Regulation, quantification and application of the effect of functional groups on anion selectivity in capacitive deionization

High-Level Disordered Metal-Organic Frameworks Synthesized by Interference-Oriented Attachment for Electrochemical Anion Sieve

Disordered MOFs seamlessly amalgamate the robust stability and pore tunability inherent in crystalline MOFs with the advantages derived from abundant defects and active sites present in amorphous structures. This study pioneers the use of the interference-oriented attachment (IOA) mechanism to meticulously craft the morphology and crystal growth of MIL-101(Cr) (Cr-MOF), resulting in the successful synthesis of a high-level disordered Cr-MOF boasting an enhanced array of active sites and exceptional electrochemical properties. The correlation between disordered structures and the electrochemical properties of MOFs are elucidated using the lattice distortion index and fractal dimension. The high-level disordered MOF electrode showcases a remarkable fluoride sieving effect, outperforming conventional fluoride removal materials with a remarkable fluoride adsorption capacity of 41.04 mgNaF gelectrodes -1. First-principles calculations, in conjunction with relevant experiments, provided further validation that the disordered structure significantly enhances the defluorination performance of the material. This study introduces a novel approach for the direct bottom-up synthesis of high-level disordered MOFs, showcasing their potential for applications in electrochemical water treatment.

Enhanced Selective Electrosorption of Nitrate from Wastewater by Controllably Doping Nitrogen into Porous Carbon with Micropores

The biological NO3- removal process might be accompanied by high CO2 emissions and operation costs. Capacitive deionization (CDI) has been widely studied as a very efficient method to purify water. Here, a porous carbon material with a tunable nitrogen configuration was developed. Characterization and density functional theory calculation show that nitrogenous functional groups have a higher NO3- binding energy than Cl-, SO42-, and H2PO4-. In addition, the selectivity of NO3- is improved after the introduction of micropores by using the pore template. The NO3- ion removal and selectivity of MN-C-12 are 4.57 and 3.46-5.42 times that of activated carbon (AC), respectively. The high NO3- selectivity and electrosorption properties of MN-C-12 (the highest N content and micropore area) are due to the synergistic effect of the affinity of nitrogen functional groups to NO3- and microporous ion screening. A CDI unit for the removal of nitrogen from municipal wastewater was constructed and applied to treat wastewater meeting higher discharge standards of A (N: 15 mg L-1) and B (N: 20 mg L-1) ((GB18918-2002), China). This work provides new insights into enhanced carbon materials for the selective electrosorption of wastewater by CDI technology.

Advanced capacitive deionization for ion selective separation: Insights into mechanism over a functional classification

Due to the diversity and variability of harmful ions in polluted water bodies, the selective removal and separation for specific ions is of great significance in water purification and resource processes. Capacitive deionization (CDI), an emerging desalination technology, shows great potential to selectively remove harmful ionic pollutants and further recover valuable ions because of the simple operation and low energy consumption. Researchers have done a lot of work to investigate ion selectivity utilizing CDI, including both theoretical and experimental studies. Nevertheless, in the investigation of selective mechanisms, phenomena where carbon materials exhibit entirely opposite selectivity require further analysis. Furthermore, there is a need to summarize the specific chemical reaction mechanisms, including the formation of hydrogen bonds, complexation reactions, and ligand exchanges, within selective electrodes, which have not been thoroughly examined in detail previously. In order to fill these gaps, in this review, we summarized the recent progress of CDI technologies for ion selective separation, and explored the selective separation mechanism of CDI from three aspects: selective physical adsorption, specific chemical reaction, and the utilization of selective barriers. Additionally, this review analyzes in detail the formation process of chemical bonds and ion conversion pathways when ions interact with electrode materials. Finally, some significant development prospects and challenges were offered for the future selective CDI systems. We believe the review will provide new insights for researchers in the field of ion selective separation.

3D-Printed river-type thick carbon electrodes for docking possible practical application-level capacitive deionization

