Efficient Photodegradation of Phenanthrene under Visible Light

Feb 22, 2008 - C. F.; Freed, J. R.; Jennings, P.; Durfee, R. L.; Whitmore, F. C.; Maestri,. B.; Mabey, W. R.; Holt, B. R.; Gould, C. U.S. Environmenta...
0 downloads 0 Views 136KB Size
Download PDF
J. Phys. Chem. C 2008, 112, 4291-4296

4291

Efficient Photodegradation of Phenanthrene under Visible Light Irradiation via Photosensitized Electron Transfer Jiahui Kou,†,‡,§ Haitao Zhang,†,‡,§ Yupeng Yuan,†,‡,§ Zhaosheng Li,†,‡,§ Ying Wang,†,‡,§ Tao Yu,†,§ and Zhigang Zou*,†,‡,§ Eco-Materials and Renewable Energy Research Center (ERERC), Department of Materials Science and Engineering, and National Laboratory of Solid State Microstructures, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: NoVember 21, 2007

Photoinduced electron transfer between transition metal ions and polycyclic aromatic hydrocarbons (PAHs) was utilized to induce the photodegradation of phenanthrene (PHE), a tricyclic aromatic hydrocarbon, under visible light irradiation (λ > 420 nm). Experimental results indicate that Fe3+ ions possess excellent ability of sensitization in the photodegradation of PHE. In solution, PHE could be oxidated completely by oxygen after 3 h irradiation. The intermediates of PHE photodegradation were detected by gas chromatographymass spectrometer. Eleven kinds of intermediates were determined, including one decomposition product, five ring-opening products, and other oxidation derivatives. Phenanthrenequinone and (1,1′-biphenyl)-2,2′dicarboxaldehyde were the two main products. Based on the experiment results, a possible mechanism was proposed. In alcohols and acetic acid, the photodegradation efficiency of PHE was low, probably because that active PHE•+ was quenched in these solvents. The impacts of variable factors including solvent component, irradiation wavelength, presence or lack of oxygen, and reaction time on the photodegradation of PHE were investigated in detail. In addition, the geometry optimization of the complex of PHE and Fe3+ ions and the calculation of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap were carried out using the B3LYP functional and 6-31 G* basis set in the Gaussian 98 program.

Introduction Polycyclic aromatic hydrocarbons (PAHs) and their derivatives are mostly produced from the incomplete combustion of some organic compounds, which occurs in many anthropogenic activities. Many of them not only have high carcinogenecity and mutagenicity for living organisms even in trace amount, but also have high stability and environmental persistence.1,2 Sixteen PAHs were therefore listed in the U.S. EPA priority pollutants.3 In recent years, the intensive industrial development aggravated the harm of PAHs to human health and the reproduction of aquatic animals.4-7 As a result, studies on the remediation of PAHs wastes are important and attractive. PAHs can transform into unsteady excited PAHs molecule (PAHs*) by absorbing UV or visible light. PAHs* easily undergo oxidation reactions, which lead to photodegradation of PAHs. Therefore, photodestruction of PAHs plays an important role in the environmental preservation.8-10 Yet most PAHs absorb only UV light and so can only utilize 4-5% of the solar energy. Photosensitization is an effective way for photoreaction to utilize the wide part of the solar spectrum.11-14 However, such reports in the field of PAHs photodegradation are scarce, and almost all of the photosensitizer are organic compounds.15,16 Nevertheless, organic compounds are not stable enough under light irradiation that they are easy to decompose * Corresponding author. Tel.: +86-25-83686304, +86-25-83686630. Fax: +86-25-83686632. E-mail: [email protected]. † ERERC. ‡ Department of Materials Science and Engineering. § National Laboratory of Solid State Microstructures.

