Multi-walled Carbon Nanotube Membranes Research Articles

Separation of hydrogen isotopic water by multi-walled carbon nanotube (MWCNT) membrane and graphene oxide (GO)-MWCNT composite membranes

Recently, membrane distillation (MD) processes using two-dimensional graphene oxide (GO) membranes exhibited good light water/deuteriated water (H2O/D2O) separation performance, which depended on membrane materials and structures. This raises the need for the development of novel membrane materials and revealing the relationship of membrane structure-separation performance to further increase separation performance. Here, one-dimensional (1D) MWCNT-based membrane was explored for the separation of H2O/D2O using the water source with specially chosen concentration of deuterium (D) in air–gap membrane distillation processes, and the resultant MWCNT membrane gave lower permeation flux and similar separation factor compared with the pristine GO membrane. Then GO-MWCNT composite membranes with different structures obtained by mixing various weight ratios of GO: MWCNTs were investigated, and their separation performance was found to be related with membrane structures. The resulted 3D hierarchical GO-MWCNT composite membrane (G/M−1:3) gave the highest permeation flux of 0.947 L·m−2·h−1 among all GO-MWCNT composite membranes, higher than that of the pristine GO membrane, while the separation factor of G/M−1:3 (1.045) and the pristine GO membrane (1.046) was comparable. Furthermore, the correlation of membrane structure-separation performance was reviewed. These results demonstrate the first example of MWCNT membrane and related composite membranes for H2O/D2O separation and suggest that H2O/D2O separation performance in membrane distillation processes can be improved by tailoring the structure of GO-MWCNT composite membranes. As the separation of H2O/D2O is usually used as a model in the laboratory for light water/tritiated water (H2O/HTO) separation process, this approach can guide for developing novel membrane materials and structures to deliver appropriate H2O/HTO separation processes needed in the nuclear energy field.

Thermally Stable, Thin, Ultralight, Reusable, and Flexible Multiwalled Carbon Nanotube Membranes for Removal of Heavy Metals, Polycyclic Aromatic Hydrocarbons, and Particulates from Coal Smoke

Thermally Stable, Thin, Ultralight, Reusable, and Flexible Multiwalled Carbon Nanotube Membranes for Removal of Heavy Metals, Polycyclic Aromatic Hydrocarbons, and Particulates from Coal Smoke

Preparation and characterization of forward osmosis cellulose acetate butyrate/OH‐functionalized multiwalled carbon nanotube membrane for the concentration of bitter orange juice

AbstractAs an essential process in food industry, fruit dewatering should have the least impact on the nutritional values of the product. In this research, bitter orange juice was concentrated via the forward osmosis (FO) process for the first time. For this purpose, cellulose acetate butyrate (CAB)/OH‐functionalized multiwalled carbon nanotubes (FMWCNTs) membranes were prepared using various nanofiller concentrations. The obtained membranes were characterized in terms of their morphology, porosity, water contact angle, chemical structure, tensile properties, and pure water flux. During the FO process, NaCl solution was applied as the draw solution (DS), and the concentration of NaCl was varied (1.2, 2.2, and 4.2 M) to assess its effect on the process performance. Increasing FMWCNTs loading in the casting solution up to 0.3 wt% boosted the membrane porosity, hydrophilicity, elastic modulus, and pure water flux. The CAB membrane incorporated with 0.3 wt% nanofiller and the DS with 4.2 M NaCl concentration represented the highest water flux (29.93 L/m2h) in the FO experiment and provided the maximum turbidity (1545 NTU) and viscosity (4.64 cP) of the treated feed. According to the obtained results, the developed CAB/FMWCNTs membrane proved promising in the FO concentration of fruit juice.Practical ApplicationsBitter orange juice was concentrated via the FO process using the developed CAB/OH‐FMWCNTs membranes. According to the results, water flux, and thus the dewatering efficiency increased and reached its highest value using the CAB membrane incorporated with 0.3% FMWCNTs. Since membrane processes operate at low temperatures and do not deteriorate the concentrate quality, FO concentration of bitter orange juice using the membrane modified in this study can be a superior alternative to the conventional thermal methods.

