LONG-TERM STABILIZATION OF URANIUM BY U(VI) PHOSPHATE MINERALIZATION: A MODEL FOR URANIUM REMEDIATION

Our research examines a natural example of the long-term sequestration of uranium by U(VI) phosphate mineral precipitation, and provides a model for remediation of uranium dispersed by accidental releases or industrial activities. The study site is an unmined, world-class uranium deposit (Coles Hill, VA) where millions of kilograms of uranium have been made available for potential groundwater transport by the oxidation and chemical weathering of an apatite-rich, coffinite-uraninite orebody. Uranium is not transported laterally away from the deposit due to the precipitation of secondary U(VI) phosphates (dominantly Ba meta-autunite). Shallow drill cores from the site preserve a sharp iron redox front that is spatially associated with a discontinuity in uranium mineralogy (U(IV) phases on the reducing side, U(VI) phosphates on the oxidizing side). This discontinuity in uranium mineralogy does not, however, represent a discontinuity in whole rock uranium concentrations as both oxidized and unoxidized samples average 1000 ppm uranium (concentration in bulk solid). This suggests that uranium remains essentially immobile during the oxidation and chemical weathering of the ore. Groundwaters from the U(VI) phosphate bearing zone at Coles Hill contain less than 15 ppb dissolved uranium, a factor of two lower than the EPA regulatory limit. These groundwaters are oxic, have a pH of approximately 6 and fall within the uranyl phosphate stability field with respect to the ratio of dissolved phosphate to carbonate (log {[H2PO4-]/[HCO3-]} > -3). Experimental results show that the precipitation of the Coles Hill U(VI) phosphates may occur rapidly (time scale of weeks) relative to groundwater transport. This study thus suggests that, in hydrologic conditions typical of the shallow subsurface in the eastern US, phosphate-based remediation of uranium may be effectively implemented by engineering critical variables to mimic those observed at Coles Hill. Furthermore, based on the rate of downward migration of the redox front, it is estimated that the oldest U(VI) phosphates within the weathered bedrock overlying the Coles Hill deposit have been stable for up to 150,000 years thus demonstrating the potential long-term effectiveness of uranium remediation by phosphate mineral precipitation.