Exfoliated Bi2Te3-enabled membranes for new concept water desalination: Freshwater production meets new routes

Fouling, performance and cost analysis of membrane-based water desalination technologies: A critical review

Membrane distillation process application using a novel ceramic membrane for Brackish water desalination

The scarcity of fresh water resources has been of great concern for contemporary society, particularly against the backdrop of worsening climate change impacts and a growing global population. Desalination of saline water resources is one of the most feasible and effective technologies for fresh water production. Membrane-based desalination processes such as reverse osmosis (RO) have risen to prominence owing to their reduced energy intensity and compact footprint. However, extensive pre-treatment is required to perform RO on chemically and thermal aggressive waters, opening the door for alternative methods. In particular, membrane distillation (MD), a process which combines the benefits of both membrane and thermal technologies has received significant research interest in the last decade. Up till now, most studies on MD have focussed on the effect of operating parameters by using the commercial available polymeric membranes, which are designed for ultra- or microfiltration and may not be suitable for MD. New membrane materials and morphologies have not been extensively explored. The key target of this work therefore is to design a suitable membrane for MD by exploring the potential of hybrid organic-inorganic materials, specifically using silica-based (as inorganic) and carbon-based (as organic) materials. Silica-based materials offer good chemical and thermal resistance, high porosity, and excellent versatility in forming various nano-sized morphologies. Unfortunately, most of the membrane-related work has focused on amorphous silica, which is sensitive to steam or water vapour and yields low water fluxes making it generally unsuitable for MD. The steam degradation can be controlled at MD relevant temperatures by incorporating organic moieties into the silica network. Meanwhile, the flux issues can be addressed by increasing porosity and pore size. This work demonstrates for the first time hybrid organic-inorganic mesoporous membranes with an ordered, narrow pore size distribution was developed by using soft-templating method and successfully applied to MD under a variety of operating conditions. Despite having a hydrophilic contact angle (i.e. l 90 d) and a pore size (2 nm) larger than hydrated salt ions (0.66-0.72 nm) which intuitively may lead to pore wetting, the membrane produced pure water (up to 13 L m-2 h-1) with g 99 % salt rejection across an extreme range of salt concentrations (10-150 g L-1 NaCl) at moderate temperature (60 dC). This major finding was complimented by the fact that no concentration polarization was observed, with fluxes effectively unchanged across the entire range of salt concentrations. Based on these results a model was proposed to explain how a hydrophilic, nanoporous membrane could operate effectively and with no observable pore wetting under vacuum MD. The model represents the second major contribution of the thesis and adapts the Lucas-Washburn equation for capillary pressure to nanopores. The model shows that the liquid/vapour interface is no longer formed in the pore entrance but shifted further into the pore channel due to the water intrusion (drawn by the capillary pressure) which balanced out by the vaporization of water from the liquid/water interface due to partial pressure difference. Crucially, fluid flow through the nanochannels experiences dramatically increased resistance, due to the sharp increase in the shear viscosity of water in nanoconfined spaces, preventing outright pore wetting. The impacts of pore size, membrane thickness, substrate thickness, concentration polarization, porosity, and contact angle on water flux and pore intrusion depth were tested using the model. The membrane hydrophilicity was found to impact on water flux and pore intrusion in a complex relationship with pore size. In order to elucidate this theory, organosilica membranes of different pore sizes and pore geometries were prepared; their performances were compared and found to be in broad agreement with the initial model. In the second part of the thesis, a different strategy was trialled for the synthesis of hybrid organic-inorganic materials, which employs the triconstituent co-assembly method using two separate precursors for the organic and inorganic compounds and a structure directing agent. Unlike the organosilica membrane where carbon and silicon atoms are covalently bonded and homogeneously distributed in atomic scale, these new carbon-silica nanocomposites are comprised of carbon and silica networks that interact physically at the molecular level. The effects of silica content, carbonization temperature, types of surfactants and coating conditions on the formation and performance of the resulting membranes were evaluated. In the third major finding of the thesis, the results demonstrated that a carbon-silica nanocomposite could be a more economically viable and promising candidate in membrane development compared to the original organosilica membranes. Furthermore, the concept of carbon-silica nanocomposite membranes is novel in MD-based water desalination and this opens up a new development pathway for hybrid organic-inorganic membranes.

