Heavy metal transformation in livestock manure (co-)pyrolysis: pathways toward safe biochar and sustainable agriculture

Livestock manure is enriched with heavy metals such as copper, zinc, and cadmium due to feed additives and intensive farming practices. Inadequate management can lead to soil accumulation, nutrient cycle disruption, and ecosystem risks. Pyrolysis, as a versatile thermochemical process, simultaneously enables pollutant control, energy recovery, nutrient recycling, and heavy metal stabilization. This review integrates mechanistic insights with sustainability-oriented evaluation, linking thermochemical transformations to agricultural applications and policy frameworks. We examine thermal-induced changes in heavy metal speciation and mobility, highlighting stabilization through encapsulation, complexation, and mineralization, while also critically assessing sequential extraction methods. The synergistic effects of co-pyrolysis and mineral additives are further discussed. By bridging molecular-scale mechanisms with sustainable resource management, this work provides a cross-disciplinary perspective to guide safe biochar reuse, integrated manure management, and broader sustainability goals. • Heavy metal transformation pathways during manure pyrolysis are systematically reviewed. • Speciation dynamics determine ecological risk and stabilization mechanisms. • Sequential extraction methods for manure-derived biochar are critically assessed. • Co-pyrolysis and additives enhance metal immobilization and reduce leaching.