21/04/2017 – Independent / Environment / Water Filtration / Desalination / Graphene
Saltwater Gets Sweeter
By 2025, the UN expects that 14 per cent of the world’s population will encounter water scarcity. As the effects of climate change continue to diminish fresh water supplies, countries are investing in desalination solutions. Unfortunately, current technological processes are expensive, energy-intensive, and involve large-scale facilities. Yet scientists have recently unveiled a new cost-effective method that utilises graphene to turn seawater into potable water – a discovery that could revolutionise water filtration across the world. Fran Roberts investigates.
Isolated and characterised by a University of Manchester-led team in 2004, graphene comprises a single layer of carbon atoms arranged in a hexagonal lattice. The nanomaterial has been posited for all sorts of potentially revolutionary uses, from faster, thinner and even transparent electronics to biotech implants and better battery capacity. Now researchers at the University of Manchester say they have come up with a method for controlling the permeation of graphene oxide membranes so they can act as a sieve to desalinate seawater. By controlling the size of the pores in the membranes, the team was able to prevent common salts passing through the material – turning seawater into drinking water. “Realisation of scalable membranes with uniform pore size down to atomic scale is a significant step forward and will open new possibilities for improving the efficiency of desalination technology,” reveals Professor Rahul Nair, lead researcher on the project. “This is the first clear-cut experiment in this regime. We also demonstrate that there are realistic possibilities to scale up the described approach and mass produce graphene-based membranes with required sieve sizes.” One of the lead authors on the study, Mr Jijo Abraham, elaborates on this: “The developed membranes are not only useful for desalination, but the atomic scale tunability of the pore size also opens new opportunity to fabricate membranes with on-demand filtration capable of filtering out ions according to their sizes.”
According to the International Desalination Association, in June 2015, 18,426 desalination plants were in operation worldwide, producing 86.8 million cubic metres per day, providing water for 300 million people. This number increased from 78.4 million cubic metres in 2013, a 10.71 per cent increase in two years. Seawater desalination technology, available for decades, has made great strides in many arid areas of the world such as the Middle East, the Mediterranean, and the Caribbean. However, the World Bank notes that desalination should remain the last resort, and should only be applied after cheaper alternatives in terms of supply and demand management have carefully been considered.
For some countries, however, institutional mismanagement of resources means that desalinisation is the best option, albeit an expensive one. Kazakhstan, for example, is looking to desalinate the Caspian Sea as an answer to its water woes. According to a report by the Asian Development Bank (ADB), before Kazakhstan’s independence, an estimated 80 per cent of all rural villages had piped potable water supply. Due to financial difficulties following the break-up of the Soviet Union, coverage was officially reported at 40 per cent, although in reality, it might be even less, with systems that were deemed to be operational having deteriorated rapidly. According to ADB estimates, the average volume of drinking water delivered to the population has been diminishing at a rate of up to five per cent a year due to the continued decline in the condition of the existing infrastructure.
It is hoped that graphene-oxide membrane systems can be built on smaller scales, making this new generation of desalination technology accessible to countries that do not have the funding available to finance large plants without compromising the yield of fresh water produced. This should prove key to supplying potable water to poorer nations struggling with water scarcity. “The selective separation of water molecules from ions by physical restriction of interlayer spacing opens the door to the synthesis of inexpensive membranes for desalination,” observes Dr Ram Devanathan from the Pacific Northwest National Laboratory. “The ultimate goal is to create a filtration device that will produce potable water from seawater or wastewater with minimal energy input.”
Of course, whilst this breakthrough holds life-changing potential for many around the world, more research is needed before graphene sieves replace polymer-based membranes in desalination plants globally. “This is our first demonstration that we can control the spacing and that we can do desalination, which was not possible before. The next step is to compare this with the state-of-the-art material available on the market,” explains Professor Nair. Given the amount of progress that has already been made with this fascinating material since its isolation over a decade ago – an event which won the scientists involved the Nobel Prize in Physics in 2010 – it seems likely that graphene could prove key to bringing us potable water in the not-too-distant future, and at a fraction of the price of existing commercially-available systems.
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