Journal of Photochemistry and Photobiology A: Chemistry 158 (2003) 63–66 Short communication Photochemical conversion of triclosan to 2,8-dichlorodibenzo- p-dioxin in aqueous solution Douglas E. Latch a , Jennifer L. Packer b , William A. Arnold b,1 , Kristopher McNeill a,∗ a Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455, USA b Department of Civil Engineering, University of Minnesota, 500 Pillsbury Dr. SE, Minneapolis, MN, USA Received 27 December 2002; received in revised form 21 February 2003; accepted 24 February 2003 Abstract The direct photolysis of triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol), an antimicrobial additive commonly detected in surface waters, is studied. It is found that 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) is produced in both buffered and natural (Mississippi River) water with yields ranging from 1 to 12% under a variety of conditions. This result indicates that triclosan is likely converted to 2,8-DCDD in sunlight-irradiated surface waters. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Triclosan; 2,8-Dichlorodibenzo-p-dioxin; Direct photolysis; Photochemical cyclization; Natural water 1. Introduction Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) is a widely employed antimicrobial that has been found as a con- taminant of rivers and lakes [1–10]. In a recent reconnais- sance for a suite of 95 pharmaceuticals, hormones and other organic wastewater contaminants, triclosan was one of the most frequently detected pollutants, being found in 57.6% of the 139 tested US streams and rivers [5]. An early set of studies in the Pawtuxet and Providence Rivers detected triclosan along with structurally related compounds, includ- ing 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD), which were hypothesized to be derived from the synthesis of triclosan [9–11]. Because of general concern about dioxins in the environment, we have become interested in the possibility that triclosan is a dioxin precursor and can be converted to 2,8-DCDD by an intramolecular photochemical substitution reaction. This hypothesis is supported by numerous exam- ples of photochemical nucleophilic aromatic substitution [12–15]. Our interest in this photoreaction is further piqued by the studies of Mueller and coworkers who concluded that photochemical transformation of triclosan accounts for up to 80% of its loss from the epilimnion in Lake Greifensee during the summer months [1,2]. ∗ Corresponding author. Tel.: +1-612-625-0781; fax: +1-612-626-7541. E-mail addresses: arnol032@umn.edu (W.A. Arnold), mcneill@chem.umn.edu (K. McNeill). 1 Co-corresponding author. The thermal cyclization of triclosan and other poly- chlorophenoxyphenols to polychlorodibenzo-p-dioxins is established and occurs readily for triclosan above 300 ◦ C [15–18]. Previous work on the photochemical cyclization has led to conflicting results. It has been reported that triclosan is relatively unique among the polychlorophe- noxyphenols in that it does not undergo cyclization to its corresponding dioxin in methanol solution [15,19]. More recent studies have shown that when irradiated by UV light in the solid state [16] or in aqueous solution [20], triclosan does convert to 2,8-DCDD. Due to the potentially impor- tant environmental and human health implications of this reaction, we have clarified the photochemical behavior of triclosan in aqueous solutions. This study has investigated the role of pH and irradiation wavelength on this reaction, and experiments in Mississippi River water have been per- formed to test if the reaction will occur in natural waters. 2. Experimental 2.1. Chemicals Triclosan and p-nitroacetophenone were purchased from Aldrich. Isoprene and m-methoxyacetophenone were obtained from Acros Organics, and 2,8-dichlorodibenzo-p- dioxin (2,8-DCDD) was purchased from NeoSyn Labo- ratories. Isoprene was purified by vacuum distillation to 1010-6030/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1010-6030(03)00103-5