Journal of Nuclear and Radiochemical Sciences, Vol. 4, No. 1, pp. A1-A8, 2003

Accounts
Technetium-99 Behavior in the Terrestrial Environment-Field Observations and Radiotracer ExperimentsKeiko Tagami*
Environmental and Toxicological Sciences Research Group, National Institute of Radiological Sciences
Anagawa 4-9-1, Inage-ku, Chiba 263-8555, Japan
Received: March 22, 2003; In Final Form: June 4, 2003
Obtaining data on 99Tc in the rice paddy field environment is important because Tc is a redox sensitive element.
The behavior of Tc is expected to be different under upland field and rice paddy field conditions since the redox
conditions in the soil environment differ. However, most of the data on the nuclide behavior in soil were obtained
under upland field conditions. To understand the global fallout 99Tc distributions in soil samples collected inJapan, a simple and rapid separation method was developed in order to determine low-levels of 99Tc in soil
samples by an inductively coupled plasma mass spectrometry. Also, radiotracer experiments using soils underaerobic and anaerobic conditions were carried out to clarify the Tc behavior under paddy field conditions. The
results of determination of global fallout 99Tc in Japanese soils indicated that the radionuclide had been accumulating in rice paddy fields. The mechanisms can be explained by the immobilization of Tc in soil under anaerobic
conditions. From the radiotracer experiments, it was clear that under waterlogged conditions, the highly mobileTcO4− in soil was readily changed to other immobilized forms, such as TcO2, TcS2 and organically bound forms.
To this immobilization, the microbial activity seemed to have an important role in Tc sorption reactions. Whenthe soil, which was once kept in anaerobic conditions, was air-dried again and kept in aerobic conditions, thechemical forms of immobilized Tc did not change remarkably. Interestingly, the similar Tc behavior wasobserved in a real wet forest near the Chernobyl Reactor.

nical report series 364.9 The transfer factor (TF) is defined as
the following equation:

1. Introduction
Technetium-99 ( 99 Tc) is of great practical importancebecause it is one of the fission products with a relatively highyield in the thermal neutron fission of 235U and 239Pu (ca. 6%)
and it has a long half-life of 211,000 y. The major source of99
Tc in the environment stems from fission in nuclear weapons
and in reactors. The quantity of 99 Tc produced by nuclear
weapon tests in the atmosphere can be estimated to be 140
TBq using an equal fission yield to that of 137Cs and actual137
Cs deposition.1, 2 The estimated release of 99Tc by nuclear
industries through 1986 was of the order of 1000 TBq. Thiswas mainly result of nuclear fuel reprocessing and most of the99
Tc was discharged into the sea. The most significant
discharge of 99Tc nowadays is from Sellafield to the Irish Sea,
total amount of 550 TBq of 99Tc was released in 1995-1999
and the annual liquid discharge limit of 90 TBq for 99Tc has
been applied from 2000.3 Compared to this, the amount introduced via nuclear medical use of 99mTc (half-life: 6.01 h) is
negligible. In Japan, it is assumed that the major source of Tc
in soils up to now arises from nuclear weapon tests in theatmosphere.
Technetium is known to exist in all valence states from +7
to –1 and the dominant species in natural aqueous solution in
equilibrium with the atmosphere is the pertechnetate, TcO4−.4, 5
In the terrestrial environment, TcO4− has a high geochemical
mobility6 and availability for plants.7, 8 Several assessments
have suggested that the Tc uptake by plants is one of the key
parameters in governing the Tc radiation dose to humans.
From experimental studies on Tc in plant-soil systems in
upland fields, it was observed that the element has hightransfer factors from soils to plants. Data for the soil-to-planttransfer of Tc in temperate area were reviewed in IAEA tech*Corresponding author. E-mail: k_tagami@nirs.go.jp. FAX: +8143-206-3267.

TF =Activity in vegetation on dry or wet weight basis (Bq/g).
Average activity in soil during planting period (Bq/g)

Transfer factors vary within and between species, with themode of contamination and with the level of administration. In
the edible parts of common plants, a TF of 5 (on wet weightbasis) was recommended as allowable by the IAEA for Tc.10
The influence of the chemical form of Tc on its uptake byspinach11 and tomato plants12 was studied by using a mathematical analysis of the uptake. When Tc was added to spinachas a Tc-cysteine complex, the Tc was absorbed by the plant,
but the TF was lowered by two orders of magnitude ascompared with TcO 4 − . From this result, it is apparent thatTcO 4 − was the most important form of Tc taken up by theplants. Although Tc was thought to be highly mobile in plants
just as water is, Gast et al.13 reported that the first and older
leaves of wheat (grown to maturity in soil experiments) accumulated much greater concentrations of Tc than the newleaves. Probably due to the fixation of Tc by a certain part ofplant, such as organic compounds, very little re-translocationof Tc took place from old tissues to new tissues. Similar
results were obtained using tomato plant by Krijger et al.12 andthey explained the Tc fate in the tomato plant.
The uptake rate might be limited to the availability andconcentration of Tc in soil; the recorded values from soil toplant TFs were accordingly high when these factors were high.
The element mobility in soil has been studied, and the extent
of the Tc-sorption is characterized by the following distribution coefficient Kd, which is defined as:
Kd (mL/g) =Activity of Tc sorbed per gram of dry soil (Bq/g).
Activity of Tc per mL of solution (Bq/mL)

