Spectroscopic investigation of cation effects in U(VI)-NO3- complexation in aqueous solutions

We report on the effect of monovalent cations (Na+, Li+, NH4+, and N(CH3)4 +) on the complexation equilibria of uranyl-nitrate in aqueous media, probed by time-resolved laser induced fluorescence spectroscopy(TRLFS). We find that N(CH3)4 + uniquely affects both the lifetime and the emission spectra of the uranyl species in nitrate solutions. Lifetime-corrected spectra, obtained by extrapolating the time-resolved spectra to t = 0, show a higher spectral shift in the case of N(CH3)4NO3 solutions. These spectra were deconvoluted into two components corresponding to UO22+ and the UO2(NO3)+ species. While adequate fits were obtained with the model for the NaNO3, LiNO3, and NH4NO3 systems, fits to the N(CH3)4NO3 data were poorer. In addition, the apparent formation constants are particularly high for the N(CH3)4+ system in comparison to the other monovalent cations. We also show that the speciation in these solutions can be modeled with the specific ion interaction theory (SIT), albeit with anomalously large interaction coefficient in the N(CH3)4+ case. We conclude that while the speciation of U(VI) in Na+, Li+, NH + 4 solutions can be described with UO22+ and the UO2(NO3)+ species, N(CH3)4+ solutions have a qualitatively different speciation, likely due to the existence of higher order uranyl-nitrate complexes.