Water desalination with graphene oxide nanoplatelets
Water security is one of the top three global risks according to the World Bank. Water is used for drinking and also for agriculture. Assuming current practises, there will be a 40% shortfall between forecast demand and water supply by 2030. So the announcement this week by a team at Manchester University in the UK that graphene can be used to remove salt from seawater (desalination) deserves our attention…
Desalination is a familiar topic for those of us who watch developments in the graphene industry. Back in 2012 a team at MIT announced they had created nano porous graphene that could filter clean water from salt water.
They made tiny holes in very small sheets of graphene, making it porous at the nanoscale. that could let water pass through but not the salt. However five years later they are no closer to launching a marketable product because this process requires large scale sheets of graphene membranes and no one has made these on a scale larger than square centimetres at the moment. So, this idea has to remain in the lab because producing graphene sheets at the large scale needed is beyond current manufacturing technology.
The team at Manchester have done something rather clever. They have invented a way of making larger composite membranes from epoxy resin and nanoplatelets of graphene oxide (GO) rather than plain graphene.
To understand what the MIT and Manchester teams have done, think about a pack of playing cards where each individual card is a tiny sheet of graphene. The MIT team made their filter by taking individual cards and punching tiny holes through the sheet. These holes let the water through but not the salt.
The Manchester team have taken the pack of cards and turned them on their edge. Now if this pack is used as a filter, the pressure of the water will force apart the cards in the pack, letting water and salt through. This is where the epoxy resin comes in. The team have made a layer cake of alternating layers of graphene oxide and epoxy resin encased in more epoxy with the GO layers exposed edge on to the flow of water. The resin keeps the whole arrangement held together and allows only water to pass through.
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All of this work is still at the lab scale at the moment. The Manchester team are now working on scaling up their invention. They have a better chance than the MIT because they are using GO nanoplatelets and don’t have to wait for large-scale sheet graphene to be made. Graphene nanoplatelets are made from commercially available graphite and these can be converted into graphene oxide using well-understood processes. This means the raw materials are freely available in commercial quantities and lays solid foundations for scaling up the design.
Let’s have a brief look at the desalination market. According to the International Desalination Association as of 30th June 2015:
- 300 million people around the world rely on desalinated water for some or all of their daily needs
- 31.6 billion cubic metres of desalinated water are produced each year
- 60% of desalination capacity is performed by the reverse osmosis (RO) method this uses membrane technology and accounts for 1.9 billion cubic metres per year. A reverse osmosis plant produces water for approximately $0.4 to $1.0 per cubic metre.
- The main input is electricity and accounts for 41% of the operating costs of a reverse osmosis plant. The membranes account for approximately 7% of the costs.
So what does all this mean? Well, the global market for desalination membranes is a significant one. The high electricity costs are due to pressurising pumps forcing water through the membrane. To be a successful product a graphene oxide membrane is therefore one that can increase the flow of water for a given level of desalination without fouling and thus reduce the overall cost of operation. If the researchers can achieve this at scale they will create a competitive advantage that can be sold into the market at a premium. This could be one to watch for the future.
Adrian Nixon began his career as a scientist and is a Chartered Chemist and Member of the Royal Society of Chemistry. As a scientist and ... <Read more about Adrian Nixon>