Biochar-assisted remediation of contaminated soils: Does reduced toxicity from PFOA immobilization outweigh impacts from metal and PAH contaminants in biochar?

Evaluation of biochar-assisted contaminant immobilization as a soil remediation technology must consider the trade-off between the positive effects of reduced toxic impacts through soil cleanup and climate change mitigation through carbon storage, and the negative impacts associated with contaminants introduced to soil with the biochar. Addressing this trade-off is challenging, as existing models and characterization factors used in life cycle impact assessment (LCIA) are generally not representative of conditions at contaminated sites, particularly in the presence of sorbents such as biochar. In addition, comparative toxicity potentials (CTPs) of most per- and polyfluoroalkyl substances (PFAS) compounds are not available for soils. To address these challenges, we applied the USEtox framework to quantify environmental fate, ecosystem exposure, and ecotoxicological effects of perfluorooctanoic acid (PFOA) as the remediation target, and several metals and polycyclic aromatic hydrocarbons (PAHs) introduced via biochar amendment. The resulting CTPs were used in the LCIA phase of a life cycle assessment (LCA) of soil remediation based on the LC-Impact methodology. Biochar had a significant influence on the CTPs of PAHs, a moderate effect on PFOA, and a limited effect on metals. The trade-off between the immobilization of PFOA and the introduction of PAHs and metals was outweighed by the benefits of carbon storage and avoided incumbent waste management. These results suggest that the implementation of biochar-assisted remediation should be primarily guided by the carbon storage potential of the biochar and the characteristics of incumbent feedstock treatment pathways, whereas the effect of biochar on PFOA immobilization was less influential.