The use of biochar reduces Pb mobility in poultry litter composting

The stabilization of potentially toxic metals during the composting of organic waste represents a central challenge for the environmentally safe management of these materials. In this study, we investigated how biochar addition modulates the structural evolution of organic matter (OM) throughout the composting of poultry litter and the implications of these changes for Pb²⁺ immobilization. Structural information obtained by solid-state ¹³C CP/MAS NMR spectroscopy was integrated with Pb²⁺ adsorption isotherms and multivariate analyses to elucidate structure-function relationships in fresh material and in composts aged for 30, 60, and 90 days and amended with biochar at rates of 5-15%. In fresh poultry litter, Pb²⁺ sorption was predominantly associated with oxygenated aliphatic domains of OM, indicating that it is controlled by relatively labile, low-energy interactions. As composting progressed, biochar addition promoted progressive structural reorganization of the organic matrix, characterized by enrichment in functionalized aromatic domains and carboxylic groups, as evidenced by the systematic increase in the 150-170 ppm region of the ¹³C CP/MAS NMR spectra. Principal component analysis revealed a gradual transition in the dominant Pb²⁺ sorption regime, shifting from associations with aliphatic structures to preferential interactions with higher surface-energy, functionalized aromatic domains. This structural evolution resulted in consistent increases in the distribution coefficients (K_d) with increasing composting time and biochar dose, reflecting enhanced Pb²⁺ stabilization through energetically stronger interactions compatible with inner-sphere complexation mechanisms. Overall, the results demonstrate that biochar acts not only as an intrinsic sorbent but also as an active agent driving OM transformation during composting, promoting the formation of more stable and reactive sorption sites. Collectively, the results of this study establish a robust mechanistic basis linking the biochar-induced structural evolution of OM to increased Pb²⁺ immobilization, supporting the use of biochar-assisted composting as an effective strategy for organic waste management and for mitigating metal mobility in agroecosystems.