Anchored Oxygen-Donor Coordination to Iron for Photodegradation of

A photocatalyst of oxygen-donor coordination to iron, complex of 5-sulfosalicylic acid (SSA) with ferric ion, supported on resin to cycle Fe3+/Fe2+ ce...
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Environ. Sci. Technol. 2007, 41, 5103-5107

Anchored Oxygen-Donor Coordination to Iron for Photodegradation of Organic Pollutants HONGWEI JI, WENJING SONG, CHUNCHENG CHEN, HONG YUAN, WANHONG MA,* AND JINCAI ZHAO* Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, The Chinese Academy of Science, Beijing 100080, China

A photocatalyst of oxygen-donor coordination to iron, complex of 5-sulfosalicylic acid (SSA) with ferric ion, supported on resin to cycle Fe3+/Fe2+ center under visible irradiation can effectively generate •OH radicals from H2O2, leading to degradation of organic pollutants in water. The higher turnover number was achieved by this catalyst for the degradation of model compound than those reported for the general N-donor ligands catalysts. The reversible “on/ off” switching of Fe3+/Fe2+ complexation with SSA, coupled with the phenol/phenoxyl radical conversion of the o-phenoxyl moiety of SSA, produces an ideal catalytic system that separates the Fenton reaction and the followed oxidations by •OH radicals (in water phase) from the regeneration of the catalytic species, Fe (SSA)2-, which occurs on the surface of resin. This system not only inhibits the undesired destruction of the ligands by •OH radicals, improving the stability of the catalyst, but also avoids the unnecessary decomposition of H2O2 into HO2• that occurs in the homogeneous Fenton system. Therefore, the system suggests an efficient utilization of H2O2 for degradation of organic pollutants.

Experimental Section

Introduction As one of the most powerful oxidants, •OH radicals have been widely used to degrade and mineralize the nonbiodegradable pollutants (1-3). Though production of •OH radicals is feasibly from decomposition of H2O2 using Fenton reactions, free iron ions usually lose their activity at high pHs or in the later stage of reactions due to the irreversible interaction with the degradation intermediates (4). It has been reported that complexation of iron with quadridentate N4-donor ligands, an analogue of cytochrome P-450, provides effective catalysts for selective oxidation of hydrocarbons and degradation of organic pollutants in the presence of H2O2 (5, 6). However, such catalysts deteriorate easily with oxidative destruction of the ligands or irreclaimable demetalation of the iron center (7-10), which are due to the rigid coordination structure. In contrast, native enzymes always work in the most efficient way in cooperating with a series of intermolecular or intramolecular interactions such as weak coordination, hydrogen bonding, and van der Waals interaction (11-14). For instance, in addition to the rigid * Address correspondence to either author. [email protected] (J. Z.); [email protected] (W. M.). 10.1021/es070021u CCC: $37.00 Published on Web 06/13/2007

