Microscopic mechanisms of dissolution and precipitation of manganese minerals.

摘要

Dissolution and precipitation of manganese minerals, especially manganese oxides and carbonates, alter the pE and pH of water, control the availability of micronutrients, and affect the fate and transport of contaminants in the aqueous environment. Therefore, a detailed understanding of microscopic mechanisms of dissolution and precipitation of manganese minerals is essential to develop quantitative geochemical models that facilitate the effective remediation of contaminated sites.; When dissolution and precipitation occur simultaneously, the macroscopic dissolution rates determined by measuring the released Mn2+(aq) concentrations underestimate the intrinsic dissolution rates from the mineral by the precipitation rates. The discrepancies can be resolved by including microscopic observations.; In this thesis, microscopic mechanisms of dissolution and precipitation of manganese minerals are proposed for three different cases, all of which are relevant to the natural environment. First, ascorbic acid significantly promotes reductive dissolution of gamma-MnIIIOOH (manganite) under oxic conditions. Increased Mn2+(aq) concentrations result in the precipitation of MnIII-(hydr)oxide at pH > 5. A two-step mechanism is proposed: (1) Reduction and release of Mn2+(aq) (thermodynamically favorable under all conditions employed). (2) Oxidation and precipitation of a MnIII-(hydro)oxide (thermodynamically possible only at pH > 5).; Second, the growth of manganese oxide coating on the (101¯4) MnCO 3 surface at circumneutral pH depends on the substrate morphology, the substrate atomic structure, and the concentration of Mn2+(aq). On terraces, rhombohedral oxide islands form with 90° rotation relative to the crystallographic axis of the underlying substrate and have self limitation in z-directional growth. Lattice mismatches can play a role in controlling the oxide coating growth. The manganese oxide coating decreases the macroscopic dissolution rates by 75% compared to bare MnCO3.; Third, foreign ions, especially Co2+(aq), affect dissolution and precipitation of manganese oxide coating on carbonate. Addition of Co 2+(aq) leads to the dissolution of the manganese oxide film. Simultaneously, there is growth of less soluble Mn-Co oxide islands having multilayer structures that are unrestrained in the z-direction.; The relaxed substrate structure can be a critical parameter for dissolution, precipitation, and sorption of metal ions. Based on X-ray scattering measurements, the cleaved (101¯4) MnCO3 surface under humid conditions has a relaxed structure with a well-ordered hydration layer.