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Efficient Photosensitized Degradation of 4-Chlorophenol over Immobilized Aluminum Tetrasulfophthalocyanine in the Presence of Hydrogen Peroxide Meiqin Hu,† Yiming Xu,*,† and Jincai Zhao‡ Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China, and Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Science, Beijing 100080, China Received February 3, 2004. In Final Form: May 10, 2004 The photosensitized degradation of 4-chlorophenol (4-CP) under visible light (λ g 450 nm) irradiation in an aerated aqueous medium at pH 12 was studied using an immobilized photosensitizer, aluminum tetrasulfophthalocyanine, on a commercial resin Amberlite IRA 400. The catalyst exhibited strong adsorption toward 4-CP, but the adsorption led to an exponential decrease in both the initial rate and the apparent first-order rate constant, as measured by 4-CP loss in the bulk solution. Several intermediates were formed from 4-CP oxidation, including fumaric acid, benzoquinone, and hydroquinone, which were adsorbed strongly on the catalyst and lowered the photosensitized reaction. Addition of H2O2 was found to be an efficient way to eliminate the colored intermediates and consequently recover the catalyst activity. The immobilized sensitizer was stable and could be used repeatedly in the presence of H2O2. The optimal loading of the photosensitizer in the catalyst was about 1.0 wt %.
Introduction Advanced oxidation processes have been studied largely for elimination of organic pollutants in water.1,2 The organic degradation is initiated by reactive oxygen species such as •OH and O2•- radicals generated from H2O2 photolysis, Fe-based (photo)-Fenton reaction, and TiO2 photocatalysis. However, the UV light required for the operation is expensive and occupies only about 5% of solar light. The exploration of an efficient system that is able to use most of the solar energy and molecular oxygen for environmental application is particularly attractive and remains a great challenge.3,4 Metallophthalocyanines and their derivatives possess an intensive absorption in the blue-green region and can collect up to 50% of the energy available in the solar spectrum.5 Some of the complexes containing nontransition metals have been proved to be efficient photosensitizers for photodynamic therapy of cancer and fine chemical synthesis.6,7 The excited complex, formed upon visible light irradiation, is sufficiently long-lived to allow for intermolecular interactions with triplet ground molecular oxygen, generating singlet oxygen via energy transfer or producing superoxide radicals via an electron transfer pathway. The singlet oxygen is ca. 1 V more oxidizing than triplet molecular oxygen and thus reacts * To whom correspondence should be addressed. E-mail: xuym@ css.zju.edu.cn. † Zhejiang University. ‡ The Chinese Academy of Science. (1) Legrini, O.; Oliveros, E.; Braun, A. M. Chem. Rev. 1993, 93, 671. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (3) Asahi, R.; Morikawa, T.; Okwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (4) Ma, W.; Li, J.; Tao, X.; He, J.; Xu, Y.; Yu, J.; Zhao, J. Angew. Chem., Int. Ed. 2003, 42, 1029. (5) Ferraudi, G. In Phthalocyanines: Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1989; pp 291340. (6) DeRosa, M. C.; Crutchley, R. J. Coord. Chem. Rev. 2002, 233234, 351. (7) Maldotti, A.; Molinari, A.; Amadelli, R. Chem. Rev. 2002, 102, 3811.
