Humic acid-mediated activation of antimony colloid during soil redox cycles: Mechanisms and environmental...

Soil redox fluctuation is a primary driver of antimony (Sb) mobilization in contaminated soils. However, the regulatory effects and mechanisms of exogenous humic acid (HA) on Sb colloids (1-450 nm) under dynamic redox conditions remain unclear. In this study, five consecutive aerobic-anaerobic incubations were performed on three Sb-contaminated soils with a contamination gradient (S1 > S2 > S3). Combining particle-size fractionation, sequential extraction, XPS, TEM-EDS, Spearman-Mantel analysis and PLS-PM, we explored the impacts of HA (10 mg C L^-1) on Sb release, speciation transformation, colloid distribution and underlying mechanisms. The results showed that HA significantly promoted total dissolved Sb (< 450 nm) release by up to 122.5% relative to the control. More importantly, HA-induced Sb activation involved both total < 450 nm Sb mobilization and size redistribution from large colloids (220-450 nm) toward small colloids (1∼3-220 nm), which maintained 20%-60% of total Sb as fine colloidal species. Under anaerobic conditions, HA facilitated Sb(V) reduction to Sb(III), increasing Sb(III) proportion by 30%-50%. HA also stabilized Sb in the reducible Fe oxide-bound fraction and decreased ion-exchangeable and adsorbed Sb. Correlation and quantitative modeling verified Fe oxides as the core inorganic carrier of Sb colloids, and HA shifted Sb colloid regulation from inorganic dominance to inorganic-organic synergistic control driven by Fe colloids and HA-derived DOM. This study clarifies HA's multifunctional regulation on Sb colloids under redox cycles and identifies colloidal Sb as the dominant mobile Sb form, providing a theoretical basis for risk assessment and in-situ remediation of Sb-contaminated soils in wet-dry alternating environments.