The Chlorination of Triclosan: A Kinetic Study

Triclosan, 5-chloro-2-(2,4 dichlorophenoxy)phenol, is an anti-microbial additive in a plethora of Pharmaceutical and Personal Care Products (PPCPs) including, toothpastes, hand creams and soaps, and acne creams. Because many triclosan containing products are topical solutions that are readily washed down the drain, significant quantities of triclosan can be introduced to wastewater treatment systems and eventually, to surface waters. Consequently, triclosan has become a contaminant of concern. The reactions between triclosan and free chlorine have been examined previously; however, no kinetic data for these reactions have been reported for conditions typical of drinking water treatment. This investigation focused specifically on the kinetics of the triclosan and free available chlorine (FAC) reactions under drinking water treatment conditions. Triclosan readily reacted with free chlorine via a second-order reaction (first order with respect to each species). No significant temperature dependency was observed from 8 to 25 °C. The reaction stoichiometry was determined to be 1:1 (triclosan oxidized per free chlorine reduced and did not vary over the pH range examined (pH 4-12). However, the reaction rate coefficients exhibited a significant pH dependency. A model that incorporates the rate coefficients for the reactions between HOCl and both neutral and anionic forms of triclosan was generated to fit the experimental data. The anionic free chlorine species hypochlorite (OCl-) was determined to play an insignificant role in the overall rate of reaction, and therefore, only the reactions involving HOCl were incorporated into the model. Additionally, a hypothesized reaction mechanism was tentatively shown to fit the collected data and its strong pH dependency.