NEPTUNIUM ADSORPTION ONTO BACILLUS SUBTILIS

Neptunium is considered the most problematic byproduct of nuclear reactors, due to its high solubility and because, after about 1000 years, Np will be the major radiation dose contributor in geologic repositories.  The mobility of Np is difficult to predict in part due to its uncertain sorption behavior in geologic systems.  In this study, we measured Np adsorption onto a common Gram-positive soil bacterium, Bacillus subtilis. We performed batch adsorption experiments with a 1 ppm Np(V) solution in contact with bacteria as a function of pH, from 2.5 to 8, and as a function of ionic strength, at 0.1M and 0.001M NaClO₄. Np adsorption exhibits an inverse relationship with ionic strength, and in general, adsorption increases with increasing pH.  For example, at pH values of 4 and 8  in the 0.001 M ionic strength system, we measured 22% and 55% removal of the total Np from solution, while at those same pH values in the 0.1M ionic strength system, Np adsorption was 3% and 17%, respectively.  We model the adsorption reaction using a surface complexation approach to yield Np-bacterial surface stability constants. The 0.1M data only require one bacterial surface complexation reaction (neptunyl-carboxylate) for the best fit: R-COO^- + NpO₂⁺=R-COO-NpO₂⁺ (log K=1.55).  Two separate adsorption reactions are needed to account for the increase in adsorption due to the decrease in ionic strength.  The two site model invokes neptunyl binding onto a deprotonated carboxyl and onto a neutral phosphoryl group: R-COO^- + NpO₂⁺=R-COO-NpO₂ (log K=2.12) and R-POH + NpO₂⁺ =R-POH-NpO₂⁺ (log K=9.05).  Our calculated neptunyl-bacterial surface stability constants are consistent with values predicted using the correlation estimation approach from Fein et al. (2001), further indicating that the binding of cations onto bacterial surfaces is largely determined by the electrostatic and chemical properties of the cations. The magnitude of the stability constants indicates that neptunyl binding onto the bacterial surface is considerably weaker than adsorption involving metals such as Cd and Pb, and is similar to metals such as Ca.  The calculated stability constants from this study may be integrated into geochemical models to help determine the effect of Np adsorption on the mobility and fate of Np in bacteria-water-rock systems.