## 3 Results and Discussion
### Effect of Adsorption Time in a Solution Mixed with EDTA and Cs Ions  
Large amounts of EDTA are used for the decontamination of RLWs, and thus large amounts of radioactive complexes are generated during the decontamination and decommissioning process of a nuclear facility. To simulate such radioactive complexes, a mixed solution consisting of EDTA and Cs ions, with a molar ratio of 1:1 was used (denoted as EDTA/Cs), for further adsorption experiments. The effect of the adsorption time on EDTA removal from the aqueous EDTA/Cs solution, with various EDTA concentrations, was investigated using an AERs of 2 g/L (Figure 6). EDTA alone showed limited solubility in an aqueous solution, about 0.5 mM, whereas when mixed with equivalent Cs ions, 1 mM EDTA solution could be prepared, indicating an increase in the solubility in water in the presence of Cs ions (pH 7). This solubility increase enabled us to investigate the effect of the adsorption time with EDTA concentrations of 0.25, 0.5, and 1 mM. A removal efficiency of more than 70% EDTA from the EDTA/Cs solution was achieved after 1 h of adsorption time, and the removal was saturated at 80% (for 1 mM) and at 100% (for 0.25 and 0.5 mM) after 2 h adsorption time (Figure 6a). The \(q_{\mathrm{e}}\) of the AER was determined as 0.125, 0.25, and 0.40 mmol/g at EDTA concentrations of 0.25, 0.5, and 1 mM, respectively (Figure 6b). This result is similar to the case when a pure EDTA solution was used (see Figure 3). It seems that the presence of Cs ions did not affect the interaction of the AERs with EDTA, because EDTA has four carboxylic acid groups; enough to chelate Cs ions and be simultaneously adsorbed by AERs. Regarding \(q_{\mathrm{t}}\), the adsorption in EDTA/Cs was enhanced to a small extent compared with the pure EDTA, presumably because of better solubility of EDTA in the presence of Cs ions. Figure 5: Changes in (**a**) the removal efficiency of EDTA and (**b**) the adsorption capacity of AERs at EDTA concentrations of 0.25 and 0.5 mM (pH = 7; adsorption time = 5 h). Figure 6: Changes in (**a**) removal efficiency of EDTA and (**b**) adsorption capacity of AERs from EDTA/Cs (1:1) with adsorption time. [AERs] = 2 g/L; [EDTA] = 0.25, 0.5, and 1 mM; pH = 7. Solid and dashed lines were fitted lines to determine adsorption order (related to Figure 12). The removal efficiency of EDTA was found to be almost 100% when more than 2 g/L of AER was used, regardless of whether the initial EDTA concentration was 0.1, 0.25, or 0.5 mM (Figure 7). This indicates that the EDTA adsorption was more efficient in the EDTA/Cs solution than in the pure EDTA solution, in which more than 90% of the EDTA was removed from the pure EDTA solution, by using more than 4 g/L of AER after 5 h (see Figure 5). In addition, EDTA adsorption by AERs from EDTA/Cs solution in 5 h adsorption time was carried out to elucidate the effect of the initial concentrations of EDTA (Figure 8). Although the removal efficiency of EDTA using 2 g/L AER decreased with the increase in the initial EDTA concentration in EDTA/Cs solution, more than 75% of EDTA was removed from 1 mM of EDTA/Cs solution. Unlike the removal efficiency of EDTA, \(q_{\mathrm{e}}\) increased continuously as the initial concentration of EDTA increased, exhibiting a maximum \(q_{\mathrm{e}}\) of 0.38 mmol/g at 1 mM of the initial EDTA concentration. In general, as large amounts of EDTA are frequently used for RLW decontamination, large amounts of radioactive isotopes are also present in the RLWs. Thus, it is important to investigate the effect of EDTA composition in an EDTA/Cs solution. The EDTA removal was investigated according to the Cs concentration (EDTA/Cs molar ratios of 1:1, 1:2, and 1:4) in the EDTA/Cs solutions (Figure 9). The removal efficiency of EDTA decreased with the increase in the Cs ion concentration under the same initial concentration of EDTA in solution, while, \(q_{\mathrm{e}}\) increased.