Reductive dissolution of 7 Å birnessite [Mn2+0.05Mn3+0.25Mn4+0.7O1.7(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.

Mn2p3/2, 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:

\[2MnO_{2}\ +\ H_{2}SeO_{3}\ +\ H_{2}O\ {\rightarrow}\ 2S-MnOOH\ +\ H_{2}SeO_{4}\]
\[MnO_{2}\ +\ H_{2}SeO_{3}\ {\rightarrow}\ S-MnO\ +\ H_{2}SeO_{4}\]

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 × 10−3M) 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.

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