Transfer Rate of Radionuclides between Chemical Species in Brackish Water
Yuichi TAKAKU, Susumu NOZOE, and Shun’ichi HISAMATSU
Department of Radioecology
Abstract
Radionuclides deposited on the surface of water expanses such as lakes and oceans are considered to be
initially in ionic forms. They are then removed from the water system as particulates after various chemicaland biological reactions. This study aims at clarifying the transformation rate of ionic radionuclides tovarious chemical forms and the effects of environmental parameters, such as salinity and microorganism
activities, etc., on this rate, by adding stable elements, Th, and U to water samples. We have already reported
results for a fresh water sample. Results for a brackish water sample are reported here.
A brackish water sample was collected from Lake Obuchi in Rokkasho where a spent nuclear fuelreprocessing plant is located. We added a mixture of lanthanoids, Th and U or just inorganic iodine as ionicforms to aliquots of the sample. After standing for a predetermined period from 10 min up to 2 weeks, theparticulate fraction in each aliquot was collected on a filter (pore size, 0.22 μm). The molecular size oforganic materials binding the elements in the filtrate was analyzed with an ultra-filter with a cut-off size of
10 kDa. The target element concentrations in the particulate and filtrate fractions were analyzed by ICP-MS.
Iodine was added to the sample as I- or IO3- to examine the effect of chemical form on I behavior.
Several percent of the I- changed to a particulate form (> 0.22 μm) 10 min after its addition, and remained asthat form during the experiment. Initially, 3% of the added I- was in the >10 kDa fraction, and then thepercentage varied from 7 to 16% after 15 min without dependency on elapsed time. Almost all added IO3was retained in the <10 kDa fraction throughout the experimental period. Most of the added lanthanoids
were in the <10 kDa fraction. The percentage of the lanthanoid particulate fraction varied from 0 to 15% at
10 min, and decreased with elapsed time. The fraction of >10kDa retained 5-10% of the original amount oflanthanoids throughout the experimental period. The relative distributions of Th in particulate and >10 kDa
fractions were larger than those of the lanthanoids, and they did not change over time. Initial percentages ofparticulate and >10 kDa fractions were 10% and 20%, respectively. However, the relative amount of natural
lanthanoids and Th in particulate form was much larger than that in our experiment; the cause of this isunknown.
The behavior of the target elements in the brackish water sample was similar to that in the fresh water
sample. However, the percentages of particulate and >10 kDa fractions of lanthanoids and I in the brackishwater sample were smaller than those in fresh water for the same fractions. From this we inferred that
dissolved salts suppressed transformation between the fractions.
Since microorganism activity is considered as one of the parameters affecting transformation ofchemical form, we have begun to investigate the effect of phytoplankton on the transformation. Species
composition of phytoplanktons in the fresh water lakes, Lake Ichiyanagi and Takahoko, was surveyed.
Dominant species were separated and their cultivation methods were established.

20100%

10 min6h1 day
15 daysIRelative distribution80%

40%

Particle (>0.22 mm)>10 kDa<10k20%particulate (%)

1560%

1050%
10 min 15 min 30 min

1h3h6h1 day 2 days 7 days

15days0Time

Relative distribution

IO3

Pr

100%
80%

Fig. 340%

Particle (>0.22 mm)>10 kDa<10k20%0%
10 min 15 min 30 min

1h3h6h1 day 2 days 7 days

15days

TimeFig. 1

Relative speciation of iodine added as Iand IO3- to the brackish water sample
collected from Lake Obuchi.

100%

80%

Relative distribution

Eu

GdTb

DyHoErTmYb

LuElements

60%

60%10 min
40%Particle (>0.22 mm)>10 kDa<10k20%0%
LaCePrNd Sm Eu

GdTb

Dy

HoEr

Tm Yb

LuElements
100%80%Relative distribution

Sm60%

15 days
40%Particle (>0.22 mm)>10 kDa<10k20%0%La

CePr

Nd Sm Eu

GdTb

Dy

HoEr

Tm Yb

Lu

Elements

Fig. 2

Relative speciation of lanthanoids added
to the brackish water sample collectedfrom Lake Obuchi.

Ratio of particulate lanthanoids from Pr to
Lu to total concentration after addition tothe brackish water sample collected fromLake Obuchi.