Membranes For Water Desalination Research Articles

Influence of acid/base activation treatment in the performance of recycled electromembrane for fresh water production by electrodialysis

In this study, an activation treatment for recycled anion exchange membranes is proposed. Following the circular economy approach, these membranes were prepared by using end-of-life reverse osmosis membranes as mechanical support. The end-of-life membrane was previously used and discarded by desalination plants after overcoming its lifespan. The activation treatment was based on the subsequent immersion of the membranes in diluted acid and alkali solutions. This treatment promoted the complete dissociation of the functional groups in the membrane, making them more reactive to the counter ions. The effects of acid and alkali concentrations and exposition times on the electrochemical properties were studied and the best combination was selected. In such a way, a decrease of 37% in membrane electrical resistance was achieved. The performance of activated and non-activated membranes in brackish water desalination by electrodialysis was compared. The results showed that the proposed activation treatment increased the flux of fresh water more than four-fold (from 1.2 to 4.9 L h−1·m−2), with a considerable reduction of energy consumption (from 5.2 to 3.0 kWh·m−3) and a great improvement in current efficiency (from 38% to 71%). In conclusion, this work shows a simple and low cost methodology for the improvement of the electrochemical properties of recycled electromembranes and thus, their performance in electrodialysis.

Synthesis, Characterization, and Evaluation of Evaporated Casting MWCNT/Chitosan Composite Membranes for Water Desalination

Fresh water scarcity and pollution turn out to be a most serious issue throughout the world due to the rapid population growth. The application of nanomaterials (NMs) for the removal of pollutants from water has attracted significant attention. The nanofiltration membrane was fabricated through the evaporative casting (EC) method using multiwalled carbon nanotubes (MWCNT) and chitosan (CHIT) as the surfactant to enable water purification. The developed EC membrane properties were characterized in mechanical, surface charging (zeta potential), surface morphology, and hydrophobicity properties. Results demonstrated that incorporation of MWCNT and the biopolymers (chitosan) resulted in suitable developments in mechanical properties of the membrane. For example, the membrane has shown values for tensile strength (28 ± 1 MPa), elongation (10.2 ± 1.2%), Young’s modulus (1.2 ± 0.1 GPa), and toughness of (1.9 ± 0.2 J/g). When more significant changes were investigated on the surface morphology of the EC membrane, it was observed that the pores on the surface morphology of the EC membrane decreased as the evaporative casting method was used. Moreover, the permeability of the membrane towards water, inorganic salts, and pH effect on salt rejections was studied using the NF/RO system. These established nanocomposite membranes signify the promising candidates for fresh water desalination and elimination of organic impurities.

Functionalization of hybrid organosilica based membranes for water desalination – Preparation using Ethyl Silicate 40 and P123

Hybrid organosilica are recency type of membranes derived from Ethyl Silicate 40 (ES40) and pluronic triblock copolymer (P123) precursors. These materials were employed to improve the hydrostability of pure silica matrices becomes stronger especially when applied for water desalination. This work shows for the first time the functionalization of hybrid ES40-P123 materials which prepare for organosilica membranes fabrication. The aim of this work is to investigate the vibration of siloxane (Si-O-Si), silanol (Si-OH) and silica-carbon (Si-C) groups of hybrid organosilica (ES40-P123) xerogels measured by Fourier Transform Infra-Red (FTIR) and fityk software. Silica sols are produced by combining ES40 and P123 materials using sol-gel method through hydrolysis and condensation reactions with acid-base catalysts. P123 was added together with base catalyst in sol mixtures to form hybrid silica P123 structures. This process was held at 50 °C for 3 h of reflux. The final molar ratios of hybrid organosilica (ES40-P123) of the ES40:ethanol:nitric acid:H2O:ammonia:P123 were calculated to be 1:120:0.0022:16:0.009:0.1, respectively. From the FTIR measurement, the functionalization of organosilica (ES40-P123) xerogels was detected as siloxane (Si-O-Si) groups at the wavelength 1067 and 1220 cm−1. Silanol (Si-OH) groups at 958 cm−1. Then, silica-carbon (Si-C) group was detected at wavelength 801 cm−1. The presence of silica-carbon indicates the organosilica structure become enhanced and more stable toward water.