in the process of photosensitization of PAHs. It is then valuable to develop stable photosensitization method of PAHs photodegradation. Photoinduced electron-transfer reaction is one of the most elementary chemical processes and plays important roles in many photosensitization phenomena.16-19 Radical cations formed by photoinduced electron transfer are believed to be reactive and important intermediates.20-24 Kotani et al.16 have shown the photooxidation of anthracene and olefins under visible light irradiation via electron-transfer photocatalyst. It is therefore very important for the photosensitizer to produce electron transfer between PAHs and them. Transition metal ions often quench the excited singlet and triplet states of aromatic hydrocarbons by forming coordination complexes.25-31 The photoinduced electron transfer or electronic energy transfer has been considered to occur in these coordination complexes under light irradiation. On the other hand, many kinds of transition metal ions can absorb visible light. So we consider that transition metal ions have the potential to sensitize the photodegradation of PAHs under visible light irradiation. However, to our knowledge, little work has been carried out to investigate the photosensitization ability of transition metal ions on PAHs degradation. Herein, we are interested in the influence of transition metal ions on the phenanthrene (PHE) under visible light irradiation. PHE is one of the tricyclic PAHs with high stability and long half-life of self-degradation32 under light irradiation in organic solvent or in aqueous solutions. The effects of variable factors including solvent species, irradiation wavelength, reaction time, presence or absence of oxygen, and transition metal ions on the photodestruction of PHE were examined in detail. Intermediates of PHE photodegradation were analyzed by gas chro-

10.1021/jp7111022 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/22/2008

4292 J. Phys. Chem. C, Vol. 112, No. 11, 2008

Kou et al.

SCHEME 1: Photooxidation Pathway of PHE in the Solution Containing Fe3+ Ions

matography-mass spectrometer (GC-MS), and the photodegradation mechanism was also discussed. The geometry optimization of the complex of PHE and Fe3+ ions and the calculation of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap were carried out using the B3LYP functional and 6-31 G* basis set in the Gaussian 98 program. Experimental Section Ultraviolet-visible absorption spectra of solvents were measured on a UV-visible spectrometer (U.S., Varian, Cary 50 Probe). Fluorescence spectra were detected by a spectrofluorometer (U.S., Varian, Cary Eclipse), and the excitation length was 344 nm. Electron paramagnetic resonance (EPR) experiment signals of radicals trapped by 5,5-dimethyl-1pyrroline-N-oxide (DMPO) were performed at room temperature on a Bruker EPR 10/12 spectrometer. The same quartz capillary tube was used for all of the measurements to minimize errors. The EPR instrument was operated with the following parameters: microwave frequency 9.76 GHz, microwave power 0.02 W, and modulation frequency 100 kHz. Products were quantitative analyzed by GC. The GC was equipped with flame ionization detection (GC/FID). It has split/splitless injector, and the injection mode is split injection with the split ratio of 16.5: 1. An HP-5 column was used for separation (30 m, 0.25 mm i.d., 0.25 µm film thickness). The temperature program started at 60 °C and was held for 5 min with the split ratio of 16.5:1. The oven was heated to 280 °C at a rate of 15 °C min-1, and the temperature was maintained for 5 min. The solvent delay time was set to 4.3 min. All quantifications and calculations were based on an external standard and the use of calibration curve. The intermediates were identified using GC-MS by comparing the mass spectra with data in the NIST 02 library. Mass spectra were recorded at 1 scan s-1 under electron impact at 70 eV, mass range 30-350 amu. The irradiation system

