Permselectivities of monovalent anions through pyridine-modified anion-exchange membranes

The permselectivities of three new series of anion-exchange membranes were evaluated by using anions such as fluoride, chloride, bromide, iodide, hydroxyl, nitrate, and nitrite ions. The membranes were prepared, respectively, via (i) quaternary amination of 100% bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) with n-butanol/pyridine solutions (A series), (ii) cross-linking of BPPO with ammonia and subsequent reaction with pyridine (B series), (iii) cross-linking of BPPO with ammonia, subsequent reaction with pyridine, and final quaternary amination of the residual bromomethyl groups with trimethylamine (C series). A stronger hydrophobicity was imparted on the anion-exchange membranes after modification with pyridine. The permselectivity tests indicate that anions with lower Gibbs free energies of hydration are more easily to permeate through a hydrophobic anion-exchange membrane generally and in the case of anions with similar Gibbs free energies of hydration, smaller Stokes radii of the anions are in favor of permeation. According to the experimental results, the B series of anion-exchange membranes have the best performances as in separation of monovalent anions.