The World Health Organization estimates that 750 million people lack access to safe water sources, while many more have insufficient supplies because of droughts like the one in California. Creating fresh water through desalination of the ocean or briny water on land is still expensive, mainly because of the energy required to push water through membranes that filter out the salt (see “How Can Desalination Become Cheaper?” and “Desalination Out of Desperation”). In recent months, scientists have published research that advances our understanding of the prospects for better membranes made out of the superstrong and lightweight material graphene.
In the March 23 issue of Nature Nanotechnology, researchers at Oak Ridge National Laboratory described how they made a graphene membrane for desalination from layering carbon atoms in a honeycomb structure. These atoms form a hexagon-shaped crystal that measures about 0.1 millimeters in width and length, with holes smaller than a nanometer designed to let water through and block salt. A desalination membrane made of this graphene crystal would ideally have to be measured in meters to work in a commercial plant, says Oak Ridge researcher Ivan Vlassiouk. He says the team has scaled this membrane material up to several millimeters so far.
MIT researchers have shown it is possible to use sheets of graphene as a desalination membrane by attaching it to a polycarbonate support structure. However, defects tend to form in the graphene, which can weaken the membrane and possibly let salt or other contaminants through. In an April 27 Nano Letters paper, the MIT researchers and authors from Oak Ridge and Saudi Arabia’s King Fahd University of Petroleum and Minerals appear to have found a way to fix this. The team filled in the larger defects with nylon and deposited hafnium metal followed by a layer of oxide to smaller defect areas.
Graphene’s strength, thinness, and chemical properties could make it the “ultimate” membrane material for desalination, MIT researchers David Cohen-Tanugi and Jeffrey Grossman wrote in a paper reviewing some of the progress researchers have made. They describe how computer simulations have helped researchers understand the chemical properties of the nanopores poked into the material; how water flows through these openings; and how well the membranes can retain their strength over time. The paper was featured in the June 15 issue of Desalination.
Oak Ridge’s Vlassiouk says it could take at least a decade to commercialize graphene desalination. While graphene could theoretically make for membranes that process water more quickly with less energy, the cost savings that would be associated with using them remain unclear.
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