rapidly with unsaturated and/or electron-rich molecules. Increasing attention has been paid recently to environmental application of the singlet oxygen. Using watersoluble metal phthalocyaninesulfonate as a photosensitizer, several toxic and recalcitrant pollutants can be easily oxidized under visible light irradiation such as phenol, chlorophenol, and nitrophenol.8-15 The ring-opening products of maleic/fumaric acid, carbon dioxide, and chloride ions have been identified from these reactions,8,13,14 illustrating the possibility for a new environmental technology. For practical application, however, the photosensitizer needs to be immobilized for easy separation and reuse.6,7 In the present work, aluminum tetraphthalocyaninesulfonate (AlPcTS) immobilized on anionic exchanger Amberlite IRA 400 has been examined as a heterogeneous sensitizer for oxidation of 4-chlorophenol in water under visible light irradiation (λ g 450 nm). AlPcTS is chosen because it is a good photosensitizer with relatively high stability against photobleaching.8,9,14 The resin has strong adsorption affinity for the target molecule in water, which enables us to evaluate the effect of adsorption. While the immobilized catalyst was stable and efficient, some colored intermediates were formed on the catalyst surface and reduced the reaction efficiency. We found that addition of H2O2 could easily eliminate the colored intermediates and thereby accelerate the photosensitized reaction. (8) Gerdes, R.; Wohrle, D.; Spiller, W.; Schneider, G.; Schnurpfeil, G.; Schulz-Ekloff, G. J. Photochem. Photobiol., A 1997, 111, 65. (9) Ozoemena, K.; Kuznetsova, N.; Nyokong, T. J. Photochem. Photobiol., A 2001, 139, 217. (10) Kasuga, K.; Fujita, A.; Miyazako, T.; Handa, M.; Sugimori, T. Inorg. Chem. Commun. 2000, 3, 634. (11) Nensala, N.; Nyokong, T. J. Mol. Catal. A: Chem. 2000, 164, 69. (12) Iliev, V.; Alexiev, V.; Bilyarska, L. J. Mol. Catal. A: Chem. 1999, 137, 15. (13) Xu, Y.; Chen, Z. Chem. Lett. 2003, 32, 592. (14) Xu, Y.; Hu, M.; Chen, Z.; Zheng, D. Chin. J. Chem. 2003, 21, 1092. (15) Sasai, R.; Sugiyama, D.; Takahashi, S.; Tong, Z.; Shichi, T.; Itoh, H.; Takagi, K. J. Photochem. Photobiol., A 2003, 155, 223.
10.1021/la049710z CCC: $27.50 © 2004 American Chemical Society Published on Web 06/22/2004
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Experimental Section 1. Materials. The sodium salt of AlPcTS was prepared as described.14,16 The anionic exchange resin Amberlite IRA 400 (ionic exchange capacity, 3.0 mmol g-1) was a commercial product from Shanghai Chemicals, Inc. Other chemicals including 4-chlorophenol (4-CP), hydrogen peroxide, hydroquinone, benzoquinone, and fumaric acid were of reagent grade and were used as received. 2. Catalyst Synthesis. The negatively charged AlPcTS was easily immobilized into the resin by anionic exchange. The resin was ground first and sieved by a 100 mesh sieve. The exchange was made at room temperature in an aqueous medium. A set of catalyst samples with different loadings of AlPcTS from 0 to 8.0 wt % were prepared and dried in the air for several days. The dye immobilized on the resin was firmly bound, and no desorption of AlPcTS into the aqueous solution was found after stirring sufficiently overnight. The diffuse reflectance spectrum of the solid sample was recorded on an Agilent 8453 UV-vis spectrometer with an attached Labsphere RSA-HP-53. 3. Reactions and Analysis. The suspension containing 0.025 g of the catalyst and 50 mL of aqueous solution of 4-chlorophenol (0.15-2.5 mM, pH 12) was equilibrated first in the dark overnight under constant shaking and then irradiated by a Halogen lamp (500 W, Shanghai Yamin) through a solution filter of dichromate (λ g 450 nm). The reactor was thermostated at 20 °C and stirred constantly during the reaction. At certain intervals, small aliquots of the suspension were withdrawn by syringe and filtered through a membrane (0.45 µm). The filtrate spectrum was recorded on an Agilent 8453 UV-vis spectrometer, and the organic substrates were analyzed on a Dionex P680 HPLC (Apollo C18 reverse column) using 50% methanol-water as an eluent (pH 4.5 by acetic acid) at a flow rate of 1.0 mL min-1. The same suspension was also set up in the dark for comparison of any catalytic reaction. The adsorption reversibility was evaluated as follows. A suspension containing 0.025 g of the adsorbent (1.0 wt %) and 50 mL of 4-CP aqueous solution (0.41 mM, pH 7 or pH 12) was equilibrated overnight. After filtration, the adsorbed sample was redispersed in 50 mL of water (pH 7 or pH 12). After equilibration for 2 h, the concentration of 4-CP in the bulk solution was analyzed. The percentage of desorption was calculated by the ratio of the amount of the desorbed 4-CP to the amount of the previously adsorbed 4-CP on the solid.
