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)(center dot-) was converted to HO(center dot) at higher pH. A hypothesis was proposed that the rate of PMS photolysis into HO(center dot) and SO(4)(center dot-) increased with pH. The hypothesis was evidenced by the measured increase of apparent-molar absorption coefficient of PMS (epsilon(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(center dot) and SO(4)(center dot-) at the pH range of 8-10. The formation of HO(center dot) and SO(4)(center dot-) in the UV/PMS system was confirmed mainly from the cooperation of the photolysis of PMS, the decay of peroxomonosulfate radical (SO(5)(center dot-)) and the conversion of SO(4)(center dot-) to HO(center dot) by simulation and experimental results. Additionally, the apparent quantum yield for SO(4)(center dot-) 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.