Comparison of the permeation of MgCl₂ versus NaCl in highly charged sulfonated polymer membranes

Sulfonated polymers have been considered for use as chlorine tolerant desalination membrane materials. The hydrophilic nature of these materials results from inclusion of sulfonate moieties on the polymer backbone. These sulfonate groups can dissociate into a fixed and a mobile ion. As such, ion exchange can occur when the polymer is contacted with an electrolyte solution. Different counter-ion forms of the same sulfonated polymer may exhibit very different transport properties. The sodium chloride permeability of a sulfonated pentablock copolymer increases as sodium chloride concentration in the upstream solution increases. However, the magnesium chloride permeability of the material does not show a strong dependence on salt concentration. These observations can be explained by Donnan theory and consideration of the effect of sodium and magnesium ions on the concentration of fixed charge groups in the polymer matrix. Greater association between magnesium ions and the polymer's sulfonate groups reduces the effective fixed charge of the polymer, and Donnan exclusion becomes less effective compared to the sodium ion case.