Characterization of Dissolved Organic Matter in Aquatic Macrophytes Derived-Biochars Using Multi-spectroscopic Analyses: Combined Effects of Pyrolysis Temperatures and Sequential Extractions

Dissolved organic matter in biochars (BDOM) influences the latter’s environmental applications. This study applied multiple spectra methods to expound the characteristics of BDOM in aquatic macrophytes ( Alternanthera philoxeroides ) biochars produced at different pyrolysis temperatures under sequential extractions (room-temperature water, hot water and a weak base). The results showed that the feedstock and the biochar produced at 200 °C, biochars produced at 300–500 °C and above 600 °C were unstable, metastable and stable, respectively, according to the release of dissolved organic carbon. One protein-like and four humic-like fluorophores were identified. The BDOM were dominated by humic-like components, and the pyrolysis of fluorescent components was hysteretic compared to the non-fluorescent materials. In addition, the relative molecular weight, aromaticity, and hydrophobicity of BDOM increased correspondingly under sequential extractions. High pH/temperature of extracting solution will lead to the BDOM with higher humification degree. This study also indicates humic acid and polysaccharide, as the fluorescent substance and the non-fluorescent substance, respectively, were more sensitive to pyrolysis temperature and changed firstly under sequential extractions. This study was beneficial for understanding the characteristics of BDOM at the molecular level and for predicting the possible effects and processes of biochars before large-scale applications. Graphical Abstract