Available data on the Kd for Tc are tabulated in Table 1.14−17
Coughtrey et al.18 concluded that, “in all aerobic soils, contam-© 2003 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/20/2003

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J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003−

inated with Tc in the TcO 4 form, 90% of added Tc can beassumed to remain in the solution either as a free ion or as
weakly absorbed to ion-exchange sites”. Routson et al.15
reported that from low K d values Tc could be assumed toadsorb poorly in soil as TcO4−. However, results of Wolfrum
and Bunzl19 indicated that the oxygen content in the leachate
affected the Kd value. It was suggested that the availability of
Tc in soils decreases with soil residence time, which is known
as ‘aging effect’.20−23
TABLE 1: Distribution Coefficients Kd for Sorption of Tc
by Soils
Sample (N)Soil (22)Soil (2)Solution

0.01M CaCl2
0.02M NaHCO3
0.2M NaHCO3
Soil (7)Not shown
Soil (129) Deionized water
Soil (2)Deionized water(pH=3−8)

Kd (mL/g)

Reference

0.007 ± 2.8−0.033 ± 0.01
+0.010 ± 0.04
0 − 0.9
0.6 ± 0.6
0 − 0.7

Wildung et al.14
Routson et al.15
Takebe and Deying16
Uchida and Tagami17
Tagami and Payne(unpublished data)

In order to improve our understanding of the behavior of
Tc in the environment and to develop appropriate waste
storage/disposal options, it is essential that we obtain morereliable information on the levels, distributions and fate of 99Tc
in the environment. The fate of Tc in the environment is influenced by combinations of chemical, physical and biologicalfactors. Most studies to obtain environmental parameters havebeen carried out for upland fields in European countries,
Canada and U.S.A., because these are in the majority in thesecountries. However, Tc is a redox sensitive element and it
could be insoluble forms under reducing conditions.4, 5, 23 The
behavior of Tc is expected to be different under upland field
and rice paddy field conditions.24 In rice paddy fields, thereducing conditions are established by waterlogging in the soil
during the cultivation period. The soil-rice plant system is an
important path for transfer of 99Tc to humans in Japan andother Asian countries because rice is the main crop in theseareas.
A series of studies including field observations and radiotracer experiments were carried out in order to clarify the fateof 99Tc in the paddy field environment. To measure low-levelsof 99Tc in environmental samples by an inductively coupledplasma mass spectrometry, ICP-MS, we developed a simple
separation method.25−30 Using the method, global fallout 99Tc
retained in rice paddy field soils collected in Japan were
measured. 25, 26, 31 Also Tc behavior in soils under aerobic
conditions and anaerobic conditions were observed because Tc
is known as a redox sensitive element.24 Further studies on Tc
behavior under anaerobic conditions by controlling microbialactivities in soil samples to find out the effect on the Tc immo99

Tagami23, 32

It was expected that Tc was immobilized in ricebilization.
paddy fields from the above mentioned studies, therefore, plantand soil samples collected in three forests (one of them was socalled “wet forest”; the groundwater-level was higher thanother two sites) within 30-km zone from the Chernobyl
Reactor were used for 99Tc measurement to obtain transfer
parameters.33−36
2. Development of a Simple Separation Method for 99Tc in
Environmental Samples
Instruments. In recent years, ICP-MS is an attractive
method for 99Tc determination because the detection limit islower than that of any radiation counting method as listed inTable 2.31, 37−42 Although ICP-MS offers several advantages for99
Tc analysis, the radioisotope is required to be concentrated insolution, and Ru, which has an abundance of 12.7% at mass99, is to be separated. Also, total elements’ concentration insolution is required to be less than 300 ppm due to instrumental limitation.
Tracers. In our experiments, the chemical recovery wasmonitored with 95mTc. The nuclide has a half-life of 61 days; it
is useful as a yield monitor in the chemical separation of Tc
compared to the other short half-life Tc isotopes such as 99mTc(T1/2 = 6.01 h). It has been reported that 95mTc is preferable asa yield monitor when ICP-MS is used because of the difference of mass between 95m Tc and 99 Tc. 43 However, somecommercially available 95m Tc solutions might contain 99 Tc
when the 95mTc was produced by 96Mo (p, 2n) 95mTc. If other
Mo isotopes were included in the target, it could be 99Tc which
was produced at the same time. Therefore, 95mTc was obtainedby irradiation of Nb foil (99.999%) using the reaction 93Nb (α,2n) 95mTc in a cyclotron at Tohoku University, Japan.29 This
reaction could not produce other long-lived Tc isotopes. Then
95m
Tc was separated in a quartz tube by heating the foil at
1000˚C for 2h under oxygen stream. Tc was evaporated andtransferred to a room temperature area of the quartz tube. The
95m
Tc was stripped from the tube with deionized water.
Sample Pretreatment. Soil samples were air-dried andpassed through a 2 mm mesh sieve. They were incinerated for
8 h at 450˚C to decompose organic matter. Soil incineration isusually performed to decompose soil organic matter that interferes with Tc chemical analysis. To clarify the Tc loss duringincineration, we measured the loss ratios from aged and freshly
contaminated soil samples during incineration. Long-contaminated soil samples (Andosol and Gray lowland soil) were contaminated with 95mTc for 1 or 6 months. Freshly contaminatedsoil samples were prepared 1-2 days before the test. Figure 1
shows loss of 95mTc by incineration as percentage of initialactivity. The weight reductions of the Andosol sample and theGray lowland soil sample were 19 - 25% and 9%, respectively.
Despite a difference in organic matter content, there was no

