Fe(iv)/Fe(v) species in iron-based advanced oxidation processes: from formation mechanisms to applications.

Iron-based advanced oxidation processes (Fe/AOPs) have been widely employed for the treatment of highly toxic and refractory organic pollutants owing to their high efficiency and environmental compatibility. Among these, the in situ generated high-valent iron species Fe(iv)/Fe(v) are recognized as key reactive intermediates. Although several reviews on Fe(iv)/Fe(v) have recently been published, most of them focus primarily on strategies to enhance oxidative performance, with limited critical insights and molecular-level discussions regarding the oxidative and reductive formation mechanisms of Fe(iv)/Fe(v) in the presence of different precursor oxidants. Moreover, the applicability of the "two-electron transfer" mechanism under varying reaction conditions remains insufficiently examined. The review critically examines the oxidative and reductive formation mechanisms of Fe(iv)/Fe(v) across diverse Fe/AOP systems, elucidating the differences in formation pathways among various reaction systems. This work further evaluates the applicability of different analytical techniques for Fe(iv)/Fe(v) identification and proposes a quantitative approach to accurately assess the contribution of these reactive species. In addition, the innovative applications of Fe(iv)/Fe(v) in the degradation of toxic organic compounds, as well as the immobilization and recovery of heavy metals, were analyzed. The review not only provides a new cognitive framework for improving the theoretical system of Fe/AOPs-based oxidation but also lays a theoretical foundation for its engineering applications in the field of water treatment.