Incorporation of\nGraphene Oxide Quantum Dots in Gradient\nLayers of Polyethersulphone Nanofiltration Membranes for Nitrate Rejection\nfrom Aqueous Solution

Graphene oxide quantum dots (GOQDs) have been widely\nused to prepare\nnanofiltration membranes due to the merits of excellent dispersity,\nultrasmall size, and unique properties related to graphene. In this\nstudy, we first prepared the polyethersulphone-based nanofiltration\n(PES-NF) membrane via an interfacial polymerization process using\na piperazine and <i>m</i>-phenylenediamine mixed solution\nas the aqueous phase. Then GOQDs were incorporated into the top-down\ngradient structured layers (i.e., ultrathin layer, interlayer, and\nsubstrate membrane layer) of the nanofiltration membrane, and subsequently\nthe effect of GOQD addition on the nitrate rejection was evaluated.\nCompared with the pristine PES-NF membrane without the incorporation\nof GOQDs, the fabricated NF membrane (GOQD/PES-NF-2) incorporating\nGOQDs at both the ultrathin layer and interlayer exhibits more remarkable\nperformances (an acceptable permeation flux of 52.2 L m<sup>–1</sup> h<sup>–1</sup> and excellent nitrate rejection of 96.3% at\n0.6 MPa), the permeation flux of this membrane increases by nearly\n2.4 times, and its nitrate rejection also shows a slight enhancement\n(∼7.6%) compared with those of PES-NF. Remarkably, at the operating\npressure much lower than that required by reverse osmosis membranes,\nthe GOQD/PES-NF-2 membrane possesses an equivalent monovalent ion\nrejection to reverse osmosis membranes but a higher permeation flux.\nFurthermore, the result of a 7 day continuous stability test validates\nthe excellent durability of the GOQD/PES-NF-2 membrane, and its antifouling\nand chlorine resistance performances also outperform those of the\nPES-NF membrane.