Direct Contact Membrane Distillation Application Research Articles

Thermal treatment enhanced the hydrophobicity of PVDF hollow fibers membrane for brine treatment using membrane distillation

The objective of this research was to explore the impact of thermal treatment on the characteristics and performance of Poly (vinylidene fluoride) (PVDF) hollow fiber (HF) membranes utilized in direct contact membrane distillation (DCMD). To assess the effects of thermal treatment, the PVDF polymer's allotropy and crystallinity were analyzed using Fourier transform infrared ATR-FTIR and Differential scanning calorimetry (DSC). The thermal treatment predominantly affected the α-phase of the membrane, leading to increased hydrophobicity. On the other hand, Atomic Force Microscopy (AFM) characterized the membrane surface morphology. The AFM characterization revealed an increment in surface membrane roughness leading to an improvement in membrane hydrophobicity. This straightforward technique of thermal treatment resulted in improved membrane properties for DCMD application, such as increased LEP, porosity, and hydrophobicity, as well as reduced water uptake. The DCMD's efficacy was then evaluated using seawater reverse osmosis (SWRO) brine and it was observed that thermal treatment improved flux while slightly reducing salt rejection. In conclusion, these findings display the potential of thermal treatment to enhance the performance of PVDF HF membranes for DCMD applications.

Enhancing Hydrophobic/Hydrophilic Dual-Layer Membranes for Membrane Distillation: The Influence of Polytetrafluoroethylene (PTFE) Particle Size and Concentration

This study assesses the effects of different polytetrafluoroethylene (PTFE) particle sizes and concentrations on the performance of dual-layer membranes in direct contact membrane distillation (DCMD). Specifically, particle sizes of 0.5 μm, 1 μm, and 6 μm were systematically evaluated at concentrations of 0 wt%, 2 wt%, 4 wt%, and 6 wt%. Comprehensive analyses, including scanning electron microscopy (SEM), liquid entry pressure (LEP), contact angle, thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP), atomic force microscopy (AFM), permeate flux, nitrogen gas permeation, and salt rejection, were employed to characterize the membranes. Under conditions of a feed temperature of 70 °C and a salt concentration of 8000 ppm for a 24 h duration, the results clearly indicated that a 0.5 μm PTFE particle size combined with a 6 wt% concentration exhibited the highest performance. This configuration achieved a permeate flux of 11 kg·m2/h and a salt rejection rate of 99.8%. The outcomes of this research have significant implications for the optimization of membranes used in DCMD applications, with potential benefits for sustainable water treatment and energy conservation.

Behavior and removal of microplastics during desalination in a lab-scale direct contact membrane distillation system

The behavior of microplastic particles (MPPs) is explored in a lab-scale direct contact membrane distillation (DCMD) system used to produce drinking water from seawater, while also analyzing the impact of their presence on the process performance. Commercial MPPs (LDPE, PET and uPVC) were first studied by temperature stress (TS) tests (using amber bottles) to mimic the exposure to the high temperatures commonly used in DCMD (ca. 80 °C), which led to uPVC being selected to be tested under different loads and aging degrees in the DCMD system. By analyzing uPVC MPPs samples before and after the TS and DCMD experiments by ATR-FTIR, UV–Visible spectroscopy and SEM, it was concluded that minor aging is expected to occur. High loads (>0.1 g L−1) of uPVC MPPs decreased the DCMD performance in terms of permeate flux (from 32.8 ± 0.3 with filtered seawater to 28.7 ± 0.3 kg m−2 h−1 with a load of 0.2 g L−1), although no signs of inferior treated water quality were found based on the conductivity and salinity results. The removal of MPPs (size > 1.2 μm) in DCMD applications with seawater was analyzed by μRaman, with all the results suggesting very high removal efficiencies (≥99 %).

