Sorption of Np(V), Pu(V), and Pu(IV) on colloids of Fe(III) oxides and hydrous oxides and MnO2

Article abstract of DOI:10.1134/S1066362207040170

Sorption of Np(V), Pu(V), and Pu(IV) on colloids of synthetic goethite (α-FeOOH), hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃), and amorphous MnO₂ was studied over wide ranges of solution pH and ionic strength by solvent extraction and X-ray photoelectron spectroscopy (XPS). Plutonium(V) is reduced upon sorption on α-FeOOH, but not on α-Fe₂O₃ and γ-Fe₂O₃. On the MnO₂ surface, Pu occurs as Pu(VI). From the pH dependences of the actinide sorption, the equilibrium constants of the reactions of Np(V)O ₂ ⁺ and Pu(V)O ₂ ⁺ with the surface hydroxy groups of the investigated colloid particles and a set of the equilibrium constants of the reactions of Pu(IV) hydroxo complexes with α-FeOOH were obtained. If no redox reactions occur on the surface of the colloid particles, these constants decrease in the order K MnO2 > K - FeOOH > K - Fe K - Fe.

Full text of DOI:10.1134/S1066362207040170

ISSN 1066-3622, Radiochemistry, 2007, Vol. 49, No. 4, pp. 419 425. Pleiades Publishing, Inc., 2007.
Original Russian Text
No. 4, pp. 367 372.
A.B. Khasanova, N.S. Shcherbina, S.N. Kalmykov, Yu.A.Teterin, A.P. Novikov, 2007, published in Radiokhimiya, 2007, Vol. 49,
Sorption of Np(V), Pu(V), and Pu(IV) on Colloids
of Fe(III) Oxides and Hydrous Oxides and MnO2
A. B. Khasanovaa, N. S. Shcherbinaa, S. N. Kalmykova,
Yu. A. Teterinb, and A. P. Novikovc
Chemistry Faculty, Lomonosov State University, Moscow, Russia
a
b
Russian Research Centre Kurchatov Institute, Moscow, Russia
c
Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences,
Moscow, Russia
Received March 20, 2006
Abstract Sorption of Np(V), Pu(V), and Pu(IV) on colloids of synthetic goethite ( -FeOOH), hematite
( -Fe2O3), maghemite ( -Fe2O3), and amorphous MnO2 was studied over wide ranges of solution pH and
ionic strength by solvent extraction and X-ray photoelectron spectroscopy (XPS). Plutonium(V) is reduced
upon sorption on -FeOOH, but not on -Fe2O3 and -Fe2O3. On the MnO2 surface, Pu occurs as Pu(VI).
From the pH dependences of the actinide sorption, the equilibrium constants of the reactions of Np(V)O+2 and
Pu(V)O+2 with the surface hydroxy groups of the investigated colloid particles and a set of the equilibrium
constants of the reactions of Pu(IV) hydroxo complexes with -FeOOH were obtained. If no redox reactions
occur on the surface of the colloid particles, these constants decrease in the order KMnO > K
>
-FeOOH
2
K
K
.
-Fe O
-Fe O
2
3
2 3
PACS numbers: 68.43.-h, 82.70.Dd, 89.60.Ec
DOI: 10.1134/S1066362207040170
The modern approaches to long-term safety assess-
ment of radioactive waste (RW) repositories in surface
and underground geological formations are based on
data on the radionuclide sorption on components of
engineered and geochemical barriers (rocks and min-
erals), and also on the solubility of radionuclides in
underground waters. Repositories for RW should meet
the criterion that radionuclides from RW must not
enter the human habitation zone in tens and hundreds
thousand years. According to most of the existing
estimates, tri- and tetravalent actinides and Tc(IV) re-
present no hazard, as having strong tendency to sorp-
tion on components of engineered barriers and host
rocks and also as having extremely low solubility.
However, recent data suggest the possibility of col-
loidal migration of these radionuclides in groundwater
demonstrated by the methods of micro- and ultrafiltra-
tion that Pu and Am are associated with colloid par-
ticles 25 450 and 2 nm in size, respectively. How-
ever, no information on the nature of these colloids
was reported.
