## 3 Results and Discussion
### Structure of the Obtained Sorbents  
The obtained sorbents were studied by structural methods using infrared (IR) spectrometry, thermogravimetric (TGA) analysis, and scanning electron microscopy (SEM) with energy dispersive analysis (EDS). In the IR spectra of the obtained sorbents (Figure 6), peaks in the range of 2000-2150 cm\({}^{-1}\) are visible, corresponding to the C\(\equiv\)N group coordinated with the metal, which indicates the presence of a complex ion [Fe(CN)\({}_{6}\)]\({}^{4-}\)[51]. In the region of 3200-3500 cm\({}^{-1}\), there is a wide absorption of the carboxyl group as well as of hydroxyl groups [52]. The absorption peak at 1730 cm\({}^{-1}\) can be associated with stretching vibrations of the carbonyl group [53], which in turn is formed due to partial oxidation of the polymer chain during the fiber modification process. The absorption bands in the region of 2940 cm\({}^{-1}\) and 1450 cm\({}^{-1}\) belong to the methyl and methylene groups of the polymer chain [53]; their positions are identical in the IR spectra. Figure 7 shows the thermograms of obtained fibrous sorbents that have the following features. Heating to 300 \({}^{\circ}\)C was accompanied by a slight weight loss up to 6% for the CoFeFC-PAN sorbent (based on fiber prepared by precipitation of iron(III) hydroxide with ammonia) and up to 15% for FeMnFC-PAN and CoMnFC-PAN sorbents (based on fiber treated potassium permanganate); up to 200 \({}^{\circ}\)C, the heating corresponded to the process of dehydration of crystallization in molecular water [54]. When heated to 300 \({}^{\circ}\)C, two parallel processes probably occurred: the beginning of the cyclization of PAN, as well as the removal of crystallization and molecular water [54]. Also, at the stage from 290 to 450 \({}^{\circ}\)C for all samples, an increase in the rate of weight loss was observed, which is characterized by thermal-oxidative destruction of mixed transition metal ferrocyanides, with a maximum rate at 350 \({}^{\circ}\)C and 390 \({}^{\circ}\)C. At this stage, the complex ion [Fe(CN)6]\({}^{4-}\) decomposed with the formation of ferrites and transition metal oxides with the release of presumably cyanogen [55]. At temperatures above 400 \({}^{\circ}\)C, there was a sharp decrease in the mass of the sample and an increase in the calorific value of the product as the temperature rose, which is associated with further carbonization and burnout of the organic residue [56]. With further heating to 800 \({}^{\circ}\)C, the weight curve gradually reached a plateau. Figure 6: IR spectra of sorbents: (a) CoFeFC-PAN; (b) CoMnFC-PAN; (c) FeMnFC-PAN. The SEM image of fibrous sorbents (Figure 8) shows the presence of interlayer spaces in which the formation of coarse-grained deposits occurs during the modification of PAN fiber. In addition, the formation of deposits also occurs on the entire surface of individual fibers, which is confirmed by the results of EDS analysis (Figure 9). Also, using energy dispersive analysis (Figure 9), the distribution of elements on the surface of composite materials was assessed, and the results obtained are presented in Table 2. It was found that during the formation of the ferrocyanide layer, the distribution of all components of the mixture occurs evenly, regardless of synthesis conditions. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sorbent** & **Fe, \%** & **Mn, \%** & **Co, \%** & **K, \%** & **C, \%** & **O, \%** & **N, \%** \\ \hline CoFeFC-PAN & 26.75 & \(-\) & 5.44 & 3.54 & 28.86 & 17.24 & 18.17 \\ CoMnFC-PAN & 7.55 & 15.23 & 4.89 & 4.01 & 32.97 & 15.32 & 20.03 \\ FeMnFC-PAN & 9.68 & 33.37 & \(-\) & 4.41 & 32.38 & 8.63 & 11.53 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Elemental composition of the surface of the resulting sorbents.