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Fluoride (F−) pollution in groundwater is an important environmental issue and capacitive deionization (CDI) holds promise for defluorination at moderate concentrations (e.g., 200 −1000 mg L−1). However, existing electrodes suffer from the overlap of electrical-double-layer (EDL) and severe co-ion effects at low (e.g., <200 mg L−1) and high sodium fluoride (NaF) concentrations (e.g., >1000 mg L−1), respectively, exhibiting poor salt adsorption capacity (SAC). Hence, a three-layer structured electrode, "membrane/carbon nanotube (CNT)/activated carbon (AC)" (CNT-MCE), was prepared through electrospinning CNT onto AC, followed by a polymer membrane coating. Compared to AC and membrane coated electrode, CNT-MCE with mesopore-dominated structure prevented EDL overlap, achieving a higher SAC of 40.8 mg g−1 at 100 mg L−1 NaF. At 1500 mg L−1 NaF, the positively charged CNT-MCE exhibited an improved SAC of 58.8 mg g−1 by inhibiting co-ion effects. Meanwhile, CNT-MCE consistently demonstrated superb SACs at 200 − 800 mg L−1 NaF and maintained excellent stability over a wide concentration gradient by inhibiting severe oxidation. Notably, CNT-MCE successfully decreased the F− concentration in simulated groundwater from 3.4 to 1.1 mg L−1. Overall, our work provides an efficient strategy of electrode design to broaden the applicability of CDI for groundwater defluorination over a wide concentration gradient. [Display omitted] • Three-layer structured membrane coating electrode was prepared for F- removal. • High F- adsorption capacities were obtained over the wide concentration gradient. • Mesopore-dominated structure overcame the overlapping of electrical double layer. • Positively charged polymer membrane inhibited the serious co-ion repulsion effect. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is surprisingly efficient to remove the aqueous Cs ion due to its small hydrated size and low hydration energy. But current experimental techniques fail in investigating deeply into the influence of some key electrode characteristics due to the difficulty in experimentally fabricating the electrodes as desired. This work presents a dynamic transport model of salt ions in a flow-by CDI cell. By using this model, the electrode thickness, macro- and micro-porosity are investigated to evaluate Cs ion removal efficiency and energy efficiency particularly from the aspect of ion transfer by the approach of decomposing energy contribution. The results indicate that the thick electrode coupled with the high current could greatly improve the effluent quality, but reduce the salt adsorption capacity (SAC). The increasement of the current density from 3 A/m2 to 6 A/m2 greatly decreases the SAC from 4.0 mg/g to 0.8 mg/g. Lower current could prolong the charging period, leading to more ions stored in the micropore. Not all the electrical energy is consumed for separating ions from the feed as desired, but some are used for driving ions diffusing in the electrodes. Consequently charging efficiency will be reduced especially when the electrodes are characterized with high porosity. It is highlighted that future work is required to further consider the complex details of porous structure and pore connectivity. [Display omitted] Radioactive cesium as the product of uranium fission in the energy industry usually exit in ionic form and could be easily dissolved in water, releasing high levels of beta as well as gamma particles and leading to cell damage. Surprisingly capacitive deionization (CDI) is efficient to remove the aqueous Cs ion due to its small hydrated size and low hydration energy. This work deeply elucidates the mechanism of Cs removal by the CDI technique from the aspect of ion transfer and energy contribution composition such as to provide theoretical guidelines for tailoring the electrode structure and tuning the operating parameters. • A CDI dynamic model was presented. • Mechanism was explained by ion transfer. • Energy was decomposed based on ion transport. [ABSTRACT FROM AUTHOR]

Cadmium pollution is a serious threat for the global drink water and natural environment. Herein, a poly-pyrrole coated dual-metal perovskite-type oxide FeMnO 3 (PFMO@PPy) was developed firstly as pseudocapacitive cathode for the reversible capture and release of cadmium ions by asymmetry pseudocapacitive deionization (APCDI) technology, extending the library of CDI electrodes. Our work highlighted several points: (i) PFMO@PPy achieved a maximum Cd-removal capacity of 144.6 mg g−1, and maintained the retention rate of 93.4% after 15-cycle CDI process for up to 150 h, far beyond other previous work. (ii) PFMO@PPy showed the superior removal ratio (~90%) under different real water environments such as tap water, lake water and the groundwater. (iii) The superior Cd(II) electrosorption and desorption behavior is ascribed to the reversible synergistic valence conversion (Fe3+/Fe0 and Mn3+/Mn2+), which is confirmed by ex-situ XPS measurement and electrochemical tests. (iv) DFT calculations confirmed the synergistic effect from Mn and Fe elements in perovskite-type bimetallic oxide FeMnO 3. This study paves a new way for promising future applications of perovskite-type oxides containing dual Faradic redox-activity for wastewater treatment and environmental remediation. [Display omitted] • PFMO@PPy was studied firstly in CDI field and achieved a maximum Cd-removal capacity of 144.6 mg g−1. • PFMO@PPy maintained the retention rate of 93.4% after 15-cycle CDI process for up to 150 h. • PFMO@PPy obtained superior Cd-removal ratio (~90%) in tap water, lake water and groundwater. • Cd(II) adsorption behavior is ascribed to synergistic valence conversion (Fe3+/Fe0 and Mn3+/Mn2+). [ABSTRACT FROM AUTHOR]