Naturally occurring minerals differ considerably in
their sorption capacity for cations, and the available
data on the preferential sorption of radionuclides on
colloid particles of various kinds are contradictory.
Duff et al. [9] studied the Pu sorption on tuff with in-
clusions of zeolite, smectite, and Fe and Mn oxides by
various microanalytical methods. They have demon-
strated that Pu is sorbed on smectites and Mn oxides
only, and in the latter case, it is oxidized to Pu(VI).
A great deal of work has been made to model
radionuclide sorption on various minerals, including
oxides and hydrous oxides of Fe [10 12] and Mn
[13 15], clay [16], calcite [15], etc. Most authors em-
[1 6]. Kersting et al. [7] reported that, at the test site
239
in Nevada,
Pu was carried with smectite colloid to
a distance of 1.3 km in 30 years. Penrose et al. [8]
have studied the behavior of Pu and Am in the Mor-
tandad canyon (Los Alamos National Laboratory)
and found that these radionuclides have migrated with
colloid particles to a distance of 3390 m from the
source. By the way, the theoretical estimates for
the migration rates of these radionuclides suggested
that their path is limited to several meters. It was
ploy the distribution coefficient K as a parameter
d
characterizing the sorption. However, the use of this
parameter is limited to specific experimental condi-
tions (pH, complexation, etc.). An alternative ap-
proach to description of the sorption involves calcula-
tion of the equilibrium constants of one or another
sorption reaction occurring on the surface of colloid
419

420
KHASANOVA et al.
-FeOOH
Table 1. Characteristics of colloid samples
Parameter
-Fe2O3
-Fe2O3
MnO2
Total specific surface area, m2/g*
Specific micropore surface area, m2/g*
Mean particle size, m**
pH(IEP) ***
41.7
7.23
0.30 0.05
8.8
6.4
3.02
0.35 0.05
9.4
20.4
9.8
182
32.9
8.9
1.5
* From BET analysis.
** From SEM data.
*** From potentiometric titration data.
particles. From the set of the equilibrium constants of
all possible reactions occurring both in solution and
on the surface of colloid particles, one may estimate
was dissolved in deionized water, concentrated HCl
was added to pH 1.5, and the solution was heated to
100 C. The resulting hot solution was diluted with
the estimated volume of hot deionized water, placed
in a Teflon vessel, and heated in a pressure vessel at
90 100 C for a week. After cooling, the suspension
was separated by centrifugation, multiply washed with
double-distilled water, and air-dried at 80 C.
K
for any system.
d
This study is aimed to examine the sorption of
Np(V), Pu(V), and Pu(IV) on various Fe(III) oxides
and hydrous oxides and MnO , to determine the
2
valence states of the actinides, and to estimate the
equilibrium constants for sorption reactions. Colloidal
iron and manganese oxides and hydrous oxides may
be formed under conditions of both near-by and dis-
tant zones of RW repositories, representing a factor
strongly influencing the behavior of radionuclides.
Furthermore, such iron oxides as hematite ( -Fe O )
Colloidal maghemite ( -Fe O ) was synthesized by
2
3
the method described in [21]. A weighed portion of
FeCl 6H O was dissolved in deionized water, 2.5 M
3
2
NaOH was added, and the mixture was heated at 70 C
for 2 days. Then the mixture was heated in a pressure
vessel at 350 C for 1 h. The suspension was separated
by centrifugation, multiply washed with double-dis-
tilled water, and dried.
2
3
and magnetite ( -Fe O ) can be formed in contour
3
4
water of nuclear reactors under the action of water
radiolysis products [17]. Transformation of magnetite
to hematite and sorption of radionuclides are the
factors determining to a considerable extent the deci-
sion on the methods for contour water management.
As mentioned above, a great body of information is
gained on the radionuclide sorption on Fe and Mn
oxides and hydrous oxides. However, data on the
equilibrium constants of sorption reactions are either
lacking at all or contradictory [9, 14, 16, 18].
Colloidal manganese dioxide was synthesized by
the method described in [22]. To a solution of KMnO
4
in NaOH, MnCl was added stepwise with magnetic
2
stirring. The resulting suspension was settled for 24 h,
filtered off, multiply washed with deionized water,
and air-dried.
