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Membrane distillation (MD) is an emerging water treatment technology that employs a microporous hydrophobic membrane as a physical barrier against liquid feed and utilizes the transmembrane vapor pressure difference as the driving force to propel volatile components transport across the membrane. MD has a great potential in various industrial applications such as desalination, food industry, drug purification, and separation of azeotropic mixtures. Although it has a bright future, the large-scale industrialization of MD is restricted by serious membrane wetting issues. This review illustrates the membrane wetting types, summarizes several wetting detections, discusses the causes of membrane wetting and eventually highlights the development of membrane synthesis for improved anti-wettability. • 5 key novel membranes for improved anti-wettability in MD are critically reviewed. • Dense membranes with hydrophilic dense layer are promising for all-around anti-wetting. • This review offers comprehensive analysis and guidelines for anti-wetting membranes. [ABSTRACT FROM AUTHOR]

This study presents the design and fabrication of a new adsorptive nanofibrous membrane incorporating metal-organic framework (MOF) by electrospinning technique for removing ammonia-nitrogen from wastewater via direct contact membrane distillation (DCMD) process. Three types of MOF particles (HKUST-1, AlFu, and UiO-66-NH 2) were used as functional nanoadditives and incorporated into the nanofibrous membrane matrix. The ammonia removal performance of these nanocomposite membranes was comprehensively investigated and compared with an activated carbon (AC) incorporated nanofibrous membrane. Various parameters such as ammonia removal efficiency, ammonia separation factor, overall ammonia mass transfer coefficient, ammonia flux, and membrane stability were evaluated. Among the tested MOFs, UiO-66-NH 2 demonstrated excellent performance and was selected for further optimization. The 5.0 wt.% UiO-66-NH 2 incorporated membrane exhibited the best performance for ammonia removal. Additionally, the mechanism of MOF incorporated nanofibrous membranes for ammonia removal from aqueous solution by DCMD was schematically analyzed. In overall, this study confirmed the feasibility of fabricating MOF incorporated electrospun adsorptive nanofibrous membranes with outstanding ammonia removal performance, demonstrating the potential of membrane distillation to remove ammonia-nitrogen from the aquatic environment. • MOF incorporated mixed-matrix membranes were fabricated by electrospinning. • Three types of MOFs with different properties were used as the nanoadditives. • MOF incorporated adsorptive membranes showed enhanced ammonia removal performance. • The MOF incorporated membranes have great potential in DCMD for ammonia removal. [ABSTRACT FROM AUTHOR]

Abstract In this study, activated carbon (AC) enhanced hydrophobic/hydrophilic dual-layer nanofiber composite membranes were prepared for direct contact membrane distillation (DCMD) by incorporating different concentrations (0–3.0 wt%) of AC nanoparticles into the polyvinylidene fluoride- co -hexafluoropropylene (PVDF-HFP) hydrophobic active layer. The membranes were composed of two layers, a thin hydrophobic PVDF-HFP/AC electrospun active layer and a thick hydrophilic support layer. Results indicated that the incorporation of AC nanoparticles have greatly increased the membrane properties and performance. The membrane containing 1.5 wt% AC nanoparticles (M3) showed the best flux performance (45.6 L/m2 h) compared to other membranes fabricated in this study and a commercial PTFE benchmark membrane (41.8 L/m2 h) without compromising the salt rejection. A two-dimensional model was developed to investigate the flux profile of the PVDF-HFP/AC membranes. The present study suggests that the incorporation of AC nanoparticles into PVDF-HFP active layer can impart additional functionalities to the composite membranes that would enhance the membranes DCMD performance. The fabricated PVDF-HFP/AC composite membranes present great potential in the application of DCMD desalination. Graphical abstract Unlabelled Image Highlights • Dual-layer nanofiber membranes were fabricated by electrospinning for DCMD. • The membranes were composed of PVDF-HFP/AC active layer and Nylon 6,6 substrate. • The incorporation of AC nanoparticles enhanced the flux performance in DCMD process. • M3 composite membrane presented excellent permeability and performance stability. • A two-dimensional model was developed to explain the effect of AC particle on flux. [ABSTRACT FROM AUTHOR]

