Quantification of thorium and uranium sorption to contaminated sediments

Desorption tests, using a sequential extraction method, were used to characterize and quantify thorium and uranium sorption to contaminated wetland sediments collected from the Department of Energy's Savannah River Site located in South Carolina. In situ distribution coefficients, or K d values (K d = C solid/C liquid), were determined. Sequential extraction data were used to assign solid-phase radionuclide concentrations (C solid) that, by definition, should represent only the reversibly sorbed fraction. A series of selective and sequential extractions was used to determine desorption K d values. Thorium K d values ranged from 115 to 2255 ml/g. Uranium K d values ranged from 170 to 6493 ml/g. Compared to sorption K d values, these desorption K d values were appreciably greater because they captured the “aging” process of the radionuclides with the sediment, making the radionuclide more refractory. Compared to nonsite-specific data, these in situ K d values improved accuracy, were more defensible, and removed unnecessary conservatism for subsequent transport and risk calculations. Additional tests were conducted to provide geochemical information relevant for selecting appropriate remediation technologies for the contaminated site. Thorium and uranium were associated with labile fractions and were not concentrated with the smaller sediment particles. These findings suggest that phytoremediation or heap leaching, and not soil washing, are viable remediation approaches for this site.