Reassessing the role of pyrolysis temperature: freeze–thaw aging challenges heavy metals stability in biochar

Abstract

Manure-derived biochar is widely used for soil remediation, yet the long-term effects of aging on the stability of its endogenous heavy metals remain unclear. This study aims to investigate how freeze–thaw (FT) cycle aging influences the physicochemical properties and endogenous heavy metal stability of chicken manure (CM)-derived biochars produced at different pyrolysis temperatures (350 °C, 550 °C, and 750 °C). By subjecting these biochars to accelerated FT aging, we compared changes in structural integrity and heavy metal speciation. FT cycles significantly induced physical fragmentation in porous biochar, reducing pH, graphitization, and stability while increasing the total specific surface area (SSA) and oxygen-containing functional groups. Biochars produced at higher pyrolysis temperatures demonstrated greater susceptibility to structural breakdown during FT aging, which led to increased leachability and phyto-availability of heavy metals. Chemical speciation analysis revealed that biochar produced at 750 °C (CMB-750) experienced a pronounced transformation of heavy metals into less stable forms during FT aging, with acid-soluble (F1) fractions of Zn, Cu, Ni, Cr, Pb, and Cd in aged biochar (ACMB-750) increasing to 34.97%, 18.06%, 18.34%, 13.16%, 31.23%, and 6.31%, respectively. These findings highlight the risks of presuming that higher pyrolysis temperatures always enhance heavy metals retention and emphasize the importance of considering the entire biochar lifecycle, from fabrication and soil remediation to aging, when optimizing its safe and sustainable agricultural use.

Keywords

Freeze–thaw aging / Manure-derived biochar / Pyrolysis temperature / Heavy metal speciation / Environmental stability

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Funding

Natural Science Foundation of Xinjiang Uygur Autonomous Region(2022D01C737)

RIGHTS & PERMISSIONS

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