A review on electrochemically modified carbon nanotubes (CNTs) membrane for desalination and purification of water

10 - Electrochemically active carbon nanotube (CNT) membrane filter for desalination and water purification

Molecular dynamics simulation of seawater reverse osmosis desalination using carbon nanotube membranes

A new class of valves for membranes is based on the formation of nanobubbles at the pore entrances of carbon nanotube (CNT) membranes. Nanobubble stabilization is achieved by electrochemically etching CNTs into a polymer matrix to form a well that can be reversibly filled. Such valves have applications in flow battery systems where high-energy chemicals can be stored indefinitely. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Application and characterization of electroactive membranes based on carbon nanotubes and zerovalent iron nanoparticles

ABSTRACTA NEMD simulation system is constructed to simulate at two-dimensional (2D) periodic boundary conditions (PBCs) and to create two different pressures on two sides of the carbon nanotube (CNT) membrane. The simulation results show that water permeation through the same CNT membrane driven by different pressure differences exhibit similar transport phenomenon including unusually fast water permeation and a periodic (non-parabolic) radial velocity distribution unlike the parabolic form characteristic of continuum flow in the CNT membrane. A three-dimensional (3D) PBC system is also constructed to simulate water permeation through the same CNT membrane at the same pressure differences, to show the effect of PBC and simulation methodologies on transport phenomenon. The two systems both show that the forward/backward water flux increases/decreases with increasing the pressure difference from 1.0 MPa to 8.0 MPa. However, the net flux is higher for the 3D PBC system, especially at higher pressure difference is high. In general, the NEMD simulation method using the 2D PBC system is shown to be a feasible and valuable tool for studying pressure-driven permeation processes such as nanofiltration through these studies with model CNT membrane.

Graphene can be considered as an ideal membrane since its thickness is only one carbon diameter. In this study, using molecular dynamics simulations, we investigate water transport through a porous graphene membrane and compare the results with water transport through thin (less than 10 nm in thickness/length) carbon nanotube (CNT) membranes. For smaller diameter pores, where a single-file water structure is obtained, CNT membranes provide higher water flux compared to graphene membranes. For larger diameter pores, where the water structure is not single-file, graphene membranes provide higher water flux compared to CNT membranes. Furthermore, in thin CNT membranes, the water flux did not vary significantly with the thickness of the membrane. We explain the results through a detailed analysis considering pressure distribution, velocity profiles, and potential of mean force. This work opens up opportunities for graphene-based membranes in molecular sieving, water filtration, fuel cells, and so forth.

Water flow rate quantification through an experimental CNT membrane: A molecular dynamics simulation approach

As a sustainable, cost-effective and energy-efficient method, membranes are becoming a progressively vital technique to solve the problem of the scarcity of freshwater resources. With these critical advantages, carbon nanotubes (CNTs) have great potential for membrane desalination given their high aspect ratio, large surface area, high mechanical strength and chemical robustness. In recent years, the CNT membrane field has progressed enormously with applications in water desalination. The latest theoretical and experimental developments on the desalination of CNT membranes, including vertically aligned CNT (VACNT) membranes, composited CNT membranes, and their applications are timely and comprehensively reviewed in this manuscript. The mechanisms and effects of CNT membranes used in water desalination where they offer the advantages are also examined. Finally, a summary and outlook are further put forward on the scientific opportunities and major technological challenges in this field.

