[math-fun] Graphene for desaliniation membranes

FYI -- We were talking about desalinization technologies & energies a few weeks ago. http://www.technologyreview.com/questionoftheweek/538596/what-will-it-take-t... What Will It Take to Commercialize Better Desalination Technologies? An update on research into whether graphene could make it easier to remove salt from seawater. By Kristin Majcher on June 18, 2015 A rendering of the structure of graphene. http://www.technologyreview.com/sites/default/files/images/graphenex299.jpg 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. http://www.technologyreview.com/questionoftheweek/532891/how-can-desalinatio... http://www.technologyreview.com/featuredstory/533446/desalination-out-of-des... Single crystals 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. http://www.nature.com/articles/nnano.2015.37 Sealing leaks 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. http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b00456 Computer design 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. http://www.sciencedirect.com/science/article/pii/S001191641500003X? This next link gets you a copy of the full paper. (The following link is correct, but the file is really a pdf file. Change the file extension to .pdf after downloading to be able to read this article. http://zeppola.mit.edu/pubs/David_Desal_2015 ) The Takeaway: 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. Do you have a big question? Send suggestions to questionoftheweek@technologyreview.com. --------------------------- https://newsoffice.mit.edu/2012/graphene-water-desalination-0702 Massachusetts Institute of Technology A new approach to water desalination Graphene sheets with precisely controlled pores have potential to purify water more efficiently than existing methods. David L. Chandler, MIT News Office July 2, 2012 The availability of fresh water is dwindling in many parts of the world, a problem that is expected to grow with populations. One promising source of potable water is the world’s virtually limitless supply of seawater, but so far desalination technology has been too expensive for widespread use. Now, MIT researchers have come up with a new approach using a different kind of filtration material: sheets of graphene, a one-atom-thick form of the element carbon, which they say can be far more efficient and possibly less expensive than existing desalination systems. When water molecules (red and white) and sodium and chlorine ions (green and purple) in saltwater, on the right, encounter a sheet of graphene (pale blue, center) perforated by holes of the right size, the water passes through (left side), but the sodium and chlorine of the salt are blocked. “There are not that many people working on desalination from a materials point of view,” says Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering in MIT’s Department of Materials Science and Engineering, who is the senior author of a paper describing the new process in the journal Nano Letters. Grossman and graduate student David Cohen-Tanugi, who is the lead author of the paper, aimed to “control the properties of the material down to the atomic level,” producing a graphene sheet perforated with precisely sized holes. They also added other elements to the material, causing the edges of these minuscule openings to interact chemically with water molecules ­ either repelling or attracting them. “We were very pleasantly surprised” by how well graphene performed compared to existing systems in computer simulations, Grossman says. One common method of desalination, called reverse osmosis, uses membranes to filter the salt from the water. But these systems require extremely high pressure ­ and hence, energy use ­ to force water through the thick membranes, which are about a thousand times thicker than graphene. The new graphene system operates at much lower pressure, and thus could purify water at far lower cost, the researchers say. While reverse osmosis has been used for decades, “really basic mechanisms of separating salt from water are not well understood, and they are very complex,” Cohen-Tanugi says, adding that it’s very difficult to do experiments at the scale of individual molecules and ions. But the new graphene-based system, he says, works “hundreds of times faster than current techniques, with the same pressure” ­ or, alternatively, the system could run at similar rates to present systems, but with lower pressure. The key to the new process is very precise control over the size of the holes in the graphene sheet. “There’s a sweet spot, but it’s very small,” Grossman says ­ between pores so large that salt could pass through and ones so small that water molecules would be blocked. The ideal size is just about one nanometer, or one billionth of a meter, he says. If the holes are just a bit smaller ­ 0.7 nanometers ­ the water won’t flow through at all. Other research groups have worked to create pores in graphene, Cohen-Tanugi says, but at very different sizes and for very different purposes ­ for example, making much bigger holes to filter large molecules such as DNA, or to separate different kinds of gases. The methods used for those processes were not precise enough to make the tiny holes needed for desalination, he says, but more advanced techniques ­ such as helium-ion bombardment to make precise holes in graphene, chemical etching and self-assembling systems ­ might be suitable. For now, Grossman and Cohen-Tanugi have been doing computer simulations of the process to determine its optimal characteristics. “We will begin working on prototypes this summer,” Grossman says. Because graphene is the subject of research into many different applications, there has been a great deal of work on finding ways of making it inexpensively and in large quantities. And for desalination, because graphene is such a strong material ­ pound for pound, it’s the strongest material known ­ the membranes should be more durable than those presently used for reverse osmosis, Grossman says. In addition, the material needed for desalination does not need to be nearly as pure as for electronic or optical uses, he says: “A few defects don’t matter, as long as they don’t open it up” so that salt could pass through. Joshua Schrier, an assistant professor of chemistry at Haverford College, says, “Previous simulations had studied the flow of water through very small holes in graphene, and the design of pores that selectively allow ion passage, but ­ despite the social and engineering relevance to desalination ­ nobody had thought to examine the intersection of these two fields.” The work by the MIT team could open a whole new approach to desalination, he says. Schrier adds, “Manufacturing the very precise pore structures that are found in this paper will be difficult to do on a large scale with existing methods.” However, he says, “the predictions are exciting enough that they should motivate chemical engineers to perform more detailed economic analyses of … water desalination with these types of materials.” The work was funded by the MIT Energy Initiative and a John S. Hennessy Fellowship, and used computer resources from the National Energy Research Scientific Computing Center. Topics: GrapheneMaterials scienceMIT Energy Initiative (MITEI)Nanoscience and nanotechnologyWaterDesalinationResearchJeffrey GrossmanResearch Laboratory of Electronics Related Paper: "Water Desalination across Nanoporous Graphene" Jeffrey Grossman David Cohen-Tanugi Research Laboratory of Electronics Department of Materials Science and Engineering MIT Energy Initiative