The Effect of Natural Humic Acid Derived from Leonardite on Uranium Species in Soil.

Humic substances (HS) consist of humin, humic acids (HA) and fulvic acids (FA), which distributed widely in soil environments and play a vital role in soil ecosystem. HA is the largest part of HS, which soluble in alkaline but insoluble in acid. HA with abundant functional groups (such as —COOH, phenolic-OH) can interact with heavy metals to influence their mobility and bioavailability and considered as one of the most important factors controlling fate and transport of heavy metals in soil environments. Uranium (U) is one of the most toxicity heavy metals with radioactivity and widely used into weapons testing and nuclear plants. In this study, we explored the effect of HA derived from leonardite on U species in soil, such as uranium-bearing cation (extracted by 0.5 M Ca(NO₃)₂), weak-acid soluble (extracted by 0.44 M CH₃COOH + 0.1 M Ca(NO₃)₂) and bioavailable ( extracted by 0.014 M NaHCO₃ + 0.0028 M Na₂CO₃) species about UO₃ and UO₂(NO₃)₂ in soil through incubation experiments. Through experiments we found that the added HA may influence the mobility of uranium species in soil: 1) uranium-bearing cation species content decreased; 2) weak-acid soluble species content increased; 3) bioavailable species of U decreased for UO₃ but has no significant influence for UO₂(NO₃)₂ in soil. Also, we found that HA greatly influence the U combined forms in soil, such as U bound to carbonate, Fe or Mn oxides and organic matter.