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.

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This paper presents an advanced oxidation process (AOP) of peracetic acid (PAA) and ruthenium(III) (Ru(III)) to oxidize micropollutants in water. Studies of PAA−Ru(III) oxidation of sulfamethoxazole (SMX), a sulfonamide antibiotic, in 0.5−20.0 mM phosphate solution at different pH values (5.0− 9.0) showed an optimum pH of 7.0 with a complete transformation of SMX in 2.0 min. At pH 7.0, other metal ions (i.e., Fe(II), Fe(III), Mn(II), Mn(III), Co(II), Cu(II), and Ni(II)) in 10 mM phosphate could activate PAA to oxidize SMX only up to 20%. The PAA−Ru(III) oxidation process was also unaffected by the presence of chloride and carbonate ions in solution. Electron paramagnetic resonance (EPR) measurements and quenching experiments showed the dominant involvement of the acetyl(per)oxyl radicals (i.e., CH 3 C(O)O • and CH 3 C(O)OO • ) for degrading SMX in the PAA−Ru(III) oxidation process. The transformation pathways of SMX by PAA−Ru(III) were proposed based on the identified intermediates. Tests with other pharmaceuticals demonstrated that the PAA−Ru(III) oxidation system could remove efficiently a wide range of pharmaceuticals (9 compounds) in the presence of phosphate ions in 2.0 min at neutral pH. The knowledge gained herein on the effective role of Ru(III) to activate PAA to oxidize micropollutants may aid in developing Ru(III)containing catalysts for PAA-based AOPs.

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