Graphene oxide (GO), owing to its atomic thickness and tunable physicochemical properties, exhibits fascinating properties in membrane separation fields, especially in water treatment applications (due to unimpeded permeation of water through graphene-based membranes). Particularly, GO-based membranes used for desalination via pervaporation or nanofiltration have been widely investigated with respect to membrane design and preparation. However, the precise construction of transport pathways, facile fabrication of large-area GO-based membranes (GOMs), and robust stability in desalination applications are the main challenges restricting the industrial application of GOMs. This review summarizes the challenges and recent research and development of GOMs with respect to preparation methods, the regulation of GOM mass transfer pathways, desalination performance, and mass transport mechanisms. The review aims to provide an overview of the precise regulation methods of the horizontal and longitudinal mass transfer channels of GOMs, including GO reduction, interlayer cross-linking, intercalation with cations, polymers, or inorganic particles, etc., to clarify the relationship between the microstructure and desalination performance, which may provide some new insight regarding the structural design of high-performance GOMs. Based on the above analysis, the future and development of GOMs are proposed.

Constructing efficient and continuous transport pathways in membranes is a promising and challenging way to achieve the desired performance in the pervaporation process. The incorporation of various metal–organic frameworks (MOFs) into polymer membranes provided selective and fast transport channels and enhanced the separation performance of polymeric membranes. Particle size and surface properties are strongly related to the random distribution and possible agglomeration of MOFs particles, which may lead to poor connectivity between adjacent MOFs-based nanoparticles and result in low-efficiency molecular transport in the membrane. In this work, ZIF−8 particles with different particle sizes were physically filled into PEG to fabricate mixed matrix membranes (MMMs) for desulfurization via pervaporation. The micro-structures and physi-/chemical properties of different ZIF−8 particles, along with their corresponding MMMs, were systematically characterized by SEM, FT-IR, XRD, BET, etc. It was found that ZIF−8 with different particle sizes showed similar crystalline structures and surface areas, while larger ZIF−8 particles possessed more micro-pores and fewer meso-/macro-pores than did the smaller particles. ZIF−8 showed preferential adsorption for thiophene rather than n−heptane molecules, and the diffusion coefficient of thiophene was larger than that of thiophene in ZIF−8, based on molecular simulation. PEG MMMs with larger ZIF−8 particles showed a higher sulfur enrichment factor, but a lower permeation flux than that found with smaller particles. This might be ascribed to the fact that larger ZIF−8 particles provided more and longer selective transport channels in one single particle. Moreover, the number of ZIF−8−L particles in MMMs was smaller than the number of smaller ones with the same particle loading, which might weaken the connectivity between adjacent ZIF−8−L nanoparticles and result in low-efficiency molecular transport in the membrane. Moreover, the surface area available for mass transport was smaller for MMMs with ZIF−8−L particles due to the smaller specific surface area of the ZIF−8−L particles, which might also result in lower permeability in ZIF−8−L/PEG MMMs. The ZIF−8−L/PEG MMMs exhibited enhanced pervaporation performance, with a sulfur enrichment factor of 22.5 and a permeation flux of 183.2 g/(m⁻²·h⁻¹), increasing by 57% and 389% compared with the results for pure PEG membrane, respectively. The effects of ZIF−8 loading, feed temperature, and concentration on desulfurization performance were also studied. This work might provide some new insights into the effect of particle size on desulfurization performance and the transport mechanism in MMMs.

Two different types of 2D nanosheets, including hydrophobic graphene (GR) and hydrophilic graphene oxide (GO), were filled into poly (vinyl alcohol) (PVA) polymers to prepare mixed matrix membranes (MMMs) for ethanol dehydration via pervaporation. The relationship between the physical/chemical properties of graphene and pervaporation performance of MMMs was investigated by a comparison of GR/PVA and GO/PVA MMMs in microstructure and PV performance. The incorporation of GO nanosheets into PVA reduced PVA crystallinity and enhanced the membrane hydrophilicity, while the incorporation of GR into PVA led to the opposite results. The incorporation of GR/GO into PVA depressed the PVA membrane swelling degree, and the incorporation of GR showed a more obvious depression effect. GR/PVA MMMs showed a much higher separation factor than GO/PVA MMMs, while they exhibited a much lower permeation flux than GO/PVA MMMs and pristine PVA membranes. The huge difference in microstructure and performance between GO/PVA and GR/PVA MMMs was strongly associated with the oxygen-containing groups on graphene lamellae. The higher permeation flux of GO/PVA MMMs was ascribed to the facilitated transport of water molecules induced by oxygen-containing groups and exclusive channels provided by GO lamellae, while the much lower permeation flux and higher separation factor GR/PVA MMMs was resulted from the smaller GR interplanar spacing (0.33 nm) and hydrophobicity as well as barrier effect of GR lamellae on the sorption and diffusion of water molecules. It was presumed that graphene intercalated with an appropriate number of oxygen-containing groups might be a good choice to prepare PVA-based MMMs for ethanol dehydration, which would combine the advantages of GR’s high interlayer diffusion selectivity and GO’s high permeation properties. The investigation might open a door to achieve both of high permeation flux and separation factor of PVA-based MMMs by tuning the microstructure of graphene.