Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 243-247, 2005

Technetium(IV) Oxide Colloids and the Precursor Produced by Bremsstrahlung
Irradiation of Aqueous Pertechnetate Solution
Muhammad Zakir,a Tsutomu Sekine,*, a Tsutomu Takayama,a Hiroshi Kudo,aMingzhang Lin,b and Yosuke KatsumurabaDepartment of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Nuclear Engineering Laboratory, School of Engineering, University of Tokyo, Ibaraki 319-1106, Japanb

Received: July 31, 2005; In Final Form: September 30, 2005
Mechanisms of the radiolytic formation of 99TcIVO2·nH2O colloids were studied by bremsstrahlung irradiation of
an aqueous pertechnetate (TcVIIO4 −) solution under different conditions; pH of the target solution, concentration of
TcO4 − and presence or absence of radical scavengers and of seed particles of TcO2·nH2O. Colloid particles (30 –130 nm) of TcO2·nH2O  a coagulate of nanoparticles with the size of 2 nm  were radiolytically produced at pH
higher than 3, whereas a soluble Tc(IV) species but colloid was formed at lower pH. The soluble Tc(IV) species
transformed to the TcO2·nH2O colloid when the pH of the solution was raised to 3 or higher after the irradiation.
This fact suggests that the soluble Tc(IV) species is a precursor of the TcO 2 ·nH 2 O colloid. The presence ofTcO2 ·nH 2O seed particles in the target solution of TcO 4 − did not enlarge of the partcile size, revealing that thesolidification would take place rather in the solution than on the surface of nanoparticles. The reduction of TcO4 −in acidic solutions proceeded mainly through processes involving a bimolecular reaction of TcO4 − with hydrogenradicals, followed by the successive disproportionation reactions of Tc(VI) and Tc(V).

1. Introduction
In terms of high-level radioactive waste disposal in deep
underground, the formation of colloids in groundwater is oneof the most important factors that promote migration of longlived radionuclides in the environment.1 A number of studies
have shown that mobile colloids, abundant in the subsurface ofearth, would facilitate the transport rate of radioactive contaminants higher than predicted by aqueous-phase transport models.2, 3 Much remains to be investigated to elucidate specificconditions necessary for stability and movement of colloids.4
Technetium (99Tc) is a long-lived (t1/2 = 2.11 × 105 y) nuclear
fission product that has entered the earth’s surface and groundwaters as a result of atmospheric nuclear testing as well as disposal of medical and nuclear waste. It also exists as an important
subsurface contaminant at numerous nuclear facilities.5 At the
Hanford Site (USA), four large 99Tc groundwater plumes exist
and new ones are forming as 99Tc released to cribs and leaked
from single-shell high-level waste tanks. The 99Tc migrates
through the vadose zone. Over 1.48 TBq of 99Tc(VII)O 4 − isenvisaged to be discharged to the Columbia River in future years,
making it one of the site’s major risk-driving contaminants.6
The environmental geochemistry of Tc is dominated by the
IV (TcO(OH) 2) and VII (TcO4 −) oxidation states. The VI, V
and III oxidation states are known, but the VI and V states are
generally unstable and disproportionate.7 The III state requires
extremely low redox potentials.8 The highly soluble pertechnetate ion (TcO4 −) would not form significant aqueous complexes
or discrete concentration-limiting solid phases under environmental conditions. This species migrates rapidly concurrent
with vadose zone water and groundwater, weakly sorbed undermost geochemical conditions.9 In contrast, Tc(IV) readily precipitates as an insoluble hydrous oxides unless complexed.10, 11
Conversion of Tc(VII)O4 − to Tc(IV)O(OH) 2, which would not
migrate due to its strong adsorption on rocks and minerals,12offers a potential capability for immobilization of Tc in the*Corresponding author. E-mail: tsekine@mail.tains.tohoku.
ac.jp. FAX: +81-22-795-6596.

