Novel PVDF membranes comprising n-butylamine functionalized graphene oxide for direct contact membrane distillation

In Situ-Grown Al2O3 Nanoflowers and Hydrophobic Modification Enable Superhydrophobic SiC Ceramic Membranes for Membrane Distillation

Membrane distillation (MD) is considered a promising technology for desalination. In the MD process, membrane pores are easily contaminated and wetted, which will degrade the permeate flux and salt rejection of the membrane. In this work, SiC ceramic membranes were used as the supports, and an Al2O3 micro-nano structure was constructed on its surface. The surface energy of Al2O3@SiC micro-nano composite membranes was reduced by organosilane grafting modification. The effective deposition of Al2O3 nanoflowers on the membrane surface increased membrane roughness and enhanced the anti-fouling and anti-wetting properties of the membranes. Simultaneously, the presence of nanoflowers also regulated the pore structures and thus decreased the membrane pore size. In addition, the effects of Al2(SO4)3 concentration and sintering temperature on the surface morphology and performance of the membranes were investigated in detail. It was demonstrated that the water contact angle of the resulting membrane was 152.4°, which was higher than that of the pristine membrane (138.8°). In the treatment of saline water containing 35 g/L of NaCl, the permeate flux was about 11.1 kg⋅m-2⋅h-1 and the salt rejection was above 99.9%. Note that the pristine ceramic membrane cannot be employed for MD due to its larger membrane pore size. This work provides a new method for preparing superhydrophobic ceramic membranes for MD.

Performance investigation of poly(vinylidene fluoride-cohexafluoropropylene) membranes containing SiO2 nanoparticles in a newly designed single vacuum membrane distillation system

The current study focuses on the development of a superhydrophobic poly(vinylidene fluoride-cohexafluoropropylene) nanocomposite membrane suitable for vacuum membrane distillation by incorporating SiO2 nanoparticles. At loading hydrophobic nano-SiO2 particle concentration (0.50-1.50 wt.%), the developed nanocomposite membranes are optimized in terms of vacuum membrane distillation performance. The influence of temperature, vacuum pressure, and feed water flow is studied for desalinating high-salinity brine. The results show that the developed vacuum distillation membrane is capable of 95% salt rejection during the treatment of a highly saline feed (65,000 ppm) at fixed flow rates of 120 L/h saline feed and different operating conditions consisting of feed inlet temperatures ranging from 40°C to 70°C and distillate inlet temperatures of 7-15°C. The vacuum membrane distillation process achieves 0.38-1.66% water recovery with increasing concentration factor, meaning that recovery is increased, and shows a specific electrical energy consumption of 5.16-23.90 kWh/m3 for product water. Overall, the newly designed membrane demonstrates suitability for a vacuum membrane distillation system. PRACTITIONER POINTS: Desalinate high-salinity brine (TDS > 35,000 ppm) using a vacuum membrane distillation system. A hydrophobic PVDF-HFP/SiO2 nanocomposite membrane development for vacuum membrane distillation. A newly designed single vacuum membrane distillation system for RO brine treatment.

Mineral scaling induced membrane wetting in membrane distillation for water treatment: Fundamental mechanism and mitigation strategies

The scaling-induced wetting phenomenon seriously affects the application of membrane distillation (MD) technology in hypersaline wastewater treatment. Unlike the large amount of researches on membrane scaling and membrane wetting, scaling-induced wetting is not sufficiently studied. In this work, the current research evolvement of scaling-induced wetting in MD was systematically summarized. Firstly, the theories involving scaling-induced wetting were discussed, including evaluation of scaling potential of specific solutions, classical and non-classical crystal nucleation and growth theories, observation and evolution of scaling-induced processes. Secondly, the primary pretreatment methods for alleviating scaling-induced wetting were discussed in detail, focusing on adding agents composed of coagulation, precipitation, oxidation, adsorption and scale inhibitors, filtration including granular filtration, membrane filtration and mesh filtration and application of external fields including sound, light, heat, electromagnetism, magnetism and aeration. Then, the roles of operation conditions and cleaning conditions in alleviating scaling-induced wetting were evaluated. The main operation parameters included temperature, flow rate, pressure, ultrasound, vibration and aeration, while different types of cleaning reagents, cleaning frequency and a series of assisted cleaning measures were summarized. Finally, the challenges and future needs in the application of nucleation theory to scaling-induced wetting, the speculation, monitoring and mitigation of scaling-induced wetting were proposed.

