The Effect and Adjustment of Ferrate Species in Ferrate-Based Advanced Oxidation Processes for Wastewater Treatment: A Systematic Review

Interest in the combination of ferrates and advanced oxidation processes (AOPs) for wastewater treatment has increased, as revealed in this systematic review. In this study, the multiple functions of Fe(VI) in ferrate-based AOPs are summarized based on the Fe species. Various enhanced oxidation pathways are achieved through electron capture by Fe(VI), oxidation by Fe(V) and Fe(IV), or the catalytic effects of Fe(III) and Fe(II). The different contributions of high-valent Fe species and general reactive oxidation species are highlighted by analyzing the results of quenching, methyl phenyl sulfoxide probing, and electron paramagnetic resonance analysis. Methods that are used to adjust the Fe species, including changing the reaction pH, oxidant dosage, dosing pattern, and the addition of reducing or complexing additives, can influence the enhancement efficiency of micropollutant treatment from the perspective of determining the transformation from Fe(VI) to Fe(V) and Fe(IV) with higher reactivity or Fe(III) and Fe(II) circulation. Future studies should focus on the in situ production of high-valent Fe and oxidation pathwaybased adjustments in Fe(VI)-AOP techniques.