Stabilization effects and mechanisms of lignin-based hydrogel-coated sulfide nano-zero-valent iron on lead and cadmium contamination in soil.

Nano zero-valent iron (nZVI) is extensively employed in soil remediation due to its superior capacity for removing heavy metals, however, issues related to its agglomeration and oxidation hinder its practical application. Therefore, in this study, lignin-based hydrogel-coated nano ferric sulfide (BLS-nZVI@LH) was synthesized and evaluated for its stabilizing effect, underlying mechanism, and influence on the soil microenvironment and health risks. The results indicate that BLS-nZVI@LH significantly mitigated nZVI agglomeration and oxidation, thereby enhancing its reactivity. The formation of FeS on the particle surface provided additional active sites for stabilizing Pb and Cd in the soil. In soil incubation experiments, BLS-nZVI@LH significantly improved the stability of Pb and Cd after 90 days. Compared to sulfide nano-zero-valent iron (S-nZVI) and ball-milled lignin sulfide nano-zero-valent iron (BLS-nZVI), BLS-nZVI@LH increased the soil's residual Cd content by 23.0 % and 31.0 %, respectively, and the oxidizable Pb content by 10.9 % and 20.8 %. Characterization analysis revealed that precipitation, redox reactions, and surface complexation primarily govern Cd stabilization by BLS-nZVI@LH, whereas complexation and reduction predominantly contribute to Pb immobilization. Furthermore, BLS-nZVI@LH improved soil pH and organic carbon content, boosting β-glucosidase and peroxidase activities. It also promoted the richness and diversity of soil microbial communities, particularly enhancing the growth of Sphingomonas, Gemmatimonas, and RB41, thereby improving the soil microenvironment and boosting remediation capacity. In continuous cropping experiments, the addition of 0.5 % and 1 % BLS-nZVI@LH significantly reduced Pb and Cd absorption and accumulation in Chinese broccoli. Notably, 1 % supplementation lowered Pb and Cd levels in edible parts below the national food safety standard (Pb ＜ 0.3, Cd ＜ 0.2 mg·kg-1), thereby effectively mitigating dietary health risks across different populations. This study offers technical insights into the development of highly active modified materials and provides scientific evidence for the application of BLS-nZVI@LH in stabilizing Pb and Cd contamination in soils, improving soil health, and reducing heavy metal accumulation in crops.