Efforts are being made to tune the
reactivity of the tetraoxy anion
of iron in the +6 oxidation state (FeVIO4
2–), 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, FeVO4
3– (FeV) and FeIVO4
4– or
FeIVO3
2– (FeIV)
species, from FeVIO4
2–, for
the treatment of polluted water with greater efficiency. Approaches
to produce FeV and FeIV species from FeVIO4
2– 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 FeVIO4
2– 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 FeVIO4
2–, 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 FeVIO4
2–. Further mechanistic elucidation of activated
ferrate systems is vital to achieve high efficiency in oxidizing emerging
pollutants in various aqueous streams.