Managing Post-Phytoremediation Biomass Within a Circular Economy Framework: Multitrophic Ecotoxicological Assessment of Biomass, Derived Biochar and Their Leachable Fractions

Featured Application: The proposed multitrophic ecotoxicological assessment framework can be directly applied by environmental practitioners, waste management operators, and decision-makers to evaluate the safety of phytoremediation-derived biomass and its conversion products prior to environmental use. The results demonstrate that pyrolysis can serve as an effective risk mitigation strategy, enabling the transformation of contaminated biomass into a more stable material suitable for soil application under controlled conditions. At the same time, the study highlights the necessity of including leachate-based aquatic toxicity tests to avoid underestimation of environmental risks. The approach supports risk-based decision-making and contributes to the development of safe circular economy pathways for biomass valorization in soil remediation systems. Phytoremediation is a sustainable approach for the remediation of heavy metal–contaminated soils; however, the management of contaminated biomass generated during this process remains an insufficiently addressed challenge. Such biomass constitutes a secondary waste stream that may release mobile pollutants and pose environmental risks. In this study, an integrated ecotoxicological assessment framework was applied to evaluate phytoremediation-derived biomass and its transformation products obtained via pyrolysis. Two types of woody biomass with different heavy metal contents and their corresponding biochars produced at 700 °C were investigated. A multitrophic battery of bioassays combining direct exposure assays using terrestrial organisms (higher plants, Eisenia fetida, and soil microbial activity) with leachate-based assays using aquatic organisms (Lemna minor, Daphnia magna, and Aliivibrio fischeri) was applied. Untreated biomass exhibited high to extreme toxicity in aquatic systems (toxic units, TU >100) and significant phytotoxic effects. Pyrolysis substantially reduced contaminant mobility and ecotoxicity of leachates, resulting in lower toxicity (TU typically <15) and no significant effects on plant growth, earthworm survival, or soil microbial functional diversity. Residual toxicity was linked to elevated pH and trace amounts of thermally generated organic substances. These results demonstrate that pyrolysis effectively reduces the environmental risk of contaminated biomass and supports the use of multitrophic ecotoxicological testing for safe waste valorization within circular economy strategies.