Innovative Membrane Technologies for the Treatment of Wastewater Polluted with Heavy Metals: Perspective of the Potential of Electrodialysis, Membrane Distillation, and Forward Osmosis from a Bibliometric Analysis

A bibliometric analysis, using the Scopus database as a source, was carried out in order to study the scientific documents published up to 2021 regarding the use of electrodialysis, membrane distillation, and forward osmosis for the removal of heavy metals from wastewater. A total of 362 documents that fulfilled the search criteria were found, and the results from the corresponding analysis revealed that the number of documents greatly increased after the year 2010, although the first document was published in 1956. The exponential evolution of the scientific production related to these innovative membrane technologies confirmed an increasing interest from the scientific community. The most prolific country was Denmark, which contributed 19.3% of the published documents, followed by the two main current scientific superpowers: China and the USA (with 17.4% and 7.5% contributions, respectively). Environmental Science was the most common subject (55.0% of contributions), followed by Chemical Engineering (37.3% of contributions) and Chemistry (36.5% of contribution). The prevalence of electrodialysis over the other two technologies was clear in terms of relative frequency of the keywords. An analysis of the main hot topics identified the main advantages and drawbacks of each technology, and revealed that examples of their successful implementation beyond the lab scale are still scarce. Therefore, complete techno-economic evaluation of the treatment of wastewater polluted with heavy metals via these innovative membrane technologies must be encouraged.

Preparation and Characterization of PVDF-TiO2 Mixed-Matrix Membrane with PVP and PEG as Pore-Forming Agents for BSA Rejection

Polymeric membranes offer straightforward modification methods that make industry scaling affordable and easy; however, these materials are hydrophobic, prone to fouling, and vulnerable to extreme operating conditions. Various attempts were made in this study to fix the challenges in using polymeric membranes and create mixed-matrix membrane (MMMs) with improved properties and hydrophilicity by adding titanium dioxide (TiO₂) and pore-forming agents to hydrophobic polyvinylidene fluoride (PVDF). The PVDF mixed-matrix ultrafiltration membranes in this study were made using the non-solvent phase inversion approach which is a simple and effective method for increasing the hydrophilic nature of membranes. Polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) as pore-forming chemicals were created. Pure water flux, BSA flux, and BSA rejection were calculated to evaluate the mixed-matrix membrane's efficiency. Bovine serum albumin (BSA) solution was employed in this study to examine the protein rejection ability. Increases in hydrophilicity, viscosity, and flux in pure water and BSA solution were achieved using PVP and PEG additives. The PVDF membrane's hydrophilicity was raised with the addition of TiO₂, showing an increased contact angle to 71°. The results show that the PVDF-PVP-TiO₂ membrane achieved its optimum water flux of 97 L/(m²h) while the PVDF-PEG-TiO₂ membrane rejected BSA at a rate greater than 97%. The findings demonstrate that use of a support or additive improved filtration performance compared to a pristine polymeric membrane by increasing its hydrophilicity.

Fluoropolymer Membranes for Membrane Distillation and Membrane Crystallization

Fluoropolymer membranes are applied in membrane operations such as membrane distillation and membrane crystallization where hydrophobic porous membranes act as a physical barrier separating two phases. Due to their hydrophobic nature, only gaseous molecules are allowed to pass through the membrane and are collected on the permeate side, while the aqueous solution cannot penetrate. However, these two processes suffer problems such as membrane wetting, fouling or scaling. Membrane wetting is a common and undesired phenomenon, which is caused by the loss of hydrophobicity of the porous membrane employed. This greatly affects the mass transfer efficiency and separation efficiency. Simultaneously, membrane fouling occurs, along with membrane wetting and scaling, which greatly reduces the lifespan of the membranes. Therefore, strategies to improve the hydrophobicity of membranes have been widely investigated by researchers. In this direction, hydrophobic fluoropolymer membrane materials are employed more and more for membrane distillation and membrane crystallization thanks to their high chemical and thermal resistance. This paper summarizes different preparation methods of these fluoropolymer membrane, such as non-solvent-induced phase separation (NIPS), thermally-induced phase separation (TIPS), vapor-induced phase separation (VIPS), etc. Hydrophobic modification methods, including surface coating, surface grafting and blending, etc., are also introduced. Moreover, the research advances on the application of less toxic solvents for preparing these membranes are herein reviewed. This review aims to provide guidance to researchers for their future membrane development in membrane distillation and membrane crystallization, using fluoropolymer materials.

