Flow-electrode Capacitive Deionization System Research Articles

Efficient removal of uranium and sulfate in acid contaminated groundwater by flow electrode capacitive deionization

In-situ leaching (ISL) causes non-negligible groundwater pollution. It is urgent to remediate the groundwater after ISL activities. In this study, we evaluated the effectiveness of flow electrode capacitive deionization (FCDI) to treat a simulated groundwater, the uranium (U) and SO42− concentration of which are comparable to groundwater in acid in-situ leaching (AISL) uranium mine for the first time. Moreover, the removal mechanism of U and SO42− were investigated in-depth. It is found that the operational mode, applied voltage and initial SO42− concentration significantly affect the removal of U and SO42− by FCDI. The removal efficiency of U and SO42− were above 98 % at 75 min under optimal condition, although U in groundwater mainly existed in the form of uncharged UO2(SO4), followed by UO22+ and UO2(SO4)22−. UO22+ and UO2(SO4)22− in groundwater migrated into the two poles and were quickly absorbed by flow electrode, which promoted the dissociation of UO2(SO4) or complexation of UO2(SO4) with SO42−. In addition, the anion exchange membrane can absorb UO2(SO4) through complexation. These resulted in the efficient removal of U(VI). FCDI can reduce the U and SO42− concentration of the contaminated water (CU = 10 mg L−1, CSO42− = 5 g L−1) to a value lower than the Chinese emission limit (U: 300 μg L−1; SO42−: 250 mg L−1) even after 18 cycles with each cycle operated for 120 min, which informed that FCDI system using activated carbon is of great potential for acidic contaminated water treatment.

Simultaneous removal and recovery of ammonium and phosphate ions using flow electrode capacitive deionization through the struvite generation mechanism

Removing and recovering nitrogen (N) and phosphorus (P) from water is essential to prevent eutrophication and support sustainable development. Flow electrode capacitive deionization (FCDI) is a promising electrosorption technology often used to remove and recover charged ions from wastewater. This study presents an FCDI system using Mg(OH)2/BC as a flow electrode in a short-circuit closed-cycle (SCC) mode, enhancing the simultaneous removal and recovery of NH4+ and phosphate ions through struvite formation. Results show improved removal of NH4+ and phosphate ions at an optimal Mg(OH)2 amount (M−BC−0.336) and an initial influent pH of 8. The NH4+ and phosphate ions adsorbed into the flow electrode solution can react with Mg2+ ions released from the electrode material, resulting in the formation of struvite nanowires on the surface of the electrode material. This process reduces the back-diffusion effect of NH4+ and phosphate ions, thereby enhancing the performance of FCDI in removing N and P. In actual domestic sewage treatment, the system achieves high removal efficiency for N (64.6 %) and P (87.3 %) while consuming low energy (0.46 kWh/kg for NH4+-N and 4.1 kWh/kg for PO43−-P). Our study demonstrates an efficient method for removing and recovering N and P from wastewater while producing struvite-rich biochar.

Sustainable approach for selective lithium recovery: Capacitive deionization integrated with novel LMO flow-electrode

Reusing materials from discarded batteries offers an environmentally friendly approach to lithium ion(Li+) recovery. Therefore, in this study, flow-electrode capacitive deionization (FCDI) was employed using lithium manganese oxide (LMO) from spent batteries to selectively recover Li+ from leachate. The Li+ selectivity of LMO enabled effective recovery from waste solutions containing various ionic substances. Delithiation of the LMO to produce λ-MnO2 enhanced its Li+ adsorption capacity. Successful synthesis was confirmed through field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) analyses, while BET analysis validated the suitability of the process for Li+ intercalation and deintercalation. XPS analysis confirmed the presence of Co2+, Ni2+, and Li+ in the electrode material at each stage, verifying the successful adsorption and recovery. CV and electrochemical impedance spectroscopy (EIS) analyses showed lower charge transfer resistance and higher ionic conductivity for the λ-MnO2 electrode as compared to activated carbon (AC), indicating its superior electrochemical performance. The λ-MnO2-based FCDI system outperformed the AC-based system; its removal rate was 3.08 times higher, specific energy consumption (SEC) was 2.6 times lower, and average salt adsorption rate (ASAR) was 5.1 times greater. Adsorption and recovery experiments indicated higher selectivity for Li+ ions as compared to Co2+ and Ni2+ ions, further highlighting the superior performance of the λ-MnO2 based system. In conclusion, the λ-MnO2 electrode, recycled from spent lithium-ion batteries (LIBs), is not only an excellent material for high-efficiency and selective lithium recovery via the FCDI process, but also holds the potential to revolutionize sustainable lithium recovery technologies. This study is a significant step towards addressing the climate crisis, promoting environmental protection, and conserving our valuable resources.

