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

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)-O-3 reaction involved Fe-II(H2O)(6)(2+) and (H2O)(5)Fe-II(OH)(+), generating Fe(IV) and HO center dot, respectively; thus, HO center dot formation increased with increasing pH at pH > 4.0. The determined pK(a) of Fe(IV) ((FeO2+)-O-IV/(OH)(FeO+)-O-IV) was 3.4, primarily modulating its pH-dependent reactivity. Fe(IV) underwent unimolecular self-decay via (FeO2+)-O-IV (k = 0.07 s(-1)) and (OH)(FeO+)-O-IV (k = 9.7 s(-1)), yielding Fe(III) and O-2, while its bimolecular decay via (OH)(FeO+)-O-IV (k = 8.1 x 10(4) 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 (FeO2+)-O-IV (k = 2.4 x 10(4) M-1 s(-1)) and (OH)(FeO+)-O-IV (k = 6.4 x 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.