Radiolysis of TcO4- in alkaline, nitrate solutions: Reduction by NO32-

Article abstract of DOI:10.1021/jp004534y

The radiation chemistry of pertechnetate has been examined in highly alkaline solution. In the presence of selected, organic O^- scavengers, radiolysis reduces TcO₄^- with varying radiation chemical yields, which depend on t

Full text of DOI:10.1021/jp004534y

J. Phys. Chem. A 2001, 105, 9611-9615
9611
-
2-
Radiolysis of TcO4 in Alkaline, Nitrate Solutions: Reduction by NO3
Wayne W. Lukens, Jr.,* Jerome J. Bucher, Norman M. Edelstein, and David K. Shuh
Chemical Sciences DiVision and The Glenn T. Seaborg Center, MS 70A-1150, Ernest Orlando Lawrence
Berkeley National Laboratory, Berkeley, California 94720
ReceiVed: December 18, 2000
The radiation chemistry of pertechnetate has been examined in highly alkaline solution. In the presence of
-
selected, organic O- scavengers, radiolysis reduces TcO4 with varying radiation chemical yields, which
depend on the reduction potentials of the organic radicals produced during radiolysis. When aminopolycar-
-
boxylates are used as O- scavengers, the radiation chemical yield for pertechnetate reduction, G(-TcO4 ) is
equal to 1/3 G(eaq-), which strongly implies that the organic radicals produced by the reaction of O- with
aminopolycarboxylates are unreactive toward the technetium species present in solution. In the presence of
-
excess nitrate, TcO4 is still efficiently reduced during radiolysis when aminopolycarboxylates are used as
-
2-
O- scavengers. This observation is consistent with the reduction of TcO4 by NO3
.
-
Introduction
TcO4 was studied in highly alkaline solutions containing
-
selected organic molecules. In addition, the radiolysis of TcO4
The radiation chemistry of nitrate solutions has recently
received increasing attention due, in part, to its relevance to
the chemistry of high-level nuclear waste.1 Specifically, the high-
level nuclear waste stored in underground tanks at the Savannah
River and Hanford Sites is highly alkaline and contains high
concentrations of nitrate and nitrite. In these tanks, the radiation
source is the decay of 137Cs and 90Sr.2 In addition, certain tanks
also contain lower concentrations of several organic ions
including formate, oxalate, glycolate, acetate, iminodiacetate
(IDA), nitrilotriacetate (NTA), ethylenediaminetetraacetate
(EDTA), 2-hydroxyethylethylenediaminetriacetate (HEDTA),
and citrate.2,3 Because of this variety of ions and molecules,
the thermal and radiation chemistries of these tanks are complex
and can produce unexpected chemical changes in the species
present.
was studied in solutions containing IDA or NTA and different
concentrations of nitrate. The radiation chemical yields for loss
of TcO4-, G(-TcO4-), reported in molecules/100 eV, was
sensitive to the reduction potentials of the radicals produced
during radiolysis. Surprisingly, in the presence of 0.2 M NaNO3
and 0.1 M NTA or IDA, G(-TcO4-) was large, 15-20% of
-
the yield when NO3 was absent. The results support a
-
2-
mechanism in which TcO4 is reduced by NO3 at a much
2-
greater rate than the reaction of NO3 with water.
Experimental Section
Caution: 99Tc is a â-emitter (Emax ) 294 keV, τ1/2 ) 2 × 105
years). All operations were carried out in a radiochemical
laboratory equipped and approved for handling this isotope.
Pertechnetate, as NH499TcO4, was obtained from Oak Ridge
National Laboratory. The solid NH499TcO4 was contaminated
with a large amount of dark, insoluble material. Prolonged
treatment of this sample with H2O2 and NH4OH did not
appreciably reduce the amount of dark material. Ammonium
pertechnetate was separated by carefully decanting the colorless
solution from the dark solid. To the colorless solution, a small
amount of NaOH was added, and the volatile components were
removed under vacuum. The remaining solid was dissolved in
water, and the colorless solution was removed from the
remaining precipitate using a cannula. The concentration of
sodium pertechnetate was determined spectrophotometrically at
289 nm (ꢀ ) 2380 M l-1 cm-1).8 UV-visible spectra were
obtained using an Ocean-Optics ST2000 spectrometer.
