Antifouling nanocellulose membranes: How subtle adjustment of surface charge lead to self-cleaning property

Membrane fouling is a major issue in wastewater treatment. In this study, a unique class of low fouling nanocellulose-enabled thin film nanofibrous composite (TFNC) ultrafiltration (UF) membranes was fabricated by coating of negatively charged TEMPO-oxidized cellulose nanofibers (CNF) on the porous electrospun polyacrylonitrile (ePAN) substrate. The surface charge density of the nanocellulose barrier layer was controlled by using CNF with different degree of oxidation (DO) and coating area density (AD, g/m²). The morphology, pore size distribution, hydrophilicity and zeta potential of these CNF-TFNC membranes were characterized, all of which exhibited excellent permeation flux (15-61 L m⁻²h⁻¹ at 0.5 psi), high rejection ratio (>98%), and good antifouling tendency against bovine serum albumin (BSA). The practical antifouling and self-cleaning characteristics of CNF-TFNC membranes were further evaluated using biotreated municipal wastewater. The best performing membrane (CNF with 0.40 AD and 1.80 DO) achieved a near total flux recovery ratio (98 ± 2%) using simple hydraulic flushing. This could be attributed to the strong electrostatic repulsions between the CNF layer and foulants, both of which were negatively charged. Conversely, the commercial polyvinylidene difluoride (PVDF) UF membrane suffered severe fouling decay and very low flux recovery ratio (33 ± 3%). The results indicated the practicality of using charged CNF as a barrier layer for antifouling ultrafiltration membranes in wastewater treatment.