Immobilization mechanisms and transport fate of neonicotinoids in biochar-amended soils: insights into sorption-desorption hysteresis.

The widespread use of neonicotinoid pesticides, characterized by high water solubility and low soil adsorption, poses a significant threat to groundwater quality due to their high leaching potential. This study investigated the efficacy of biochar as a soil amendment for immobilizing thiamethoxam (TMX) and clothianidin (CLO). Through a combination of batch adsorption-desorption experiments and dynamic soil column studies, we demonstrate that biochar amendment induces strong and nearly irreversible sorption, increasing the soil's adsorption capacity by over 20-fold at a 3% application rate. The adsorption process was governed by chemisorption and best described by the pseudo-second-order kinetic and Freundlich isotherm models, indicating multi-mechanistic synergy including pore-filling and π-π interactions. Critically, column leaching and elution experiments identified a dosage threshold of ≥1% (w/w) biochar, which achieved complete interception of freshly applied pesticides and eliminated the mobilization risk of aged residues. Mechanistic analysis revealed that adsorption promoted the formation of larger, stabilized soil-biochar-pesticide aggregates, a key colloidal-scale process for co-immobilization. These findings underscore the potential of biochar as an effective in-situ strategy to mitigate the environmental mobility of neonicotinoids and protect water resources.