Study of Membranes with Nanotubes to Enhance Osmosis Desalination Efficiency by Using Machine Learning towards Sustainable Water Management.

Water resources management is one of the most important issues nowadays. The necessity of sustainable management of water resources, as well as finding a solution to the water shortage crisis, is a question of our survival on our planet. One of the most important ways to solve this problem is to use water purification systems for wastewater resources, and one of the most necessary reasons for the research of water desalination systems and their development is the problem related to water scarcity and the crisis in the world that has arisen because of it. The present study employs a carbon nanotube-containing nanocomposite to enhance membrane performance. Additionally, the rise in flow brought on by a reduction in the membrane's clogging surface was investigated. The filtration of brackish water using synthetic polyamide reverse osmosis nanocomposite membrane, which has an electroconductivity of 4000 Ds/cm, helped the study achieve its goal. In order to improve porosity and hydrophilicity, the modified raw, multi-walled carbon nanotube membrane was implanted using the polymerization process. Every 30 min, the rates of water flow and rejection were evaluated. The study's findings demonstrated that the membranes have soft hydrophilic surfaces, and by varying concentrations of nanocomposite materials in a prescribed way, the water flux increased up to 30.8 L/m2h, which was notable when compared to the water flux of the straightforward polyamide membranes. Our findings revealed that nanocomposite membranes significantly decreased fouling and clogging, and that the rejection rate was greater than 97 percent for all pyrrole-based membranes. Finally, an artificial neural network is utilized to propose a predictive model for predicting flux through membranes. The model benefits hyperparameter tuning, so it has the best performance among all the studied models. The model has a mean absolute error of 1.36% and an R2 of 0.98.

Effects of modification groups and defects on the desalination performance of multi-walled carbon nanotube (MWNT) membranes

Like single-wall carbon nanotubes (SWNTs), multi-wall carbon nanotubes (MWNTs) have channels for selective separation of ions. Moreover, MWNTs have the advantages of simpler preparation and lower cost, potentially allowing wider applications in the desalination industry. In this study, the transport of water and ions inside two types of MWNTs, i.e., double-wall carbon nanotubes (DWNTs) and tri-wall carbon nanotubes (TWNTs) were investigated using non-equilibrium molecular dynamics simulations. Modification groups, including carbon chains of different lengths and types with hydrophilic groups like carboxylate anion (COO–), and ammonium (NH3+), were used to improve the salt rejection performance. The simulation results suggest that 100% salt rejection could be achieved by adding different modification groups to various MWNTs ranging from 13 to 20 Å. More importantly, this study demonstrates that MWNT membranes with 100% desalination have water fluxes that are 10–30% greater than that of SWNT membranes with 100% desalination, implying the greater potential of MWNTs as building blocks towards next-generation desalination membranes. Additionally, even though all real-world carbon nanotubes (CNTs) have a certain amount of defects from the production process, most previous molecular model studies of water and ions transport in CNTs assumed that CNTs were perfect and defect-free. This study shows for the first time that CNTs with defects have improved salt rejection rates than perfect carbon nanotubes, although the former have lower water fluxes than the latter.

Development and Characterization of Silver-Doped Multi-Walled Carbon Nanotube Membranes for Water Purification Applications.

A unique approach was utilized to develop multi-walled carbon nanotube (MWCNT) silver (Ag) membranes. MWCNTs were impregnated with 1 wt% Ag loading, which resulted in a homogeneous dispersion of Ag in MWCNTs. MWCNTs impregnated with Ag were then uniaxially compacted at two different pressures of 80 MPa and 120 MPa to form a compact membrane. Compacted membranes were then sintered at two different temperatures of 800 °C and 900 °C to bind Ag particles with MWCNTs as Ag particles also act as a welding agent for CNTs. The powder mixture was characterized by FESEM, thermogravimetric analysis, and XRD, while the developed samples were characterized by calculating the porosity of membrane samples, contact angle, water flux and a diametral compression test. The developed membranes showed overall large water flux, while maximum porosity was found to decrease as the compaction load and sintering temperature increased. The mechanical strength of the membranes was found to increase as the compaction load increased. The hydrophilicity of the membranes remained unchanged after the addition of Ag particles. The developed membranes would be useful for removing a variety of contaminants from water.

