CDI System Research Articles

Electrochemical elegance: A review on capacitive deionization for fluoride and arsenic mitigation in brackish water

Selective ion extraction from aqueous solutions is crucial for both water purification and resource recovery. This review presents an innovative approach to address the selective removal of fluoride and arsenic in brackish water. In this venture, we explore the elimination of fluoride and arsenic through the integration of tailored electrode materials and electrochemical methods in conjunction with CDI technology. We also describe the future potential of selective CDI, covering aspects such as performance evaluation, objectives, application range, and challenges. This information aims to guide future researchers in addressing knowledge gaps in the development and application of selective CDI systems. The findings of this study set the stage for further exploration and practical implementation of CDI as a robust water purification technique for brackish water sources.

Effect of a solvothermal method using DMF on the dispersibility of rGO, application of rGO as a CDI electrode material, and recovery of sp2-hybridized carbon.

Graphene is prized for its large surface area and superior electrical properties. Efforts to maximize the electrical conductivity of graphene commonly result in the recovery of sp2-hybridized carbon in the form of reduced graphene oxide (rGO). However, rGO shows poor dispersibility and aggregation when mixed with other materials without hydrophilic functional groups, This could lead to electrode delamination, agglomeration, and reduced efficiency. This study focuses on the impact of solvothermal reduction on the dispersibility and capacitance of rGO compared with chemical reduction. The results show that the dispersibility of rGO-D obtained through solvothermal reduction using N,N-dimethylformamide improved compared to that obtained through chemical reduction (rGO-H). Furthermore, when utilized as a material for CDI, an improvement in deionization efficiency was observed in the AC@rGO-D-based CDI system compared to AC@rGO-H and AC. However, the specific surface area, a key factor affecting CDI efficiency, was higher in rGO-H (249.572 m2 g-1) than in rGO-D (150.661 m2 g-1). While AC@rGO-H is expected to exhibit higher deionization efficiency due to its greater specific surface area, the opposite was observed. This highlights the effect of the improved dispersibility of rGO-D and underscores its potential as a valuable material for CDI applications.

Efficient removal of short-chain perfluoroalkyl substances (PFAS) using asymmetric membrane capacitive deionization

Concerns have been raised over the persistence and potential health risks of perfluoroalkyl substances (PFAS). PFAS pose significant challenges in water treatment owing to their chemical stability and the difficulty of treating them with conventional purification methods. Although traditional water treatment methods for contaminant removal have been explored, their effectiveness against PFAS remains limited. This gap in knowledge highlights the need for innovative approaches. This study introduces asymmetric membrane capacitive deionization (ACDI) as a novel method for short-chain PFAS removal to address the shortcomings of existing techniques. ACDI refers to a configuration in which an anion exchange membrane is removed from a conventional membrane capacitive deionization with both a cation exchange membrane and an anion exchange membrane. Among the PFAS, perfluorobutanesulfonic acid (PFBS), which is known to be difficult to remove because of its short chain, has been the focus. The ACDI system demonstrated superior deionization capacity (28.75 mg/g) and energy efficiency (energy consumption of 0.46 Wh/g) compared to conventional CDI and membrane CDI systems. In addition, the effects of key operating parameters, such as voltage (0.5 ∼ 1.2 V), initial concentration (50 ∼ 500 mg/L), and flow rate (1 ∼ 5 mL/min), on the PFBS removal were systematically investigated and superior at 1.2 V, 500 mg/L, and 2 mL/min. This study suggests the need for further exploration of real-world applications considering the presence of competing ions in wastewater. This study provides valuable insights into advancing CDI technology for the sustainable treatment of industrial wastewater containing PFAS.

Electric double layer capacitive adsorption and faradaic pseudo-capacitance behavior of ZnFe-PANI/CNT electrode for phosphate removal in capacitive deionization

Capacitive deionization technology possesses puissant competitiveness in the field of water treatment due to its low cost and high efficiency. In this study, polyaniline (PANI) doped and carboxyl intercalated metal hydroxide composite (ZnFe-PANI/CNT) was developed by a simple co-precipitation method, and used as anode electrode for phosphate removal in sewage. The results show that ZnFe-PANI/CNT electrode can efficiently remove phosphate at concentration of 2–10 mg/L at a wide range of pH (3–10) by electric double layer capacitive adsorption and pseudocapacitive adsorption. The concentration of phosphate in the effluent was as low as 0.095 mg/L, and the coexisting anions (Cl−, NO3−, SO42− and HCO3−) hardly inhibited the phosphate adsorption by ZnFe-PANI/CNT composite electrode. In addition, under the optimal phosphate adsorption conditions (pH = 7 and applied voltage of 1.2 V), the energy consumption of CDI system was calculated as low as 0.00274 kWh/g P and 0.017 kWh/m3 water, realizing the expectation of low cost and high efficiency operation.

