Tokunaga, and two co-authors, reported mid-2009 that methods for remediating groundwater contaminated with uranium (U) through precipitation under oxidizing conditions are needed because bioreduction-based approaches require indefinite supply of electron donor. Although strategies based on precipitation of some phosphate minerals within the (meta)autunite group have been considered for this purpose, thermodynamic calculations for K- and Ca-uranyl phosphates, meta-ankoleite and autunite, predict that U concentrations will exceed the Maximum Contaminant Level (MCL ) 0.13 µM for U) at any pH and pCO2, unless phosphate is maintained at much higher concentrations than the sub-µM levels typically found in groundwater.
The authors hypothesized that potassium uranylvanadate will control U(VI) concentrations below regulatory levels in slightly acidic to neutral solutions based on thermodynamic data available for carnotite, K2(UO2)2V2O8. The calculations indicate that maintaining U concentrations below the MCL through precipitation of carnotite will be sustainable in some oxidizing waters having pH in the range of 5.5 to 7, even when dissolution of this solid phase becomes the sole supply of sub-µM levels of V. Batch experiments were conducted in solutions at pH 6.0 and 7.8, chosen because of their very different predicted extents of U(VI) removal. Conditions were identified where U concentrations dropped below the MCL within 1-5 days of contact with oxidizing solutions containing 0.2-10 mM K, and 0.1-20 µM V(V). This method may also have application in extracting(mining)U and V from groundwater where they both occur at elevated concentrations.
Implications and Future Studies.
These equilibrium calculations and experiments on U removal from aqueous solutions indicate that precipitation of a carnotite-like phase is a potentially viable strategy for treating some U-contaminated groundwater. Because this approach does not rely on maintaining reduction of uraninite, the need for an
indefinite supply of electron donor is circumvented. At pH∼6, additions of moderate levels of V(V) (<2 µM) and K+ (<2 mM) can precipitate with U(VI) in oxidizing solutions, leaving dissolved U concentrations below the MCL. The results from our batch experiments further demonstrate that the precipitation reaction could also be employed to extract (mine) U and V from groundwaters where they both occur at elevated concentrations. From the work presented here, a combination of K+ addition and adjustment to pH ∼6.0 will precipitate carnotite-like solids, yielding about 10 kg U and 2 kg V per hectare-meter of groundwater when initial concentrations of these elements are 2 µM. More U-contaminated solutions and sediments are being tested in batch and column
experiments over a range of pH (5 to 7.5) and solution compositions (complexing agents and ionic strength) in our next stages of exploring the potential for K- and Ca-vanadate-based in situ U remediation.
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