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

Abstract

Environmentally persistent free radicals (EPFRs) are emerging contaminants of increasing concern due to their toxicity for life and ecosystems, yet their formation, behavior and fate are poorly known. In particular, there is actually no knowledge on the formation of those radicals during the thermal treatment of soils containing polycyclic aromatic hydrocarbons. Such knowledge is important because thermal treatment is a remediation method used to decontaminate soils by removing and degrading PAHs. Here, we studied the formation of radicals in three types of cultivated soils, bauxite soil, fluvo-aquic soil and chernozem soil, artificially contaminated by benzo[a]pyrene, during thermal treatment from 100 to 200 °C for 1 h, using electron paramagnetic resonance. Results show spins densities of radicals up to of 2.079 × 10 17 spins/g for bauxite soil, 1.481 × 10 17 spins/g for fluvo-aquic soil and 8.592 × 10 16 spins/g for chernozem soil at 175 °C. The formed radicals exhibited multiple decays during their observable time and the shortest 1/e lifetimes of radicals up to 757.58 h. These findings are strengthened by EPFR-induction of reactive oxygen species (ROS), O 2 •− and • OH, which increased in concentrations from 100 to 200 °C. Overall, our results demonstrates for the first time that thermal treatment of PAHs-contaminated soils induces the formation of EPFRs and suggests that thermal treatment might not be a fully clean remediation method for soils as thermal treatment creates new contaminants.

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

“…Results show strong PFRs signals in heated soils, maximizing at 300 °C, and almost no signal for the unheated anthracene-spiked soils (Figure S3) and heated raw soils without anthracene (Figure S1). This finding confirms that PFRs signals are induced by both heating and the presence of PAHs, as shown for benzo[a]pyrene-spiked soils heated at 100-200 °C (Liu et al 2020). Rising the temperature was also necessary to form PFRs during adsorption of dichlorobenzene on cupric oxidecontaining particles at 100-400 °C (Lomnicki et al 2008).…”

“…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). Since thermal treatment is commonly applied to clean PAH-contaminated soils (Kuppusamy et al 2016), this remediation technique may pose potential health risks due to the formation of PFRs.…”

“…1a). The increase in PFRs from 100 to 300 °C is explained partly by the decreasing concentration of anthracene (Figure S4), which suggests the transformation of anthracene into anthracene-PFRs (Liu et al 2020). A decline of PFRs above 300 °C may be due to radical decomposition, recombination and transformation , c hydroxyl radical •OH, d hydrogen peroxide H 2 O 2 , e oxidative potential, and f growth of the green algae Chlorella.…”

“…Anthracene was below detection limits in original soils. The preparation of anthracene-contaminated soil was previously described (Liu et al 2020). Briefly, 1 g of soil sample was mixed with 1 mL of acetone solution containing 1000 mg L −1 of anthracene.…”

“…Thermally treated soils and control were extracted under various conditions to analyze persistent free radicals (PFRs) and reactive oxygen species (ROS) as described by Zhao et al (2020) and Liu et al (2020). PFRs and ROS were analyzed by an electron paramagnetic resonance (EPR) spectrometer (Bruker EMXmicro-6/1/P/L, Karlsruhe, Germany) at room temperature, about 23 °C.…”