Saikat Sinha Ray

A comprehensive review: electrospinning technique for fabrication and surface modification of membranes for water treatment application

In this world of nanotechnology, nanofibrous structures offer specialized features, such as mechanical strength and a large surface area, which makes them attractive for many applications. Their large surface area to volume ratio also makes them highly efficient. Among all the techniques for generating nanofibers, electrospinning is an emerging and efficient process. Additionally, the electrospinning technique allows a uniform pore size, which is considered to be one of the important characteristics of membranes. Therefore, electrospun nanofibrous membranes have been used in water purification applications. Furthermore, the technique is widely utilized for generating membranes for membrane distillation and nanofiltration processes, for the removal of contaminants. However, in this review paper, more emphasis is given to the optimization of specific parameters and the preparation of polymeric solutions for fabricating specialized nanofibrous non-woven membranes, and surface modification for application in water treatment technology. Other issues, such as technology limitations, research challenges, and future perspectives, are also discussed.

A novel osmosis membrane bioreactor-membrane distillation hybrid system for wastewater treatment and reuse

A novel approach was designed to simultaneously enhance nutrient removal and reduce membrane fouling for wastewater treatment using an attached growth biofilm (AGB) integrated with an osmosis membrane bioreactor (OsMBR) system for the first time. In this study, a highly charged organic compound (HEDTA(3-)) was employed as a novel draw solution in the AGB-OsMBR system to obtain a low reverse salt flux, maintain a healthy environment for the microorganisms. The AGB-OsMBR system achieved a stable water flux of 3.62L/m(2)h, high nutrient removal of 99% and less fouling during a 60-day operation. Furthermore, the high salinity of diluted draw solution could be effectively recovered by membrane distillation (MD) process with salt rejection of 99.7%. The diluted draw solution was re-concentrated to its initial status (56.1mS/cm) at recovery of 9.8% after 6h. The work demonstrated that novel multi-barrier systems could produce high quality potable water from impaired streams.

Exploring high charge of phosphate as new draw solute in a forward osmosis–membrane distillation hybrid system for concentrating high-nutrient sludge

For the first time, a high charge of phosphate was used as the draw solute in a forward osmosis-membrane distillation (FO-MD) hybrid system for concentrating high-nutrient sludge. A high water flux (12.5L/m(2)h) and a low reverse salt flux (0.84g/m(2)) were simultaneously achieved at pH9 by using 0.1M Na3PO4 as the draw solute and deionized water as the feed solution in the FO process. The specific reverse salt flux of 0.1M Na3PO4 (Js/Jw=0.07g/L) was considerably less than that of 0.1M NaCl (Js/Jw=0.37g/L) because the complexion between Na(+) and HPO4(2-) at pH9 led to the reduction of free Na(+) ions, which subsequently reduced the reverse salt diffusion substantially. Moreover, for a feed solution with an initial sludge concentration of 3500mg/L, the sludge concentration could be concentrated to 19,800 and 22,000mg/L in the pressure-retarded osmosis (PRO) and FO membrane orientations, respectively, after 15h of operation. Four types of MD membranes were selected for draw solution recovery; of these, a polytetrafluoroethylene membrane with a pore size of 0.45μm was the most effective in achieving a high water flux (10.28L/m(2)h) and high salt rejection (approximately 100%) in a diluted Na3PO4 draw solution.

