DIVALENT MONOVALENT CATION UPTAKE SELECTIVITY IN A NAFION CATION-EXCHANGE MEMBRANE - EXPERIMENTAL AND MODELING STUDIES/

The equilibrium uptake of monovalent/divalent cation salt mixtures by a Nafion perfluorosulfonic acid cation-exchange membrane has been inve stigated using experimental measurements and a theoretical model. Memb rane concentrations were determined for Nafion equilibrated in 0.15 M solutions containing Ni2+, CU2+, Ca2+, or Mg2+ with a co-absorbed mono valent cation (Li+, K+, or Cs+). An equilibrium ion absorption model t hat accounts for ion hydration effects, the dielectric saturation of w ater molecules in a membrane pore, and the neutralization of tired-cha rges by ion pairing with divalent cations, was matched to the experime ntal data. When the extent of ion-pair formation was used as an adjust able parameter, the model predicted accurately both the monovalent and divalent cation concentrations (with an average error of 7.7%). Both theory and experiments showed that the monovalent cation selectivity w as in the same order as observed previously during the uptake of monov alent/monovalent cation salt mixtures (i.e. the monovalent cation with the larger hard sphere radius was preferentially absorbed). The compu ted mobile divalent cation concentration in a Nafion pore was found to be dependent on the extent of monovalent cation absorption. The numbe r of divalent cation/sulfonate fixed-charge-site ion pairs was found t o be independent of the divalent cation type, but was controlled by th e type and concentration of the co-absorbed monovalent cation. The fra ction of ion-paired fixed charges was correlated with the membrane por e concentration of mobile divalent cations via a Frumkin adsorption is otherm.