Aging Effect on Speciation and Soil-to-Plant Transfer of Cs and I in Soil
Akira TAKEDA, Hirofumi TSUKADA, Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
Aging effect of radionuclides in soil after deposition is an important factor for their speciation andsoil-to-plant transfer. Radiocaesium and radioiodine released from nuclear fuel reprocessing plants areimportant radionuclides for the assessment of radiation exposure for the public. We have already reported
the aging effect of Cs and I in an Andosol soil sample collected from grassland in Rokkasho where alarge-scale spent nuclear fuel reprocessing plant is located. From FY 2009, the effect of fertilizers on theaging was examined using the soil sample. The soil sample was mixed with a composite chemical fertilizer,
which had the recommended N-P-K composition, and then stable Cs or I was added. The sample was put
into plastic pots, and then stored in an artificial climate chamber. Orchardgrass (Dactylis glomerata L.) wascultivated in the pots for 28 d several times during the storage period, and the soil-to-plant transfer factors ofCs and I were determined. Extractability of the target elements from a portion of the soil in the cultivation
pots by pure water or 1 M ammonium acetate solution for Cs, and only pure water for I was occasionallydetermined.
In FY 2009, the soil-to-plant transfer factors and extractability of Cs and I were measured up to 113 dafter their addition. In FY 2010, we continued the experiment until 512 d after the addition. The transfer
factor of both target elements rapidly decreased with time up to approximately 100 d after the addition, andthen the decrease became slower thereafter. At the early stage, the transfer factor of I added in the form of Idecreased faster than when added as IO3-. The decreasing patterns of the transfer factor were approximatedwith a combination of two exponential functions, and regression results, which are usable for modeling of
temporal change of the transfer factor, were obtained for both target elements. The decreasing patterns of theextraction yield with pure water and ammonium acetate solution for Cs, and with pure water for I, weresimilar to those of their transfer factors.
Although the transfer factors of Cs observed in the soil with the fertilizer application were generally
lower than those without the application, a statistically significant difference was observed in them only at
86 - 113 d after the addition of Cs. The effect of the fertilizer application on the transfer factor of I after
addition as both I- and IO3- was opposite that on Cs. The transfer factors of I with the fertilizer application
were generally higher than those without the application during the experiment. Although the statisticallysignificant differences at several cultivation stages after the addition were observed, further study is required
to elucidate the effect of fertilizer on behavior of Cs in soil.

Fig. 1

Temporal changes of soil-to-grass transfer factor ofCs after addition of Cs to soil○: Without fertilizer application△: With fertilizer application
Results for the soil without fertilizer application
include those obtained in the long-term experimentfrom FY 2007. The solid and broken lines indicate
regression curves for the soil without and withfertilizer application, respectively.
Fig. 2

Table 1

Parameters in the regression curves for the temporalchanges of soil-to-grass transfer factor of Cs and I
after the addition.
a1a2
λ1λ2CsWithout fertilizer application 3.1E-1 4.0E-2 4.2E-2 9.9E-5
With fertilizer application
4.6E-1 2.6E-2 4.4E-2 9.9E-5
I-Added as I
Without fertilizer application 5.6E+0 6.7E-2 9.5E-2 8.6E-4
With fertilizer application
2.9E+0 1.0E-1 6.7E-2 4.8E-4-Added as IO3Without fertilizer application 1.9E+0 7.0E-2 3.3E-2 8.2E-4
With fertilizer application
9.7E-1 9.2E-2 2.2E-2 2.8E-4

TF = a1 exp(-λ1t) + a2 exp(-λ2t)
TF, Transfer factort, Elapsed time after the addition (d)Temporal changes of soil-to-grass transfer factor of I
after addition of I- or IO3- to soil○: Without fertilizer application△: With fertilizer application
Results for the soil without fertilizer application
include those obtained in the long-term experimentfrom FY 2007. The solid and broken lines indicate
regression curves for the soil without and withfertilizer application, respectively.