Insights into toxic elements mobilization in Karstic paddy soil of southwest China: The overlooked significance of iron-organic matter colloids

Abstract

Due to their large specific surface area, variable surface charge, and abundant reactive functional groups, soil colloids are important “carriers” for the migration of HMs (heavy metals) in the soil. This study systematically investigates the migration-transformation mechanisms of colloidal As, Cd, Tl and their driving factors in paddy soils of the Karst region in southwest China. Results show that colloidal Fe and OM are the primary environmental factors influencing the formation and distribution of these three colloidal heavy metals, with significant positive correlations (correlation coefficients r² = 0.56–0.79). TEM-EDS and XRD analyses confirm that As/Cd are closely associated with Fe oxides (e.g., magnetite, goethite) at the nanoscale. AF4-UV-ICP-MS technology reveals that colloidal HMs primarily occur in the 100–350 nm size range, and anthropogenic activities in artisanal smelting areas promote the formation of smaller-sized colloids (20–350 nm), enhancing their migration potential. Metagenomic analysis indicates that N/S metabolic genes (e.g., narA, cysN) are significantly correlated with colloidal HMs concentrations, and microbial metabolites affect the binding of HMs to soil colloids. Traditional assessments overlook the high mobility and stability of colloidal HMs (e.g., 100–350 nm), leading to underestimation of potential risks to paddy ecosystems and adjacent water bodies. Future biogeochemical research should prioritize colloid - and nanoparticle - bound HMs to improve risk assessment and remediation strategies.