there are two types of retention curves with
increase of pH: quasi-symmetric rejection curves for the symmetric
salts (KCl, LiCl and MgSO4) and highly asymmetric S-shaped curves
for the asymmetric salts (K2SO4 and MgCl2), and it is clear that this
membrane shows an amphoteric behavior with an IEP close to 6 in
these salt solutions [37]. For symmetric salts, the salt rejection is
governed by co-ions (ions showing the same charge sign as the
membrane) due to electrostatic repulsion and Donnan equilibrium.
With increasing pH, the cations (Kþ, Liþ and Mg2þ) are dominant
below IEP (membrane is positively charged) while the anions
(Cl− and SO4
2−
) determine the salt retention above IEP (membrane is
negatively charged), thus inducing V-shaped rejection curves for
symmetric salts. On the other hand, for asymmetric salts, the salt
retention is dominated by the ions with higher valence (SO4
2− for
K2SO4 and Mg2þ for MgCl2) because they hold more charges.