Thermodynamics of Neptunium(Y) in Concentrated Salt Solutions: Chloride Complexation and Ion Interaction (Pitzer) Parameters for the NpO⁺ ₂ Ion

Abstract

The interaction between NpOJ and chloride ions has been investigated at 25 °C by means of absorption spectroscopy and solvent extraction with NaDNNS. The experimental results are discussed by two different approaches: (1) the Np(V) chloride complexation and (2) the strong ion-ion interaction. The formation constants of the complexes Np0 2 Cl and Νρ0 2 0 2 are determined by solvent extraction experiments at different ionic strength, I = 1.0, 2.0, 3.0 and 5.0 mol/L (NaC10"/NaCl). The complexation constants extrapolated to / = 0 are found to be very low, i.e. logyS? = -0.29±0.05 and log/g < -1. The spectroscopic study shows no evidence for the formation of strong inner sphere complexes. Therefore, the interaction between NpOJ and CI" is considered as strong ion-ion interaction without invoking Np(V) chloro complexes. Applying the Pitzer equations, the interaction parameters β°\ β" and C* for NpOJ/ClO; and NpOJ/ CI" have been evaluated from trace activity coefficients of the Np0 2 ion in 0.2-5 M NaC10 4 and 0.2-5 M NaCl, respectively. The results are applied for calculating the solubility products of Np0 2 0H(s), NaNp0 2 C0 3 (s) and Na 3 Np0 2 (C0 3 ) 2 (s) as a function of the NaC10 4 and NaCl concentration. In both media, there is excellent agreement between experimental and predicted solubility products.

Solubility and hydrolysis of Np(V) in dilute to concentrated alkaline NaCl solutions: formation of Na–Np(V)–OH solid phases at 22 °C

“…The total concentration of Np(V) hydrolysis species under the alkaline conditions is estimated by evaluating the differences in the prominent · cm −1 (the uncertainty of the ε values is about ±20, corresponding to an absolute analytical error of 5% in the LSC determination of [Np]). Similar results for Np(V) in NaCl and LiCl solutions were reported in [54] and [55], respectively. The slight changes indicate a rather small contribution of Np(V) innersphere chloro species.…”

Neptunium(V) Complexation by Acetate, Oxalate and Citrate in NaClO4 Media at 25°C

“…The stability constants measured with the organic ligands in Perchlorate media are higher than those measured by the same method in NaCl solutions in our laboratory [9]. This may be attributable to formation of Np0 2 CU~* species in the NaCl solution since such chloro species would be included in the "free" neptunyl cation concentration in the calculation of the stability constants [13].…”

“…Some deviation (ca. 0.1 in log β) of the model fit from the experimental values occurred at ionic strengths above 7 m, reflecting that the Pitzer parameters available for HC10 4 -NaC10 4 binary system [12] and Np0 4 -C10 4 interaction [13] are valid only to 6-7 m ionic strength.…”

“…Standard chemical potentials and Pitzer ion interaction parameters for the H 2 0/(Na,Np0 2 )C10 4 system were taken from the literature [12][13][14].…”

Thermodynamic Modeling of Neptunium(V)-Acetate Complexation in Concentrated NaCl Media

“…Model parameterizations exist for brine evaporite systems (Harvie et al, 1984;Felmy and Wean, 1986) and the Pitzer formalism is increasingly being used to interpret and describe the chemical behavior of +m, +IV, and +V actinides in concentrated electrolytes (Fanghbel et al, 1995;Felmy et al, 1989;1990;1991;Felmy and Rai, 1992;Neck et al, 1995;Novak et al, 1995;Novak and Roberts, 1995 The concentration dependence of the activity coefficient for neutral species is (Felmy and Weare, 1986), where hc and ha are neutral-cation and neutral-anion interaction parameters, respectively, and rNca is a neutral-cation-anion interaction parameter.…”