Sorption of americium / europium onto magnetite under saline conditions: Batch experiments, surface complexation...

This study investigates the adsorption of americium and its chemical analogue europium on magnetite, which is expected to form as a major long-term steel canister corrosion product under anoxic and highly saline conditions.

The sorption of europium on magnetite (solid/liquid ratio = 0.5 g/L) was investigated batch wise in NaCl brines with ionic strength I = 1 m, 3.5 m, and 6.67 m, as a function of pH_m for two europium concentrations (6 × 10^-10m, 1.2 × 10^-5m). Information on the chemical nature of the surface species was obtained by X-ray absorption spectroscopy (XAS) at the americium L₃-edge.

Retention of europium by magnetite of >99.5% was found above pH_m 6.4 for all ionic strengths for europium concentration of 6 × 10^-10m. No ionic strength effect was observed in this pH_m range. At 1.2 × 10^-5m europium concentration, 95 ± 4% sorption was found above pH_m 7.5 for I = 1 m and above pH_m 8.0 for I = 3.5 m and 6.67 m. A small ionic strength effect was observed in this case. X-ray absorption spectroscopy (XAS) results are consistent with the batch sorption experiment outcomes, showing an insignificant effect of ionic strength on the pH_m dependent sorption. Results from potentiometric titrations of the solid phase, batch sorption experiments and spectroscopy were interpreted consistently with a charge distribution multi-site (CD-MUSIC) triple layer surface complexation model assuming surface coordination of the metal ion via a tridentate binding mode.