Abstract:
Reductive dissolution of synthetic birnessite (MnO1.7(OH)0.25 or MnO1.95) by arsenious acid (H3AsO3) proceeds in two steps. The first entails reduction of Mn(IV) to Mn(III), with stoichiometry: 2MnO2+H3AsO3=2MnOOH*+H3AsO4H3AsO3 then attacks MnOOH* according to the stoichiometric reaction: 2MnOOH*+H3AsO3=2MnO+H3AsO4+H2O, where MnOOH* is an intermediate reaction product. Mn(II) is released ultimately to solution. Most importantly, one electron is transferred to each metal ion per reaction step. A Mn(III) component of the original, synthetic birnessite also undergoes reductive dissolution independently of, and at a different rate than, reduction of MnOOH*.X-ray Photoelectron Spectroscopy (XPS) demonstrates formation of an intermediate reaction product composed of Mn(III), hydroxyl, and H2O (here represented as MnOOH*). MnOOH* increases to a maximum value and subsequently decreases, as expected of an intermediate reaction product of a consecutive reaction scheme. Seven reactions are required to represent adequately reductive dissolution of birnessite. These include redox and sorption reactions. A Monte Carlo simulation successfully reproduces the major features of both XPS and previously published leach-rate results.Reductive dissolution of birnessite may proceed either via a classic electron transfer mechanism by which a bidentate surface complex forms, or via a substitution reaction mechanism, by which a monodentate surface complex forms. X-ray absorption spectroscopic (XAS) studies may be used to identify the appropriate mechanism.