Janus membranes for membrane distillation: Recent advances and challenges

© 2021 Elsevier B.V. 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

Polyvinylidene fluoride-based dual-layer hollow fibre nanocomposite membranes for membrane distillation

Membrane distillation (MD) is a technology that makes use of the temperature difference created across the membrane. One of the main challenges in MD research is to fabricate membranes with high wetting resistance and high permeate flux. To date, very limited work has reported on the use of both hydrophobised carbon-based nanoparticles and hydrophobicity gradient in achieving those qualities. Thus, this work was focused on the fabrication of dual-layer hollow fibre polyvinylidene fluoridebased nanaocomposite membranes with hydrophobic gradient. The nanocomposite membranes were incorporated with hydrophobised nanoparticles, i.e. multi-walled carbon nanotube (MWCNT) and graphene nanoplatelet (GNP). The effect of different concentrations of nanoparticles (1 and 2 wt%) on the performance of the membranes was investigated as well. The nanoparticles were oxidised with concentrated nitric acid and sulphuric acid, hydrophobised with 1H,1H,2H,2H-perfluorodecyltriethoxysilane and characterised to confirm the hydrophobisation and determine their specific surface area. The hydrophobicity gradient was created by introducing the hydrophobised nanoparticles in the outer layer dope solution and hydrophilic polyethylene glycol in the inner layer dope solution. The dual-layer membranes were then fabricated and characterised to determine their wetting resistance, chemical composition, surface and cross-sectional morphology, pore size, porosity and contact angle before their performances were tested in a direct contact membrane distillation (DCMD) set-up. MWCNT and GNP were successfully oxidised and hydrophobised as confirmed by energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR) results. Both the hydrophobised nanoparticles had smaller Brunauer-Emmett-Teller (BET) specific surface area as compared to the pristine nanoparticles. All the fabricated membranes had an asymmetrical structure with finger-like and sponge-like pores in the inner and outer layers respectively. The outer layers of the nanocomposites membranes were hydrophobic and the membrane with 2 wt% of hydrophobised GNP had the highest contact angle of 111.1°. All the inner layers of the membranes were found to be hydrophilic, thus proving that hydrophobicity gradients were achieved in the membranes. The surface roughness, contact angle and wetting resistance of all the nanocomposite membranes were higher than the neat membrane and the values increased with increasing concentration of nanoparticles. Membranes incorporated with GNP showed better performance in terms of contact angle and wetting resistance. All the nanocomposite membranes showed better DCMD performance than the neat membrane. The membrane incorporated with 2 wt% of hyrophobised GNP achieved the highest flux of 8.27 kg/(m2h) at feed temperature of 80?. All the membranes achieved a salt rejection of more than 99%. Nonetheless, the flux of the membranes were quite low compared to the flux of membranes reported in the literature, likely due to small pore sizes and dense interfaces

A review of nanocellulose adsorptive membrane as multifunctional wastewater treatment

Dyes, inorganic and organic solvents, heavy metals and oils represent a substantial danger to water supplies, which is a major global problem. Advanced research and development in the manufacture of green-adsorptive membranes as well as simple operation, high separation efficiency, low energy consumption, eco-friendly and affordable cost have led the way to the development of sophisticated treatments for water remediation. To date, nanocellulose has been extensively investigated as excellent biomaterials in membrane filtration due to their exceptional properties such as large specific surface area, anti-fouling behaviour, high aspect ratio, high thermal resistance, outstanding mechanical properties, biodegradability and biocompatibility. The large surface area of nanocellulose contains a large number of free hydroxyl groups, which are easily modified and functionalized has been discussed. In addition, recent progresses in the application of modified nanocellulose for heavy metal removal, oily water separation and dye extractions are surveyed, since they are potentially useful as adsorbents in the filtration membrane to enhance its performance

Novel Photothermal Materials and Thermally Engineered Membranes for Solar Desalination