Highly porous polyvinylidene fluoride (PVDF) hollow fiber membranes are of great interest for membrane contactor applications such as sea water desalination by membrane distillation in order to enhance the flux of the process. For the first time in this paper, porous hydrophobic PVDF asymmetric hollow fiber membranes with a high outer surface porosity for use in vacuum membrane distillation (VMD) were successfully fabricated by applying a simultaneous co-extrusion spinning process through the non-solvent induced phase separation (NIPS) method. In this approach, five kinds of dual-layer composite membranes were prepared by employing different inner and outer polymer dopes. Single-layer membrane was also fabricated through the conventional dry-jet wet phase inversion method. Results revealed that the introduction of the simultaneous co-extrusion spinning process greatly increased the outer surface porosity of the dual-layer composite membranes. It was also observed that the VMD flux of the dual-layer membranes spun through the simultaneous co-extrusion spinning process was 54% higher than the single-layer membrane fabricated by the conventional dry-jet wet phase inversion method. A VMD flux as high as 61.8 kg/(m 2 h) at 70 °C was obtained for the novel dual-layer composite membranes. [ABSTRACT FROM AUTHOR]

In this study, a new simultaneous heat and mass transfer theoretical model of the membrane distillation process operating under direct contact membrane distillation (DCMD) mode has been developed for the hydrophobic/hydrophilic dual-layer composite membranes. Using the proposed model, a criterion parameter (β) was derived to determine whether the flux will be increased for the prepared hydrophobic/hydrophilic composite membranes. A flux ratio ( γ DL/SL M ) of dual-layer to single-layer membrane was also developed to predict the DCMD flux performance of the composite membranes. To validate the theoretical model, three sets of dual-layer hydrophobic/hydrophilic composite membranes and single-layer hydrophobic membranes with defined total thicknesses (240 μm, 260 μm, and 285 μm) and different thickness ratios (δ / δ o = 3.69, 2.48 and 2.19) were purposefully fabricated using electrospinning technique. Theoretical modelling results were compared with the experimental data in terms of DCMD flux ratio. The experimental data of flux ratios for the three sets of fabricated membranes at 60 °C, γ DL/SL E , are 2.75, 1.97 and 1.80, respectively, which fit well with the model prediction values ( γ DL/SL M = 2.96, 2.18, and 1.97) within high agreement level at a relative standard deviation of 9.6%. Furthermore, the flux ratio of DL2 to SL2 membranes (dual-layer and single-layer membranes with 260 μm total thicknesses and δ / δ o = 2.48 thickness ratio) at different feed inlet temperatures still maintain a high degree of consistency with model values. Overall, the results show the experimental data agree well with the theoretical model prediction for the flux ratio within relative standard deviation of 10.1%, thereby confirming the validity of the proposed model. Additionally, a critical value of the thickness ratio (χ) was calculated χ = 1 + 1 β , this value could provide useful guidance to design a highly efficient hydrophobic/hydrophilic dual-layer composite membrane for DCMD applications in the future work. Image 1 • Dual-layer nanofiber membranes were fabricated by electrospinning for DCMD. • The membrane was composed of PVDF-HFP nanofibers and Nylon 6,6 membrane substrate. • A mathematical model was developed to predict the flux of dual-layer membranes. • The theoretical model was tested and validated by the experimental results. • The experimental data resemble well with the theoretically calculated values. [ABSTRACT FROM AUTHOR]

