Abstract
The effects of Nd(III)/Am(III) complexation with sulfate were studied by 1) re-examining existing data for the Am−SO4 system using more, advanced aqueous electrolyte models valid to high concentration to obtain reliable thermodynamic data for SO 2−4 complexes or ion interactions with Nd3+ and Am3+ and 2) conducting experimental solubility studies of NdPO4(c), an analog phase of AmPO 4 (c), a possibly important phase in high level nuclear wastes, in the presence of SO 2− 4 to test the newly developed thermodynamic model and show the possible influence of sulfate in a repository environment. The data showed that the increase in the solubility of NdPO 4 (c) resulted primarily from the increase in ionic strength. Slightly higher observed Nd concentrations in the presence of sulfate, as compared with concentrations predicted at the experimental ionic strengths, resulted from the weak complexes or ion interactions involving Nd3+−SO 2− 4 . The Pitzer ion interaction parameters, applicable to 0.5m sulfate, were obtained for Am3+−SO 2− 4 from a reinterpretation of known solvent extraction data. These parameters are also consistent with literature data for Am3+/Na+ exchange and solvent extraction in the presence of sulfate. When used for the analogous Nd3+−SO 2− 4 system to predict NdPO 4 (c) solubility in the presence of sulfate, they provided excellent agreement between the predicted and the observed solubilities, indicating that they can be reliably used to determine Nd3+ or Am3+ ion interactions with SO 2− 4 in all ground waters where SO 2− 4 is less than 0.5m
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Rai, D., Felmy, A.R. & Fulton, R.W. Nd3+ and Am3+ ion interactions with sulfate ion and their influence on NdPO4(c) solubility. J Solution Chem 24, 879–895 (1995). https://doi.org/10.1007/BF00973443
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DOI: https://doi.org/10.1007/BF00973443