Transfer Rate of Radionuclides between Chemical Species in Sea Water,
and Their 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 they are removed
from the water system as particulates after various chemical and biological reactions. This study aims toclarify the transformation rate of ionic radionuclides to various chemical forms and the effects ofenvironmental parameters, such as salinity and microorganism activities, etc., on this rate, by adding thestable elements, Th and U to water samples. We have already reported results for fresh water and brackish
water samples. Results for a seawater sample are presented here.
A seawater sample was collected from the coast at Rokkasho. An aliquot of a solution of lanthanoids
and Th as ionic forms was added to the sample. After being allowed to stand for a predetermined period
from 10 min up to 2 weeks, the particulate fraction in the sample was collected through a filter which had apore size of 0.22 μm. The molecular size of organic materials binding the elements in the filtrate wasanalyzed with an ultra-filter with a cut-off size of 10 kDa. The target element concentrations in theparticulate and filtrates fractions were analyzed by ICP-MS. Most of the added lanthanoids were found in<10 kDa fractions. The proportion of the particulate fraction of lanthanoids was less than 1 % in all time
periods. Initial proportions of Th in particulate and >10 kDa fraction were 6% and 40%, respectively. The
particulate fraction then increased to 17% at 1 d after the addition, and then decreased to 6% at 14 d.
Since microorganism activity is considered as one of the parameters affecting transformation ofchemical form, the effect of phytoplankton on the transformation was investigated by adding target elements
in ionic forms to aliquots of the incubation medium for Anabaena curva, which is the predominant speciesof the fresh water lake Ichiyanagi. An aliquot of a solution of ionic lanthanoids, Th and U or inorganic iodine
as I- or IO3- was added to the medium sample. After allowing this to stand for a predetermined period from 1
to 9 d, the medium samples were separated into three fractions using the method described above. Most of
the added light lanthanoids and Th were changed to a particulate fraction less than 1 d after the addition. On
the other hand, almost all of the added I- and IO3- was retained in the <10 kDa fraction throughout theexperimental period. Although the chemical form of iodine added as IO3- was retained as IO3- in the <10 kDa
fraction, the added I- was gradually oxidized to IO3-, and after 9 days, this reached 15% of the amount ofadded I.
The same experiment will be carried out for phytoplanktons in brackish water and seawater in the next
financial year. To decide species to be used for the experiment, the species composition of phytoplanktons
was surveyed in the brackish lake, Obuchi, and along the seacoast at Rokkasho. Dominant species in each ofthese water expanses were separated and their cultivation methods were established.

100%

Relative distribution

1 hour
80%

60%

40%

Particle
>10k
<10k

20%

0%
La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Elements

Fig. 1

Relative speciation of lanthanoids added tothe sea water sample collected from thecoast at Rokkasho

100%

9 daysRelative distribution

80%

60%

40%

20%

Particle>10 kDa<10 kDa

0%
La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Th

U

Elements

Fig. 2

Relative speciation of lanthanoids and Th, U
9 days after adding to the incubation mediumof Anabaena curva

120

Concentration (ng g -1 )

I100
80
60
40

IO3
I

20
0
Control

1d

2d

5d

9d

Time

Fig. 3

Concentration of iodine and iodate in the
incubation medium of Anabaena curva
with added I-