Toward dioxin-free chars: comparative toxicity of biomass valorization pathways under CO2, N2, and hydrothermal conditions

Low-temperature torrefaction converts biomass into high-carbon chars with valuable potential for a circular bioeconomy and diverse industrial applications. However, the concurrent formation of toxic polychlorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDF), and dioxin-like polychlorinated biphenyls (dl-PCB) severely constrains their use, as these contaminants threaten to enter the environment and the food chain. In this article, we assess how different thermochemical pathways for valorization of bark rhytidome shape the toxicity of the resulting material, focusing on the evolving load of PCDD/F, dl-PCB, and indicator PCB. The valorization treatments encompass mild-oxidative torrefaction (MOT, under CO₂), inert torrefaction (IT, under N₂), and wet subcritical torrefaction (hydrothermal carbonization, HTC), each conducted at four temperatures-200 °C, 240 °C, 280 °C, and 320 °C. Wet torrefaction yields the most toxic chars-(1.74 ± 0.26) ng TEQ·kg^-1 (88 % DM) at 200 °C and (3.44 ± 0.52) ng TEQ·kg^-1 (88 % DM) at 240 °C-based on toxicity equivalent (TEQ) of PCDD/F/dl-PCB; 88 % DM refers to 88 % dry-matter basis. Dry-run torrefactions under CO₂ and N₂ produce less toxic chars, with elevated toxicity over raw biomass only at 200 °C. In chars generated by wet torrefaction, we discover a distinct dechlorination pattern from less-toxic octa-substituted PCDD/F (OCDD/F) to more-toxic penta- to tetra-substituted PCDD/F (PeCDD/F to TCDD/F). Density functional theory (DFT) calculations provide supporting evidence for the water-catalyzed 1,2-Cl positional shift-a newly identified pathway that converts non-toxic congeners of PCDD/F into toxic ones during wet torrefaction. We also determine that temperature exerts a statistically significant influence on the abundance of detectable PCDD/F/dl-PCB congeners across all thermochemical valorization treatments. Overall, at higher temperatures, these treatments provide an effective means of detoxifying the biomass.