Direct Contact Membrane Distillation Tests Research Articles

Novel post-heat treatment green biodegradable PLA@SiO2 nanocomposite membrane for water desalination

Membrane distillation (MD) has received significant attention because of its energy efficiency and ability to treat industrial wastewater and salty water. However, the synthetic nature of commonly used membranes raises environmental disposal concerns, requiring the development of alternative eco-membranes. To address this issue, a novel, eco-friendly membrane using polylactic acid (PLA) coated with green silica nanoparticles (SiO2 NPs), modified by post-heat treatment, was developed for effective direct contact membrane distillation (DCMD). Herein, two nanofibrous membranes were fabricated via electrospinning/electrospraying: one, PLA, with a 12.5 wt% PLA solution, and another, PLA@SiO2, with 2 % (w/v) SiO2 NPs coating. The PLA@SiO2 membrane demonstrated enhanced hydrophobicity. Various post-heat treatments were applied to optimize membrane properties, such as structure, pore size, hydrophobicity, liquid entry pressure (LEP), thickness, and thermal stability. In addition, the desalination performance of all membranes was evaluated using a DCMD unit for 6 h. Results showed that the heat-pressed then annealed PLA@SiO2 membrane (denoted M44) exhibited notable improvements, including an LEP of 128.7 kPa, a tunable pore size of 0.21 μm, and a contact angle of 135.9°. Furthermore, the M44 membrane demonstrated a porosity of 80.5 %, a stable permeate flux of 14.3 kg.m−2.h−1, and an exceptional salt rejection of 99.99 % over a 24 h DCMD test. This novel eco-membrane shows promising potential for efficient desalination and represents a significant advancement in sustainable MD applications.

Engineered eco-friendly composite membranes with superhydrophobic/hydrophilic dual-layer for DCMD system

Considerable advancements have been made in the development of hydrophobic membranes for membrane distillation (MD). Nonetheless, the environmentally responsible disposal of these membranes poses a critical concern due to their synthetic composition. Herein, an eco-friendly dual-layered biopolymer-based membrane was fabricated for water desalination. The membrane was electrospun from two bio-polymeric layers. The top hydrophobic layer comprises polycaprolactone (PCL) and the bottom hydrophilic layer from cellulose acetate (CA). Additionally, silica nanoparticles (SiO2 NPs) were electrosprayed onto the top layer of the dual-layered PCL/CA membrane to enhance the hydrophobicity. The desalination performance of the modified PCL-SiO2/CA membrane was compared with the unmodified PCL/CA membrane using a direct contact membrane distillation (DCMD) unit. Results revealed that silica remarkably improves membrane hydrophobicity. The modified PCL-SiO2/CA membrane demonstrated a significant increase in water contact angle of 152.4° compared to 119° for the unmodified membrane. In addition, PCL-SiO2/CA membrane has a smaller average pore size of 0.23 ± 0.16 μm and an exceptional liquid entry pressure of water (LEPw), which is 3.8 times higher than that of PCL/CA membrane. Moreover, PCL-SiO2/CA membrane achieved a durable permeate flux of 15.6 kg/m2.h, while PCL/CA membrane showed unstable permeate flux decreasing approximately from 25 to 12 kg/m2.h over the DCMD test time. Furthermore, the modified PCL-SiO2/CA membrane achieved a high salt rejection value of 99.97% compared to a low value of 86.2% for the PCL/CA membrane after 24 h continuous DCMD operation. In conclusion, the proposed modified PCL-SiO2/CA dual-layer biopolymeric-based membrane has considerable potential to be used as an environmentally friendly membrane for the MD process.

A superhydrophobic nanofiber membrane and its application to dye filtration using membrane distillation

Direct contact membrane distillation tests are carried out on four kinds of dye solutions using the superhydrophobic polyvinylidene difluron-copolymerized hexafluoro-propylene nanofiber membrane (PH-E) and its modification coated with polydime-thylsiloxane (PDMS) nanoparticles (PDMS-PH-E membrane). The test results show that PDMS-PH-E membrane has higher water flux, and the removal efficiency of four dyes reaches 100%. While the removal efficiency of the PH-E membrane reaches 100% only for two dyes. The loosely absorbed dye structure on PDMS-PH-E membrane roughens the surface, and stimulates the negatively charged droplets to bounce, resulting in an effective anti-fouling and self-cleaning effect and a long-term scaling prevention.

