Sorption and oxidation of tetravalent plutonium on Mn oxide in the presence of citric acid

Article abstract of DOI:10.1246/cl.2009.1032

Sorption experiments of Pu^IV on synthetic Mn oxide were made in 0.1 M NaCl + 0.1 mM sodium citrate solutions under acidic to alkaline pH conditions. As the results of the sorption experiments, Pu was efficiently removed from the solutions under neutral pH conditions, where Pu forms the stable 1:2 PuIVcitrate complex. Furthermore, it was demonstrated that Pu ^IV was oxidized to Pu^v and Pu^VI on Mn oxide. Copyright

Full text of DOI:10.1246/cl.2009.1032

1032
Chemistry Letters Vol.38, No.11 (2009)
Sorption and Oxidation of Tetravalent Plutonium on Mn Oxide
in the Presence of Citric Acid
Kazuya Tanaka,ꢀ Yoshinori Suzuki, and Toshihiko Ohnuki
Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Naka-gun, Ibaraki 319-1195
(Received July 14, 2009; CL-090657; E-mail: tanaka.kazuya24@jaea.go.jp)
Sorption experiments of PuIV on synthetic Mn oxide were
(half-life: 87.7 year) as a radionuclide in this study. The stock
solution of 238Pu (1:0 ꢃ 10ꢁ8 M) was prepared in 1 M HNO3.
Plutonium in the stock solution was placed in the tetravalent state
electrochemically. Sorption experiments of 238Pu on Mn oxide
were carried out using 0.1 M NaCl solutions with 0.1 mM sodium
citrate and different pH conditions at room temperature. After ad-
dition of the 238Pu stock solution and synthetic Mn oxide pow-
ders, pH of the solutions was adjusted with drops of HCl or
NaOH solution. Initial 238Pu concentration was 2 ꢃ 10ꢁ10 M in
all the experiments. The concentration of the Mn oxide was
0.167 g Lꢁ1, corresponding to 4.63 m2 Lꢁ1 as the surface area.
After sorption experiments, the Mn oxide and solution were
separated by centrifugation. The supernatant was used for deter-
mination of the remaining 238Pu concentration in the solutions.
The redox speciation of Pu sorbed on the Mn oxide was
determined using solvent extraction with thenoyltrifluoro-
acetone (TTA) and di(2-ethylhexyl)phospharic acid (HDEHP)
(Figure S2),15 which have been frequently used in previous stud-
ies.4–6,8,9 Sample solutions were added to liquid scintillation
cocktails (Ultima-Gold AB and FG, Packard Instruments Com-
pany, Meriden, CT, USA), and then 238Pu concentrations were
measured by a combination of liquid scintillation with alpha/
beta discrimination (Tri-Carb 2550TR/AB, Packard Instruments
Company, Meriden, CT, USA).16
The results of the sorption experiments are shown in
Figure 1. Sorption of Pu on Mn oxide increased with increasing
reaction time at pH 5.6, and 85% of Pu added was removed from
aqueous phase after 52 h (Figure 1a). At pH higher than 6.6,
sorption was fairly rapid, and most PuIV added was sorbed on
Mn oxide within several hours. The fractions of Pu sorbed on
Mn oxide after 52 h were plotted as a function of pH in
Figure 1b. Sorption of Pu increased with increasing pH, and
the sorption edge was observed in the pH region between 4
and 6. Such sorption edge is typically observed in partitioning
of Pu between oxide minerals and aqueous solutions.4,7–9 At
pH higher than 6.6, most of Pu was removed from the solutions.
made in 0.1 M NaCl + 0.1 mM sodium citrate solutions under
acidic to alkaline pH conditions. As the results of the sorption
experiments, Pu was efficiently removed from the solutions un-
der neutral pH conditions, where Pu forms the stable 1:2 PuIV–
citrate complex. Furthermore, it was demonstrated that PuIV was
oxidized to PuV and PuVI on Mn oxide.
Plutonium (Pu) is a radioactive element contained in radio-
active wastes. If Pu is released from radioactive wastes, the fate
of Pu in natural environments would be a serious problem for hu-
man activities. Therefore, it has been of great concern to predict
the behavior of Pu in natural environments.1–3 The behavior of
Pu in natural water systems is governed by solubility, redox,
complexation, and sorption. Since the oxidation state of Pu sig-
nificantly affects solubility, complexation, and sorption proper-
ties, knowledge of the redox chemistry of Pu is essential for
modeling the behavior of Pu in aqueous systems, which contrib-
utes to design of efficient remediation procedures for Pu conta-
minated areas.
Interaction of dissolved Pu species with solid materials such
as Fe and Mn oxides is an important process which can change
the chemical species of Pu.4–9 In particular, MnIV oxide has re-
ceived considerable attention because of its sorption and oxidiz-
ing properties.10 In addition, Mn oxide is ubiquitous in natural
environments. Nevertheless, little work on Pu oxidation by Mn
oxide has been reported.6,9 Previous studies on PuIV oxidation
by Mn oxide were carried out using solutions with low initial
Pu concentrations of 10ꢁ8–10ꢁ11 M, in which Pu was dissolved
