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XPS study of reductive dissolution of birnessite by H₂SeO₃ with constraints on reaction mechanism

Department of Earth Sciences, University of Western Ontario, London, Ontario, N6A 5B7 Canada

Reductive dissolution of 7 Å birnessite [Mn²⁺_0.05Mn³⁺_0.25Mn⁴⁺_0.7O_1.7(OH)_0.25] by selenious acid (H₂SeO₃) produces Mn³⁺ and Mn²⁺ surface reaction products (here represented as S-MnOOH and S-MnO, respectively) and Mn²⁺,³⁺-selenite surface complexes at the solution-mineral interface.

Mn2p_3/2, Se3d, and O1s X-ray photoelectron spectra of reacted surfaces reveal that Mn⁴⁺ of birnessite is reduced simultaneously to Mn³⁺ and Mn²⁺ while Se⁶⁺ is oxidized to Se⁴⁺ according to the probable stoichiometric reactions:

There is no XPS evidence for reduction of surface Mn³⁺ to Mn²⁺ in the presence of selenite. Whereas this reaction proceeds in the presence of arsenite, selenite apparently inhibits reduction of Mn³⁺, perhaps through formation of a strong Mn-selenite surface complex.

The rate of release of Mn²⁺ to dilute selenious acid (1.5 x 10^–3 M) is considerably lower than the rate of release to aerated, distilled water at similar pH. This behavior suggests that adsorbed selenite complexes impede the proton promoted dissolution of the soluble Mn²⁺ component of birnessite.