Fabrication of hollow fiber nanofiltration separation layer with highly positively charged surface for heavy metal ion removal

Efficient concentration and separation of heavy metal ions from wastewater are crucial for environmental protection and resource recovery. In this research, a double highly positively charged nanofiltration (NF) separation layer was constructed on the outer surface of hollow fiber membranes for the first time via surface grafting and iodomethane (CH3I) quaternization. The separating properties for Ni2+, Cu2+ and Cr3+ were measured at different concentrations and pressures, and the long-term stability and anti-fouling property of the membrane were investigated. The relationship between the positively charged degree of surface, pore size, and rejection of heavy metal ions were interpreted by comparing the discrepancies in microstructures and separating properties of membranes. The membrane can efficaciously remove Ni2+ (99.4%), Cu2+ (99.6%), and Cr3+ (99.9%) while maintaining a high permeability (30.9, 29.6 and 28.5 L m−2 h−1 bar−1). The high permeability (about 27.9 L m−2 h−1 bar−1) and rejection (>99%) were retained even after 72 h of operation. Moreover, the membrane exhibited a good anti-fouling property for heavy metal ions. This study offers the reference for preparing hollow fiber NF membrane with stable microstructure, highly positively charged surface and high efficiency for the removal of heavy metal ions. [Display omitted] •The NF membrane with highly positively charged surface was prepared.•The double positively charged layer enhances the density of positive charge.•Ni2+, Cu2+, Cr3+ can be removed while maintaining a high permeability.•The relationship between the charged degree, pore size and rejection was interpreted.•The membrane showed long-term stable and efficient removal of heavy metal ions.