SAN-including PVDF membranes with improved permeability for hypersaline water desalination using DCMD process

Styrene-acrylonitrile (SAN) was added to polyvinylidene fluoride (PVDF)/N-methyl-2 pyrrolidone (NMP) system to fabricate SAN-modified PVDF membranes using non-solvent induced phase separation (NIPS). By increasing the SAN concentration in casting solution, number of microbeads started to grow. However, by further increasing the amount of SAN, size of microbeads was also increased. Surface hydrophobicity of neat ethanol-induced composite membrane was increased from 139.2 ± 1 to 148.7° ±1, since SAN polymer incorporation bestowed another texture to whole structure. Different salty feed solutions, i.e., 35 and 105 g/L, were used as feed by direct contact membrane distillation (DCMD) process. Permeate flux and salt rejection were high enough with stable permeation using 35 g/L feed solution. Neat and well-modified PVDF/SAN membranes were then applied for hypersaline DCMD test. During 38 h desalination test using SAN-modified membrane, permeate flux and salt rejection of 29.74 ± 1 kg/m2 h and >99.9 % were achieved, respectively. Also, due to multi-textured structure, permeate flux enhancement and better scaling resistance were observed using SAN/PVDF membrane. A sufficient way was proposed to improve desalination performance using a straightforward fabrication process.

Preparation and characterization of PPO/PS porous membrane for desalination via direct contact membrane distillation (DCMD)

The non-solvent induced phase separation (NIPS) method was used to prepare novel polyphenylene oxide and polystyrene blend (PPO/PS) flat-sheet porous membranes. The performance of prepared samples was studied in a direct contact membrane distillation (DCMD) experimental set-up. The prepared membranes were characterized using field emission scanning electron microscopy (FESEM), attenuated total reflection Fourier transform infrared (ATR-FTIR), atomic force microscopy (AFM), and zeta potential. A number of other properties, such as porosity, contact angle, and liquid entry pressure (LEP), were measured. Permeate flux has also been evaluated at different feed concentrations (0, 3500, and 35000 ppm NaCl) and temperatures (0, 60, and 80 °C). According to the results, by increasing feed temperature from 40 to 80 °C and maintaining permeate temperature constant at 22 °C, the permeate flux was increased from 5.53 to 31.36 kg/m2h. Altering the feed concentration from 0 to 35000 ppm has resulted in a reduction of permeate flux from 31.36 to 27.87 kg/m2h. In all DCMD tests, high rejection was observed (99.9%). Moreover, the anti-fouling and anti-scaling properties of the prepared membrane were investigated. Anti-scaling properties were examined by adding CaCl2 and Na2SO4 to the feed solution; the permeate flux did not change significantly and rejection was maintained above 99.8%. Humic acid (HA) in feed solution led to fouling, resulting in a 20% reduction in flux, but the rejection was maintained above 99.4%. Furthermore, SDS and CTAB surfactants (below CMC) as foulant agents were added to feed and due to the wetting phenomenon, the rejection dropped to 99.11 and 99.28%, respectively, while permeate flux decreased very slightly after 4 h operation.

Performance and economic analysis of a solar membrane distillation pilot plant under various operating conditions

Although membrane distillation (MD) is a promising desalination process, its use is limited because it requires a large amount of thermal energy. To reduce the thermal energy consumption during MD, studies combining various renewable energy sources (e.g., solar, geothermal, and waste heat) are currently being conducted. Therefore, pilot plant experiments combining solar energy with an MD system were conducted. In particular, a module that could be changed into different MD configurations, including direct contact MD (DCMD) and air gap MD (AGMD), was reviewed. The performance of the pilot plant was analyzed for each configuration under various operating conditions and according to onsite weather conditions. Because DCMD can obtain a 43% higher water flux than can AGMD, a long-term DCMD test was conducted to achieve a high water flux. The specific energy consumption (SEC) and gained output ratio (GOR) were compared in the presence and absence of solar energy. A 30% decrease in the SEC and 17% increase in the GOR were observed for the sunny days compared with when no solar energy was used. These results indicate that combining the MD with solar energy can improve its performance during long-term operation. In addition, the cost per unit volume of product water was estimated based on the designed solar MD pilot plant.

