Selective Capacitive Removal of Pb2+ from Wastewater over Redox-Active Electrodes

Efficient separation and extraction of copper ions from electroplating wastewater by capacitive deionization using chalcopyrite (CuFeS2) electrode

As a hot issue, the scientific and effective separation and extraction of heavy metal ions from complex industrial effluent deserves wide investigation. Copper is an important valuable heavy metal in industrial wastewater. Selective extraction of copper ion (Cu2+) from effluent not only alleviates the shortage of resources, but also has economic and social benefits. In this study, we used capacitive deionization (CDI) to selectively extract Cu2+ from electroplating wastewater using the bimetallic sulfide CuFeS2 electrode. The CuFeS2 electrode showed good Cu2+ extraction performance and selectivity in a variety of cationic coexistence systems and real electroplating wastewater, indicating its potential for practical application. Specifically, the adsorption capacity of Cu2+ was up to 208.3 mg g-1 in a 100 mg L-1 CuSO4 solution. The CuFeS2 electrode removed more than 90 % of Cu2+ and had the highest partition coefficient (Kd) in a variety of cationic coexistence systems. The characterizations demonstrated that the excellent performance of CuFeS2 electrode during the adsorption process was mainly due to the Faraday redox reaction. This work offers a novel approach and theoretical basis for extracting Cu2+ from electroplating wastewater by capacitive deionization.

Sustainable Pb(II) Removal and Recovery from Wastewater Using a Bioinspired Metal-Phenolic Hybrid Membrane with Efficient Regeneration

High-performance adsorbents often require efficient selectivity in wastewater, recoverability, and ease of multiple regeneration cycles, but achieving this remains a significant challenge. We report a new strategy for the efficient removal of lead (Pb(II)) from contaminated water streams using an innovative tannic acid (TA)-Fe(III)-based metal-phenolic network (MPN) hybrid membrane (MPN-PAM). This novel membrane exploits the tunable pH-sensitive coordination structure of the MPN to achieve selective removal and recovery of Pb(II) while enabling efficient membrane regeneration by filtration. This membrane demonstrates superior selectivity for Pb(II) with a removal efficiency of up to 98 % and an adsorption capacity of approximately 117.58 mg/g, even in the presence of high salinity, as well as coexisting heavy metals. The membrane maintains high Pb(II) removal efficiency over 20 consecutive cycles and 95 % efficiency over 10 regeneration cycles. Under continuous operation, it treats approximately 85 L per m2 of membrane, reducing Pb(II) concentrations to trace levels (~40 μg/L), meeting electroplating wastewater standard (GB21900-2008). Additionally, even low concentrations of Pb(II) (<5 mg/L) are efficiently purified to below WHO drinking water standard (10 μg/L). The operational cost for treating Pb(II)-contaminated wastewater is about $0.13 per ton, highlighting the cost-effectiveness and potential for large-scale application in wastewater treatment.

Enhanced hybrid capacitive performance for efficient and selective potassium extraction from wastewater: Insights from regulating electrode potential

Prussian blue analogues hold great promise for directly extracting potassium resource from wastewater via hybrid capacitive deionization (HCDI). However, there remain unresolved scientific issues regarding low efficiency and selectivity arising from asymmetric potential distribution induced by spontaneous charge matching. This work systematically investigated the underlying mechanisms for enhancing the storage capacity and specific affinity of representative Berlin Green towards K+ through precise regulation of insertion potential during HCDI operation. Empowered by controlling electrochemical intercalation behaviors, the compatibility between ionic and electronic kinetics was significantly enhanced. Impressive values of 160.12 mg/g, 61.27 %, and 0.07 kWh/mol were achieved under potentiostatic mode (0.1 V vs. Ag/AgCl) for insertion capacity, charge efficiency, and energy consumption, respectively. These results significantly outperformed the optimal levels obtained under constant cell voltage (0.9 V), which were 128.52 mg/g, 47.50 %, and 0.12 kWh/mol, respectively. In both aqueous solution with binary components and urine, the results emphasized the potential of the synergy effect between lattice hindrance and insertion chemistry in promoting intercalation selectivity, with the highest selectivity coefficients of 28.35 (K+/Na+), 76.22 (K+/Ca2+) and 175.12 (K+/Mg2+), respectively. The presented concept-to-proof offers a versatile approach for the advancement of high-performance HCDI and paves the way towards its sustainable application in nutrient recycling from natural waters or wastewaters.

Design of Carbon Materials with Selective Ion Separation in Capacitive Deionisation and Their Applications

Capacitive deionization (CDI) is a novel, cost-effective and environmentally friendly desalination technology that has garnered significant attention in recent years. Carbon materials, owing to their excellent properties, have become the preferred electrode materials for CDI. Given the significant differences between different ions, ion-selective performance has emerged as a critical aspect of CDI applications. However, comprehensive reviews on the selective ion separation capabilities of carbon materials for CDI remain scarce. This review examines the progress in developing carbon materials for ion-selective separation in CDI, focusing on regulatory mechanisms and representative materials. It also discusses the applications of selective CDI carbon materials in areas such as heavy metal removal, nutrient recovery, seawater desalination resourcing, and water softening. Furthermore, the challenges and future prospects for advancing carbon materials in CDI are explored. This review aims to provide theoretical insights and practical guidance for utilising carbon materials in wastewater treatment and resource recovery.

