Catalytic Wet Peroxide Oxidation Technique for the Removal of

Dec 16, 2008 - Department of Chemistry, Anna University, Chennai 600 025, India, and Centralized Waste Management Facility, BARC Facilities, Kalpakkam...
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Ind. Eng. Chem. Res. 2009, 48, 1556–1561

Catalytic Wet Peroxide Oxidation Technique for the Removal of Decontaminating Agents Ethylenediaminetetraacetic Acid and Oxalic Acid from Aqueous Solution Using Efficient Fenton Type Fe-MCM-41 Mesoporous Materials Narasimhan Gokulakrishnan,† Arumugam Pandurangan,*,† and Pradeep Kumar Sinha‡ Department of Chemistry, Anna UniVersity, Chennai 600 025, India, and Centralized Waste Management Facility, BARC Facilities, Kalpakkam 603 102, India

Catalytic wet peroxide oxidation (CWPO) of organic contaminants may have a possible path for the treatment of decontamination waste containing chelating agents that cause a complex with radioactive cations leading to interferences in their removal by various techniques and may pose a hazard to the environment. The aim of the present work was to synthesize and characterize the mesoporous Fe-MCM-41 with various Si/Fe ratios for evaluating the removal of organic contaminants such as ethylenediaminetetraacetic acid (EDTA) and oxalic acid in aqueous solution. The influence of reaction parameters such as time, pH, and temperature was studied to achieve efficient removal of organic contaminant. Among the catalysts studied, Fe-MCM-41 (Si/ Fe ) 25) provides high-percentage removal of total organic carbon in aqueous solution. The mineralization of organic contaminants was found to be higher in heterogeneous catalysis than homogeneous catalysis (Fe3+). Further, after the reaction, only a minimum percentage (2%) of Fe was leaching from Fe-MCM-41 under optimized condition and the catalyst remained unaltered in its structure. Introduction Wastewater from nuclear power stations is a major source of waste causing potential danger to the environment because of their organic contaminants and heavy metal ions, which are shown to be radioactive in nature. EDTA (ethylenediaminetetraacetic acid) and oxalic acid, organic chelants, have been used in decontamination processes for the removal of surface acitivity.1 However, the presence of organic chelants in decontamination wastes2 form complexes with the radioactive cations resulting in interferences in their removal by conventional techniques, namely, chemical precipitation and ion exchange, etc., which are generally employed for the removal of radioactivity. The presence of a complexing agent in the effluent is likely if a decontamination operation was conducted elsewhere and the waste streams were mixed together. The radioisotopes complexed with the above-mentioned complexing agent would be present in a solubilized form. At the Centralised Waste Management Facility (CWMF), Kalpakkam, India, chemical precipitation is used to treat low-level and intermediate level radioactive liquid wastes. During chemical treatment the complexing agent present at a concentration g 10 ppm interferes with the removal of activity by complexing with the precipitant and the separation of phases are adversely affected.3 Consequently, suitable ecofriendly techniques need to be explored to destruct the organic contaminants in a safe way to protect the green environment. Wet air oxidation (WAO) could be considered as an attractive and useful technique for reducing the total organic carbon (TOC) in industrial wastewater,4,5 since it has proven its worth for wastewater treatment, particularly with low or moderate levels of organic contaminants using pure oxygen or air as oxidant; however, it requires very high temperature (100-300 °C) and pressure (1-10) MPa. On the other hand, such reaction conditions can lead to high installation * To whom correspondence should be addressed. Tel.: 91-4422203158. Fax: 91-44-22200660. E-mail: [email protected]. † Anna University. ‡ BARC Facilities.

