Enhanced Membrane Distillation Research Articles

MOF-based photothermal Janus membrane with robust anti-fouling performance for enhanced membrane distillation

As a promising desalination technology, photothermal membrane distillation (PMD) is confronted with challenges such as low freshwater yield and membrane fouling. Herein, we have introduced a copper-based metal-organic framework (Cu-CAT) into a hydrophilic/superhydrophobic polyvinylidene fluoride (PVDF) Janus tree-like nanofiber membrane (TLNMs) to construct photothermal Janus TLNMs (P-Janus TLNMs) for enhanced PMD. Given the exceptional photothermal properties of the hydrophilic Cu-CAT layer, coupled with the fine pore structure and elevated porosity characteristics of the superhydrophobic PVDF TLNMs, P-Janus TLNMs have demonstrated outstanding performance when integrated into a PMD system. Upon exposure to solar illumination of 1 kW·m−2, the P-Janus TLNMs achieved a remarkable surface temperature of 88 °C, yielding a permeation flux of 1.46 L m−2 h−1, a salt rejection rate exceeding 99.99 %, and an energy utilization rate of 81 %. Furthermore, after 20 days of continuous operation with simulated seawater containing oil and surfactant, the P-Janus TLNMs presented a salt rejection of 99.9 % and a stable permeation flux of 1.71 L m−2 h−1 at the feed/permeate temperatures of 24/20 °C. This work introduces a novel membrane with immense potential for boosting permeation flux and enhancing anti-fouling properties in PMD, thereby advancing the frontier of sustainable seawater desalination.

Holistic approach for fabrication of superhydrophobic MXene-based membrane for enhanced membrane distillation

There is growing interest in membrane distillation (MD) as a means of desalinating seawater with 100 % theoretical salt rejection which has the capacity to address freshwater scarcity. MD has huge potential for commercial applications owing to its low operating temperature and pressure requirements. However, wetting polymeric membranes inhibits water permeance and lowers salt rejection. MXene nanosheet-incorporated polyvinylidene (PVDF) membranes have been constructed for enhanced vapor transport with high water repellence and antiwetting properties. MXene nanosheets in PVDF polymeric matrices are responsible for their high super-hydrophobicity, which can mitigate membrane fouling and wetting during the MD process. As far as experimental outcomes are concerned, the pristine PVDF membrane exhibited severe wetting with salt leakage. Ti3C2Tx MXene nanosheets allow the formation of hierarchical polymeric micro/nanostructures, changing the intrinsic hydrophilicity to super-hydrophobicity. Surface engineering of a PVDF membrane with MXene nanosheets enables efficient saline desalination during the MD process. The surface-engineered MXene/PVDF membrane demonstrated a high-water contact angle of 143° with extremely high self-cleaning characteristics as compared to that of pristine PVDF membrane. As far as the performance of the membrane is concerned, the water flux of the pristine PVDF membrane decreased from 6.5 LMH to 6 LMH after the 14th hour which can be attributed to partial pores wetting during MD operation. Eventually, the pristine PVDF membrane exhibited a continuous salt flux (SF) increase. However, MXene/PVDF membrane showed stable water flux (8 LMH) and negligible SF. The study therefore demonstrates a facile and holistic approach for constructing MXene-based PVDF membranes that exhibit superior antiwetting performance during MD operation.

Mitigating near-surface polarizations in membrane distillation via membrane surface decoration

Temperature and concentration polarizations near the membrane surface zone can significantly affect the performance of membrane distillation, resulting in reduced transmembrane flux and overall efficiency. The fundamental approach to mitigating near-surface polarizations involves enhancing turbulence flow near the membrane surface to minimize the formation of polarization layers. This study focuses on modifying the membrane surface via surface decoration to reduce the polarization layers. The effects of membrane surface decoration on the performance of membrane distillation were systematically investigated through an air gap membrane distillation module under various operating conditions. The hydrodynamic characteristics of the flow near the membrane surface were studied using computational fluid dynamics simulations. The characteristics of both bottom-up and top-down disturbing configurations were analyzed. In addition, we investigated the influence of various flow disturbing methods on the mass transfer coefficient in membrane distillation processes. The membrane distillation test results show that at feed temperatures of 50 °C and 70 °C, there are respective increases in transmembrane flux of 30 % and 12 %. The results provide insights into optimizing the flow state near the membrane surface for enhanced membrane distillation performance.

