Photosensitized Oxidation of Emerging Organic Pollutants by Tetrakis

ARTICLE pubs.acs.org/est

Photosensitized Oxidation of Emerging Organic Pollutants by Tetrakis C60 Aminofullerene-Derivatized Silica under Visible Light Irradiation Jaesang Lee,† Seokwon Hong,† Yuri Mackeyev,‡ Changha Lee,§ Eunhyea Chung,† Lon J. Wilson,‡ Jae-Hong Kim,|| and Pedro J. J. Alvarez*,^ †

Water Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea Department of Chemistry, Rice University, Houston, Texas 77005, United States § Department of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea Department of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ^ Department of Civil and Environmental Engineering, Rice University, Houston, Texas 77005, United States

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bS Supporting Information ABSTRACT: We recently reported that C60 aminofullerenes immobilized on silica support (aminoC60/silica) efficiently produce singlet oxygen (1O2) and inactivate virus and bacteria under visible light irradiation.1 We herein evaluate this new photocatalyst for oxidative degradation of 11 emerging organic contaminants, including pharmaceuticals such as acetaminophen, carbamazepine, cimetidine, propranolol, ranitidine, sulfisoxazole, and trimethoprim, and endocrine disruptors such as bisphenol A and pentachlorophenol. Tetrakis aminoC60/silica degraded pharmaceuticals under visible light irradiation faster than common semiconductor photocatalysts such as platinized WO3 and carbon-doped TiO2. Furthermore, aminoC60/silica exhibited high target-specificity without significant interference by natural organic matter. AminoC60/silica was more efficient than unsupported (water-suspended) C60 aminofullerene. This was attributed to kinetically enhanced 1O2 production after immobilization, which reduces agglomeration of the photocatalyst, and to adsorption of pharmaceuticals onto the silica support, which increases exposure to 1O2 near photocatalytic sites. Removal efficiency increased with pH for contaminants with a phenolic moiety, such as bisphenol A and acetaminophen, because the electron-rich phenolates that form at alkaline pH are more vulnerable to singlet oxygenation.

’ INTRODUCTION Many previous studies have attempted the degradation of various organic contaminants using photosensitizers that produce reactive oxygen species (ROS) upon irradiation by light.2,3 Recently, increasing attention has been given to the development of photosensitizers that utilize visible light4 6 to overcome limitations of commonly used metal oxide photocatalysts such as TiO2 and ZnO that require UV irradiation for activation. Such efforts include enhancing the susceptibility of metal oxide photocatalysts to visible light by introducing various dopants,4,5,7 anchoring of organic photosensitizers,8,9 and hybridization with other semiconductors. New semiconductor photocatalysts (e.g., BiVO4, PbBi2Nb2O9)10 12 and macrocyclic functional dyes (e.g., porphyrins and phthalocyanines)13 that exhibit inherent visible light activity have also been considered as photocatalysts for water or wastewater treatment. C60 fullerene and C60 derivatives with various surface functional groups undergo facile photoexcitation under irradiation by UV and visible light (λ < 550 nm). The resulting photoexcited r 2011 American Chemical Society

triplet state of fullerenes efficiently mediates the transfer of energy to molecular oxygen to produce singlet oxygen (1O2), as well as the transfer of electrons to produce superoxide radical anions (O2• ).14,15 Although C60 is extremely hydrophobic, C60 derivatives with hydrophilic surface functional groups enable the application of the above phenomena in the aqueous phase.1,16 19 For water and wastewater treatment applications, photochemically produced 1O2 is a primary oxidant for electron-rich moieties including polycyclic aromatic rings,20 benzene rings activated with electron-donating substituents,21 and conjugated double bonds22 that are commonly found in organic contaminants and cellular components (e.g., proteins, lipids, and DNA). Accordingly, multiple hydroxylated C60, commonly known as fullerenol, Received: August 25, 2011 Accepted: November 5, 2011 Revised: November 4, 2011 Published: November 05, 2011 10598

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Environmental Science & Technology has been shown to photochemically oxidize 2-chlorophenol17 and inactivated MS-2 bacteriophage16 in water. We previously developed C60 adducts with amine-, carboxylic-, and hydroxyl functional groups as a photocatalyst to inactivate Escherichia coli and MS-2 phage via photochemically produced 1 O2.19 We also demonstrated that cationic C60 aminofullerene, immobilized onto silica beads for effective catalyst recycling, efficiently inactivated MS-2 phage under natural sunlight irradiation through rapid singlet oxygenation of viral capsid proteins.18 Whereas these studies verified the high efficacy of functionalized C60 for photodynamic microbial inactivation, there is a need to delineate the applicability and limitations of these novel nanoparticles as visible-light-activated 1O2 photosensitizers for broader water treatment applications. In particular, there is a need for a multiactivity assessment with multiple emerging pollutants to discern susceptible chemical structures, and to compare functionalized fullerene performance versus other photocatalysts under various irradiation conditions. Pharmaceuticals and endocrine disruptors are emerging environmental contaminants with significant concerns of interfering with hormones and reproductive systems, and thus causing severe adverse biological effects.23 The present study explores the potential use of tetrakis C60 aminofullerene immobilized onto silica beads (referred to herein as aminoC60/silica) for oxidative degradation of various pharmaceuticals and endocrine disruptors under fluorescent and visible light irradiation. Photochemical reactivity for 1O2 yield is measured as a function of fullerene content in the presence and absence of natural organic matter (NOM) or L-histidine (a 1O2 quencher). The efficacy of aminoC60/silica is also compared to that of other widely studied photocatalysts, namely, TiO2, carbon-doped TiO2 (C-TiO2), and platinized WO3 (Pt/WO3).

