Influence of pH on the Formation of Sulfate and Hydroxyl Radicals in the UV/Peroxymonosulfate System

TLDR

The study examined how pH influences benzoic acid degradation in a UV(254 nm)/peroxymonosulfate system and proposed that higher pH increases the photolysis rate of PMS into hydroxyl and sulfate radicals. Degradation proceeds via PMS photolysis, SO5•– decay, and SO4•– conversion to HO•, with higher pH raising the apparent absorption coefficient and photolysis rate, thereby elevating quasi‑stationary concentrations of both radicals. Benzoic acid removal was markedly improved at pH 8–11, a result supported by the observed rise in PMS absorption, photolysis rate, and radical concentrations, and a calculated SO4•– quantum yield of 0.52 ± 0.01 at pH 7, offering kinetic expressions for UV/PMS applications.

Abstract

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.