consisted of a 300 W Xe arc lamp, a cutoff filter, and a water cooler trough (preventing the thermal catalytic effect) that was used to locate the reaction cell. Irradiation experiments were carried out for 1.68 × 10-5 mol of PHE and 1.20 × 10-4 mol of transition metal salts (FeCl3, CuCl2, MnCl2, Cr(NO3)3, NiCl2, ZnCl2, CoCl2) dissolved in 60 mL of solution. The glass reactor with reaction solution was immersed in an ice-water mixture, and then was irradiated with light from a 300 W xenon arc lamp through a cutoff filter. After reaction, the reaction mixture was extracted by dichloromethane, and the dichloromethane layer was dried by anhydrous sodium sulfate for GC and GC-MS analysis. If no special explanation was presented, irradiation wavelength was longer than 420 nm; irradiation time was 4 h, and the solvent was the air-saturated mixture of 40 mL of acetone and 20 mL of H2O; the Fe3+ ions came from FeCl3. To clarify the valence of Fe after irradiation reaction, Fe2+ ions were determined by a colorimetric method using o-phenanthroline as a reagent. To examine the role of O2, experiments were carried out in a vacuum system to ensure the absence of O2, and free-radical in system of abounding with O2 was determined by the EPR DMPO spin-trap technique under visible light irradiation. The fluorescence of PHE in mixture solvent (acetone:H2O ) 2:1) was detected in the presence of Fe3+ ions with excitation wavelength of 344 nm. The concentration of PHE was fixed at 2.80 × 10-4 mol L-1, and the ratio of Fe3+ ions to PHE changed from 1:6 to 9:1. The cumulative time of PHE exposure to UV radiation was less than 5 min for each fluorescent measurement; no PHE self-photodegradation was observed with controls for these conditions. In the investigation of cutoff wavelength dependence of PHE degradation, the cutoff filters of 420, 440, 480, and 520 nm were used. In the study of time courses, the irradiation time transferred from 1 to 6 h. In experiments involving the effect of solvent component on the degradation of PHE, 40 mL of methanol, ethanol, isopropanol, acetonitrile, acetone, butanone, and acetic acid were used to

Efficient Photodegradation of Phenanthrene

Figure 1. Time courses for the degradation of PHE under visible light irradiation. Reaction conditions: λ > 420 nm, solvent was 40 mL of acetone and 20 mL of H2O, the concentration of FeCl3 was 2.00 × 10-3 mol L-1, and the concentration of PHE was 2.80 × 10-4 mol L-1.

mix with 20 mL of water, respectively. The geometry optimization of the complex of PHE and Fe3+ ions and the calculation of the HOMO-LUMO gap were carried out using the B3LYP functional and 6-31 G* basis set33-36 in the Gaussian 98 program.37 Results and Discussion In the Fe3+-acetone-water system, 11 products were identified by GC-MS, as listed in Scheme 1. The total ion chromatograms and the mass spectra of products are shown in Figures S1 and S2, respectively. All products (1-11) are decomposition products. Excluding product 11, all others are oxidation products. In addition, the quantitative analysis results indicate that compounds 8 and 9 were the most abundant among the products. However, no product was determined in the systems containing Cu2+, Co2+, Ni2+, Zn2+, Mn2+, and Cr3+ ions under visible light irradiation in solvent of acetone-water, which was reasonably attributed to the weak optical absorption of such ions, as can be seen from the Supporting Information (Figure S3). Consequently, we wish to focus attention particularly on the effect of Fe3+ ions in the subsequent experiments. Figure 1 displays time courses for the concentration of PHE and the yield of product 8 and 9 in the presence of Fe3+ ions. It can be found that the conversion of PHE increased with the prolonging of reaction time, and PHE was decomposed completely after 3 h reaction. According to the GC-MS analysis, the photodegradation products of PHE changed at different intervals. Only the products of 3, 8, and 11 were detected after 1 h irradiation, while a new compound 9 was produced after 2 h illumination. Furthermore, all of the intermediates (1-11) appeared in the reaction solution when the irradiation was conducted in the range from 3 to 6 h. The yields of product 8 and 9 were enhanced in the time range of 1-3 h, and they reached the maximum of 55.6% and 28.6% after 3 h irradiation, respectively. Further prolonging reaction time, the yield of compounds 8 and 9 decreased as a result of further degradation. According to the yield of these intermediates, the ring-opening product 8 was easier to degrade than product 9 in this system. The experiment without Fe3+ ions was carried out to clarify the role of Fe3+ ions on the photodegradation of PHE. After 6 h reaction, PHE was not degraded at all under visible light irradiation. For the investigation of the role of anion, Fe2(SO4)3 solution with the identical Fe3+ ions concentration was employed for the degradation of PHE. Results similar to those of FeCl3 solution were obtained, indicating that the degradation of PHE was independent of anion. In addition, the valence state of iron was also analyzed after reaction by a colorimetric method using o-phenanthroline as a reagent. The absorbance of the Fe2+-