Detection of formic acid and acetic acid gases by a QCM sensor coated with an acidified multi-walled carbon nanotube membrane

ABSTRACT In this paper, the acidified multi-walled carbon nanotubes (MWCNTs) film was coated on the quartz crystal microbalance (QCM) to prepare a high-performance sensor for the real-time detection of organic acid gases. The material characteristics of the thin films were analysed by field emission scanning electro microscopy (FESEM), Raman spectra and X-ray photoelectron spectroscopy (XPS). The organic acid vapours’ sensing results indicated that acidized-MWCNTs thin film exhibited good frequency response, repeatability, reversibility and stability. There is a clear linear relationship between the frequency offset and the organic acid vapours with concentration below 5.0 ppm, and the detection limit of 0.77 and 0.73 ppm for formic and acetic acid vapours, respectively. The sensor shows the highest response to formic acid vapour than acetic acid vapour which may be ascribed to molecular polarity. Furthermore, a sensing mechanism model was introduced to understand the adsorption reaction between organic acid molecules and acidized-MWCNTs. This paper proves that acidized-MWCNTs is a potential and suitable material for organic acid vapour detection when combined with a QCM sensor.

Calcium-enhanced retention of humic substances by carbon nanotube membranes: Mechanisms and implication

Efficient removal of humic substances (HS) in natural water is of importance to drinking water treatment. In this study, the adsorption and retention of HS by multi-walled carbon nanotube (MWCNT) membranes were systematically investigated by dynamic filtration of synthetic and natural surface water. In the absence of Ca(II), HS were dominated by small, dissolved species albeit the varying pH. Accordingly, HS retention by the MWCNT layers only ranged from 15%-65% at the end of the filtration. In contrast, the presence of Ca(II) in the feed water partially transformed HS molecules into colloidal aggregates as found by light scattering analyses. Furthermore, molecular dynamics (MD) simulation results reveal that Ca(II) complexation with -COO- on MWCNT and humic acid (HA) not only leads to HA aggregation in the feed solution, but also promotes HA adsorption onto carboxylated MWCNT. The modeling results are consistent with the high retention of HS by the carboxylated MWCNT membrane, i.e., >90% for the synthetic model water and >85% for the natural water, at a moderate calcium concentration range of 0.5–2.0 mM. Considering the widespread presence of calcium in natural water, these findings suggested that carboxylated MWCNT has a potential for effective adsorptive filtration of HS in drinking water.

The preparation and characterization of buckypaper made from carbon nanotubes impregnated with chitosan

AbstractBiopolymer chitosan was incorporated into a thin multiwalled carbon nanotube membrane (MWNT buckypaper) via filtration and soaking in 0.1% (w/v) of low‐molecular‐weight (MW) chitosan. The properties of the buckypaper membrane before and after annealing and after soaking were characterized by measurement of their electrical conductivities (19 ± 2 to 42 ± 2 S/cm), contact angles (31 ± 4° to 71 ± 4°), and mechanical properties (tensile strength, small‐ranging between 1.4 ± 0.1 and 4.2 ± 0.7 MPa; Young's modulus: 85 ± 4 to 443 ± 20 MPa). Moreover, the morphological properties, surface area, and permeability toward water of these buckypaper membranes were characterized and compared with corresponding carbon nanotube membranes prepared with Triton X‐100 (Trix) as the surfactant. Scanning electron microscopic (SEM) images and Brunauer, Emmett, and Teller (BET) data of MWNT‐annealing buckypaper membranes revealed that the diameters of their surface pores were significantly higher than that of the corresponding buckypaper membranes soaked in chitosan solution. The solution of chitosan incorporated inside the porous structure of the annealed MWNT membrane led to a significantly reduced surface area and pore size distribution of the composite membrane, revealing that this could be a useful method for desalination.