A Zero-Brine Discharge Seawater Desalination Using a Pilot-Scale Membrane Distillation System Integrated with Crystallizer

The management of brines generated from reverse osmosis operation remains a critical challenge requiring new approaches and processes to limit the impact of brine discharge onto ecosystems and to enhance both water and valuable resource recovery. The treatment of real seawater reverse osmosis (SWRO) brines (45,000 ppm TDS) obtained from a local Singaporean desalination plant with a crystallizer integrated pilot-scale membrane distillation unit (MDC) was studied. Commercial STOMATE® hollow fiber membranes were used in vacuum membrane distillation (VMD) configuration, leading to an average flux of around 3.7 L/m2-h at a permeate vacuum of 80 kPa and an average feed temperature of 65 °C. Consistent separation operations were achieved for the treatment of real SWRO brine over a period of 280 h; this led to a water recovery of >95% and to the collection of salt slurries, containing up to ~10−20 wt% of moisture, from the crystallizer. This approach demonstrates the potential of MDC systems to achieve zero brine discharge efficiently from seawater desalination systems, providing an environmentally friendly alternative to manage brines by increasing water recovery and generating salt slurries of economic value.

Spectral and Structural Properties of High-Quality Reduced Graphene Oxide Produced via a Simple Approach Using Tetraethylenepentamine

A simple temperature-assisted solution interaction technique was used to functionalize and reduce graphene oxide (GO) using tetraethylenepentamine (TEPA) with less chemicals, low temperature, and without using other reducing agents. GO nanosheets, produced using a modified Hummers’ method, were functionalized using two different GO:TEPA ratios (1:5 and 1:10). The reduction of GO was evaluated and confirmed by different spectroscopic and microscopic techniques. The FTIR and XPS spectra revealed that most of the oxygenated groups of GO were reduced. The emergence of amide groups in the XPS survey of the rGO-TEPA samples confirmed the successful reaction of TEPA with the carboxyl groups on the edges of GO. The replacement of the oxygenated groups increased the carbon/oxygen (C/O) ratio of GO by approximately 60%, suggesting a good reduction degree. It was found that the I_2D/I_D+D′ ratio and the relative intensity of the D″ band clearly increased after the reduction reaction, suggesting that these bands are good estimators for the reduction degree of GO. The morphological structure of GO was also affected by the reaction with TEPA, which was confirmed by SEM and TEM images. The TEM images showed that the transparent GO sheets became denser and opaque after functionalization with TEPA, indicating an increase in the stacking level of the GO sheets. This was further confirmed by the XRD analysis, which showed a clear decrease in the d-spacing, caused by the removal of oxygenated groups during the reduction reaction.

Free-standing hybrid film for separation of dye pollutant with self-cleaning ability under visible light

The development of low cost and environmental-friendly materials has long been an ambition for effective removal of dye pollutants in complex water environments. In this study, a free-standing separation film of bacterial cellulose reinforced/functionalized by graphitic phase carbon nitride is developed by a facile suction filtration strategy, of which the former is precoated by polypyrrole, and the latter is pre-doped by oxygen to endow the as-obtained film an enhanced photocatalytic performance and self-cleaning ability. The as-obtained film exhibits a high tensile stress of 51.8 ± 1.1 MPa, and a high resistance to cold, heat, acid and alkali. For typical dyes of methylene blue and rhodamine B, a high dye rejection rate of 99.9% at 138 L/m²•h•bar feed flux is obtained by the as-obtained film. Even at a salt concentration higher than 5%, it still maintained high dye removal rates and achieves effective separation of dye and salt. Simultaneously, a high dye photocatalytic degradation of the composite films rates up to 98% in only 90 min, and a high self-cleaning ability demonstrated by recovery of flux after light treatment in cyclic tests. The density functional theory calculation validates the beneficial effects of improved light response range and separated photogenerated electron/holes for the effective degradation of dyes by oxygen-doped carbon nitride coupled with one-dimensional polypyrrole chains. Overall, this study proposes a new direction for the separation of dye pollutants with a high visible-light self-cleaning capacity by structural tailoring of bacterial cellulose with carbon nitride.

