Improving nanofiltration performance using modified cellulose nanocrystal-based TFN membranes

Heavy metal ions are one of the principal contaminants in industrial wastewater. Their removal in nanofiltration processes using thin-film nanocomposite (TFN) membranes is a promising treatment method. l-cysteine functionalized cellulose nanocrystals (CysCNCs) and acetylated CNCs (ACNCs) were synthesized via an iodine-catalyzed method. The successful modification of CNCs was confirmed using ATR-FTIR and 13C NMR spectroscopy. The modified CNCs were used in the synthesis of TFN membranes. Prior to interfacial polymerization, the CNCs were dispersed into the organic monomer solution (trimesoyl chloride in n-hexane) rather than in the aqueous monomer solution (piperazine), contrary to conventional practice. Membrane performance was assessed for thin film composite (TFC) and CNC, ACNC and CysCNC-TFN membranes. A simultaneous increase in water permeability/salt and heavy metal rejection of ACNC-TFNs and CysCNC-TFNs was observed. In heavy metal removal tests, the CysCNC-TFN0.2 membranes exhibited the highest water permeability (16 L/m2 h bar) compared to the ACNC-TFN0.1 membranes (14.5 L/m2 h bar), the CNC-TFN0.1 membranes (11.5 L/m2 h bar), and the TFC membranes (6 L/m2 h bar). The heavy metal rejections ranged from 89.2 to 98.4% for the copper ions and 87.0–95.2% for the lead ions, in the same order as for the water permeability results. Therefore, the water permeability/selectivity trade-off was overcome. The TFN membrane performance improvements were mainly attributed to both the nanoparticle functionality and their uniform distribution in the polyamide layer via their dispersion in the organic monomer solution.