Although 71% of earth surface is covered with water, more than 97% of it is saltwater, and freshwater is limited to only about 2.5%. The freshwater shortage has been exacerbated due to the environmental pollution, increased agriculture needs, socio-economic development, and population growth. Among various desalination technologies, membrane distillation has gained wide attention due to its ability to treat highly saline water utilizing waste heat from industrial processes. However, the implementation of conventional membrane distillation is hindered in the remote regions and disaster-struck communities where the low-grade thermal energy from industrial plants and electricity are not readily available. To address this problem, photothermal driven membrane distillation (PMD), where membrane distillation is integrated with photothermal materials that can effectively convert light to thermal energy, has been recognized as an attractive and sustainable technology for freshwater generation. The overall objective of this work is to overcome several fundamental scientific challenges in realizing efficient PMD by exploring two key components of photothermal membranes, namely, substrates and photothermal materials.In the first part of this work, we have designed and investigated environmentally benign substrates (hydroxyapatite (HA) nanowires) for efficient PMD. Their structure and properties of this novel substrate material have been systematically investigated to reveal their potential in replacing the widely used polymeric substrates for efficient PMD. In the second part, we have investigated high-performance photothermal materials (polydopamine (PDA), MXene and polypyrrole (PPy)) for efficient PMD. Their excellent photothermal property, abundant functional groups and facile processability make them highly appealing in achieving high-performance PMD membranes. Taken together, this work further our understanding of the structure, properties, stability and performance of these novel materials and open up novel avenues in designing and realizing highly efficient PMD systems

Nano-gefunctionaliseerde membraandistillatiemembranen voor drinkwaterproductie uit zout of brak water

