Enhanced permselective separation of per-fluorooctanoic acid in graphene oxide membranes by a simple PEI modification†
Perfluoroalkyl compounds of various molecular weights have emerged as a class of persistent contaminants with profound negative impact on human health and the environment. The selectivity and permeance of Graphene Oxide (GO) and amine surface functionalised GO nanofiltration membranes were assessed for the removal of perfluorooctanoic acid (PFOA ∼400 Da) in concentration ranges relevant to availability in wastewater. GO nanofiltration membranes demonstrated a reasonable efficiency of 74.3% at 50 ppm of PFOA and a water permeance of 10 ± 2.1 L m−2 h−1 bar−1. By functionalising the top surface with polyethyleneimine (PEI), the GO membrane underwent reduction and cross-linking reactions. The modified membranes demonstrated improved mechanical stability and an enhanced retention of 96.5% at 50 ppm PFOA and a permeance of 15.9 ± 1.3 L m−2 h−1 bar−1. The electron rich PEI deoxygenates GO leading to a smaller interlayer spacing, but also increases the surface hydrophilicity—the combination of both these properties leads to increased PFOA retention by steric hindrance and enhanced water permeation. As steric hindrance effects were the dominant mechanism of retention, the GO-PEI membranes operated more effectively (retention >90%) in a much wider range of concentrations (100 ppb to 100 ppm), compared to the GO membrane. Our research demonstrates that strategic surface-modification techniques can tailor the effectiveness of GO-based loose nanofiltration membranes for the retention of emerging contaminants while maintaining lower osmotic pressure effects through salt retention minimisation.