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

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 (F_vs) were the ratio of the activity concentrations of the radionuclides in crops to those in soil, both as dry weight (Bq kg^-1). F_vs of ¹³⁷Cs to lettuce decreased significantly with ageing; geometric means for the 1^st, 2^nd and 10^th year contaminated soil were 0.114, 0.030 and 0.013, respectively. However, a significant decline of F_vs for ¹³⁷Cs was only seen between the 1^st and 2^nd year for both wheat compartments (straw and grains) which disappeared thereafter. The dynamic of ¹³⁷Cs F_vs may be explained according to the distribution coefficient experiment (K_d) 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. F_vs 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 K_d, i.e. 10 L kg^-1. In general, higher F_vs 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.