coordination with N-donor ligands (2-histidine), most mononuclear iron enzymes also involve flexible coordination with an O-donor ligand (1-carboxylate ligand). This flexible coordination enables exquisite translocation of metal center between two phases or different coordination environments, thus preventing organic ligands from attack by active species (15, 16). Inspired by the precious nature of iron enzymes, here we report a new photocatalyst (1, Figure 1) that takes advantages of flexible and reversible complexation of Fe2+/Fe3+ with an Oxygen-donor ligand, 5-sulfosalicylic acid (SSA). When supported on resin, such photocatalyst can effectively degrade organic pollutants in water in the presence of H2O2 under visible irradiation. We select SSA as the ligand because it possesses two unique characteristics: first, the complex of Fe3+ with SSA has strong absorption in visible region due to the ligand-to-metal charge-transfer transition, assuring potential utilization of the visible light of solar irradiation(17); second, SSA has an o-hydroxyl arm of HQ-like structure, which can effectively facilitate the Fe3+/Fe2+ cycling through the reversible conversion of phenol/phenoxyl radical(2, 3, 18). Consequently, the reversible “on/off” switching of Fe3+/ Fe2+ complexation with SSA, coupled with the phenol/ phenoxyl radical conversion of the o-phenoxyl moiety of SSA, produces an ideal catalytic system that separates the Fenton reaction and the followed oxidations by •OH radicals (in water phase) from the regeneration of the catalytic species, Fe(SSA)2- complex, which occurs on the surface of resin. Such a system inhibits the potential destruction of the ligands by •OH radicals, and thus improves the stability of the catalyst. For comparison, two other ligands, 3-sulfobenzoic acid (SA) and 4-sulfophthalic acid (SPA), which are in similar structure as SSA but without phenol structure, were also employed to prepare the photocatalysts (2 and 3, Figure 1) to compare the properties for photodegradation. As expected, we found that only partial degradation of model substrates was observed when 2 and 3 were used as catalysts under the otherwise identical conditions, and these two catalysts lost their activities remarkably, even after the first cycle, which further implies that the existence of the HQ-like structure in Fe(SSA)2- is indispensable in the catalytic cycle.

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Materials. The Amberlite resin IRA900 was purchased from Aldrich Chemical Co. Horseradish peroxidase (POD), which was used for the measurement of H2O2, was purchased from the Huamei Biologic Engineering Co. (China), whereas the N, N-dialkyl-p-phenylenediamine (DPD) reagent was from Merck (p.a.). The spin trap reagent 5, 5-dimethyl-1-pyrrolineN-oxide (DMPO) was purchased from the Sigma Chemical Co. Orange II and other chemicals were all of analytic grade and were used without further purification. Deionized and doubly distilled water was used throughout this study. The pH of the solutions was adjusted by dilute aqueous solutions of either NaOH or HClO4. Preparation of the Photocatalyst. Amberlite IRA 900 resin was pulverized and sieved (200 meshes), followed by repeated rinsing using alcohol, HCl solution, NaOH solution, and water, to remove the impurities. Catalyst 1 was prepared as follows: 5 mL of Fe(ClO4)3 solution (0.01 mol/L) was mixed with 10 mL of 5-sulfosalicylic acid (SSA) solution (0.01 mol/ L), followed by dilution with water to 100 mL and pH adjustment to 5.0. A certain amount of pretreated resin was added to the solution, leading to a loading ratio of 0.5 mmol Fe (III)/g dry resin. After 24 h of mild stirring, the loading process (ion exchange) was completed as verified by deterVOL. 41, NO. 14, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Sketch map of the structures of the three photocatalysts

FIGURE 2. Diffusion reflectance UV/vis spectra for the anchored 1, 2, 3, and blank Amberlite resin. mining the remaining Fe (III) and SSA in the solution using spectrophotometry. The resin particles obtained were filtered out and repeatedly washed with water, followed by drying at room temperature and then at 50 °C for 24 h. The photocatalyst 1 thus prepared was a red color powder. Catalysts 2 and 3 were prepared following the same procedure as used for 1. The catalyst 1 was found to have strong absorption in the visible region (λmax ) 427 nm,  ) 1.64 × 103 M-1cm-1) as compared with the other catalysts 2, 3 and the blank resin (Figure 2) due to the ligand-to-metal chargetransfer transition of the Fe3+-SSA complex, and this assures the potential utilization of the visible light of solar irradiation. Photodegradation Processes. A 500 W halogen lamp as a visible light source was positioned inside a cylindrical Pyrex vessel surrounded by a Pyrex glass jacket with circulating water to cool the lamp. A cutoff filter (5 × 5 cm) was used to ensure irradiation only by visible light (λ > 420 nm). The distance between the reaction vessel and light source was adjusted to about 3 cm. Unless otherwise noted, all the experiments were carried out in aqueous solutions (pH ) 3) in a 60 mL Pyrex vessel. 3 mL samples were collected at given irradiation time intervals and analyzed immediately by using a Hitachi 3010 UV/vis spectrophotometer. UV-vis diffuse reflectance spectra of the blank resin, 1, 2, and 3 were recorded using the same spectrophotometer equipped with an integraph (Φ 150 mm). X-ray photoelectron spectroscopy (XPS) was carried out with an ESCALAB 2202-XL spectrometer. The photodegradation of DCP was analyzed by HPLC on an ODS-3 5 µm column (250 × 4.6 mm) with an eluent of methanol/water (70/30% v/v, 1.0 mL/min). Intermediates were identified by comparing the standard samples, and further analyzed by GC-MS according to a previously established procedure (19). The amount of Cl- ion was analyzed by a DX-120 ion chromatograph (DIONEX) using Na2CO3 (1.8 mM) and NaHCO3 (1.7 mM) solution as eluent. An Apollo 9000 TOC was used for measuring the total organic carbon (TOC) values of the 5104