J. Phys. Chem. C, Vol. 112, No. 11, 2008 4293

Figure 2. Fluorescence spectra of PHE in solution (acetone:water ) 2:1) in the presence of different concentration of Fe3+ ions. The concentration of PHE was 2.80 × 10-4 mol L-1. λex ) 344 nm. Inset: fluorescene intensity at λmax(em) as a function of Fe3+ ions concentration.

phenanthroline was not detected in the reaction residua. The result suggests that all of the ferric ions still existed in the form of trivalence. It can be concluded from the results mentioned above that Fe3+ ions were indispensable in the photodegradation of PHE and it played a catalytic role in this process. The dependence of PHE photodegradation on oxygen was also examined. It is worthwhile noting that no product except a trace amount of 11 was detected in the oxygen-free solution, suggesting that oxygen was crucial in the photodegradation of PHE. Free radical was determined by EPR to confirm the form of oxygen in photooxidation under visible light irradiation. However, the signal of the DMPO-oxygen-free-radical adduct was not detected, implying that oxygen did not exist in the form of radical in the experiments. As a result, the photooxidation of PHE was not due to the existence of strong oxidant OH•, and oxygen probably participated in the degradation of PHE in the form of molecule oxygen. To investigate the complexation between Fe3+ ions and PHE, response of PHE fluorescence toward Fe3+ ions was detected (Figure 2). A characteristic fluorescence emission maximum center was at about 380 nm. With the increment of Fe3+ ions concentration, the fluorescence intensity decreased linearly with the amount of Fe3+ ions when the molar ratio of Fe3+ ions to PHE was in the range of 1:6 to 1:2, as the inset of Figure 2 displays. According to the linearly dependent coefficient, the fluorescence of PHE in the mixture solvent (acetone:water ) 2:1) should be quenched completely when the molar ratio of PHE to Fe3+ ions was 1:1. However, the fluorescence peaks of PHE actually disappeared entirely when the molar ratio of Fe3+ ions to PHE was beyond 6:1, because the complex reaction was reversible. The quenching phenomenon by Fe3+ ions can be ascribed to the relaxation of the excited states to the ground states of the separated molecules, through photoinduced electron transfer between d orbitals of Fe3+ ions and π* orbitals of PHE, as well as energy transfer within the Fe3+‚‚‚PHE complex.25,26,30 Photoinduced electron transfer and electronic energy transfer were considered as the two main deactivation pathways responsible for efficient fluorescent inhibition. The optimized structure of complex of PHE and Fe3+ is displayed in Figure 3, and the coordinates of atoms are shown in Table S1. Fe3+ ion is situated at the center of two PHE molecules, which is similar to the structure of ferrocene. However, the molecules of PHE are not parallel to each other but form an angle. The calculated HOMO-LUMO gap of PHE in the presence of Fe3+ ions is 1.91 eV, while the value is 7.96 eV without Fe3+ ions. The HOMO-LUMO gap is generally used as a direct indicator of kinetic stability.38-44 A compound

4294 J. Phys. Chem. C, Vol. 112, No. 11, 2008

Kou et al.

Figure 5. Effect of irradiation wavelength on the photodegradation of PHE. Reaction conditions: solvent was 40 mL of acetone and 20 mL of H2O, the concentration of FeCl3 was 2.00 × 10-3 mol L-1, the concentration of PHE was 2.80 × 10-4 mol L-1, and reaction time was 4 h. Inset: UV-vis absorption spectra of the reaction mixture. Figure 3. The structure of PHE-Fe3+ optimized using Gaussian 98 program: (1) and (2) represent the structure observed from different angles.

Figure 4. The effect of Fe3+ ions amount on PHE photodegradation. Reaction conditions: λ > 420 nm, solvent was 40 mL of acetone and 20 mL of H2O, the concentration of PHE was 2.80 × 10-4 mol L-1, and reaction time was 4 h.