costs, and thus practical applications of this process are limited. In addition, if oxygen is used as oxidant, the problems of oxygen low solubility in water and the hindered oxygen supply to the liquid phase due to the gas-liquid mass-transfer resistances are encountered.6,7 Conclusively, a economical alternative path to supercritical wet oxidation is to choose a nontoxic liquid-phase oxidant, hydrogen peroxide, that would allow the (wet peroxide oxidation) conditions to decrease significantly to atmospheric pressure and subboiling water temperature.8,9 The oxidation of organic contaminants by homogeneous catalysts, such as Fe2+/Fe3+, or other dissolved transition metal cations, e.g., Cu2+, Mn2+, and Co2+, is recognized as an efficient method to decompose the organics into harmless H2O and CO2,10 but this requires tight pH control to avoid precipitation and extra steps for the recuperation and the reuse of the catalyst.11 The development of Fenton type solid-phase catalysts, i.e., heterogeneous catalysts such as Al-Fe pillared bentonite,9 Fe pillared laponite,12 and Fe containing zeolites8 which feature efficiency as well as stability toward the reaction condition, can be an alternative to avoid such drawbacks. However, these materials also possess resistance to the diffusion of organic contaminants, if the molecule size of organic contaminants is larger than the pore size of the above materials. Consequently, a material with a mesopore region is required to overcome zeolite limitations when dealing with large contaminants such as EDTA. Mobil researchers invented a new family of mesoporous MCM-41 materials (Mobil Composition of Matter No. 41), which has attracted worldwide interest in many areas of physical, chemical, and engineering sciences13 in 1992. The important characteristics of this novel material are its large BET (Brunauer-Emmett-Teller) surface area, high porosity, and controllable and narrowly distributed pore sizes, manifesting this material as a very promising candidate as a catalyst or catalyst support,14-19 adsorbent,20,21 and host for host-guest encapsulation in the development of advanced composite materials.22 The substitution of iron(III) species in the framework of MCM-41, referred to as Fe-MCM-41,23,24 tends to have redox properties25 which could participate in the conventional

10.1021/ie800907y CCC: $40.75  2009 American Chemical Society Published on Web 12/16/2008

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Fenton/Haber-Weiss homogeneous reaction pathways, and it is expected to enhance intraparticle diffusion when compared to microporous zeolite materials due to its special characteristics such as higher surface area together with large pore size. In the present study, we report here the removal of EDTA and oxalic acid through catalytic wet peroxide oxidation (CWPO) using nontoxic hydrogen peroxide as oxidant at low temperature in the presence of Fenton type mesoporous FeMCM-41 catalysts with various ratios. The effect of parameters such as time, pH, Si/Fe ratios of Fe-MCM-41, and temperature has been investigated to yield higher removal of organics. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis confirmed the absence of structural collapse of FeMCM-41 after the reaction. Experimental Section Materials and Methods. Mesoporous Fe-MCM-41 molecular sieves with various Si/Fe ratios (25, 50, 75, and 100) were synthesized using the gel composition of 1:0.2:X:0.89:120 SiO2: CTAB:Fe2O3:H2SO4:H2O (X varies with the Si/Fe ratio).13,26,27 Sodium metasilicate and iron(III) nitrate were used as the sources for silicon and iron, respectively. Cetyltrimethylammonium bromide (CTAB) was used as the structure-directing agent. EDTA disodium salt and oxalic acid dihydrate, and hydrogen peroxide (6%, 2.54 M) were used as organic contaminants and oxidant, respectively. All of the above chemicals were purchased from E-Merck. Characterization. The XRD powder diffraction patterns of the calcined mesoporous Fe-MCM-41 molecular sieves (Si/Fe ratio ) 25, 50, 75, and 100) were obtained with a Stereoscan diffractometer using nickel-filtered Cu KR radiation (λ ) 1.54 Å) and a liquid nitrogen-cooled germanium solid-state detector. Surface area, pore volume, and pore size distribution were measured by nitrogen adsorption at 77 K using an ASAP-2010 porosimeter (Micromeritics Corp.). The coordination environment of Fe in Fe-MCM-41 samples was examined by diffuse reflectance UV-visible spectroscopy with a Shimadzu UVvisible spectrophotometer (Model 2101 PC; Shimadzu, Tokyo, Japan) in the wavelength range of 200-400 nm. The coordination environment of Fe in Fe-MCM-41 samples was further confirmed by electron paramagnetic resonance (EPR) analysis recorded on a JEOL (Peabody, MA) EPR spectrometer (JESREIXM) operating in the X-band region. TEM was performed using a Philips (Eindhoven, The Netherlands) CM30 ST electron microscope operated at 300 kV. Experimental Procedure. Prior to the experiment, the FeMCM-41 catalyst was ground into small particles, kept at 500 °C for 3 h in a furnace for activation, and then cooled to room temperature. Organic contaminants oxidation with hydrogen peroxide was carried out in a glass batch reactor of 100 cm3 capacity equipped with a condenser, stirrer, and thermocouple, etc., under air atmosphere using Fe-MCM-41 as catalyst. Aqueous 50 cm3 (1 mM) solutions of EDTA and oxalic acid were prepared in distilled water, and their pH was adjusted with H2SO4 and NaOH (the pH of the solution was measured by a calibrated pH meter (ELICO, model LI 120 with electrode)) and then added to the reactor together with the Fe-MCM-41 catalyst (0.2 g) individually. Subsequently, the EDTA and oxalic acid solution was heated to the desired temperature within the range of 30-70 °C and 17 and 1 mM H2O2 was added to those mixtures respectively at the stoichiometric amount required to oxidize 1 mM of organic contaminants completely to CO2 and H2O. At different time intervals, samples were taken out from the flask and one drop of hydrazine was added to prevent H2O2