TABLE 2: Analytical Methods for 99Tc Determination
Detection limit
1 – 5 mBq
3 – 5 mBq
1 – 25 mBq
0.002 – 4 mBq
3.8 mBq/mL
1.1 mBq/mL
0.03 mBq/mL
0.0021 mBq/mL
0.005 mBq/mL
0.003 mBq/mL<0.5 pg (0.3 mBq)

Type of apparatus
Anti coincidence shielded gas flow counting
Neutron activation (n, γ)
Liquid scintillation counting
Inductively coupled plasma mass spectrometry
Q-ICP-MSQ-ICP-MSQ-ICP-MSQ-ICP-MS with ultrasonic nebulizer
HR-ICP-MS with ultrasonic nebulizer
HR-ICP-MS
Accelerator mass spectrometry

Reference
Holm37

Crain and Gallimore38
Morita et al.39
Tagami and Uchida31
Morita et al.40
Kim et al.41
Tagami (unpublished data)
McAninch et al.42

Note) Q-ICP-MS: quadrupole ICP-MS, HR-ICP-MS: high resolution (or double focusing) ICP-MS.

Technetium-99 Behavior in the Terrestrial Environment

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003
1

0.8

Recovery

4

0

0

3

6

9

12

Figure 1. Loss of 95mTc from soil samples as a function of heatingtime when incinerated at 450˚C.
Freshly contaminated soil (■); 1-month contaminated Andosol (○);
1-month contaminated Gray lowland soil (◇); 6-month contaminatedsoil (●). Bars indicated counting errors.

100

80

60

40

20

0

0.6

0.4

0.2

Heating time/hPercent of accumulative volatilizedTc-95m from soil / %

Mo-98
Tc-99
Ru-106895m

Loss of
Tc in percent ofinitial activity / %

12

-4

A3

500

600

700

800

900

1000A 1000B

Temperature of the ovens in the combustion apparatus / ˚CFigure 2. Temperature effect on Tc volatilization from a soil sample.
The soil sample was volatilized for 1 h at each temperature. 1000A
and 1000B: First and second volatilizations at 1000˚C.

loss of Tc in either 1-month contaminated soil during incineration. The results were the same as for 6-month contaminatedsoil. Loss of 95mTc gradually increased in freshly contaminatedsoil by incineration whereas the soil samples contaminatedmore than 1 month by 95mTc showed no increase.26 This result
suggested that most soil samples could be incinerated without
any loss of fallout Tc. Thus the soil samples were incinerated
at 450˚C for 8 hours. However, the loss must be consideredwhen adding Tc freshly to soil samples as a yield monitor.
Combustion Apparatus. The volatility of 95mTc from theAndosol soil sample is shown in Figure 2. The volatility
increased with temperature. However, second heating at
1000˚C showed no remarkable increase of Tc volatilization. It
was suggested that Tc in the soil sample was not completely
volatilized even if the temperature of 1000˚C was maintained
for a long time. Probably, the Tc volatility was affected by thesoil volume, soil components, etc. Although Tc volatility did
not reach to 100%, the method could separate Tc from soil
matrices. Consequently, the following separation steps can be
simplified.
The percentages of 99Tc and 95mTc volatilized from the soil
were almost the same (data not shown); these Tc isotopes
showed the same volatility behavior although their contamination periods differed.28 Therefore, the freshly added 95m Tc
could be used as a yield monitor of originally included 99Tc.

0
Load
Wash-1(200 mL) (20 mL)

Wash-2(20 mL)

Strip-1(5 mL)

Strip-2(5 mL)

VolumeFigure 3. Recoveries of 99Tc, Ru and Mo in load (0.1M HNO3), wash
(2M HNO3) and strip (8M HNO3) solutions. The sample solutions
were pure water.

