While the impregnation of SiO The present study suggests that SiO 2 , Al 2 O 3 and carbon in sewage sludge-derived char can act as efficient promoters in heterogeneous Fenton-like reaction. Therefore, more efficient catalysts can be developed by changing the composition and proportion of inorganic and organic phases in the support.

INTRODUCTION

It is known that water-soluble organic dyes are bio-recalcitrant compounds existing with a wide concentration range in effluents from the textile and photographic industries. 1,2 In recent years, extensive research has investigated the use of visible irradiation for the degradation of organic pollutants. It was found that the photo-Fenton reaction can be driven with low energy photons in the visible part of the spectrum by enhancing the photo reduction of Fe 3+ to Fe 2+ . This provides a good approach to the treatment of dye-containing water using visible light or sunlight. 3,4 However, there are some drawbacks of the homogeneous photo-Fenton treatment under visible light irradiation, because it leads to high iron concentration in the final effluent so that the removal of large quantities of ferric hydroxide sludge is costly in terms of chemicals and manpower. 5 To avoid these limitations, some efforts have been made to develop heterogeneous catalysts for photo-Fenton-like reactions by immobilizing Fe ions on a suitable substrate. In photochemical studies, many inorganic or organic materials have been used as catalyst supports by immobilizing iron ions or iron oxides on them, such as rectorite, 6 alumina, 7 silica gel, 8 cation transfer membranes. 9 Recently, development of heterogeneous photo-Fenton-like catalysts from solid waste materials has occurred. For example, Li and Zhang 10 developed an amorphous FeOOH catalyst from high iron-containing fly ash and immobilized FeOOH on the precursor, then used it for the degradation of Methyl Orange (MO) dye in the presence of H 2 O 2 /sunlight.

It is known that sewage sludge is mainly a mixture of exhausted biomass generated in the aerobic or anaerobic digestion of the organic contaminants in municipal sewage along with inorganic materials such as SiO 2 , Al 2 O 3 , Fe 2 O 3 and CaO. 11 After being pyrolyzed at high temperature, these oxides will become highly thermal stable and could play a role as catalyst or catalyst promoter in Fenton reactions. 12,13 Sludge derived carbon (SC) formed after pyrolysis of sewage sludge have been widely used as an adsorbent for organic pollutants and heavy metals. 14 work centered on the application of sludge derived products as efficient catalysts has been reported. For example, sewage sludge based carbon was used in catalytic wet air oxidation of phenolic compounds, 17,18 as a catalyst to enhance the removal of oxalic acid in ozonation, 19 and as the support for a heterogeneous TiO 2modified photoccatalyst. 20 However, there is no report of SC being used as supports in photo-Fenton-like catalysts. On the other hand, since SC is an inorganic-organic composite, it could exhibit specific properties compared with other commonly employed inorganic or organic supports. This situation encouraged us to investigate the potential of SC as support for heterogeneous photo-Fenton-like catalysts. And a representative azo dye, Acid Orange II (AOII), was used as a model target pollutant in our study.

The objectives of this study were: (i) to synthesize and to characterize the sewage sludge-derived carbon-supported iron oxide catalyst (named FeSC); (ii) to investigate the effects of various operating parameters on the activity of the FeSC catalysts; and (iii) to evaluate the mineralization of Acid Orange II (AOII) and the stability of FeSC.

MATERIALS AND EXPERIMANTS

Materials

The dewatered sewage sludge was obtained from the Lie De municipal wastewater treatment plant in Guangzhou, China. The sludge was dried at 105 • C to constant weight, subsequently ground, sifted through a 100-mesh sieve, and finally stored in a desiccator at room temperature. The dried sample was labeled SS.

Analytical grade ferrous sulfate (FeSO 4 •7H 2 O) and hydrogen peroxide solution (30%, w/v) were obtained from Guangzhou Chemical Reagent Company, China. Ferroferric oxide and commercial activated carbon prepared from coconut husk were obtained from Tianjin Kermel Chemical Reagent Co. Ltd, China. The AOII was a commercial dye (Hengrun Dyestuff Chemical Co., Ltd, Guangzhou) which was used without any further purification.

Preparation of the catalysts

The sewage sludge-derived carbon-supported iron oxide catalyst (named FeSC) was prepared via a one-step method combining synthesis of SC with loading of the iron. First, 20 g dried sludge was added into a 150 mL iron solution (0.5 mol L -1 FeSO 4 , pH = 2.5). The suspension was stirred for 24 h slowly, evaporated in a rotary evaporator at 60 • C and dried at 105 • C for 12 h. Subsequently, the dried solid was pyrolyzed under flowing nitrogen (40 mL min -1 ) in a horizontal furnace at 800 • C for 2 h (20 • C min -1 ). Two additional pyrolysis temperature (600 • C and 700 • C) were also checked. The pyrolyzed samples were washed with deionized water several times until the conductivity became constant, then dried at 105 • C overnight and ground to less than 200-mesh. A sludge derived carbon (SC) sample, formed after pyrolysis at 800 • C for 2 h, was also used for comparison.

Characterization of catalysts

The average particle size was obtained using a Malvern MasterSizer 2000 Particle Size Analyzer. The ash content of the catalysts was determined using the standard ASTM D2866-94 method. The specific surface area (S BET ) was determined using an Autosorb-iQ-MP gas sorption analyzer (Quantachrome Instruments, USA) via nitrogen adsorption at 77 K. The isoelectric point values (pH IEP ) were determined from electrophoretic mobility measurements using a zeta potential analyzer (Zetasizer Nano ZS, Malvern).

The iron content in the synthesized samples was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, optima 5300DV, Perkin-Elmer). Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-6330 F field emission scanning electron microscope equipped with an energydispersive X-ray (EDX) detector (JED-2300). The chemical state of iron on the catalysts was followed by X-ray diffraction (XRD) using a D/max 2200 vpc Diffratometer (Rigaku Corporation, Japan) with Cu Kα radiation at 40 kV and 30 mA. The TGA-FTIR analyses were performed using a Netzsch TG 209 thermobalance coupled with a Bruker Vector-22 (Bruker Company, Germany). The samples were heated from room temperature to 900 • C at a heating rate of 20 • C min -1 under flowing nitrogen (40 mL min -1 ). The data were processed using 'OPUS 1.0'.

Experimental procedures

All batch experiments were carried out in a 1 L cylindrical reactor equipped with a water cooled quartz jacket inserted in the center. Irradiation was provided by a 150 W short-arc Xe lamp (Sankyo, Japan) located in the center of the quartz jacket. Short-arc Xe lamp energy output at 320-780 nm, hence a 420 nm cutoff filter was used to remove the ultraviolet radiation. Air was bubbled from the bottom of the reactor (4 L min -1 ) to hydraulically suspend the catalyst particles. The reaction temperature was kept at 30 • C by a thermostatic bath. The reaction suspension was prepared by adding a given amount of catalyst into 600 mL of a 100 mg L -1 AOII solution. The solution pH was adjusted from 6.8 to a given value using either NaOH or H 2 SO 4 . After the adsorption/desorption equilibrium was established in the dark for 30 min, 500 mL suspension was used for the photo-Fenton-like reaction and initiated by turning on the Xe lamp. According to the Equation ( 1

Analytical methods

The pH of the AOII solutions was measured with a PHS-3C pH meter (Rex Instrument Factory, Shanghai, China). The concentration of AOII was analyzed using a UV-vis spectrophotometer (UV-3150, Shimadzu) with maximum absorbance at 484 nm. The Fe concentration in the solution was measured by inductively coupled plasma optical emission spectrometry (ICP-OES, optima 5300DV, Perkin-Elmer). The total organic carbon (TOC) of the reaction mixture was measured on a TOC analyzer (Shimadzu, Japan).

RESULTS AND DISCUSSION

Characterization of catalysts

The basic physical and chemical characteristics or properties of the SC and FeSC samples are presented in Table 1. The average particle size in volume, BET surface area and pH IEP of the SC and FeSC sample are similar. The ash content in the FeSC sample is c. 18.4 wt% higher than that of the SC sample since iron has been impregnated. The iron content in SC and FeSC, determined by ICP-OES, was 3.9 and 12.9 wt%, respectively. The iron chemical state in FeSC was studied by XRD (Fig. 1). Three broad bands at 35.5 • , 62.6 • and 30.1 • are observed on the FeSC sample XRD pattern compared with the SC sample XRD pattern. These diffraction peaks are respectively assigned to the (3 1 1), (4 4 0) and (2 2 0) planes of Fe 3 O 4 (JCPDS 88-0866). 21 The Fe 3 O 4 particles were 12 nm in size based on the Debye-Scherrer formula. According to the XRD results, other elements in FeSC, such as Si, Al, Ca and Na were present as SiO 2 , Al 2 O 3 , CaSO 4 and NaPO 3 , respectively. The SEM micrographs of SC and FeSC are shown in Fig. 2. Compared with SC, the surface of FeSC appears a little more rough. In addition, the Fe content on the surface of FeSC measured by EDX analysis is 15.3%, in agreement with the result obtained by ICP-OES. Therefore, it can be deduced that the iron particles are almost uniformly distributed on the SC support.