Enhanced desalination performance of flow capacitive deionization with the addition of conductive polymer in redox couples and activated carbon

Freshwater scarcity is a critical global issue and desalination of brackish water and seawater technologies are regarded as effective solution to mitigate the increasing severity of water shortage. Flow-electrode capacitive deionization (FCDI) is an emerging electrochemical desalination technology capable of continuous deionization behavior. However, reducing energy consumption and enhancing desalination rate are now markedly needed for its advancement. Herein, we propose a FCDI system with energy consumption as low as 88.08 kJ mol−1 and a desalting rate of 1.75 μg cm−2 s−1. This is achieved by using flow electrodes containing 0.03125 wt% conductive polymer, 5 wt% activated carbon/carbon black and 80 mM/80 mM ferricyanide/ferrocyanide at a current density 3 mA cm−2 (3.36 mA current for a 1.12 cm2 active area). We further investigate the effects of polymer content, redox pair content, salt content and current densities on desalination performance. Seawater with a conductivity of 52.78 mS cm−1 was successfully desalinated to 0.50 mS cm−1 in continuous process. This research provides a promising approach to enhance FCDI system by achieving a low energy consumption and high salt removal rate, representing a significant advancement in continuous electrochemical desalination technology.

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

The expansion of industrial facilities poses a significant environmental risk due to the generation of extensive wastewater containing complex components that effluent human health. Flow-electrode Capacitive Deionization (FCDI) technology has emerged as a promising solution for wastewater treatment due to its unlimited adsorption capacity and continuous long-term operations compared to conventional systems. This study investigates the efficiency of the FCDI system in purifying industrial wastewater by extracting complex heavy metal ions, intending to achieve complete recycling and zero discharge. Different electrolyte/electrode materials were investigated to achieve high removal efficiency, including electrolytes such as sodium chloride (NaCl) and sodium sulfate (Na2SO4), and electrodes such as active carbon (AC) and C65 carbon black (C65 CB) used in lithium-ion batteries. We found that the innovative combination of Na2SO4 electrolyte with 0.1 g of C65 CB exhibited an exceptional salt removal efficiency of 90 %, surpassing NaCl electrolyte which showed 68 % efficiency. Additionally, we observed higher adsorption with C65 CB slurry compared to AC when using Na2SO4 electrolyte. To deepen our insights, Molecular Dynamics (MD) simulations were conducted to explore heavy metal ions interactions with electrode/electrolyte and membrane materials under an electric field. The results indicated that Ni2+ ions had the highest passage rate of 80 % across the membrane, followed by Cr2+, Cu2+, and Zn2+. Furthermore, superior adsorption of Ni2+ ions onto the C65 CB electrode was noted, contributing to enhanced charge transfer and overall conductivity. Furthermore, we evaluated the economic and environmental implications associated with implementing the FCDI system in practical. This study presents a novel approach by using C65 carbon black (CB) with Na2SO4 as the electrolyte, indicating high adsorption efficiency at low carbon loading electrode. This combination offers a cost-effective and sustainable solution for improving complex heavy metal ion removal in FCDI systems. Overall, our study’s findings significantly contribute to offering crucial insights into optimizing FCDI technology for efficient heavy metal ions removal in industrial wastewater treatment using lithium-ion battery anode/cathode electrode material.

Enhancing selective NH4+ recovery from wastewater using modified zeolite-based flow electrode capacitive deionization

Despite flow-electrode capacitive deionization (FCDI) is an emerging technology for desalination, contaminant removal, and resource recovery, the application of conventional FCDI in wastewater treatment is hindered by the electrode selectivity and material costs. In this study, we synthesized a low-cost ammonium (NH4+) adsorption electrode material by modifying zeolite using ethylenediaminetetraacetic acid disodium salt (EDTA-2Na). The flow electrode prepared by the mixture of EDTA-zeolite and carbon black exhibits a high selectivity and adsorption capacity for the recovery of NH4+ from wastewater. The NH4+ in wastewater passes through the ion exchange membrane and is rapidly adsorbed by the modified zeolite through ion exchange, while Na+ is retained in the electrolyte. The decrease in NH4+ concentration and the increase in Na+ concentration in the catholyte lead to a significant change in ion concentration gradient across the membrane. Consequently, the transmembrane selectivity between NH4+ and Na+ reached 3.46. We validated the feasibility of NH4+ recovery using FCDI with food waste fermentation supernatant. Under optimal operating conditions, 99.15 % of the NH4+ in the fermentation supernatant was removed, and 95.92 % of the NH4+ in the electrolyte was stored in the EDTA-zeolite. By gravitational settling, the NH4+-rich modified zeolite was separated from carbon black and could be utilized as nitrogen fertilizer. Meanwhile, the mixture of carbon black and brine was used to prepare a fresh electrode suspension. In brief, the FCDI system exhibits a satisfying NH4+ recovery performance and demonstrates a sustainable wastewater resource recovery strategy.

