Coupling of the Biogeochemical Cycles of Uranium and Manganese: Implications for the Fate and Transport of Uranium in Subsurface Environments

Organic electron donor stimulated microbial reduction of U(VI) to U(IV) has been proposed as a strategy for the in situ immobilization of uranium contamination in the subsurface. The success of the bioremediation of uranium relies on the stability of the reduced U(IV) species: e.g., UO2) with respect to reoxidation and/or remobilization under groundwater conditions. Manganese is present at appreciable concentrations at several uranium-contaminated sites, and the redox cycling of manganese may significantly impact uranium\u27s fate and transport. The biogeochemical coupling of uranium and manganese involves multiple interaction pathways that occur in the aqueous phase as well as at solid-water interfaces. A mechanistic and quantitative understanding of these processes is needed to establish input parameters for reactive transport models and to enable decision-making for remediation strategies. Coupling of the biogeochemical cycles of uranium and manganese involves various interfacial reactions that occur between UO2 and Mn species of various oxidation states: +IV, +III and +II). This study investigated the physical and chemical factors controlling the interactions between uraninite: UO2) and manganese oxide: MnO2), which are both poorly soluble minerals. A multi-chamber reactor with a permeable membrane was used to simulate a barrier for direct contact of the two solids. The results suggested that an effective redox reaction between UO2 and MnO2 requires physical contact. Continuously-stirred tank reactors: CSTRs) were used to evaluate the dissolution rates of UO2. MnO2 dramatically promoted UO2 dissolution, but the degree of promotion leveled off once the MnO2:UO2 ratio exceeded a critical value. The fate of uranium and manganese after the reaction was investigated by chemical extraction and X-ray absorption spectroscopy: XAS). Substantial amounts of U(VI) and Mn(II) were retained on MnO2 surfaces, and the fate of Mn products may involve Mn(III) phases. A conceptual model was proposed to describe the oxidation of UO2 by MnO2, which is potentially applicable to other environmental redox processes involving two poorly soluble minerals. Although MnO2 can oxidize UO2, the U(VI) produced may not be readily released into the aqueous phase due to its strong adsorption to MnO2. This study integrated batch experiments of U(VI) adsorption to synthetic and biogenic MnO2, surface complexation modeling: SCM), and molecular-scale characterization of adsorbed U(VI) with extended X-ray absorption fine structure: EXAFS) spectroscopy. The surface complexation model incorporated the surface complexes that are consistent with EXAFS analysis, and it could successfully simulate adsorption results over a broad range of pH and dissolved inorganic carbon concentrations. The description of bidentate surface complexes, which are widely observed for contaminant adsorption to metal oxides including the U(VI)-MnO2 system, is a subject with considerable confusion in the literature. Consequently, a critical review was prepared that discussed the theoretical and practical aspects of mass action expressions for bidentate surface complexation reactions. Suggestions were provided for handling bidentate reactions and publishing results without ambiguity or confusion. The effects of soluble Mn species: +III and +II oxidation states) on UO2 dissolution were also investigated. Soluble Mn(III) species were recently identified as important intermediates in Mn biogeochemical cycling. This study evaluated the kinetics of oxidative UO2 dissolution by soluble Mn(III) stabilized by pyrophosphate: PP) and desferrioxamine B: DFOB). The Mn(III)-PP complex was a potent oxidant that induced rapid UO2 dissolution at a rate higher than by a comparable concentration of dissolved O2. However, the Mn(III)-DFOB complex was not able to induce oxidative dissolution of UO2. The potency of Mn(III) with respect to oxidizing UO2 was governed by the identity of the ligand and water chemistry parameters that affect the speciation of the Mn(III). The effect of soluble Mn(II) was more complicated than that of non-redox-active divalent cations: e.g., Ca and Zn). Under anoxic conditions, Mn(II) inhibited UO2 dissolution, which may be attributed to both Mn(II) adsorption to the UO2 surface and precipitation of MnCO3, both of which could decrease the exposure of U(IV) surface sites. In contrast to the anoxic conditions, Mn(II) promoted UO2 dissolution under oxidizing condition. The promotional effect was likely due to Mn redox cycling in which oxidized forms of Mn species were: re)generated as oxidants of UO2 that were more potent than O2. The observed effects of soluble Mn(II, III) species on UO2 dissolution highlighted the need to consider Mn redox intermediates in facilitating electron transfer processes in subsurface biogeochemical cycles