Nonequilibrium molecular dynamics simulation for studying the effect of pressure difference and periodic boundary conditions on water transport through a CNT membrane

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

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.

Nonequilibrium molecular dynamics (NEMD) simulations are used to investigate pressure-driven water flow passing through carbon nanotube (CNT) membranes at low pressures (5.0 MPa) typical of real nanofiltration (NF) systems. The CNT membrane is modeled as a simplified NF membrane with smooth surfaces, and uniform straight pores of typical NF pore sizes. A NEMD simulation system is constructed to study the effects of the membrane structure (pores size and membrane thickness) on the pure water transport properties. All simulations are run under operating conditions (temperature and pressure difference) similar to a real NF processes. Simulation results are analyzed to obtain water flux, density, and velocity distributions along both the flow and radial directions. Results show that water flow through a CNT membrane under a pressure difference has the unique transport properties of very fast flow and a non-parabolic radial distribution of velocities which cannot be represented by the Hagen-Poiseuille or Navier-Stokes equations. Density distributions along radial and flow directions show that water molecules in the CNT form layers with an oscillatory density profile, and have a lower average density than in the bulk flow. The NEMD simulations provide direct access to dynamic aspects of water flow through a CNT membrane and give a view of the pressure-driven transport phenomena on a molecular scale.

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

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

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

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

Recently extraordinary breakthroughs have been made towards applying nano-structured materials such as carbon nanotubes (CNTs) and porous graphene membranes in water purification and desalination applications. In this regard, the potential of the electrochemically active carbon nanotube (CNTs) membrane has been highly strengthened for the last few decades. One of the main advantages for such approach is the capability of CNT channel to permit the water flow easily. The perusal of the literature showed that, the performance of CNT based membrane can be three times higher than that of the conventional membrane devices. The unique and excellent characteristics of CNT membrane can outperform the conventional polymer membranes. CNT membrane has been widely used to adsorb chemical and biological contaminants as well as ion separation from sea water due to their high stability, great flexibility, and large specific surface area. Electrochemically active CNT filters deliver further Electro-oxidation of the adsorbed contaminants. Usually polymeric membranes have flexible chains for which it fails to have well-defined pores necessary for filtration. On the contrary CNTs based filters can provide pores with appropriate sizes and configurations by tailoring the growth parameters. The narrow pores of CNTs are capable of filtering water while eliminating ions (Na+/Cl–). Even this type of membranes is capable of removing bacteria from water and heavy hydrocarbon from petroleum. However, the success of desalination entirely depends on the basic design of the CNT-based filter with detailed optimization of the process parameters. Polymer filters cannot be recurrently used through several cycles since elimination of fouling ingredients is difficult. Even though electrochemically active CNT-based membranes have lot of advantages due to their hydrophobic nature, high porosity and specific area; there are numerous traits, which are yet to be considered and optimized. Thus the intrinsic properties of CNT as well as the fabrication of the membrane could be a critical factor for their applicability in various water treatment processes. This chapter provides an explicit and systematic overview of the recent progress of electrochemically active CNT membranes addressing the current prevalent problems associated with water treatment and desalination. The physio-chemical aspect including the working principles of this type of membrane have been discussed. The prevailing challenges and future perceptions are also discussed.

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.

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

The flow rate of water through carbon nanotube (CNT) membranes is considerably large. Hence, CNT membranes can be used in nanofluidic applications. In this work, we performed a molecular dynamics (MD) simulation of the introduction of water into CNTs in the CNT membranes, especially in vertically aligned CNT forests. The results showed that the Knudsen number (Kn) increased with an increasing volume fraction of CNT (VC) and was greater than 10−3 for each VC. Beyond this value, the flow became a slip flow. Further, the permeability increased as VC increased in the actual state calculated by the MD simulation, whereas the permeability in the no-slip state predicted by the Hagen–Poiseuille relationship decreased. Thus, a clear divergence in the permeability trend existed between the states. Finally, the flow enhancement ranged from 0.1 to 23,800, and the results show that water easily permeates as VC increases.

Chapter 8 - Prospects and State-of-the-Art of Carbon Nanotube Membranes in Desalination Processes

Remediating acidic mine drainage is essential to mitigate environmental concerns due to its acidity and elevated heavy metal concentration. This study investigates the performance of capacitive deionization (CDI) and membrane CDI (MCDI) for remediating acidic zinc mine tailing leachate from Australian mining sites, specifically focusing on the roles of fabricated carbon-based electrodes and carbon nanotube (CNT) membrane. Among the four fabricated electrodes, activated carbon + carbon nanotubes (AC + CNT) electrode achieved the highest metal removal at a rapid rate (30–65% Al 3+ , Fe 3+ , Zn 2+ , salt adsorption rate 0.78 mg·g −1 ·min −1 ). This was attributed to AC + CNT electrode's superior capacitance (9.0 F·g −1 ), and markedly reduced internal resistances (148.7 Ω, vs AC's 208.2 Ω,), contributing to enhanced ion transport pathways compared to AC and carbon black electrodes. While polymer-coated membrane electrodes initially hindered ion transport, embedding CNTs within the membrane restored charge-transfer pathways, enabling 67–88% removal of major metals (Al 3+ , Fe 3+ , Zn 2+ ) from zinc tailing. Membrane reuse evaluation indicated inevitable metal oxidation, leading to precipitation on membrane. To address this, a granular activated carbon (GAC) pretreatment step was implemented upstream, reducing bulk metal loading by ~62% The integrated configuration of MCDI with CNT membrane upon GAC pretreatment maintained high metal removal efficiencies (97–98%) across multiple cycles of adsorption and chemical-free desorption, while producing an acidic stream (pH 1.1 ± 0.2) suitable for mining reuse. Overall, these findings demonstrate the potential of electrochemical processes for rapid acid mine drainage (AMD) remediation, while concurrently supporting acid recovery. • Combined AC + CNT carbon electrode exhibited superior metal removal in CDI. • CNT membrane MCDI achieved high metal removal from acidic zinc mine tailing. • GAC pretreatment prevented CNT membrane fouling by lowering feed ion concentrations. • GAC–MCDI process achieved 98% metal removal, yielding an acidic effluent. • The electrochemical approach enabled rapid and low-chemical AMD remediation.