Reduction of Pu(IV) and Np(VI) with carbohydrazide in nitric acid solution

Article abstract of DOI:10.1134/S1066362212020087

The reduction of Pu(IV) and Np(VI) with carbohydrazide (NH ₂NH)₂CO in 1-6 M HNO₃ solutions was studied. The Pu(IV) reduction is described by a first-order rate equation with respect to Pu(IV). At [HNO₃] ≥ 3 M, the reaction becomes reversible. The rate constants of the forward and reverse reactions were determined, and their activation energies were estimated. Neptunium(VI) is reduced to Np(V) at a high rate, whereas the subsequent reduction of Np(V) to Np(IV) is considerably slower and is catalyzed by Fe and Tc ions. The possibility of using carbohydrazide for stabilizing desired combinations of Pu and Np valence states was examined. Pleiades Publishing, Inc., 2012.

Full text of DOI:10.1134/S1066362212020087

ISSN 1066-3622, Radiochemistry, 2012, Vol. 54, No. 2, pp. 143–148. © Pleiades Publishing, Inc., 2012.
Original Russian Text © V.I. Volk, V.I. Marchenko, K.N. Dvoeglazov, V.N. Alekseenko, S.I. Bychkov, E.Yu. Pavlyukevich, V.V. Bondin, A. S. D’yachenko,
2012, published in Radiokhimiya, 2012, Vol. 54, No. 2, pp. 133–138.
Reduction of Pu(IV) and Np(VI) with Carbohydrazide
in Nitric Acid Solution
V. I. Volka, V. I. Marchenkoa, K. N. Dvoeglazov*a, V. N. Alekseenkob, S. I. Bychkovb,
E. Yu. Pavlyukevicha, V. V. Bondinb, and A. S. D’yachenkob
а Bochvar High-Tech Institute of Inorganic Materials, Public Joint-Stock Company,
ul. Rogova 5a, Moscow, 123060 Russia; * e-mail: march@bochvar.ru
b Mining and Chemical Combine, Federal State Unitary Enterprise, Zheleznogorsk, Krasnoyarsk krai, Russia
Received April 25, 2011; in final form, October 5, 2011
Abstract—The reduction of Pu(IV) and Np(VI) with carbohydrazide (NH2NH)2CO in 1–6 M HNO3 solutions
was studied. The Pu(IV) reduction is described by a first-order rate equation with respect to Pu(IV). At [HNO3] ≥
3 M, the reaction becomes reversible. The rate constants of the forward and reverse reactions were determined,
and their activation energies were estimated. Neptunium(VI) is reduced to Np(V) at a high rate, whereas the
subsequent reduction of Np(V) to Np(IV) is considerably slower and is catalyzed by Fe and Tc ions. The possibil-
ity of using carbohydrazide for stabilizing desired combinations of Pu and Np valence states was examined.
Keywords: carbohydrazide, nitric acid, solutions, plutonium, neptunium, ions, reduction, kinetics, catalysis,
technetium, iron
DOI: 10.1134/S1066362212020087
EXPERIMENTAL
One of important lines of improving aqueous tech-
nology for reprocessing irradiated fuel, primarily high-
burn-up spent nuclear fuel, is search for new reagents,
more convenient for performing redox operations in-
volving Pu and Np ions. Organic compounds show
promise for this purpose. Their use allows the amount
of salt-containing radioactive wastes to be sharply re-
duced [1]. Carbohydrazide (NH2NH)2CO can be one of
such reagents. Preliminary experiments showed that it
rapidly reduces Pu(IV) and Np(VI) in nitric acid solu-
tions at ambient temperature.
Procedure. A temperature-controlled cell of a
spectrophotometer was charged with the calculated
amounts of stock solutions of HNO3 and carbohy-
drazide and kept to attain the preset temperature. After
that, a small aliquot of a Pu(IV) or Np(VI) solution
was added, and the absorption spectra of the working
solution were recorded at definite time intervals in the
wavelength (λ) range 350–1100 nm. The concentration
of the Pu and Np valence forms was calculated from
the optical density (D) at λ = 475, 602, 980, and
710 nm, corresponding to the absorption maxima of
the Pu4+, Pu3+, NpO2+, and Np4+ ions, respectively.
