Temperature and soil pollution affect dissolved and colloidal antimony release from flooded shooting range soils

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 Sb^III. 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.