High-Valent Iron Species (Fe(IV)/Fe(V)) for Selective Oxidation of Emerging Contaminants: Mechanisms, Applications, and Future Directions

The increasing release of emerging contaminants (ECs) into aquatic systems threatens ecosystems and public health, necessitating advanced water treatment solutions. High-valent iron species (Fe(IV) and Fe(V)) have gained significant attention as highly selective oxidants for degrading ECs. These species present notable advantages over conventional oxidizing radicals (e.g., HO^• and SO₄^•-), including selective oxidation of electron-rich organic contaminants, prolonged persistence in aqueous environments, and greater resistance to scavenging by water matrices. These unique characteristics make Fe(IV)/Fe(V)-based oxidation processes particularly effective for trace-level ECs removal, achieving a superior treatment efficiency while significantly reducing undesirable byproduct formation. However, their moderate oxidative power compared to radicals demands strategic optimization to balance selectivity and reactivity. This critical review systematically evaluates Fe(IV)/Fe(V)-based oxidation processes, focusing on mechanistic insights into their selective chemistry, advanced generation and characterization approaches, and performance in complex water matrices. Key strategies to enhance their oxidative contributions, including reaction condition optimization, ligand facilitation, and hybrid catalytic activation, are critically examined. Particular emphasis is placed on small-scale practical applications, with a focus on overcoming challenges like competitive inhibition, pH sensitivity, and effluent toxicity. Finally, knowledge gaps are identified, and future research directions are proposed to advance Fe(IV)/Fe(V)-based technologies toward more efficient water treatment applications.