An insight into the improved capacitive deionization performance of activated carbon treated by sulfuric acid

The Cd immobilization mechanisms in paddy soil through ureolysis-based microbial induced carbonate precipitation: Emphasis on the coexisting cations and metatranscriptome analysis

Microbial induced carbonate precipitation (MICP) can immobilize metals and reduce their bioavailability. However, little is known about the immobilization mechanism of Cd in the presence of soil cations and the triggered gene expression and metabolic pathways in paddy soil. Thus, microcosmic experiments were conducted to study the fractionation transformation of Cd and metatranscriptome analysis. Results showed that bioavailable Cd decreased from 0.62 to 0.29 mg/kg after 330 d due to the MICP immobilization. This was ascribed to the increase in carbonate bound, Fe-Mn oxides bound, and residual Cd. The underlying immobilization mechanisms could be attributed to the formation of insoluble Cd-containing precipitates, the complexation and lattice substitution with carbonate and Fe, Mn and Al (hydr)oxides, and the adsorption on functional group on extracellular polymers of cell. During the MICP immobilization process, up-regulated differential expression urease genes were significantly enriched in the paddy soil, corresponding to the arginine biosynthesis, purine metabolism and atrazine degradation. The metabolic pathway of bacterial chemotaxis, flagellum assembly, and peptidoglycan biosynthesis and the expression of cadA gene related to Cd excretion enhanced Cd resistance of soil microbiome. Therefore, this study provided new insights into the immobilization mechanisms of Cd in paddy soils through ureolysis-based MICP process.

Kinetic-Thermodynamic Promotion Engineering toward High-Density Hierarchical and Zn-Doping Activity-Enhancing ZnNiO@CF for High-Capacity Desalination

Despite the promising potential of transition metal oxides (TMOs) as capacitive deionization (CDI) electrodes, the actual capacity of TMOs electrodes for sodium storage is significantly lower than the theoretical capacity, posing a major obstacle. Herein, we prepared the kinetically favorable ZnxNi1 - xO electrode in situ growth on carbon felt (ZnxNi1 - xO@CF) through constraining the rate of OH- generation in the hydrothermal method. ZnxNi1 - xO@CF exhibited a high-density hierarchical nanosheet structure with three-dimensional open pores, benefitting the ion transport/electron transfer. And tuning the moderate amount of redox-inert Zn-doping can enhance surface electroactive sites, actual activity of redox-active Ni species, and lower adsorption energy, promoting the adsorption kinetic and thermodynamic of the Zn0.2Ni0.8O@CF. Benefitting from the kinetic-thermodynamic facilitation mechanism, Zn0.2Ni0.8O@CF achieved ultrahigh desalination capacity (128.9 mgNaCl g-1), ultra-low energy consumption (0.164 kW h kgNaCl-1), high salt removal rate (1.21 mgNaCl g-1 min-1), and good cyclability. The thermodynamic facilitation and Na+ intercalation mechanism of Zn0.2Ni0.8O@CF are identified by the density functional theory calculations and electrochemical quartz crystal microbalance with dissipation monitoring, respectively. This research provides new insights into controlling electrochemically favorable morphology and demonstrates that Zn-doping, which is redox-inert, is essential for enhancing the electrochemical performance of CDI electrodes.

Development of nitric acid-modified activated carbon electrode for removal of Co2+/Mn2+/Ni2+ by electrosorption

In this paper, nitric acid-modified activated carbon was used as an electrode in the electrosorption process for the removal of Co²⁺, Mn²⁺, and Ni²⁺ from wastewater. The effects of applied voltage, initial pH, and coexisting ions on removal efficiency were investigated. The adsorption process was evaluated by adsorption isotherm models. The results indicated that the electrosorption process was consistent with the Langmuir model, proving that the electrosorption process was a monolayer adsorption process. The maximum adsorption capacities of Co²⁺, Mn²⁺, and Ni²⁺ were 131.58 mg/g, 102.04 mg/g, and 103.09 mg/g. Electrochemical tests revealed that the specific capacitance of AC-HNO₃ was 54.11 F/g when the scanning rate was 5 mV/s, while the specific capacitance of AC was 36.51 F/g. The Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirmed that the content of oxygen groups on the surface of activated carbon increased after modification, which provided more adsorption sites for electrosorption. When the selected concentration of HCl was used as the eluent, the elution efficiency of Co²⁺, Mn²⁺, and Ni²⁺ could reach 94.23%, 93.65%, and 90.61%. The removal efficiency could reach more than 95% after three cycles. The results of the study can be used as a reference significance for the removal of cobalt, manganese, and nickel ions from heavy metal wastewater by electrosorption.

