## 3 Results and Discussion
### Selection of Optimal Materials and Conditions for Sorbent Synthesis
#### 3.1.1 Assessment of the Effect of Potassium Ferrocyanide Concentration and pH  
When obtaining sorbents, the prepared support was treated at the first stage with a solution of potassium ferrocyanide with different concentrations and at different pH levels. Two types of sorbents were obtained:  
* KFeFC-PAN--sorbents based on potassium ferrocyanide and fiber prepared by precipitation of iron(III) hydroxide with ammonia (PAN-Fe(OH)\({}_{3}\));
* KMnFC-PAN--sorbents based on potassium ferrocyanide and fiber treated with potassium permanganate (PAN-MnO\({}_{2}\)).  
Figure 2 shows the dependences of the cesium distribution coefficients on the concentration of potassium ferrocyanide in a solution with pH 1, used in the preparation of sorbents.  
From the data obtained, it is seen that in both cases the optimal concentration of potassium ferrocyanide solution is 0.1-0.2 mol/L, which generally corresponds to data from other studies on the production of sorbents based on ferrocyanide [26; 49].  
Thus, according to [26], at high content of potassium ferrocyanide in the initial modifying solution, an unstable solid phase of variable composition is formed (KFe[Fe(CN)\({}_{6}\)]: \(n\)K[Fe(CN)\({}_{6}\)], where \(n\) is a variable value depending on the concentration of K\({}_{n}\)[Fe(CN)\({}_{6}\)] in the initial solution), which, unlike normal and mixed iron(III) ferrocyanides, is soluble in water. Therefore, in cases of its formation, a certain fraction of iron(III) ferrocyanide passes from the solid phase into solution, which is the reason for the decrease in the sorption capacity of ferrocyanide ions during the modification process.  
Figure 3 shows the dependence of distribution coefficients of cesium on the pH of a potassium ferrocyanide solution with a concentration of 0.1 mol/L used in the preparation of sorbents.  
Figure 2: 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 concentration of potassium ferrocyanide solution used at the first stage in sorbent preparation: (\(\blacklozenge\)) KFeFC-PAN; (\(\blacklozenge\)) KMnFC-PAN.  
Fororbents obtained based on PAN fiber prepared by precipitation of iron(III) hydroxide with ammonia (PAN-Fe(OH)\({}_{3}\)) [45], it was found that the best cesium distribution coefficients are achieved when using a ferrocyanide solution with pH of 1-1.5.  
This is explained by the fact that at this pH, iron(III) hydroxide dissociates according to the main type [50]. At high pH, the dehydroxylation of iron(III) hydroxide is insignificant, so the modification of the fiber is less active. The data acquired coincide with the conclusions reached when obtaining the Anfezh sorbent [26]. The production of sorbents at a pH of less than 1 was not carried out, because according to [26], under these conditions, peptization of the sorbent occurs, accompanied by the transition of part of the ferrocyanide component into a solution.  
In the case of using PAN fiber treated with potassium permanganate, the sorption characteristics of the resulting sorbents do not depend on the pH of the potassium ferrocyanide solution in the range from one to five, which is consistent with the data of other researchers [49].