Figure 1. Nitrogen sorption isotherms of Fe-MCM-41 molecular sieves:

from reacting with organic substrates and centrifuged. Then the supernatant solution was analyzed for total organic carbon (TOC) using a TOC analyzer (Shimadzu, TOC-V CPN) using the nonpurgeable organic carbon method.28 Leaching Tests. Leaching tests were carried out in two ways. The aim of the first method was to establish whether small amounts of the dissolved iron were responsible for the observed catalytic activity. After wet oxidation with H2O2, the Fe-MCM41 was filtered at the temperature of the catalytic tests, to prevent the possible readsorption of any leached iron during the cooling of the solution. The substrate and H2O2 were then added to the solution at the same concentrations (1 mM of substrate was maintained by adding the remaining moles of organics in the resultant solution) as used in the catalytic tests. The TOC of the substrate was then measured at the same temperature as the catalytic tests as a function of time, but in the absence of the solid Fe-MCM-41. If dissolved iron ions were responsible for the catalytic behavior, reactivity similar to that shown in the presence of the Fe-MCM-41 would be expected. The second method was used to check potential leaching of the iron from the Fe-MCM-41 and involved analysis of the solution by ICPAES. In this case also, the solid was filtered at the temperature of the catalytic reaction, to prevent possible readsorption. This method allowed direct determination of the presence of iron ions in solution. Results and Discussion Characterization. The XRD powder diffraction patterns of the calcined mesoporous Fe-MCM-41 show that materials possess an intense diffraction peak at 1.4-3.2 (2θ) due to the [100] plane, confirming the hexagonal mesophase of the material.29,30 Figure 1 represents the type IV nitrogen sorption isotherm with a hysteresis loop indicating capillary condensation of uniform mesopore materials at p/po ∼ 0.3-0.4.31 There was a slight shift in the inflection step toward lower p/po upon the introduction of metal. BET surface area, pore size, and pore volume for calcined materials are presented in Table 1. The diffuse reflectance UV-visible spectra were recorded for calcined Fe-MCM-41 catalysts to study the coordination environment of Fe in which a broad peak around 275 nm and a small peak around 215 indicates the iron in tetrahedral and octahedral coordination, respectively (Figure 2).32,33 Figure 3 illustrates the EPR spectra of calcined Fe-MCM-41 samples with two signals at g ) 4.3 and 2.0 could be attributed to Fe(III) ion in tetrahedral coordination with a strong rhombic distortion34