Figure 1. Diffuse reflectance spectra of the immobilized AlPcTS solids: (a) unbound AlPcTS (29 µM) in water at pH 7; (b) the resin; (c-g) the immobilized samples containing AlPcTS at 0.05, 0.10, 0.25, 0.50, 1.0, and 5.0 wt %, respectively, all prepared from an aqueous solution; (i) the sample corresponding to curve g, but prepared from an alcoholic solution.
Results and Discussion 1. The Visible Reflectance Spectra. For the photosensitized oxidation of 4-CP in water, the monomer of AlPcTS has been shown to be more active than the corresponding aggregate.8,9,14 Thus, the visible reflectance spectrum was examined first as a function of the amount of AlPcTS loaded into the resin (Figure 1). The complex before loading was highly aggregated in aqueous solution, as evidenced by a strong absorption band at 618 nm (Figure 1a). After immobilization on the resin, interestingly, an obviously increased portion of the monomeric species was exhibited in the spectrum at 673 nm (Figure 1c-h). The interaction between the negatively charged AlPcTS and the positive charged resin is electrostatic in nature. During the exchange process, the entrance of monomeric species into the porous resin, due to steric factors, would be easier than that of the corresponding aggregates, so that the bulk equilibrium was shifted toward monomer formation. As the percentage of the dye in the catalyst increased, however, the relative monomer concentration decreased slightly (Figure 1c-h). This could be attributed to the dispersion effect of the resin that was not sufficient to minimize the interactions among the immobilized dyes. Further evidence was seen from one immobilized sample, prepared from an alcoholic solution (CH3OH/H2O ) 1:1, (16) Martin, P. C.; Gouterman, M.; Pepich, B. V.; Renzoni, G. E. Inorg. Chem. 1991, 30, 3305.
Figure 2. (a) Adsorption isotherm of 4-CP on the immobilized catalyst (1.0 wt %) at pH 12. (b) Effect of the immobilized AlPcTS percentage on the 4-CP adsorption (the initial concentration of 4-CP was [4-CP]0 ) 0.41 mM, and that of the catalyst was 0.50 g L-1).
v/v). The dye in such a parent solution existed almost totally in the monomeric form,14 and consequently the resulting composite catalyst displayed a spectrum with a substantially higher concentration of the monomeric species (Figure 1i) than the corresponding sample synthesized from an alcohol-free aqueous solution (Figure 1g). 2. Adsorption. The target organic substrate 4-CP was strongly adsorbed on the catalyst. After the adsorbed sample was redispersed in a blank solution, only about 1% was desorbed into the solution (see the Experimental Section). Figure 2a shows the adsorption isotherm of 4-CP from aqueous solution at pH 12 on the adsorbent (1.0 wt %, corresponding to 10.6 µmol AlPcTS per gram). The isotherm was not the Langmuir type,17 as clarified by a plot of C2b/n2s versus C2b. Instead, the amount of adsorp(17) Xu, Y.; Langford, C. H. Langmuir 2001, 17, 897.
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Figure 3. The changes of 4-CP concentration with time under the conditions of (a) w1.0 in the dark, (b) resin + hν, (c) w1.0 + hν, and (d) mw1.0 + hν. The symbols w1.0 and mw1.0 represent the samples prepared from an aqueous and 50% CH3OH aqueous solution, respectively (each of the solids contains the same amount of AlPcTS at 1.0 wt %). Conditions: [4-CP]0 ) 0.41 mM; pH 12; catalyst, 0.50 g/L.
