Full text of DOI:10.1134/S1066362217030043

ISSN 1066-3622, Radiochemistry, 2017, Vol. 59, No. 3, pp. 233–239. © Pleiades Publishing, Inc., 2017.
Original Russian Text © A.V. Gogolev, V.P. Shilov, V.P. Perminov, A.M. Fedoseev, 2017, published in Radiokhimiya, 2017, Vol. 59, No. 3, pp. 204–209.
Reaction of Ozone with Uranium(IV) Oxalate in Water
A. V. Gogolev*, V. P. Shilov, V. P. Perminov, and A. M. Fedoseev
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, korp. 4, Moscow, 119071 Russia
*e-mail: Gogolev@ipc.rssi.ru
Received August 8, 2016
Abstract—Decomposition of aqueous suspensions of uranium(IV) oxalate under the action of an ozone–
oxygen mixture was studied. The process occurs in two steps. In the first step, the U(IV) oxidation with the for-
mation of oxalic acid uranyl solutions prevails. The second step involves decomposition of oxalate ions and hy-
drolysis of uranyl ions. An increase in temperature accelerates the transformation of uranium(IV) oxalate into
uranium(VI) hydroxide compounds. In solutions containing KBr or UO2Br2, the following reaction occurs: O3 +
Br– → O2 + BrO–. The arising hypobromite ions and hypobromous acid oxidize uranium(IV) oxalate extremely
efficiently. The possible mechanism of ozonation of aqueous uranium(IV) oxalate suspensions is discussed.
Keywords: uranium, ozone, uranium oxalate
DOI: 10.1134/S1066362217030043
Precipitation of tri- and tetravalent f elements in the
form of oxalates allows deep purification of rare earth
elements and actinides to remove the majority of
d elements [1]. It seems promising to use oxalates of
these elements as starting compounds for the synthesis,
because they contain no other metals, in contrast to
using hydroxides of f elements in various valence states,
which are precipitated most frequently with alkali
metal and ammonium hydroxides. However, high sta-
bility of simple and complex oxalates complicates their
further use for preparing compounds with other anions.
The problem of removing organic anions and preserv-
ing the purity of f elements can be solved by using
ozone as a salt-free oxidant. The use of gaseous ozone
and low solubility of oxalates of tri- and tetravalent
f elements in water complicate decomposition of ox-
alate anions. For choosing the optimum conditions, it
is necessary to use experimental data, lacking today.
ozone–oxygen mixture allows acceleration of the oxi-
dation, does not introduce additional substances into
the reaction mixture, and is used for oxidation of vari-
ous compounds in aqueous and nonaqueous media
[3, 4].
EXPERIMENTAL
We used HClO4, KHCO3, K2C2O4, and H2C2O4 of
chemically pure or analytically pure grade. Potassium
bromate (analytically pure grade) was recrystallized
twice from water. Uranium(IV) oxalate U(C2O4)2·
nH2O was prepared by adding H2C2O4 to a hydrochlo-
ric acid U(IV) solution heated to 90°С. The precipitate
was heated for 30 min on a water bath [1]. The product
composition was in good agreement with the formula
U(C2O4)2·6H2O. Uranyl oxalate UO2C2O4·3H2O was
prepared from an aqueous solution of uranyl nitrate
and oxalic acid and was dried in air. Uranium(IV)
tetrafluoride was precipitated from a solution of U(IV)
in 0.2 M HCl with hydrofluoric acid, washed three
times with cooled water to neutral pH value, and dried
at 100°С in air. An ozone–oxygen mixture containing
~4 vol % O3 was bubbled through aqueous suspensions
and solutions at a rate of 10–15 L h–1. The suspensions
were clarified on a PE-6910 laboratory centrifuge.
Oxidation of poorly soluble U(IV) compounds can
favor the formation of more soluble U(VI) compounds.
Therefore, knowledge of the specific features of the
U(C2O4)2·xH2O oxidation in neutral or close-to-neutral
media can be useful for predicting the uranium migra-
tion in the environment [2]. Oxidation of uranium(IV)
oxalate in neutral aqueous media was not studied in
detail. The use of oxygen as oxidant is inefficient be-
cause of low solubility of uranium(IV) oxalate. The
The pH values were measured with an OP-211/1
pH meter equipped with a glass combined electrode.
