Please cite this article in press as: H.-S. Son, et al., Kinetics and mechanism of photolysis and TiO 2 photocatalysis of triclosan, J. Hazard. Mater. (2009), doi:10.1016/j.jhazmat.2008.11.107 ARTICLE IN PRESS G Model HAZMAT-9212; No. of Pages 7 Journal of Hazardous Materials xxx (2009) xxx–xxx Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Research article Kinetics and mechanism of photolysis and TiO 2 photocatalysis of triclosan Hyun-Seok Son, Gwangpyo Ko, Kyung-Duk Zoh ∗ Institute of Health & Environment, Seoul National University, Seoul 110-799, Republic of Korea article info Article history: Received 23 April 2008 Received in revised form 27 November 2008 Accepted 29 November 2008 Available online xxx Keywords: TiO2 Langmuir–Hinshelwood Radical scavenger Dibenzo-dichloro-p-dioxin Dibenzo-p-dioxin abstract The degradations of triclosan (5-chloro-2-(2,4-dichlorophenoxy)-phenol), a potent broad-spectrum antimicrobial agent, were compared in TiO 2 -only in the dark condition, photolysis, and TiO 2 photo- catalysis with a UV-A lamp. TiO 2 photocatalysis more effectively degraded and mineralized triclosan compared to TiO 2 -only and photolysis conditions. While triclosan removed only 30% by TiO 2 -only con- dition within 20 min, the triclosan degradation in photolysis and photocatalysis at the same time was 75 and 82%, respectively, and TOC removal was significantly higher in photocatalysis than in photolysis. The data of kinetics showed that triclosan adsorption onto TiO 2 was fitted to Langmuir isotherm, and TiO 2 photocatalysis was fitted to Langmuir–Hinshelwood model (b = 27.99 mM -1 , K triclosan = 9.49 mM -1 ). The neutral range of pH was favorable to photocatalysis due to the charge effect between TiO 2 and triclosan. The addition of 2-propanol, a radical scavenger, significantly reduced the degradation of triclosan both in photolysis and photocatalysis. Dioxin-type intermediates such as dibenzo-dichloro-p-dioxin (DCDD), dibenzo-p-dioxin were produced in photolysis with and without 2-propanol, and also in photocatalysis with 2-propanol, but these intermediates were not detected in photocatalysis without 2-propanol. This result indicates that the photocatalytic degradation of triclosan is mainly achieved by radicals, and these radicals can further degrade dioxin-type intermediates once they are produced in photocatalysis. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Triclosan (5-chloro-2-(2,4-dichlorophenoxy)-phenol) is a potent broad-spectrum antimicrobial agent that is included as an additive in many products [1]. Triclosan resistance in bacteria may arise by inhibiting the enzyme enoyl-acyl carrier protein reductase, blocking lipid biosynthesis in E. coli, and promoting a mutation in the FabI gene [2]. Triclosan in wastewater treatment plant is primarily subjected to biological degradation, sorption to sludge, or discharged to receiving surface water untreated [3,4]. Triclosan discharged to surface water, which has the typical pH between 7 and 9, is resistant to ionization since the pK a of triclosan is 7.9–8.1 [5]. Although the adsorption of triclosan onto particulate matter can be expected because of its high octanol–water partition coefficient (log K ow = 5.4) [6,7], it requires the post-treatment for treated water as well as adsorption process. Since the biological degradation of triclosan has been found to be inefficient [8], an alternative or post- treatment is required after adsorption and biological treatment processes. Singer et al. [7] reported a significant removal rate (0.03 day -1 ) of triclosan by photolysis in the epilimnion of Greifensee lake in ∗ Corresponding author. Tel.: +82 2 745 9104; fax: +82 2 745 9104. E-mail address: zohkd@snu.ac.kr (K.-D. Zoh). Switzerland. Since Tixier et al. [9] reported a high quantum yield (0.31) of triclosan for light at 313nm, therefore photolysis may be main reason of the elimination of triclosan in the lake. Photolysis can be a promising method to supplement the exist- ing treatment of triclosan in wastewater treatment plants. However, one of the major drawbacks of using photolysis to treat triclosan is the production of harmful intermediates. It was found that the pho- tolysis can produce dioxin- and phenol-type intermediates in the presence or absence of chloride ions, as a result of the structural similarities between triclosan and dioxin compounds [10]. Dioxin and phenol are environmentally important materials because of their high toxicity and potential for bioaccumulation; therefore, preventing their production may be more important than degrading triclosan by photolysis. Recent research performed in our laboratory [11] showed that dibenzo-dichloro-p-dioxin (DCDD) and dibenzo-p-dioxin were detected in the photolysis of triclosan at the low light intensity at UV-A condition (maximum wavelength = 365 nm), where OH radical contribution is lower than at UV-C condition (maximum wavelength = 254 nm). However, DCDD and dibenzo-p-dioxin were not detected in photolysis of triclosan at UV-C. This result indi- cates that oxidants such as OH radicals during photolysis are the key factors in preventing the production of dioxin-type interme- diates in the photolysis of triclosan. Photocatalytic reaction can be the alternative to maximize production of oxidants such as OH radicals [12–14]. Photocatalysis may completely mineralize a 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.11.107