Graphene-coated membranes for membrane distillation have been fabricated by using a wet-filtration approach. Graphene nanoplatelets have been deposited onto PVDF membrane surfaces. Morphology and physicochemical properties have been explored to evaluate the changes in the surface topography and related effects on the membrane performance in water desalination. The membranes have been tested in membrane distillation plants by using mixtures of sodium chloride and humic acid. The multi-scale rough structure of the surface has been envisaged to amplify the wetting and fouling resistance of the graphene-coated membranes so that a better flux and full salt rejection have been achieved in comparison with pristine PVDF. Total salt rejection and an increase of 77% in flux have been observed for coated membrane with optimized graphene content when worked with NaCl 0.6 M (DCMD, ΔT ≈ 24 °C) over a test period of 6 h. The experimental findings suggest these novel graphene-coated membranes as promising materials to develop functional membranes for high-performing water desalination.

Reverse osmosis (RO) technology is currently the most progressive, energy-saving and efficient membrane separation technology . Meanwhile, graphene becomes a promising candidate for fabricating the RO membranes in water desalination due to its high salt rejection and water flux. The concept of “temporal selectivity” is first proposed in our previous work in terms of the time difference between the penetration time of an ion passing through the pore and the tangential slipping time for the ion sliding across the pore. Nevertheless, the temporal selectivity mechanism of multilayered graphene membrane remains ambiguous. In this paper, the RO process of saltwater through porous graphene column RO membrane is studied by using molecular dynamics (MD) simulations method, and the effects of rotating angular velocity and the thickness of RO membrane on desalination performance of seawater are considered first. The MD results show that the salt rejection increases with the rotation speed of porous membrane increasing while the water flux initially increases and then decreases . Meanwhile, the interfacial slip velocity increases linearly with angular velocity increasing. On the other hand, the increasing thickness of porous graphene membrane can enhance the selectivity and reduce the permeability of water molecules. As expected, the tri-layered porous graphene RO membrane can achieve high salt rejection at low interfacial slip velocity. In order to ensure high selectivity and energy conservation and efficient, the pore structure of the porous graphene RO membrane is optimized. The results show that the optimized nanopores can increase the water flux significantly, whereas the salt rejection is not changed appreciably. It is found that the pore size of the innermost layer membrane near the feed region has the most significant effect on the water flux. The water flux increases sharply with the increase of pore diameter and the salt rejection remains totally higher than 80%. Moreover, the RO membrane with a special Type 3 structure exhibits excellent performance in seawater desalination, specifically, the ultrahigh water flux reaches 20029 L·cm<sup>–2</sup>·d<sup>–1</sup> and the super salt rejection arrives at 94%. The research results further clarify and verify the mechanism of the temporal selectivity in RO process, and improve the water flux under the condition of the same membrane thickness by designing gradient hole. The findings can conduce to the in-depth theoretical understanding of porous graphene-based membranes and designing and developing the large-scale seawater desalination devices and water filtration equipment.

Despite the widespread use of reverse osmosis (RO) membranes in water desalination, the role of solute-membrane interactions in solute transport remains complex and relatively not well understood. This study elucidates the relationship between solute-membrane electrostatic interactions and solute permeability in RO membranes. The transport of salt and neutral molecules through charged polyamide (PA) and uncharged cellulose triacetate (CTA) RO membranes was examined. Results show that salt rejection and salt permeability in the PA membrane are highly dependent on the solution pH due to the variations of membrane charge density and the Donnan potential at the membrane-solution interface. Specifically, a higher salt rejection (and hence lower salt permeability) of the PA membrane is observed under alkaline conditions compared to acidic conditions. This observation is attributed to the enhanced Donnan potential at higher solution pH, which hinders co-ion partitioning into the membrane. In contrast, for salt transport through the CTA membrane and neutral solute transport through both membranes, solute permeability is independent of the solution pH and solute concentration due to the negligible Donnan effect. Overall, our results demonstrate the important role of solute-membrane electrostatic interactions, combined with steric exclusion, in regulating solute permeability in RO membranes.

