Publications

Prediction of metal sorption in soils

Westrich, Henry R.; Arthur, Sara E.; Brady, Patrick V.; Cygan, Randall T.; Zhang, Pengchu Z.

Radionuclide transport in soils and groundwaters is routinely calculated in performance assessment (PA) codes using simplified conceptual models for radionuclide sorption, such as the K{sub D} approach for linear and reversible sorption. Model inaccuracies are typically addressed by adding layers of conservativeness (e.g., very low K{sub D}'s), and often result in failed transport predictions or substantial increases in site cleanup costs. Realistic assessments of radionuclide transport over a wide range of environmental conditions can proceed only from accurate, mechanistic models of the sorption process. They have focused on the sorption mechanisms and partition coefficients for Cs{sup +}, Sr{sup 2+} and Ba{sup 2+} (analogue for Ra{sup 2+}) onto iron oxides and clay minerals using an integrated approach that includes computer simulations, sorption/desorption measurements, and synchrotron analyses of metal sorbed substrates under geochemically realistic conditions. Sorption of Ba{sup 2+} and Sr{sup 2+} onto smectite is strong, pH-independent, and fully reversible, suggesting that cation exchange at the interlayer basal sites controls the sorption process. Sr{sup 2+} sorbs weakly onto geothite and quartz, and is pH-dependent. Sr{sup 2+} sorption onto a mixture of smectite and goethite, however, is pH- and concentration dependent. The adsorption capacity of montmorillonite is higher than that of goethite, which may be attributed to the high specific surface area and reaction site density of clays. The presence of goethite also appears to control the extent of metal desorption. In-situ, extended X-ray absorption fine structure (EXAFS) spectroscopic measurements for montmorillonite and goethite show that the first shell of adsorbed Ba{sup 2+} is coordinated by 6 oxygens. The second adsorption shell, however, varies with the mineral surface coverage of adsorbed Ba{sup 2+} and the mineral substrate. This suggests that Ba{sup 2+} adsorption on mineral surfaces involves more than one mechanism and that the stability of sorbed complexes will be affected by substrate composition. Molecular modeling of Ba{sup 2+} sorption on goethite and Cs{sup +} sorption on kaolinite surfaces were performed using molecular dynamics techniques with improved Lennard-Jones interatomic potentials under periodic boundary conditions. Ba{sup 2+} was observed to have a preference for inner sphere sorption onto goethite, with the (101) and (110) surfaces representing the controlling sorption surfaces for bulk K{sub D} measurements. Large-scale simulations of Cs{sup +} sorption on kaolinite (1000's of atoms) provide a statistical basis for the theoretical evaluation and prediction of Cs{sup +} K{sub D} values. Results suggest the formation of a strong inner sphere complex for Cs{sup +} on the kaolinite edge surfaces and a weakly bound outer sphere complex on the hydroxyl basal surface.