Janus membrane prepared via one step depositing coatings onto PVDF/PDMS membrane for simultaneous antiwetting and antifouling in DCMD

A Janus membrane with antiwetting and antifouling properties was prepared for complicated wastewater treatment via direct contact membrane distillation (DCMD) technology. The membrane was fabricated by directly depositing gallic acid (GA) and polyethyleneimine (PEI) onto one side of the PVDF/polydimethylsiloxane (PDMS) blend membrane. The results from different characterization methods demonstrate the successful fabrication of the Janus membrane. The obtained Janus membrane performs excellent antiwetting and antifouling properties because of its asymmetric wettability. When the 36 h DCMD test was carried out using the Janus membrane to treat feed solution containing 1000 mg/L crude oil, 0.3 mM SDS and 3.5 wt% NaCl, a relatively stable flux of 15.7 ± 0.2 Kg·m−2·h−1 was maintained, as well as the very small conductivity of 0.3 ± 0.0 μS/cm in the permeate side was kept. In addition, the as-prepared Janus membrane maintained stable when pH ranged from 1.0 to 11.0. Therefore, this Janus membrane can be a strong candidate in DCMD process to treat wastewater containing complicated contaminants.

Hypersaline drilling mud water treatment using pretreatment-free DCMD process

Membrane distillation (MD) can be applied to treat highly polluted saline wastewaters. This work is an attempt to treat drilling mud water (DMW) directly without any pre-treatment processes. In this regard, commercial polytetrafluoroethylene (PTFE) and nanofibrous styrene-acrylonitrile (SAN) membranes were employed. Nanofibrous SAN membranes were fabricated via the high-throughput electroblowing process. A water contact angle (WCA) of 144.2° was obtained for the hot-pressed SAN membrane, which was higher than that of the PTFE membrane (128.3°). Two treatment cycles were performed for each membrane using direct contact membrane distillation (DCMD) process. In the first cycle, hydrophobic membranes were subjected to the DMW. The removed membranes were washed. Then, for the second cycle, were subjected to the diluted DMW of the first cycle. The distillate water quality showed a declining trend using the PTFE membrane at the end of the second cycle. However, due to higher wetting resistance, distillate water with a better quality and a more stable flux was achieved for the SAN membrane. The DCMD tests were terminated when water recovery reached around 60 %. This work demonstrated that the MD process is a suitable option for treating complex and highly saline wastewaters.

Fouling studies on hydrophobic PVDF-bentonite hollow fiber membrane during membrane distillation of palm oil mill effluent

Membrane distillation (MD) is one of the emerging methods that can be adopted for oily wastewater treatment. However, the problems associated with membrane surface fouling may impede its separation efficiency. In this work, we performed a fouling study using self-synthesized polyvinylidene difluoride (PVDF)-bentonite hollow fiber membrane (HFM) to treat palm oil mill effluent (POME) via direct contact membrane distillation (DCMD). To date, there is no report on POME treatment using MD technology based on PVDF-bentonite HFM. The fouling tendency was identified based on the membrane's permeate flux and pollutant rejection. The fouled membranes after POME treatment were characterized based on its morphology, functional group, water contact angle, surface roughness and mechanical strength. The results showed that the average permeate flux of MD was 3.34 kg/m2·h over 72-h operation. The membrane showed good performance in separating pollutants with >95 % removal rate recorded for chemical oxygen demand, nitrate nitrogen, total suspended solid, total dissolved solid, color and turbidity. However, based on the post-characterization analysis, the used membranes still suffered from fouling after 72-h DCMD test. Thus, an in-depth investigation into the prevention and control of membrane fouling is still necessary for the POME treatment using DCMD.