In the literature, carbohydrazide was mainly exam-
ined as ligand for bi- and trivalent element ions [2–5].
Its reducing properties have been studied in considera-
bly less detail. It is known that carbohydrazide reduces
the Cu2+ and Fe3+ ions to Cu+ and Fe2+, respectively.
Both reactions occur at high rate and are accompanied
by the evolution of gases: CO2 and N2 [6]. We found
no data in the literature on interaction of actinide ions
with carbohydrazide in nitric acid solutions. In this
study we measured the rate of reduction of Pu(IV) and
Np(VI) with carbohydrazide in HNO3 solutions and
examined the possibility of stabilizing Pu and Np in
desirable valence state combinations using this re-
agent.
Chemicals and analysis. The Pu stock solution
was prepared by dissolving PuO2 in a boiling mixture
of HNO3 and HF. After the complete dissolution of
PuO2, Al(NO3)3 was added to bind F– anions in a com-
plex. The resulting solution was acidified with HNO3
to a concentration of 7 M and purified by sorption on
VP-1Ap anion exchanger. The desorbate was evapo-
rated, and Pu was stabilized in the tetravalent state by
adding H2O2. The solution thus prepared contained
440 g l–1 Pu and 6.6 M HNO3.
The 90 g l–1 Np stock solution was prepared by dis-
143

144
VOLK et al.
of its weighed portion with a 0.1 M alkali solution, and
in working solutions containing U and Pu, by potenti-
ometric titration with ammonium oxalate.
RESULTS AND DISCUSSION
Reduction of Pu(IV)
In solutions of low acidity (<1 M HNO3), carbohy-
drazide fully converts Pu(IV) to Pu(III) within 1–2 min
at room temperature. The Pu(III) formed remains sta-
ble against oxidation, including solutions containing
50 g l–1 U(VI) and 100 mg l–1 Tc(VII). Experiments at
decreased temperature (~10°C) showed that the reac-
tion is described by a first-order rate equation with re-
spect to Pu(IV) (Fig. 1) and allowed estimation of the
rate constant in the rate equation
Fig. 1. Kinetic curve of Pu(IV) reduction and its lineariza-
tion in semilog coordinates. [Pu(IV)] = 0.010, [HNO3] = 1,
and [(NH2NH)2CO] = 0.05 M; ~10°С.
–d[Pu(IV)]/dt = k1[Pu(IV)],
(1)
equal to 0.02 min–1 at [HNO3] = 1 M and [(NH2NH)2CO] =
0.05 M.
At higher acidity, the reaction does not go to com-
pletion, and equilibrium between Pu(III) and Pu(IV) is
attained (Fig. 2). The equilibrium constant
Fig. 2. Kinetic curves of Pu(IV) reduction with carbohy-
drazide. [Pu(IV)] = 0.012 and [(NH2NH)2CO] = 0.2 M; 35°С.
[HNO3], M: (1) 1, (2) 4, (3) 3 {+ 5 × 10–3 M [Tc(VII)]}, and
(4) 6.
K = [Pu(III)]e/[Pu(IV)]e
(2)
solving a weighed portion of 98% pure NpO2 in boil-
ing 7 M HNO3, followed by reduction of Np(VI) to
Np(IV) with hydrazine and Np purification by sorption
on VP-1Ap anion-exchange resin. In the purified solu-
tion, Np(IV) was oxidized to Np(VI) by repeated
evaporation to wet salts. The Pu and Np concentrations
in the stock solutions were determined by γ-ray spec-
trometry, and the content of valence forms of these
elements in working solutions was found from the ab-
sorption spectra recorded with a UV-1201 spectropho-
tometer (Shimadzu).
{[Pu(III)]e are [Pu(IV)]e are the equilibrium concentra-
tions of Pu(III) and Pu(IV)} decreases with an increase
in the solution acidity and increases with temperature.
