Revelation of Fe(V)/Fe(IV) Involvement in the Fe(VI)-ABTS System: Kinetic Modeling and Product Analysis.
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To quantitatively probe iron intermediate species [Fe(V)/Fe(IV)] in Fe(VI) oxidation, this study systematically investigated the reaction kinetics of Fe(VI) oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic)acid (ABTS) at different ratios of [ABTS]0/[Fe(VI)]0 (i.e., >1.0, =1.0, and <1.0) in pH 7.0 phosphate (10 mM)-buffered solution. Compared to the literature, a more comprehensive and robust kinetic model for the Fe(VI)-ABTS system including interactions between high-valent iron species [Fe(VI), Fe(V), and Fe(IV)], ABTS, and the ABTS+ radical was proposed and validated. The oxidation of ABTS by Fe(VI) (k = (5.96 0.9%) 105 M-1 s-1), Fe(V) (k = (2.04 0.0%) 105 M-1 s-1), or Fe(IV) (k = (4.64 13.0%) 105 M-1 s-1) proceeds via one-electron transfer to generate ABTS+, which is subsequently oxidized by Fe(VI) (k = (8.5 0.0%) 102 M-1 s-1), Fe(V) (k = (1.0 40.0%) 105 M-1 s-1), or Fe(IV) (k = (1.9 17.0%) 103 M-1 s-1), respectively, via two-electron (oxygen atom) transfer to generate colorless ABTSox. At [ABTS]0/[Fe(VI)]0 > 1.0, experimental data and model simulation both indicated that the reaction stoichiometric ratio of Fe(VI)/ABTS+ increased from 1.0:1.0 to 1.0:1.2 as [ABTS]0 was increased. Furthermore, the Fe(VI)-ABTS-substrate model was developed to successfully determine reactivity of Fe(V) to different substrates (k = (0.7-1.42) 106 M-1 s-1). Overall, the improved Fe(VI)-ABTS kinetic model provides a useful tool to quantitatively probe Fe(V)/Fe(IV) behaviors in Fe(VI) oxidation and gains new fundamental insights.