Sorption of Eu(III) on C–S–H phases in the presence of gluconate: A molecular dynamics study

Cement is a key barrier material in radioactive waste repositories, where calcium-silicate-hydrate (C–S–H) phases play a central role in immobilizing cationic radionuclides. However, organic ligands, originating from additives or waste, can enhance radionuclide mobility by forming soluble complexes and competing for surface sorption. In this study, a surface model was developed that combines experimental observations with theoretical insights into C–S–H structure, enabling detailed sampling of the most probable sorption sites. Molecular dynamics (MD) simulations and potential of mean force (PMF) calculations were used to develop molecular-scale understanding of how organic additives influence the adsorption and mobility of trivalent actinides and lanthanides in cementitious materials. Eu(III) was considered as a model of key trivalent radionuclides expected in nuclear waste, i.e. Pu(III) and Am(III), based on their similar charge-to-size (z/d) ratios, and gluconate was chosen as a model organic ligand. The results from the Eu(III)/C–S–H binary system confirmed strong sorption and showed that the most common sorption sites are the deprotonated silanol groups of the surface. Results obtained for the binary system are in line with Time Resolved Laser Fluorescence Spectroscopy data available for Eu(III) and Cm(III). Depending on gluconate concentration, two main effects on Eu(III) uptake on the C–S–H phases have been found: (a) sorption of the 1:1 Eu(III)-GLU complex at low ligand concentration; (b) formation of a stable ternary Ca–Eu(III)-GLU aqueous complex that does not sorb at high ligand concentration. It is important to consider formation of the ternary complex C–S–H/Eu(III)/GLU for the overall interpretation and understanding of the system.