The samples were characterized by X-ray diffrac-
tion (XRD), scanning electron microscopy (SEM),
and potentiometric titration [21]. The BET surface
area was determined from the nitrogen adsorption. No
foreign phases were found in Fe-containing samples.
Manganese dioxide samples are amorphous. Charac-
teristics of the samples are summarized in Table 1.
EXPERIMENTAL
Preparation and Characterization of Colloid Particles
of Fe(III) and Mn(IV) Oxides and Hydrous Oxides
Colloidal goethite ( -FeOOH) was synthesized by
the procedure described in [19]. A weighed portion of
Fe(NO ) 9H O was dissolved in deionized water,
Aqueous solutions for the sorption experiments
were prepared with deionized water (Milli-Q) treated
to remove CO by purging ultrapure grade N . Solu-
tion pH values were measured with a combined glass
electrode [Mettler Toledo InLab (Switzerland)].
3 3
2
2
2
2.5 M KOH was added to pH 12, and the reaction
mixture was vigorously stirred for 24 h at 60 C.
The resulting suspension was dialyzed until the total
nitrate concentration in the outer solution was
Preparation and Purification of Pu(IV), Pu(V),
and Np(V)
4
<10 M. Finally, the colloid suspension was multiply
washed with acetone and dried at 40 C for 48 h.
Since the Pu sorption experiments were planned to
carry out at various total Pu concentrations, Pu iso-
Colloidal hematite ( -Fe O ) was synthesized ac-
2
3
cording to [20]. A weighed portion of FeCl 6H O
3
2
RADIOCHEMISTRY Vol. 49 No. 4 2007

SORPTION OF Np(V), Pu(V), AND Pu(IV) ON COLLOIDS
242
421
topes of different specific activity were used ( Pu,
An + colloid
239
238
Pu, and
Pu). The radioactivity of solutions was
measured using liquid scintillation spectrometry
(TriCarb 2700TR, Packard Ind.).
Dilution (1 M HClO4, pH 1.5)
Filtration
Plutonium in the oxidation state 4+ was stabilized
Dilution (0.6 M HNO3).
Dilution (1.0 M HCl).
by adding a small amount of NaNO to the stock
2
Extraction (0.05 M HDEHP
in heptane)
Extraction (0.5 M TTA
in cyclohexane)
nitric acid solution [23] and was monitored by the
UV spectra (Shimadzu UV-3100).
Solutions of Pu(V) were prepared by electrolytic
method [23]. The completeness of Pu(IV) oxidation to
Pu(V) was controlled by the UV spectra. Just prior to
the sorption experiments, the Pu(V) solution was
treated with 0.05 M HDEHP (Merck) in heptane to
remove traces of Pu(IV) and Pu(VI) formed through
disproportionation of Pu(V) [24].
Organic
phase:
Organic
phase:
An(IV)
Aqueous
phase:
An(V)
Aqueous
phase:
An(V), An(VI)
An(IV), An(VI)
Fig. 1. Flowsheet of extraction separation of actinides to
study their redox speciation.
237
233
Purification of
Np to remove the daughter
Pa
was performed by adding 0.5 M TTA (Lancaster) in
cyclohexane to the stock Np solution in 1 M HNO .
3
The oxidation state of Np was monitored by the
UV spectra.
Sorption Experiment
To eliminate the sorption of the actinides on the
walls and carbonate complexation, the sorption ex-
periments were carried out in plastic vials (20 and
50 ml in volume) under nitrogen atmosphere. All the
experiments were performed at 25 2 C. Goethite,
hematite, maghemite, and manganese dioxide were
used as colloidal suspensions at concentrations of
Fig. 2. Sorption kinetics from Pu(V) solution [c(Pu) =
8
9
10 10
M, c(NaClO ) = 0.1 M] on (1) -FeOOH
4
2
1
2
1
(15 m l ), pH 5.5; (2) -Fe O (10 m l ), pH 5.8;
2
3
2
1
2
1
(3) -Fe O (8 m l ), pH 4.5; and (4) MnO (20 m l ),
2
3
2
pH 4.9.
corded on an MK II VG Scientific electrostatic spec-
1
0.36, 0.44, 0.38, and 0.30 g l , respectively. The sup-
trometer [AlK exciting radiation (1486.6 eV)] in
porting electrolyte was 0.001 0.1 M NaClO . After
7
4
a vacuum (10 Pa) at room temperature [25].
adding the colloid to an actinide-containing solution,
the desired pH was adjusted with dilute NaOH or
RESULTS AND DISCUSSION
HClO . After sorption, the liquid phase was separated
4
by filtration through a 170-nm filter, and its radioac-
tivity was measured. The sorption efficiency was
estimated from the difference between the radionu-
clide concentrations in the initial solution and filtrate.
