Reactive High-Valent Iron Intermediates in Enhancing Treatment of Water by Ferrate

Efforts are being made to tune the reactivity of the tetraoxy anion of iron in the +6 oxidation state (FeVIO₄²⁻), commonly called ferrate, to further enhance its applications in various environmental fields. This review critically examines the strategies to generate highly reactive high-valent iron intermediates, FeVO₄³⁻ (FeV) and FeIVO₄⁴⁻ or FeIVO₃²⁻ (FeIV) species, from FeVIO₄²⁻, for the treatment of polluted water with greater efficiency. Approaches to produce FeV and FeIV species from FeVIO₄²⁻ include additions of acid (e.g., HCl), metal ions (e.g., Fe(III)), and reductants (R). Details on applying various inorganic reductants (R) to generate FeV and FeIV from FeVIO₄²⁻ via initial single electron-transfer (SET) and oxygen-atom transfer (OAT) to oxidize recalcitrant pollutants are presented. The common constituents of urine (e.g., carbonate, ammonia, and creatinine) and different solids (e.g., silica and hydrochar) were found to accelerate the oxidation of pharmaceuticals by FeVIO₄²⁻, with potential mechanisms provided. The challenges of providing direct evidence of the formation of FeV/FeIV species are discussed. Kinetic modeling and density functional theory (DFT) calculations provide opportunities to distinguish between the two intermediates (i.e., FeIV and FeV) in order to enhance oxidation reactions utilizing FeVIO₄²⁻. Further mechanistic elucidation of activated ferrate systems is vital to achieve high efficiency in oxidizing emerging pollutants in various aqueous streams.