Radiofrequency-Triggered Surface-Heated Laser-Induced Graphene Membranes for Enhanced Membrane Distillation

Membrane distillation (MD) has attracted significant research interest for desalinating hypersaline brine. However, the lack of robust hydrophobic membrane and lower energy efficiency requirements restrict its true potential. Designing and...

Tailored PVDF membrane with coordinated interfacial nano/micro-structure for enhanced membrane distillation

Membrane distillation (MD) has attracted growing attention as a sustainable green membrane process technology for seawater and high saline water desalination, yet seriously challenged by the potential membrane wetting and fouling. Herein, a controllable approach to fabricate tailor-made MD membranes was proposed via combining in-situ growth of SiO2 nanoparticles with membrane surface activation and low surface energy substances. SiO2 nanoparticles distributed uniformly on the membrane surface and combined with the intrinsic microstructure of the membrane, a hybrid macro/micro-structure was constructed, further endowing the membrane with superhydrophobicity. The modified membranes exhibited excellent wetting-resistance properties and over 99.9 % salt rejection during vacuum membrane distillation (VMD) process. The surface wetting and anti-nucleation theory special for MD membranes was uncovered. This theory and VMD experiments synchronously indicated that the hybrid macro/micro structure with optimized nanoparticle size rendered the resulting fabricated membranes enhanced permeate flux and anti-fouling properties. The optimal L-F-3 membrane achieved high flux (16.08 kg/(m2·h)) under feed temperature of 60 °C with a water contact angle of 150° and LEP (liquid entry pressure) of 2.9 bar. Finally, the modified membranes possessed the enlarged effective evaporative surface and intensified the nucleation barrier as well as the interfacial nanoscale turbulent flow without potential scaling.

Solar enhanced membrane distillation for ammonia recovery

Directly recovering ammonia from waste streams is a sustainable approach for ammonia management since it saves energy from both the Haber-Bosch process, the major industrial method for ammonia synthesis, and wastewater treatment. Membrane distillation (MD), an evaporation-based membrane separation process, has been employed to recover ammonia from ammonia-rich wastewater due to the high volatility of ammonia. In this study, the photothermal effect is incorporated into MD to enhance the ammonia recovery from ammonia-rich wastewater. Carbon black particles are coated on the membrane surface to increase its absorption of solar irradiation at the solution-membrane interface and facilitate the ammonia transport across the membrane. We demonstrate that the system can recover ammonia at a maximum ammonia flux of 4.52 g-N·m−2·h−1 with a solar intensity of 1.7 kW·m−2. The estimated mass transfer coefficient of carbon black coated membrane is 2.67 × 10−2 m·h−1 with solar irradiation, enhanced by 30.8% when compared to that in a pristine membrane. We also confirm that the improvement of ammonia flux by photothermal effect is equivalent to heating the feed solution by 20–30 °C. Our study demonstrates a promising pathway for utilizing solar energy by photothermal effects to enhance MD for ammonia recovery from ammonia-rich wastewater.