Robust ultrathin nanoporous MOF membrane with intra-crystalline defects for fast water transport

Rational design of high-performance stable metal–organic framework (MOF) membranes is challenging, especially for the sustainable treatment of hypersaline waters to address critical global environmental issues. Herein, a molecular-level intra-crystalline defect strategy combined with a selective layer thinning protocol is proposed to fabricate robust ultrathin missing-linker UiO-66 (ML-UiO-66) membrane to enable fast water permeation. Besides almost complete salt rejection, high and stable water flux is achieved even under long-term pervaporation operation in hash environments, which effectively addresses challenging stability issues. Then, detailed structural characterizations are employed to identify the type, chemical functionality, and density of intra-crystalline missing-linker defects. Moreover, molecular dynamics simulations shed light on the positive atomistic role of these defects, which are responsible for substantially enhancing structural hydrophilicity and enlarging pore window, consequently allowing ultra-fast water transport via a lower-energy-barrier pathway across three-dimensional sub-nanochannels during pervaporation. Unlike common unfavorable defect effects, the present positive intra-crystalline defect engineering concept at the molecular level is expected to pave a promising way toward not only rational design of next-generation MOF membranes with enhanced permeation performance, but additional water treatment applications.

The development of highly water-permeable membranes is key for the treatment of high salinity waters. Here the authors enhance the water permeability of a metal-organic framework nanoporous membrane via an intra-crystalline defect engineering strategy.

Functional GO-based membranes for water treatment and desalination: Fabrication methods, performance and advantages. A review

Graphene oxide (GO) and GO-based materials have gained a significant interest in the membrane synthesis and functionalization sector in the recent years. Inspired by their unique and tuneable properties, several GO-based nanomaterials have been investigated and utilized as effective nanofillers for various membranes in the water treatment, purification and desalination sectors. This paper comprehensively reviews the recent advances of GO utilization in pressure, concentration and thermal-driven membrane processes. A brief overview on GO particles, properties, synthesis and functionalization methods was provided. The conventional and the state-of-art fabrication methods of GO-based membranes were summarized and discussed, and consequently the GO-based membranes were classified into different categories. The applications, types, and the performance in terms of flux and rejection were summarized and reviewed. The advantages of GO-based membranes in terms of antifouling properties, bactericidal effects, mechanical strength and stability have been reviewed, too. The review gives insights on the future perspectives of GO functional materials and their potential use in the various membrane processes discussed herein.

Preparation of perovskite CsPb(Br x I1-x )3 quantum dots at room temperature

Here, we propose a method for preparing red perovskite CsPb(Br_xI_1−x)₃ quantum dots (QDs) at room temperature. The PL emission peak of the QDs is close to 650 nm, and the full width at half maximum reaches 61 nm. The red CsPb(Br_xI_1−x)₃ QDs with higher water stability are obtained successfully at room temperature by adjusting the proportion of halogen elements and incorporating polystyrene. At the same time, the red QDs are combined with a blue light-emitting diode to prepare a white light-emitting device (WLED). Compared with the WLED based on the Ce-doped Yttrium Aluminum Garnet (YAG:Ce) phosphor alone, the new WLED based on QDs exhibits a reduced correlated color temperature of 2976 K, while achieving a lumen efficiency of 55.3 lm W⁻¹, which provides a feasible solution for future research on light-emitting diodes (LEDs).

Here, we propose a method for preparing red perovskite CsPb(Br_xI_1−x)3 quantum dots (QDs) at room temperature and successfully applied to WLEDs.

Advances in seawater membrane distillation (SWMD) towards stand-alone zero liquid discharge (ZLD) desalination

Seawater membrane distillation (SWMD) is a promising separation technology due to its ability to operate as a stand-alone desalination unit operation. This paper reviews approaches to improve laboratory-to-pilot-scale MD performance, which comprise operational strategies, module design, and specifically tailored membranes. A detailed comparison of SWMD and sea water reverse osmosis is presented to further analyze the critical shortcomings of SWMD. The unique features of SWMD, namely the ability to operate with extremely high salt rejection and at extreme feed concentration, highlight the SWMD potential to be operated under zero liquid discharge (ZLD) conditions, which results in the production of high-purity water and simultaneous salt recovery, as well as the elimination of the brine disposal cost. However, technical challenges, such as thermal energy requirements, inefficient heat transfer and integration, low water recovery factors, and lack of studies on real-case valuable-salt recovery, are impeding the commercialization of ZLD SWMD. This review highlights the possibility of applying selected strategies to push forward ZLD SWMD commercialization. Suggestions are projected to include intermittent removal of valuable salts, in-depth study on the robustness of novel membranes, module and configuration, utilization of a low-cost heat exchanger, and capital cost reduction in a renewable-energy-integrated SWMD plant.

