Equivalent film-electrode model for flow-electrode capacitive deionization: Experimental validation and performance analysis

Enhanced capacitive deionization for Cr(VI) removal from electroplating wastewater: Efficacy, mechanisms, and high-voltage flow electrodes

Redox flow deionization using Prussian blue and functionalized ion exchange membrane for enhanced selective ion recovery

Flow electrode capacitive deionization (FCDI) is a newly developed water treatment technology that offers better performance than the original, solid electrode capacitive deionization (CDI). By eliminating the need for discharging process, FCDI provides much higher desalination capacity with a continuous desalination operation. Along with applications for salt removal, selective removal and recovery of specific ions are also prominent features of FCDI technology. Characteristics of electrodes and ion exchange membranes play a crucial role in controlling the selectivity of ions in an FCDI system. In this study, we demonstrated selective and continuous ion recovery by combining redox-active Prussian blue as flow electrodes and functionalized ion exchange membranes in FCDI configuration. Prussian blue selectively absorbs monovalent Na+ ions from Na+/Ca2+ mixtures through intercalation reaction, in which a smaller Stokes radius of Na+ ion is more favored to fit in the crystal lattice. Furthermore, such selectivity is much more enhanced through functionalization of a cation-exchange membrane (CEM) with polymer multilayers. A layer-by-layer method was used to deposit poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) on the CEM. Results showed that polymer multilayers changed the CEM from divalent to monovalent ion-affinity due to different ion sizes and charge density. The combination of Prussian blue and functionalized CEM raised the selectivity for sodium around 17 times compared to a control system (0.2 to 3.35). Furthermore, our results showed a highest Li selectivity of 12.88 with an extremely low energy consumption of 0.57 Wh/molLi. We believe our approach can lead to new technologies that address the shortcomings of existing lithium recycling method and expand the application not only to lithium recycling, but also to future lithium production technologies and the removal of specific ions, such as toxin removal.

This study demonstrates the efficacy of a symmetrically designed flow-electrode capacitive deionization (FCDI) system for the electrochemical defluorination of photovoltaic (PV) wastewater, with a systematic investigation conducted to optimize operational parameters and analyze key factors influencing system performance, offering valuable insights into enhancing FCDI efficiency. The results revealed that an optimal applied voltage of 1.2 V yielded a fluoride removal efficiency of 92.9% with an energy consumption of 6.49 kWh/mol. Increasing the electrode content to 0.75 wt% enhanced the removal efficiency to 98.3%; however, further increases in electrode content led to higher energy consumption due to elevated viscosity. Optimizing the flow rate to 45 mL/min resulted in a removal efficiency of 98.6%, accompanied by improved adsorption rates and reduced energy consumption. Adding 1 g/L of electrolyte substantially enhanced system performance, achieving a fluoride removal efficiency of 92.9%. In mixed-ion wastewater, competitive adsorption between NO 3 − and F⁻ was observed. Doubling the NO 3 − concentration relative to F⁻ decreased the F⁻ removal efficiency from 96.2% to 87.3%. Nonetheless, the FCDI system demonstrated robust fluoride removal performance under high ion concentrations and complex matrix conditions, offering an efficient and sustainable approach for industrial wastewater treatment.

A novel two-stage continuous capacitive deionization system with connected flow electrode and freestanding electrode

Influence of particle size distribution on carbon-based flowable electrode viscosity and desalination efficiency in flow electrode capacitive deionization

Towards long-term operation of flow-electrode capacitive deionization (FCDI): Optimization of operating parameters and regeneration of flow-electrode

Enhanced salt removal performance using nickel hexacyanoferrate/carbon nanotubes as flow cathode in asymmetric flow electrode capacitive deionization

Superiority of a novel flow-electrode capacitive deionization (FCDI) based on a battery material at high applied voltage

Pine-derived porous carbon for efficient capacitive deionization and the role of its hierarchical pore structure

Effects of electrode/electrolyte materials on heavy-metal removal in industrial wastewater with flow capacitive deionization method