Hierarchical structure design of electrospun membrane for enhanced membrane distillation treatment of shrimp aquaculture wastewater

Electrospun fibrous membranes with superhydrophobicity can reduce membrane wettability and maintain a long-term stable desalination process during membrane distillation (MD). In this study, we developed a novel approach to prepare superhydrophobic poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrospun membranes (EMs) by constructing hierarchical microtextures on membrane surfaces via a template printing strategy. PVDF-HFP membranes with printed surface microtextures were successfully prepared using a stainless-steel mesh (SSM) template as a fiber collector. For the injection rate of 1.0 mL/h and the solvent weight ratio of N-methyl-2-pyrrolidone (NMP) and tetrahydrofuran (THF) of 1:1, the membrane exhibited optimal printed microtextures when electrospun at the temperature of 30 ± 2 °C and relative humidity of 70 ± 5 %. The microtexture template structure provides hierarchical micro- and nano-roughness on the membrane surface. The optimal membrane exhibits superhydrophobicity, with a static contact angle of water (WCA) of 158° and a low water sliding angle (WSA) of 8.5°. In the direct contact membrane distillation (DCMD) for treating shrimp farm wastewater, the initial permeation flux of the optimal membrane was as high as 33.45 kg·m−2·h−1. The superhydrophobic membrane exhibited a stable permeation flux of 29.06 kg·m−2·h−1 (only a 13 % reduction in flux) after long-term DCMD for 72 h, which portrays great potential for practical wastewater treatment, especially in nutrient-rich aquaculture wastewater.

Carbon nanotube enhanced membrane distillation for salty and dyeing wastewater treatment by electrospinning technology

Membrane distillation (MD) is considered as a promising and attractive technology due to its effective production of fresh water. However, the low permeability and easy wetting of MD membranes limit its practical applications. Herein carbon nanotubes (CNTs) and polyvinylidene fluoride-co-hexafluoropropylene (PcH) were used to fabricate nanofiber membranes by electrospinning. Effects of heat-press temperature and CNTs concentration on the morphology and performance of the as-fabricated membranes were systematically investigated. Dye rejections of CNTs/PcH membranes were also studied and role of CNTs played in the as-prepared MD membranes were analyzed. Results suggest that heat-press treatment effectively improved the mechanical strength as well as liquid entry pressure of membranes, and the optimal heat-press temperature was 150 °C. CNTs were proved to be successfully blended in nanofibers. Hydrophobicity and mechanical strength of membranes increased with CNTs incorporation. The 0.5 wt % CNTs loaded membrane heat-pressed at 150 °C exhibited the highest permeate flux (16.5–18.5 L m−2 h−1), which signified an increase of 42–50 % compared to the commercial MD membrane (11–13 L m−2 h−1) when 35 and 70 g L−1 NaCl solutions were used as feed solutions, respectively. It was noteworthy that salt rejection efficiencies of tested membranes achieved more than 99.99 %. When CNTs/PcH nanofiber membrane was applied to the treatment of dyeing wastewater, the removal rates of acid red and acid yellow reached 100 %. The removal rates of methylene blue and crystal violet were 99.41 % and 99.91 %, respectively. The present study suggested that the as-prepared membranes showed high potential towards MD application.

Enhanced Membrane Distillation Water Flux through Electromagnetism

Enhancing the efficiency of membrane distillation (MD) is crucial for implementing this desalination technology on a large scale for freshwater production. Herein, an interesting opportunity of enhancing membrane distillation performance by applying an external electromagnetic field to the feed stream is demonstrated. It has been proven that electromagnetic fields can affect water evaporation rate, which is primarily related to the effect of the electromagnetic field on the hydrogen bonds of water clusters. However, as of yet, the enhancement in MD performance through the assistance of electromagnetic fields has not been previously recognized. Herein, it is demonstrated that the permeate flux of membrane distillation can be increased by up to 10% through exposing the saline feed water to an external electromagnetic field (EMF). It was found that the presence of divalent ions in the feed solution during the application of EMF resulted in a higher increase in permeate flux than when only NaCl was present. The electromagnetic treatment is non-destructive to current MD and thus can be easily adapted to large-scale water desalination processes.

