Effective fluoride removal from brackish groundwaters by flow-electrode capacitive deionization (FCDI) under a continuous-flow mode

Single modular flow-electrode capacitive deionization using modified Prussian blue analogues as cation intercalation electrode for continuous water desalination

Ion back-diffusion hinders the practical application of conventional flow-electrode capacitive deionization (FCDI) under long-term operational conditions. To address this challenge, the present study integrated cation intercalation deionization (CID) with FCDI. A novel PFCDI-CID system was developed by utilizing a modified Prussian blue analogues owing to their enhanced rheological and electrochemical properties. The PFCDI-CID system achieved a high charge efficiency of 89.77% and an energy-normalized removal salt of 0.69 mol kJ-1 in single-cycle (SC) mode with the flow electrodes mass fraction of 2% and a desalinized water chamber-to-concentrated saline water chamber ratio of 2:1. Furthermore, under continuous operation for 12 h in SC mode, the PFCDI-CID system maintained stable desalination performance within the first 2 h. Over an extended duration, the average charge efficiency of the PFCDI-CID system was maintained at 88.44%, with an average energy-normalized removal salt of 0.65 mol kJ-1. The mechanism revealed that the desalination process involving the Prussian blue analogues primarily involves Na+ intercalation, accompanied by a small amount of electro-sorption process. This system exhibits the characteristics of conventional FCDI while enabling desalination and concentration of simulated saline water during brine discharge, thereby mitigating the impact of ion back-diffusion and broadening the application scope of FCDI.

Experimental and DFT calculation study on the efficient removal of high fluoride wastewater from metallurgical wastewater by kaolinite

Fluoride pollution and water scarcity are urgent issues. Reducing fluoride concentration in water is crucial. Kaolinite has been used to study adsorption and fluoride removal in water and to characterize material properties. The experimental results showed that the adsorption capacity of kaolinite decreased with increasing pH. The highest adsorption of fluoride occurred at pH 2, with a capacity of 11.1 mg/g. The fluoride removal efficiency remained high after four regeneration cycles. The fitting results with the Freundlich isotherm model and the external diffusion model showed that the non-homogeneous adsorption of kaolinite fit the adsorption behavior better. Finally, the adsorption mechanism was analyzed by FT-IR and XPS. The binding energies of various adsorption sites and the chemical adsorption properties of atomic states were discussed in relation to DFT calculations. The results showed that Al and H sites were the main binding sites, and the bonding stability for different forms of fluoride varies, with the size of Al-F (-7.498 eV) > H-F (-6.04 eV) > H-HF (-3.439 eV) > Al-HF (-3.283 eV). Furthermore, the density of states and Mulliken charge distribution revealed that the 2p orbital of F was found to be active in the adsorption process and was the main orbital for charge transfer.

Hierarchically porous biochar derived from aerobic granular sludge for high-performance membrane capacitive deionization

Membrane capacitive deionization (MCDI) is a cost-effective desalination technique known for its low energy consumption. The performance of MCDI cells relies on the properties of electrode materials. Activated carbon is the most widely used electrode material. However, the capacitive carbon available on the market is often expensive. Here, we developed hierarchically porous biochar by combining carbonization and activation processes, using easily acquired aerobic granular sludge (AGS) from biological sewage treatment plants as a precursor. The biochar had a specific surface area of 1822.07 m2 g-1, with a micropore area ratio of 58.65% and a micropore volume of 0.576 cm3 g-1. The MCDI cell employing the biochar as electrodes demonstrated a specific adsorption capacity of 34.35 mg g-1, comparable to commercially available activated carbon electrodes. Our study presents a green and sustainable approach for preparing highly efficient, hierarchically porous biochar from AGS, offering great potential for enhanced performance in MCDI applications.

The function of doping nitrogen on removing fluoride with decomposing La-MOF-NH2: Density functional theory calculation and experiments

Fluoride is an important pollutant in wastewater, and adsorption is an effective way to remove fluoride. Because nitrogen plays an important role in adsorbent materials, computational models were developed to understand the changes in work function resulting from nitrogen doping. La-N-C-800°C, was prepared by pyrolyzing La-MOF-NH2 to verify the influence on the performance of removing fluoride by electrosorption. Material and electrochemical performance tests were performed to characterize La-N-C-800°C. Adsorption kinetics, adsorption thermodynamics, initial concentrations, pH, and ions competition were investigated using La-N-C-800°C for fluoride removal. In addition, density functional theory was applied to evaluate the function of nitrogen. When nitrogen atoms were added, the density of states, partial density of states, populations, and different orbits of charge were calculated to discover deep changes. Nitrogen strengthened the carbon structure and La2O3 structure to remove fluoride. In addition, nitrogen can also act as an adsorption site in the carbon structure. These results provide design ideas for improving the performance of adsorbent materials by doping elements.

