Uptake of Eu(III) by C-S-H phases in CaCl2 solution in presence of EDTA: A molecular dynamics study

The presence of organic ligands in radioactive waste influences radionuclide mobility and sorption in cement. Although EDTA is often assumed to have minimal impact, recent experiments indicate reduced Eu(III)/Cm(III) uptake by calcium silicate hydrate (C-S-H) at high EDTA concentrations in CaCl₂ solutions. We used molecular dynamics (MD) simulations and potential of mean force calculations to investigate complexation and sorption mechanisms. Focusing on the (001) surface of C-S-H, we analyzed radionuclide behavior in the presence of organic ligands under repository-relevant conditions. Solution-phase simulations identified three probable Eu/EDTA/Ca complexes. Sorption at two common sites was examined. Site-specific analysis shows that Ca–Eu(III)–EDTA complexes remain stable in solution, significantly reducing Eu(III) uptake by C-S-H. These findings align with experiments and highlight the role of ternary complex formation in limiting radionuclide retention in cementitious environments. • Eu(III) forms highly stable complexes with EDTA and Ca 2+ in solution on the C-S-H interface. • Three of the most probable ternary complexes in CaCl 2 solution are Eu(EDTA) − , Ca[Eu(EDTA)] + , and Ca 2 [Eu(EDTA)] 3+ . • In the presence of Ca and EDTA, Eu(III) sorption is decreased at bridging sites and is not detected at defect sites. • Eu(III)/EDTA/Ca complexes are stabilized in solution by additional Ca 2+ , which weakens their affinity for adsorption.