We conducted batch-reactor experiments to measure the reductive dissolution of pyrolusite-coated (β-MnO{sub}2) quartz by Fe(II) under conditions representative of an acid mine-drainage subsurface plume. The results reveal that reductive dissolution rates were initially rapid but declined considerably as Fe(III)(aq), a product of the reductive-dissolution reaction, was removed from solution by heterogeneous precipitation. The inhibition of reductive-dissolution was attributed to blocking of the β-MnO{sub}2 surface sites by the Fe(III)(s) precipitate. Calculations of a simple model that accounts for the effects of Fe(III)(s) precipitate formation on reductive dissolution rates closely match temporal changes in Mn(II), Fe(II), and Fe(III) concentrations measured in 10 experiments, distinguished on the basis of the initial Fe(II)-to-Mn(IV) mole ratio and the initial Fe(III)(aq) concentration. The model-data comparisons reveal that the initial reaction rate on a clean β-MnO{sub}2 surface exceeds the long-term reaction rate by 3 orders of magnitude, highlighting the importance of linking Fe(III) precipitation with the reductive dissolution of β-MnO{sub}2 by Fe(II).
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