Separation of nutrients and acetate from sewage sludge fermentation liquid in flow-electrode capacitive deionization system: Competitive mechanisms of ions and influence of activated carbon

Effective separation of volatile fatty acids (VFAs), ammonia (NH4+-N) and reactive phosphorous (RP) generated from anaerobic fermentation liquid is critically important for efficient resource recovery. Flow-electrode capacitive deionization (FCDI) is proven to be capable of efficient removal of ions, environmentally friendly and cost-effective in operation. The performances of FCDI system in the separation of NH4+-N, RP, and acetate and mechanism of pHs and activated carbon on their performances were investigated. Results showed that a pH of 5.0 promoted the removal of NH4+-N (53.1 %) and RP (39.5 %), and 72.0 % of acetate was retained in the solution, which revealed that removal of NH4+-N and RP, and retention of acetate were evidently affected by speciation of ions. Furthermore, the recovery of NH4+-N and RP was undermined by the adsorption of ions on activated carbon. This study provides a novel insight of ion selective mechanism during the operation of the FCDI system.

Simultaneous desalination and molecular resource recovery from wastewater using an electrical separation system integrated with a supporting liquid membrane

Separating molecular substances from wastewater has always been a challenge in wastewater treatment. In this study, we propose a new strategy for simultaneous desalination and selective recovery of molecular resources, by introducing a supported liquid membrane (SLM) with molecular selectivity into an asymmetric flow-electrode capacitive deionization. Salts and molecular substances in wastewater are removed after passing through the ion separation chamber and the molecular separation chamber, respectively. Faradaic reactions, i.e., the electrolysis of water with OH-, occurred in the electrochemical cathode electrode provides a sufficient and continuous chemical potential gradient for the cross-SLM transport of phenol (a model molecule substance). By optimizing the formulation of the liquid membrane and the pore size of the support membrane, we obtained the SLM with the best performance for separating phenol. In continuous experiment tests, the electrochemical membrane system showed stable separation performance and long-term stability for simultaneous salts removal and phenol (sodium phenol) recovery from wastewater. Finally, we demonstrate the potential application of this technology for the recovery of different carbon resources. Overall, the electrochemical system based on SLM is suitable for various wastewater treatment needs and provides a new approach for the recovery of molecular resources in wastewater.

Electron Transfer of Activated Carbon to Anode Excites and Regulates Desalination in Flow Electrode Capacitive Deionization

The desalination performance of flow electrode capacitive deionization (FCDI) is determined by the ion adsorption on the powdered activated carbon (PAC) and the electron transfer between the current collector and PAC. However, a comprehensive understanding of rate-limiting steps is lacking, let alone to enhance FCDI desalination by regulating the PAC characteristics. This study showed that the electron transfer between PAC and the current collector on the anode side was the rate-limiting step of FCDI desalination. Compared with W900, the desalination performance of FCDI decreased by 95% when W1200 with weak electron transfer ability was used as a flow electrode. The PAC selected in this study transferred electrons directly through the conductive carbon matrix in FCDI and was mainly affected by graphitization. The desalination performance of FCDI was improved by 20 times when the graphitization degree of PAC increased from 0.69 to 1.03. The minimum energy required for electrons to escape from the PAC surface was reduced by the high degree of graphitization, from 4.27 to 3.52 eV, thus improving the electron transfer capacity of PAC on the anode side. This study provides a direction for the optimization of flow electrodes and further promotes the development of FCDI.

Preparation of MOF/polypyrrole and flower-like MnO2 electrodes by electrodeposition: High-performance materials for hybrid capacitive deionization defluorination

Fluorine pollution has become a global public health problem due to its adverse health effects. Adsorption is the primary method for removing fluoride from drinking water. However, the adsorption method has disadvantages such as difficulty in recovering the adsorbent, and the need to add additional chemicals for regeneration, thereby causing secondary pollution, which limits further industrial applications. Capacitive deionization (CDI), as an emerging water treatment technology, has attracted widespread attention due to its advantages of simple operation, low energy consumption and less environmental impact. In this study, a polypyrrole (PPy) film was prepared on a graphite substrate by electrodeposition, and then metal-organic framework Ce/Zn-BDC-NH₂ (CZBN) was deposited on the PPy film by electrophoretic deposition to obtain CZBN/PPy electrode was obtained. The CZBN/PPy anode was then coupled with the MnO₂ cathode for capacitive removal of fluoride in a CDI cell. Both CZBN/PPy and MnO₂ electrodes exhibit pseudocapacitive behavior, which can selectively and reversibly intercalate F^- (CZBN/PPy) and Na⁺ (MnO₂) ions. As expected, the CZBN/PPy-MnO₂ system exhibits excellent fluorine removal performance. In 1.2 V, 100 mg/L F^- solution, the F^- removal capacity can reach 55.12 mg/g. It has high F^- selectivity in the presence of some common anions, and can maintain high F^- removal ability even after five adsorption regeneration processes. The mechanism of F^- removal was studied by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). F^- was mainly removed by electrostatic interaction and ion exchange with hydroxyl. The excellent defluorination performance of the CZBN/PPy-MnO₂ system makes it have good practical application prospects.

