Adsorption of Aqueous Uranyl Complexes onto Bacillus subtilis Cells

In oxygenated, CO2-rich systems, negatively charged uranyl complexes dominate the aqueous uranium speciation, and it is commonly assumed that these complexes exhibit negligible adsorption onto negatively charged surfaces such as bacteria. We measured the adsorption of 4.2 × 10-6 M aqueous uranium onto Bacillus subtilis from pH 1.5 to 9 and with wet weight bacterial concentrations from 0.125 to 0.5 g/L. Experiments were performed in the presence and absence of dissolved CO2, and additional experiments were performed in the presence of dissolved CO2 and Ca. We observed extensive uranium adsorption onto the bacterial surface under all conditions. Thermodynamic modeling of the data suggests that uranyl−hydroxide, uranyl−carbonate, and calcium−uranyl−carbonate species each can form stable surface complexes on the bacterial cell wall. These results could dramatically alter predictions of uranium mobility in near-surface environments.