In situ remediation of uranium is showing increased promise as a means of efficiently and cost-effectively meeting water quality standards downgradient of uranium-impacted sites, including inactive mines, ore milling sites, and processing facilities.  Effective in situ treatment involves decreasing uranium solubility, thereby immobilizing dissolved uranium within relatively insoluble precipitates.  Chemical and microbial reduction approaches (which involve reduction of U(VI) to U(IV) and precipitation as uraninite) have proven effective; however, this strategy may not be appropriate in environments prone to reoxygenation due to the rapid kinetics of uranium reoxidation in the presence of dissolved oxygen.  Direct-precipitation as U(VI) is viable, yet challenging, particularly in alkaline groundwater environments where calcium-uranyl-carbonate complexes typically dominate U(VI) speciation.  Recently, the application of soluble phosphates (including inorganic mono- and polyphosphate salts) has been identified as a potential strategy for treating soluble uranium via the formation of low-solubility uranium phosphate minerals.  Results of laboratory bench and field pilot-scale tests will be presented, demonstrating efficacy and scalability of the approach.  Bench tests performed using groundwater and aquifer solids collected from different sites highlight the importance of understanding site-specific geochemical characteristics before implementation, particularly as they relate to understanding and quantifying secondary effects (e.g., arsenic release).  Field-scale results further demonstrate the need to understand site-specific factors as they relate to the geochemical strategy as well as the engineering implementation, including such factors as reagent distribution and wash-out.  Overall, the results show significant success for soluble phosphates in treatment of dissolved uranium, and provide insight into optimized large-scale application of phosphate-based technologies as an alternative to long-term groundwater extraction and above-ground treatment.

Treatment of uranium in groundwater with soluble phosphate: Successes and challenges at the field-scale

253rd National Meeting (2017)

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