Ageing impact on the transfer factor of 137Cs and 90Sr to lettuce and winter wheat

Abstract

The study focuses on long-term (extending from 1 to 10 years) lysimeter experiments of the transfer factor of 137Cs and 90Sr to lettuce and winter wheat crops. Transfer factors (Fvs) were the ratio of the activity concentrations of the radionuclides in crops to those in soil, both as dry weight (Bq kg−1). Fvs of 137Cs to lettuce decreased significantly with ageing; geometric means for the 1st, 2nd and 10th year contaminated soil were 0.114, 0.030 and 0.013, respectively. However, a significant decline of Fvs for 137Cs was only seen between the 1st and 2nd year for both wheat compartments (straw and grains) which disappeared thereafter. The dynamic of 137Cs Fvs may be explained according to the distribution coefficient experiment (Kd) which had a value of 3600 L kg−1 showing a high affinity of the clay minerals for caesium. Desorption data revealed that Cs fixation enhanced with ageing. The mechanism involved may be an initial sorption of caesium species to the surface soil particles followed by progressive irreversible fixation to the interlayer of the porous clay minerals. Fvs of 90Sr were high and showed trivial variation for both crops for the time course studied. Sorption of Sr2+ species to the clay mineral may be the governing process, which was supported by high desorption percentage (ranged 77%) with low Kd, i.e. 10 L kg−1. In general, higher Fvs of 137Cs and 90Sr for lettuce was observed in comparison to winter wheat. The diversity of plant species and root systems would play essential roles for such behaviours.