Cooperative reinforcement of photocatalysis-coupled capacitive deionization in fructose intercalated MoS2 for removal of tetracycline

CDI technology have attracted attention as a promising next generation water remediation technology, while the sustainability and the removal capacity inhibit the application of CDI for organic pollutant removal. Herein, we introduced photocatalysis coupled with CDI technology for removal of tetracycline (TC) by using MoS2/C electrode as a bifunctional catalyst, which could achieve the self-cleaning of the working electrode through in-situ degrading the adsorbed TC. On account of the introduction of fructose, the restrained ultrathin MoS2/C nanosheets composite with enhanced ion storage capacity and promoted photo-generated electron-hole pairs separation efficiency was constructed. By using TC as a model molecule for mimicking organic contaminants, MoS2/C electrode in the photocatalysis-coupled CDI system exhibited a superior TC removal capacity of 1302.5 mg g−1, TC removal efficiency of 84.8 %, and fast removal kinetics. Moreover, it achieved six cycles through “self-cleaning” based on the cooperative effect of photocatalytic degradation and electrosorption. Finally, FT-IR and radical scavenger experiments were utilized to demonstrate the mechanism of TC degradation. This study paves a way of photoelectronic degradation of antibiotics through a synergetic effect of electrosorption and photocatalysis.

Fabrication and utilization of ternary cubical nanostructure modified activated carbon cloth electrodes for capacitive deionization system

Cubical nanostructure zinc tin oxide (ZTO) was in-situ deposited on activated carbon cloth (ACC) substrate via a co-precipitation method. The amount and size of ZTO on the ACC substrate were varied by changing the different overall metal ion (Sn4+ and Zn2+) concentrations. The surface morphology and crystalline analysis revealed the well distributed cubic shape perovskite ZTO on the ACC substrates. The ZTO-deposited ACC electrodes showed superior electrochemical performances with higher specific capacitance and better ion transfer activity than pristine ACC. ZTO deposited ACC substrates were used as electrodes in a capacitive deionization system. ACC substrate coated with 0.01 M Sn and Zn metal concentrations achieved the best desalination efficiency of 52%, salt adsorption capacity of 45.0 ± 2 mg/g and faster salt adsorption rate of 0.14 mg/g/s compared to the other tested electrodes using 500 ppm NaCl solution. Further desalination of real groundwater (1000 ppm) achieved with ions adsorption improvement of 36% for Na+, 48% for Ca2+, 60% for Mg2+, and 46% for Cl- compared to the pristine ACC. Additionally, compared to the pristine ACC, the ZTO-coated electrode enhanced the ions adsorption rate and reduced energy consumption by 40%. Post surface characterization of used pristine ACC displayed substantial scaling of the minerals from the ground water but ZTO-II electrodes exhibited better stability with less scaling. The outcomes of this study provide a stepping stone for the modification and utilization of electrodes for the real brackish water in the CDI system.

Chicken feather-derived carbon electrodes for capacitive deionization using poly(vinyl alcohol)-glutaraldehyde as the binder

Abstract Capacitive deionization (CDI) is an emerging technology which is being developed as a promising desalination alternative for charged species from salt water, being electrode materials as key drivers for highly efficient process. Here, we describe synthesis of highly porous activated carbons from chicken feathers using pyrolysis followed by chemical activation with potassium hydroxide in a 1:4 ratio to produce sustainable, scalable, and sustainable carbon electrodes for CDI. Poly (vinyl alcohol) (PVA) was used as binder to modify chicken feather activated carbons (CF-AC), which were subsequently crosslinked with glutaraldehyde (GA) to produce CF-AC-PVA-GA polymers through acetylation reaction. This resulted from improving hydrophilicity of CF-AC-PVA-GA polymers to raise the electrodes' resistance. Prepared materials were characterized using scanning electron microscopy, Fourier transform infrared, X-ray diffraction, Brunauer-Emmett-Teller and cyclic voltammetry. Fabricated CDI electrodes were used to investigate their performance for desalination, and exhibited different electrosorption capacity at different applied potentials. Modified electrodes possessed good stability in shear conditions and CDI process was stable and reproducible around 16 electrosorption cycles. Salt removal capacity of the modified electrodes (CF-AC-PVA-GA) was found to be 3.89 mg g−1. The obtained outcomes offer important considerations of ions electrosorption and help advancing CDI system for water treatment and desalination.