TEVA extraction. Each solution obtained by the above
mentioned extraction procedure was adjusted to 0.1 M HNO 3
and passed through a TEVA resin column (Eichrom Industries
Inc.) to separate and concentrate Tc isotopes.27 Because ICPMS cannot differentiate between 99Ru and 99Tc, it is necessary
to remove all Ru from the final sample solution prior to ICPMS measurement. Also, if the final solution for ICP-MS
measurements contains large amounts of stable Mo, theisotopes 98 Mo and 100 Mo might interfere with 99 Tc countalthough there is no natural abundance at 99 for Mo. Figure 3
shows the elution behaviors of Tc, Ru and Mo in a 200-mL ofdeionized water sample adjusted to 0.1 M HNO 3. From theresults, Ru and Mo present in the sample solution are noteffectively retained on the TEVA resin, while Tc was extracted
on the resin.30
Then, the column was washed with 2 M HNO3 to remove
any remaining traces of Ru. Technetium isotopes were eluted
with 5 mL of 8 M HNO3. The separation steps completely
removed Ru; only < 0.1% was found in the 8 M HNO3 eluate.
The eluate containing Tc was evaporated to dryness at lower
than 70˚C, and the residue was dissolved in 5 mL of 2% HNO3
for ICP-MS.
Measurements. After the separation procedure, the chemical recovery was obtained by counting 95mTc in the sample on
a NaI (Tl) scintillation counter (Aloka, ARC-380). The 99Tc
content of the sample solution was determined by ICP-MS(Yokogawa, PMS-2000) with 180 s counting time at mass 99.
To check levels of potential interference elements (e.g., Ru,
Mo), m/z = 98, 101 and 102 were also scanned at the sametime. The instrumental detection limit for 99Tc by ICP-MS was
0.03 mBq/mL of a sample solution, which corresponds to0.014 mBq/g dry weight of soil sample under the operational
conditions.
Fallout 137 Cs was used as an indicator for 99 Tc behavior
because of large number of data and the fission yields of both
radionuclides from 235U and 239Pu are close. The 137Cs activities in the soil samples were measured by a Ge detector (Seiko
EG&G Ortec) coupled with a multi channel analyzer (Seiko
EG&G, Model 7800). A 100-mL of each soil sample wastransferred into a plastic vessel and measured for 80000 s.
3. Distributions of 99Tc in Japanese Agricultural Soils
The 99Tc distribution in Japanese soils is shown in Figure 4.
The ranges of 99Tc and 137Cs concentrations are <3.6 – 110
mBq/kg-dry and <1.1 –143 Bq/kg-dry, respectively. The
average concentrations of 99Tc and 137Cs in paddy fields are 34

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TagamiJ. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003
8 10

Paddy Field

Upland Field& Other

<10
10-19
20-29
30-39
40 <
(mBq/kg-d)

Activity ratio of Tc-99 to Cs-137

7 10
6 10
5 10
4 10
3 10
2 10
1 10

−3

−3

−3

−3

−3

*4.8 × 10

−3

−3

−3

−3

*1.9 × 10−3−4

Theoretical ratio (3.3 × 10 )0 10

*3.6 × 10−40Paddy field

Upland field99

Figure 5. Activity ratios of Tc to
Japan. *: geometric mean

Others137

Cs in soil samples collected in

4. Radiotracer Experiments on Tc Immobilization in Soil
Figure 4. Global fallout 99 Tc concentrations (mBq/kg-dry) inJapanese agricultural soils.

mBq/kg-dry and 7.8 Bq/kg-dry, respectively, those in uplandfields are 5.9 mBq/kg-dry and 3.2 Bq/kg-dry, respectively, andthose in other soils are 16.3 mBq/kg-dry and 44.8 Bq/kg-dry,
respectively. 2 5 , 2 6 , 3 1 When the 9 9 Tc concentration wascompared on the basis of several characteristics of the soils,
the results showed that there is no correlation between 99Tc
concentrations and soil characteristics for paddy field soils, butslightly high correlations (r>0.5) between 99Tc concentrations
and some soil properties, such as cation exchange capacity,
act-Fe, total-C, organic-C, and total-N, for upland field soils
and other soils are found.
The activity ratios of 99Tc to 137Cs are given in Figure 5 andthe geometric means are 4.8 × 10−3 for paddy field soils, 1.9 ×
10 −3 for upland filed soils and 3.6 × 10 −4 for other soils.
Theoretically, the activity ratio from nuclear fission yield ispresently calculated as 3.3 × 10−4 with correction for radioactive decay, because it is assumed that the major source of 99Tc
in Japan arises from fallout. Compared to the theoretical ratio,
the activity ratios in the paddy field soils are one order ofmagnitude higher. The average activity ratios for upland fieldsoils are also higher than the theoretical ratio but those forother soils are close to the theoretical one. The ratios in paddy
field soils are twice as much as those in upland field soils andone order of magnitude higher than those in other soils maybe
because of different degrees of the soil reduction.
The higher or similar activity ratios found in this study ascompared to the theoretical one suggest 99Tc accumulation in
the soils, even under aerobic conditions. Generally, the most
preferable chemical form of Tc is TcO4− which shows a small
possibility for changing to a lower oxidation state or lesssoluble form under aerobic conditions. Also, the reported
distribution coefficient (Kd) of 99Tc in Japanese soil samples
generally showed low values as shown in Table 1, that is, nosignificant Tc fixation to surface soils is expected when Tc isadded as TcO4−. Subsequently, TcO4− is thought to migrate
much faster than cations, e.g. 137 Cs + , because of its highly
soluble chemical form. However, the studied soils seemed toretain 99Tc. It is necessary to identify chemical forms of Tc innatural soil conditions.

