NUREG/CR-6893, "Modeling Adsorption Processes: Issues in Uncertainty, Scaling, and Prediction."

Adsorption of contaminant species to mineral surfaces is largely responsible for the retardation of radionuclides in the subsurface environment. However, despite much research effort, the advancement of models that can be used to successfully calculate or predict adsorption is still somewhat limited. This report covers three different aspects of modeling adsorption of radioruclides with an emphasis on the use of surface complexation models (SCM). The methods provide a rigorous and thermodynamic-based alternative to the more conventional and empirical Ko approach often used inappropriately in the performance assessment of nuclear waste sites. The fist study provides an example of how adsorption constant uncertainty propagates through a one-dimensional reactive-transport code and can strongly influence the calculated aqueous metal (i.e., uranyl) concentrations as a function of distance and time from a contaminant source. In this study, the hydrology and mineralogy of the Naturita uranium mill tailings site in Colorado are used to establish initial conditions and processes to incorporate into a onedimensional (1-D) reactive-transport model. An electrostatic surface complexation model is used lo describe adsorption onto smectite, an abundant clay mineral at the Naturita site. A probabilistic investigation demonstrates that uncertainty in adsorption constants can dramatically change the calculated shape of contaminant concentration profiles. This study demonstrates the importance of selecting appropriate adsorption constants when using reactive-transport models in performance assessment to evaluate risk and pollution attenuation at contaminated sites. Adsorption processes at the solid-water interface can be investigated at different levels of chemical detail: electronic, atomistic, and thermodynamic. The second study addresses this scaling issue by describing how electronicand atomic-scale investigations provide useful insight for the development of accurate bulk thermodynamic models (for example, SCM). Molecular modeling can be used to investigate the stoichiometries and relative adsorption energies of possible surface complexes. Both quantum and molecular mechanics calculations that focus on the submicroscopic details of the adsorption process can provide us with new, more quantitative ways to bound the uncertainties associated with "averaging" surface site characteristics and for selecting only one or two surface reactions to describe the adsorption of a contaminant over a range of environmental conditions. These atomic-scale studies may provide us with a more definitive appreciation for how detailed an SCM is necessary for accurate reactive-transport simulations of contaminant migration. The third study reviews recent progress in developing an internally-consistent database to describe adsorption over a wide range of solution and solid compositions. Substantial progress has been made to establish a database for a specific SCM (triple-layer model). Newly-defined standard states for surface species allow us to normalize and compare experimental adsorption data collected using different solid to liquid ratios. X-ray standing-wave measurements, X-ray absorption spectroscopy, molecular modeling, and ab initio modeling all contribute to a greater understanding of surface complexation, and in particular, to the nature of contaminant surface species that need to be incorporated into larger-scale thermodynamic models. Combining approaches ranging from bulk adsorption measurements to ab initio quantum calculations in our investigation of processes at the solid-water interface and synthesizing information for different interfacial systems, may lead to major breakthroughs in adsorption modeling in the next decade.