Studies on the electrostatic effects of stretched PVDF films and nanofibers

The electroactive β-phase in Poly (vinylidene fluoride, PVDF) is the most desirable conformation due to its highest pyro- and piezoelectric properties, which make it feasible to be used as flexible sensors, wearable electronics, and energy harvesters etc. In this study, we successfully developed a method to obtain high-content β-phase PVDF films and nanofiber meshes by mechanical stretching and electric spinning. The phase transition process and pyro- and piezoelectric effects of stretched films and nanofiber meshes were characterized by monitoring the polarized light microscopy (PLM) images, outputting currents and open-circuit voltages respectively, which were proved to be closely related to stretching ratio (λ) and concentrations. This study could expand a new route for the easy fabrication and wide application of PVDF films or fibers in wearable electronics, sensors, and energy harvesting devices.

Polyvinylidene fluoride nanocomposite super hydrophilic membrane integrated with Polyaniline-Graphene oxide nano fillers for treatment of textile effluents

Water pollution from the fashion industries containing dyes has become a major source of water pollution. These anthropogenic contaminated waters directly enter irrigation and drinking water systems, causing irreversible environmental damage to human health. Nanomembrane technology has attracted extensive attention to remove these toxic chemicals but new approaches are still required for improving removal efficiency and control the channel size. The work deals with the fabrication of a novel hybrid polyvinylidene fluoride (PVDF)-polyaniline (PANI) membrane with graphene oxide (GO). Incorporation of PANI-GO as a nanofiller has significantly improved antifouling properties and a solvent content of the fabricated membrane. Besides, pure water flux also increases from 112 to 454 L m^-2 h^-1 indicating the hydrophilic nature of the nanocomposite membrane. Among various compositions, the nanocomposites membrane with 0.1 %w/v GO demonstrated a maximum of 98 % dye rejection at 0.1 MPa operating pressure. After multiple testing of the membrane, the flux recovery ratio reached about 94 % and dyes rejection improved with the addition of PANI-GO. The removal efficiency of the composite membrane for Allura red is 98 % and for methyl orange is 95 %. Based on the above results the PVDF/PANI/GO membranes are recommended for practical use in wastewater treatment, particularly for anionic dyes removal from textile effluents.

Polydopamine Functionalized Graphene Oxide as Membrane Nanofiller: Spectral and Structural Studies

High-degree functionalization of graphene oxide (GO) nanoparticles (NPs) using polydopamine (PDA) was conducted to produce polydopamine functionalized graphene oxide nanoparticles (GO-PDA NPs). Aiming to explore their potential use as nanofiller in membrane separation processes, the spectral and structural properties of GO-PDA NPs were comprehensively analyzed. GO NPs were first prepared by the oxidation of graphite via a modified Hummers method. The obtained GO NPs were then functionalized with PDA using a GO:PDA ratio of 1:2 to obtain highly aminated GO NPs. The structural change was evaluated using XRD, FTIR-UATR, Raman spectroscopy, SEM and TEM. Several bands have emerged in the FTIR spectra of GO-PDA attributed to the amine groups of PDA confirming the high functionalization degree of GO NPs. Raman spectra and XRD patterns showed different crystalline structures and defects and higher interlayer spacing of GO-PDA. The change in elemental compositions was confirmed by XPS and CHNSO elemental analysis and showed an emerging N 1s core-level in the GO-PDA survey spectra corresponding to the amine groups of PDA. GO-PDA NPs showed better dispersibility in polar and nonpolar solvents expanding their potential utilization for different purposes. Furthermore, GO and GO-PDA-coated membranes were prepared via pressure-assisted self-assembly technique (PAS) using low concentrations of NPs (1 wt. %). Contact angle measurements showed excellent hydrophilic properties of GO-PDA with an average contact angle of (27.8°).

