## 3 Results and Discussions  
Scanning electron microscope images of the synthesised carbon nanocomposites are shown in Figure 3a-d.  
From the obtained images it can be seen that all the synthesised composites are quite homogeneous; moreover, elements showing the macroporous structure of the original biowastes are also observed. It should be added that organic raw materials often contain various additional atoms in addition to carbon, the type and amount of which depends on the type of raw material. These atoms combine with oxygen or carbon atoms during high-temperature synthesis and can form water-insoluble particles. Therefore, such particles do not dissolve during water rinsing and remain in the synthesised carbon material. However, as can be seen from Figure 3, such additional (non-carbon) particles are practically absent in any of the synthesised composites. X-ray diffraction methods were used to analyse the internal structural features of the synthesised samples. X-ray diffractograms of the obtained samples are shown in Figure 4c-f. As can be seen, wide scattering maxima are observed in the diffractograms of all materials, indicating the amorphous structure of the samples [36; 37; 38]. For comparative sake, the positions of polycrystalline graphite reflections corresponding to three-dimensional (002) and two-dimensional (100), (110) diffraction from graphene layers (Figure 4b) are given, and red lines (Figure 4a) correspond to Fe\({}_{3}\)O\({}_{4}\) magnetite. In addition, all diffractograms show a number of small peaks of crystalline phases, which are most likely associated with impurity elements present in the initial raw material. In the work [39], it was established that the raw materials (beet pulp and corn stalks) do not contain Fe\({}_{3}\)O\({}_{4}\) magnetite particles. Therefore, they are introduced into the carbon structure during activation with ferric chloride. Figure 3: SEM images of beet pulp carbon composite using a one-step synthesis (**a**) and using a two-step synthesis (**b**), corn carbon composite using a one-step synthesis (**c**) and using a two-step synthesis (**d**). During the carbonisation of raw materials in the presence of iron salts, processes occur that lead to the formation of iron oxides and the development of porosity [40; 41]. First of all, Fe\({}^{3+}\) ions are hydrolys to Fe(OH)\({}_{3}\) at a temperature of 350 \({}^{\circ}\)C:  
\[\mathrm{Fe}^{3+}+3\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{Fe}(\mathrm{OH}) _{3}+3\mathrm{H}^{+} \tag{1}\]  
When further heated in an inert atmosphere, the hydroxide is dehydrated and converted to Fe\({}_{2}\)O\({}_{3}\) hematite at 400 \({}^{\circ}\)C:  
\[\mathrm{Fe}(\mathrm{OH})_{3}\rightarrow\mathrm{FeO}(\mathrm{OH})\rightarrow \mathrm{Fe}_{2}\mathrm{O}_{3} \tag{2}\]  
At higher temperatures (from 500 \({}^{\circ}\)C to 700 \({}^{\circ}\)C), Fe\({}_{2}\)O\({}_{3}\) hematite is reduced to Fe\({}_{3}\)O\({}_{4}\) magnetite by amorphous carbon and CO gas:  
\[3\mathrm{Fe}_{2}\mathrm{O}_{3}+\mathrm{C}(\mathrm{CO})\to 2\mathrm{Fe}_{3} \mathrm{O}_{4}+\mathrm{CO}(\mathrm{CO}_{2})\uparrow \tag{3}\]  
A number of studies [42; 43] have reported that FeCl\({}_{3}\) promotes the development of pores in carbon-containing materials. Unfortunately, the mechanism of FeCl\({}_{3}\) activation is not well understood. But, for example, the reduction reaction given in Equation (3) can promote the development of pores in samples of synthesised nanocomposites by converting C into CO and removing carbon in the form of CO or CO\({}_{2}\), resulting in certain cavities within the material structure. Nitrogen adsorption/desorption isotherms are shown in Figure 5. All the isotherms can be classified as type II isotherms [44]. The isotherms of composites synthesised in one step are characterised by a discrepancy between the adsorption and desorption curves. The main reason for this behaviour may be the irreversible retention of adsorbate molecules in pores whose size is close to the size of the adsorbate molecules.