A luminescent thermosensitive coating for a non-invasive and in-situ study of thermal polarization in hollow fiber membranes

A new non-invasive method to evaluate the thermal polarization is herein proposed for hollow fiber (HFs) membranes during the Direct Contact Membrane Distillation (DCMD) process. The goal was reached by using a temperature-sensitive phosphorescent molecular probe such as Tris(phenanthroline) ruthenium(II) chloride (Ru(phen)3) and an infrared camera (IR) camera, to map, in-situ, the thermal profile directly at the membrane surface which is in contact with the hot feed water solution. The molecular probe was immobilized, by means of a perfluoropolyether (PFPE)-based coating, on the surface of a polypropylene HF in order to obtain a thermosensitive polymerized coating. The produced HFs were fully characterized in order to demonstrate the successful embodiment of the molecular probe on the surface of the membrane and their suitability, in terms of pore size, hydrophobicity and stability, for DCMD application. The luminescent activity of the immobilized Ru(phen)3 complex was measured by a bifurcated optical fiber, one branch was used to excite the molecular probe and the other branch was able to constantly monitor the molecular emission which was dependent on the temperature along the HF surface. The HFs were tested in DCMD at different feed temperatures (40 °C, 50 °C, 60 °C) and constant conditions of streams flow rate and distillate temperature (Qf = 90 L/h; Qp = 24 L/h; Tp = 13 °C). The obtained results clearly demonstrated the possibility of studying the thermal polarization phenomenon occurring during DCMD, with the temperature profile developed on the HF surface monitored, for the first time, at molecular level using a non-invasive technique. This approach can shed the light on better understanding this phenomenon and the possible strategies for its mitigation and counteraction.

Electrical impedance spectroscopy for non-destructive detection of wetting, fouling and scaling in membrane distillation

Wetting, fouling, and scaling of hydrophobic membranes are significant constraints in membrane distillation (MD) technology. In recent research trends, novel membrane fabrication has been given utmost priority to address these constraints, majorly induced by the low surface tension chemicals like surfactants and dissolved organic matters present in the feed water. In this research, we fabricated a hydrophobic membrane using phase inversion technique with polyvinylidene fluoride (PVDF) polymer under an alcohol coagulation bath (ethanol). The wetting, fouling, and scaling of the membrane were monitored using electrochemical impedance spectroscopy (EIS) incorporated with direct contact membrane distillation (DCMD). The surface tension (52.14–27.5 mN m−1) of the synthetic brine water was reduced by adding surfactant, whereas model algal organic matters were dissolved to study the organic fouling. In the fabricated PVDF membrane, an asymmetric interconnected pore structure was obtained with a high hydrophobicity (water contact angle 146°). A narrow pore size distribution (∼0.43 μm), raised liquid entry pressure (∼1.3 bar), porosity (∼75 %), high surface roughness (1.06 μm), and low surface energy (∼28 mN m−1) were noticed with the same membrane. Hence, it qualified for DCMD application under different feed compositions. Different stages of pore wetting and their progress were assessed instantaneously by adopting EIS approach. Further, the permeate side conductivity was correlated with impedance measured during EIS. Advanced proficiency in distinguishing intrusion and different stages of wetting was illustrated.

Modification of PVDF hollow fiber membrane by co-deposition of PDA/MPC-co-AEMA for membrane distillation application with anti-fouling and anti-scaling properties

In this study, a self-prepared polyvinylidene fluoride (PVDF) hollow fiber membrane with a hierarchical structure was used for facile modification by co-deposition of polydopamine (PDA) and poly(MPC-co-2-aminoethyl methacrylate hydrochloride) (MPC-co-AEMA) for application in direct contact membrane distillation (DCMD). The facile formation of the PDA/MPC-co-AEMA layer on the membrane outer surface drastically changed the chemical and physical properties of the membrane surface. Although the membrane became hydrophilic and smooth with a low porosity and small pore size, the bulk structure of the membrane remained almost intact. For a saline solution including protein and Ca2+ as organic fouling and scaling agents, the deposition of the hydrophilic PDA/MPC-co-AEMA layer on hydrophobic PVDF led to the alleviation of the membrane fouling and scaling during long-term DCMD application. Whereas the formation of a hydrated layer on the membrane surface avoided BSA fouling by avoiding hydrophobic–hydrophobic interactions, a combination of several chemo-physical properties of the modified membrane surface resulted in anti-scaling properties.