Efficient capacitive deionization with hierarchical porous carbon flow electrodes

Flow-electrode capacitive deionization (FCDI) represents an innovative, chemically-free, and environmentally-friendly approach for desalination. However, the long-standing challenge of constrained electronic and ionic transport within its flow-electrode has impacted overall desalination performance. In this study, we explore novel concepts and advancements in capacitive deionization by utilizing hierarchical porous carbon (HPC) flow electrodes. By employing widely available sodium carboxymethyl cellulose as a carbon precursor and NaCl as an activator, we have successfully synthesized a carbon-based particle electrode featuring a hierarchical porous structure. The HPC electrode was further optimized through precise adjustments to its pore structure. The optimized electrode exhibited excellent dispersion, viscosity, and remarkable specific capacitance of 283.96 F g−1. Under optimal conditions of a 10% mass loading and an applied voltage of 1.2 V, the electrode demonstrated an average salt removal rate of 82.11 μg cm−2 min−1, a charge efficiency of 101.44%, and an energy consumption of 9.32 × 10−3 kWh mol−1. Furthermore, we investigated the optimal operating parameters for the FCDI device equipped with the HPC flow electrode. Impressively, the HPC flow electrode achieved a salt removal efficiency exceeding 80% over five cycles, maintaining an average salt removal rate of 54.00 μg cm−2 min−1 during 12 h of continuous operation. Detailed textural analysis and electrochemical characterizations revealed that the superior desalination performance is attributed to the hierarchical porous structure of the electrode. This unique structure significantly enhances both electronic and ionic transport within the FCDI system. Our study underscores the critical role of hierarchical porous carbon flow electrodes in capacitive deionization, paving the way for novel strategies to optimize ion and electron transport.

Ion-Selective Metathesis Design of Flow-Electrode Capacitive Deionization for Energy-Saving and Anti-Scaling Softening of Brackish Water.

While flow-electrode capacitive deionization (FCDI) is recognized as an attractive desalination technology, its practical implementation has been hindered by the ease of scaling and energy-intensive nature of the single-cell FCDI system, particularly when treating brackish water with elevated levels of naturally coexisting SO42- and Ca2+. To overcome these obstacles, we propose and design an innovative ion-selective metathesis FCDI (ISM-FCDI) system, consisting of a two-stage tailored cell design. Results indicate that the specific energy consumption per unit volume of water for the ISM-FCDI is lower (by up to ∼50%) than that of a conventional single-stage FCDI due to the parallel circuit structure of the ISM-FCDI. Additionally, the ISM-FCDI benefits from a conspicuous disparity in the selective removal of ions at each stage. The separate storage of Ca2+ and SO42- by the metathesis process in the ISM-FCDI (46.25% Ca2+, 14.25% SO42- in electrode 1 and 4.75% Ca2+, 35.25% SO42- in electrode 2) can effectively prevent scaling. Furthermore, configuration-performance analysis on the ion-selective migration suggests that the properties of the ion exchange membrane, rather than the carbon species, govern the selectivity of ion removal. This work introduces system-level enhancements aimed at enhancing energy conservation and scaling prevention, providing critical optimization of the FCDI for brackish water softening.

