Environ. Sci. Technol. 2008, 42, 908–912
Removing Organic Compounds from Aqueous Medium via Wet Peroxidation by Gold Catalysts Y I - F A N H A N , * ,† NOPPHAWAN PHONTHAMMACHAI,‡ K A N A P A R T H I R A M E S H , †,‡ Z I Y I Z H O N G , † AND TIM WHITE‡ Institute of Chemical and Engineering Sciences, 1, Pesek Road, Jurong Island, Singapore, 627833, and School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798
Received May 14, 2007. Revised manuscript received August 10, 2007. Accepted November 6, 2007.
A new heterogeneous Fenton-like system, consisting of supported Au catalysts and hydrogen peroxide, was proved to be effective in removing low level organic compounds (ca. 100 ppm) such as phenol, ethanol, formaldehyde, and acetone in aqueous solution. Among all gold catalysts the Au/ hydroxyapatite (Au/HAp) exhibits the highest activity, and even better than the conventional iron ions exchanged zeolite (Fe/ ZSM-5) catalyst. In particular, unlike the limited operational pH range (pH: 2∼5) for the other heterogeneous Fenton catalysts such as Fe/ZSM-5, Au/HAp shows higher stability even in strong acid solution (pH ∼ 2), due to almost no leaching of active metal from supports into solution. It can be potentially applied in treating the industrial wastewaters with strong acidity and purifying drinking water. In addition, in the case of complete oxidation of phenol, a plausible route was suggested for deep understanding of this process.
Introduction Industrial pollutants such as phenol, formaldehyde, and alcohols significantly contaminate ground and surface water. The stochastic influence of their toxic on the human health is not well understood, and a fact increasing concern as water resources became increasingly stressed due to the climate change. The treatment of water and wastewaters to destroy or remove organic pollutants and deliver a portable product is therefore the subject of wide studying. Thus, developing simple, safe and economic processes to purify contaminated waste streams is of great importance. Methods under long development for the treatment of wastewaters containing toxic organic compounds include wet oxidation with or without solid catalysts (1–4), biological oxidation, adsorption, and supercritical oxidation (5, 6). Generally, the treatment of effluents that contain organic pollutants at low concentrations, e.g., 573 K) and high pressures (0.5–10 MPa); (ii) Catalytic peroxidation (CPO) explores hydroxyl radicals under mild conditions ( Fe2O3 > TiO2 > C. Obviously, the catalytic performance varied significantly depending on the properties of supports even through the average size of gold particles (dAu) was very close, generally acetone > ethanol > formaldehyde. A common kinetic profile suggests that the compounds were largely oxidized in the first halfhour, after which oxidation slowed, possibly, due to the reduced concentration of reactants remaining in the solution. In the case of phenol oxidation over the Au/HAp catalyst, the rate of peroxidation in aqueous media was profoundly influenced by pH and temperature. The conversion of phenol varied reversely with pH over the range of 2.0-9.0, with most experiments conducted at pH 6.5, which is close to the pH of drinking water (Figure 3a). Reactivity significantly accelerated with temperature (from 298 to 343 K) as illustrated by Figure 3b. Most importantly, there was almost no loss of Au in quite acidic conditions when pH value approached to ∼2.0, see Figure 4; while the leaching of Fe in Fe/ZSM-5 increased with gradually decreasing the pH value, a loss of 50% Fe was measured at pH ∼ 2.0. These results suggest that the Au (2.4 wt%)/HAp catalyst is very effective at removing organic compounds from aqueous solution by wet peroxidation at 343 K. In addition, as shown in Figure 5a, the Au/ HAp catalyst not only showed a comparable stability with Fe/ZSM-5 at pH ∼ 5.0 after several recycles for the removal of phenol, but also exhibited a high stability in the strong acidic solution (pH ∼ 2.0) (Figure 5b), and almost no decay after recycling five times. However, poor stability was observed for the Fe/ZSM-5 catalyst at strong acidic solution, the conversion of phenol was decreased from 60 to 10% after recycling five times, probably due to the leaching of Fe, as depicted by Figure 4. Noting that by XPS and TEM the features of Au nanoparticles on HAp were little changed after reaction. Several mechanisms for the production of various radicals through the aqueous decomposition of H2O2 over Fe-based heterogeneous/homogeneous catalysts have been demonstrated (8, 20). More interestingly, Lunsford and co-workers (21) have investigated H2O2 decomposition on a Au/SiO2 catalyst with comparing to oxides, such as R-Al2O3, TiO2, and SiO2, and found that Au possessed the highest reactivity due to strong bonding of the hydroxyl group to Au surface. In reference with those studies and in term of the positively charged Au particles on HAp, we propose that a redox process Au(0)T Au(&+) likely takes place during the wet peroxidation
(eqs 1 and 2). In general, the wet peroxide oxidation of organic constitutes can be described in four steps (8): activation of H2O2 to produce · OH, oxidation of organic compounds with · OH, recombination of · OH to form O2, and wet oxidation of organic compounds with O2. Catalyst/Temperature
H2O2 98 2 · OH
(3)
Temperature
·OH + organic compounds 98 CO2 + H2O2
(4)
Temperature
2 · OH 98 1 ⁄ 2O2 + H2O
(5)
Temperature/pressure
O2 + organic compounds 98 CO2 + H2O (6) According to reactions in eqs 3-6, the reaction rate depends on the concentration of hydroxyl radicals in solution, which is controlled by the rate of H2O2 decomposition and a selfscavenging of radicals, the latter mechanism being beyond the scope of this study. However, the production of radicals and subsequent oxidation of organic compounds (eqs 3 and 4) are strongly temperature-dependent (Figure 3b), and favored by acidic conditions (Figure 3a). In addition, the protons are believed capable suppressing the scavenging of radicals (8). The mechanism for the total oxidation those organics may vary depending on the molecular structure. In the case of total oxidation of phenol, the possible mechanism can be illustrated as Scheme 2 (18), the final products are essentially CO2 and H2O through the formation of several intermediate. Note that neither intermediate nor byproduct was detected, perhaps, due to short lifetime or extremely low concentration for those species. As shown in Figure 2, the oxidation rates for ethanol and formaldehyde are clearly lower than that for phenol, in part, possibly due to those molecules bonding to the catalyst surface stronger than phenol and retarding the reactions in eqs 3 and 5. And in accordance with blank tests and previous studies (7, 18), the oxidation by molecular oxygen (eq 6) is not operative as these take place only under high pressures and temperatures. This study shows that the Au/HAp-H2O2 regent has great potential for the total oxidation of organic pollutants in aqueous media by peroxidation, which therefore avoids the need of secondary-treatments. The approach described could compete with other methods to remove organic compounds from drinking water or industrial effluents, especially with strong acidity (pH 2). Future studies should demonstrate if this novel system can destroy more complex organic comVOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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pounds, which are difficult to transform or degrade. The long-term stability of heterogeneous gold catalyst will require the enhancement and role of HAp better understood.