environment. At present, however, knowledge on the formationand stability of Tc(IV) colloids is limited.
In addition to general routes of colloid formation, a radiolytic effect on the formation of colloids should be taken into
account in this case, because an extremely high radiation dosefrom the spent nuclear fuel will be continuously given to thesurroundings for a long period of time.
The radiolytic colloid formation has been extensively investigated,13, 14 because it is one of the unique means to prepare
nanoparticles of noble metals and bimetals in terms of technological applications of nanoparticles. In radiolyses of aqueous
solution, hydrated electrons (eaq −) and hydrogen radicals (H),which are radiolytic products of water, are effective reductantsfor metal cations to produce nanoparticles. As for technetium,
Said et al.15 reported that a brown turbid solution was obtainedby γ-irradiation (60Co source) of an aqueous solution of pertechnetate (Tc(VII)O4−). This was the first evidence for the formation of colloidal Tc(IV)O2 ·nH 2O by radiolysis. In contrast,
Lefort reported the redissolution of TcO2·nH2O by irradiation of60
Co γ-rays.16 Recently, we have found the formation oftechnetium(IV) oxide (Tc(IV)O2·nH2O) colloids by bremsstrahlung irradiation of aqueous pertechnetate solution. The color ofirradiated solutions gradually changed to brownish black, suggesting the formation of the colloids.17 Here, we report characteristic features of the radiolytically produced colloid and its
formation processes.
2. Experimental
Ammonium pertechnetate (NH499TcO4) crystals were thermally decomposed under oxygen gas flow (flow rate = 100 cm3
min−1) in a glass tube heated with an electric furnace to 300 ˚C.
Technetium heptaoxide (Tc2O7) that was vaporized and deposited on the inner surface of the glass tube was dissolved in triply distilled water. This solution was kept as a stock solutionof pertechnetic acid (HTcO4). The TcO4 − concentration of thesolution was adjusted to the range of 2.56 × 10−6 – 1.12 × 10−4
M. The target solutions were bubbled with Ar, O2, N2, or H 2gas, and sealed in polypropylene vials (5 mL in volume). The© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

ZakirC KX rays

244 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005

Figure 1. Energy dispersive X-ray spectrometry (EDXS) of
TcO2·nH2O colloids on copper grid.

target solutions of pH 1.8 and 2.7 were prepared by acidifying
with sulfuric acid.
The target solutions were irradiated with bremsstrahlung
from an electron linear accelerator of the Laboratory of Nuclear
Science, Tohoku University. The irradiation setup was described
elsewhere in detail.17 The irradiation time was ranged from
5 min to 2 h. The pulse duration was 3 µs and the repetition
rate of electron pulses per second (pps) was varied from 6 –300 pps. Target temperature was also varied from 8–40 ˚C.17
The irradiated solutions were kept in an ice reservoir until chemical analysis.
The amount of Tc(IV) colloids was determined by ultrafiltration. One milliliter of the irradiated solution was passed
through an ultrafilter (Millipore, nominal molecular weight
limit = 3,000 or 10,000) by centrifugation (4000 rpm, 2 h).
The concentration of 99Tc was measured with a liquid scintillation counter (Aloka LSC 5100). UV–vis spectra were recorded
on a Shimadzu MultiSpec-1500 in the range of 190–800 nm
before and after ultrafiltration. Separation of soluble Tc species was performed by paper chromatography (Chromatography
papers No. 51B, Advantec Toyo Co.). The chromatogram wasdeveloped for 20 cm length with an aqueous eluent having thesame acidity of the target solution. Technetium species on thepaper were visualized by using an imaging plate (Fuji Photo
Film, BAS-IP SR-2005) and analyzed by a Bio Imaging Analyzer(Fuji Photo Film, BAS-5000).
A transmission electron microscope (TEM, JEOL JEM2000 EXII) was used for evaluation of a mean size of colloids.
Chemical composition of the colloids was confirmed by theenergy dispersive X-ray spectroscopy (EDXS; Oxford Instruments
Link-ISIS).
3. Results
Characterization by energy dispersive X-ray spectrometry(EDXS) of one colloid particle revealed that the colloid consisted of technetium and oxygen (Figure 1). The observed Xrays peaks were assigned to technetium, oxygen, copper andcarbon; Kα 1 (Tc) 18.41 keV, Lα 1 (Tc) 2.424 keV, Kα 1 (O) 0.523
keV, Kα1 (Cu) 8.047 keV, Kα1 (Cu) 8.904 keV and Kα (C) 0.282
keV. The X-rays of copper and carbon arose from a copper gridcoated with graphite used as a sample holder in the EDXS measurement.
As reported in the previous paper,17 the size of colloid particles distributes around 30–130 nm in diameter. The shape of

Figure 2. Fraction of TcO 4 − in the irradiated solution against
absorbed dose. The initial concentration of TcO4 − was 1 × 10−4 M and
pH = 6/7 (neutral). The target solution was saturated with: £, H2 ;¢, Ar; r, N2O; p, O2.

