Ion interaction (SIT) coefficients for the Th4+ion and trace activity coefficients in NaClO4, NaNO3and NaCl solution determined by solvent extraction with TBP

Abstract

Trace activity coefficients of the Th4+ion in dilute to concentrated NaClO4, NaNO3and NaCl solutions ([H+]=0.01−0.02 M) have been determined at 22 °C from liquid-liquid phase equilibria with 10−50 vol.% TBP inn-dodecane. Using the specific ion interaction theory (SIT) to describe these equilibria as a function of the medium electrolyte concentration, the following ion interaction coefficients are calculated: ε(Th4+, ClO4-)=0.70±0.06 kg/mol and ε(Th4+, NO3-) = 0.31±0.12 kg/mol. The latter value differs considerably from ε(Th4+, NO3-) = 0.11±0.02 kg/mol used in the NEA-TDB. The low distribution coefficients atmNaCl<3 mol/kg do not allow the linear SIT extrapolation toI=0, but the equilibrium constants in 2.5−5.0 m NaCl are compatible with the NEA-TDB value of ε(Th4+, Cl-)=0.25±0.03 kg/mol.The SIT coefficients determined for the Th4+ion follow the linear correlations between known values of ε(MZ+, NO3-), ε(MZ+, Cl-) and ε(MZ+, ClO4-) for non-complexed cations MZ+withZ=1−4, including ε(Pu4+, ClO4-)=0.82±0.07 kg/mol and ε(Pu4+, Cl-)=0.37±0.05 kg/mol calculated from data accepted in the NEA-TDB. The interaction coefficients in the series of the tetravalent actinide ions show a slight systematic dependence on the ionic radius.