Novel flow-electrode capacitive deionization system employing modified MOF-derived carbon electrodes for metal ion removal

Activated carbon (AC) as an electrode for capacitive deionization (CDI) is constrained by its low specific surface area and specific capacitance, leading to electrical double layer overlap (EDL). To address this issue, a flow electrode capacitive deionization system (MDC-FCDI) was developed, which used modified MOFs-derived carbon (MDC) as the electrode. The MDC electrode exhibited enhanced specific capacitance (145.78 F/g) and specific surface area (2458.44 m2/g). Compared to conventional FCDI, the MDC-FCDI system demonstrated a 35–39 % improvement in heavy metal removal efficiency from wastewater containing Pb2+, Cu2+, Cd2+, and Zn2+. The selectivity of MDC-FCDI for heavy metals exhibited a unique “two-step phenomenon” that differed from what was observed in conventional CDI systems. In the MDC-FCDI system, high-valent ions experienced stronger electric field forces and preferentially occupied active sites on the electrode surface prior to the adsorption of ions with lower valence. The selectivity of the MDC-FCDI system for heavy metal ions is determined by three factors in the following order of priority: ionic charge, hydration radius, and ion feed concentration. This study offers a practical approach for expanding the range of MDC-FCDI applications.

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

In recent years, researchers have increasingly focused on flow electrode capacitive deionization (FCDI) for heavy metal removal. However, the practical application of FCDI has been limited because of its low removal efficiency at low voltages. To address this limitation, we investigated Cr(VI) removal using FCDI in high-voltage mode (≥1.23 V). We aimed to clarify the mechanism and capabilities of high-voltage FCDI, including the removal energy efficiency, ion valence changes, migration process, and practical applications. Our findings demonstrate that even at high voltages (≤1.6 V), FCDI maintains efficient Cr(VI) removal and charge efficiency. This success was attributed to the significant IR drop in the FCDI system. Additionally, Cr (VI) removal by high-voltage FCDI results from a synergistic effect combining electroporation and electroreduction. The higher the applied voltage, the stronger was the electroporation capacity and the reduction ratio. Under the voltage condition of 2.2 V, the maximum electroporation capacity reached 172.82 mg g−1. However, excessively high-voltage can also lead to a more pronounced side reaction, resulting in a slow improvement in the removal efficiency. We further optimized the process parameters of the FCDI using response surface methodology (RSM). The optimized conditions yielded impressive results, a 99.96 % removal efficiency and low energy consumption (0.104 kWh/m3) in actual Cr(VI)-containing wastewater. Additionally, even in the 19th cycle of continuous operation, Cr(VI) removal by FCDI exceeded 90 %. This study unveils the efficacy and mechanism of Cr(VI) removal by high-pressure FCDI, which is expected to advance industrial applications of FCDI for treating heavy metal-containing wastewater.

Optimization of the intermediate chamber improves desalination performance in flow electrode capacitive deionization (FCDI): A comparative study

Flow electrode capacitive deionization (FCDI), a novel ion removal technology, is influenced by various factors such as electrode material properties, fluid flow rate, and device structure. In the FCDI system, the intermediate chamber is the core element where ions migrate towards the cathode and anode to complete the adsorption process. In this study, we designed a hollow plastic plate with channels (PP) and a hollow graphite plate with channels (GP) as the intermediate chamber, compare and study with conventional plastic mesh (PM) chambers. CFD simulations revealed that the newly designed PP and GP chambers significantly reduced the hydraulic stagnation zone. The results of the desalination experiments revealed that the desalination efficiency of the GP-FCDI was 1.5 and 1.9 times higher than that of the PP-FCDI and PM-FCDI, respectively. At the same time, in situ potential measurement (ISPM) was used to monitor the voltage drop of each component in FCDI, thereby accurately calculated the proportion of energy consumption in the intermediate chamber. The results indicate that the energy consumption of the intermediate chamber of GP-FCDI was 54 % less than that of PM-FCDI. The current work will help us provide new insights for the design and optimization of FCDI systems.

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

The desalination of seawater/wastewater utilizing flow-electrode capacitive deionization (FCDI) has received significant interest. However, challenges like the low electrical conductivity of flow electrode and excessive energy consumption have prevented the scaling up of desalination operations. To overcome these issues, this study introduces a new FCDI system termed the two-stage FCDI (TS-FCDI) system, in which desalination modules are equipped with freestanding and flow electrode. The TS-FCDI system offers a graded desalination module design that produces an average salt removal rate (ASRR) of 113 µg cm2·min−1. However, the TS-FCDI system requires substantially reduced infrastructure investment, device size, and energy expenses. Notably, the design allows for the simultaneous operation of freestanding and flow electrode and enables the transport of ions and charges in the electrode region. Experimental evidence from multiple device configurations (CDI, FCDI, and TS-FCDI) supports this claim. Notably, the TS-FCDI system shows about twice the desalination performance over FCDI, with approximately 50 h of single-cycle (SC) operation. This enhancement links the freestanding electrode with the flow electrode, enabling the device to be well-suited for practical applications. The continuous flow of electrode particles in contact with the freestanding electrode initiates secondary ion transport, merging two different types of electrodes to enhance the active space of the electrode, effectively improving ion adsorption performance. In summary, the results of this study indicate the potential of the TS-FCDI system as a suitable desalination technology for scaling up applications.