Performance analysis of reverse osmosis, membrane distillation, and pressure-retarded osmosis hybrid processes

In this century, water scarcity is one of the most crucial issues to be resolved. A practical substitute for resolving this problem is seawater desalination. Membrane-based technologies (e.g., membrane distillation, reverse osmosis, and pervaporation) are compelling and sufficiently proposed for water desalination purposes. However, polymers face some issues like degradation and low penetrability of water and increase energy consumption and overall water desalination costs. 2D nanoporous materials such as graphene oxide, MXenes, metal organic frameworks (MOFs), transition metal dichalcogenides (TMDCs), boron nitrides nanosheets, zeolite, MoS2, etc., with large surface area, mechanical strength, and having atomically thin structure are regarded to be the ideal substitution for water purification and desalination. 2D nanomaterials-based membranes have been used to increase the membrane's overall performance in desalination and water purification. Nevertheless, these nanomaterials’ exceptional properties can lower the energy consumption and increase the efficiency for desalination, which led to the immense attempt in fabrication and commercialization. Here, we have discussed the synthesis, properties, and water purification/desalination performance of 2D nanomaterials-based membranes.Keywords2D nanomaterialsMembranesWater desalinationWater purification

Bioinspired humic acid-based membranes for water desalination: Mechanistic insights from molecular simulations

Membranes are widely used in industrial separation processes, particularly for gas separation and desalination processes. To develop membrane materials with improved permeability, selectivity can achieve more energy-efficient membrane separations and reduce costs. Since composite membranes offer improved performance, the aim of this research is to develop polymer-based composite membranes with improved performance for gas separation and water desalination applications. First, in order to obtain a composite membranes with high chlorine tolerance, a carbonaceous poly(furfuryl alcohol) (PFA) composite membrane was synthesized at a low temperature carbonation by formation and post-treatment of a thin PFA layer on porous polymer substrates. The carbonaceous PFA membrane exhibits high selectivity and excellent chemical stability in seawater desalination. The low-temperature carbonization method developed in this study is promising for developing a wide range of other carbonaceous polymer composite membranes for water desalination. Next, in order to apply PFA to other applications, understanding the effects of polymerization conditions on the properties of the PFA composite membrane is required. The PFA membrane was fully characterized in terms of microstructure and separation properties. Suitable synthesis conditions for the preparation of PFA composite membranes with smooth surfaces and uniform structure were (1) FA/ H2SO4 molar ratios: 74-300, (2) polymerization temperatures: 80-100°C and (3) solvents: ethanol and acetone. The preparation conditions were also optimized. The PFA composite membrane prepared with a FA/ H2SO4 molar ratio of 250, a polymerization temperature of 80°C and with ethanol as the solvent exhibited the highest H2/N2 ideal selectivity (αH2/N2=24.9), and a H2 permeability of 206 Barrers. This work led to a better understanding of the effect of the preparation procedures on the membrane performance. In order to investigate the effects of the incorporation of molecular sieve nanoparticles on the membrane structure and membrane performance, silicalite-poly(furfuryl alcohol) (PFA) mixed matrix composite membranes were successfully synthesized based on the best synthesis condition obtained previously. The silicalite-PFA mixed matrix composite membrane with 20% w/w silicalite loading had a high ideal selectivity (αo2/N2= 3.5 and αco2/N2= 5.4) and a good permeability (Po2= 821.2, Pco2= 1263.7, PN2= 233.3 Barrers) at room temperature. This membrane can be a good candidate for oxygen enrichment applications. Finally, in order to investigate the effects of the incorporation of silicalite nanocrystals on the desalination property of polyamide membranes, silicalite nanocrystals were also incorporated into polyamide matrix to synthesize silicalite-polyamide mixed matrix membranes. With an increase in the loading of silicalite nanocrystals, the water flux of silicalite-polyamide mixed matrix composite membranes increased whereas the salt selectivity significantly decreased. The silicalite-polyamide mixed matrix composite membrane prepared from TMC-hexane with 0.5% (w/v) silicalite had water flux of 2.7×10-6 m3/m2·s and NaCl rejection of 50% at a feed pressure of 34.5 bar which 2000 ppm salt solution was used as the feed. The silicalite-polyamide mixed matrix composite membrane is promising for developing high water flux composite membranes for water desalination. In this research, composite membranes with improved permeability, selectivity and chemical resistance were successfully synthesized for desalination and gas separation. For desalination, carbonaceous PFA composite membranes with high chlorine tolerance and silicalite-PA mixed matrix composite membranes with high salt rejection and water flux were successfully obtained. For gas separation, an optimized composite membranes PFA synthesis condition was found and silicalite-PFA mixed matrix composite membranes with high O2/N2 separation were successfully synthesized.

