Mechanistic insights into pyrolysis temperature-dependent lead (Pb) stabilization in phytoremediation residue-derived biochar

The substantial generation of hazardous, metal-enriched biomass residues poses significant risks of secondary contamination, presenting a critical bottleneck to the broader implementation of phytoremediation that urgently requires effective treatment solutions. This study addressed this challenge by pyrolyzing Pb-enriched biomass (BM_Pb) across a temperature range (300 °C-700 °C) to produce Pb-enriched biochar (BC_Pb), evaluating its efficacy for safe residue management. The results demonstrated that pyrolysis effectively reduced the volume of BM_Pb, and the produced BC_Pb significantly enriched and immobilized Pb. Element analysis revealed distinct stabilization mechanisms: Pb₂(P₄O₁₂) and PbCO₃ precipitation dominated Pb immobilization at 400 °C, whereas Pb₃(CO₃)₂(OH)₂, Pb₂(P₄O₁₂), and NaAlSiO₄ became predominant at temperatures ≥500 °C. Sequential extraction of Pb (BCR) demonstrated a consistent decline in the more labile Pb fractions (exchangeable, F1, and reducible, F2) with increasing pyrolysis temperature, concurrent with a significant increasing in the stable fractions (oxidizable, F3, and residual, F4). Notably, the combined F1+F2 fraction decreased substantially (17% at 700 °C), while the stable F3+F4 fraction increased correspondingly (83% at 700 °C), indicating markedly reduced Pb bioavailability and ecological risk at elevated temperatures. Leaching tests confirmed that Pb release from all BC_Pb samples remained well below relevant regulatory thresholds when the pH higher than 2 (<9.98 mg·g^-1 vs. 10.0 mg·g^-1), with leaching susceptibility inversely related to pyrolysis temperature. Soil simulation experiments further indicated a conversion of bioavailable Pb (F1+F2) in BC_Pb-amended systems towards stable forms (F3+F4), confirming low ecological risk. Overall, these findings suggested that pyrolysis of BM_Pb at temperatures above 500 °C shows great promise as an effective and safe method for treating phytoremediation residues, demonstrating high stability and low ecological risk to both water and soil environments under most natural conditions, though careful management is required under extreme acidic scenarios.