Combined macroscopic and pore scale modeling of direct contact membrane distillation with micro-porous hydrophobic membranes

The transport processes of a direct contact membrane distillation (DCMD) module are investigated with a multiscale approach in this study. A stochastic, numerical reconstruction algorithm is first developed to generate 3D virtual membranes based on the membrane's pore size and fiber orientation distributions. Pore-scale simulations are then performed on the reconstructed membrane model. The membrane's effective thermal conductivity and mass diffusivity are computed using AVIZO software and OpenFOAM solver, respectively. A 2D macroscopic, multiphysics model considering the conservation of mass, species, momentum, and energy is developed, where the effective transport properties obtained from pore-scale simulations are utilized, to study the heat and mass transfer through the membrane. Two commercially available polytetrafluoroethylene (PTFE) membranes (GE-Osmonics/0.22 μm and MS-3010/0.45 μm) are chosen as examples for the present multiscale approach. The computed transport properties of the membranes are found to yield good distillate flux predictions with data reported in the literature. The MS-3010/0.45 μm membrane is found to produce 24% higher freshwater on average than GE-Osmonics/0.22 μm membrane. Finally, the performance of DCMD of three hypothetical membranes with different specifications is compared to the reconstructed MS-3010/0.45 μm membrane to study the factors that need to be taken into account for optimal design of the membrane for DCMD applications.

Development of non-woven fabric-based ECTFE membranes for direct contact membrane distillation application

In this work, a lower melting point of HALAR® Ethylene-Chlorotrifluoroethylene (LMP ECTFE) was used to prepare non-woven fabric-based porous membranes, using a polyester non woven (NWPET) as support, via dip-coating. Di-ethyl Adipate (DEA) and di-ethylene Glycol (DEG) were selected as primary and secondary diluent, respectively. SEM analyses showed that in all cases the coating layer was homogeneous, covering all the NWPET support and leading to an interconnected sponge-like microstructure. Membrane characterization evidenced the effects of the DEG concentration, which plays the role of pore former. The effect of the immersion time was examined and studied, too. The produced membranes were fully characterized in terms of water contact angle, porosity and pore size. Direct contact membrane distillation (DCMD) experiments using water and salty solution 0.6 M (NaCl) as feed, were performed for selected supported membranes. The obtained fluxes ranged between 3 and 22 kg/m2h, depending on DEG concentration, at the same immersion time, and on the feed temperatures (40–60 °C). Rejection (R%) was comprised from 94.95 to 99.82%. In particular, the N-ECTFE 7-0-7 membrane, with the highest R%, showed a stable deionized water flux before and after the experiments, indicating its ability to recover its original performance.

Predicting the performance of direct contact membrane distillation (DCMD): Mathematical determination of appropriate tortuosity based on porosity

Direct contact membrane distillation (DCMD) is a promising novel membrane technology for management of waste streams from food manufacturing operations, and for concentration of value-added components from the waste streams. Due to the complexity of technology, the development of prediction models to describe mass transfer during DCMD presents a significant contribution. The overall objective of this investigation was to propose models for prediction of membrane tortuosity based on measured porosity, and to confirm the models by experimental measurements of water flux for DCMD applications. Three types of commercial membranes with porosities ranging from 69.8% to 87.9% were investigated. Eleven (11) different models for prediction of tortuosity from porosity were identified from published literature and evaluated. Statistical comparisons of predicted and experimental water flux were conducted using root-mean-square-error, accuracy factor, and bias factor to identify the models providing the closest agreement between predicted and experiment outcomes. The statistical comparisons indicated that models based on the fractal theory provide the closest agreement between predicted and experimental water flux for DCMD applications. In addition, the proposed model provided best agreement between predicted and experimental temperatures. The simulations of water flux were used to evaluate the influence of membrane surface area on water flux, and capabilities of the tortuosity models. As the width and length of membrane were increased, the differences in water flux predicted by proposed model, as compared to the traditional model, increased significantly. These outcomes confirm the importance using appropriate models for prediction of tortuosity for DCMD applications. • 11 different models provide the different magnitude of estimation for tortuosity. • Estimated tortuosity provides a well-matched prediction for water flux. • Accurate prediction is useful for large scale designs.

