Spectroluminescence measurements of stability constants of CanUO2(CO3)3(4-2n)- complexes in NaClO4 medium and investigation of interaction effects.

The stability constants of ternary calcium uranyl tricarbonate complexes, CaUO2(CO3)32- and Ca2UO2(CO3)3(aq), were determined in NaClO4 medium at various ionic strengths using time-resolved laser-induced luminescence spectroscopy (TRLS) – also known as time-resolved laser-induced fluorescence spectroscopy (TRLFS). As in a previous study, the potential precipitation of schoepite (UO3·2H2O) and calcite (CaCO3) were avoided via titration of triscarbonatouranyl complex by Ca2+ at varying pH values. The Ringbom coefficients relative to UO2(CO3)34- were individually evaluated for test sample conditions. Steadily enhanced luminescence intensity and increased decay-times were representative of complexation processes. The stoichiometry of calcium was quantified by slope analysis, the measured intensity of which was corrected by the corresponding Ringbom coefficient. The conditional formation constants, i.e. log10 Kn.1.3, were then assessed after rounding off the slope values to their nearest integers. Determinations of cumulative formation constants at infinite dilution log10 β°n.1.3, and specific ion interaction parameters e were based on the experimental origin and slope values, respectively, over the range 0.1-2.46 mol kgw-1 NaClO4 using the specific ion interaction theory (SIT) approach. The cumulative stability constants – log10 β°(CaUO2(CO3)32-) = 27.26 ± 0.04 and log10 β°(Ca2UO2(CO3)3(aq)) = 30.53 ± 0.06. The specific ion interaction coefficients are estimated to be e(CaUO2(CO3)32-,Na+) = (0.02 ± 0.04) kgw mol-1 and e(Ca2UO2(CO3)3(aq),NaClO4) = (0.18 ± 0.07) kgw mol-1. These latter values are different from the ones that were our previously obtained in NaCl, and underlying causes are discussed from different aspects. This work provides valuable information to address the interaction effects between Ca-UO2-CO3 species and 1:1 type electrolytes. It is suggested that the affinity of the cation in background electrolyte with CanUO2(CO3)3(4-2n)- (n = {1;2}) are to be taken into consideration at high ionic strengths, especially for the globally non-charged species.