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The membrane market has grown rapidly over the past several decades, supported by continuous improvements in membrane performance, module and process design, and fouling control. However, such growth will be unsustainable with current membrane fabrication methods that employ significant amounts of toxic solvents (e.g., N-methylpyrrolidone, dimethylacetamide, and dimethylformamide), thereby producing billions of liters of contaminated wastewater each year. A possible solution is to identify greener alternatives with appropriate properties that are compatible with conventional polymers. In this work, we employed a novel green solvent, Rhodiasolv PolarClean®, that is less toxic than current solvents and eco-friendly, while exhibiting the necessary properties to be employed as a solvent for membrane preparation via the nonsolvent-induced phase separation (NIPS) method. Rhodiasolv PolarClean® has been successfully applied to membrane preparation for water desalination and reclamation by ultrafiltration (UF) and nanofiltration (NF) with conventional polymers, including polysulfone (PSF), polyethersulfone (PES), and cellulose acetate (CA). The UF membranes prepared from PES/Pluronic F127 and PSF/polyvinylpyrrolidone exhibited pure water permeabilities greater than 314.5 ± 57.8 L m−2 h−1 bar−1 and tensile strength 3.78 ± 0.12 MPa with BSA rejection of 98.1 ± 0.4%. Cellulose acetate membrane used for NF applications demonstrated pure water permeability of 1.5 ± 0.25 L m−2 h−1 bar−1 with NaCl and MgCl2 rejection of 85.1 ± 5.7% and 93.2 ± 4.7%, respectively. The performance of the prepared membranes was competitive with current state-of-the-art membranes across all applications, indicating immediate applicability to improving the sustainability of membrane fabrication processes.

In the field of membrane technology, the combined nonsolvent-thermally induced phase separation (N-TIPS) method is a versatile technique to prepare macroporous membranes with narrow pore size distribution. However, its full potential in hollow fiber preparation has been hindered due to the formation of a dense skin layer via rapid mass exchange at the solvent-nonsolvent interface. In this study, a simple and effective solution is proposed to eliminate the dense skin layer and prepare a highly porous, macroporous membrane. We employed a triple spinneret to apply a transient coating layer to prevent the polymer dope solution from directly contacting the nonsolvent, which is regarded as the main cause of dense skin layer formation. Interestingly, depending on the choice of transient coating fluid, the fluid acts as a selective layer to facilitate unidirectional diffusion of the solvent into the nonsolvent while blocking the nonsolvent inflow. By controlling the solvent-nonsolvent exchange and thermal gradient in tandem, we show that a convenient control over NIPS and TIPS kinetics can be achieved. Several solvents, ranging from conventional solvents such as dibutyl phthalate (DBP) and N-methyl-2-pyrrolidone (NMP) to environmentally-friendly solvents such as acetyl tributyl citrate (ATBC) and methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (PolarClean®), were employed as the transient coating layer to investigate the effects of different solvents on the formation of macroporous hollow fiber membranes. It was observed that as the hydrophobicity of the coating layer increased, the prepared membrane exhibited a more porous morphology with a larger pore size. Moreover, the tensile strength of the prepared fibers improved from 3.4 ± 0.1 MPa to 7.7 ± 0.3 MPa, the pure water permeability increased from 1 to 988 L m−2 h−1 bar−1, and the membrane exhibited a narrow pore size distribution.