Tailoring the electrode material and structure of rocking-chair capacitive deionization for high-performance desalination

With the gradually increasing requirement for freshwater, capacitive deionization (CDI) as a burgeoning desalination technique has gained wide attention owing to its merits of easy operation, high desalination efficiency, and environmental friendliness. To enhance the desalination performance of CDI, different CDI architectures are designed, such as membrane CDI, hybrid CDI, and flow-electrode CDI. However, these CDI systems have their own drawbacks, such as the high cost of membranes, capacity limitation of carbon materials and slurry blockage, which severely limit their practical application. Notably, rocking-chair CDI (RCDI) composed of symmetric electrode materials delivers excellent desalination performance because of its special dual chamber structure, which can not only break through the capacity limitations of carbon materials, but also deliver a continuous desalination process. Although RCDI showcases high promise for efficient desalination, few works systematically summarize the advantages and applications of RCDI in the desalination field. This review offers a thorough analysis of RCDI, focusing on its electrode materials, structure designs and desalination applications. Furthermore, the desalination performances of RCDI and other CDI architectures are compared to demonstrate the advantages of RCDI and the prospect of RCDI is elucidated.

Selective Capacitive Recovery of Rare-Earth Ions from Wastewater over Phosphorus-Modified TiO2 Cathodes via an Electro-Adsorption Process

Large amounts of wastewater containing low-concentration (<10 ppm) rare-earth ions (REIs) are discharged annually in China's rare-earth mining and processing industry, resulting in severe environmental pollution and economic losses. Hence, achieving efficient selective recovery of low-concentration REIs from REIs-containing wastewater is essential for environmental protection and resource recovery. In this study, a pseudocapacitance system was designed for highly efficient capacitive selective recovery of REIs from wastewater using the titanium dioxide/P/C (TiO2/P/C) composite electrode, which exhibited over 99% recovery efficiency for REIs, such as Eu3+, Dy3+, Tb3+, and Lu3+ in mixed solution. This system maintained high efficiency and more than 90 times the enrichment concentration of REIs even after 100 cycles. Ti4+ of TiO2 was reduced to Ti3+ of Ti3O5 under forward voltage in the system, which trapped the electrons of phosphorus site and caused it to be oxidized to phosphate with a strong affinity for REIs, thus improving the selectivity of REIs. Under reverse voltage, Ti3O5 was oxidized to TiO2, which transferred electrons to phosphate and transformed to the phosphorus site, resulting in the desorption and enrichment of REIs and the regeneration of the electrode. This study provides a promising method for the efficient recovery of REIs from wastewater.

Achieving Exceptional Volumetric Desalination Capacity Using Compact MoS2 Nanolaminates

Capacitive deionization (CDI) has emerged as a promising technology for freshwater recovery from low-salinity brackish water. It is still inapplicable in specific scenarios (e.g., households, islands, or offshore platforms) due to too low volumetric adsorption capacities. In this study, a high-density semi-metallic molybdenum disulfide (1T'-MoS2) electrode with compact architecture obtained by restacking of exfoliated nanosheets, which achieve high capacitance up to ≈277.5 F cm-3 under an ultrahigh scan rate of 1000 mV s-1 with a lower charge-transfer resistance and nearly tenfold higher electrochemical active surface area than the 2H-MoS2 electrode, is reported. Furthermore, 1T'-MoS2 electrode demonstrates exceptional volumetric desalination capacity of 65.1 mgNaCl cm-3 in CDI experiments. Ex situ X-ray diffraction (XRD) reveal that the cation storage mechanism with the dynamic expansion of 1T'-MoS2 interlayer to accommodate cations such as Na+, K+, Ca2+, and Mg2+, which in turn enhances the capacity. Theoretical analysis unveils that 1T' phase is thermodynamically preferable over 2H phase, the ion hydration and channel confinement also play critical role in enhancing ion adsorption. Overall, this work provides a new method to design compact 2D-layered nanolaminates with high-volumetric performance for CDI desalination.

Coordination Confined Silver-Organic Framework for High Performance Electrochemical Deionization

Silver (Ag) is deemed a promising anode material for capacitive deionization (CDI) due to its high theoretical capacity and efficient selectivity to Cl-. However, the strong volume change during the conversion reaction significantly undermines the cycling performance of the Ag electrode. Additionally, achieving well-dispersed Ag in the active matrix is challenging, as Ag electrodes prepared by conventional thermal reduction tend to agglomerate. Herein, the organic linker confinement strategy is proposed, applying metal-organic framework (MOF) chemistry between Ag nodes and organic ligands to construct Ag-based MOF. The uniform dispersion of Ag at the molecular level, confined in the organic matrix, efficiently enhances the utilization of active sites, and strengthens the interfacial stability of Ag. Consequently, the Ag-MOF for the CDI anode exhibits an excellent Cl- removal capacity of 121.52 mg g-1 at 20 mA g-1 in 500 mg L-1 NaCl solution, and a high Ag utilization rate of 60.54%. After 100 cycles, a capacity retention of 96.93% is achieved. Furthermore, the Cl- capture mechanism of Ag-MOF is elucidated through density functional theory (DFT) calculations, ex situ XRD, ex situ Raman and XPS. This ingenious electrode design can offer valuable insights for the development of high-performance conversion electrodes for CDI applications.