Novel PVDF membrane with sandwich structure for enhanced membrane distillation

Membrane distillation (MD) holds great potential in the desalination of high concentration brines but need improvement to achieve sustained permeate flux and durable performance in long-term MD operation. In this study, a novel Janus membrane with sandwich structure was fabricated. Detailly, the microporous PVDF membrane was single-side modified with a bio-inspired hydrophilic polydopamine/polyethylenimine (PDA/PEI) coating, then a hydrophobic nanofiber layer was constructed via forcespinning on the other side as an anti-wetting layer. As the PDA/PEI deposition time increases from 0 to 1.5 h, the contact angle of the modified PVDF membrane surface decreases gradually from 124.6° to 31.3° with little change in membrane morphology and pore structures, resulting in an upward trend of permeate flux up to 50.9 LMH at a feed temperature of 70 °C. The rough forcespun nanofiber layer endows the membrane surface with good hydrophobicity, which brings improved LEP (136.4 kPa) and operation durability for the membrane during MD process. Moreover, numerical simulations were conducted to verify and predict the permeate flux variation of MD membranes under different operation conditions, revealing the superiority of the sandwich-structured Janus membrane combined with the experimental results. Finally, the optimized PDA-PVDF-NF membrane was used for the desalination of model inland brine, exhibiting stable MD performance with high permeate flux of 25 LMH, ~100% salt rejection, and excellent water recovery during a continuous operation as long as 96 h.

Application of flying jet plasma for amelioration of polyvinylidene fluoride membrane properties towards enhanced membrane distillation

AbstractThis study explored the feasibility of applying sequential argon‐CF4 plasmas treatment as a featured approach towards enhancement of polyvinylidene fluoride (PVDF) membrane structure and surface topography for application in membrane distillation (MD). Systematic procedures were conducted to synthesize membranes of different combinations composed mainly from PVDF and assisted by addition of polyvinylpyrrolidone (PVP) at different percentages. Upon casting four classes of the membranes, the analysis results via Scanning Electron Microscopy (SEM), Energy Dispersive X‐Ray Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), Thermal Gravimetric Analysis (TGA), as well as the contact angle measurements have shown that the membrane surface topography was mediated in terms of appearance of clear bumps on the surface. Upon implementing the sequential treatment cycles for several durations, the surface topography has considerably changed at the end of argon treatment cycle in terms of appearance of pores at small size. Further changes on the surface topography were detected at the end of CF4 treatment cycle elucidated by increase in the pore size and uniform distribution of the pores along the examined membranes. The function of the sequential plasma treatment was become certain upon implementing MD experiments for dilute solution of ethanol/water, in which higher permeation water flux and permeate separation index were observed for membranes of higher PVP content and sequentially treated for 6 min.

Incorporation of nanosized LTL zeolites in dual-layered PVDF-HFP/cellulose membrane for enhanced membrane distillation performance

We report here the fabrication of a membrane with dual wettability capable of desalinating oily saline water using membrane distillation (MD). The fabrication process involves modification of electrospun PVDF-HFP (PH) using a nanozeolite-cellulose surface coating followed by stacking unto unmodified electrospun PH to form a dual-layered membrane. Cross-sectional EDS mapping of the modified PH membranes indicates that as the concentration of nanoparticles in the coating solution increases, they become less uniformly distributed across the membrane thickness. Direct contact membrane distillation (DCMD) of oily saline feed tests show that the membrane performance is optimized when 1 wt % of nanoparticles is embedded in the cellulose coating. The composite membrane with 1 wt % nanosized zeolite loading maintains a flux 38% higher than that of the dual-layered membrane without zeolite due to the increase in the feed side membrane porosity from 41.59% to 49.71%. This study concludes that the incorporation of an optimum amount of nanosized Linde type L (LTL) zeolite into the cellulose surface coating can alter the characteristics of the PH membrane enabling achievement of higher flux in MD without compromising the oil and salt rejection.

Review on Blueprint of Designing Anti-Wetting Polymeric Membrane Surfaces for Enhanced Membrane Distillation Performance.