The kinetic data were processed using equations for
reversible first-order reactions [8]. In the case when
both the forward [Pu(IV) reduction] and reverse
[Pu(III) oxidation] reactions are first-order with respect
to Pu ions, the kinetic curves should be linearized in
the coordinates ln[A/(A – x)]–time, where A = Ka/(K +
1), a is the initial Pu(IV) concentration, and x is the
Pu(III) concentration at time t.
The slope of the straight line in these coordinates is
numerically equal to the sum of the rate constants of
the forward (k1) and reverse (k–1) reactions. Knowing
the sum k1 + k–1 and the quantity K equal to the ratio
k1/k–1, we can calculate the rate constants of both reac-
tions. An example of processing experimental kinetic
data is given in Fig. 3, and the values of k1 and k–1 for
separate experiments are given in Table 1.
The Tc(VII) stock solution was prepared by dis-
solving in water a weighed portion of 97 wt % pure
KTcO4. The Tc content in solution was determined by
inductively coupled plasma mass spectrometry.
The carbohydrazide stock solution was prepared by
dissolving in 0.1 M HNO3 a weighed portion of the
97 wt % pure product (Acros Organics, USA). The
carbohydrazide concentration in stock and working
solutions was determined by potentiometric titration
with potassium iodate [7].
Table 1 shows that an increase in the HNO3 con-
centration leads to a decrease in the rates of both for-
ward and reverse reactions. The rates of both reactions
increase with temperature, but for the forward reaction,
Nitric acid was of ultrapure grade. The acid concen-
tration in the stock solution was determined by titration
RADIOCHEMISTRY Vol. 54 No. 2 2012

REDUCTION OF Pu(IV) AND Np(VI) WITH CARBOHYDRAZIDE
145
Pu(IV) reduction, to a greater extent than for the reverse
reaction. Estimation of the activation energies from these
data gives the values of ~88 and ~34 kJ mol–1 for the for-
ward and reverse reactions, respectively. It should be
noted that, in a 3 M HNO3 solution containing Tc ions,
the Pu(III) formed is unstable and starts to be oxidized
already in the course of the experiment (Fig. 2).
Reduction of Np(VI) and Np(V)
Fig. 3. Plot of ln[A/(A – x)] vs. time in reduction of Pu(IV)
At low acidity ([HNO3] < 1 M) and room tempera-
ture, carbohydrazide rapidly reduces Np(VI) to Np(V).
In a 0.3 M HNO3 solution, even at low temperature
(10–15°С), Np(VI) is fully reduced to Np(V) within
the time of mixing the reactants. The rate decreases
with an increase in the HNO3 concentration and, e.g.,
in a 1 M HNO3 solution at [(NH2NH)2CO] = 0.05 M
and ~10°С, the reaction is complete in ~8 min.
with carbohydrazide. [Pu(IV)] = 0.012, [(NH2NH)2CO] =
0.2, and [HNO3] = 6 M. T, °C: (1) 35 and (2) 50.
With an increase in the solution acidity and tem-
perature, Np(V) undergoes further reduction to Np(IV),
accelerated by Fe and Tc ions (Fig. 4). A characteristic
feature of the catalytic process is an induction period
in the Np(IV) formation curve. A separate study is re-
quired to elucidate the origin of the induction period.
Fig. 4. Kinetic curves of Np(IV) formation in reduction of
Np(VI) with carbohydrazide. [Np(VI)] = 8.4 × 10–3,
[(NH2NH)2CO] = 0.2, and [HNO3] = 3 M. Additives:
(1) none, (2) 9 × 10–4 M Fe(III), and (3) 1 × 10–3
M
In experiments on Np(V) reduction with carbohy-
drazide, the Np(IV) formation was monitored by an
increase in the optical density of the solution at λ =
710 nm (Np4+ absorption maximum). Experimental
data were processed assuming that the reaction follows
a first-order rate equation with respect to Np
Tc(VII); the same for Fig. 5.
–d[Np(V)]/dt = d[Np(IV)]/dt = k[Np(V)].
