Advances in ef ﬁ cient desalination technology of capacitive deionization for water recycling

Available freshwater resources are becoming harder to obtain due to climate change, population growth, industrial development, and water pollution. The main technologies in the ﬁ eld of wastewater desalination include reverse osmosis, electrodialysis, thermal distillation, and adsorption etc . Capacitive deionization technology (CDI) belongs to a novel electrochemical desalination technology with low energy consumption and low environmental impact, simple equipment structure and convenient operation. With the importance of wastewater desalination highlighted, some great technological progress of CDI has been made in electrode materials, reactor structure and the hybrid process. In this paper, the development of CDI technology was expounded from three aspects to achieve the goal of strong adaptability, low cost and strong adsorption capacity by analysis of the latest research papers. Corresponding improved methods of CDI are summarized to solve the main technology bottlenecks such as the inef ﬁ cient and vulnerable electrode materials, low selectivity and unreasonable unit structure, and limitations of single CDI unit for promoting the continuous development of CDI technology. The main technology bottlenecks of were analyzed from three aspects. Review the of CDI based on electrode material, novel process and hybrid process. New electrode materials are introduced and analyzed for the future development of CDI. The energy consumption of novel FCDI-NF hybrid process is signi ﬁ cantly reduced.


INTRODUCTION
Freshwater resources are essential for the sustainable development of human society. Rapid population growth, urbanization, and climate change are putting tremendous stress on freshwater resources (Fan et al. ); approximately one-fifth and one-third of the world's population live in areas of water scarcity and in countries with moderate to high water stress, respectively, according to the report of UN Water (Chen et al. ). In the past decades, the usage and treatment of water in China has grown tremendously due to the large population and rapid economic development (Xu et al. ). Water recycling can be an effective option for saving water resources, reducing the environmental impacts from the discharge of treated wastewater, and reducing the cost and energy involved in water resource management (Lyu et al. ; Takeuchi & Tanaka ). The development of novel water regeneration technology is of great significance to alleviate the crisis of freshwater resources.
Traditional nonthermal distillation methodologies, such as reverse osmosis (RO), adsorption/ion exchange, and membrane desalination (MD) (Burakov et al. ; Folaranmi et al. ; Shi et al. ), have wide applications in desalination and water purification, while membrane fouling, adsorbent regeneration and high energy consumption are always problems to be solved (Naushad et al. ). Capacitive deionization (CDI) is a promising new technology for water desalination and ions removal compared to other traditional desalination methodologies, due to its low energy consumption (i.e. room temperatures, low pressures and low voltages), low capital and processing costs, easy operation and maintenance processes, and environmentally friendly characteristics (i.e. without chemicals or hazardous substances) (Khalil et al. ). CDI has emerged as a promising alternative desalination technology of reverse osmosis, electrodialysis and thermal distillation (Zhang et al. ; Tang et al. ). The electrosorption/desorption processes of CDI for deionization is based on electric double layer theory. Ions in the feed water will be attracted on the electrode with opposite charge by an external electrical potential (very low voltage, 1.2 V), and desorbed into the concentrate from the electrode by a reversing potential for a continuous desalination process (Faisal et al. ).
The technical principle of CDI is shown in Figure 1.
Ions are removed by transferring from aqueous solution to the electrode surface, the charge balance is achieved on the electric double layer, and the effluent is purified (Figure 1(a)). The ions or charged particles adsorbed on the surface of the electrode material are released into solution to realize the electrode regeneration through reverse connection or short-circuit voltage (Figure 1(b)). During the charging and discharging process of the CDI electrodes, the absorption and desorption of ions are completed, respectively. Therefore, the electrode materials with huge surface, ultra-high capacitance and high desalination efficiency are essential to CDI technology for water resources recycling.
Porous carbon electrodes materials are widely used in electrostatic adsorption of ions in the CDI process. Significant efforts have been made in recent decades in the development of customized porous carbon materials for CDI including carbon nanotubes, graphene, MXene, aerogels, xerogels, mesoporous carbon, and activated carbons.
Although posing unique benefits, so far these materials have generally failed to provide a significant competitive advantage to traditional adsorption and RO in either capital or operating cost due to low salt adsorption capacity and carbon electrode oxidation (Landon et al. ). Nowadays, how to promote the desalination efficiency of CDI device is still a difficult problem for many researchers to develop materials with ultra-high capacitance or new processes with more efficient desalination performance (Zhao et al.

).
The desalting performance of CDI also depends on the operating parameters including external voltage, inflow velocity, thickness of interlayer between two electrodes and design of flow channel, and quality, species and concentration of solution ions etc. (Chen et al. ). Commonly used CDI desalination performance evaluation indicators mainly include electroadsorption capacity, charge efficiency, average desalination rate, and long-term stable desalting belonging to reactor structure optimization (Yasin et al. ; Maheshwari et al. ; Wang et al. ).
In this paper, the three main technology bottlenecks have been analyzed for the desalination performance of CDI device such as the inefficient and vulnerable electrode materials with low selectivity, unreasonable unit structure and limitations of single CDI unit ( Figure 2). Furthermore, the corresponding improved methods of CDI were

Novel 3D graphene electrode
In order to achieve optimal desalination during CDI, CDI electrodes should possess high electrical conductivity, large surface area, good wettability to water, narrow pore size distribution and efficient pathways for ion and  constructing interconnected graphene sheets with in-plane nanopores (NP-3DG). As compared to 3DG, NP-3DG features a larger specific surface area and therefore higher specific capacitance ( Figure 4). The NP-3DG electrode can be used as a promising electrode material for highperformance CDI applications. The results of NP-3DG exhibit ultrahigh CDI performance with a superior electrosorption capacity of 22.09 mg/g, high adsorption rate, good deionization cyclic stability and high salt removal percentage. Furthermore, due to its excellent electrosorption performance, it is expected that NP-3DG will also play an important role in the removal of charged hazardous species and other heavy metal ions (Khan et al.

