As the global population continues to grow and the demand for clean water increases, the development of efficient, cost-effective, and environmentally friendly water treatment technologies has become increasingly important. Capacitive deionization (CDI) is an emerging desalination technology that has gained significant attention in recent years due to its potential to address these challenges. CDI is an electrochemical process that uses porous carbon electrodes to remove dissolved ions from water, producing purified water suitable for various applications.
The basic principle of CDI involves applying an electrical potential difference across two porous carbon electrodes, which are separated by a spacer and immersed in a solution containing dissolved ions. When a voltage is applied, the electrodes become polarized, attracting oppositely charged ions from the solution and storing them in the electrical double layers (EDLs) formed at the electrode-solution interface. As a result, the concentration of ions in the solution decreases, producing purified water. Once the electrodes become saturated with ions, the voltage is removed or reversed, releasing the ions back into a concentrated brine solution, which can be disposed of or further processed for resource recovery.
CDI offers several advantages over conventional desalination technologies, such as reverse osmosis (RO) and electrodialysis (ED). These advantages include lower energy consumption, the ability to operate at low voltages (typically less than 1.5 V), the absence of high-pressure pumps and membranes, and the potential for selective removal of specific ions. Additionally, CDI systems can be easily scaled up or down, making them suitable for a wide range of applications, from small-scale, point-of-use water treatment to large-scale, industrial desalination plants.
This comprehensive article aims to provide an in-depth overview of capacitive deionization, covering its principles, applications, and future perspectives. The article will discuss the fundamental mechanisms underlying the CDI process, the key components and design considerations of CDI systems, and the various operational modes and configurations that have been developed to enhance the performance and efficiency of CDI. Furthermore, the article will explore the potential applications of CDI in various sectors, including drinking water production, wastewater treatment, and resource recovery, and highlight the challenges and opportunities for the future development and implementation of this promising technology.
Principles of Capacitive Deionization
Electrical Double Layer Formation and Ion Removal
The fundamental principle behind capacitive deionization is the formation of electrical double layers (EDLs) at the interface between the porous carbon electrodes and the electrolyte solution. When a voltage is applied across the electrodes, the electrode surfaces become charged, attracting oppositely charged ions from the solution and forming a layer of counterions in the vicinity of the electrode surface. This layer of counterions, along with the charged electrode surface, constitutes the EDL.
The structure of the EDL can be described by the Gouy-Chapman-Stern (GCS) model, which consists of two regions: the Stern layer and the diffuse layer. The Stern layer is a compact layer of specifically adsorbed ions that are in direct contact with the electrode surface, while the diffuse layer is a more loosely bound region of ions that extends into the bulk solution. The thickness of the EDL depends on several factors, including the applied voltage, the concentration and valence of the ions, and the properties of the electrode material.
As the applied voltage increases, more ions are attracted to the electrode surface, leading to an increase in the charge storage capacity of the EDL. This process continues until the electrodes become saturated with ions, at which point the CDI system reaches its maximum salt removal capacity. The amount of salt removed from the solution can be quantified by measuring the change in the solution's conductivity or by analyzing the concentration of specific ions using analytical techniques such as ion chromatography or inductively coupled plasma mass spectrometry (ICP-MS).
The ion removal process in CDI is governed by several transport mechanisms, including diffusion, migration, and convection. Diffusion is driven by concentration gradients and plays a dominant role in the transport of ions from the bulk solution to the electrode surface. Migration, on the other hand, is driven by the applied electric field and is responsible for the selective removal of ions based on their charge and mobility. Convection, which can be induced by flow or stirring, helps to enhance the mass transfer of ions and prevent the formation of concentration gradients near the electrode surface.
Electrode Materials and Properties
The performance of CDI systems largely depends on the properties of the electrode materials used. Ideal electrode materials for CDI should have a high specific surface area, good electrical conductivity, high charge storage capacity, and good stability and durability under operating conditions. Porous carbon materials, such as activated carbon, carbon aerogels, and carbon nanotubes, have emerged as the most promising electrode materials for CDI due to their unique combination of these properties.
Activated carbon is the most widely used electrode material in CDI systems due to its high specific surface area (up to 3000 m²/g), low cost, and good availability. Activated carbon electrodes can be produced from various precursors, such as coconut shells, wood, and coal, using physical or chemical activation methods. The pore size distribution and surface chemistry of activated carbon electrodes can be tailored to optimize their performance for specific CDI applications.
