High contribution of hydrocarbon transformation during the removal of polycyclic aromatic hydrocarbons from soils, humin and clay by thermal treatment at 100-200 °C

Polycyclic aromatic hydrocarbons (PAHs) are major pollutants in air, soils and sediments. PAH-polluted soils can be cleaned rapidly by thermal treatment. PAH volatilization is considered as the main process explaining PAH removal at low temperature, yet other processes may occur. Particularly, we hypothesize that thermal transformation can also explain PAH removal, where transformation refers to both degradation and formation of bound PAHs. We thus studied the removal of spiked benzo[a]pyrene at 0.5 mg/g in bauxite soil, fluvo-aquic soil, chernozem soil, montmorillonite, humin, and quartz sand as control, from 100 to 200 degrees C. We measured concentrations of benzo[a]pyrene in the volatilized fraction and solid residues by high-performance liquid chromatography. We identified transformation products by gas chromatography-mass spectrometry. Results show that the contribution of thermal transformation to the removal of benzo[a]pyrene increased from 24.7 to 58.4 wt% for bauxite soil, from 4.4 to 38.2 wt% for fluvo-aquic soil, and from 11.5 to 35.9 wt% for chernozem soil, with temperature increasing from 100 to 200 degrees C. Transformation such as oxidation occurred in all samples except in benzo[a]pyrene-spiked quartz sands. Transformation of benzo[a]pyrene was thus partly explained by the presence of clay minerals, as evidenced for the montmorillonite assay where transformation contributed 74.6 wt% to the total removal of benzo[a]pyrene at 200 degrees C. Overall, our findings demonstrate a major overlooked contribution of transformation to PAH removal at low temperature.