Influence of pH on the formation of sulfate and hydroxyl radicals in the UV/peroxymonosulfate system

The influence of pH on the degradation of refractory organics (benzoic acid, BA) in UV(254 nm)/Peroxymonosulfate (UV/PMS) system was investigated. The degradation of BA was significantly enhanced at the pH range of 8-11, which could not be explained only by the generally accepted theory that SO(4)(?-) was converted to HO(?) at higher pH. A hypothesis was proposed that the rate of PMS photolysis into HO(?) and SO(4)(?-) increased with pH. The hypothesis was evidenced by the measured increase of apparent-molar absorption coefficient of PMS (ε(PMS), 13.8-149.5 M(-1)·cm(-1)) and photolysis rate of PMS with pH, and further proved by the increased quasi-stationary concentrations of both HO(?) and SO(4)(?-) at the pH range of 8-10. The formation of HO(?) and SO(4)(?-) in the UV/PMS system was confirmed mainly from the cooperation of the photolysis of PMS, the decay of peroxomonosulfate radical (SO(5)(?-)) and the conversion of SO(4)(?-) to HO(?) by simulation and experimental results. Additionally, the apparent quantum yield for SO(4)(?-) in the UV/PMS system was calculated as 0.52 ± 0.01 at pH 7. The conclusions above as well as the general kinetic expressions given might provide some references for the UV/PMS applications.