Carbon aerogels are another class of porous carbon materials that have been extensively studied for CDI applications. Carbon aerogels are produced by the sol-gel polymerization of resorcinol and formaldehyde, followed by pyrolysis and activation. The resulting materials have a highly porous, three-dimensional network structure with a high specific surface area (up to 1000 m²/g) and good electrical conductivity. Carbon aerogels can be produced in various forms, such as monoliths, powders, and films, and their properties can be tuned by adjusting the synthesis conditions.
Carbon nanotubes (CNTs) are one-dimensional carbon nanomaterials that have recently gained attention as potential electrode materials for CDI. CNTs have a high specific surface area (up to 1000 m²/g), excellent electrical conductivity, and good mechanical stability. They can be produced by various methods, such as arc discharge, laser ablation, and chemical vapor deposition, and can be functionalized with various surface groups to enhance their adsorption capacity and selectivity for specific ions.
In addition to these conventional carbon materials, novel electrode materials such as graphene, metal-organic frameworks (MOFs), and conductive polymers have also been explored for CDI applications. These materials offer unique properties and functionalities that can potentially enhance the performance and versatility of CDI systems. For example, graphene-based electrodes have been shown to exhibit high salt removal capacities and fast ion transport kinetics due to their two-dimensional structure and high electrical conductivity. MOFs, on the other hand, can provide high surface areas and tunable pore sizes and chemistries, enabling the selective removal of specific ions or contaminants.
System Design and Operational Modes
Capacitive deionization systems typically consist of several key components, including porous carbon electrodes, a spacer material, current collectors, and a power supply. The electrodes are usually arranged in a parallel plate configuration, with the spacer material separating the electrodes to prevent direct contact and allow for the flow of the electrolyte solution. The current collectors, which are typically made of a conductive material such as graphite or metal foil, are used to supply the electrical current to the electrodes. The power supply is used to apply the voltage across the electrodes and control the charging and discharging processes.
CDI systems can be operated in various modes, depending on the specific application and desired outcome. The most common operational modes are:
- Batch mode: In batch mode, a fixed volume of the electrolyte solution is treated in a single cycle. The solution is fed into the CDI cell, and the voltage is applied until the electrodes become saturated with ions. The purified water is then collected, and the electrodes are regenerated by removing the voltage or reversing the polarity. Batch mode is simple and easy to implement but may have limited throughput and efficiency.
- Continuous mode: In continuous mode, the electrolyte solution is continuously fed into the CDI cell, and the purified water is continuously collected. The electrodes are periodically regenerated by removing the voltage or reversing the polarity, and the concentrated brine solution is collected separately. Continuous mode can provide higher throughput and efficiency than batch mode but requires more complex system design and control.
- Flow-through mode: In flow-through mode, the electrolyte solution is passed through the porous electrodes, rather than being stored in a spacer between the electrodes. This configuration can enhance the mass transfer of ions and reduce the resistance to flow, leading to improved salt removal efficiency and faster kinetics. However, the flow-through mode may require more complex electrode fabrication and assembly processes.
- Membrane capacitive deionization (MCDI): MCDI is a variant of CDI that incorporates ion-exchange membranes between the electrodes to enhance the selectivity and efficiency of ion removal. The membranes allow the passage of counterions while blocking the transport of co-ions, reducing the co-ion expulsion effect and improving the charge efficiency of the system. MCDI has been shown to achieve higher salt removal capacities and lower energy consumption than conventional CDI but may have higher material and manufacturing costs.
The choice of operational mode and system design depends on several factors, including the feed water composition, the desired product water quality, the available energy and infrastructure, and the economic and environmental considerations. Optimizing the CDI system design and operation for specific applications requires a thorough understanding of the underlying principles and trade-offs involved, as well as extensive experimentation and modeling efforts.
Applications of Capacitive Deionization
Drinking Water Production
One of the most promising applications of capacitive deionization is in the production of drinking water from brackish water sources. Brackish water, which has a salinity between that of freshwater and seawater (typically 1-10 g/L of total dissolved solids), is a widely available but underutilized resource that could potentially provide a sustainable and affordable source of drinking water for many communities around the world. However, the high energy consumption and cost of conventional desalination technologies, such as reverse osmosis, have limited the widespread adoption of brackish water desalination.
CDI has emerged as a potential alternative to conventional desalination technologies for brackish water treatment, offering several advantages such as lower energy consumption, simple and compact system design, and the ability to operate at low pressures and voltages. Several studies have demonstrated the feasibility and effectiveness of CDI for brackish water desalination, achieving salt removal efficiencies of up to 90% and producing water with a salinity of less than 500 mg/L, which is suitable for drinking and other potable uses.
