## 4 Conclusions  
In this study, we investigated the parameters for the effective removal of Cs\({}^{+}\) ions from an aqueous solution using an AC-CPL by a CDI process. The CEL-AC electrode confirmed higher SAC and removal efficiency for Cs\({}^{+}\) ions than the AC electrode. The ion adsorption capacity of the AC-CPL was strongly dependent on the applied potential, flow rate, initial concentration of Cs\({}^{+}\), and initial pH value. When the potential increased, the adsorption capacity increased, and the charge efficiency decreased at 1.4 V. The maximum adsorption capacity decreased with increasing flow rate in 50 and 100 mg L\({}^{-1}\) Cs\({}^{+}\) influent water, respectively. On the other hand, the maximum adsorption capacity increased with the increase of the flow rate in 200 mg L\({}^{-1}\) Cs\({}^{+}\) influent water. The effect of pH on the electrospray removal of Cs\({}^{+}\) ions was examined and showed that Cs\({}^{+}\) ions were preferentially adsorbed to the electrode in the early stage of adsorption. However, the adsorbed Cs\({}^{+}\) ions were gradually replaced by H\({}^{+}\) ions, especially in the case of low pH. A greater presence of H\({}^{+}\) ions reduced the adsorption capacity of the Cs\({}^{+}\) ions. These findings indicate that, with the exception of highly acidic solutions, the electrospray removal method using an AC-CPL can be applied successfully for the removal of Cs\({}^{+}\) ions from acidic waste-water solutions.  
Figure 8: Changes in Cs\({}^{+}\) concentration (**a**), and pH of effluent (**b**) according to pH in the influent (Experimental conditions: Influent of 200 mg L\({}^{-1}\) Cs\({}^{+}\) ions concentration was fed with a flow rate of 20 mL min\({}^{-1}\) at 1.2 V for 60 min adsorption).  
Figure 9: Ragone plots of effluent according to the concentration and pH of Cs\({}^{+}\) ions in the influent (Experimental conditions: Influent of pH 3 or pH 4 was fed with a flow rate of 20 mL min\({}^{-1}\)).  
**Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/app112110042/s1](https://www.mdpi.com/article/10.3390/app112110042/s1)]([https://www.mdpi.com/article/10.3390/app112110042/s1](https://www.mdpi.com/article/10.3390/app112110042/s1)), Figure S1: Changes in concentration and pH of Cs\({}^{+}\) ions of effluent during the adsorption process (Experimental conditions: Influent of 50 mg L\({}^{-1}\) Cs\({}^{+}\) ions at pH 2 was fed with a flow rate of 20 mL min\({}^{-1}\) at 1.2 V for 60 min).  
**Author Contributions:** Conceptualization, S.-H.L., M.C. and S.L.; methodology, S.-H.L., S.L. and J.-K.M.; validation, S.K.; formal analysis, S.-H.L.; investigation, S.-H.L. and S.L.; resources, S.K.; data curation, S.-H.L. and M.C.; writing--original draft preparation, S.-H.L. and M.C.; writing--review and editing, S.-H.L., M.C. and J.-K.M.; supervision, J.C. and J.-K.M.; project administration, J.-K.M. and S.K. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Acknowledgments:** This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government, Ministry of Science and ICT (No. 2017M28A5015144).  
**Conflicts of Interest:** The authors declare no conflict of interest.