Antimony (Sb) mobility in soils, whether dissolved or colloidal, is an emerging concern, yet its geochemical behavior, particularly under future climatic conditions, remains poorly understood. In a 28-day microcosm experiment, we investigated Sb release from two shooting range soils under flooded conditions at 20°C and 25°C. Sequential extractions and pore water analyses were conducted to assess Sb release at different temperatures. Sb size distribution was examined across pore water filtrates and characterized using asymmetric flow field-flow fractionation (AF4). The soils had different levels of Sb (58 ± 9 and 422 ± 53 mg kg-1) and Pb (2.0 ± 0.9 and 14.1 ± 3.0 g kg-1) pollution and microbial biomass (microbial carbon: 1005 ± 48 and 196 ± 62 mg kg-1), which greatly affected Sb release. Elevated temperature decreased Sb release in the less contaminated soil with higher microbial biomass (max: 85 ± 4 vs. 125 ± 3 µg L-1), but increased it in the more contaminated soil with lower microbial biomass (max: 492 ± 10 vs. 410 ± 11 µg L-1). This contrast was attributed to differences in soil microbial biomass, which impacted the effect of temperature on microbially-mediated redox processes, such as the reduction to less mobile SbIII. A substantial fraction (13-36 %) of total soil pore water Sb was released as colloids (0.02-10 µm). AF4 analysis identified two colloidal Sb populations: organic colloids or Sb microcrystals (∼2.5 nm) and mineral nanoparticles (15-20 nm). This study underscores the microbial control of redox transformations and Sb release and provides new evidence on the importance of colloidal transport of Sb.
