Integrating source stabilization to ecological exposure: A new framework to reassess sludge biochar potential toxic elements risk driven by pyrolysis temperature

Pyrolysis is a promising strategy for sewage sludge valorization, yet potential toxic elements (PTEs) limit the safe application of sludge-derived biochar (SBC). Conventional assessments rely on source-based indicators (e.g., BCR sequential extraction method, the risk assessment code), assuming that higher pyrolysis temperatures improve PTE stability and reduce risk. However, this assumption lacks validation in real soil-plant systems. Here, the behavior of Cr, Ni, Pb, and Zn was tracked across SBCs produced at 400, 600, and 800 °C using oxidative aging, soil incubation (60% vs. 100% water-holding capacity), and pot experiments. High-temperature SBCs enhanced Cr and Pb stabilization, but plant uptake revealed metal-specific trade-offs: Ni accumulation increased under SBC800, while Zn peaked under SBC400. Soil moisture strongly modulated Ni and Zn bioavailability, and SBCs disturbed microbial communities and enzyme activity. We developed SEIR-a source-stabilization-to-ecological-exposure integrated risk assessment framework-to quantify source reactivation, soil mobilization, plant exposure, and process hazard. SEIR demonstrates that no universal "optimal" temperature exists. Instead, safe SBC use depends on PTE identity and site-specific water conditions. This exposure-oriented framework enables scenario-specific temperature selection, advancing risk assessment from material stabilization toward system-level ecological safety.