Recently, membrane distillation (MD) has emerged as a versatile technology for treating saline water and industrial wastewater. However, the long-term use of MD wets the polymeric membrane and prevents the membrane from working as a semi-permeable barrier. Currently, the concept of antiwetting interfaces has been utilized for reducing the wetting issue of MD. This review paper discusses the fundamentals and roles of surface energy and hierarchical structures on both the hydrophobic characteristics and wetting tolerance of MD membranes. Designing stable antiwetting interfaces with their basic working principle is illustrated with high scientific discussions. The capability of antiwetting surfaces in terms of their self-cleaning properties has also been demonstrated. This comprehensive review paper can be utilized as the fundamental basis for developing antiwetting surfaces to minimize fouling, as well as the wetting issue in the MD process.

Hierarchically Structured Janus Membrane Surfaces for Enhanced Membrane Distillation Performance

Commercial hydrophobic poly(vinylidene fluoride) (PVDF) membranes are vulnerable to membrane fouling and pore wetting, hampering the use of membrane distillation (MD) for the treatment of surfactant- and oil-containing feed streams. To address these challenges, we designed novel Janus membranes with multilevel roughness to mitigate foulant adhesion and prevent pore wetting. Specifically, fouling- and wetting-resistant Janus MD membranes with hierarchically structured surfaces were tailored via a facile technique that involved oxidant-induced dopamine polymerization followed by in situ immobilization of silver nanoparticles (AgNPs) on commercial PVDF hollow fiber substrates. These membranes demonstrated outstanding antifouling properties and salt rejection performances in comparison to membranes with single-level structures. We ascribed the membranes' excellent performances to the coupled effects of improved surface hydrophilicity and self-healing mechanism brought about by AgNPs. Furthermore, the newly engineered membranes exhibited antibacterial properties in Bacillus acidicola solutions as evidenced by clear inhibition zones observed on a confocal laser scanning microscope. The development of hierarchically structured Janus MD membranes with multilevel roughness paves a way to mitigate membrane fouling and pore wetting caused by low-surface-tension feed streams in the MD process.

One-step tailoring surface roughness and surface chemistry to prepare superhydrophobic polyvinylidene fluoride (PVDF) membranes for enhanced membrane distillation performances

Superhydrophobic polyvinylidene fluoride (PVDF) membrane is a promising material for membrane distillation. Existing approaches for preparing superhydrophobic PVDF membrane often involve separate manipulation of surface roughness and surface chemistry. Here we report a one-step approach to simultaneously manipulate both the surface roughness and surface chemistry of PVDF nanofibrous membranes for enhanced direct-contact membrane distillation (DCMD) performances. The manipulation was realized in a unique solvent-thermal treatment process, during which a treatment solution containing alcohols was involved. We demonstrate that by using different chain-length alcohols in the treatment solvent, surface roughness can be promoted by creating nanofin structures on the PVDF nanofibers using an alcohol which has moderate affinity with PVDF. Meanwhile, surface chemistry can be tuned by adjusting the fraction distribution of crystal phases (nonpolar α phase and polar β phase) in the membrane using different alcohols. PVDF membranes with different surface wettabilities were used to evaluate the effects of surface roughness and surface energy on the DCMD performances. Combining both low surface energy and multi-scale surface roughness, pentanol-treated PVDF membrane achieved best anti-water property (water contact angle of 164.1° and sliding angle of 8.1°), and exhibited superior water flux and enhanced anti-wetting ability to low-surface-tension feed in the DCMD application.

Enhanced membrane distillation of organic solvents from their aqueous mixtures using a carbon nanotube immobilized membrane

In this paper, we report for the first time the application of carbon nanotube immobilized membrane (CNIM) for enhanced separation of organic solvents from their aqueous mixtures via sweep gas membrane distillation. The presence of carbon nanotubes (CNTs) on the hydrophobic membrane surface significantly altered the liquid–membrane interactions to promote isopropanol (IPA) transport in IPA-water mixture by inhibiting water penetration into the membrane pores. The preferential sorption of IPA on the CNIM was evident from the contact angle measurements. The isopropanol flux, selectivity and mass transfer coefficient obtained with CNIM were significantly higher than the corresponding unmodified PTFE membrane at different isopropanol concentrations and temperatures with enhancement in separation factor reaching as high as 350% at 70 °C. An increase in mass transfer coefficient of about 132% was also observed. Performance enhancement in CNIM was mainly attributed to the preferential sorption on the CNTs followed by rapid desorption from its surface.

