Based on a porous carbon electrode, capacitive deionization (CDI) is a promising desalination technology in which ions are harvested and stored in an electrical double layer. However, the ion removal capacity of CDI systems is not sufficient for desalting high-concentration saline water. Here, we report a novel desalination technique referred to as “hybrid capacitive deionization (HCDI)”, which combines CDI with a battery system. HCDI consists of a sodium manganese oxide (Na₄Mn₉O₁₈) electrode, an anion exchange membrane, and a porous carbon electrode. In this system, sodium ions are captured by the chemical reaction in the Na₄Mn₉O₁₈ electrode, whereas chloride ions are adsorbed on the surface of the activated carbon electrode during the desalination process. HCDI exhibited more than double the ion removal sorption capacity (31.2 mg g⁻¹) than a typical CDI system (13.5 mg g⁻¹). Moreover, it was found that the system has a rapid ion removal rate and excellent stability in an aqueous sodium chloride solution. These results thus suggest that the HCDI system could be a feasible method for desalting a highly concentrated sodium chloride solution in capacitive techniques.

Fishing ions: this review provides a comprehensive analysis of different approaches in utilizing capacitive deionization (CDI) for selective ion separations and ion removal.

Despite a wealth of experimental evidence concerning the efficacy of the biocidal action associated with the TiO ₂ photocatalytic reaction, our understanding of the photochemical mechanism of this particular biocidal action remains largely unclear. It is generally accepted that the hydroxyl radical (·OH), which is generated on the surface of UV-illuminated TiO ₂ , plays the main role. However, our understanding of the exact mode of action of the hydroxyl radical in killing microorganisms is far from complete, and some studies report that other reactive oxygen species (ROS) (H ₂ O ₂ and O ₂ · ⁻ , etc.) also play significant roles. In particular, whether hydroxyl radicals remain bound to the surface or diffuse into the solution bulk is under active debate. In order to examine the exact mode of action of ROS in inactivating the microorganism, we tested and compared the levels of photocatalytic inactivation of MS-2 phage and Escherichia coli as representative species of viruses and bacteria, respectively. To compare photocatalytic microbial inactivation with the photocatalytic chemical degradation reaction, para -chlorobenzoic acid, which rapidly reacts with a hydroxyl radical with a diffusion-limited rate, was used as a probe compound. Two different hydroxyl radical scavengers, tert -butanol and methanol, and an activator of the bulk phase hydroxyl radical generation, Fe ²⁺ , were used to investigate their effects on the photocatalytic mode of action of the hydroxyl radical in inactivating the microorganism. The results show that the biocidal modes of action of ROS are very different depending on the specific microorganism involved, although the reason for this is not clear. It seems that MS-2 phage is inactivated mainly by the free hydroxyl radical in the solution bulk but that E. coli is inactivated by both the free and the surface-bound hydroxyl radicals. E. coli might also be inactivated by other ROS, such as O ₂ · ⁻ and H ₂ O ₂ , according to the present results.

The demand for fresh water has been increasing, caused by the growing population and industrialization throughout the world. In this study, we report a capacitive-based desalination system using Prussian blue materials in a rocking chair desalination battery, which is composed of sodium nickel hexacyanoferrate (NaNiHCF) and sodium iron HCF (NaFeHCF) electrodes. In this system, ions are removed not only by charging steps but also by discharging steps, and it is possible to treat actual seawater with this system because the Prussian blue material has a high charge capacity with a reversible reaction of alkaline cations. Here, we demonstrate a rocking chair desalination battery to desalt seawater, and the results show that this system has a high desalination capacity (59.9 mg/g) with efficient energy consumption (0.34 Wh/L for 40% Na ion removal efficiency).

A novel concept to evaluate desalination performance in capacitive deionization (CDI) is proposed called the CDI Ragone plot. The plot can allow for intuitive acquisition of deionization capacity (mg g⁻¹), rate (mg g⁻¹ s⁻¹), and time (s) and thus, will work as a functional tool to evaluate desalination performance in CDI.