Herein, we introduce a novel membrane-based process for lithium recovery and compare it to the conventional solar evaporation followed by chemical precipitation process. Conventional technologies have limitations to meet the recent demand for massive lithium production due to several drawbacks of solar evaporation. Recently, in order to reduce the dependency of solar evaporation, several technologies have been proposed such as precipitation, ion-exchange, liquid-liquid extraction, adsorption, and electrodialysis. We suggest a novel membrane-based lithium recovery process by combining membrane distillation (MD) and nanofiltration (NF) to concentrate a brine solution containing lithium and to remove divalent ions. The proposed membrane-based process was demonstrated to concentrate 100 ppm lithium solution in artificial brine to 1200 ppm lithium solution within several days and exhibited up to 60 times higher water flux (22.5 L m−2 h−1) than that of solar evaporation (0.37 L m−2 h−1 at 30 °C and 0.56 L m−2 h−1 at 50 °C). Moreover, the NF process can suppress crystal formation to prevent process failure while alleviating the massive chemical usage of the conventional process. As a result, the proposed membrane-based process showed a possibility to utilize the low concentration of lithium brine with one-tenth of capital cost, process time, and foot-print of the conventional process, and represented a competitive operating cost with the conventional process which can be reduced further by harnessing the waste heat from the industrial plants and solar energy.

Membrane-integrated gas separation is of great interest due to its energy-saving and economic merits. Although easy-to-process polymer membranes have shown potential, insufficient gas permeation and low stability in harsh environments limit their use in practical applications. Here, we demonstrate nanoporous and rigid semi-interpenetrating polymer networks (SIPNs) by incorporating crosslinked network into polymer matrices, accompanying interpenetration and thermal rearrangement (TR) to construct an optimized microporous structure where nanometric and sub-nanometric pores coexist parallel to the gas transport direction. The resulting TR-SIPN improves gas transport without sacrificing separation efficiency since the nanometric and sub-nanometric pores serve as molecular highways and selective bottlenecks, respectively. Furthermore, the plasticization resistance against condensable gases was enhanced due to improved polymer rigidity of the TR-SIPNs. Our study suggests wide applicability of polymer membranes for aggressive gas separations such as natural gas sweetening and olefin/paraffin separation.

Microporous polyvinylidene difluoride (PVDF) hollow fiber membranes were fabricated via a thermally-induced phase separation (TIPS) method using an environmental-friendly hydrophobic solvent, acetyl tributyl citrate (ATBC, tradename Citroflex ® A4 ). To maximize membrane tensile strength, the TIPS method was fully utilized by spinning fibers with high polymer content. It was observed that the fiber quality was significantly affected by the dope and bore flow rates and compositions, and an appropriate spinning range was established. The prepared membranes were subsequently stretched to tune the porosity, mean pore size, permeability, tensile strength, and fiber strain. A design of experiment (DOE) analysis was conducted using a 3-factor quadratic model to optimize the stretching conditions and to understand the effects of the parameters and interactions thereof. The permeability of the stretched membranes improved by a factor of 35 (15.1–538 L m −2 h −1 bar −1 ), and the tensile strength increased from 7.2 MPa to 8.4 MPa at the expense of the fiber strain. The DOE analysis revealed that the stretching ratio positively affects the permeability and porosity but decreases the fiber strain. On the other hand, it was determined that the stretching temperature positively influences the permeability and fiber strength. The stretched membranes exceeded the PVDF performance upper bound prepared by the TIPS method. The membranes were primarily in the α-phase polymorph, and stretching the fibers up to 40% at 90 °C did not induce any detectable β-phase crystals. The proposed preparation method offers a feasible and sustainable alternative to fabricate hollow fibers membranes with high tensile strength and high permeability.

In this study, we report for the first time a pioneering method to prepare membranes with a fibrous morphology via phase inversion with extremely high pore connectivity. Microfiber aligned hollow fiber membranes were prepared via a polymer blend solution consisting of poly (vinylidene fluoride) with polysulfone in PolarClean® as a green solvent. The microfiber structure appeared only when controlled phase separation was performed with polymer blends. A systematic analysis has revealed that the shear rate and the reduced capillary number (α) of a blend solution significantly affected the morphology of the final membrane. The fabricated fibrous membranes exhibited six-fold higher water productivity compared to the best electrospun membranes reported in the literature, yet they remain highly scalable.