Sorption Kinetics
We found that the time of establishment of the
dynamic sorption equilibrium for Pu and Np is in-
fluenced by the redox reactions during sorption and by
the diffusion to micropores of the sorbent. For exam-
Pu and Np Redox Speciation
ple, in sorption of Pu(V) at pH 5.0 0.5 on MnO and
2
The actinide redox speciation on the colloid surface
was studied using solvent extraction with TTA and
HDEHP after desorption of the radionuclides with 1 M
goethite, i.e., when redox reactions can occur [10, 12,
14], the dynamic equilibrium is established in several
hours, while in the case of hematite and maghemite
[no redox reactions of Pu(V)], this time increases to
more than 300 h (Fig. 2). In studying the sorption of
HClO according to the scheme presented in Fig. 1
4
[14]. Since the redox speciation of the actinides can
change during the desorption and solvent extraction
stages, the samples containing microgram amounts of
Pu and Np occurring initially as Pu(V) and Np(V)
were examined by XPS. The XPS spectra were re-
2+
U(VI)O on synthetic goethite, amorphous hydrous
2
Fe oxide, hematite, and on natural hematite, Hsi
and Langmuir [26] have demonstrated that the dynam-
ic sorption equilibrium is established in 4 h for
RADIOCHEMISTRY Vol. 49 No. 4 2007

422
KHASANOVA et al.
-FeOOH and Fe(OH) (amorph.) and in 3 days for
3
-Fe O .
2
3
The sorption of Np(V), Pu(V), and Pu(IV) proved
to proceed in two stages. The fast stage involves the
sorption on the outer surface of the sorbents, and the
slow stage, diffusion to the micropores (Figs. 3 and
4). Similar results were obtained in [26 28] for the
metal sorption on minerals. From the published data
+
on the sorption kinetics of NpO on low-crystalline
2
Fe oxide [16], we estimated the diffusion coefficient
13
2
1
7
Fig. 3. Sorption kinetics of Np(V) [c(Np) = 1.5 10 M,
of Np (D = 2.0 10
cm s ), and found that the
s
2
1
c(NaClO ) = 0.1 M] on MnO (55 m l ). pH: (1) 6.0 0.1,
sorption proceeds in two stages, the primary dynamic
equilibrium being established in less than an hour.
4
2
(2) 4.0 0.1, and (3) 2.0 0.1.
In all the systems studied, the sorption rate was
pH-dependent. In sorption of Pu(IV) on MnO from
2
neutral solutions, the equilibrium is established
slightly more slowly as compared to acidic solutions
(Fig. 5). In sorption of Pu(V) on -FeOOH, the sorp-
tion rate increases with increasing pH (Fig. 6). Powell
et al. [12] reported that, in sorption of Pu(V) on goe-
thite at pH 8, the maximal sorption (>90%) is reached
in 30 min, while at pH 6, in 400 min. The authors
also indicated that sorption of Pu(V) on synthetic
hematite was slower than on goethite.
2
1
Fig. 4. Sorption kinetics of Pu(V) on hematite (10 m l
)
8
[c(Pu) = 1.8 10 M, c(NaClO ) = 0.1 M, pH 5.8 0.1].
4
With increasing Pu(V) concentration by three
10
7
orders of magnitude (from 10
to 10 M), the time
of establishment of the dynamic equilibrium of sorp-
tion on maghemite increases, indirectly suggesting
the effect of diffusion of radionuclides to micro- and
mesopores. Similar results were also reported in [15]
for sorption of Pu(V) on MnO from deionized water.
2
The authors have found that, with decreasing Pu(V)
7
10
concentration from 10 to 10
M, the sorption of
Pu increased from 14 to 40% at a sorption time of 2 h,
the time of attainment of the maximal sorption chang-
ing insignificantly.
