Aqueous curium(III) phosphate species characterized by time-resolved laser-induced fluorescence spectroscopy

The formation of aqueous Cm(III) phosphate complexes was studied at room temperature by time-resolved laser-induced fluorescence spectroscopy (TRLFS) in 0.1 M NaClO₄ solutions. The experiments were perfomed at a fixed total Cm(III) concentration of 3x10^-7 or 2x10^-8 M by varying the phosphoric acid concentration (3x10^-5 - 0.1 M) and the pH (1.4 - 6.0). The red shift of the excitation and emission spectra, as well as the increase of luminescence lifetimes clearly showed the influence of phosphate on the aqueous Cm(III) speciation. In acidic phosphate solutions ([H₃PO₄] < 0.1 M, pH 1.4 - 2.6) an increase in luminescence intensity was detected due to complexation with H₂PO₄ -. At [H₃PO₄] > 4x10^-4 M and between pH 4.0 and 6.0 in general a decrease in luminescence intensity affiliates the complexation with HPO₄ ^2-. Two Cm(III)-phosphate complexes could be identified from the emission data, CmH₂PO₄ ²⁺ and CmHPO₄ ⁺, having peak maxima at 599.6 and 600.8 nm, respectively. TRLFS in combination with ultra-filtration (1 kD) showed that the formation of CmHPO₄ ⁺ is affiliated by the generation of Cm(III)-phosphate colloids especially at [H₃PO₄] > 0.002 M and pH > 5. Cm(III)-phosphate colloids formed at pH 5 and 6 are characterized by an emission maximum at 603.1 nm. Based on the factor analysis of the emission data the stability constants of the two complexes were calculated to be log ß₁₂₁ = 20.23 ± 0.13 and log ß₁₁₁ = 16.54 ± 0.80 at an ionic strength of 0.1 M (NaClO₄).