colloid produced depends on the temperature of target solutions
during irradiation. Spherical particles were formed by the irradiation at 40 ˚C (3–7 h), while irregular-shaped colloid particles
composed of tiny particles (2 nm in diameter) were formed at 17
˚C. The irregular surface feature was independent of the kind of
dissolved gases and the absorbed dose. This is true for almost
all particles produced by irradiation at 17 ± 3 ˚C. Assumingthat the tiny spherical particles has the same density as the TcO2
crystal (3.02 g cm−3),18 we can describe that about 30000 of tiny
particles (2 nm in diameter) coagulate to form colloids in alarger size. The colloids afforded by bremsstrahlung irradiation
were amorphous.
The fraction of TcO4 − in the irradiated solution are plottedagainst the absorbed dose up to 50 kGy in Figure 2 for samples
with an initial concentration of 0.1 mM. The fraction of TcO4 −sharply decreased in the H2- and Ar-saturated solutions even in
the low dose range. In the O2- and N2O-saturated target solutions, the colloid formation was not found up to 200 kGy.
The formation of TcO2.nH 2O colloid was only observed in
the solutions irradiated at pH 6–7 (neutral) and pH 4.6 (Figure3(a)). The polymeric Tc(IV) species, the presence of which
was suggested by Vichot et al.19 was formed only in the solution
at pH 1.8, acidified with sulfuric acid. Both Tc(IV) colloids
and the soluble Tc(IV) species were formed at pH 2.7. Here,the polymeric Tc(IV) species was formed first in the low dose
region followed by generation of TcO2·nH2O colloids (Figures
3(a) and 3(b)). The effect of the presence of sulfate ions was
examined by adding Na2SO4 (0.01 M) to the target solution (pH4.6). Only the TcO2·nH2O colloids but polymeric Tc(IV) species were formed in the solution.
The presence of own seed particles which might enhance theformation rate of colloids by γ-radiolysis was also examined inthis experiment. The fraction of TcO4 − was determined for theirradiated TcO4 − solution in the presence and absence of seed
TcO2.nH2O colloids. The results are shown in Figure 4 for the
lower dose range up to 8 kGy. There is no significant difference in the TcO4 − fraction, irrespective of the irradiation withor without TcO2.nH2O seed particles.
In the neutral target solutions (pH 6–7), the TcO2·nH2O colloid yield or the decrease in TcO 4 − fraction in the solution
depends on the initial concentration of TcO4 − as summarized inTable 1. The TcO2·nH2O colloid was not formed at 2.56 × 10−6
M of the initial concentration, the TcO 4 − fraction remaining

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 245

Technetium(IV) Oxide Colloids and the Precursor Produced

TABLE 1: Pertechnetate Fraction (%) in the IrradiatedSamples for Different Initial Concentrations and Absorbed
Doses. The Target Solutions were Neutral (pH 6–7)

Fraction of Tc(IV) colloids / %(a) 100

80

60[TcO4 − ] initial / M

Dose / kGy

TcO4 − fraction / %1.12 × 10 −4

0.18
0.24
0.5
0.69
1.6
2.9
6.8
13.7
23.8
30.7

62.9
10.2
8.2
6.5
6.5
6.7
7.1
6.6
6.9
6.2

7.86 × 10 −5

0.15
0.28
0.44
0.68
1.8
2.6
9.7
20.5
28.4
1.7
6.3
5.8

50.9
14
14.7
13
11.4
12
9.8
10.2
8.7
100
100
100

40

20

0

0

2

4

6

8

10

Dose / kGy

Fraction of soluble Tc(IV) species / %(b) 100

80

60

40

2.56 × 10 −6
20

0
0

2

4

6

8

10

Dose / kGy

Figure 3. Fraction of Tc(IV) colloids (a) and soluble Tc(IV) species(b) in the irradiated TcO4 − solutions as a function of dose. pH of thetarget solutions was: r, neutral (6/7); ¢, pH 4.6; £, pH 4.6 + 0.01
M Na2SO4; ò, pH 2.7; ô, pH 1.8. Solid lines are a guide for eyes.

Fraction of TcO4 in solution / %

100

80

60

40

20

0
0

2

4

6

8

Absorbed dose / kGy

Figure 4. TcO4 − fraction in the solution against absorbed dose. The
solution was Ar-saturated and the initial concentration of TcO4 − was 1
× 10−4 mol dm−3, in the presence (ô) and absence (ò) of TcO2·nH2O
colloid. The temperature was 8 ± 2 ˚C during irradiation.

