Optimization of salt adsorption rate in membrane capacitive deionization

Development and environmental performance of a pilot-scale membrane capacitive deionization system for wastewater reclamation: Long-term operation and life cycle analysis

Wastewater reclamation is regarded as a primary solution for efficient water resource management because of its environmental friendliness and energy efficiency. Membrane capacitive deionization (MCDI) has shown great promise as a practical technology for wastewater reclamation, but challenges remain for the large-scale deployment of this technology due to gaps in understanding its technical and environmental performance. This study presents a pilot-scale MCDI-based wastewater treatment and reclamation system that includes sand filtration (SF), ultrafiltration (UF), MCDI, and ultraviolet (UV) units. Additionally, this research aims to investigate the overall environmental impacts and trade-offs of the system through a life cycle assessment (LCA) approach to identify impact hotspots with the potential for system improvement. Over long-term operation, the water quality characteristics showed significant improvements in conductivity, ammonia-N content, and total hardness, satisfactorily meeting the standards for wastewater reclamation. Results from the impact assessment revealed that the production of 1 m3 of desalinated water for reclamation in the MCDI-based system generates a global warming potential of approximately 2.77 kg CO2 eq, primarily due to electricity consumption and the use of high-impact chemicals. Electricity and chemical consumption contribute nearly 81 % and 15 %, respectively, to the overall impacts. These inputs also have remarkable impacts on marine aquatic ecotoxicity, human toxicity and abiotic depletion. The impacts from material and chemical usage are average out during the scaling-up process due to the increase in water productivity. As demonstrated, the integration of emerging water treatment technologies with high energy efficiency could significantly improve the environmental performance of the system. The results from the present study can offer valuable insights for advancing future wastewater reclamation systems aimed at improving environmental outcomes.

A preliminary attempt at capacitive deionization with PVA/PSS gel coating as an alternative to ion exchange membrane

PVA/PSS composite gel membrane electrode for membrane capacitive deionization (MCDI) was fabricated and characterised in the present study. The composite electrode with ion exchange surface is prepared by coating glutaraldehyde cross-linked polyvinyl alcohol (PVA) composite hydrogel, with Poly (Sodium 4-Styrenesulfonate) (PSS) added into the network, on the surface of activated carbon (AC) electrode. The feasibility of the gel membrane is analyzed by rheological, swelling rates and ion exchange capacity tests. Then electrochemical test and desalination test are used to study the performance of the MCDI electrode. The results show that coating of composite hydrogel layer improved the hydrophilicity, specific capacitance and lower interfacial electron transfer resistance of the electrode. Finally, we assemble the asymmetrical CDI cell with PVA/PSS composite gel electrode and AC electrode. Compared with the AC electrode, the salt adsorption capacity of PVA6-PSS15 can reach 18.9 mg g-1 and stable charge efficiency at 73.0% at operating voltage of 1.2 V. The decrease in specific capacitance of PVA6-PSS15 after 50 cycles is 1.33%, indicating that the electrode has a good cycling life. The gel membrane coated electrode prepared by PSS provides a new idea for the development of MCDI.

Upcycling Waste Polyethylene Terephthalate to Produce Nitrogen-Doped Porous Carbon for Enhanced Capacitive Deionization

Porous carbon with a high surface area and controllable pore size is needed for energy storage. It is still a significant challenge to produce porous carbon in an economical way. Nitrogen-doped porous carbon (N-PC) was prepared through carbonization of a mixture of waste PET-derived metal-organic frameworks (MOFs) and ammonium. The obtained N-PC exhibits a large surface area and controlled pore size. When utilized as an electrode material for supercapacitors, the N-PC exhibits a specific capacitance of 224 F g-1, significantly surpassing that of commercial activated carbon (AC), which has a capacitance of 111 F g-1. In the subsequent capacitive deionization (CDI) tests, the N-PC demonstrated a maximum salt adsorption capacity of 19.9 mg g-1 at 1.2 V in a NaCl electrolyte (0.5 g L-1), and the salt adsorption capacity increased to 24.7 mg g-1 at 1.4 V. The N-PC electrode also exhibited superior regeneration. The present work not only presents a potential approach to develop cost-effective electrodes for seawater purification but also paves the way for recycling of waste plastics into high value-added products.

Autonomous real-time control for membrane capacitive deionization

Artificial intelligence has been employed to simulate and optimize the performance of membrane capacitive deionization (MCDI), an emerging ion separation process. However, a real-time control for optimal MCDI operation has not been investigated yet. In this study, we aimed to develop a reinforcement learning (RL)-based control model and investigate the model to find an energy-efficient MCDI operation strategy. To fulfill the objectives, we established three long-short term memory models to predict applied voltage, outflow pH, and outflow electrical conductivity. Also, four RL agents were trained to minimize outflow concentration and energy consumption simultaneously. Consequently, actor-critic (A2C) and proximal policy optimization (PPO2) achieved the ion separation goal (<0.8 mS/cm) as they determined the electrical current and pump speed to be low. Particularly, A2C kept the parameters consistent in charging MCDI, which caused lower energy consumption (0.0128 kWh/m3) than PPO2 (0.0363 kWh/m3). To understand the decision-making process of A2C, the Shapley additive explanation based on the decision tree model estimated the influence of input parameters on the control parameters. The results of this study demonstrate the feasibility of RL-based controls in MCDI operations. Thus, we expect that the RL-based control model can improve further and enhance the efficiency of water treatment technologies.