with a small HOMO-LUMO gap is always kinetically unstable and has high chemical reactive because electrons are energetically favorable to add to a high-lying LUMO or to extract electrons from a low-lying HOMO.38-40 The HOMO-LUMO gap was pointed out to represent the chemical hardness of a molecule.39,40,43 Therefore, the small HOMO-LUMO gap of PHE in the presence of Fe3+ ions should enhance the activation of the photodegradation of PHE. The calculated results are in agreement with the experimental results. The effect of Fe3+ ions dose on PHE photodegradation was studied, and the results are given in Figure 4. When the concentration of Fe3+ ions was 2.80 × 10-4 mol L-1, PHE did not degrade after 4 h irradiation. Also, the conversion of PHE was 8.7% as the concentration of Fe3+ ions was 5.60 × 10-4 mol L-1. With the increase of Fe3+ ions amount, degradation efficiency of PHE was enhanced. When the concentration of Fe3+ ions was higher than 1.68 × 10-3 mol L-1, PHE could be completely degraded at 4 h. The yields of compounds 8 and 9 were enhanced when the concentration of Fe3+ ions was in the range from 0 to 1.12 × 10-3 mol L-1. However, the yields of compounds 8 and 9 decreased with addition of Fe3+ ions amount when the concentration of Fe3+ ions was more than 1.12 × 10-3 mol L-1, which was attributed to the rapid degradation rate of compounds 8 and 9. As the Supporting Information (Figure S4) displays, the visible light absorption of Fe3+-acetone-water system was enhanced with the increment of Fe3+ ions, which possibly was the reason for the enhancement of PHE degradation efficiency with increase of Fe3+ ions dose. Figure 5 illustrates the wavelength dependence of the photodegradation of PHE in the system of Fe3+-water-acetone,

TABLE 1: Conversion of PHE in Different Organic-Aqueous Solvents and the Polarity of the Corresponding Organic Solventsa organic solvent

polarity

conversion of PHE (%)

ethanol isopropanol butanone methanol acetone acetonitrile acetic acid

4.3 4.3 4.7 5.1 5.1 5.8 6.0

0 0 7.9 0 100 100 33.4

a Reaction conditions: λ > 420 nm, solvent was 40 mL of organic solvent and 20 mL of H2O, reaction time was 4 h, the concentration of FeCl3 was 2.00 × 10-3 mol L-1, and the concentration of PHE was 2.80 × 10-4 mol L-1.

which was generally to prove if a reaction was driven by light. When the cutoff wavelength was 420 nm, all of the PHE was converted. With the increase of cutoff wavelength, the conversion of PHE decreased. This tendency in the photodegradation of PHE to the wavelength of irradiation light is well in agreement with the UV-vis absorption of reaction mixture, which declined also with incident light wavelength in the range from 420 to 520 nm, as shown in the inset of Figure 5. The yield maximum of products 8 and 9 was at 440 nm, and was 40.3% and 35.6%, respectively. The low yields of compounds 8 and 9 when the cutoff filter of 420 nm was used denoted the further degradation of them. However, the low conversion of PHE should be responsible for the low yield of compounds 8 and 9 when the irradiation wavelength was longer than 480 nm. Solvation effects, which are important for the interaction between PAHs and metal ions, should be taken into account in the photoreaction. Table 1 lists the conversions of PHE in different organic-aqueous solvent, including the polarity of corresponding pure organic solvents. All of the conversions of PHE photodegradation were 100% in the solvent of acetonewater and acetonitrile-water. The conversion of PHE in the mixture solvent of butanone-water and acetic acid-water was 7.9% and 33.4%, respectively. Intermediates in butanone-water, acetonitrile-water, and acetic acid-water were similar to that in acetone-water as shown in Scheme 1. However, in the systems of methanol-water, ethanol-water, and isopropanolwater, no product was detected. The polarity of pure ethanol, isopropanol, butanone, methanol, acetone, acetonitrile, and acetic acid was 4.3, 3.9, 4.7, 5.1, 5.1, 5.8, and 6.0, respectively.45 These results suggest that organic solvent polarity has little effect on the conversion of PHE. The reason for the solvent effect will be discussed in detail in the discussion of the mechanism. The effects of proportion of acetone and water were also investigated

Efficient Photodegradation of Phenanthrene

J. Phys. Chem. C, Vol. 112, No. 11, 2008 4295 involving vibronic coupling to the ground state via the hydrogen bond.51 According to the mechanism mentioned above, the low efficiency of PHE photodegradation in alcohols and acetic acid was probably because that PHE•+ was quenched in these solvents. However, low conversion of PHE in butanone should be attributed to the low solubility of butanone in water, which resulted in the insufficient contact of Fe3+ ions and PHE in the mixture solvent. Conclusion

Figure 6. Influence of solvent component on degradation of PHE under visible light irradiation. Reaction conditions: λ > 420 nm, the concentration of Fe3+ was 2.00 × 10-3 mol L-1, the concentration of PHE was 2.80 × 10-4 mol L-1, total amount of acetone and H2O was 60 mL, and reaction time was 4 h.