The low carbon mass loading along with serious imbalance between the carbon mass loading and the electrode performance greatly hinders practical applications of capacitive deionization (CDI). Traditional thick bulk-type (BT) carbon electrodes often suffer from extremely limited active sites, thereby being vital to explore a basic strategy to unlock the performance. Herein, 3D-printed thick carbon electrodes were utilized for CDI desalination for the first time. The experimental outcomes revealed that BT electrodes existed a serious salt adsorption capacity (SAC) drop under variable mass loading of 3-30 mg/cm2. In contrary, 3D-printed river-type (RT) electrodes acquired a superior SAC of 10.67 mg/g and achieved 54.1 % SAC rise compared with that of BT electrodes (500 mg/L; 1.0 V; 30 mg/cm2). Meanwhile, RT electrodes took only 12 min to reach the equilibrium SAC of BT electrodes, being 44 min faster. Further, RT electrodes with diverse mass loading of 30-45 mg/cm2 were investigated, and it still kept 7.13 mg/g SAC under ultrahigh mass loading of 45 mg/cm2. This strategy has been successfully extended and carbons with proper micro-meso pore distribution, high specific capacitances and low resistance may be a better selection. Besides, the impact of electrode channel structure on the desalting performance was investigated, and the influence mechanism was revealed via COMSOL simulation. Overall, this work demonstrates the splendid feasibility of utilizing 3D-printed thick carbon electrodes for possible practical application-level CDI desalination.

Emerging electro-driven technologies for phosphorus enrichment and recovery from wastewater: A review

The recovery of phosphorus from wastewater is a critical step in addressing the scarcity of phosphorus resources. Electro-driven technologies for phosphorus enrichment have gathered significant attention due to their inherent advantages, such as mild operating conditions, absence of secondary pollution, and potential integration with other technologies. This study presents a comprehensive review of recent advancements in the field of phosphorus enrichment, with a specific focus on capacitive deionization and electrodialysis technologies. It highlights the underlying principles and effectiveness of electro-driven techniques for phosphorus enrichment while systematically comparing energy consumption, enrichment rate, and concentration factor among different technologies. Furthermore, the study provides a thorough analysis of the capacity of various technologies to selectively enrich phosphorus and proposes several methods and strategies to enhance selectivity. These insights offer valuable guidance for advancing the future development of electrochemical techniques with enhanced efficiency and effectiveness in phosphorus enrichment from wastewater.

Amino acids functionalized vascular-like carbon fibers for efficient capacitive deionization

Porous carbons have attracted great attention in capacitive deionization (CDI), benefiting from their high surface areas and abundant adsorption sites. However, the sluggish adsorption rate and poor cycling stability of carbons are still concerns, which are caused by the insufficient ion-accessible networks and the side reactions (the co-ion repulsion and oxidative corrosion). Herein, inspired by the blood vessels in organisms, mesoporous hollow carbon fibers (HCF) were successfully synthesized via a template assisted coaxial electrospinning strategy. Subsequently, the surface charge of HCF was modified by various amino acids (arginine (HCF-Arg) and aspartic acid (HCF-Asp)). Combining structure design and surface modulation, these freestanding HCFs present enhanced desalination rate and stability, in which the hierarchal vasculature facilitates electron/ion transport, and the functionalized surface suppresses the side reactions. Impressively, when HCF-Asp and HCF-Arg serve as cathode and anode respectively, the asymmetric CDI device provides an excellent salt adsorption capacity of 45.6 mg g^-1, a fast salt adsorption rate of 14.0 mg g^-1 min^-1 and a superior cycling stability up to 80 cycles. In short, this work evidenced an integrated strategy to exploiting carbon materials with outstanding capacity and stability for high-performance capacitive deionization.

Nanoarchitectonics of heteroatom-doped hierarchical porous carbon derived from carboxymethyl cellulose carbon aerogel and metal-organic framework for capacitive deionization

Capacitive deionization (CDI) using porous materials offers a sustainable solution for providing affordable freshwater, but the low salt adsorption rate of benchmark carbon materials significantly limit the practical implementation. Herein, we utilized carboxymethyl cellulose sodium (CMC) as the carbon skeleton to produce a composite carbon aerogel loaded with ZIF-8 (ZIF-8/CMC-CA). The presence of ZIF-8 nanoparticles improved the pore structure of the material and provides a certain pseudo capacitance by introducing N. Compared with ZIF-8 derived carbons (ZIF-8-C), the CMC provided a good three-dimensional structure for the dispersion of ZIF-8 nanoparticles, reduced the agglomeration of particles. Furthermore, numerous carboxyl and hydroxyl groups on CMC enhanced the hydrophilicity of materials. Due to the interconnected structure, ZIF-8/CMC-CA exhibited excellent conductivity, a high specific surface area, and offered suitable channels for the rapid entry and exit of ions. In a three-electrode system, the total specific capacitance of the ZIF-8/CMC-CA electrode was 357.14 F g^-1. The adsorption rate of ZIF-8/CMC-CA was 2.02 mg g^-1 min^-1 in a 500 mg L^-1 NaCl solution. This study may provide new insight for modifying and fabricating electrode materials for practical CDI applications.