Oxidation of the antimicrobial agent triclosan by aqueous ozone (O-3) was investigated to determine associated reaction kinetics, reaction site(s), and consequent changes in antibacterial activity of triclosan. Specific second-order rate constants, k(O3), were determined for reaction of O-3 with each of triclosan's acid-base species. The value of k(O3) determined for neutral triclosan was 1.3 (+/- 0.1) x 10(3) M-1 s(-1), while that measured for anionic triclosan was 5.1 (+/- 0.1) x 10(8) M-1 s(-1). Consequently, triclosan reacts very rapidly with O-3 at circumneutral pH (the pH-dependent, apparent second-order rate constant, k(app,O3), is 3.8 x 10(7) M-1 s(-1) at pH 7). The pH-dependence of k(app,O3) and comparison of triclosan reactivity toward O-3 with that of other phenolic compounds indicates that O-3 reacts initially with triclosan at the latter's phenol moiety. k(O3) values for neutral and anionic triclosan were successfully related to phenol ring substituent effects via Brown-Okamoto correlation with other substituted phenols, consistent with electrophilic attack of the triclosan phenol ring. Biological assay of O-3-treated triclosan solutions indicates that reaction with O-3 yields efficient elimination of triclosan's antibacterial activity. In order to evaluate the applicability of these observations to actual wastewaters, triclosan oxidation was also investigated during ozonation of effluent samples from two conventional wastewater treatment plants. Nearly 100% triclosan depletion was achieved for a 4 mg/L (8.3 x 10(-5) mol/L) O-3 dose applied to a wastewater containing 7.5 mg/L of DOC, and degrees similar to 58% triclosan depletion for dosage of 6 mg/L (1.3 x 10(-4) mol/L) O-3 to a wastewater containing 12.4 mg/L of DOC. At O-3 doses greater than 1 mg/L (2.1 x 10(-5) mol/L), hydroxyl radical reactions accounted for < 35% of observed triclosan losses in these wastewaters, indicating that triclosan oxidation was due primarily to the direct triclosan-O-3 reaction. Thus, ozonation appears to present an effective means of eliminating triclosan's antibacterial activity during wastewater treatment. (c) 2007 Elsevier Ltd. All rights reserved.