Advanced redox‐polymer materials offer a powerful platform for integrating electroseparations and electrocatalysis, especially for water purification and environmental remediation applications. The ...selective capture and remediation of trivalent arsenic (As(III)) is a central challenge for water purification due to its high toxicity and difficulty to remove at ultra‐dilute concentrations. Current methods present low ion selectivity, and require multistep processes to transform arsenic to the less harmful As(V) state. The tandem selective capture and conversion of As(III) to As(V) is achieved using an asymmetric design of two redox‐active polymers, poly(vinyl)ferrocene (PVF) and poly‐TEMPO‐methacrylate (PTMA). During capture, PVF selectively removes As(III) with exceptional uptake (>100 mg As/g adsorbent), and during release, synergistic electrocatalytic oxidation of As(III) to As(V) with >90% efficiency can be achieved by PTMA, a radical‐based redox polymer. The system demonstrates >90% removal efficiencies with real wastewater and concentrations of arsenic as low as 10 ppb. By integrating electron‐transfer through the judicious design of asymmetric redox‐materials, an order‐of‐magnitude energy efficiency increase can be achieved compared to non‐faradaic, carbon‐based materials. The study demonstrates for the first time the effectiveness of asymmetric redox‐active polymers for integrated reactive separations and electrochemically mediated process intensification for environmental remediation.
Redox‐active polymer materials are exploited for the reactive separation of arsenic oxyanions. The molecular selectivity of a metallopolymer is combined with the electrocatalytic properties of a radical‐based organic electrode to achieve exceptional separation factors and redox‐mediated transformation. This work paves the way for advanced redox‐materials to be used in synergistic electrochemical processes for water purification, chemical and environmental process intensification, and electrocatalysis.
•Enhancement of arsenic removal was achieved by capacitive deionization.•Electrosorption of As(V) is ascribed to electrical double-layer charging.•Removal of As(III) may involve its oxidation to ...As(V) on the anode electrode.•The presence of NaCl or natural organic matter reduces the degree of arsenic removal.
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 (0.0–1.2V) and initial concentration (0.1–200mgL−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 0.2mgL−1 on the activated carbon electrode at 1.2V was determined to be 2.47×10−2mgg−1, which is 1.8-fold higher than that of As(III) (1.37×10−2mgg−1). Notably, the possible transformation of arsenic species was further characterized. The higher effectiveness of As(V) removal via electrosorption at 1.2V 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 1.2V further demonstrated that capacitive deionization is a potential means of effectively removing arsenic from aqueous solutions.
•A hybrid CDI with NiHCF and AC electrodes was utilized for selective NH4+ removal.•The optimized operating voltage of the NiHCF electrode was discovered.•A high intercalation selectivity of NH4+ ...over cations was observed.•Selective capture of NH4+ from WWTP effluents was demonstrated.
Currently, intercalation materials such as Prussian blue analogs have attracted considerable attention in water treatment applications due to their excellent size-based selectivity toward cations. This study aimed to explore the feasibility of using a nickel hexacyanoferrate (NiHCF) electrode for selective NH4+ capture from effluent from a municipal wastewater treatment plant. To assess the competitive intercalation between NH4+ and other common cations (Na+, Ca2+), a NiHCF//activated carbon (AC) hybrid capacitive deionization (CDI) cell was established to treat mixed-salt solutions. The results of cyclic voltammetry (CV) analysis showed a higher current response of the NiHCF electrode toward NH4+ ions than toward Na+ and Ca2+ ions. In a single-salt solution with NH4+, the optimized operating voltage of the hybrid CDI cell was 0.8 V, with a higher salt adsorption capacity (51.2 mg/g) than those obtained at other voltages (0.1, 0.4, 1.2 V). In a multisalt solution containing NH4+, Na+, and Ca2+ ions, the selectivity coefficients of NH4+/Ca2+ and NH4+/Na+ were 9.5 and 4.9, respectively. The feasibility of selective NH4+ capture using the NiHCF electrode in a hybrid CDI cell was demonstrated by treating the effluent from a municipal wastewater treatment plant (WWTP). The intercalation preference of the NiHCF electrode with the WWTP effluent was NH4+>K+>Na+>Ca2+>Mg2+, and NH4+ showed the highest salt adsorption capacity among the cations during consecutive cycles. Our results revealed that cations with smaller hydrated radii and lower (de)hydration energies were more favorably intercalated by the NiHCF electrode. The results provide important knowledge regarding the use of intercalation-type electrodes for selective nutrient removal and recovery from wastewater.
