Sorption of neptunium in the highest oxidation states from alkaline solutions with complexing fibrous "filled" sorbents

Article abstract of DOI:10.1134/S1066362207040169

Sorption of Np(VII), Np(VI), and Np(V) from 1 M NaOH by complexing fibrous "filled" sorbents was examined. POLIORGS 33-n and 34-n sorbents containing amidoxime and hydrazidine groups efficiently recover Np in the highest oxidation states and exhibit good

Full text of DOI:10.1134/S1066362207040169

ISSN 1066-3622, Radiochemistry, 2007, Vol. 49, No. 4, pp. 415 418. Pleiades Publishing, Inc., 2007.
Original Russian Text S.A. Perevalov, N.P. Molochnikova, G.V. Myasoedova, B.F. Myasoedov, 2007, published in Radiokhimiya, 2007, Vol. 49, No. 4,
pp. 364 366.
Sorption of Neptunium in the Highest Oxidation States
from Alkaline Solutions with Complexing Fibrous
Filled Sorbents
S. A. Perevalov, N. P. Molochnikova, G. V. Myasoedova, and B. F. Myasoedov
Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia
Received April 14, 2006
Abstract Sorption of Np(VII), Np(VI), and Np(V) from 1 M NaOH by complexing fibrous filled sorbents
was examined. POLIORGS 33-n and 34-n sorbents containing amidoxime and hydrazidine groups efficiently
recover Np in the highest oxidation states and exhibit good kinetic properties. During sorption, Np(VII) is
reduced with the sorbent to Np(VI) having a higher, under the actual conditions, distribution coefficient.
1
The distribution coefficients, ml g , were estimated at 4.6 103 for Np(VII), 1.4 104 for Np(VI), and
3.6 102 for Np(V).
PACS numbers: 68.43.-h
DOI: 10.1134/S1066362207040169
In recent years, nuclear fuel reprocessing generated
huge amounts of alkaline slurries, whose precipitates
and mother liquors are abundant in various radionu-
clides, including Np, which require reprocessing and
further disposal. The most promising way to solve this
urgent problem is sorption recovery using fibrous
filled complexing sorbents comprised of thin porous
polyacrylonitrile fibers with finely dispersed filler
strongly fixed in their pores. Various sorption materi-
als are suitable as filler: polymeric and mineral ion
exchangers, complexing sorbents, natural tuffs, etc.
Advantages offered by fibrous filled sorbents are
high sorption capacity, fast sorption kinetics ensured
by the porous structure, and selectivity of recovery,
determined by the filler properties [1]. We examined
previously the sorption of actinides, including Np(V),
with fibrous filled sorbents containing amidoxime
and hydrazidine groups from aqueous media and
multicomponent salt-containing alkaline solutions
[2 4]. Depending on the conditions, the oxidation
state of neptunium in solutions varies from III to VII.
In alkaline solutions, Np occurs in the highest oxida-
tion states only, and compounds of Np(VII) are the
most readily soluble in alkalis [5]. Sorption recovery
of Np(VII) and Np(VI) from alkaline solutions is
studied inadequately. Examination of sorption of Np
in the highest oxidation states from strongly alkaline
solutions is of both scientific and practical impor-
tance.
EXPERIMENTAL
237
In our experiments we used
Np containing
241
0.03 wt %
Am. A solution of Np(V) in 1 M NaOH
was prepared by the following procedure: 0.3 ml of
a 48.1 mg ml Np(IV) solution in 4 M HCl was
diluted to 2 ml with distilled water; the resulting solu-
tion was heated on a water bath to 90 C, after which
1
several NaNO crystals were added. The solution
2
acquired an emerald-green color typical for solutions
of Np(V) in acidic media. Then 1 M NaOH was added
in small portions to the resulting solutions to precipi-
tate NpO OH. To achieve complete precipitation, the
2
NaOH solution was added in a small excess. The pre-
cipitate was separated from the mother liquor by
centrifugation and washed with NaOH solution, after
which 10 ml of 1 M NaOH was added. The resulting
suspension was stirred for 3 days using a shaker. Then
the solution containing soluble neptunium hydroxide
was separated from the precipitate by centrifugation.
The Np(V) concentration in the resulting solution was
5
5.49 10 M.
The solution of Np(VI) in 1 M NaOH was prepared
by evaporating virtually to dryness 0.5 ml of a Np(IV)
solution with 3 ml of concentrated HNO added.
The dry residue was dissolved in 3 ml of concentrated
HNO , and the resulting solution was again evap-
orated to wet salts. This yielded a red-brown salt, to
which 1 M NaOH was added. The resulting light pink
3
3
415

