Capacitive deionization: Processes, materials and state of the technology

Capacitive deionization (CDI) is a novel technology that has been successfully utilized for many water treatment/purification applications. In many cases, the CDI technology has shown increased efficiency compared to the other contemporary technologies. There have been some recent developments in this field that have enhanced the overall adsorption capacity and thus performance of CDI. Different types of carbon electrodes, such as conventional and latest forms of porous-carbon and graphene based electrodes have been used in the CDI technology. This has facilitated mass transfer in the form of adsorption of salt ions onto modified electrodes and increased the effectiveness of CDI operations. This paper aims to assess the overall state of the CDI technology and review basic concepts, saturation-regeneration of CDI electrodes, and various types of materials used for electrodes. In addition, this article reviews the recent progress made in electrode surface modifications by metals or metal oxides, membrane, polymer, organics and other compounds that have resulted in increased efficiency of ion extraction from the water. The evaluation of past and present published research on use of low-cost carbon forms as CDI electrodes to reduce the overall cost and eliminate the need of regeneration of saturated electrodes is also a part of the article.