Model and simulations of the effects of polyelectrolyte-coated electrodes in capacitive deionization

The problem of ion transport in porous media is fundamental to many practical applications such as capacitive deionization, where ions are electrostatically attracted to a porous electrode and stored in the electric double layer, leaving a partially desalinated solution. These electrodes are functionalized to achieve maximum efficiency: it is intended that for each depleted electron one ion is removed. For this purpose, the surface is coated with a polyelectrolyte layer of the same sign as the electronic charge. In this work, the movement of ions from the solution to the soft or polyelectrolyte-coated electrodes is studied. For this purpose, a one-dimensional model is used to study the electric and diffusive fluxes produced by the application of an electric field and the storage of these ions in the micropores. The partial differential equations governing the process are numerically solved using the explicit Euler method. The results of the model indicate that the number of ions removed using soft electrodes is approximately 15% greater than that achieved with bare electrodes. Ion adsorption kinetics show that coated electrodes provide slightly slower adsorption compared to bare electrodes. Regarding the charging time of the micropores, it can be seen that it is a faster process (characteristic time of 100 s) compared to the time in which the ion concentration reaches equilibrium: electromigration is faster than diffusion. Comparing the situations with and without polyelectrolyte coating, it is observed that saturation in the micropores is reached earlier when the electrodes are coated. Concerning the cell geometry, it has been found that the characteristic time is proportional to the length of the spacer and inversely proportional to the length of the electrodes. With regard to microporosity, the rate of the process is approximately constant, irrespective of the number of micropores. Moreover, the number of adsorbed ions strongly depends on their initial concentration. Finally, the analysis of the ionic diffusion coefficient is determinant in the kinetics of the process: Taking into account the tortuosity of the porous electrode, which directly affects the diffusion in the channel, is fundamental to obtain model predictions close to reality.

Membrane capacitive deionization (MCDI): A flexible and tunable technology for customized water softening

There is a growing demand for water treatment systems for which the quality of feedwater in and product water out are not necessarily fixed with "tunable" technologies essential in many instances to satisfy the unique requirements of particular end-users. For example, in household applications, the optimal water hardness differs for particular end uses of the supplied product (such as water for potable purposes, water for hydration, or water for coffee or tea brewing) with the inclusion of specific minerals enhancing the suitability of the product in each case. However, conventional softening technologies are not dynamically flexible or tunable and, typically, simply remove all hardness ions from the feedwater. Membrane capacitive deionization (MCDI) can potentially fill this gap with its process flexibility and tunability achieved by fine tuning different operational parameters. In this article, we demonstrate that constant-current MCDI can be operated flexibly by increasing or decreasing the current and flow rate simultaneously to achieve the same desalination performance but different productivity whilst maintaining high water recovery. This characteristic can be used to operate MCDI in an energy-efficient manner to produce treated water more slowly at times of normal demand but more rapidly at times of peak demand. We also highlight the "tunability" of MCDI enabling the control of effluent hardness over different desired ranges by correlating the rates of hardness and conductivity removal using a power function model. Using this model, it is possible to either i) soften water to the same hardness level regardless of the fluctuation in hardness of feed waters, or ii) precisely control the effluent hardness at different levels to avoid excessive or insufficient hardness removal.

Emerging Frontiers in Multichannel Membrane Capacitive Deionization: Recent Advances and Future Prospects

Capacitive deionization (CDI) has emerged as a promising desalination technology and recently promoted the development of multichannel membrane capacitive deionization (MC-MCDI). In MC-MCDI, the independent control of multiflow channels, including the feed and electrolyte channels, enables the optimization of electrode operation in various modes, such as concentration gradients and reverse voltage discharge, facilitating semicontinuous operation. Moreover, the integration of redox couples into MC-MCDI has led to advancements in redox-mediated desalination. Specifically, the introduction of redox-active species helps enhance the ion removal efficiency and reduce energy consumption during desalination. This systematic approach, combining principles from CDI and electrodialysis, results in more sustainable and efficient desalination. These advancements have contributed to improved desalination performance and practical feasibility, rendering MC-MCDI an increasingly attractive option for addressing water scarcity challenges. Despite the considerable interest in and potential of this process, there is currently no comprehensive review available that covers the operational features and applications of MC-MCDI. Therefore, this Review provides an overview of recent research progress, focusing on the unique cell configuration, vital operation principles, and potential advantages over conventional CDI. Additionally, innovative applications of MC-MCDI are discussed. The Review concludes with insights into future research directions, potential opportunities in industrial desalination technology, and the fundamental and practical challenges for successful implementation.

Sustained Phosphorus Removal and Enrichment through Off-Flow Desorption in a Reservoir of Membrane Capacitive Deionization

In this study, we used a membrane capacitive deionization device with a reservoir (R-MCDI) to enrich phosphorus (P) from synthetic wastewater. This R-MCDI had two small-volume electrode chambers, and most of the electrolyte was contained in the reservoir, which was circulated along the electrode chambers. Compared with conventional MCDI, R-MCDI exhibited a phosphate removal rate of 0.052 μmol/(cm2·min), approximately double that of MCDI. This was attributed to R-MCDI's utilization of OH- alternative adsorption to remove phosphate from the influent. Noticing that around 73.9% of the removed phosphate was stored in the electrolyte in R-MCDI, we proposed a novel off-flow desorption operation to enrich the removed phosphate in the reservoir. Exciting results from the multicycle experiment (∼8 h) of R-MCDI showed that the PO43--P concentration in the reservoir increased all the way from the initial 152 mg/L to the final 361 mg/L, with the increase in the P charge efficiency from 5.5 to 22.9% and the decrease in the energy consumption from 28.2 to 6.8 kW h/kg P. The P recovery performance of R-MCDI was evaluated by viewing other similar studies, which revealed that R-MCDI in this study achieved superior P enrichment with low energy consumption and that the off-flow desorption proposed here considerably simplified the operation and enabled continuous P enrichment.

