Bromide accelerates oxidation of selenite by unactivated peroxymonosulfate: PH-dependent kinetics, mechanism and...

Selenium (Se) is an essential trace element that is toxic to humans in a relatively small excess. In natural waters it occurs mainly in inorganic form as Se(IV) and Se(VI) oxyanions with the former being more toxic at high levels. With the increasing use of advanced oxidation processes in drinking water treatment, the oxidation of Se(IV) with unactivated peroxymonosulfate (PMS) has been investigated, but the role of bromide (Br^-) on the oxidation of Se(IV) during reaction with unactivated PMS remains unknown. In the present work, several influencing factors on this reaction are reported, including PMS and Se(IV) concentrations, pH, Br^-, and natural organic matter (NOM), on the oxidation of Se(IV), as well as the influence of different water matrices. Results show that the second-order rate constant for reaction of Se(IV) with PMS increases with increasing pH (5.0-10.0) from 0.02 to 0.33 M^-1s^-1, and that Se(IV) oxidation occurs mainly via a direct oxidation pathway. This increases with increasing initial concentrations of PMS and Se(IV), but is inhibited by the presence of NOM. However, the presence of Br^- significantly enhances Se(IV) oxidation at circumneutral pH, but has negligible effect in alkaline conditions. It is proposed that Se(IV) oxidation by PMS involves formation of a hypobromous acid/hypobromite (HOBr/OBr^-) intermediate in the presence of Br^-, and its formation is supported by DFT calculations. Based on these results, a kinetics model for Se(VI) formation in bromide-containing water has been developed. Also, compared to the Br^-/NOM/PMS system, the presence of Se(IV) inhibited the formation of brominated disinfection by-products (i.e., bromform and tribromoacetic acid). Overall, these results help improve our understanding of the behavior of selenium in water containing Br^- during a common oxidative treatment process.