Reverse osmotic characteristics and mechanism of hydrogenated porous graphene

Graphene-based materials have aroused great interest for their potential applications in water desalination and purification membranes attributed to their ultrathin thickness, high mechanical strength, and anti-foiling properties. Reverse osmosis (RO) technology is currently the most progressive, energy-saving and efficient separation technology by membranes, therefore the new materials with high strength, strong pollution resistance and excellent performance are urgently needed. The ability of porous graphene to serve as a kind of novel advanced RO membrane is due to two major potential strengths of this atomically thin two-dimensional material, i.e., ultrahigh permeability and super selectivity. Thus, the reverse osmotic properties of the porous graphene membranes should be further investigated theoretically. In this paper, classical molecular dynamics method is used to investigate the reverse osmosis characteristics of brine in hydrogenated porous graphene reverse osmosis membrane. The results show that the water permeation rate increases with the driving force, pore size and temperature increasing, for the pore diameter larger than the hydration radius. The ion rejection rate decreases with the driving force and temperature increasing. Interestingly, as the porous graphene moves in the tangential direction to perform a shearing process, the interception rate of the salt ions can be effectively improved and the concentration difference polarization phenomenon can be reduced with the tangential velocity increasing, although the water flux decreases slightly. The influence mechanism of each parameter on permeability and on water flux are explored by analyzing the hydrogen bond distribution, the ionic hydration in feed solution, and the energy barrier of the water molecules in penetrating process. In order to further evaluate the effects of various parameter changes on the benefits of reverse osmosis membranes, both the selectivity and permeability are calculated to evaluate the tradeoff between permeability and selectivity, indicating that the increase of the pore diameter can obtain both high permeability and selectivity under the shearing circumstance of the membrane. The research results in this paper will provide a theoretical understanding of porous graphene-based desalination membrane and also may be helpful in designing the shearing graphene-based water filtration devices.

From a bulk to nanoconfined water chain: bridge water at the pore of the (6,6) carbon nanotube.

Hydrophobic porous materials with nano-pores are critical in many processes such as water desalination and biological membrane transportation. Herein, we performed molecular dynamics (MD) simulations on a prototypical hydrophobic nanochannel consisting of a (6,6) carbon nanotube (CNT) of 4.12 Å in radius and 13.72 Å in length immersed in water. The simulation shows that there are two major filling numbers of water N = 5 and N = 6, with the former being the most stable one. The confined waters form a single-file water chain with two hydrogen bonds per water. An extending water chain is formed for N = 5, with a bridge water near the pore of the CNT linking the water confined inside the CNT and hydration layer around the pore of the CNT. The bridge water can be considered as intermediate water characterized by three hydrogen bonds that distinguish from the confined water and bulk water. On the other hand, the hydration layer is depleted from the pore when N = 6. The analyses of the correlation of the bond order for the adjacent hydrogen bond pair of the hydration layer around the pore of the CNT does not show apparent difference from that of bulk water, though the former is slightly ordered. van Hove analysis of the bridge water shows that it tends to move inside the CNT when N < 5, in order to maintain the chemical equilibrium between the confined water and bulk water. This study highlights the unique structure of water around the hydrophobic pore of a sub-nanometer nanochannel.

Atomistic insights into the separation mechanism of multilayer graphene membranes for water desalination.

Graphene-based membranes have been extensively explored owing to their excellent separation properties. In this paper, multiple factors regarding desalination performance were investigated by molecular dynamics (MD) simulations. These factors include the interlayer spacing distance (H), the gap width (dG), offset (O), and the number of gaps and layers in a multilayer graphene membrane (MGM). It is found that salt rejection is influenced significantly by the interlayer spacing distance owing to the largest free energy between ions and graphene sheets as well as the relatively larger size of the hydration layer around the ions. The optimal desalting parameter (dG = 1 nm, H = 0.8 nm) was selected; MGM systems based on the optimized parameter exhibited excellent salt rejection for NaCl, MgCl2 and CaCl2 solutions. These results can provide some ideas for the future design of graphene-based membranes.

