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Updated: Jan 10, 2026

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Formation and Decay of Aqueous Fe(IV) during Fe(II) Ozonation under Acidic Conditions: Kinetics and Mechanistic
Zhen Wang1, Jaedon Shin2, Jaesang Lee3
1Department of Environment and Energy Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea.
Abstract:
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 FeII(H2O)62+ and (H2O)5FeII(OH)+, generating Fe(IV) and HO•, respectively; thus, HO• formation increased with increasing pH at pH > 4.0. The determined pKa of Fe(IV) (FeIVO2+/(OH)FeIVO+) was 3.4, primarily modulating its pH-dependent reactivity. Fe(IV) underwent unimolecular self-decay via FeIVO2+ (k = 0.07 s-1) and (OH)FeIVO+ (k = 9.7 s-1), yielding Fe(III) and O2, while its bimolecular decay via (OH)FeIVO+ (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 FeIVO2+ (k = 2.4 × 104 M-1 s-1) and (OH)FeIVO+ (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.
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