One of the key challenges in using CDI for drinking water production is the presence of other contaminants, such as organic compounds, heavy metals, and pathogens, which may require additional treatment steps to ensure the safety and quality of the product water. To address this challenge, researchers have developed various strategies, such as the use of modified electrode materials, the incorporation of adsorptive or reactive layers, and the integration of CDI with other treatment processes, such as filtration, disinfection, and advanced oxidation.
For example, Liu et al. (2016) developed a novel CDI system with activated carbon electrodes modified with silver nanoparticles, which demonstrated enhanced removal of bacteria and viruses from brackish water, in addition to salt removal. The silver nanoparticles, which were deposited on the electrode surface using a simple chemical reduction method, provided a biocidal effect that inactivated the pathogens and prevented their proliferation in the system. The modified CDI system achieved a salt removal efficiency of 85% and a bacterial removal efficiency of 99.9%, producing water that met the drinking water standards for both salinity and microbial safety.
In another study, Gu et al. (2019) developed a hybrid CDI-filtration system that incorporated a porous carbon electrode coated with a zirconium-based metal-organic framework (MOF) layer, which provided selective adsorption of heavy metal ions, such as lead and copper, from brackish water. The MOF layer, which was grown on the electrode surface using a hydrothermal synthesis method, had a high specific surface area and a strong affinity for heavy metal ions, enabling their effective removal from the water. The hybrid system achieved a salt removal efficiency of 80% and a heavy metal removal efficiency of over 95%, producing water that met the drinking water standards for both salinity and heavy metal content.
These examples demonstrate the potential of CDI as a versatile and effective technology for the production of safe and affordable drinking water from brackish water sources, particularly in remote and resource-limited settings where conventional desalination technologies may not be feasible or sustainable. However, further research and development are needed to optimize the performance and scalability of CDI systems for drinking water production and to validate their long-term reliability and cost-effectiveness under real-world conditions.
Wastewater Treatment and Resource Recovery
Another important application of capacitive deionization is in the treatment of industrial and municipal wastewater streams, which often contain high levels of salts, heavy metals, and other dissolved contaminants that can pose environmental and health risks if discharged without proper treatment. Conventional wastewater treatment technologies, such as chemical precipitation, ion exchange, and membrane separation, can be effective in removing these contaminants but often generate large volumes of waste sludge or brine that require further treatment and disposal, leading to high operational costs and environmental impacts.
CDI has been investigated as a potential alternative or complementary technology for wastewater treatment, offering several advantages such as high removal efficiency, low energy consumption, and the ability to recover valuable resources from the wastewater. By selectively removing and concentrating the dissolved ions from the wastewater, CDI can produce a purified water stream that can be reused or safely discharged, while also generating a concentrated brine stream that can be further processed to recover valuable materials, such as metals, salts, and nutrients.
Several studies have demonstrated the feasibility and effectiveness of CDI for the treatment of various types of wastewater, including industrial wastewater from the textile, mining, and electroplating industries, as well as municipal wastewater from sewage treatment plants. For example, Huang et al. (2018) developed a CDI system with activated carbon electrodes modified with a polyaniline (PANI) conductive polymer layer, which demonstrated enhanced removal of heavy metal ions, such as copper and zinc, from electroplating wastewater. The PANI layer, which was deposited on the electrode surface using an electrochemical polymerization method, provided additional adsorption sites and redox activity that improved the selectivity and capacity of the electrodes for heavy metal removal. The modified CDI system achieved a heavy metal removal efficiency of over 90% and a salt removal efficiency of 80%, producing water that met the discharge standards for electroplating wastewater.
In another study, Wang et al. (2019) developed a CDI system with graphene-based electrodes for the treatment of textile wastewater, which contained high levels of salts, dyes, and organic compounds. The graphene electrodes, which were prepared by a simple hydrothermal method, had a high specific surface area and excellent electrical conductivity, enabling fast and efficient ion removal from the wastewater. The CDI system achieved a salt removal efficiency of 85% and a dye removal efficiency of over 95%, producing water that met the discharge standards for textile wastewater. Furthermore, the concentrated brine stream generated by the CDI system was found to contain high levels of salts and dyes that could be potentially recovered and reused in the textile production process, reducing the overall environmental impact and cost of the wastewater treatment.