(3)
In this case, with the reductant taken in excess, the
kinetic curves should be linearized in the coordinates
ln(Dfin – D)–time, where Dfin and D are the final and
running optical densities of the solution. Indeed, as
seen from Fig. 5, the dependences in these coordinates
are linear. From their slopes, we calculated the rate
constants k (Table 2).
It follows from Table 2 that the Fe and Tc ions con-
siderably accelerate the Np(V) reduction, with Tc(VII)
being approximately 4 times more effective than
Fe(III). The catalytic effect of the Fe ions can be ac-
counted for by the occurrence of the reactions
Fig. 5. Plot of ln(Dfin – D) vs. time in reduction of Np(V)
with carbohydrazide. [Np] = 8.4 × 10–3, [(NH2NH)2CO] =
0.2, and [HNO3] = 3 M.
We also should note strong (approximately quad-
ratic) dependence of the rate constant on the HNO3
concentration: In the presence of Fe ions, k increases
by a factor of ~4.5 in going from 3 to 6 M HNO3.
Table 1. Rate constants of forward (k1) and reverse (k–1)
reactions of Pu(IV) reduction with carbohydrazide.
[Pu(IV)] = 0.012, [(NH2NH)2CO] = 0.2 M
[HNO3], M T, °C [Tc(VII)], M k1, min–1 k–1, min–1
Fe(III) + (NH2NH)2CO → Fe(II) + products,
Np(V) + Fe(II) → Np(IV) + Fe(III).
(4)
(5)
3
4
6
6
35
35
35
50
0.005
0.15
0.12
0.011
0.054
0.020
0.016
0.0064
0.012
0
0
0
Apparently, similar scheme can be suggested for
catalysis with Tc ions.
RADIOCHEMISTRY Vol. 54 No. 2 2012

146
VOLK et al.
Table 2. Rate constants of Np(V) reduction with carbohydrazide. [(NH2NH)2CO] = 0.2 M, T = 35°С
[Np] × 103
[HNO3]
[Fe(III)] × 104
M
0
[Tc(VII)] × 103
k × 103, min–1
8.4
8.4
8.4
4.2
3
3
3
6
0
0
1.0
0
0.18
1.15
4.5
9.0
0
9.0
5.1
Use of Carbohydrazide for Stabilizing Desired
Combinations of Pu and Np Valence States
τ ~3.5 h, i.e., the shorter time than that found in the
experiment. However, taking into account the fact that
the kinetic data given in [9] were obtained in solutions
with a higher ionic strength (μ = 2), we can conclude
that the calculated and experimental estimations of τ
are reasonably consistent.
It is of interest to examine the possibility of using
carbohydrazide for stabilization of desired combina-
tions of Pu and Np valence states, in particular, of the
Pu(III)–Np(V) couple suitable in the extraction tech-
nology for separation of these elements from U and of
the Pu(III)–Np(IV) couple suitable for mutual separa-
tion of Pu and Np by extraction or ion exchange.
In any case, the very fact of the Np(IV) formation
indicates that, under the examined conditions, the use
of carbohydrazide does not allow stabilization of the
Pu(III)–Np(V) couple. This may become possible at
lower acidity (0.5 M) and lower temperature (20–
25°C), because in this case the rate of the Np(IV) for-
mation by reaction (7) will considerably decrease
(according to [9], by a factor of ~14 in going to
[HNO3] = 0.5 M and 25°C).
Apparently, the Pu(III)–Np(V) couple should be
prepared at a low acidity ([HNO3] ≤ 1 M), because
under these conditions both Pu(IV) and Np(VI) are
rapidly reduced with carbohydrazide and the products,
Pu(III) and Np(V), should be stable for a long time.1
We attempted to stabilize the Pu(III)–Np(V) couple at
35°C in a 1.1 M HNO3 solution containing (M) 0.012
Pu(IV), 2.1 × 10–3 Np(VI), and 0.2 (NH2NH)2CO. As
seen from Fig. 6, Pu(IV) is very rapidly (within less
than 1 min) reduced to Pu(III), whereas Np(VI) is re-
duced to Np(V) more slowly that it was expected from
the results of experiments without Pu and does not re-
main in this valence state, gradually transforming into
Np(IV) (to ~30% within 5.5 h).
