Time-resolved laser fluorescence spectroscopy (TRLFS) is used to study Cm(III) solutions ([Cm]tot=2×10−7 M) in 0.1−3.5 M CaCl2 at pHc≈11.7 and in three sets of experiments at constant ionic strength (1.0, 2.5 and 3.5 M CaCl2) with pHc varying from 10.8 to 11.9. The fluorescence spectra show Cm(III) emission bands in the range 605−620 nm. Increasing the CaCl2 concentration strongly enhances the intensity of the observed bands. The concentration of aqueous Cm(III) species determined after 10 kD ultrafiltration increases as well, indicating that the Cm(III) hydroxide complexes are stabilized by a strong interaction with Ca2+ ions. In 5 M NaCl-NaOH solutions studied for comparison, the curium is almost completely precipitated as Cm(OH)3 colloids which show no fluorescence emission bands.
Peak deconvolution and the evaluation of the pH dependence of the spectroscopic data show that three emission bands with λmax=607.5, 609.9 and 614.7 nm come from Cm(III) complexes with three, four and six OH− ligands, respectively. The tri- and tetrahydroxide complexes are dominant in 0.1−1.0 M CaCl2, the hexahydroxide complex dominates at higher CaCl2 concentrations and pHc>11.5. As a consequence of the strong dependence on the Ca2+ concentration and the absence of these complexes in NaOH-NaCl solutions, thermodynamic modelling of the apparent equilibrium constants for stepwise complex formation and the solubility of colloidal Cm(OH)3(am) requires that the observed species are treated as ternary Ca−Cm(III)−OH complexes: Cax[Cm(OH)3]2 x, Cay[Cm(OH)4]2 y −1 and Caz[Cm(OH)6]2 z −3 where x may be 0 or 1, y=2 and z=3.