Quantifying the contributions of Fe(VI), Fe(V), and Fe(IV) in contaminant oxidation by ferrate: A kinetic insight

Pivotal roles of high-valent intermediate iron species [Fe(V) and Fe(IV)] in ferrate system have been widely recognized, yet their individual contributions remain unclear. Herein, we conducted systematic investigations to differentiate Fe(VI), Fe(V) and Fe(IV) in ferrate systems. By using pyrophosphate to sequester Fe(V), we observed three distinct variation trends in the apparent rate constant (k_app,OCs,PP) with respect to the reactant molar ratio: positive correlation, negative correlation and no correlation (remaining constant). Kinetic model simulations revealed that the variation trends of k_app,OCs,PP depended on the relative reactivity of Fe(IV), and further established a relationship between k_app,OCs,PP and the actual rate constants for the reactions of Fe(VI) species with contaminants (k_Fe(VI)-OCs). Based on these findings, we proposed a generalized procedure to determine k_Fe(VI)-OCs. Subsequently, we developed a novel approach, termed the targeted sequestered kinetic method, to evaluate the individual contributions of Fe(VI), Fe(V), and Fe(IV). Experimental verification using methyl phenyl sulfoxide as a model compound demonstrated that the targeted sequestered kinetic method reliably quantifies the contributions of individual high-valent iron species. Overall, this study offers a robust method to identify high-valent iron species and advances the in-depth understanding of ferrate chemistry.