XPS STUDY OF REDUCTIVE DISSOLUTION OF 7#9-BIRNESSITE BY H3ASO3, WITH CONSTRAINTS ON REACTION MECHANISM - PART 1. EXAFS STTUDIES OF THE GEOMETRY OF COPRECIPITATED AND ADSORBED ARSENATE

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.