Sorption of caesium, iodine and sulphur in solution to the adaxial leaf surface of broad bean (Vicia faba L.)

The interception by crop canopies of radionuclides in rainfall can be important in determining radiation exposures to animals and man. Data were obtained on the sorption and desorption of radionuclides on the adaxial surfaces of fully expanded bean leaves by exposing them to ionic forms of caesium (Cs+), iodine (I−) or sulphur (SO42−) over a six order of magnitude concentration range. The accumulation of each element was determined as a time course over a 48 h period using radioactive labels (137Cs, 125I or 35S, respectively). Time- and concentration-dependent sorption of each element to the leaf surface was analysed to determine: (a) the leaf surface-solution distribution coefficient (Kd) at equilibrium and (b) the sorption and desorption rate coefficients for each element over the range of concentrations investigated. It was expected that Cs+ would show a stronger tendency to sorb to the leaf surface than both I− and SO42− because of the cation exchange properties of the cuticular membrane. The Kd for Cs+ was approximately 90× greater than that for SO42− but 5× less than that for I−. This is thought to be due to either (a) the highly organophilic nature of iodide and the relatively high iodine number of cuticular waxes on plant leaf surfaces or (b) the possible oxidation of I− to I0 or IO3−, with consequently enhanced leaf surface sorption. Based on data obtained in this study, ranges and best estimates of sorption and desorption rate coefficients are presented for Cs+, I− and SO42− for use in modelling the interception of radioactive Cs, I and S in rainfall by crops.