Biochar-mediated immobilization of aged napropamide in soil: reduced bioavailability and underlying sequestration mechanisms.

Biochar has emerged as a promising material for immobilizing organic contaminants in agricultural soils. This study investigated how different biochars influence the adsorption, distribution, and bioavailability of aged napropamide during 112 d of aging in sterilized typical farmland soils across China. Aging significantly enhanced napropamide adsorption, with adsorption coefficients increasing by 4-16-fold compared to freshly spiked pesticide. This enhancement was further amplified when red soil was amended with three types of biochar (0.1-1%, w/w), particularly coconut shell-derived biochar (CSC), which demonstrated superior performance. Biochar addition substantially reduced the dissolved fraction of napropamide while promoting the formation of non-extractable residues through various interaction mechanisms. The residual napropamide content in 1% CSC-treated soil remained remarkably low (0.46-1.34%) compared to 0.1% CSC treatment (2.15-5.32%). Mechanistic studies revealed that while bamboo-derived and tobacco stem-derived biochars involved both physical and chemical interactions with aged napropamide, CSC functioned predominantly through physical sequestration. Most importantly, CSC amendment substantially reduced the bioavailability of aged napropamide, as evidenced by over 85% reduction in Capsicum annuum uptake, minimal accumulation in Eisenia fetida tissues (0.05 mg/kg), and significantly decreased degradation residues by Bacillus cereus LGY06 (0.70 mg/kg). These comprehensive findings provide crucial insights into how biochar modifies the environmental behavior and reduces the ecological risk of aged pesticides in soil systems, offering valuable guidance for developing effective soil remediation strategies.