Novel electrodialysis membranes with hydrophobic alkyl spacers and zwitterion structure enable high monovalent/divalent cation selectivity

Development of monovalent cations perm-selective membranes (MCPMs) for electrodialysis with the ability to improve cation flux, selectivity and reduce membrane swelling are the major concerns in different industrial applications, such as seawater desalination and wastewater purification. We designed and fabricated the novel MCPMs of quaternized poly(2,6-dimethyl phenylene oxide)s containing different lengths of alkyl spacers which were grafted directly to the nitrogen-centered cations connected to hydrophilic carboxylic and sulfonic acids groups. Pendant chains were in charge of tuning the membrane hydrophobicity and promoting the microphase separation. We observed that during changing the hydrophobicity of MCPMs, higher cation flux, greater perm-selectivity and more dimensional stability of monovalent cations perm-selective membrane can be achieved. The cations selectivity of prepared MCPMs was measured from electrodialysis in a Na+/Mg2+ system and compared with the commercial monovalent cation selective NeoseptaTM CIMS membrane. Highest perm-selectivity (PMg+2Na+) that reached up to 25.26 was observed for the MCPMs with the longest hydrophobic pendant chain which demonstrated that enhancement of membranes hydrophobicity considerably improved the monovalent cation perm-selectivity. These observations are quite interesting to expedite the improvement of monovalent cations perm-selective membranes for electrodialysis.