The Determinants of Divalent/monovalent Selectivity in Anion Exchangers

This research demonstrates that it is the distance of fixed-charge separation in the resin which is the primary determinant of divalent/monovalent selectivity. Anion resins, particularly acrylics and epoxies, with closely spaced N-containing (amine) functional groups are inherently divalent-ion selective. This is because the uptake of a divalent anion, e.g. sulfate, requires the presence of two closely-spaced positive charges. Results from this study of 30 commercially-available, strongand weak-base, anion resins indicate that in order to bring positive charges into close proximity within a resin one can: (1) incorporate the amine functional groups into the polymer chains as opposed to having them pendant on the chains," (2) minimize the size and number of organic ("R") groups attached to the N atom, i.e., minimize the size of the amine," and (3) maximize the resin flexibility, i.e., its ability to reorient to satisfy divalent counterions, by minimizing the degree of crosslinking. The distance-of-charge-separation theory is not restricted to divalent anion exchangers but also applies to cation exchangers and to polyvalent ions in general.