Simultaneous generation of free radicals, Fe(IV) and Fe(V) by ferrate activation: A review

Ferrate (FeVIO42-, Fe(VI)) is a green and effective oxidant that has been extensively studied for removing organic pollutants in water and wastewater. However, practical applications of ferrate are limited due to self-decomposition at nearly neutral pH and low removal efficiency for electron-deficient pollutants. Therefore, recent research has focused on activating Fe(VI). Although various methods for enhancing Fe(VI) oxidation efficacy have been explored and reviewed extensively, there remains a gap in the systematic comparison of the different highly reactive species involved in the Fe(VI) activation process. To bridge this gap, this review aims to comprehensively present the mechanisms, strategies, and efficiencies of generating key reactive species, including sulfate radicals (SO4•-), hydroxyl radicals (•OH), and intermediate iron species (Fe(IV) and Fe(V)), in the Fe(VI) activation system. We observed that both iron intermediates and free radicals react at much higher rate constants than their Fe(VI) precursor, resulting in rapid abatement of organic pollutants. The oxidation characteristics of these active species in Fe(VI) activation are discussed. Finally, the practicability of each Fe(VI) activating strategy is evaluated for rapid and efficient removal of organic pollutants in water and wastewater.