Alleviating Pb Toxicity in Rice–Soil Systems by GSNO-Encapsulated Chitosan–Biochar/S-nZVI Composites: Sustained Release of NO and the Stabilization of Pb

Lead (Pb) accumulation in paddy soils promotes its transfer to rice grains, posing persistent risks to food safety and human health. Here, we developed a GSNO-encapsulated chitosan–biochar-supported sulfidized nanoscale zero-valent iron composite (GSNO-CS@S-nZVI/BC) to integrate sustained nitric oxide (NO) delivery with Pb immobilization in rice–soil systems. The novelty of this strategy lies in coupling a biocompatible NO donor with a Pb-reactive CS@S-nZVI/BC matrix to simultaneously alleviate Pb-induced physiological stress in rice and suppress Pb migration from soil to edible grains. In hydroponic experiments, Pb exposure reduced rice fresh weight, dry weight, and chlorophyll levels by 71.2%, 55.6%, and 75.1%, respectively. Compared with Pb treatment alone, GSNO-CS@S-nZVI/BC increased rice dry weight and chlorophyll levels by 68.5% and 260%, respectively, while decreasing Pb concentrations in roots and shoots by 78.2% and 85.8%. In soil pot experiments, GSNO-CS@S-nZVI/BC reduced Pb concentrations in roots, shoots, panicles, and grains by 57.6%, 57.0%, 40.3%, and 64.3%, respectively. It also decreased the grain Pb bioconcentration factor by 80.7%, reduced the soil-to-root Pb translocation factor by 84.41%, and lowered the grain Pb bioconcentration factor to 0.043 ± 0.007. Mechanistically, GSNO-CS@S-nZVI/BC promoted root iron plaque formation by 176.1% and increased Pb adsorption by root iron plaque by 156.9%, while decreasing the oxidizable Pb fraction in soil by 11% and increasing the reducible Pb fraction by 10%. These results indicate that GSNO-CS@S-nZVI/BC mitigates Pb toxicity through sustained NO release, enhanced root iron plaque-mediated Pb sequestration, and soil Pb stabilization. Overall, this nano-enabled amendment provides a promising strategy for reducing Pb transfer from contaminated paddy soils to rice grains and improving the safe utilization of Pb-contaminated farmland.