Reductive dissolution of manganese(IV) oxides and precipitation of iron(III): Implications for redox processes in an alluvial aquifer affected by acid mine drainage

The processes that control the reductive dissolution of MnO₂ by Fe(II) under conditions simulating the effects of acid mine drainage on subsurface environments and the subsequent precipitation of Fe(III) has been investigated. Results from real-time, in situ X-ray absorption spectroscopy (XAS) flow-through reaction cell studies indicate that a mixed Fe/Mn solid phase with the local structure of the spinel mineral jacobsite (MnFe₂O₄) is formed after the MnO₂ surfaces are coated with ferric precipitates. In the absence of previously precipitated Fe(III), no reduced manganese solid is formed. The ferric precipitates do not incorporate significant quantities of Mn(II) down gradient from the reactive Fe(II) front. The maximum amount of the original MnO₂ incorporated into this jacobsite-like solid is 5%. Results from batch experiments showed similar results compared to the flow-through experiments, with an initially fast rate of Mn(II) release, followed by a much slower release after 5–10 min had elapsed. The reaction products, Fe(III)(aq) and Fe(III)(s) were found to decrease the initial reaction rate. A simple model was developed to describe the temporal concentrations of Mn(II)(aq), Fe(II)(aq), and Fe(III)(aq) that include a Langmurian blocking function to describe the effects of the ferric reaction products on the reaction rate. The model also allowed for a second order process to occur at long time that was dependent solely on the aqueous concentrations of Fe(II) and MnO₂. The formation of the ferric reaction products were found to transform from aqueous sulfate complexes to (presumable) ternary surface complexes with sulfate. Within 10 h, these precipitates may have formed chains of edge-sharing octahedra on the order of 60 Å. The precipitates have large amount of sulfate associated with them, which may preclude the formation of ferrihydrite, and may indicate the formation of schwertmannite. The average Fe:SO₄ ratio was 4.4 ± 1.0, a value similar to that reported for schwertmannite. The presence of goethite was identified by X-ray diffraction as early as 50 d, indicating that sulfate is being excluded from the precipitates. The release of Mn(II), Fe_T, and sulfate was controlled by diffusion, which may also be the process that controls the rate of transformation.