Synthesis of a monovalent selective cation exchange membrane to concentrate reverse osmosis brines by electrodialysis

In this work, novel monovalent selective cation exchange membranes (CEMs) with different mixtures of polyvinylidene fluoride (PVDF) and sulfonated PVDF (S-PVDF) were synthesized. The selected composite membranes were modified by surface polymerization of polyaniline (PANi) to improve their monovalent cation selectivity. PANi was doped with p-toluene sulfonic acid (pTSA) or L-valine (2-amino-3-methylbutanoic acid) and the selectivity of each CEM was determined. Membrane properties, such as chemical composition, water uptake, ion exchange capacity, permselectivity, contact angle and crystallinity were measured. The influence of the applied voltage was also studied. The newly developed membranes were used for electrodialytic concentration of NaCl from synthetic reverse osmosis (RO) brine. Doped membranes with pTSA (SMg = 0.13, SCa = 3.59) and Na Na valine (SMg = 0.09, SCa = 0.8) have a higher selectivity for sodium than the composite ones (SMg = 0.63, Na Na Na SCa = 6.82). Moreover, the increase of applied voltage results in an increase of the selectivity for monovalent ions. Na © 2015 Elsevier B.V. All rights reserved.

Novel hybrid cation exchange membranes were synthesized by surface polymerization of polyaniline (PANi) on S-PVDF/PVDF composite membranes with different doping agents. FTIR proved the formation of a PANi layer on the membrane surface, leading to a change in functional groups. XRD showed that after surface polymerization, the hybrid membrane structure becomes more ordered than that of the composite membrane, especially for pTSA doping agent. The combination of structure information with other membrane properties such as water uptake, IEC and permselectivity helped to determine the relation between membrane properties and ED performance. To synthesize a membrane with excellent sodium selectivity among the mixture of three cations in the solution (Na+, Mg2+ and Ca2+) for the concentration of SWRO brine as feed for the chlor-alkali process, S-30 was selected to be modified by PANi surface polymerization. These membranes were doped with pTSA and valine and the results showed that for both doping agents the selectivity of sodium will increase. A remarkable result was achieved for S30-valine, which showed a very good selectivity for monovalent ions compared to both bivalent cations. This membrane had a very high Na+ to Mg2+ selectivity and also good Na+ to Ca2+ selectivity. This is valuable because the surface polymerization of S-30 by PANi-valine did not decrease the concentration of Na+ in the concentrate and kept the flux of Na+ as high as S-30. Valine is an amino acid that can keep the surface hydrophobicity as well as membrane IEC high and provide a selective barrier for Na+ because of its special chemical structure after doping on coated PANi. The influence of the applied voltage was also evaluated on hybrid membranes; it was found that by increasing the voltage from 5 V to 7 V, the monovalent cation selectivity increases. This result can be explained by the lower flux of bivalent cations at 7 V which is due to more powerful attraction of cations by the cathode at higher voltage.