In order to characterize the chemical changes which take place during the pyrolysis process, a TGA-FTIR study was performed. The TG curves of SS and FeSS are shown in Fig. 3. A larger mass loss was observed on the TG curve of FeSS between 470 o C and 600 o C compared with that of SS. The mass loss of SS and FeSS ended at 550 and 600 o C, respectively. As shown on the three-dimensional (3D) diagrams obtained during the pyrolysis of SS and FeSS (Fig. 4 According to Fig. 4(A1), Three main kinds of volatile compounds were generated during the thermal treatment: H 2 O (bending and stretching vibrations at 1350-1700 cm -1 and 3500-3700 cm -1 ), CO 2 (asymmetric stretching mode at 2250-2392 cm -1 ) and C-H n (asymmetric vibration of the methylene group at 2250-2392 cm -1 ). While on Fig. 4(B1), in addition to the three main products mentioned above, some extra SO 2 (asymmetric stretching vibration at 1300-1420 cm -1 ) was observed on the 3D diagram corresponding to FeSS.

To go further, the evolution versus temperature of the intensity of the bands associated with CO 2 (2359 cm -1 ), SO 2 (1375 cm -1 ), C-H n (2933 cm -1 ) and H 2 O (1508 cm -1 ) were plotted on Fig. 4(A2) and (B2). Differences in the evolution of these bands between the two samples can clearly be observed. The amount of CO 2 evolved from FeSS is much higher than from SS. The evolution of SO 2 from FeSS is also much better defined and ends around 600 o C on Fig.

4(B2).

It is worth mentioning that, in terms of intensities, CO 2 and SO 2 showed equivalent evolutions with temperature as shown in Fig. 4(B2). This is because the thermal decomposition of ferrous sulfate produces SO 3 , which decomposes to SO 2 and O 2 . 22 The O 2 produced reacts with the carbon support and results in the formation of CO 2 . The iron sites trapped into the carbon can then be exposed, which means that more reactive sites were accessible for the heterogeneous photo-Fenton-like reaction.

Influence of the experimental conditions on the FeSC catalyst performances Effect of the pyrolysis temperature

According to the TGA-FTIR results, the pyrolysis process should finish at temperature higher than 600 o C, hence 600, 700 and 800 o C were chosen as the maximum temperatures for the catalyst preparation. The corresponding catalysts are referred to as FeSC 600 , FeSC 700 and FeSC 800 . As shown in Fig. 5, about 13% of AOII was removed by FeSC in adsorption phase. While in the photo-Fenton-like oxidation process, the discoloration efficiency of AOII increased with increase of pyrolysis temperature. As shown in Fig. 5, the apparent rate constant (k app ) of the AOII degradation was obtained by fitting the results between 30 and 90 min, using a pseudo-first-order kinetic equation by plots of ln (C 0 /C t ) against reaction time, which has been widely applied in the literature for the description of the heterogeneous photo-Fenton oxidation of organic pollutants. 23,24 The k app were calculated to be 6.52 × 10 -2 , 7.46 × 10 -2 and 9.24 × 10 -2 min -1 for FeSC 600 , FeSC 700 and FeSC 800 , respectively. As a result of the increase of mass loss during pyrolysis at higher temperatures, the iron content in the FeSC catalysts increased slightly (12.3 wt%, 12.6 wt% and 12.9 wt% for catalysts pyrolyzed at 600, 700 and 800 o C). Hence there is more iron oxide in catalysts pyrolyzed at higher temperatures, which means there are more active sites for accelerating the decomposition of H 2 O 2 in the presence of visible light. Further experiments were performed on the sample pyrolyzed at 800 o C as the heterogeneous photo-Fenton-like catalyst.

Effect of the catalyst concentration

The effect of FeSC catalyst dosage on the discoloration of AOII was studied and the results are presented in Fig. 6. During the adsorption phase, when the amount of FeSC catalyst increased, more AOII was adsorbed on the catalyst surface. The maximum adsorption was determined to be 13.4 mg g -1 by fitting the experimental isotherm data to the Langmuir isotherm model (R 2 = 0.9967), which is much lower than that of commercial activated carbon (purchased from Tianjin Chemical Reagent Co. Ltd, China, S BET = 605 m 2 g -1 , Q max = 345.0 mg g -1 ). During the oxidation phase, as shown in Table 2, when the catalyst concentration increased from 0.5 g L -1 to 1.5 g L -1 , the initial reaction rates, expressed in milligrams of AOII converted per gram ), the initial reaction rate and pseudofirst-order degradation kinetic constant all decreased significantly. This change is because the increase in catalyst dosage makes the solution very turbid and decreases light penetration into the solution, resulting in a decrease in the amount of •OH produced and a less efficient process. Therefore, subsequent runs were performed using 1.5 g L -1 catalyst.

Influence of initial solution pH

The removal of azo dye, catalyzed by Fe-catalysts, was earlier shown to be a pH-dependent process, 25,26 and so the effect of the initial solution pH was studied. As shown in Fig. 7(A), after 120 min reaction, over 99.9% AOII was transformed at pH 3.0, 99.7% at pH 4.0, and 95.3% at pH 5.0. As the pH of the solution was increased from 3.0 to 4.0 to 5.0 (insert on Fig. 7(A)), the pseudo-firstorder kinetic constant decreased from 11.63 × 10 -2 to 9.24 × 10 -2 and to 5.17 × 10 -2 min -1 , respectively. These results indicate that the higher the initial solution pH, the slower the reaction rate.

The iron leaching from the catalyst was also checked throughout the experiments. It can be seen from Fig. 7(B) that the iron concentration in solution after 90 min reaction was 2.5, 0.6 and 0.2 mg L -1 after reaction at pH 3.0, 4.0 and 5.0, respectively. A similar trend was observed by Ramirez et al. 27 using Fe/active carbon as catalyst. The leaching of Fe at pH 3.0 is not desirable since it is higher than the EU discharge limits (<2 ppm). Considering the high inorganic components in SC support, the concentration of the other mental elements in the filtrate was also analyzed by ICP-OES. For the FeSC system with initial pH = 4.0, little Al, Ca, Mg and Cu were detected in the filtrate; their concentrations were 0.6, 0.5, 0.5 and 0.03 mg L -1 , respectively, and the concentrations of Zn, Cr, Mn, Ni and Pb were below the detection limit of ICP-OES. In order to determine the influence of homogeneous phase reaction, we also tested the photo-Fenton catalytic activity of the filtrate after the reaction and only 24.7% degradation of AOII is observed at the end of the reaction. In conclusion, the optimal initial solution pH was 4.0, since the AOII removal efficiency was as high as at pH 3.0 but iron leaching was lower. It is worth mentioning that the amount of iron lost from the FeSC catalyst, even at pH 3.0 is only 1.3 wt% of the total iron content initially incorporated in the catalyst, significantly lower than in the case of the Fe/active carbon prepared by classical incipient wetness impregnation (c. 2.6 wt%). 27

Comparison of the catalytic activity of FeSC with different catalyst or different operating conditions

