A humic acid loaded nano-zero-valent iron composite for enhanced immobilization of chromium and cadmium from soil.

INTRODUCTION: As the global demand for soil pollution control becomes increasingly urgent, innovative material application strategies are essential for remediating soils co-contaminated with chromium (Cr) and cadmium (Cd) by efficiently immobilizing heavy metal ions and promoting their stabilization. This study evaluated humic acid-loaded nanoscale zero-valent iron (nZVI@HA) for the remediation of Cr- and Cd-contaminated soil. METHODS: The remediation performance and underlying mechanisms of nZVI@HA were systematically investigated through soil incubation experiments, metal speciation analysis, high-throughput sequencing, and quantitative real-time PCR. RESULTS: nZVI@HA achieved immobilization efficiencies of 58.19% for available Cr and 35.84% for available Cd. On day 50 of remediation, exchangeable Cr and Cd were markedly transformed into residual, Fe-Mn oxide-bound, and carbonate-bound fractions, substantially reducing their mobility and bioavailability. Concurrently, nZVI@HA alleviated Cr(VI) and Cd(II) stress, increased microbial community diversity, and improved soil fertility. The enrichment of taxa associated with ChrA, CzcA, and NitR suggests that nZVI@HA may enhance microbial resistance to Cr(VI) and Cd(II), potentially contributing to Cr and Cd immobilization. DISCUSSION: The immobilization mechanisms primarily involved chemical reduction, adsorption, ion exchange, and microbially mediated processes. The synergistic effects between nZVI@HA and soil microorganisms make nZVI@HA an efficient and environmentally benign remediation material, providing an effective strategy for treating soils co-contaminated with heavy metals.