Effect of Manganese Oxide Aging and Structure Transformation on the Kinetics of Thiol Oxidation

The kinetics and mechanism of thiol oxidation by Mn oxides undergoing dynamic structural transformation under environmentally relevant conditions remain poorly understood. In this study, thiol/disulfide pair concentrations were simultaneously determined in situ using voltammetric microelectrodes during the interaction of four common thiols (cysteine, homocysteine, cysteamine, and glutathione) with fresh and aged delta-MnO2 at pH 7.0. The reaction kinetics was first order with respect to thiol and zero order with respect to Mn oxides. A transient intermediate sulfur surface species observed during the reaction provides evidence for a mechanism involving two successive one-electron transfer steps. The reaction kinetics of fresh and aged delta-MnO2 was investigated with cysteine and compared to that of manganite, a Mn(III) oxyhydroxide phase. The reactivity of aged delta-MnO2 decreased as a result of structural transformation to cryptomelane but remained higher than that of manganite, suggesting the potential roles of transient Mn(III) surface intermediate in promoting the reduction of Mn(IV) in delta-MnO2 and cryptomelane (compared to Mn(III) in manganite). This study demonstrates the importance of correlating Mn oxide mineral structure and redox reactivity and extends the potential for thiols commonly found in sedimentary environments to be utilized as electron shuttles during dissimilatory Mn reduction.