Novel Janus composite hollow fiber membrane-based direct contact membrane distillation (DCMD) process for produced water desalination

Enhanced solar-driven evaporation and mineral extraction from hypersaline produced water using low-cost microporous photothermal foam

The beneficial reuse of produced water (PW) holds significant promise to alleviate water scarcity. However, it still suffers major limitations associated with the high cost of treatment due to energy consumption, economics of scale, and the complex physiochemical constituents. PW is a hypersaline (TDS ∼ 250,000 mg/l) oilfield water with bio-species, organic matter, anions, divalent cations, and radioactive elements. A sustainable treatment option is solar-driven floating photothermal evaporation (PTE), a desalination technology implemented for seawater characterized by simpler chemical compositions and low salinity. In this work, the photothermal evaporator for PW was fabricated using low-cost commercially available charcoal polyurethane foam. The engineered macrochannels and structural alterations created unique pathways for salt extraction and evaporation; and ensured hydrodynamic balance between the rates of capillary flow and evaporation. This novel design mitigated flooding or dry out on the evaporating surface and kept the system running steadily while simultaneously harvesting freshwater and valuable salts. The key findings from this work are (a) the development of a novel temperature ratio-based method to determine optimum PTE thickness that results in maximum evaporation and thermal localization, (b) the development of the empirical correlation between the rate of thermal localization, evaporation rate, and PTE thickness. It combines the interplay of convection, evaporative flux, conduction, heat capacitance, and thickness on the thermal response of PTE foam to incident solar flux, and (c) experimental evidence revealing efflorescence and subflorescence salt on the evaporating surface and pore, and (d) enhanced evaporation rate of 118 % or 71.6 kg/day-m2 of clean water from chemically complex hypersaline produced water. These findings are significant for the engineering design and estimation of the performance of a PTE in a solar-driven evaporation system.

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.

Chitosan-based hollow nanofiber membranes with polyvinylpyrrolidone and polyvinyl alcohol for efficient removal and filtration of organic dyes and heavy metals

Due to their large specific surface area and numerous diffusion channels, hollow fibers are widely used in wastewater treatment. In this study, we successfully synthesized a chitosan (CS)/polyvinylpyrrolidone (PVP)/polyvinyl alcohol (PVA) hollow nanofiber membrane (CS/PVP/PVA-HNM) via coaxial electrospinning. This membrane demonstrated remarkable permeability and adsorption separation. Specifically, the CS/PVP/PVA-HNM had a pure water permeability of 4367.02 L·m^-2·h^-1·bar^-1. The hollow electrospun nanofibrous membrane exhibited a continuous interlaced nanofibrous framework structure with the extraordinary advantages of high porosity and high permeability. The rejection ratios of CS/PVP/PVA-HNM for Cu²⁺, Ni²⁺, Cd²⁺, Pb²⁺, malachite green (MG), methylene blue (MB) and crystal violet (CV) were 96.91 %, 95.29 %, 87.50 %, 85.13 %, 88.21 %, 83.91 % and 71.99 %, and the maximum adsorption capacities were 106.72, 97.46, 88.10, 87.81, 53.45, 41.43, and 30.97 mg·g^-1, respectively. This work demonstrates a strategy for the synthesis of hollow nanofibers, which provides a novel concept for the design and fabrication of highly efficient adsorption separation membranes.

New Materials and Phenomena in Membrane Distillation

In recent decades, membrane-based processes have been extensively applied to a wide range of industrial processes, including gas separation, food industry, drug purification, and wastewater treatment. Membrane distillation is a thermally driven separation process, in which only vapour molecules transfer through a microporous hydrophobic membrane. At the operational level, the performance of membrane distillation is negatively affected by wetting and temperature polarization phenomena. In order to overcome these issues, advanced membranes have been developed in recent years. This review, which focuses specifically on membrane distillation presents the basic concepts associated with the mass and heat transfer through hydrophobic membranes, membrane properties, and advances in membrane materials. Photothermal materials for solar-driven membrane distillation applications are also presented and discussed.

Simulation and Experimental Investigation of the Vacuum-Enhanced Direct Membrane Distillation Driven by a Low-Grade Heat Source

Vacuum-enhanced direct contact membrane distillation (VEDCMD) has been proven experimentally to improve the permeate flux, compared with direct contact membrane distillation (DCMD). However, the theoretical mechanism for its transmembrane transfer process has not been revealed sufficiently. In this paper, with full consideration of the different driving forces of diffusion and Poiseuille flow under the vacuum enhancing condition, a theoretical transmembrane model for mass and heat transfer in VEDCMD is proposed. The CFD model and experimental platform are established to verify the theoretical model. The simulated results agree with the experimental data well, and nearly 200% improvement of the permeate flux is obtained when the permeate pressure drops to 30 kPa. The flow fields of the flow along the membrane surface are obtained and analyzed, with good consistency in the variation of the permeate flux. Since all the parameters of the proposed model are independent of the operating condition, the model is much easier for use and has better adaptability to fluctuating operating conditions.

