Aqueous Iron(IV)–Oxo Complex: An Emerging Powerful Reactive Oxidant Formed by Iron(II)-Based Advanced Oxidation Processes for Oxidative Water Treatment

High-valent iron(IV)-oxo complexes are of great significance as reactive intermediates implicated in diverse chemical and biological systems. The aqueous iron(IV)-oxo complex (FeaqIVO²⁺) is the simplest but one of the most powerful ferryl ion species, which possesses a high-spin state, high reduction potential, and long lifetime. It has been well documented that FeaqIVO²⁺ reacts with organic compounds through various pathways (hydrogen-atom, hydride, oxygen-atom, and electron transfer as well as electrophilic addition) at moderate reaction rates and show selective reactivity toward inorganic ions prevailing in natural water, which single out FeaqIVO²⁺ as a superior candidate for oxidative water treatment. This review provides state-of-the-art knowledge on the chemical properties and oxidation mechanism and kinetics of FeaqIVO²⁺, with special attention to the similarities and differences to two representative free radicals (hydroxyl radical and sulfate radical). Moreover, the prospective role of FeaqIVO²⁺ in Feaq²⁺ activation-initiated advanced oxidation processes (AOPs) has been intensively investigated over the past 20 years, which has significantly challenged the conventional recognition that free radicals dominated in these AOPs. The latest progress in identifying the contribution of FeaqIVO²⁺ in Feaq²⁺-based AOPs is thereby reviewed, highlighting controversies on the nature of the reactive oxidants formed in several Feaq²⁺ activated peroxide and oxyacid processes. Finally, future perspectives for advancing the evaluation of FeaqIVO²⁺ reactivity from an engineering viewpoint are proposed.