Kinetics and mechanism of photolysis and TiO 2 photocatalysis of triclosan

Abstract

The degradations of triclosan (5-chloro-2-(2,4-dichlorophenoxy)-phenol), a potent broad-spectrum antimicrobial agent, were compared in TiO 2-only in the dark condition, photolysis, and TiO 2 photocatalysis with a UV-A lamp. TiO 2 photocatalysis more effectively degraded and mineralized triclosan compared to TiO 2-only and photolysis conditions. While triclosan removed only 30% by TiO 2-only condition within 20 min, the triclosan degradation in photolysis and photocatalysis at the same time was 75 and 82%, respectively, and TOC removal was significantly higher in photocatalysis than in photolysis. The data of kinetics showed that triclosan adsorption onto TiO 2 was fitted to Langmuir isotherm, and TiO 2 photocatalysis was fitted to Langmuir–Hinshelwood model ( b = 27.99 mM −1, K triclosan = 9.49 mM −1). The neutral range of pH was favorable to photocatalysis due to the charge effect between TiO 2 and triclosan. The addition of 2-propanol, a radical scavenger, significantly reduced the degradation of triclosan both in photolysis and photocatalysis. Dioxin-type intermediates such as dibenzo-dichloro- p-dioxin (DCDD), dibenzo- p-dioxin were produced in photolysis with and without 2-propanol, and also in photocatalysis with 2-propanol, but these intermediates were not detected in photocatalysis without 2-propanol. This result indicates that the photocatalytic degradation of triclosan is mainly achieved by radicals, and these radicals can further degrade dioxin-type intermediates once they are produced in photocatalysis.