Abstract:

Taking the calcareous soils from Wujiang River catchments of Southwest China as an example, the study investigated the speciation of elements such as uranium (U) and discussed the mobility and release behavior of U from the studied calcareous soils with a technology of Sequential Extraction (SEE) in combination with chemical composition and related parameters of calcareous soils. The main purpose of the study is to improve our understanding on biogeochemical cycling of mineral elements such as U and set a basis for water environmental protection against U pollution from natural sources especially from natural weathering of Urich bedrocks or Urich soils by taking proper measures at a catchment's scale especially for Wujiang River catchments. The investigation results show that the element Mn in the studied calcareous soils is mainly bound in hydroxides of manganese, Ca is mainly bound in carbonate and silicate minerals, P is mainly hosted in silicate minerals and organic matters, U is mainly hosted in residual phases such as silicate minerals, secondly hosted in organic matters and carbonate minerals. The percentage of mobile U in calcareous soils is 10% to 30%, with an average of 17%, indicating that at least part of U in the studied calcareous soils can be released and transferred under certain acidic and oxidationreduction surface conditions, that is, at least part of the U in calcareous soils can be released and transferred into water bodies or absorbed by low to high plants under the above surface conditions. The potential release amount of U in calcareous soils can reach to a value of 036~150 g/t, with an average of 076 g/t, so contribution of U flux in Wujiang River from calcareous soils under the role of accelerating soil erosion and atmospheric acid precipitation in Wujiang River catchments should never be ignored.

Key words: uranium/ speciation/ mobility/ calcareous soils/Wujiang River catchments