Graphene for Water Purification
Graphene is an extremely thin 2D layer of carbon atoms arranged in a honeycomb structure. A single sheet of grapheme is just one atom thick and in order to get 1mm thick layer it requires stacking of millions of individual graphene sheets. There are some fascinating properties of graphene that make it extremely useful material in many-many applications as depicted in Figure 1. It is extremely light and extremely strong material with very high conductivity for heat and electricity. In the early days since its discovery in 2000 it was very costly and complicated to create pure graphene, but in last decade or so research in graphene has gained a lot of momentum. Many research groups around the globe have proved & explored the potential of graphene for variety of applications like building aerospace parts & building materials; heat spreading solutions; efficient LED lighting; energy storage in batteries & supercapacitors; flexible electronics; drug delivery; filtering at atomic level and lot more.
Figure 1: Applications of graphene in different areas (Ref. 1)
In this article filtration using graphene especially for water treatment is discussed. Like many other natural occurring resources water is also limited in supply and its scarcity is evident from the fact that there are a few parts in India where no adequate drinking & irrigation water is available, also some states are having dispute over river water sharing etc. Water treatment is a process where used water or sea water is made potable and useable by removing harmful bacteria, algae, inorganic compound by the means of filtration and sedimentation. Among many methods of water treatment one of them is use of polymeric membranes for water filtration. Polymeric membrane should have high efficiency, selectivity, resistance to temperature and extremely small defects. Graphene possess all these properties and in addition to this it is hydrophobic i.e. it repels water. This is very important trait that makes graphene an ideal choice for water filtering. Graphene naturally is impermeable to water but if micro pores are made in it, then a very fast permeation of water is achieved that separates it from impurities & salts.
Figure 2: The structure of graphene oxide (Ref. 3)
Dr. Yang Su of the University of Manchester's prestigious National Graphene Institute along with his team is conducting research on graphene oxide membranes that contain several pores as shown in Figure 2. Selectivity and permittivity of membrane depends upon shape and size of these pores. Graphene oxide (GO) membranes consists of numerous layers of 2-D graphene oxide layers stacked on top of each other. The pores are consistent in size and their width is only 0.9 nm. It means the ions or molecules of size greater than 0.9nm will be filtered out by the process called Physical Extrusion as shown in Figure 3. In comparison to polymer membranes, GO membranes are chemically inert that ensures better lifetime.
Figure 3: Physical extrusion process (Ref. 3)
Using chemical processes and physical pore size control GO membrane structure can be modified and can be used for super-efficient atomic level filtering in gases, for example hydrogen purification, CO2 capture, drying out and filtration of biofuels etc.
By: Ms. Manisha Aggarwal, Assistant Professor, ECE, Chitkara University, H.P.
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