Modified-graphene quantum dot thin-film nanocomposite membrane for enhanced nanofiltration performance and antifouling properties: Minimal incorporation

Nitrogen-doped graphene quantum dot (NGQD) with both oxygen-containing and amine functional groups was employed in this work to enhance the surface chemistry of thin film nanocomposite (TFN) membrane and cross-linking degree of polyamide (PA) layer for real wastewater treatment. The incorporation of NGQD has enhanced the hydrophilicity of all TFN membranes, as reflected by the decreased water contact angle (WCA) from 44.5 to 31.1 degrees. Besides, the incorporation of NGQD interfered the interfacial polymerization (IP) reaction, and at the same time affected the surface tension between aqueous and organic phase, which caused the formation of thinner and rougher PA. The increase in hydrophilicity and surface roughness resulted in 17.5-65.5 % improvement of water permeability for TFN membrane which was attributed to the encouragement of water molecules attachment and transportation by incorporating hydrophilic NGQD. The membrane incorporated with minimal concentration (0.01 wt%) of NGQD (TFN2) has shown the optimum results. The TFN membranes fabricated displayed enhanced divalent salt (Na2SO4) rejection at above 95 %, especially TFN2 has shown 3.4 % higher rejection than un-modified thin film composite (TFC) membrane. For humic acid (HA) and congo red (CR) antifouling test, the flux recovery ratio (FRR) was improved by 3-14.4 %, while flux decline rate (FDR) was reduced by 5.4-22.1 % even after 3 cycles of filtration (FRR3). For real wastewater, aerobically-treated palm oil mill effluent (AT-POME) antifouling test, TFN2 has achieved the highest FRR3 at 75 % compared to TFC and commercial NF2001. With minimal incorporation of NGQD, antifouling properties were enhanced due to the increase of PA cross-linking degree and surface negativity by both its amine groups and oxygen-containing functional groups that repel and prevent the deposition of pollutants on the membrane surface.