Up-shifting the desalination rate limit of capacitive deionization via integrating chloride-capturing Bi nanocluster with flow-through cell architecture

Faradic electrode-directed capacitive deionization (faradic-based CDI) as a green and eco-friendly technique is particularly promising in addressing the freshwater crisis, but its practical applications are plagued by the sluggish desalination kinetic. After deciphering its dynamic desalination process, two rate-determine steps (RDS) are identified, i.e., the ion diffusion inside the faradic electrode (RDS #1) and mass transfer process in the electrolyte (RDS #2). Therefore, we propose a synergetic strategy to upshift the desalination rate limit of faradic-based CDI through the integration of delicate material design with rational cell architecture. Specifically, deploying ultrasmall Bi nanoclusters impregnated carbon nanofibers (Bi NCs@CNF) as chloride-capturing electrodes and constructing a flow-through CDI (FT-CDI) system to accelerate the diffusion process (RDS #1) and shorten the mass transfer pathway (RDS #2), respectively. As expected, Bi NCs@CNF-based FT-CDI performs super-fast desalinization (0.56 mg g−1 s−1) and excellent cyclic stability. Taken together, our study opens an avenue in upshifting the desalination kinetics of faradic-based CDI based on the joint power of both “material design” and “cell architecture”, which may motivate the development of highly-efficient desalination systems.

Nitrogen-Doped Core-Shell Mesoporous Carbonaceous Nanospheres for Effective Removal of Fluorine in Capacitive Deionization

Fluorine pollution of wastewater is a global environmental problem. Capacitive deionization has unique advantages in the defluorination of fluorine-containing wastewater; however, the low electrosorption capacity significantly restricts its further development. To overcome this limitation, nitrogen-doped core-shell mesoporous carbonaceous nanospheres (NMCS) were developed in this study based on structural optimization and polarity enhancement engineering. The maximal electrosorption capacity of NMCS for fluorine reached 13.34 mg g−1, which was 24% higher than that of the undoped counterpart. NMCS also indicated excellent repeatability evidenced by little decrease of electrosorption capacity after 10 adsorption-regeneration cycles. According to material and electrochemical measurements, the doping of nitrogen into NMCS resulted in the improvement of physicochemical properties such as conductivity and wettability, the amelioration of pore structure and the transformation of morphology from yolk-shell to core-shell structure. It not only facilitated ion transportation but also improved the available adsorption sites, and thus led to enhancement of the defluorination performance of NMCS. The above results demonstrated that NMCS would be an excellent electrode material for high-capacity defluorination in CDI systems.

Pressure-induced decomposition of cadmium iodide

A static pressure-induced decomposition of cadmium iodide into cadmium and iodine solids is reported in this work using a structure prediction approach combined with first-principles calculations. By compression, CdI2 decomposes into Cd and I at 61.5 GPa, which goes against the common intuition that applying pressure makes the material stable and dense. The increase in the term and with pressure between the compound and the element contributes to the increase in the enthalpy difference, leading to the decomposition of CdI2. On the other hand, the analyses of interatomic interaction demonstrate that under the action of pressure, the decrease of charge transfer between atoms leads to the decrease of the Coulomb interaction, which finally induces CdI2 to decompose into Cd and I solids. Our work represents a significant step toward an understanding of the high-pressure behaviors of Cd-I systems and draws attention to the influence of pressure parameters in certain materials.