Carbon Material-Based Flow-Electrode Capacitive Deionization for Continuous Water Desalination

Flow-electrode capacitive deionization (FCDI) offers an electrochemical, energy-efficient technique for water desalination. In this work, we report the study of carbon-based FCDI, which consists of one desalination chamber and one salination chamber and applies a carbon nanomaterials-based flow electrode that circulates between the cell anode and cathode, to achieve a fast, continuous desalination process. Five different carbon nanomaterials were used for preparing the flow electrode and were studied for the desalination performance, with properties including average salt removal rate (ASRR), salt removal efficiency (SRE), energy consumption (EC) and charge efficiency (CE) being quantitatively determined for comparation. Different FCDI parameters, including carbon concentration and flow rate of the flow electrode and cell voltage, were investigated to examine the influences on the desalination. Long-term operation of the carbon-based FCDI was evaluated using the optimal results found in the conditions of 1.5 M concentration, 1.5 V cell voltage, and 20 mL min−1 flow rate of electrode and water streams. The results showed an ASRR of 63.7 µg cm−2 min−1, EC of 162 kJ mol−1, and CE of 89.3%. The research findings validate a good efficiency of this new carbon-based FCDI technology in continuous water desalination and suggest its good potential for real, long-term application.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

Precise manipulation of the charge percolation networks of flow-electrode capacitive deionization using a pulsed magnetic field

Magnetic field is a simple and powerful means that enables controlled the transport of electrode particles in flow electrode capacitive deionization (FCDI). However, the magnetic particles are easily stripped from hybrid suspension electrodes and the precise manipulation of the charge percolation network remains challenging. In this study, a programmable magnetic field was introduced into the FCDI system to enhance the desalination performance and operational stability of magnetic FCDI, with core-shell magnetic carbon (MC) used as an alternative electrode additive. The results showed that the pulsed magnetic field (PMF) was more effective in enhancing the average salt removal rate (ASRR) compared to the constant magnetic field (CMF), with 51.6% and 67.7% enhancement, respectively, compared to the magnetic field-free condition. The outstanding advantage of the PMF lies in the enhancement in the trapping and mediating effects in the switching magnetic field, which keeps the concentration of the electrode particles near the current collector at a high level and greatly facilitates electron transport. In long-term operation (20,000 cycles), the pulsed magnetic FCDI achieved a stable desalinating rate of 0.4-0.68 μmol min^-1 cm^-2 and a charge efficiency of >96%. In brief, our study introduces a new approach for the precise manipulation of charge percolation networks of the suspension electrodes and provides insight into the charging mechanism of the magnetic FCDI.

Metal-organic framework derived carbon nanoarchitectures for highly efficient flow-electrode CDI desalination

Flow-electrode capacitive deionization (FCDI) has shown a robust desalination performance, in which the electrode materials play a crucial role. However, commercial activated carbon (AC) commonly with relatively poor conductivity, which can be a limit to the desalination process. To address this issue, we successfully prepared ZIF-8 derived nanocarbon materials (Zx, X = 0, 1, 2, 3, the number representing the activator ratio) via a pyrolysis activation procedure as electrode materials for FCDI desalination. The results manifested that Z3 achieved desalination rates of 0.0403 and 0.094 mg min^-1 cm^-2 in the isolated closed cycle (ICC) and the short-circuited closed cycle (SCC) mode, respectively, at 1.2 V with only 5 wt% carbon loading. The desalination rate of Z3 in the SCC mode was improved with flow rates and influent salt concentrations increase, reaching 0.278 mg min^-1 cm^-2 under a continuous operation. In the ICC mode, it was found that the adsorption capacity of the Zx sample was positively correlated with its specific surface area. The superior performance of Z3 could be attributed to the high conductivity, large specific surface area and well-developed pores. Overall, this work provided new insights and references for electrode material's application to FCDI desalination.