Construction of a gradient cross-linked nanofiltration membrane with quaternized surface for efficient heavy metal removal

The global generation of industrial wastewater has caused substantial heavy metal contamination, posing a severe threat to human health and environment. Nanofiltration (NF) membranes have gained widespread application in water treatment. However, traditional NF membranes typically feature the mixed separation zone and permeation zone, which hinder the simultaneous improvement of separation and permeation performance. Herein, we developed a gradient cross-linking NF membrane with a customized quaternized surface for the efficient removal of heavy metal ions. The loose upper layer of the gradient cross-linking structure was responsible for the high water flux, while the dense lower layer was accounted for rejecting heavy metal ions. The quaternized surface (glycidyl trimethyl ammonium chloride (GTMAC) grafted surface) endowed membrane with an enhanced positive charge characteristic, thereby improving its removal performance by repelling positively charged metal ions. Meanwhile, GTMAC rendered the membrane surface with enhanced water affinity, thus further enhancing the water permeance. Concretely, the resultant GTMAC-PEI/TMA membrane achieved satisfactory rejection rates for Mn 2+ (96.7 %), Cu 2+ (95.2 %), Ni 2+ (96.2 %) and Zn 2+ (93.3 %), with high water flux of 19.1–20.6 L m −2  h −1  bar −1 . This strategy provides an insight for the development of NF membranes with simultaneously improved permeability and rejection performance in heavy metal removal application.