Capacitive deionization (CDI) has been considered as the most promising and environmentally friendly electrical desalination technology owing to its low energy consumption and no secondary pollution. ...CDI is based on the principle of electric double layer for salt ion adsorption, but the existence of co-ions repulsion reduce the charge efficiency, leading to the low salt adsorption capacity. To prevent the intrinsic “co-ion effect” inside the porous carbon electrodes, membrane capacitive deionization (MCDI) by applying an ion-exchange membrane (IEM) to the surface of electrode is of increasing interest. However, MCDI brings various resistances, such as the internal and interface resistances of membrane, as well as the contact resistance between membrane and carbon electrode. More recently, by integrating “membrane” with carbon electrode without the introduction of free-standing IEM, integrated-MCDI has shown great merits to enhance counter-ion selectivity of electrode and decrease resistance, resulting in improving adsorption rate and reducing energy consumption. In this review, the preparation methods of innovative electrodes, the working mechanisms and performances of integrated-MCDIs, and the advanced advantages are summarized. It is hoped that this work will provide insight into integrated-MCDI and shed light on the future direction of water desalination based on CDI technology.
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•Integrated-MCDI has been developed without using free-standing IEM on electrode.•Integrated-MCDI outperforms MCDI on energy consumption, cost, and kinetic rate.•Integrated-MCDI has the potential of anti-organic pollution and ion-selective removal.•Review summarized current status of integrated-MCDI for water desalination.•The mechanisms of integrated-MCDI with different fabrication methods are analyzed.
•LSTM-based simulation models were established to simulate the actual MCDI operation.•RL agents were examined in the actual MCDI system using a telecommunication system.•A2C was the best agent, with ...the largest desalination goal and longer operation.•SHAP analysis explained the contribution of input parameters to the model decisions.
Artificial intelligence has been employed to simulate and optimize the performance of membrane capacitive deionization (MCDI), an emerging ion separation process. However, a real-time control for optimal MCDI operation has not been investigated yet. In this study, we aimed to develop a reinforcement learning (RL)-based control model and investigate the model to find an energy-efficient MCDI operation strategy. To fulfill the objectives, we established three long-short term memory models to predict applied voltage, outflow pH, and outflow electrical conductivity. Also, four RL agents were trained to minimize outflow concentration and energy consumption simultaneously. Consequently, actor-critic (A2C) and proximal policy optimization (PPO2) achieved the ion separation goal (<0.8 mS/cm) as they determined the electrical current and pump speed to be low. Particularly, A2C kept the parameters consistent in charging MCDI, which caused lower energy consumption (0.0128 kWh/m3) than PPO2 (0.0363 kWh/m3). To understand the decision-making process of A2C, the Shapley additive explanation based on the decision tree model estimated the influence of input parameters on the control parameters. The results of this study demonstrate the feasibility of RL-based controls in MCDI operations. Thus, we expect that the RL-based control model can improve further and enhance the efficiency of water treatment technologies.
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Membrane-based pre-concentration (MPC) process has been regarded as an attractive approach to sustainable municipal wastewater (MW) treatment. The aim of this study is to verify the techno-economic ...feasibility of pilot-scale MPC demonstrations and to design a complete technical route for MPC-based MW treatment and resource recovery. Three pilot MPC demonstrations (50–500 t/d) were operated for several months with different strategy combinations of chemicals and aeration. The filtration performance, pollutant removal performance, and concentrate characteristics were investigated and a techno-economic assessment was conducted. Based on the characteristics of each MPC effluent, biofilters, reverse osmosis (RO) and membrane capacitive deionization (MCDI) were selected as post-treatment units. Comparisons were carried out from 6 perspectives, including effluent quality, energy consumption, capital investment, operating cost, by-products, and recovery potential. In MPC_C, the combination of intermittent aeration and “50 mg/L Polyaluminium chloride +25 mg/L activated carbon” ensured a stable filtration (flux >10 LMH over 3 months) and an exceptional pollutant removal (soluble organics removal rate >75%) performance. The highest energy recovery potential (0.35 kWh/m3) was also achieved in MPC_C with the lowest energy consumption (0.12 kWh/m3) and an acceptable operating cost (0.53 CNY/m3). As for post-treatment units, RO showed advantages in effluent quality, technological maturity, and economic cost. An “MPC (intermittent aeration + high strength dosage) + RO + anaerobic digestion” system was proposed for synchronous water-energy reclamation. The net energy consumption and operating cost were estimated as 0.273 kWh/m3 and CNY 1.132/m3, respectively. This study will open new possibilities for the establishment of revolutionary wastewater treatment routes for carbon neutrality and sustainable development.