Enhanced Desalination Performance Using Phosphate Buffer-Mediated Redox Reactions of Manganese Oxide Electrodes in a Multichannel System

Water desalination mediated by electrochemical reactions to directly capture and release salt at electrode materials offers a low-voltage method for producing freshwater. Developing new system designs has allowed electrode materials to maximize their capacity for salt separation, especially when a multichannel system is used to introduce a separate electrode rinse solution. Here, we show that the use of an additive can provide a new strategy for improving electrode capacity and, hence desalination performance, which so far has been limited to increasing the electrolyte concentration. A custom-built, 2/2-channel flow cell divided by two cation exchange membranes and an anion exchange membrane was fed with 50 mM NaCl as the feed (two inner channels) and 0.5 M NaCl containing up to 0.1 M phosphate as the electrode rinse (two outer channels). Using manganese oxide electrodes with phosphate buffer-mediated redox reactions exhibited an improved desalination capacity of 68.0 ± 5.2 mg g-1 (0.55 mA cm-2) and a rate of 5.6 ± 1.3 mg g-1 min-1 (0.96 mA cm-2). The improvement was attributed to the buffer that served as a proton donor for promoting the H+ insertion reaction of amorphous or poorly crystalline MnO2. Additionally, the buffering capacity against acidification and the creation of insoluble manganese phosphate on the electrode surface prevented the dissolution of Mn2+, which could otherwise occur at the anode due to a decrease in the local pH upon H+ deinsertion. Thus, the use of manganese oxide electrodes coupled with phosphate provides a new strategy of increasing electrode capacity for water desalination.

Controllable Preparation of a N-Doped Hierarchical Porous Carbon Framework Derived from ZIF-8 for Highly Efficient Capacitive Deionization

Capacitive deionization (CDI) is a promising desalination technology, and metal-organic framework (MOF)-derived carbon as an electrode material has received more and more attention due to its designable structure. However, MOF-derived carbon materials with single-pore structures have been difficult to meet the technical needs of related fields. In this work, the ordered hierarchical porous carbon framework (OMCF) was prepared by the template method using zeolitic imidazolate frameworks-8 (ZIF-8) as a precursor. The pore structures, surface properties, electrochemical properties, and CDI performances of the OMCF were investigated and compared with the microporous carbon framework (MCF), also derived from ZIF-8. The results show that the hierarchical porous carbon OMCF possessed a higher specific surface area, better hydrophilic surface (with a contact angle of 13.45°), and higher specific capacitance and ion diffusion rate than those of the MCF, which made the OMCF exhibit excellent CDI performances. The adsorption capacity and salt adsorption rate of the OMCF in a 500 mg·L-1 NaCl solution at 1.2 V and a 20 mL·min-1 flow rate were 12.17 mg·g-1 and 3.34 mg·g-1·min-1, respectively, higher than those of the MCF. The deionization processes of the OMCF and MCF closely follow the pseudo-first-order kinetics, indicating the double-layer capacitance control. This work serves as a valuable reference for the CDI application of N-doped hierarchical porous carbon derived from MOFs.

Induced-charge membrane capacitive deionization enables high-efficient desalination with polarized porous electrodes

Exposure of a conducting porous material to an electric field in electrolytes induces an electric dipole, which results in capacitive charging of cations and anions at opposite poles. In this letter, we investigate a novel desalination method using this induced-charge capacitive deionization (ICCDI). To do this, we devise a microscale ICCDI platform that can visualize in situ ion concentrations, pH shifts, and fluid flows, and study ion transport dynamics and desalination performances compared to conventional CDI with unipolar / bipolar connections. Similar ion concentration and fluid flow characteristics were observed in Ohmic, limiting, and over-limiting regimes, but variations in desalination performance trends were noted based on the number of stacks. In a single cell, ICCDI generates a higher electric field at the opposite poles of porous electrodes than simple conducted electrodes in CDIs with unipolar/bipolar connections, leading to superior salt removal and/or lower ionic current at a given applied voltage. This marks a clear contrast from CDI with bipolar connection, which lacks any advantage over CDI with unipolar connection in a single cell. These metrics of ICCDI however deteriorated as the stack number increased, likely due to short-circuiting between the dipoles. As a result, ICCDI in current form shows higher desalination efficient than conventional CDIs with low stack numbers (< 6), so we offer the scale-up module by repeating 4-stack ICCDI units. Our study enhances comprehension of ion transport dynamics and desalination performance in ICCDI, and the results could aid in the development of ICCDI for energy/cost-efficient desalination.

Insufficient desorption of ions in constant-current membrane capacitive deionization (MCDI): Problems and solutions

Membrane capacitive deionization (MCDI) is a water desalination technology that involves the removal of charged ions from water under an electric field. While constant-current MCDI coupled with stopped-flow during ion discharge is expected to exhibit high water recovery and good performance stability, previous studies have typically been undertaken using NaCl solutions only with limited investigation of MCDI performance using multi-electrolyte solutions. In the present work, the desalination performance of MCDI was evaluated using feed solutions with different levels of hardness. The increase of hardness resulted in the degradation of desalination performance with the desalination time (Δtd), total removed charge, water recovery (WR) and productivity decreasing by 20.5%, 21.8%, 3.8% and 3.2%, respectively. A more serious degradation of WR and productivity would be caused if Δtd decreases further. Analysis of the voltage profiles and effluent ion concentrations reveal that the insufficient desorption of divalent ions at constant-current discharge to 0 V was the principal reason for the degradation of performance. The Δtd and WR can be improved by discharging the cell using a lower current but the productivity decreased by 15.7% on decreasing the discharging current from 161 to 107 mA. Discharging the cell to a negative potential was shown to be a better option with the Δtd, total removed charge, WR and productivity increasing by 27.4%, 23.9%, 3.6% and 5.3%, respectively, when the cell was discharged to a minimum voltage of - 0.3 V. Use of such a method should be feasible for operation of full scale MCDI plants and would be expected to lead to better regeneration of the electrode, improved desalination performance and, potentially, a significant reduction in the need for use of clean-in-place procedures.

