Carbonate Complexation of Neptunium(IV) and Analogous Complexation of Ground-Water Uranium

Abstract

The solubility of Np(IV) in a carbonate solution in the pH range of 8 -12.5 was studied. The total carbonate concentration was varied between 5xl0~4 -5xl0~3 M. The result was interpreted by considering two kinds of reactions:a. below pH 10 where bicarbonate ion was the dominant species,or, Np(OH) 4(am) + 2HCO3-= Np(0H) 4 (HC0 3 )r, jq-0.35±033b. above pH 10 where carbonate ion was considered predominant, Np(OH) 4(am) + 2CO3 = Np(0H) 4 (C0 3 )r K=XQ -1.43 + 0.33The solubility of uranium in Swedish ground-water was analyzed by assuming the presence of a typical complex of U(0H) 2 (C0 3 )2~, and a good agreement was achieved with the data observed. The recommended stability constant of U(0H) 2 (C0 3 )2~ was estimated to be >10 41 · 70 . This suggests the importance of such complex species in the natural environment.

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

“…In a carbonatecontaining solution, the solid Np(IV) hydroxo-carbonato complex, Np(OH)2(CO3)(s), is considered the most thermodynamically stable solid phase. However, a concentration ratio of 10 -9.4 between Np(OH)2(CO3)(s) and Np(OH)4 (am) was derived in a carbonate concentration range of 5 × 10 -4 M to 5 × 10 -3 M [22]. Therefore, Np(OH)4 (am) is highly abundant and can consequently be considered a solubility-limiting phase for a carbonate concentration range of 3.1 × 10 -5 M to 1.3 × 10 -3 M in the YS-1 groundwater condition.…”

“…For a comparison of the neptunium solubility with that from various experimental results, the calculation of solubility and the neptunium species was performed with the YS-1-7 groundwater [26]. The geochemical properties of YS-1-7 are analogous to other experimental conditions in the literature [19,21,22,26,27]. The Eh value of the YS-1-7 groundwater, with a higher carbonate concentration of 78.4 mg/L at a depth of 140 m to 150 m, is -200 mV.…”

“…In the YS-1 groundwater, the total carbonate concentration is much lower the experimental value reported in the literature [19,21,22,26,27]. For the purpose of comparison between calculated and experimental solubility, the YS-1-7 groundwater, which has the highest carbonate concentration of all the groundwater samples, was used for the solubility calculation.…”

“…The experimental solubility study was performed with YS-1-7 groundwater at a total carbonate concentration of about 1. Generally, it is difficult to perform a solubility experiment in the only tetravalent oxidation state of neptunium, particularly under conditions of a possible oxidizing geologic environment [20,22,28]. Accordingly, a reducing agent or potentiometer is used to control the Eh value throughout the experiments for the purpose of maintaining the redox environment.…”

The effects of the glass surface area/solution volume ratio on glass corrosion: A critical review

“…Notice that the boron continues to be released from the glasses doped with U, 237 Np, and 239 Pu through 364 days, but reaches a nearly constant value in tests with glasses doped with 238 Pu and 24 1 Am. The normalized mass losses of 237 Np increases slightly with the reaction time, but the mass losses of the other actinides remain low at all test times. The lower corrosion rates measured in tests at 50*C (as measured by the boron release) correspond to the much lower solution pH values attained in tests with 2 3 8 Pu and 24 1 Am compared to tests with U, 23 7 Np, and 239 Pu.…”