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Due to the increasing worldwide water scarcity associated with the climate change there is the need to increase the injection of wastewater into the overall water cycle. As a consequence wastewater treatment is within the largest energy use sector in many developed countries. In recent years, capacitive deionization (CDI), which is based on the principle of electrosorption of ions on charged high surface area electrodes, the same as charging and discharging an electrochemical double-layer capacitor, has been reported to be a promising technology which is an alternative to other classical water treatment methods such as reverse osmosis (RO) and evaporation processes. We see that this technology is gaining increased scientific interest since 2006. However, not too many publications indicate the feasibility of the kWh/m3consumption in CDI systems for brackish water treatment (500–30,000 ppm). The common assumption proposes that the main problem may be the ions adsorption capability of the electrode material during discharging. However, in energy efficiency terms, desorption processes may be even more difficult since experience diffusional complications mainly when high currents are used. This presentation will provide an overview of current strategies for operating these systems aiming to improve energy recovery and lead this process to the point of making these systems an option for use in water treatment plants. [ABSTRACT FROM PUBLISHER]

• Theoretical capacitive ion-exchange mechanism is validated in a CDI pilot plant. • Mechanism for modifying mono and divalent cation concentrations was explained. • Solution with lower conductivity and SAR values were consistently produced. • R Mg/Na > 5–6 and water production rate of 0.5–0.8 L m−2 h−1 were achieved. • Robustness of the mechanism was confirmed in long-term cycling. Soil salinization poses a significant challenge to agricultural activities. To address this, the agricultural industry seeks an irrigation water solution that reduces both ionic conductivity and sodium adsorption rate (SAR), thereby diminishing the risks of soil sodification and fostering sustainable crop production. Capacitive deionization (CDI) is an attractive electrochemical technology to advance this search. Recently, a one-dimensional transient CDI model unveiled a capacitive ion-exchange mechanism presenting the potential to adjust the treated water composition by modifying monovalent and divalent cation concentrations, thereby influencing the SAR index. This behavior would be achieved by using electrodes rich in surface functional groups able to efficiently capture divalent cations during conditioning and releasing them during charging while capturing monovalent ions. Beyond the theoretical modelling, the current experimental research demonstrates, for the first time, the effectiveness of the capacitive ion-exchange mechanism in a CDI pilot plant using real water samples spiked with solutions containing specific mono and divalent ions. Electrosorption experiments and computational modeling, specifically Density-Functional Theory (DFT), were used along with the analysis of the surface functional groups present in the electrodes to describe the capacitive ion-exchange phenomenon and validate the steps involved on it, highlighting the conditioning as a critical step. Various operational and flow modes confirm the versatility of CDI technology, achieving separation factors (R Mg/Na) of 5–6 in batch, raising production from 0.5 to 0.8 L m−2 h−1 (batch) to 8.0–8.1 L m−2 h−1 when using single pass although reducing R Mg/Na to 2. The reliability of the CDI technology in reducing SAR was also successfully tested with different influent compositions, including magnesium and calcium. Finally, the robustness of the capacitive ion-exchange mechanism was validated by a second CDI laboratory 9-cell stack cycled over 350 cycles. Our results confirm the reported theoretical model and expands the conclusions through the experiments in a pilot plant showing direct implications for employing CDI in agricultural applications. [Display omitted] [ABSTRACT FROM AUTHOR]

While Electrical Double-Layer Capacitors (EDLC's) have been proven to achieve round-trip efficiencies of over 95%, electrical efficiencies of Capacitive Deionization (CDI) systems are still far from those values. Since this issue seems to be crucial for CDI to commercially compete with other water treatment processes including Reverse Osmosis (RO) it is essential that these efficiencies be improved. In this work, the electrical response of a commercial EDLC's and a CDI system are compared. Results of asymmetric constant current experiments reveal that the main electrical inefficiency occurs during the discharge stage (i.e. in CDI this refers as salt desorption or electrode's regeneration). Here, we propose an operational mode of CDI focused on improving performance of regeneration by simply conducting this stage in a more concentrated solution (3.5M NaC) brine water. The experimental findings suggest that this operational strategy led to 1) an increase from ≈40% to 75% in the round-trip efficiency of CDI, approaching (although admittedly not reaching) those of EDLC's; 2) a drop in ohmic resistance in the discharge cycle of 92% (from 19.77 Ω to 1.45 Ω) consequently increasing the amount of energy that could be potentially recovered from 0.24 J to 0.50 J, and thus, reducing significantly the energy consumption of the process; 3) a wider range of deionization/regeneration current density ratios (0.1–1.25) in which high efficiency values (65–75%) are still maintained. In general, we find that this operational mode might have practical relevance as it implies that CDI systems may be operated with a higher degree of flexibility allowing them to be coupled more readily with renewable energy technologies. Furthermore, the use of a highly concentrated electrolyte (i.e. not deionized water) during regeneration offers an innovative solution for brine concentration. Image 1 • The electrical performance of a CDI flow-cell and an EDLC were compared. • Low round-trip efficiency of CDI cell was mainly associated to discharge step. • New operational mode based on electrolyte replacement was successfully tested. • Round-trip efficiency of CDI can be boosted up to 75%. • Wider range of charge/discharge current density ratios can be used. [ABSTRACT FROM AUTHOR]