Removal of Trace Uranium from Groundwaters Using Membrane Capacitive Deionization Desalination for Potable Supply in Remote Communities: Bench, Pilot, and Field Scale Investigations

The performance of membrane capacitive deionization (MCDI) desalination was investigated at bench, pilot, and field scales for the removal of uranium from groundwater. It was found that up to 98.9% of the uranium can be removed using MCDI from a groundwater source containing 50 μg/L uranium, with the majority (94.5%) being retained on the anode. Uranium was found to physiochemically adsorb to the electrode without the application of a potential by displacing chloride ions, with 16.6% uranium removal at the bench scale via this non-electrochemical process. This displacement of chloride did not occur during the MCDI adsorption phase with the adsorption of all ions remaining constant during a time series analysis on the pilot unit. For the scenarios tested on the pilot unit, the flowrate of the product water ranged from 0.15 to 0.23 m³/h, electrode energy consumption from 0.28 to 0.51 kW h/m³, and water recovery from 69 to 86%. A portion (13-53% on the pilot unit) of the uranium was found to remain on the electrodes after the brine discharge phase with conventional cleaning techniques unable to release this retained uranium. MCDI was found to be a suitable means to remove uranium from groundwater systems though with the need to manage the accumulation of uranium on the electrodes over time.

Comparison of Pilot-Scale Capacitive Deionization (MCDI) and Low-Pressure Reverse Osmosis (LPRO) for PV-Powered Brackish Water Desalination in Morocco for Irrigation of Argan Trees

The use of saline water resources in agriculture is becoming a common practice in semi-arid and arid regions such as the Mediterranean. In the SmaCuMed project, the desalination of brackish groundwater (TDS = 2.8 g/L) for the irrigation of Argan trees in Essaouira, Morocco, to 2 g/L and 1 g/L (33% and 66% salt removal, respectively) using low-pressure reverse osmosis (LPRO) (p < 6 bar) and membrane capacitive deionization (MCDI) was tested at pilot scale. MCDI showed 40-70% lower specific energy consumption (SEC) and 10-20% higher water recovery; however, the throughput of LPRO (2.9 m³/h) was up to 1.5 times higher than that of MCDI. In addition, both technologies were successfully powered by PV solar energy with total water costs ranging from EUR 0.82 to EUR 1.34 per m³. In addition, the water quality in terms of sodium adsorption ratio was slightly higher with LPRO resulting in higher concentrations of Ca²⁺ and Mg²⁺, due to blending with feed water. In order to evaluate both technologies, additional criteria such as investment and specific water costs, operability and brine disposal have to be considered.

A Comparison of Capacitive Deionization and Membrane Capacitive Deionization Using Novel Fabricated Ion Exchange Membranes

Another technique for desalination, known as membrane capacitive deionization (MCDI), has been investigated as an alternative. This approach has the potential to lower the voltage that is required, in addition to improving the ability to renew the electrodes. In this study, the desalination effectiveness of capacitive deionization (CDI) was compared to that of MCDI, employing newly produced cellulose acetate ion exchange membranes (IEMs), which were utilized for the very first time in MCDI. As expected, the salt adsorption and charge efficiency of MCDI were shown to be higher than those of CDI. Despite this, the unique electrosorption behavior of the former reveals that ion transport via the IEMs is a crucial rate-controlling step in the desalination process. We monitored the concentration of salt in the CDI and MCDI effluent streams, but we also evaluated the pH of the effluent stream in each of these systems and investigated the factors that may have caused these shifts. The significant change in pH that takes place during one adsorption and desorption cycle in CDI (pH range: 2.3-11.6) may cause problems in feed water that already contains components that are prone to scaling. In the case of MCDI, the fall in pH was only slightly more noticeable. Based on these findings, it appears that CDI and MCDI are promising new desalination techniques that has the potential to be more ecologically friendly and efficient than conventional methods of desalination. MCDI has some advantages over CDI in its higher salt removal efficiency, faster regeneration, and longer lifetime, but it is also more expensive and complex. The best choice for a particular application will depend on the specific requirements.

An open-source deep learning model for predicting effluent concentration in capacitive deionization

To effectively evaluate the performance of capacitive deionization (CDI), an electrochemical ion separation technology, it is necessary to accurately estimate the number of ions removed (effluent concentration) according to energy consumption. Herein, we propose and evaluate a deep learning model for predicting the effluent concentration of a CDI process. The developed deep learning model exhibited excellent prediction accuracy for both constant current and constant voltage modes (R² ≥ 0.968), and the accuracy increased with the data size. This model was based on the open-source language, Python, and the code has since been distributed with proper instructions for general use. Owing to the nature of the data-oriented deep learning model, the findings of this study are not only applicable to conventional CDI but also to various types of CDI (membrane CDI, flow CDI, faradaic CDI, etc.). Therefore, by referring to the examples shown in this study, we hope that this open-source deep learning code will be widely used in CDI research.

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.