Casting of a superhydrophobic membrane composed of polysulfone/Cera flava for improved desalination using a membrane distillation process

Superhydrophobic membranes are necessary for effective membrane-based seawater desalination. This paper presents the successful fabrication of a novel electrospun nanofibrous membrane composed of polysulfone and Cera flava, which represents a novel class of enhanced performance membranes consisting of a superhydrophobic nanofibrous layer and hydrophobic polypropylene (PP). Cera flava, which helps lower the surface energy, was found to be the ideal additive for increasing the hydrophobicity of the polysulfone (PSF) polymeric solution because of its components such as long-chain hydrocarbons, free acids, esters, and internal chain methylene carbons. In the fabricated membrane, consisting of 10 v/v% Cera flava, the top PSF–CF nanofibrous layer is active and the lower PP layer is supportive. The hybrid membrane possesses superhydrophobicity, with an average contact angle of approximately 162°, and showed high performance in terms of rejection and water flux. This work also examined the surface area, pore size distribution, fiber diameter, surface roughness, mechanical strength, water flux, and rejection percentage of the membrane. The salt rejection was above 99.8%, and a high permeate flux of approximately 6.4 LMH was maintained for 16 h of operation.

Enhanced desalination using a three-layer OTMS based superhydrophobic membrane for a membrane distillation process

Superhydrophobic membranes are essential for improved seawater desalination. This study presents the successful casting of a three-layered membrane composed of a top superhydrophobic coating onto a polypropylene (PP) mat through simple sol–gel processing of octadecyltrimethoxysilane (OTMS), and the bottom layer was casted with hydrophilic poly(vinyl alcohol) (PVA) by using a knife casting technique; this membrane represents a novel class of improved-performance membranes consisting of a top superhydrophobic coating onto a hydrophobic PP mat and a hydrophilic layer (PVA) at the bottom. OTMSs are well known low-surface-energy materials that enhance superhydrophobicity, and they were observed to be the ideal chemical group for increasing the hydrophobicity of the PP mat. The PVA layer acted as base layer absorbing the condensed vapor and thus enhancing the vapor flux across the membrane. The hybrid three-layered membrane exhibited superhydrophobicity, with an average contact angle of more than 160°, and demonstrated high performance in terms of rejection and water flux. This study also examined the pore size distribution, surface roughness, surface area, tensile strength, water flux, and salt rejection of the fabricated membrane. The salt rejection level was calculated to be 99.7%, and a high permeate flux of approximately 6.7 LMH was maintained for 16 h.

Sustainable Desalination Process and Nanotechnology

A wide variety of methods are used for water treatment and purification. The use of membranes allows efficient treatment by reverse osmosis, nanofiltration, ultrafiltration, microfiltration, membrane distillation and forward osmosis. Membrane technology has been researched extensively for water treatment and desalination. Desalination is the technology predominantly used to solve water scarcity. The sustainability of desalination processes aims at reducing energy costs and increasing water recovery. In recent years numerous large-scale seawater desalination plants have been built in water-stressed countries. Construction of new desalination plants with the latest emerging technology is expected to increase in the future. Despite major advances in desalination technologies, seawater desalination is still more energy intensive compared to conventional processes uszd for the treatment of fresh water. However, forward osmosis and membrane distillation are emerging for sustainable desalination.

Anti-wetting behaviour of a superhydrophobic octadecyltrimethoxysilane blended PVDF/recycled carbon black composite membrane for enhanced desalination

A novel superhydrophobic octadecyltrimethoxysilane (OTMS)-modified composite membrane was fabricated by incorporating recycled carbon black (CB) into a polyvinylidene fluoride (PVDF) membrane. Waste tires have been treated with extraction and high-temperature calcination to obtain recycled CB. After incorporating CB into the PVDF solution, a steep increase of 18–20% was observed in the loss and storage moduli. CB was observed to be an ideal filler for doping the PVDF solution to enhance the mechanical properties of the material. OTMSs are known for their low surface energy, which improves hydrophobicity. Moreover, OTMSs were found to be ideal agents for enhancing the superhydrophobicity of PVDF membranes. The results of surface treatment with OTMS, CB loading, particle size, and membrane properties and their effect on desalination performance were thoroughly studied. The composite membrane was observed to be superhydrophobic with a contact angle of 160° and exhibited high desalination performance in terms of permeate water flux and salt rejection. The fabricated membrane has been reused after physical cleaning in order to analyse the anti-wetting and long-term performance.