Abstract in English and GermanThe reported PhD research study was conceived from real water problems experienced by a rural community in South Africa (SA). Specifically, water quality in the Nandoni Dam situated in the Vhembe District, Limpopo Province, South Africa was assessed in order to determine its fitness for use, following complaints by community members using this water for drinking and domestic purposes. The dam supplies water to 55 villages with approximately 800 000 residents. At the inception of the study, there was little scientific information relating to the quality of the water in the dam. Water samples from various sites across the Nandoni Dam, a primary source of domestic water supply in the region, were collected through each season of the year over a period of 12 months to ascertain the concentrations of dissolved salts in the dam. Additionally, harmful polycyclic aromatic hydrocarbons (PAHs) and phenols were assessed. The concentrations of the ions contributing to water salinity were generally lower than the brackish water bracket (i.e. 500 – 30 000 mg/L) but too high for potable water. The concentration of the phenols was relatively higher than the threshold limit of drinking water. Therefore, the water sourced from the Nandoni Dam was found not suitable for human consumption and therefore required integrated water resource management, as well as robust and cost-effective water treatment especially since the salinity of the water was high even after treatment by a water treatment plant sourcing water from the dam. In an attempt to develop a suitable energy-efficient technology or system for complete removal of salts (desalination) from the salty water (including brackish water), electrospun polyvinylidene fluoride (PVDF) nanofibre membranes were synthesised and evaluated for removal of salts using the Direct Contact Membrane Distillation (DCMD) process. The nanofibre membranes were synthesised with combined high mechanical stability, porosity, and superhydrophobicity to prevent fouling and wetting while maintaining high salt rejection and water flux. Organically functionalised silica nanoparticles (f-SiO2NPs) were embedded on PVDF nanofibre membranes using an in-situ electrospinning technique for superhydrophobicity enhancement. These modified membranes displayed Young’s modulus of ~43 MPa and showed highly porous properties (~80% porosity, 1.24-1.41 μm pore sizes) with superhydrophobic surfaces (contact angle >150°). Membranes embedded with octadecyltrimethoxysilane (OTMS), and chlorodimethyl-octadecyl silane (Cl-DMOS), octadecyltrimethoxysilane (ODTS)-modified SiO2NPs were the most efficient; rejecting >99.9% of NaCl salt, with a water flux of approximately 30.7-34.2 LMH at 60°C, thus indicating their capacity to produce potable water. The superhydrophobic membranes were coated with a thin layer consisting of carboxylated multiwalled carbon nanotubes (f-MWCNTs) and silver nanoparticles (AgNPs) to reduce membrane fouling. The AgNPs and f-MWCNTs were uniformly distributed with size diameters of 28.24±1.15 nm and 6.7±2.1 nm respectively as evidenced by transmission electron microscopy (TEM) micrographs. The antibacterial AgNPs embedded in the PVDF nanofibre membranes inhibited the growth of Gram-positive Geobacillus stearothermophilus and Staphylococcus aureus as well as Gram-negative Pseudomonas aeruginosa and Klebsiella pneumoniae indicating their potential to prevent biofilm formation. Fouling tests were conducted using bovine serum albumin (BSA), sodium alginate, colloidal silica, and thermophilic bacteria effluent as model organic, inorganic, and bio-foulants, respectively, using DCMD. The uncoated membranes were characterised by a flux decays ranging from 30% to 90% and salt rejection decays ranging from 1.4% to 6.1%. Membrane coating reduced the flux and salt rejection decays to 10–24% and 0.07–0.75%, respectively. Although the initial flux decreased from 42 to 16 LMH when using coated membranes, the resistance of these coated membranes to water flux and salt rejection decays indicated that coating could be a suitable one-step solution for fouling mitigation in DCMD. The major challenge would be to design the MD membranes with architectures that allow a high-water flux to be maintained i.e., a highly porous layer. Furthermore, the volatile compounds bearing hydrophobic groups were pretreated to reduce their fouling capacity on PVDF nanofibre membranes. In this study, polyacrylonitrile (PAN) and polyethylene-imine (PEI) functionalised-PAN nanofibre membranes were synthesised and evaluated as a pretreatment for the removal of chlorophenol and nitrophenol from solutions. Under optimised experimental conditions, adsorption capacities ranging from 27.3 – 38.4 mg/g for PAN and PEI-modified nanofibres, respectively, were recorded. The PEI-functionalised nanofibres showed a high potential as a pretreatment step to be integrated to MD process. Ultimately an integrated water desalination system was developed. This involved a pretreatment filter (pore size ~100 μm) containing PEI-functionalised PAN nanofibre materials to reduce particulates and large molecules of dissolved organic/inorganic compounds from the water to be treated. In this research, it was observed that the pre-treatment step was not sufficient in removing all traces of compounds causing fouling of the superhydrophobic PDVF nanofibre membranes. As such, coating of the membranes with a thin hydrophilic layer and coupled with the filtration pretreatment step was found to provide fouling-resistance properties, high salt rejection, and low flux decays on brackish water collected at an estuary in Belgium and the Nandoni Dam in South Africa, demonstrating the potential of the MD separation process towards potable water recovery from brackish water.Het onderzoek in dit proefschrift was gebaseerd op concrete waterproblemen die een landelijke gemeenschap in Zuid-Afrika (SA) ervaart. In het bijzonder werd de waterkwaliteit in het Nandoni-reservoir in het Vhembe-district in de provincie Limpopo in Zuid-Afrika onderzocht, om te bepalen of dit water geschikt is voor gebruik, na klachten van leden van de gemeenschap die dit water gebruiken als drinkwater en voor huishoudelijk gebruik. Het reservoir levert water aan 55 dorpen