Infrared (IR) irradiation was applied to prepare thermally rearranged (TR) and carbon molecular sieve (CMS) membranes based on the polymer precursor of HPI-HD5. The effects of the IR treatment conditions such as temperature, time, and environment on the thermal conversion of TR and CMS membranes were investigated for the first time. The membranes subjected to the IR furnace exhibited the same changes of the chemical structure and comparable gas transport behaviors as the membranes treated in the electric furnace. Surprisingly, the maximum treating temperature and processing time were reduced by ~100 °C and a factor of ~20, respectively, in the IR furnace due to the energy-intensive heat radiation and instant temperature control. By replacing the electric furnace with the IR furnace, energy savings by a factor of ~10 were obtained at the same thermal conversion rate and consequently, the overall productivity was improved by more than a factor of 100. The applicability and advantages of efficient IR irradiation on TR and CMS membrane fabrication were confirmed in this study.

A new class of thermally rearranged (TR) polymers was prepared via copolymerization of TR-able o-hydroxyl polyimide and non-TR-able polyimide incorporating highly rigid Troger's Base (TB) units. The effect of TB content, type of TR-able diamine, and TR protocols on polymer properties and gas transport behaviors were thoroughly studied. TB moieties in the copolymers efficiently enhanced polymer rigidity and induced high Tg and TR temperature as confirmed by thermogravimetric analysis and dynamic mechanical analysis. Additionally, as the TB molar ratio increased, the interchain distances of precursor polyimides increased from 0.545 to 0.585 nm. The most important finding in this work was the synergistic effect between TR conversion and TB segments, which provide the optimum contents of TB in the copolymers. As a result, 6F6FTB-0.3-450 presented a maximum d-spacing value of 0.609 nm and excellent gas separation performance of 1567 Barrer for H2 and 1944 Barrer for CO2 along with a selectivity of 18.6 for H2/CH4 and 23.07 for CO2/CH4, surpassing the corresponding 2008 upper bounds. In addition, 6F6FTB-0.3 exhibited good plasticization resistance under CO2/CH4 mixed gas up to CO2 fugacity of ~15 bar.

Membrane fouling is a common phenomenon in anaerobic membrane bioreactor (AnMBR), deteriorating membrane performance and increasing the operation costs of AnMBR. In this study, β-phase polyvinylidene fluoride (PVDF) flat sheet membranes were successfully prepared by first mixing PVDF powder with ionic liquid [BMIM]PF6 and subsequently casting them using thermally induced phase separation (TIPS), followed by high voltage polarization treatment to endow the PVDF membranes with piezoelectric properties. For the first time in AnMBR, we reported in this study on the piezoelectric PVDF membrane that generates in-situ vibrations to reduce membrane fouling by applying a given frequency and alternating voltage (AV). The results suggested that the steady-state water flux through the piezoelectric membrane in AnMBR operation increased along with the AV, enhancing it by up to 72.6% (at 20 V) compared to that of the membrane without applying AV. In addition, high water flux was obtained at the optimal operating frequency of 10 kHz when the cross-flow mode was taken into consideration in AnMBR operation in this study, although the resonance frequency with maximum amplitude of the membrane was 600 kHz. These results indicate that the piezoelectric β-PVDF membrane is effective at minimizing or alleviating the membrane fouling when the electric field simultaneously applied during AnMBR operation. During the 30-day operation of AnMBR with the β-PVDF membrane with poling, the COD removal rate was maintained at over 90%, even if the applied frequencies and voltages were changed during the operation.

The thermally induced phase separation (TIPS) method is one of the most versatile techniques to fabricate polymeric membranes, with distinct advantages such as a narrow pore size distribution and high reproducibility. Selecting the right solvent is arguably the most important decision to be made for fabricating high-performance membranes via TIPS. Hence, it is crucial to understand the intricate relationship between the solvent and the polymer during TIPS. In this article, we discuss the fundamental aspects of TIPS solution thermodynamics and phase inversion kinetics. Recent advances in the TIPS process and new green solvents that have been identified for fabrication of membranes are discussed in depth.