mainly as free aquo ions, to prevent PuIV precipitation.6,9 Al-
though solubility of PuIV is lower than that of PuV and PuVI,
PuIV forms strong complexes with organic acids such as citric
acid, which are present in transuranic (TRU) wastes.11–13 Com-
plexation of PuIV with organic ligands results in the stabilization
of PuIV in aqueous phase and further enhances the mobility of
Pu. Therefore, it is necessary to evaluate the influence of organic
ligands on sorption and oxidation processes at solid–water inter-
faces. In particular, citric acid forms fairly strong complexes
with PuIV.11–13 In this study, we investigated sorption and oxida-
tion of PuIV on Mn oxide in the presence of citric acid, which
was not considered in previous work on PuIV oxidation.6,9
Manganese(IV) oxide was synthesized as reported by
Murray, using KMnO4 and MnCl2.14 After drying at 40 ꢂC, ag-
gregated Mn oxides were crushed and pulverized with an agate
mortar and subsequently sieved to collect the <125-mm size
fraction. XRD analysis identified the synthetic product as poorly
crystalline Mn oxide (Figure 1S).15 The specific surface area of
the synthetic Mn oxide (27.8 m2 gꢁ1) was determined with a
Brunauer–Emmett–Teller (BET) analyzer (BELSORP-mini,
BEL JAPAN, INC) at Hiroshima University. We used 238Pu
Figure 1. (a) Sorption kinetics of PuIV on Mn oxide in 0.1 M
NaCl solution with 0.1 mM sodium citrate and (b) sorption of
PuIV on Mn oxide as a function of pH after 52 h.
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.11 (2009)
1033
Since we used solutions with the initial citrate/Pu ratio of
5 ꢃ 105, Pu possibly formed citrate complexes in the solu-
tions.11–13 According to speciation calculation, 1:2 PuIV–citrate
complex (Pu4þ þ 2cit3ꢁ ! Pucit22ꢁ) is dominant in the solu-
tion at pH 2–7.5 (Figure S3).15 At pH 8–9, hydrolysis of Pu to
form Pu(OH)4aq is important. The carbonate complexes of Pu-
(CO3)44ꢁaq and Pu(CO3)56ꢁaq are dominant under alkaline con-
ditions at pH higher than 9.5.
The extent of Pu sorption on Mn oxide depends on the pro-
tonation of surface functional groups, Pu species in solution and
sorbed Pu–surface complexes. It has been shown that the proton-
ation on surfaces of Mn oxide is due to hydroxy groups as well as
those of other oxide minerals.17 At low pH values, the surface
has a net positive charge due to the protonation of the hydroxy
groups, whereas the deprotonation of the hydroxy groups results
in a net negative surface charge at higher pH. Therefore, sorption
of cation is favored by higher pH values, whereas that of anion is
favored by lower pH values.17 The surface charge of Mn oxide is
negative at pH higher than 3. Sorption of negatively charged
metal–citrate complexes on negatively charged oxide minerals
was suppressed.18 Our results show that sorption of Pu on Mn
Khasanova et al. reported PuIV oxidation by Mn oxide, but
unfortunately they did not show any quantitative data on the
PuIV oxidation.9 Morgenstern and Choppin conducted sorption
experiments of PuIV on synthetic MnIV oxide in 1 M NaCl solu-
tions without organic acids as chelating agents.6 Their experi-
mental results indicated that the oxidation of PuIV occurred on
Mn oxide under pH conditions between 2 and 8.1. Consequently,
it is consistent with the previous work that PuIV oxidation was
observed in the pH range between 5.6 and 9.7 in our sorption ex-
periments (Figure 2).
This work is the first report that shows that Mn oxide can re-
move Pu efficiently from aqueous phase even in the presence of
citrate under neutral pH conditions, where 1:2 PuIV–citrate com-
plex is dominant (Figure 1). Furthermore, we demonstrated that
PuIV oxidation occurs on Mn oxide surface in the presence of cit-
rate (Figure 2). This suggests that Mn oxide can change the
chemical species of Pu because of its strong oxidizing properties,
even if PuIV forms strong complexes with organic acids in natu-
ral water systems. The results of this study are important in terms
of modeling the behavior of Pu in natural environments and give
fundamental information to future work on interaction of Pu be-
tween solid and aqueous phases in the presence of strong chelat-
ing agents.
2ꢁ
oxide increased with increasing pH from 3 to 7 where Pucit2
is dominant (Figures 1 and S315). These facts suggest that sorp-
tion of Pu on Mn oxide was accompanied with dissociation of
2ꢁ
Pu from Pucit2 and that positively charged Pu aquo ions were
References and Notes
1
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Pu(CO3)44ꢁaq and Pu(CO3)56ꢁaq are dominant (Figures 1 and
S315).
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Figure 2. Fractions of PuIV, PuV, and PuVI on Mn oxide as a
function of pH after 52 h.
Published on the web (Advance View) October 3, 2009; doi:10.1246/cl.2009.1032