Figure 8 shows the removal efficiency of AOII versus time under different conditions. Compared with the discoloration rates of AOII in H 2 O 2 -visible light system (curve A), the SC-H 2 O 2 -visible light system (curve B) and FeSC-H 2 O 2 system (curve C), the FeSC-H 2 O 2 -visible light system (curve E) had the highest discoloration efficiency. The apparent rate constant for the FeSC-H 2 O 2 -visible light system and the FeSC-H 2 O 2 system were calculated to be 9.24 × 10 -2 min -1 and 2.64 × 10 -2 min -1 , respectively. The former was about 3 times higher than the latter. These results clearly indicated the effect of light enhancement. That is because dyes can absorb visible light and become excited under visible irradiation, Equation ( 1), which will lead to electron transfer between the dye molecule and ferric ion on the catalyst and then accelerate the wileyonlinelibrary.com/jctb c 2013 Society of Chemical Industry J Chem Technol Biotechnol (2013) photo-Fenton-like process, Equations ( 2)-( 5). than FeSC. The k app for the Fe 3 O 4 oxide was calculated to be 3.80 × 10 -2 min -1 , which is less than 40% of the k app for the FeSC-H 2 O 2 -visible light system. There are many reasons for the better photocatalytic performance of the FeSC system compared with the Fe 3 O 4 oxide. First, in the adsorption phase, the AOII removal efficiency was 3 and 12% for the Fe 3 O 4 -H 2 O 2 -visible light system and FeSC-H 2 O 2 -visible light system, respectively. This indicated that the porous carbon matrix in the SC can facilitate the adsorption of dyes. 30 Second, the carbon support, as a photosensitizer and a conductive material, can absorb light and produce some photo-induced electrons (e -), transfer them to the iron particles and accelerate the reduction of Fe 3+ to Fe 2+ , 31,32 and thereby promote the photocatalytic process. Furthermore, some inorganic components in the SC support (SiO 2 and Al 2 O 3 ) may also act as co-catalysts in the degradation of AOII during the heterogeneous photo-Fenton-like process. For example, SiO 2 (i) can facilitate the adsorption of H 2 O 2 due to the formation of hydrogen bonds between H 2 O 2 and Si-O-Si, 33 and (ii) increase the activity of •OH for the formation of an acidic microenvironment near the silica phase. Furthermore, the basic sites on Al 2 O 3 can facilitate the adsorption of AOII, activate the H 2 O 2 degradation and accelerate the reduction of Fe 3+ by H 2 O 2 due to the Lewis sites on the alumina. 34 Finally the hydroxyl groups on the surface of SiO 2 or Al 2 O 3 , acting as electron donors for the photo-generated H + , can also be oxidized into hydroxyl radicals (•OH). 35 Therefore, it can be deduced that SC is a favorable photo-Fenton-like catalyst support for the degradation of AOII.

Durability of the FeSC catalyst and the mineralization of AOII

For industrial practical applications, the long-term stability of the catalyst is a key issue. A direct study of the stability of FeSC catalyst in the catalytic degradation of AOII was carried out through successive batch experiments using the same sample recovered by filtration under identical reaction conditions. As shown in Fig. 9, the degradation of AOII decreased by only 7% after the 10th run. These results demonstrated that FeSC is durable and can be reused without loss of catalytic activity.

It is known that complete discoloration of AOII does not mean that the dye is completely oxidized. So, the mineralization of AOII, in terms of TOC removal, was investigated in parallel (the reaction conditions were: FeSC: 1.5 g L -1 ; H 2 O 2 : 15 mmol L -1 ; AOII: 100 ppm; reaction temperature: 30 o C; 150 W short-arc Xe lamp). As shown in Fig. 9, compared with the AOII removal, it can be seen that the mineralization of AOII is slower than the discoloration. This is due to the fact that some partially-oxidized refractory intermediates are generated during AOII degradation. 36 The results shown in wileyonlinelibrary.com/jctb Fig. 9 also indicate that FeSC as a heterogeneous photo-Fentonlike catalyst can lead to rapid discoloration of AOII as well as a remarkable degree of mineralization of the solution.

Application in natural river water

To obtain further insight into the catalytic behavior of FeSC catalyst in a practical application, natural river water was used instead of UP water to prepare the simulated dye water. The river water was collected from Pearl River (Guangzhou), some characteristics of the natural river water are listed in Table S1 (Supplementary Materials). The river water was filtered through a 0.22 μm Millipore membrane filter to remove any suspended particles, and then the AOII sample was added and the pH of the solution was adjusted to 4.0. The result showed that after 30 min adsorption and 90 min oxidation, 10.3 and 87.5% of AOII, respectively, was removed, and TOC removal at the end of reaction was 53%. Compared with the result using UP water, a detrimental effect of river water component was observed. This result may due to the presence of natural radical scavengers in river water such as Cl -, CO 3 2-, HCO 3 -and some organic compounds. 37,38

CONCLUSIONS

A novel heterogeneous photo-Fenton-like catalyst was successfully prepared for the first time, employing SCs as the catalyst support. The results showed that almost 100% AOII discoloration and 67% mineralization could be achieved after 30 min adsorption and 60 min photo-Fenton-like oxidation using a FeSC catalyst prepared by pyrolysis at 800 o C for 2 h (1.5 g L -1 catalyst, initial solution pH 4, 15.0 mmol L -1 H 2 O 2 and under the irradiation of a 150 W short-arc Xe lamp). Compared with a commercial pure Fe 3 O 4 oxide, the FeSC catalyst presented higher catalytic efficiency because of the promoting effect of the SC support components. The FeSC catalyst also exhibited very limited iron leaching and good stability under appropriate reaction conditions.

Introduction

Municipal sewage sludge are produced in huge quantity in conventional wastewater treatment plants. In 2010 for instance, the total production of dry sewage sludge in the United States, the European Union and China were ca. 8.2, 10.0 and 8.0 million tons, respectively. Furthermore, it is predicted that the production of sludge all over the world will continue to rise with an annual increase of about 2 vol.%. Therefore, the disposal of sewage sludge is an issue of serious environmental concern [1,2]. Compared with the traditional methods of sewage sludge disposal, such as ocean dumpling, landfilling and incineration, pyrolysis could be considered as an economically feasible and eco-friendly route [3]. Sludge derived carbons (SC) produced upon pyrolysis of sewage sludge have been used as adsorbents for organic pollutants and heavy metals [4,5]. On the other hand, the application of sewage sludge derived materials as efficient catalysts in the wastewater treatment have been recently reported. For example, sewage sludge based carbons were used in the catalytic wet air oxidation of phenolic compounds [6], in the catalytic ozonation of oxalic acid [7], and as supports for heterogeneous Fenton-like catalysts [8,9].

In the wastewater treatment area, 2-chlorophenol (2-CP), which is widely used in the paper, the pulp, the pesticide and the herbicide industries, is unambiguously identified as a severe http://dx.doi.org/10.1016/j.jhazmat.2014.05.024 0304-3894/© 2014 Elsevier B.V. All rights reserved. environmental pollutant of major concern due to its high toxicity and low biodegradability. The maximum allowable concentration of 2-CP in drinking water was fixed at 10 g L -1 [10]. Among the different techniques applied to date for the treatment of 2-CP, the catalytic wet air oxidation (CWAO) has attracted some interest. Compared to the biological, electrochemical or photocatalytic treatments, CWAO is more efficient at treating high concentrations of organic pollutants [11]. Compared to the incineration and the supercritical water oxidation, CWAO might further be considered as an energy-saving catalytic method [12].

In the past years, a series of noble metal catalysts were used in the CWAO and exhibited highly activities [13,14], but they are quite expensive. Therefore, transition metal oxide based catalysts were used as an alternative in the CWAO process [15][16][17]. Since sewage sludge are rich in iron, it was decided to test them directly in the CWAO of 2-CP. Furthermore, sewage sludge derived carbon supported iron oxide catalysts (named as FeSC) were also prepared, following an optimized method, in order to stabilize the iron. Compared with the costly noble-metal catalysts, the prepared FeSC catalysts are potentially low cost. Since the metal leaching remains the main drawback for an extensive application of transition-metal based catalysts in CWAO processes, the reasons and effects of the iron leaching upon the reaction were investigated in this study. And different attempts to prevent the iron leaching were also tested.

Experimental

Preparation of the catalysts

The dewatered sewage sludge was obtained from the Lie De municipal wastewater treatment plant in Guangzhou, China. The sludge was dried at 105 • C to constant weight, grinded and sieved (100-mesh). Subsequently stored in a desiccator at room temperature.

The sewage sludge-derived char supported iron oxide catalyst (named as FeSC) was prepared following a one-step method combining the carbon synthesis with the Fe loading. At first, 20 g dried sludge were added into 150 mL of a ferrous sulfate solution (0.5 mol L -1 FeSO 4 , pH = 2.5). After the suspension was stirred for 24 h, it was slowly evaporated in a rotary evaporator at 60 • C under reduced pressure, and then dried at 105 • C for 12 h. Subsequently, the received solid was pyrolyzed under flowing nitrogen (40 mL min -1 ) in a horizontal furnace at 800 • C for 2 h (heating rate: 20 • C min -1 ). The pyrolyzed sample was washed several times with deionized water until the conductivity became constant in order to make sure there is no soluble iron salts anymore on the catalyst surface. Then, the sample was dried at 105 • C overnight and grinded to less than 200-mesh size.