All operations were carried out in air except as noted. Water
was deionized, passed through an activated carbon cartridge to
remove organic material, and then distilled. Iminodiacetic acid
was recrystallized three times from water. All other chemicals
were used as received.
One example is the discovery of reduced technetium species
in high-level waste.4 Although the most stable form of techne-
tium at pH > 10 is pertechnetate, TcO4-,5 a large fraction of
lower-valent technetium complexes was found in certain waste
tanks. These tanks contain relatively high concentrations of 137Cs
and aminopolycarboxylates including HEDTA, NTA, and IDA.
-
This combination has aroused suspicion that radiolysis of TcO4
was responsible for the presence of reduced technetium species.
However, these tanks contain concentrations of nitrate and nitrite
up to 5 orders of magnitude greater than that of TcO4-.2 The
large excess of nitrate prevents the reducing primary radiolysis
-
product, eaq-, from reducing TcO4 at an appreciable rate. In
-
addition, TcO4 cannot be readily reduced by organic radicals
produced by radiolysis since nitrite reacts quickly with reducing
-
organic radicals.6 For these reasons, direct reduction of TcO4
by the primary radiolysis products seems unlikely in the presence
of high concentrations of nitrate and nitrite. In addition,
-
reduction of TcO4 by certain organic chemicals in alkaline
solution can be catalyzed by the colloids of the fission products
Pt, Rh, and Ru.7
Radiolysis Experiments. Solutions for radiolysis experiments
were freshly prepared by weighing the appropriate amounts of
sodium hydroxide, organic compound, and sodium nitrate or
sodium nitrate solution into a volumetric flask then preparing
the sample solution. To a known volume of sample solution,
To improve the understanding of fundamental technetium
chemistry relevant to high level tank waste, the radiolysis of
* To whom correspondence should be addressed.
10.1021/jp004534y CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/26/2001

9612 J. Phys. Chem. A, Vol. 105, No. 41, 2001
Lukens et al.
NaTcO4 (3.9 × 10-2 M) was added to give the desired
concentration of TcO4-; an identical volume of water was added
to the reference solution. A Cu(II)/Fe(II) (Hart) dosimeter with
0.002 M Fe(II), 0.010 M Cu(II), and 0.010 M H2SO4, or an
Results and Discussion
To interpret the radiation chemical yield for the reduction of
pertechnetate, G(-TcO4-), the oxidation state of the technetium
radiolysis product must be identified to determine the number
of reducing equivalents needed to remove a pertechnetate ion
oxygen-saturated Fricke dosimeter was used to record the
radiation dose.9,10 A set of three tubes (sample with TcO4
,
-
-
from solution. Based on known chemistry, radiolysis of TcO4
chemical dosimeter, sample without TcO4-) was positioned
equidistant from a 600 Ci 60Co source. In a given experiment,
three or five different sets of tubes were placed at varying
distances from the 60Co source and irradiated for the same period
of time. The tubes were contained in an aluminum box with a
0.25 in. thick polycarbonate window. For radiolysis experiments
with added nitrate, the samples were purged with argon by filling
the headspace (∼1 mL) with argon and vigorously shaking the
tube for 10 s. This process was repeated three times.
