Reaction of Fe_aq^II with Peroxymonosulfate and Peroxydisulfate in the Presence of Bicarbonate: Formation of Fe_aq^IV and Carbonate Radical Anions

Many advanced oxidation processes (AOPs) use Fenton-like reactions to degrade organic pollutants by activating peroxymonosulfate (HSO5 -, PMS) or peroxydisulfate (S2O82-, PDS) with Fe(H2O)62+ (FeaqII). This paper presents results on the kinetics and mechanisms of reactions between Feaq II and PMS or PDS in the absence and presence of bicarbonate (HCO3-) at different pH. In the absence of HCO3 -, FeaqIV,rather than the commonly assumed SO4 center dot-, is the dominant oxidizing species. Multi-analytical methods verified the selective conversion of dimethyl sulfoxide (DMSO) and phenyl methyl sulfoxide (PMSO) to dimethyl sulfone (DMSO2) and phenyl methyl sulfone (PMSO2), respectively, confirming the generation of FeaqIV by the Feaq II-PMS/PDS systems without HCO3-. Significantly, in the presence of environmentally relevant concentrations of HCO3 -, a carbonate radical anion (CO3 center dot-) becomes the dominant reactive species as confirmed by the electron paramagnetic resonance (EPR) analysis. The new findings suggest that the mechanisms of the persulfate-based Fenton-like reactions in natural environments might differ remarkably from those obtained in ideal conditions. Using sulfonamide antibiotics (sulfamethoxazole (SMX) and sulfadimethoxine (SDM)) as model contaminants, our study further demonstrated the different reactivities of FeaqIV and CO3 center dot- in the FeaqII-PMS/PDS systems. The results shed significant light on advancing the persulfate-based AOPs to oxidize pollutants in natural water.