XPS STUDY OF REDUCTIVE DISSOLUTION OF 7-ANGSTROM-BIRNESSITE BY H3ASO3, WITH CONSTRAINTS ON REACTION-MECHANISM

Reductive dissolution of synthetic birnessite (MnO1.7(OH)(0.25) or MnO 1.95) by arsenious acid (H3AsO3) proceeds in two steps. The first enta ils reduction of Mn(IV) to Mn(III), with stoichiometry: 2MnO(2) + H3As O3 = 2MnOOH + H3AsO4 H3AsO3 then attacks MnOOH* according to the stoi chiometric reaction: 2MnOOH + H3AsO3 = 2MnO + H3AsO4 + H2O, where MnO OH is an intermediate reaction product. Mn(II) is released ultimately to solution, Most importantly, one electron is transferred to each me tal ion per reaction step. A Mn(III) component of the original, synthe tic 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 there represented as Mn OOH). MnOOH* increases to a maximum value and subsequently decreases, as expected of an intermediate reaction product of a consecutive reac tion scheme. Seven reactions are required to represent adequately redu ctive dissolution of birnessite. These include redox and sorption reac tions. A Monte Carlo simulation successfully reproduces the major feat ures of both XPS and previously published leach-rate results. Reductiv e 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 comp lex forms. X-ray absorption spectroscopic (XAS) studies may be used to identify the appropriate mechanism. Copyright (C) 1998 Elsevier Scien ce Ltd.