Application of Surface Complexation Modeling to Selected Radionuclides and Aquifer Sediments (NUREG/CR-6959)
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Manuscript Completed: January 2008
Date Published: April 2008
U.S. Geological Survey
Menlo Park, Ca 94025
M. Fuhrmann, NRC Project Manager
NRC Job Code Y6462
Office of Nuclear Regulatory Research
Several different surface complexation modeling approaches exist to describe the adsorption of solutes from water onto the surfaces of mineral phases. The Generalized Composite surface complexation modeling approach considers the surfaces of a mixed mineral assemblage, such as soils or sediments, to contain generic functional groups with averaged properties (see demonstration of this approach in USNRC, 2003). The objective of the study described in this report was to extend the Generalized Composite modeling approach to other complex geomaterials and other radionuclides. The modeling approach provides the methodology to estimate Kd values and potential retardation of sorbing radionuclides (Np, U, Ni) with complex aqueous chemistry, and as a function of aqueous chemical conditions in groundwaters. Laboratory batch experiments with natural sediments illustrate that the adsorption (and Kd values) are sensitive to aqueous chemical conditions, including pH and the dissolved carbonate concentration (which equilibrates with a given partial pressure of carbon dioxide in the gas phase). Complex geomaterials in this study included aquifer sediments from: 1) the Naturita (Colorado) UMTRA site, 2) the Forty-Mile Wash (Nye County, Nevada) aquifer, and 3) the USGS research site at Cape Cod (Massachusetts).
Generalized Composite surface complexation models (GC-SCM) were developed for the following radionuclide/geomaterial pairs, as examples of the modeling approach: 1) Np(V)/Naturita aquifer sediments, 2) U(VI)/Forty-Mile Wash aquifer sediments, and 3) Ni/Cape Cod aquifer sediments. In each case the models were calibrated by fitting batch adsorption data as a function of chemical conditions. In all cases, the GC-SCM were developed without electrical double layer terms.
In the case of Np(V)/Naturita aquifer sediments, the GC-SCM was able to accurately simulate Kd values for Np(V) adsorption on the sediments over a range of pH and dissolved carbonate and Np(V) concentrations using three Np(V) surface reactions and two surface sites for each reaction (strong- and weak-binding). Unlike U(VI), the sorption of Np(V) was found to be much less sensitive to the dissolved carbonate concentration. Part of the reason for this limited dependence on dissolved carbonate may be the Np(V) forms both aqueous and surface (ternary) carbonate complexes. The effect of humic acids on Np(V) sorption was also studied. Like carbonate, Np(V) sorption was not influenced (decreased) as much by aqueous Np(V)-humic aqueous complexation as might have been expected, indicating that ternary surface-Np(V)-humic complexes may form on mineral surfaces.
In the case of the U(VI)/Forty-Mile Wash aquifer sediments, the GC-SCM produced good agreement with the experimental data for U(VI) adsorption as a function of pH, dissolved carbonate, and U(VI) concentration with only one U(VI) surface reaction with two surface sites that formed strong and weak complexes. The Kd values decreased significantly with increasing carbonate or U(VI) concentrations, but the GC-SCM was able to simulate the dependence of Kd on these variables. In the case of the Ni/Cape Cod aquifer sediments, Ni Kd values increased with increasing pH and with decreasing Ni concentration. A very simple GC-SCM was calibrated to the experimental data, with one Ni surface species and only one site type. Addition of more reactions or more sites did not improve the goodness-of-fit to the data collected. Aqueous complexation of Ni with sulfate ions in the groundwater was considered as a part of the model calculations.
The challenge in applying the surface complexation concept in the environment is to simplify the SCM, such that predicted adsorption is still calculated with mass laws that are coupled with aqueous speciation, while lumping parameters that are difficult to characterize in the environment in with other parameters. In order to be applied by solute transport modelers and within PA applications, the complexity of the adsorption model needs to be balanced with the goal of using the simplest model possible that is consistent with observed data. This can be achieved with the semi-empirical, site-binding GC modeling approach used in this report and previously demonstrated for modeling U(VI) retardation at the Naturita UMTRA site (Curtis et al., 2006; USNRC, 2003). The GC-SCM is a compromise between the simple constant-Kd approach and more complex SCM that are difficult to apply to the environment at present. The GC modeling approach is preferable to completely empirical approaches, such as the constant-Kd model or adsorption isotherms, because the important linkage between surface and aqueous species (and associated thermodynamic data) is retained in the modeling through the coupling of mass action equations. This linkage also provides a framework for conducting uncertainty analyses that is based on process level parameters rather than on ranges of Kd values that result from lumping together multiple processes. Uncertainty in radionuclide retardation predictions may be reduced, quantified, and more completely understood in the future with the use of the GC-SCM modeling approach.