Deconvolution of carbon silica templated thin film using ES40 and P123 via rapid thermal processing method

This work aims to fabricate silica-P123 (Pluronic-123) xerogels and investigates the vibrational carbon groups of ES40-P123 xerogels using FTIR analysis and fityk software. In this work, ES40 (ethyl silicate 40) was used as the silica precursor, and P123 as the template agent. Meanwhile, HNO3 and NH3 take a role as acid and base catalyst. The combination of silica materials between of ES40 and P123 as an organic polymer was prepared via sol–gel process. The obtained of silica-P123 sol derived from ES40 was then dried at 33.15 K for 172,800 s which named xerogel. The molar ratio for ES40: ethanol: nitric acid: ammonia: aquadest: P123 were 1: 38: 0.0007: 0.003: 5: 0.0207, respectively. Rapid Thermal Processing (RTP) technique is favorable for calcination at 723 K for 3600 s. Then, FTIR analysis was applied to analyze the xerogel samples. The wavelength results were 800; 975; and 1080 (x10-2 m−1) for Si-C; Si-OH, and Si-O-Si, respectively. In addition, the area of pure ES40 for Si-O = 9.21 × 10−5; Si-O-Si = 1.0344 × 10−3; Si-C = 8.28 × 10−5; and for ES40-P123 template were 9.51 × 10−5; 1.0264 × 10−3; 8.10 × 10−5 in m2 unit. Hence, the peak area ratios of pure ES40 and ES40-P123 template were 0.089 and 0.093, respectively. It is clearly shown that the polymerisation by templating carbon groups into silica matrices was successfull and promising potential to apply in RTP technique as well as to then fabricate organosilica membrane for water desalination.

Features of pulsed photon annealing of graphene oxide membranes for water desalination

Fabrication of reduced graphene oxide/polyvinylidene fluoride (rGO/PVDF) membranes by coating the surface of standard polymeric PVDF membranes with thin graphene oxide (GO) film is demonstrated. rGO/PVDF membranes have a better filtration efficiency than standard polymer PVDF membranes. For stabilization of rGO/PVDF membranes, a pulsed photon annealing has been used. Increase of water flow through the rGO/PVDF membranes with the decrease of the nanochannel sizes has experimentally been shown. Water transportation via planar nanochannels of rGO membranes depends on an rGO reduction time, which is explained by a decrease of the water shear viscosity activation energy in nanochannels.

Controlled Pore Generation in Single-Layer Graphene Oxide for Membrane Desalination.

Nanoporous graphene was proposed as an efficient material for reverse osmosis water desalination membranes because it allows water molecules to pass at high flux while rejecting hydrated salt ions. Nevertheless, from an experimental point of view it is still difficult to control the pore size. A scalable method to generate pores is urgently required for the diffusion of this technology. We propose, by theoretical calculations, an innovative and scalable strategy to better control the dimension of the pores in graphene-based membranes by reduction of single-layer graphene oxide (GO). The latter is first annealed at a controlled mild temperature to induce the aggregation of its randomly distributed oxygen-containing functional groups into small nanometric clusters. The layer then undergoes a high-temperature reducing treatment that causes the desorption of the functional groups along with carbon removal only in the oxidized areas, producing subnanometric pores while leaving unchanged the remaining pristine graphene areas.

Electrospun Nanofibrous Polyphenylene Oxide Membranes for High-Salinity Water Desalination by Direct Contact Membrane Distillation

Desalination by the membrane distillation (MD) process has been proposed to significantly increase the water recovery ratio, especially treating with highly saline waters, for example, seawater and...