In order to reduce the ash content, 5 g of the sludge-derived carbon (SC), formed after pyrolysis at 800 • C for 2 h, was treated in a 80 mL acid solution (25 vol.% HCl + 25 vol.% HF) at 70 • C for 2 h before, washing with distilled water and drying. The sample was named as SC-A.

Characterizations

The ash content of the catalysts was determined using the standard ASTM D2866-94 method. The metal content in the synthesized samples was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, Horiba Jobin-Yvon).

The specific surface area (S BET ) was determined using an Autosorb-iQ-MP gas sorption analyzer (Quantachrome Instruments, USA) via nitrogen adsorption at 77 K.

The pH at the point of zero charge (pH PZC ) of the catalyst was measured according to the method developed by Brunelle [18].

Increasing amounts of solid were added sequentially to a 50 mL 0.0005 M NaCl solution until the pH reached a plateau. The pH at the plateau corresponds to the pH PZC .

The chemical state of iron on the catalysts was followed by Xray diffraction (XRD) using a D/max 2200 vpc Diffratometer (Rigaku Corporation, Japan) with a Cu K␣ radiation at 40 kV and 30 mA.

Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-6330F field emission scanning electron microscope equipped with an energy-dispersive X-ray (EDX) detector (JED-2300).

Catalytic tests

The catalytic wet air oxidation of 2-chlorophenol over the SCbased catalysts was studied in a conventional batch reactor made of Hastelloy C22 (Model 4836, Parr Instrument Inc). The experimental apparatus is described in Fig. S1. In each run, the 300 mL autoclave was loaded with 150 mL of a 2 g L -1 2-CP aqueous solution (ca. 2.3 mmol 2-CP in the reactor) and a defined amount of catalyst. After the reactor was repeatedly outgassed with argon at ambient temperature, the reactor was heated up to the desired temperature (120 • C). When the temperature was stabilized, synthetic air (20 vol.% O 2 + 80 vol.% N 2 ) was introduced into the reactor until the total pressure reached 5 MPa. This point was taken as "zero" time for the reaction. The oxygen partial pressure was ca. 0.9 MPa (ca. 42.8 mmol O 2 in the reactor), corresponding globally to a large excess of oxygen according Eq. ( 1). Noteworthy, the oxygen solubility in water under such conditions was only ca. 0.

2 g O 2 L -1 . C 6 H 5 ClO + 6.5O 2 = 6CO 2 + 2H 2 O + HCl (1)
Furthermore, to avoid any mass transfer limitation in the liquid phase, the stirrer speed was fixed at 1300 rpm [19]. Liquid samples were taken from the reactor at regular time intervals and analyzed after centrifugation.

Analytical methods

The concentrations of 2-chlorophenol and the oxidation intermediates were measured by HPLC using a UV-detector set at 210 and 281 nm (Shimadzu). The HPLC system was equipped with a Kinetex PFP 100A 2.6 m 100 mm × 4.6 mm column (Phenomenex), operated at constant temperature (40 • C). The mobile phase was a mixture of 4 vol.% methanol and 96 vol.% water, which pH was acidified down to 2.0 using a 85 wt.% H 3 PO 4 solution. The flow rate was fixed at 0.6 mL min -1 .

The total organic carbon (TOC) in the liquid samples was determined by using a Shimadzu 5050 TOC analyzer. The pH of the solution was measured with a Radiometer Analytical PHM240 pH meter. The Fe concentration in the liquid samples was further analyzed by ICP-OES to check for lixiviation.

Results and discussion

Characterization of the catalysts

The basic physical and chemical characteristics of the SC and FeSC samples are summarized in Table 1.

The ash content values and the results of the ICP-OES analysis indicated a high content in inorganic components, such as iron, silicon and aluminum. After the acidic washing, the ash fraction was partial removed. The iron content in SC, FeSC and SC-A, determined by ICP-OES, was 4.5, 12.0 and 0.4 wt.%, respectively. SC and FeSC exhibited quite a low specific surface area, while SC-A showed a higher surface as a result of the acidic washing and the efficient ash removal. As indicated in Table 1, the pH PZC value decreased upon incorporation of iron in the SC support. This variation could be attributed to the higher amount of acidic oxygen surface functional groups induced by the presence of iron, which could be beneficial for the CWAO reaction [20].

The iron phase in the FeSC sample could be further characterized using XRD (Fig. 1 Finally, as shown in Fig. 2, the SEM micrographs indicated that the surface of FeSC is relatively rough. In addition, the mean Fe content (for the zone shown in Fig. 2B) measured by EDX was ca. 15.3 wt.%, which is in good agreement with the results obtained by ICP-OES. It could be deduced that the iron particles are quite uniformly distributed on the SC support.

CWAO of 2-chlorophenol over the different catalysts

The wet air oxidation of 2-CP, in absence of any catalyst, was shown to be very slow. Upon reaction at 120 • C under 0.9 MPa oxygen partial pressure, 6% 2-CP conversion was achieved via thermal decomposition at the very beginning of the oxidation reaction (t = 0). After 24 h of reaction, the 2-CP and TOC removal were lower than 15% (Fig. 3), indicating that the 2-CP molecule was thermally stable under the applied reaction conditions.

The 2-CP molecule was observed to adsorb on the SC based catalysts. The initial adsorption, before the introduction of air (t = 0), accounted for ca. 3% after subtraction of the 6% 2-CP conversion due to thermal decomposition. The untreated SC sample appeared to be only moderately active in the CWAO of 2-CP. After 24 h of reaction, the 2-CP conversion was only 50% and TOC abatement reached 39%.

After acidic treatment (HCl + HF), the 2-CP adsorption increased up to 9%. This observation could be simply explained by the higher specific surface area of the SC-A sample compared to the initial SC (33 vs. 19 m 2 g -1 ). On the contrary, the catalytic activity decreased upon removal of the ash fraction. The initial reaction rate decreased from 0.4 to 0.1 mmol 2-CP g catalyst -1 h -1 . The much smaller amount of iron in the treated sample (0.4 vs. 4.5 wt.%) could be responsible for this difference.

After incorporation of iron on the SC support, a substantial increase of the 2-CP conversion was observed. The complete removal of 2-CP could be reached within 5 h of reaction. The initial reaction rate was calculated to be 3.0 mmol 2-CP g catalyst -1 h -1 , which was almost 7 times higher than the rate measured on the untreated SC, while the iron concentration in the catalyst was only tripled. Compared with the classical Ru/ZrO 2 catalyst [21], which is known as a kind of high efficient noble metal catalyst, the FeSC catalyst displayed higher catalytic activity under identical reaction conditions. As shown in Fig. 3B, the TOC removal also remarkably increased upon addition of the FeSC catalyst, reaching 91% at the end of the reaction (24 h). By comparison with the evolution of the 2-CP conversion, it could be deduced that the mineralization of the 2-CP molecule was not complete and occurred at a slower rate. Several partially-oxidized intermediates formed upon 2-CP degradation and appeared to be resistant toward further conversion to carbon dioxide.

Furthermore, as the decomposition of 2-CP occurred, the pH of the reaction mixture decreased from an initial value of 5.5 to very acidic values (Fig. 3C). This increase of the acidity of the reaction mixture could be attributed to (i) the production of HCl as a result of the initial dechlorination of the 2-CP molecule; and (ii) the formation of some small chain organic acids such as maleic acid, succinic acid, formic acid and oxalic acid, as detected by HPLC (Fig. S2).

Heterogeneous vs. homogeneous reaction

Considering the very low pH achieved at the end of the reaction, the leaching of metal (Fe, Ca, Al, etc.) was suspected. Therefore, the concentration of iron, as the most notably catalytically active metals [1], in the liquid phase at the end of the reaction (24 h) was systematically measured. As shown in Table 2, 27 mg L -1 of iron was detected in the reaction mixture after reaction over the FeSC system, which corresponded to ca. 7 wt.% of the total amount of iron initially incorporated in the FeSC catalyst. Significant amounts of metallic ions were also detected in the liquid phase upon reaction over different catalysts based on transition metals such as Cu, Zn, Mn, Ni [22][23][24]. For example, Posada et al. [24] have studied the catalytic wet air oxidation of substituted phenols using 1.5 g L -1 of a 4 wt.% Cu/CeO 2 catalyst. 54.5 ppm Cu were detected at the end of reaction, corresponding to ca. 91 wt.% of the total amount of copper in the catalyst. Therefore, leaching is a major issue when using transition metal oxide based catalysts under acidic conditions.

