Synergistic enhancement of neonicotinoid removal at paddy soil-water interface via biochar and nanoscale zero-valent iron

Continuous pesticide application in agricultural systems poses significant risks to soil quality, yet strategies for maintaining sustainable pesticide cycling remain underexplored. Using pot experiments, this study simulates the entry of exogenous pollutants into paddy soils by combining biochar (BC) and nanoscale zero-valent iron (nZVI) to control and remove neonicotinoid pesticides. Soil extract experiments demonstrated complete degradation of thiamethoxam (20 mg/L) within 20 days. The combined treatment enhanced degradation efficiency by at least 55 % compared to the cumulative degradation by separate treatments, indicating significant synergistic effects. Amendment with 10 g/kg biochar immobilized 68.5 % of exogenous thiamethoxam in surface soil (1–2 cm) but concurrently prolonged its persistence. In contrast, nZVI incorporation enhanced thiamethoxam and intermediate degradation by 21.5 % (90 d) in biochar-amended soil, demonstrating superior efficacy to other iron-based materials. Microbial activity dominated thiamethoxam degradation at soil-water interfaces, while phototrophic biofilms primarily drove aqueous-phase degradation. The synergistic interactions between biochar and nZVI regulated microbial functionality, significantly enhancing adsorption and degradation efficiency for exogenous thiamethoxam. This work systematically elucidates adsorption-degradation mechanisms within the BC-nZVI-microbe ternary system, establishing a theoretical framework for optimizing material-biological interfacial kinetics in soil remediation. These findings provide novel strategies for pesticide control in paddy fields to benefit soil safety.