Dual-layer membranes with a thin film hydrophilic MOF/PVA nanocomposite for enhanced antiwetting property in membrane distillation

Membrane distillation (MD) has been considered as a promising separation technology for desalination due to its ultrahigh rejection towards nonvolatile salts. However, pore wetting is a long-standing problem for the practical deployment of conventional hydrophobic MD membranes. Herein, hydrophilic/hydrophobic dual-layer membrane consisting of a thin hydrophilic, aluminum fumarate (AlFu) metal−organic framework (MOF) doped poly (vinyl alcohol) (PVA) dense layer on top of a hydrophobic microporous polytetrafluoroethylene (PTFE) substrate layer was prepared via a facial solution casting procedure. Adjusted by the nanoporous AlFu MOF, the thin PVA based hydrophilic layer (~800 nm) exhibited increased water vapor flux, reaching 45.1 kg/m2 h while maintaining nearly complete salt rejection (99.9%) in desalting NaCl (35,000 ppm) solution (50 °C and 10 °C in the feed side and permeate side, respectively) via direct contact membrane distillation (DCMD) process. Compared with the PTFE membrane, the dual-layer membranes possessed enhanced wetting resistance towards low-surface-tension solution by exhibiting stable throughput and salt rejection until 65% water recovery whereas the permeate conductivity of PTFE membrane increased substantially to 145 μS/cm at only 20% water recovery. Furthermore, the stability of the dual-layer membrane was assessed using real seawater in a 48 h DCMD test, showing no decline in water vapor flux and salt rejection while the permeate conductivity increased to 100 μS/cm for PTFE membrane. This study demonstrated the feasibility of using mixed matrix material as the top layer for designing high-performance MD membranes with outstanding antiwetting property.

Comparison of calcium scaling in direct contact membrane distillation (DCMD) and nanofiltration (NF)

Calcium scaling is commonly encountered in membrane separation processes with a profound impact on permeate flux and energy efficiency of these processes. Direct contact membrane distillation (DCMD) and nanofiltration (NF) were selected to study calcium scaling impact on thermal and pressure driven membrane processes. Calcium sulfate (CaSO4) and calcium carbonate (CaCO3) solutions with salinity in the range from 3000 to 30,000 mg/L were used as the feed. All DCMD and NF tests were performed at identical feed temperature (i.e., 40 °C) and shear conditions (i.e., Re = 771 ± 28). The average size of CaSO4 and CaCO3 crystals formed in the DCMD system was larger and the scale was more porous than in the NF system. The permeate flux in NF system was impacted by both calcium scales regardless of the feed salinity, while only CaSO4 scale formed at low salinity affected the performance of DCMD system. Surface crystallization in NF systems is likely extended into the membrane pores, which lead to permanent reduction in membrane permeability. This study demonstrated that the impact of calcium scaling is affected by both feed salinity and separation driving force, and is much less severe in thermal than pressure driven membrane processes.

Fabrication of omniphobic PVDF composite membrane with dual-scale hierarchical structure via chemical bonding for robust membrane distillation

In recent years, omniphobic membranes have drawn much attention due to the strong repellence to various liquids. In this study, a PVDF composite membrane with omniphobic characteristics was prepared by constructing dual-scale hierarchical structure and introducing long chemical chains with the low surface energy. The special architecture was derived from the immobilization of TiO2@PDA@Cu composite nanoparticles on the surface via chemical bonding, and the fluorosilanization treatment with 1H,1H,2H,2H-perfluorodecyltriethoxysilane for membrane surfaces. A series of characterizations were implemented to explore composite nanoparticles’ morphology and composition, as well as membrane morphology and surface analysis. In addition, anti-fouling and anti-wetting behaviors of pristine and composite membranes were investigated by successive direct contact membrane distillation (DCMD) experiments, in which different contaminations containing inorganic salts, organic matter, surfactants and mineral oil were chosen as the feed. The results displayed that composite nanoparticles were obtained through the chelating effect of polydopamine (PDA) and the dual-scale structure was realized with the assistance of trimesoyl chloride (TMC). The omniphobic membrane exhibited strong repulsive forces to various liquids, such as water, SDBS solution (0.4 mM), CTAB solution (0.4 mM), tween-20 solution (0.4 mM) and mineral oil emulsion (0.2% v/v) with the contact angles of 168.0 ± 2.0°, 157.1 ± 1.5°, 160.0 ± 1.2°, 158.3 ± 1.5° and 152.5 ± 3.0°, respectively. Furthermore, compared to pristine membrane, the omniphobic membrane presented superior stability, higher salt rejection rate and excellent anti-wetting and anti-fouling performances in the DCMD tests. Thus, the composite membrane with superhydrophobic and superoleophobic features possessed great application potential for high salinity wastewater treatment in membrane distillation.

