Transfer Rate of Radionuclides between Chemical Species in Sea Water, and Incorporation
into Phytoplankton
Yuichi TAKAKU, Atsushi NAKAO, and Shun’ichi HISAMATSU
Department of Radioecology
Abstract
Radionuclides, which are released from nuclear industry facilities and deposited on the surface of water
expanses such as lakes and oceans, are considered to be initially in ionic forms, and then the radionuclides
are removed from the water system as particulates after various chemical and biological reactions. This study
aims to clarify the transformation rate of ionic radionuclides to various chemical forms and the effects ofenvironmental parameters, such as salinity and microorganism activities, on the rate. The effect ofphytoplankton on the transformation was investigated by adding target elements in ionic forms to f/2 culture
seawater medium of a phytoplankton, Skeletonema costatum or Nitzschia sp., which are predominant speciesof brackish Lake Obuchi and the Rokkasho seacoast.
An aliquot of solution containing ionic lanthanoids and Th was added to the medium sample. After
standing for a predetermined period from 1 to 7d, the medium samples were separated into the particulate
fraction, which was collected with a filter which had a pore size of 0.22μm, and the dissolved fraction. Mostof the added light lanthanoids and Th were found in the particulate fraction less than 1 d after their addition.
An aliquot of 125I solution in the form of I- or IO3- was added to the medium sample of thephytoplanktons, and incubated for 12 d. The plankton cells sorbed 125I in an amount up to about 10% of theadded 125I-, while they did not apparently sorb 125I added in the form of IO3-. In the medium samples ofSkeletonema costatum, valences of both added 125I- and 125IO3- did not change significantly. However, theconcentration of 125I- added to the medium samples of Nitzschia sp. was gradually decreased, suggesting that
I was oxidized to 125IO3- or bound to organic compounds, while some of the 125IO3- was reduced to 125I-125 -during the incubation. Since the cell numbers of the phytoplanktons varied during the experiments, the effectof the growth stage of the phytoplanktons should be examined further.
The chemical forms of iodine of both phytoplanktons incubated in the medium with 500 mg I L-1 addedin the form of I- or IO3- were examined by using K-XANES in the large synchrotron radiation facility,SPring-8. The incubated phytoplankton cells were separated from the medium by filtration, and then driedby lyophilization. The iodine sorbed to both phytoplanktons grown in the media with added I- remained as Iwith no valence change of iodine. Both I- and IO3- were detected for both phytoplanktons grown in themedium with added IO3-, and it showed that the added IO3- was reduced to I- on/in the planktons and/or inthe medium. The percentage of I- to the total iodine of the phytoplankton cells of Nitzschia sp. was 61% andlarger than the 28% of Skeletonema costatum.

-(b) IO3

I concentration
in medium (Bq/mL)/
培地試料1 mlあたりの各形態の-1cell number 125
in I量medium(Bq mL (cell/mL)
)

(a) I

-

800

600

40000246),

12508 10 12246Nitzschia sp.

1200(c) I-

800600400
200008 10 122

Incubation period (d)Fig. 1TotalI(468 10 12 14 02468 10 12 14Incubation
period (d)培養期間（日）培養期間（日）125(d) IO3-1000

125200125

I concentration
in medium (Bq/mL)/
培地試料1 mlあたりの各形態の-1
cell number125
inI量medium(Bq mL (cell/mL))

Skeletonema costatum

1000-I in the form of I (), and125) in medium. (a) 125I-

I accumulated by phytoplankton cells (was added to the medium of Skeletonema costatum, (b) 125IO3- was added to the medium of Skeletonema
costatum, (c) 125I- was added to the medium of Nitzschia sp, (d) 125IO3- was added to the medium of Nitzschia sp.

Skeletonema costatum

33100

33150

33200

33250

X-ray energy (eV)Fig. 2

33300

33100

0.1M KIO3 (72%)
0.1M KI (28%)33150(d) IO3-(c) I-

33200

33250

X-ray energy (eV)

33300

33100

0.1M KI (100%)

33150

33200

33250

33300

Relative absorbance

0.1M KI (100%)

Nitzschia sp.

-

Relative absorbance(b) IO3
Relative absorbance

Relative absorbance(a) I-3 3100

X-ray energy (eV)0.1M KI (61%)

0.1M KIO3 (39%)

331 5033200

3325 03 3300

X-ray energy (eV)K-XANES spectra of I sorbed by phytoplanktons (red solid line) in f/2 medium containing 500 mg I L-1.
Short-dashed lines represent the K-XANES spectra of standard I- (0.1 M KI solution), and long-dashed lines
represent those of standard IO3- (0.1 M KIO3 solution). Percentages shown in the parentheses were obtained byspectrum resolving. (a) Skeletonema costatum grown in the medium with added I in the form of I-. (b)Skeletonema costatum grown in the medium with added I in the form of IO3-. (c) Nitzschia sp. grown in themedium with I added as I-. (d) Nitzschia sp. grown in the medium with I added as IO3-.