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

The question of whether Fe(IV) or SO₄ ^{â -} is the dominant intermediate in the Fe(II)-Activated peroxydisulfate process [Fe(II)/PDS process] remains unanswered. In this study, besides Fe(IV), SO₄ ^{â -} and HO^â 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 ^{â -}/HO^â with an increase in the PDS/Fe(II) molar ratio at pH 3.0. Fe(IV), SO₄ ^{â -}, and HO^â 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 ^{â -}, and HO^â 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 ^{â -}, and HO^â ] 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.