Efficacy of Fe/Mn (hydro)oxide-phosphate composites for synergistic immobilization of Cd, Pb, Cu, and Zn and microecological regulation in mining soils

Abstract

In situ stabilization is a practical strategy for mitigating multi-metal contamination while restoring soil ecological functions. In this study, Fe/Mn (hydr)oxide-phosphate composites (FMPs) were synthesized and applied to highly Cd–Pb-Cu–Zn co-contaminated mining soil. After 60 days of FMPs application (5 wt.%), DTPA-extractable Cd, Pb, Cu, and Zn were effectively reduced by 70.10%, 99.82%, 68.30%, and 75.05%, respectively, successfully transforming target metals from labile (F1/F2) to stable (F3/F4) fractions. Physicochemically, FMPs applications elevated soil pH and essential nutrients (TP, AP, NH4+-N), which directly correlated with the significant enhancement of soil enzymatic activities (S_ACP, S_CL and S_CAT). High-throughput sequencing further revealed that FMPs reconstructed the soil microbiome. While overall bacterial diversity decreased and fungal diversity increased, ecologically beneficial and metal-resistant taxa, including Chloroflexi, Bacteroidota and Pseudaleuria were significantly enriched. Moreover, molecular ecological network (MEN) analysis indicated that FMPs induced more tightly connected and stable bacterial networks while simplifying fungal networks, with low-abundance taxa primarily acting as keystone connectors. Furthermore, neutral community model (NCM) predictions verified that stochastic processes dominantly governed the assembly mechanisms of both bacterial and fungal communities during the restoration process. These multidimensional results systematically demonstrate that FMPs not only achieve highly efficient chemical stabilization of multiple heavy metals but also facilitate robust ecological restoration by optimizing microbial community.