To determine the contribution of the homogeneous phase reaction, 0.5 g FeSC catalyst was first introduced into the reaction mixture for 10 min, after that the hot reaction mixture was filtrated in order to remove any catalyst particle. From Fig. 4, it is observed that the conversion of 2-CP continues, even the absence of any solid catalyst, indicating that part of the active phase which has been leached into the reaction mixture is active in the homogeneous phase reaction.

To further check the contribution of the homogeneous phase reaction, the evolution of the 2-CP conversion as a function of time was studied at different catalyst loading (Fig. 5A). The iron concentration in the filtrate was systematically measured after 10, 30 and 60 min by ICP-OES. As shown in Fig. 5B, The linear variation

Table 2

Iron leaching upon catalytic wet air oxidation (CWAO) of 2-chlorophenol over different catalysts (120 • C, PO 2 : 0.9 MPa, 1300 rpm, 150 mL [2-CP]0 =2 g L -1 , 0.5 g catalyst).

Sample

Iron concentration in the reaction mixture after 24 h (mg L -1 )

Percentage of iron leached in the solution after 24 h (wt.%) between the instantaneous reaction rate and the amount of iron detected in solution at the same time clearly indicated that (i) the reaction definitively occurred in the homogeneous phase since the initial reaction rate did not correlate with the amount of catalyst but with the volume of the reaction mixture and (ii) the reaction was strongly dependent on the iron concentration in the liquid phase.

To explain further the irreversible leaching of iron in the solution, the potential taken by the reaction mixture was systematically measured as a function of time and recalculated with respect to the standard hydrogen electrode (SHE). According to the pH and the potential of each liquid sample shown in the Pourbaix diagram (Fig. 5C) [25], we could find that the most stable form of iron in the aqueous phase all along the reaction would be Fe(II). This result could be explained by the low solubility of oxygen in water under the applied reaction conditions (ca. 0.2 g L -1 ) and the rapid consumption of oxygen, resulting in anoxic conditions in the liquid phase. Noteworthy, the potential of the solution increased slightly as the reaction proceeded, indicating the Fe(II) could be progressively converted to Fe(III). Considering the excellent redox property of Fe(II), the formation of free radicals, such as alkyl peroxy radicals, oxygen radicals and hydroxyl radicals would be promoted [23,26,27] and the CWAO reaction could be initiated.

Since the leached iron existed as Fe 2+ in the liquid phase, according to Eq. ( 2),

Fe 2+ + 2OH -↔ Fe(OH) 2 (2)
The ion product data (IP) for Fe 2+ and OH -was calculated as follows:

IP = [Fe 2+ ] [OH -] 2 (3)
The IP value got by curvilinear regression analysis was 1.39 × 10 -26 , and good correlation coefficient (R 2 = 0.9900) was gotten. It should be noted that the solubility product constant (K sp ) for Fe(OH) 2 was 3.16 × 10 -14 [28]. Therefore, when under the same pH condition, the dissolved ferrous iron concentration in the FeSC system is much lower than the value got from theoretic solubility calculation, indicating the using of SC support could stabilize the iron and reduce the leaching problem. Furthermore, from the result of curve fitting, in order to meet the EU discharge standards for Fe (<2 ppm) [29], the solution pH need to be kept higher than 3.29. Conclusively, the 2-CP conversion in the homogeneous phase, the pH of the solution and the amount of iron leached in the reaction mixture were shown to be interconnected.

Minimization of the iron leaching

Based on the first results presented above, the minimization of the iron leaching from the FeSC catalysts upon CWAO of 2chlorophenol clearly appeared as a key point. Since the iron leaching was shown to be mainly driven by the acidic conditions generated during the initial 2-CP degradation process, the pH of the reaction mixture was tentatively controlled following different approaches; i.e. adjusting the initial pH of the reaction mixture or using a buffer solution to keep the pH constant all along the reaction.

Alkaline agents, such as NaOH, are often used to neutralize the acids produced upon treatment of chlorinated organics via supercritical water oxidation [30] or wet air oxidation [31]. In our study, NaOH was used to adjust the initial pH value of the reaction mixture from 5.6 to 10.0. As shown in Fig. 6A, when NaOH was initially added into the solution but without using any catalyst, the 2-CP conversion and TOC abatement after 24 h of reaction increased from ca. 14 to 68% and from 7 to 40%, respectively. These results are similar to what was reported by Pintar et al. [32] in the WAO of substituted phenols (p-chlorophenol, p-nitrophenol) in the absence of any catalyst. When the pH value is higher than the pKa values of the substituted phenol, the phenolate anions (ArO -) will yield phenoxy radicals (ArO • ) which are considered to be much more reactive than the protonated form (ArOH) [31]. In our case, 2-CP would also yield phenoxy radicals since the pKa of the 2-CP molecule is 8.6. However, almost no additional catalytic activity was observed under such reaction conditions in the presence of the FeSC catalyst (64% 2-CP conversion and 42% TOC removal after 24 h). While, the iron concentration in the liquid at the end of the reaction was only 0.7 mg L -1 , indicating that the contribution from the homogeneous reaction could be somehow neglected. This could be the main explanation for the much lower performances of the FeSC catalyst under alkaline conditions.

The addition of a buffer solution to the reaction mixture was tested as a second attempt to avoid the iron leaching. Firstly, the impact of a phosphate buffer (pH = 6.8) was investigated. The amount of iron leached after 24 h reaction markedly decreased down to 0.4 ppm. However, compared to the experiment performed in the absence of any catalyst, no extra 2-CP conversion or TOC removal could be observed over the FeSC catalyst (Fig. 6B). This was tentatively related to the poisonous effect of phosphates on the iron oxide catalyst with the formation of iron phosphate chelates [33]. Conclusively, the phosphate buffer was not appropriate in this specific case.

Alternatively, an acetate buffer (pH = 4.5) could be reliably used to control the pH since acetic acid was shown to be refractory toward wet air oxidation up to 200 • C [34]. As presented in Fig. 6C, in the absence of any catalyst, the 2-CP conversion and the TOC removal after 24 h reaction did not change too much by the using of acetate buffer, ca. 17% 2-CP conversion and 8% TOC removal (the effective TOC removal was calculated after subtracting the TOC content due to the acetate buffer). When the FeSC catalyst was introduced, the 2-CP conversion was 42% and the TOC abatement reached 33% at the end of the reaction. In comparison with the results obtained when the same amount of FeSC catalyst was used but in the absence of any buffer, a detrimental effect of the acetate buffer was observed. Nevertheless, the amount of iron leached in the filtrate at the end of reaction decreased from 30 to only 0.9 ppm in the presence of the acetate buffer. Furthermore, the contribution of homogeneous phase containing 0.9 ppm iron in the solution was also checked, and no further CWAO catalytic activity was observed.

In conclusion, the use of the acetate buffer (pH = 4.5) could effectively help to prevent the iron leaching while preserving part of the catalytic activity of the catalyst. From a practical point of view, although the TOC content of the acetate buffer is high (ca. 4.5 g L -1 ) and the acetic acid is refractory toward further oxidation the implementation of such a buffer for the treatment of 2-CP via CWAO might still be viable since acetic acid is non toxic and would even be easily biodegraded in a conventional wastewater treatment plant.

Stability tests

It is known that dissolved iron in the effluent could generate an unwanted secondary pollution. To prevent this, the iron leached upon CWAO was tentatively recovered at the end of the reaction via pH adjustment (precipitation) by using NaOH. Bernata et al. [35] revealed that the iron recovery efficiency strongly depends on the pH since it strongly impacts on the iron (III) retention. In our case, the pH of the liquid was adjusted to 6.0, then the solid catalyst was recovered by filtration, washed with distilled water and dried at 105 • C before any subsequent re-utilizations.

Four successive runs were carried out using the same catalyst sample. As shown in Fig. 7, an initial induction period appeared from the second run and in subsequent runs the pH decrease was not as sharp as in the first run. This observation is consistent with the differences observed in the amount of the iron which is leached after 1 or 24 h of reaction (Table S1). However, even after 4 runs, the 2-CP conversion and the TOC removal were still very high. Only a small decrease in activity was observed between the first run and the subsequent ones. molecular weight acids and the aromatic compounds which were only detected in trace amounts.

