Experimental Study of Uranyl Adsorption onto Bacillus subtilis

Uranyl adsorption onto the Gram-positive soil bacterium Bacillus subtilis was measured using batch experiments in 0.1 M NaClO 4 as a function of pH, time, and solid:solute ratio at 25 °C. The experimental data were interpreted using a surface complexation approach. The experimental measurements constrain the stoichiometry and thermo- dynamic stabilities of the important uranyl-surface complexes. The U adsorption data require two separate adsorption reactions, with the uranyl ion forming surface complexes with the neutral phosphate functional groups and the deprotonated carboxyl functional groups of the bacterial cell wall: R−POH 0 + UO 2 2+ ⇔ R−POH−UO 2 2+ (log K = 11.8 ± 0.2) and R−COO - + UO 2 2+ ⇔ R−COO−UO 2 + (log K = 5.4 ± 0.2). These new stability constants, in conjunction with other experimental and predicted stability constants, may be incorporated in surface complexation models to determine the mobility and fate of U in bacteria-bearing water−rock systems.