Fe(III)-Activated Sulfite Revisited: The Overlooked Role of High-Valent Iron(IV)-Oxo Complex (Fe(IV)) in Water Treatment

Abstract

Sulfate radical (SO4•−) and its secondary radical (hydroxyl radical, •OH) are commonly recognized as the primary reactive intermediates formed by Fe(III)/sulfite system. However, it still remains unknown whether Fe(IV) is involved in this system where the well documented Fe(IV)-precursors (i.e., Fe(II) and persulfates) were in-situ generated. Intriguingly, we observed that methyl phenyl sulfone (PMSO2), indicative of Fe(IV) formation, was formed during methyl phenyl sulfoxide (PMSO) transformation in Fe(III)/sulfite system, which unprecedently verified that Fe(IV) played a crucial role in it. In parallel, the involvement of SO4•− and •OH in this system were also identified, but the limited •OH was proposed to be derived from hydrolysis of both Fe(IV) and SO4•−, rather than by self-decay of SO4•− alone. Moreover, the contribution of Fe(IV) relative to it of free radicals was explored by monitoring the yield of PMSO2. It was disclosed that the relative contribution of Fe(IV) was progressively promoted as Fe(III)-sulfite reaction proceeding with an upper limit of 80%-90%, and it was accelerated by promoting Fe(III) and sulfite dosages, while was declined with increasing pH. Furthermore, a kinetic model was developed, which precisely simulated kinetic traces of PMSO transformation and dissolved oxygen evolution in Fe(III)/sulfite system. More importantly, the kinetic model offered the first insight into the evolution of Fe(IV), SO4•−, and •OH, which provided in-depth mechanistic understanding of the iron-catalyzed sulfite auto-oxidation process. Considering the different chemical properties between Fe(IV) and free radicals, it is urgent to re-evaluate the decontamination process by iron/sulfite system.