Water. desalination + reuse

RESEARCH May-June 2014 | Desalination & Water Reuse | 35 | GE ecomagination and Aramco Entrepreneurship launched a worldwide open technology challenge on 15 April 2014 to speed up efforts to improve the energy-effi ciency of seawater desalination. Four winners will be awarded a prize of US$ 50,000 each, and further investments towards commercialization of the best ideas will be considered. The last major effort in this direction was the Affordable Desalination Collaboration in California, which demonstrated in 2009 total energy consumption for seawater desalination at 11.28 kWh/kgal (2.98 kWh/m 3 ) at a projected total cost of US$ 3.00/kgal (US$ 0.79/m 3 ). The goal of this challenge is to identify novel ways to lower these costs around the world, either through technology advances, process improvements or both. Nabil Al-Khowaiter, Aramco Entrepreneurship's director of special projects, stated "Finding a more effi cient method of desalinating seawater will be a game-changer in our collective pursuit of a more sustainable energy future across the globe. Aramco Entrepreneurship is partnering with GE not only to identify new solutions to lowering desalination costs, but also to invest in and attract new technologies and industries to Saudi Arabia." Solutions must be innovative, impactful, feasible and scalable across the globe. Entries are being immediately accepted at www.ninesights.com/community/ecomagination. The deadline is 16 July 2014, and winners will be announced in November 2014 GE and Aramco launch desalination energy challenge A new method for controlling the creation of subnanometer-scale pores in graphene sheets could make all the difference in the use of the technology in desalination, according to a research team at the Massachusetts Institute of Technology. Following on from previous research, graduate student Sean O'Hern and associate professor of mechanical engineering Rohit Karnik say that their new work demonstrates a method for actually producing such material with dense concentrations of nanometer-scale holes over large areas. First, the graphene is bombarded with gallium ions, which disrupt the carbon bonds. Then, the graphene is etched with an oxidizing solution that reacts strongly with the disrupted bonds — producing a hole at each spot where the gallium ions struck. By controlling how long the graphene sheet is left in the oxidizing solution, the MIT researchers can control the average size of the pores. The permeability of such graphene fi lters, according to computer simulations, could be 50 times greater than that of conventional membranes, as demonstrated earlier by a team of MIT researchers led by graduate student David Cohen-Tanugi of the Department of Materials Science & Engineering. But producing such fi lters with controlled pore sizes has remained a challenge. With this technique, the researchers were able to control the fi ltration properties of a single, centimeter-sized sheet of grapheme. Without etching, no salt fl owed through the defects. With just a little etching, the membranes started allowing positive salt ions to fl ow through. With further etching, the membranes allowed both positive and negative salt ions to fl ow through, but blocked the fl ow of larger organic molecules. With even more etching, the pores were large enough to allow everything to go through. Scaling up the process to produce useful sheets of the permeable graphene, while maintaining control over the pore sizes, will require further research, O'Hern says. The team's paper, Selective Ionic Transport through Tunable Subnanometer Pores in Single-Layer Graphene Membranes, was published in Nano Letters on 3 February 2014. MIT team makes progress in graphene fi ltration control Graphene frameworks 'can make desalination 100x faster' New computational research on graphene sheets is showing that, when oxidized sheets are linked together as graphene oxide frameworks (GOFs), they can be used as a tunable desalination membrane. Researchers at Oak Ridge National Laboratory (ORNL) and Rensselaer Polytechnic Institute (RPI) in the USA used supercomputer simulations to explore the purifi cation potential of GOFs. After developing computational models to describe the interactions among the material's atoms, Dr Bobby G Sumpter, director of ORNL's Nanomaterials Theory Institute, set out with RPI's Prof Vincent Meunier and Dr Adrien Nicolaï to compute the ideal confi guration for a GOF desalination membrane. They used high-performance computers to simulate how layer thickness, the density of the linking pillars, and applied pressure affect the material's performance. The simulations revealed that fi ne-tuning the GOF structure results in the ability to remove all the ions from saltwater at a much quicker rate -- approximately 100 times faster than the materials currently used as reverse osmosis membranes. The use of water-repellent graphene as part of the porous membrane contributes to the increased performance by forcing the water into channels. The coupling between water permeability and salt rejection of GOF membranes is studied as a function of linker concentration n, thickness h and applied pressure ΔP. The simulations reveal that water permeability in GOF-(n,h) membranes can be tuned from ~5 (n = 32 and h = 6.5 nm) to 400 L cm −2 day −1 MPa −1 (n = 64 and h = 2.5 nm) and follows a Cnh − αn law. For a given pore size (n = 16 or 32), water permeability of GOF membranes increases when the pore spacing decreases, whereas for a given pore spacing (n = 32 or 64), water permeability increases by up to two orders of magnitude when the pore size increases. Furthermore, for linker concentrations n ≤ 32, the high water permeability corresponds to a 100% salt rejection, elevating this type of GOF membrane as an ideal candidate for water desalination, the authors claim in their paper Tunable water desalination across graphene oxide framework membranes in the journal Physical Chemistry Chemical Physics. The researchers also observe that, as well as for salt ions, the GOF material could be used as fi ltration membranes for other contaminants such as bacteria. Since GOFs are made with abundant, inexpensive materials through a standard fabrication process, the researchers also believe that the GOF-based membranes could help make desalination more economically viable.

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