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FIGURE 3. Degradation profile of Orange II (0.4 mM, 50 mL) in the presence of H2O2 (2 mM) and different photocatalysts (2 mg): (a) dark reaction in the presence of 1, (b) visible irradiation but no catalyst, (c) visible irradiation in the presence of 2, (d) visible irradiation in the presence of 3, and (e) visible irradiation in the presence of 1. Insert: Stability of catalyst 1 in repeated photodegradation reactions of Orange II (0.1 mM, 100 mL/run) in the presence of H2O2 (2 mM/run) and 1 (2 mg). degraded solutions. The concentration of H2O2 was measured by the previously established method (20). A Brucker EPR spectrometer (model E500) equipped with a Quanta-Ray Nd: YAG laser (355 and 532 nm) was used for measuring the electron paramagnetic resonance (EPR) signals of free radicals and those spin-trapped by DMPO. Quartz EPR tubes of Φ ) 4 mm were used, and DPPH (2, 2-diphenyl-1-picrylhydrazyl) was used as a reference (g ) 2.0037). To minimize experimental errors, the same quartz capillary tube was used for all EPR measurements. The temperature was controlled with a standard temperature accessory and was monitored before and after each measurement.

Results and Discussion Photodegradation of Orange II. Figure 3 shows the photodegradation profile of Orange II, a widely used azo dye, in an aqueous solution in the presence of 1 and H2O2 under visible irradiation (λ > 420 nm). In controlled experiments, little degradation occurred in the presence of H2O2 alone under visible irradiation (curve b) or in the presence of both H2O2 and catalyst 1 but without irradiation (curve a). Effective degradation of Orange II with both H2O2 and 1 was observed under visible irradiation (curve e). A high turn-over number (TON) of 1840 was achieved for the degradation of Orange II in 20 h of irradiation. The TON obtained here was much higher than those reported for the iron complexes of N-donor ligands (21). In contrast, only partial degradation of Orange II was observed when catalysts 2 and 3 were used under the otherwise identical conditions (Figure 3, curves c and d). The activity of both the 2 and 3 lost significantly even after the first cycle. Moreover, no significant loss of the activity of 1 was observed even after seven consecutive runs of photodegradation (Inset of Figure 3), meanwhile neither the iron ions Fe(III/II) nor SSA ligands were detected in the bulk solution. The photodegradation mediated by the Fe(III)-SSA complex was also carried out in homogeneous solutions under the otherwise identical conditions. It was found that both the substrates and SSA ligands underwent degradation. The red color of the Fe(III)-SSA complex solution disappeared irreversibly during the photoreaction process, consistent with the general mechanism for homogeneous photoFenton reactions. In contrast, the red color of the anchored Fe(SSA)2- complex (catalyst 1) remained unchanged throughout the photodegradation process, and XPS measurements also showed that the oxidation state of iron in the complex matrix adsorbed on the resin persists as Fe(III) before and after use in the photocatalytic reaction (Figure 4). The signal intensity for both Fe and S was decreased after photoreaction, likely due to adsorption of substrate and degradation

FIGURE 4. XPS spectra of 1 before and after use in photodegradation of DCP under visible irradiation: (a) for Fe (III) and (b) for S (VI).

