TOWARD AN ACCURATE MODEL OF METAL SORPTION IN SOILS

摘要

Radionuclide transport in soils and groundwaters is routinely evaluated in performanceassessment (PA) using simplified conceptual models (e.g., KD method) to describe radionuclidesorption. However, the KD approach with linear and reversible sorption of metal cations is rarelyobserved in the field. Inaccuracies of this model are typically addressed by conservativeness inthe use of the chemical partitioning parameters, and often result in failed transport predictions orin increased costs for the cleanup of a site. Realistic assessments of radionuclide transport over awide range of environmental conditions can proceed only from accurate and mechanistic modelsof the metal sorption process.Our research has recently examined the sorption mechanisms and partition coefficients for Ba2+(analog for 226Ra2+) onto soil minerals (iron oxides and clay phases) using a combination ofisothermal sorption/desorption measurements, synchrotron spectroscopic analyses of metalsorbed substrates, and computer molecular modeling simulations. Research goals include 1)evaluation and quantification of the critical mechanisms and geochemical parameters that controlthe retardation of radionuclides on the sorbing phases in near-field soils, 2) use of atomisticcomputer simulations to predict radionuclide KD values based on the partitioning of the metalcations between the solution and mineral surface, and 3) identification of the general trends inmetal plume length associated with field sites. Results should improve our ability to estimateradionuclide migration at contaminated sites.