Enhanced virus filtration in hybrid membranes with MWCNT nanocomposite.

Membrane separation is proved to be a powerful tool for several applications such as wastewater treatment or the elimination of various microorganisms from drinking water. In this study, the efficiency of inorganic composite-based multi-walled carbon nanotube (MWCNT) hybrid membranes was investigated in the removal of MS2 bacteriophages from contaminated water. With this object, multi-walled carbon nanotubes were coated with copper(I) oxide, titanium(IV) oxide and iron(III) oxide nanoparticles, respectively, and their virus removal capability was tested in both batch and flow experiments. Considering the possible pH range of drinking water, the filtration tests were carried out at pH 5.0, 7.5 and 9.0 as well. The extent of MS2 removal strongly depended on the pH values for each composite, which can be due to electrostatic interactions between the membrane and the virus. The most efficient removal (greater than or equal to 99.99%) was obtained with the Cu2O-coated MWCNT membrane in the whole pH range. The fabricated nanocomposites were characterized by X-ray diffraction, specific surface area measurement, dynamic light scattering, zeta potential measurement, Raman spectroscopy, transmission electron microscopy and scanning electron microscopy. This study presents a simple route to design novel and effective nanocomposite-based hybrid membranes for virus removal.

Multiwalled carbon nanotube membranes for water purification

Abstract For the development of advanced membrane technologies, a good understanding of the materials properties and their transport mechanisms, as well as the realization of innovative functional materials with improved properties, are key issues. Due to their attractive permeability characteristics, various nano-structured RO membranes were proposed with the incorporation of carbon nanotubes. The growth in computational power has now made possible simulating tubes with characteristics closer to the real material, providing novel insights in the water flux to tube structure relationship. However, the understanding of the effect of the different number of walls, remains challenging and as a result, a deeper understanding is needed of how these changes can affect the performance of the membrane at the molecular level. To address this issue, molecular dynamic simulation (MD) is well known to be a powerful tool to enhance the understanding of nanoscale systems. This theoretical work provides new insights on the effect of walls in Multi Walled Carbon nanotube membranes, as model case of a CNT nanocomposite RO membrane, for desalination applications. Specifically, two types of vertically aligned multi Walled Carbon Nanotube membranes, MWCNT (6,6) and MWCNT (8,8), were analyzed theoretically by means of non-equilibrium Molecular Dynamics simulations, in order to study the influence of the number of walls on permeation of reverse osmosis simulations. A comparison of the two membranes formed by using differently sized tubes give us the estimation of the level of desalination and efficiency. The carbon nanotube membranes were modeled using two graphene sheets and single walled (SW), double walled (DW), three walled (TW) for both (6,6) and (8,8) configurations, and six walled (6W) for (6,6). Simulations were conducted with a hydrostatic pressure difference to investigate the ion rejection and water conductance in each membrane system. MWCNT (6,6) are very selective to ions whilst MWCNT (8.8) were more permeable to water. The different behaviour was justified in terms of the different size of entrance of CNT with a cooperative effect due to numbers of walls of CNT and the hydrophobic effect of the graphene layers. A good agreement is found if we compare our system with some functionalized SWCNT (8,8) (Corry, 2011) and with that of protein aquaporin-1 (Zhu et al., 2004). As the number of walls is augmented, the water conductance followed the same trend with a general performance greater than commercial reverse osmosis membranes (Corry, 2011). Under the conditions of our simulations, it appears that the MWCNT (8,8), particularly with DW (8,8) and TW(8,8) could offer an improvement over current generation membranes in terms of water conductance with a relatively good compromise between water conductance and salt rejection.

