Radionuclide sorption groundwater composition

Radionuclide transport in natural waters is strongly dependent on sorption, desorption, dissolution, and precipitation processes. The first two sections discuss laboratory investigations of these processes. Descriptions of sorption and desorption behavior of important radionuclides under a wide range of environmental conditions are presented in the first section. Among the sorbents studied are basalt interbed solids, granites, clays, sediments, hydrous oxides, and pure minerals. Effects of redox conditions, groundwater composition and pH on sorption reactions are described. 

The groundwater transport of radionuclides through waterbearing interbed layers in the Columbia River basalt formation will be controlled by reactions of the radionuclides with groundwater and interbed solids. These interactions must be understood to predict possible migration of radionuclides from a proposed radioactive waste repository in basalt. Precipitation and sorption on interbed solids are the principle reactions that retard radionuclide movement in the interbeds. The objective of the work described herein was to determine the sorption and desorption behavior of radionuclides important to safety assessment of a high-level radioactive waste repository in Columbia River basalt. The effects of groundwater composition, redox potential, radionuclide concentration, and temperature on these reactions were determined. 

Sorption isotherms were also measured for sorption of selenium, technetium, tin, radium, uranium, neptunium, plutonium, and americium on the reference Mabton Interbed solids. The GR-1A groundwater composition was used in these experiments. Two temperatures (23°C 2°C and 60°C 1°C) were used, and both oxidizing and reducing conditions were used for each radionuclide. 

Effects of Groundwater Composition and Eh. Radionuclide sorption on geologic solids is dependent on the chemical composition of the groundwater solution and the redox potential (Eh) of the solid-groundwater system. Aquifers at various depths in the Columbia Plateau formation have -been observed to have significant differences in composition. To accurately model radionuclide migration, it is necessary to understand the effects of chemical components and Eh on sorption and solubility of key radionuclides. An additional benefit of this work is to better understand the mechanisms of sorption and desorption of the radionuclides. 

Sorption onto rock samples pretreated with simulated ENF groundwater (pH 12.5 at 25°C). The solutions used in the experiments simulate the solution composition measured at the end of the pretreatment process, and have pH in the range 8-10. These experiments simulate the case where radionuclides migrate into the geosphere after development of the ADZ. 

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