Objectives The reversible electrical modulation of chemical delivery is crucial for various biomedical engineering applications, enabling a tailored drug delivery profile. Electroosmosis flow (EOF) represents a significant electrokinetic phenomenon where a bulk flow is induced through narrow channels including capillaries or microfluidic devices or porous medium, under the influence of an applied electric field (Fig. 1a). Since the velocity of EOF is directly proportional to the applied current density ( I ) and the EOF strength ( K eo ), a standardized indicator related to the zeta potential of the channel walls, it provides the possibility of controlling the direction and velocity of EOF by simply adjusting the surface zeta potential.[1] Herein, a novel concept of electrically modulated delivery utilizing EOF generated within the porous network of the carbon nanotube (CNT) membrane is reported. The charge sign and magnitude of the micropores inside the CNT membrane are altered by applying varying voltages, leading to corresponding changes in the direction and velocity of the EOF. Results and Discussion The multi-walled carbon nanotubes powered are first treated with ozone and dispersed in ethanol. Then, the suspension is formed into the random alignment and cohesive CNT membrane by vacuum-assisted filtration (Fig. 1b-c). The experimental setup consists of symmetrically positioned reservoirs at both ends and a CNT membrane placed between the two reservoirs filled with 0.1 M KCl solution. The electrically modulated system is comprised of two circuits: the main circuit that generates an axial electric field to drive the EOF and the sub-circuit that varies the sign and magnitude of charges on the CNT membrane. The controllability of the EOF direction and velocity is evaluated by observing the movement of the liquid column in the reservoirs (Fig. 1d -e). Based on the established experimental setup, the voltage at which EOF is unobserved is referred to as the neutral voltage (or bias voltage) and is used as the reference for electrical modulation. Significant differences in the direction and velocity of EOF can be observed and measured on the CNT membrane when a positive (or negative) voltage relative to the bias voltage is applied. The EOF under the main circuit of + 0.25 mA and the sub-circuit voltage of + 0.5 V and - 0.5 V relative to the bias voltage, indicating a mobility rate of - 0.781 μL/mm2·min and + 0.972 μL/mm2·min, respectively (Fig. 1f). The plus and minus means that opposite directions of EOF are generated with applied voltages. Next, to demonstrate the effect of applied voltages on the EOF rate, the sub-circuit voltage of + 0.5 V and + 0.25 V relative to the neutral voltage was applied (with the same main circuit of + 0.25 mA), generating a mobility rate of - 0.781 μL/mm2·min and - 0.431 μL/mm2·min. The difference in velocity is due to the differential surface zeta potential within the microchannels of the CNT membrane. Conclusion Here, this study introduces a novel concept of a chemical delivery system by focusing on the electrically modulated EOFs generated in the microchannels of the CNT membrane. According to the results of the experiments, it can be demonstrated that altering the direction and velocity of EOF through electrical modulation is achievable. Additionally, the electrical modulation on EOF is feasible under varying axial electric field strengths and concentration solution environments. Reference [1] Kusama, S., Sato, K., Matsui, Y., Kimura, N., Abe, H., Yoshida, S., & Nishizawa, M. (2021). Transdermal electroosmotic flow generated by a porous microneedle array patch. Nature communications , 12 (1), 658. Fig. 1 Experimental setup and electrical modulation performance. (a) The mechanism of EOF. (b) Optical and SEM images of CNT membrane. (c) Schematic illustration of EOF generated in the micropores of CNT membrane. (d) An experimental setup consisting of Franz Cells with insertion of CNT membrane and the EOF direction and velocity were analyzed by observing the liquid movement. (e) Electrical modulation. (f) The relationship between the applied voltages (relative to the neutral voltage) and flow rates. Figure 1

Nonequilibrium molecular dynamics (NEMD) simulations are presented to investigate the effect of water-membrane interactions on the transport properties of pressure-driven water flow passing through carbon nanotube (CNT) membranes. The CNT membrane is modified with different physical properties to alter the van der Waals interactions or the electrostatic interactions between water molecules and the CNT membranes. The unmodified and modified CNT membranes are models of simplified nanofiltration (NF) membranes at operating conditions consistent with real NF systems. All NEMD simulations are run with constant pressure difference (8.0 MPa) temperature (300 K), constant pore size (0.643 nm radius for CNT (12, 12)), and membrane thickness (6.0 nm). The water flow rate, density, and velocity (in flow direction) distributions are obtained by analyzing the NEMD simulation results to compare transport through the modified and unmodified CNT membranes. The pressure-driven water flow through CNT membranes is from 11 to 21 times faster than predicted by the Navier-Stokes equations. For water passing through the modified membrane with stronger van der Waals or electrostatic interactions, the fast flow is reduced giving lower flow rates and velocities. These investigations show the effect of water-CNT membrane interactions on water transport under NF operating conditions. This work can help provide and improve the understanding of how these membrane characteristics affect membrane performance for real NF processes.