Selective Capacitive Removal of Pb2+ from Wastewater over Biochar Electrodes by Zinc Regulation

Biochar materials have shown great potential for broad catalytic application. However, using these materials in the capacitive deionization technology (CDI) system for heavy metal removal still faces a significant challenge due to their low specific capacity and removal capability. Here, a comprehensive regulation on the interfacial/bulk electrochemistry of biochar by Zn doping is reported, which suggests a high renewable capacity (20 mg g-1) and outstanding selective capacitive removal ability (SCR) of Pb2+ from leachate. The SCR efficiency of Pb2+ is as high as 99% compared to K+ (8%), Na+ (13%), and Cd2+ (37%). This work proves that the doped Zn on the biochar can combine with OH- generated by water splitting to form M─OH bonds, which is beneficial for improving the specific capacity. Significantly, the relationship between double-layer capacitance and pseudo-capacitance can also be optimized by regulating the content of Zn, leading to different removal abilities of heavy metals. Therefore, this work offers insights into charge-storage kinetics, which provide valuable guidelines for designing and optimizing the biochar electrode for broader environmental applications.

Insights into the Mechanism of Selective Removal of Heavy Metal Ions by the Pulsed/Direct Current Electrochemical Method

Heavy metal pollution treatment in industrial wastewater is crucial for protecting biological and environmental safety. However, the highly efficient and selective removal of heavy metal ions from multiple cations in wastewater is a significant challenge. This work proposed a pulse electrochemical method with a low-/high-voltage periodic appearance to selectively recover heavy metal ions from complex wastewater. It exhibited a higher recovery efficiency for heavy metal ions (100% for Pb2+ and Cd2+, >98% for Mn2+) than other alkali and alkaline earth metal ions (Na+, Ca2+, and Mg2+ were kept below 3.6, 1.3, and 2.6%, respectively) in the multicomponent solution. The energy consumption was only 34-77% of that of the direct current electrodeposition method. The results of characterization and experiment unveil the mechanism that the low-/high-voltage periodic appearance can significantly suppress the water-splitting reaction and break the mass-transfer limitation between heavy metal ions and electrodes. In addition, the plant study demonstrates the feasibility of treated wastewater for agricultural use, further proving the high sustainability of the method. Therefore, it provides new insights into the selective recovery of heavy metals from industrial wastewater.

Fluorescence biosensor for ultrasensitive detection of the available lead based on target biorecognition-induced DNA cyclic assembly

A fluorescence biosensor was developed for the ultrasensitive detection of the available lead in soil samples by coupling with DNAzyme and hairpin DNA cyclic assembly. The biorecognition between lead and 8-17 DNAzyme will cleave the substrate strands (DNA2) and release the trigger DNA (T), which can be used to initiate the DNA assembly reactions among the hairpins (H1, H2, and H3). The formed Y-shaped sensing scaffold (H1-H2-H3) contains active Mg2+-DNAyzmes at three directions. In the presence of Mg2+, the BHQ and FAM modified H4 will be cleaved by the Mg2+-DNAyzme to generate a high fluorescence signal for lead monitoring. The linear range of the fluorescence biosensor is from 1 pM to 100 nM and the detection limit is 0.2 pM. The biosensor also exhibited high selectivity and the nontarget competing heavy metals did not interfere with the detection results. Compare with the traditional method (DTPA+ICP-MS) for the available lead detection, the relative error (Re) is in the range from -8.3 % to 9.5 %. The results indicated that our constructed fluorescence biosensor is robust, accurate, and reliable, and can be applied directly to the detection of the available lead in soil samples without complex extraction steps.

A Study on the Impermeability of Nanodispersible Modified Bentonite Based on Colloidal Osmotic Pressure Mechanisms and the Adsorption of Harmful Substances

With the growing demands of human beings, sanitary landfill, along with the increase in landfill depth and leachate water pressure, has put forward new and higher requirements for the impermeable layer. In particular, it is required to have a certain adsorption capacity of harmful substances from the perspective of environmental protection. Hence, the impermeability of polymer bentonite-sand mixtures (PBTS) at different water pressure and the adsorption properties of polymer bentonite (PBT) on contaminants were investigated through the modification of PBT using betaine compounded with sodium polyacrylate (SPA). It was found that the composite modification of betaine and SPA could reduce the average particle size of PBT dispersed in water (reduced to 106 nm from 201 nm) and enhance the swelling properties. As the content of SPA increased, the hydraulic conductivity of PBTS system decreases and the permeability resistance improves, while the resistance to external water pressure increases. It is proposed a concept of the potential of osmotic pressure in a constrained space to explain the impermeability mechanism of PBTS. The potential of osmotic pressure obtained by linear extrapolation of the trendline of colloidal osmotic pressure versus mass content of PBT could represent the external water pressure that the PBT resist. Additionally, the PBT also has a high adsorption capacity for both organic pollutants and heavy metal ions. The adsorption rate of PBT was up to 99.36% for phenol; up to 99.9% for methylene blue; and 99.89%, 99.9%, and 95.7% for low concentrations of Pb²⁺, Cd²⁺, and Hg⁺, respectively. This work is expected to provide strong technical support for the future development in the field of impermeability and removal of hazardous substances (organic and heavy metals).

