Shock Electrodialysis As an Aqueous Separation Platform

Abstract

Efficiently separating water from its dissolved contents remains a pressing challenge with far-reaching impacts. The most natural application is the removal of salt and other toxins in water to make it suitable for drinking. In addition to removing impurities, separation technologies can be used to concentrate valuable species in an aqueous effluent stream in an industrial process. Shock electrodialysis (SED), a recently developed separation platform from our lab1–4, is well equipped to concentrate and separate salts, nanoparticles, wastes, charged species, and many uncharged particles in low salinity water. SED, much like electrodialysis (ED), passes an electric current perpendicular to flow in order to guide ions out of a purified stream of water and into a concentrated brine. To contrast ED, the separation of fresh and brine is not achieved by a physical barrier like a membrane. Instead, the streams are separated by a stable concentration “shock” that arises from passing overlimiting current (defined as current requiring ion transport faster than is possible by diffusion alone) between the electrodes. This shock is stable in flow and possible only because of the SED device’s carefully chosen porous media, which must be “leaky membranes” whose surface charge per volume is large enough to sustain over-limiting current (faster than diffusion) by electro-migration (surface conduction) and electro-osmotic flow (surface convection) but small enough to allow for strong salt depletion and “wave breaking” of the concentration profile. Using a combination of this technology with shock electrodeposition, another recent advance from our research lab5, it is possible to achieve the deionization of water without the costly water splitting reaction at the electrodes while collecting a significant portion of the impurities in a solid waste stream when removing certain metallic ions.