Cyclic olefin polymer as a novel membrane material for membrane distillation applications

A first attempt was made to prepare cyclic olefin polymer/copolymer (COP/COC) flat-sheet porous membranes by the well-known non-solvent induced phase separation method. In this study, two solvents (chloroform and 1,2,4-trichlorobenzene), different additives (polyvinylpyrrolidone, PVP, polyethylene glycol, PEG400, polyethylene oxide, PEO, and Sorbitan monooleate, Span 80) and coagulants (acetone and 70/30 wt% acetone/water mixture) were employed. The prepared membranes were characterized in terms of the thickness (70–85 μm), porosity (~50–80%), liquid entry pressure (1.16–4.55 bar), water contact angle (~86° - 111°), mean pore size (158–265 nm), mechanical properties (tensile strength: 0.74–5.51 MPa, elongation at break: 3.34%–7.94% and Young's modulus: 29–237 MPa), morphological and topographical characteristics. Short-term direct contact membrane distillation (DCMD) tests showed maximum permeate fluxes of 20 kg m−2h−1 and 15 kg m−2h−1 when using as feed distilled water and 30 g/L sodium chloride aqueous solution, respectively, with a high salt separation factor (99.99%). Long-term DCMD tests of some selected membranes carried out during 24–50 h showed that the membranes prepared with PEG additive exhibited more stable DCMD performance. In general, it was proved that COP can be successfully used as a novel polymer candidate in membrane distillation (MD) applications.

Spent caustic wastewater treatment using direct contact membrane distillation with electroblown styrene-acrylonitrile membrane

Spent caustic is classified as a highly polluted and high-risk hazardous wastewater, which severely threatens the environment. In this work, a novel approach was proposed to treat the neutralized spent caustic wastewater using an electroblown nanofibrous styrene-acrylonitrile (SAN) membrane for direct contact membrane distillation (DCMD). The obtained results revealed that the permeate flux of the SAN membrane was lower compared to the commercial polytetrafluoroethylene (PTFE) membrane owing to the thicker structure. The higher surface hydrophobicity of the SAN membrane limited the intrusion of the feed water into the membrane pores. Therefore, extremely pure permeate water was produced. Stable permeate flux (12.03 kg/m2 h) and rejection factor (99.98%) were achieved for the hot-pressed SAN membrane during 96 h continuous DCMD test. Water contact angle of ˃ 90° was remained for both PTFE and SAN membranes, showing that DCMD performance was considerable even after 96 h operation. Besides, the chemical oxygen demand and sulfide removal factor for the fabricated membrane were 99.2 and 99.99%, respectively. Overall, the neutralization followed by the DCMD process can be considered as a proper alternative for treating the spent caustic wastewater.

Fluorinated silica–modified anti–oil-fouling omniphobic F–SiO2@PES robust membrane for multiple foulants feed in membrane distillation

Direct–contact membrane distillation (DCMD) can be eminent solution for oily wastewater treatment if the membrane provided is slippery and tolerant to low surface tension complex solutions. This study describes preparation of an anti–oil–fouling omniphobic polyethersulfone membrane using fluorinated silica nanoparticles (F–SiO2@PES) combined with perfluorodecyl triethoxysilane and polydimethylsiloxane for application against oil–In–water (o/w) emulsions. Feed solutions consist of different concentrations of oil (hexadecane), different charge surfactants (anionic sodium dodecyl benzenesulfonate, non-ionic Tween 20, and cationic hexadecyltrimethylammonium bromide, and salt (NaCl). The hierarchical re-entrant micro structured surface of the omniphobic F–SiO2@PES membrane and functional groups are confirmed by atomic force microscopy, scanning electron microscopy, and Fourier-transform infrared spectroscopy. The anti–oil-fouling and anti–wetting performance of omniphobic F–SiO2@PES membranes are investigated using contact-angle, sliding angles, DCMD tests with multiple foulants of surfactants. Omniphobic F–SiO2@PES membrane exhibited effective anti–oil–fouling and anti–wetting performance against emulsions as no severe fouling and a conductivity rises were evident regardless of surfactant charge and the concentration of components. Flux reduction and rejection rates for the omniphobic F–SiO2@PES membranes are in a range of 5–15% (only) and >99%, respectively, for various combinations of feed solution components.

