Effect of major cation water composition on the ion exchange of Np(V) on montmorillonite: NpO₂⁺–Na⁺–K⁺–Ca²⁺–Mg²⁺ selectivity coefficients

Highlights: • Determined Np(V)-montmorillonite ion exchange constant, applicable in a wide range of conditions. • Developed a model for Np(V) ion exchange which can be readily applied in thermodynamic databases. • Identified solution conditions at which Np(V) ion exchange will play a significant role. - Abstract: Np(V) sorption was examined in pH 4.5 colloidal suspensions of nominally homoionic montmorillonite (Na-, K-, Ca- and Mg-montmorillonite). Ionic exchange on permanent charge sites was studied as a function of ionic strength (0.1, 0.01 and 0.001 M) and background electrolyte (NaCl, KCl, CaCl₂ and MgCl₂). An ion exchange model was developed using the FIT4FD program, which considered all experimental data simultaneously: Np sorption data, major cation composition of the electrolyte and associated uncertainties. The model was developed to be consistent with the ion exchange selectivity coefficients between the major cations reported in the literature and led to the following recommended selectivity coefficients for Np(V) ion exchange according to the Vanselow convention: log(^NpO₂+_Na⁺K_V)=-0.20,log(^NpO₂+_K⁺K_V)=-0.46,log(^NpO₂+_Ca²⁺K_V)=-0.57,log (^NpO₂+_Mg²⁺K_V)=-0.57. Both the experimental data and the estimated selectivity coefficients in this study are consistent with the limited Np(V) ion exchange and sorption data reported in the literature. The results indicate that, as expected, low ionic strengths favor Np(V) sorption when ion exchange is the main sorption mechanism (i.e. acidic to neutral pHs) and that the divalent cations Ca²⁺ and Mg²⁺ may be important in limiting Np(V) ionic exchange on montmorillonite