CDI has also been explored for the recovery of valuable resources from municipal wastewater, such as nutrients (e.g., nitrogen and phosphorus), which are essential for agricultural production but can cause eutrophication and other environmental problems if discharged in excess. Tong et al. (2020) developed a CDI system with activated carbon electrodes modified with a magnesium-based layered double hydroxide (LDH) layer, which demonstrated selective removal and recovery of phosphate ions from municipal wastewater. The LDH layer, which was deposited on the electrode surface using a co-precipitation method, had a high affinity and capacity for phosphate adsorption, enabling its effective removal and concentration from the wastewater. The CDI system achieved a phosphate removal efficiency of over 90% and a salt removal efficiency of 80%, producing a concentrated phosphate solution that could be potentially used as a fertilizer or raw material for the phosphate industry.
These examples highlight the potential of CDI as a sustainable and cost-effective technology for wastewater treatment and resource recovery, providing a closed-loop solution that can minimize the environmental impact and maximize the value of wastewater. However, the application of CDI in wastewater treatment is still in the early stages of development, and further research is needed to address the challenges related to the complexity and variability of wastewater composition, the long-term stability and regenerability of the electrodes, and the integration of CDI with other treatment processes and resource recovery technologies.
Challenges and Future Perspectives
Electrode Fouling and Regeneration
One of the main challenges in the application of CDI for water treatment is the fouling of the porous carbon electrodes, which can occur due to the accumulation of inorganic, organic, and biological foulants on the electrode surface and pores. Electrode fouling can lead to a decrease in the salt removal capacity and efficiency of the CDI system, as well as an increase in energy consumption and maintenance requirements. The type and extent of electrode fouling depend on several factors, such as the composition and concentration of the feed water, the operating conditions (e.g., voltage, flow rate, and cycle time), and the electrode material and morphology.
Inorganic fouling is caused by the precipitation and scaling of sparingly soluble salts, such as calcium carbonate and calcium sulfate, on the electrode surface, which can block the pores and reduce the active surface area of the electrodes. Organic fouling is caused by the adsorption and deposition of organic compounds, such as humic acids and proteins, on the electrode surface, which can form a resistive layer that hinders ion transport and charge storage. Biological fouling is caused by the growth and attachment of microorganisms, such as bacteria and algae, on the electrode surface, which can form a biofilm that clogs the pores and consumes the organic and inorganic nutrients in the water.
To mitigate electrode fouling, various strategies have been proposed and investigated, including:
- Pretreatment of the feed water: Removing or reducing the foulants in the feed water before it enters the CDI system can help prevent or delay the onset of electrode fouling. Pretreatment methods such as filtration, coagulation, and oxidation can be used to remove suspended solids, organic matter, and microorganisms from the water.
- Modification of the electrode surface: Modifying the surface chemistry and morphology of the porous carbon electrodes can help reduce their affinity and susceptibility to foulants. For example, incorporating hydrophilic or charged functional groups on the electrode surface can help repel organic and biological foulants, while creating a smooth and homogeneous surface can help prevent the nucleation and growth of inorganic scales.
- Optimization of the operating conditions: Adjusting the operating conditions of the CDI system, such as the voltage, flow rate, and cycle time, can help minimize the accumulation and impact of foulants on the electrodes. For example, using a lower voltage and shorter cycle time can help reduce the electrostatic attraction and deposition of foulants, while using a higher flow rate can help shear off the loosely bound foulants from the electrode surface.
- Periodic cleaning and regeneration of the electrodes: Cleaning and regenerating the electrodes periodically can help restore their salt removal capacity and efficiency, and extend their lifespan. Various cleaning methods, such as hydraulic backwashing, chemical cleaning, and electrochemical regeneration, have been explored for the in-situ or ex-situ removal of foulants from the electrodes. For example, Gao et al. (2015) developed an electrochemical regeneration method that uses a low-voltage, polarity-reversal current to desorb and oxidize the organic foulants from the electrodes, achieving a regeneration efficiency of over 90% and a salt removal capacity recovery of over 80%.
Despite these efforts, electrode fouling remains a significant challenge for the long-term operation and scalability of CDI systems, particularly for the treatment of complex and variable water sources. Further research is needed to develop more effective and sustainable strategies for the prevention, monitoring, and control of electrode fouling, as well as to improve the fundamental understanding of the fouling mechanisms and their impact on CDI performance.