2
1
Fig. 5. Sorption from Pu(IV) solution on MnO (55 m l
)
2
12
as a function of pH [c(Pu) = 5.9 10
M, c(NaClO ) =
0.1 M] at various sorption times (h): (1) 0.5, (2) 2.5, (3) 25,
(4) 49, and (5) 120.
4
Sorption of Pu(IV)
Figure 7 shows the sorption of Pu(IV) on -FeOOH,
-Fe O , -Fe O , and MnO as influenced by pH.
2
3
2
3
2
The sorption curve on MnO is shifted to the range of
2
acidic solutions against the other sorbents.
The sorption of trace amounts of Pu(IV) on MnO
2
from 0.1 M NaClO was found to be accompanied by
4
slow oxidation to Pu(VI), whereas in the sorption
on the Fe(III) colloids, no change in the Pu(IV) oxida-
tion state was observed. Similar result was obtained
by Morgenstern and Choppin [14] for Pu(IV) sorption
Fig. 6. Sorption from Pu(V) solution on -FeOOH
(10 m l ) as a function of pH [c(Pu) = 4 10 M,
2
1
9
on MnO at pH 8.1. They have demonstrated that,
c(NaClO ) = 0.1 M] at various sorption times (days): (1) 1,
2
4
(2) 3, and (3) 5 (equilibrium).
with increasing concentration of the solid phase in
RADIOCHEMISTRY Vol. 49 No. 4 2007

SORPTION OF Np(V), Pu(V), AND Pu(IV) ON COLLOIDS
the solution, the oxidation rate of Pu increases. This is
423
consistent also with data of [9] on oxidation of Pu(V)
to Pu(VI) in sorption on mixed-valence Mn oxide,
II
IV
ranseite (Ca,Mn )O 4Mn O .
2
Sorption of Np(V) and Pu(V)
The pH dependences of the sorption of Np(V) and
Pu(V) from 0.1 M NaClO are shown in Figs. 8 and 9.
4
The sorption curves of Np on MnO are shifted to
Fig. 7. Sorption from Pu(IV) solutions on (1) -FeOOH
2
2
1
2
1
2
1
the acidic range against those on -FeOOH, -Fe O ,
(15 m l ), (2) -Fe O (10 m l ), (3) MnO (55 m l ),
2 3 2
2
3
2
1
and (4) -Fe O (10 m l ) as a function of pH [c(NaClO )=
and -Fe O , suggesting tighter binding of Np to
2
3
4
2
3
11
0.1 M] at various Pu concentrations (M): (1) 1.2 10
(2) 6.4 10 , (3) 5.9 10 , and (4) 9.3 10
,
MnO . The similar trends are observed for Pu, except
2
12
12
10
.
for the sorption on -FeOOH (in this case, the sorp-
tion curve is also shifted to the acidic range). The lat-
ter fact indirectly reveals the possibility of occurrence
of redox reactions involving Pu(V) in sorption on
-FeOOH.
The liquid extraction data showed that, in sorption
on all the investigated colloids, Np(V) changed its
oxidation state neither in the solution nor on the sorb-
ent surface. In sorption on -FeOOH, Pu(V) retained
its oxidation state in the solution, being reduced to
Pu(IV) on the sorbent surface. Figure 6 shows the
sorption of Pu(V) on goethite as influenced by pH.
With time, the sorption curve drifts toward lower
pH values, which can be considered as an indirect
evidence in favor of a slow change in the Pu oxida-
tion state. In sorption of Pu(V) on hematite and mag-
hemite, the initial oxidation state did not change.
The results obtained by solvent extraction were con-
firmed by the XPS spectra. It was demonstrated that
Np(V) retained its initial oxidation state during sorp-
tion. As for the samples with adsorbed Pu [initially
Pu(V)], we found that it was reduced to Pu(IV) on
the surface of -FeOOH. The XPS experiments are
described in more detail in our previous paper [25].
Similar results were reported in [12]. However, the
mechanism of Pu(V) reduction on goethite and hema-
tite remains to be understood. We observed the reduc-
tion of Pu(V) to Pu(IV) on hematite also, but at a con-
siderably lower rate than on goethite. We have not
established the mechanism of Pu(V) reduction, but
suggested that this reaction could be caused by trace
amounts of Fe(II) occurring in the investigated Fe
oxides. Another suggestion is that both hematite and
goethite, being semiconductors, can carry electrons,
thus affecting the Pu oxidation state.
