A speciated, conventional Pitzer ion-interaction model for the aqueous Nd3+–H+–Na+–K+–Ca2+–Cl−–OH− system at 298 K and 0.1 MPa

Abstract

A conventional, speciated Pitzer ion-interaction model at 298 K and 0.1 MPa is presented for the NdCl3 + H2O system and mixtures containing HCl, NaCl, KCl, and CaCl2. The model uses Nd3+, NdOH2+, Nd(OH)2+, Nd(OH)30, NdCl2+, and NdCl2+ solute species and was fitted to isopiestic, electromotive force (emf), and NdCl3·6H2O solubility data. The model was then extended to pH values at which Nd(OH)3, Nd(OH)2Cl, and a speculative double salt, Nd(OH)3·NdCl3, are stable. The two hydroxychloride salts enabled the model to join the stability fields of NdCl3·6H2O and Nd(OH)3 ensuring continuity in the range 2 < pH < 13. The activity coefficient model developed from the low pH [NdCl3 + NaCl](aq) system reproduces the solubility of solid Nd(OH)3(s) in water and NaCl(aq) with only the addition of standard state free energy terms for crystalline (cr) and amorphous (am) NdOH3(s) and the Nd(OH)2+ and Nd(OH)30 solute species. No Pitzer parameters involving NdOH2+, Nd(OH)2+ and Nd(OH)30 were necessary to reproduce the full body of thermodynamic data over an ionic strength range from infinite dilution to the solubility limits of NaCl, KCl, CaCl2·6H2O, Nd(OH)3(s), the hydroxychloride salts, and for mixtures containing up to 4.5 molal HCl(aq). The model adds only five ‘binary’ and three ‘ternary’ Pitzer ion-interaction parameters to the H-Na-K-Ca-Cl-OH parameters in Harvie et al. (1984; hereafter HMW). The model is simpler, compositionally broader, and more accurate than previously published Pitzer ion-interaction models for aqueous neodymium-chloride and -hydroxide solutions containing HCl, NaCl, KCl or CaCl2. The model has ramifications for geochemical modeling and uncertainty quantification of An(III) solubilities used to support certification of evaporite hosted nuclear waste repositories.