Inhibitory effects of natural organic matter on methyltriclosan photolysis kinetics.

This study evaluated the effects and related mechanisms of natural organic matter (NOM) on the photolysis of methyltriclosan (MTCS), a metabolite of triclosan. Addition of two representative NOM isolates, Pony Lake fulvic acid (PLFA-microbial origin) and Suwannee River fulvic acid (SRFA-terrestrial origin), significantly inhibited the direct photolytic rate of MTCS by ∼70%. The MTCS photolytic rate in the presence of PLFA was greater than for SRFA. NOM not only suppressed photolysis by light-shielding, but also produced ROS to oxidatively degrade MTCS and/or triplet NOM ((3)NOM*) to sensitize degradation. The dual effects of light-screening and photo-sensitization led to an overall decrease in photolysis of MTCS with a positive concentration-dependence. Upon addition of NOM, EPR documented the occurrence of (1)O(2) and ˙OH in the photolytic process, and the bimolecular k value for the reaction of (1)O(2) with MTCS was 1.86 × 10(6) M(-1) s(-1). ROS-quenching experiments indicated that the contribution of ˙OH (19.1-29.5%) to indirect photolysis of MTCS was lower than for (1)O(2) (38.3-58.7%). Experiments with D(2)O further demonstrated that (1)O(2) participated in MTCS photodegradation. Moreover, the addition of sorbic acid and O(2) gas to the reaction confirmed the participation of (3)NOM* as a key reactant in the photochemical transformation of MTCS. This is the first comprehensive analysis of NOM effects on the indirect photolysis of MTCS, which provides new insights for understanding the environmental fate of MTCS in natural environments.