Energy Efficiency and Cost Considerations
Another important challenge for the widespread adoption of CDI technology is the need to improve its energy efficiency and cost-effectiveness, particularly in comparison to other established water treatment technologies, such as reverse osmosis and electrodialysis. Although CDI has been shown to have lower energy consumption than these technologies for the treatment of low-salinity water (< 3 g/L), its energy efficiency and cost competitiveness for higher-salinity water (> 3 g/L) are still limited by several factors, such as the low salt removal capacity and charge efficiency of the porous carbon electrodes, the high resistive losses in the electrodes and electrolyte, and the incomplete utilization of the electrical double layer capacity.
To enhance the energy efficiency and cost-effectiveness of CDI, various strategies have been proposed and investigated, including:
- Development of advanced electrode materials: Developing novel electrode materials with higher salt removal capacity, charge efficiency, and electrical conductivity can help reduce the energy consumption and cost of the CDI process. For example, using graphene-based electrodes with a high specific surface area and electrical conductivity has been shown to achieve a salt removal capacity of up to 15 mg/g and a charge efficiency of up to 95%, which are significantly higher than those of conventional activated carbon electrodes.
- Optimization of the system design and operation: Optimizing the design and operation of the CDI system, such as the electrode configuration, flow pattern, and regeneration mode, can help improve its energy efficiency and cost-effectiveness. For example, using a flow-through electrode configuration with a short flow path and a high flow rate has been shown to reduce the pumping energy and increase the salt removal rate, while using a closed-loop regeneration mode with energy recovery has been shown to reduce energy consumption and waste generation.
- Integration with renewable energy sources: Integrating CDI with renewable energy sources, such as solar, wind, and geothermal energy, can help reduce its reliance on fossil fuels and lower its operating costs. For example, using a solar-powered CDI system with battery storage has been shown to achieve a specific energy consumption of less than 1 kWh/m³ and a water production cost of less than $1/m³, which are competitive with those of reverse osmosis for the treatment of brackish water in remote and off-grid areas.
- Hybridization with other water treatment technologies: Hybridizing CDI with other water treatment technologies, such as membrane filtration, ion exchange, and advanced oxidation, can help improve its overall performance and cost-effectiveness, by leveraging the strengths and overcoming the limitations of each technology. For example, using a CDI-reverse osmosis hybrid system has been shown to achieve a higher water recovery and lower energy consumption than either technology alone, by using CDI to pretreat the feed water and reduce the osmotic pressure for reverse osmosis.
Despite these promising developments, the energy efficiency and cost-effectiveness of CDI are still limited by several technical and economic barriers, such as the low technology readiness level, the lack of industry standards and regulations, and the high capital and maintenance costs. Further research, demonstration, and commercialization efforts are needed to advance the CDI technology and make it more competitive and sustainable for the treatment of various water sources and applications.
Environmental Impact and Life Cycle Assessment
Another important aspect to consider for the development and deployment of CDI technology is its environmental impact and sustainability, particularly from a life cycle perspective. Although CDI has been shown to have several environmental advantages over other water treatment technologies, such as lower energy consumption, lower chemical usage, and lower waste generation, its overall environmental impact and sustainability are still not well understood and quantified, due to the lack of comprehensive and consistent life cycle assessment (LCA) studies.
LCA is a systematic and standardized method for evaluating the environmental impact of a product or process throughout its entire life cycle, from raw material extraction and processing to manufacturing, use, and end-of-life disposal. LCA can provide valuable insights into the environmental hotspots, trade-offs, and improvement opportunities of CDI technology, and help inform its sustainable design, operation, and management.
Several LCA studies have been conducted on CDI technology, focusing on different aspects and applications, such as:
Energy and carbon footprint
Assessing the energy consumption and greenhouse gas emissions of CDI in comparison to other water treatment technologies, such as reverse osmosis and electrodialysis, for the treatment of brackish water and seawater. For example, a study by Kim et al. (2019) showed that CDI has a lower energy consumption and carbon footprint than reverse osmosis for the treatment of brackish water with a salinity of less than 3 g/L, but a higher energy consumption and carbon footprint for the treatment of seawater with a salinity of more than 30 g/L, due to the limitations of the electrode capacity and stability.
Resource and waste management
Assessing the resource consumption and waste generation of CDI in comparison to other water treatment technologies, such as ion exchange and chemical precipitation, for the treatment of industrial wastewater and brine. For example, a study by Choi et al. (2018) showed that CDI has a lower resource consumption and waste generation than ion exchange for the treatment of textile wastewater, due to the higher regeneration efficiency and lower chemical usage of the electrodes, but a higher resource consumption and waste generation than chemical precipitation for the treatment of brine, due to the lower salt removal capacity and higher electrode replacement rate.
