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

Abstract

AbstractThe formation of aqueous Cm(III) phosphate complexes was studied at room temperature by time-resolved laser-induced fluorescence spectroscopy (TRLFS) in 0.1 M NaClO4solutions. The experiments were perfomed at a fixed total Cm(III) concentration of 3 × 107or 2 × 108 M by varying the phosphoric acid concentration (3 × 105–0.1 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] ≥4 × 104 M and between pH 4.0 and 6.0 in general a decrease in luminescence intensity affiliates the complexation with HPO42−. Two Cm(III)-phosphate complexes could be identified from the emission data, CmH2PO42+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 accompanied 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 logβ121= 20.23 ± 0.13 and logβ111= 16.54 ± 0.80 at an ionic strength of 0.1 M (NaClO4).