Physically Coating Nanofiltration Membranes with Graphene Oxide Quantum Dots for Simultaneously Improved Water Permeability and Salt/Dye Rejection

A physically bound coating of graphene oxide quantum dots (GO QDs) on thin-film composite polyamide nanofiltration (NF) membranes is prepared through a pressure-assisted filtration method to simultaneously improve the membranes' water permeability and salt/dye rejection. It is the first attempt to use carbon-based nanoparticles as surface coating to improve the efficiency of water filtration membranes. Influences of the GO QDs' coating on the hydrophilicity and surface charge of the membranes are investigated by water contact angle and zeta potential measurements. With the GO QDs' coating, the surface roughness and hydrophilicity of the membranes increase, and the membrane surface becomes more negatively charged. Water permeability and salt/dye rejection of GO QD-coated NF membranes increase simultaneously in model salt/dye aqueous solutions, and negatively charged methyl orange. The increased water permeability can be ascribed to the improved membrane surface hydrophilicity, while the increased salt/dye rejection can be attributed to the improved surface electronegativity. The detachment of GO QDs during testing is negligible, which is confirmed by a long-time stability test with transmission electron microscopy (TEM) characterizations. The interactions between GO QDs' and NF membranes' surface are strong enough so that the coating exhibits good durability.