Dual effects of schwertmannite pre-/post-loaded with nano zero-valent iron on Cr(VI) stabilization and microbial community in contaminated soils.

Hexavalent chromium (Cr(VI)) contamination in soil presents a severe environmental hazard due to its high toxicity and mobility. This study investigates the stabilization efficiency, mechanisms, and short-term microbial response of Schwertmannite pre-/post-loaded nano zero-valent iron (Sch and nZVI@Sch) in real Cr(VI) contaminated soil. Sch represents the unloaded material, whereas nZVI@Sch is the Fe0-loaded composite. Compared with Sch, nZVI@Sch provides additional reductive reactivity for Cr(VI) stabilization. Response surface model (RSM) was applied to optimize treatment conditions. Under the optimized conditions, nZVI@Sch achieved Cr(VI) stabilization rates of 99.97% and 90.38% for lightly and heavily contaminated soils, respectively, significantly outperforming Sch alone. Changes in Cr(VI), Eh, and material characteristics were consistent with the possible involvement of reduction, adsorption, ion exchange, and Cr/Fe-associated solid-phase formation. From a soil microbial perspective, Sch treatment increased soil enzyme activities and microbial diversity, whereas high-dose nZVI@Sch caused stronger short-term microbial inhibition, likely due to combined redox, iron-related, nanoparticle associated, and Cr speciation stresses. The associated decline in microbial richness and diversity may indicate the loss of sensitive taxa and reduced functional redundancy and short-term community resilience. This study provides an integrated assessment of nZVI@Sch for real Cr(VI)-contaminated soils and highlights a potential trade-off between stabilization efficiency and short-term microbial ecological responses, which should be carefully considered in practical applications.