Three-Dimensional Cobalt Hydroxide Hollow Cube/Vertical Nanosheets with High Desalination Capacity and Long-Term Performance Stability

Faradaic electrode materials have significantly improved the performance of membrane capacitive deionization, which offers an opportunity to produce freshwater from seawater or brackish water in an energy-efficient way. However, Faradaic materials hold the drawbacks of slow desalination rate due to the intrinsic low ion diffusion kinetics and inferior stability arising from the volume expansion during ion intercalation, impeding the engineering application of capacitive deionization. Herein, a pseudocapacitive material with hollow architecture was prepared via template-etching method, namely, cuboid cobalt hydroxide, with fast desalination rate (3.3 mg (NaCl)·g^-1 (h-Co(OH)₂)·min^-1 at 100 mA·g^-1) and outstanding stability (90% capacity retention after 100 cycles). The hollow structure enables swift ion transport inside the material and keeps the electrode intact by alleviating the stress induced from volume expansion during the ion capture process, which is corroborated well by in situ electrochemical dilatometry and finite element simulation. Additionally, benefiting from the elimination of unreacted bulk material and vertical cobalt hydroxide nanosheets on the exterior surface, the synthesized material provides a high desalination capacity (117 ± 6 mg (NaCl)·g^-1 (h-Co(OH)₂) at 30 mA·g^-1). This work provides a new strategy, constructing microscale hollow faradic configuration, to further boost the desalination performance of Faradaic materials.

A review of modification of carbon electrode material in capacitive deionization

Capacitive deionization (CDI) technology has attracted wide attention since its advent and is considered as one of the most promising technologies in the field of desalination and ion recycling. It is constructed with an electric field by applying a low voltage of direct-current to make ions migrate directionally in solution to achieve the purpose of ion separation and removal. The performance of CDI is heavily dependent on the electrode material. Carbon is widely used as CDI electrode material because of its lower price and better stability. To enhance the adsorption capacity, extensive research efforts have been made for the modification of carbon material. In this review, we enumerate and analyze four modification methods of carbon material including element doping, metal oxide modification, chemical treatment and surface coating. The influence of each modification method on CDI performance is concluded in the perspective mechanism and some constructive advice is put forward on how to effectively enhance the performance of CDI by the decoration of carbon materials.

Grafting the Charged Functional Groups on Carbon Nanotubes for Improving the Efficiency and Stability of Capacitive Deionization Process

In the capacitive deionization (CDI) process, the degradation of desalting performance is predominantly due to the co-ion expulsion effect and electrode oxidation. To overcome these complications, carbon nanotubes grafted with amine and sulfonic functional groups respectively were prepared and used as the CDI electrodes. The structural characterizations and performance tests confirmed that a uniform functional layer was formed on the surface of the modified electrodes and it enhanced the ion selectivity and wettability of the electrode surface. Moreover, the effects of the functional layer on the electrode stability were investigated by circulating CV tests and desalination tests. The positive shift value of the potential of zero charge (PZC) for the as-prepared electrodes was tested as a quantitative indication for their possible surface oxidation during cyclic tests. Analysis of the PZC variation and desalting performance demonstrated that the excellent desalting stability was achieved by the Cell N-S assembled with the ammoniated CNTs electrode as anode and sulfonated CNTs electrode as cathode. Because the functional layer could preserve the pores system on the modified electrodes and diminish the parasitic reactions that exacerbate the electrode oxidation. This work provides an effective strategy for promoting the electrode performance and prolonging the life of the electrode.

Chemical activation of biochar for energy and environmental applications: a comprehensive review

Biochar (BC) generated from thermal and hydrothermal cracking of biomass is a carbon-rich product with the microporous structure. The graphene-like structure of BC contains different chemical functional groups (e.g. phenolic, carboxylic, carbonylic, etc.), making it a very attractive tool for wastewater treatment, CO2 capture, toxic gas adsorption, soil amendment, supercapacitors, catalytic applications, etc. However, the carbonaceous and mineral structure of BC has a potential to accept more favorable functional groups and discard undesirable groups through different chemical processes. The current review aims at providing a comprehensive overview on different chemical modification mechanisms and exploring their effects on BC physicochemical properties, functionalities, and applications. To reach these objectives, the processes of oxidation (using either acidic or alkaline oxidizing agents), amination, sulfonation, metal oxide impregnation, and magnetization are investigated and compared. The nature of precursor materials, modification preparatory/conditions, and post-modification processes as the key factors which influence the final product properties are considered in detail; however, the focus is dedicated to the most common methods and those with technological importance.