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•Techno-economic feasibility of “membrane-based pre-concentration (MPC) + post-treatment” was verified.•Intermittent aeration + high-strength chemical dosage is a promising operating strategy in MPC.•Reverse osmosis (RO) is a favorable technology for treating MPC permeate currently.•A combined system (MPC-RO) was proposed for synchronous water-energy reclamation.
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•A kind of industrialized carbon aerogel (CA) was used for MCDI electrodes;•The CA has high specific surface area and high conductivity;•The CA electrodes have excellent fluoride ...removal properties (24.44 mg g−1);
Membrane Capacitive deionization (MCDI) technology has excellent performance in treating fluorine pollution in brackish groundwaters. Traditional carbon-based materials for MCDI electrodes have weakness in cost and toxicity. Here, we demonstrated a kind of industrially-prepared carbon aregol (CA) can be used as an effective MCDI electrode material. The results of characterization and electrochemical measurements showed that this material has huge specific surface area (1632.01 m2g-1), hierarchical porous structure, good conductivity and stable electrical double-layer capacitance. CA shows excellent defluorination capacity of 24.44 mgF- gCA electrode-1 and good desalination capacity of 20.24 mgNaCl gelectrodes-1. High specific surface coupling good conductivity enhanced fluoride removal performance of CA electrode. Our work demonstrates industrialized CA that can be applied to MCDI to achieve efficient fluoride removal, which will further advance the expansion of MCDI in practical applications.
Global demand for lithium (Li) resources has dramatically increased due to the demand for clean energy, especially the large-scale usage of lithium-ion batteries in electric vehicles. Membrane ...capacitive deionization (MCDI) is an energy and cost-efficient electrochemical technology at the forefront of Li extraction from natural resources such as brine and seawater. In this study, we designed high-performance MCDI electrodes by compositing Li+ intercalation redox-active Prussian blue (PB) nanoparticles with highly conductive porous activated carbon (AC) matrix for the selective extraction of Li+. Herein, we prepared a series of PB-anchored AC composites (AC/PB) containing different percentages (20%, 40%, 60%, and 80%) of PB by weight (AC/PB-20%, AC/PB-40%, AC/PB-60%, and AC/PB-80%, respectively). The AC/PB-20% electrode with uniformly anchored PB nanoparticles over AC matrix enhanced the number of active sites for electrochemical reaction, promoted electron/ion transport paths, and facilitated abundant channels for the reversible insertion/de-insertion of Li+ by PB, which resulted in stronger current response, higher specific capacitance (159 F g−1), and reduced interfacial resistance for the transport of Li+ and electrons. An asymmetric MCDI cell assembled with AC/PB-20% as cathode and AC as anode (AC//AC-PB20%) displayed outstanding Li+ electrosorption capacity of 24.42 mg g−1 and a mean salt removal rate of 2.71 mg g min−1 in 5 mM LiCl aqueous solution at 1.4 V with high cyclic stability. After 50 electrosorption-desorption cycles, 95.11% of the initial electrosorption capacity was retained, reflecting its good electrochemical stability. The described strategy demonstrates the potential benefits of compositing intercalation pseudo capacitive redox material with Faradaic materials for the design of advanced MCDI electrodes for real-life Li+ extraction applications.
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•Redox-active PB intercalation material with AC for efficient Li+ extraction.•AC facilitate electron and Li + transport to redox-active sites of PB.•AC/PB exhibit high specific capacitance (159 F g−1) and rapid Li+ diffusion.•Excellent Li + extraction of 24.42 mg g−1 in Asymmetric AC//AC/PB-20 MCDI cell.•Excellent cyclic stability and Li + retention of 95.11% even after 50 cycles.
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.
•Data were collected from capacitive and pseudocapacitive deionization experiments.•A random forest model predicted the membrane capacitive deionization effluent.•The concentration of each ion in ...effluent was accurately predicted.•An appropriate sampling interval was proposed for each process.
Owing to its simplicity of measurement, effluent conductivity is one of the most studied factors in evaluations of desalination performance based on the ion concentrations in various ion adsorption processes such as capacitive deionization (CDI) or battery electrode deionization (BDI). However, this simple conversion from effluent conductivity to ion concentration is often incorrect, thereby necessitating a more congruent method for performing real-time measurements of effluent ion concentrations. In this study, a random forest (RF)-based artificial intelligence (AI) model was developed to address this shortcoming. The proposed RF model showed an excellent prediction accuracy when it was first validated in predicting the effluent conductivity for both CDI (R2 = 0.86) and BDI (R2 = 0.95) data. Moreover, the RF model successfully predicted the concentration of each ion (Na⁺, K⁺, Ca2⁺, and Cl⁻) from the conductivity values. The accuracy of the ion concentration prediction was even higher than that of the effluent conductivity prediction, likely owing to the linear correlation between the input and output variables of the dataset. The effect of the sampling interval was also evaluated for conductivity and ion concentrations, and there was no significant difference up to sampling intervals of <80 s based on the error value of the model. These findings suggest that an RF model can be used to predict ion concentrations in CDI/BDI, which may be used as core indicators in evaluating desalination performance.
