URANIUM SOLUBILITY OF CARBONATE-RICH URANIUM-CONTAMINATED SOILS

A 300 d solubility study involving two carbonate-rich, uranium-contami nated soils from the Department of Energy's Fernald Environmental Mana gement Project site was conducted to predict the behavior of uranium d uring on-site remediation of these soils. Geochemical modeling was per formed on the aqueous species dissolved from these soils following the solubility measurements to predict the on-site uranium leaching and t ransport potential. Results showed that the soluble levels of the majo r components (total uranium, calcium, magnesium, and carbonate) increa sed continually for the first 4 weeks. After the first 4 weeks, these components either reached a steady-state or continued to increase line arly throughout the study. Soluble uranium levels of both soils and th eir correlation with alkalinity was strongly mediated by the source te rm of the contamination. Geochemical modeling predicted and anion exch ange experiments confirmed that uranyl-carbonate complexes were the mo st stable and abundant complexes. Further modeling showed that uranium solubility in these soils and in onsite groundwater wells is controll ed by UO2(H2PO4)(2), but is also mediated by complexation with carbona te and the oxidation state of the uranium. For assessing the risk rela ted to off-site transport of uranium, it should be recognized that the solubility of uranium-bearing minerals is the critical factor in cont rolling uranium solubility of these soils rather than sorption/desorpt ion processes as measured by the uranium distribution coefficient (K-d ) in these soils.