Water Desalination By Shock Electrodialysis

The development of cost and energy efficient water purification systems, specifically desalination systems, is one of the most critical engineering challenges today. Recently, desalination has been used to solve the drought crisis in California, but it is costly and energy intensive [1]. Shock electrodialysis is a newly developed method that uses ion concentration polarization (ICP) zones and deionization shocks in porous media near an ion selective membrane to purify water.  In this project, we demonstrate experimentally the feasibility of shock electrodialysis in water desalination. An overlimiting current, a current that is more than the diffusion limited current, is applied through a weakly charged porous medium (a silica glass frit with pore sizes ranging from 0.75 to 1 micron) sandwiched between two cation-selective membranes (Nafion), producing a sharp concentration gradient or “shock”[2]. The cations move through the cation-selective membrane toward the cathode while the anions get trapped on the cation-selective membrane on the anode side. This movement of ions produces a sharp concentration gradient with high ion concentration near the anode side and low ion concentration near the cathode side. Flowing saltwater through the porous medium and adding a splitter that splits the flow into two streams at the end can separate the high ion concentration (brine) stream and the low ion concentration (fresh) stream. This separation allows for continuous desalination. Using this device, we were able to remove up to 99+% of ions from NaCl feed solutions of various concentrations (1 mM, 10mM, and 100mM) at a flowrate of 75 uL/min. Keeping the flow rate constant at 75 uL/min and varying the concentrations revealed that plots of the percentage of ion removal versus the applied current divided by the limiting current for all three concentrations overlapped, agreeing with the theory.  By varying the flow rate and keeping the concentration of NaCl constant at 10mM, it was seen that at lower flowrates, there was more ion removal at the same applied current, agreeing with the theory because a lower flowrate allows the solution to spend more time in the device which allows for more ion removal.  The 37.5 uL/min and 75 uL/min flowrates achieved 99+% ion removal at the highest applied current (4080 uA) while the 150 uL/min flowrate achieved 90% ion removal.  Overall, preliminary results show good desalination factors, reasonable efficiency, and unique separation capabilities. These unique separation capabilities stem from the feed water flowing through the glass silica and the large electric field applied. The small pore size in the glass silica frit is able to separate out large particles and the large electric field has the potential for disinfection; both of these qualities may find applications in compact water treatment and brine concentration.

[1] Roth, S.; California's last resort: Drink the Pacific; Desert Sun; 2015. http://www.desertsun.com/story/news/environment/2015/04/20/californias-l...

[2] Mani, A.; Zangle, T.; Santiago, J.G. On the Propagation of Concentration Polarization from Microchannel-Nanochannel Interfaces Part I: Analytical Model and Characteristic Analysis. Langmuir 2009, 25 (6), 3898-3908