Stepwise synthesis of polyacrylonitrile-supported oligoamide membranes with selective dye–salt separation

Production processes of the dye and textile industries release toxic organic dyes that are detrimental to the environment. The treatment of industry waste effluents using membranes is difficult as membrane recovery is hampered by high salt contents in dyes. This work reports a molecular layer-by-layer approach for preparing oligoamide membranes on a poly(acrylonitrile) (PAN) support capable of effective dye–salt separation, where the oligoamide film is covalently linked to a functionalized PAN support. Membranes with improved water permeance, molecular weight cut-off (MWCO), and dye–salt selectivity were synthesized by screening different PAN supports and investigating their effect on performance, varying the number of oligoamide synthesis cycles, and tuning the monomer coupling reaction conditions. Microscopic and spectroscopic techniques confirmed the structure of the oligoamide membranes. Water permeance was directly correlated to the number of synthesis cycles, the MWCO of the oligoamide membranes varied between 104.5 and 2.0 kDa for 2.5 to 10.5 synthesis cycles, respectively. Synthesis with 10.5 cycles gave an oligoamide membrane with >99% rejection of Congo Red with a high permeate flux of 87.5 ± 14.8 L m−2 h−1 bar−1 and 99%–100% salt passage, demonstrating its promising potential of fractionating dye–salt mixtures. The synthesized membranes showed excellent stability during long-term (one week) filtration of Congo Red effluents containing 20 g/L NaCl. The reported membranes hold great promise for dye–salt separation and treatment of wastewater from the dye and textile industries.