Selenium speciation and mobility at trace level in soils: contribution to long term risk assessment

A radioactive isotope of selenium, ⁷⁹Se, was shown to be among the most critical radionuclide for safety assessment of high level and long lived nuclear waste repository in case of hypothetical soil contamination. A methodology was thus developed to determine Se species (speciation) present in soils at trace level (μg kg^-1; radiological context) in assessing their leaching and retention phases (distribution). Combined to a ⁷⁷Se(IV) tracer, kinetically limited processes were shown to be involved in Se retention and chemical transformations in soils, suggesting that native Se behavior is particularly relevant for its long term mobility assessment and modeling. The investigation of native Se speciation and distribution in 29 soils has finally highlighted that soil organic matter impact the nature of Se species susceptible to be leached (organic and colloidal compounds) and the Se mobility (Se stabilization in solid phase via organo-mineral associations). (author)