tion, n2s, changed almost linearly with the equilibrium concentration, C2b, in the bulk solution. The result implied that the adsorption of 4-CP (pKa ) 9.5) from alkaline solution followed the anionic exchange mechanism, similar to the case of AlPcTS. The parent resin used in this study has an ionic exchange capacity of 3.0 mmol g-1, which is sufficient for both 4-CP and AlPcTS to be exchanged. As the dye immobilized into the resin increased from 0.025 to 8.0 wt %, the amount of 4-CP adsorption was decreased accordingly from 0.484 to 0.390 mmol g-1 (Figure 2b). Except the anionic exchange mechanism, other pathways such as hydrophobic interactions could not be neglected. While the adsorption of 4-CP at pH 12 was 51%, the corresponding data at pH 7 counted up to 33% (the fraction of 4-CP dissociation at this pH in solution is about 0.3%). The adsorbed 4-CP on the catalyst from a solution at pH 7 existed indeed in its molecular form, as revealed by photosensitization experiments. In an irradiated aqueous suspension of the immobilized catalyst, the 4-CP oxidation was much slower at pH 7 than at pH 12, consistent with the early conclusion that the protonated phenol, for energetic reasons, is less oxidizable than the deprotonated phenol by singlet oxygen.8,9 3. Photosensitization Activity. The catalyst activity was evaluated in an air-saturated aqueous suspension at pH 12. Before initiation of visible light irradiation (λ g 450 nm), the suspension was equilibrated sufficiently in the dark overnight. While no reaction was observed in the presence of bare resin, the photosensitized oxidation of 4-CP occurred efficiently over the resin-supported sensitizer (Figure 3). The concentration of 4-CP in the bulk solution decreased exponentially with irradiation time. Moreover, the catalyst prepared from an alcoholic aqueous solution was more active than the one prepared from an alcohol-free aqueous solution, displaying an apparent firstorder rate constant of 0.0182 and 0.0112 min-1, respectively (note that both of the catalysts contained the same amount of AlPcTS at 1.0 wt %). This difference in the photoactivity was correlated well to the relative monomeric concentration in the solid (see Figure 1). The result demonstrated that the immobilized monomer was also more photoactive than the corresponding aggregates, similar to the case observed in a homogeneous solution, and was attributed to the enhanced self-quenching by the dye aggregation.8,9,14 However, the overall efficiency of the immobilized system was lower than that of the unbound sensitizer. An
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Figure 4. Effect of AlPcTS loading in the catalyst on the apparent rate constant measured from the photosensitized oxidation of 4-CP. The experimental conditions were the same as in Figure 3.
attempt to extract 4-CP off the catalyst by organic solvents failed, due to its strong adsorption, but in the extract the substrate concentration was higher than that in a parallel homogeneous photoreaction (the reaction was performed in the solution containing equal amounts of AlPcTS and 4-CP). The decrease in efficiency could be attributed in part to the diffusion limitation of oxygen into and off the polymer matrix, where it was sensitized and reacted nearby with the target molecules. 4. Effect of the Immobilized Dye Concentration. The photosensitized reaction was expected to be influenced by the amount of the sensitizer immobilized in the catalyst. It was observed that under comparable conditions, the apparent rate constant increased sharply at low loading but then slowly decreased at a high percentage of AlPcTS in the catalyst (Figure 4). This behavior indicated that all light was harvested by the immobilized AlPcTS at 1.0 wt % (corresponding to 5.3 µM of net AlPcTS in solution) and further increase in the dye concentration would lead to partial shielding of the entrance light on the surface. On the other hand, both the monomeric concentration (Figure 1) and the substrate adsorption (Figure 2) decreased slightly as well with the increased percentage of AlPcTS. These two factors put an opposite impact on the reaction. While the dye aggregation led to decreased photosensitization activity, the decrease in the adsorption of 4-CP on the catalyst, as will be shown below, was beneficial to the photosensitized oxidation. We prefer the first explanation, the effect of light absorption. Therefore, the optimal sample of 1.0 wt % was selected for further studies in the following. 5. Effect of the Substrate Adsorption. The initial purpose of this study aimed to improve the photosensitized reaction by concentration of the substrate around the photosensitizer, since singlet oxygen once formed would not go far away from the immobilized photosensitizer. Disappointingly, both the apparent rate constant (kapp) and the initial rate (r), as determined by 4-CP loss in the bulk solution, decreased exponentially with the initial amount of 4-CP adsorption, n2s, and/or the initial equilibrium concentration of 4-CP, C2b, in the bulk solution (Table 1). This behavior was different from the one observed commonly in the TiO2 photocatalysis. Whereas the apparent first-order rate constant, determined also from the bulk solution, decreases with the initial equilibrium concentration, C2b, the initial rate increases, obeying the L-H (Langmuir-Hinshelwood) equation.17 Obviously, the present system of heterogeneous photosensitization did not follow the L-H model. The L-H plot of C2b/r versus
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Table 1. Influence of Initial 4-CP Concentration on the Photosensitized Oxidation of 4-CPa no.