Oxidation-reduction reactions play an important role incontrolling the mobilization and biological accumulation ofsome toxic trace elements from soils. The migration of Tc in
soil and its absorption by plant roots are related to the sorption
behavior in the soil-water system. Technetium is expected to
be in a soluble form as pertechnetate under an aerobic condition, such as in surface soils and water. However, Tc sorption
on soil or mineral has been reported under relatively low redox
conditions.23, 44−46 Even in surface soil, the reducing conditionis generated by microbial activity and/or chemical reactions.
For example, paddy fields are generally waterlogged during
the planting period and subsequently, the redox potentials (Eh)decrease because microorganisms use up the oxygen present in
the water or trapped in the soil.47 The relatively low redox
condition is common in paddy fields during the planting
period. Thus, we carried out radiotracer experiments to obtain
the influence of microbial activity on Tc behavior in surface
soil under anaerobic conditions and to measure the change ofTc amounts in several soil fractions as a function of time.
4.1. Microbial Role in Immobilization of Tc Under
Anaerobic Conditions. To clarify the influence of microbialactivity on Tc behavior in surface soil under anaerobic conditions, glucose was added to the air-dried soil sub-samples from0 to 0.5% on an air-dried soil basis to enhance microbial activities in soil, and a sterile soil sub-sample was prepared fromthe air-dried soil (without glucose) by autoclaving. Seven soil
samples were used in this experiment. These pretreated airdried and sterile soil samples were placed in polystyrene vessels along with a porous cup at the bottom. They were carefully waterlogged with 60 mL of deionized water contaminatedwith 95mTcO4−. The surface water depth (from the soil surface)
was 1-2 cm. The vessels were sealed with polyethylene film toprevent microorganisms in the air from entering. At each sampling time, aliquots of the surface solution and the bottom
solution (from the porous cup) were collected. The Eh and pH
values of the surface soil (depth: ca. 5 mm) were also measured at each sampling time with a platinum complex electrode(Horiba pH meter, F-13). Activities of 95mTc in the collected
solutions were measured with the well type NaI (Tl) scintillation counter.
The Eh values and relative concentrations (RCs) of surface
and bottom solutions of one of the soil samples are shown inTechnetium-99 Behavior in the Terrestrial Environment

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003

Figure 6 as a function of time. The RCs of the surface solutions and also the Eh values decreased with increasing glucose
contents, while the RCs of S-0 (Sterile soil with glucose 0%)
were almost constant and had hardly any decrease. The RC for
the surface solution of AD-0.5 (Air-Dried soil with addition of
0.5% glucose on an air-dried soil weight basis) was almost
zero at the end of the experiment and the RCs for the bottom
solutions of AD-0 (AD with 0% glucose), AD-0.05 (AD with
0.05% glucose) and AD-0.5 decreased to almost 0 within 3
days. The data suggested that not only reducing conditions,but also microbial activity seemed to have an important role inTc sorption reactions. It was also found that the microbialactivity could be controlled by adjusting the nutrient concen-

RC (C/C0) of 95mTc in
surface solution

1
0.8
0.6

AD-0
AD-0.05
AD-0.5
S-0

0.4
0.2

RC (C/C0) of 95mTc inbottom solution

0
1
0.8
0.6
0.4
0.2
0
0.6

tration in soil. The Tc decrease in the surface and bottom solutions could be explained by the lowered Eh values and consequently, changing the chemical form of Tc in the surface soil.
The E h of AD-0.5 was lower than that of AD-0; microbialactivity in the surface solution of AD-0.5 was higher than thatof AD-0 and Tc(+7) as TcO4− of AD-0.5 might reduce faster to
Tc(+4) and be sorbed on the soil surface.23
4.2. Identification of the Adsorption Behavior of Tc inSoils. For some nuclides, the amount of the available fraction
may change substantially over time through a process sometimes called “aging”. Although a strict delineation of specificchemical species or binding forms is difficult, speciation of
toxic heavy metals in the environment has been carried out toassess the potential effects of soil contamination instead of
total concentration. 48−52 These studies designed single orsequential extraction schemes to remove “exchangeable” or“bioavailable” fraction in soil and sediment, but the extraction
methods have not been used to follow the change ofextractable fraction as a function of time.
Two typical upland field and paddy field soils in Japan,
classified as Andosol and Gray lowland soil (GLS), were used.
The air-dried soils (ø<2-mm) were placed into plastic vessels
and 95mTcO4− was uniformly mixed with each sample. Then
deionized water was added to adjust the soil water content to adesired level as follows: 1) waterlogged upland field soil(FW); 2) upland field soil with a typical upland moisture
content (FO); 3) waterlogged paddy field soil (PW); and 4)
paddy field soil with an upland field moisture content (PO).
The water contents for numbers 1 and 3 were similar to thosein paddy fields, that is, the waterlogged condition. All samples
were stored in the dark while being kept at room temperaturefor 6 months. At 1, 3, 7, 14, 30, 60, and 180 days, 20 to 25g
amounts from each soil sample were used for the extractions.
The reagents used are listed in Table 3, which are modified
from the method of McLaren and Crawford 48 to extract thefractions.24
Figure 7 shows the Tc fixation pattern in the soil samples
TABLE 3: Reagents Used in the Extraction