Mixed matrix membranes combining the processibility of polymers with the properties of porous nano-additives is an effective method to enhance the performance of membrane distillation (MD) process. In this work, a new type of hydrophobic hybrid PVDF hollow fiber membranes doped with aluminum fumarate metal-organic frameworks (AlFu MOF) was fabricated and their performance in direct contact membrane distillation were studied experimentally and theoretically. The results showed that the addition of MOF particles efficiently enlarged the effective porosity of membrane and increased the water flux as well as the thermal efficiency of MD process. At 1 wt% MOF loading, the effective porosity of membrane was enlarged by 52.4%, which induced 55.9% increment in overall mass transfer coefficient of the hybrid membrane, and the thermal conductivity of the membrane was decreased by 38.6%, which contributed to the reduction of sensible heat loss of MD. Correspondingly, the experimental water flux of the hybrid 1 wt% MOF/PVDF membrane was improved by 50.5% and the thermal efficiency increased by 46.2% (0.58 vs 0.31) at a feed temperature of 40 °C. Moreover, the MOF/PVDF membrane exhibited stable flux and retained high salt rejection (>99.9%) for 3.5 wt% NaCl solution over a 50 h desalination test period. Overall, this study provides an insight into the positive effects of AlFu MOF additives on the enhancement of membrane performance in distillation process. Image 1 • Aluminum fumarate MOF was blended with PVDF to fabricate hollow fiber membranes. • Effects of nanoparticle on membrane structure and performance were investigated. • Hybrid membranes showed 50.5% higher flux and 46.2% higher thermal efficiency. • The mechanism on the enhancement of heat and mass transfer performance were revealed. [ABSTRACT FROM AUTHOR]

Air gap width has an important influence on the performance of air gap membrane distillation (AGMD) process. In this study, a novel double-pipe AGMD module (DP-AGMD-M) consisted of PVDF hollow fiber membrane and heat exchange capillary copper tubes was successfully developed. The evenly distribute and easily control of the air gap width in DP-AGMD-M can be implemented. The effect of the air gap width ( d a ), hot feed temperature ( T 1 ), hot feed flow rate ( Q ), temperature difference (Δ T ) between the hot feed outlet temperature and the cold feed inlet temperature, and the effective membrane module length ( L ) on the performance of DP-AGMD-M were experimental studied. The concept of the equivalent membrane distillation flux ( J AGMD ) was firstly introduced in this paper and used to evaluate the comprehensive performance of DP-AGMD-M. The optimal performances, including membrane distillation flux ( J ), gained output ratio ( GOR ) and J AGMD were obtained. Within the experimental range, the maximum J arrived at 11.4 kg/(m 2 · h), the maximum GOR reached 6.6 and the maximum J AGMD was 29.6 kg/(m 2 · h). [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is a promising technology for treating saline industrial wastewater, but current hydrophobic MD membranes suffer significant wetting issues due to contaminants commonly present in wastewater. In this study, we report an effective method to fabricate a polyvinylidene fluoride- co -hexafluoropropylene (PVDF-HFP) electrospun nanofiber membrane with improved anti-wetting property against low-surface-tension substances. Without surface activation, the pristine electrospun nanofiber PVDF-HFP membrane with intrinsic re-entrant structure was directly fluorinated by 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) employing vapor deposition (VD). The fluorinated membrane exhibited excellent surface omniphobicity with high water and ethanol contact angle of 154.1 ± 0.1° and 122.6 ± 1.7°, respectively. The fluorinated membrane showed highly stable omniphobicity and mechanical properties at harsh conditions such as ultrasonic, boiling water, acid and base treatment. More importantly, the resultant omniphobic membrane exhibited robust wetting resistance to the saline feed containing sodium dodecyl sulfate (SDS, 0.4 mM) in an 8-h dynamic direct contact MD (DCMD) test. This study provides an effective and benign approach to fabricate omniphobic nanofiber membranes which have great potential in treating saline wastewater containing low surface tension substances. • An omniphobic electrospun nanofiber membrane was fabricated by an effective method. • No need to activate the surface and creat secondary re-entrant structures before fluorination. • The fluorination was on the basis of physical interaction between PVDF-HFP and FDTS. • The obtained omniphobicity was very stable under harsh conditions. • The omniphobic membrane showed the robust anti-wetting property to surfactants in DCMD. [ABSTRACT FROM AUTHOR]