Corn Stalk-Derived Carbon Quantum Dots with Abundant Amino Groups as a Selective-Layer Modifier for Enhancing Chlorine Resistance of Membranes

Low permeability and chlorine resistance of normal thin-film composite (TFC) membranes restrict their practical applications in many fields. This study reports the preparation of a high chlorine-resistant TFC membrane for forward osmosis (FO) by incorporating corn stalk-derived N-doped carbon quantum dots (N-CQDs) into the selective polyamide (PA) layer to construct a polydopamine (PDA) sub-layer (PTFCCQD). Membrane modification is characterized by surface morphology, hydrophilicity, Zeta potential, and roughness. Results show that TFCCQD (without PDA pretreatment) and PTFCCQD membranes possess greater negative surface charges and thinner layer-thickness (less than 68 nm). With N-CQDs and PDA pretreatment, the surface roughness of the PTFCCQD membrane decreases significantly with the co-existence of microsized balls and flocs with a dense porous structure. With the variation of concentration and type of draw solution, the PTFCCQD membrane exhibits an excellent permeability with low J(reverse salt flux)/J(water flux) values (0.1-0.25) due to the enhancement of surface hydrophilicity and the shortening of permeable paths. With 16,000 ppm·h chlorination, reverse salt flux of the PTFCCQD membrane (8.4 g m–2 h–1) is far lower than those of TFCCQD (136.2 g m–2 h–1), PTFC (127.6 g m–2 h–1), and TFC (132 g m–2 h–1) membranes in FO processes. The decline of salt rejection of the PTFCCQD membrane is only 8.2%, and the normalized salt rejection maintains 0.918 in the RO system (16,000 ppm·h chlorination). Super salt rejection is ascribed to the existence of abundant N—H bonds (N-CQDs), which are preferentially chlorinated by free chlorine to reduce the corrosion of the PA layer. The structure of the PA layer is stable during chlorination also due to the existence of various active groups grafted on the surface. This study may pave a new direction for the preparation of durable biomass-derivative (N-CQD)-modified membranes to satisfy much more possible applications.