One of the most important challenges that engineers face in this world today is that of providing access to clean water around the world. The irony behind it is that water is actually found in abundance on the earth, but it is distributed unevenly amongst all countries. Additionally, about 97% of all water on earth is found in the salty ocean, which is unable to be consumed without purification. This issue hits the developing world the hardest. Lack of clean water causes diseases like diarrhoea and cholera, and a huge number of people die every year due to this problem.
A number of methods have been developed for providing access to clean water in the developing world, such as filtration, sedimentation, chlorination, and coagulation, but many of these prove to be too expensive or unreliable. One interesting method of providing clean water is water desalination. This is the process by which salt, toxins, and other minerals are removed from dirty water, usually from the ocean, in order to obtain clean, purified water. This is commonly done by a process called reverse osmosis. To understand how reverse osmosis works, it is important to first know the process of natural osmosis. In osmosis, solvent molecules from a dilute solution pass through a semipermeable membrane to a concentrated salt solution, thereby diluting the latter and making both solutions equal in concentration. Osmosis is a natural process and requires no energy. In contrast, reverse osmosis requires energy to force the solvent from the latter solution back into the originally dilute solution, thereby leaving behind salts and resulting in a pure water solution. The processes of osmosis and reverse osmosis are illustrated in Figures 1 and 2, respectively. (“What is Reverse Osmosis?”)
Although this seems like an easy method for providing clean water, it actually is not used all that much. In fact, less than 1% of drinking water comes from reverse osmosis. (Perlman) Reverse osmosis requires a great amount of force and pressure to push water through the membrane—therefore a huge supply of energy—and the more concentrated the solution is, the more pressure is required. This proves to be a costly process, especially for use in developing countries.
Currently, the semipermeable membranes for use in reverse osmosis desalination plants are commonly made with cellulose acetate or aromatic polyamide, with aromatic polyamide usually being the preferred material. The disadvantages of using aromatic polyamide in desalination plants include it not completely filtering all salt particles from the water, as well as its relatively high pressure requirement to filter the salt water solution, resulting in a high cost for the overall process. (Martinez) New materials for cheaper and more effective desalination are always on the lookout for use in the membranes of reverse osmosis plants.
Graphene
Graphene is a recently discovered two-dimensional material formed from the isolation of a single, atom-thin sheet of graphite—a brittle and common carbon allotrope used in the fillings of pencils. Graphite occurs naturally and has a basic atomic arrangement, or crystalline structure, that consists of a large, planar chain of strongly bonded carbon atoms organized in a hexagonal cell as shown in Figure 3. This cell is known as graphene. (Kielmas)
It has proven to have amazing physical and chemical properties, such as extremely high tensile strength and flexibility, making it a convenient choice for a variety of necessities. However, being a new material, it has few commercial uses, yet a large amount of ongoing research and promising potential applications in almost every discipline. (De la Fuente) Right now, it is being used to a large extent in the optoelectronic industry, specifically in the production of touchscreens and liquid crystal displays (LCD), due to its ability to transmit light and its transparent and conductive properties.
Among the many potential uses of graphene, such as in flexible electronics, semiconductors, and superconductors, the one that stands out is its application in water filtration. Because of its tight atomic bonds, graphene is impermeable to most gases and liquids, yet it allows water to pass through it. This, added to the fact that it is extremely thin, could make of graphene a “great filtration medium acting as a barrier between two substances”. (Nicol)
With this in mind, graphene can play a really important role in the desalination of water, probably replacing the current material used, aromatic polyamide, as it is stronger, has a smaller diameter (graphene has a diameter of approximately 0.34 nm while aromatic polyamide has a diameter of approximately 100 nm), a higher chlorine tolerance and chemical resistance, a substantially lower energy required for reverse osmosis, and therefore a lower cost. This difference in needed energy by the system between these two materials is due to the fact that water flux is inversely proportional to the thickness of the membrane, making the graphene membrane more permeable to water, therefore needing less pressure to be applied and lower energy overall required. (Homaeigohar, Elbahri)
These properties of graphene make it a more suitable material for desalinating water. However, researchers found that when putting it into practice, some common salts found on sea water can still pass through. Current research has found a large variety of solutions to this problem, and it is thought that eventually, graphene will become the key of effective and cheaper water desalination.
History of Graphene
As explained above, graphene is in fact a relatively new material. However, graphite has been present in our lives for thousands of years, and ,therefore, there is a wide knowledge of its uses and properties. By observing these properties, particularly its crystal structure, the concept of graphene was idealized, and many attempts for obtaining it have taken place– the earliest record of such attempt being in the mid 19th century– yet none of them succeeded.
For years, graphene was exclusively a theory, as the idea of isolating a single layer of graphite seemed extremely complicated as it might need the most advanced and expensive technology and equipment. However, in 2004 the first physical existing sample of graphene and a simple technique for its collection was discovered by Professor Sir Konstantin Novoselov and Professor Sir Andre Geim at the University of Manchester in England.
The procedure employed was as simple as using scotch tape to polish a sample of graphite and peeling the material layer by layer. They noticed that little particles remained in the tape, this being the layer of graphene. This procedure was so simple that the scientific community was skeptical at first. However, by 2010, they received the Nobel Prize in Physics for their breakthrough.
Despite its short history, graphene has gained a lot of popularity in most of disciplines, particularly in the material science field. Its importance will increase exponentially as new potential applications are proven real and new properties of the material are discovered.
Ongoing Research
Graphene is currently not being used industrially as a material in desalination plant membranes, but there is a great deal of research in the lab going on to perfect the material for this certain application. In order to be useful in this process, graphene has to be the perfect amount of semipermeable. This means that its pores have to be big enough to allow water molecules through, but small enough to keep out salts and other impurities. One group of researchers at the University of Manchester have found that when graphene membranes are submerged in water, they tend to swell a small amount, which opens up the pores and allows certain small salts through. This is a setback in the use of this material for desalination properties. This group, however, has developed a strategy to prevent the swelling of graphene membranes when exposed to water by using walls of epoxy resin on both sides of the membrane. This method has allowed for the ability to control the size of pores in graphene membranes, effectively trapping every salt and impurity.
Another area of research that is being focused on to perfect graphene membranes is testing how durable these membranes will be under desalination conditions and how long they will last before replacement is needed. The polymer-based membranes used today tend to wear out after some use, thereby causing some salt particles to be filtered into the water. However, graphene is experimentally tested to be a strong and durable material, so the potential for longer durability is high, and research is being done to test this application.
Additionally, graphene has potential to be used in many other applications in relevance to filtration. These includes pharmaceutical filtration, gas separation, and dialysis, among others.