’ MATERIALS AND METHODS Preparation of Tetrakis C60 Aminofullerenes and Immobilization onto Silica Support. Tetrakis C60 aminofullerene was

synthesized and purified as previously described.19 A brief description of the methods and chemicals is given in the Supporting Information (Text S1, Figure S1). The C60 aminofullerene was further immobilized onto 3-(2-succinic anhydride)propyl functionalized silica gel through amide bond. Detailed procedures for synthesis and purification are available in our previous publication1 and a brief summary is included in the Supporting Information (Text S2, Figure S2). Characterization by FIB/SEM and AFM. Cross-sectioning and electron beam imaging were performed using a dual beam focused ion beam/scanning electron microscopy (FIB/SEM) system coupled with an energy dispersive X-ray spectrometer (EDS) (Quanta 3D FEG, FEI Co., USA). Thin platinum and carbon layers were deposited on the surface of the materials (silica gel and aminoC60/silica) before ion milling to protect the area of interest and enhance charge and heat transfer. The FIB milling was performed by a focused gallium ion (Ga+) beam at 30 kV and 50 pA. SEM images of the FIB-prepared section were taken at an accelerating voltage of 5 kV. Topographic surface images of the silica support and aminoC60/silica were produced by atomic force microscopy (AFM, Dimension edge, Bruker AXS) in tapping mode. Preparation of Platinized WO3 and Carbon-Doped TiO2. Platinization of WO3 was performed using a photodeposition method.6 Briefly, an aqueous suspension containing 0.5 g/L

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Figure 1. (a) Photosensitized 1O2 production (i.e., degradation of furfuryl alcohol, FFA) by 50 μM tetrakis C60 aminofullerene versus tetrakis aminoC60/silica with a fullerene content of 0.05 mmol/g under fluorescent light irradiation, and effects of L-histidine and Suwannee River natural organic matter; and (b) FFA degradation by tetrakis aminoC60/silica as a function of fullerene content. Dashed curves represent nonlinear regression fits to pseudo-first-order decay ([aminoC60/silica]0 = 1 g/L; [FFA]0 = 100 μM; [L-histidine]0 = 0.1 M; [SRNOM]0 = 10 mg/L; [phosphate]0 = 10 mM; pHi = 7).

WO3 (nanopowder ( 400 nm) and fluorescent lamp light were very similar (i.e., k = 1.378 ( 0.015 h 1 for fluorescent light irradiation, versus 1.248 ( 0.024 h 1 for visible light-irradiation) (Figure 7a). Figure 7a and b compare the photocatalytic activity of aminoC60/ silica to those of the widely researched semiconducting photocatalysts TiO2, C-TiO2, and Pt/WO3 in terms of FFA, ranitidine, and cimetidine degradation under fluorescent and visible light irradiation. Whereas TiO2 catalyzed significant FFA oxidation under fluorescent light, FFA was not degraded when a UV cutoff filter was placed, confirming the lack of visible light activity by TiO2 (Figure 7a). Similarly, unmodified TiO2 degraded ranitidine and 10602

