Synergistic nZVI-biochar and biofilm remediation of thiamethoxam at soil-water interfaces

The widespread use of neonicotinoid insecticides such as thiamethoxam poses significant ecological risks due to their persistence and mobility, particularly at the soil-water interface (SWI). This study investigated a nanoscale zero-valent iron-loaded biochar composite (nZVI-BC) synergized with indigenous phototrophic biofilms to enhance in situ degradation. While biochar alone immobilized thiamethoxam, it also prolonged its persistence in soil. The nZVI-BC composites significantly reduced pesticide leaching (49.7–62.6%) and accelerated its soil degradation, achieving 85.3–96.6% removal within 40 days, which was 2.1–2.4 times greater than biochar alone, while reducing the pesticide’s half-life to 6.73–14.6 days. Phototrophic biofilms were the primary driver at the SWI, mediating aqueous-phase degradation (half-life of 0.67 days). Integrated analysis revealed that nZVI provided electrons for reductive transformation and enriched functional microbes, while biochar acted as a stable carrier and electron shuttle, facilitating microbial co-metabolism. This work demonstrates a synergistic “adsorption-reduction-biodegradation” strategy, effectively overcoming the limitation of biochar’s contaminant retention and offering a sustainable approach to pesticide remediation at critical environmental interfaces.