met ongeveer 800.000 inwoners. Bij het begin van het onderzoek was er weinig wetenschappelijke informatie over de kwaliteit van het water in het reservoir. Watermonsters van verschillende locaties in het reservoir, dat een primaire bron van drinkwater is in de regio, werden gedurende verschillende seizoenen van het jaar verzameld over een periode van 12 maanden, om de concentraties van de meest voorkomende ionen in het reservoir te bepalen. Bovendien werden de concentraties van schadelijke polycyclische aromatische koolwaterstoffen (PAK's) en fenolen gemeten. De concentraties van de ionen die bijdroegen aan het zoutgehalte van het water waren in het algemeen lager dan de drempel om het water als brak water te bestempelen (dat wil zeggen 500 – 30 000 mg/l), maar waren te hoog voor drinkwater. De concentratie van de fenolen was hoger dan de limiet voor drinkwater. Daarom bleek het water afkomstig van het Nandoni reservoir niet geschikt voor menselijke consumptie. Een beter geïntegreerd waterbeheer is dus nodig om deze bron voor drinkwater te beschermen, naast een robuuste en kosteneffectieve waterbehandeling. Deze waterbehandeling moet vooral het zoutgehalte van het water naar beneden halen, maar ook de concentraties van fenolen. In een poging om een geschikte energie-efficiënte technologie of een systeem voor de volledige verwijdering van zouten (~ontzilting) uit brak water te ontwikkelen, werden elektrisch gesponnen polyvinylideenfluoride (PVDF) nanovezelmembranen gesynthetiseerd en beoordeeld op verwijdering van zouten met behulp van Direct Contact Membraandestillatie (DCMD). De nanovezelmembranen hadden een gecombineerde hoge mechanische stabiliteit, porositeit en superhydrofobiciteit, die hielp om vervuiling (fouling) en vloeistofintrede in de poriën (wetting) te voorkomen, terwijl een hoge zoutverwijdering en hoge waterflux doorheen de membranen gehandhaafd bleven. Organische gefunctionaliseerde silica-nanodeeltjes (f-SiO2NP's) werden nadien geïncorporeerd in de PVDF nanovezelmembranen met behulp van een in-situ elektrospinning techniek om zo een nog grotere superhydrofobiciteit te bekomen. Deze gemodificeerde membranen hadden een degelijke treksterkte (Young's modulus van ~ 43 MPa) en waren zeer poreus (~ 80% porositeit, 1.24-1.41 μm poriegrootte). Het oppervlak van de membranen vertoonde inderdaad superhydrofobe eigenschappen (contacthoek met water > 150 °). De membranen ingebed met octadecyltrimethoxysilaan (ODTS) SiO2NP's waren het meest efficiënt: ze toonden een zoutretentie van> 99.9% voor NaCl, bij een waterflux van ongeveer 30.7-34.2 l/(m².h) bij 60 ° C (ten opzichte van 20°C in het permeaat), wat aangeeft dat ze in staat zijn om drinkbaar water te produceren. De superhydrofobe membranen werden nadien ook gecoat met een dunne laag bestaande uit gecarboxyleerde multiwall-carbon nanotubes (f-MWCNT's) en zilver nanodeeltjes (AgNP's), in een poging om membraanvervuiling te verminderen. De AgNP's en f-MWCNT’s hadden uniforme diameters van respectievelijk 28,24 ± 1,15 nm en 6,7 ± 2,1 nm (zoals bleek uit transmissie-elektronenmicroscopie (TEM)). De antibacteriële AgNP's ingebed in de PVDF-nanovezelmembranen remden de groei van Gram-positieve Geobacillus stearothermophilus en Staphylococcus aureus bacteriën, evenals Gram-negatieve Pseudomonas aeruginosa en Klebsiella pneumoniae bacteriën. Dit toont het potentieel van deze membranen om biofilmvorming te voorkomen. Vervuilingsproeven (in DCMD) werden uitgevoerd met behulp van runderserumalbumine (BSA), natriumalginaat, colloïdaal silica, en thermofiele bacteriën - als respectievelijk organische, anorganische en biologische vervuiling. De niet-gemodificeerde membranen werden gekenmerkt door een fluxverval, met een daling van de flux met 30% tot 90%, naast een daling van de zoutretentie met 1.4% tot 6.1%. Bij de gecoate membranen daalde de flux slechts met 10-24% en de zoutverwijdering slechts met 0.07-0.75% respectievelijk. Hoewel de initiële flux ook afnam (van 42 naar ± 16 l/(m².h)) bij het gebruik van gecoate membranen, toonde de hogere weerstand tegen vervuiling van deze gecoate membranen aan dat deze coating een geschikte oplossing zou kunnen zijn tegen vervuiling in DCMD. Bovendien kan de synthese in één stap verlopen. De grootste uitdaging zal echter zijn om MD-membranen te ontwerpen waarbij de coating de oorspronkelijke waterflux/de porositeit van de membranen niet teveel verlaagt. Daarnaast werden gemodificeerde PVDF nanovezels geproduceerd om de verwijdering van vluchtige, hydrofobe stoffen (zoals fenolen) door adsorptie aan deze vezels te verhogen. Er werden polyacrylonitril (PAN) en polyethyleen-imine (PEI) gefunctionaliseerde PAN nanovezels gesynthetiseerd, waarna deze geëvalueerd werden als adsorbens (en dus voorbehandeling voor de membraanstap) voor chloorfenol en nitrofenol. Onder geoptimaliseerde experimentele omstandigheden werden adsorptiecapaciteiten tussen respectievelijk 27.3 en 38.4 mg / g voor PAN- en PEI-gemodificeerde nanovezels gemeten. De PEI-gefunctionaliseerde nanovezels vertoonden een hoog potentieel als een voorbehandelingsstap voor de hierboven beschreven DCMD. Tenslotte werd ook een geïntegreerd waterontziltingssysteem ontwikkeld. Dit systeem bestond uit een voorbehandelingsstap met PEI-gefunctionaliseerde PAN-nanovezels (in de vorm van een membraan met poriegrootte ~100 μm), gevolgd door een gemodificeerde DCMD stap. De voorbehandeling diende om deeltjes en grote opgeloste organische verbindingen uit het te behandelen water te verwijderen voor de DCMD-stap. In dit onderzoek werd waargenomen dat de voorbehandelingsstap niet voldoende was om alle organische contaminanten te verwijderen die vervuiling veroorzaakten op de superhydrofobe PDVF nanovezelmembranen in de DCMD-stap. Toch bleek coating van de DCMD membranen met een dunne hydrofiele laag (gekoppeld aan de voorbehandelingsstap) een voldoende bescherming tegen vervuiling te bieden zodat de zoutretentie en waterflux van deze membranen hoog bleef. De combinatie van voorbehandeling – gemodificeerde DCMD werd succesvol getest op water uit de Schelde en uit het Nandoni reservoir, waarmee het potentieel van de technologie om drinkwater uit brak water te produceren werd aangetoond.School of SciencePh.D. (Applied Biological Science : Environmental Technology