Eh / V

0.4

Reagent

Soil Solution
weight amount Code/g/mL

Fraction

0.2

0.05 M calcium chloride
0

0

1

2

3

4

5

6

7

Time / day

Figure 6. Time dependence for relative counts of 95mTc in surface andbottom solutions and Eh (V) of air-dried soil with addition of 0, 0.05
and 0.5% glucose on an air-dried soil weight basis (AD-0, AD-0.05
and AD-0.5) and sterile soil without glucose (S-0).

Exchangeable & water soluble 2
0.5 M acetic acid
Specifically adsorbed
2
0.1 M sodium pyrophosphate Organically bound2
Sorbed onto the residue

CA
AA
PY
Res

Technetium-Andosol

1.2

Aerobic

Anaerobic

40
40
200

Note: All soils were extracted at their experimental moisture contents forwhich the mass of the leached soil was chosen to be equivalent(±5%) to the air-dry weight listed above.

Technetium-GLS

1.2

Aerobic

Anaerobic

1

1

Relative amount (Q ra )Relative amount (Q ra )

A5

0.8
0.6
0.4
0.2

PW-ResPO-Res

PW-PY

PO-PY

PW-AA

PO-AA

PW-CA

PO-CA

3

7

14 30 60 180

1Time /day

3

7

0.6
0.4

FW-Res
FW-PY
FW-AA
FW-CA

0.2

0
1

0.8

14 30 60 180

0

1

3

7

FO-Res
FO-PY
FO-AA
FO-CA

14 30 60 180

1

3

7

14 30 60 180

Time /dayFigure 7. Time dependence on the amounts of CA-, AA- and PY-extractable Tc in GLS and Andosol under waterlogged and upland field waterconditions. PW: GLS under waterlogged conditions, PO: GLS under upland filed conditions, FW: Andosol under waterlogged conditions and FO:
Andosol under upland field conditions. CA, AA. PY and Res are explained in Table 3.

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TagamiJ. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003

under upland field and waterlogged conditions. The relative
amount (Qra) was defined as “the total activity in extracted
solution from unit soil sample at each sampling time” divided
by “ the total activity of the subjected unit soil sample”. The
original extraction method was used sequential,48, 50 however,in this study, the reagents were used individually because thesingle use showed less damage to the soils.24 Thus, the extractions with AA and PY include CA and CA+AA, respectively.
To calculate PW-AA, PO-AA, FW-AA, or FO-AA in Figure 7,
the extracted fraction with CA (hereinafter this is designated as
Qra of CA-Tc) was subtracted from the actually extracted fraction with acetic acid (Qra of AA-Tc) and, similarly, to get PWPY, PO-PY, FW-PY, or PO-PY, Qras of CA-Tc was subtracted
from Qras of PY-Tc. Residual part was calculated to subtract
Qra of PY from 1. As can be seen from the Figure 7, the Qrasof 95mTc in soil samples under aerobic and anaerobic conditions
differed. The Qra values of CA-Tc and AA-Tc were almost thesame. It seems that the Tc which strongly adsorbed on the
Andosol or GLS samples might be negligible. Under the
aerobic condition, Qra of CA-Tc and Qra of AA-Tc for Andosol
samples kept at a constant value of around 0.8 during theexperiment while that for GLS dropped to about 0.5. Under
the waterlogged condition, Qra of CA-Tc and Qra of AA-Tc forAndosol were decreased to about 0.2 and that for GLS
decreased over time and became almost 0. One day after Tc
was initially added to the aerobic and anaerobic soils, the Qraof PY-Tc was negligible compared to those of CA-Tc or AATc. Then, in the anaerobic soils, the PY fraction gradually
increased for 30 days then became almost constant, while no
Qra of PY-Tc increments was found in aerobic soils. From theresult, it was found that some part of the non-CA extractable
Tc transformed to PY extractable, that is, organically boundforms.
These Tc reductions under the anoxic condition may be
attributed to the transformation of Tc under the low redox
potentials in the soil produced by waterlogging. The pertechnetate was transformed to lower oxidation forms, sulfates ororganically bound. The sulfides might occur because H2S gas
can be produced in rice paddy fields under the waterlogged
condition. As mentioned above, TcO4− is the most available
form for plants.11, 12 Thus, these changes of Tc in its chemical
form under the anoxic condition affected less Tc transfer fromsoil to plants. This suggested that Tc transformation from soil
to plants (not only rice but also any other higher plants grownunder the anaerobic condition) became less with time after Tc
addition to the soil.
4.3. Possibility of Change in Tc Extractability After AirDrying. Technetium extractions in air-dried soils were carried

