Predicting ion selectivity in water purification by capacitive deionization: electric double layer models

Thermodynamic Evaluation of Electrode Storage for Capacitive Deionization

This study details the development of a computational adsorption model for predicting thermodynamic adsorption parameters for capacitive deionization (CDI) processes. To do this, multiple starting concentrations and temperatures are needed to predict the best fit value. This is first demonstrated experimentally using an in-house CDI cell with custom heaters, and determining maximum adsorption capabilities for a selected range of conditions. This has been done previously for CDI in the published literature, but here, experimental results are incorporated to provide the best fit to a computational model, which runs transient CDI tests in batch mode over multiple concentrations and temperatures to determine adsorption parameters. This saves the eventual challenge of having to run many different experiments independently to determine such adsorption parameters, the accuracy of which may be questionable subject to different experimental errors. With the model, many parameters can be quickly scanned at once, and adsorption parameters can be determined based on the concentration and temperature values selected, as well as other operating conditions, such as voltage and cell resistance. The computational isotherms are generated using the Gouy-Chapman-Stern (GCS) model, which is common for the lower concentration values used for CDI. The model also considers fixed and mobile chemical charges for enhanced CDI (ECDI) and Faradaic CDI (FaCDI), respectively, which have been examined as alternatives to improve CDI performance. While primarily proof-of-concept, the results obtained here demonstrate the benefits in adsorption capabilities, and energy savings obtained here demonstrate benefits in adsorption capabilities and energy savings for FaCDI, coinciding with higher enthalpies and entropies of adsorption. The model also serves as a benchmark in the future for how the results can be further explored and better fits can be obtained experimentally to confirm stability in the thermodynamic values.

Electrostatics, Hydration, and Chemical Equilibria at Charged Monolayers on Water

The chemistry and physics of soft matter interfaces, especially aqueous-organic interfaces, are centrally important to many areas of science and technology. Often, the thermodynamics, kinetics, and selectivity of reactions are modified at interfaces. Here, we review the electrostatics and hydration at charged monolayers on water and their influence on interfacial chemical equilibria. First, we provide an understanding of interfaces as a conceptual continuation of the solvation shell of small molecules, along with recent relevant experimental work. Then, we provide a summary of models for describing the electrostatics of aqueous interfaces. While we will discuss a range of new developments, our focus will be on systems where the electrostatics of the surface is controllable by the choice of relatively simple insoluble surfactants. New insights into the molecular structure of the double layer, with particular attention on the knowledge gained from spectroscopy will be reviewed. Our approach is to familiarize the reader with simple models, followed by discussion of models with further complexity for explaining interfacial phenomena. Experiments that test the limits of such models will also be discussed. Finally, we will provide an outlook on engineering the interfacial environment for tailored reactivity, along with the anticipated experimental advancements and potentials impacts.

Selective adsorption in ion exchange membranes: The effect of solution ion composition on ion partitioning

Electrodialysis is a water desalination technology that enables selective separation of ions, making it a promising solution for sustainable water reuse. The selectivity of the process is mainly determined by the properties of ion exchange membranes that can vary depending on the composition of ions in water, such as water uptake and charge density. In this work, we studied selective adsorption of Na+ and K+ ions in various ion exchange membranes considering the effect of solution ion composition on membrane water volume fraction. For that purpose, we conducted membrane adsorption experiments using solutions with Na+ and K+ ions with different ion compositions including Li+, Ca2+ or Mg2+ ions at different concentrations (0.001 - 0.25 M). The experiments showed that with the total ion concentration and the amount of divalent ions in solution, the membrane water volume fraction decreases while the selective adsorption of the smaller (hydrated) K+ ions over the Na+ ions in the membrane increases. We developed a theoretical framework based on Boublik-Mansoori-Carnahan-Starling-Leland (BMCSL) theory to describe the effect of membrane water volume fraction on selective adsorption of the ions by including volumetric effects, such as size exclusion. The developed framework was used to describe ion partitioning results of the membrane adsorption experiments. In addition, the effect of solution ion composition on selective ion removal during electrodialysis operation was evaluated using experimental data and theoretical calculations. The results of this study show that considering volumetric effects can improve the ion partitioning description in ion exchange membranes for solutions with various ion compositions.

Hydrolyzed Hydrated Titanium Oxide on Laser-Induced Graphene as CDI Electrodes for U(VI) Adsorption

Recently, laser-induced graphene (LIG), which has been successfully applied in CDI technology (directly without a complex preparation process), has gained considerable attention. However, the raw LIG electrode with a limited number of active sites exhibits low adsorption efficiency. Therefore, the search for a suitable and effective method to modify LIG to improve its electroadsorption performance is significant. Herein, a very simple titration hydrolysis method is adopted to modify LIG, resulting in a layer of hydrated titanium oxide (HTO) being synthesized on the surface of LIG. The LIG/HTO composites possess a good adsorption property since covering the surface of LIG with a layer of HTO can greatly improve the adsorption capacity of LIG. Moreover, with the addition of HTO, not only the proton transfer ability of LIG has been enhanced but also considerable specific capacitance has been enlarged. As a result, LIG/HTO composite as CDI electrode displays a maximum theoretical adsorption capacity of 1780.89 mg/g at 1.2 V, and the capacitance of LIG/HTO composite material is 4.74 times higher than LIG. During the electroadsorption process, Ti4+ is reduced to Ti3+ under external voltage, and O2- is produced through oxidation. Meanwhile, part of the U (VI) is hydrolyzed into UO3·2H2O under the action of -OH, and some combine with O2- to produce UO4·4H2O.

Automation of membrane capacitive deionization process using reinforcement learning

Capacitive deionization (CDI) is an alternative desalination technology that uses electrochemical ion separation. Although several attempts have been made to maximize the energy efficiency and productivity of CDI with conventional control methods, it is difficult to optimize the CDI processes because of the complex correlation between the operational conditions and the composition of feed water. To address these challenges, we applied deep reinforcement learning (DRL) to automatically control the membrane capacitive deionization (MCDI) process, which is one of the representative CDI processes, to accomplish high energy efficiency while desalinating water. In the DRL model, the numerical model is combined as the environment that provides states according to the actions. The feed water conditions, that is, the input state of the DRL, were assumed to have a random salt concentration and constant foulant concentration. The model was constructed to minimize energy consumption and maximize desalted water volume per cycle. After training of 1,000 episodes, the DRL model achieved a 22.07% reduction in specific energy consumption (from 0.054 to 0.042 kWh m^-3) and 11.60% increase in water desalted water volume per cycle (from 1.96×10^-5 to 2.19×10^-5 m³), achieving the desired degree of desalination, compared to the first episode. This improved performance was because the trained model selected the optimized operating conditions of current, voltage, and the number and intensity of flushing. Furthermore, it was possible to train the model depending on demand by modifying the reward function of the DRL model. The fundamental principle described in this study for applying the DRL model in MCDI operations can be the cornerstone of a fully automated water desalination process.