Carbon Nanotubes: The Future of the Planet’s Freshwater

Carbon Nanotubes: The Future of the Planet’s Freshwater
Abstract
As a new wonder material, the carbon nanotube has found a wide range of applications. This review highlights its use in desalination of seawater that includes its concept, functioning principles, advantages to current reverse osmosis technologies, and challenges. In case you need it, we have written an article about when is the best time to change or replace your carbon filters for your space.
Introduction
The Earth is experiencing an urgent freshwater crisis. With the amount of water use rising twice as fast as the global population, the demand for freshwater has greatly exceeded its supply. It is estimated that by 2025, 1.8 billion people will live with complete water shortages, and by 2030, nearly half of the planet will be living in water-insufficient conditions.^{((Powers, M. “Multiple Ways of Assessing Threats to Water: Supply-Side and Demand-Side Problems.” http://www.fewresources.org/water-scarcity-issues-were-running-out-of-water.html.))} To further exacerbate the situation, accessible freshwater consists of less than one percent of the Earth’s water, whereas above 95% of the water comes from the ocean.^{((Powers, M. “Multiple Ways of Assessing Threats to Water: Supply-Side and Demand-Side Problems.” http://www.fewresources.org/water-scarcity-issues-were-running-out-of-water.html.))} The seawater is composed of 96.5% pure water and 3.5% dissolved salts (such as sodium chloride) that are detrimental to human health.^{((“Ocean Health”. http://oceanplasma.org/documents/chemistry.html.))} As a result, researchers have been working on developing various desalination technologies to effectively utilize the planet’s saltwater resources.
Current Methods
Desalination, or the purification of water from its salt and mineral components, can be completed with methods such as thermal distillation, reverse osmosis and electrodialysis. Among these methods, reverse osmosis is most commonly used and accounts for 60% of the global desalination capacity.^{((“Water Desalination Using Renewable Energy.” https://iea-etsap.org/E-TechDS/PDF/I12IR_Desalin_MI_Jan2013_final_GSOK.pdf.))} Reverse osmosis (RO) purifies saltwater by forcing it through a semipermeable membrane, which requires large amounts of energy because high pressure is needed as a driving force. In addition, the operating pressure has to be increased with the rise of salt concentration of the feed solution. Thus, a high volume of salt water is wasted to avoid the over-pressurization that could cause the malfunction of the membrane. Finally, RO not only removes salt from water, but also filters out healthy minerals. This diminishes the quality of water and makes it less suitable for drinking purposes. Therefore, a more efficient and cost-effective technology needs to be developed to replace RO. With the advent of a carbon nanotube (CNT) system, this will be made possible.
What is a carbon nanotube?
The carbon nanotube, discovered by Sumio Iijima in 1991, is one of the allotropes of carbon, apart from diamond, graphite, and fullerenes.^{((“Water Desalination Using Renewable Energy.” https://iea-etsap.org/E-((Iijima, S. “Helical microtubules of graphitic carbon.” Nature 354 (1991): 56-58.)),[5]} The simplest or single-walled carbon nanotube (SWCNT) is formed by rolling up a sheet of a honeycomb network of carbon atoms, or graphene, seamlessly into a tubular form (Figure 1).^{((Iijima, S., “The Discovery of Carbon Nanotubes.” http://www.balzan.org/en/prizewinners/sumio-iijima/the-discovery-of-carbon-nanotubes-iijima.))} The CNT is one of the lightest and strongest known materials in the world,with excellent heat and electrical conductivity abilities.^{((Jacobs, M. M. et al. “Precarious Promise: A Case Study of Engineered Carbon Nanotubes.” http://www.sustainableproduction.org/downloads/ECN_casestudy_0325.pdf.))} Due to these unique properties, CNTs have shown great potential in a variety of applications such as sporting goods, electronic devices, batteries, and chemical sensors.^{((Jacobs, M. M. et al. “Precarious Promise: A Case Study of Engineered Carbon Nanotubes.” http://www.sustainableproduction.org/downloads/ECN_casestudy_0325.pdf.))}

Figure 1. Graphene and single-walled carbon nanotube
How do CNTs work in desalination?
A typical carbon nanotube with positive (-NH³⁺) and negative (-COO^–) functional groups is shown in Figure 2.^{((Das, R. et al. “Carbon nanotube membranes for water purification: a bright future in water desalination.” Desalination 336 (2014): 97-109.))} These functional groups are very hydrophilic (i.e. prefer to be close to water) and can attract water molecules to the opening of the tube. However, the CNT itself is very hydrophobic (i.e. prefers to be away from water), so once water molecules enter the opening, they are able to flow through very quickly because of the repulsion from the walls of the CNT. On the other hand, the dissolved salts, mainly in the form of cations (e.g. Na⁺) and anions (e.g. Cl^–) are captured by the functional groups due to the electrostatic attractions between oppositely charged ions (Na⁺ with -COO^– and Cl^– with -NH³⁺). Consequently, the saline water is purified by CNTs by the rejection of the salt pollutants.

Figure 2. Desalination with functionalized carbon nanotubes (adapted from Ref. 8)
Advantages
Compared to RO, the CNT desalination process has several major advantages. First of all, the frictionless property of CNT allows extremely rapid water flow, making the process more efficient with much less energy needed.^{((“Can Engineered Carbon Nanotubes Help to Avert Our Water Crisis?”))} Additionally, water recovery amounts are expected to rise. It has been reported recently that the combination of CNTs with membrane distillation can increase water recovery to nearly 100%.^{((“New Membrane Could Limit Corrosive Hot Brines in Water Treatment.” http://www.materialsperformance.com/news/2017/06/new-membrane-could-limit-corrosive-hot-brines-in-water-treatment.))} Furthermore, it is possible to selectively capture unwanted salts and retain beneficial minerals by adjusting the ratios of functional groups that exhibit higher affinity to specific ions on a CNT.^{((Gracia-Espino, E.; López-Urías, F.; Terrones, H.; Terrones, M. “ Novel Nanocarbons for Adsorption” in Novel Carbon Adsorbents, edited by Tascón, J. M. D.; 2012 Elsevier Ltd.))} This results in a better drinking water quality in comparison with the RO process. CNTs are even capable of self-regenerating. For example, when a part of a tube is broken, its carbon atoms naturally reconnect to close the gap.^{((Debroy, S. et al. “Graphene heals thy cracks.” Computational Material Science 109 (2015): 84-89.))} Lastly, CNTs can kill microbes to prevent the damaging of its surface.^{((“Can Engineered Carbon Nanotubes Help to Avert Our Water Crisis?”))} As a result, the CNT desalination system is more reusable and long-lasting than the current RO technologies.
Challenges
Although the use of CNTs has many benefits in desalination, there are still hurdles to overcome before the process can be commercialized. One challenge is the high cost of producing CNTs, which has dropped significantly during the past ten years,^{((DeVolder, M. F. et al. “Carbon nanotubes: Present and future commercial applications.” Science 339 (2013): 535–539))} but is still higher than the regular polymer membrane materials. Another issue is the controlled (size, shape, etc.) growth of SWCNTs with uniform structures, especially for bulk quantity synthesis.^{((Liu, C. and Cheng, H. M. “Carbon nanotubes: controlled growth and application.” Materials Today 16 (2013): 19-28.))} Researchers and scientists will continue to work on and improve this technology.
Conclusion
CNTs have demonstrated to be a promising substitute for RO in desalination. The commercial application will be available once the obstacles of their large scale production are overcome. This system’s efficiency and advantages means thatit not surprising that it has become an increasingly important fieldin the research of nanomaterials.