Distribution characteristics and ecotoxicity assessment of polycyclic aromatic hydrocarbons (PAHs) in hydrothermal carbonization products of corn stalks

Hydrothermal carbonization (HTC) of corn straw causes hydrolysis and pyrolytic reorganization of the carbon skeleton, leading to the formation of polycyclic aromatic hydrocarbons (PAHs). When used as a soil amendment, hydrothermal carbon can lead to soil contamination, increased biotoxicity, and potential harm to ecosystem health. To systematically evaluate PAHs formation mechanisms, single-factor experiments were carried out by treating corn straw under varying temperatures (180 to 300 °C) and durations (2 to 6 h) in a closed batch reactor. PAHs were quantified via gas chromatography-mass spectrometry (GC-MS) with deuterated internal standards. Results revealed that total PAHs concentrations increased by 409%, 66.5%, and 68.3% at 180 °C, 210 °C, and 240 °C (4 h and 2 h), respectively, attributed to intensified dehydration and aromatization reactions under subcritical conditions. Conversely, PAHs levels decreased by 80.4% and 78.1% at 270 °C and 300 °C (4 h and 2 h), likely due to thermal cracking of PAHs macromolecules into low-molecular-weight fragments. Prolonged treatment (6 h and 4 h) reduced PAHs by 62.9 to 70.8% at ≤240 °C, suggesting oxidative degradation pathways dominate over pyrolysis under extended residence time. Mechanistic analysis indicated that optimizing HTC at 270 °C for 4 h achieves a critical balance between carbonization efficiency and PAHs suppression, providing a feasible strategy to mitigate ecotoxicological risks of hydrothermal carbon in soil remediation.