100%. The yield of colloid was fluctuated between 6 to 10% at
the absorbed dose above 0.8 kGy. The final concentration of
TcO 4 − reached 7.44 × 10 −6 M for [TcO 4 − ] initial = 1.12 ×10 −4 M
and 7.51 × 10−6 M for [TcO4 −] initial = 7.86 × 10−5 M.
4. Discussion
Pertechnetate (99TcO4 −) ions were radiolytically reduced totetravalent technetium before the TcO2·nH2O colloid formation.
In general, the reduction reaction in radiolysis is brought about
by reducing agents arising from radiolysis of water:
H2O → eaq−, H, OH, H2, H2O2, O2, H3O +.
The reducing agents are hydrated electrons (eaq−, G-value =2.7) and H radicals (G-value = 0.6) 20 in neutral solution. The
hydrated electron (eaq−) is a reducing agent stronger than the H
radical. In the O2-saturated solution, eaq− and H are effectively
scavenged by O2 to give O2−. In the N2O-saturated solution, N2O
rapidly reacts with eaq − giving rise to an OH radical that is astrong oxidizing agent. In H2- or Ar-saturated solution, scavenging of eaq− and H radicals would be minimized because of the
low O2 concentration (< 0.2 ppm). Moreover, the presence of H
radicals in the H2-saturated solution will scavenge the existingO2 molecules. Since the eaq− (stronger reducing agents) is effectively scavenged by dissolved O2 and N2O, the formation of
TcO2·nH2O colloids does not take place (Figure 2). These facts
suggest that eaq− plays a fatal role at an initial stage in the reduction of TcO4−. Details of the role of eaq− in the radiolytic reduction of TcO 4 − were discussed in the previous paper.17 Other
possible reduction reactions of TcO4 − ions could be initiated by
H radicals. In addition, hydrated electrons are converted to H
radicals in acidic water,21 and the contribution of H radicals in
the radiolytic reduction of TcO4 − should be effective in acidic
solutions of the target .
Pertechnetate (TcO4−) solution was electrochemically reduced
at pH 1.5 as reported by Vichot et al.22, 23 They suggested theoccurrence of soluble polymeric Tc(IV) species (TcnOp (4n−2p)+with n > 2) in acidic solution. The species was proposed to be
Tc 3 O 4 4 + (Ref. 23) from the similarity of proper ties toMo(IV) 3O 4 4+ clusters24 observed in acidic media. They also
reported that the polymeric Tc(IV) species was soluble in anacidic solution and would aggregate each other at higher pH,
resulting in TcO2·nH2O.
When the acidic solution (pH 1.8) was irradiated in the present experiment, the colloids were not formed (Figure 3(a)),
although a soluble Tc(IV) species were formed (Figure 3(b)).
This result suggests that H radicals effectively reduce TcO4 − toform soluble Tc(IV) species, because eaq − reacts with proton(H+) to generate H radicals in acidic water.21 The soluble Tc(IV)species showed a strong absorption peak (ε = 6800 M−1 cm−1) at
320 nm and a weak peak (ε = 700 M−1 cm−1) at 500 nm in theUV–vis spectra.25 These absorption peaks could be assigned topolymeric Tc(IV) species from the similarity of the spectra andof the molar extinction coefficients reported by Vichot et al.19, 22, 23

