241Am, 237Np, and 99Tc Sorption on Two United States Subsoils from Differing Weathering Intensity Areas

Abstract

For the purpose of assessing the consequences of a potential release event from a high-level waste repository, the sorption of ²⁴¹Am, ²³⁷Np, and ⁹⁹Tc was measured on two sons as a function of solution concentration of Na and Ca from extremes of weathering intensity areas. Distribution coefficients (Kd values) were determined on subsoil samples from Washington and South Carolina for ²⁴¹Am, ²³⁷Np, and ⁹⁹Tc, as a function of equilibrium solution concentration of calcium (Ca⁺²) and of sodium (Na⁺μ). Kd values decreased in all cases with increasing solution concentrations of (Ca⁺²) and (Na⁺). For the South Carolina subsoil, Kd values ranged from 1.0 to 67 ml/g for ²⁴¹Am as a function of (Ca⁺²), from 0.2 to 0.002_M, respectively; 1.6 to 280 ml/g for ²⁴¹Am as a function of (Na⁺); 0.43 to 0.66 ml/g for ²³⁷Np as a function of (Ca⁺²); and 0.16 to 0.25 ml/g for ²³⁷Np as a function of (Na+), from 3.0 to 0.015_M, respectively. For the Washington soil, Kd values were > 1200 ml/g for ²⁴¹Am and ranged from 0.36 to 2.37 ml/g as a function of (Ca⁺²) and from 3.10 to 3.90 ml/g for ²³⁷Np as a function of (Na⁺), over the above concentration ranges. Kd values for ⁹⁹Tc were essentially zero at all NaHCO₃ concentrations on the South Carolina subsoil. This study demonstrated that sorption assessments based on plutonium data are inadequate for estimating other actinide elements sorption. ²⁴¹Am and ²³⁷Np were found to be poorly sorbed in comparison to Pu.