Copper removal and recovery from electroplating effluent with wide pH ranges through hybrid capacitive deionization using CuSe electrode

In modern industry, selective extraction and recovery of Cu from strongly acidic electroplating effluent are crucial to reduce carbon emissions, alleviate resource scarcity, and mitigate water pollution, yielding considerable economic and environmental benefits. This study proposed a high-efficiency CuSe electrode to selectively remove Cu from electroplating effluent via hybrid capacitive deionization (HCDI). The potential of this electrode was thoroughly evaluated to assess its effectiveness. The CuSe electrode exhibited superior deionization performance in terms of Cu adsorption capacity, selectivity, and applicability in various water matrices. Specifically, under strong acid conditions (1 M H⁺), the CuSe electrode maintained an optimal adsorption capacity of 357.36 mg g^-1 toward Cu²⁺. In systems containing salt ions, heavy metals, and actual electroplating wastewater, the CuSe electrode achieved a remarkable removal efficiency of up to 90% for Cu²⁺ with a high distribution coefficient K_d. Notably, the capacitive deionization (CDI) system demonstrated the simultaneous removal of Cu-EDTA. The removal mechanism was further revealed using ex-situ X-ray diffraction and X-ray photoelectron spectroscopy analyses. Overall, this study presents a practical approach that extends the capabilities of CDI platforms for effectively removing and recovering Cu from acidic electroplating effluent.

Ultrasensitive and on-site eDNA detection for the monitoring of crown-of-thorns starfish densities at the pre-outbreak stage using an electrochemical biosensor

The coral reef crisis has significantly intensified over the last decades, mainly due to severe outbreaks of crown-of-thorns starfish (COTS). Current ecological monitoring has failed to detect COTS densities at the pre-outbreak stage, thus preventing early intervention. In this work, we developed an effective electrochemical biosensor modified by a MoO₂/C nanomaterial, as well as a specific DNA probe that could detect trace COTS environmental DNA (eDNA) at a lower detection limit (LOD = 0.147 ng/μL) with excellent specificity. The reliability and accuracy of the biosensor were validated against the standard methods by an ultramicro spectrophotometer and droplet digital PCR (p > 0.05). The biosensor was then utilized for the on-site analysis of seawater samples from SYM-LD and SY sites in the South China Sea. For the SYM-LD site suffering an outbreak, the COTS eDNA concentrations were 0.33 ng/μL (1 m, depth) and 0.26 ng/μL (10 m, depth), respectively. According to the ecological survey, the COTS density was 500 ind/hm² at the SYM-LD site, verifying the accuracy of our measurements. At the SY site, COTS eDNA was also detected at 0.19 ng/μL, but COTS was not found by the traditional survey. Hence, larvae were possibly present in this region. Therefore, this electrochemical biosensor could be used to monitor COTS populations at the pre-outbreak stages, and potentially serve as a revolutionary early warning method. We will continue to improve this method for picomolar or even femtomolar detection of COTS eDNA.

Enhanced Pseudo-Capacitance Process in Nanoarchitectural Layered Double Hydroxide Nanoarrays Hollow Nanocages for Improved Capacitive Deionization Performance

Layered double hydroxides (LDHs) are perceived as a hopeful capacitive deionization (CDI) faradic electrode for Cl^- insertion due to its tunable composition, excellent anion exchange capacity, and fast redox activity. Nevertheless, the self-stacking and inferior electrical conductivity of the two-dimensional structure of LDH lead to unsatisfactory CDI performance. Herein, the three-dimensional (3D) hollow nanocage structure of CoNi-layered double hydroxide/carbon composites is well designed as a CDI anode by cation etching of the pre-carbonized ZIF-67 template. C/CoNi-LDH has a unique 3D hollow nanocage structure and abundant pore features, which can effectively suppress the self-stacking of LDH sheets and facilitate the transport of ions. Moreover, the introduced amorphous carbon layer can act as a conductive network. When employed as the CDI anode, C/CoNi-LDH exhibited a high Cl^- removal capacity of 60.88 mg g^-1 and a fast Cl^- removal rate of 18.09 mg g^-1 min^-1 at 1.4 V in 1000 mg L^-1 NaCl solution. The mechanism of the Cl^- intercalation pseudo-capacitance reaction of C/CoNi-LDH is revealed by electrochemical kinetic analysis and ex situ characterization. This study provides vital guidance for the design of high-performance electrodes for CDI.

Controlled fabrication of nitrogen-doped porous carbon foam with refined hierarchical architectures for desalination via capacitive deionization

Porous carbon materials have been regarded as a promising alternative to activated carbon for desalination via capacitive deionization (CDI) due to refined architectures and functionalities. However, it is still challenging to obtain a controlled hierarchical pore structure and considerable nitrogen-doped content by convenient method. Herein, nitrogen-doped hierarchical porous carbon foams (NHCFs) with different microstructural features, nitrogen contents and nitrogen species were successfully fabricated via a stepwise pyrolysis carbonization strategy using easily available melamine foam. Due to the synergistic effect of hierarchical porous structure and doped nitrogen, the optimized NHCF sample carbonized at 800℃ (NHCF-800) exhibited a maximum desalination capacity of 30.1 mg g^-1 at the optimal operating parameters (500 mg/L NaCl solution, 1.2 V) and an excellent regeneration performance after 50 continuous adsorption-desorption cycles. Furthermore, density functional theory (DFT) was also conducted to elaborate the disparity of sodium adsorption energy among the nitrogen species for in-depth understanding, and it mainly benefits from the ascendency of the pyrrolic-N and pyridinic-N over the graphitic-N dopant. This work paves the way of rational regulation of nitrogen-doped process and hierarchical porous structure carbon as CDI electrode materials for desalination.

