Membrane Technlogy – Polymer Membranes Reinforced With Carbon Based Nano Materials For Water Purification
Fresh water is not only important to human health but it also plays a very crucial role in biological, chemical and physical processes. About 71% of Earth’s surface, and the oceans hold about 96. 5% of all water. Out of the remaining 3. 5%, 2. 5% is fresh water in which only 0. 3% is avalilable in liquid form for consumption. As per a report from the United Nations, by 2025 two-thirds of the world population could be under water stress conditions and 1800 million people wil be facing absolute water scarcity. As stated by Coleridge in his poem “Water water everywhere all the boards did shrink, water water everywhere nor any drop to drink”, there is less availability of water which can be used for various domestic and industrial purposes. Increasing efforts are going on to avert this looming crisis. The reuse, recovery and recycling of water has also proved useful, but not for maintaining public health. Membrane filtration is considered to be the most widely used and compromising process for purification of waste water, sea water and brackish water.
As a low cost, easy to scale up, simple in operation and efficient method membrane based separations have been widely used in many separation processes e. g. gas separation water treatments (desalination and waste water treatment).
Separation through membrane is widely used and is achieved through the selective transport of the molecules through the membrane matrix. Membranes are classified into different categories based on their configurations and their molecular weight cutoff (MWCO). Membrane processes such as micro-filtration (MF), ultra filtration (UF), nano- filtration (NF), reverse osmosis(RO), prevaporation(PV) and membrane distillation are widely employed. Reverse osmosis and nano filtration membranes are widely used for water purification. But, the thick active separating layer of the pressure driven membranes restricts the water flux. The solution to this problem is through the reinforcement of nano materials in the membrane matrix to enhance the flux while achieving the targeted molecules to be separated. These membranes are usually prepared from ceramic, polymeric or hybrid materials. Ceramic membranes are used for challenging water purification and desalination due to their good thermal and chemical stabilities.
On the other hand polymeric membrane are used due to their excellent mechanical strength and high selectivity. Among various nano materials, carbon nano materials (carbon nanotube, graphene) are having huge applications. Depending on the number of shell of graphene, carbon nanotube (CNT) can be classified as single walled carbon nanotube (SWNT) with single shell of graphene, similarly double walled carbon nanotube (DWNT) and multiwalled carbon nano tube (MWNT). Graphene is a one atom thick sheet of graphite which contain sp-2 hybridized carbon atoms arranged in hexagonal honey comb lattices and graphene oxide (GO) nano sheets are oxygen functional groups containing graphene which is obtained by treating graphene with strong oxidizer.
CNMs are widely used as reinforcing materials in the matrix due to their unique physical, mechanical and other functional properties. The properties of the nanocomposite membranes depend on the dimensions, dispersion, distribution and interfacial interaction of the nanofillers with matrix. CNMs are extensively used for elimination of the impurities from the aqueous media. The advantages of using nano materials in water purification and desalination is that the devices have ease of functionalization, high aspect ratio and faster water transport. These can be used either as direct filters or as a filler to improve the membrane surface. These carbon based nano materials have been in tremendous use as fillers because of their antifouling properties, strength, rejection, disinfection, ultra high water flux and permeability. These have recently attracted considerable attention for the synthesis of novel membranes with attractive features for water purification.