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Environmental Science & Technology cimetidine under fluorescent lamp light, but at a much slower rate compared to aminoC60/silica under visible light irradiation (Figure 7b). C-TiO2 did not catalyze FFA degradation under either fluorescent or visible light (Figures 7a), while it exhibited moderate efficacy in ranitidine and cimetidine degradation under visible light irradiation (Figures 7b). These results are likely due to reduced oxidizing power of the holes generated in midgap states of C-TiO240,41 and accelerated charge recombination in dopinginduced intermediate states.42,43 Pt/WO3 degraded FFA faster than the other photocatalysts tested under both fluorescent and visible light conditions, as Pt/WO3 effectively produces •OH via multiple electron transfer to oxygen.39 However, ranitidine and cimetidine degradation was faster with aminoC60/silica than with Pt/WO3 under visible light irradiation (Figure 7b). Although such comparisons should recognize that these photoactive materials generate different oxidizing species (e.g., mainly 1O2 for aminoC60, •OH for Pt/WO3 6,39 and TiO2, and valence band hole for C-TiO2), results suggest that aminoC60/silica has a potential for application as an alternative environmental photocatalyst. Potential Applications. This study demonstrates that the photochemical activity of aminoC60/silica results in efficient 1O2 production, which could be used to degrade emerging organic micropollutants such as pharmaceuticals and endocrine disruptors. This newly developed photocatalyst has several advantages over existing technologies. First, aminofullerene immobilized onto relatively large support media facilitates separation for recycling. Second, it functions efficiently under visible light irradiation, overcoming a major drawback for many semiconductor-based photocatalysts that require UV irradiation and its associated infrastructure. Third, 1O2 is highly selective toward organic contaminants containing electron-rich moieties (e.g., furan and imidazole),32 which would enhance its efficacy when such pollutants are present in complex water matrices with relatively high concentrations of background organics (e.g., wastewater). In contrast, •OH (which is the major oxidant in semiconductor photocatalyst) has poor selectivity that offsets its higher oxidation capacity, and is wastefully consumed by nontarget substrates (including natural organic matter) and byproducts.44,45 It is noteworthy that even though complete oxidation of target micropollutants by 1O2 is unlikely to occur, slight oxidative modifications often cause drastic reduction of biological/estrogenic activities of pharmaceuticals,46 elimination of hazardous effects of algal toxins,47 and effective decolorization of dyes.48 Whereas these results suggest several potential advantages of aminoC60/silica for water pollution control applications, more studies are needed to identify potential critical limitations associated with scale-up issues and long-term operation on a larger scale under real world conditions. Further photocatalyst development should also consider different types of functionalized C60, preferably with improved visible light quantum yield, modifying the physical and chemical properties of catalyst support to increase surface area available for the photocatalytic reaction, and introducing high-affinity adsorption sites to enhance photocatalytic degradation.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional figures and text. This information is available free of charge via the Internet at http://pubs.acs.org/.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; phone: (713) 348-5903; fax: (713) 348-5203.

’ ACKNOWLEDGMENT This study was supported by the U.S. National Science Foundation (Award CBET-0932872). Partial funding was also provided by the Korea Ministry of Environment as “Eco-Innovation program (Environmental Research Laboratory)” (414-111011) and as “Converging Technology Project” (191-101-001), by the Robert A. Welch Foundation (Grant C-0627), and by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (200110005647). We thank Dr. Jungwon Kim and Dr. Yiseul Park at POSTECH for their assistant on synthesis of platinized WO3 and carbon-doped TiO2. ’ REFERENCES (1) Lee, J.; Mackeyev, Y.; Cho, M.; Wilson, L. J.; Kim, J.-H.; Alvarez, P. J. J. C60 aminofullerene immobilized on silica as a visible-lightactivated photocatalyst. Environ. Sci. Technol. 2010, 44, 9488–9495. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69–96. (3) Mills, A.; LeHunte, S. An overview of semiconductor photocatalysis. J. Photochem. Photobiol., A 1997, 108, 1–35. (4) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible light photocatalysis in nitrogen doped titanium oxides. Science 2001, 293, 269–271. (5) Choi, J.; Park, H.; Hoffmann, M. R. Effects of single metal ion doping on the visible light photoreactivity of TiO2. J. Phys. Chem. C 2010, 114, 783–792. (6) Kim, J.; Lee, C. W.; Choi, W. Platinized WO3 as an environmental photocatalyst that generates OH radicals under visible light. Environ. Sci. Technol. 2010, 44, 6849–6854. (7) Park, Y.; Kim, W.; Park, H.; Tachikawa, T.; Majima, T.; Choi, W. Carbon-doped TiO2 photocatalyst synthesized without using an external carbon precursor and the visible light activity. Appl. Catal. B: Environ. 2009, 91, 355–361. (8) Park, Y.; Lee, S. H.; Kang, S. O.; Choi, W. Organic dye-sensitized TiO2 for the redox conversion of water pollutants under visible light. Chem. Commun. 2010, 46, 2477–2479. (9) Li, G. S.; Zhang, D. Q.; Yu, J. C. A new visible light photocatalyst: CdS quantum dots embedded mesoporous TiO2. Environ. Sci. Technol. 2009, 43, 7079–7085. (10) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. Photophysical and photochemical aspects of coupled semiconductors. Charge transfer processes in colloidal CdS-TiO2 and CdS-AgI systems. J. Phys. Chem. 1990, 94, 6435–6440. (11) Kim, H. G.; Hwang, D. W.; Lee, J. S. An undoped, single phase oxide photocatalyst working under visible light. J. Am. Chem. Soc. 2004, 126, 8912–8913. (12) Li, G.; Zhang, D.; Yu, J. C. Ordered mesoporous BiVO4 through nanocasting: A superior visible light driven photocatalyst. Chem. Mater. 2008, 20, 3983–3992. (13) Kim, W.; Park, J.; Jo, H. J.; Kim, H. J.; Choi, W. Visible light photocatalysts based on homogeneous and heterogenized tin porphyrins. J. Phys. Chem. C 2008, 112, 491–499. (14) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. Photophysical properties of C60. J. Phys. Chem. 1991, 95, 11–12. (15) Yamakoshi, Y.; Umezawa, N.; Ryu, A.; Arakane, K.; Miyata, N.; Goda, Y.; Masumizu, T.; Nagano, T. Active oxygen species generated 10603

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