Geochemical evaluation of uranium sequestration from field-scale infiltration and injection of polyphosphate solutions in contaminated Hanford sediments

Abstract

To remediate persistent groundwater uranium contamination in the 300 Area of the Hanford Site, located in southeast Washington State, U.S.A., approximately 7 million liters of polyphosphate solutions (mixture of 90% orthophosphate and 10% pyrophosphate) were delivered in high concentrations to the vadose zone and top of the aquifer over a 13-day period via infiltration and injection. The persistence of uranium in the aquifer is attributed to leaching of residual uranium contamination from the vadose zone by periodic rewetting due to fluctuating water table controlled by daily and seasonal variations of nearby Columbia river stage.The remedy relies on in-situ availability of Ca2+, primarily from cation-exchange and dissolution of carbonate mineral phases, to form calcium phosphate (Ca-P) bearing precipitates that incorporate and adsorb available uranium and form a relatively insoluble coating on sediments thereby reducing the dissolution of residual uranium bearing mineral phases. During injection and infiltration some dissolution of mineral phases is expected to occur along with rapid nucleation of Ca-P and the resulting precipitates are likely to incorporate components from the dissolving phases.The phosphate loading following treatment is calculated to be about 1000–2500 μg/g of sediment surface exposed. Of this amount, approximately 50–650 μg/g is associated with the solid phase that is extractable with acetic acid indicating association with carbonate phase. The observations at the monitoring wells during remedy implementation indicate an initial decrease in pH and an increase in dissolved uranium concentration with delayed breakthrough of phosphate due to reactions with mineral surfaces. Following the initial breakthrough, as the phosphate concentrations increase the uranium concentrations show a sharp decline and then remain low afterwards. Comparison of total uranium concentration profiles from collocated pre- and post-treatment borehole sediment samples indicate limited uranium leaching to the aquifer. The total uranium concentrations remained within the range of variability expected in the field.Sequential extraction tests conducted on sediment samples indicate that prior to treatment uranium is associated primarily with crystalline oxides of iron and clay minerals and to a lesser extent with carbonate minerals. In the post-treatment samples, appreciable reduction in uranium fraction associated with crystalline oxides of iron and clay minerals is observed indicating that uranium has been remobilized due to mineral dissolution, which later complexed with Ca-P-carbonate phases. The formation of surface complex of calcium-carbonate-phosphate likely structurally incorporated or surface adsorbed any available uranium in solution. Surface complexation with oxyhydroxides of iron (and other trace metals) also occurred.To evaluate the significant reactions resulting from introduction of phosphate bearing solutions a reactive transport model is developed to simulate the infiltration event. The results indicate that the extent of HPO42− reaction front is a net result of several reactions. As HPO42− is added to the sediments, the resulting deprotonation reactions lead to excess H+ and pH reduction that are buffered via surface complexation reactions and mineral phase dissolution. The cycle of deprotonation followed by consumption of H+ will continue as long as supply of both phosphate and reacting iron oxyhydroxide surfaces and minerals, primarily uranium bearing carbonates, uranium silicates, and calcite, are maintained. Due to excess supply, the phosphate will react with the available calcium (primarily from ion-exchange reactions) and start forming Ca-P bearing precipitates. In this process, any uranium in the solution will adsorb or get bound to forming precipitate and be sequestered.