Both Fe(IV) and Radicals Are Active Oxidants in the Fe(II)/Peroxydisulfate Process

The question of whether Fe(IV) or SO4•– is the dominant intermediate in the Fe(II)-activated peroxydisulfate process [Fe(II)/PDS process] remains unanswered. In this study, besides Fe(IV), SO4•– and HO• were shown to be produced in the Fe(II)/PDS process by using multiple probes [dimethyl sulfoxide, methyl phenyl sulfoxide, p-nitrobenzoic acid (p-NBA), and benzoic acid (BA)]. The removal of p-NBA and BA and the influence of BA on the yield of methyl phenyl sulfone (PMSO2) indicated that the major oxidizing intermediate changed from Fe(IV) to SO4•–/HO• with an increase in the PDS/Fe(II) molar ratio at pH 3.0. Fe(IV), SO4•–, and HO• were all involved in this process at pH 3.0–6.5, but their available amounts that contributed to abating organic contaminants decreased with an increase in pH considering the influence of pH on the generation of PMSO2 and p-hydroxybenzoic acid. Furthermore, Fe(IV), SO4•–, and HO• contributed differently to abating different organic contaminants because of the different reactivities of these oxidizing oxidants toward different organic contaminants. Overall, this study demonstrates that multiple oxidizing species [Fe(IV), SO4•–, and HO•] are generated in the Fe(II)/PDS process, which was significant for the application of this process and understanding the mechanisms of Fe(II)-activated peroxide processes.