Functionalized nanoporous graphene membrane for water desalination; Effect of feed salinity on permeability and salt rejection, a molecular dynamics study

Feed water composition are sometimes instable and thus affecting the desalination processes. The effect of feed water salinity on permeability and salt rejection of graphene-based desalination membranes as novel desalination material, was investigated using molecular dynamics simulation to examine the performance of the membrane and permeate water quality. Hydroxylated (OH) pore was tested with pore diameter of 7.4 Å. Three feed water salinity of 33.4 (sea water), 50.1, 66.8, and 83.5 g/L was studied. The result indicated that best water permeability was observed for 33.4 g/L of water salinity while salt rejection of this membrane was lowest for this salinity. Concentration polarization graphs were obtained for desalination process indicating a concentrated area before the membrane as salty as more than 1000 g/L of while the highest off-membrane salinity reached to utmost 2 g/L. Density map plots for water molecules near the nanopores clearly displayed the orientation of water molecules as function of time and coordinate which proves that higher salinity causes lower congestion of water molecules near the pore as result of hydration effects or decreased water permeability as a result of a considerable drop in water entropy. This observation was approved by radial distribution graphs of water molecules around Na and Cl ions that displayed higher bounding at higher salinity. The effect of hydrated ions on water transport and salt rejection revealed the importance of the finely manipulating the feed composition.

Relating the performance of sulfonated thin-film composite nanofiltration membranes to structural properties of macrovoid-free polyethersulfone/sulfonated polysulfone/O-MWCNT supports

Sulfonated thin-film composite nanofiltration (TFC NF) membranes were produced on macrovoid-free polyethersulfone/sulfonated polysulfone/oxidised multi-walled carbon nanotube (PES/SPSf/O-MWCNT) supports via an interfacial polymerisation method. The aqueous phase solution consisted of a mixture of piperazine (PIP) and 2,4-diaminobenzene sulfonic acid (2,4-DABSA) and the organic phase solution consisted of trimesoyl chloride (TMC). The TFC NF membranes fabricated on the PES/SPSf/O-MWCNT supports showed a 35% improvement in pure water flux with comparable salt rejections from those prepared on PES/SPSf supports. The presence of hydrophilic O-MWCNTs in the PES/SPSf supports resulted in the formation of a thin polyamide film on the surface of the support with enhanced water permeability and salt rejection. The addition of low monomer weight ratio of 2,4-DABSA in the diamine aqueous mixture led to the generation of sulfonated TFC NF membranes with superior membrane performance in terms of pure water permeability (30.2 L/m2·h·bar), monovalent/bivalent salt selectivity (αNaCl/Na2SO4 = 25.0) at low operating pressures (3 bar) and at salt concentrations in the range of brackish waters. Moreover, the fabricated sulfonated TFC NF membranes exhibited good antifouling properties against bovine serum albumin, with a flux recovery ratio of 96.4% obtainable after three cycles of fouling and cleaning. The performance of the sulfonated TFC NF membranes remained fairly stable over a 10-day period of pure water flux and Na2SO4 salt rejection testing. The fabricated sulfonated TFC NF membranes displayed promising features for use as pre-treatment membranes for lower-pressure water softening and desalination of brackish water.

Interaction between water and carbon nanostructures: How good are current density functional approximations?

Due to their current and future technological applications, including realization of water filters and desalination membranes, water adsorption on graphitic sp2-bonded carbon is of overwhelming interest. However, these systems are notoriously challenging to model, even for electronic structure methods such as density functional theory (DFT), because of the crucial role played by London dispersion forces and noncovalent interactions, in general. Recent efforts have established reference quality interactions of several carbon nanostructures interacting with water. Here, we compile a new benchmark set (dubbed WaC18), which includes a single water molecule interacting with a broad range of carbon structures and various bulk (3D) and two-dimensional (2D) ice polymorphs. The performance of 28 approaches, including semilocal exchange-correlation functionals, nonlocal (Fock) exchange contributions, and long-range van der Waals (vdW) treatments, is tested by computing the deviations from the reference interaction energies. The calculated mean absolute deviations on the WaC18 set depend crucially on the DFT approach, ranging from 135 meV for local density approximation (LDA) to 12 meV for PBE0-D4. We find that modern vdW corrections to DFT significantly improve over their precursors. Within the 28 tested approaches, we identify the best performing within the functional classes of generalized gradient approximated (GGA), meta-GGA, vdW-DF, and hybrid DF, which are BLYP-D4, TPSS-D4, rev-vdW-DF2, and PBE0-D4, respectively.

