Abstract :
The U.S. Bureau of Mines has performed aqueous SO2 leaching studies for β-MnO2, γ-MnOOH, and α-Fe2O3 under surface-chemistry-limited conditions, revealing substantially greater dissolution and S2O62− formation rates for β-MnO2 than for γ-MnOOH. Rate differences can be explained by an inner-sphere electron transfer mechanism at manganese oxides surfaces, featuring (a) chemisorption of S02 at different mineral surface sites to form SO32− and S02 precursor complexes, (b) metal reduction to form.SO3− or.SO2+ radicals, and (c).SO3− dimerization to S2O62− or oxidation to SO42−, depending on the rate of.SO3− formation at the mineral surface. Assuming electron transfer to be rate-limiting, molecular modeling and frontier molecular orbital theory explain fasterʹ S03 formation at β-MnO2 Mn4+ centers than γ-MnOOH Mn3+ ones. S02 reductive dissolution is not favorable for α-Fe2O3 which has no vacant Fe3+ eg* 3d orbitals to receive electrons.
