Solution equilibria of uranyl minerals: Role of the common groundwater ions calcium and carbonate

JOURNAL OF HAZARDOUS MATERIALS

卷:377

Article

Stanley, Dovie M.¹  Wilkin, Richard T.² 

[1] US EPA, Oak Ridge Associated Univ, Off Res & Dev, Natl Risk Management Res Lab,Groundwater Watershe, Ada, OK 74820 USA

[2] US EPA, Off Res & Dev, Natl Risk Management Res Lab, Groundwater Watershed & Ecosyst Restorat Div, Ada, OK 74820 USA

关键词: Uranium;    Groundwater;    Remediation;    Metaschoepite;    Uranophane;   

DOI  :  10.1016/j.jhazmat.2019.05.101

来源: Elsevier

【 摘 要 】

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 center dot(2H(2)O)] and uranophane [Ca(UO2)(2)(SiO3OH)(2)center dot 5H(2)O] 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 greater than or similar to 8.5 were approximately equivalent to the reference value for safe drinking water established by the EPA (30 mu g/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 < 75 mu g/L at typical groundwater concentrations of calcium and dissolved silica (pH > 7). The diversity of uranyl minerals that possibly form in the presence of common groundwater species: Ca-Mg-Na-K-Si-bicarbonate/carbonate-sulfate-chloride, has not been fully explored with respect to understanding potential mineral transformations and impacts on uranium solubility and mobility.

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