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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.

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