Hierarchically textured superhydrophobic polyvinylidene fluoride membrane fabricated via nanocasting for enhanced membrane distillation performance

The further advance of membrane distillation (MD) is hampered due to inadequate membrane durability challenged by wetting-induced performance deterioration. The current study presents a novel approach to fabrication of superhydrophobic polyvinylidene fluoride (PVDF) membranes via construction of hierarchical textures on membrane surfaces based on a nanocasting strategy. Two types of PVDF membranes with different surface textures were prepared, separately using a stainless-steel mesh (SSM) or its negative polydimethylsiloxane (PDMS) template as a fabrication substrate. The obtained membranes were systematically characterized in terms of membrane morphology, wetting behavior, MD performance, and antifouling performance. Consequently, the PDMS-based membrane obtained a plaited surface texture, resulting in a high static contact angle of ~153° and a high sliding angle of >90°. The resultant parahydrophobic membrane surface resembled a petal surface. The SSM-based membrane obtained a wave-like pattern, resulting in a high static contact angle of ~164° and a low sliding angle of ~6.8°. The resultant superhydrophobic membrane surface resembled a lotus leave surface. Moreover, both membranes achieved higher water flux and enhanced antifouling performance with no significant compromise in salt rejection. Particularly, the SSM-based membrane achieved the highest flux and best antifouling performance, suggesting promising applications in various fields.

Electrospun porous poly(tetrafluoroethylene-co-hexafluoropropylene-co-vinylidene fluoride) membranes for membrane distillation

Novel THV electrospun membrane exhibits enhanced membrane distillation performance.

Vacuum enhanced membrane distillation for trace contaminant removal of heavy metals from water by electrospun PVDF/TiO2 hybrid membranes

Electrospun hybrid membranes were synthesized using electrospinning of Poly (vinylidenefluoride) - titanium tetraisopropoxide (PVDF-TTIP) sol. Asymmetric post-treatment of membrane conducted for deprotonation of titanate and making hydrophilic/hydrophobic dual characteristics. The membranes were characterized by various methods such as wettability, scanning electron microscopy, infrared spectroscopy, X-ray diffraction and liquid entry pressure tests. For evaluating the separation performance, these membranes were applied in the VMD process to treat water heavy metal contaminants. The effects of operating parameters such as flow rate, temperature and membrane properties as porosity, on contaminant removal and producing ultra-pure water have been studied.

CFD Study of Heat Transfer Enhanced Membrane Distillation Using Spacer-Filled Channels

Membrane distillation (MD) can utilize low level thermal energy and holds high potential to replace conventional energetically intensive separation technologies. Direct contact membrane distillation (DCMD) is suitable for the applications of desalination and concentration of aqueous solutions. Employing spacer-filled channels can enhance the mass flux of the DCMD modules, which can further result in the increase of energy utilization efficiency of the separation. The trans-membrane mass flux is controlled by the boundary layer heat transfer of both fluid channels. The estimation of heat transfer coefficients is critical to the analysis and design of MD modules. This paper presents the results of a comprehensive 3-D computational fluid dynamics (CFD) simulation which covers the entire length of the module and takes into account the trans-membrane heat and mass transfer. The model was verified with experimental data in the literature. The contour maps show that spacers create high velocity regions in the vicinity of the membrane. The trans-membrane heat and mass fluxes both show fluctuating patterns corresponding to the repetitive structure of the spaces and the fluxes are much higher than that of the modules using empty channels. The heat transfer coefficient enhancement factors obtained from CFD simulation are significantly higher than the predictions from literature correlations. The model can serve as an effective tool for developing correlations of heat transfer coefficients and optimal design of spacer-filled MD modules.