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Fe2+ + H2O2 f Fe3+ + OH· + OH-

Table 1. Textural Properties of the Catalysts unit surface pore pore ICP d100 cell area size volume (Si/Fe) (Å) a0 (Å) (m2 g-1) (Å) (cm3 g-1)

catalysts Fe-MCM-41 Fe-MCM-41 Fe-MCM-41 Fe-MCM-41

(Si/Fe (Si/Fe (Si/Fe (Si/Fe

) ) ) )

100) 106 41.62 48.06 75) 79 40.10 46.30 50) 55 38.31 44.24 25) 32 39.09 45.14

982 937 865 824

25.91 25.54 24.38 23.99

0.932 0.929 0.923 0.915

and Fe(II) in octahedral coordination.35,36 TEM images of FeMCM-41 show organized uniform pores. The iron content in Fe-MCM-41 with various ratios was determined using ICP-AES (Allied Analytical ICAP 9000), and their results are given in terms of Si/Fe ratio (Table 1). The detailed characterization study of Fe-MCM-41 is provided in the literature.27 Organic Chelants Oxidation over Fe-MCM-41. Influence of pH. To achieve the effective removal of total organic carbon of EDTA and oxalic acid in aqueous solution, the influence of pH was investigated. The removal of organic carbon from aqueous solutions of EDTA and oxalic acid was carried out over Fe-MCM-41 (Si/Fe ) 100) at 30 °C with a stoichiometric quantity of H2O2 in a range from pH 3 to 5 for 4 h. The optimum value of EDTA and oxalic acid was found to be 4.5 and 4, respectively. The percentage removal of organic carbon from the aqueous solution of EDTA at pH 3, 4, 4.5, and 5 is 18.3, 22.1, 27.3, and 23.5%, respectively, whereas for oxalic acid at pH 3, 4, 4.5, and 5 it is 4.1, 5.6, 4.9, and 3.8%, respectively. From the above result, it is observed that considerable removal is feasible only at pH 4-4.5 which clearly points out the possibility of iron leaching from Fe-MCM-41 at high acidic pH (4.5), adsorption of organics is very low because of less electrostatic attraction between ionized organic chelants and adsorbent surface.37 Influence of Fe-MCM-41 with Various Si/Fe Ratios. The contribution of Fe content in the MCM-41 plays a vital role in determining the removal of total organic carbon from aqueous solution of organic chelants. Hence, the degradation of EDTA and oxalic acid with H2O2 over Fe-MCM-41 at Si/Fe ratios of 25, 50, 75, and 100 was carried out at 30 °C at pH 4.5 and 4, respectively, as a function of time. The percentage removal of organic carbon at different time intervals for the various ratios is illustrated in Figure 1, which indicates the gradual increase in the removal of organic carbon with time for each ratio. Among the ratios studied, Fe-MCM-41 (Si/Fe ) 25) was found to be more active than the others, which is probably due to the iron content of the catalyst as degradation occurs via the Fenton reaction. The following Fenton reaction mechanism38,39 can be predicted for the Fe(III) active species in the framework of MCM-41.

