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Membranes 2020, 10, 347 2 of 15 In the past decades, there has been great progress made in this cause. Obviously, the development of water treatment technologies has focused on the desalination of sea and brackish waters, which provide the largest, although in the raw state generally not useful, source of water. Membrane technologies quickly became the dominant processes offering high production capacities and feasible operational requirements thanks to intensive research and development in the past years. With a 69% share of the desalinated water produced worldwide in 2019 and the total capacity of 65.5 million m3/day, reverse osmosis (RO) is currently the most advantageous technology to operate in large scale desalination plants [2] without any doubt. However, being a pressure-driven process, RO still encounters its limits connected with fouling and scaling, which adds significant maintenance costs. The necessity to overcome the osmotic pressure, which in the case of seawater (with the salinity of approx. 35 g/L TDS), is about 2.3 MPa, significantly drives up the energy requirements and/or lowers the water recovery of the systems [3]. On top of that, RO removes practically every non-water substance from the feed stream, which makes it unsuitable for selective decontamination [4]. Electrodialysis (ED), which has been commercially exploited for 70 years now, is another water treatment method, using selective transport of ionic species in an electric field across ion-exchange membranes [5]. This process offers high energy efficiency for desalination in the range of concentrations between ca. 500 and 5000 ppm and is, therefore, more suitable for low-salinity brackish waters [6,7]. Thanks to being able to selectively separate charged species from non-ionic ones, ED finds use in the treatment of industrial wastewaters, including removal and recycling of heavy metals (nickel) from rinse waters, inorganic acid regeneration in the chemical industry, reacidification of fruit juices, desalination of whey in the food industry, and more [8,9] On the other hand, the effectivity of ED falls significantly as the feed stream becomes more diluted (<100 ppm). The high ohmic resistivity drives the energy consumption up. Therefore, ED cannot be used for complete water purification purposes. In electrodeionization (EDI), a related method, this is solved by filling the dilute chambers with ion-exchange beads. Ion-exchangers are characterized by relatively high conductivity and provide a large active ion-exchange area. As a result, it is possible to obtain a concentration of ionic and ionizable species far below ppb levels. Therefore, EDI is a method that is used in the electronic, pharmaceutical, and chemical industries, where ultra-pure water with conductivity lower than 0.1 μs/cm is needed. Although further development of these currently quite well-understood processes continues, there is only so much that can be done to optimize the components and materials used for certain applications before reaching the physical limitations each of the current technologies entails. The other way to look at the research for new and more effective methods is to revise the fundamentals of the current technologies using state-of-the-art tools that modern science provides. Shock electrodialysis (SED) is one of the rising technologies to be developed using this approach by overcoming the limits of classical ED and practically using them to one’s advantage. SED utilizes ion concentration polarization (ICP) to produce fresh, ion-free water by deionization shocks in porous microstructures, which extend the boundary layers adjacent to the ion-exchange element in the overlimiting current (OLC) region. Development and Theory of Shock Electrodialysis Deionization shocks were first described and experimentally confirmed on nano-microfluidic lab-on-a-chip devices by [10,11]. At the interface of a negatively charged microchannel and a nano-channel filled with electrolyte, a propagation of ICP from the ion-exchange element (nano-channel) in both directions was observed. Forming a sharp concentration gradient, this manifested itself as a deionization shock. To utilize this mechanism at a larger scale, Mani and Bazant [12] extended the models to complex microstructures with many interconnected micro- and nano-channels, where the overlimiting current to propagate the deionization shocks was predominantly driven by the surface conduction (SC) in smaller (around 1 μm) and electroosmotic flow (EOF) in larger (around 100 μm) channels, as) described in [13]. A device for water purification based on these principles was proposed in patents by Bazant’s group at the Massachusetts Institute of Technology (MIT) [14,15]. The term Shock electrodialysis was first used in the work of Deng et al., where a laboratory-scale water purificationPDF Image | Desalination Performance Assessment Anion-Exchange Membranes
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