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 SO4•- was converted to HO• at higher pH. A hypothesis was proposed that the rate of PMS photolysis into HO• and SO4•- increased with pH. The hypothesis was evidenced by the measured increase of apparent-molar absorption coefficient of PMS (εPMS, 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 SO4•- at the pH range of 8–10. The formation of HO• and SO4•- in the UV/PMS system was confirmed mainly from the cooperation of the photolysis of PMS, the decay of peroxomonosulfate radical (SO5•-) and the conversion of SO4•- to HO• by simulation and experimental results. Additionally, the apparent quantum yield for SO4•- 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.