Energy consumption and salt adsorption in capacitive deionization
Keywords: Mechanical Engineering.

Challenges for clean water are global and diverse, and flexible water treatment approaches are needed. To date, over 4 billion people, more than half of the world's population, live in water scarce areas where withdrawn water surpasses the amount the region can sustainably support for at least one month per year. Slightly under 4 billion live in areas of severe scarcity conditions. Aggravating the situation further, naturally occurring toxins, chemical contamination introduced by human activities, and high salinity make many already limited water sources unsafe for consumption. Water desalination and disinfection provides viable solution to deficit of clean water. Seawater desalination is currently the main source of clean water production, however, availability of seawater is geographically uneven and thus not an available option targeted towards inland regions. Brackish water (water with a low to moderate salt content), however, is relatively common and can provide an appealing source of potable water with appropriate treatment technologies. Capacitive desalination or capacitive deionization (CDI) is an electrosorptive desalination method that leverages porous and conductive electrodes for electrostatic ion adsorption. Upon application of a small voltage (order 1 V) across each electrode pair, salt ions are removed from feed water and electrostatically held within the electrode pores. The CDI cell is then regenerated by removing or reversing the voltage, which spontaneously releases the ions and forms brine solution. CDI has a number of advantages over common desalination techniques. Most importantly, it does not require high pressure or temperature to operate, is widely scalable, and thus relevant for distributed applications (as investment and infrastructure cost is low and is directly proportional to plant capacity). CDI is potentially energy efficient and cost effective for brackish water desalination, since the energy cost per volume of treated water roughly scales with the amount of removed salt (rather than volume of treated water). CDI is thus the most advantageous in brackish water desalination as well as water recycling and reuse where salt content is far below that of seawater. In addition, CDI has the great potential for selective removal of ionic species based on ion valence, hydrated ion size and pore size, surface chemistry, and pH environments. We first focus on electrosorptive desalination energy, in both theory and practice. We present a general top-down approach to show minimum energy of ion separation is indeed Gibbs free energy of separation for most known EDLs irrespective of EDL geometry and thickness. We fabricate a low series resistance CDI cell, operate the cell at various current and flow rates, and demonstrate low-energy desalination with unprecedented 9% thermodynamic efficiency and only 4.6 kT energy requirement per removed ion. We further experimentally quantify individual loss mechanisms and show resistive and Faradaic losses as two main loss mechanisms. We show the two loss mechanisms favor different charging rates: resistive losses are dominant at high charging currents, but Faradaic losses are dominant at low charging rates, as the cell spends longer time at high voltage. Our results provide a powerful tool for optimizing CDI operation. In addition to study of desalination energy, we study charge and species transport of electrosorption process. We formulate and solved the first two-dimensional model of a CDI cell coupling external electrical network, charge conservation, and mass conservation in bimodal pore structure electrodes. We fabricate a lab-scale CDI cell, experimentally calibrate the model, and show a good agreement between model results and experimental data. Our results show CDI process has two distinct phases: a fast adsorption step at the beginning of charging followed by a slow salt removal step. Finally, we study the effect of surface functional groups on pH dependent salt adsorption and ion selectivity by developing theory and performing controlled experiments. To this end, we expanded the current surface charge models by coupling a double layer model with acid-base equilibria theory and further validate the model by well-controlled titration experiments. The fitted model with one acidic and one basic surface group showed a very good agreement with the experiments. Our results show (1) specific adsorption of cations and expulsion of anions at electrolyte pH values higher than pK of acidic groups, and (2) specific adsorption of anion and expulsion of cations at pH values lower than pK of basic groups.