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

Abstract

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 °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 °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 °C. Overall, our findings demonstrate a major overlooked contribution of transformation to PAH removal at low temperature.

Formation of environmentally persistent free radicals and reactive oxygen species during the thermal treatment of soils contaminated by polycyclic aromatic hydrocarbons

“…The negligible EPR signals observed for quartz may be also explained by the absence of clay minerals and organic matter. Our finding agrees with our previous report showing that benzo[a]pyrene is mainly removed by volatilization in quartz sands, whereas benzo[a]pyrene is mainly removed by thermal transformation when clay minerals and organic matter are present in soils, thus generating radicals (Jia et al 2020).…”

Plant and algal toxicity of persistent free radicals and reactive oxygen species generated by heating anthracene-contaminated soils from 100 to 600 °C

“…Transformation, e.g. oxidation, has also been evidenced recently during the removal of benzo [a]pyrene-spiked soils at 100-200 °C, despite desorption being considered as the main removal mechanism (Jia et al 2020). We also found that the transformation of benzo[a]pyrene in heated soils induces the formation of persistent free radicals (PFRs) (Liu et al 2020).…”

Roles of Natural Phenolic Compounds in Polycyclic Aromatic Hydrocarbons Abiotic Attenuation at Soil–Air Interfaces through Oxidative Coupling Reactions

“…The phenolic compounds were able to transfer electrons to oxidants (i.e., the minerals, O 2 ), and thus PFRs such as semiquinone radicals and phenoxy radicals were formed . These PFRs could induce a sustained generation of ROS through the redox cycle, ,− acting on PAHs oxidation and even mineralization.…”