Revelation of Fe(V)/Fe(IV) Involvement in the Fe(VI)-ABTS System: Kinetic Modeling and Product Analysis

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%) x 10(5) M-1 s(-1)), Fe(V) (k = (2.04 +/- 0.0%) x 10(5) M-1 s(-1)), or Fe(IV) (k = (4.64 +/- 13.0%) x 10(5) M-1 s(-1)) proceeds via one-electron transfer to generate ABTS+, which is subsequently oxidized by Fe(VI) (k = (8.5 +/- 0.0%) x 102 M-1 s(-1)), Fe(V) (k = (1.0 +/- 40.0%) x 105 M-1 s(-1)), or Fe(IV) (k = (1.9 +/- 17.0%) x 10(3) 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(center dot+) 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) x 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.