Photolysis of 2-mercaptobenzothiazole in aqueous medium: Laboratory and field experiments

We first investigated the direct photolysis of 2-mercaptobenzothiazole (pKa=6.94±0.05) using 313 nm radiation. The anionic form was found to be photoconverted into benzothiazole and 2-hydroxybenzothiazole when irradiated in aerated medium (Φ=0.02). Transient species assigned to the triplet excited state (λmax=510 nm, τ=0.2 μs), the solvated electron and the benzothiazolyl radical (λmax=350 nm, τ≈100 ms) were detected by laser flash photolysis (λexc=266 nm). A mechanism is proposed to rationalise all these observations. In a second step, 2-mercaptobenzothiazole was dissolved either in Milli-Q purified water buffered at pH 8.0 or in a natural water sampled in a lake and exposed to solar light. Desulfurisation into benzothiazole was clearly favoured in the latter case and the disappearance of 2-mercaptobenzothiazole was found to be four-fold faster in the natural than in the Milli-Q water. It shows that sensitised reactions mediated by the chromophoric components of natural waters significantly contribute to the transformation of 2-mercaptobenzothiazole in the aquatic environment.