Humic acid-supported nanoscale zero-valent iron for sustainable cadmium remediation and crop safety in farmland soil.

The immobilization efficacy of most existing stabilizers for cadmium (Cd)-contaminated soil is highly sensitive to ecological and environmental fluctuations. These fluctuations can induce the reactivation of Cd, thereby restricting the long-term applicability of stabilization technologies. In this study, we synthesized a humic acid (HA)-supported nanoscale zero-valent iron (nZVI) particle (nZVI@HA). The roles of nZVI@HA in Cd sequestration and rice production were investigated throughout the entire rice life cycle using pot experiments with separate and combined applications of pristine nZVI and HA, as well as varying dosages of nZVI@HA. The application of 500-1000 mg/kg nZVI@HA significantly reduced the available Cd content to 37.85 %-55.24 % of that in the control group at 140 days. Analysis of the dynamic relationship between Cd species and solid-phase iron revealed that HA modification increased electronegativity and inhibited Fe0 oxidation, enabling nZVI@HA to passivate Cd through electrostatic attraction and complexation. Furthermore, the grain Cd content decreased from 0.78 to 0.16-0.19 mg/kg, whereas the grain yield increased from 3.59 to 6.54-8.24 g/plant after treatments with 500-1000 mg/kg nZVI@HA. The stimulatory effects of nZVI@HA on both soil ammonia nitrogen (NH4+-N) content and iron plaque formation were identified as specific functions. Additionally, 16S rRNA sequencing indicated that the simultaneous promotion of Cd sequestration and safe crop production relied on the nZVI@HA-facilitated coupling process of microbial iron-metabolism, Cd species transformation, and nitrogen cycling.