Construction of TpPa-COF interlayered positive nanofiltration membrane for efficient removal of high-concentration heavy metal ions

Abstract

Polyamide nanofiltration membranes often exhibit reduced rejection rates and flux decline under high ionic strength conditions. In order to efficiently remove heavy metal ions (HMIs) from water, a TpPa-COF interlayered positive nanofiltration membrane was proposed. TpPa-1 COF was synthesized and then reacted with 1,3,5-Benzenetricarbonyl trichloride (TMC) to narrow the pore size. The narrowed pore size from 18 Å to 5.4 Å was designed for rejection of heavy metal ions (>5.8 Å). Then, the membrane was functionalized with polyethyleneimine (PEI) grafting to impart positive surface charges for enhanced HMIs removal. The developed membrane was systematically characterized and evaluated for its physicochemical properties and performance through attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), cold-field emission scanning electron microscope (SEM), and atomic force microscopy (AFM). The fabricated membrane achieved exceptional rejections of Cr³⁺ (99.04 %), Mn²⁺ (97.03 %), Co²⁺ (97.74 %), Ni²⁺ (98.43 %), Zn²⁺ (98.85 %), and Cd²⁺ (99.07 %) at a feed concentration of 1000 ppm. Notably, the membrane maintained the rejection efficiency of 90 % under a higher feed concentration of 6000 ppm, which is better than the commercial NF3 membrane. After 7 days of continuous operation, the rejection of HMIs exceeded 98.4 %, demonstrating the great stability. When treating actual electroplating wastewater, the membrane showed good performance. The removal rate for major ions (Cr(VI), Co²⁺, Cu²⁺, Mn²⁺, Ni²⁺, Zn²⁺) reached from 97.4 % to 99.99 %. The developed membrane exhibited better performance than commercial membrane, demonstrating the practical potential in treating actual wastewater.