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

TL;DR

This study shows that heating metal-rich plant waste at high temperatures can safely lock in lead, reducing its risk to the environment.

Contribution

The study identifies specific chemical mechanisms by which pyrolysis stabilizes lead in biochar at different temperatures.

Findings

Pyrolysis at 400°C stabilizes lead mainly through Pb2(P4O12) and PbCO3 formation.

At 500°C and above, Pb3(CO3)2(OH)2 and other compounds dominate lead immobilization.

Higher pyrolysis temperatures significantly reduce bioavailable lead fractions and ecological risk.

Abstract

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 (BMPb) across a temperature range (300 °C–700 °C) to produce Pb-enriched biochar (BCPb), evaluating its efficacy for safe residue management. The results demonstrated that pyrolysis effectively reduced the volume of BMPb, and the produced BCPb 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 BCPb 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 BCPb-amended systems towards stable forms (F3+F4), confirming low ecological risk. Overall, these findings suggested that pyrolysis of BMPb 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.

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