Approaches to sorption modeling for high-level waste performance assessment

Available experimental data indicate that radionuclide sorption is a complex function of mineral and solution chemistry. Surface complexation models (SCM's) can be used to describe this sorption behavior, but comparison of model results requires interpretation of these data using a consistent set of model parameters. Based on available potentiometric titration data, a uniform approach is used to develop a set of consistent parameters for three SCM's and nine simple (hydr)oxides. Using these parameters, all three models prove capable of reproducing observed charge data, but each model is subject to limitations. Binding constants are developed for the U(VI)-H2O-CO2-goethite system. In the absence of CO2, all three models are able to reproduce observed trends for a given data set assuming a single surface complex. Two or more surface complexes were required to model observed sorption behavior in the presence of CO2/CO32-. The experimental data from the two studies considered differ, and binding constants based on combining modeling results do not perform as well, exhibiting bias towards the larger data set. For more complex minerals, sorption of Np(V) on biotite and U(VI) on kaolinite were well modeled using the simplest SCM and assuming stoichiometric proportions of silanol (SiOH0) and aluminol (AlOH0) sites.