Investigations on the Interaction of EDTA with Calcium Silicate Hydrate and Its Impact on the U(VI) Sorption

The interaction of EDTA with calcium silicate hydrate (C-S-H) and its impact on the sorption of U(VI) by C-S-H in the presence of EDTA at varying concentrations has been investigated under N2 and ambient atmosphere. The solid phase characterization was performed by FTIR, XRD and TGA measurements and the uranium concentration in solution has been determined by alpha-spectroscopy. At increased EDTA concentrations ([EDTA] > 0.1 M) calcium is complexed and extensively extracted from the solid resulting in a quantitative dissolution of the Ca(OH)2 phase and deterioration of C-S-H. At lower EDTA concentrations ([EDTA] ≤ 0.01 M), EDTA is sorbed into the solid phase and the associated adsorption capacity (qmax = 0.67 mol/kg) has been evaluated by fitting the corresponding data with the Langmuir isotherm model. The incorporation of EDTA in the C-S-H matrix was corroborated by FTIR, XRD and TGA measurements. Regarding the effect of EDTA on the U(VI) sorption by C-S-H, evaluation of the experimental data reveal a significant decrease of the Kd values in the presence of EDTA most probably due to the stabilization of U(VI) in the form of U(VI)-EDTA complexes in solution. Under ambient conditions a further decrease of the Kd values is observed because of the formation of U(VI)-carbonato complexes related to CO2 dissolution and hydrolysis.