Aminoimidazole ionic liquid (IL) embellishment of nanofiltration (NF) membrane to simultaneously enhance heavy metals rejection and permeability

Most commercial nanofiltration (NF) membranes carry negative charges, which is unfavorable for rejecting positively charged impurities according to the Donnan effect. Two ionic liquids (ILs) with the same cation, namely 1-aminopropylimidazolium hexafluorophosphate ([C₃NH₂MIm]PF₆) and 1-aminopropyl-3-methylimidazolium bromide ([C₃NH₂MIm]Br), were employed to improve membrane surface charge by surface modification. With the ILs regulating the membrane structure, the resultant membranes (IL-PA) displayed a rough surface with higher hydrophilicity, larger pores, enhanced surface charge and structure stability than the blank PA membrane. Specifically, with IL concentration increasing, the [C₃NH₂MIm]PF₆ modified (PF6/PA) membranes showed enhanced isoelectric point (IEP) from 4.5 to 6.5, while that of the Br/PA membranes was decreased from 3.7 to 3.1. It indicates the anionic part of ILs determined its distribution in the PA layer. High P_o/w of [C₃NH₂MIm]PF₆ enabled high diffusivity in the PA layer, and thus, more [C₃NH₂MIm]⁺ grafted on the membrane surface, leading to high IEP and contact angle. Overall, the optimized 0.5PF6/PA membrane exhibited a high pure water flux of 62.2 L·m⁻²·h⁻¹ and enhanced heavy metal removal efficiency (Cu²⁺ 97.1 %, Ni²⁺ 94.6 %, Cd²⁺ 95.5 %, Zn²⁺ 96.4 %). This work demonstrates the feasibility of specific functionalized ILs in heavy metal wastewater treatment.