Electric Potential of Ions in Electrode Micropores Deduced from Calorimetry

The internal energy of capacitive porous carbon electrodes was determined experimentally as a function of applied potential in aqueous salt solutions. Both the electrical work and produced heat were measured. The potential dependence of the internal energy is explained in terms of two contributions, namely the field energy of a dielectric layer of water molecules at the surface and the potential energy of ions in the pores. The average electric potential of the ions is deduced, and its dependence on the type of salt suggests that the hydration strength limits how closely ions can approach the surface.

Knowledge and Technology Used in Capacitive Deionization of Water

The demand for water and energy in today’s developing world is enormous and has become the key to the progress of societies. Many methods have been developed to desalinate water, but energy and environmental constraints have slowed or stopped the growth of many. Capacitive Deionization (CDI) is a very new method that uses porous carbon electrodes with significant potential for low energy desalination. This process is known as deionization by applying a very low voltage of 1.2 volts and removing charged ions and molecules. Using capacitive principles in this method, the absorption phenomenon is facilitated, which is known as capacitive deionization. In the capacitive deionization method, unlike other methods in which water is separated from salt, in this technology, salt, which is a smaller part of this compound, is separated from water and salt solution, which in turn causes less energy consumption. With the advancement of science and the introduction of new porous materials, the use of this method of deionization has increased greatly. Due to the limitations of other methods of desalination, this method has been very popular among researchers and the water desalination industry and needs more scientific research to become more commercial.

Asymmetric and Symmetric Redox Flow Batteries for Energy-Efficient, High-Recovery Water Desalination

Electrochemical separation offers an energy-efficient means to desalinate brackish water, a relatively untapped but increasingly utilized water source for freshwater supply. Several electrochemical techniques are being developed to enable low-energy desalination combined with energy storage. We report a new approach that produced a peak power density of 6.0 mW cm^-2 from the energy stored in iron cyanide (Fe-CN) and iron citrate (Fe-Cit) redox couples during water desalination, using asymmetric redox flow batteries (RFBs). Desalination and the charging of the redox couples occurred in a four-channel RFB cell. The stored energy was extracted in a two-channel RFB cell. Desalination of model brackish water (2.9 g L^-1) to freshwater (0.5 g L^-1) was also studied in a symmetric system using the environmentally benign Fe-Cit. The process was characterized by low energy consumption (0.56 kW h m^-3), high productivity (41.1 L freshwater m^-2 area h^-1, representing practical operating conditions for brackish water desalination), and high water recovery (91% product-to-intake water ratio, addressing the environmental and economic challenges of brine disposal). The low cell voltage (<0.5 V) required in the reported system is ideally suited for developing modular desalination systems powered by renewables, including solar energy. Collectively, water-based RFBs for desalination and power production would lead to sustainable water-energy infrastructure.

Microscopic study of ion transport in the porous electrode of a desalination battery based on the lattice Boltzmann method

Cation Intercalation Desalination (CID).

Electrode for selective bromide removal in membrane capacitive deionisation

Due to the shortage of freshwater around the world, seawater is becoming an important water source. However, seawater contains a high concentration of bromide that can form harmful disinfection by-products during water disinfection. Therefore, the current seawater reverse osmosis (SWRO) has to adopt two-pass reverse osmosis (RO) configuration for effective bromide removal, increasing the overall desalination cost. In this study, a bromide selective composite electrode was developed for membrane capacitive deionisation (MCDI). The composite electrode was developed by coating a mixture of bromide selective resin and anion exchange polymer on the surface of the commercial activated carbon electrode, and its performance was compared to that of conventional carbon electrode. The results demonstrated that the composite electrode has ten times better bromide selectivity than the conventional carbon electrode. The study shows the potential application of MCDI for the selective removal of target ions from water sources and the potential for resource recovery through basic modification of commercial electrode.

Unravelling pH Changes in Electrochemical Desalination with Capacitive Deionization

Membrane capacitive deionization (MCDI) is a water desalination technology employing porous electrodes and ion-exchange membranes. The electrodes are cyclically charged to adsorb ions and discharged to desorb ions. During MCDI operation, a difference in pH between feed and effluent water is observed, changing over time, which can cause the precipitation of hardness ions and consequently affect the long-term stability of electrodes and membranes. These changes can be attributed to different phenomena, which can be divided into two distinct categories: Faradaic and non-Faradaic. In the present work, we show that during long-term operation, as the electrodes age over time, the magnitude and direction of pH changes shift. We studied these changes for two different feed water solutions: a NaCl solution and a tap water solution. Whereas we observe a pH decrease during the regeneration with a NaCl solution, we observe an increase during regeneration with tap water, potentially resulting in the precipitation of hardness ions. We compare our experimental findings with theory and conclude that with aged electrodes, non-Faradaic processes are the prominent cause of pH changes. Furthermore, we find that for desalination with tap water, the adsorption and desorption of HCO₃^-and CO₃^2- ions affect the pH changes.

Novel Inorganic Integrated Membrane Electrodes for Membrane Capacitive Deionization

In capacitive deionization (CDI), coion repulsion and Faradaic reactions during charging reduce the charge efficiency (CE), thus limiting the salt adsorption capacity (SAC) and energy efficiency. To overcome these issues, membrane CDI (MCDI) based on the enhanced permselectivity of the anode and cathode is proposed using the ion-exchange polymer as the independent membrane or coating. To develop a novel and cost-effective MCDI system, we fabricated an integrated membrane electrode using a thin layer of the inorganic ion-exchange material coated on the activated carbon (AC) electrode, which effectively improves the ion selectivity. Montmorillonite (MT, Al₂O₉Si₃) and hydrotalcite (HT, Mg₆Al₂(CO₃)(OH)₁₆·4H₂O) were selected as the main active anion- and cation-exchange materials, respectively, for the cathode and anode. The HT-MT MCDI system employing HT-AC and MT-AC electrodes obtained a CE of 90.5% and an SAC of 15.8 mg g^-1 after 100 consecutive cycles (50 h); these values were considerably higher than those of the traditional CDI system employing pristine AC electrodes (initially, a CE of 55% and an SAC of 10.2 mg g^-1, which attenuated continuously to zero, and even "inverted work" occurs after 50 h, i.e., desorption during charging and adsorption during discharging). The HT-MT MCDI system showed moderate tolerance to organic matters during desalination and retained 84% SAC and 89% CE after 70 cycles in 50-200 mg L^-1 sodium alginate. This study demonstrates a simple and cost-effective method for fabricating high-CE electrodes for desalination with great application potential.