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Capacitive de-ionization (CDI) systems are well-known for their low energy consumption making them suitable for applications powered by renewable energy. In this study, CDI technology is, for the ...first time, integrated with a suitably-scaled, stand-alone, renewable power system comprising photovoltaic panels and battery storage. Guidelines for designing and sizing such power systems are proposed including determining electrode charging current, PV panels and battery capacity. A 1 kW pilot plant was designed, constructed and operated to verify the proposed guidelines. Using the pilot plant, the total energy consumption of the system has been evaluated with different electrode charging currents and influent flow rates and the relationship between these parameters analyzed. This analysis has enabled the development of practical design guidelines for bulk water treatment with MCDI electrodes. The results of this study show that use of photovoltaic-powered MCDI water treatment, particularly when combined with energy recovery, is competitive against more mature water-treatment technologies for particular applications and at particular locations.
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•Use of photovoltaics for powering an MCDI unit is described in this work.•Principles for design of such PV-powered MCDI units is presented.•Energy recovery can be implemented rendering MCDI very energy efficient.
Introduction of new nanomaterials with conductivity, salt adsorption capacity (SAC) and rate (SAR) exceeding that of carbon electrodes may greatly improve capacitive deionization of water. However, ...those materials show a different electrochemical behavior, which must be studied and optimized for practical use. Here, effects of operating conditions on desalination performance of pre-conditioned Ti3C2Tx-MXene-based electrodes in a symmetric membrane capacitive deionization (MCDI) system were investigated. Specifically, influences of discharge potential, half-cycle length (HCL), and flow rate were systematically studied. Results showed different degrees of performance dependence on operating conditions. For instance, lower discharge potentials increased SAC and SAR by 152%. However, longer HCL increased SAC by 32% while decreasing SAR by 54%. Finally, faster flow rates decreased both SAC and SAR by 20%. Desalination performances of symmetric pre-conditioned MXene and activated carbon cloth (ACC) electrodes were gravimetrically and volumetrically compared in MCDI system. Pre-conditioned MXene electrodes gravimetrically performed 30% lower than ACC due to their notably higher density. Yet, pre-conditioned MXene electrodes volumetrically outperformed ACC by 162%. Results suggest that although MXenes offer high electrochemical activity and hydrophilicity, making them promising candidates for CDI applications, the strong dependence of desalination performance of MXenes on operating conditions requires in-depth understanding and warrants further research.
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•Pre-conditioned MXene outperforms ACC by 162% due to bulk volume ion storage.•Pre-conditioned MXene MCDI system has shown steady SAC for 100 cycles.•Lower discharge potentials enable more effective ion deintercalation.•Improved ion deintercalation leads to up to 152% increase in SAC and SAR.•Longer half-cycle length increases SAC by 32% and decreases SAR by 54%.
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•S and N co-doped porous carbon nanofibers were obtained by the electrospinning process.•Flexible and monolithic SCNFs can be used directly as MCDI electrode materials.•SCNFs-6 ...exhibited a salt adsorption capacity as high as 27.48 mg g−1, twice that of the pristine CNFs.•Theoretical calculations demonstrated the synergistic effect of S，N co-doping.
Capacitive deionization (CDI), especially membrane capacitive deionization (MCDI) technology, is an energy-efficient and environmentally friendly water desalination technology. Tremendous efforts have been devoted to designing and preparing high-performance and applicable electrode materials for CDI and MCDI. In this work, sulfur and nitrogen co-doped carbon nanofibers (SCNFs) are prepared by a facile co-electrospinning strategy coupled with subsequent high-temperature heat treatment, and directly used as monolithic and binder-free electrodes for MCDI. Benefiting from the integrated structural and compositional merits, i.e. 3D network composed of disorderly stacked 1D nanofibers and uniform distribution of nitrogen and sulfur atoms, the as-obtained SCNFs exhibit superior electrosorption ability and excellent cycling stability, with the salt adsorption capacity up to 29.50 mg g−1 and the current efficiency higher than 80% when desalting 500 mg L−1 NaCl solution at the voltage of 1.4 V. The doped nitrogen and sulfur atoms in SCNFs not only effectively improve their electrical conductivity and wettability but also tailor the pore structure, thus providing more active sites for ions adsorption. The heteroatom doping strategy could shed new light on the design of high-performance carbon-based electrodes for the electrochemical MCDI application.