233

234
GOGOLEV et al.
RESULTS AND DISCUSSION
Bubbling of ozone–oxygen mixture through an
aqueous suspension of uranium(IV) oxalate leads to its
complete dissolution and formation of a transparent
yellow solution. Further ozonation leads to precipita-
tion of yellow substances. The time required for disso-
lution of the gray-green suspension and precipitation
of yellow substances depends on the uranium(IV) ox-
alate amount, temperature, and presence of additives.
Our results show that two steps can be distinguished in
ozonation of an aqueous suspension of uranium(IV)
oxalate. In the first step, U(C2O4)2 is predominantly
oxidized to UO2C2O4, with partial decomposition of
oxalate ions also possible. In the second step, oxalate
ions and oxalic acid decompose with the precipitation
of uranyl hydrolysis products.
Fig. 1. Optical density at 419 nm of a solution initially
containing 25.5 mg of U(C2O4)2·6H2O in 5 mL of H2O as a
function of ozonation time at 22°С. The pH values of the
solution are indicated in parentheses.
The ozonized solutions were stored in the dark bet-
ween the analytical operations.
The first step of oxidation of U(C2O4)2 with ozone
can be described by the following reaction equation:
The concentration of uranium ions was monitored
with a Shimadzu UV 3100 spectrophotometer using a
cell with an optical path length of 1 cm.
U(C2O4)2 + O3 + H2O → UO2C2O4 + H2C2O4 + O2. (1)
A typical dependence of the concentration of
UO2C2O4 formed in reaction (1) on the ozonation time
is shown in Fig. 1.
Preliminary experiments have shown that ozonation
of U(C2O4)2·xH2O suspensions occurs in weakly acidic
and close-to-neutral aqueous media. U(VI) ions exist
in such solutions mainly in the form of oxalate com-
plexes or in the hydrolyzed form. For a 0.013 M
UO2C2O4 + 0.013 M H2C2O4 solution, D419 = 0.23,
ε419 = 17.7 L mol–1 cm–1 (Fig. 1). The observed value
of ε is the mean of the ε values of 16.4 and
19.7 L mol–1 cm–1, estimated from the figure and cal-
culated in [5] for the hydrated complexes UO2C2O4
and UO2(C2O4)22–, respectively.
Dissolution of uranium(IV) oxalate is accompanied
by the release of oxalic acid and decrease in pH. For
ozonized solutions after the dissolution of the sus-
pended material, pH depends on the amount of ura-
nium(IV) oxalate taken and approximately corre-
sponds to pH of the model solutions. For 0.01 M
H2C2O4, pH is 1.97; 0.01 and 0.005 M UO2C2O4 solu-
tions have pH 4.06 and 4.08, respectively. The 9.7 ×
10–3 M UO2C2O4 + 0.01 M H2C2O4 solution has
pH 1.82. The transparent yellow solutions formed by
ozonation of the suspension have the electronic ab-
sorption spectra typical of uranyl complexes (Fig. 2).
The extinction coefficient for hydrolyzed uranyl
species strongly depends on the experimental condi-
tions and can vary in the course of slow hydrolysis and
polymerization reactions. Therefore, for monitoring
the concentration of uranyl ions, we recorded their
electronic absorption spectra after introducing an ali-
quot of the ozonized solution into 1.2 M HClO4 or into
a saturated KHCO3 solution. The extinction coeffi-
cients ε of U(VI) ions in perchloric acid and bicarbon-
ate solutions for the spectrophotometer used were de-
termined in special experiments. For UO2(H2O)25+ and
UO2(CO3)34–, we obtained the ε values equal to 8.3 and
28.5 L mol–1 cm–1 for λ = 414 and 449 nm, respectively.
These values virtually coincide with those known from
the literature, 8.1 [6] and 29 ± 0.7 L mol–1 cm–1 [7].
Close value of ε for UO2(H2O)52+ is also given in the
figure in [8].
The maximum of the strongest absorption band is
shifted to 419 nm, and the spectra can be assigned to
uranyl oxalate complexes [5, 9]. Complete dissolution
of hydrated U(C2O4)2 occurs also when the solubility
of the formed UO2C2O4 is exceeded, suggesting the
formation of soluble oxalate complexes UO2(C2O4)22–.
