## 3 Results and Discussion
If the sponge contains carbonate, then in the temperature range of 550-830 \({}^{\circ}\)C should produce endothermic bands in the temperature range of 550-830 \({}^{\circ}\)C. The temperature range of 550-830 \({}^{\circ}\)C is 1000 \({}^{\circ}\)C, which is consistent with the temperature range of 550-830 \({}^{\circ}\)C.  
Figure 4: TG (1), DTGA (2), and DCS (3) curves of thermal modification of marine sponge. mic decomposition into carbon dioxide and calcium oxide according to the following equation [53; 54; 55; 56]:  
\[\ce{CaCO_{3}\to CaO+CO_{2}} \tag{2}\]  
Therefore, the partial mass loss in this interval and the partial contribution to the first endothermic band at a temperature of 880 \({}^{\circ}\)C observed in the DTGA dependence can be attributed to reaction (2). In this case, the high-temperature shift of the maximum in the DTGA may be due to the presence of carbon dioxide since the decomposition of CaCO\({}_{3}\) shifts to the region of higher temperatures when the partial pressure of CO\({}_{2}\) increases [57; 58]. As a result of the decomposition reactions (1) and (2) at temperatures above 900 \({}^{\circ}\)C, only CaO oxide will remain from the calcium compounds, and carbon dioxide CO\({}_{2}\) and H\({}_{2}\)O will be formed. It is essential to note that according to reference [24], the qualitative analysis by means of quadrupole mass spectrometry of the evolved gases during heat treatment clearly showed the evolution of H\({}_{2}\)O and CO\({}_{2}\) in this temperature range. It is known that under these conditions there is a high-temperature reversible decomposition reaction associated with the release of carbon dioxide in accordance with the reaction  
\[\ce{CaCO_{3}\leftrightarrow CaO+CO_{2}} \tag{3}\]  
The carbonate decomposition reaction is endothermic and the reverse reaction is exothermic [58; 59]; thus, they should appear as minima and maxima in the DSC dependence, respectively. It is obvious that this process must proceed in cycles and, as the carbon dioxide evaporates, it ends in an exothermic reaction with the formation of calcium oxide. Exactly such an alternating sequence of peaks of different polarity is observed in the DSC experimental dependence. When the temperature reaches \(\sim\)1100 \({}^{\circ}\)C, the carbon dioxide volatilizes from the material. As a result, carbon, carbon oxides, and calcium oxide remain in the carbonized sponge after pyrolysis. The EDX analysis of carbonized sponge showed that carbon dominated in the carbonized sponge volume (\(\sim\)97 at.%). In addition, it contained oxygen (\(\sim\)2 at.%) and traces of calcium and silicon. However, according to the XPS data, the oxygen content in the surface layer of carbonized sponge was slightly higher than in the volume (\(\sim\)8-10 at.%), the carbon content was \(\sim\)90-98 at.%, the calcium content was \(\sim\)1-2 at.%, and there was an insignificant amount of silicon atoms. Figure 1e-1h show photos and SEM images of carbonized sponge, from which it is clear that its original 3D structure was preserved. It should be noted that the 3D structure of the native sponge existed solely due to the spongin scaffold, which had already completely decomposed at 450 \({}^{\circ}\)C. Therefore, from the end of thermal treatment stage II, the first steps of graphitization of the substance surrounding the sponge fibrils had already taken place. This was convincingly demonstrated in our previous work [21] by the characteristic changes in the C 1s NEXAFS spectra and in the Raman spectra with successive increases in pyrolysis temperature at 400, 600, 800, 1000, and 1200 \({}^{\circ}\)C. This process proceeded uniformly up to thermal treatment stage III and continued further after the decomposition of calcium compounds. Figure 3 shows the C 1s NEXAFS spectra of the sponge after pyrolysis in an argon flow at 1200 \({}^{\circ}\)C in comparison with the data for the native sponge and multiwalled carbon nanotube (MWCNT) [38; 39].