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

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.

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.

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.

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.