A versatile technique to transfer multi-walled carbon nanotubes membranes to surfaces

We present a simple and versatile technique to fabricate and transfer multi-walled carbon nanotube (MWCNT) membranes to virtually any substrate, including devices and a large variety of materials with different shapes. First, a mesoporous silica (Mobil Composite material MCM-41)/MWCNT composite is deposited by spin coating on SiO2. Immersion of the wafer in HF solution, releases a floating MWCNT membrane that strongly adheres to any surface that makes contact with it. The process does not require functionalization or surfactants, thus avoiding contamination and additional cleaning steps, and has potential to be extended to allow the fabrication and transfer of membranes of other carbon materials, including other CNT types, fullerenes and graphene.

Electrowetting on a multi-walled carbon nanotube membrane with different droplet sizes in an electric field

An electric field was applied across a deionized water droplet placed on a multi-walled carbon nanotube (MWCNT) membrane. Droplets of different size were tested by varying the voltage applied from 3 to 25 V during the electrowetting process. After electrowetting, it was observed that the topography of the membrane deformed from vertical alignment to a grouped pattern due to bubbles appearing during the electrowetting process, indicating the occurrence of electrolysis. For the droplet size ranging from 2 to 3.5 μl, the contact angle (CA) for smaller droplets reduced more dramatically than for larger droplets at the same voltage, and the contact angle saturation condition also varied in response to the droplet size. To reveal the droplet size effect, the Young-Lippmann equation is modified to simulate CA reduction on a MWCNT membrane.

Immobilization of laccase on functionalized multiwalled carbon nanotube membranes and application for dye decolorization

Myceliophthora thermophilalaccase was covalently immobilized on functionalized multiwalled carbon nanotubes (MWNT) arranged over a supporting membrane to obtain a permeable bio-barrier that could be applied in multibatch or continuous processes.

Nanoscale Bubble Valves on MWCNT Membranes for Chemical Energy Storage

Nanoscale Bubble Valves on MWCNT Membranes for Chemical Energy Storage

Potential of Polarized PVDF/Carbon Nanotube Nanocomposite Scaffolds for Cell Growth

ABSTRACTWe investigated the effects of varying the multiwalled carbon nanotube (MWCNT) contents, as well as the additional use of drawing and poling on the polymorphic behavior and electroactive (piezoelectric) properties of the cast poly(vinylidene fluoride) (PVDF)/MWCNT membranes. Dramatic changes occurred in the polar β-phase crystal contents with the MWCNT loading. An optimum concentration of MWCNT exists for PVDF film polarization. On the other hand, films prepared by electrospinning process exhibited almost constant amount of β-phase with the MWCNT concentration. In this process, polymer fibers with diameters down to the nanometer range, or nanofibers, are formed by subjecting a fluid jet to a high electric field. The remanent polarization and piezoelectric response increased with the β-phase crystals. Cell adhesion and proliferation measured with MTT (Methylthiazolyl diphenyl-tetrazolium bromide) assay coincidentally responded to the polarized PVDF films (β-phase amount).

Characterization and performance study on chitosan-functionalized multi walled carbon nano tube as separator in microbial fuel cell

The influence of Multi Walled Carbon Nano Tube (MWCNT) in oxygen cross over, water absorption and performance of chitosan (CHIT) in Single Chamber Microbial Fuel Cell (SCMFC) was studied. CHIT and non covalently functionalized MWCNT with CHIT (CHIT–MWCNT) were prepared in the form of membranes and characterized using FTIR, XRD, DRS UV, CD, Raman, SEM and TEM to study the interaction of MWCNT with chitosan. CHIT–MWCNT showed lesser water absorption compared to that of CHIT. CHIT–MWCNT membrane showed oxygen mass transfer coefficient (2.66×10−4cm/s) one order less than that of CHIT (2.28×10−3cm/s). The prepared membranes were continuously tested in an in-house built Single Chamber Microbial Fuel Cell (SCMFC). The maximum power density obtained by CHIT–MWCNT composite membrane was higher compared to that of CHIT membrane obtained was 38.16mW/m2 and 46.94mW/m2 for CHIT and CHIT–MWCNT, respectively. The results of the study showed that the MWCNT incorporated chitosan composite membrane had strong influence on the properties and performance of CHIT in microbial fuel cell. Hence, CHIT–MWCNT is a promising candidate for application in microbial fuel cell.

