Photo-persulfate oxidation and mineralization of benzoic acid: Kinetics and optimization under UVC irradiation

The strong oxidant, persulfate (PS, S2O82-), was applied to treat the synthetic wastewater of benzoic acid (BA) under UV irradiation. UVC light initiated a chain reaction that derived the sulfate radical (SO4•-) and hydroxyl radical (HO•) from S2O82- ion. The experiment parameters, including light irradiation (UVA and UVC), pH, dose ratio ([PS]0/[BA]0), initial concentration ([BA]0, mg/L), was optimized based on degradation efficiency and total organic carbon (TOC) removal of BA, which reached up to 100% and 96%, respectively, under pH 3.0. The best dose ratio was close to equivalent stoichiometry (and [PS]0/[BA]0 = 15) for the treatment of 100 mg-BA/L, suggesting that UV/S2O82- was able to completely convert BA to carbon dioxide and water. The scavenging test showed that SO4•- contributed to about 60% of degradation rate, which the HO• predominated the mineralization rate, i.e., TOC removal. A consecutive kinetic model was proposed to clarify the reaction sequence and rate-determining factor of photo-persulfate oxidation for benzoic acid.