Owing to the increasing need to mitigate excessive organic solvent waste, the efficient separation and recovery of organic solvents have received major research attention in recent years. The ...membrane‐based organic solvent nanofiltration (OSN) process has demonstrated its feasibility in addressing this problem with low energy costs, compared to conventional separation techniques, such as adsorption, liquid–liquid extraction, and solvent evaporation. Recently, membranes made of 2D graphene‐based materials have shown great promise because they attain high solvent flux and solute rejection using easy processing methods. Thus, this paper focuses on state‐of‐the‐art studies of graphene‐based membranes used in OSN processes, which include syntheses, characterizations, performance evaluations, membrane fouling, and simulation studies, in combination with the development of the “upper‐bound” line to indicate the performance of graphene‐based membranes. In this paper, critical challenges involved in the development of graphene‐based membranes are also focused on and discussed to map out the future directions of these membranes in industrial OSN processes. In addition to OSN, this paper pertains to a broader audience in other separation processes, particularly in the fields of gas separation and water treatment.
Graphene‐based membranes have attracted substantial attention among researchers in the field of molecular separation. Due to the great stability of graphene and its derivatives, the potential of graphene‐based membranes in organic solvent nanofiltration has been promising, as high solute (organic dyes, active pharmaceutical ingredients and drugs) rejection is achievable, creating high‐purity organic solvent in the permeate stream.
MXenes are emerging rapidly as a new family of multifunctional nanomaterials with prospective applications rivaling that of graphenes. Herein, a timely account of the design and performance ...evaluation of MXene‐based membranes is provided. First, the preparation and physicochemical characteristics of MXenes are outlined, with a focus on exfoliation, dispersion stability, and processability, which are crucial factors for membrane fabrication. Then, different formats of MXene‐based membranes in the literature are introduced, comprising pristine or intercalated nanolaminates and polymer‐based nanocomposites. Next, the major membrane processes so far pursued by MXenes are evaluated, covering gas separation, wastewater treatment, desalination, and organic solvent purification. The potential utility of MXenes in phase inversion and interfacial polymerization, as well as layer‐by‐layer assembly for the preparation of nanocomposite membranes, is also critically discussed. Looking forward, exploiting the high electrical conductivity and catalytic activity of certain MXenes is put into perspective for niche applications that are not easily achievable by other nanomaterials. Furthermore, the benefits of simulation/modeling approaches for designing MXene‐based membranes are exemplified. Overall, critical insights are provided for materials science and membrane communities to navigate better while exploring the potential of MXenes for developing advanced separation membranes.
The current status and future potential of MXene‐based separation membranes is reviewed. After providing a basis on MXene synthesis and membrane design, separation applications of MXene‐based membranes are introduced, namely gas separation, water treatment (e.g., solute removals and bacterial disinfection), desalination, and organic solvent purification. As future perspectives, polymer nanocomposites, electroresponsive/reactive membranes, and simulation‐driven membrane design are discussed.
Co-solvent assisted interfacial polymerization (CAIP) has been widely used to increase the water permeability of thin-film composite (TFC) reverse osmosis (RO) membranes. However, its outcomes are ...often poorly understood or unpredictable. To bridge the gap between conventional wisdom and the real effects of the co-solvent, we report—for the first time, to the best of our knowledge—empirical evidence in terms of the actual interfacial tension between two immiscible solutions used in CAIP. According to our results, dimethyl sulfoxide (DMSO), which is frequently used as a co-solvent, influences IP by interacting with trimesoyl chloride (TMC). The dipole-dipole interaction between DMSO and TMC was estimated to increase the TMC concentration at the interface and, thereby, the reaction rate. Due to the fast reaction, the diffusion barrier forms quickly, reducing the thickness and roughness of the active layer. The cross-linking degree was also determined to decrease due to the incomplete reaction that occurs when one of three acyl chloride groups interacts with Sδ+−Oδ− electrostatic dipoles of DMSO at the interface, as evidenced by the variation in unreacted acyl chloride groups in the active layer and by the nitrogen/oxygen ratio. Such morphological changes were consistent with the trend in the performances of the RO membranes prepared with different amounts of DMSO, and were used to interpret the possible transport phenomena.
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•The interaction between DMSO and TMC was considered as a parameter dictating IP.•DMSO increased the TMC concentration at the interface during IP.•The increase in the TMC concentration led to a faster but incomplete IP reaction.•The faster and incomplete IP changed the morphological features of polyamide films.•A new mechanism was proposed to clarify the DMSO effect based on empirical evidence.
Well matched: Submicrometer‐sized metal–organic framework (MOF) crystals (ZIF‐90) were synthesized by a nonsolvent‐induced crystallization technique and incorporated in mixed‐matrix gas‐separation ...membranes. ZIF‐90/6FDA‐DAM membranes (empty pink circle; beyond the upper bound for polymer membranes) show unprecedented high performance for CO2/CH4 separation by a MOF‐based membrane. The key is the combination of the highly selective MOF and a highly permeable polymer.
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•An overview of polymer-based membranes for membrane contacting process is presented.•Current challenges include limited performances, pore wetting and membrane ...fouling.•Membrane-centred strategies to high-performing antiwetting/fouling membranes are proposed.•Biocatalytic, mixed-matrix, composite, Janus and omniphobic membranes are promising.•Future research is on fouling, materials development and process design optimizations.
