THE USE OF CARBON NANOTUBES FOR WATER PURIFACTION By Ben Posey If carbon nanotubes are to be used for water treatment, they must be sustainable. One definition is that sustainability is the endurance of systems and processes, and carbon nanotubes do exemplify this trait. Another definition of sustainability is providing for the current generation while also being considerate of future generations. Variations of the nanotubes are economically sustainable as they can be reused with little to no effect on their adsorptive properties. Even if one were to burn the oil off of the CNT sponge, it can still be reused with little effect on the sponge’s adsorptive properties. As it can become rather expensive to mass produce the nanosponges, the most logical and effective way to utilize them is to keep reusing them until they are no longer able to adsorb a certain amount of oil. This is similar to the filters that remove viruses and bacteria wherein the CNTs can be disinfected and subsequently reused, making this method sustainable. One issue of environmental sustainability arises with the hazard of toxicity from the CNTs as the potential effects could be detrimental for future generations. CNTs are hydrophobic, so they are insoluble in water which can cause them to leave particulates over time. It is very bad to have these particulates in water, because the CNTs are highly toxic to living systems. After water treatment, the length of CNTs allows them to more easily react with cellular membranes while containing toxic substances, effectively carrying them into living organisms and posing a threat to their health. The main purpose of using CNTs for water filtration is to provide clean and safe water for people around the world. If these health effects are a serious problem, they are essentially nullifying the primary goal. Although CNTs can effectively treat water and serve their purpose for the current generation, it is important to invest in improving the safety of the technology so it can be continued to be used for future generations. Sustainability Nanosponges collecting oil from water (a and b) and the subsequent harvesting of the absorbed oil (d) Need for Water Purification Roughly 780 million people throughout the world do not have access to clean drinking water. Furthermore, the World Water Council predicts that by 2025, three billion people will be struggling to survive with water scarcity. Every year, 1.5 million people die due to waterborne diseases. The diseases can result in diarrhea, malnutrition, skin infections and organ damage, but the leading cause of death is dehydration. These diseases are caused by pathogens in the water. The cruel realization that a significant portion of the global population will be living with water scarcity is reason enough to merit the necessity of research into the use of carbon nanotubes (CNTs) for water purification. Conventional water purification methods, such as sizeable, industrialized plants, utilize technologies that expend a large amount of energy, require steep operational costs, and cannot be easily utilized in developing countries and resource-deficient areas. Thus, point of use (POU) water treatment technologies are better suited to resolve the water shortage problem around the world. These technologies are smaller and more transportable, so they can easily be applied to communities and households, making them a much better option for developing countries. CNTs demonstrate usefulness as a water treatment technology with high efficiency and easy distribution. These attributes are promising for making them a viable solution to the global water scarcity problem. Carbon nanotubes can be utilized in many scenarios to treat water. They can purify water that has been contaminated with oil, bacteria and viruses, and even be used to desalinate salt water. These different applications are particularly useful for the functionality of CNTs, since they provide the option of purifying water in many different scenarios. This option is useful for making CNTs a viable water treatment technology that can applied worldwide. The nanotubes can be made into membranes to filter water or into sponges to adsorb contaminants. Carbon nanotubes, by nature, are hydrophobic, thereby permitting the adsorption of oil in water. The CNTs that make up these sponges are exceptionally porous, which warrants the ability to adsorb a large volume of substance. Studies have shown that a CNT sponge of 1.5mg can hold 150 times its own weight of contaminants. The adsorbed oil can be reused by squeezing out the sponges, or burning them. Even after the nanotube conglomerates have been burned, they can be reused immediately. This property means that CNT sponges can be reused more efficiently, making them a viable water treatment technology. CNT sponges can become magnetized if the nanotubes are introduced to a magnetic material during their synthesis, which would allow for easier removal of the CNTs from the water. This magnetism is very important as the extraction of the nanotubes from water is often the most difficult step in the process of using carbon nanotubes for water treatment. Oil Filters can be made out of carbon nanotubes to purify water. This process can either be done by placing numerous individual nanotubes next to each other, or creating a thin sheet-like filter. Due to the combination of the porosity and the relative microscopic size of the nanotubes (dimensions of 1 to tens of nanometers), these filters can effectively sift out bacteria, viruses, and most harmful metals of a few millimeters in size. Hydrophobic relationship between water and CNTs means the water can flow freely through the porous CNTs and results in a clog-resistant filter. Bacteria, Viruses, and Debris Carbon nanotubes can be made into membranes that have proven to be effective for water desalination. CNT membranes are composed of hexagonally packed nanotubes. Even when they are very narrow, they still allow for the rapid flow of water, while preventing ions from entering the tubes. This function means the CNT filters can effectively separate salt from salt water to create fresh water. Even when they are very narrow, they still allow for the rapid flow of water, while preventing ions from entering the tubes. Furthermore, a CNT membrane can be expected to work with 95% desalination and a flow rate that is over 1500 times that of a typical membrane used in reverse osmosis. This exceptional flow rate means that CNTs could function at the same speed or faster than existing membranes, while undergoing a much smaller pressure. The lower pressure that is needed to run this process allows for a much better operational cost and a decreased environmental impact by saving a large amount of energy. Desalination There are three primary methods for CNT synthesis, known as the Arc- discharge method, chemical vapor deposition (CVD), and the laser-vaporization method. CVD is the oldest of the three methods of carbon nanotube synthesis, but it is the most cost-effective method as it produces the largest quantity of CNTs with very few impurities. By using the CVD method it is possible to get a production rate of 595 kg/h with the cheap cost of $10/kg. This high production rate and low cost means that the CVD technique is extremely efficient as a way of mass producing CNTs. CVD is a very versatile process in that a variety of substrates can be used and it can produce many different types of nanotubes. Some examples of these different types of nanotubes are powders, films with differing viscosities, and ones that are entangled, coiled or in a desired structure. The substrate (underlying material) is what determines the type of nanotube that is produced. The process involves sending a hydrocarbon vapor through a reactor at a high temperature with a catalyst present. The CNTs form on the catalyst and are later extracted once they are cooled. Synthesis An elementary schematic depiction of CVD Microscopic representation of a carbon nanotube water filter (Water with contaminants on right side) Table of waterborne diseases