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)–O 3 reaction involved Fe II (H 2 O) 6 2+ and (H 2 O) 5 Fe II (OH) +, generating Fe(IV) and HO •, respectively; thus, HO • formation increased with increasing pH at pH > 4.0. The determined p K a of Fe(IV) (Fe IV O 2+ /(OH)Fe IV O + ) was 3.4, primarily modulating its pH-dependent reactivity. Fe(IV) underwent unimolecular self-decay via Fe IV O 2+ ( k = 0.07 s –1 ) and (OH)Fe IV O + ( k = 9.7 s –1 ), yielding Fe(III) and O 2, while its bimolecular decay via (OH)Fe IV O + ( k = 8.1 × 10 4 M –1 s –1 ) produced Fe(III) and H 2 O 2 . The Fe(IV)–Fe(II) reaction exhibited marked sensitivity to pH and ionic strength, driven by electrostatic interactions. Fe(IV) reacted with H 2 O 2 via Fe IV O 2+ ( k = 2.4 × 10 4 M –1 s –1 ) and (OH)Fe IV O + ( k = 6.4 × 10 4 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.