XPS study of reductive dissolution of 7A-birnessite by H_3AsO_3, with constraints on reaction mechanism

摘要

Reductive dissolution of synthetic birnessite (MnO_(1.7)(OH)_(0.25) or MnO_(1.95)) by arsenious acid (H_3AsO_3) proceeds in two steps. The first entails reduction of Mn(IV) to Mn(III), with stoichiometry: 2MnO_2 + H_3AsO_3 = 2MnOOH~* + H_3AsO_4 H_3AsO_3 then attacks MnOOH~* according to the stoichiometric reaction: 2MnOOH~* + H_3AsO_3 = 2MnO + H_3AsO_4 + H_2O, 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 H_2O (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.