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Theory of shock electrodialysis

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Theory of shock electrodialysis ( theory-shock-electrodialysis )

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Theory of shock electrodialysis I: Water dissociation and electrosmotic vortices Huanhuan Tianb, Mohammad A. Alkhadrab, Martin Z. Bazantb,1 aDepartment of Mathematics, Massachusetts Institute of Technology. Massachusetts 02139, USA bDepartment of Chemical Engineering, Massachusetts Institute of Technology, Massachusetts 02139, USA Abstract Shock electrodialysis (shock ED), an emerging electrokinetic process for water purification, leverages the new physics of deionization shock waves in porous media. In previous work, a simple leaky membrane model with surface conduction can explain the propagation of deionization shocks in a shock ED system, but it cannot quantitatively predict the deionization and conductance (which determines the energy consumption), and it cannot explain the selective removal of ions in experiments. This two-part series of work establishes a more comprehensive model for shock ED, which applies to multicomponent electrolytes and any electrical double layer thickness, captures the phenomena of electroosmosis, diffusioosmosis, and water dissociation, and incorporates more realistic boundary conditions. In this paper, we will present the model details and show that hydronium transport and electroosmotic vortices (at the inlet and outlet) play important roles in determining the deionization and conductance in shock ED. We also find that the results are quantitatively consistent with experimental data in the literature. Finally, the model is used to investigate design strategies for scale up and optimization. Keywords: Shock electrodialysis, water disscoiation, electroosmotic vortices, nonlinear electrokinetics 1. Introduction The ability to efficiently remove ions and ionic compounds from a dilute feed is central to a sustainable future with clean water. With growing industrial development, for example, toxic heavy metals, radioactive ions, and inorganic compounds are increasingly discharged into the environment and into our sources of drinking water. These contaminants are hazardous even when present in trace quantities, and it remains a challenge to remove them affordably and reliably [1, 2]. This challenge creates an urgent need for the development of systems by which charged species are selectively separated from dilute feeds. Traditional membrane processes for water treatment such as reverse osmosis (RO) and electrodialysis (EO)—which have proven successful for desalination of concentrated solutions like seawater—are not selective and have limited utility when the objective is to remove target ions in the presence of a competing electrolyte like sodium [3, 4]. Over the past decade, Bazant and coworkers have developed, patented, and characterized an emerging electrokinetic process called “shock electrodialysis (shock ED)” for water purification [5, 6, 7, 8, 9, 10]. As shown in Fig.1, the main component of the shock ED prototype is a weakly charged macroporous material sandwiched between two ion exchange membranes. In the current prototype, the macroporous material and both ion exchange membranes have negatively charged surfaces. When current is applied to the system, the membranes block the transport of coions (i.e., anions) and cause ion concentration polarization, which leads to extreme concentration gradients in the macroporous material. As the concentration is reduced to nearly zero on the depleted side, the current reaches the so-called diffusion limited current. However, Email address: bazant@mit.edu (Martin Z. Bazant) Preprint submitted to Elsevier December 11, 2020 arXiv:2012.05431v1 [physics.flu-dyn] 10 Dec 2020

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