Is Sulfate Radical Really Generated from Peroxydisulfate Activated by Iron(II) for Environmental Decontamination?

It is well documented that the traditional Fenton reagent (i.e., the combination of Fe(II) and H₂O₂) produces hydroxyl radical (^•OH) under acidic conditions, while at near-neutral pH the reactive intermediate converts to ferryl ion (Fe(IV)) that can oxidize sulfoxides to produce corresponding sulfones, markedly differing from their ^•OH-induced products. However, it remains unclear whether Fe(IV) is generated in the Fe(II) activated peroxydisulfate (PDS) process, where sulfate radical (SO₄^•-) is long recognized as the dominant intermediate in literature. Here we demonstrated that SO₄^•- oxidized methyl phenyl sulfoxide (PMSO, a model sulfoxide) to produce biphenyl compounds rather than methyl phenyl sulfone (PMSO₂). Interestingly, the formation of PMSO₂ was observed when PMSO was treated by the Fe(II)/PDS system over a wide pH range, and the yields of PMSO₂ were quantified to be ∼100% at acidic pH 3-5. The identification of Fe(IV) in the Fe(II)/PDS system could also reasonably explain the literature results on alcohol scavenging effect and ESR spectra analysis. Further, a Fe(IV)-based kinetic model was shown to accurately simulate the experimental data. This work urges re-evaluation of the Fe(II)/PDS system for environmental decontamination, given that Fe(IV) would have different reactivity toward environmental contaminants compared with SO₄^•- and/or ^•OH.