The gas–liquid membrane contactor (GLMC) is a promising technology for gas absorption and stripping with many competitive advantages over conventional contacting processes. At the centre of this technology, membranes are instrumental in offering a high contact area at the gas-liquid interface. Today, polymer-based membranes are the mainstream for GLMC applications owing to their well-established fabrication techniques and economic advantages. Herein, we aim to review the current progress in polymer-based membranes for GLMC processes. Specifically, we first provide a basic overview of the principles and background of GLMC. Next, we identify three of the biggest Achilles heels of polymer-based membranes for GLMC processes, namely, (1) membrane performances, (2) pore wetting, and (3) membrane fouling. To a certain extent, we contend that a large part of these challenges can be resolved by optimizing the membrane designs. As such, in the next section, we focus our attention on high-performance, antiwetting and antifouling membranes as potential solutions to address these challenges. From a membrane-centred perspective, the scope of our discussion covers extensively from membrane materials, morphologies, structures and configurations, surface modifications to novel membrane designs including composite and mixed-matrix membranes. Notably, as membrane fouling studies are still in the preliminary stage, attractive membrane designs such as Janus and omniphobic membranes are borrowed from membrane distillation process to gain greater insights into the potential solutions that can be adapted for GLMC processes.
Two-dimensional (2-D) CuBDC nanosheets (ns-CuBDC) with high-aspect-ratios were deliberately paired with polymers possessing high free volumes to fabricate high performance gas separation membranes. ...Owing to the molecular sieving effect of the filler, a small ns-CuBDC loading (2-4 wt%) could significantly improve the CO
/CH
selectivities of membranes, resulting in performances that surpass the upper bound limit for polymer membranes.
This work presents an energy analysis and optimization of the hollow fiber membrane contactors for the recovery of dissolved methane (CH4) in effluents of anaerobic membrane bioreactor wastewater ...treatment processes. The obtained CH4 could be merged with biogas for further purification or used with a micro-turbine for electricity generation to achieve an energy self-sufficient wastewater treatment process. A mathematical model considering simultaneous CH4 and carbon dioxide (CO2) desorption was used to estimate the membrane area required to remove the dissolved CH4, as well as quality of the outlet gas from the membrane contactor. Energy balance between electrical energy obtained from the recovered CH4 and energies consumed by vacuum and liquid pumps for the operation of membrane contactor were investigated and reported as a Net Electricity obtained per m3 of effluent or simply Net E. Results revealed that a combination of a high strip gas flow rate and slightly low vacuum condition closed to the atmospheric pressure can provide the highest Net E at 0.178 MJ/m3. This value is 85.37% of the total electrical energy that can generated from a 90% recovery of dissolved CH4 using an effluent saturated with a 60 vol% CH4 biogas and flow rate at 2 m3/day. The calculation was made based on the assumptions that 1) the membrane contactor is operated in a non-wetting mode where membrane properties remain constant, 2) flux decline due to the membrane fouling is not considered and 3) the energy required for membrane cleaning and other relevant activities are not factored into the energy analysis. Based on our results, to obtain a high CH4 mole fraction at the gas outlet, a low strip gas flow rate is recommended, however, the operating gas pressure needs to be lowered by applying a vacuum condition to improve the Net E. In addition, it was found that the Net E could be improved by increasing the number of membrane fibers, and lowering the liquid flow rate. The CH4 recovery efficiency could also be optimized to obtain an optimal Net E.
•An energy analysis of the membrane contactor for the recovery of CH4 was performed.•A mathematical model was used to estimate membrane area required.•Gas pressure and flow rate were optimized to obtain a maximum net energy obtained.•Effects of operation parameters on energy and produced gas obtained were discussed.
Carbon nanotubes (CNTs) with hydrophobic and atomically smooth inner channels are promising for building ultrahigh‐flux nanofluidic platforms for energy harvesting, health monitoring, and water ...purification. Conventional wisdom is that nanoconfinement effects determine water transport in CNTs. Here, using full‐atomistic molecular dynamics simulations, it is shown that water transport behavior in CNTs strongly correlates with the electronic properties of single‐walled CNTs (metallic (met) vs semiconducting (s/c)), which is as dominant as the effect of nanoconfinement. Three pairs of CNTs (i.e., (8,8)met, 10.85 Å vs (9,7)s/c, 10.88 Å; (9,8)s/c, 11.53 Å vs (10,7)met, 11.59 Å; and (9,9)met, 12.20 Å vs (10,8)s/c, 12.23 Å) are used to investigate the roles of diameter and metallicity. Specifically, the (9,8)s/c can restrict the hydrogen‐bonding‐mediated structuring of water and give the highest reduction in carbon–water interaction energy, providing an extraordinarily high water flux, around 250 times that of the commercial reverse osmosis membranes and approximately fourfold higher than the flux of the state‐of‐the‐art boron nitrate nanotubes. Further, the high performance of (9,8)s/c is also reproducible when embedded in lipid bilayers as synthetic high‐water flux porins. Given the increasing availability of high‐purity CNTs, these findings provide valuable guides for realizing novel CNT‐enhanced nanofluidic systems.
Water transport in single‐walled carbon nanotubes strongly correlates with their electronic properties (metallic vs semiconducting), which is as dominant as the effect of nanoconfinement. Specifically, the semiconducting (9,8) gives the highest reduction in carbon–water interaction energy, providing an extraordinarily high water flux.
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