Exploring competing surface reactions in radionuclide retention: Experimental and modelling insights

We are convinced that a sound understanding of retardation and migration processes is essential for the development of any sorption databases. In this context discussions often raise the question, "Haven't we already collected enough data and knowledge on sorption processes over the past decades?" Indeed, as an example, many studies^[1,2] have focused on the sorption of trivalent actinides (e.g., Am, Cm) and lanthanides (e.g., Eu, Y) onto various mineral surfaces. However, these studies typically investigate geochemically simple systems, often in binary configurations with single surfaces and single sorbates, to gain a fundamental understanding of surface processes.

Natural systems are far more complex, with the possibility of e.g. competitive sorption, bulk and surface precipitation, incorporation, and co-precipitation processes that all potentially affect radionuclide (RN) retardation, not to mention possible organic interactions. Therefore, in an ongoing research project we focus on retardation and mobilization processes under more complex, closer-to-nature geochemical conditions. Here, the challenge lies in the unique combination of spectroscopic, microscopic, chemical, and electrochemical techniques, as along with the corresponding thermodynamic modeling. Consequently, we couple batch, column, and surface charge (streaming potential) experiments with atomic force microscopy (AFM), crystal truncation rod scattering/resonant anomalous X-ray reflectivity (CTR/RAXR), powder X-ray diffraction (PXRD), and quartz-crystal-microbalance (QCM) measurements. Our focus is on competing surface reactions, dissolution kinetics, and surface precipitation. The systems investigated involve trivalent lanthanides competing with Al(III) and Ga(III) on K- and Ca-rich feldspars, as well as on hematite and quartz surfaces.

So far, AFM measurements have confirmed the presence of an Al layer on the surface of K-rich feldspar (orthoclase), revealing the formation of a low-density amorphous Al layer, which varies with pH. It is assumed that this layer might influence the mineral’s surface charge behavior as previously reported in [2]. In terms of hematite, competitive sorption studies revealed that Al(III) does not significantly affect Eu(III) retention, except for a slight increase at low pH, suggesting limited competition under environmentally relevant conditions. For the investigated quartz surfaces, Al(III) retardation is relevant at lower pH-values compared to Eu(III) sorption processes resulting in potential competing surface reactions under relevant pH conditions which will be studies in detail. QCM measurements with feldspar-coated sensors have shown that it is possible to observe feldspar dissolution and Al as well as Eu uptake, albeit for relatively high solute concentrations. A new cell design was developed offering the opportunity to directly couple QCM and streaming potential measurements.

The experimental data will be used to develop sound and robust thermodynamic surface complexation models (SCMs) consistent with dissolution/precipitation patterns. These models will then be validated using data from column experiments to assess the applicability, advantages, and limitations of derived parameters.

[1] J. Neumann et al., J. Colloid Interface Sci. (2020) (https://doi.org/10.1016/j.jcis.2020.11.041) [2] J. Lessing et al., Colloids and Surfaces A (2024) (https://doi.org/10.1016/j.colsurfa.2024.133529)