Multilayered PEC nanofiltration membranes based on SPEEK/PDDA for anion separation

Layer-by-layer (LBL) deposition of polyelectrolytes (PEs) on porous substrates has emerged as a versatile and inexpensive method to form high flux nanofiltration (NF) membranes. PE complexes (PECs) were prepared by alternative deposition of poly(diallyldimethylammonium) chloride (PDDA) and sulfonated poly(ether ether ketone) (SPEEK) on hydrolyzed polyacrylonitrile (PAN-H) supports. PDDA/SPEEK multilayers consisting of 5, 10, 15 and 20 bilayers were deposited from solutions with different NaCl concentrations. Film thickness was studied by AFM. Thicker films were formed with increasing the bilayer number and the salt concentration. For the first time, multilayered PDDA/SPEEK PEC membranes were applied as NF membrane in an anion separation application. The NF performance of PDDA/SPEEK membranes was studied for single salt solutions (chloride, sulfate and phosphate) as well as mixed-salt (binary and ternary). Membranes showed different retentions and fluxes for anions in single and mixed-salt solutions. When using mixed-salt solutions, the retentions were found to be higher compared to the single salt NF experiment results. HPO42− ions showed a remarkable influence on the performance of the PEC membranes via formation of intermolecular bonds within the polymer chains. In the best case, membranes showed Cl−/SO42− and Cl−/HPO42− selectivities of 20 in the separation of mixed-salt solutions.