Divalent vs monovalent ion sorption and transport in ion exchange polymers based on cross-linked polystyrene with fixed sulfonate anions
Michele Galizia, Francesco M. Benedetti, Donald R. Paul, Benny D. Freeman
J.McKetta Department of Chemical Engineering, 200 E. Dean Keeton Street, 78712 Austin,
and Center for Energy and Environmental Resources, 10100 Burnet Road, 78758 Austin,
The University of Texas at Austin, Texas (USA)
Sorption and transport of alkali-chlorides (NaCl, KCl, LiCl) and alkaline earth-chlorides (CaCl2, MgCl2) in a commercial cation exchange membrane based on cross-linked polystyrene with fixed sulfonate ions was investigated. Salt transport parameters are correlated to the ion size and valency, as well to the membrane water content. It is found that divalent salts show higher sorption and permeability coefficients than monovalent salts. These differences are due to the stronger interaction between divalent counter-ions and the polymer’s sulfonate ions, which significantly reduces the actual fixed charge of the polymer. Due to this partial neutralization of fixed charges, the Donnan exclusion  becomes less effective in the case of divalent salts, thus determining the observed increases in permeability and sorption coefficients. Moreover, water content in membranes in Ca++ of Mg++ form is significantly less than in membranes in Na+, K+ or Li+ form. Divalent ions tend to more strongly screen the polymer fixed charges, thus reducing the hydrophilicity of the membrane.
Thermodynamics of sorption was also investigated by calculating the salt activity coefficients in membrane, by using a Donnan-type treatment. The results confirm that divalent ions can strongly interact with fixed sulfonate ions relative to monovalent ions.
 F.G. Donnan, Chem. Rev. 1924 1 73-90