300 Area Uranium Leach and Adsorption Project

Abstract

The objective of this study was to measure the leaching and adsorption characteristics of uranium in six near-surface sediment samples collected from the 300 Area of the Hanford Site. Scanning electron micrographs of the samples showed that the uranium contamination in the sediments is most likely present as co-precipitates and/or discrete uranium particles. Molecular probe techniques also confirm the presence of crystalline discrete uranium bearing phases. In all cases, the uranium is present as oxidized uranium (uranyl [U(VI)]). Results from the column leach tests showed that uranium leaching did not follow a constant solubility paradigm. Four of the five contaminated sediments showed a large near instantaneous release of a few percent of the total uranium followed by a slower continual release. Steady-state uranium leachate concentrations were never measured and leaching characteristics and trends were not consistent among the samples. Dissolution kinetics were slow, and the measured leach curves most likely represent a slow kinetically controlled desorption or dissolution paradigm. Batch adsorption experiments were performed to investigate the effect of pH and uranium and carbonate solution concentrations on uranium adsorption onto the uncontaminated sediment. Uranium adsorption Kd values ranged from 0 to > 100 ml/g depending on which solution parameter was being adjusted. Results of the experiments showed that carbonate solution concentration has the greatest impact on uranium adsorption in the 300 Area. Solution pH was shown to be important in laboratory tests; however, the sediment will dominate the field pH and minimize its overall effect in the 300 Area sediments. Results also showed that uranium sorption onto the background sediment is linear up to uranium concentrations of 3 mg/L, well above the values found in the upper unconfined aquifer. Therefore, the linear Kd model is defensible in predicting the fate of uranium in the 300 Area aquifer.