Nanotechnology in water and wastewater treatment. Graphene – the nanomaterial for next generation of semipermeable membranes

In recent decades, novel and promising graphene-based materials (i.e., carbon nanotubes, nanoporous graphene and graphene oxide) suitable to be used in preparation of high-capacity membranes for water desalination and water and wastewater treatment have been developed. After detail recognition of properties of nanoporous graphene and graphene oxide-based materials, the general conclusion on preparation of membranes made of only graphene or only graphene oxide as well as other composite materials involving these nanomaterials has been given. Novel composite membranes containing nanoporous graphene and, first of all, graphene oxide can be divided into two groups: (i) freestanding membranes made only of graphene or only of graphene oxide, and (ii) polymeric/ceramic membranes modified with the use of graphene-based nanomaterials. The modification of polymeric membranes can be achieved either by introduction of a nanomaterial onto a membrane surface or its addition to a membrane casting solution followed by membrane formation from a polymer and a nanomaterial mixture. Membranes based on nanomaterials enable to obtain significantly higher water/permeate fluxes than currently used thin film composite polyamide membranes. The future prospect of membranes based on nanoporous graphene and graphene oxide in regard to the final separation efficiency and commercial scaling up has been discussed.

Tailoring the internal void structure of polyamide films to achieve highly permeable reverse osmosis membranes for water desalination

The considerable permeability of fully-aromatic polyamide reverse osmosis (RO) membranes is believed to be strongly dependent on the presence of a mount of internal voids within the polyamide separation layer, which provides additional permeation channels in addition to diffusion through the polyamide matrix. Based on this understanding, herein, we reported a novel strategy of developing highly permeable RO membranes via introducing a void-tailoring agent, (3,3,3-trifluoropropyl)trichlorosilane (TFPTCS) into the traditional interfacial polymerization (IP) system composed of m-phenylenediamine and trimesoyl chloride. During IP process, the fast and violent hydrolysis and aminolysis of TFPTCS caused a significantly increasing pore size and porosity of the incipient polyamide film, which resulted in the formation of double- or multi-layer void structures with higher void fraction in the polyamide film. As a result, the TFPTCS added membrane showed a dramatic enhancement in water permeability, and the optimum condition was observed at the 0.03 wt% of TFPTCS concentration with 3.74 L m−2 h−1 bar−1, which was 2.8 times higher than that of the pristine membrane, while the salt rejection was almost unchanged (around 97.0%). These findings demonstrate the feasibility of void-tailoring inside the polyamide film, and encourage further exploration of new alternative void-tailoring agents to prepare highly permeable polyamide RO membranes for water desalination, which were seldom studied previously.

High performance cross-linked dehydro-halogenated poly (vinylidene fluoride-co-hexafluoro propylene) based anion-exchange membrane for water desalination by electrodialysis

We are reporting synthesis of novel cross-linking agent 1,4-bis(4-vinylbenzyl)-1,4-diazabicyclo[2,2,2]octane-1,4-diium) and cross-linked dehydro-halogenated poly (vinylidene fluoride-co-hexafluoro propylene) (DHPVDF-co-HFP) based anion exchange membranes (AEMs). Reported di-quaternized fluorinated polymer based AEMs are analysed for stabilities, and different properties to assess their suitability for electrodialysis. Attractive structural features such as multi-cationic side chain, fluorinated main polymer chain, absence of SN2 mechanism, and β-hydrogen elimination, are responsible for their excellent performance. Optimized splendid AEM (Cr-AEM/1.02) exhibited 23.5% water content, 1.02 × 10−3 equiv/g ion-exchange capacity, and 5.7 × 10−2 S/cm conductivity at 30 °C, which are superior than other 1,4-diazabicyclo[2,2,2]octane/imidazolium functional groups containing AEMs reported in the literature, or commercialized AEMs. Because of di-quaternized nature, high permselectivity (0.94–0.96) of the reported membrane (Cr-AEM/1.02) has been obtained chronopotentiometric and membrane potential studies. Relatively high limiting current density (Ilim) (2.36 mA/cm2) of Cr-AEM-1.02 provides a broad window to achieve fast electrodialysis under relatively high applied voltage/current without any electrode reaction. Further, about 95% current efficiency and 3.1 kWhr kg−1 energy consumption confirmed efficient nature of Cr-AEM/1.02 for salt removal by electrodialysis.