C0 (×10-4 M)
C2b (×10-4 M)
n2s (×10-4 mol g-1)
kapp (×10-2 min-1)
r (×10-6 M min-1)
1 2 3 4 5 6
1.64 2.64 3.28 4.10 4.92 6.56
0.72 1.12 1.53 1.85 2.28 3.40
1.84 2.69 3.49 4.50 5.29 6.31
5.66 2.68 1.36 1.12 0.81 0.52
5.05 2.73 1.49 1.10 0.94 0.57
a Here C , C b, and n s represent the initial concentration, the equilibrium concentration of 4-CP in the bulk solution, and the corresponding 0 2 2 amount of 4-CP adsorption before light irradiation, respectively. kapp and r represent the apparent first-order rate constant and the initial rate, respectively. All the reactions were carried out in a reactor containing 0.025 g of catalyst (1.0 wt %) and 50 mL of 4-CP aqueous solution at pH 12.
Figure 5. Effect of initial H2O2 concentration on the photosensitized oxidation of 4-CP. Conditions: [4-CP]0 ) 0.41 mM; catalyst, 0.50 g/L; pH 12. H2O2 was added after dark adsorption and before irradiation.
C2b, if applied, was highly scattered. And importantly, the adsorption isotherm was not of the Langmuir type as well (Figure 1). It has been shown that the photosensitized degradation of 4-CP in aqueous solution is initiated by singlet oxygen, which is generated from energy transfer reaction between the excited sensitizer and triplet ground oxygen. While the lifetime of singlet oxygen is strongly influenced by solvent properties such as polarity,7 the singlet oxygen would not diffuse too far away from where it was formed in the immobilized catalyst. As a result, the oxidation of 4-CP by singlet oxygen would occur preferably on the catalyst surface. In this regard, the increase in the substrate surface coverage is expected to accelerate the reaction. However, it is mostly likely that the excited AlPcTS is also physically quenched by 4-CP adsorbed nearby, lowering the efficiency of singlet oxygen formation. Nevertheless, the adsorption and/or diffusion of molecular oxygen might decline because of the increased substrate adsorption on the catalyst. 6. Effect of H2O2 Addition. During the photoreaction at pH 12, the catalyst surface was observed to become orange to brown and then return to its original pale blue. No color change was observed in the absence of 4-CP, indicating that the colored intermediates were formed definitely from the photosensitized oxidation of 4-CP. The colored intermediates, once formed, were strongly bound on the catalyst and experienced slow reactions to form other colorless intermediates or products. In the filtrate, no intermediates were found by HPLC, because of the adsorption by the support. But in a corresponding homogeneous reaction, several peaks appeared at a retention time (RT) shorter than 3 min (4-CP at RT ) 12.5 min). The observation appeared consistent with our previous work in a homogeneous solution where 99.7% of chloride ions and 16.6% of carbon dioxide were determined.14
Figure 6. HPLC graphs of the homogeneous solution at different irradiation times (a) in the absence and (b) in the presence of H2O2 (9.7 mM). Conditions: [4-CP]0 ) 0.164 mM, [AlPcTS] ) 5.3 µM, pH 12. The detection wavelength was set at 250 nm.
The colored intermediates could be eliminated easily by addition of hydrogen peroxide. During the reaction in the presence of H2O2, the catalyst was always a pale blue color. Importantly, the reaction became substantially faster, as compared to the one in the absence of H2O2. Moreover, the photosensitized reaction increased with the initial concentration of H2O2 and approached a limit at 5 mM H2O2 present initially in the suspension (Figure 5). No corresponding dark reaction was observed even after 2 h. The enhanced photosensitized oxidation of 4-CP by addition of H2O2 was also observed in a homogeneous reaction. When the solution was irradiated for 20 min in the presence of H2O2 (9.7 mM), the percentage of 4-CP oxidation increased from 60% to 94%. No effect of H2O2 on the AlPcTS spectrum was observed in the visible range. Moreover, the intermediates detected in the irradiated
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Figure 7. The repeated experiments (a) in the absence and (b) in the presence of H2O2. Conditions: [4-CP]0 ) 0.164 mM; [H2O2] ) 3.9 mM; catalyst, 0.50 g/L; pH 12. After 60 min of irradiation for each run, 1 mL of 4-CP stock solution (8.2 mM) and 1 mL of H2O2 stock solution (0.194 M) were added.