Relative extracted amount

0.5

Andosol
Gray lowland soil

0.4

out following 52 days of waterlogging of the soil. Qra values
for extractions by CA as a function of time are shown in
Figure 8. Extractions with AA and 1 M CH3COONH4 (pH=7)
were also carried out to check the change of the amount ofreadily extractable Tc with time. The Qras were approximately
equal to that of CA, so that the data are not shown. Even
though the soil was air-dried and kept in aerobic conditions, Tc
extractability did not change significantly from the initialvalue, that is, the value at just the end of the waterlogging
period. If all the Tc that adsorbed on to the soil was Tc(+4)
oxides, such as TcO2 and TcO(OH)2, it could be transformed toa higher oxidation state, TcO4− though its rate would be slow.
Even in the aerobic condition, other stable chemical forms ofTc, e.g. organically bound forms, sesquioxides bound forms
and sulfides, probably were presented. The immobilized Tc
might change to TcO4− in the presence of oxidants; however,such chemicals are never used in paddy field soils so that aquick reaction to TcO4− would not occur after the harvest.
From these radiotracer experiments, it is predicted that Tc
transformation to lower oxidation states would be encouraged
by waterlogging in an actual rice paddy field, and also microbial activity seemed to have an important role in Tc sorption
reactions. Besides, the reaction of the immobilized Tc form to
TcO4− should be slow.32 When there is no potential process forremoval of the 99Tc from soil (leaching and harvesting), there
was a possibility that Tc had been accumulating in the surface
layer of paddy fields, although the fields would keep uplandconditions from the harvest to the transplanting season (Figure
9).
Although paddy field soils could immobilize global fallout
99
Tc that was deposited directly onto them, the relatively high99
Tc concentrations in paddy fields observed in our field study
suggest other 99Tc sources. Probably, the paddy field soils had
trapped 99Tc not only from deposition, but also from irrigationwater through the mechanism as mentioned above. The
average Japanese paddy field receives 1800 mm rainfall equivalent of irrigation water during the irrigation season, which is
twice as much as the water supply from precipitation. The irrigation water would contain 99Tc leached from other soil sites,
e.g. forest soils. Some part of the 99Tc in the water could betrapped onto the paddy field soil surface.
5. Transfer factors of Tc obtained in Chernobyl Area
For realistic assessment, it is necessary to obtain transfer
parameters, such as TFs and Kds, under natural conditions, but
the data have been limited due to trace levels of 9 9 Tc.
Obtaining TFs using global fallout 99Tc in fields is difficult,because the concentration of 99Tc is very low. Therefore, we
focused on the Chernobyl site where 99Tc concentration levels
have already been shown to be higher than those from global
fallout Tc.33
Soil and plant samples were collected by the Federal Officefor Radiation Protection, Germany, in 1994 and 1995, assisted

0.3

0.2
O2

TcO4−

Air-dry

0.1

Immobilization of Tc(possible transformation)

0.0

TcO4−

0

20

40

60
Time / day

80

100

Figure 8. Time dependence on the amounts of CA-extractable Tc in
the air-dried Andosol and GLS following waterlogging.
The soil samples were waterlogged for 52 days then air-dried for 86
days. Each extraction was carried out in duplicate.

Time (after waterlogging)Numbers of microorganisms
Aerobic
Anaerobic
Redox potential (Eh)H2S gas concentration

Reduction

Tc(+4)

e.g. TcO2 and TcO(OH)2

H2S gas

TcS 2
Tc2S7

Bound toOrganic matter / Sesquioxide orFix to other soil solid phasere-oxidation reaction
SLOW

TcO4−

Tc accumulation inrice paddy fields

Figure 9. Immobilization mechanism of Tc in paddy field soil.