Highly antibacterial rGO/Cu2O nanocomposite from a biomass precursor: Synthesis, performance, and mechanism

Reduced graphene oxide (rGO) has been widely used to fabricate electronics, sensors, photodetectors,and in other applications. However, the antibacterial performance of pristine rGO is relatively weak. The application of rGO in biomedical devices, smart food packaging, and water desalination membranes requires further improvement of rGO’s antibacterial abilities. Copper(I) oxide (Cu2O) is an effective antibacterial agent, which denatures protein and enhances the permeability of cell membranes. In this work, we report a simple method of synthesizing a highly antibacterial rGO/Cu2O nanocomposite from cellulose acetate, a derivative of abundant natural cellulose. The synthesized rGO/Cu2O nanocomposite was thoroughly characterized by Raman spectroscopy, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and scanning transmission electron microscopy (STEM). Then, the antibacterial abilities of rGO/Cu2O nanocomposite were evaluated and a bactericidal mechanism was revealed from the molecular biology perspective. Results indicate that our synthesized rGO/Cu2O nanocomposite owns strong antibacterial activity, mainly stemming from the uniformly incorporated Cu2O nanocrystals with a lateral size of 5–40 nm.

Designing an All-Carbon Membrane for Water Desalination

We design an all-carbon membrane for the filtration and desalination of water. A unique layered assembly of carbon nanostructures including graphite oxide (GO), buckypaper consisting of carbon nanotubes, and a strong carbon fabric provides high mechanical strength and thermal stability, resilience to harsh chemical cleaning agents and electrical conductivity, thus addressing major shortcomings of commercial reverse osmosis membranes. We use ab initio density functional theory calculations to obtain atomic-level insight into the permeation of water molecules in-between GO layers and across in-layer vacancy defects. Our calculations elucidate the reason for selective rejection of solvated Na$^+$ ions in an optimized GO membrane that is structurally stabilized in a sandwich arrangement in-between layers of buckypaper, which are protected on both sides by strong carbon fabric layers.

Role of Cellulose Micro and Nano Crystals in Thin Film and Support Layer of Nanocomposite Membranes for Brackish Water Desalination.

Reverse osmosis is a major process that produces soft water from saline water, and its output represents the majority of the overall desalination plants production. Developing efficient membranes for this process is the aim of many research groups and companies. In this work, we studied the effect of adding cellulose micro crystals (CMCs) and cellulose nano crystals (CNCs) to the support layer and thin film nanocomposite (TFN) membrane on the desalination performance. SEM, TEM, ATR-FTIR, and contact angle measurements were used to characterize the membrane’s properties; and membrane’s performance were evaluated by water flux and NaCl rejection. Filling 2% of CNCs gel in the support layer improved the water flux by +40%, while salt rejection maintained almost the same, around 95%. However, no remarkable improvement was gained by adding CNCs gel to m-phenylenediamine (MPD) solution, which was used in TFN membrane preparation. Filling CMCs powder in TFN membrane led to a slight improvement in terms of water flux.

Enhancement of desalination performance of thin-film nanocomposite membrane by cellulose nanofibers

The incorporation of nanoparticles in a polymer membrane has been recognised as an effective strategy to improve the membrane performance in water processing. However, the leaching of nanoparticles from the membrane is potentially a concern as it may lead to the change of membrane properties and contamination of treated water. In this work, 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibers (CNFs) were for the first time incorporated into the polyamide layer of thin film composite (TFC) reverse osmosis (RO) membrane during interfacial polymerization to improve its membrane performance. CNFs are made from renewable and sustainable resources and they are non-toxic and environmentally friendly. The optimal loading of CNFs within the TFC membrane enhanced the permeance of the fabricated membranes by more than 50% (29.8 L m−2 h−1 at operating pressure of 1.5 MPa), whilst retaining high NaCl rejection at 96.2%. Chlorine stability test results indicated that the nanocomposite membranes showed better chlorine resistance performance than bare TFC membranes. This work successfully demonstrated the potential use of CNFs as a low cost and non-toxic filler to improve the performance of RO membranes for water desalination.