Highly efficient and selective extraction of phosphorous from wastewater as vivianite in a strategically operated four-chamber flow electrode capacitive deionization

Phosphorus (P) is not only one of the main contaminants in wastewater but also an essential nutrient for all living organisms. In this study, we demonstrate a highly efficient and selective process for extraction of P from wastewater in the form of solid vivianite using a strategically operated four-chamber flow electrode capacitive deionization (FC-FCDI). The separated diluate and concentrate chambers in FC-FCDI enables continuous production of dephosphorylated water and P-rich brine, realizing P removal rate (0.22 μmol/(cm2·min)) and enrichment efficiency (concentration factor of 17.2) much higher than previous studies. The spontaneously enriched competing ions in the concentrate mitigate their further migration due to concentration countering effect, realizing to date the highest selectivity for P over SO42− (4.4) in CDI systems. Most importantly, the precipitated vivianite has high P content (12.4 %) and much smaller volume compared with the P-recovered solution in previous studies. The recovery of P in the solid state prevents the production of brine wastes, requires no water input for concentrate replenishment, and is conducive to transport, storage, and direct usage for various purposes. This work suggests a novel method for highly selective and efficient extraction of P from wastewater using tailor-designed and strategically-operated FC-FCDIs.

Synthesis and characterization of Bi2O3/ZIF-67 nanocomposite integration with capacitive deionization system in the degradation tetracycline

A Bi2O3/ZIF-67 heterostructure was synthesized as a novel and highly active photocatalyst by the hydrothermal method. The as-synthesized catalysts were identified by XRD, SEM, EDX, FTIR, TEM, UV–Vis DRS, BET, and PL analyses. The two-component composite showed a significant improvement in photoactivity compared to Bi2O3 and pristine ZIF-67 due to its unique characteristics such as increased specific surface area and visible light adsorption, as well as reduced bandgap and recombination rate of electron-hole. The optimum values of the processing variables for the contaminant degradation efficiency were determined for photocatalytic process (catalyst concentration= 0.4 g/L, pH= 9, tetracycline (TC) initial concentration= 20 mg/L, and time= 120 min) and capacitive deionization system (CDI) (L=5 mm, voltage=1.8 V, and TC concentration=20 mg/l) and the maximum pollutant removal efficiency under optimal conditions for catalyst adsorption, photocatalytic process, and CDI systems were obtained 12%, 84% and 69%, respectively. The identification of a kinetic study confirmed that the TC removal followed the pseudo-first-order model. According to kinetic studies, the combination of photocatalysis and CDI system (PCS) has a significant effect on the efficiency of the contaminant removal, and the reaction rate constant is increased by about 1.5 and 2.4 times compared to the photodegradation process and the CDI system, respectively. As the results show, the PCS system is a promising way of removing contaminants and dramatically increasing the removal efficiency. Experimental results showed a great chemical stability and reusability of the photocatalyst and the graphite electrode after five cyclic usage.

A low-cost and eco-friendly fabrication of an MCDI-utilized PVA/SSA/GA cation exchange membrane

Abstract The alternative desalination technique of membrane capacitive deionization (MCDI) has emerged in the last 15 years and received a lot of research attention since then. By using a voltage applied between two electrodes covered with ion-exchange membranes, MCDI has attempted to challenge established methods such as reverse osmosis or electrodialysis. In this study, through the crosslinking of sulfosuccinic acid (SSA) and glutaric acid (GA) with polyvinyl alcohol (PVA), cation exchange membrane preparation and characterization were introduced. For the CDI system, membranes were chosen based on their water absorption and ion exchange properties. The PVA/SSA/GA composite (mass ratio of 100:5:5) had the best water adsorption and charge efficiencies that could be utilized for CDI application. The membrane’s ability to desalinate water was assessed using electrical properties such as total resistance, specific capacitance, and electro adsorption coated with the best ratio composite CEM. The salt absorption capacity of 19.06 mg·g−1 with stable performance was found to be encouraging.

A dynamic intercalation mechanism in pre-intercalation carbon nanosheets for capacitive deionization cells