The feasibility of the electro-removal of arsenate (As(V)) and arsenite (As(III)) from aqueous solutions via capacitive deionization was investigated. The effects of applied voltage (0.0–1.2 V) and initial concentration (0.1–200 mg L −1 ) on arsenic removal were examined. As evidenced, an enhancement of arsenic removal can be achieved by capacitive deionization. The capacity to remove As(V) at an initial concentration of 0.2 mg L −1 on the activated carbon electrode at 1.2 V was determined to be 2.47 × 10 −2 mg g −1 , which is 1.8-fold higher than that of As(III) (1.37 × 10 −2 mg g −1 ). Notably, the possible transformation of arsenic species was further characterized. The higher effectiveness of As(V) removal via electrosorption at 1.2 V was attributed to the formation of an electrical double layer at the electrode/solution interface. The removal of As(III) could be achieved by the oxidation of As(III) to As(V) and subsequent electrosorption of the As(V) onto the electrode surface of the anode. The presence of sodium chloride or natural organic matter was found to considerably decrease arsenic removal. Single-pass electrosorption-desorption experiments conducted at 1.2 V further demonstrated that capacitive deionization is a potential means of effectively removing arsenic from aqueous solutions. [ABSTRACT FROM AUTHOR]

This study presents the viability and preference of capacitive deionization (CDI) for removing different heavy metal ions in various conditions. The removal performance and mechanisms of three ions, cadmium (Cd 2+ ), lead (Pb 2+ ) and chromium (Cr 3+ ) were investigated individually and as a mixture under different applied voltages and ion concentrations. It was found that CDI could effectively remove these metals, and the performance was positively correlated with the applied voltage. When 1.2 V was applied into solution containing 0.5 mM individual ions, the Cd 2+ , Pb 2+ , and Cr 3+ removal was 32%, 43%, and 52%, respectively, and the electrosorption played a bigger role in Cd 2+ removal than for the other two ions. Interestingly, while the removal of Pb 2+ and Cr 3+ remained at a similar level of 46% in the mixture of three ions, the Cd 2+ removal significantly decreased to 14%. Similar patterns were observed when 0.05 mM was used to simulate natural contaminated water condition, but the removal efficiencies were much higher, with the removal of Pb 2+ , Cr 3+ , and Cd 2+ increased to 81%, 78%, and 42%, respectively. The low valence charge and lack of physical sorption of Cd 2+ were believed to be the reason for the removal behavior, and advanced microscopic analysis showed clear deposits of metal ions on the cathode surface after operation. [ABSTRACT FROM AUTHOR]

90Sr-containing radioactive wastewater during Fukushima nuclear accident (FNA) aroused extensive consideration for its disposal. Massive coexisted Na+ ions seriously inhibited Sr2+ removal, aggravating the expenditure of radioactive wastewater treatment. Herein, a chestnut shell derived porous carbon material modified with aryl diazonium salt (ADS) of sodium 4-aminoazobenzene-4′-sulfonate (SPAC) was developed as capacitive deionization electrode for selective removal of Sr2+ from saliferous radioactive wastewater. Based on ADS modification, the Sr2+ electrosorption capacity of SPAC electrode was improved to 33.11 mg g−1 with fast ion removal rate of 2.89 mg g−1 min−1, comparing with only 16.10 mg g−1 before modification. The isothermal adsorption and kinetics by SPAC electrode fitted well with Langmuir and pseudo-second-order model, achieving a maximum Sr2+ electrosorption capacity of 58.21 mg g−1, superior cycling stability, and excellent charge efficiency (77.63%). Fascinatingly, the SPAC electrode exhibited superhigh Sr2+ selectivity of 70.65 against Na+ in Na+–Sr2+ mixed solution with molar ratio of Na+:Sr2+ as 20:1. Density functional theory (DFT) simulation, combining with electrochemical and spectral analyses, revealed that the high overlap of electron cloud between Sr2+ ion and anionic sulfonic group (–SO 3 -) provided SPAC with remarkable selectivity of Sr2+ ion, and illustrated the ion-swapping mechanism of Sr2+ selectivity. [Display omitted] • SPAC electrode exhibited Sr2+ selectivity of 70.65 against Na+ in molar ratio of Na+:Sr2+ as 20:1. • A maximum Sr2+ electrosorption capacity of 58.21 mg g−1 was achieved in certain condition. • High overlap of electron cloud between Sr2+ ion and –SO 3 - group provided Sr2+ selectivity of SPAC. [ABSTRACT FROM AUTHOR]