Carbon nanotube membranes – Strategies and challenges towards scalable manufacturing and practical separation applications

Poly(vinyl alcohol)/carbon nanotube (CNT) membranes for pervaporation dehydration: The effect of functionalization agents for CNT on pervaporation performance

We report the development of a simple microfluidic device, which uses a carbon nanotube (CNT) membrane (CNTM) to separate two proteins based on their molecular weights (MW). The device was fabricated by traditional microfabrication techniques and chemical vapor deposition (CVD). Protein A (MW= 42 kDa) and aprotinin (MW= 6.5 kDa) were flowed through the CNTM in separate identical electrophoresis runs. The larger protein was observed to accumulate ahead of the CNTMs, whereas in the case of the smaller protein, no accumulation was observed, indicating that the buildup was due to the larger size of the higher MW protein. Electrophoresis on protein mixtures of 1) protein A and aprotinin and 2) protein A and lysozyme (MW= 17 kDa) was also performed and it was observed that only Protein A formed bands, wheras, the smaller proteins did not.

The adsorption equilibrium isotherms as well as gas sensing of three chlorinated phenolic compounds (CPCs) including phenol, 2‐chlorophenol, and 2,4‐dichlorophenol by acid‐functionalized carbon nanotube (CNT) membrane were studied. CNTs were synthesized via floating catalyst chemical vapor deposition method at 800°C and then functionalized with a mixture of HNO3 and H2SO4. The functionalized CNTs were vacuum filtered to form CNT membranes. The adsorption isotherm followed the Langmuir adsorption model better than the Freundlich model, suggesting monolayer adsorption. The maximum adsorption capacities were found to be 61.35, 93.46, and 104.17 mg g−1 for phenol, 2‐chlorophenol, and 2,4‐dichlorophenol, respectively. The desorption energy determined via thermogravimetric analysis indicates that adsorption of all three CPCs onto the CNT membranes belonged to chemisorption type. For the gas sensing test, the CNT membrane showed fast response to the tested CPCs with good stability and repeatability. The sensitivity values obtained were 5.39 × 10−2, 3.35 × 10−2 and 2.58 × 10−2 for 2,4‐dichlorophenol, 2‐chlorophenol, and phenol, respectively. It is noteworthy that the adsorption capacity and sensitivity of the CNT membrane to the target gases increased with the increase in the number of chlorine atoms in the phenolic compounds. © 2018 American Institute of Chemical Engineers Environ Prog, 38: S315–S322, 2019

Gated ion diffusion is found widely in hydrophobic biological nanopores, upon changes in ligand binding, temperature, transmembrane voltage, and mechanical stress. Because water is the main media for ion diffusion in these hydrophobic biological pores, ion diffusion behavior through these nanochannels is expected to be influenced significantly when water wettability in hydrophobic biological nanopores is sensitive and changes upon small external changes. Here, we report for the first time that ion diffusion through highly hydrophobic nanopores (approximately 3 nm) showed a gated behavior due to change of water wettability on hydrophobic surface upon small temperature change or ultrasound. Dense carbon nanotube (CNT) membranes with both 3-nm CNTs and 3-nm interstitial pores were prepared by a solvent evaporation process and used as a model system to investigate ion diffusion behavior. Ion diffusion through these membranes exhibited a gated behavior. The ion flux was turned on and off, apparently because the water wettability of CNTs changed. At 298 K, ion diffusion through dense CNT membranes stopped after a few hours, but it dramatically increased when the temperature was increased 20 K or the membrane was subjected to ultrasound. Likewise, water adsorption on dense CNT membranes increased dramatically at a water activity of 0.53 when the temperature increased from 293 to 306 K, indicating capillary condensation. Water adsorption isotherms of dense CNT membranes suggest that the adsorbed water forms a discontinuous phase at 293 K, but it probably forms a continuous layer, probably in the interstitial CNT regions, at higher temperatures. When the ion diffusion channel was opened by a temperature increase or ultrasound, ions diffused through the CNT membranes at a rate similar to bulk diffusion in water. This finding may have implications for using CNT membrane for desalination and water treatment.