(Figure 6). In the pure water medium, PHE was not degraded. Also, the conversion of PHE was enhanced with the increase of acetone content in the solution. After 4 h photoreaction, PHE was destructed completely in the solution, the volume ratio of acetone of which was 50%. Further increasing the acetone content of the solution, the conversion of PHE kept the constant at 100%. As shown in Figure 2, the fluorescence of PHE was quenched completely in the mixture solvent of acetone and water when the molar ratio of Fe3+ ions to PHE was 6:1. However, in pure acetone solution, the fluorescence of PHE vanished when the molar ratio of Fe3+ ions to PHE was 3:1, as shown in the Supporting Information (Figure S5). These results indicate that hydration of Fe3+ ions strongly diminished the fluorescence quenching of the PHE, implying that the complexation of PHE to Fe3+ ions was favorable when the amount of acetone increased. Therefore, stronger complexation between Fe3+ ions and PHE molecule46,47 may be an important factor for enhancing the photodegradation rate of PHE. A possible reaction pathway was proposed, as shown in Scheme 1. In the mixture of organic solvent and water, PHE and Fe3+ ions involved that the electron transferred from the highest occupied molecular orbitals of the PHE to the lowest unoccupied molecular orbitals of the Fe3+ ions and formed an acting force. The acting force shortened the distance between PHE and Fe3+ ions, which was of great importance to the increase of collision rate. As a result, the complex, Fe3+‚‚‚PHE, and spin coupling was formed.25,46 Under visible light irradiation, Fe3+ ions absorbed photons to conduce the transition of d to d and then the highest occupied molecular orbitals of Fe3+ ions were unoccupied. As a result, the π electrons of PHE were more facile to transfer to d orbitals of Fe3+ ions. In the photoionization of PAH, electron was transferred from the PAH to O2 following light absorption by a ground-state complex, thus resultingintheformofcharge-transfercomplex[PAH•+‚‚‚O2•-].48-50 Accordingly, electron transfer from the PHE to Fe3+ ions possibly led to the formation of Fe2+‚‚‚PHE•+. The resultant charge-transfer complex may undergo solvent separation, forming PHE•+ and Fe2+ ions. Fe2+ ions were rapidly oxidized by the molecule oxygen to produce Fe3+ ions again. A small amount of PHE•+ was decomposed directly to generate product 11, whereas most PHE•+ was further oxidized by molecule oxygen from medium-ring structure to form intermediates 8-10. Also, the degradation of compound 10 was another source of products 8 and 9. Intermediate 8 can be further destructed to yield products 1-4. In addition, the products 1-4 may also derive from compound 9 via intermediates 7, 6, and 5, sequentially; compounds 1-4 were finally oxidized to small molecules, such as CO2. Alcohols and weak acids often quench the excited state, which was attributed to a physical mechanism,

