Holey Reduced Graphene Oxide Nanofiltration Membranes: Microstructure–Transport Relations and Enhanced Fractionation of Complex Streams

Abstract Membranes based on two-dimensional nanomaterials such as graphene oxide (GO) and reduced GO (rGO) are increasingly investigated for nanofiltration applications involving complex biomass-derived streams. However, GO membrane performance is limited by extended transport pathways and restricted flux arising from the tortuous alignment of impermeable nanosheets. We report a systematic study of reduced “holey” graphene oxide (rhGO) membranes, fabricated by controlled oxidative etching of GO nanosheets, to investigate the effect of in-plane nanopores on water and solute transport. Membranes with varying degrees of hole sizes and densities, as well as two distinct film thicknesses (70 and 130 nm), were evaluated systematically for pure water permeation and rejection of ionized and neutral solutes. Concurrently, the microstructural features of the nanoflakes and membranes (hole size distribution, flake size distribution) were characterized quantitatively with extensive AFM and TEM statistical measurements. The membrane permeation and microstructural data allowed the parametrization and validation of an extended transport model that allows the prediction of flux and solute rejection trends that account for the presence of holes in the membranes. Compared to nonholey rGO membrane controls, rhGO membranes demonstrated enhanced water flux and 3-fold increase in throughput during nanofiltration of a complex feedstock (kraft black liquor) while maintaining high lignin rejection (>95%) and similar divalent salt exclusion as nonholey rGO membranes. This study underscores the potential of nanosheet defect engineering as a scalable strategy for tuning microstructure–transport relationships in 2D-material-based membranes for harsh-environment separations.