## 1 Introduction  
Nuclear energy provides a more steady and efficient supply of powerful energy than traditional energy. However, a large amount of radioactive waste is generated in nuclear power plants during their daily operations, maintenance, and decommissioning. In 2011, the nuclear power plant accident in Fukushima Daiichi, Japan, released large amounts of radioactive elements into the atmosphere, causing serious damage to the environment. Among the several nuclides, cesium (137 Cs+) is one of the most dangerous radioactive elements. This material has a comparatively long half-life of 30 years and emits high-energy gamma radiation during the decay process [1]. Cs+ is chemically similar to sodium and potassium. It can easily enter the human body and may induce severe diseases, such as cancer [2, 3]. In addition, it can be dispersed in a variety of environments and integrate easily into soil and water, destroying eco-systems [4, 5]. Therefore, effective techniques that remove Cs+ are essential for solving these problems and sustaining the development of nuclear power plants. Various technologies, including chemical precipitation, adsorption, membrane separation, and solvent extraction, have been widely studied for the removal of Cs+ from wastewater [6, 7, 8, 9]. However, these methods use excessive amounts of chemicals or consume high amounts of energy. Meanwhile, electrochemical methods have been developed and used as alternative options for the remediation of water and wastewater because of a relatively simple process, ease of deposition on a large area, and the potential to serve as a low-cost commercial process that minimizes the generation of secondary radioactive waste. Moreover, the adsorption/desorption of ions can be easily controlled by the electrochemical method. Among the electrochemical methods, capacitive deionization (CDI) is reportedly one of the most promising electrochemical methods for removing ions from aqueous solutions [10; 11; 12; 13; 14; 15]. CDI is also an environmentally friendly process because it does not produce any additional contaminants during the operation. In the CDI process, the charged ions from the feed solution are adsorbed onto the electrical double layer (EDL) formed on the electrode surface [16; 17]. CDI performance depends on the applied potential, flow rate, concentration of feed solution, and electrode material. The CDI process has been extensively applied for many types of ions, including Na\({}^{+}\), NH\({}^{4+}\), Cu\({}^{2+}\), Zn\({}^{2+}\), Cl\({}^{-}\), F\({}^{-}\), and NO\({}^{3-}\)[18; 19; 20; 21]. CDI can also be used to remove radionuclides from contaminated wastewater from nuclear power plants prior to release. Recently, it was reported that the activated carbon cloth (ACC)-based CDI electrode exhibited the Sr\({}^{2+}\) removal efficiency of 41.45% for an applied potential of 1.2 V [22]. In another study, the CDI process was used for the removal of Co\({}^{2+}\) and Cs\({}^{+}\) from aqueous solutions. The ACC-based CDI electrode achieved the maximum adsorption capacity of Co\({}^{2+}\) and Cs\({}^{+}\), which was 16.7 mg g\({}^{-1}\) and 22.9 mg g\({}^{-1}\) at 1.2 V and 20 mg L\({}^{-1}\), respectively [23]. It is necessary to study the electrosorption performance under various experimental conditions, particularly the removal of Cs\({}^{+}\) under acidic solutions, [PERSON] et al. [24] investigated electrosorption in influent with a pH of 2.6 and reported that H\({}^{+}\) derived from acetic acid competed for Na\({}^{+}\) during the electrosorption process. In addition, [PERSON] et al. [25] reported that when H\({}^{+}\) ions are present in higher concentrations than In\({}^{3+}\) ions, H\({}^{+}\) ions are preferentially adsorbed to the electrode in the early stage of charging. Thus, it is important to understand the fluctuating performance according to operating conditions in aqueous solutions for the removal of Cs\({}^{+}\) during the CDI process.