In summary, the photoinduced electron transfer between transition-metal ions and PAHs was utilized to induce the photodegradation of PHE under visible light irradiation. Fe3+ ions exhibited excellent sensitization ability for PHE photodegradation in the solvent without alcohols and acetic acid. Under visible light irradiation, PHE could be oxidated absolutely by oxygen after 3 h in acetone-water. As far as we are concerned, a complex, Fe3+‚‚‚PHE, was formed first in this reaction. Under visible light irradiation, Fe3+ ions absorbed photons to conduce the transition of d to d, and then the highest occupied molecular orbitals of Fe3+ ions were unoccupied. As a result, π electrons of PHE were more facile to transfer into d orbitals of Fe3+ ions, which possibly led to the form of the active intermediates, Fe2+‚‚‚PHE•+. The calculated structure of the complex of Fe3+‚‚‚PHE indicated that Fe3+ is situated at the center of two PHE molecules, which is similar to the structure of ferrocene. However, the molecules of PHE are not parallel to each other but form an angle. The HOMO-LUMO gap of PHE is small in the presence of Fe3+, which may be one of the reasons for the enhancement of the photodegradation of PHE. In alcohols and acetic acid, the photodegradation efficiency of PHE was low probably because PHE•+ was quenched in these solvents. Cu2+, Cr3+, Mn2+, Ni2+, Zn2+, and Co2+ were of no avail for PHE degradation at the same amount as Fe3+, probably ascribed to the low visible light absorption. Fe3+ ions are nontoxic, abundant, and stable. This study might open a new environmentally benign approach for the photooxidation of PAHs and their derivatives under visible light irradiation. Further work is in progress on the photooxidation of other PAHs by this method. Acknowledgment. Financial support from the National Natural Science Foundation of China (Nos. 20603017 and 20528302), the National High Technology Research and Development Program of China (No. 2006AA05Z113), the Science and Technology Research Program of the Ministry of Education (MOE) of China (No. 307012), the National Basic Research Program of China (973 Program, 2007CB613301, 2007CB613305), as well as the Jiangsu Provincial Natural Science Foundation (Nos. BK2006718 and BK2006127) and the Jiangsu Provincial High Technology Research Program (No. BG3006030) is gratefully acknowledged. Z.G.Z. and T.Y. would like to thank the Jiangsu Provincial Talent Scholars Program. Supporting Information Available: Ion chromatographs and mass spectra after the PHE photooxidation, UV-vis spectra of solvents, fluorescence spectra of PHE, and coordinates of the atoms in the optimized structure. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) El-Alawi, Y. S.; McConkey, B. J.; Dixon, D. G.; Greenberg, B. M. Ecotoxicol. EnViron. Saf. 2002, 51, 12-21. (2) Fouillet, B.; Chambon, P.; Castegnaro, M.; Weill, N. Bull. EnViron. Contam. Toxicol. 1991, 47, 1-7.

4296 J. Phys. Chem. C, Vol. 112, No. 11, 2008 (3) Callahan, M. A.; Slimak, M. W.; Gabelc, N. W.; May, I. P.; Fowler, C. F.; Freed, J. R.; Jennings, P.; Durfee, R. L.; Whitmore, F. C.; Maestri, B.; Mabey, W. R.; Holt, B. R.; Gould, C. U.S. Environmental Protection Agency, Washington, DC, 1979. (4) Yan, J.; Wang, L.; Fu, P. P.; Yu, H. Mutat. Res. 2004, 557, 99. (5) Gocke, E. J. EnViron. Pathol. Toxicol. Oncol. 2001, 20, 285. (6) Baudouin, C.; Charveron, M.; Tarroux, R.; Gall, Y. Cell Biol. Toxicol. 2002, 18, 341. (7) Arfsten, D. P.; Schaeffer, D. J.; Mulveny, D. C. Ecotoxicol. EnViron. Saf. 1996, 33, 1. (8) Miller, J. S. Water Res. 2001, 35, 233. (9) Sabate´, J.; Bayona, J. M.; Solanas, A. M. Chemosphere 2001, 44, 119. (10) Barbas, J. T.; Sigman, M. E.; Dabestani, R. EnViron. Sci. Technol. 1996, 30, 1776. (11) Rose, M. J.; Oimstead, M. M.; Mascharak, P. J. Am. Chem. Soc. 2007, 129, 5342. (12) Hu, M.; Xu, Y.; Zhao, J. Langmuir 2004, 20, 6302. (13) Sudeep, P. K.; Kamat, P. V. Chem. Mater. 2005, 17, 5404. (14) Mccullagh, C.; Robertson, P. K. EnViron. Sci. Technol. 2006, 40, 2421. (15) Zhao, X.; Hu, X.; Hwang, H.-M. Chemosphere 2006, 63, 1116. (16) Kotani, H.; Ohkubo, K.; Fukuzumi, S. J. Am. Chem. Soc. 2004, 126, 15999. (17) Kochi, J. K. Angew. Chim. 1988, 27, 1227. (18) Mariano, P. S., Ed. AdVances in Electron Transfer Chemistry; JAI Press: Greenwich, 1991, 1992; Vols. 1 and 2. (19) Fujita, M.; Ishida, A.; Takamuku, S.; Fukuzumi, S. J. Am. Chem. Soc. 1996, 118, 8566. (20) Mariano, P. S. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.;; Elsevier: Amsterdam, 1988; Part C, p 372. (21) Albini, A.; Mella, M. Tetrahedron 1986, 42, 6219. (22) Albini, A.; Fasani, E.; Mella, M. J. Am. Chem. Soc. 1986, 108, 4119. (23) Fasani, E.; d’Alessandro, N.; Albini, A.; Mariano, P. S. J. Org. Chem. 1994, 59, 829. (24) Davis, H. F.; Chattopadhyay, S. K.; Das, P. K. J. Phys. Chem. 1984, 88, 2798. (25) Linschitz, H.; Pekkarinen, L. J. Am. Chem. Soc. 1960, 82, 2411. (26) Bodenant, B.; Fages, F.; Delville, M.-H. J. Am. Chem. Soc. 1998, 120, 7511. (27) Grieser, F.; Tausch-Treml, R. J. Am. Chem. Soc. 1980, 102, 7258. (28) Nosaka, Y.; Kira, A.; Imamura, M. J. Phys. Chem. 1981, 85, 1353. (29) Sabbatini, N.; Indelli, M. T.; Gandoifi, M. T.; Baizani, V. J. Phys. Chem. 1982, 86, 3585.

