Constructing positively charged acid-resistant nanofiltration membranes via surface postgrafting for efficient removal of metal ions from electroplating rinse wastewater

To efficiently recycle metal ions from electroplating wastewater, a positively charged acid-resistant nanofiltration membrane was prepared by coupling interfacial polymerization and postgrafting modification with an ethanol grafting solution containing N-(3-aminopropyl)-imidazole (ANPI). Ethanol, as the grafting solvent, not only slightly swelled the original polyamide (PA) layer but also is mutually miscible with n-hexane, which prevented the formation of additional PA layers. ANPI, as the grafting monomer, was covalently bound to the -COCl groups of the PA layer to guarantee grafting stability. Due to the inhibited hydrolysis of the -COCl group by ethanol, more ANPI was grafted on the PA layer, which could be easily protonated under low pH. This resulted in a looser, thinner and positively charged selectivity layer. The membrane permeability was improved by 56.0 ± 0.55% with high interception performance (> 98% for Cu²⁺ and Ni²⁺ ions) in electroplating rinse wastewater. The formed positive protective layer prevented H⁺ ions from attacking the –CO-NH- bonds, resulting in strong stability after immersion in 10 wt% H₂SO₄ and H₃PO₄. Therefore, the obtained membrane shows promise for application in highly acidic wastewater.