416
PEREVALOV et al.
carried out until a maximal D was achieved. The
4
Np(VII) concentration in solution was 8.64 10 M.
The electronic absorption spectra of the Np solu-
tions were taken on Cary 100 (Varian) and UV300
(Unicam Instruments) spectrophotometers.
Figure 1 shows the absorption spectra of the solu-
tions of Np in different oxidation states. The spectra
contain absorption bands characteristic only for
Np(V), Np(VI), and Np(VII).
We used the following fibrous filled sorbents:
POLIORGS 34-n with amidoxime groups, POLIORGS
33-n with amidoxime and hydrazidine groups, and
AV-17-n with AV-17 anion exchanger as filler. The
degree of filling of the fiber with sorbents was 50%.
Sorption was run under static conditions with stirring
of the Np(V), Np(VI), and Np(VII) solutions in 1 M
NaOH with sorbents on a LAB-PU-02 stirring device
for 1 60 min. The solution volume to the sorbent
1
mass ratio (V/m) was 100 ml g .
Fig. 1. Electronic absorption spectra of solutions of Np(V),
The Np concentration in the solutions was deter-
mined by -ray spectrometry. To this end, aliquots of
the solution under study were applied to stainless-steel
targets. The targets were dried under an IR lamp, cal-
cined over an alcoholic burner flame to red heat, and
analyzed on an Alpha Analyst (Canberra) -ray spec-
trometer. The latter was preliminarily calibrated using
5
Np(VI), and Np(VII). [NaOH] = 1, [Np(V)] = 5.49 10
[Np(VI)] = 3.87 10 , and [Np(VII)] = 8.64 10 M.
,
4
4
solution was separated from a scanty precipitate by
centrifugation. The Np(VI) concentration in the solu-
4
tion was 3.87 10 M.
A solution of Np(VII) in 1 M NaOH was prepared
by electrochemical oxidation of Np(VI) solution. To
this end, the solution of Np(VI) in 1 M NaOH, pre-
pared by the above-described technique, was elec-
trolyzed for 40 60 min in an electrochemical cell
at the anode potential of +1.094 V vs. standard hy-
drogen electrode. A platinum grid served as anode,
and a platinum wire, as cathode. The cathode space
was separated from the anode space by a glass frit.
A mercury oxide electrode served as reference elec-
trode; its potential in 1 M NaOH is +0.0984 V vs.
standard hydrogen electrode. The degree of oxidation
of Np was monitored spectrophotometrically from
the optical density D at = 412 nm. Electrolysis was
239
243
242
Pu (5155 keV),
Am (5275 keV), and
Cm
(6112 keV). The background of the -ray spectrom-
eter at energies within 3 7 MeV is 5 10 Bq.
5
The degree of recovery, %, of Np was determined
from its content in the solution before and after sorp-
tion; the distribution coefficients K of Np were calcu-
d
lated by the standard formula.
The reagents were of no less than analytically pure
grade.
RESULTS AND DISCUSSION
The focus of this study was sorption of Np(VII)
from alkaline solutions, which was studied to the least
extent compared to Np in other oxidation states.
The most probable species of Np(VII) and Np(VI)
Table 1. Sorption of Np(VII), %, with filled complexing
sorbents in relation to the phase contact time (V /m =
100 ml g , [NaOH] = 1 M)
1
3
ions in strongly alkaline solutions are NpO (OH)
4
2
2
Time,
min
POLIORGS
34-n
POLIORGS
33-n
and NpO (OH) , respectively [6, 7]. Presumably,
AV 17-n
2
4
Np(V) also forms a similar anionic hydroxo complex
NpO (OH) . It was of interest to examine the sorption
2
2
5
10
30
60
86
89
98
99
84
87
98
98
0
4
18
18
recovery of Np in the highest oxidation states from
alkaline solutions with POLIORGS 34-n and 33-n
complexing sorbents, as well as with AV-17-n highly
basic anion-exchanging sorbent.
RADIOCHEMISTRY Vol. 49 No. 4 2007