Kou et al. (30) Varnes, A. W.; Dobson, R. B. E. L. W. J. Am. Chem. Soc. 1972, 94, 946. (31) Nithipatikom, K.; McGown, L. B. Anal. Chem. 1988, 60, 1043. (32) Chen, J.; Peijnenburg, W. J. G. M.; Quan, X.; Chen, S.; Martens, D.; Schramm, K.-W.; Kettrup, A. EnViron. Pollut. 2001, 114, 137. (33) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (34) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (35) Poater, J.; Fradera, X.; Duran, M.; Sola`, M. Chem.-Eur. J. 2003, 9, 400. (36) Hehre, W. J.; Radom, L.; Schleyer, P. V. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; AlLaham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998. (38) Manolopoulos, D. E.; May, J. C.; Down, S. E. Chem. Phys. Lett. 1991, 181, 105. (39) Aihara, J. J. Phys. Chem. A 1999, 103, 7487. (40) Ruiz-Morales, Y. J. Phys. Chem. A 2002, 106, 11283. (41) Hess, B. A., Jr.; Schaad, L. J. J. Am. Chem. Soc. 1971, 93, 2413. (42) Schmalz, T. G.; Seitz, W. A.; Klein, D. J.; Hite, G. E. J. Am. Chem. Soc. 1988, 110, 1113. (43) Zhou, Z.; Parr, R. G. J. Am. Chem. Soc. 1989, 111, 7371. (44) Zhou, Z.; Parr, R. G. J. Am. Chem. Soc. 1990, 112, 5720. (45) Gupta, M. N.; Batra, R.; Tyagi, R.; Sharma, A. Biotechnol. Prog. 1997, 13, 284. (46) Naqvi, R.; Melø, T. B. Chem. Phys. Lett. 2006, 428, 83. (47) Wijnja, H.; Pignatello, J. J.; Malekani, K. J. EnViron. Qual. 2004, 33, 265. (48) Fasnacht, M. P.; Blough, N. V. EnViron. Sci. Technol. 2003, 37, 5767. (49) Vialation, D.; Richard, C.; Baglio, D.; Paya-Perez, A.-B. J. Photochem. Photobiol. A 1999, 123, 15. (50) Slgman, M. E.; Schuler, P. F.; Ghosh, M. M.; Dabestani, R. T. EnViron. Sci. Technol. 1998, 32, 3980. (51) Biczo´k, L.; Be´rces, T.; Linschitz, H. J. Am. Chem. Soc. 1997, 119, 11071.