The U.S. Department of Energy is currently investigating Yucca Mountain, Nevada as a potential site for geologic disposal of high-level nuclear waste. Studies have suggested that Np-237 sorption on calcite, a mineral with widespread occurrence at Yucca Mountain, is enhanced relative to sorption on other common mineral phases. An improved understanding of the mechanisms and magnitude of calcite uptake of Np-237 from solution is important to evaluate the potential transport of Np-237 through the saturated zone at Yucca Mountain.

Batch experiments were conducted using calcite substrate in solutions at or near saturation with respect to calcite over a pH range of 7.25 to 9.0. The solutions were at equilibrium with atmospheric PCO₂, and the initial concentration of Np-237 was ~1.6×10^–6 M. Results of the experiments indicate that calcite is an effective sorber of Np-237 over the pH range studied and that the magnitude of sorption is dependent on pH. The pH dependence of sorption suggests that the speciation of Np-237 in solution plays a large role in the observed sorption behavior.

The experimental data are reasonably modeled using a simple ion exchange model, but a constant capacitance surface complexation model better reproduces the sorption data. Model results indicate that Np-237 uptake is reduced by sorption of Ca²⁺ onto the calcite surface and by formation of neptunyl carbonate complexes in solution; however, constant capacitance model results also indicate that some neptunyl carbonate sorption on calcite may occur at high pH.

DISCLAIMER: This abstract was prepared to document work performed for the U.S. Nuclear Regulatory Commission (NRC) under Contract No. NRC-02-97-009. This work is an independent product of the Center for Nuclear Waste Regulatory Analyses and does not necessarily reflect the views or regulatory position of the NRC.