Homogeneous voltammetric sensing strategy for lead ions based on aptamer gated methylthionine chloride@UiO-66-NH2 framework as smart target-stimulated responsive nanomaterial

Herein an innovative electrochemical method is proposed for the determination of lead ions (Pb²⁺) based on a homogeneous voltammetric (HVC) sensing strategy using an aptamer gated methylthionine chloride@UiO-66-NH₂ framework as a smart target-stimulated responsive material. The proposed HVC sensor exhibits excellent sensing performance: ultralow detection limit (0.166 pM) and wide linearity (5.0 pM-500.0 nM), simultaneously, it avoids electrodeposition processes and it is simple to modify the electrode compared to previous electrochemical methods for Pb²⁺ detection. Thus our method shows great potential in the highly efficient detection of Pb²⁺ and other heavy metal ions by simply altering the related specific aptamer.

Insight into the role of nitrogen in N-doped ordered mesoporous carbons for the spontaneous non-covalent attachment and electrografting of redox-active materials

The role of nitrogen functional groups in nitrogen-doped ordered mesoporous carbons (OMCs) toward the spontaneous non-covalent and electrografting was investigated using two home-made ionic liquid-derived ordered mesoporous carbons having different nitrogen concentrations (guanine-rich ionic liquid-derived ordered mesoporous carbon (GIOMC) and ionic liquid-derived ordered mesoporous carbon (IOMC)). The carbonaceous materials were fabricated by the carbonization of a mixture of ionic liquid (1-methyl-3-phenethyl-1H-imidazolium hydrogen sulfate) as a carbon source using SBA-15 as a hard template. Guanine was used during the carbonization of GIOMC as a nitrogen source. The electrode was modified with either GIOMC or IOMC followed by electrochemical surface functionalization with a few electro-active precursors as redox-active molecular models bearing different substituents and electronic properties. The high surface coverage of 5.6(±0.3) × 10^-9 mol cm^-2 for 4,4-biphenol was obtained for the GIOMC-modified electrode. We seek to explain whether the nitrogen content could indeed exert a dramatic impact on loading electroactive species on the electrode surface. The non-covalent anchoring studies indicated that at higher pH values the loading of electro-active moieties was significantly influenced by the content of nitrogen on the employed OMCs. The adsorption capacity (mg g^-1) of the OMCs was studied for catechol as a typical electro-active species in the range of 0.050-0.165 mg ml^-1. The adsorption capacity of 0.11 mg g^-1 catechol was 42(±4) and 26(±3) mg g^-1 for GIOMC and IOMC, respectively. In addition, our observations revealed that electro-grafting efficiency via diazonium ion was restricted by the protonation of nitrogen in the reaction media. Further, the fabricated redox-active/N-doped OMC electrodes showed sensitivity to pH, which was accompanied by either a Nernstian shift of the redox peak potentials (60(±3) mV per pH) in the pH range of 2-13 in the buffer solutions or variations of the redox peak currents (9.7(±0.3) μA per pH) in the pH range of 1-5.5 in the unbuffered situations. The resulting electrodes as voltammetric pH probes showed a simple response to pH in both buffer and unbuffered solutions. In addition, we introduced the fabricated electrode as a zero-gap generator/collector electrode system using a single electrode to recognize proton-dependent electron transfer from the proton-independent electrode process by detecting pH changes quite close to the surface of the electrode. The detailed descriptions are outlined.

"Blockchain-Like" MIL-101(Cr)/Carbon Black Electrodes for Unprecedented Defluorination by Capacitive Deionization

Metal-organic frameworks (MOF) have attracted extensive attention due to their ultra-high specific surface area and tunable structure, the mechanism of direct utilization for capacitive deionization (CDI) defluorination remains undefined. Here, MIL-101(Cr) with ultra-high specific surface area, high water stability, and open metal sites (OMSs) is prepared by a hydrothermal method for defluorination of CDI. Carbon black is used as a "chain" to connect F-stored in the holes of MIL-101(Cr) (Cr-MOF)as "blocks" to enhance the conductivity and ion storage capacity of MIL-101(Cr)/carbon black electrodes (Cr-MOF electrodes). This simple construction method avoids the process complexity of in situ synthesis and performs better. These easily constructed "blockchain-like" Cr-MOF electrodes exhibit excellent defluorination capacity (39.84 mg_NaF g_electrodes ^-1 ), low energy consumption (1.2 kWh kg_NaF ^-1 ), and good stability. The coupling of the electrochemical redox reaction of Cr³⁺ /Cr⁴⁺ with confined water is investigated using in situ and ex situ analysis methods combined with density functional theory (DFT), resulting in an unprecedented defluorination mechanism for Cr-MOF electrodes. This study opens up new ideas for the application of MOF in CDI, clarifies the removal mechanism of MOF, and lays a foundation for further promoting the application of raw materials with poor conductivity in the field of CDI.

Electrosorption performance on graphene-based materials: a review

Due to its unique advantages such as flexible planar structure, ultrahigh specific surface area, superior electrical conductivity and electrical double-layer capacitance in theory, graphene has unparalleled virtues compared with other carbon materials. This review summarizes the recent research progress of various graphene-based electrodes on ion electrosorption fields, especially for water desalination utilizing capacitive deionization (CDI) technology. We present the latest advances of graphene-based electrodes, such as 3D graphene, graphene/metal oxide (MO) composites, graphene/carbon composites, heteroatom-doped graphene and graphene/polymer composites. Furthermore, a brief outlook on the challenges and future possible developments in the electrosorption area are also addressed for researchers to design graphene-based electrodes towards practical application.

