Enhanced Oxidation of Organic Contaminants by Iron(II)-Activated Periodate: The Significance of High-Valent Iron–Oxo Species

Potassium periodate (PI, KIO₄) was readily activated by Fe(II) under acidic conditions, resulting in the enhanced abatement of organic contaminants in 2 min, with the decay ratios of the selected pollutants even outnumbered those in the Fe(II)/peroxymonosulfate and Fe(II)/peroxydisulfate processes under identical conditions. Both ¹⁸O isotope labeling techniques using methyl phenyl sulfoxide (PMSO) as the substrate and X-ray absorption near-edge structure spectroscopy provided conclusive evidences for the generation of high-valent iron-oxo species (Fe(IV)) in the Fe(II)/PI process. Density functional theory calculations determined that the reaction of Fe(II) with PI followed the formation of a hydrogen bonding complex between Fe(H₂O)₆²⁺ and IO₄(H₂O)^-, ligand exchange, and oxygen atom transfer, consequently generating Fe(IV) species. More interestingly, the unexpected detection of ¹⁸O-labeled hydroxylated PMSO not only favored the simultaneous generation of ^·OH but also demonstrated that ^·OH was indirectly produced through the self-decay of Fe(IV) to form H₂O₂ and the subsequent Fenton reaction. In addition, IO₄^- was not transformed into the undesired iodine species (i.e., HOI, I₂, and I₃^-) but was converted to nontoxic iodate (IO₃^-). This study proposed an efficient and environmental friendly process for the rapid removal of emerging contaminants and enriched the understandings on the evolution mechanism of ^·OH in Fe(IV)-mediated processes.