Investigation of bromide removal and bromate minimization of membrane capacitive deionization for drinking water treatment

The ubiquitous bromide (Br^-) poses a challenge to current drinking water treatment schemes due to the formation of brominated disinfection by-products, especially bromate (BrO₃^-). A cost-effective and energy-efficient technology to remove Br^- before disinfection is highly desired. In this work, the application of membrane capacitive deionization (MCDI) for the removal of Br^- and BrO₃^- minimization for drinking water treatment was systematically investigated. Results showed that the removal of Br^- by MCDI followed the pseudo-second-order kinetics, in which kinetics was faster at lower Br^- concentration. Additionally, Br^- displayed a preferential electrosorption over Cl^- in MCDI despite the relatively smaller amounts. Due to high removal performance of Br^-, 99.49% of BrO₃^- minimization can be achieved. Moreover, the presence of humic acid (HA) had a negative effect on the removal of Br^- and BrO₃^- minimization. However, Br^- could be more preferentially removed than Cl^- in the presence of HA due to the weak interaction with HA. Finally, by treating an actual surface water sample, it was found that the removal rate of Br^- was 91.80%, and 83.97% of BrO₃^- minimization can be achieved. BrO₃^- concentration of effluent meets the control standard. Overall, these results prove the feasibility of MCDI for practical drinking water treatment.

Optimization of constant-current operation in membrane capacitive deionization (MCDI) using variable discharging operations

Membrane capacitive deionization (MCDI) is an emerging electric field-driven technology for brackish water desalination involving the removal of charged ions from saline source waters. While the desalination performance of MCDI under different operational modes has been widely investigated, most studies have concentrated on different charging conditions without considering discharging conditions. In this study, we investigate the effects of different discharging conditions on the desalination performance of MCDI electrode. Our study demonstrates that low-current discharge (1.0 mA/cm²) can increase salt removal by 20% and decrease volumetric energy consumption by 40% by improving electrode regeneration and increasing energy recovery, respectively, while high-current discharge (3.0 mA/cm²) can improve productivity by 70% at the expense of electrode regeneration and energy recovery. Whether discharging electrodes at the low current or high current is optimal depends on a trade-off between productivity and energy consumption. We also reveal that stopped flow discharge (85%) can achieve higher water recovery than continuous flow discharge (35-59%). However, stopped flow discharge caused a 20-30% decrease in concentration reduction and a 25-50% increase in molar energy consumption, possibly due to the higher ion concentration in the macropores at the end of discharging step. These results reveal that an optimal discharging operation should be obtained from achieving a balance among productivity, water recovery and energy consumption by varying discharging current and flow rate.

Pharmaceutical removal at low energy consumption using membrane capacitive deionization

The performance of the membrane capacitive deionization (MCDI) system was evaluated during the removal of three selected pharmaceuticals, neutral acetaminophen (APAP), cationic atenolol (ATN), and anionic sulfamethoxazole (SMX), in batch experiments (feed solution: 2 mM NaCl and 0.01 mM of each pharmaceutical). Upon charging, the cationic ATN showed the highest removal rate of 97.65 ± 1.71%, followed by anionic SMX (93.22 ± 1.66%) and neutral APAP (68.08 ± 5.24%) due to the difference in electrostatic charge and hydrophobicity. The performance parameters (salt adsorption capacity, specific capacity, and cycling efficiency) and energy factors (specific energy consumption and recoverable energy) were further evaluated over ten consecutive cycles depending on the pharmaceutical addition. A significant decrease in the specific adsorption capacity (from 24.6 to ∼3 mg-NaCl g^-1) and specific capacity (from 17.6 to ∼2.5 mAh g^-1) were observed mainly due to the shortened charging and discharging time by pharmaceutical adsorption onto the electrode. This shortened charging time also led to an immediate drop in specific energy consumption from 0.41 to 0.04 Wh L^-1. Collectively, these findings suggest that MCDI can efficiently remove pharmaceuticals at a low energy demand; however, its performance changes dramatically as the pharmaceuticals are present in the target water.

Faradic capacitive deionization (FCDI) for desalination and ion removal from wastewater

Capacitive deionization (CDI) is one of the emerging desalination technologies that attracted much attention in the last years as a low-cost, energy-efficient, and environmentally-friendly alternative to other desalination technologies, such as multi-stage flash desalination (MSF) and multiple effect distillation (MED). The implementation of faradaic electrode materials is a promising method for enhancing CDI systems' performance by achieving higher salt removal characteristics, lower energy consumption, and better ion selectivity. Therefore, a novel CDI technology named Faradaic CDI (FCDI) that implements faradaic electrode materials arose as a high-performance CDI cell design. In this work, the application of FCDI cells in desalination and wastewater treatment systems is reviewed. First, the progress done on using various FCDI systems for saline water desalination is summarized and discussed. Next, the application of FCDI in wastewater treatment applications and selective ion removal is presented. A thorough comparison between FCDI and conventional carbon-based CDI is carried out in terms of working principle, electrode material's cost, salt removal performance, energy consumption, advantages, and disadvantages. Finally, future research consideration regarding FCDI technology is included to drive this technology closer towards practical application.