SORPTION OF NEPTUNIUM IN THE HIGHEST OXIDATION STATES
Table 2. Stability of Np(VII) in contact with the polyacrylonitrile matrix
Phase contact time, min
417
Parameter
0
5
10
20
30
60
120
Np concentration, mM
1.76
2.078
100
1.74
1.892
89.3
1.73
1.731
80.1
1.76
1.539
69.1
1.74
1.378
59.9
1.78
0.927
34.0
1.73
0.333
0
Optical density at
= 412 nm
Np(VII) content in solution, rel. %
1
Table 3. Distribution coefficients, ml g , of Np(VII),
Np(VI), and Np(V) in sorption on POLIORGS 34-n from
1 M NaOH in relation to the phase contact time. V /m =
rapid reduction of Np(VII) on the sorbent. Therefore,
in subsequent experiments we used POLIORGS 34-n.
Also, we examined how the stability of Np(VII) is
influenced by the polyacrylonitrile matrix of the
sorbent without filler. Table 2 presents the data char-
acterizing the stability of Np(VII) in 1 M NaOH in
the presence of polyacrylonitrile fiber. We found that
the latter reduces Np(VII) to Np(VI), but much more
slowly in the case of the fiber without filler compared
to the filled fiber, and the green color of the solutions
is preserved for 1 h. As seen form Table 2, Np is not
sorbed by the polyarcylonitrile fiber (the Np concen-
tration in solution remains unchanged), and the optical
density of the solution at = 412 nm tends to mono-
tonically decrease, which suggests reduction of
Np(VII) to Np(VI).
1
100 ml g
Phase contact time,
Np(VII)
Np(VI)
Np(V)
min
1
2
102
225
180
500
680
33
39
3
372
42
5
596
2550
4600
10470
13970
50
10
30
60
807
4220
4621
130
252
361
Table 1 presents the results of sorption of Np(VII)
from 1 M NaOH on POLIORGS 34-n, POLIORGS
33-n, and AV-17-n polymeric sorbents. It is seen that
high degrees of recovery of Np(VII) by sorption with
the complexing sorbents were achieved within 5
10 min, whereas the Np(VII) sorption by AV-17-n
anion-exchanging sorbent in this period is negligible.
In the case of sorption on POLIORGS 34-n sorbent,
a green color of the solution and the sorbent, typical
for Np(VII) in 1 M NaOH, is preserved during 5
10 min, whereas in the case of POLIORGS 33-n con-
taining hydrazidine groups the green color of the solu-
tion and sorbent disappears virtually immediately after
the phases are brought into contact. This suggests
We examined the degree of recovery of Np(VII),
Np(VI), and Np(V) with POLIORGS 34-n sorbent
from 1 M NaOH as influenced by the phase contact
time. Figure 2 and Table 3 show that, under the actual
conditions, Np(VI) is sorbed most efficiently. This
suggests that, under these conditions, POLIORGS
34-n sorbs Np(VI) preferably to Np(VII) and especial-
ly to Np(V). This is also confirmed by the fact that,
after a 10-min contact of the solutions with the sorb-
1
ent, the distribution coefficient, ml g , of Np(VI),
3
4.6 10 , is higher than those of Np(VII) and Np(V):
2
2
8.1 10 and 1.3 10 , respectively (Table 3).
The experiments on sorption of Np in the highest
oxidation states from 1 M NaOH with fibrous filled
sorbents POLIORGS 34-n and 33-n showed that high
distribution coefficients of Np can be achieved within
the phase contact time of 5 10 min. Sorption of
Np(VII) under identical conditions with highly basic
anion exchanger AV-17-n is insignificant. During
sorption, Np(VII) is reduced to Np(VI) with the filler
and the polyacrylonitrile matrix simultaneously.
The reduction with the unfilled matrix is much slower
than that with the filled matrix, and no sorption of Np
with the unfilled matrix is observed.
Fig. 2. Sorption, %, of Np in different oxidation states in
Our data indicate that complexing fibrous filled
sorbents with amidoxime and hydrazidine groups
relation to the phase contact time. [NaOH] = 1 M, V/m =
1
100 ml g
.
RADIOCHEMISTRY Vol. 49 No. 4 2007

418
PEREVALOV et al.
show promise for sorption recovery of Np in the
highest oxidation states from strongly alkaline solu-
tions.
va, N.P., and Lileeva, L.V., Zh. Anal. Khim., 2000,
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5. Krot, N.N., Gel’man, A.D., Mefod’eva, M.P., et al.,
Semivalentnoe sostoyanie neptuniya, plutoniya, ameri-
tsiya (Heptavalent State of Neptunium, Plutonium, and
Americium), Moscow: Nauka, 1977.
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RADIOCHEMISTRY Vol. 49 No. 4 2007