Solid-liquid equilibria of neptunium(V) in carbonate solutions of different ionic strengths: II. Stability of the solid phases

Effect of Carbonate on the Solubility of Neptunium in Natural Granitic Groundwater

“…Nevertheless, for a reasonable account of the experimentally measured solubility data from the literature, the formation of Np(IV) hydroxocarbonato complexes needs to be considered. In contrast, spectroscopic investigations have been undertaken to ascertain the nature of Np(V) hydroxo-carbonato complexes, particularly NpO2(OH)y(CO3)z 1-y-2z [1,9,29]. In the case of Np(V) solubility, the calculated Np(V) solubility agrees well with the measured solubility [1].…”

“…In carbonate free solutions, NpO2OH(s) and Np2O5(s) are considered stable solid phases [1,[3][4][5][6], whereas MNpO2CO3⋅xH2O or M3NpO2(CO3)2 (M = Na + , K + ) or both are the solubility-limiting solid phases based on the X-ray powder diffraction in an equilibrium state containing carbonate. Therefore, one of the characteristics of the solubility-limiting solid phases of Np(V) is their strong reliance on the alkali carbonate concentration [1,[7][8][9][10][11][12]. Additionally, many calculation studies have reported that the solubility of Np(IV) is 5 × 10 -9 M in a neutral to highly alkaline solution [13] and 3 × 10 -9 M for a pH range of 8.5-12.5 [14].…”

Precipitation of neptunium dioxide from aqueous solution

“…Significantly higher solution concentrations of both Na + and CO 3 2-are needed for the precipitation of NaNpO 2 CO 3 to occur in preference to the precipitation of Np(V) hydroxides or oxides as stated in the literature by Neck et al 46 Also, the patterns obtained in this work do not resemble the powder pattern for Np 3 O 8 in the ICDD database. 39 Additionally, the existence of a discrete Np 3 O 8 solid phase has been discounted.…”

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

“…This extention depends critically upon the database used. 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).…”

“…This letter report deliverable has been designed to support the following level 3 reports: (Neck et al, 1995;Runde et al, 1996) High sodium carbonate concentration support the formation of Na3Np02(C03)2*nH20, while Na-Np02 (CO3).nH2O is stable at low sodium carbonate concentrations. However, at low ionic strength and thus low sodium concentrations the formation Np(V) carbonates is suppressed and the formation of NpO2OH favoured.…”