Water desalination using a multilayer graphene oxide membrane: a molecular dynamics study.

Evaluation of membrane-based desalting processes for RO brine treatment

The essence of water in our everyday activities cannot be overemphasized. The major source of water in the Middle East is seawater and the most widely used technique for water treatment is Reverse osmosis (RO). However, the major challenge in the use of RO is the high-energy consumption resulting from the need for pumping at very high pressure. In this research work, the capability of a low-pressure Membrane Distillation (MD) technique as a replacement for RO was evaluated. A comparative study of MD and RO was done using process intensification, cost estimation, and process economic approach. The study was performed using process intensification metrics including mass intensity; waste intensity; productivity/size ratio; productivity/weight ratio; flexibility and modularity. The cost estimation involving the capital and operating expenses for RO and MD desalination plants was also determined based on the productivity of the plants. Moreover, process economic factors including profits, cash flow, and cumulative cash flow were also evaluated. The preliminary results obtained showed that the MD and RO possess the same capability to be used in the desalination plant based on process intensification. In addition, MD can be preferred in a situation where waste heat can be harnessed from neighboring industries. MD can also be better than RO with respect to the resistance of MD membrane materials to fouling. On the other hand, MD is more expensive than RO based on cost estimation and process economic results obtained.

Membrane distillation is an emerging separation technology with a high separation factor in water desalination. Ceramic membranes are increasingly used in membrane distillation because of high thermal and chemical stabilities. Coal fly ash is a promising ceramic membrane material with low thermal conductivity. In this study, three hydrophobic coal-fly-ash-based ceramic membranes were prepared for saline water desalination. The performances of different membranes in membrane distillation were compared. The effects of membrane pore size on permeate flux and salt rejection were researched. The coal-fly-ash-based membrane showed both a higher permeate flux and a higher salt rejection than the alumina membrane. As a result, using coal fly ash as the material for membrane fabrication can effectively increase the performance when applied to MD. Increasing the membrane pore size improved the permeate flux, but reduced the salt rejection. When the mean pore size increased from 0.15 μm to 1.57 μm, the water flux rose from 5.15 L·m−2·h−1 to 19.72 L·m−2·h−1, but the initial salt rejection was reduced from 99.95% to 99.87%. The hydrophobic coal-fly-ash-based membrane with a mean pore size of 0.18 μm exhibited a water flux of 9.54 L·m−2·h−1 and a salt rejection of higher than 98.36% in membrane distillation.

