Search

New and previously reported data on the extraction of water (W), HNO3 (HX), and UO2(NO3)2 (MX2) from aqueous mixtures by DEHSO (B) in dodecane are combined to develop a comprehensive model of U(VI) extraction from nitric acid by DEHSO. The computer program SXFIT used for this modeling estimates solute activity coefficients and solvent activities in the aqueous phase by the Pitzer treatment and in the nonaqueous phase by the Hildebrand-Scott treatment. Included is a critical evaluation of the Pitzer parameters needed and an estimate of the smaller effect of the solubility parameters assigned to the nonaqueous solutes. The resulting model indicates that: (1) The small amount of water extracted by dodecane alone is monomolecular (W). (2) The additional water extracted by B requires three hydrated species (BW, B2W, and an aggregate assigned as B2W5). (3) The extraction of HX by B requires four species (BHX, B(HX)2W5, and two species of B2HX, probably B2HX, and B2HXW2). (4) Finally, the extraction of MX2 yields two additional species (principally MB2X2 and a hydrate of MB3X2). To account adequately for all the data, no less than 11 nonaqueous solute species are required as well as the adjustment of two Pitzer parameters, ψ[UO22+ -H+ -NO3-] and ψ[Na+ -H+ - NO3-]. The treatment of non-ideality, the low interdependence of the adjusted parameters, the diversity of the data (fitted in stages), and the consistency of the resulting model with the behavior of related neutral, oxygen-donor extractants strongly suggest that the complexity of this system is real, rather than an artifact of over interpretation. [ABSTRACT FROM AUTHOR]

SXFIT is the latest in a series of programs designed to model solvent extraction systems of increasing complexity. Unlike its predecessors, SXFIT permits the user complete freedom to define the constituents from which the composition of each phase is to be specified in the data and from which all species of a model for a system are to be formed. This paper describes the manner in which the physical chemistry of the nonaqueous and aqueous solutions involved has come to be treated in this modelling effort. Included are: (1) solute activity coefficients and solvent activity in the nonaqueous phase, estimated by the Scatchard-Hildebrand-Scott treatment; (2) the effect of ionization on these quantities; (3) solute activity coefficients and water activity in the aqueous phase, estimated by the Pitzer treatment; (4) the effect of non ideal behavior on the total concentration of nonaqueous solute species as rendered by a vapor pressure osmometer; (5) a similar treatment of heats of mixing of nonaqueous solutions; (6) the estimation of apparent molar volumes of aqueous species and, from these, the solution density, needed for the conversion of concentrations from the molarity to the molality scale; (7) the conversion of equilibrium constants from one concentration scale to another. [ABSTRACT FROM AUTHOR]