Composite electrodes for selective bromide recovery using membrane capacitive deionization: Optimizing selectivity and performance

Nitrite desorption from activated carbon fiber during capacitive deionization (CDI) and membrane capacitive deionization (MCDI)

Capacitive deionization (CDI) technologies couple electronic and ionic charge storage, enabling improved thermodynamic efficiency of brackish desalination by recovering energy released during discharge. However, insight into CDI has been limited by discrete experimental observations at low desalination depths (Δ c, typically reducing influent salinity by 10 mM or less). In this study, the performance and sensitivity of three common CDI configurations [standard CDI, membrane CDI (MCDI), and flowable electrode CDI (FCDI)] were evaluated across the operational and material design landscape by varying eight common input parameters (electrode thickness, influent concentration, current density, electrode flow rate, specific capacitance, contact resistance, porosity, and fixed charge). All combinations of designs were evaluated for two influent concentrations with a calibrated and validated one-dimensional (1-D) porous electrode model. Sensitivity analyses were carried out via Monte Carlo and Morris methods, focusing on six performance metrics. Across all performance metrics, high sensitivity was observed to input parameters which impact cycle length (current, resistance, and capacitance). Simulations demonstrated the importance of maintaining both charge and round-trip efficiencies, which limit the performance of CDI and FCDI, respectively. Accounting for energy recovery, only MCDI was capable of operating at thermodynamic efficiencies similar to reverse osmosis.

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.

Capacitive deionization (CDI) is a novel technique to eliminate ions present in the solution. This study was designed to evaluate the impact of cation exchange resin loading on the membrane using the nonsolvent-induced phase inversion method. After optimizing the resin loading on the membrane, the prepared membrane was employed for the membrane capacitive deionization (MCDI). A concentration of 500 mg.L-1 Cu2+ was used as a test contaminant in the CDI system to study Cu2+ removal and recovery at different voltages. Results showed that resin loading substantially influenced the membrane structure and enhanced the Cu2+ removal by the CDI technique. The presence of cation exchange resin on the membrane was found by the appearance of a peak at 1,007 cm-1, 1,035 cm-1, and 1,128 cm-1 in the FTIR spectra of the –SO3 group, which became more significant as the amount of resin increased. The prepared membrane’s water uptake and ion exchange capacity increased as the quantity of resins loaded onto the membrane increased. When monitoring a current-time graph during CDI tests, an adsorption amount of 6.9 mg.g-1 of Cu2+ and recovery efficiency of 51.7 % at the 10th cycle in the MCDI cell was observed for M20 (20% by wt resin). Using the prepared membrane in the CDI system has the potential to remove Cu2+ from the solution selectively.

Novel composite electrodes were developed for application in membrane capacitive deionization (MCDI). Activated carbon (AC) was dispersed in a solution of poly(vinyl alcohol) (PVA) mixed with polyacrylic acid (PAA) or poly dimethyl diallyl ammonium chloride (PDMDAAC), and cast onto the surface of an AC‐based modified graphite electrode, prepared by phase inversion, to form a composite membrane further cross‐linked with glutaraldehyde (GA). The effect of the cross‐linking on the chemical structure of the PVA‐based membranes was determined by attenuated total reflectance Fourier‐transform infrared (ATR‐FTIR) spectroscopy. Cyclic voltammetry was conducted to examine the specific capacitance of the composite electrodes. Desalination experiments were then performed with MCDI unit cells to study the effect of cross‐linking on the desalination efficiency. It was proved that after optimization, the synthesized composite electrodes exhibited one and a half times higher NaCl removal capacity and three times higher adsorption rate as compared to that of a conventional CDI cell using commercial ion exchange membranes (IEMs) with almost the same energy consumption. The enhanced desalination performance was attributed to the optimized properties of the selected polymers and the improved adhesion of IEMs to the electrodes. This research paves the way for the application of new materials in MCDI processes for improved water desalination.