Small-scale membrane-based arsenic removal for decentralized applications-Developing a conceptual approach for future utilization

Various technologies are used for the treatment of arsenic (As) contaminated water, but only a few seem to be suitable for small-scale applications; these are mostly used in rural communities where the access to potable water is the most vulnerable. In this review paper, the salient advantages and most notable challenges of membrane-based technologies for the removal of arsenate As(V) and arsenite As(III) are evaluated and systematically compared to alternative technologies such as e.g. adsorption. A comparison of different scientific papers, case studies and pilot trials is used to discuss the most important aspects when evaluating As mitigation technologies, including the ability to comply with the stringent WHO drinking water guideline limit value of 10 µg/L As and the safe disposal of produced As-laden waste. The use of renewable energies such as solar power in small-scale (<10 m³/day) membrane applications is evaluated. Finally, a conceptual approach for holistic As mitigation is proposed as an important approach to prevent exposure to As by providing a safe water supply.

Faradaic Electrodes Open a New Era for Capacitive Deionization

Capacitive deionization (CDI) is an emerging desalination technology for effective removal of ionic species from aqueous solutions. Compared to conventional CDI, which is based on carbon electrodes and struggles with high salinity streams due to a limited salt removal capacity by ion electrosorption and excessive co-ion expulsion, the emerging Faradaic electrodes provide unique opportunities to upgrade the CDI performance, i.e., achieving much higher salt removal capacities and energy-efficient desalination for high salinity streams, due to the Faradaic reaction for ion capture. This article presents a comprehensive overview on the current developments of Faradaic electrode materials for CDI. Here, the fundamentals of Faradaic electrode-based CDI are first introduced in detail, including novel CDI cell architectures, key CDI performance metrics, ion capture mechanisms, and the design principles of Faradaic electrode materials. Three main categories of Faradaic electrode materials are summarized and discussed regarding their crystal structure, physicochemical characteristics, and desalination performance. In particular, the ion capture mechanisms in Faradaic electrode materials are highlighted to obtain a better understanding of the CDI process. Moreover, novel tailored applications, including selective ion removal and contaminant removal, are specifically introduced. Finally, the remaining challenges and research directions are also outlined to provide guidelines for future research.

Temperature and desorption mode matter in capacitive deionization process for water desalination

Literature reporting temperature and desorption mode as factors of capacitive deionization (CDI) process for water desalination is rare. This study explored the impacts of four water temperatures (15°C, 25°C, 35°C, and 45°C), three salt concentrations (350, 1260, and 3100 mg/L), and three desorption modes (potential removal, short circuit, and polarity reversal) on performance of a 'closed-loop' CDI system. Results showed that a higher temperature promoted adsorption and desorption rates but impaired adsorption capacity. Polarity reversal could greatly expedite the desorption process compared to short circuit and potential removal. A promotional impact of concentration on CDI desalination could be explained by the formation of electrical double layers. The research also noted the earlier occurrence of re-adsorption at higher temperatures during polarity-reversal desorption. Strategies of increasing water temperature on short adsorption cycles and using an adjustable combination mode of polarity reversal and short circuit for desorption are implied for improving desalination efficiency and water recovery of CDI systems.

Exploring the Function of Ion-Exchange Membrane in Membrane Capacitive Deionization via a Fully Coupled Two-Dimensional Process Model

In the arid west, the freshwater supply of many communities is limited, leading to increased interest in tapping brackish water resources. Although reverse osmosis is the most common technology to upgrade saline waters, there is also interest in developing and improving alternative technologies. Here we focus on membrane capacitive deionization (MCDI), which has attracted broad attention as a portable and energy-efficient desalination technology. In this study, a fully coupled two-dimensional MCDI process model capable of capturing transient ion transport and adsorption behaviors was developed to explore the function of the ion-exchange membrane (IEM) and detect MCDI influencing factors via sensitivity analysis. The IEM enhanced desalination by improving the counter-ions’ flux and increased adsorption in electrodes by encouraging retention of ions in electrode macropores. An optimized cycle time was proposed with maximal salt removal efficiency. The usage of the IEM, high applied voltage, and low flow rate were discovered to enhance this maximal salt removal efficiency. IEM properties including water uptake volume fraction, membrane thickness, and fixed charge density had a marginal impact on cycle time and salt removal efficiency within certain limits, while increasing cell length and electrode thickness and decreasing channel thickness and dispersivity significantly improved overall performance.

Treatment of industrial brine using capacitive deionization (CDI) towards zero liquid discharge - challenges and optimization

Thermal-based Zero Liquid Discharge (ZLD) process has been used for managing industrial brine. However, conventional thermal ZLD process is very energy intensive. In view of this, pre-concentration techniques have been applied prior to thermal process to reduce energy consumption of ZLD systems. Capacitive Deionization (CDI) is an emerging desalination technique and has yet to be extensively explored for the treatment of industrial brine especially for ZLD applications. High concentration of total dissolved solids (TDS) and high fouling potential of industrial brine are two major challenges in CDI process. This paper reviews the possible factors for optimizing CDI process in industrial brine treatment, namely, cell architectures, strategies in operation and fouling control. Cell architectures of membrane CDI (MCDI) and flow-electrode CDI (CDI) are preferred options for treating industrial brine compared with classic CDI in terms of energy consumption and fouling propensity. There are other operational strategies that could enhance the feasibility of using CDI process for ZLD application. These include reversed voltage desorption, multi-stage operation, brine recirculation and fouling control. Fouling control methods comprise pretreatment, antifouling modification, antiscalant and chemical cleaning. These methods could be integrated to optimize fouling mitigation. In addition to providing insights on feasibility of using CDI to concentrate industrial brines, this review also proposed guidelines for optimizing CDI process applied to treat industrial brines for ZLD applications.