As seen from Table 1, oxidation of relatively small
amounts of aqueous suspensions of uranium(IV) ox-
alate with ozone occurs slowly even at elevated tem-
peratures.
To compare, we performed ozonation of a suspen-
sion of one more insoluble U(IV) compound, UF4.
Oxidation of 0.0068 g of UF4 was complete in 60 min.
RADIOCHEMISTRY Vol. 59 No. 3 2017

REACTION OF OZONE WITH URANIUM(IV) OXALATE IN WATER
235
The solution spectra corresponded to the spectra of
uranyl fluoride complexes; the solutions had pH ≈
2.87. Because fluoride ions do not react with ozone,
we can conclude that UF4 quantitatively transformed in
accordance with Eq. (2):
UF4 + O3 + H2O → UO2F2 + 2HF + O2,
(2)
(3)
UO2F2 + 2 HF ҙ UO2Fn(2–n)+ + 2H+ + (4 – n)F–.
The positions of the maxima in the electronic ab-
sorption spectra of the solutions (Fig. 2) prove the
presence of uranyl fluoride complexes in the solution,
and the spectrum agrees with that of a mixture of com-
plex species UO2Fn(2–n)+ (n = 1, 2, 3, 4), given in [8].
Fig. 2. Spectra of (1) a model solution containing 0.013 M
UO2C2O4 and 0.013 M H2C2O4 and solutions obtained after
ozonation of suspensions of (2) 25.5 mg of U(C2O4)2·6H2O
in 5 mL of H2O and (3) 11 mg of UF4·H2O in 4.5 mL of
H2O.
Calculations show that oxidation of weighable
amounts of uranium(IV) oxalate requires a long time
even at elevated temperatures. Furthermore, as will be
shown below, further oxidation of oxalic acid and ox-
alate ions in aqueous solutions requires a still longer
time. Therefore, we checked the possibility of acceler-
ating the decomposition of uranium oxalates by intro-
ducing bromide or chloride ions into the solution.
Additional experiments on U(IV) oxidation in po-
tassium oxalate solutions with potassium bromate have
shown that the consumption of U(IV) oxalate com-
plexes in a homogeneous solution is also slow. For
example, in a solution containing 0.0052 M U(C2O4)2,
0.05 M K2C2O4, and 0.05 M KBrO3, a pseudo-first-
order reaction with the first half-conversion time of
150 min occurred at 22°С. Deviations from the first
order were observed only in the first 15 min, despite a
uniform decrease in pH from 6.6 to 5.16 that we noted.
The observed rate constant of the reaction U(C2O4)22– +
BrO3– at 22.4°С is 4.2 × 10–3 min–1. The U(IV) concen-
tration was monitored using the strongest absorption
band with a maximum at 663 nm in the spectrum of
As seen from Table 1, uranium(IV) oxalate rapidly
dissolves in solutions containing bromide ions, and
decreasing temperature to room temperature deceler-
ates the process insignificantly. Apparently, Br– ions
react with O3, and products of this reaction efficiently
oxidize U(C2O4)2. It is known that the products of the
reaction of ozone with bromide ions in aqueous solu-
tions are hypobromite, BrO–, and bromate, BrO3–, ions
[10]. An experiment on oxidation of a suspension of
22.3 mg of uranium(IV) oxalate in 4 mL of a 0.1 M
KBrO3 solution showed that uranium(VI) oxalate com-
plexes did not appear in the solution in 45 min at room
temperature. Further storage of this solution without
stirring for 24 h leads to the appearance of 0.0035 M
U(VI) oxalate complexes in the solution. This solution
has pH 2.81 and a spectrum characteristic of U(VI)
oxalate complexes. Hence, U(IV) is oxidized with bro-
mate ions preferentially with simultaneous release of
oxalic acid. Apparently, the reaction with bromate ions
cannot ensure the observed rapid oxidation of a
U(C2O4)2 suspension with ozone in approximately
neutral KBr solutions. Therefore, we conclude that the
suspension dissolves owing to the occurrence of the
reactions
Table 1. Time τs of dissolution of uranium(IV) oxalate sus-
pensions in aqueous solutions under the action of ozone, pH
values of the resulting solutions, and positions of the most
intense optical absorption maximum λmax
U(C2O4)2·nH2O,
λmax,
nm
Electrolyte, M T, °C τs, min
pH
mg mL–1
5.1
UF4, 2.4
6.25
11.8
5.5
–
–
–
22
22
70
80
80
70
19
22
540
60
60
120
10
10
10
5
418 2.22
420 2.87
419
418
448
–
–
–
–
KBr, 0.1
KBr, 0.01
KBr, 0.1
KBr, 0.04
5.6
5.5
4.5
≥7
449 7.57
7.48
5.25
5.1
4.6
UO2Br2, 0.04 24
15
25
150
–
4.26
5.46
1.83
O3 + Br– → O2 + BrO–,
(4)
NaCl, 0.1
KCl, 0.04
80
22
U(C2O4)2 + BrO– + H2O → UO2C2O4 + H2C2O4 + Br–. (5)
–
RADIOCHEMISTRY Vol. 59 No. 3 2017

236
GOGOLEV et al.
