Capacitive deionization of a RO brackish water by AC/graphene composite electrodes

Pseudocapacitive Deionization of Saltwater by Mn3O4@C/Activated Carbon

Capacitive deionization (CDI), a m ethod with notable advantages of relatively low energy consumption and environmental friendliness, has been widely used in desalination of saltwater. However, due to the weak electrical double-layer electrosorption of ions from water, CDI has suffered from low throughput capacity that may limit its commercial applications. Thus, it is of importance to develop a high-efficiency and engineering-feasible CDI process. Manganese and cobalt and their oxides, being faradic materials, have a relatively high pseudocapacitance, which can cause an increase of positive and negative charges on opposing electrodes. However, their low conductivity properties limit their electrochemical applications. Pseudocapacitive Mn₃O₄ nanoparticles encapsulated within a conducting carbon shell (Mn₃O₄@C) were prepared to enhance charge transfer and capacitance for CDI. Desalination performances of the Mn₃O₄@C (5-15 wt %) core-shell nanoparticles on activated carbon (AC) (Mn₃O₄@C/AC) serving as CDI electrodes have thus been investigated. The pseudocapacitive Mn₃O₄@C/AC electrodes with relatively low diffusion resistances have much greater capacitance (240-1300 F/g) than the pristine AC electrode (120 F/g). In situ synchrotron X-ray absorption near-edge structure spectra of the Mn₃O₄@C/AC electrodes during CDI (under 1.2 and -1.2 V for electrosorption and regeneration, respectively) demonstrate that reversible faradic redox reactions cause more negative charges on the negative electrode and more positive charges on the positive electrode. Consequently, the pseudocapacitive electrodes for CDI of saltwater ([NaCl] = 1000 ppm) show much better desalination performances with a high optimized salt removal (600 mg/g·day), electrosorption efficiency (48%), and electrosorption capacity (EC) (25 mg/g) than the AC electrodes (288 mg/g·day, 23%, and 12 mg/g, respectively). The Mn₃O₄@C/AC electrode has a maximum EC of 29 mg/g for CDI under +1.2 V. Also, the Mn₃O₄@C/AC electrodes have much higher pseudocapacitive electrosorption rate constants (0.0049-0.0087 h^-1) than the AC electrode (0.0016 h^-1). This work demonstrates the feasibility of high-efficiency CDI of saltwater for water recycling and reuse using the low-cost and easily fabricated pseudocapacitive Mn₃O₄@C/AC electrodes.

Electrosorption performance on graphene-based materials: a review

Due to its unique advantages such as flexible planar structure, ultrahigh specific surface area, superior electrical conductivity and electrical double-layer capacitance in theory, graphene has unparalleled virtues compared with other carbon materials. This review summarizes the recent research progress of various graphene-based electrodes on ion electrosorption fields, especially for water desalination utilizing capacitive deionization (CDI) technology. We present the latest advances of graphene-based electrodes, such as 3D graphene, graphene/metal oxide (MO) composites, graphene/carbon composites, heteroatom-doped graphene and graphene/polymer composites. Furthermore, a brief outlook on the challenges and future possible developments in the electrosorption area are also addressed for researchers to design graphene-based electrodes towards practical application.

Enhanced Electrodesorption Performance via Cathode Potential Extension during Capacitive Deionization

Complete desorption of contaminants from electrode materials is required for the efficient utilization and long service life of capacitive deionization (CDI) but remains a major challenge. The electrodesorption capacity of CDI in the conventional electrode configuration is limited by the narrow electrochemical stability window of water, which lowers the operating potential to approximately 1.2 V. Here, we report a graphite anode–titanium cathode electrode configuration that extends the cathode potential to −1.7 V and provides an excellent (100%) electrodesorption performance, which is maintained after five cycles. The improvement of the cathode potential depends on the redox property of the electrode. The stronger the oxidizability of the anode and reducibility of the cathode, the wider the cathode potential. The complete desorption potential of SO42− predicted by theoretical electrochemistry was the foundation for optimizing the electrode configuration. The desorption efficiency of Cl− depended on the ionic strength and was negligibly affected by circulating velocities above 112 mL min−1. This work can direct the design optimizations of CDI devices, especially for reactors undergoing chemisorption during the electrosorption process.

