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

Persistent free radicals (PFRs) are emerging contaminants of increasing concern, yet their formation, fate, toxicity and health risk are poorly known. Thermal treatment, a common remediation technique to clean industrial soils, induces the formation of PFRs, which could paradoxically increase soil toxicity, contrary to the original objective of remediation. Actually, there is little knowledge on the formation and toxicity of PFRs in soils contaminated by polycyclic aromatic hydrocarbons (PAHs). Here we studied the generation of PFRs of soils spiked with anthracene and heated 1 h from 100 to 600 degrees C, using electron paramagnetic resonance. We also investigated the formation of reactive oxygen species (ROS), e.g. superoxide radical (O-2(center dot-)), hydrogen peroxide (H2O2) and hydroxyl radical (center dot OH), in the aqueous phase of thermal-treated soil, and the impact of heating on soil oxidative potential, wheat growth and green algae activity. Results showed that PFRs, ROS, soil oxidative potential, plant toxicity and algal toxicity show a similar trend with an increase from 100 to 300 degrees C, followed by a decrease to 600 degrees C. Scavenger trapping tests reveal that algal toxicity is mainly due to the generation of O-2(center dot-), center dot OH and H2O2 induced by anthracene-PFRs and that anthracene and anthracene-PFRs have negligible direct algal toxicity. Overall, our findings reveal the unintended formation of toxic compounds peaking at 300 degrees C during the thermal remediation of PAH-contaminated soils. These results should help to assess the environmental risk of thermally treated PAH-contaminated soils.

Automated summary

PFRs generated during thermal treatment of contaminated soils may lead to the formation of toxic compounds, with limited understanding of their formation and toxicity in soils contaminated by PAHs. Results show that toxic compounds peak at 300 degrees Celsius during thermal treatment, with algal toxicity mainly due to the generation of O-2(center dot-), center dot OH, and H2O2.