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

• Published in: Environmental science & technology Vol. 56; no. 3; pp. 1492 - 1509
• Main Authors: Wang, Zhen, Qiu, Wei, Pang, Su-yan, Guo, Qin, Guan, Chaoting, Jiang, Jin
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01.02.2022
• Subjects: Chemical properties; Coordination compounds; Electron transfer; Ferrous Compounds; Free Radicals; Hydroxyl radicals; Intermediates; Iron; Organic compounds; Oxidants; Oxidants - chemistry; Oxidation; Oxidation-Reduction; Oxidative Stress; Oxidizing agents; Oxygen - chemistry; Prospective Studies; Reaction kinetics; State-of-the-art reviews; Water Purification; Water treatment; Advanced oxidation processes; Ferryl ion species; Aqueous iron(IV)−oxo complex; Free radical; Oxidative water treatment
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.1c04530

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 (Feaq IVO2+) 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 Feaq IVO2+ 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 Feaq IVO2+ 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 Feaq IVO2+, with special attention to the similarities and differences to two representative free radicals (hydroxyl radical and sulfate radical). Moreover, the prospective role of Feaq IVO2+ in Feaq 2+ 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 Feaq IVO2+ in Feaq 2+-based AOPs is thereby reviewed, highlighting controversies on the nature of the reactive oxidants formed in several Feaq 2+ activated peroxide and oxyacid processes. Finally, future perspectives for advancing the evaluation of Feaq IVO2+ reactivity from an engineering viewpoint are proposed.