Fe2+ + H2O2 f [FeIVO]2+ + H2O Hence, it can be clearly noted that both framework and nonframework iron sites are capable of decomposing H2O2 to produce hydroxyl and perhydroxyl radicals by correlating the removal of organic carbon for all the catalysts. It should be noted that the difference in removal of organic chelants between Fe-MCM-41 (Si/Fe ) 100) and Fe-MCM-41 (Si/Fe ) 25) might be due to nonframework iron since Fe-MCM-41 (Si/Fe ) 100) possesses a maximum amount of framework irons confirmed from EPR and DRS analysis. Furthermore, the reason behind the less nonframework iron sites in Fe-MCM-41 (Si/Fe ) 100) might be due to much more dimensions than that of Fe-MCM41 (Si/Fe ) 25) and it can also considerably contribute to the decomposition of organic chelants. From the above observation, it can be concluded that Fe-MCM-41 possessing a higher amount of iron, i.e., Si/Fe ) 25, is a superior catalyst than the others. The removal of organic chelants would be possible through adsorption since Fe-MCM-41 is a mesoporous material with large pore dimension and surface area. The amounts of 4.2 and 6.3% of organic carbon of EDTA and oxalic were removed by adsorption, respectively. Hence, it was subtracted from the percentage removal of EDTA and oxalic acid in aqueous solution. Influence of Temperature. A study on the influence of temperature on the degradation of organic chelants was performed over Fe-MCM-41 (Si/Fe ) 25) using a stoichiometric quantity of H2O2 in temperature ranging from 30 to 70 °C for 4 h. As can be seen from Figure 2, the percentage removal of total organic carbon for EDTA and oxalic acid increases significantly with temperature until 50 °C, beyond which a considerable decrease in total organic carbon removal is observed. Therefore 50 °C can be considered as the optimum temperature to achieve maximum removal of total organic carbon. It is worth noting from the above observation that the formation of hydroxyl radicals from the decomposition of H2O2 by iron is temperature-dependent and the decrease in organic

Fe3+ + H2O2 f [FeIIIOOH]2+ + H+ [FeIIIOOH]2+ f Fe2+ + OOH· [FeIIIOOH]2+ f [FeIVO]2+ + OH· [FeIIIOOH]2+ f [FeVO]3+ + OH[FeIIIOOH]2+, [FeIVO]2+, and [FeVO]3+ are active intermediates. The above mechanism illustrates that Fe3+ species are involved in the formation of various active intermediates to produce perhydroxyl and hydroxyl radicals to decompose organics. However, Fe(II) species in extraframework also produce hydroxyl radical when treated with H2O2, and its mechanism40,41 follows:

Figure 2. Diffuse reflectance UV-visible spectra of Fe-MCM-41: (a) FeMCM-41 (Si/Fe ) 25); (b) Fe-MCM-41 (Si/Fe ) 50); (c) Fe-MCM-41 (Si/Fe ) 75); (d) Fe-MCM-41 (Si/Fe ) 100).

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Figure 6. Percentage of iron leached from Fe-MCM-41 (Si/Fe ) 25) after reaction.

Figure 3. EPR spectra of Fe-MCM-41: (a) Fe-MCM-41 (Si/Fe ) 25); (b) Fe-MCM-41 (Si/Fe ) 50); (c) Fe-MCM-41 (Si/Fe ) 75); (d) Fe-MCM-41 (Si/Fe ) 100).

Figure 4. Influence of Si/Fe ratios of Fe-MCM-41 on removal of organic chelants.

Figure 5. Influence of temperature on removal of organic chelants over Fe-MCM-41 (Si/Fe ) 25).

carbon removal above 50 °C could be ascribed to the decomposition of H2O2 into water without producing any radicals.42,43 Performance of Heterogeneous Fe-MCM-41 over Homogeneous Fenton Fe3+: A Comparative Study. To evaluate the activity, a comparative study of homogeneous Fe3+ with H2O2 was carried out under the following reaction conditions of temperature ) 50 °C, time ) 4 h, pH ) 4-4.5 using Fe3+ (51 mg in 5 cm3 solution equivalent to an iron content of 0.2 g of Fe-MCM-41 (Si/Fe ) 25)), a stoichiometric quantity of H2O2 for EDTA and oxalic acid decomposition and compared with the given heterogeneous Fe-MCM-41 (Si/Fe ) 25) catalyst. It

Figure 7. TEM images of Fe-MCM-41: (a) before reaction and (b) after reaction.

should be noted that in homogeneous catalysis, the percentage of total organic carbon removal for EDTA and oxalic acid is found to be 23.8 and 8.1%, respectively, but in the case of heterogeneous catalysis, 50 and 20.1% of removal are observed. From the above points, it is clearly evident the superior performance of Fe-MCM-41 (Si/Fe ) 25) by achieving more mineralization of organic chelants over homogeneous catalyst. It is also estimated that mineralization of EDTA and oxalic acid

