Regulating Heavy Metal Mobility in Soil Landscapes: Mechanistic Insights into Selective Sequestration by Recalcitrant Humin

The long-term persistence and migration of heavy metals within soil profiles pose significant threats to landscape ecological integrity. Understanding the role of stable soil organic matter (SOM) fractions in regulating metal mobility is crucial for establishing effective biogeochemical barriers. In this study, humin (HUM), the most recalcitrant fraction of SOM, was isolated from soil as a representative model to evaluate its sequestration potential in complex soil environments systematically. Transitioning from idealized single-solute models to competitive multi-metal systems, this research simulates the biogeochemical behavior of co-contaminants typical of anthropogenic landscapes. Kinetic and thermodynamic analyses revealed that sequestration on HUM is spontaneous, endothermic, and chemisorption-dominated. Notably, HUM exhibited a distinct preferential retention for Pb(II) over Cu(II) and Cd(II), with the maximum sequestration capacity for Pb(II) reaching 242.41 mg/g. Molecular insights from FTIR, XRD, and XPS analyses demonstrated that the selective immobilization is driven by the formation of robust inner-sphere complexes and surface precipitation, primarily involving oxygen-containing functional groups and mineral-organic interactions. These findings provide a mechanistic understanding of how stable organic-mineral complexes dictate the spatial distribution and geochemical fate of heavy metals in soil landscapes, offering a scientific basis for enhancing the soil's natural attenuation capacity through organic matter management.