This NSF award by the Chemical and Biological Separations program supports work by Professor Juan Santiago at Stanford University to develop highly efficient sea and brackish water desalination (ion removal) devices. Currently, the US faces simultaneous population growth and a decline in freshwater resources, so that desalination will become increasingly important towards realizing sustainable water management. However, current desalination methods suffer from high energy and/or infrastructure costs. In an effort to develop new methods which reduce these costs, Santiago's group is researching ion concentration shock waves created by concentration polarization. Concentration polarization arises when an electric field is applied across specialized membranes called ion exchange membranes. This concentration polarization describes the creation of a region of enriched ion concentration on one side of the membrane, and the associated creation of a region of depleted (desalinated) ion concentration on the opposite side of the membrane. The waves of interest are sharp, moving interfaces which bound growing regions of ion enrichment and depletion. The Santiago group will focus on performing controlled experiments and developing models of key phenomena, which will be utilized to develop highly efficient devices. The group will develop prototype desalination devices which generate small, test-amounts of desalinated water. The group will engage undergraduate students and incorporate material from this research into engineering curriculum courses.
Electrokinetic shockwave desalination method: We have discovered a new method which achieves desalting concentration shock waves by coupling electromigration processes to replace ions in a water volume with ions which then undergo a precipitation reaction. The scheme that we investigate in detail involves exchanging chloride ions for carbonate ions and sodium ions for calcium ions. Carbonate ions then react with calcium ions to form calcium carbonate (limestone) precipitate. Calcium carbonate can be settled or filtered out, producing fresh water. As an initial test system, we have demonstrated this method to desalinate 100 to 600 mM sodium chloride solutions, experimentally achieving roughly 1 mM ionic strength product water. The additional energy required to desalinate over and above that necessary to generate hydrogen via electrolysis is on par with the energy consumed by state-of-the-art desalination methods. We therefore believe that the process can be feasible as a part of a co-generation facility: generating fresh water and hydrogen (and chlorine gas). The method's permeate recovery ratio decreases linearly with feed water concentration and is also on par with that for reverse osmosis. Lastly, the method employs no special membranes, which contribute significantly to the cost of reverse osmosis and electrodialysis technologies. Flow through capacitive desalination method: We have developed a new capacitive desalination geometry by leveraging the combined micro - nano scale pore size conductive aerogels. In our geometry feed saline water flows directly through the electrodes in the direction parallel to the primary electric field. This is unlike traditional capacitive desalination where feed water flow is perpendicular to the electric field direction. Our geometry allows us to overcome limitations of traditional capacitive desalination such as inability to desalinate moderate brackish water feeds in a single charge and slow, diffusion-limited desalination. We analyzed this novel capacitive desalination method using porous electrode theory and experimentally demonstrated the method in a bench scale setup. Our desalination cell had a mean sorption rate of nearly 1 mg of NaCl per g of electrode material per min, which is 4 to 10 times higher than traditional capacitive desalination cells. Our cell also desalinated the feed at the cell's RC time scale, which is significantly faster than the diffusion timescale, the time scale of traditional capacitive desalination cells. Key outcomes or Other achievements: Our work has led to several key outcomes which we summarize here: 1. We invented two novel desalination technologies which avoid the use, expense, and infrasture costs associated with traditional membrane technologies. Each of these inventions has led to a disclosure to Stanford Office of Technology license, and at least one US Patent Application. 2. We published our work in eight (8) journal papers in top journals (see list below). 3. Mathew Suss, a student formerly working on the project, has now graduated with his PhD and started a postdoctoral researcher position at Massachusetts Institute of Technology working with Prof. Martin Bazant. Suss has also received a faculty offer from a top university which he is considering. Yatian Qu and Viktor Shkonikov, both students working on the project, completed their Masters of Science in Mechanical Engineering and are now pursuing their PhD in my group (both working on desalination technologies). 4. Daniel Strickland, a student formerly working on the project, graduated with his PhD and started his career as a professor at Santa Clara University. 5. Supreet Bahga, a student formerly working on the project, graduated with his PhD and started his career as a professor at Indian Institute of Technology, Delhi. 6. We initiated an ongoing collaboration with Lawrence Livermore National Labs (primarily working with Dr. Michael Stadermann of LLNL). This collaboration has directly led to 4 of the publications above and to, so far, three other proposals around our technology. 7. Santiago has incorporated theory and examples of water desalination into two of his graduate courses at Stanford. 8. Our work has been featured in popular press articles including articles by Research Media Ltd. and PhysOrg.com Other outcomes include: Internet Dissemination: http://microfluidics.stanford.edu/spresso NSF funds from this project have helped contribute to SPRESSO. SPRESSO is a unique, free, open-source simulation tool which we have developed for the simulation of a wide range of electrokinetic (including electrophoresis) phenomena. Contributions: The project funded the research of Viktor Shkolnikov, Matt Suss, Daniel Strickland, and Supreet Bahga. I have been able to fund three students as Suss and Bahga currently hold fellowships. Suss's thesis was centered on desalination using electric capacitance as a form of salt removal from water. Contributions Beyond Science and Engineering: We have filed two disclosures which we hope will lead to a US patent in the area of water desalination, using the techniques developed as part of this project. The university is proceeding with at least one of these toward a US patent application. Potentially, this work may lead to commercialization of our method.