A ship is torn apart by a terrifying storm when sailing in the very center of the ocean. Survived people are struggling on a lifeboat drifted by the ocean current. They are dehydrated because of lack of fresh water. The sun is up and high in the sky and there is no sigh of raining. People are desperate at the edge of death. At this point, a box of packaged goods from the crushed ship drifted by the sea passes by. People grab it and find some sort of magic membranes in it. They scoop up seawater, pour it through the membrane, taste the exudative water, and find out it is surprisingly fresh. And it only takes minutes to turn salty nasty seawater into lovely sweet freshwater with such membrane. What a magic.
It seems that this scene only exists in some science fiction or sci-fi movie. But now it can be turned into reality thanks to the newly research progress in graphene oxide membranes. In Feb. 14th, an article named Precise and Ultrafast Molecular Sieving through Graphene Oxide Membranes was published on Science and drew a lot of attention since then. It is about the recent research findings made by Prof. WU HengAn and Dr. WANG FengChao from the Chinese Academy of Sciences Key Laboratory of Mechanical Behavior and Design of Materials, Department of Modern Mechanics, University of Science and Technology of China. They cooperate with Prof. A. K. Geim, winner of the 2010 Nobel Prize in Physics from University of Manchester, and made a breakthrough in the rapid permeation of ions and neutral molecules through a graphene oxide (GO) membrane in an aqueous solution. Together they identified that membranes based on graphene can simultaneously block the passage of very small molecules with hydrated radii smaller than 0.45 nanometers while allowing the rapid permeation of water several orders of magnitude faster than predicted, based on diffusion theory.
Although graphene-based materials have been known for their unique features in filtration and separation, their interrelationship with water hasn’t been unveiled until the discovery of WU and the groups. Graphene-based materials can have well-defined nanometer pores and can exhibit low frictional water flow inside them. In the dry state, they are vacuum-tight. But once immersed in water, a network of nanocapillaries open up, then they act as molecular sieves, blocking all solutes with hydrated radii larger than 4.5 angstroms and accepting only species that fit in. The anomalously fast permeation is attributed to a capillary-like high pressure acting on ions inside graphene capillaries. These properties of GO membranes promise an interesting material to consider for separation and filtration technologies, particularly those that target extraction of valuable solutes from complex mixtures. It then may be possible that one monolayer of water would still go through a GO membrane, but even the smallest salts would be rejected. In which case, salty seawater can be desalinated and turned into sweet freshwater with a single GO membrane.
The research result is very promising and Science even published an article to assess its future development and application. GO membranes are particularly attractive because they are easy to fabricate and mechanically robust and should be amenable to industrial-scale production.
With the prosperous development in science and technology, many scenes you may have imagined or seen in science fictions or movies will actually come true someday, and the scene described in the very beginning of this report would mostly become reality.
(YANG Lixia, USTC News Center）
