Transfer of radionuclides to plants: Influence on the speciation of radionuclides in soil

Abstract

The quantification of the soil-to-plant transfer by means of transfer factor or concentration ratios values presents high range of variation, about 4-5 orders of magnitude for radiocesium. This range can be partially explained by the different association of radionuclides to soil particles, which can be assessed by speciation procedures. The fact that there are a lot of speciation procedures in the literature may, in some occasions, make its interpretation difficult. The source of radionuclides is a major factor influencing the speciation. The anthropogenic radionuclides associated with fuel particles present usually low mobility, although they can be weathered with time. Regarding the radionuclides released in the global fallout, the 90 Sr is the most bioavailable. Plutonium is usually associated with organic matter. The mobility and bioavailability of radiocesium depends on the soil clay content, and is also time dependent. The naturally occurring radionuclides in soil are mainly associated with fractions strongly fixed, because they mainly occur in minerals forming part of soil particles. The speciation of soil can also be modified by agricultural procedures, such as the addition of fertilizers or phosphogypsum. The fertilization can be used to reduce the soil-to-plant transfer of radiocesium and radiostrontium by supplying stable elements, potassium, and calcium, respectively, so that their content in soil solution decreased. The phosphate-based fertilizers have also naturally occurring radionuclides, which can be transferred to plants. Phosphogypsum, which can contain high levels of radium, is used as soil amendment. However, its radium content is mainly associated with immobile fractions, and its transfer to plants is of the same order of magnitude than without phosphogypsum amendment.