Experimental and Theoretical Insights into the Involvement of Radicals in Triclosan Phototransformation Sarah Kliegman, † Soren N. Eustis, †,⊥ William A. Arnold, ‡ and Kristopher McNeill* ,† † Institute for Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland ‡ Department of Civil Engineering, University of Minnesota, 500 Pillsbury Drive SE, Minneapolis, Minnesota 55455, United States * S Supporting Information ABSTRACT: The phototransformation of triclosan has been a matter of longstanding interest due to both its prevalence in the environment and the discovery of 2,8-dichlorodibenzodiox- in as a photoproduct. In this study, photolysis of triclosan resulted in several primary photoproducts including the following: 2,8-dichlorodibenzodioxin (4%), 4,5′-dichloro- [1,1′-biphenyl]-2,2′-diol (10%), 5-chloro-2-(4-chlorophenoxy)- phenol (0.5%), and 2,4-dichlorophenol (7%). Trapping studies using d 8 -isopropanol showed deuterium incorporation in 5- chloro-2-(4-chlorophenoxy)phenol, providing strong evidence for the involvement of organic radicals in this reaction. Density functional calculations of the excited states of triclosan support the involvement of a radical intermediate in the mechanisms responsible for the dioxin, biphenyl, and phenoxyphenol photoproducts. The pathways for C-Cl bond cleavage and cyclization reactions are discussed. ■ INTRODUCTION Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol), a widely used additive in consumer products due to its antibacterial and antifungal properties, has become a frequently detected contaminant in wastewater effluents, 1,2 surface waters, 2,3 and sediments. 2,4,5 Triclosan is known to undergo a variety of partitioning and degradation processes in the aquatic environ- ment, including direct (unsensitized) photochemical degrada- tion leading to a variety of photoproducts. 2,6,7 Concern about the potential ecotoxicological effects of triclosan and its degradation products in the environment encouraged efforts to identify these products and quantify their toxicity. 8 Direct photolysis has been found to be an important loss process for triclosan in the environment. 2,6,7 Due in part to higher light absorption in the near-UV region of the solar spectrum, triclosan (pK a 8.1) is more photochemically labile in the phenolate form 7,9,10 and degrades readily yielding a range of products (Figure 1). There are four major photoproducts that have been identified previously: 2,8-dichlorodibenzodioxin (2,8-DCDD) resulting from cyclization (Figure 1, process I); 9-14 4,5′-dichloro-[1,1′-biphenyl]-2,2′-diol ((OH) 2 PCB-13) involving a skeletal rearrangement from a diphenyl ether to a hydroxylbiphenyl core structure (II); 10 lower chlorinated triclosan derivatives resulting from hydrodehalogenation reactions or replacement of a chlorine substitutent by a hydrogen atom, (III); 15 and, dichlorophenol resulting from ether cleavage (IV). 15 In addition, polymerization (V) occurs at high triclosan concentrations (100 μM). 9,16 In the aquatic environment, where triclosan concentrations are in the subnanomolar range, 2 processes I-IV, which involve only a single triclosan molecule, are expected to dominate. The mechanisms of the transformation processes are not yet fully understood. For example, although photocyclization of halogenated phenoxyphenols leading to dioxins has been observed with triclosan, 9-14 more highly chlorinated phenox- Special Issue: Rene Schwarzenbach Tribute Received: October 12, 2012 Revised: December 27, 2012 Accepted: January 2, 2013 Published: January 2, 2013 Figure 1. Products observed from photochemical degradation of triclosan under basic conditions. Article pubs.acs.org/est © 2013 American Chemical Society 6756 dx.doi.org/10.1021/es3041797 | Environ. Sci. Technol. 2013, 47, 6756-6763