Oilfield-produced water treatment using conventional and membrane-based technologies for beneficial reuse: A critical review

Oilfield produced water (OPW) is one of the most important by-products, resulting from oil and gas exploration. The water contains a complex mixture of organic and inorganic compounds such as grease, dissolved salt, heavy metals as well as dissolved and dispersed oils, which can be toxic to the environment and public health. This article critically reviews the complex properties of OPW and various technologies for its treatment. They include the physico-chemical treatment process, biological treatment process, and physical treatment process. Their technological strengths and bottlenecks as well as strategies to mitigate their bottlenecks are elaborated. A particular focus is placed on membrane technologies. Finally, further research direction, challenges, and perspectives of treatment technologies for OPW are discussed. It is conclusively evident from 262 published studies (1965-2021) that no single treatment method is highly effective for OPW treatment as a stand-alone process however, conventional membrane-based technologies are frequently used for the treatment of OPW with the ultrafiltration (UF) process being the most used for oil rejection form OPW and oily waste water. After membrane treatment, treated effluents of the OPW could be reused for irrigation, habitant and wildlife watering, microalgae production, and livestock watering. Overall, this implies that target pollutants in the OPW samples could be removed efficiently for subsequent use, despite its complex properties. In general, it is however important to note that feed quality, desired quality of effluent, cost-effectiveness, simplicity of process are key determinants in choosing the most suitable treatment process for OPW treatment.

An environmentally friendly fluorine-free sandwich coating based on a nonwoven fabric for efficient unidirectional water transport

Medical research reveals that keeping the skin surface dry can effectively prevent a variety of fungal skin diseases. Therefore, it is of great practical significance to develop a light, thin, comfortable and breathable functional garment fabric with efficient one-way water transmission for people's requirements. Here, we successfully prepared a three-layer stacking sandwich coating to construct a Janus nonwoven material with efficient unidirectional water transport function, viz. PDMS-ormosil-PDMS@nonwoven fabric (PDMS-ormosil-PDMS@NF). Theoretical analysis showed that the loading of ormosil particles could enhance the surface roughness, while PDMS could strengthen the hydrophobic properties and bondability between the ormosil particles and fabric, thus acting as an environmentally friendly fluorine-free treating agent. The results revealed that the hydrophilic-hydrophobic PDMS-ormosil-PDMS@nonwoven fabric maintained excellent hydrophobic stability and good air permeability even after abrasion, which solved the existing problems of aerogel poor deposition and PDMS poor permeability, benefiting the application of textiles with unidirectional water transport performance.

Janus membranes for membrane distillation: Recent advances and challenges

Membrane distillation (MD) is a promising hybrid thermal-membrane separation technology that can efficiently produce freshwater from seawater or contaminated wastewater. However, the relatively low flux and the presence of fouling or wetting agents in feed solution negate the applicability of MD for long term operation. In recent years, 'two-faced' membranes or Janus membranes have shown promising potential to decrease wetting and fouling problem of common MD system as well as enhance the flux performance. In this review, a comprehensive study was performed to investigate the various fabrication, modification, and novel design processes to prepare Janus membranes and discuss their performance in desalination and wastewater treatment utilizing MD. The promising potential, challenges and future prospects relating to the design and use of Janus membranes for MD are also tackled in this review.

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

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.

Progress on the Fabrication and Application of Electrospun Nanofiber Composites

Nanofibers are one of the most attractive materials in various applications due to their unique properties and promising characteristics for the next generation of materials in the fields of energy, environment, and health. Among the many fabrication methods, electrospinning is one of the most efficient technologies which has brought about remarkable progress in the fabrication of nanofibers with high surface area, high aspect ratio, and porosity features. However, neat nanofibers generally have low mechanical strength, thermal instability, and limited functionalities. Therefore, composite and modified structures of electrospun nanofibers have been developed to improve the advantages of nanofibers and overcome their drawbacks. The combination of electrospinning technology and high-quality nanomaterials via materials science advances as well as new modification techniques have led to the fabrication of composite and modified nanofibers with desired properties for different applications. In this review, we present the recent progress on the fabrication and applications of electrospun nanofiber composites to sketch a progress line for advancements in various categories. Firstly, the different methods for fabrication of composite and modified nanofibers have been investigated. Then, the current innovations of composite nanofibers in environmental, healthcare, and energy fields have been described, and the improvements in each field are explained in detail. The continued growth of composite and modified nanofiber technology reveals its versatile properties that offer alternatives for many of current industrial and domestic issues and applications.