Sequestration and retention of uranium(VI) in the presence of hydroxylapatite under dynamic geochemical conditions

Abstract

Environmental context. Contamination of surface and subsurface geologic media by heavy metals and radionuclides is a significant problem within the United State Department of Energy complex as a result of past nuclear operations. Numerous phosphate-based remediation strategies have been proposed to introduce hydroxylapatite directly or indirectly (i.e. through in situ precipitation) into subsurface regimes to act as an efficient sorbent for sequestration of metals and radionuclides such as uranium. Results presented here illustrate the importance of variable geochemical conditions on the mechanism of sequestration and long-term retention of uranium in the presence of hydroxylapatite. Abstract. Numerous solid- and aqueous-phase phosphate-based technologies for remediating heavy metals and radionuclides have the common premise of sequestration by hydroxylapatite. Complexation reactions and hydrolysis generally preclude actinides from incorporation into intracrystalline sites; rather, surface sorption and precipitation are significant mechanisms for the sequestration of actinides. The effect of pH, aqueous speciation, and the availability of reactive surface sites on minerals such as hydroxylapatite have a significant impact on the mechanism and degree of sequestration and retention of variably charged contaminants such as uranium. Yet, little attention has been given to the sequestration and retention of uranium by hydroxylapatite under dynamic geochemical conditions that may be encountered during remediation activities. We present the results of an investigation evaluating the removal of uranium by hydroxylapatite in systems near equilibrium with respect to hydroxylapatite, and the effect of dynamic aqueous geochemical conditions, such as those encountered during and subsequent to remediation activities, on the retention of uranium. Results presented here support previous investigations demonstrating the efficiency of hydroxylapatite for sequestration of uranium and illustrate the importance of geochemical conditions, including changes to surface properties and aqueous speciation, on the sequestration and retention of uranium.