Flow electrode capacitive desalination of industrial RO reject.

Flow electrode capacitive deionization (FCDI) is a promising technology for efficiently treating industrial brine with high salt content. However, its desalination performance is currently limited by internal resistance. Achieving an effective FCDI system relies on active electrode materials with high conductivity. This study compares the desalination performances of the widely used flow electrode activated carbon (AC) with more conductive materials, reduced graphene oxide (rGO), and ZnO/rGO composite. Additionally, the lack of particle-to-particle contact in the flow electrode contributes to internal resistance and to address this, a cationic surface-active agent is introduced. This agent forms a stable dispersion, creating a space for enhanced mass loading of the active material. This modification enhances the conductive network and particle contact, reducing the diffusion path and promoting rapid ion transport. With a 5 wt% loading, ZnO/rGO achieved a 73% salt removal efficiency, surpassing AC at 63%. Furthermore, the surfactant-modified ZnO/rGO flow electrode with a 7 wt% loading demonstrated an 81% salt removal efficiency.

Energetic Comparison of Flow-Electrode Capacitive Deionization and Membrane Technology: Assessment on Applicability in Desalination Fields.

Flow-electrode capacitive deionization (FCDI) is a promising technology for sustainable water treatment. However, studies on the process have thus far been limited to lab-scale conditions and select fields of application. Such limitation is induced by several shortcomings, one of which is the absence of a comprehensive process model that accurately predicts the operational performance and the energy consumption of FCDI. In this study, a simulation model is newly proposed with initial validation based on experimental data and is then utilized to elucidate the performance and the specific energy consumption (SEC) of FCDI under multiple source water conditions ranging from near-groundwater to high salinity brine. Further, simulated pilot-scale FCDI system was compared with actual brackish water reverse osmosis (BWRO) and seawater reverse osmosis (SWRO) plant data with regard to SEC to determine the feasibility of FCDI as an alternative to the conventional membrane processes. Analysis showed that FCDI is competent for operation against brackish water solutions under all possible operational conditions with respect to the BWRO. Moreover, its distinction can be extended to the SWRO for seawater conditions through optimization of its total effective membrane area via scale-up. Accordingly, future directions for the advancement of FCDI was suggested to ultimately prompt the commercialization of the FCDI process.

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

Capacitive deionization (CDI) is a low-energy, eco-friendly, and economically viable technology for seawater desalination. In the desalination process of CDI, the pore structure characteristics of electrode materials play a crucial role. In this study, the carbon materials with a hierarchical pore structure were prepared through high-temperature carbonization and KOH activation using pine powder as raw material. When the pine-derived porous carbon (PPC) was used as an electrode material, it was found that its specific capacitance is remarkably consistent with the specific surface area of the mesopores. This discovery provides guidance for further optimization and regulation of biomass-derived carbon electrode materials. The desalination performance of the PPC electrode was studied in detail by CDI and flow-electrode capacitive deionization (FCDI) devices. In NaCl solution with an initial concentration of 700 ppm, the equilibrium adsorption capacities in CDI and FCDI systems are 24.96 mg g−1 and 23.12 mg g−1, respectively. Particularly in FCDI tests, the average adsorption rate reaches 23.3 μg cm−2 min−1, and the desalination rate reaches 98.96 %. The PPC shows an excellent desalination performance, making it a promising efficient electrode material for electrochemical desalination.

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). We believe that our approach can provide insight into various areas including resource production, recovery, removal of toxic ions, and material recycling using an FCDI system.

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

This study systematically optimized the key operating parameters and interpreted their effecting mechanisms in a flow-electrode capacitive deionization (FCDI) system. The optimal voltage, activated carbon electrode content, electrolyte concentration, feedwater flowrate, and electrode flowrate for desalinating low salinity feedwater (1.0 g L−1 NaCl) were determined to be 1.8 V, 2.0 wt%, 10.0 g L−1, 80 mL min−1, and 60 mL min−1, respectively. The variations of the above parameters can affect the system conductivity, the thickness and stability of the electric double layers, and/or the degree of concentration polarization, thereby influencing the desalination performance. Moreover, a sensitivity analysis identified the operating voltage as the dominant parameter with the most significant influence on the FCDI system. Subsequently, a long-term operation was carried out under single-pass mode. The results showed that the lab-scale FCDI system was able to constantly maintain the desalination efficiency of 1.0 g L−1 feedwater (NaCl) at 40–60 % for multiple operating cycles. Over 99.8 % of electrode material regeneration and desalination efficiency recovery was able to be obtained during a 60-h operation, demonstrating that the FCDI system showed strong stability and long-term operation potential.