2
1
Fig. 8. Sorption of Np(V) on (1) -FeOOH (15 m l ),
2
1
2
1
(2) -Fe O (10 m l ), (3) -Fe O (10 m l ), and
(4) MnO (55 m l ) as a function of pH [c(NaClO ) =
0.1 M] at various Np concentrations (M): (1, 2) 7 10
2
2
3
2 3
2
1
4
7
,
7
7
(3) 3 10 , and (4) 1.5 10
.
Fig. 9. Sorption from Pu(V) solutions on (1) -FeOOH
(15 m l ), (2) -Fe O (10 m l ), and (3) MnO
(55 m l ) as a function of pH [c(NaClO ) = 0.1 M] at
various Pu concentrations (M): (1, 3) 1.8 10 and (2) 2
2
2
1
1
2
1
2
3
2
4
8
8
10
.
showed that the ionic strength has no effect on the
sorption curves of Np(V), Pu(V), and Pu(IV), suggest-
ing that inner-sphere complexes are formed on the
colloid surface [29].
Equilibrium Constants of the Sorption Reactions
Sorption of Np and Pu as Influenced by Ionic Strength
Using FITEQL program pack [3], based on Gibbs
energy minimization, we have found that the diffusion
layer model is best suited to reproduce our experi-
The results obtained in the sorption experiments at
various ionic strengths [c(NaClO ) = 0.01 0.5 M)
4
RADIOCHEMISTRY Vol. 49 No. 4 2007

424
KHASANOVA et al.
Table 2. Stability constants of surface complexes of actinides adsorbed on colloids, as estimated by FITEQL
Phase
Reaction
Fe O NpO2 + H+
Fe O Pu3+ + H+
logK
-FeOOH
-FeOOH
-FeOOH
-FeOOH
-FeOOH
-Fe2O3
Fe OH + NpO+2
Fe OH + Pu4+
1.41
2.64
1.93
5.87
11.9
2.12
2.14
2.49
2.37
0.25
Fe OH + Pu4+ + H2O
Fe OH + Pu4+ + 2H2O
Fe OH + Pu4+ + 3H2O
Fe O Pu(OH)2+ + 2H+
Fe O Pu(OH)+2 + 3H+
Fe O Pu(OH)3 + 4H+
Fe OH + NpO+2
Fe OH + PuO+2
Fe OH + NpO+2
Fe OH + NpO+2
Mn OH + NpO+2
Fe O NpO2 + H+
Fe O PuO2 + H+
Fe O NpO2 + H+
Fe O PuO2 + H+
Mn O NpO2 + H+
-Fe2O3
-Fe2O3
-Fe2O3
MnO2
Note: Tilde denotes the surface of colloid particles.
mental data on actinide sorption on the colloidal
oxides. All the possible complexation reactions were
included in calculation, but the formation of mono-
dentate actinide complexes was found to be the most
probable. We estimated the stability constants of
the Np(V) complexes formed in sorption on all the
oxides studied, those of Pu(IV) on goethite, and of
Pu(V) on hematite and maghemite (Table 2). For Np,
the equilibrium constants decrease in the order
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> K
> K
K
. Therefore,
-Fe O
3
Radiochim. Acta, 1996, vol. 74, pp. 173 179.
MnO
-FeOOH
-Fe O
2
2
3
2
the behavior of actinides in the environment is con-
trolled to a considerable extent by the sorption on Mn
oxides. Data on the equilibrium constants may be
used for estimation of the distribution coefficients
in the actual underground water colloid systems.
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The stability constants of the complexes Fe O
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2
tite, are well consistent with those reported by Kohler
et al. [18] (logK = 1.565 and 2.090, respectively).
ACKNOWLEDGMENTS
11. Waite, T.D., Davis, J.A., Payne, T.E., et al., Geochim.
Cosmochim. Acta, 1994, vol. 58, no. 24, pp. 5465
5478.
The study was financially supported by the DOE
RAS Joint Program (project RUC2-20008-MO-04)
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