Unveiling molecular transformations and mechanisms of soil organic matter during thermal remediation across temperatures

Thermal treatment is a commonly applied soil remediation technique that can also change the properties of soil. However, few studies looked into the transformation and mechanism of soil organic matter (SOM) at the molecular level when contaminated soil undergoes thermal remediation. Herein, FT-ICR MS, pyrolysis gas chromatography mass spectrometry (Py-GC–MS), and size exclusion chromatography-diode array detector-fluorescence detector-organic carbon detector (SEC-DAD-FLD-OCD) were used to unveil the molecular transformation mechanism of SOM under 100 °C and 400 °C treatments. The macromolecules transformed to humic and fulvic components after 400 °C treatment, while few transformations occurred after 100 °C treatment. The molecules decreased with the increase in temperature. After 400 °C treatment, over 99 % of the molecules were newly produced, which were mainly aromatic (66 %) and lignin-like (19 %) components. After 100 °C treatment, most of the molecules (73.6 %) were reserved. The unsaturation and aromatic structures of SOM increased significantly after the 400 °C treatment, while the changes were relatively minor after the 100 °C treatment. The fracture of the carbon skeleton, cyclization, and H abstraction reactions were the main mechanisms for the molecular transformation of SOM during 400 °C treatment, while volatilization was the main transformation scheme during 100 °C. Statistical results of pyrolysates show the increase of aromatic components, and the molecular markers reflected the increase of condensed organic carbon and the decrease of phenols, alkene components during 400 °C treatment. The sorption of PAHs for soil decreased after treatment, owing to the loss and transformation of SOM. This study advances the mechanistic understanding of transformation in SOM molecular signatures during thermal treatment.