A computational assessment of the permeability and salt rejection of carbon nanotube membranes and their application to water desalination.

Fabrication of nanofiltration membrane on MoS2 modified PVDF substrate for excellent permeability, salt rejection, and structural stability

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.

Composite reverse osmosis membrane with a selective separation layer of double-layer structure for enhanced desalination, anti-fouling and durability properties

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.

Hydrophilic-hydrophobic heterogeneous interface enables the formation of a high-performance polyamide membrane for water purification

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.

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

Recent applications of nanomaterials in water desalination: A critical review and future opportunities

Functionalised carbon nanotube thin film nanocomposite membranes: A comparison study on the role of backbone monomers and hydraulic pressure on membrane's performance and fouling

Nowadays, reverse osmosis is the most widely utilized strategy in membrane technology due to its continuous improvement. Recent studies have highlighted the importance of the surface characteristics of support layers in thin-film membranes to improve their reverse osmosis performance. In this study, interfacial polymerization was used to generate the membranes by employing polyamide as a selective layer on top of the polysulfone supporting sheet. Different membranes, varying in terms of the concentrations of unfunctionalized and functionalized multiwalled carbon nanotubes (MWCNTs), as well as ethanol, have been fabricated. The efficiency of the membrane has been increased by increasing its permeability towards water with high salt rejection. Different characterization techniques were applied to examine all of the fabricated membranes. PA-EtOH 30% (v/v), as a selective layer on polysulfone sheets to enhance the membrane’s salt rejection, was shown to be the most efficient of the suggested membranes, improving the membrane’s salt rejection. The water permeability of the polyamide membrane with EtOH 30% (v/v) was 56.18 L/m2 h bar, which was more than twice the average permeability of the polyamide membrane (23.63 L/m2 h bar). The salt rejection was also improved (from 97.73% for NaCl to 99.29% and from 97.39% for MgSO4 to 99.62% in the same condition). The PA-MWCNTs 0.15% membrane, on the other hand, had a reduced surface roughness, higher hydrophobicity, and higher water contact angle readings, according to SEM. These characteristics led to the lowest salt rejection, resulting from the hydrophobic nature of MWCNTs.

Reverse osmosis (RO) has become a premier technology for desalination and water purification. The need for increased selectivity has incentivized research into novel membranes, such as biomimetic membranes that incorporate the perfectly selective biological water channel aquaporin or synthetic water channels like carbon nanotubes. In this study, we consider the performance of composite biomimetic membranes by projecting water permeability, salt rejection, and neutral-solute retention based on the permeabilities of the individual components, particularly the water channel, the amphiphilic bilayer matrix, and potential support layers that include polymeric RO, nanofiltration (NF), and porous ultrafiltration membranes. We find that the support layer will be crucial in the overall performance. Selective, relatively low-permeability supports minimize the negative impact of defects in the biomimetic layer, which are currently the main performance-limiting factor for biomimetic membranes. In particular, RO membranes as support layers would enable >99.85% salt rejection at ∼10000-fold greater biomimetic-layer defect area than for porous supports. By fundamentally characterizing neutral-solute permeation through RO and NF membranes, we show that RO membranes as support layers would enable high rejection of organic pollutants based on molecular size, overcoming the rapid permeation of hydrophobic solutes through the biomimetic layer. A biomimetic membrane could also achieve exceptionally high boron rejections of ∼99.7%, even with 1% defect area in the biomimetic layer. We conclude by discussing the implications of our findings for biomimetic membrane design.

Polyamide reverse osmosis (RO) membranes with carbon nanotubes (CNTs) are prepared by interfacial polymerization using trimesoyl chloride (TMC) solutions in n-hexane and aqueous solutions of m-phenylenediamine (MPD) containing functionalized CNTs. The functionalized CNTs are prepared by the reactions of pristine CNTs with acid mixture (sulfuric acid and nitric acid of 3:1 volume ratio) by varying amounts of acid, reaction temperature, and reaction time. CNTs prepared by an optimized reaction condition are found to be well-dispersed in the polyamide layer, which is confirmed from atomic force microscopy, scanning electron microscopy, and Raman spectroscopy studies. The polyamide RO membranes containing well-dispersed CNTs exhibit larger water flux values than polyamide membrane prepared without any CNTs, although the salt rejection values of these membranes are close. Furthermore, the durability and chemical resistance against NaCl solutions of the membranes containing CNTs are found to be improved compared with those of the membrane without CNTs. The high membrane performance (high water flux and salt rejection) and the improved stability of the polyamide membranes containing CNTs are ascribed to the hydrophobic nanochannels of CNTs and well-dispersed states in the polyamide layers formed through the interactions between CNTs and polyamide in the active layers.

A novel high temperature resistance thin film composite polyamide reverse osmosis membrane with covalent organic frameworks intermediate layer

Carbon nanotube arrays as multilayer transverse flow carbon nanotube membrane for efficient desalination

Novel thin-film reverse osmosis membrane with MXene Ti3C2Tx embedded in polyamide to enhance the water flux, anti-fouling and chlorine resistance for water desalination