Severe nuclear accidents, including the Fukushima nuclear power plant accident, can lead to the discharge of large amounts of radionuclides into aquatic environments. Some radionuclides such as 137Cs and 90Sr have long half-lives and can remain in the environment for a long period of time. These radionuclides can be detrimental to humans and the environments because they can cause human and environmental exposure to ionizing radiation and because they can be bioaccumulated and biomagnified. Therefore, in case of a severe accident and a radioactive spill, it is important to quickly clean up and treat radioactively contaminated water to protect public health and aquatic ecosystems from unwanted radiation exposure. Ion separation technologies can play a critical role in the cleanup and treatment of radioactive water because most radionuclides in water are present as radioactive ions. Conventional ion separation technologies are typically used to decontaminate radioactive water, but many of these technologies are energy-consuming. Capacitive deionization (CDI) and membrane CDI (MCDI) are alternatives to the conventional technologies. CDI and MCDI are based on electrosorption and are less energy-intensive. However, because they have been designed and investigated for brackish water desalination, CDI and MCDI are not normally considered for the removal of radioactive ions from aqueous solutions. Toward more efficient and effective cleanup and treatment of radioactively contaminated water, this study aims at investigating the potential of CDI and MCDI for the removal of radioactive ions from aqueous solutions. Modeling and experimental investigations are performed along with techno-economic analysis to assess the applicability of CDI and MCDI for radionuclide separation. Carbon electrodes covered with carboxylic functional groups are employed to enhance their removal efficiency. This study contributes toward better protection of the public health and aquatic environments against radioactive contamination in water and the developed CDI and MCDI can be used to separate radioactive ions from aqueous environments. The results are also useful in expanding the applicability of CDI and MCDI to the treatment of various wastewater and high-salinity solutions.

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.

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.

The polymeric conformational effect on capacitive deionization performance of graphene oxide/polypyrrole composite electrode

Electro-driven\ntechnologies are viewed as a potential alternative\nto the current state-of-the-art technology, reverse osmosis, for the desalination of brackish waters.\nCapacitive deionization (CDI), based on the principle of electrosorption,\nhas been intensively researched under the premise of being energy\nefficient. However, electrodialysis (ED), despite being a more mature\nelectro-driven technology, has yet to be extensively compared to CDI\nin terms of energetic performance. In this study, we utilize Nernst–Planck\nbased models for continuous flow ED and constant-current membrane\ncapacitive deionization (MCDI) to systematically evaluate the energy\nconsumption of the two processes. By ensuring equivalently sized ED\nand MCDI systemsin addition to using the same feed salinity,\nsalt removal, water recovery, and productivity across the two technologiesenergy\nconsumption is appropriately compared. We find that ED consumes less\nenergy (has higher energy efficiency) than MCDI for all investigated\nconditions. Notably, our results indicate that the performance gap\nbetween ED and MCDI is substantial for typical brackish water desalination\nconditions (e.g., 3 g L<sup>–1</sup> feed salinity, 0.5 g L<sup>–1</sup> product water, 80% water recovery, and 15 L m<sup>–2</sup> h<sup>–1</sup> productivity), with the energy\nefficiency of ED often exceeding 30% and being nearly an order of\nmagnitude greater than MCDI. We provide further insights into the\ninherent limitations of each technology by comparing their respective\ncomponents of energy consumption, and explain why MCDI is unable to\nattain the performance of ED, even with ideal and optimized operation.

Desalination of brackish water containing oil compound by capacitive deionization process