Dye synthetic solution treatment by direct contact membrane distillation using commercial membranes

The reuse of treated dyeing wastewater has become a viable option to minimizing water scarcity problems and environmental impacts in the textile industry. The potentiality of commercial flat sheet membranes of polytetrafluoroethylene (PTFE) and polypropylene (PP) in direct contact membrane distillation (DCMD) for dye synthetic solution treatment has been explored in this work. DCMD is interesting for the textile industry since a recovery of heat by hot dyeing wastewater for thermal energy is possible. Moreover, DCMD enables water and dye reclamation with possible reuse in the textile process. The commercial availability of membranes may expedite the DCMD commercialization in the textile industry. Experiments were conducted in a laboratory-scale circulating unit with synthetic solutions containing reactive or disperse dye. High mean permeate flux up to 18.8 kg m^-2 h^-1 with complete colour rejection was obtained. The dyes tested in this study are not able to completely wet the membranes and the increase of the permeate flux when compared to distilled water is attributed to electrostatic interactions between the dyes and the membranes. Moreover, a partial wetting reduced vapour diffusion path and the permeate flux was increased. PP membrane showed higher performance due to higher porosity when compared to the PTFE membrane. In addition, an influence of dye class on permeability was observed. The results were promising when compared to other studies, which used commercial or lab-scale membranes.

Macrovoid-Inhibited PVDF Hollow Fiber Membranes via Spinning Process Delay for Direct Contact Membrane Distillation

A polyvinylidene fluoride (PVDF) hollow fiber membrane was fabricated through water-induced dope crystallization by allowing a facile spinning process delay (SPD) in the nonsolvent-induced phase separation (NIPS) process for direct contact membrane distillation (DCMD). The SPD was achieved by the addition of a small amount of water to the PVDF dope solution that was held in a closed container for a particular time. The crystalline property of the PVDF dope solution was investigated by differential scanning calorimetry. The obtained PVDF hollow fiber membranes were characterized with different techniques, including field emission scanning electron microscopy, X-ray diffraction, and the mechanical strength. Both the formation mechanism and properties were studied for the membranes with different SPD times. The results showed that macrovoid-inhibited PVDF membranes were obtained from 12 days of the SPD via the crystallization-dominated membrane formation process. The obtained membrane 4D-12 exhibited desirable membrane structure and properties for DCMD, which includes an improved liquid entry pressure of 2.25 bar, a surface water contact angle of 129°, a maximum pore size of 0.40 μm, and a mean pore size of 0.34 μm. The membrane 4D-12 possessed a twofold increase in both energy efficiency and permeate water flux in DCMD and stable permeate water flux and salt rejection through 224 h of continuous desalination operation. Compared to the commonly used approach by adding chemicals to the external coagulant, the SPD method provided a low-cost and environmentally friendly alternative to pursuing the macrovoid-free PVDF membranes for DCMD.

PFOM fillers embedded PVDF/cellulose dual-layered membranes with hydrophobic-hydrophilic channels for desalination via direct contact membrane distillation process

In this research work, novel perfluorooctanoic acid-modified melamine (PFOM) was synthesized as a hydrophobic filler using a facile one-pot synthesis. PFOM incorporating polyvinylidene fluoride (PVDF) solution was cast on a cellulose sheet to prepare a dual-layered membrane employing the phase-inversion technique for direct contact membrane distillation (DCMD) application. The influence of PFOM to tailor the dual-layered membrane performance was then investigated. The long perfluoro chain in PFOM hydrophobic fillers increased the surface roughness of the membranes due to its random overlapping with PVDF backbone, and these membranes exhibited a higher water contact angle value. The increase in pore size and membrane porosity did not significantly influence the liquid entry pressure of water (LEPw). The microporous membranes displayed good mechanical strength for use in the test setup. Pure water permeation was the highest (6.9 kg m-2 h-1) for membrane (M1) with 1 wt% of PFOM when tested with a simulated sea-water solution (3.5% w/v NaCl) in the direct contact distillation mode. These membranes also achieved the theoretical salt-rejection of 99.9%, thus confirming the potential of these membranes to be investigated for large scale membrane distillation (MD) applications like desalination of seawater.