Reactions (4) and (5) can be performed without
introducing additional cations into the solutions. For
example, ozonation of 4.25 mL of a solution contain-
ing 0.04 M UO2Br2 and 21 mg of U(C2O4)2·6H2O, per-
formed at 25°С for 15 min, leads to a change in the
suspension color from gray-green to light yellow. Fur-
ther ozonation for 15 min allowed preparation of
a bright yellow opalescent solution with the spectrum
characteristic of hydrolyzed U(VI) (Fig. 3, spec-
trum 1). In storage, a light yellow substance precipi-
tated from the solution.
The rate of the reaction of O3 with Cl– increases
with a decrease in pH of solutions. Therefore, dissolu-
tion of uranium(IV) oxalate accelerates in chloride so-
lutions. However, the performance of chloride addi-
tives is low and was not considered in this study in
detail.
Fig. 3. Spectra of a solution obtained (1) after ozonation of
a suspension containing 21 mg of U(C2O4)2·6H2O in 4 mL
of 4.25 × 10–2 M UO2Br2 for 30 min and (2) after precipi-
tated formation. (3) Spectrum of aqueous suspension that
initially contained 25.2 mg of U(C2O4)2·6H2O in 0.1 M
KBr after ozonation for 90 min.
In our experiments on ozonation of U(C2O4)2 sus-
pensions without additives, the solution spectra after
the dissolution completion corresponded to the pub-
lished spectra of oxalate complexes and in some cases
to the spectra of hydrolysis products of uranyl ions
[12–14]. Because of the release of oxalic acid in reac-
tion (1), such solutions had pH from 1.8 to 3.
the initial solution, coinciding with the published spec-
trum [11]. The spectrum of the oxidized solution had
the shape characteristic of U(VI) oxalate complexes [5,
9] with the most pronounced maximum at approxi-
mately 419 nm.
It should be noted that ozonation of aqueous
U(C2O4)2 suspensions containing Br– occurs virtually
without intermediate formation of a transparent yellow
solution. Precipitation of a yellow substance starts im-
mediately after dissolution of the initial gray-green
suspended material. In some experiments, the gray-
green suspended substance transformed into a yellow
precipitate without intermediate formation of a trans-
parent solution. Hence, in KBr solutions hypobromite
ions efficiently react with oxalate ions.
In the second step of the ozonation, oxalic acid and
oxalate ions decompose, which leads to an increase in
pH. Ozonation of oxalic acid solutions of U(VI) with-
out additives takes a long time (Table 2).
Ozonation leads to the decomposition of oxalate
ions in reactions (6)–(8),
H2C2O4 + O3 → 2CO2 + O2 + H2O,
(6)
HC2O4– + O3 → 2CO2 + O2 + OH–,
C2O42– + O3 + H2O → 2CO2 + O2 + 2OH–,
(7)
(8)
Table 2. Total time τ of transformation of aqueous urani-
um(IV) oxalate suspensions into uranium(VI) hydroxide
compounds under the action of ozone
and to removal of CO2 with a stream of the ozone–
oxygen mixture. Therefore, the [С2O42–]/[UO22+] ratio
decreases and pH increases, which leads to the forma-
tion of polymeric hydroxo oxalate complexes. Without
complexing agents at [UO22+] ~0.01 M, hydrolysis oc-
curs at pH > 3. Manfredi et al. [14] studied the hy-
drolysis of UO22+ at pH in the interval 2.5–4.5 and for-
mation of hydroxo oxalate complexes at pH in the in-
terval 4.5–8.5. The process completes in precipitation
of yellow substances and disappearance of the optical
absorption caused by uranium compounds in solution.