Demand for freshwater supplies is continuously increasing globally to the extent where some parts of the world became highly water stressed. In particular, the Arabian Gulf states rely heavily on seawater desalination for their freshwater supply which is met using commercial seawater desalination technologies like thermal and reverse osmosis (RO) desalination processes. However, these technologies require considerable power input and actually do have a negative impact on the environment in terms of carbon footprint.An alternative technology to the conventional desalination processes with potentially lower environmental impacts is the Membrane Distillation (MD) process. Membrane Distillation is a thermally-driven process that utilizes a hydrophobic micro-porous membrane and can utilize low grade heat and solar energy. The driving force of the process is the vapor pressure difference between the sides of the membrane that is induced by the temperature difference between the feed and distillate. However, one of the challenges facing the deployment of MD in large commercial scale desalination of seawater is membrane fouling.The objective of this study is to investigate and compare the fouling characteristics of three different commercial membranes (PP membrane of 0.22 μm, PP membrane of 0.45 μm, and PTFE membrane of 0.22 μm) using two feed solutions (seawater from the Arabian Gulf and synthetic 100,000 ppm NaCl solution) using a bench-scale direct contact membrane distillation (DCMD) flat sheet module at hot water inlet temperature of 75 °C, cold water inlet temperature of 20 °C, and hot and cold water flow rate of 1.5 L/min. The study was performed by evaluating the distillate flux performance of the various membranes, measuring their contact angle before and after fouling, testing the quality of the distillate produced and examining the salt rejection, and interpreting membrane surface analysis using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS) in order to study the morphology and the composition of the fouling layer.Initially, the average flux obtained was 50.5 L/m2h, 50.3 L/m2h, and 38.3 L/m2h for PP membrane of 0.22 μm, PP membrane of 0.45 μm, and PTFE membrane of 0.22 μm, respectively. Therefore, PP membrane generated a higher flux than PTFE membrane. In terms of membrane pore size, the results showed that a larger pore size membrane is more prone to fouling and flux decay. In terms of membrane material, PP membrane showed a more rapid flux decline than PTFE membrane. Moreover, the percentage of drop in the average flux was more than 60%, 97%, and 94% for PP membrane of 0.22 μm, PP membrane of 0.45 μm, and PTFE membrane of 0.22 μm, respectively, after almost 19 h, 30 h, and 25 h of operation, respectively.In terms of feed solution, a lower flux was obtained with the higher salinity feed, the 100,000 ppm NaCl solution. However, the difference was not very large, indicating that initially salinity does not have a great impact on the distillate flux.The results also showed that fouling/scaling causes the quality of the distillates to deteriorate and that membrane wetting has occurred. A salt rejection of more than 99.9% was achieved initially; however, with continuous operation, a salt rejection as low as 83.5% and 69.9% was achieved when the seawater and the 100,000 ppm NaCl solution were used, respectively. PP membrane of 0.22 μm gave a better salt rejection followed by PTFE membrane of the same size then by PP membrane of 0.45 μm.The contact angle of a clean PP membrane of 0.22 μm, PP membrane of 0.45 μm, and PTFE membrane of 0.22 μm, was found to be 134.8°, 133.2°, and 136.7°, respectively. However, after fouling, the contact angle dropped to 40.5°, 36.1°, and 13.8° for PP membrane of 0.22 μm, PP membrane of 0.45 μm, and PTFE membrane of 0.22 μm, respectively, indicating significant loss of hydrophobicity.SEM-EDS analysis showed that the salt layer formed on the membranes was not uniform and that the major foulants were CaCO3 and CaSO4. In addition, membrane pore blocking by salts and a cake layer formation (which was a result of the elevated feed temperature that resulted in the formation of temperature polarization) were observed.The results of the study show that fouling needs to be more investigated in MD process to be practically implemented and considered as competitive to the conventional desalination technologies. An optimum temperature and flow rate should be explored; however, the results of the study urge the need for developing new membranes and improved membrane modules and MD configurations as well as finding optimum procedures for membrane cleaning.