Chapter 6 - Nanofluidic Carbon Nanotube Membranes: Applications for Water Purification and Desalination

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.

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

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.

Development of technologies for water desalination and purification is critical to meet the global challenges of insufficient water supply and inadequate sanitation, especially for point-of-use applications. Conventional desalination methods are energy and operationally intensive, whereas adsorption-based techniques are simple and easy to use for point-of-use water purification, yet their capacity to remove salts is limited. Here we report that plasma-modified ultralong carbon nanotubes exhibit ultrahigh specific adsorption capacity for salt (exceeding 400% by weight) that is two orders of magnitude higher than that found in the current state-of-the-art activated carbon-based water treatment systems. We exploit this adsorption capacity in ultralong carbon nanotube-based membranes that can remove salt, as well as organic and metal contaminants. These ultralong carbon nanotube-based membranes may lead to next-generation rechargeable, point-of-use potable water purification appliances with superior desalination, disinfection and filtration properties.

Passive water transport in biological pores

Italicized carbon nanotube facilitating water transport: a molecular dynamics simulation

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.

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.

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

Size effect in determining the water diffusion rate in carbon nanotubes

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

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

In recent years, carbon nanotubes (CNTs) are attracting researchers’ attention as they are used globally in wide-ranging applications such as solar and hydrogen storage, fuel cells, lithium batteries, supercapacitors, nanocomposites, gas sensors, pathogens, heavy metals, dyes, pesticides, and water desalination as well as decontamination. This chapter elaborates characteristics and fascinating structures of CNTs, activation of CNT membranes, mechanism of pollutants, and salt removal from water. The CNTs can be effectively used for deionizing light metal salts, to take away organic dyes and enriching heavy metal ions. The CNTs with well-aligned structures can be employed as effective pores in membranes to desalinate and purify the water. The CNTs pores can be modified on the need basis for sensing and refusing the ions. The distinctive nonpolar tip functionalized interior structure of CNTs interacts with the polar water (H2O) molecules and inhibits the pollutants and salts. The noteworthy properties such as energy efficiency, antifoulant, and self-cleaning nature have proven CNTs as a strong candidate compared to the conventional ones. Such distinguished properties and the reusable nature of CNTs, assisted with membrane technology make them a promising candidate in 192the area of water purification and desalination. This chapter will also focus on the current challenges in the commercialization of CNT membranes.

Globally, the scarcity of pure water has been an alarming threat for the survival of ecosystem since fresh water is the necessity of every being and is one of the most important factors in a healthy ecosystem. Potable or drinking water is meant to be free from all kinds of unwanted toxic chemicals, and its pH should always lie between 6.5 to 8.5. However, even today, knowingly or unknowingly, almost 30% in the world are drinking water with a pH below 6.5, which means it is acidic and contaminated with various kinds of pollutants. Even though, in some areas, freshwater is available, it is not affordable for the poor people in need. So, in the current situation, the most essential step is to adapt new trends and technologies for water purification using nature friendly and renewable resources. Emphasis is to be given to cost-effective, easy-to-use solutions for water purification, i.e., purifying water using carbon-based materials. In the present era, nanomaterials have emerged as the most sought-after and desirable solutions for various industrial problems and domestic applications. The carbon-based nanomaterials, including single and multi-walled carbon nanotube (CNTs), graphene oxide, activated carbons, carbon dot, and fullerenes are proposed as eco-friendly option. The introduction of carbon nanoparticles in water treatment is able to enhance the properties, such as fouling-resistance, permeability, catalytic activity, and stabilized water chemically and thermally. CNTs have been established as a great solution for water purification and treatment applications in recent times. Activated nano-carbon is widely employed in water purification because of good absorbing capacity and cost-effectiveness. It can absorb contaminants, including organic, inorganic, and biological from the water. Due to this adsorption property of the activated carbon, it plays a vital role in binding chemicals. Activated carbon takes away one or more atoms, molecules, or ions of contamination, when water passes through its surface. Molecular dynamics (MD) simulation facilitates such molecular-scale interaction for further scale-up of the treatment process. All these methods of water purification are gaining huge popularity due to their control over biofouling, superior structural properties, renewable, and eco-friendly nature.

Chapter 26 - Challenges and Opportunities of Graphene-Based Materials in Current Desalination and Water Purification Technologies