# A Thermogravimetric Analysis of Biomass Conversion to Biochar: Experimental and Kinetic Modeling
## 3 Results and Discussion
### Effect of Temperature on Biomass Waste Surface Structure
#### 3.3.2 SEM Analysis Results  
The surface morphologies of both samples are influenced by the pyrolysis temperature, as depicted in Figure 9. When the apple and potato peels were heated to 250 \({}^{\circ}\)C, the resulting image displayed a smooth surface with minimal visible pores. At a temperature of 800 \({}^{\circ}\)C, the biochar exhibited the formation of deep porous channels, which were a result of the significant elimination of volatile organic material during the pyrolysis process. [PERSON] et al. [77] suggest that higher pyrolysis temperatures can lead to the destruction of aliphatic alkyl and ester groups and of the aromatic lignin. These observations are in correlation with the FTIR analysis, which indicates the disappearance of the bands characteristic of hydroxyl, aliphatic, ether, and ester functional groups. The elemental analysis conducted by the Energy Dispersive Spectrometer (EDS) shows that potato peel biochar pyrolyzed at 800 \({}^{\circ}\)C and has 66.52% carbon and 29.60% oxygen, while apple peel biochar consists of 77.78% carbon and 16.35% oxygen (Figure 10). In comparison to the ultimate analysis of potato peel, which indicates a carbon content of 43.9% and oxygen content of 46.8%, and apple waste [78; 79], which exhibits a chemical composition ranging from 46.8% to 47.1% carbon and 43.6% to 46% oxygen, the reduced oxygen content in biochar confirms the loss of functional groups caused by dehydration and deoxygenation reactions during thermal treatment. This process may result in an increase in surface area and consequently enhance the adsorption capacity of the material, as stated by [PERSON] et al. [80] and [PERSON] et al. [81]. The cavity's diameter measured less than 40 \(\upmu\)m for the potato peel and less than 30 \(\upmu\)m for the apple peel. Figure 8: FTIR curves of food waste biomass and biochar. Figure 10: EDS images of (**A**) apple peel pyrolyzed at 250 \({}^{\circ}\)C; (**B**) apple peel pyrolyzed at 800 \({}^{\circ}\)C (the + sign in the figures is the cursor position where the sample was analyzed). Figure 9: SEM images of (**A**) apple peel pyrolyzed at 250 \({}^{\circ}\)C; (**B**) apple peel pyrolyzed at 800 \({}^{\circ}\)C; (**C**) potato peel pyrolyzed at 250 \({}^{\circ}\)C; (**D**) potato peel pyrolyzed at 800 \({}^{\circ}\)C.  
A preliminary study regarding the possible uses of the obtained biochars  
Biochar has been tested as an adsorbent in numerous studies to remove NH\({}_{4}\)+, NO\({}_{3}\)-, and PO\({}_{4}\)3- from water either separately or in mixtures [82; 83]. According to these reports, the types and characteristics of biochar as well as the environmental conditions of the aqueous phase affect the removal efficiency [84]. The surface morphology (porosity, surface area, etc.) and surface chemistry (functional group existence) of the biochar are influenced by the pyrolysis temperature, which influence the biochar's sorption capacity [85]. The feedstock characteristics also have an impact on NO\({}^{3-}\) adsorption potentials [86], so more research is needed to better understand how feedstock characteristics determine NO\({}^{3-}\) adsorption potentials. Our search in the Web of Science (WOS) database using terms \"(biochar) AND (apple peel) OR (potato peel) AND (nitrate removal)\" yielded no documents, indicating that this combination has not yet been explored. A broader search for biochar produced from apple or potato peel resulted in only 52 documents, consisting of 44 research papers and 8 review papers. Using VOSviewer software, the co-occurrence analysis was performed again, including author keywords, setting the minimum number of keyword occurrences to 5, resulting in a map with 14 thresholds (Figure 11). The density visualization map highlights the research landscape surrounding biochar, with \"biochar\" positioned as the central, most densely colored node, indicating that it is the primary focus and most studied area. This cluster reflects strong interest and connectivity with other related topics.