10

3

10

2

10

1

10

0

−1

−2

D3-r3

D3-r2

D3-r1

D3-f2

D3-f1

D1-r2

D1-r1

D1-f2

D1-f1

−3

K2-r2

10

For all sites, TFs for Tc observed in this work are two or
three orders of magnitude lower than those reported for otherplants.9 However, recently Green et al.54−56 performed uptake
experiments using contaminated soil of marine origin in anarea of reclaimed land to obtain Tc-TFs under the field conditions. The TFs they obtained were less than 2.0 for manykinds of vegetables based on dry weight of the vegetable,similar to those obtained in our study.
6. Summary

K2-f2

10

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Green et al (1995)

K2-f1

10

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003

IAEA-TRS-364 (1994)

K2-r1

Transfer factor for Tc-99

Technetium-99 Behavior in the Terrestrial Environment

SamplesFigure 10. TFs of Tc from soil to strawberry and raspberry leaves
collected from forest sites within the 30-km zone around theChernobyl Reactor.

by Moscow State University, Russia, and the Research andIndustrial Association ‘Pripyat’, Ukraine. The sampling was
carried out in three forests around Chernobyl. Forests D1 andD3 were 28.5 km and 26 km to the south of the Chernobyl
Reactor, respectively, while forest K2 was 6 km to the southeast of the Reactor. The sites D1 and D3 are mixed forests
while K2 is a pine forest. Surface organic soil layers (L, Of
and Oh) collected at each site were mixed and measured fortheir 99Tc activity. The D1 samples, collected in 1994 and1995, were mixed together, because of their small volumes. It
is noted that forest understorey plants are expected to take upradionuclides mainly from the organic layers53 and therefore,
concentration of 99Tc in organic soil would be important for
estimation of its uptake. For this reason, we calculated TFs interms of activity concentration in plant relative to activity
concentration in organic soil. Plant samples used were strawberry and raspberry leaves since these species were found eachyear at each site, and possible differences of the TFs fromdifferent cites could be investigated.
The details of the 99Tc concentrations in soils and plants
were reported elsewhere,34−36 therefore, only calculated TF is
discussed. The expected TF values for grass and spinach are
7.6 × 101 and 2.6 × 103, respectively.9 Comparing with the TF
for Cs, that for Tc is about three to five orders of magnitude
higher. For this reason, 99 Tc is one of the very importantradionuclides for the assessment of internal dose by ingestionof the contaminated vegetables. However, attention should be
paid to the fact that most of the TF values for Tc reported wereobtained by tracer experiments in which Tc was applied to soil
in a soluble form, e.g. as TcO4−. Because the chemical formsof Tc are variable under natural conditions, TF values obtained
from laboratory experiments need not necessarily be the sameas those obtained in the field.
The TF data for Tc obtained are shown in Figure 10. The
TFs for Tc and Cs (not shown) obtained in this study are range
from 0.09 to 0.45 and from 0.06 to 0.46, respectively, based on
dry weight.36 The obtained TFs for Cs are almost the same as
those previously reported for strawberry leaves samples at theinvestigated sites.53 The corresponding TFs for Tc are between
factors two to five larger compared to those for Cs for the sites
K2 and D1. On the contrary, at D3 the TF for Tc is lower by afactor of about 5 compared to that for Cs. This might be due tothe fact that D3 is a so-called wet forest, and accordingly Tc
might exist in the TcO2 form, which is much less plant-available (see above).

Tc is known to have high mobility and availability to plantin the soil environment because the dominant oxidation state ofTc is heptavalent, TcO 4−. However, because Tc is a redox
sensitive element, it might change to lower oxidation forms
under anaerobic conditions. The behavior of Tc is expected tobe different under upland field and rice paddy field conditions,but available data have been too limited to understand the Tc
behavior under waterlogged conditions. Therefore, it was
important to obtain some knowledge of 99Tc in the soil-to-rice
plant system.
To determine low-levels of 99Tc in environmental samples,
we developed a unique analytical procedure based on ICP-MS.
Then, 99Tc concentrations in Japanese rice paddy field soils
were analyzed. The average concentration of 99Tc in paddy
fields was measured to be 34 mBq/kg-dry. The range ofactivity ratios of 99Tc/137Cs in the soil samples was (2.0 – 5.2)
× 10−3; these ratios were one order of magnitude higher than
the theoretical one, which is calculated now to be about 3.3 ×
10−4. Such high activity ratios indicated that 99Tc had been
accumulating in rice paddy soils like 137Cs.
A series of radiotracer experiments were carried out tounderstand the accumulation mechanisms of Tc in soil. It was
found that Tc transformed from TcO4− to other chemical forms
because it was immobilized in soil under waterlogged conditions. The reaction of the immobilized Tc to TcO4− should be
slow. On this Tc immobilization, microbial activity seemed tohave an important role. We are working on radiotracer experiments to find out a possibility of Tc immobilization by aerobic
bacteria under waterlogged conditions.57 When Tc was immobilized, leaching and harvesting, those are thought to be main
Tc removal paths, could only remove a small part of Tc fromthe soil and, thus, Tc had been accumulating in the surface
layer of paddy fields. The results from the wet forest within
30-km zone from the Chernobyl Reactor can be a real exampleof this series of our experiments: the Tc in the forest was lessavailable to plants than those in other two forests.
99
Tc would retain in the terrestrial and the marine environments for many generations, further studies on 99Tc behavior in
the environment is necessary, from the viewpoint of the longterm dose effect on humans.
Acknowledgement. The author is grateful to Dr. T. Sekine,
Tohoku University, for kindly providing us with the Tc-95m
tracer solution.
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