Electrochemically mediated seawater desalination.

With global demand rising faster than availability, fresh water is quickly becoming a limited resource. In fact, the United Nations estimates one third of the world s population is living in water-stressed regions, and by 2025 this number is expected to double. Seawater desalination is an attractive solution to this problem because seawater accounts for more than 97% of the world s water supply. Currently, the primary limitation preventing the widespread use of seawater desalination as a fresh water supply is the immense amount of energy required to drive the process. Here, we describe a new, electrochemically mediated desalination (EMD) method for membraneless seawater desalination. Our approach for desalination is illustrated in Figure 1a. A seawater feed is separated into brine and desalted water streams at the junction of a branched microchannel where a bipolar electrode (BPE) is present. The anodic pole of the BPE generates an ion depletion zone, and hence a local electric field gradient that redirects ions present in seawater to the brine channel. Importantly, this device operates with an energy efficiency of 25 mWhL 1 (25 5% salt rejection, 50% recovery), which is near the theoretical minimum amount of energy required for this process (ca. 17 mWhL ). In addition to this energy efficiency, the approach provides three other important benefits relative to currently available desalination methods. First, EMD does not require a membrane, thereby eliminating a major drawback of reverse osmosis (RO), the most widespread method for desalination. Second, EMD requires only a simple 3.0 V power supply to operate and therefore, in the future, may be employed in resource-limited settings with a battery or lowpower, renewable energy source. Third, the EMD platform may be prepared with little capital investment and could be implemented in a massively parallel format. Our groups have developed microfluidic technologies using BPEs for the enrichment, separation, depletion, and controlled delivery of charged analytes. In all cases, the approach relies on the formation of a locally generated electric field gradient and control over convection. The basic operating principles of BPEs and how they are able to generate electric field gradients have been previously described. Briefly, if a sufficiently high potential bias (Etot) is applied across a microfluidic channel in which a BPE is present, faradaic reactions will occur at the BPE poles. In seawater, these faradaic reactions result in the formation of an ion depletion zone (region of high solution resistivity), thus producing a local electric field gradient and providing a means for controlling the movement of ions. Several other techniques, including dynamic field gradient focusing and electric field gradient focusing, also rely on a gradient in the electric field to control the transport of charged analytes. Most relevant to this work is a phenomenon called ion concentration polarization (ICP), which generates an ion depletion zone when a potential bias applied across two fluidic channels causes a large proportion of ionic Figure 1. Schematic illustrations of a) the BPE desalination device and b) the region of interest near the BPE anode and ion depletion zone depicting the net velocity vectors of a cationic species under the combined forces of electromigration and convection. Gnd=ground.