Asymmetrically charged polyamide nanofilm of intrinsic microporosity containing quaternary ammonium groups for heavy metal removal

High-performance nanofiltration (NF) is a highly hopeful approach for removing heavy metals from water, which can effectively solve the hazards of heavy metals to the public health and environment security. In this study, a polyamide (PA) nanofilm of intrinsic microporosity was fabricated through interfacial polymerization of the novel spirocyclic diamine monomer (SBI) with trimesoyl chloride (TMC). The PA nanofilm demonstrated high microporosity due to the porous structure formed by the covalent cross-linking of SBI with TMC and the stacking of the SBI monomers due to its rigidly-contorted structure. Consequently, its free volume was high with the water permeance of 19.3 L·m−2 h−1·bar−1. Meanwhile, the PA nanofilm exhibited asymmetrical charges on both sides with a positively charged bottom and a negatively charged top. The quaternary ammonium moieties in SBI produced positively charged nanochannels in the PA nanofilm, reinforcing the Donnan exclusion against multivalent cations. The proposed SBI-TMC membrane was able to effectively remove heavy metals from water, with rejections of Cu2+, Ni2+, Zn2+, and Pb2+ at 93.3%, 93.0%, 91.8%, and 88.3%, respectively. This study confirmed that the fit-for-purpose design of monomer with rigidly-contorted structure is a feasible approach to fabricate NF membrane with enhanced separation performance.