).
As a new type of 3D carbon nanomaterial, NP-3DG has incomparable advantages for use in CDI electrodes.
Because it is still in the initial stage of development, there are some problems such as high cost, imperfect theoretical system and unsatisfactory treatment effect (Zhang et al. ). In recent years, although some progress has been made in the design and preparation of graphene CDI electrode materials, further research on the construction of graphene pore structure and the comprehensive performance of CDI graphene electrode materials is needed to   promote the large-scale practical development and application of graphene electrode materials in CDI.
High efficiency particle free porous silicon network electrode An energy saving water desalination process using low-cost materials is a key step for the sustainable demand of clean water resources in the future. Here, porous silicon has been proven to be a kind of material with rich content, low cost and strong biocompatibility ( According to Figure 6, the porous silicon network electrode described in this section has a specific salt removal rate of 3.8-5.2 mg/g, ranging from fresh water to seawater salt concentration, and only 1.45 Wh/L of energy input is needed to convert seawater into drinking water (Yu et al. ; Zhao et al. a, b).
In summary, compared with the traditional carbonbased electrode, the main advantage of a porous silicon network is that the silicon content is huge, resulting in lower cost, and it has a tunable pore morphology and structure that can optimize CDI performance in different flow architectures and mitigate the effect of fouling. Therefore, a porous silicon network can be used in developing countries on a large scale, thus becoming a low-cost and easy to operate basic seawater purification equipment.

NEW CDI PROCESS
Although CDI has made great breakthroughs in electrode materials, architecture and system integration, it has no advantages over traditional desalination technologies in terms of energy efficiency, water recovery rate, desalination concentration range and capital cost. However, CDI has illustrated great potential for selective removal of target ions because of its tailorable electrode materials and ability to couple with selective ion exchange membranes (Zhang et al. ; Park et al. ). In this section, two novel CDI processes were introduced for their higher ability of selective ions removal, and the application feasibility was explored.
Novel CDI using a pulsed power supply In recent years, capacitive deionization technology (CDI) has been widely used in many fields due to its environmental  In all, compared with the traditional direct current-CDI, novel pulsed-CDI has better ion removal rate and lower energy consumption, which proves that pulsed CDI can be an excellent alternative to traditional DC-CDI. However, due to the high cost and complex operation of a pulsed electric field, and the diversity of capacitive deionization electrode materials, repeated testing is needed to find the best voltage condition. Therefore, in the future, researchers should focus on enhancing the compatibility of the pulsed electric field so that it can better adapt to various CDI electrode materials, so as to realize large-scale industrial application.

Selective ion separation by CDI based technologies
Selective ion extraction from aqueous solution is of great significance for water purification as well as resource recovery (Kavitha et al. ). However, despite the great   ). The processing route of FCDI-ED is shown in Figure 9.
However, the disadvantages of FCDI-ED technology are complex process, high cost and easy failure, so it is difficult to carry out large-scale industrial application at this stage.
Therefore, researchers should focus on improving the stability of the process and using renewable materials, so as to make the FCDI-ED coupling process better developed. The schematic of nanofiltration and membrane capacitive deionization (NF-MCDI) hybrid system configuration is shown in Figure 10. The location and sequence of NF unit and CDI unit should be comprehensively considered by the influent quality, requirements of effluent and energy consumption.
After study, researchers found that the standalone NF system was insufficient for salty water desalination due to its comparatively low removal rate of <60% (NaCl). However, it was improved through combination with the MCDI unit to adsorb the remaining salts in the NF-treated water, thereby achieving a total salt removal rate of up to 95% ( Jeon et al. ). Therefore, compared with the single NF or FCDI process, the desalination rate of FCDI-NF is significantly improved, and the energy consumption of the process is also significantly reduced.
However, this hybrid system may require a higher capital cost than the typical RO membrane system because of the price of the electrodes and ion-exchange membranes (Marchetti et al. ). Also, the NF-MCDI system performance is influenced by the feed concentration, MCDI flow rate and the NF recovery rate. Therefore, the complexity of the system will result in a significant increase in the capital cost and maintenance complexity of the specific equipment used to couple the MCDI system with the NF system. In future work, how to overcome the limitations of industrial applications brought about by the high cost and high complexity of FCDI-NF devices is very important.

CONCLUSION AND PROSPECTS
In less than 20 years CDI technology has developed into the most attractive research field and it is expected that in the system not only has strong salt adsorption capacity to treat high salinity water, but also can continuously produce fresh water and regenerate electrodes by using non-charged carbon-based materials to continuously supplement the electroactive region. However, FCDI is still in the initial development stage, and there are many possibilities for its future development. One of the main current challenges for FCDI is the low efficiency of charge transfer between collector and flow electrode, and much work needs to be done to solve this problem.
In conclusion, despite the challenges, CDI is still a promising water treatment technology and exciting research field. With the continuous solution to these challenges and the emergence of new CDI units, CDI may be regarded as the key to solve the global water shortage and water pollution in the future.