Enhancing the decomposition of refractory contaminants on SO42--functionalized iron oxide to accommodate surface SO4- generated via radical transfer from OH

OH or SO4− are powerful oxidants that efficiently degrade recalcitrant contaminants. The productions of OH and SO4− via activation of their precursors (H2O2 and Na2S2O8), however, can be sustainable only after continuously feeding such precursors into the activators. Motivated by the advantages of SO4− over OH as an environmental cracker, this study highlighted a simple and proficient way to persist solid-supported SO4− species used to accelerate the decomposition of recalcitrants in the presence of an electric potential. While using ubiquiotous iron oxide as a platform to accomodate SO4−, we functionalized iron oxide surface with SO42- species, which could be transformed into surface SO4− species via radical transfer from aqueous OH species. Specifically, a series of SO42--modified iron oxide catalysts were synthesized using SO2 and O2 at 300–600 °C in order to vary their surface properties such as the contents of surface Feδ+ species acting as H2O2 activators to form OH, the contents of surface SO42- species functioning as surface SO4− precursor, and the character of SO bonds innate to SO42- functionalities linked to their long-term stability. In addition to the comparison of energetics between SO42- functionalities and their SO4- analogues via computation, a kinetic assessment of reaction runs were conducted under controlled environments to gather convincing evidence that the formation of surface SO4− via its radical interconversion with aqueous OH was highly plausible and that surface SO4− would be the major decomposer of phenol (model compound of recalcitrants). In addition, 500 °C was found to be the optimized temperature to greatly populate Feδ+ and SO42- species rigidly immobilized on iron oxide surface among all temperatures studied, thereby providing the greatest activity and recyclability to degrade phenol.