Synthesis of aerogels, nanocomposites and lightweight silica aerogel superinsulation nanocomposites by ambient pressure drying method

Ph. D ThesisThis thesis mainly investigates the improvement of the new ambient pressure approach used to synthesise aerogels by using a solvent comprising of sodium bicarbonate and water instead of a low surface tension solvent. Firstly, to improve the efficiency of thermal insulation, the sodium bicarbonate approach is utilised to synthesise cost effective ceramic blanket silica aerogels (CBSA) and short ceramic fibres silica aerogel composites (CSSA). To reduce the manufacturing cost and scalable of silica aerogels, we propose applying the sodium bicarbonate approach to synthesis silica aerogels from sodium silicate (water glass) precursor. In addition, the approach is used to synthesise alumina-based aerogel (dawsonite-sodium aluminium carbonate hydroxide) from Aluminium sec-butoxide precursor (ASB). To mimic the structure and thickness of the wings of the damselfly, which was the main source of inspiration for this study, multi-layered silica aerogel films with a thickness of 0.3 mm were synthesised using the bicarbonate approach. Finally, wavy nickel nanowires (NiNWs) were synthesise and immobilised on mesoporous silica (SiO2) aerogels by the sol-gel method. In addition, nickel nanoparticles (NiNPs) were immobilised in silica aerogels to do a comparative study between the catalytic activity of immobilised NiNWs and NiNPs in silica aerogels for CO2 hydration reaction (CHR) in gaseous phase. Dynamic vapour sorption (DVS) analysis is used for that purpose. The analysis is performed at levels of 50% CO2 and 50% H2O vapour for SiO2 aerogels, immobilised nickel nanoparticles (NiNPs) on silica aerogels and NiNWs-SiO2 aerogels composites. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), uniaxial compression test, Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods are used to characterise the synthesised materials

Recent progress on nanomaterial-based membranes for water treatment

Nanomaterials have emerged as the new future generation materials for high-performance water treatment membranes with potential for solving the worldwide water pollution issue. The incorporation of nanomaterials in membranes increases water permeability, mechanical strength, separation efficiency, and reduces fouling of the membrane. Thus, the nanomaterials pave a new pathway for ultra-fast and extremely selective water purification membranes. Membrane enhancements after the inclusion of many nanomaterials, including nanoparticles (NPs), two-dimensional (2-D) layer materials, nanofibers, nanosheets, and other nanocomposite structural materials, are discussed in this review. Furthermore, the applications of these membranes with nanomaterials in water treatment applications, that are vast in number, are highlighted. The goal is to demonstrate the significance of nanomaterials in the membrane industry for water treatment applications. It was found that nanomaterials and nanotechnology offer great potential for the advancement of sustainable water and wastewater treatment.Internal Qatar University grant QUCG-CENG-21/22-4 and Qatar National Research Fund grant NPRP12S-0306-190247.Scopu