requires three reducing equivalents and produces Tc(IV) as the
radiolysis product:13-18
TcO4- + eaq- f TcO4
k3 ) 2.5 × 1010 M-1 s-1 (3)
2-
2TcO42- f TcO4- + Tc(V) k4 ) 1.5 × 105 M-1 s-1 (4)
2Tc(V) f TcO42- + Tc(IV) k5 ) 2.4 × 103 M-1 s-1 (5)
Radiolysis Data Treatment. All radiation-chemical yields
are reported as molecules/100 eV. The radiation doses absorbed
by the samples were determined from the Hart dosimeters and
were corrected for the different compositions of the dosimeter
and sample solutions, for the relative positions of the dosimeter
and sample to the source. In addition, the response of the Fe(II)/
Cu(II) dosimeter with a 5:1 Fe/Cu ratio was found to be slightly
nonlinear in comparison to the oxygen-saturated Fricke dosim-
eter; this nonlinearity was corrected. At low absorbed doses,
the absorbed dose was calculated using oxygen-saturated Fricke
dosimeters. UV-visible spectra were collected from the pertech-
netate-containing solutions, using the solutions without pertech-
netate as references. The concentration of pertechnetate was
determined by fitting the spectra. Unirradiated samples were
Tc(V) + TcO42- f Tc(IV) + TcO4
(6)
-
Pertechnetate reacts very quickly with hydrated electrons,
2- 13-15
yielding technetate, TcO4
.
Technetate disproportionates
with a bimolecular rate constant of 1.5 × 105 M-1 s-1 in alkaline
solution.16 Similarly, in the absence of stabilizing ligands, Tc(V)
is known to disproportionate rapidly with a bimolecular rate
constant of 2.4 × 103 M-1 s-1 17
.
Alternatively, Tc(V) species
could be reduced by TcO42-, which is a moderately strong
reducing agent, E°(TcO4-/TcO42-) ) -0.64 V vs NHE.14,18 For
these reasons, Tc(IV), as TcO2‚xH2O, is the likely radiolysis
product in alkaline solutions in the absence of ligands capable
of forming Tc(V) or Tc(IV) complexes that are stable at high
pH. The supposition that Tc(IV) is the final oxidation state is
strongly supported by the observation that the major radiolysis
product is a dark precipitate in the absence of diolate ligands,
which can form soluble, lower valent technetium complexes in
highly alkaline solution.19,20 The dark precipitate is spectro-
scopically identical to TcO2‚xH2O produced by hydrolysis of
-
used to determine the initial concentration of TcO4 and the
-
position and line width of the TcO4 peaks. Only the heights
of the TcO4- peaks were allowed to vary when fitting the spectra
of irradiated samples. The presence of additional radiolysis
products was treated by including additional peaks in the fit.
The radiation chemical yields for the primary radiolysis
radicals were calculated using the method derived by Schul-
er.11,12 The yield of hydrogen atoms g(H‚) is 0.55. The yield of
hydrated electrons g(eaq-) was determined using eq 1
2-
TcCl6 as determined by X-ray absorption fine structure and
electron paramagnetic spectroscopy.21 In addition to the in-
soluble radiolysis product, a minor, soluble radiolysis product
is usually observed. However, the principal radiolysis product
is TcO2‚xH2O, as expected.
g(H ‚)kH‚+OH-[OH-]
H‚+OH-[OH-] + kH‚+RH[RH]
g(eaq-) )
+
The effect of selected organic molecules upon the radiolytic
reduction of TcO4- was examined in 2 M NaOH; these results,
plus data previously reported by Pikaev and co-workers,22 are
reported in Table 1. In 2 M NaOH, hydroxyl radicals are rapidly
converted to oxide radical anions, O-; therefore, O- can be
treated as the primary oxidizing radical produced by radiolysis.23
The organic molecules studied rapidly scavenge O- radicals
yielding organic radicals and preventing the oxidation of the
reduced technetium species by O-.23 The hydrated electrons
produced during radiolysis reduce TcO4-, removing it from
solution with a radiation chemical yield, G(-TcO4-), of 1/3
g(eaq-). The deviation of G(-TcO4-) from this value can be
explained using the reduction potentials of the organic radicals.
When the organic scavengers are primary or secondary
alcohols, R-hydroxyalkyl radicals are produced by H-abstraction
from the alcohol. These radicals are strongly reducing, e.g.,
E°[(CH3)2CO,H+/(CH3)2C•OH] ) -1.7 V, and are even more
strongly reducing when deprotonated.24 In fact, 2-hydroxy-2-
k
k[S]/λ
x
∑
x
2.55 + 2.33
(1)
1 +
k[S]/λ
∑
where the left-hand term is the fraction of hydrogen atoms that
react with hydroxide to form hydrated electrons (kH‚+OH- is the
rate constant for the reaction of H‚ with OH- and kH‚+RH is
the rate constant for the reaction of H‚ with the organic species),
and the right-hand side is the yield of hydrated electrons
corrected for scavenging from spurs; k[S] is the rate of reaction
of substrate S with hydrated electrons, and λ is 8 × 108 s-1 11
.