Uranium capture by a layered 2D/2D niobium phosphate/holey graphene architecture via an electro-adsorption and electrocatalytic reduction coupling process

As an energy-efficient and eco-friendly technique, capacitive deionization (CDI) has shown great potential for uranium (U(VI)) capture recently. However, extracting U(VI) with high kinetics, capacity and selectivity remains a major challenge due to the current surface active sites-based material and co-existing ions in aqueous solution. Here we rationally designed a layered 2D/2D niobium phosphate/holey graphene (HGNbP) electrode material, and originally demonstrated its efficient U(VI) capture ability via an electro-adsorption and electrocatalytic reduction coupling process. The less-accumulative loose layered architecture, open polycrystalline construction of niobium phosphate with active phosphate sites, and rich in-plane nano-pores on conductive graphene nanosheets endowed HGNbP with fast charge/ion transport, high electroconductivity and superior pseudocapacitance, which enabled U(VI) ions first to be electro-adsorbed, then physico-chemical adsorbed, and finally electrocatalysis reduced/deposited onto electrode surface without the limitation of active sites under a low potential of 1.2 V. Based on these virtues, the HGNbP exhibited a fast adsorption kinetics, with a high removal rate of 99.9% within 30 min in 50 mg L^-1 U(VI) solution, and a high adsorption capacity up to 1340 mg g^-1 in 1000 mg L^-1 U(VI) solution. Furthermore, the good recyclability and selectivity towards U(VI) were also realized.

Occurrence characteristics of uranium mineral-related substances in various environmental media in China: A critical review

The high demand and extensive exploitation of uranium resources resulted in the ubiquity and high detection levels of uranium mineral-related substances in various environment media in China. The potential adverse effects of uranium mineral-related substances on environment and human health have received extensive attention. Therefore, we reviewed the occurrence and spatial distribution of uranium mineral-related substances in various basins and environmental media in China to obtain an overall understanding. We collected information from over 70 papers reporting the occurrence and distribution of uranium mineral-related substances in multiple environments and 183 articles on the genesis of uranium deposits in China from 2001 to 2021. Then the occurrence of uranium mineral-related substances and corresponding correlation in different basins, environmental media and depth ranges were compared in detail. And this review assessed the uranium mineral-related pollution in China based on various environmental quality standards of China, EPA and WHO, and proposed the priority uranium mineral-related heavy metals and radioactive substances based on cluster analysis. This review showed that there were obvious differences in the occurrence characteristics of various uranium mineral-related substances in different environmental media, especially in the surrounding environment of sandstone type and hard rock type uranium deposits. These results will guide us to tackle the challenge of uranium mineral-related pollution in China. The correlation analysis of uranium mineral-related pollutants in different environmental media and the identification of priority pollutants will also provide instructions for us to control uranium mineral-related pollution. Finally, we put forward a series of urgent and practical suggestions on risk management and control of uranium mining according to the current situation of uranium mining environment in China, which is of guiding significance for the realization of "green uranium mining".

Ion Exchange Conversion of Na-Birnessite to Mg-Buserite for Enhanced and Preferential Cu2+ Removal via Hybrid Capacitive Deionization

Layered manganese oxides (LMOs) have recently been demonstrated to be one of the most promising redox-active material platforms for electrochemical removal of heavy metal ions from solution via capacitive deionization (CDI). However, the impact of interlayer spacing of LMOs on the deionization performance of electrodes in a hybrid capacitive deionization (HCDI) system with an LMO cathode and a carbon anode (i.e., LMO/C electrodes), and their phase transformation behaviors, particularly during the desalination operations, have yet to be extensively evaluated. In this study, we thoroughly evaluate Mg-buserite obtained by ion exchange of fresh Na-birnessite and Na- and K-birnessite as HCDI electrodes to remove copper ions (Cu²⁺) from saline solutions. Among the three LMO/C electrodes, the Mg-buserite/C (MgB/C) electrodes demonstrate the best deionization performance in terms of salt adsorption capacity (SAC), electrosorption rate, and cycling stability, followed by K-birnessite/C (KB/C) and Na-birnessite/C (NaB/C). More importantly, MgB/C exhibits the highest Cu²⁺ ion adsorption capacity (IAC) of 89.3 mg Cu²⁺ per gram electrode materials at a cell voltage of 1.2 V in 500 mg L^-1 CuCl₂ solution, with an IAC retention as high as 96.3% after 60 charge/discharge cycles. Given that electrosorption of Cu²⁺ ions is often competed by alkali and alkaline earth metal ions, our data reveal that the MgB/C electrodes demonstrate selectivities of 4.7, 7.7, and 8.1 for Cu²⁺ over Na⁺, Ca²⁺, and Mg²⁺, respectively. Moreover, X-ray diffraction and spectroscopic analyses show that the enhanced deionization performance and preference for Cu²⁺ are mainly attributed to the expanded interlayer spacing of LMO minerals. This study provides a promising strategy for tailoring LMO minerals for improving their electrosorption capacity and preference for copper ions from a multivalent-ion solution via an HCDI platform.