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while the cycles increase. The percentage removal of TOC for cycles 1-3 was found to be 50, 47.8, and 43.7%, respectively. Conclusion

Figure 8. X-ray diffraction patterns of parent Fe-MCM-41 and Fe-MCM41 after reaction.

is 2.1- and 2.5-fold times higher than that obtained by homogeneous Fe3+ catalyst, respectively. Leaching and Structure Retainment Study of Fe-MCM41. The leachout study was conducted to be aware of the higher reactivity of the heterogeneous Fe-MCM-41 (Si/Fe ) 25) catalyst with respect to homogeneous Fe3+ ions, which confirms the catalytic activity of Fe-MCM-41 (Si/Fe ) 25) cannot be due to the dissolution of iron on the MCM-41. The removal of total organic carbon was found to be approximately 2% for 4 h at 50 °C in the absence of the catalyst (filtered solution). As can be seen from the above observation, it demonstrates beyond a doubt that the activity of the solid catalyst is not due to the iron ions leached from the Fe-MCM-41. Furthermore, since organic carbon removal in the absence of a catalyst is negligible, the activity during the catalytic run is not due to other potential reagents able to activate the hydrogen peroxide (such as impurities in the reactor or light, etc.). The percentage of iron leached after 4 h of reaction, with respect to the initial amount of iron in the Fe-MCM-41 catalyst at pH ) 4 is illustrated in Figure 3 at different temperatures ranging from 30 to 70 °C. Even though it is observed that the leaching of iron increases with respect to temperature, previous tests showed that this amount of iron does not influence the overall reactivity. Apart from this, to check the retainment of the crystallinity of the catalyst after the reaction, the Fe-MCM-41 (Si/Fe ) 25) was filtered out and dried at 80 °C for 6 h and the catalyst was then subjected to TEM and XRD analysis. TEM analysis was employed for Fe-MCM-41 (Si/Fe ) 25) catalyst before and after the reaction. From Figure 4a, the presence of organized uniform pores can be clearly seen in parent Fe-MCM-41 (Si/Fe ) 25). It is surprising to note from Figure 4b that the appearance of organized uniform pores as parent material undoubtedly confirms the absence of structural collapse of Fe-MCM-41. In addition, XRD analysis was also performed to investigate the structural alteration in catalyst after the reaction. The X-ray diffraction patterns of Fe-MCM-41 before and after reaction are shown in Figure 5, where a strong peak at 1.4-3.2 and a weaker peak at 3.2-5 (2θ) due to d100 and d110 lattice planes were typical of hexagonal mesoporous structure. From the figure, it is clearly evident that Fe-MCM-41 catalyst after the reaction has strong and weak peaks the as same as parent Fe-MCM-41, illustrating the retainment of ordering. To know the reusability of the catalyst, Fe-MCM-41 (Si/Fe ) 25) was activated at 500 °C for 5 h in a furnace with air and tested for three cycles using EDTA under optimized condition (temperature ) 50 °C, time ) 4 h, pH ) 4.5). It has been observed that TOC removal for EDTA is slightly decreased