FIGURE 6. DMPO-trapping ESR spectra of •OH radicals in the photodegradation of 2, 4-DCP by H2O2/1. The ESR parameters were the same as used in ref 25.

SCHEME 1. Redox Cycle for Phenol/phenoxyl Radical Coupled with the Cycle of Fe2+/Fe3+.

FIGURE 5. HPLC distribution of intermediates produced in the photodegradation of 2, 4-DCP (DCP: 0.2 mM, H2O2: 2 mM, Catalyst 1: 2 mg, in 50 mL solution).

TABLE 1. 2, 4-DCP and the Intermediates Produced in the Photodegradation of 2, 4-DCP

intermediates on the surface of the catalyst. These results suggest that catalyst 1 can be potentially developed as an effective, stable photocatalyst for degradation of organic pollutants. Photodegradation of 2, 4-Dichlorophenol. For better understanding of the photoreaction mechanism, the photodegradation of colorless compounds such as 2, 4-dichlorophenol (DCP) and 2, 4, 6-trichlorophenol (TCP) was also carried out under the otherwise identical conditions. These two compounds represent a typical class of the organic pollutants found in aqueous systems. About 75% dechloration and 25% TOC removal were obtained for DCP after 1 h of photoreaction under visible irradiation (Figure 5, Table 1). Each intermediate (b, c, d, or e) was identified by GC-MS using a previously established methods (19). All these intermediates were degraded and finally disappeared with further irradiation. Unlike other photodegradation systems reported for chlorophenols, toxic chlorinated dibenzodioxins and dibenzofurans were not detected in the present system. Since DCP itself does not have absorption in visible region, the observed photocatalysis can be attributed to the visible light response of photocatalyst 1. Similar catalytic activity of 1 was also observed for the photodegradation of TCP. Discussion on Mechanism: Scheme 1 illustrates the possible mechanism of photocatalysis by 1. Photoexcitation of the ligand-to-metal-charge-transfer (LMCT) band of Fe (SSA)2- complex leads to efficient intramolecular electron

transfer, producing a phenoxyl radical of SSA and a Fe2+ ion. The latter will be released from the chelator due to the weak complexation of oxidized SSA. The released Fe2+ ions will rapidly react with H2O2 to yield •OH radicals, which are responsible for the subsequent degradation of the substrate compounds. The formation of •OH radical was confirmed by the spin-trapping ESR measurements of DMPO-•OH radicals under the visible light irradiation (a 1:2:2:1 quartet pattern, aN ) aH ) 14.9 G (22)) (Figure 6). Due to the separated twophase traction system, the •OH radicals preferably attack the molecules in the solution rather than the SSA ligands, strongly bound to the resin surface. Such a reaction system protects the chelating ligands from oxidative destruction by •OH radicals. Some similar release/capture switchable systems with iron ions have also been reported for the Ferritin-like system using phenylazomethine dendrimer on electrode (23). The generation of released Fe2+ ions by such an intramolecular electron transfer was supported by the fact that the photodegradation of substrates was significantly suppressed upon addition of R, R-bipyridine, an efficient chelator for Fe2+. Concurrent with the generation of Fe2+ ions, the production of phenoxyl radicals (g ) 2.003) was observed by direct ESR measurement of the powder of 1 at room temperature (Figure. 7a). However, almost no signal could be detected for 2, 3, Fe3+-SSA complex powder or the SSAbound resin. From the control experiments on photolysis of Fe(III)SSA complexes and the catalyst 1 alone, respectively, we observed the formation of Fe(II) in our catalytic system under the visible light irradiation, while no Fe(II) was detected during irradiation of Fe(III)-SSA complexes in homogeneous solution. It is another direct support of mechanism presented by Scheme 1. The rapid reaction of Fe2+ with H2O2 will regenerate Fe3+ and thus recover the catalyst. Efficient recapturing of Fe3+ ions by chelation with SSA on resin is crucial not only for facilitating the recovery of catalyst, but also for preventing the undesired decomposition of H2O2 by Fe3+ (via HaberWeiss reaction(2)), which leads to formation of HO2•, rather VOL. 41, NO. 14, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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estimate absorbance spectra of this free radical, which showed significant absorbance in 420∼460 nm (λmax ) 430 nm). Therefore, the phenoxyl radicals can easily be excited under visible light irradiation, and the photoexcited state of phenoxyl radicals (at higher energy level) turned out to be more favorable to facilitate the redox reaction between phenoxyl radicals and substrates (28). Thus, we propose that visible light irradiation as driving force and the substrates as electron donor promote the catalytic cycles. FIGURE 7. (a) X-band ESR spectra for as-prepared photocatalysts (1, 2, 3), Fe3+-SSA complex and SSA-resin. Frequency ) 9.611GHz, center field ) 3432 G, modulation ) 100 kHz, powder ) 6.34 mW. (b) X-band ESR spectra of photocatalyst 1 powders without Orange II (curve A), after adsorption of Orange II (curve B), ESR parameters were the same as in (a).

