ENGINEERED IN-SITU URANIUM PRECIPITATION: AN EMERGING APPROACH TO GROUNDWATER REMEDIATION

Uranium contamination of soil and groundwater due to mining and chemical refining operations is often persistent and difficult to remediate. Uranium is highly soluble under oxic conditions due to the propensity of the uranyl cation (UO₂²⁺) to form complexes with a variety of ligands of environmental significance, specifically carbonate, phosphate, and hydroxide as well as a multitude of low molecular weight organics and natural organic matter. The geochemistry of uranium is complex because these soluble forms persist in oxidizing environments and can be resistant to surface reactions. An emerging approach to remediation is the in-situ precipitation of insoluble forms of uranium under anaerobic and reducing conditions. A variety of microbial processes can transform soluble uranium to uraninite; the most promising from a remediation geochemistry perspective are those that create conditions that buffer the reducing environment to mitigate reoxidation of the precipiated uranium. Engineering a successful immobilization/sequestration strategy requires that consideration be given to reactons among all of the insoluble mineral phases formed and the native mineralogy of the aquifer. In addition, an understanding of the chemical speciation of the dissolved and solid form of uranium in a contaminated system is often required for the development of appropriate in-situ treatment strategies that can provide stability for the immobilized form. Hydrogeologic features of the impacted aquifer or surface water system must also be considered including an assessment of transient recharge and groundwater flow conditions. Through the integration of biogeochemical, mineralogical, and hydrological design aspects an engineered in-situ approach to uranium contamination is feasible and sustainable.