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 Savannah River Site located in South Carolina. In situ distribution coefficients, or Kd values (Kd equal to Csolid/Cliquid), were determined. A problem associated with determining desorption Kd values is that it is difficult to identify Csolid, because by definition it must comprise only the fraction that is reversibly (and linearly) sorbed. A series of selective and sequential extractions was used to determine desorption Kd values. Thorium Kd values ranged from 115 to 2255 mL/g. Uranium Kd values ranged from 170 to 6493 mL/g. Compared to sorption Kd values, these desorption Kd 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 Kd values improved accuracy, were more defensible, reduced uncertainty, 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 U 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