THE INFLUENCE OF CITRIC ACID, EDTA, AND FULVIC ACID on UO22+ SORPTION TO KAOLINITE

The mobility of aqueous UO₂²⁺ in near surface environments can be significantly influenced by adsorption on solids. Organic acids also modify UO₂²⁺ mobility, potentially enhancing adsorption by formation of ternary complexes and diminishing it via competition with strong aqueous complexes. In this study, UO₂²⁺ sorption on kaolinite (KGa-1B) was investigated as a function of pH (2-12), sorbate/sorbent ratio (10^-6 to 10^-4 M UO₂²⁺ with 2 g/L kaolinite), ionic strength (0.001-0.1 M NaNO₃), and pCO₂ (0-5%) in the presence or absence of 0.01 M citric acid, 0.01 M EDTA, and 10 or 20 mg/L fulvic acid. Experiments were conducted under room T and P in 50 mL polypropylene VWR centrifuge tubes. Individual tubes were titrated with NaOH or HNO₃ to achieve a pH range of 2-12, equilibrated for 24 hrs, and the pH remeasured. Next, unless specified, samples were centrifuged, filtered, acidified with 5% HNO₃ and analyzed for U by ICP-OES. No-solid experiments with 10^-6 to 10^-4 M UO₂²⁺ in 0.01 M NaNO₃ were used to evaluate sorption to the container wall and precipitation of U phases as a function of pH. In these experiments, aliquots of solutions were removed and analyzed before and after centrifugation and filtration. After removal of the remaining solution, UO₂²⁺ sorbed on the container wall was extracted by adding 50 mL of 0.5 M NaHCO₃ to each tube and agitating for 24 hrs. This solution was removed without filtration, acidified with 12% HNO₃ and analyzed for U using matrix-matched standards. These experiments demonstrate significant loss (up to 50%) of U from solution. Although some loss, particularly in 10^-5 and 10^-4 M U experiments, is expected due to precipitation of schoepite, adsorption on the container walls is significant, particularly in 10^-6 M U experiments. In the absence of ligands, UO₂⁺² sorption on kaolinite increases from pH ~3 to 7 and decreases from pH ~7.5 to 12. Increasing ionic strength from 0.001 to 0.1 M produces only a slight decrease in UO₂⁺² sorption at pH<7, whereas 5% pCO₂ greatly diminishes UO₂⁺² sorption between pH ~5.5 to 11. Addition of fulvic acid produces a small increase in UO₂⁺² sorption, whereas EDTA or citric acid decreases UO₂⁺² sorption. This suggests that fulvic acid enhances UO₂⁺² sorption slightly via formation of ternary ligand bridges, whereas EDTA and citric acid do not form ternary surface complexes with the UO₂⁺².