Formation of Chloroform and Chlorinated Organics by Free-Chlorine-Mediated Oxidation of Triclosan

The widely used antimicrobial agent triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) readily reacts with free chlorine under drinking water treatment conditions. Overall second-order kinetics were observed, first-order in free chlorine and first-order in triclosan. Over the pH range of 4−11.5, the kinetics were pH sensitive as a result of the pH dependent speciation of both triclosan and free chlorine. Using a Marquardt−Levenberg routine, it was determined that this pH effect indicates that the dominant reaction in this system is between the ionized phenolate form of triclosan and hypochlorous acid (HOCl). The overall second-order rate coefficient was determined to be k_ArO^- = 5.40 (±1.82) × 10³ M^-1 s^-1. Three chlorophenoxyphenols and two chlorophenols were identified by gas chromatographic−mass spectroscopic analysis. The chlorophenoxyphenol compounds include two monochlorinated triclosan derivatives (5,6-dichloro-2-(2,4-dichlorophenoy)phenol and 4,5-dichloro-2-(2,4-dichlorophenoxy)phenol) and one dichlorinated derivative (4,5,6-trichloro-(2,4-dichlorophenoxy)phenol); these species form via bimolecular electrophilic substitution of triclosan. 2,4-Dichlorophenol was detected under all reaction conditions and forms via ether cleavage of triclosan. In experiments with excess free chlorine, 2,4,6-trichlorophenol was formed via electrophilic substitution of 2,4-dichlorophenol. Chloroform formation was observed when an excess of free chlorine was present. A Hammett-type linear free-energy relationship (LFER) using Brown−Okamoto parameters (σ⁺) was established to correlate the reactivity of HOCl and the phenolate forms of triclosan and other chlorophenols (log k_ArO^- = −(10.7 ± 2.2)Σσ⁺_o,m,p + 4.43). This LFER was used to obtain estimates of rate coefficients describing the reactivity of the intermediates 5,6-dichloro-2-(2,4-dichlorophenoy)phenol (k_ArO^- ≈ 6 × 10²), 4,5-dichloro-2-(2,4-dichlorophenoxy)phenol (k_ArO^- ≈ 3 × 10²), and 4,5,6-trichloro-(2,4-dichlorophenoxy)phenol (k_ArO^- ≈ 4 × 10¹).