Radionuclides sorption

By direct measurements of the radionuclide sorption on exposed rocks, surface related distribution coefficients K can be determined and conversion factors calculated according to )... 

The chapters of this volume are organized into sections that cover the chemical aspects that are important to understanding the behavior of disposed radioactive wastes. These aspects include radionuclide sorption and desorption, solubility of radionuclide compounds, chemical species of radionuclides in natural waters, hydrothermal geochemical reactions, measurements of radionuclide migration, solid state chemistry of wastes, and waste-form leaching behavior. The information in each of these sections is necessary to predict the transport of radionuclides from wastes via natural waters and thus to predict the safety of the disposed waste. 

Radionuclide Sorption and Desorption Reactions with Interbed Materials from the Columbia River Basalt Formation... 

Geochemical models of sorption and desorption must be developed from this work and incorporated into transport models that predict radionuclide migration. A frequently used, simple sorption (or desorption) model is the empirical distribution coefficient, Kj. This quantity is simply the equilibrium concentration of sorbed radionuclide divided by the equilibrium concentration of radionuclide in solution. Values of Kd can be used to calculate a retardation factor, R, which is used in solute transport equations to predict radionuclide migration in groundwater. The calculations assume instantaneous sorption, a linear sorption isotherm, and single-valued adsorption-desorption isotherms. These assumptions have been shown to be erroneous for solute sorption in several groundwater-soil systems (1-2). A more accurate description of radionuclide sorption is an isothermal equation such as the Freundlich equation ... 

Three interbed materials from the Columbia River Basalt Group have been investigated in the radionuclide sorption experiments. Interbeds are porous sedimentary layers located between many of the basalt flows in the Columbia River Basalt Group and comprise a potential preferential pathway for groundwater and, therefore, radionuclide transport. 

Freundlich constants and ranges of Kd values for radionuclide sorption on the Rattlesnake Ridge sandstone are given in Table III. The constants K and N were calculated using linear regression. Linear sorption isotherms (N = 1.0) are observed only for strontium, selenium, and radium. 

Table III. Freundlich Constants for Radionuclide Sorption (Molar Basis) on Sandstone...

Table IV. Freundlich Constants (Molar Basis) for Radionuclide Sorption on Mabton Interbed Solids...

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. 

Barney, G. S. "FY 1981 Annual Report Radionuclide Sorption on Basalt Interbed Materials, RHO-BW-ST-35 P, Rockwell Hanford Operations, Richland, Washington, 1981. 

Radionuclide Sorption Mechanisms and Rates on Granitic Rock... 

A variety of chemical extraction techniques has been developed by soil scientists to determine quantitatively the amount of trace metals bound to soil particles by various mechanisms. Multimechanism sorption has been suggested in order to explain partial irreversibility of radionuclide sorption and increased sorption with exposure time. The five main sorption mechanisms that have been reported are ... 

In order to assess the feasibility of any nuclear waste disposal concept, mathematical models of radionuclide sorption processes are required. In a later section kinetic descriptions of the three common sorption isotherms (3) are compared with experimental data from the mixing-cell tests. For a radionuclide of concentration C in the groundwater and concentration S on the surface of the granite, the net rate of sorption, by a first-order reversible reaction, is given by... 

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