Microbial desalination cell (MDC) was considered inefficient to desalinate salt water with low salt concentration, therefore, the feasibility of using capacitive deionization (CDI) and membrane capacitive deionization (MCDI) as a post-processing technologies for MDC was investigated in this study, as well as the possibility of using MDC as the power supply for CDI and MCDI. The internal resistances of MDC with different salt concentration, the desalination rate and fresh water yield during a typical desalination cycle under initial salt concentration of 35 g/L were investigated in order to find out the deadline salt concentration for the MDC to desalinate effectively. The internal resistance increased from 21.7 to 602 Ω as the concentration of salt water decreased from 35 g/L to 0.1g/L. The salt water volume increased from 42 to 48 ml when the salt concentration decreased from 35 to 15 g/L, then decreased to 38 ml at the end of one desalination cycle when the salt concentration achieved 0.05 g/L due to the salt gradient (osmotic pressure). The maximum desalination rate during one typical desalination cycle in our experiment reached 5.65 mg/h when salt concentration decreased from 27.26 to 26.32 g/L, while the minimum desalination rate was 0.534 mg/h when salt concentration decreased from 0.38 to 0.05 g/L. It was concluded that MDC was not suitable to desalinate salt water with salt concentration less than 1 g/L. When CDI and MCDI were used as the post-processing technologies for MDC, a better performance in term of electrosorption capacity was obtained from MCDI with an influent salt concentration of 1 g/L. The experimental result also showed that the electrosorption capacity of MCDI with MDC as power supply was more than that with potentiostat as power supply at 0.8V, this suggests that MDC could be an alternative power supply for MCDI.

Comparison of Faradaic reactions in capacitive deionization (CDI) and membrane capacitive deionization (MCDI) water treatment processes

Brackish water resources may be an attractive option for human consumption, agriculture, and industry if efficient water purification can be implemented. In the past few decades, research and development of various desalination technologies have been carried out, among which distillation, reverse osmosis, and electrodialysis are the most commonly known and widespread.1 Capacitive deionization (CDI) is an alternative, emerging, and energy-efficient technology for water desalination, which employs an electrochemical flow cell configured with polarized porous carbon electrodes to remove ionized salts in a stream with low molar concentration. Briefly, by regulating an external voltage to a CDI cell, ionized salts are electrostatically captured (or released) in the pores of the carbon electrodes, resulting in the stream being deionized (or the electrodes being regenerated).2-4 Recent studies have found that the salt adsorption capacity (SAC) could be substantially improved by using surface modified carbon electrodes resulting from nitric acid and ethylenediamine treatments.5 Combined with the modified Donnan model including a term of chemical surface charge, this improved SAC was accounted for by enhancement of the chemical charges immobilized in the carbon micropores, validating both enhanced CDI (e-CDI) and extended-voltage CDI (eV-CDI) effects in the CDI literature (Fig. 1).6In summary, it is considered that, for the carbon electrodes used in a CDI cell, an increase in the chemical surface charges makes the pores more readily available for salt adsorption under proper applied voltages. In addition to the surface modified carbon electrodes, immobilized chemical charges can be found in ion-exchange materials. For instance, a well-known cation-exchange polymer, Nafion, contains the negative chemical charges, -SO3 -, while an anion-exchange polymer typically holds positive chemical charges, e.g., NR4 + and NR3 +. As a consequence, together with the knowledge gained above, ion-exchange polymers coating were used in our current studies to explore new composite carbon electrodes for CDI cycling tests. As shown in an initial test (Fig. 2), the addition of ion-exchange polymers results in the SAC not only being increased but also being stabilized with operational time when NaCl solution was used. In this presentation, the preparation and characterizations of composite carbon electrodes will be detailed including comparisons to conventional CDI and membrane capacitive deionization cells. Furthermore, these composite electrodes will be configured into a CDI cell to investigate both e-CDI and eV-CDI effects in various salt solutions such as CaCl2, Na2SO4, and NH4NO3. In addition, the relevant charge efficiency and cycling longevity will be reported and discussed. Figure 1. Demonstration of both enhanced CDI (e-CDI) and extended voltage CDI (eV-CDI) effects using the modified Donnan model with the addition of chemical surface charge. The parameters used in the model can be found in ref. (5 and 6). Figure 2. Improved salt adsorption capacity and operational stability using cation- and anion-exchange polymers added to the carbon cathode and anode, respectively, in a CDI cell. The CDI cell was operated using 1 V charging and at 0 V discharging in ~31 L of ~7 mM deaerated NaCl solution.

Various cell architectures of capacitive deionization: Recent advances and future trends