XPS study of dissolution of birnessite by humate with constraints on reaction mechanism

Abstract

The present study is an investigation into the surface reactions of synthetic birnessite [Mn(II) 0.05Mn(III) 0.25Mn(IV) 0.7O 1.7(OH) 0.25] when reacted with dilute (5 × 10 −4 M) Na-humate solution using X-ray Photoelectron Spectroscopy (XPS). Mn2p 3/2, C1s and O1s spectra of reacted surfaces reveal that Mn(IV) of birnessite is reduced to Mn(III) while humate is oxidized to CO 2. Initial rapid production of CO 2 results in accumulation of CO 2 at the reaction interface. After approximately 15 min, the reaction rate decreases to the point where desorption keeps pace with accumulation until after 5 h of reaction, when CO 2 is no longer detected at the reaction surface. Reduction of Mn(IV) of birnessite to Mn(III) indicates that the MnO 2-oxalate redox reaction proceeds as a transfer of one electron per metal centre. There is no XPS evidence for reduction of Mn(III) of the solid to Mn(II) in the presence of humate. The carboxyl group of humate apparently inhibits this step, most likely through formation of strong Mn(III)-carboxyl surface complexes. This postulate is consistent with Mn 3+(aq) stabilization by oxalate in aqueous solutions. Further study using X-ray absorption spectroscopy (XAS) is required for a better understanding of the structure of the surface complexes. The rate of release of soluble Mn(II) into dilute humate solutions (5 × 10 −4 M) is lower by more than two orders of magnitude compared with rate of release to aerated, distilled water at similar pH. The process of proton promoted dissolution of the soluble Mn(II) component of birnessite in distilled water would appear to be impeded by the addition of humate. This could be attributed to the formation of strong, multinuclear surface complexes between Mn(II, III) and the adsorbed carboxyl groups.