Similarly, the yield of oxide radical ions was determined using
eq 2
k[S]/λ
x
∑
x
g(O-) ) 2.7 + 1.5
(2)
-
propyl radical reduces TcO4 with a rate constant of 7 × 108
M-1 s-1 15 Since these radicals reduce TcO4-, the predicted
.
1 +
k[S]/λ
∑
value of G(TcO4-) is 1/3[g(eaq-) + g(R‚)], where g(R‚) is the
yield of organic radicals produced from H-abstraction by O-
and H‚. The first two entries in Table 1 show that the observed
G(-TcO4-) values are in good agreement with this prediction.
where k[S] is the rate of reaction of substrate S with hydroxyl
radicals, and λ is 4.7 × 108 s-1 12
. Hydroxide was not included
as a substrate in the calculation of g(O-).

-
Radiolysis of TcO4
J. Phys. Chem. A, Vol. 105, No. 41, 2001 9613
-
TABLE 1: Observed Radiation-Chemical Yields for the Loss of TcO4 in the Presence of Selected Organic Molecules
-
-
organic
ethanol
methanol
EDTA
NTA
IDA
acetate
citrate
ethylene glycol
[TcO4-] mM
[organic] M
Gobs(-TcO4
)
Gcalc(-TcO4
)
ref
5
5
5
0.2
0.2
1.2
1.2
1.2
0.1
0.1
0.04
0.1
0.1
0.5
0.5
0.5
2.7
2.5
1.2
1.0
0.9
0.1
0.5
0.4
2.4a
2.5a
1.2b
1.0b
1.0b
22
22
22
this work
this work
this work
this work
this work
b
a Gcalc(-TcO4-) ) (g(eaq-) + g(R‚))/3 Gcalc(-TcO4-) ) g(eaq-)/3
TABLE 2: Observed and Calculated Radiation-Chemical
On the other hand, if the organic radicals are capable of
-
Yields for the Loss of TcO4 in 2 M NaOH, 0.1 M IDA or
-
oxidizing reduced technetium species, the observed G(-TcO4
)
NTA at Various Nitrate Concentrations
values will be smaller than 1/3 g(eaq-). For example, the
calculated standard potential of the radical produced by H-
abstraction from acetate, E°(‚CH2CO2-,H+/CH3CO2-) ) 1.7
-
[NO3
]
G(-TcO4-) NTAa,b
G(-TcO4-) IDAa,b
0.20
0.10
0.05
0.01
0.005
0.001
0
-
0.13(1)
0.12(2)
0.13(1)
0.17(1)
0.21(1)
0.50(3)
0.90(3)
V,25 is similar to that of O-.26 Consequently, this radical can
0.16(1)
0.17(3)
0.22(1)
0.29(7)
0.50(3)
1.0(1)
-
oxidize reduced technetium species. As a result, G(-TcO4
)
will be smaller than 1/3 g(eaq-), although the value of
G(-TcO4-) cannot be predicted without knowing the rates of
reaction of the organic radicals with each other and with the
reduced technetium species. As shown by the lower entries in
Table 1, H-abstraction from certain organic molecules produces
radicals capable of oxidizing reduced technetium species. The
fact that radicals produced from citrate are oxidizing is not
surprising since the carbon-centered radical of citrate is R to
the carboxylate group as in the acetate radical. Similarly, the
1,2-dihydroxyethyl radical, produced by H-atom abstraction
from ethylene glycol, rapidly dehydrates in alkaline solution to
form the oxidizing formylmethyl radical.27
a Standard error from fitting the radiolysis data is given in paren-
theses. b Radiation-chemical yields in ions/100 eV.