solutions displayed a different distribution (Figure 6). Whereas several intermediates were found from the irradiated solution without H2O2 present (Figure 6a), species 7 was the dominant one in the presence of H2O2 (Figure 6b). The HPLC trace changed in some regular way with time, forming the intermediates from species 1 to species 7 (Figure 6a). But these intermediates were degraded quickly by H2O2, resulting in species 7. During this process, H2O2 was consumed gradually (Figure 6b, the trace at 1.5 min), indicating that H2O2 did participate in the intermediate degradation. The structure of species 7 was not identified, but it appeared to be one of the intermediates from benzoquinone dark reaction in an aerated alkaline solution (HPLC graphs changed with time similarly to Figure 6a). Benzoquinone has been identified as the intermediate from the reaction performed at acidic or neutral pH.9,14 By comparison of retention time with the standard, components 1, 2, and 6, numbered in the figure, were assigned to p-benzoquinone, hydroquinone, and fumaric acid, respectively. In addition, the alkaline solution of benzoquinone in the presence of H2O2 was always colorless, indicating that intermediate 7 was a colorless compound. The evidence obtained here suggested that the colored intermediates competed with 4-CP for singlet oxygen, but they could be eliminated quickly by H2O2 to form species 7 as the final product. This was suggested as the reason the heterogeneous reaction was significantly accelerated by H2O2. 7. Recycling Experiments. The stability of the supported catalyst is very important for its application in environmental technology. Figure 7 shows the results from six repeated experiments, carried out under the same conditions but with or without the addition of H2O2. In the absence of H2O2, both the adsorption and the oxidation
Figure 8. The HPLC graphs of the corresponding filtrates in Figure 7 after each run was irradiated for (a) 30 min and (b) 20 min.
of 4-CP decreased cumulatively from the first run to the last (Figure 7a). In the presence of H2O2, however, this was not seen (Figure 7b). Instead, the catalyst remained almost unchanged in its activity for 4-CP adsorption and oxidation, except the last run. The HPLC graphs, shown in Figure 8 only for the last irradiated sample for each run, demonstrated the gradual accumulation of the intermediates in the bulk solution (note that the signal at 0.94 min was some stuff leached off from the resin). The intermediates once formed from 4-CP oxidation were adsorbed on the catalyst, but the number of intermediates and their concentration in the bulk solution increased gradually from the first run to the last, due to the catalyst possessing a limited capacity of adsorption. In the last run, the number of intermediates in the presence of H2O2 (Figure 8b) was much less than that in the corresponding reaction in the absence of H2O2 (Figure 8a), consistent with those exhibited in a homogeneous system (Figure 6). The slightly decreased activity shown in Figure 7b was a reflection that the amount of H2O2 added into the suspension was not sufficient. The repeated experiments demonstrate clearly that addition of H2O2 is an efficient way to improve the photosensitized degradation of 4-CP over the immobilized catalyst. Conclusion The water-soluble sensitizer AlPcTS is immobilized easily and firmly on the anionic resin via electrostatic interactions. Although the activity is lower than that of the unbound sensitizer, the immobilized catalyst is efficient to sensitize the degradation of 4-CP in an alkaline
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aqueous medium under visible light irradiation. If the H2O2 concentration present in the system is sufficient, the catalyst can be used repeatedly with almost unchanged activity. It is highly possible to improve further the catalyst activity by means such as increasing the monomeric concentration of the immobilized sensitizer. A preliminary experiment has shown that this system is also efficient for the photosensitized degradations of 2,4-dichlorophenol and 2,4,6-trichlorophenol. The work has illustrated a
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potential application as a new method for wastewater treatment of chlorophenols by using visible light or solar energy. Acknowledgment. This work was supported by NFSC (No. 299779019, 20273060) and the Zhejiang Provincial Natural Science Foundation of China (No. 299033). LA049710Z
