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Solution equilibria of uranyl minerals: Role of the common groundwater ions calcium and carbonate
Citation:
Stanley, D. AND Richard T. Wilkin. Solution equilibria of uranyl minerals: Role of the common groundwater ions calcium and carbonate. JOURNAL OF HAZARDOUS MATERIALS. Elsevier Science Ltd, New York, NY, 377:315-320, (2019). https://doi.org/10.1016/j.jhazmat.2019.05.101
Impact/Purpose:
Uranium is a naturally occurring metal contained in bedrock and aquifer solids which can be dissolved into groundwater through water-rock interactions. The transport and fate of uranium in groundwater environments depends on geochemical parameters such as pH, redox conditions, and the presence of ligand-forming species such as dissolved carbonate, Ca2+, and Mg2+ ions. This study examines the solubility of uranyl minerals in the presence of common groundwater ions to better understand the long-term fate of uranium and possible attenuation processes in moderately oxidizing groundwater systems.
Description:
Understanding the factors that govern aqueous solubility of uranyl minerals is important for predicting uranium mobility in groundwater and for designing effective remediation strategies. The uranyl-containing minerals metaschoepite [UO3∙(2H2O)] and uranophane [Ca(UO2)2(SiO3OH)2·5H2O] were synthesized and evaluated in batch solubility experiments conducted in the presence of common groundwater ions: calcium, bicarbonate/carbonate, and dissolved silica. Solid-phase characterization revealed the expected structural and thermogravimetric properties of metaschoepite and uranophane. Metaschoepite solubility in carbonate-free water followed a u-shaped pH dependency with minimum solubility near pH 8.5; uranium concentrations at pH ≳ 8.5 were approximately equivalent to the reference value for safe drinking water established by the EPA (30 µg/L). With increasing bicarbonate/carbonate concentration (1 mM – 50 mM) the solubility of metaschoepite increased, presumably due to the formation of uranyl-carbonate complexes. However, the experimental concentrations of uranium were lower than concentrations predicted from accepted complexation constants. For uranophane, equilibrium uranium concentrations were 7). The diversity of uranyl minerals that possibly form in the presence of common groundwater species: Ca-Na-Mg-K-Si-bicarbonate/carbonate-sulfate-chloride, has not been fully explored with respect to understanding potential mineral transformations and impacts on uranium solubility.
URLs/Downloads:
DOI: Solution equilibria of uranyl minerals: Role of the common groundwater ions calcium and carbonate
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