Weathering of Iodine Deposited on Grass Leaf Surfaces
Hitoshi KAWABATA, 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 (I) on the surface of grasses
was launched in FY 2011. In FY 2014, we studied the following subjects: 1) the effect of the growth stage ofgrasses on the foliar uptake and volatilization of iodate applied as liquid droplets onto leaf surfaces; and 2)
the effect of fog on the weathering of I deposited on leaf surfaces as dry aerosol, solution or inorganic I
vapor (I2).
After applying droplets of iodate solution onto the leaf surfaces of Orchard grass (Dactylis glomerata L.
var. Akimidori II) at different growth stages, we placed each plant within an acrylic chamber that was placedin an artificial climate chamber where the plants were cultivated for 7 d. The plant leaves were periodicallycollected, and the leaf surfaces were washed with solution containing detergent. The foliar uptake of I wasmeasured by analyzing the plant leaf and the solution samples obtained by washing the surface. The I
volatilized from the leaf surface was calculated by subtracting the sum of the I amounts in the plants and thewashing solution from the amount of I initially applied on the leaf surface. At two growth stages, 39 d and
49 d after sowing, more than about 80% of the I applied at 7 d remained on the leaf surface of the plants, andfurthermore the I applied was not volatilized during the 7-d period after application. The behavior of I wasalmost the same at the two growth stages of the plant. These behaviors of iodate were different from those ofiodide studied in FY 2013 showing that most of the I was absorbed into the leaf interior and partly
volatilized to the atmosphere.
After applying 1) gaseous I2, 2) liquid droplets containing NaI or NaIO3, or 3) dry aerosol containing
NaI or NaIO3, onto the leaf surfaces of Orchard grass, the plants were exposed to fog with different
atmospheric liquid water contents and exposure durations. The exposed plant leaves were collected andtreated in the same manner as mentioned for the first study. The I applied as I2 was not removed from theleaf surfaces by the exposure to fog with the atmospheric liquid water content of 47 mg m-3 for 24 h. On theother hand, the I applied as iodate of a dry aerosol or liquid droplets was removed by fog with 47 mg m-3liquid water content, and decreased with a second order exponential function of fog duration. The effect ofatmospheric water content of fog on the leaf surface retention of I applied as iodide of the dry aerosol orliquid droplets was studied at fixed fog duration of 16 h. The retention after fog exposure decreased withatmospheric liquid water content, then reached an almost fixed value in high atmospheric liquid watercontent.

1010▲：Particle◆：Liquid

0.1
0.01
0.001

0.1
0.01
0.001

0Fig. 1

1

Retention

Retention

1●：Particle■：Liquid

5101520Duration of fog (h)25Retention of I and duration of fog. Retention

0102030

4050-3Atmosphericliquidwatercontent(mgAtmosphericliquidwater(mgm-3m) )Fig. 2Retention of I and atmospheric liquid waterof I was defined as the ratio of the amounts oncontent of fog. Retention of I was definedleaf surfaces to that loaded initially on the leaf

as the ratio of the amounts on leaf surfaces

surfaces. Vertical bars indicate a standard

to that loaded initially on the leaf surfaces.

deviation of 3 samples. The lines show least

Vertical bars indicate a standard deviationsquare approximation by a function with aof 3 samples. The lines show least squaresecond order exponential term of duration ofapproximation by a function with a secondfog.

order exponential term of water content offog.