Reaction intermediates

As reported earlier, the dechlorination reaction also appeared in the present study as the first and essential step in the 2-CP decomposition route [36]. As shown in Fig. 8, 93% of the chlorine atoms in the 2-CP molecule are released in the reaction mixture as HCl by the end of the reaction. At the beginning 2 h, the accumulation of HCl in the reaction mixture was very rapid and the pH dropped drastically, then the formation of HCl markedly slowed down. Indicating that in the first step of the reaction the chlorine atom on the aromatic ring was removed and released in the solution as HCl. In this later period, HCl probably originated from partially oxidized chlorinated intermediates.

The concentrations of the different aromatic by-products reached a maximum at different reaction times before being totally oxidized by the end of the reaction (Table 3). The selectivity at the maximum was calculated based on the number of carbon atoms.

The selectivity of all aromatic intermediates was very low, indicating that (i) under the applied reaction conditions, most of the 2-CP molecules are directly mineralized and (ii) the oxidation of these aromatic intermediates took place very rapidly.

As shown in Fig. 8, some other short chain acids were also detected, The time at the maximum concentration for these acids appeared to be delayed compared to the aromatic intermediates, offering an extra indication that they are produced in a second step only from the aromatic ring opening.

From the difference between the measured and the calculated TOC values, we obtained a maximum discrepancy in the carbon balance of 26% (Fig. S3). This discrepancy only appeared at the end of the reaction, indicating that some short chain carboxylic acids remained unidentified or that they were inappropriately quantified.

Finally, from the identified reaction products, a simplified reaction pathway for the degradation of 2-CP upon CWAO over FeSC was proposed (Fig. 9). In the first step, the 2-CP molecule was predominantly dechlorinated and both dechlorinated aromatic intermediates and HCl were produced. The 2-CP molecule might also be partly oxidized before dechlorination occurs and partly-oxidized chlorinated aromatic intermediates were generated. Further oxidation of the aromatic intermediates led to ring opening and short chain carboxylic acids were formed. At last, all oxidized intermediates gradually converted into CO 2 and H 2 O.

Conclusions

While the catalytic wet air oxidation of phenol using heterogeneous transition metal catalysts has been extensively studied in the literature, little attention has been paid to the use of this kind of catalysts for the treatment of 2-chlorophenol. In this work a sewage sludge derived carbon supported iron oxide catalyst (FeSC) was prepared and used in catalytic wet air oxidation of 2-chlorophenol. After 24 h reaction, high 2-CP conversion and TOC removal could be reached even at moderate reaction temperature (120 • C) and low oxygen partial pressure (0.9 MPa). However, iron leaching rapidly occurred upon decrease of the pH, promoting the activity in the homogeneous phase. The pH evolution essentially resulted from the release of HCl after dechlorination of 2-CP and the formation of small chain carboxylic acids at the end of the reaction. Much attention was paid to the iron leaching and different attempts to prevent the iron leaching were tested, such as the use of alkaline reagents or buffer solutions. Among the different approaches, the use of the acetate buffer was successful in minimizing the iron leaching while keeping some catalytic activity.

Therefore, the use of this type of catalyst in the CWAO of 2-CP has to be carefully considered. The iron leaching would indeed require an additional treatment before discharge. In our study, the iron could be recovered by adjusting the pH to neutral pH at the end of the reaction. Only a very slight catalyst deactivation was observed upon successive batch experiments.

Fig. 2

 2 Fig. 2-1.

Fig. 2 - 6 Fig. 2 - 7 . 2 . 3 . 3 . 4 .Fig. 2 - 9 2. 3 . 5 .

 262723342935 Fig.2-6

Fig. 2

 2 Fig. 2-20.

Fig. 3

 3 Fig. 3-3.

Fig. 3

 3 Fig. 3-7.

Fig. 3

 3 Fig. 3-12.

Fig. 4

 4 Fig. 4-2

  Fig. 4-9.

6 .

 6 Fig. 4-20.

Fig. 1 .

 1 Fig. 1. Schematic diagram of the submerged MFR reactor.

Fig. 2 .

 2 Fig. 2. XRD patterns of FeWC, FeSC and their corresponding supports.

Fig. 3 .

 3 Fig. 3. SEM images of FeSC (A) and FeWC (B).

Fig. 4 .

 4 Fig. 4. Degradation of AOII through adsorption and oxidation by hydrogen peroxide and different catalysts: (A) H2O2; (B) SC + H2O2, (C) FeWC + H2O2, (D) FeSC + H2O2 (catalysts: 2.0 g/L; H2O2: 15 mM; AOII: 100 mg/L; reaction pH: 4.0 ± 0.1; and reaction temperature: 30 • C).

Fig. 5 .

 5 Fig. 5. Effect of SiO2 content in FeSC on AOII removal (A) and H2O2 consumption (B) (the iron concentrations are all 0.2588 g/L: (A) SiSC-A: 1.6085 g/L, (B) SC-A: 1.2882 g/L, (C) FeSC-A: 1.3306 g/L, (D) FeSiSC-A1: 1.2699 g/L, (E) FeSiSC-A2: 1.3251 g/L, (F) FeSiSC-A3: 1.4042 g/L; H2O2: 15 mM; AOII: 100 mg/L; reaction pH: 4.0 ± 0.1; and reaction temperature: 30 • C).

Fig. 7 .

 7 Fig. 7. Degradation of AOII through adsorption and oxidation in the presence of Al2O3-containting catalysts (the iron concentrations are all 0.2588 g/L: (A) AlSC-A: 1.9540 g/L, (B) FeAlSi: 0.8682 g/L; (C) FeAlSC-A1: 1.5012 g/L, (D) FeAlSC-A2: 1.5377 g/L, (E) FeAlSiSC-A: 1.6256 g/L; H2O2: 15 mM; AOII: 100 mg/L; reaction pH: 4.0 ± 0.1; and reaction temperature: 30 • C).

Fig. 6 .

 6 Fig. 6. H2O2 adsorb over siloxane by hydrogen bonds (A) and acidic microenvironment form near the surface of SiO2 particle (B).

Fig. 8 .

 8 Fig. 8. Effect of consecutive experiments with the FeWC (A), FeAC (B) and FeSC (C) catalysts on the degradation of AOII solution (catalysts: 2.0 g/L; H2O2: 15 mM; AOII: 100 mg/L; reaction pH: 4.0 ± 0.1; reaction temperature: 23 • C; total volume of reaction solution: 2.4 L and residence time: 120 min).

  2 and Al 2 O 3 resulted in substantial promoting effects on discoloration of AOII. The addition of SiO 2 will facilitate the adsorption of H 2 O 2 due to the formation of hydrogen bonds between H 2 O 2 and Si O Si, and increase the activity of • OH for the formation of acidic microenvironment near the silica phase. The insertion of Al 2 O 3 as basic sites can facilitate the adsorption of AOII, activate H 2 O 2 degradation and accelerate the reduction of Fe 3+ to Fe 2+ by H 2 O 2 due to the Lewis property of alumina.

  -16 Recently, www.soci.org Y Tu et al.

Figure 1 .

 1 Figure 1. XRD patterns of the SC and the FeSC sample pyrolyzed at 800 • C.

  (A1) and (B1)), more visible peaks appeared in the 3D diagram of FeSS compared with SS, especially in the temperature range from 470 o C to 600 o C.

Figure 2 .

 2 Figure 2. SEM images of the SC (A) and FeSC (B) samples pyrolyzed at 800 • C for 2 h.

Figure 3 .

 3 Figure 3. TG curves of SS and FeSS (heating rate: 20 o C min -1 , nitrogen flow rate: 40 mL min -1 ).

Figure 4 .

 4 Figure 4. 3D diagrams corresponding to the gases evolved from SS (A1) and FeSS (B1). Evolution versus temperature of the bands characteristic of CO 2 (2359 cm -1 ), SO 2 (1375 cm -1 ), C-H (2933 cm -1 ) and H 2 O (1508 cm -1 ) for SS (A2) and FeSS (B2).

Figure 5 .

 5 Figure 5. Effect of pyrolysis temperature on the adsorption and the oxidation of AOII during the photo-Fenton-like process (catalyst: 1.5 g L -1 ; H 2 O 2 : 15 mmol L -1 ; AOII: 100 ppm; initial solution pH: 4.0; reaction temperature: 30 o C; 150 W short-arc Xe lamp).

Figure 6 .Figure 7 .

