Environ. Sci. Technol. 2009, 43, 460–465
Photocatalytic Oxidation of Paracetamol: Dominant Reactants, Intermediates, and Reaction Mechanisms L I M I N G Y A N G , * ,† L I Y A E . Y U , † A N D MADHUMITA B. RAY‡ Division of Environmental Science and Engineering, National University of Singapore, Singapore 119260, and Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada
Received July 19, 2008. Revised manuscript received October 06, 2008. Accepted October 27, 2008.
The role of primary active species (ecb-, hvb+, •OH, HO2•, O2•-, and H2O2) during photocatalytic degradation of paracetamol (acetaminophen) using TiO2 catalyst was systematically investigated. Hydroxyl radicals (•OH) are responsible for the major degradation of paracetamol with a second-order rate constant (1.7 × 109 M-1 s-1) for an •OH-paracetamol reaction. A total of 13 intermediates was identified and classified into four categories: (i) aromatic compounds, (ii) carboxylic acids, (iii) nitrogen-containing straight chain compounds, and (iv) inorganic species (ammonium and nitrate ions). Concentration profiles of identified intermediates indicate that paracetamol initially undergoes hydroxylation through •OH addition onto the aromatic ring at ortho (predominantly), meta, and para positions with respect to the -OH position of paracetamol. This initial •OH hydroxylation is followed by further oxidation generating carboxylic acids. Subsequent mineralization of smaller intermediates eventually increases ammonium and nitrate concentration in the system.
Introduction Paracetamol (acetaminophen), a common analgesic and antipyretic drug, is heavily used all over the world. It has been found with a concentration of up to 6 µg L-1 in European STP effluents (1), up to 10 µg L-1 in natural waters in the United States (2) and more than 65 µg L-1 in the Tyne river in the United Kingdom (3). Various advanced oxidation processes such as electrochemical (4-6), ozonation, and H2O2/UV oxidation (7-9) have been employed to remove aqueous paracetamol. In particular, TiO2 photocatalysis, one of the most promising advanced oxidation processes, receives significant attention for the treatment of contaminated streams because of its chemical stability, nontoxicity, and low cost (10). While our earlier work (11) shows that TiO2 photocatalysis effectively removes paracetamol from aquatic environments, better understanding of reaction mechanisms of photocatalytic degradation of paracetamol in water is required for effective design of treatment processes. Reaction mechanisms of photocatalytic processes using TiO2 have been discussed extensively in literature (10, 12, 13) * Corresponding author phone: +65 6516 8478; fax: +65 6774 4202; e-mail:
[email protected]. † National University of Singapore. ‡ University of Western Ontario. 460
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 2, 2009
and are summarized in Table 1. As reactions R1-R10 (Table 1) show, an irradiation energy greater than the band gap energy (Ebg) of TiO2 (hν > Ebg ) 3.2 eV in the case of anatase TiO2) generates various active species in reaction systems, including valence band holes (hvb+), conduction band electrons (ecb-), hydroxyl radicals (•OH), superoxide ions (O2•-), hydrogen peroxide radicals (HO2•), and hydrogen peroxide (H2O2). These active species then undergo redox reactions to mineralize organic compounds (R11-R15, Table 1). However, dominant reactants during various photocatalytic reactions need to be determined; for example, by examining a similar TiO2 photocatalytic oxidation of arsenite system, Yoon and Lee (14) concluded hvb+ as the major oxidants, whereas Lee and Choi (15) proposed O2•- as the dominant oxidant irrespective of the fact that O2•--arsenite reaction (3.0 × 106 M-1 s-1) is slower than •OH-arsenite reaction (8.5 × 109 M-1 s-1) (16). By excluding oxidation effects of hvb+, Dutta et al. 16 concluded •OH as the major oxidants for arsenite. To systematically determine the most dominant reactants among hvb+, ecb-, •OH, O2•-, HO2•, and H2O2 during photocatalytic degradation of paracetamol in aqueous TiO2 suspension, this work first examines the combined degradation effects of five reactive components (hvb+, •OH, O2•-, HO2•, and H2O2) and the effects of ecb-. Measuring combined effects of ecb-, O2•-, HO2•, and H2O2 is equivalent to assessing the extent of degradation in absence of both hvb+ and •OH. By further differentiating between •OH and hvb+, this work finds that •OH appeared to be the major reactant. It should be noted that, although many reactants in the system can indirectly (through contributing to •OH) degrade paracetamol, • OH is the most dominant species directly decomposing paracetamol. On the basis of chemical structures and concentration profiles of identified intermediates, possible reaction pathways during the photocatalytic degradation of paracetamol are proposed.
Materials and Methods Photocatalytic Reaction System. Photooxidation of paracetamol was conducted in an annular cylindrical reactor with a quartz sleeve at the center of the reactor to house a UV light source. Experiments were conducted in a semibatch fashion; a 150-mL suspension of TiO2 and paracetamol (SigmaAldrich) in Millipore Direct-Q water was circulated at a flow rate of 0.15 L min-1, with a residence time outside of the reactor for less than 30 s. Aliquots were sampled every 45 min and filtered through a 0.2-µm PTFE syringe filter (SMILabHut) to remove TiO2 particles before analyses. This filtration step consistently removes all TiO2. Total volume of sample withdrawn from each experiment was less than 5% (by volume) of the circulated solution. Temperature (26 ( 2 °C), pH (using Thermo Orion 720A + pH meter), and oxygen concentration (using SCHOTT Handylab OX12) of reaction media were measured online throughout each experiment. Nonporous titanium dioxide (TiO2, P25, Degussa AG) with a primary particle diameter of 21 nm, specific surface area of 50 ( 15 m2 g-1, and crystal distribution of 80% anatase and 20% rutile was used as the photocatalyst. A UV lamp with a principal output at 254 nm accompanied with a small amount of radiation at 185 nm (Ster-L-Ray, Atlantic Ultraviolet Corp.) was used as the light source. The radiation of 185 nm can be absorbed by water to directly produce hydroxyl radicals (•OH). While paracetamol can be photolyzed at 254 nm (the molar extinction coefficient ε is around 750 M-1 cm-1), our laboratory tests showed that, with TiO2 suspension, direct photolysis of paracetamol is insignificant because of turbidity 10.1021/es8020099 CCC: $40.75
2009 American Chemical Society
Published on Web 12/15/2008
TABLE 1. Summary of Potential Reactions in Photocatalytic Degradation of Paracetamol generation of charge carriers and photooxidants λ