represent excellent scavengers for use in studying the radiolysis
of technetium in the presence of nitrate. The basic radiation
chemistry of nitrate is given in eqs 10-12.1,30,31
NO3- + eaq- f NO3
k10 ) 9.7 × 109 M-1 s-1 (10)
2-
The radicals produced by the reaction of O- with the
aminopolycarboxylates EDTA, IDA, and NTA appear to be
unreactive toward both pertechnetate and reduced technetium
species. In the presence of these molecules, the G(-TcO4-) can
NO32- + H2O f NO2 + 2HO- k11 ) 1 × 103 M-1 s-1
•
(11)
-
be attributed to reduction by eaq alone, as shown in Table 1.
2NO2 (+ H2O) f NO3- + NO2- + 2H+
•
However, a similar radiation-chemical yield could be obtained
if radicals produced from the aminopolycarboxylates both
reduced TcO4- and oxidized reduced technetium species. Such
a scenario is suggested by the known radiation chemistry of
glycine.28
k12 ) 6 × 107 M-1 s-1 (12)
-
-
In solutions containing both NO3 and TcO4-, NO3 acts as a
scavenger of eaq-, in competition with TcO4-. Since the reaction
rates for NO3- and TcO4- with eaq- are known, the fraction of
•
Strongly reducing H2N-CH2 radicals result from decar-
boxylation of the H2N+•-CH2-CO2 radical produced by
-
-
-
eaq that react with TcO4 can be determined.
oxidation of glycine anion, eq 7, and oxidizing HN•-CH2-
The observed radiation chemical yields for TcO4- reduction
in solutions of 2 M NaOH, 0.1 M NTA or IDA, and various
concentrations of nitrate are listed in Table 2. At high nitrate
concentrations, G(-TcO4-) is much larger than can be explained
-
CO2 radicals are produced by H-atom abstraction from the
amino group, eq 8. However, the reduction potential for the
H2N+•-CH2-CO2 radical is calculated to be 1.6 V,29 which
-
is much greater than the 0.94 V reduction potential of O- in 2
M NaOH.26 Therefore, reaction 7 will not occur at high pH
where O-, rather than HO•, is present. The calculated reduction
potential of the principal radical produced during radiolysis of
-
by the reaction of TcO4 with eaq-. For example, in 0.1 M
NaNO3 and 0.1 M NTA, G(-TcO4-) is 0.19; if only eaq- were
reducing TcO4-, the radiation-chemical yield would be 0.01.
Since the organic radicals produced from aminopolycarboxylates
do not react with technetium species, the only radiolysis product
capable of reducing TcO4- is NO32- (E°(NO3-/NO32-) ) -0.89
V),1 eq 13.
glycine, H2N-C•H-CO2-, eq 9, is 0.26 V in 2 M NaOH, which
-
is insufficient to reduce TcO4
.
H2NCH2CO2- + HO• f ΗÃ-+ H2NCH2CO2 f
•
•
Η2NCH2 + CO2 (7)
-
TcO4- + NO32- f TcO42- + NO3
(13)
H2NCH2CO2- + HO• f Η2Ã + HN•CH2CO2
(8)
(9)
-
2-
-
If only NO3 and eaq reduce TcO4-, the mechanism for
H2NCH2CO2- + HO• f Η2Ã + H2NC•HCO2
-
-
-
-
radiolysis of TcO4 is simple: TcO4 and NO3 compete for
-
eaq-, and TcO4 and H2O compete for NO32-. The expected
radiation chemical yield G(-TcO4-) is then given by eq 14,
-
Radiolysis of TcO4 in the Presence of NO3-. Since the
C-centered radicals produced during radiolysis of IDA and NTA
are unreactive toward technetium species, these molecules
where kn is the rate constant for the reaction given in equation
n.

9614 J. Phys. Chem. A, Vol. 105, No. 41, 2001
Lukens et al.
Figure 1. Radiation-chemical yield for loss of TcO4- in 2 M NaOH with (a) 0.1 M NTA and (b) 0.1 M IDA as a function of nitrate concentration.