High performance capacitive deionization using modified ZIF-8-derived, N-doped porous carbon with improved conductivity

Zeolitic imidazolate framework (ZIF) composite-derived carbon exhibiting large surface area and high micropore volume is demonstrated to be a promising electrode material for the capacitive deionization (CDI) application. However, some inherent serious issues (e.g., low electrical conductivity, narrow pore size, relatively low pore volume, etc.) are still observed for nitrogen-doped porous carbon particles, which restrict their CDI performance. To solve the above-mentioned problems, herein, we prepared gold-nanoparticle-embedded ZIF-8-derived nitrogen-doped carbon calcined at 800 °C (Au@NC800) and PEDOT doped-NC-800 (NC800-PEDOT). The newly generated NC800-PEDOT and Au@NC800 electrodes exhibited notably increased conductivity, and they also achieved high electrosorption capacities of 16.18 mg g-1 and 14.31 mg g-1, respectively, which were much higher than that of NC800 (8.36 mg g-1). Au@NC800 and NC800-PEDOT can be promisingly applicable as highly efficient CDI electrode materials.

Facile synthesis of TiO2/ZrO2 nanofibers/nitrogen co-doped activated carbon to enhance the desalination and bacterial inactivation via capacitive deionization

Capacitive deionization, as a second generation electrosorption technique to obtain water, is one of the most promising water desalination technologies. Yet; in order to achieve high CDI performance, a well-designed structure of the electrode materials is needed, and is in high demand. Here, a novel composite nitrogen-TiO₂/ZrO₂ nanofibers incorporated activated carbon (NACTZ) is synthesized for the first time with enhanced desalination efficiency as well as disinfection performance towards brackish water. Nitrogen and TiO₂/ZrO₂ nanofibers are used as the support of activated carbon to improve its low capacitance and hydrophobicity, which had dramatically limited its adequacy during the CDI process. Importantly, the as-fabricated NACTZ nanocomposite demonstrates enhanced electrochemical performance with significant specific capacitance of 691.78 F g⁻¹, low internal resistance and good cycling stability. In addition, it offers a high capacitive deionization performance of NACTZ yield with electrosorptive capacity of 3.98 mg g⁻¹, and, good antibacterial effects as well. This work will provide an effective solution for developing highly performance and low-cost design for CDI electrode materials.

Polymer Dehalogenation-Enabled Fast Fabrication of N,S-Codoped Carbon Materials for Superior Supercapacitor and Deionization Applications

Doped carbon materials (DCM) with multiple heteroatoms hold broad interest in electrochemical catalysis and energy storage but require several steps to fabricate, which greatly hinder their practical applications. In this study, a facile strategy is developed to enable the fast fabrication of multiply doped carbon materials via room-temperature dehalogenation of polyvinyl dichloride (PVDC) promoted by KOH with the presence of different organic dopants. A N,S-codoped carbon material (NS-DCM) is demonstratively synthesized using two dopants (dimethylformamide for N doping and dimethyl sulfoxide for S doping). Afterward, the precursive room-temperature NS-DCM with intentionally overdosed KOH is submitted to inert annealing to obtain large specific surface area and high conductivity. Remarkably, NS-DCM annealed at 600 °C (named as 600-NS-DCM), with 3.0 atom % N and 2.4 atom % S, exhibits a very high specific capacitance of 427 F g^-1 at 1.0 A g^-1 in acidic electrolyte and also keeps ∼60% of capacitance at ultrahigh current density of 100.0 A g^-1. Furthermore, capacitive deionization (CDI) measurements reveal that 600-NS-DCM possesses a large desalination capacity of 32.3 mg g^-1 (40.0 mg L^-1 NaCl) and very good cycling stability. Our strategy of fabricating multiply doped carbon materials can be potentially extended to the synthesis of carbon materials with various combinations of heteroatom doping for broad electrochemical applications.