Insights into the characteristics and toxicity of microalgal biochar-derived dissolved organic matter by spectroscopy and machine learning.

Microalgal biochar has potential applications in various fields; however, there is limited research on the properties and risks of microalgal biochar-derived dissolved organic matter (MBDOM). This study examined how different pyrolysis temperatures (200 °C and 500 °C) and extraction solutions (0.1 mol/L HCl, Milli-Q water, and 0.1 mol/L NaOH) affect the characteristics and toxicity of MBDOM from three microalgae using multi-spectroscopy methods. Results showed that higher pyrolysis temperature reduced dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorus (TP) but increased total potassium (TK) in the MBDOM. Alkaline solution promoted DOC and TN dissolution, while acidic solutions enhanced TP and TK release from biochar. The molecular weight, aromaticity, and fluorescent composition of MBDOM varied based on pyrolysis temperature, extraction solution, and microalgae species. MBDOM from low pyrolysis temperature and alkaline extraction exhibited significant toxicity to Photobacterium phosphoreum T. Correlation analysis and machine learning revealed that pyrolysis temperature had a greater influence on the characteristics and toxicity of MBDOM than the extraction solution. The toxicity of MBDOM was primarily associated with TN and DOC contents and also influenced by molecular weight, aromaticity, and humification. These findings are essential for optimizing microalgal biochar production and application.