Simultaneous removal of tetracycline and copper ions from wastewater by flow-electrode capacitive deionization

ABSTRACT To effectively solve the problem of tetracycline (TC) and Cu2+ contamination in wastewater, this study innovatively proposed a low-energy flow-electrode capacitive deionization (FCDI) technology to simultaneously remove TC and Cu2+ from wastewater. The removal efficiencies of TC and Cu2+ using FCDI was investigated under various voltages, electrode flow rates, influent flow rates, and electrode liquid concentrations. The results showed that the removal efficiency of TC and Cu2+ was 60.78% and 84.43%, respectively. The energy consumption for TC and Cu2+ removal was only 1.76 and 1.10 kWh kg−1, which was lower than other electrochemical systems. The ion removal performance of the FCDI system remained stable after six cycles of continuous operation. These findings demonstrated the promising potential of FCDI as an innovative technology for the simultaneous removal of TC and Cu2+, presenting a significant prospects for application in the water treatment field.

New insights into the performance analysis of flow-electrode capacitive deionization using ferri/ferrocyanide redox couples for continuous water desalination

Flow Electrode Capacitive Deionization (FCDI) is a promising technology that uses carbon materials as flow electrodes in aqueous conditions to enable continuous desalination. In this work, the parametric investigation of the FCDI cell was carried out for low and high surface area activated carbon. When 1 mM/1 mM ferri-/ferrocyanide redox couple in the 1 M NaCl supporting electrolyte containing the 5 wt% high surface area activated carbon and 0.5 wt% MWCNT is circulated as flow electrode at the flow rate of 8 mL/min in the electrode chamber and 0.1 mM NaCl feed water is circulated in single-pass mode at 8 mL/min, the FCDI system achieved a significant improvement in desalination performance up to 76.38 % of SRE, with a high average salt removal rate of 43.925 µg/cm2s @ 2 V, charge efficiency of 20.11 % and average energy consumption of 0.27 kWh/mol, through a thorough parametric investigation. When the FCDI cell was tested with tap water, the average salt removal rate, average energy consumption, and salt removal efficiency were calculated at 6.11 μg/cm2s, 2.46 kWh/mol, and 46.61 % respectively, resulting in the reduction of total dissolved solids of 600 mg/L at the inlet to 365 mg/L at the outlet. The combination of microporous high surface area carbon and ferro-ferri redox couple propelled the desalination performance in comparison with the macroporous low surface area carbon, maximizing the ion removal rate, which further reduced the energy consumption and operating voltage for the process.

Enhanced nitrate removal from groundwater using a conductive spacer in flow-electrode capacitive deionization

Flow-electrode capacitive deionization (FCDI) represents a promising approach for ion separation from aqueous solutions. However, the optimization of spacer, particularly for nitrate-contaminated groundwater systems, has often been overlooked. This research comprehensively investigates the influence of using a conductive (carbon cloth, CC) spacer on nitrate removal performance within FCDI system, comparing it to a non-conductive (nylon net, NN) spacer. In both CC and NN FCDI systems, it is unsurprisingly that nitrate removal efficiency improved notably with the increasing current density and hydraulic retention time (HRT). Interestingly, the specific energy consumption (SEC) for nitrate removal did not show obvious fluctuations when the current density and HRT varied in both systems. Under the auspiciously optimized process parameters, CC-FCDI attained a 20% superior nitrate removal efficiency relative to NN-FCDI, accompanied by a notably diminished SEC for CC-FCDI, registering at a mere 28% of NN-FCDI. This great improvement can be primarily attributed to the decrement in FCDI internal resistance after using conductive spacer, which further confirmed by electrochemical tests such as linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Upon prolonged continuous nitrate removal at the optimized conditions, the CC-FCDI system achieved a consistent 90% nitrate removal efficiency with a low SEC of 2.7–7.8 kWh/kg NO3-N, underscoring its steady performance. Overall, this study highlights the pivotal importance of careful spacer design and optimization in realizing energy-efficient groundwater treatment via FCDI.