Treatment of dye wastewater by direct contact membrane distillation using superhydrophobic nanofibrous high-impact polystyrene membranes

A superhydrophobic and nanofibrous high-impact polystyrene (HIPS) membrane was fabricated using electroblowing technique. The neat HIPS membrane was then subjected to the hot-pressing process to increase mechanical strength, narrowing pore size distribution, and also improving liquid entry pressure of water (LEP). The membrane characteristics (surface morphology, hydrophobicity, LEP, porosity, as well as mechanical properties) and direct contact membrane distillation (DCMD) performance (permeate flux and dye rejection factor) of the fabricated membranes were assessed using synthetic disperse dye solutions (0.4, 0.8 and 1.2 g/L) under constant DCMD condition. HIPS membranes maintained their durability even after operating at the highest concentration. The obtained results demonstrated that the dye removals were higher than 99.8% when different concentrations of dye were used as the feed. The permeate flux decline and surface fouling were increased by increasing the feed concentration. However, the surface contact angle of membranes subjected to DCMD tests was still in the range of hydrophobic surface (˃ 90°). Given these attributes, nanofibrous HIPS membrane can be considered as a potential alternative for membrane distillation in dying water treatment. Furthermore, DCMD can utilize the high temperature of dyeing baths as a viable heat source.

Direct contact membrane distillation for effective concentration of perfluoroalkyl substances – Impact of surface fouling and material stability

Polyfluoroalkyl and perfluoroalkyl substances (PFAS) are ecotoxic amphiphilic compounds containing alkyl-fluorinated chains terminated with weak acid moieties, and hence difficult to be degraded or removed from water sources. Direct contact membrane distillation (DCMD) was used for concentrating and removing of perfluoropentanoic acid (PFPeA) compounds from model contaminated water using commercially available poly (tetrafluoroethylene) (PTFE) membranes. The membranes were characterised for surface morphology, roughness, contact angle and pore size distribution before and after the DCMD test to investigate and evaluate membrane fouling. During the DCMD test performed for 6 h using 10 ppm PFPeA solution, the membrane exhibited progressive increased flux (from 17 to 43 kg m−2 h−1) and decreased PFPeA rejection (from 85 to 58%), as the feed temperature was increased from 50 to 70 °C. Further, the feed/retentate side showed a 1.8, 2.1 and 2.8-fold increase in PFPeA concentration tested at feed temperatures 50, 60, and 70 °C, respectively. The permeate side contained less than 1 ppm of PFPeA revealing that the PFPeA moved across the PTFE membrane during DCMD, which is attributed to progressive surface diffusion over time. This study opens a new route to concentrate and remove amphiphilic molecules, such as PFAS, from source points, relevant to landfill leachates or surface waters. The study also points at gaps in materials science and surface engineering to be tackled to deal with PFAS compounds efficiently.

Preparation of re-entrant and anti-fouling PVDF composite membrane with omniphobicity for membrane distillation