Given that carbon-based electrodes suffer from poor long-term stabilities and limited desalination capacity, the optimized charge storage mechanism used for improving desalination performance remains requisite. In this work, the KC-pre-intercalated carbon nanosheets (PCNs) with abundant conjugated carbonyl groups were designed via a one-step smelting method for the first time as a novel intercalation electrode in the symmetric capacitive deionization. Interestingly, below 500 mg/L, the physical electrical double-layer mechanism is mainly influential. Benefitting from the proposed dynamic intercalation mechanism, a remarkable ion removal capacity of 1.65 g g−1 and impressive cyclability of 300 cycles (97.96%) were observed in the high salt concentrations (>1000 mg/L) and simulated seawater system rather than pure NaCl solution. The ultrahigh adsorption capacity and prominent cycling stability make the PCNs-based device rank top three among these of all the symmetric CDI systems reported in the literatures so far. The Na+ ions are intercalated in the enlarged interlayer spacing without a host structure deformation, thus offering a high ion removal rate (2.04 mg g−1 min−1) and a low energy consumption (0.28 Wh g−1). Our work exemplifies the importance of engineering conjugated carbonyl groups and pre-intercalated structure to capacitive deionization for brine.

The impact of collective and individual drip irrigation systems on fertilizer use intensity and land productivity: Evidence from rural Xinjiang, China

Drip irrigation is claimed to save water and fertilizers and improve land productivity. Considerable recent evidence supports the water-saving effect of drip irrigation techniques, but little attention has been given to the effect on fertilizer savings and land productivity improvement. This study examines the effects of collective and individual drip irrigation (CDI and IDI, respectively) on fertilizer use intensity and land productivity. Using a multinomial endogenous switching treatment regression and a survey dataset collected in Awati County, Xinjiang, we found that (1) on average, CDI reduces fertilizer use intensity by 10.31%, but IDI increases it by 5.57%; and (2) compared to IDI, CDI improves land productivity by more than 13.65%. Moreover, based on quantitative analyses, we found that (3) a high frequency of mutual adjustment of fertigation (an information coordination mechanism) gives CDI these advantages. The policy implications are to continue the top-down promotion of CDI technology and strengthen the information coordination on fertigation during the operation of CDI systems.

Integrated Capacitive Deionization and Humidification-Dehumidification System for Brackish Water Desalination

A hybrid capacitive deionization and humidification-dehumidification (CDI–HDH) desalination system is theoretically investigated for the desalination of brackish water. The CDI system works with two basic operations: adsorption and regeneration. During adsorption, water is desalted, and during the regeneration process the ions from electrodes are detached and flow out as wastewater, which is higher in salt concentration. This wastewater still contains water but cannot be treated again via the CDI unit because CDI cannot treat higher-salinity waters. The discarding of wastewater from CDI is not a good option, since every drop of water is precious. Therefore, CDI wastewater is treated using waste heat in a process that is less sensitive to high salt concentrations, such as humidification-dehumidification (HDH) desalination. Therefore, in this study, CDI wastewater was treated using the HDH system. Using the combined system (CDI–HDH), this study theoretically investigated brackish water of various salt concentrations and flow rates at the CDI inlet. A maximum distillate of 1079 L/day was achieved from the combined system and the highest recovery rate achieved was 24.90% from the HDH unit. Additionally, two renewable energy sources with novel ideas are recommended to power the CDI–HDH system.

The role of reversible and polarizable surface charge in the electro-sorption of NaCl electrolyte onto activated carbon-graphite electrode

An electrode made of activated carbon that had high-specific-surface area and low pHzpc supported on graphite sheet was fabricated. The electrode, NSA/G, was used to study the effect of reversible surface charge on the adsorption of simple electrolyte, i.e., NaCl, on polarizable electrode. Polarizable surface charge is originated from externally applied potential whereas reversible surface charge is derived from change of activity of potential-determining ions, i.e., H+ or OH– owning to hydrous surface, which undergoes protonation and deprotonation of surface hydroxyl groups. Ion adsorption density was obtained from direct analytical measurement of corresponding ion studied instead of conductivity measurement. Langmuir adsorption isotherm was applied to estimate the monolayer ion adsorption density. Electrosorption/desorption kinetics was fitted well by the first-order rate law. In the applied potential range of −0.3 to −1.0 V (vs. SCE) and pH of 3 to 10, the pH-determining surface, i.e., reversible charge, accounted for Na+ sorption intensity was estimated based on the Gouy-Chapman version of electrical double layer theory. Results revealed that the two major surface charges, i.e., polarizable (σE) and reversible (σpH), contributed equally to the total charge density (σT). At high pH (pH 10) and low applied voltage (-0.3 V vs. SCE), the fraction of pH-determining contribution to total surface charge with respect to Na+ sorption, i.e., σpH/σT, was around 0.50. Results reaffirmed the importance of reversible surface charge together with polarizable surface charge on the design and operation of CDI system for electrosorption of ions from water.

