Deep eutectic solvent functionalized graphene oxide nanofiltration membranes with superior water permeance and dye desalination performance

Graphene oxide (GO) lamellar membranes have been used for dye desalination; however, low water flux and quite high Na2SO4 salt rejection (>95%) hinder their applicability. Here, we used a choline chloride and ethylene glycol based novel deep eutectic solvent (DES) for low-temperature functionalization of GO membranes to improve their dye desalination performance. Characterization techniques including TEM, SEM, AFM, DLS, zeta potential, contact angle, UV–Vis, XRD, Raman, and FTIR spectroscopy were used to determine the physical and chemical properties of DES/GO nanomaterials and membranes. The effect of DES functionalization time, i.e. 1 h and 2 h, on the physicochemical properties of DES/GO membranes along with their filtration performance were systematically investigated. The DES functionalization resulted in wrinkled and reducedGO nanosheets, decreased their lateral size, increased the interlayer spacing, and decreased their wettability (i.e., increased contact angle) of the lamellar membrane all of which in turn provided superior water permeability. DES/GO-1hr and DES/GO-2hr membranes showed water permeance of 124.8 ± 3.1 and 94.2 ± 17.7 L m−2h−1 bar−1, respectively, which were about 7.1 and 5.4 times that of the unmodified GO membrane. DES/GO-1hr membrane not only provided the highest water permeability but also lowered divalent Na2SO4 salt rejection while keeping the dye rejection high towards different dyes i.e., 99.4 ± 0.8% for Congo Red, 96.7 ± 1.2% for Methyl Blue, 98.5 ± 0.8% for Evan Blue, and 99.3 ± 0.6% for Direct Red. The long-term performance of dye/salt filtration showed that the DES/GO-1hr membrane achieved 74–100% flux recovery for four filtration cycles and also provided ~99.9% dye rejection and 5.4% Na2SO4/NaCl salt rejection.