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•Electrosorption performance of activated carbons was examined for the desalination.•The effect of pore structure on capacitive characteristics was investigated.•High specific ...capacitance suggested high salt adsorption capacity.•CDI Ragone provided comprehensive view on desalination capacity and rate.•Single-pass mode CDI was desirable to estimate desalination performance.
In the present work, the electrosorption performance of activated carbon-base capacitive deionization (CDI) was evaluated by batch and single-pass mode experiments. Two commercial activated carbon electrodes with raw materials of coal (AC1) and wood (AC2) were selected and their physical and electrochemical characteristics were determined by Brunauer–Emmett–Teller (BET) and cyclic voltammetry experiments. The results indicated that micropores and mesopores are well-balanced in both AC1 and AC2. The specific surface of AC1 (940 m2/g) was higher than that of AC2 (662 m2/g), and the AC1 showed a higher specific capacitance from cyclic voltammetry curves. CDI characteristics could be determined from the batch-mode and single-pass experiment, both AC1 and AC2 showed good electrosorption performance. The batch-mode, however, can only serve as evaluation of basic CDI performance. As evidenced in single-pass experiments with 2 mM NaCl at 1.0 V, AC1 exhibited a higher salt adsorption capacity of 5.08 mg/g-carbon than that of AC2 (2.39 mg/g-carbon). The CDI Ragone plot from single-pass experiments was further used as a functional tool to provide the key performance indicators: salt adsorption capacity and mean deionization rate. In the CDI Ragone plot, AC1 appeared towards the top and right side, showing that AC1 was more well-developed than AC2. In addition, the impact of pore structure on electrosorption performance was clearly observed. Overall, this study can provide a fundamental basis for understanding the estimation of CDI performance.
The electrosorption selectivity of nitrate (NO3−) over chloride (Cl−) was investigated in capacitive deionization (CDI) and membrane capacitive deionization (MCDI). In CDI, the selectivity depended ...on several key operational factors, including the charging time, initial NO3− and Cl− concentration ratio (CNO3−/CCl−) and applied voltage. The NO3− selectivity increased with prolonged charging time and in proportion to the initial CNO3−/CCl−, suggesting that the differences in ion-carbon affinity results in the preferential electrosorption of NO3−. Increasing the applied voltage decreased the NO3− selectivity, revealing that the electrical force kinetically controlled the competitive electrosorption of anions. Therefore, our results indicate that the electrosorption selectivity for NO3− ions in CDI was determined by the contributions of ion-carbon affinity and electrical force during the charging period. In comparison, the NO3− selectivity was found to be significantly reduced in MCDI due to the presence of an ion-exchange membrane controlling ion kinetics on the basis of charge rather than affinity. The electrosorption selectivity of NO3− over Cl− in CDI was 2.44, which was approximately 1.9-fold higher than that in MCDI (1.28). These results provide a practical understanding of the NO3− selectivity in the studied electrosorption processes.
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•Preferential electrosorption of NO3− over Cl− is discovered in both CDI and MCDI.•The ion-carbon affinity effect determines NO3− selectivity in CDI.•The ionic charge promotion effect governs NO3− selectivity in MCDI.•A high initial concentration ratio of NO3− promotes preferential electrosorption.•Increasing the applied voltage reduces NO3− selectivity.
