New insights into the environmental photochemistry of 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan): Reconsidering the importance of indirect photoreactions

Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) is a widely used antimicrobial agent that undergoes fairly slow biodegradation. It is often found in surface waters in both the acidic (HTric) and basic (Tric(-)) forms (pK(a) similar to 8), and it can undergo direct photodegradation to produce several intermediates including a dioxin congener (2,8-dichlorodibenzodioxin, hereafter 28DCDD). The latter is formed from Tric(-) and causes non-negligible environmental concern. Differently from current literature reports, in this paper we show that the direct photolysis would not be the only important transformation pathway of triclosan in surface waters. This is particularly true for HTric, which could undergo very significant reactions with (OH)-O-center dot and, if the laser-derived quenching rate constants of this work are comparable to the actual reaction rate constants, with the triplet states of chromophoric dissolved organic matter ((CDOM)-C-3*). Model calculations suggest that reaction with (CDOM)-C-3* could be the main HTric phototransformation pathway in deep waters with high dissolved organic carbon (DOC), while reaction with (OH)-O-center dot could prevail in low-DOC waters. In the case of Tric(-) the direct photolysis is much more important than for HTric, but triplet-sensitised transformation could produce 28DCDD + 27DCDD with higher yield compared to the direct photolysis, and it could play some role as dioxin source in deep waters with elevated DOC.