Many industrial and agricultural applications require the treatment of water streams containing high concentrations of ionic species for closing material cycles. High concentration factors are often desired, but hard to achieve with established thermal or membrane-based water treatment technologies at low energy consumptions. Capacitive deionization processes are normally assumed as relevant for the treatment of low salinity solutions only. Flow-electrode capacitive deionization (FCDI), on the other hand, is an electrically driven water desalination technology, which allows the continuous desalination and concentration of saline water streams even at elevated salinities. Ions are adsorbed electrostatically in pumpable carbon flow electrodes, which enable a range of new process designs.In this article, it is shown that continuously operated FCDI systems can be applied for the treatment of salt brines. Concentrations of up to 291.5 g/L NaCl were reached in the concentrate product stream. Based on this, FCDI is a promising technology for brine treatment and salt recovery. Additionally, a reduction of the energy demand by >70% is demonstrated by introducing multiple cell pairs into a continuous FCDI system. While the economic feasibility is not investigated here, the results show that FCDI systems may compete with established technologies regarding their energy demand.

The increasing shortage of potable water in many part of the world is leading the development of various approaches to water purification, with desalination of sea water the prime target. The most common approach to desalination is reverse osmosis (RO), being used in 65% of currently installed plants[1], but its high costs due to the use of membranes, leading to high-energy requirements, is pushing the desalination industry to find other solutions. An alternative to the high-energy requirements of RO is a method called capacitive deionization (CDI). In this process, two porous electrodes are arranged parallel to one to the other, letting brackish water flow between them. When a voltage is applied, the ions from the feed water migrate to the surface of the electrodes and are adsorbed capacitively, reducing the overall concentration of salts in the water stream. When the voltage is removed, or reversed, the ions desorb from the surface of the electrodes, resulting in a by-product of this process, brine. One of the greatest advantages of this system is that the applied energy can be recovered during the discharge step, just like an electrochemical capacitor, that is used to store energy by means of a pair of electrodes[2]. There are a number of recent reports in the literature describing an innovative CDI technique that focuses on improving the operation of these desalination systems. A suspended electrode material flowing in a path carved on the current collector plays the same role as the porous carbon electrodes fixed on the current collector in a typical CDI process, and this has been termed “flow-electrode capacitive deionization (FCDI)”[3].The flow operates continuously by providing fresh flow electrodes with increased ion capacitance, showing a continuous desalination behaviour and high desalting efficiency that originates from this. In this study, we have tested FCDI systems using graphene and other 2D materials in suspension, as well as optimizing the FCDI system parameters, to try to improve the desalination performance of the system. The goal is to treat feed water with high salinity, close to that of seawater (35 g/L approximately). Studies of FCDI to date have used activated carbon and carbon black suspensions [4,5,6,7,8,9,10]. Graphene and 2D materials are expected to improve the overall desalination capacity of FCDI systems because of their high theoretical surface area and superior electrical conductivity. In the case of traditional CDI setups, graphene was able to increase the salt adsorption capacity 2.3 times[11], compared to previous studies made on activated carbon and carbon black electrodes.

While flow-electrode capacitive deionization (FCDI) operated in short-circuited closed cycle (SCC) mode appears to hold promise for removal of salt from brackish source waters, there has been limited investigation on the removal of other water constituents such as nitrate, fluoride or bromide in combination with salt removal. Of particular concern is the effectiveness of FCDI when ions, such as nitrate, are recognized to non-electrostatically adsorb strongly to activated carbon particles thereby potentially rendering it difficult to regenerate these particles. In this study, SCC FCDI was used to desalt source waters containing nitrate at different concentrations. Results indicate that nitrate can be removed from source waters using FCDI to concentrations <1 mg NO3-N L−1 though a lower quality target such as 10 mg L−1 would be more cost-effective, particularly where the influent nitrate concentration is high (50 mg NO3-N L−1). Although studies of the fate of nitrate in the FCDI system show that physico-chemical adsorption of nitrate to the carbon initially plays a vital role in nitrate removal, the ongoing process of nitrate removal is not significantly affected by this phenomenon with this lack of effect most likely due to the continued formation of electrical double layers enabling capacitive nitrate removal. In contrast to conventional CDI systems, constant voltage mode is shown to be more favorable in maintaining stable effluent quality in SCC FCDI because the decrease in electrical potential that occurs in constant current operation leads to a reduction in the extent of salt removal from the brackish source waters. Through periodic replacement of the electrolyte at a water recovery of 91.4%, we show that the FCDI system can achieve a continuous desalting performance with the effluent NO3-N concentration below 1 mg NO3-N L−1 at low energy consumption (~0.5 kWh m−3) but high productivity.

Efficient capacitive deionization with hierarchical porous carbon flow electrodes