As for the Pu(III)–Np(IV) couple, it is appropriate
to stabilize it at a high acidity. In this case, the rate of
the Np(V) reduction to Np(IV) under the action of car-
bohydrazide increases, as well as the contribution of
reaction (7), whose rate increases with the HNO3 con-
centration [9], to the Np(V) reduction. However, an
increase in the HNO3 concentration inevitably leads to
a decrease in the rate and degree of Pu(IV) reduction to
Pu(III). This problem can be solved by elevating the
solution temperature simultaneously with increasing
the acidity. Indeed, as shown above, the activation en-
ergy of the forward reaction [Pu(IV) reduction] consid-
erably exceeds that of the reverse reaction [Pu(III) oxi-
The observed decrease in the rate of the Np(VI)
reduction to Np(V) can be accounted for by the occur-
rence of fast Np(V) reproportionation in weakly acidic
solutions:
Np4+ + NpO22+ + 2Н2O ҙ 2NpO2+ + 4H+,
(6)
and Np(IV) is apparently formed in the reaction
NpO2+ + Pu3+ + 4H+ ҙ Np4+ + Pu4+ + 2Н2O.
(7)
Estimation, based on data of [9], of the time τ re-
quired for 30% completion of the reaction under the
conditions of our experiment {[Pu(IV)] = 0.012,
[Np(VI)] = 8.4 × 10–3, [HNO3] = 1.1 M, 35°С} gives
1
If we assume that the reaction order of Np(V) reduction with
carbohydrazide with respect to HNO3 without Fe ions is the
same as in their presence, calculation from data in Table 2 shows
that the conversion of only 10% of Np(V) to Np(IV) requires
97 h at [HNO3] = 1, [(NH2NH)2CO] = 0.2 M and 35°C.
Fig. 6. Kinetic curves of formation of Pu(III) and Np(V) in
reduction of Pu(IV) and Np(VI) with carbohydrazide.
[HNO3] = 1.1, [Pu(IV)] = 0.012, [Np(VI)] = 2.1 × 10–3, and
[(NH2NH)2CO] = 0.2 M; 35°С.
RADIOCHEMISTRY Vol. 54 No. 2 2012

REDUCTION OF Pu(IV) AND Np(VI) WITH CARBOHYDRAZIDE
147
dation], and hence the rate of the forward reaction will
increase with temperature considerably more rapidly
than the rate of the reverse reaction, which will result
in more complete reduction of Pu(IV) to Pu(III).
We performed two experiments on stabilization of
the Pu(III)–Np(IV) couple with carbohydrazide in a
6 M HNO3 solution at two temperatures: 35 and 50°C.
The Pu(IV) and Np(VI) concentrations in the starting
solution were 0.012 and 2.1 × 10–3 M, respectively,
and the carbohydrazide concentration was 0.2 M. The
spectrophotometric measurements showed that, in both
experiments, Pu(IV) was reduced incompletely, al-
though, as expected, the reduction was more complete
at 50°C (81%, against 63% at 35°C) (Fig. 7). On fur-
ther keeping at room temperature for 18 h, a part of
Pu(III) (about 25%) was oxidized back to Pu(IV).
Fig. 7. Kinetic curves of Pu(III) formation in reduction of
Pu(IV) and Np(VI) with carbohydrazide. [HNO3] = 6, [Pu(IV)] =
0.012, [Np(VI)] = 2.1 × 10–3, and [(NH2NH)2CO] = 0.2 M.
T, °C: (1) 35 and (2) 50.
The Np(IV) formation occurred without induction
period at a rate exceeding that in the solution without
Pu by 1.5–2 orders of magnitude. In the experiment at
35°C, Np transformed into Np(IV) incompletely, be-
cause a small peak at ~980 nm, belonging to Np(V)
and corresponding to its concentration of ~8 × 10–5 M
(about 4% of total Np concentration), was recorded
throughout the observation period. Keeping the solu-
tion after the experiment for 18 h at room temperature
led to an increase in the Np(V) fraction. In the experi-
ment at 50°C, Np was fully reduced to Np(IV), but
after keeping for 18 h at room temperature approxi-
mately 1.5% of Np(IV) passed into the pentavalent
state.
