Use of thermodynamic sorption models to derive radionuclide K d values for performance assessment: selected results and recommendations of the NEA sorption project

Uranium adsorption – a review of progress from qualitative understanding to advanced model development

Surface adsorption has a major influence on the environmental mobility of radionuclides, including uranium. Six decades ago, the description of the sorption process relied predominantly on simple descriptive parameters of solid–liquid partitioning (such as Kd values). There have since been numerous systematic investigations of the processes controlling U adsorption, including the affinity of U for different types of geologic materials, the influence of factors such as pH, the effects of complexing ligands, and the role of microorganisms. Mathematical descriptions of sorption processes have adopted various models – including sorption isotherms, surface complexation models and other types of modelling approaches, aided by advances in computational and analytical techniques. In recent years, mechanistic models have incorporated structural insights gained from spectroscopic techniques (such as EXAFS and TRLFS). Throughout the period, the nuclear waste community has sought to develop models for U sorption in complex systems associated with radioactive waste disposal, involving a range of mineral surfaces and incorporating numerous interactions and processes. To some extent, the ongoing questions concerning U adsorption can be considered as being common to many environmental metal contaminants. However, uranium is a unique and significant case, particularly for the radiochemical community, where the long-term behaviour of actinides is a central issue.

Surface complexation model for strontium sorption to amorphous silica and goethite

Strontium sorption to amorphous silica and goethite was measured as a function of pH and dissolved strontium and carbonate concentrations at 25 degrees C. Strontium sorption gradually increases from 0 to 100% from pH 6 to 10 for both phases and requires multiple outer-sphere surface complexes to fit the data. All data are modeled using the triple layer model and the site-occupancy standard state; unless stated otherwise all strontium complexes are mononuclear. Strontium sorption to amorphous silica in the presence and absence of dissolved carbonate can be fit with tetradentate Sr2+ and SrOH+ complexes on the beta-plane and a monodentate Sr2+complex on the diffuse plane to account for strontium sorption at low ionic strength. Strontium sorption to goethite in the absence of dissolved carbonate can be fit with monodentate and tetradentate SrOH+ complexes and a tetradentate binuclear Sr2+ species on the beta-plane. The binuclear complex is needed to account for enhanced sorption at hgh strontium surface loadings. In the presence of dissolved carbonate additional monodentate Sr2+ and SrOH+ carbonate surface complexes on the beta-plane are needed to fit strontium sorption to goethite. Modeling strontium sorption as outer-sphere complexes is consistent with quantitative analysis of extended X-ray absorption fine structure (EXAFS) on selected sorption samples that show a single first shell of oxygen atoms around strontium indicating hydrated surface complexes at the amorphous silica and goethite surfaces. Strontium surface complexation equilibrium constants determined in this study combined with other alkaline earth surface complexation constants are used to recalibrate a predictive model based on Born solvation and crystal-chemistry theory. The model is accurate to about 0.7 log K units. More studies are needed to determine the dependence of alkaline earth sorption on ionic strength and dissolved carbonate and sulfate concentrations for the development of a robust surface complexation database to estimate alkaline earth sorption in the environment.