Table 3 shows how pH of the solution varies in the
course of ozonation.
U(IV)0,
Electrolyte, M
T, °C
τ
pH
–
mg mL–1
6.25
11.8
5.5
–
–
70
80
7 h
10 h 5.55
KBr, 0.1
80 16 min 8.45
80 1.5 h 5.46
70 11 min
19 10 min 7.57
24 30 min 4.26
5.3
5.6
5.7
5.3
NaCl, 0.1
KBr, 0.01
KBr, 0.1
UO2Br2, 0.042
≤7
–
UO2С2О4, 0.013 + H2C2O4, 23
>21 h >3.2
8 min 7.5
0.013
4.5
KBr, 0.04
22
RADIOCHEMISTRY Vol. 59 No. 3 2017

REACTION OF OZONE WITH URANIUM(IV) OXALATE IN WATER
Table 3. Ozonation of a 0.013 M UO2C2O4 + 0.013 M H2C2O4 solution at 23°С
237
Value at indicated τ, h
Parameter
0
1.8
0.23
1
1.8
0.23
3.5
1.86
0.23
6
8.5
1.96
0.238
11.5
–
0.246
15.5
2.65
0.234
16.5
2.8
0.24
18
2.99
0.241
18а
3.3
0.241
21
3.2
–
рН
D419
1.89
0.236
a The solution storage in the dark for 21 h led to a change in pH without changes in the optical absorption spectrum.
Owing to decomposition of oxalate ions in reac-
tions (6)–(8) and dissociation of oxalate complexes,
Apparently, uranyl ions in the dark do not catalyze
the reaction of ozone with oxalic acid and oxalates.
Evolution of the spectrum of a 0.01 M UO2C2O4 solu-
tion under the action of an ozone–oxygen mixture and
of UV radiation (VIO-1 lamp) in a glass bubbler shows
that the initial spectrum of uranyl oxalate complexes
transforms into the spectrum of hydrolyzed uranium
(similar to spectra 1 and 2 in Fig. 3) within 2 h. In the
process, pH increases from 4.11 to 4.92. In the subse-
quent 2 h, the solution spectrum does not change in
shape but considerably increases in the intensity, and
pH increases to 5.6. Hydrolysis of uranyl ions occurs
relatively slowly. The subsequent storage of this solu-
tion for 17.5 h in the dark led to precipitation of a yel-
low substance and to a decrease in pH to 5.04. Similar
but faster changes occur in the course of joint photoly-
sis and ozonolysis of a 0.01 M UO2C2O4 solution in a
quartz bubbler. After the lapse of 1 h, the solution
spectrum (pH of solution 4.94) contained only the ab-
sorption band of hydrolyzed U(VI). In the subsequent
2 h, the intensity of the absorption band of hydrolyzed
U(VI) considerably increased, and a fine suspension
was formed. After storage of this solution in the dark
for 20 h, a pale yellow precipitate was formed. The
intensity of the spectrum decreased by a factor of ap-
proximately 3, but its shape did not change.
UO2(C2O4)22– → UO2(C2O4) + C2O42–.
(9)
the solubility of uranyl oxalate can be exceeded, which
will cause partial precipitation of UO2C2O4·3H2O. In
ozonation of solutions with [UO2C2O4] ≤ 0.01 M, we
recorded the spectra of hydrolyzed U(VI) species
(Fig. 3), similar to those presented previously in nu-
merous papers, e.g., in [13, 14].
According to [15], the reaction of O3 with C2O42–
ions is so slow that the reaction rate at room tempera-
ture cannot be measured. Hydrolysis of uranyl ions
occurs at pH in the range 4–5, i.e., in solutions contain-
ing predominantly HC2O4– ions. In the slow ozonation
process in solutions with pH < 5, the most probable
limiting step is the reaction
O3 + HС2O4– → O2 + OH + С2O4–,
(10)
followed by a chain of radical reactions
OH + HC2O4– → H2O + C2O4–,
O3 + С2O4– → O3– + 2СO2,
(11)
(12)
O3– + H+ → O2 + OH.