Hierarchical porous carbon (HPC) has attracted increasing research interest for energy and environmental applications. HPC is conventionally fabricated by activated carbon, which potentially causes ...hidden environmental burdens. To overcome this issue, biochar, a promising renewable precursor, offers an attractive raw material substitute and has already been explored for the preparation of low-cost HPC. Recent studies have demonstrated that HPC exhibited great applications in capacitive energy storage, owning to its easily tuned physicochemical and electrochemical properties. Besides, biochar-based HPC with a three-dimensional (3D) interconnected controllable pore structure, high specific surface area (SSA), and pore volume (PV) can provide smaller resistance and shorter diffusion pathways for the transport of ions. Importantly, most recent research efforts have been made on the synthesis of biochar-based engineered hierarchical porous carbons (EHPCs) from biomass/biochar or developed from the HPC. A templating technique, heteroatom, and metal oxides doping have been applied to develop the biochar-based EHPC to improve 3D pore structure or/and expose abundant active sites and subsequently enhance the capacitive charge storage performance. In this review, recent advances in the applications of biochar-based HPC or EHPC for capacitive charge storage, e.g., capacitive deionization (CDI) and a supercapacitor (SC) are summarized and discussed. This review concludes with several perspectives to provide possible future research directions for the preparation and applications of biochar-based EHPC for capacitive charge storage.
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•Biochar-based HPC is an efficient sustainable material for capacitive charge storage.•Biochar-based HPC contains a 3D pore structure and abundant functional groups.•Engineered HPC can be obtained with controlled porosity and surface modification.•Electrode architecture design can ensure the fast transport of ions.•Heteroatom and electroactive particle doping can enhance charge storage capability.
•Engineered biochar was decorated with MnO2 as a composite material.•As(III) can be oxidized to As(V) and subsequently adsorbed by MBC.•pH has a strong effect on arsenic removal due to electrostatic ...repulsion.•Redox transformation plays a crucial role in enhancing the removal of As(III).
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In the present work, an active MnO2/rice husk biochar (BC) composite (MBC) was prepared to enhance As(III) removal for groundwater remediation. The MBC material obtained an improved porous structure (i.e., specific surface area, pore volume and mesoporosity) with MnO2, providing abundant reaction or interaction sites for surface or interface-related processes such as redox transformation and adsorption of arsenic. As a result, a significant enhancement in arsenic removal can be achieved by using MBC. More specifically, MBC showed a high removal capacity for As(III), which was tenfold higher than that of BC. This improvement can be ascribed to the redox transformation of As(III) via MnO2, resulting in the more effective removal of As(V) species. In addition, pH was an important factor that could influence the As(III) removal capacity. Under alkaline conditions, the As(III, V) removal capacity of MBC was clearly lower than those under acidic and neutral conditions due to the negative effects of electrostatic repulsion. Importantly, a powerful transformation capability of As(III) via MBC was presented; namely, only 5.9% As(III) remained in solution under neutral conditions. Both MnO2 and the BC substrate contributed to the removal of arsenic by MBC. MnO2 delivered Mn-OH functional groups to generate surface complexes with As(V) produced by As(III) oxidation, while the reduced Mn(II) and As(V) could precipitate on the MBC surface. The BC substrate also provided COOH and OH functional groups for As(III, V) removal by a surface complexation mechanism. Note that the application of MBC in the treatment of simulated groundwater demonstrated an efficient arsenic removal of 94.6% and a concentration of arsenic as low as the 10 µg L–1 WHO guideline.
Oxygen- and nitrogen-doped porous oxidized biochar (O,N-doped OBC) was fabricated in this study. Biochar (BC) can be enriched in surface functional groups (O and N) and the porosity can be improved ...by a simple, convenient and green procedure. BC was oxidized at 200 °C in an air atmosphere with quality control via oxidation time changes. As the oxidation time increased, the O and N contents and porosity of the materials improved. After 1.5 h of oxidation, the O and N contents of O,N-doped OBC-1.5 were 54.4% and 3.9%, higher than those of BC, which were 33.4% and 1.8%, respectively. The specific surface area and pore volume of O,N-doped OBC-1.5 were 88.5 m2 g−1 and 0.07 cm3 g−1, respectively, which were greater than those of BC. The improved surface functionality and porosity resulted in an increased heavy metal removal efficiency. As a result, the maximum adsorption capacity of Cu(II) by O,N-doped OBC was 23.32 mg L−1, which was twofold higher than that of pristine BC. Additionally, for a multiple ion solution, O,N-doped OBC-1.5 showed a greater adsorption behavior toward Cu(II) than Zn(II) and Ni(II). In a batch experiment, the concentration of Cu(II) decreased 92.3% after 90 min. In a filtration experiment, the O,N-doped OBC-based filter achieved a Cu(II) removal capacity of 12.90 mg g−1 and breakthrough time after 250 min. Importantly, the chemical mechanism was mainly governed by monolayer adsorption of Cu(II) onto a homogeneous surface of O,N-doped OBC-1.5. Surface complexation and electrostatic attraction were considered to be the chemical mechanisms governing the adsorption process.