This study was performed to determine the feasibility of electrosorptive removal of copper ions from aqueous solutions using a capacitive deionization process. The electrosorptive potential of copper ions was determined using cyclic voltammetry measurements, and copper electrodeposition could be suppressed at a voltage less than 0.8 V. Importantly, the experimental results demonstrated a significant enhancement of electrosorption capability of copper ions using the activated carbon electrodes under electro-assistance, associated with electrical double-layer charging. At 0.8 V, the equilibrium electrosorption capacity was enhanced to 24.57 mg/g based on the Langmuir model, and the electrosorption constant rate was increased to 0.038 min-1 simulated by a first-order kinetics model. Moreover, the activated carbon electrode showed great regeneration performance for the removal of low level copper ions. Additional experiments regarding electrosorption selectivity were performed in the presence of sodium chloride, natural organic matter, or dissolved silica. Copper ions that were preferentially electroadsorbed on the electrode surface can be effectively removed in a competitive environment. Therefore, the electrosorption process using activated carbon electrodes can be recommended to treat copper solutions at low concentrations for wastewater treatment and water purification. [ABSTRACT FROM AUTHOR]

MXenes are a new type of two-dimensional (2D) material which are rapidly gaining traction for a range of environmental, chemical and medical applications. MXenes and MXene-composites exhibit high surface area, superlative chemical stability, thermal conductivity, hydrophilicity and are environmentally compatible. Consequently, MXenes have been successfully employed for hydrogen storage, semiconductor manufacture and lithium ion batteries. In recent years, MXenes have been utilized in numerous environmental applications for treating contaminated surface waters, ground and industrial/ municipal wastewaters and for desalination, often outperforming conventional materials in each field. MXene-composites can adsorb multiple organic and inorganic contaminants, and undergo Faradaic capacitive deionization (CDI) when utilized for electrochemical applications. This approach allows for a significant decrease in the energy demand by overcoming the concentration polarization limitation of conventional CDI electrodes, offering a solution for low-energy desalination of brackish waters. This article presents a state-of-the-art review on water treatment and desalination applications of MXenes and MXene-composites. An investigation into the kinetics and isotherms is presented, as well as the impact of water constituents and operating conditions are also discussed. The applications of MXenes for CDI, pervaporation desalination and solar thermal desalination are also examined based on the reviewed literature. The effects of the water composition and operational protocols on the regeneration efficacy and long-term usage are also highlighted. [Display omitted] • MXenes can be used for drinking / wastewater treatment and desalination applications. • Electrostatic and chemical interactions play a key role in adsorption via MXenes. • CDI electrodes with MXenes offer exceptionally high salt adsorption capacity. • MXenes also show intriguing results for pervaporation and solar desalination. • Assessment of the toxicity of MXenes is warranted before their widespread use. [ABSTRACT FROM AUTHOR]

Three-stage operation system (3S A–C system) combining adsorption and CDI is efficient in recovering liquid fertilizers from wastewater and is a promising technology for simultaneously addressing nutrient crises and environmental pollution. • Hydrogel adsorbents have good separation ability for metal cation and anion. • Hydrogel adsorbents showed little removal of nutrients. • 3S A–C system was presented for nutrient recovery and wastewater treatment. • Plants using recovered liquid fertilizers revealed excellent values. The recovery of nitrogen (N) and phosphorus (P) from wastewater is of great importance in addressing the global nutrient crisis. The limitations of existing methods require the development of effective technology. Here, two different hydrogel adsorbents were fabricated with good separation ability for metal cation (M+) and metal anion (M−) but showed little removal of nutrients. Based on the materials, a novel three-stage operation system combining adsorption and capacitive deionization (CDI) technology was presented for nutrient recovery and wastewater treatment. In the first two stages, mixed metals in wastewater were successfully separated (Cu2+: 144.6 mg/g; Cr 2 O 7 2−: 167.0 mg/g), and nutrients were retained (N and P < 1 mg/g). In the third stage, the residual trace metal ions in the solution were removed (2.0 mg/L to N/A), and the nutrients were enriched through electroadsorption and desorption processes by CDI. Plants using recovered liquid fertilizers revealed similar values for height, root length, and chlorophyll compared with those obtained using actual fertilizers. The results indicated that this novel three-stage operation system (3S A–C system) combining adsorption and CDI is efficient in recovering liquid fertilizers from wastewater and is a promising technology for simultaneously addressing nutrient crises and environmental pollution. [ABSTRACT FROM AUTHOR]