Revealing the Kinetics of Fe(V/IV) Intermediates in Ferrate-Driven Contaminant Oxidation Using Dual-Probe Compounds

Ferrate (Fe(VI)) is a promising oxidant for removing trace organic contaminants (TrOCs) in water treatment, yet the reaction kinetics of its short-lived intermediates, Fe(V) and Fe(IV), remain insufficiently resolved. A dual-probe kinetic framework employing methyl phenyl sulfoxide (PMSO) and diphenyl sulfoxide (Ph₂SO) was established to independently resolve the reactivity of these coexisting species. The second-order rate constants of Fe(VI), Fe(V), and Fe(IV) with the probes were first determined, revealing that buffer composition exerted a pronounced influence on Fe(V) reactivity, whereas effects on Fe(VI) and Fe(IV) were substantially weaker. Using the dual-probe approach, the kinetic parameters, including time-integrated exposures (CT values), rate constants, and relative oxidation contributions, of high-valent iron species were determined in the oxidation of two representative TrOCs, sulfamethoxazole (SMX) and carbamazepine (CBZ). Fe(V) was the dominant oxidant in borate buffer, while phosphate complexation markedly suppressed its reactivity, increasing the relative contribution of Fe(VI); Fe(IV) contributed minimally in both systems. This study provides mechanistic insight into the distinct roles of high-valent iron species and establishes a methodologically transferable kinetic framework for resolving Fe(V) and Fe(IV) reactivity under controlled aqueous conditions.