A dual-layer, nanofibrous styrene-acrylonitrile membrane with hydrophobic/hydrophilic composite structure for treating the hot dyeing effluent by direct contact membrane distillation

A robust, nanofibrous membrane with dual-layer composite structure and remarkable features has been developed in this work. The new membrane is based on the affordable styrene-acrylonitrile (SAN) polymer and a commercial hydrophilic nonwoven for the support layer. This can help to step forward the industrialization of the direct contact membrane distillation (DCMD) process for treating the hot dyeing effluent in the textile industry. The modified electrospinning with a gas-jet surrounding the Taylor cone was used for fabricating the membrane samples. Using the gas-assisted electrospinning, a nanofibrous nonwoven structure embedded with micro-cups and micro-spheres can be achieved in a one-step, gas-assisted electrospinning. The hydrophilic substrate was used for enhancing the permeate flux of the resultant composite membrane. The fabricated membrane showed remarkable features in comparison with a commercial polytetrafluoroethylene (PTFE) membrane, including high surface hydrophobicity (≥148°), higher porosity (≥81%), and smaller tortuosity factor (1.71). The unique properties of the membrane with the best performance provided a promising permeate flux (28.31 kg.m−2.h−1) and contaminant rejection (RCOD: 98.15%) compared to the commercial membrane (flux: 18.50 kg.m−2.h−1 and RCOD: 97.10%), without considerable pore wetting. However, the new membrane possessed lower liquid entry pressure (LEP) owing to its higher porosity, larger maximum pore size (0.76 μm), and thinner structure (150 μm). Results indicated that the decline in the permeate flux was mainly caused by the cake layer formation on the surface of the membrane rather than the partial pore wetting during the long-term operation for 48 h. Therefore, the newly developed nanofibrous SAN membrane with hydrophobic/hydrophilic composite structure is a promising, robust candidate for DCMD application in the textile wastewater treatment.

Technical and economic potential of high-temperature NF and DCMD for gold mining effluent reclamation

Direct contact membrane distillation (DCMD) and high-temperature nanofiltration (NF) were compared in terms of technical and economic aspects for the treatment of pressure oxidation process (POX) real effluent. Tests were carried out in bench scale and differences in fouling behavior and its impacts on NF and MD performance were evaluated. High pollutants rejection rates were observed for both systems (>92.4 %). However, DCMD had better permeate quality and lower energy requirement, which contributed significantly to its lower operational costs due to use of the effluent residual heat. CAPEX and OPEX were estimated at US$/m3 2.251 for NF and US$/m3 0.233 for DCMD. DCMD application represents a saving of US$ 45,138.3 annually supplying 250,768.3 m3 of high-quality reuse water. At the end, this research allowed for a better comprehension of these membrane technologies, preventing the production of waste and increasing efficiencies in the uses of energy, water, and resources.

Development of hierarchical surface roughness on porous poly (vinylidene fluoride) membrane for membrane distillation process

AbstractThe surface hydrophobicity of a poly (vinylidene fluoride) membrane was increased through the phase separation of polycarbonate (PC) solution on the membrane surface. A variety of characterization techniques including surface contact angle measurement, scanning electron microscopy, liquid entry pressure measurement, atomic force microscopy, porosity measurement, pore size measurement, and attenuated total reflectance‐Fourier transform infrared spectroscopy were performed to investigate the membrane surface modification. According to the results, the formation of PC deposits on the membrane surface provided hierarchical structures consisting of micro and nanoscale roughness which enhanced the surface hydrophobicity. Direct contact membrane distillation (DCMD) experiments, in which an aqueous solution of sodium chloride (3.5 wt%) resembling the seawater was used as feed, were carried out to assess the performance of membranes. This surface modification technique led to the improved pore wetting resistance of the coated membrane in DCMD application. This novel method is believed to have a great potential to apply on a wide variety of membranes for MD application.