Concentration of 1,3-dimethyl-2-imidazolidinone in Aqueous Solutions by Sweeping Gas Membrane Distillation: From Bench to Industrial Scale

Sweeping gas membrane distillation (SGMD) is a useful option for dehydration of aqueous solvent solutions. This study investigated the technical viability and competitiveness of the use of SGMD to concentrate aqueous solutions of 1,3-dimethyl-2-imidazolidinone (DMI), a dipolar aprotic solvent. The concentration from 30% to 50% of aqueous DMI solutions was attained in a bench installation with Liqui-Cel SuperPhobic^® hollow-fiber membranes. The selected membranes resulted in low vapor flux (below 0.15 kg/h·m²) but were also effective for minimization of DMI losses through the membranes, since these losses were maintained below 1% of the evaporated water flux. This fact implied that more than 99.2% of the DMI fed to the system was recovered in the produced concentrated solution. The influence of temperature and flowrate of the feed and sweep gas streams was analyzed to develop simple empirical models that represented the vapor permeation and DMI losses through the hollow-fiber membranes. The proposed models were successfully applied to the scaling-up of the process with a preliminary multi-objective optimization of the process based on the simultaneous minimization of the total membrane area, the heat requirement and the air consumption. Maximal feed temperature and air flowrate (and the corresponding high operation costs) were optimal conditions, but the excessive membrane area required implied an uncompetitive alternative for direct industrial application.

Wetting phenomena in membrane distillation: Mechanisms, reversal, and prevention

Membrane distillation (MD) is a rapidly emerging water treatment technology; however, membrane pore wetting is a primary barrier to widespread industrial use of MD. The primary causes of membrane wetting are exceedance of liquid entry pressure and membrane fouling. Developments in membrane design and the use of pretreatment have provided significant advancement toward wetting prevention in membrane distillation, but further progress is needed. In this study, a broad review is carried out on wetting incidence in membrane distillation processes. Based on this perspective, the study describes the wetting mechanisms, wetting causes, and wetting detection methods, as well as hydrophobicity measurements of MD membranes. This review discusses current understanding and areas for future investigation on the influence of operating conditions, MD configuration, and membrane non-wettability characteristics on wetting phenomena. Additionally, the review highlights mathematical wetting models and several approaches to wetting control, such as membrane fabrication and modification, as well as techniques for membrane restoration in MD. The literature shows that inorganic scaling and organic fouling are the main causes of membrane wetting. The regeneration of wetting MD membranes is found to be challenging and the obtained results are usually not favorable. Several pretreatment processes are found to inhibit membrane wetting by removing the wetting agents from the feed solution. Various advanced membrane designs are considered to bring membrane surface non-wettability to the states of superhydrophobicity and superomniphobicity; however, these methods commonly demand complex fabrication processes or high-specialized equipment. Recharging air in the feed to maintain protective air layers on the membrane surface has proven to be very effective to prevent wetting, but such techniques are immature and in need of significant research on design, optimization, and pilot-scale studies.

Ultrastable Quantum Dot Composite Films under Severe Environments

Semiconductor quantum dots (QDs) have attracted extensive attention because of their remarkable optical and electrical characteristics. However, the practical application of QDs and further the QD composite films have greatly been hindered mainly owing to their essential drawbacks of extreme unstability under oxygen and water environments. Herein, one simple method has been employed to enhance enormously the stability of Cd _xZn_{1- x}Se _yS_{1- y} QD composite films by a combination of Cd _xZn_{1- x}Se _yS_{1- y} QDs and poly(vinylidene) fluoride (PVDF), which is characteristic of closely arranged molecular chains and strong hydrogen bonds. There are many particular advantages in using QD/PVDF composite films such as easy processing, low cost, large-area fabrication, and especially extreme stability even in the boiling water for more than 240 min. By employing K₂SiF₆:Mn⁴⁺ as a red phosphor, a prototype white light-emitting diode (WLED) with color coordinates of (0.3307, 0.3387), T_c of 5568 K, and color gamut 112.1NTSC(1931)% at 20 mA has been fabricated, and there is little variation under different excitation currents, indicating that the QD/PVDF composite films fabricated by this simple blade-coating process make them ideal candidates for liquid-crystal display backlight utilization via assembling a WLED on a large scale owing to its ultrahigh stability under severe environments.