246 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
No changes were observed in the spectra before and after ultrafiltration, indicating no colloid formation. When an NaOH
solution was added to the solution containing polymeric Tc(IV)species, TcO2·nH2O colloids appeared eventually. In the target
solution with the initial pH at 2.7, both TcO2·nH2O colloids andpolymeric Tc(IV) species were formed by the irradiation(Figures 3(a) and 3(b)). It can be seen in the figures that thepolymeric Tc(IV) species is produced early in the low dose
region and followed by generation of the TcO2·nH2O colloids.
Namely, the polymeric Tc(IV) species partly changed to
TcO2·nH2O colloids at pH 2.7 as the dose increases. Hence, it
seems reasonable to consider that the polymeric Tc(IV) species
should be a precursor of the TcO2·nH2O colloids, although anexact stoichiometry of the polymeric species has not been clarified yet.
Reduction of TcO4 − in the presence of colloidal TcO2·nH2O
seed particles was also investigated. The own seed particles
are known to enhance the rate of metal reduction in solution by
γ-radiolysis. It is reported, for instance, that the reduction rateof Ag+ was largely enhanced when colloidal silver seed particles are present in the solution, and larger colloidal silver particles with a narrow size distribution are produced.26, 27, 28 The
reduction mechanism proposed is that reducing radicals transfer electrons to the seed particles, and the stored electrons
finally reduce Ag + directly on the surface of the seeds. The
metal nanoparticles in the solution principally work as a catalyst to enhance the coalescence to form larger particles. In thepresent experiment, however, no effects of colloidal TcO2·nH2O
seed particles were observed as shown in Figure 4. The factsuggests that the TcO2 ·nH 2O colloids are produced throughsolidification of Tc(IV) species in the solution, irrespective of thepresence or absence of own seeds. Reoxidation of TcO2·nH2O
colloids by irradiation, as reported for the dissolution of
TcO2·nH2O by 60Co γ-rays,16 might also occur.
As reported in the previous paper,17 a tiny TcO2·nH2O particle
2 nm in diameter must be a nanoparticle that is stabilized in thesolution immediately after the colloid generation. The tiny particle generation can be explained by a mechanism proposed by
LaMer.29 The mechanism describes that solidification of solutein a solution is initiated at the solute concentration much higher
than the solubility; so-called supersaturation.
Based on the LaMer’s model, Sugimoto proposed a colloid
generation mechanism that dealt with the emergence of nuclei
with sizes distributed around a thermodynamically stable critical size under the supersaturation condition.30 Nuclei smaller
than the critical size quickly dissolve again, while larger nuclei
grow through the Ostwald ripening, resulting in colloids withsuitable sizes stabilized in the solution. In our experiment,
amounts of the polymeric Tc(IV) species increased with anincrease of absorbed dose, and 80% of TcO4 − was converted tothe polymeric Tc(IV) species at 6.5 kGy.25 The stabilized colloids or nanoparticles of TcO2·nH2O (2 nm in diameter) appear
through the solidification process of the colloid precursor,
being followed by the growth of nuclei as mentioned above.
As pointed out in the previous section, the final concentration of TcO4 − is 7.5 × 10−6 M. This should be a concentration
where the solidification of TcO2·nH 2O colloid is initiated. Itmeans that only 90% of the TcO4 − available in the target solution is converted to the TcO2·nH2O colloid. The target solution
below this concentration (7.5 × 10−6 M) would not afford colloids but soluble Tc(IV) species as can be seen in Table 1 for[TcO 4 − ] initial = 2.56 × 10 −6 M. When the Tc(VII) reached 7.5
×10−6 M in the irradiated solution, the Tc(IV) concentration ismuch higher than the reported solubility19 of TcO(OH) 2, leading to the supersaturation condition.29
The chemical structure of this highly soluble Tc(IV) species
is unknown, and further investigation is required to elucidate
more details for conversion of the polymeric Tc(IV) species to
TcO2 ·nH 2O colloids. At present, the overall formation pro-

Zakir
TcVIIO4– + eaq –TcVIO42–

Neutral2Tc(VI)

Tc(VII) + Tc(V)

2Tc(V)

Tc(VI) + Tc(IV)

TcVIIO4 – + H

TcVIO42–

AcidicTc (IV) polymer
Generation of TcO2.nH2O nuclei
by hydrolysis
Formation of TcO2.nH2O
nanoparticles (~2 nm)

TcO2.nH2O colloid (30 - 130 nm)

Scheme 1. Formation processes of TcO2·nH 2O colloids by bremsstrahlung irradiation.

cesses of TcO2 ·nH 2O colloids by bremsstrahlung irradiation
can be summarized in the Scheme 1.
5. Conclusions
Characteristic features of 99TcIVO2·nH2O colloids formed bybremsstrahlung irradiation of aqueous pertechnetate (TcVIIO4 −)solution are as follows. The diameter of the colloids distributes
around 30 –130 nm. Irradiation at 40 ˚C gave round-shaped
colloids, whereas irregular-shaped colloid particles composedof nanoparticles (2 nm in diameter) were formed at 17 ˚C. The
colloid formation was observed only in the target solutions at
pH higher than 3. At pH lower than 3, the dominant productwas the polymeric Tc(IV) species that should be a precursor of
TcO2·nH2O colloids as proved by its transformation to the colloid at higher pH.
The reduction of TcO 4 − to afford TcO2 ·nH 2O colloids proceeded mainly through the processes involving a bimolecular
reaction of TcO4 − with hydrated electrons (eaq−) followed by the
successive disproportionation reactions of Tc(VI) and Tc(V) to
Tc(IV). In the acidic target solutions, TcO 4 − was effectively
reduced by H radicals to produce polymeric Tc(IV) species.
When the Tc(VII) reached 7.5 × 10−6 M in the irradiated solution, concentration of the polymeric Tc(IV) species should bemuch higher than the reported solubility of TcO(OH) 2, leading
to the supersaturation condition. The stabilized colloids or
nanoparticles of TcO2·nH2O (2 nm in diameter) appear throughthe solidification process of the polymeric Tc(IV) species.
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