Water desalination using graphene-enhanced electrospun nanofiber membrane via air gap membrane distillation

Diffusion behavior of humic acid during desalination with air gap and water gap membrane distillation

Desalination and water reuse are important means to resolve local water scarcity and security issues worldwide where membrane distillation (MD) may be part of a solution. Natural organic matter and in particular, humic acids (HA), are widely present in water supplies to be treated but exhibit little understood behavior to diffuse through MD membranes into permeate. In this work, air gap (AGMD) and water gap (WGMD) were utilized to study HA behavior in MD using seawater and synthetic water over a range of typical MD temperatures, flow rates and membrane types. HA diffusion was first shown with seawater feed then on synthetic solutions at all process conditions. While electrical conductivity rejection was always above than 99%, HA rejection showed values of 33% and 90% for AGMD and 68% and 93% for WGMD with seawater and synthetic water, respectively. Analytical techniques were used to perform a preliminary organic matter characterization in permeate, obtaining clear differences between the feed and permeate HA property. Compared to hydrophobic membranes, uniquely oleophobic membranes inhibit HA diffusion suggesting hydrophobic surface diffusion of HA through the membrane. HA flux as well as potential undesirable effects of the organic matter in permeate should be considered for MD applications.

Electrospinning of Cellulose Nanocrystal-Filled Poly (Vinyl Alcohol) Solutions: Material Property Assessment

Poly (vinyl alcohol) (PVA) and cellulose nanocrystals (CNC) random composite mats were prepared using the electrospinning method. PVA/CNC mats were reinforced with weight concentrations of 0, 20 and 50% CNC (w/w) relative to PVA. Scanning electron microscopy was used to measure the fiber diameter, which ranged from 377 to 416 nm. Thermogravimetric analysis (TGA) confirmed the presence of CNC fibers in the mat fibers which were not visible by scanning electron microscope (SEM). Mechanical testing was conducted using ASTM D 638 on each sample group at 10 mm min-1. Neat PVA and PVA/CNC mats were heat treated at 170 °C for 2h hours, and the morphological structure was maintained with some fiber diameter reduction. Mechanical property results after heat treatment showed a decrease in tensile strength, an increase in tensile stiffness and a decrease in strain to yield (%). This effect was attributable to enhanced diffusion bonding of the mat fiber intersections. The CNC fibers also increased mat stiffness, and reduced strain to yield in non-treated mats. The use of CNCs show potential for compounding into bulk polymer composites as a reinforcement filler, and also show promise for chemical crosslinking attributable to the -OH groups on both the PVA, in addition to esterification of the vinyl group, and CNC.

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.

Hierarchical Composite Membranes with Robust Omniphobic Surface Using Layer-By-Layer Assembly Technique

In this study, composite membranes were fabricated via layer-by-layer (LBL) assembly of negatively charged silica aerogel (SiA) and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FTCS) on a polyvinylidene fluoride phase inversion membrane and interconnecting them with positively charged poly(diallyldimethylammonium chloride) (PDDA) via electrostatic interaction. The results showed that the PDDA-SiA-FTCS coated membrane had significantly enhanced the membrane structure and properties. New trifluoromethyl and tetrafluoroethylene bonds appeared at the surface of the coated membrane, which led to lower surface free energy of the composite membrane. Additionally, the LBL membrane showed increased surface roughness. The improved structure and property gave the LBL membrane an omniphobic property, as indicated by its good wetting resistance. The membrane performed a stable air gap membrane distillation (AGMD) flux of 11.22 L/m² h with very high salt rejection using reverse osmosis brine from coal seam gas produced water as feed with the addition of up to 0.5 mM SDS solution. This performance was much better compared to those of the neat membrane. The present study suggests that the enhanced membrane properties with good omniphobicity via LBL assembly make the porous membranes suitable for long-term AGMD operation with stable permeation flux when treating challenging saline wastewater containing low surface tension organic contaminants.

Recent developments in polymeric electrospun nanofibrous membranes for seawater desalination

Seawater desalination is a promising strategy that offers an abundant and reliable source of clean fresh water. Nanotechnology, in terms of nanoparticles or electrospun nanofibrous membranes, for water-treatment or desalination applications, is a new concept that has rapidly grown in interest as a method for improving performance by enhancing the surface properties of membranes. Here, we report a critical review on recent developments in membrane-fabrication methods for seawater desalination technologies, focusing mainly on the electrospinning technique. High-performance membranes that address ongoing permeability concerns, while maintaining membrane selectivity, need further study and development. Considering that the world today is faced with energy-shortage crises, these membranes also need to be energy efficient. As electrospinning is considered to be a feasible method for the production of desalination membranes, this technique requires appropriate optimization and the structural properties of the membranes produced need to be controlled in order to tailor their properties to those desired for well-known desalination technologies, such as reverse osmosis and membrane distillation. Moreover, there is a need to understand the influence of membrane structure on performance, and the latest trends in their use as high-performance desalination membranes.

Seawater desalination is a promising strategy that offers an abundant and reliable source of clean fresh water.

One-Step Fabrication of Fe(OH)3@Cellulose Hollow Nanofibers with Superior Capability for Water Purification

The conventional strategies employed for the synthesis of hollow nanofibers (HNFs) require either multistep treatments or special design of the equipment. An additional annealing process is always required, which inevitably consumes more energy and raises the manufacturing cost. In addition, the annealing process may also cause a waste of the matrix materials and the release of toxic gases. Herein, we report for the first time a novel one-step synthesis of hollow hybrid nanofibers via electrospinning. Cellulose was chosen as the polymer matrix, and Fe(OH)₃ nanoparticles were grown in situ on the nanofibers during electrospinning. There was no need to remove cellulose to create the hollow nanofiber structure. This can significantly simplify the fabrication process without any negative influence to the air. The obtained Fe(OH)₃@cellulose HNF membranes exhibited great mechanical properties and an extremely high water flux of 11 200 L m^-2 h^-1 bar^-1. They could effectively remove various pollutants in water, including phosphate, heavy metal ions, and organic dyes, with excellent reusability. Importantly, this approach could also be applied for the fabrication of other hybrid HNFs, which may serve in a broad range of scientific and engineering applications, including water purification, energy conversion and storage, catalysts, sensors, and so on.

A Capacitive Humidity Sensor Based on an Electrospun PVDF/Graphene Membrane

Humidity sensors have been widely used in areas such as agriculture, environmental conservation, medicine, instrumentation and climatology. Hydrophobicity is one of the important factors in capacitive humidity sensors: recent research has shown that the inclusion of graphene (G) in polyvinylidene fluoride (PVDF) improves its hydrophobicity. In this context, a methodology to fabricate electrospun membranes of PVDF blended with G was developed in order to improve the PVDF properties allowing the use of PVDF/G membrane as a capacitive humidity sensor. Micrographs of membranes were obtained by scanning electron microscopy to analyze the morphology of the fabricated samples. Subsequently, the capacitive response of the membrane, which showed an almost linear and directly proportional response to humidity, was tested. Results showed that the response time of PVDF/G membrane was faster than that of a commercial DHT11 sensor. In summary, PVDF/G membranes exhibit interesting properties as humidity sensors.