Concentration Coefficients of Radioiodine in Different Chemical Forms
from Sea Water to Fishery Products
Masaomi TAKAHASHI, Yuichi TAKAKU, and Shun’ichi HISAMATSU
Department of Radioecology
Abstract
Radioiodine discharged from the first Japanese commercial nuclear fuel reprocessing plants located inRokkasho has been found in different chemical forms such as I- and IO3- in the ocean. Since theconcentration coefficient of iodine from seawater to marine products strongly depends on the chemical formof the iodine, it is necessary for realistic assessment of radiation dose from the discharged radioiodine via
marine products to use the concentration coefficient of each chemical form of iodine. This study aims toestablish the concentration coefficient of radioiodine in I- and IO3- for marine products (seaweed, shellfishand benthos). In FY 2011, the following experimental methods for getting the concentration coefficientswere decided: 1) the incubation methods of seaweeds for experiments using an iodine radiotracer; 2) thecultivation method of phytoplankton which has a high concentration of iodine; and 3) the analytical methodfor determining the chemical form of stable iodine in marine products using X-ray absorption fine structure(XAFS) analysis.
Incubation methods of Ulva pertusa and Sargassum siliquastrum for a few days and weeks were
established by using a culture flask and a marine flask, respectively. The iodine volatilized from the
incubation vessel was captured with activated carbon filters with the collecting efficiency of >99%. The
concentration coefficient of I- or IO3- for Ulva pertusa and Sargassum siliquastrum was preliminary
measured with stable iodine in different concentrations in incubation seawater. The seaweed samples wereincubated for 3 d at 15°C in the media with added iodine to get concentrations of 0, 100, 500 and 1000 ng g-1
seawater. Iodine concentration in the seaweed samples increased in proportion to that in incubation media
excluding that in Ulva pertusa incubated in the media with added IO3-. The iodine concentration of Ulva
pertusa exposed to IO3- did not show any significant increase. The concentration coefficients of I- for Ulva
pertusa and Sargassum siliquastrum were found as 310 and 660, respectively, while that of IO3- forSargassum siliquastrum was 330. Those results showed that the iodine concentration coefficients ofseaweeds depend on not only the chemical form of iodine but also the seaweed species.
The incubation methods of Skeletonema costatum and Nitzschia sp with a high concentration of iodine
were established. The concentration coefficient of iodine depended on chemical forms, and increased in theorder of IO3-< I- < organic iodine (thyroxin).
The analysis method of the chemical form of stable iodine (I-, IO3- and organic iodine) in the marine
products was established using XAFS analysis. Most of the iodine in Sargassum siliquastrum incubated inseawater without added iodine was found to be organic iodine.

-1Concentration of iodine in Ulva (μg g )

500

Ulva
400

I-y = 3.4E+2x + 2.3E+1R2 = 9.8E-1300

200

y = 1.8E+1x + 9.3E+0R 2 = 5.4E-1

100

IO 3 -00

0.2

0.4

0.6

0.8

11.2-1-1

Concentration of iodine in Sargassum (μg g )Added iodine (μg g )800

SargassumI600

y = 6.2E+2x - 3.2E+1R2 = 9.7E-1
400

IO 3 -200y = 3.3E+2x + 5.6E-1
R2 = 1.0E+000

0.2

0.4

0.6

0.8

1-1

Added iodine (μg g )Fig. 1

Concentration of iodine to seaweed.

1.2