 67 Figure 6. Effect of the catalyst concentration on the adsorption and the oxidation of AOII during photo-Fenton-like process (H 2 O 2 : 15 mmol L -1 ; AOII: 100 ppm; initial solution pH: 4.0; reaction temperature: 30 o C; 150-W short-arc Xe lamp). HO • +AOII ads or AOII +• ads → degraded products (5)To evaluate the role of the SC support in the heterogeneous photo-Fenton-like reaction, a commercial pure Fe 3 O 4 oxide (S BET = 7.0 m 2 g -1 ) was used as a catalyst, using the same amount of iron as in the FeSC catalyst (116.1 mg Fe). As shown in Fig.8curves D and E, the Fe 3 O 4 oxide showed less catalytic activity

Figure 8 .

 8 Figure 8. Removal of AOII (initial solution pH 4.0, 30 o C). (A) 15 mmol L -1 H 2 O 2 + visible light; (B) 1.5 g L -1 SC + 15 mmol L -1 H 2 O 2 + visible light; (C) 1.5 g L -1 FeSC + 15 mmol L -1 H 2 O 2 ; (D) 0.2682 g L -1 Fe 3 O 4 + 15 mmol L -1 H 2 O 2 + visible light; (E) 1.5 g L -1 FeSC + 15 mmol L -1 H 2 O 2 + visible light.

Figure 9 .

 9 Figure 9. Discoloration and mineralization of AOII after 90 min reaction upon reuse of FeSC as Fenton-like catalyst (FeSC: 1.5 g L -1 ; H 2 O 2 : 15 mmol L -1 ; AOII: 100 ppm; reaction temperature: 30 o C; 150 W short-arc Xe lamp).

Fig. 1 .

 1 Fig. 1. XRD patterns of FeSC, SC and SC-A.

  ). Compared with the diffractogram of the pure SC and SC-A sample, the XRD pattern for FeSC exhibited several extra diffraction lines at 2Â equal 35.5 • , 62.6 • and 30.1 • . These lines could respectively be assigned to the (3 1 1), (4 4 0) and (2 2 0) planes of Fe 3 O 4 (magnetite, JCPDS n • 88-0866).

Fig. 2 .

 2 Fig. 2. SEM images of the SC (A) and FeSC (B) samples pyrolyzed at 800 • C for 2 h, and the result of EDX analysis for FeSC (C).

Fig. 3 .

 3 Fig. 3. Evolution of the 2-chlorophenol conversion (A), the TOC removal (B) and the pH (C) upon wet air oxidation (WAO) of 2-CP over FeSC ( ), SC ( ), SC-A ( ) and Ru/ZrO2 ( ) or in the absence of catalyst ( ) (0.5 g catalyst, 120 • C, PO 2 : 0.9 MPa, 1300 rpm, 150 mL [2-CP]0 =2 g L -1 ).

Fig. 4 .

 4 Fig. 4. 2-CP conversion attained by heterogeneous: 0.5 g FeSC ( ) and homogeneous reaction: hot filtrate from 0.5 g FeSC system after 10 min reaction ( ) (120 • C, PO 2 : 0.9 MPa, 1300 rpm, 150 mL [2-CP]0 = 2 g L -1).

FeSCFig. 5 .

 5 Fig. 5. Contribution of the homogeneous phase reaction. (A) 2-CP conversion during the first 2 h of reaction in the presence of different concentration in FeSC; (B) Correlation between the 2-CP conversion rate and the amount of iron leached in the reaction mixture; (C) Pourbaix diagram for iron and evolution of the potential and pH of the reaction mixture as a function of time; (D) Correlation between the solution pH and the amount of iron leached in the reaction mixture [120 • C, PO 2 : 0.9 MPa, 1300 rpm, 150 mL [2-CP]0 = 2 g L -1 , catalyst dosage: 1.0 g L -1 ( ), 1.5 g L -1 ( ) and 2.0 g L -1 ( )].

Fig. 6 .

 6 Fig. 6. Evolution of the 2-CP conversion upon wet air oxidation (WAO) in the absence of any catalyst ( ) or in the presence of the FeSC catalyst ( ) and with the initial addition of NaOH to pH = 10.0 (A) or in the presence of a phosphate buffer at pH = 6.8 (B) or in the presence of an acetate buffer at pH = 4.5 (C) (120 • C, PO 2 : 0.9 MPa, 1300 rmp, 150 mL [2-CP]0 = 2 g L -1 , catalyst dosage: 2.0 g L -1 ).

Fig. 7 .

 7 Fig. 7. Evolution of the 2-CP conversion (A), the TOC removal (B) and the pH (C) upon wet air oxidation (WAO) of 2-CP while recycling a 0.3 g sample of the FeSC catalyst (120 • C, PO 2 : 0.9 MPa, 1300 rpm, 150 mL [2-CP]0 = 2 g L -1 , 1st run ( ), 2nd run ( ), 3rd run ( ), 4th run ( )].

Fig. 8

 8 Fig. 8 shows the evolutions of the concentrations of 2chlorophenol and the different reaction intermediates we could identify upon CWAO of 2-CP over the FeSC catalyst. These intermediates have been classified in two different groups: the low

Fig. 8 .

 8 Fig. 8. Concentration profiles of 2-CP and the identified intermediates formed during the catalytic wet oxidation of 2-CP over FeSC (2 g L -1 FeSC, 120 • C, PO 2 : 0.9 MPa, 1300 rpm, 150 mL [2-CP]0 = 2 g L -1 ).

Fig. 9 .

 9 Fig. 9. Simplified reaction pathway upon catalytic wet air oxidation of 2-chlorophenol over FeSC catalysts.

Table 2 -2 Proximate analysis Elemental analysis Ash analysis (wt.%) Ash analysis (mg/kg) 2.2.2. Preparation of the catalystsTable 2 -3. 2.2.3. Characterization of catalysts 2.2.4. Catalytic activity 2.2.5. Analytical methods

 22

Table 2 -4.Table 2 -5.

 22

	0	10 20 30 40 50 60 70

Table 2 -7.

 2

Table 3 -

 3

1.

Table 3 -

 3

		Ash content	Fe	Si	Al	Ca	S BET
	Sample	(wt.%)	(wt.%)	(wt.%)	(wt.%)	(wt.%)	(m 2 g -1 )	pH PZC
	SC	74.6	4.5	23.4	10.6	1.7	19	7.5
	FeSC	93.0	12.0	19.9	9.1	1.8	14	4.2
	SC-A	56.1	0.4	28.6	1.8	<0.1	33	3.7

1. Physicochemical properties of the SC, FeSC and SC-A samples.

Table 3 -3.

 3

Table 3 -4. 3.3.7. Reaction intermediates Fig. 3-11.

 3

Table 3 -5.

 3

Table 4 -1.

 4

Table 4 -2.Table 4 -3. 4.2.3. Characterization of the catalysts 4.2.4. Catalytic ozonation experiments 4.2.5. Analytical methods

 44

Table 4 -4.

 4

	Intensity (a.U.)					% 10 % 0 %
						%
						%
	10	20	30	40	50	60
			2 (deg)
	Fig. 4-6.

d

Table 4 -5.

 4

	4000 3500 3000 2500 2000 1500 1000	500

Table 1

 1 Labels of prepared SC-A based catalysts and specification of their preparation conditions.

	Sample	SS-A (g)	TEOS (mmol)	Ethanol (mmol)	Fe (mmol)	Al (mmol)	Fe/Si/Al
	FeSC-A	2.5	0	0	3.60	0	1/0/0
	SiSC-A	2.5	3.60	3.60	0	0	0/1/0
	AlSC-A	2.5	0	0	0	3.60	0/0/1
	FeSiSC-A1	2.5	1.80	1.80	3.60	0	2/1/0
	FeSiSC-A2	2.5	3.60	3.60	3.60	0	1/1/0
	FeSiSC-A3	2.5	7.20	7.20	3.60	0	1/2/0
	FeAlSC-A1	2.5	0	0	3.60	1.80	2/0/1
	FeAlSC-A2	2.5	0	0	3.60	3.60	1/0/1
	FeAlSiSC-A	2.5	3.60	3.60	3.60	3.60	1/1/1
	FeAlSi	0	3.60	3.60	3.60	3.60	1/1/1
	The reaction tank was made of polymethylmethacrylate (PMMA).
	An Al 2 O 3 ceramic membrane with the bore diameter of 3 m was
	used in the submerged membrane module. Before reaction, 2.4 L
	AOII solution (100 mg/L), 4.8 g catalyst and 36 mmol H 2 O 2 (30%,
	w/v) were added into the reaction tank. At the same time, two
	pumps start to supply AOII (100 mg/L) and H 2 O 2 (1.02%, w/v) to
	the tank continuously. Their flow rate was control to be 20 mL/min
	and 1 mL/min respectively. And the third pump was used to con-
	trol the flux speed of the effluent from the reactor. The hydraulic
	residence time was controlled to be 120 min. The air flow rate was
	kept at 5 L/min to hydraulically suspend the catalyst particles and
	to clean the membrane surface by the shear force of air bubble. All
	experiments were conducted at ambient temperature (23 • C), and
	the pH was kept at 4.0 ± 0.1 during the reaction.

