IN-SITU IMMOBILIZATION OF DISSOLVED URANIUM FOR RESTORATION OF URANIUM MINING SITES

In-situ immobilization of dissolved uranium in groundwater associated with uranium mining sites offers advantages over ex-situ treatment, notably elimination of the generation of treatment residues requiring management as radioactive waste. The in-situ method utilizes selective ion exchange processes to immobilize dissolved uranium in the aquifer matrix, thereby reducing potential risk to human health and the environment at lower cost than conventional ex-situ groundwater treatment technologies. To achieve these benefits at uranium mining and other facilities, commercially available anion exchange resins can be placed within the subsurface via soil mixing, permeable barrier wall construction, or hydraulic fracturing and injection techniques.

We conducted bench-scale studies to compare two methods of ion exchange resin placement for uranium removal. The slurry method involved pumping a powdered mixture of strong base anion (SBA) resin particles and water into the simulated aquifer sand. The simulated hydraulic fracturing method involved injection of fracking gel followed by a mixture of 10% resin beads and 90% proppant sand. Measurements included uranium removal efficiency, effect of resin on permeability of the simulated aquifer, influence on treatment capacity of feed-water sulfate and post-treatment washing fluids used to remove fracking gel, and the desorption of uranium from the exhausted media.

Treatment of a simulated aquifer matrix by the slurry method resulted in very effective uranium removal owing to the exceptionally high affinity of anionic uranyl carbonate complexes for SBA resin. The slurry method produced columns capable of reducing uranium concentrations from 0.100 mg/L to less than 0.001 mg/L for more than 100,000 bed volumes of throughput. Resin particles were tested in sizes from 85-115 mesh to 200-400 mesh, and the largest resin size resulted in a maximum reduction in permeability of 15%. The presence of 100 mg/L of sulfate and 0.100 mg/L of uranium reduced the column run length to 12,000 bed volumes. Deionized water served as an effective washing fluid for removal of sulfate (as a surrogate for uranyl carbonate).