Theoretical guidance for fabricating higher flux hydrophobic/hydrophilic dual-layer membranes for direct contact membrane distillation

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 (γDL/SLM) 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/δo=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, γDL/SLE, are 2.75, 1.97 and 1.80, respectively, which fit well with the model prediction values (γDL/SLM= 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/δo=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 calculatedχ=1+1β, this value could provide useful guidance to design a highly efficient hydrophobic/hydrophilic dual-layer composite membrane for DCMD applications in the future work.

Effect of ionic liquid on the structure and desalination performance of PVDF‐PTFE electrospun membrane

ABSTRACTThe relatively large pore size of electrospun membranes might limit their application for direct contact membrane distillation (DCMD). Incorporation of ionic liquid is a potential approach to decrease the pore size of electrospun membranes, which was attributed to the increased conductivity of electrospinning solution. In this study, a novel nanofibrous membrane based on the blends of poly(vinylidene fluoride) (PVDF), polytetrafluoroethylene (PTFE) and ionic liquid (BMIMPF6) was fabricated and applied for the DCMD. The effects of the BMIMPF6 on the morphology, pore size and DCMD performance of the PVDF‐PTFE nanofibrous membrane were investigated. Compared with neat (PVDF‐PTFE) membranes (average pore size: 0.93 μm), the incorporation of BMIMPF6 resulted in a smaller mean pore diameter (0.58 μm). The liquid entry pressure value of the modified composite membrane also increased from 62.75 kPa (neat) to 83 kPa, due to the decreased pore size. The composite membrane exhibited a longer lifespan (about 26 h) than neat membrane during long‐term DCMD process, which makes this composite membrane a promising candidate for DCMD application. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48467.

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.

Direct Contact Membrane Distillation Applied to Colored Reactive or Disperse Dye Solutions

AbstractThe influence of dye class and chromogen groups of dyeing wastewaters composed by reactive or disperse dyes with different colors on direct contact membrane distillation (DCMD) was investigated. The state‐of‐the‐art shows that disperse dyes and different dye colors are not studied in DCMD and the application of DCMD for dyeing wastewater treatment is limited to a few publications. High color rejection was obtained and water was recovered at the permeate side for feed solutions containing reactive or disperse dye. Influence of chromogen groups (colors) was not observed, whereas the class dye was decisive for the process performance. Reactive dyes showed higher mean permeate flux due to the repulsion between their anionic groups and the negative surface of the membrane. Partial wetting of larger pores allowed the passage of small disperse dye molecules, reducing color rejection.

Optimization of membrane modification using SiO2 for robust anti-fouling performance with calcium-humic acid feed in membrane distillation

The goal of this study was to prepare a robust anti-wetting and anti-fouling polyethersulfone (PES) membrane for the rejection of a highly saline (NaCl and CaCl2·2H2O) feed solution containing humic acid (HA) in direct contact membrane distillation (DCMD). Response surface methodology (RSM) was used to determine the optimum formulation of the used materials. The variable factors selected were polydimethyl siloxane (PDMS) and silica (SiO2); liquid entry pressure (LEP) and contact angle (CA) were selected as responses. Scanning electron microscopy (SEM) analysis confirmed the SiO2 deposition and Fourier-transform infrared spectroscopy (FTIR) test evidenced the new functional groups i.e., Si–OH, siloxane, and C–F bond vibrations at 3446, 1099 cm−1, and 1150–1240 cm−1 respectively on the membrane surface. The average roughness (Ra) was increased four times for the coated membranes (0.202–0.242 µm) as compared to that for pristine PES membrane (0.053 µm). The optimum PES-13 membrane exhibited consistent flux of 12 LMH and salt rejection (> 99%) with anti-fouling characteristic in DCMD using the feed solution of 3.5 wt% NaCl + 10 mM CaCl2·2H2O + 10 mg L−1 HA. The PES-13 membrane may therefore be a key membrane for application in DCMD against CaCl2·2H2O-containing salty solutions with HA.