Acknowledgments This work was supported financially by 973 project (no. 2003CB415006), NSFC (nos. 20537010, 20520120221 and 50436040) and CAS.

Literature Cited

FIGURE 8. (a) DMPO-trapping ESR spectra of •OOH radicals in the photodegradation of 2,4-DCP in the presence of H2O2 and 1. (b) DMPO-trapping ESR spectra of •OOH radicals in the photodegradation of 2,4-DCP in the presence of H2O2 and SSA-Fe3+ complex in a homogeneous solution under otherwise identical conditions as (a). The ESR parameters for (a) and (b) were the same as used in ref (25). than •OH radical. Indeed, DMPO-HO2• adduct was not detected by ESR in the system of 1, whereas strong ESR signals of DMPO-HO2• were detected in the homogeneous aqueous system of Fe3+ or Fe3+-SSA complex (Figure 8). No formation of HO2• radical suggests efficient utilization of H2O2 in the catalytic system of 1. Moreover, no obviously consumption of H2O2 was observed in the absence of substrate or after the substrate was completely decomposed. This is consistent with our previous observation that anchored Fe3+ can hardly decompose H2O2 into HO2• and O2 due to the more negative redox potential of the bound Fe3+ (compared with the free Fe3+) (24, 25). Phenol/phenoxyl radical conversion of our catalytic cycle is very similar to the reported hydroquinone/semiquinone (HQ/SQ) conversion, and both of these two systems can accelerate the Fe(III)/Fe(II) cycling (2, 3, 18). Reduction of SQ back to HQ is usually due to either rapid disproportionation or by accepting electron from substrate compounds (26-31). In our system, the SSA ligands are immobilized on the surface of resin, and the formed phenoxyl radicals are separated from each other, preventing the disproportionation of itself. Indeed, significant ESR signals for phenoxyl radicals (g ) 2.003) were observed with catalyst 1 in the absence of substrate (Figure 7a). Subsequently, the ESR signals for phenoxyl radicals observed with 1 were drastically decreased upon adsorption of Orange II onto the catalyst (Figure 7b). This observation suggests an efficient redox reaction between phenoxyl radicals and substrates, which leads to recovery of the phenol structure of SSA. On the other hand, visible light irradiation plays an important role in the process of phenoxyl radicals back to phenol. It was reported that these kinds of radicals showed significant absorbance in visible region extending to 500 nm (λmax∼430 nm) (4, 32, 33). We used TD-DFT theory (the calculations for both geometry optimization and excitation energy were performed at UB3LYP/ 6-311++G(d) level by Gaussian 03 program package) to 5106

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Received for review January 4, 2007. Accepted May 10, 2007. ES070021U

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