The data are represented by the circles with the standard error represented by vertical lines. A least-squares fit of the data to eq 14 is represented
by the solid line.
k11[H2O] . k13[TcO4-], G(-TcO4-) can be approximated using
-
g(eaq
-
)
-
eq 15.32
G(-TcO4 ) )
×
-
3(k3[TcO4 ] + k10[NO3 ])
g(eaq-)k13[TcO4 ]
-
-
-
k13[TcO4 ]
G(-TcO4 ) ≈
(15)
-
-
k3[TcO4 ] + k10[NO3 ]
(14)
3k11[H2O]
-
{
}
(k13[TcO4 ] + k11[H2O])
Conclusion
The results in Table 2 are presented graphically in Figure 1
along with a least-squares fit of the data to eq 14 using k13, the
rate constant for the reaction of TcO4- with NO32-, as the only
variable. The agreement between the data and the fit is good;
however, the two sets of experiments give slightly different
-
The γ-radiolysis of TcO4 in highly alkaline solution in the
presence of selected organic compounds and nitrate has been
examined. In highly alkaline solution, the radicals produced by
radiolysis from the aminopolycarboxylates, EDTA, NTA, and
IDA, are unreactive toward the technetium species present in
solution. These observations strongly suggest that only unre-
active C-centered radicals are produced during radiolysis of
aminopolycarboxylates under these conditions. In the presence
values for k13. Radiolysis in the presence of NTA gives k13
)
2.9(2) × 107 M-1 s-1, while radiolysis in the presence of IDA
gives the slightly lower value of 2.2(2) × 107 M-1 s-1; the
standard errors are in parentheses. Although these experiments
do yield slightly different rate constants, rate constants derived
from γ-radiolysis data are inherently less accurate and less
precise than rates determined using pulse radiolysis.23 For this
reason, the rates are not sufficiently different to warrant
discussion.
of O- scavengers, radiolysis of TcO4 in nitrate solutions
-
-
2-
proceeds through reduction of TcO4 by NO3
.
Acknowledgment. The authors thank Prof. Dan Meisel, Dr.
Don Camaioni, Prof. K. Dieter Asmus, and Prof. Heino Nitsche
for helpful discussions and suggestions regarding the radiation
chemistry of nitrate and glycine and electrochemistry. We further
thank Prof. Dan Meisel and Dr. Don Camaioni for making two
manuscripts about nitrate and nitrite radiation chemistry avail-
able to us prior to publication. This work is supported by the
Environmental Management Science Program of the Office of
Science and Technology of the U. S. Department of Energy.
This research was performed at the Ernest O. Lawrence Berkeley
National Laboratory, which is operated by the U. S. DOE under
Contract No. DE-AC03-76SF00098. Part of this work was
performed at the Stanford Synchrotron Radiation Laboratory,
which is operated by the Director, U. S. DOE, Office of Science,
Office of Basic Energy Sciences, Division of Chemical Sciences.
The reduction of TcO4- by NO32- is considerably slower than
the reduction of TcO4 by 2-hydroxy-2-propyl radical.15 The
-
slower rate is consistent with the smaller equilibrium constant
of reaction 13, based on the reduction potentials of the species
involved, and with the fact that reaction 13 involves two
negatively charged substrates. The reaction of NO3 with
TcO4 may also be compared with other reactions of NO3
2-
-
2-
.
2-
NO3 reduces O2, benzoquinone, and methyl viologen with
rate constants of 2.3 × 108, 7.6 × 108, and 3.3 × 109 M-1 s-1
,
respectively.31 Again, these rate constants are considerably
greater than k13, for the reasons noted above.
Although the reaction of NO32- with TcO4- is considerably
slower than the other electron-transfer reactions of NO32-, the
reaction of NO32- with TcO4- is much faster than the hydrolysis
References and Notes
2- 1,31
of NO3
.
The main effect of this difference in rate is the
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-
Radiolysis of TcO4
J. Phys. Chem. A, Vol. 105, No. 41, 2001 9615
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