Basis and Prospects of Combining Electroadsorption Modeling Approaches for Capacitive Deionization

Electrically driven adsorption, electroadsorption, is at the core of technologies for water desalination, energy production, and energy storage using electrolytic capacitors. Modeling can be crucial for understanding and optimizing these devices, and hence different approaches have been taken to develop multiple models, which have been applied to explain capacitive deionization (CDI) device performances for water desalination. Herein, we first discuss the underlying physics of electroadsorption and explain the fundamental similarities between the suggested models. Three CDI models, namely, the more widely used modified Donnan (mD) model, the Randles circuit model, and the recently proposed dynamic Langmuir (DL) model, are compared in terms of modeling approaches. Crucially, the common physical foundation of the models allows them to be improved by incorporating elements and simulation tools from the other models. As a proof of concept, the performance of the Randles circuit is significantly improved by incorporating a modeling element from the mD model and an implementation tool from the DL model (charge-dependent capacitance and system identification, respectively). These principles are accurately validated using data from reports in the literature showing significant prospects in combining modeling elements and tools to properly describe the results obtained in these experiments.

MXene as a Cation-Selective Cathode Material for Asymmetric Capacitive Deionization

Capacitive deionization (CDI) has become a promising method to solve the shortage of freshwater resources recently. However, the co-ion expulsion effect obviously hinders electrosorption capacity and charge efficiency of CDI. In this work, an asymmetric CDI cell is assembled in which Na⁺-intercalated Ti₃C₂T_x (NaOH-Ti₃C₂T_x) serves as a cation-selective cathode, while the activated carbon (AC) serves as the anode. The NaOH-Ti₃C₂T_x with negatively charged surface groups (-OH, -O, and -F) is adopted to weaken the co-ion expulsion effect. Benefited from the synergistic effect of the reduced co-ion expulsion effect and expanded interlayer space, the asymmetric CDI cell achieves a higher electrosorption capacity of 12.19 mg g^-1 and a higher charge efficiency of 0.826 compared with the symmetric one composed of AC (4.55 mg g^-1 and 0.306) in 100 mg L^-1 NaCl solution. High cyclic stability of the as-prepared asymmetric CDI cell is also observed. The improved desalination performance indicates that NaOH-Ti₃C₂T_x is a promising alternative as cation-selective cathode material for asymmetric CDI cells. The desalination mechanism is discussed in detail to lay the foundation for further improvement of the CDI performance of other 2D materials like MXene.

Energy Efficiency of Electro-Driven Brackish Water Desalination: Electrodialysis Significantly Outperforms Membrane Capacitive Deionization

Electro-driven technologies are viewed as a potential alternative to the current state-of-the-art technology, reverse osmosis, for the desalination of brackish waters. Capacitive deionization (CDI), based on the principle of electrosorption, has been intensively researched under the premise of being energy efficient. However, electrodialysis (ED), despite being a more mature electro-driven technology, has yet to be extensively compared to CDI in terms of energetic performance. In this study, we utilize Nernst-Planck based models for continuous flow ED and constant-current membrane capacitive deionization (MCDI) to systematically evaluate the energy consumption of the two processes. By ensuring equivalently sized ED and MCDI systems-in addition to using the same feed salinity, salt removal, water recovery, and productivity across the two technologies-energy consumption is appropriately compared. We find that ED consumes less energy (has higher energy efficiency) than MCDI for all investigated conditions. Notably, our results indicate that the performance gap between ED and MCDI is substantial for typical brackish water desalination conditions (e.g., 3 g L^-1 feed salinity, 0.5 g L^-1 product water, 80% water recovery, and 15 L m^-2 h^-1 productivity), with the energy efficiency of ED often exceeding 30% and being nearly an order of magnitude greater than MCDI. We provide further insights into the inherent limitations of each technology by comparing their respective components of energy consumption, and explain why MCDI is unable to attain the performance of ED, even with ideal and optimized operation.

Electrochemical Desalination Using Intercalating Electrode Materials: A Comparison of Energy Demands

One approach for desalinating brackish water is to use electrode materials that electrochemically remove salt ions from water. Recent studies found that sodium-intercalating electrode materials (i.e., materials that reversibly insert Na⁺ ions into their structures) have higher specific salt storage capacities (mg_salt/g_material) than carbon-based electrode materials over smaller or similar voltage windows. These observations have led to the hypothesis that energy demands of electrochemical desalination systems can be decreased by replacing carbon-based electrodes with intercalating electrodes. To test this hypothesis and directly compare intercalation materials, we examined nine electrode materials thought to be capable of sodium intercalation in an electrochemical flow cell with respect to volumetric energy demands (W·h·L^-1) and thermodynamic efficiencies as a function of productivity (i.e., the rate of water desalination, L·m^-2·h^-1). We also examined how the materials' charge-storage capacities changed over 50 cycles. Intercalation materials desalinated brackish water more efficiently than carbon-based electrodes when we assumed that no energy recovery occurred (i.e., no energy was recovered when the cell produced electrical power during cycling) and exhibited similar efficiencies when we assumed complete energy recovery. Nickel hexacyanoferrate exhibited the lowest energy demand among all of the materials and exhibited the highest stability over 50 cycles.