Coupled reaction/transport modeling of a chemical barrier for controlling uranium(VI) contamination in groundwater

Adsorption of uranium onto amorphous ferric oxyhydroxide can be used for in situ containment of uranium(VI) present in mill tailings and for prevention of potential groundwater contamination. Adsorption of uranium(VI) is strongly influenced by groundwater chemistry, especially pH and total dissolved carbonate concentration. To evaluate the effectiveness of adsorption onto amorphous ferric oxyhydroxide as a containment barrier, the extent of uranium adsorption was quantified as a function of solution chemistry and other adsorption parameters. The adsorption model was used within a coupled hydrogeochemical transport model to evaluate the conditions under which amorphous ferric oxyhydroxide can be effective in preventing groundwater contamination. The modeling results depicted spatial and temporal evolution of the tailings-groundwater system, and permitted delineation of the uranium(VI) plume in groundwater under different scenarios. The analysis showed that adsorption of uranium(VI) onto amorphous ferric oxyhydroxide can be an effective mechanism for preventing release of uranium(VI) into groundwater especially for neutral-pH mill tailings. It was also found that essentially no retardation occurred in tailings with alkaline pore fluids. An economic analysis showed that the amorphous ferric oxyhydroxide treatment can be applied to neutral-pH mill tailings in a cost-effective manner. The results obtained in the study support the conceptual foundation of geochemical engineering in waste management.