Sustainable spray coating fabrication of acid-resistant polyamine nanofiltration membrane to separate heavy metal ions and treat acid wastewater

The fabrication of a scalable and economic acid-resistant polyamine-based NF membrane by a spray coating polymerization-based technique for heavy metals removal and acid wastewater treatment is herein reported for the first time. The process was optimized to produce a well crosslinked selective layer network membrane (named SC-TFC-N) with controlled thickness of ∼75 nm and effective mean pore radius of ∼6.2 Å. Two membranes having similar chemical compositions, christened SC-TFC-R and DC-TFC-N were also developed by reverse interfacial polymerization (RIP) spray coating and conventional dip-coating IP techniques, respectively for comparison. All the fabricated membranes displayed high rejections to divalent cations such as MgCl₂ (2000 ppm) reaching rejection up to 98.6 %, but the optimized SC-TFC-N membrane achieves a water permeance of 9.1 L m⁻² h⁻¹ bar⁻¹, which was about 1.7 and 1.4 times that of SC-TFC-R and DC-TFC-N membranes, respectively. Specifically, the SC-TFC-N membrane exhibits great stability under high salt concentrations, continuous filtration performance, and excellent fouling resistance after five-filtration stages. Consequently, the membranes displayed over 90 % rejections of the heavy metal ions, in the order of Pb²⁺ > Zn²⁺ > Cu²⁺, and also exhibit excellent acid resistance by maintaining high divalent cations salt rejection performances after being subjected to different acidic conditions. In addition, density functional theory (DFT) calculations carried out proved the stability of the polyamine NF membrane against nucleophilic and electrophilic attacks. This work thus demonstrates an efficient and sustainable approach for producing NF membrane with good potential application in heavy metal ions removal and acid mine drainage wastewater treatment.