Effect of phosphate on peroxymonosulfate activation: Kinetics, mechanism and implication to in-situ chemical oxidation for water decontamination

Peroxymonosulfate (PMS, HSO5-) is an increasingly popular oxidant for groundwater remediation via in situ chemical oxidation (ISCO). However, it is desired to evaluate the effect of phosphate on the PMS oxidation of organics in detail due to the saline characteristics of groundwater. In this study, ofloxacin (OFX) as a typical antibiotic was chosen as a model pollutant. The results indicated that 64% of OFX was degraded by PMS in the presence of HPO42- (PMS/HPO42- system), and the efficiency increased from 64% to 80% when the HPO42- concentration was increased from 0.1 mol/L to 0.3 mol/L. The corresponding pseudo-first-order reaction constants were 0.0086 and 0.02 min-1, respectively. In comparison, PO43- and H2PO4- showed almost no reactivity. Electron spin response (ESR) analysis and quenching tests proved that SO4·- and 1O2 were the main oxidizing species for OFX degradation in PMS/HPO42- system. Hence, HPO42- was the most effective form in PMS activation and its performance was highly pH and ionic strength sensitive. Given the ambient pH and phosphate concentration in saline groundwater, a considerable amount of HPO42- can be produced, which then serves as a homogeneous PMS activator in ISCO process. This finding provides new insights into ISCO process using PMS.