Formation and Decay of Aqueous Fe(IV) during Fe(II) Ozonation under Acidic Conditions: Kinetics and Mechanistic Insights into pH-Dependent Behavior

Aqueous Fe(IV) is a crucial oxidant in iron-mediated oxidation processes relevant to water purification and atmospheric aqueous systems; yet, its chemical behavior under environmentally relevant pH conditions remains poorly understood. This study elucidated the pH-dependent kinetics and mechanisms of Fe(IV) formation, self-decay, and secondary reactions during Fe(II) ozonation at pH 1.0-5.0. The Fe(II)-O₃ reaction involved Fe^II(H₂O)₆²⁺ and (H₂O)₅Fe^II(OH)⁺, generating Fe(IV) and HO^•, respectively; thus, HO^• formation increased with increasing pH at pH > 4.0. The determined pK_a of Fe(IV) (Fe^IVO²⁺/(OH)Fe^IVO⁺) was 3.4, primarily modulating its pH-dependent reactivity. Fe(IV) underwent unimolecular self-decay via Fe^IVO²⁺ (k = 0.07 s^-1) and (OH)Fe^IVO⁺ (k = 9.7 s^-1), yielding Fe(III) and O₂, while its bimolecular decay via (OH)Fe^IVO⁺ (k = 8.1 × 10⁴ M^-1 s^-1) produced Fe(III) and H₂O₂. The Fe(IV)-Fe(II) reaction exhibited marked sensitivity to pH and ionic strength, driven by electrostatic interactions. Fe(IV) reacted with H₂O₂ via Fe^IVO²⁺ (k = 2.4 × 10⁴ M^-1 s^-1) and (OH)Fe^IVO⁺ (k = 6.4 × 10⁴ M^-1 s^-1), forming Fe(III) and Fe(II), respectively. A kinetic model incorporating these reactions accurately predicted Fe(IV) pH-dependent behaviors and determined the Fe(IV) reaction kinetics with methyl phenyl sulfoxide. These findings significantly advance our understanding of the pH-dependent fate of Fe(IV) in iron-based oxidation processes.