From this investigation, it could be concluded that Fenton type mesoporous Fe-MCM-41 catalysts have proven their worth toward the catalytic wet peroxide oxidation of organic chelants by achieving higher removal of organics as compared to homogeneous catalyst. Among the catalyst studied, the FeMCM-41 (Si/Fe ) 25) provides higher removal compared to other ratios due to higher iron content. Moreover, during the reaction, only a minimum percentage (2%) of iron was leached from Fe-MCM-41 (Figure 6) under optimized condition without altering the structure of the material, confirmed by TEM (Figure 7) and XRD (Figure 8) analysis. Consequently, this material can be applied for successive oxidation and may provide an economical and permanent solution in the nuclear industry to protect the green environment. Acknowledgment The authors thank the Board of Research in Nuclear Science (2001/36/8-BRNS/662) for providing financial support. Literature Cited (1) William, J. C.; Randy, D. C.; Kevin, E. O. Organics in mixed nuclear wastes: Gamma-radiolytic degradation of chelating and complexing agents, In EnVironmental Applications of Ionizing Radiation; Anthony, P. T., Ed.; John Wiley & Sons: New York, 1998; pp 429-449. (2) Riley, R. G.; Zachara, J. M.; Wobber, F. H. Chemical contaminants on DOE lands and selection of contaminant mixture for subsurface science research, DOE/ER-0547T; Department of Energy: Washington, DC, 1992. (3) Sinha, P. K.; Amalraj, R. V.; Krishnasamy, V. Flocculation studies on freshly precipitated copper ferrocyanide for the removal of caesium from radioactive liquid waste. Waste Manage. 1993, 13, 341–350. (4) Matatov-Meytal, Y. I.; Sheimtuch, M. Catalytic abatement of water pollutants. Ind. Eng. Chem. Res. 1988, 37, 309–326. (5) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet air oxidation. Ind. Eng. Chem. Res. 1995, 34, 2–48. (6) Ding, Z. Y.; Frisch, M. A.; Li, L.; Gloyna, E. F. Catalytic oxidation in supercritical water. Ind. Eng. Chem. Res. 1996, 35, 3257–3279. (7) Wandleler, R.; Baiker, A. Supercritical fluids: Opportunities in heterogenous catalysis. CATTECH 2000, 4, 128–143. (8) Fajerwerg, K.; Foussard, J. N.; Ferraed, A.; Debellefontaine, H. Wet oxidation of phenol by hydrogen peroxide: The key role of pH on the catalytic behaviour of Fe-ZSM-5. Water Sci. Technol. 1997, 35, 103–110. (9) Barrault, J.; Abdellaoui, M.; Bouchoule, C.; Majesste, A.; Tatibouet, J. M.; Louloudi, A.; Papayannakos, N.; Gangas, N. H. Catalytic wet peroxide oxidation over mixed (Al-Fe) pillared clays. Appl. Catal., B 2000, 27, L225– L230. (10) Falcon, M.; Fajerweg, K.; Foussard, J. N.; Peuch-Costes, E.; Maurette, M. T.; Debellefontaine, H. Wet oxidation of carboxylic acids with hydrogen peroxide. Wet peroxide oxidation (WPO) process. Optimal ratios and role of Fe:Cu:Mn metals. EnViron. Technol. 1995, 16, 501–513. (11) Plant, L.; Jeff, M. Hydrogen peroxide: a potent force to destroy organics in wastewater. Chem. Eng. 1994, E16–EE20. (12) Sum, O. S. N.; Feng, J.; Hu, X.; Yue, P. L. Pillared laponite clay based Fe nanocomposites as heterogeneous catalysts for photo-Fenton degradation of acid black 1. Chem. Eng. Sci. 2004, 59, 5269–5275. (13) Beck, J. S.; Vartuli, C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmit, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A new family of mesoporous molecular sieves prepared with liquid crystal templates. J. Am. Chem. Soc. 1992, 114, 10834–10843. (14) Sayari, A. Catalytic by crystalline mesoporous molecular sieves. Chem. Mater. 1996, 8, 1840–1852. (15) Han, P.; Wang, X.; Qiu, X.; Xiangling, J.; Gao, L. One step synthesis of palladium/SBA-15 nanocomposites and its catalytic application. J. Mol. Catal. A: Chem. 2007, 272, 136–141. (16) Samanta, S.; Mal, N. K.; Bhaumik, A. Mesoporous Cr-MCM-41: An efficient catalyst for selectie oxidation of cycloalkanes. J. Mol. Catal. A: Chem. 2005, 236, 7–11.

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ReceiVed for reView June 9, 2008 ReVised manuscript receiVed November 5, 2008 Accepted November 10, 2008 IE800907Y