Table 2

 2 Physicochemical properties of FeSC, FeWC and the corresponding supports.

	Sample	Fe (wt.%)	Ash (wt.%)	dV	a (m)	SBET (m 2 /g)	pHIEP
	SC	3.91	74.63	23.08	18.9	3.57
	FeSC	12.94	93.01	27.14	14.3	3.39
	WC	0.52	4.59	29.58	10.4	3.09
	FeWC	16.24	26.31	26.66	12.1	3.13
	a dV , volume average particle diameter.

Table 3

 3 Discoloration of AOII with H2O2 in the presence of different catalysts.

	Sample	Discoloration efficiency of AOII (%)		Pseudo first-order model parameters
		By adsorption, t = 60 min	By oxidation, t = 180 min	kap (×10 -3 min -1 )	R 2
	H2O2	0	5.1	0.44	0.9850
	SC	26.9	48.8	2.98	0.9973
	FeSC	22.8	94.6	22.45	0.9972
	FeWC	17.8	66.2	7.31	0.9987
	SC-A	5.0	11.2	0.57	0.9975
	FeSC-A	19.8	51.3	4.21	0.9875
	SiSC-A	2.8	8.6	0.63	0.9964
	AlSC-A	6.6	14.8	0.81	0.9904
	FeSiSC-A1	14.2	59.8	6.00	0.9804
	FeSiSC-A2	13.8	68.8	8.44	0.9885
	FeSiSC-A3	13.4	80.5	12.41	0.9965
	FeAlSC-A1	22.1	65.7	6.71	0.9988
	FeAlSC-A2	39.7	77.7	8.17	0.9870
	FeAlSiSC-A	34.8	89.9	15.11	0.9917
	FeAlSi	8.5	39.6	3.39	0.9977

Table 4

 4 Physicochemical properties of different catalysts.

	Sample	Fe (wt.%)	Al (wt.%)	Si (wt.%)	Ash (wt.%)	SBET (m 2 /g)	Zeta potential at pH 4.0 (mV)	pHIEP
	SC-A	1.50	0.13	2.50	10.02	13.7	-17.16	3.13
	FeSC-A	19.45	0.06	1.21	39.97	21.1	-16.84	3.21
	SiSC-A	0.93	0.08	13.68	39.32	12.2	-26.44	2.64
	AlSC-A	0.82	9.66	1.29	28.65	18.3	-11.81	3.51
	FeSiSC-A1	20.38	0.07	6.31	48.89	19.2	-20.15	3.06
	FeSiSC-A2	19.53	0.06	11.09	49.92	16.5	-21.46	2.94
	FeSiSC-A3	18.43	0.06	16.28	55.42	13.1	-24.53	2.37
	FeAlSC-A1	17.24	4.04	1.07	44.92	15.8	-8.16	3.56
	FeAlSC-A2	16.83	7.88	1.04	45.99	21.9	-0.85	3.82
	FeAlSiSC-A	15.92	7.45	8.70	55.03	17.3	-10.74	2.98
	FeAlSi	29.81	14.40	15.01	90.35	6.1	4.12	5.24

  ) C 16 H 11 N 2 NaO 4 S + 42 H 2 O 2 → 16 CO 2 + 46 H 2 O

	+ 2 HNO 3 + NaHSO 4	(1)
	12 mmol L -1 of H 2 O 2 are theoretically required to completely
	degrade 100 mg/L AOII solution. In our case, 15 mmol L -1 H 2 O 2
	was used to reach the maximum degradation of AOII. Samples were
	taken at predetermined time intervals and analyzed immediately
	after filtration through a 0.22 μm Millipore membrane filter to
	remove any suspended particles. All experiments were repeated
	twice and the results given are the average values with a relative
	standard deviation of less than 5%.

Table 1 .

 1 Physicochemical properties of SC and FeSC pyrolyzed at 800

	SC	3.9	74.6	23	19	3.6
	FeSC	12.9	93.0	27	14	3.4

• C for 2 h Sample Fe (wt%) Ash (wt%) d V a (μm) S BET (m 2 g -1 ) pH IEP a d V , average particle diameter in volume.

Table 2 .

 2 Discoloration

		Initial		Pseudo-first-order
		reaction rate	model parameters
	FeSC (g/L)	(mg AOII • g FeSC	-1 h -1 )	k app (×10 -2 min -1 )	R 2
	0.5	179.1		2.8	0.9720
	1.0	170.8		6.7	0.9859
	1.5	152.4		9.2	0.9922
	2.0	99.7		7.9	0.9874
	of FeSC catalyst per hour, remained almost constant, which means
	the reaction was operated under chemical control. At higher
	catalyst dosage (c. 2 g L -1

of AOII in photo-Fenton-like process with different dosage of FeSC catalyst. (H 2 O 2 : 15 mmol L -1 ; AOII: 100 ppm; initial solution pH: 4.0; reaction temperature: 30 o C; 150 W short-arc Xe lamp)

Table 1

 1 Physicochemical properties of the SC, FeSC and SC-A samples.

	Sample	Ash content (wt.%)	Fe (wt.%)	Si (wt.%)	Al (wt.%)	Ca (wt.%)	SBET (m 2 g -1 )	p H PZC
	SC	74.6	4.5	23.4	10.6	1.7	19	7.5
	FeSC	93.0	12.0	19.9	9.1	1.8	14	4.2
	SC-A	56.1	0.4	28.6	1.8	<0.1	33	3.7

Table 3

 3 The maximum selectivities in the different reaction intermediates calculated on a carbon basis.

	Intermediate	Formula	Maximum time	Max selectivity
			(h)	(%)
	Chlorohydroquinone	C6H3(OH)2Cl	1.70	5.4
	4-Chloroesoreinol	C6H5O2Cl	1.77	3.7
	Catechol	C6H6O2	2.12	2.8
	Hydroquinone	C6H6O6	2.33	5.6
	Maleic acid	C4H4O4	2.37	0.6
	Formic acid	CH2O2	2.82	35.3
	Oxalic acid	C2H2O4	3.60	10.2
	Succinic acid	C4H6O4	3.63	6.3

c 2013 Society of Chemical Industry J Chem Technol Biotechnol (2013)

Acknowledgments

This research was supported by the Nature Science Foundations of China (20977117), Science and Technology Research Programs of Guangdong Province (0711220600311), Nature Science Key Foundations of Guangdong Province (92510027501000005), Foundation of Industry-Education-Academy Cooperation from Gongdong province (2010B090486, 2009A090100047), and Education Department of Chinese Government (2009A090100047, 2010B090486), New Process and Technology Project of Guangzhou EPA (2009-03) and Project of Education Bureau of Guangdong Province (2010-275). Research Fund Program of Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (2011K0008) and the National Natural Science Foundation of China (21107146). ACKNOWLEDGEMENTS This research was supported by Nature Science Foundations of China (20977117), Science, Nature Foundations of Guangdong Province (92510027501000005), Foundation of Industry-Education-Academy Cooperation from Gongdong Province and Education Department of Chinese government (2009A090100047, 2010B090486), Project of Education Bureau of Guangdong Province (cgzhzd1001), New Process and Technology Project of Guangzhou EPA (2009-03), Oversea study program of Guangzhou Elites scholarship, Technology Research Programs of Dongguan City (0711220600311), the Open Foundation of the Labs in Sun Yat-sen University (KF200924) and the Research Fund Program of Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (2011 K0008). Acknowledgements This research was supported by the Nature Science Foundations of China (21107146), the Nature Foundations of Guangdong Province (92510027501000005), the Science and Technology Research Programs of Guangzhou City (2012J4300118), and the Innovative Talents Training Funding of Doctoral Students of Sun Yat-sen University. Ms. Yuting Tu gratefully acknowledges the Oversea Study Program of Guangzhou Elites Scholarship for supporting her 18 month research training in France. The authors also thank G. Aubert for his precious technical help.

SMZ removal efficiency (%)

Run times