Electrospraying Zwitterionic Copolymers as an Effective Biofouling Control for Accurate and Continuous Monitoring of Wastewater Dynamics in a Real-Time and Long-Term Manner

Long-term continuous monitoring (LTCM) of water quality can provide high-fidelity datasets essential for executing swift control and enhancing system efficiency. One roadblock for LTCM using solid-state ion-selective electrode (S-ISE) sensors is biofouling on the sensor surface, which perturbs analyte mass transfer and deteriorates the sensor reading accuracy. This study advanced the anti-biofouling property of S-ISE sensors through precisely coating a self-assembled channel-type zwitterionic copolymer poly(trifluoroethyl methacrylate-random-sulfobetaine methacrylate) (PTFEMA-r-SBMA) on the sensor surface using electrospray. The PTFEMA-r-SBMA membrane exhibits exceptional permeability and selectivity to primary ions in water solutions. NH₄⁺ S-ISE sensors with this anti-fouling zwitterionic layer were examined in real wastewater for 55 days consecutively, exhibiting sensitivity close to the theoretical value (59.18 mV/dec) and long-term stability (error <4 mg/L). Furthermore, a denoising data processing algorithm (DDPA) was developed to further improve the sensor accuracy, reducing the S-ISE sensor error to only 1.2 mg/L after 50 days of real wastewater analysis. Based on the dynamic energy cost function and carbon footprint models, LTCM is expected to save 44.9% NH₄⁺ discharge, 12.8% energy consumption, and 26.7% greenhouse emission under normal operational conditions. This study unveils an innovative LTCM methodology by integrating advanced materials (anti-fouling layer coating) with sensor data processing (DDPA).

Heterointerface optimization in a covalent organic framework-on-MXene for high-performance capacitive deionization of oxygenated saline water

Capacitive deionization (CDI) provides a promising option for affordable freshwater while simultaneously storing energy, but its large-scale application is usually limited owing to the poor performance of conventional materials in natural (oxygenated) saline water. Herein, we report heterointerface optimization in a covalent organic framework (COF)-on-MXene heterostructure achieving a high CDI performance for desalination of oxygenated saline water. The 2D heterostructure with the optimal core-shell architecture inherits the high conductivity and reversible ion intercalation/deintercalation ability of MXene, and the hierarchical porous structure, large porosity, and extraordinary redox capacity of COFs. Thanks to the heterointerface optimization, the MXene@COF heterostructure exhibits a very stable cycling performance over 100 CDI cycles with a maximum NaCl adsorption capacity of 53.1 mg g^-1 in oxygenated saline water, among the state-of-the-art values for CDI electrodes and also exceeding those of most MXene-based or 2D materials. This study highlights the importance of heterointerface optimization in MXene-organic 2D heterostructures to promote CDI of natural (oxygenated) saline water.

Selective removal of Sr(II) from saliferous radioactive wastewater by capacitive deionization

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 (-SO₃^-) provided SPAC with remarkable selectivity of Sr²⁺ ion, and illustrated the ion-swapping mechanism of Sr²⁺ selectivity.

Selective Ion Removal by Capacitive Deionization (CDI)-Based Technologies

Severe freshwater shortages and global pollution make selective removal of target ions from solutions of great significance for water purification and resource recovery. Capacitive deionization (CDI) removes charged ions and molecules from water by applying a low applied electric field across the electrodes and has received much attention due to its lower energy consumption and sustainability. Its application field has been expanding in the past few years. In this paper, we report an overview of the current status of selective ion removal in CDI. This paper also discusses the prospects of selective CDI, including desalination, water softening, heavy metal removal and recovery, nutrient removal, and other common ion removal techniques. The insights from this review will inform the implementation of CDI technology.

Carbon Nanoarchitectonics with Bi Nanoparticle Encapsulation for Improved Electrochemical Deionization Performance

Electrochemical deionization (EDI) is hopefully the next generation of water treatment technology. Bismuth (Bi) is a promising anode material for EDI, due to its high capacity and selectivity toward Cl^-, but the large volume expansion and severe pulverization aggressively attenuated the EDI cycling performance of Bi electrodes. Herein, carbon-layer-encapsulated nano-Bi composites (Bi@C) were prepared by a simple pyrolysis method using a Bi-based metal-organic framework as a precursor. Bi nanoparticles are uniformly coated within the carbon layer, in which the Bi-O-C bond enhances the interaction between Bi and C. Such a structure effectively relieves the stress caused by volume expansion by the encapsulation effect of the carbon layer. Moreover, the introduction of a carbon skeleton provides a conductive network. As a consequence, the Bi@C composite delivered excellent electrochemical performance with a capacity of 537.6 F g^-1 at 1 mV s^-1. The Cl^- removal capacity was up to 133.5 mg g^-1 at 20 mA g^-1 in 500 mg L^-1 NaCl solution. After 100 cycles, the Bi@C electrode still maintains 71.8% of its initial capacity, which is much higher than the 26.3% of the pure Bi electrode. This study provides a promising strategy for improving EDI electrode materials.

Selective adsorption of phosphate by carboxyl-modified activated carbon electrodes for capacitive deionization

Capacitive deionization (CDI) has been considered as a promising technology for removing phosphate from water but suffer inferior selectivity and electrosorption performances for phosphate of current carbon electrodes in CDI. Herein, we achieved highly selective phosphate removal from a ternary effluent of Cl^-, PO₄^3-, and SO₄^2- by using nitric acid-treated activated carbon (AC) with various modification times and pure AC as the anode and cathode, a novel phosphate selective asymmetric CDI reactor. The results showed that carboxyl groups greatly grafted on the materials after modification (varying from 0.00084 to 0.0012 mol g^-1). The phosphate selectivity of the present research was higher than that of unmodified CDI, and it increased with the increase of carboxyl groups content. The highest phosphate selectivity (2.01) in modified materials is almost six times higher than that of pure AC. Moreover, the modified electrodes exhibited good regenerative ability with a phosphate desorption efficiency of around 72.12% during the adsorption/desorption process and great stability during the cycling experiment. These results demonstrated that the innovative application of nitric acid-modified AC can effectively selectively remove phosphate from mixed anion solution, opening a hopeful window to selective adsorption in water treatment by CDI.