Photo-regenerable multi-walled carbon nanotube membranes for the removal of pharmaceutical micropollutants from water

Pharmaceutical micropollutants fall in the category of "emerging contaminants" in water because of their prevalence and persistence in the aqueous environment, and because of a poor understanding of their low-dose exposure effects on human and animal populations. In this study, photo-regenerable multiwalled carbon nanotube membranes with variable water permeabilities were produced by embedding hierarchical TiO2 structures (having porous, spherical morphology) onto a pre-deposited bed of multi-walled carbon nanotubes (MWNTs) using a modified sol-gel technique. These MWNT-TiO2 composites and their constituent materials were characterized by analytical electron microscopy, surface charge measurement, thermogravimetric analysis, and hydrophobicity determination. The adsorption removal potential of MWNT-TiO2 membranes was demonstrated for three representative pharmaceuticals: acetaminophen, carbamazepine and ibuprofen. The peak initial removal percentages of the pharmaceuticals by the MWNT-TiO2 membranes were 80%, 45%, and 24% for carbamazepine, ibuprofen, and acetaminophen, respectively. The ability of the membranes to be regenerated, once they were saturated with the pharmaceutical compounds, was verified by repeating the adsorption removal experiment on the same membranes after exposure to UV light at 254 nm. Peak removal efficiencies after regeneration were 55%, 32%, and 19% for carbamazepine, ibuprofen, and acetaminophen, respectively, indicating some loss in sorptive capacity upon regeneration. Furthermore, the effect of pH on adsorption of ibuprofen, the pharmaceutical that attained the highest mass loading on the sorbent at equilibrium saturation, was studied and its mechanism of adsorption was proposed at pH below pKa.

Enhanced Condensation, Agglomeration, and Rejection of Water Vapor by Superhydrophobic Aligned Multiwalled Carbon Nanotube Membranes

The separation of gas molecules and water vapor has become increasingly important for electronic, energy, and environmental systems. Here we demonstrate a new mechanism of enhanced condensation, agglomeration, and rejection of water vapor by superhydrophobic aligned multiwalled carbon nanotubes with the intertube distance of 73 nm, channel aspect ratio of ~5.5 × 10(4), and tortuosity of 1.157. The array with the characteristic channel dimension some 300 times greater than the target molecule size effectively suppressed water molecular transport at room temperature with the selectivity as high as ~2 × 10(5) (H(2)/H(2)O). The flow through the interstitial space of nanotubes allowed high permeability of other gas molecules (2.1 × 10(-9) to 3.8 × 10(-8) mol · m/m(2) · s · Pa), while retaining high selectivity, which is orders of magnitude greater than the permeate flux of polymeric membranes used for the water-gas mixture separation. This new separation mechanism with high selectivity and permeate flux, enabled by the unique geometry of aligned nanotubes, can provide a low-energy and cost-effective method to control humidity.

Electrophoretic Transport of Biomolecules through Carbon Nanotube Membranes

Electrophoretic transport of proteins across electrochemically oxidized multi-walled carbon nanotube (MWCNT) membranes has been investigated. A small charged protein, lysozyme, was successfully pumped across MWCNT membranes by an electric field while rejecting larger bovine serum albumin (BSA). Transport of lysozome was reduced by a factor of about 30 in comparison to bulk mobility and consistent with the prediction for hindered transport. Mobilities between 0.33 and 1.4 × 10(-9) m(2) V(-1) s(-1) were observed and are approximately 10-fold faster than comparable ordered nanoporous membranes and consistent with continuum models. For mixtures of BSA and lysozyme, complete rejection of BSA is seen with electrophoretic separations.