## 3 Results and Discussion
### Selection of Optimal Conditions for Cesium Sorption  
The study on the cesium sorption efficiency from solutions of various compositions (pH, concentration of Na\({}^{+}\), K\({}^{+}\), NH\({}_{4}\)\({}^{+}\) ions) with the sorbents obtained in this work was also carried out.  
The following results assessment of the effect of solution pH on cesium sorption were obtained, presented in Figure 10.  
Based on the results of assessing the influence of solution pH, it is seen that the produced ferrocyanide sorbents are characterized by the independence of cesium distribution coefficients from pH in the range from one to eight, which is associated with the high selectivity of the ferrocyanide phase to cesium. At the same time, a decrease in the distribution coefficients of cesium in an alkaline medium at pH above nine was observed for sorbents, associated with peptization and partial destruction of the ferrocyanide phase. Overall, the results show that the sorbents obtained are potentially suitable for the sorption and concentration of cesium from liquid media in the pH range from one to nine.  
Figure 11 shows the results of assessing the dependence of the cesium sorption efficiency on the type and concentration of the cation, which can have a significant competing effect.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sorbent**} & \multicolumn{2}{c}{\(K_{d}\)\({}^{137}\)Cs (mL/g) for Various Sorbents in Solution of} \\ \cline{2-4}  & **0.1 mol/L NaNO\({}_{3}\)** & **1.0 mol/L NaNO\({}_{3}\)** & **S seawater** \\ \hline CoFeFC-PAN & (1.4 \(\pm\) 0.4) \(\times\) 10\({}^{5}\) & (1.0 \(\pm\) 0.2) \(\times\) 10\({}^{5}\) & (5.3 \(\pm\) 1.7) \(\times\) 10\({}^{5}\) \\ CoMnFC-PAN & (1.5 \(\pm\) 0.7) \(\times\) 10\({}^{5}\) & (4.5 \(\pm\) 0.1) \(\times\) 10\({}^{4}\) & (2.0 \(\pm\) 0.2) \(\times\) 10\({}^{4}\) \\ FeMnFC-PAN & (7.4 \(\pm\) 0.3) \(\times\) 10\({}^{4}\) & (1.1 \(\pm\) 0.3) \(\times\) 10\({}^{5}\) & (1.6 \(\pm\) 0.3) \(\times\) 10\({}^{5}\) \\ FNS-10 & (8.4 \(\pm\) 0.3) \(\times\) 10\({}^{4}\) & (7.3 \(\pm\) 0.2) \(\times\) 10\({}^{4}\) & (1.1 \(\pm\) 0.3) \(\times\) 10\({}^{4}\) \\ Termoksid-35 & (1.2 \(\pm\) 0.4) \(\times\) 10\({}^{5}\) & (8.1 \(\pm\) 0.2) \(\times\) 10\({}^{4}\) & (3.1 \(\pm\) 0.3) \(\times\) 10\({}^{4}\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Elemental composition of the surface of obtained sorbents.  
Figure 11: Dependence of the cesium distribution coefficients during sorption by produced sorbents CoFeFC-PAN (\(\bullet\)), FeMnFC-PAN (\(\blacktriangle\)), and CoMnFC-PAN (\(\blacklozenge\)) on the presence of cations in the test solution: (**a**) sodium; (**b**) potassium; (**c**) ammonium.  
Figure 10: Dependence of cesium distribution coefficients during sorption by produced sorbents CoFeFC-PAN (\(\bullet\)), FeMnFC-PAN (\(\blacktriangle\)), and CoMnFC-PAN (\(\blacklozenge\)) on the pH of the test solution.  
It is seen that the results obtained are in good agreement with the literature data on the sorption of alkali metal and ammonium ions by transition metal ferrocyanides [57]. A decrease in the values of cesium distribution coefficients is expressed in the series of cations present: Na\({}^{+}\) > K\({}^{+}\) > NH4\({}^{+}\). Ferrocyanides sorb alkali metal ions due to the zeolite effect, which is more pronounced the closer the diameter of the sorbed ion is to the cross-sectional dimensions of the channels in the ferrocyanide structure. The maximum zeolite effect is observed for the Cs\({}^{+}\) cation; the NH4\({}^{+}\) ion is closest in size to the Cs\({}^{+}\) ion. Thus, the effective diameter of the hydrated Cs\({}^{+}\) cation is 0.25 nm; for comparison, the diameter of the NH4\({}^{+}\) ion is also 0.25 nm, K\({}^{+}\)--0.3 nm, Na\({}^{+}\)--0.4-0.45 nm; therefore, the NH4\({}^{+}\) ion has a greater effect on Cs\({}^{+}\) sorption than the cations of other elements under consideration [58].