Supporting Information Available Information about the details of the preparation of the Au/ HAp catalyst, characterization of catalyst. This material is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgments This research was partly supported by A*Star, Singapore.
Literature Cited (1) Matatov-Meytal, Y. I.; Sheintuch, M. Catalytic abatement of water pollutants. Ind. Eng. Chem. Res. 1988, 37, 309. (2) Donglagic, J.; Levec, J. Comparison of catalyzed and noncatalyzed oxidation of azo dye and effect on biodegradability. Environ. Sci. Technol. 1998, 32, 1294. (3) Levec, J.; Pintar, A. Catalytic oxidation of aqueous solutions of organics. An effective method for removal of toxic pollutants from waste waters. Catal. Today y 1995, 24, 51. (4) Santos, A.; Yustos, P.; Qiutanilla, A.; Ruiz, G.; Garcia-Ochoa, F. Study of the copper leaching in the wet oxidation of phenol with CuO-based catalysts: Causes and effects. Appl. Catal., B 2005, 61, 323. (5) Gopalan, S.; Savage, P. E. A reaction network model for phenol oxidation in supercritical water. AIChE J 1995, 41, 1864. (6) Thornton, T. D.; Savage, P. E. Phenol oxidation pathways in supercritical water. Ind. Eng. Chem. Res. 1992, 31, 2451. (7) Pintar, A. Catalytic processes for the purification of drinking water and industrial effluents. Catal. Today 2003, 77, 451. (8) Miller, C. M.; Valentine, R. L. Catalytic fixed bed reactor systems for the destruction of contaminants in water by hydrogen peroxide. U.S. Patent. 5817240, 1998. (9) Haruta, M.; Yamada, N.; Kobayashi, T.; Iilima, S. Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxidation. J. Catal. 1989, 115, 301.
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(10) Hutchings, G. J.; Haruta, M. A golden age of catalysis: A perspective. Appl. Catal., A 2005, 291, 2. (11) Chen, M. S.; Goodman, D. W. The structure of catalytically active gold on titania. Science 2004, 306, 252. (12) Carretin, S.; McMorn, P.; Johnston, P.; Griffin, K.; Kiely, C. J.; Hutchings, G. J. Selective oxidation of glycerol to glyceric acid using a gold catalyst in aqueous sodium hydroxide. Chem. Commun. 2002, 696. (13) Abad, A.; Concepcion, P.; Corma, A.; Garcia, H. A collaborative effect between gold and a support induces the selective oxidation of alcohols. Angew. Chem., Int. Ed. 2005, 44, 4066. (14) Hughes, M. D. Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions. Nature 2005, 437, 1132. (15) Christensen, C. H.; Jφrgensen, B.; Rass-Hansen, J.; Egeblad, K.; Madsen, R.; Klitgaard, S. K.; Hansen, S. M.; Hansen, M. R.; Anderson, H. C.; Riisager, A. Formation of acetic acid by aqueousphase oxidation of ethanol with air in the presence of a heterogeneous gold catalyst. Angew. Chem., Int. Ed. 2006, 45, 4648. (16) Han, Y.-F.; Lunsford, J. H. A comparison of ethanol and water as the liquid phase in the direct formation of H2O2 from H2 and O2 over a palladium catalyst. Catal. Lett. 2005, 99, 13. (17) Han, Y.-F.; Zhong, Z.; Ramesh, K.; Chen, F.; Chen, L. Effects of different types of γ-Al2O3 on the activity of gold nanoparticles for CO oxidation at low-temperatures. J. Phys. Chem. C 2007, 111, 3163. (18) Forty, F.; Ferry, C.; Bengoa, C.; Font, J.; Fabregat, A. Catalytic removal of phenol from aqueous phase using oxygen or air as oxidant. Catal. Today 1995, 24, 79. (19) Phu, N. H.; Hoa, T. T. K.; Tan, N. V.; Thang, H. V.; Ha, P. L. Characterization and activity of Fe-ZSM-5 catalysts for the total oxidation of phenol in aqueous solutions. Appl. Catal., B 2001, 34, 267. (20) Kremer, M. L. Mechanism of the Fenton reaction. Evidence for a new intermediate. Phys. Chem. Chem. Phys. 1999, 1, 3595. (21) Suh, M.; Bagus, P. S.; Pak, S.; Rosynek, M. P.; Lunsford, J. H. Reactions of hydroxyl radicals on titania, silica, alumina, and gold surfaces. J. Phys. Chem. B 2000, 104, 2736.
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