General, Metal-free Synthesis of Carbon Nanofiber Assemblies from Plant Oils.

We designed a metal‐free synthesis of carbon nanofiber based on ketene chemistry using phosphorus pentoxide (P2O5) and vegetable oil. Based on the characterization of intermediates, P2O5‐oil reaction yielded most possibly alkylketenes, which polymerized into poly(ketene) with abundant enol groups. The enol groups further reacted with P2O5, forcing the poly(ketene) to assemble into a nano‐sized preassembly structure. Moderate heating transforms these structures into carbonaceaus nanofibers. This approach is applicable to other chemicals with similar structure to vegetable oil. The carbon nanofibers with P‐O‐C functionalization show relatively high graphitization degree and promising textural properties. The C‐O‐P environment accounts for 66 at % of the total P and creates a superior thermal stability. As a model application, a CDI system built of a carbon‐nanofiber‐based electrode countered by an activated carbon‐based electrode exhibited exceptional performance.

General, Metal‐free Synthesis of Carbon Nanofiber Assemblies from Plant Oils

AbstractWe designed a metal‐free synthesis of carbon nanofiber based on ketene chemistry using phosphorus pentoxide (P2O5) and vegetable oil. Based on the characterization of intermediates, P2O5‐oil reaction yielded most possibly alkylketenes, which polymerized into poly(ketene) with abundant enol groups. The enol groups further reacted with P2O5, forcing the poly(ketene) to assemble into a nano‐sized preassembly structure. Moderate heating transforms these structures into carbonaceaus nanofibers. This approach is applicable to other chemicals with similar structure to vegetable oil. The carbon nanofibers with P‐O‐C functionalization show relatively high graphitization degree and promising textural properties. The C‐O‐P environment accounts for 66 at % of the total P and creates a superior thermal stability. As a model application, a CDI system built of a carbon‐nanofiber‐based electrode countered by an activated carbon‐based electrode exhibited exceptional performance.

Genetic Evidence for SecY Translocon-Mediated Import of Two Contact-Dependent Growth Inhibition (CDI) Toxins.

The C-terminal (CT) toxin domains of contact-dependent growth inhibition (CDI) CdiA proteins target Gram-negative bacteria and must breach both the outer and inner membranes of target cells to exert growth inhibitory activity. Here, we examine two CdiA-CT toxins that exploit the bacterial general protein secretion machinery after delivery into the periplasm. A Ser281Phe amino acid substitution in transmembrane segment 7 of SecY, the universally conserved channel-forming subunit of the Sec translocon, decreases the cytotoxicity of the membrane depolarizing orphan10 toxin from enterohemorrhagic Escherichia coli EC869. Target cells expressing secYS281F and lacking either PpiD or YfgM, two SecY auxiliary factors, are fully protected from CDI-mediated inhibition either by CdiA-CTo10EC869 or by CdiA-CTGN05224, the latter being an EndoU RNase CdiA toxin from Klebsiella aerogenes GN05224 that has a related cytoplasm entry domain. RNase activity of CdiA-CTGN05224 was reduced in secYS281F target cells and absent in secYS281F ΔppiD or secYS281F ΔyfgM target cells during competition co-cultures. Importantly, an allele-specific mutation in secY (secYG313W ) renders ΔppiD or ΔyfgM target cells specifically resistant to CdiA-CTGN05224 but not to CdiA-CTo10EC869, further suggesting a direct interaction between SecY and the CDI toxins. Our results provide genetic evidence of a unique confluence between the primary cellular export route for unfolded polypeptides and the import pathways of two CDI toxins.IMPORTANCE Many bacterial species interact via direct cell-to-cell contact using CDI systems, which provide a mechanism to inject toxins that inhibit bacterial growth into one another. Here, we find that two CDI toxins, one that depolarizes membranes and another that degrades RNA, exploit the universally conserved SecY translocon machinery used to export proteins for target cell entry. Mutations in genes coding for members of the Sec translocon render cells resistant to these CDI toxins by blocking their movement into and through target cell membranes. This work lays the foundation for understanding how CDI toxins interact with the protein export machinery and has direct relevance to development of new antibiotics that can penetrate bacterial cell envelopes.