Thermodynamic Modeling of Neptunium(V) Solubility in Concentrated Na-CO 3 -HCO 3 -Cl-ClO 4 -H-OH-H 2 O Systems
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THERMODYNAMIC MODELING OF NEPTUNIUM(V) SOLUBILITY IN CONCENTRATED Na-C0 3-HCO 3 -CI-C10 4 -H-OH-H 2 0 SYSTEMS, Craig F. Novak* and Kevin E. Roberts** *
Sandia National Laboratories, MS 1320, P.O. Box 5800, Albuquerque, NM 87185-1320 USA
** Lawrence Berkeley Laboratory, MS 70A-1 115, 1 Cyclotron Road, Berkeley, CA 94270 USA
ABSTRACT Safety assessments of nuclear waste repositories often require estimation of actinide solubilities as they vary with groundwater composition. Although a considerable amount of research has been done on the solubility and speciation of actinides, 1,2 relatively little has been done to unify these data into a model applicable to concentrated brines. Numerous authors report data on the aqueous chemical properties of Np(V) in NaClO4, Na2 CO 3 , and NaCl media, but a consistent thermodynamic model for predicting these properties is not available. To meet this need, a model was developed to describe the solubility of Np(V) in Na-CI-C10 4 -C0 3 aqueous systems, based on the Pitzer activity coefficient formalism for concentrated electrolytes. Hydrolysis and/or carbonate complexation are the dominant aqueous reactions with the neptunyl ion in these systems. Literature data for neptunyl ion extraction and solubility are used to parameterize an integrated model for Np(V) solubility in the Np(V)-Na-C0 3 -HCO 3 -CI-C10 4-H-OHH20 system. The resulting model is tested against additional solubility and extraction data, and compared with Np(V) solubility experiments in complex synthetic brines. THE PITZER ACTIVITY COEFFICIENT MODEL The Pitzer activity coefficient model 3 is a semi-empirical formalism describing the thermodynamics of electrolyte systems. Model parameterizations exist for brine evaporite systems 4 ,5 and, in a more limited fashion, for +111 and +IV actinides in brines. 6,7,8, 9 The model is general enough to describe solutions from dilute to high concentration, and is formalized for complex electrolytes such as natural brines. Activity coefficients are represented by a virial expansion which, truncated to binary interactions, has the form Na Inym =ZmF +2 1 ma Brna(1)
for cation "m"
a=1 where F represents the Debye-Htickel term, Na is the number of anions, and zm is the charge on cation m. A similar expression holds for anions. The complete forms of the equations and extensive discussion are given elsewhere. 3,4 The second virial coefficients Bma (I) are functions of the ionic strength, and are given by the relationship Bma(1) ma()-ma+ma
(0)
r (2) Pm(1) ma g(0Cma\Fi) + Prna a g(12Vr)
for cation "m," anion "a"
(2) w ee(0) (1) where 0ma, and t )maare parameters that are important at high, intermediate, and low ionic strength, respectively, and g(y) is a decaying exponential function of the argument. When only the P(') coefficient is nonzero, this model is similar to SIT, 10 although a species molality, not the total ionic strength, this coefficient. The following sections discuss the process of deemnn (0) an multiplies (1) determining 0 ma and 0(ma values and standard chemical potent
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