Polyvinylidene fluoride membrane modification via oxidant-induced dopamine polymerization for sustainable direct-contact membrane distillation

Porous hydrophobic polyvinylidene fluoride (PVDF) membranes have been extensively used in direct-contact membrane distillation (DCMD) processes. However, these PVDF membranes are vulnerable to membrane fouling and pore wetting in low surface tension feeds, restricting its application for water recovery from challenging industrial wastewaters. Therefore, it is of paramount importance to engineer fouling- and wetting-resistant MD membranes for robust long-term applications. In this study, a superoleophobic composite hollow fiber membrane with sandwich structure has been developed via accelerated oxidant-induced polydopamine (PDA) deposition on both the outer and inner surfaces of a commercial hydrophobic PVDF membrane under slightly acidic conditions (pH = 5). The modified surface prevents organics adhesion ascribing to its underwater superoleophobicity while the unmodified pores remain hydrophobic for vapor transport. The long-term robustness of the PDA-decorated membrane in highly saline feeds containing low surface tension contaminants has been evaluated via bench-scale DCMD experiments. In contrast to the pristine PVDF membrane, the PDA-decorated membrane exhibits excellent fouling- and wetting-resistant properties in different surfactant solutions as well as oil-in-water emulsion. The PDA-decorated membrane has also been used for seawater desalination, during which it maintains a stable flux and high salt rejection rate. Furthermore, the PDA-decorated membrane presents a flux enhancement of up to 70% over the pristine PVDF membrane in 3.5 wt% NaCl solution at 333 K. This study demonstrates the potential of the PDA-decorated membrane for extended DCMD applications such as water recovery from industrial wastewater containing low surface tension substances.

Preparation of PVDF/PTFE hollow fiber membranes for direct contact membrane distillation via thermally induced phase separation method

Polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) hollow fiber membranes were developed via thermally induced phase separation (TIPS) method for direct contact membrane distillation (DCMD). The effects of PTFE addition on the thermal behavior of the dope mixtures and membrane formation were investigated. It was found that the crystallization of PVDF was significantly enhanced with increased nucleation sites provided by PTFE particles, leading to promoted formation of smaller spherulites in a greater density. Furthermore, the improved uniformity and increased amount of cavity between the spherical crystallites coherently facilitated the formation of smaller pores ranging from 0.08 to 0.12μm. With certain PTFE loading, the membranes exhibited improved porosity, water permeability and hydrophobicity as well as enhanced tensile strength of 9.4±0.3MPa. To examine the DCMD performance, the membranes were tested under various conditions using 3.5wt% NaCl solution. A stable permeation flux of 28.3kgm−2h−1 at the feed temperature of 60°C with 99.99% NaCl rejection for over 50h of operation was achieved, which is comparable with similar type of PVDF membranes while the newly developed membrane exhibited better mechanical strength. This study suggests that the as-spun PVDF/PTFE hollow fiber membranes have potential for DCMD applications.

TiO2-FTCS modified superhydrophobic PVDF electrospun nanofibrous membrane for desalination by direct contact membrane distillation

The major challenges for electrospun membranes used in direct contact membrane distillation (DCMD) process are insufficient pore wetting resistance related with hydrophobicity and pore structure of membrane surface. A novel super-hydrophobization method containing first coating TiO2 nanoparticles on membrane surface by the low temperature hydrothermal process and then the TiO2 coated membrane being fluorosilanized with low surface energy material of 1H, 1H, 2H, 2H-perfluorododecyl trichlorosilane (FTCS) was employed to modify the virgin polyvinylidene fluoride electrospun nanofiber membrane (PVDF ENM). Results showed that this TiO2-FTCS modified membrane possessed high hydrophobicity (157.1°), high mean roughness (4.63μm), considerable wetting resistance (158kPa), well-distributed pore size (0.81μm), reasonable surface porosity (57%) and modest membrane thickness (55μm). These combined properties made the modified PVDF ENM an attractive candidate for DCMD. High flux and stable desalination performances were achieved during short-term DCMD process (73.4LMH permeate flux, 99.99% salt rejection) using 3.5wt% NaCl solution. Long-term DCMD process with actual reverse osmosis brine as feed solution also showed high performance (40.5LMH, 99.98%). These results exceeded those of commercial PVDF membrane and unmodified PVDF ENM significantly, suggesting the potential for PVDF ENM in DCMD applications.