Capacitive Removal of Fluoride Ions via Creating Multiple Capture Sites in a Modulatory Heterostructure

Fluoride pollution has become a major concern because of its adverse effects on human health. However, the removal capacity of defluorination agents in traditional methods is far from satisfactory. Herein, capacitive removal of F^- ions via creating multiple capture sites in a modulatory heterostructure has been originally demonstrated. The heterostructure of uniformly dispersed Al₂O₃ coating on hollow porous nitrogen-doped carbon frameworks was precisely synthesized by atomic layer deposition. An exceptional F^- ion removal efficiency at 1.2 V (95.8 and 92.9% in 5 and 10 mg/L F^- solutions, respectively) could be finally achieved, with a good regeneration ability after 20 consecutive defluorination cycles. Furthermore, we investigated the removal mechanisms of F^- ions by in situ Raman, in situ X-ray diffraction, and ex situ X-ray photoelectron spectroscopy measurements. The promotional removal capacity was realized by the multiple capture sites of the reversible conversion of Al-F species and the insertion of F^- ions into the carbon skeleton. This work offers an important new pathway and deep understanding for efficient removal of F^- ions from wastewater.

Hybrid capacitive deionization of NaCl and toxic heavy metal ions using faradic electrodes of silver nanospheres decorated pomegranate peel-derived activated carbon

Capacitive deionization (CDI) is an evolving technology for eradicating salt and toxic heavy metal ions from brackish wastewater. However, traditional CDI electrodes have lower salt adsorption capacity and inadequate adsorption of selective metal ions for long-term operations. Herein, Ag nanospheres incorporated pomegranate peel-derived activated carbon (Ag/P-AC) was prepared and implied to the CDI process for removing NaCl, toxic mono-, di-, and trivalent metal ions. Morphological analysis revealed that the 80-100 nm-sized Ag nanospheres were uniformly decorated on the surfaces of P-AC nanosheets. The Ag/P-AC has a higher specific surface area (640 m² g^-1), superior specific capacitance (180 F g^-1 at 50 mV s^-1) and a lower charge transfer resistance (0.5 Ω cm²). CDI device was fabricated by Ag/P-AC as an anode, which adsorbed anions and P-AC as cathode for adsorption of positively charged ions at 1.2 V in an initial salt concentration of 1000 mg L^-1. An asymmetric Ag/P-AC//P-AC exhibited a maximum NaCl adsorption capacity of 36 mg g^-1 than symmetric P-AC//P-AC electrodes (22.7 mg g^-1). Furthermore, Pb(II), Cd(II), F^-, and As(III) ions were successfully removed from simulated wastewater by using Ag/P-AC//P-AC based CDI system. These asymmetric CDI-electrodes have an excellent prospect for the removal of salt and toxic contaminants in industrial wastewater.

Selective Capacitive Removal of Heavy Metal Ions from Wastewater over Lewis Base Sites of S-Doped Fe-N-C Cathodes via an Electro-Adsorption Process

The pollution of toxic heavy metals is becoming an increasingly important issue in environmental remediation because these metals are harmful to the ecological environment and human health. Highly efficient selective removal of heavy metal ions is a huge challenge for wastewater purification. Here, highly efficient selective capacitive removal (SCR) of heavy metal ions from complex wastewater over Lewis base sites of S-doped Fe-N-C cathodes was originally performed via an electro-adsorption process. The SCR efficiency of heavy metal ions can reach 99% in a binary mixed solution [NaCl (100 ppm) and metal nitrate (10 ppm)]. Even the SCR efficiency of heavy metal ions in a mixed solution containing NaCl (100 ppm) and multicomponent metal nitrates (10 ppm for each) can approach 99%. Meanwhile, the electrode also demonstrated excellent cycle performance. It has been demonstrated that the doping of S can not only enhance the activity of Fe-N sites and improve the removal ability of heavy metal ions but also combine with heavy metal ions by forming covalent bonds of S^- clusters on Lewis bases. This work demonstrates a prospective way for the selective removal of heavy metal ions in wastewater.

Capacitive Removal of Heavy Metal Ions from Wastewater via an Electro-Adsorption and Electro-Reaction Coupling Process

Heavy metals widely exist in wastewater, which is a serious threat to human health or water environment. Highly efficient removal of heavy metal ions from wastewater is a major challenge to wastewater treatment. In this work, capacitive removal of heavy metal ions from wastewater via an electro-adsorption and electro-reaction coupling process was originally demonstrated. The removal efficiency of heavy metal ions in the binary-component solutions containing metal nitrate (10 mg/L) and NaCl (100 mg/L) can reach 99%. Even the removal efficiency of heavy metal ions can be close to 99% in the multi-component solution containing all the seven metal nitrates (10 mg/L for each) and 100 mg/L NaCl. Meanwhile, the electro-adsorption and electro-reaction coupling process maintained excellent regeneration ability even after 20 cycles. Furthermore, the heavy metal ions removal mechanism was proven to be the pseudocapacitive intercalation of heavy metal ions into the layered structure of the employed W₁₈O₄₉/graphene in the electro-adsorption and electro-reaction coupling process. This work demonstrates great potential for general applicability to wastewater treatment.