Weathering of Iodine Deposited on Grass Leaf Surfaces
Hitoshi KAWABATA, Hirofumi TSUKADA, Masumi YANAI,
Yuichi TAKAKU, Shun'ichi HISAMATSU
Department of Radioecology
Abstract
Radionuclides released into the atmosphere are deposited on the leaf surfaces of crop plants, taken up
by the plants, and translocated from the leaves to other parts. Some amount of the radionuclides deposited
onto the leaves is removed from the surface by the environmental process called weathering, i.e. removal byrain, wind, etc. Although weathering, foliar uptake and translocation are important processes involved in theradiation dose assessment of radionuclides from crops, parameters to describe those processes for 129I, which
is an important radionuclide for the safety assessment of the nuclear fuel reprocessing plant in Rokkasho,
have not yet been elucidated. Since grasses are cultivated for livestock farming which is one of the important
industries in Rokkasho, a research project to determine the behavior of iodine on the surface of grasses waslaunched in FY 2011. In FY 2012, we studied: 1) the effect of the growth stage of grasses on the foliar
uptake and volatilization of iodide deposited on leaf surfaces as dry aerosol; 2) the effect of light intensity on
the volatilization of iodide deposited on leaf surfaces as dry aerosol; and 3) the effect of wind on theweathering of iodine deposited on leaf surfaces as dry aerosol, solution or inorganic I vapor (I2).
After applying a dry aerosol of NaCl containing NaI onto the leaf surfaces of Orchard grass (Dactylis
glomerata L. var. Akimidori II) at different growth stages, we placed each plant within an acrylic chamberand cultivated them for 7 d in the artificial climate chamber. The plant leaves were periodically collected,
and the leaf surfaces were washed with solution containing detergent. The foliar uptake of I was measured
by analyzing the plant leaf and the solution samples obtained by washing the surface. The volatilized I fromthe leaf surface was collected on activated carbon paper filters. The amounts of I on the leaf surface of plants
at all growth stages removable by washing decreased with time, while the amount of the volatilized fraction
increased. The I content in the plant was almost constant throughout the experiment. The time constant of
the I volatilization rate was not affected by the growth stage of the plant.
After applying the dry aerosol of NaCl containing NaI onto the leaf surfaces of the Orchard grass at
32th day after sowing, each plant was placed within an acrylic chamber and cultivated them for 3 d at
different light intensities. The volatilized fraction of applied I was almost constant and had no dependence
on the examined light intensities.
After applying 1) gaseous I2, 2) liquid droplets containing NaI or NaIO3, 3) dry aerosol containing NaIor NaIO3, onto the leaf surfaces of Orchard grass, the plants were exposed to a wind with a speed of 2 m s-1
for 1 h. These plants were treated after the wind exposure in the same manner as mentioned above. The I
contents in the plants applied as various physical and chemical forms were not statistically different fromthose for the control plants not exposed to the wind. Therefore, the exposure to the 2 m s-1 wind for 1 h did
not affect the weathering of iodine.

Relative distribution

10.1
Leaf surface
LeafVolatilization
0.01
0Fig. 112345
6Days after application of I (d)78Relative distribution of iodide applied on the leaf surface of Orchard grass on the32th day after sowing. Vertical bars indicate standard deviation of three samples.