## 3 Results and Discussion
The peak in the O 1s PE spectrum was asymmetric and could be decomposed into two components at the BEs of 532.9 eV and 531.45 eV, which corresponded to the BEs of O 1s electrons in the C-OH group and atomic groups containing the double C=O bond, which also include peptide and carboxyl groups. It should be noted that the possible presence of water adsorbed on the sponge surface, for which the O 1s BE lies in the range of 532-534 eV, may have contributed to the formation of the peak at the BE of 532.9 eV [40; 48; 49; 50]. The atomic content of oxygen in these groups was 13.7% (C=O) and 2.3% (C-OH) of the total number of atoms in the sponge. In the carbon spectrum, the peak consisted of three components with BEs of 284.6 eV (C-C, C=C), 286.1 eV (C-OH), and 288.2 eV (C=O). The percentages of carbon atoms in these groups, as determined by the peak areas, were 63.2% (C-C and C=C), 6.3% (C-OH), and 7.3% (C=O). Taking into account the earlier assessment of the percentage of nitrogen, carbon, and oxygen atoms in the marine sponge spongin, the content of atomic groups in the sponge around the spongin fibrils was estimated. The carbon atom content was obtained for the following groups: C-C and C=C -61%, C=O -5%, and C-OH -5% (taking into account the presence of such a group in the spongin). Similar estimates for the oxygen atom content were as follows: C=O -12% and C-OH -2%. The fact that there was twice as much oxygen  
Figure 3: C 1s NEXAFS spectra of native sponge, carbonized sponge, and MWCNT. as carbon per double bond suggested the dominance of carboxyl O=C-OH groups. In turn, about 2.5-fold higher oxygen content relative to carbon in the C-OH groups suggested a significant number of epoxy C-O-C bonds in the sponge. To study the carbonization process of the native sponge, TGA, DTGA, and DSC were performed by heating the sample in an argon flow in the temperature range of 24-1200 \({}^{\circ}\)C.  
The thermographic dependencies in Figure 4 show that the thermal modification of the sponge proceeded in three stages. The first stage was accompanied by approximately 10% mass loss in the temperature range of 80-140 \({}^{\circ}\)C, which is characteristic of collagen, spongin, and other biomaterials [51; 52], and is caused by the evaporation of water adsorbed on the surface and involved in hydrogen bonding in the volume of the sponge. The second stage was observed in the temperature range of 210-450 \({}^{\circ}\)C and was accompanied by a significant (more than 40%) mass loss. The DTGA minimum was observed at a temperature of 302 \({}^{\circ}\)C. According to previous studies, spongin decomposes in this temperature range and the peptide bond is broken [21; 51]. This process is accompanied by the release of nitrogen-rich compounds and partial burnout of carbon with the release of CO\({}_{2}\)[24]. The third stage occurred at high temperatures of 800-1100 \({}^{\circ}\)C with a loss of about 20% mass and was accompanied by two broad peaks in the DTGA at temperatures of 880 \({}^{\circ}\)C and 1014 \({}^{\circ}\)C, as well as a sequence of alternating exothermic and endothermic narrow peaks in the DSC curve in the temperature range from 920 \({}^{\circ}\)C to 1080 \({}^{\circ}\)C. Taking into account the presence of a small (\(\sim\)2 at.%) content of calcium in the native sponge in such probable compounds as calcium carbonate (limestone CaCO\({}_{3}\)), calcium oxide (CaO), and calcium hydroxide (Ca(OH)\({}_{2}\)), the following interpretation of the observed changes in the thermographic dependences may be given. If there is Ca(OH)\({}_{2}\) in a sponge, then in a range of temperatures from 370 to 440 \({}^{\circ}\)C, there will be an endothermic decomposition [53] according to the equation  
\[\text{Ca(OH)}_{2}\rightarrow\text{CaO}+\text{H}_{2}\text{O} \tag{1}\]  
Due to the above reactions, the appearance of a wide endothermic band in the DTGA dependence at \(\sim\)400 \({}^{\circ}\)C can be associated with this decomposition.