INFLUENCE OF WATER CHEMISTRY ON URANIUM RELEASE FROM NATURAL SEDIMENTS: POTENTIAL CONTRIBUTIONS FROM MINERAL DISSOLUTION AND DESORPTION

Mineral dissolution and uranium (U) desorption from mineral surface play significant roles in controlling U contamination in subsurface environment. To predict and control U contamination, the knowledge of release mechanism and extent of release under water chemistry conditions relevant to natural soil water and groundwater is essential. In this study, a glacial till sample was collected, characterized, and examined with batch leaching experiments to investigate U release mechanism and the effects of water chemistry conditions on U and major element release. SEM-EDX and sequential extraction showed that carbonate minerals, Fe-Mn oxyhydroxides, and silicate minerals are the major U hosting minerals, and substantial amounts of U exit as absorbed uranyl ion. Water chemistry conditions significantly influence the mechanism and extent of U release: at high pH (8 and 10), U release was high and dissolution of Fe-bearing minerals (i.e., Fe oxyhydroxides and/or Fe-containing silicate minerals) contributed to U release. At low pH (3 and 5), U release was low and desorption of exchangeable U (i.e., U desorption from mineral surface) was the dominant mechanism of U release. Redox potential (Eh) has strong influence on U release: at high Eh (+200 to +300 mV), U release pattern was similar to that of major elements; indicating U release was related to mineral dissolution. At low Eh (-150 to +50 mV), U release was low and the pattern of U release and major element release was different, indicating reductive precipitation of U. Citrates, bicarbonates, and natural organic matter are common natural chemicals found in soil pore water and groundwater. We found citrate and bicarbonate greatly facilitated U release due to the formation of low-adsorbing aqueous U complexes, and natural organic matter moderately enhanced U release via the same mechanism. By analyzing the trend and pattern of U and major element release, we concluded that U desorption is the dominant U release mechanism under most of the water chemistry conditions tested in our leaching experiments, although mineral dissolution can be important at high pH.