Electrode material and manufacturing
Assessing the environmental impact and sustainability of different electrode materials and manufacturing processes for CDI, such as activated carbon, graphene, and carbon nanotubes, and their production methods, such as steam activation, chemical vapor deposition, and arc discharge. For example, a study by Liu et al. (2021) showed that activated carbon electrodes have a lower environmental impact and higher sustainability than graphene and carbon nanotube electrodes for CDI, due to their lower energy and resource consumption, and higher renewability and recyclability, but a lower performance and durability, due to their lower specific surface area and electrical conductivity.
Despite these studies, the LCA of CDI technology is still in its early stages, and more research is needed to address the challenges and limitations, such as the lack of standardized and comprehensive LCA methodology and data, the variability and uncertainty of the CDI performance and operating conditions, and the complexity and diversity of the CDI applications and contexts.
Further LCA studies are needed to provide a more holistic and reliable assessment of the environmental impact and sustainability of CDI technology and to guide its sustainable development and implementation for various water treatment needs and scenarios.
Conclusion
Capacitive deionization (CDI) is an emerging and promising technology for the sustainable and cost-effective treatment of water and wastewater, offering several advantages over conventional water treatment technologies, such as lower energy consumption, lower chemical usage, and lower waste generation. CDI uses the principles of electrosorption and electrical double-layer formation to remove and recover dissolved ions from water, using porous carbon electrodes and low-voltage electrical current.
This comprehensive article provided an in-depth overview of the principles, applications, and future perspectives of CDI technology, covering the fundamental mechanisms of ion removal and storage, the key factors and challenges affecting the CDI performance and efficiency, the potential applications and benefits of CDI for drinking water production, wastewater treatment, and resource recovery, and the research and development needs and opportunities for advancing and optimizing the CDI technology.
The article highlighted the importance of the electrode material and properties, the system design and operation, and the feed water composition and quality, for the effective and efficient performance of CDI, and reviewed the various strategies and approaches for enhancing the salt removal capacity, charge efficiency, and fouling resistance of the electrodes, such as surface modification, hybridization, and regeneration.
The article also discussed the energy efficiency, cost-effectiveness, and environmental impact of CDI, in comparison to other water treatment technologies, and emphasized the need for more comprehensive and consistent life cycle assessment studies, to evaluate and improve the sustainability and competitiveness of CDI for various water treatment applications and scenarios.
Despite the significant progress and potential of CDI technology, several challenges and limitations still need to be addressed, such as the low salt removal capacity and charge efficiency of the electrodes, the high fouling and scaling propensity of the feed water, the incomplete understanding and control of the ion transport and adsorption mechanisms, and the lack of standardized and optimized system design and operation.
To overcome these challenges and limitations, further research and development efforts are needed, in areas such as:
- Novel electrode materials and architectures, with higher specific surface area, electrical conductivity, chemical stability, and lower cost and environmental impact, such as graphene, carbon nanotubes, and metal-organic frameworks.
- Advanced characterization and modeling techniques, to elucidate the fundamental mechanisms and kinetics of ion transport and adsorption, and to predict and optimize the CDI performance and efficiency, such as in-situ spectroscopy, electrochemical impedance spectroscopy, and molecular dynamics simulation.
- Innovative system designs and operation modes, to enhance the salt removal capacity, charge efficiency, and fouling resistance of CDI, and to reduce energy consumption and waste generation, such as flow-through electrodes, membrane capacitive deionization, and closed-loop regeneration.
- Integrated and sustainable CDI systems, to combine and synergize the benefits of CDI with other water treatment and resource recovery technologies, and to improve the overall performance, cost-effectiveness, and environmental impact of water treatment, such as CDI-reverse osmosis, CDI-electrodialysis, and CDI-membrane distillation.
- Comprehensive and consistent life cycle assessment and techno-economic analysis, to evaluate and compare the environmental impact, energy efficiency, and cost-effectiveness of CDI with other water treatment technologies, and to identify the key drivers and barriers for the sustainable and competitive development and deployment of CDI.
In conclusion, capacitive deionization is a promising and versatile technology for the sustainable and cost-effective treatment of water and wastewater, with significant potential for addressing the global challenges of water scarcity, quality, and security. With further research and development, optimization and integration, and demonstration and commercialization, CDI can become a key enabler and accelerator for the transition towards a more sustainable, resilient, and circular water economy, and contribute to the achievement of the United Nations Sustainable Development Goals, particularly SDG 6 on clean water and sanitation for all.