Report on neptunium speciation by NMR and optical spectroscopies

Kinetic Studies of the [NpO2(CO3)3]4– Ion at Alkaline Conditions Using 13C NMR

“…The rate of exchange from the solvation shell is inferred from the effect on the bulk solution 17 O peak. 13 C NMR spectra for the [NpO 2 (CO 3 ) 3 ] 4− ion carbonate solution contain two wellseparated peaks, one for the free carbonate (166 ppm) and one for the bound carbonate (84 ppm), which is consistent with the spectra reported by Stout et al and Tait et al 1,24 (Figure 2). The presence of two individual signals, one assignable to the bound carbonate ligand and one assignable to free carbonate, indicate that ligand exchange between bound and free carbonate is slow on the NMR time scale.…”

“…Tait et al used 13 C NMR spectroscopy to determine the speciation of the Np(VI)−carbonate solutions. 24 The monomeric [NpO 2 (CO 3 ) 3 ] 4− species exhibits a single 13 C NMR peak attributable to bound carbonate with a chemical shift around ∼80 ppm (Figure 2), while the trimeric species exhibits two peaks at approximately 8 and −89 ppm, again corresponding to bound ligands.…”

Pressure Dependence of Carbonate Exchange with [NpO2(CO3)3]4– in Aqueous Solutions

“…At higher and lower pH conditions, appreciable amounts of either partly hydrolyzed neptunium complexes or the trimeric neptunyl-carbonato complex can form, respectively. 20 These were avoided. No precipitation was observed in any of the NMR samples.…”

“…Samples were prepared at pH 9–10 to maintain a simple speciation where the [NpO 2 (CO 3 ) 3 ] 4– ion was the dominant aqueous neptunium species. At higher and lower pH conditions, appreciable amounts of either partly hydrolyzed neptunium complexes or the trimeric neptunyl-carbonato complex can form, respectively . These were avoided.…”

Impact of the formation of NpO{sub 2} and Np{sub 2}O{sub 5} on the Np solubility in YM waters

“…Np(V) solubility-limiting solids include Np2O5(c), NpOzOH(am), and, in high ionic strength carbonate media, the so-called double carbonate salts Nal-2xNp02(C03)x (x=l to 3) (Volkov et al, 1979(Volkov et al, , 1981Neck et al, 1994Neck et al, , 1995. In the absence of carbonates, the solution speciation of Np(V) is dominated by the highly soluble Np02+, which does not hydrolyze readily below a pH of 10 (Moslwin, 1971; Rosch et al, 1987;Neck et al, 1992;Tait et al, 1995). In 5-13 type waters, the carbonate-complexed Np(V) species of importance includes NpO2CO3-, where the higher carbonate complexes are not strong enough to be predominant, even at higher temperatures .…”