Superhydrophobic and oleophobic membrane applied in membrane distillation was fabricated by constructing a re-entrant composite structure via immobilizing silica nanoparticles (NPs) on the polyvinylidene fluoride (PVDF) membrane. The silica coated membrane with the assistance of polydopamine (PDA) was then fluorosilanized by means of a silane coupling agent 1H,1H,2H,2H-perfluorooctyltrichlorosilane. A series of characterizations were implemented to investigate the effects of modification process on the structure and performance of the PVDF composite membranes. The fouling and wetting behaviors of original and modified membranes were explored by direct contact membrane distillation (DCMD) tests using a mixed solution of inorganic salts, organic matter and surfactant as the feed. The results indicated that silica NPs and fluorinated silane were presented on the PVDF membrane substrate. After surface modification, the liquid entry pressure (LEP) increased by 120%, in addition, the water contact angle (WCA) increased from 127.0° to 169.0°, and the oil contact angle (OCA) increased from 2.8° to 112.1°. The modification process showed no effect on the pore size and pore structure. Furthermore, the composite membrane exhibited excellent chemical stability and prominent mechanical stability. The re-entrant structure resulting from nanoparticles and microspheres and the omniphobic feature were conferred on the composite membrane. The DCMD tests showed that the composite membrane kept a stable permeate flux in the long-term concentration experiment, with the salt rejection rate approximately 100%. What's more, the composite membrane did not suffer from fouling and wetting, and still dry and clean after the test. In comparison, the pristine membrane was seriously polluted during the test, and the permeate flux decreased to 28.3% of the initial value. Overall, the novel modified PVDF composite membrane reported in this work presents outstanding anti-fouling performances.

Preparation of omniphobic PVDF membranes with silica nanoparticles for treating coking wastewater using direct contact membrane distillation: Electrostatic adsorption vs. chemical bonding

In order to reduce the wetting and fouling propensity of polyvinylidene fluoride (PVDF) membrane during coking wastewater treatment by membrane distillation, two kinds of omniphobic PVDF composite membranes were prepared by coating SiO2 nanoparticles on PVDF membrane surface via electrostatic adsorption or chemical bonding respectively, and subsequently fluorination. The coated SiO2 nanoparticles observed as amounts of nano-scale protrusions by scanning electron microscope constructed a re-entrant structure and enhanced membrane surface roughness. The omniphobicity of the modified membranes was confirmed by high contact angles (> 150°) for water, diiodomethane and ethylene glycol on the membrane surface. The omniphobic layer with SiO2 nanoparticles coated via chemical bonding was more stable than electrostatic adsorption. Both the omniphobic PVDF membranes exhibited stable and high rejections of the inorganic and organic contaminants, which were determined by conductivity, total organic carbon and three-dimensional excitation-emission fluorescence spectroscopy of the distillate. The modified omniphobic membranes also showed much less flux decline than the pristine PVDF membrane in direct contact membrane distillation tests (120 h), suggesting their excellent anti-fouling performance. This study indicates that coating silica nanoparticles via chemical bonding is an effective and promising method to fabricate omniphobic PVDF membrane for coking wastewater treatment.

Novel technique for fabrication of electrospun membranes with high hydrophobicity retention

In a bid to mitigate the problem of pore wetting in membrane distillation, a superhydrophobic membrane was prepared by in situ deposition of teflon oligomer microparticles on poly (vinylidenefluoride-co-hexafluoropropylene) nanofibers during electrospinning. This new approach relies on dusting in the chamber where the threads of polymer solution are spun in the zone of fiber transition between liquid to solid. In addition to distributing the particles across the electrospun mat, the novel technique allows bonding of the particles to the solidified fibers. The prepared membrane was characterized and tested in direct contact membrane distillation (DCMD). The results show that the membrane, which was about 30 μm thick, was superhydrophobic and displayed a water contact angle of 155 ± 1°. The membrane also supported a contact angle of 153 ± 1° for a 3 mM SDS solution having lower surface tension (49 mN/m) than water (72 mN/m). The membrane had nominal and maximum pore diameters of 0.57 μm and 1.93 μm respectively, and was about 91% porous. The membrane also supported a liquid entry pressure of about 22 psi. Furthermore, the membrane gave average permeate fluxes of 7 kg m−2 h−1, 10 kg m−2 h−1 when tested in DCMD at 60 °C and 70 °C feed temperatures respectively, with a salt rejection of 99.97%. Finally, the membrane supported a water contact angle of 140 ± 1° after it had been used in DCMD test, while the mean pore diameter, maximum pore diameter and porosity decreased to 0.32 μm, 0.83 μm and 86.5% respectively after test.