Stability of U(VI) and Cm(III) doped calcium silicate hydrate phases in high saline brines

U(VI) and Cm(III) doped calcium silicate hydrate (CSH) phases with different C/S ratios (1.0-2.0) were synthesized directly in presence of either U(VI) or Cm(III) and characterized by time-resolved laser-induced fluorescence spectroscopy (TRLFS), infrared (IR) spectroscopy, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The time-dependent release of Ca, Si, U and Cm from CSH phases into brines that contained either 2.5 M NaCl, 2.5 M NaCl/0.02 M Na2SO4, 2.5 M NaCl/0.02 M NaHCO3 or 0.02 M NaHCO3 for U(VI) doped CSH phases or 2.5 M NaCl/0.02 M NaHCO3 or 0.02 M NaHCO3 for Cm(III) doped CSH phases was monitored in batch leaching experiments for 30 or 60 days, respectively. Subsequently, leaching induced changes of the CSH structure and of the U(VI) or Cm(III) coordination environment were investigated with TRLFS, IR spectroscopy and XRD.
Site-selective TRLFS studies of the Cm(III)/CSH binding at 4 K revealed a fluent transition between two sorption sites causing a luminescence line-narrowing effect. The leached CSH phases showed pronounced differences in terms of decomposition behavior and actinide release depending on their C/S ratio and type of incorporated actinide. CSH phases with a lower C/S ratio were influenced strongly by NaHCO3 and showed a mobilization of U(VI) as Ca2UO2(CO3)3(aq). In contrast, Cm(III) was not leached out but as detected by site-selective TRLFS it is incorporated into calcite and vaterite (Fig. 1 (r.), 608 and 612.8 nm) formed during leaching in NaHCO3. The comparison between leaching experiments performed in 0.02 M NaHCO3 and 2.5 M NaCl/0.02 M NaHCO3 revealed that the presence of 2.5 M NaCl increases the U(VI) mobilization for CSH phases with high C/S ratios while no influence on the Cm(III) release was detectable.