## 3 Results and Discussion
### Selection of Optimal Materials and Conditions for Sorbent Synthesis
#### 3.1.2 Assessment of the Influence of Transition Metal Salt Type and Concentration  
At the second stage of the synthesis, solutions of cobalt(II), nickel(II), zinc(II), copper(II), and iron(III) chlorides with a concentration of 0.01-0.1 mol/L were used for subsequent processing of sorbents. Pre-treatment of the prepared PAN-Fe(OH)\({}_{3}\) and the PAN-MnO\({}_{2}\) fiber (first stage of synthesis) was carried out with a 0.1 mol/L solution of K\({}_{4}\)Fe(CN)\({}_{6}\) with pH 1.  
Results obtained for sorbents based on KFeFC-PAN (PAN fiber prepared by precipitation of iron(III) hydroxide with ammonia PAN-Fe(OH)\({}_{3}\), followed by treatment with potassium ferrocyanide) and KMnFC-PAN (PAN fiber treated with potassium permanganate PAN-MnO\({}_{2}\), followed by treatment with potassium ferrocyanide) are shown in Figures 4 and 5, respectively. More detailed dependences of the cesium distribution coefficients on the concentration of the transition metal salt, taking into account the concentration of potassium ferrocyanide used in the preparation of sorbents, are given in Supplementary Materials in Figures S1 and S2.  
Figure 3: Dependences of the cesium distribution coefficients upon sorption from 0.1 (\(\bullet\)) and 1 (\(\blacklozenge\)) mol/L solutions of sodium nitrate with pH 6 and seawater (\(\blacktriangle\)) on the pH of potassium ferrocyanide solution used at the first stage in sorbent preparation: (\(\blacklozenge\)) KFeFC-PAN; (\(\blacklozenge\)) KMnFC-PAN.  
It was found that for sorbents based on PAN fiber prepared by precipitation of iron(III) hydroxide with ammonia (PAN-Fe(OH)\({}_{3}\)), the best cesium distribution coefficients are achieved when cobalt(II) chloride is used for modification with a concentration of 0.02 mol/L--CoFeFC-PAN sorbent.  
For sorbents based on fiber treated with potassium permanganate (PAN-MnO\({}_{2}\)), the best cesium distribution coefficients are achieved at a concentration of cobalt(II) chloride of 0.02-0.05 mol/L and iron(III) chloride of 0.02 mol/L--sorbents CoMnFC-PAN and FeMnFC-PAN, respectively.  
Figure 4: Dependence of the cesium distribution coefficients upon sorption from 0.1 (\(\bullet\)) and 1 (\(\blacklozenge\)) mol/L solutions of sodium nitrate with pH 6 and seawater (\(\blacktriangle\)) on the concentration of the transition metal salt used in sorbent preparation: (**a**) CoFeFC-PAN, (**b**) NiFeFC-PAN, (**c**) CuFeFC-PAN, (**d**) ZnFeFC-PAN.  
Figure 5: Dependence of the cesium distribution coefficients upon sorption from 0.1 (\(\bullet\)) and 1 (\(\blacklozenge\)) mol/L solutions of sodium nitrate with pH 6 and seawater (\(\blacktriangle\)) on the concentration of the transition metal salt used in sorbent preparation: (**a**) FeMnFC-PAN, (**b**) CoMnFC-PAN, (**c**) NiMnFC-PAN, (**d**) CuMnFC-PAN, (**e**) ZnMnFC-PAN.  
In general, the degree of influence of the transition metals used on the cesium sorption can be expressed as follows:  
* fororbents based on PAN-Fe(OH)\({}_{3}\) support: \[\text{Co}^{2+}>\text{Ni}^{2+}>\text{Zn}^{2+}>\text{Cu}^{2+};\]
* fororbents based on PAN-MnO\({}_{2}\) support: \[\text{Fe}^{3+}>\text{Co}^{2+}>\text{Ni}^{2+}>\text{Zn}^{2+}>\text{Cu}^{2+}.\]  
The analysis of work [38] devoted to the study of sorbents based on transition metal ferrocyanides shows that sorption characteristics largely depend on the type of transition metal in the inorganic phase. At the same time, various authors [13; 38] obtained different series of decreases in the capacity of ferrocyanide sorbents concerning cesium. The authors associate such discrepancies with the peculiarities of the formation and binding of the matrix surface with the sorption-active phase, the properties of which depend on the method of composite sorbent obtaining [37; 38].