Preparation of ultrathin-film composite membrane use as nanofiltration materials

The hydrophilic copolyamide (Co-PA) composite membranes for nanofiltration (NF) system were developed. The composite membranes were fabricated as a selective layer on the outer surface of polyvinylidene fluoride (PVDF) hollow fiber membrane substrate. The fabrication process was carried out by the dip-coating method to create two layers. The first layer is tannin to generate hydroxyl groups on the porous surface of a support. The Co-PA as a final selective layer is prepared through interfacial polymerization via the combination of aliphatic diamine (i.e., 1,4-diaminobutane, DABT) and conventional aromatic diamine monomer (i.e., m-phenylenediamine, MPD) in the aqueous phase with 1,3,5-benzenetricarbonyl trichloride (TMC) in the organic phase. The composite membranes were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), a water contact angle analyzer, scanning electron microscopy (SEM), gel permeation chromatography (GPC), UV-vis spectroscopy, and nanofiltration (NF) performance. The NF performance of the TFC membrane was tested with various salts and dyes at the operating pressure of 3 bars. In addition, DFT was applied to study the reaction mechanisms, including defining the configurations, the energetics of transition states, and the separation mechanism of the membrane and its important interactions that occur in the separation process. The results show that sample 9a-2 enhances surface hydrophilicity, molecular weight cutoff, and NF membrane performance compared to the conventional PA membrane. The water contact angle (CA) of sample 9a-2 decreased by 61.76% compared to the pristine PVDF support. Moreover, sample 9a-2 membrane showed a water permeation flux of 2.45 Lm-2h-1. Especially, sample 9a-2 exhibits the best salt rejection toward MgSO4 (80.38%), Na2SO4 (77.84%), MgCl2 (67.78%) and NaCl (34.78%) rejections. Further investigations to evaluate dye removal of sample 9a-2 show that sample 9a-2 membrane removes methylene blue at 93.43% and congo red at 99.98%. In addition, the isoelectric point of sample 9a-2 is found to be 7.03. This result confirms that the Co-PA layer is positively charged. Furthermore, the results from calculating the absorption energy of various salt ions. The result shows that sulfate ions (SO42-) can absorb on sample 9a-2 surfaces better than chloride ions (Cl-) due to the lowest absorption energy. The major factor influencing the absorption of salt ions to the membrane surfaces is electrostatic adsorption. Sample 9a-2 surfaces with polar functional groups are relatively presented as an effective membrane surface for salt rejection. Based on these results, the hydrophilic Co-PA composite membrane is a promising membrane for its high hydrophilicity and selectivity of the nanofiltration membrane.