Faradaic deionization (FDI) is an emerging water treatment technology based on electrodes able to remove ionic species from water by charge transfer reactions. It is a young and promising technology that has attracted much attention due to its large capacity to store ions and the high selectivity of the faradaic electrode materials. This study reviews published papers on FDI from different angles: data mining, bibliometric and machine learning. Metrics such as annual growth rate, most important journals, relevant authors, collaborations maps, sentiment and subjectivity analysis, similarity and clustering analysis were performed. The results indicated that the strong interest in FDI really started in 2016, China is the most active country in FDI, and Desalination is the most important journal publishing FDI articles. The word cloud method showed that the most preferred adopted words are deionization, capacitive, electrode, material. Sentiment analysis results indicated that most of the researchers are optimistic about FDI technology. The title similarity method revealed that FDI researchers were successful in proposing unique and appropriate titles. The clustering approach stressed that FDI literature is concentrated on electrode material production, desalination application, lithium recovery and comparison with CDI. [Display omitted] • Faradaic Deionization (FDI) dataset includes 170 papers downloaded from Scopus. • Annual growth rate of FDI research based on published articles is 55.12 %. • Desalination , J. Mat. Chem. and ACS Appl. Mater. Inter. are the core sources of FDI. • China and the USA are the most active countries on FDI research. • Deionization , capacitive and desalination are frequent words in FDI articles titles. [ABSTRACT FROM AUTHOR]

In order for capacitive deionization (CDI) as a water treatment technology to achieve commercial success, substantial improvements in the operational aspects of the system should be improved in order to efficiently recover the energy stored during the deionization step. In the present work, to increase the energy efficiency of the adsorption-desorption processes, we propose a new operational procedure that utilizes a concentrated brine stream as a washing solution during regeneration. Using this approach, we demonstrate that by replacing the electrolyte during regeneration for a solution with higher conductivity, it is possible to substantially increase round-trip energy efficiency. This procedure was experimentally verified in a flow cell reactor using a pair of carbon electrodes (10² cm geometric area) and NaCl solutions having concentrations between 50 and 350 mmol·L-1. According to experimental data, this new operational mode allows for a better utilization of the three-dimensional structure of the porous material. This increases the energetic efficiency of the global CDI process to above 80% when deionization/regeneration currents ratio are optimized for brackish water treatment. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is a rapidly emerging desalination technology that promises to deliver clean water while storing energy in the electrical double layer (EDL) near a charged surface in a capacitive format. Whereas most research in this subject area has been devoted to using CDI for removing salts, little attention has been paid to the energy storage aspect of the technology. However, it is energy storage that would allow this technology to compete with other desalination processes if this energy could be stored and reused efficiently. This requires that the operational aspects of CDI be optimized with respect to energy used both during the removal of ions as well as during the regeneration cycle. This translates into the fact that currents applied during deionization (charging the EDL) will be different from those used in regeneration (discharge). This paper provides a mechanistic analysis of CDI in terms of energy consumption and energy efficiencies during the charging and discharging of the system under several scenarios. In a previous study, we proposed an operational buffer mode in which an effective separation of deionization and regeneration steps would allow one to better define the energy balance of this CDI process. This paper reports on using this concept, for optimizing energy efficiency, as well as to improve upon the electro-adsorption of ions and system lifetime. Results obtained indicate that real-world operational modes of running CDI systems promote the development of new and unexpected behavior not previously found, mainly associated with the inhomogeneous distribution of ions across the structure of the electrodes. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI), a promising ion removal technology, has shown encouraging results in the last decade. In this work a comprehensive characterization of a CDI prototype equipped with 3D graphite felt electrodes is shown. Initially, a study of different flow distributor designs, analyzed by Computational Flow Dynamic (CFD), determined the importance of elements such as small inlet cavities or perforated channels to achieve a uniform fluid flow distribution. Then, an evaluation under different operational parameters and modes (batch and single pass, SP) demonstrated the strong flexibility in terms of performance that the CDI system is able to offer. Thus, in general terms, SP experiments were able to produce larger quantities of water (14–47 L h−1 m−2) than batch experiments (2–24 L h−1 m−2) although obtaining lower salt concentration reductions (1–2 mM vs 6–10 mM, respectively). The CDI stack was tested for 350 galvanostatic cycles (700 h of operation) showing an exceptional robustness in performance. Energy consumption values below 200 Wh m−3 were obtained for SP experiments, whereas that value increased to 600 Wh m−3 for batch tests. Nevertheless, by implementing an energy recovery system these values could be reduced by an average of 40–50%. Unlabelled Image • A CDI stack equipped with 3D GF electrodes was tested. • SP experiments produced 14–47 L h−1 m−2 with concentration reductions of 1–2 mM and energy consumption below 100 Wh m−3. • Batch experiments produced 2–24 L h−1 m−2 with a Δ c of 6–10 mM, and below 400 Wh m−3. • Proper selection of current density and electrolyte flow are critical to optimize performance. • The CDI system was run for 350 galvanostatic cycles (700 h of operation). [ABSTRACT FROM AUTHOR]