Effects of the selectivity coefficient on the kinetics of Donnan separation

Abstract

The objective of this study was to enable precise evaluation of Donnan separations, under diffusion controlled conditions, using the ion exchange membrane selectivity coefficient, Kc. This was accomplished by experimental measurement of the selectivity of Selemion® AMV anion–exchange membrane toward nitrate, bicarbonate, phosphate and sulfate, and analyzing batch Donnan separations data of these target ions with NaCl stripping solution according to flux equations which take into account the membrane selectivity. By fitting the experimental separation data to the model equations, kinetic coefficients characterizing the Donnan dialysis were determined for both the simplified case of Kc = 1 and for the more precise case based on the experimental Kc value. The difference in the kinetic coefficient based on the simplified case of Kc = 1 and on the experimental Kc value is around 10% for sulfate ions, 20% for the nitrate and bicarbonate ions and approximately 40% for the phosphate. These differences may cause deviation in evaluating the time required to achieve a desired target ion removal.