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 ((FeaqO2+)-O-IV) 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 (FeaqO2+)-O-IV 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 (FeaqO2+)-O-IV 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 (FeaqO2+)-O-IV, with special attention to the similarities and differences to two representative free radicals (hydroxyl radical and sulfate radical). Moreover, the prospective (FeaqO2+)-O-IV in Fe-aq(2+) role of activation-initiated advanced oxidation processes (AOPs) has been intensively investigated over the past 20 aq years, which has significantly challenged the conventional recognition that free radicals dominated in these AOPs. The latest progress in identifying the contribution of (FeaqO2+)-O-IV in Fe-aq(2+)-based AOPs is thereby reviewed, highlighting controversies on the nature of the reactive oxidants formed in several Fe-aq(2+) activated peroxide and oxyacid processes. Finally, future perspectives for advancing the evaluation of (FeaqO2+)-O-IV reactivity from an engineering viewpoint are proposed.

Automated summary

High-valent iron(IV)-oxo complexes play a significant role as reactive intermediates in various chemical and biological systems. Among them, the aqueous iron(IV)-oxo complex ((FeaqO2+)-O-IV) is the simplest but also the most powerful ferryl ion species, showing high reactivity and selectivity in oxidizing water treatment. This review provides a comprehensive understanding of the chemical properties, oxidation mechanisms, and kinetics of (FeaqO2+)-O-IV, with comparisons to other free radicals. Additionally, the potential role of (FeaqO2+)-O-IV in Fe-aq(2+)-based advanced oxidation processes (AOPs) is discussed, challenging the conventional view that free radicals dominate these AOPs.