New insight into Cm(III) interaction with kaolinite – Influence of mineral dissolution

Cm(III) speciation in natural kaolinite (St. Austell, UK) suspensions under alkaline conditions was studied by time-resolved laser fluorescence spectroscopy (TRLFS). The spectroscopic investigations were performed under argon atmosphere (O2 < 1 ppm) using a constant curium concentration, ionic strength and mineral content of 2 × 10−7 M, 1 mM NaClO4 and 0.25 g/L, respectively, throughout the study. The impact of kaolinite mineral dissolution on the speciation of the trivalent actinide was investigated in oversaturation experiments where excess amounts of aluminum and/or silicon were added to alkaline kaolinite suspensions. Only silicon addition was found to influence the curium ligand-field under the experimental conditions indicating the formation of a curium-silicate complex in the kaolinite environment. In experiments with 10−3 M added silicon but no solid phase for curium attachment only the hydrolysis species Cm(OH)2+ could be detected at pH 10. Thus, the formation of colloidal silicate species for the attachment of curium could be excluded and the observed species in alkaline kaolinite environments could be assigned to a ternary kaolinite/curium/silicate complex forming between adsorbed curium at the mineral surface and dissolved silicates in solution. A similar curium-silicate complex with identical spectroscopic features was also found in investigations with α-alumina as sorbent phase upon addition of silicon to the mineral suspensions, suggesting that silicon complexation with surface-bound curium is independent of the sorbent material.