Experimental and theoretical study of a new CDI device for the treatment of desulfurization wastewater

According to the characteristics of desulfurization wastewater, A new capacitive deionization (CDI) device was designed to study the desalination characteristics of desulfurization wastewater in this paper. The experiments investigated the desalination efficiency under different conditions which find that the best desalination efficiency is achieved at a voltage of 1.2V, pH=11 and 50°C. Besides, ion adsorption is more favorable under acidic and alkaline conditions. The anion and cation removal performance experiments showed that the order of cation removal is Mg²⁺>Na⁺>Ca²⁺>K⁺ and the order of anion removal is Cl^->CO₃^2->NO₃^->SO₄^2->HCO₃^-. The mechanism of CDI was studied and analyzed by the isothermal adsorption model and COMSOL simulation software. It was found that the Freundlich model and Redlich-Peterson model have a good fit with the experimental results. The experiments show that the CDI device has excellent stability. CDI device was used to treat actual desulfurization wastewater. Furthermore, the study provides theoretical support for the industrial application of CDI for desulfurization wastewater treatment in the future. Graphical abstract.

Recent progress in the conversion of biomass wastes into functional materials for value-added applications

The amount of biomass wastes is rapidly increasing, which leads to numerous disposal problems and governance issues. Thus, the recycling and reuse of biomass wastes into value-added applications have attracted more and more attention. This paper reviews the research on biomass waste utilization and biomass wastes derived functional materials in last five years. The recent research interests mainly focus on the following three aspects: (1) extraction of natural polymers from biomass wastes, (2) reuse of biomass wastes, and (3) preparation of carbon-based materials as novel adsorbents, catalyst carriers, electrode materials, and functional composites. Various biomass wastes have been collected from agricultural and forestry wastes, animal wastes, industrial wastes and municipal solid wastes as raw materials with low cost; however, future studies are required to evaluate the quality and safety of biomass wastes derived products and develop highly feasible and cost-effective methods for the conversion of biomass wastes to enable the industrial scale production.

Physico-chemical processes

The review scans research articles published in 2018 on physico-chemical processes for water and wastewater treatment. The paper includes eight sections, that is, membrane technology, granular filtration, flotation, adsorption, coagulation/flocculation, capacitive deionization, ion exchange, and oxidation. The membrane technology section further divides into six parts, including microfiltration, ultrafiltration, nanofiltration, reverse osmosis/forward osmosis, and membrane distillation. PRACTITIONER POINTS: Totally 266 articles on water and wastewater treatment have been scanned; The review is sectioned into 8 major parts;  Membrane technology has drawn the widest attention from the research community.

Effect of the chemical bond on the electrosorption and desorption of anions during capacitive deionization

This paper is concerned with the effect of the chemical bond on the electrosorption and desorption of anions during capacitive deionization (CDI). An empirical equation developed firstly based on the experimental data in the electrosorption of Cl^-. The empirical equation shows that the electrosorption capacity exhibits a logarithmic relationship with anode potential. Electrosorption of ReO₄^- and NO₃^- at different anode potentials were studied and the obtained data used to compare with empirical equation. The empirical equation provided results that were in good agreement with experimental data. According to parameters (A and b) of empirical equation, the chemical bond mainly affected adsorption mechanism and desorption performance. For Cl^- with the weaker chemical bond, the main mechanism of electrosorption of Cl^- is formation of electrical double layers and the desorption is easier, while for ReO₄^-, the main adsorption mechanism is chemical bonding and adsorbed anion basically fail to desorb at the equal conditions with Cl^-. The revelation of all the CDI performance here would appear to be a useful tool for selection of more suitable electrodes materials to improve adsorption capacity and desorption efficiency of anions.