Ferrate(VI)-based synergistic oxidation processes (Fe(VI)-SOPs): Promoted reactive species production, micropollutant/microorganism elimination, and toxicity reduction

Studies have proposed diverse strategies to activate ferrate(VI) (Fe(VI)) to compensate for the deficiencies of Fe(VI) alone oxidation. This review elucidates the reaction mechanisms of Fe(VI)-based synergistic oxidation processes (Fe(VI)-SOPs), including Fe(VI)/NaClO, Fe(VI)/H2O2, Fe(VI)/PMS, Fe(VI)/O3, Fe(VI)/PAA and Fe(VI)/UV processes. Compared to Fe(VI) alone, Fe(VI)-SOPs provide additional advantages in terms of reactive oxygen species, reactive iron species, and other free radicals actively participating in the oxidation process. In addition, micropollutant oxidation, e.g., ·OH in Fe(VI)/H2O2 and Fe(VI)/O3, ·OH and SO4∙- in Fe(VI)/PMS, ·O2- in Fe(VI)/UV, and the organic free radical in Fe(VI)/PAA. This leads to an enhanced removal rates of micropollutants and the mineralization of TOC. The reaction rate constant of Fe(VI)/PAA was increased by a maximum 143.8 times. Fe(VI)-SOPs create more active sites and pathways for micropollutant oxidation, generally forming more hydroxylation products, thereby facilitating the electron-deficient pollutants to be attacked by high-valent iron. The rate constants for pollutants containing electron-deficient groups can be increased by 1.4 to 143.8 times, thus compensating for the limited oxidation capability of electron-deficient pollutants solely through Fe(VI). Furthermore, Fe(VI)-SOPs exhibited a higher bacterial inactivation capacity but a lower production of byproducts, via the pathways of quenching reactive halogen and promoting precursor oxidation. Additionally, Fe(VI)-SOPs exhibit stronger detoxification effects on comprehensive wastewater toxicity and transformation products of micropollutants by effectively destroying highly toxic groups. This work offers guidance for future research and applications of Fe(VI)-SOPs.