Correlations between slow pyrolysis characteristics and organic carbon structure of aquatic plant biomass

Thermal analysis techniques have been widely used to characterize natural organic matter; in particular, thermal oxidation has been used to examine soil and sediment organic matter. However, few studies have characterized natural organic matter (NOM) by using slow thermal degradation under a N₂ atmosphere. ¹³C nuclear magnetic resonance (NMR) spectroscopy, UV-Vis spectroscopy, and three-dimensional excitation and emission matrix (EEM) fluorescence spectroscopy were used to characterize aquatic plant biomass for the detailed interpretation of the structures of organic carbon during slow pyrolysis. There was a significant linear correlation between the absorption of heat (99–110 °C) and the loss of mass (110–160 °C) (r² = 0.507, p = 0.01), which indicates that the initial slight loss in mass of the plant materials was due to the loss of less thermally stable components. The release of heat (277–311 °C) and the ratio of the specific absorbances at 253 and 203 nm (A_253/203) were also correlated (r² = 0.388, p = 0.008), which suggests that the release of plant biomass upon heating was associated with the proportion of substituent groups on aromatic rings and that the release of heat increased with the amount of substitution. The coefficient of determination (r²) between fulvic acid-like fluorescence peaks and the loss of mass (230–340 °C) was 0.236 (p = 0.048). This result indicates that the loss of mass in the plant material samples was related to fulvic acid-like substances. More specifically, the reason for this result was the splitting of some aromatic functional groups, such as ether bonds, carbonyl groups, and oxygen heterocycles. In conclusion, these results suggest that the developed correlations between slow pyrolysis characteristics and organic carbon structures contribute to the investigation of the inner chemical structures of natural organic matter.