Sorption of americium / europium onto magnetite under saline conditions: Batch experiments, surface complexation modelling and X-ray absorption spectroscopy study

Abstract

HypothesisThis 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. ExperimentsThe 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 pHm 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. FindingsRetention of europium by magnetite of >99.5% was found above pHm 6.4 for all ionic strengths for europium concentration of 6 × 10−10m. No ionic strength effect was observed in this pHm range. At 1.2 × 10−5m europium concentration, 95 ± 4% sorption was found above pHm 7.5 for I = 1 m and above pHm 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 pHm 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.