Kinetic modeling and selectivity of anion exchange in Donnan dialysis

Abstract

The objective of this work was to compare selective sorption and transport behavior of a Selemion AMV membrane for different anions with a theoretically derived kinetic model describing the Donnan dialysis (DD) process. This analysis resulted in a suggested relation for the diffusivity of small ions through "nanochannels" of ion exchange membranes. Mass transfer through boundary layers and membrane diffusion were modeled on the basis of the Nernst-Planck equation by introducing constant diffusivity ratios between the exchanging counter ions. To indentify the kinetic and selectivity coefficients, DD batch experiments with sodium nitrate, sulfate or dihydrogen phosphate as feed electrolytes and sodium chloride as receiver electrolyte were conducted. The derived kinetic model simulated the measured concentration changes very precisely after fitting three concentration-independent parameters and the concentration-dependent permeability coefficient. The selectivity sequence was found to be nitrate>sulfate>dihydrogen phosphate>chloride, while this sequence is strongly connected to activity in solution and in the membrane. This influence was very significant for sulfate, which resulted in higher removal efficiency than expected. Regarding diffusivity the identified sequence was nitrate>sulfate>chloride>dihydrogen phosphate. These results led to a correlation that describes diffusivity of counter ions through nanochannels as a function of hydrated cross section area and valence.