(13)
The use of uranyl oxalate solutions for measuring
the luminous flux intensity is known for a long time
[16]. In particular, photolysis of weakly acidic uranyl
oxalate solutions was studied [17]. The possibility of
photochemical decomposition of oxalate ions in ura-
nium-containing liquid radioactive waste was consid-
ered [18]. Therefore, we attempted to combine ozona-
tion with photochemical decomposition of oxalates,
sensitized with uranyl ions. We have found that such
combination can be efficient in the step of transparent
solutions. In this case, oxidation of oxalate ions with
excited uranyl ions occurs simultaneously with ozone-
initiated reactions:
The efficiency of the action of UV radiation de-
creases owing to precipitation of uranyl oxide–
hydroxide compounds in the solution volume and on
bubbler walls. Therefore, photolysis does not ensure
complete decomposition of oxalic acid and oxalates at
acceptable rate, and the use of accelerating additives is
necessary. We have studied the effect of bromide and
chloride ions.
Decomposition of oxalic acid and its anions effi-
ciently occurs in solutions containing KBr and
UO2Br2. Hypobromite ions arise in reaction (4) and are
consumed in parallel reactions (17)–(19):
UO22+ + hν → UO22+*,
UO22+* + C2O42– → UO2+ + C2O4–,
UO2+ + O3 → UO22+ + O3–.
(14)
(15)
(16)
H2C2O4 + HOBr → 2CO2 + HBr + H2O,
C2O42– + HOBr → 2CO2 + Br– + OH–,
HC2O4– + HOBr → 2CO2 + Br– + H2O.
(17)
(18)
(19)
RADIOCHEMISTRY Vol. 59 No. 3 2017

238
GOGOLEV et al.
The observed process rate is controlled by reac-
As seen from Table 2, chloride ions also accelerate
decomposition of oxalate ions and oxalic acid, but
their performance is considerably lower than that of
KBr. This may be due to considerably lower rate of
the reaction of O3 with Cl–, especially in solutions with
pH > 3. Therefore, we have not studied the ozonation
of U(VI) chloride–oxalate solutions in detail.
tion (19) [19]. According to [20], in solutions with
pH 6 the pseudo-first-order rate constants for the reac-
tions of oxalate ions with O3 and HOBr are <8 × 10–7
and 4 × 10–5 s–1, respectively. Also, the following equi-
librium occurs in bromide solutions:
Br2 + H2O ҙ H+ + Br– + HOBr.
(20)
The results of our study show that ozonation of sus-
pensions of uranium(IV) oxalate can yield oxalic acid
U(VI) solutions. The decomposition considerably ac-
celerates in the presence of KBr. The UF4 suspension
also reacts with an ozone–oxygen mixture to form hy-
drofluoric acid U(VI) solutions. Decomposition of ox-
alic acid and uranium(VI) oxalate also considerably
accelerates in solutions containing KBr or UO2Br2.
The ozonation completes in hydrolysis of uranyl ions
and precipitation of U(VI) oxide–hydroxide com-
pounds. Prolonged ozonation of aqueous suspensions
of uranium(IV) oxalate and of solutions of U(VI) ox-
alate can be used in the laboratory practice for prepar-
ing U(VI) hydroxide compounds free of foreign metal
cations.
Molecular bromine Br2 (or tribromide anion Br3–) is
less efficient in oxidation of oxalic acid and oxalates.
Increasing pH enhances the bromine hydrolysis and
accelerates the decomposition. The suspensions and
solutions containing KBr after the ozonation comple-
tion have pH > 7. On the other hand, in solutions con-
taining UO2Br2, the process completes in precipitation
at pH < 7. The electronic absorption spectra suggest
the presence of small amounts of uranyl carbonate
complexes in such solutions. The most probable expla-
nation to this fact is based on the occurrence of equi-
librium reaction (20) in aqueous solution.
After the decomposition of oxalate ions, at pH > 5,
the reaction between bromide and bromate ions be-
comes very slow, and bromate ions are accumulated in
the solution. Upon acidification of such solutions, the
following reactions occur:
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