Fig. 8. Kinetic curves of Pu(III) and Np(V) formation in re-
duction of Pu(IV) and Np(VI) with carbohydrazide. [P(IV)] =
0.012, [Np(VI)] = 2.1 × 10–3, [(NH2NH)2CO] = 0.2, [HNO3] =
3, and [Fe(III)] = 9 × 10–4 M; 50°С.
ess solutions and originates from equipment corrosion
and impurities in chemicals. The Pu(IV) and Np(VI)
concentrations were the same as in the previous experi-
ments (0.012 and 2.1 × 10–3 M, respectively).
The kinetic curves of the Pu(III) accumulation and
Np(V) loss are shown in Fig. 8. As can be seen, under
these conditions Pu(IV) and Np(V) are reduced to
Pu(III) and Np(IV), respectively, to ~97–98% in ~1.5–
2 h, and the content of these forms of Pu and Np does
not change in the course of subsequent storage of the
solution for, at least, 70 h at room temperature.
Presumably, heating to a higher temperature (~70°C)
would allow more complete conversion of Pu(IV) to
Pu(III), but the instability of Pu(III) and, though to a
lesser extent, Np(IV) to oxidation in the course of solu-
tion storage makes apparently impossible stabilization
of the Pu(III)–Np(IV) couple at very high acidity (6–
7 M HNO3).
Thus, carbohydrazide rapidly reduces Pu(IV) at low
acidity (≤1 M). The rate of this reaction is comparable
with the rate of the reduction with the most kinetically
effective reductants of the class of organic derivatives
of hydrazine, hydroxylamine, and oximes, which are
considered as alternatives to traditional reagents for Pu
stripping [1]. Calculation based on the results of this
study and on data of [10, 11] shows that, at [HNO3] =
1 M and 35°С, the time of 99% reduction of Pu(IV) is
1.3, 3.7, 1.3, and 1.1 min with carbohydrazide, hy-
droxyethylhydrazine, acetaldoxime, and dimethylhy-
droxylamine, respectively (at reductant concentration
of 0.5 M). The remaining Pu(III) is stable against oxi-
dation at low acidity (including solutions containing
Because the degree of Pu(IV) reduction with carbo-
hydrazide and the stability of the Pu(III) formed in-
crease with a decrease in the HNO3 concentration, it
seemed appropriate to decrease the solution acidity for
stabilization of the Pu(III)–Np(IV) couple, but the
acidity should be, at the same time, sufficient to ensure
acceptable rate of the Np(V) reduction to Np(IV).
Therefore, in the next experiment the HNO3 concentra-
tion was decreased to 3 M, but the solution tempera-
ture was left on the level of 50°C to accelerate the
Np(V) reduction. In addition, we added to the starting
solution Fe(III) (9 × 10–4 M), which is present in proc-
RADIOCHEMISTRY Vol. 54 No. 2 2012

148
VOLK et al.
Tc ions); hence, carbohydrazide acts as antinitrite sta-
bilizer protecting Pu(III) from oxidation with nitrous
acid. This conclusion is confirmed by the fact that car-
bohydrazide reacts with HNO2 at a high rate (at ~20°C,
[(NH2NH)2CO] = 0.1, [HNO2] = 0.014, and [HNO3] =
0.5 M, the reaction is complete within the time of reac-
tant mixing). Preliminary data show that the reaction
products contain no HN3. At a high acidity (no less
than ~3 M HNO3), Pu(IV) is reduced with carbohy-
drazide incompletely, and the forming Pu(III) is unsta-
ble and is gradually oxidized in Tc(VII)-containing
solutions.
Fe(III)] can be considered as positive: up to ~98% of
Pu and Np passed into the required valence states
whose content did not change in the course of pro-
longed storage of the solution.
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