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•O, N-doped OBC was fabricated by a simple air oxidation process.•Air can provide natural and green O and N sources for the doping process.•Air oxidation also improved the porous structure of O, N-doped OBC.•O, N doping played an important role in improved heavy metal removal efficiency.
The detection of ultratrace analytes is highly desirable for the non‐invasive monitoring of human diseases. However, a major challenge is fast, naked‐eye, high‐resolution ultratrace detection. ...Herein, a rectangular 3D composite photonic crystal (PC)‐based optoelectronic device is first designed that combines the sensitivity‐enhancing effects of PCs and optoelectronic devices with fast and real‐time digital monitoring. A crack‐free, centimeter‐scale, mechanically robust ellipsoidal composite PCs with sufficient hardness and modulus, even exceeding most plastics and aluminum alloys, are developed. The high mechanical strength of ellipsoidal composite PCs allows them to be hand‐machined into rectangular geometries that can be conformally covered with the centimeter‐scale flat light‐detection area without interference from ambient light, easily integrating 3D composite PC‐based optoelectronic devices. The PC‐based device's signal‐to‐noise ratio increases dramatically from original 30–40 to ≈60–70 dB. Droplets of ultratrace analytes on the device are identified by fast digital readout within seconds, with detection limits down to 5 µL, enabling rapid identification of ultratrace glucose in artificial sweat and diabetes risk. The developed 3D PC‐based sensor offers the advantages of small size, low cost, and high reliability, paving the way for wider implementation in other portable optoelectronic devices.
A photonic crystal‐based optoelectronic device is developed for rapid and real‐time digital monitoring of ultratrace analytes with detection limits down to 5 µL, which is highly desirable for the non‐invasive monitoring of human diseases, such as the quick identification of diabetes from ultratrace glucose in sweat.
Microbial desalination cells (MDCs) are promising bioelectrochemical systems that are being investigated for simultaneous seawater desalination, electricity generation, and wastewater treatment. ...Anode materials play an important role in determining the performance of MDCs. In this study, a three-dimensional (3D) macroporous sponge was coated with compatible and conductive carbon nanotube-chitosan (CNT-CS) as a composite electrode for MDCs. Experimental results showed that the flexible CNT-CS sponge exhibited a high capacitance (159.4F/g at 20mVs−1), good cycling stability (96% specific capacitance retention after 1000 cyclic voltammetry cycles) and low resistance. Moreover, the MDC with a CNT-CS sponge anode generated a high power density of 1776.6mW/m2 (per electrode area) and desalination rate of 16.5mgh−1, which are significantly higher than those of commercial carbon felt electrodes under the same conditions. The improved MDC performance can be attributed to the continuous 3D macroporous structure of the sponge anode promoting the bacterial loading capacity on the electrode surface. Moreover, the presence of CNTs also further enhances extracellular electron transfer. Our results demonstrate that an MDC operating with a 3D CNT-CS sponge anode offers an effective means for manufacturing high-performance MDCs with wide applicability to bioelectrochemical systems.
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•A 3D macroporous electrode was developed by coating a CNT-CS composite on a sponge.•The CNT-CS sponge exhibited great cycle stability and electrochemical performance.•Water desalination and electricity generation in the MDC were improved.•The 3D macroporous architecture provides sufficient space for bacterial growth.•The presence of CNTs is beneficial for charge transfer and electricity generation.
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