Science in China Series B: Chemistry
© 2007
SCIENCE IN CHINA PRESS
Springer
Properties of TRPO-HNO3 complex used for direct
dissolution of lanthanide and actinide oxides in
supercritical fluid CO2
DUAN WuHua†, ZHU LiYang, JING Shan & ZHU YongJun
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 102201, China
The mixed trialkylphosphine oxide-nitric acid (TRPO-HNO3) complex prepared by contacting pure TRPO
with concentrated HNO3 may be used as additives for direct dissolution of lanthanide and actinide oxides in the supercritical fluid carbon dioxide (SCF-CO2). Properties of the TRPO-HNO3 complex have
been studied. Experimental results show when the initial HNO3/TRPO volume ratio is varied from 1:7 to
5:1, the concentration of HNO3 in the TRPO-HNO3 complex changes from 2.12 to 6.16 mol/L, the
[HNO3]/[TRPO] ratio of the TRPO-HNO3 complex changes from 0.93 to 3.38, and the content of H2O in
the TRPO-HNO3 complex changes from 0.97% to 2.70%. All of the density, viscosity and surface tension
of the TRPO-HNO3 complex change with the concentration of HNO3 in the complex. The protons of
HNO3 and H2O in the complex undergo rapid exchange to exhibit a singlet resonance peak in NMR
spectra with D2O insert. When the TRPO-HNO3 complex dissolves in a low dielectric constant solvent,
small droplets of HNO3 appear which can be detected by NMR.
TRPO-HNO3 complex, property, supercritical fluid carbon dioxide, dissolution
Nuclear energy generation is primarily based on uranium fuels, and U-235 is the energy-producing fission
source. Fission products and transuranium elements Np,
Pu, Am, and Cm are formed during nuclear energy generation. Reprocessing of the spent nuclear fuel for extraction of plutonium (Pu) and uranium (U) is conventionally carried out by the PUREX process, which involves the dissolution of the spent nuclear fuel in nitric
acid followed by solvent extraction of U and Pu from
the acidic solution using tri-n-butyl phosphate (TBP) as
extractant in kerosene dilutent. However, a large amount
of high-level radioactive liquid wastes containing fission
products and actinide elements is produced in the
PUREX process. The cost for treatment of the liquid
wastes is high.
A mixed trialkylphosphine oxide (TRPO) has been
used as extractant to extract the actinides together with
lanthanides fission products from high-level radioactive
－
liquid wastes in the TRPO process developed in China[1 4].
www.scichina.com
www.springerlink.com
The TRPO extractant has good physical properties, good
separation factors, high radiation stability and sufficiently high loading capacity. In addition, it is commercially available, and its cost is low[5].
Recently, TBP-HNO3 complex has been found to be
effective for the dissolution of solid UO2 and U3O8 as
well as lanthanide oxides in supercritical fluid carbon
－
dioxide (SCF-CO2)[6 9]. The U (Ⅳ) in solid UO2 is possibly oxidized to U (Ⅵ) by HNO3 in the TBP-HNO3
complex and dissolves in SCF-CO2 as UO2(NO3)2·2TBP.
The metallic nitrate complex with TBP is finally recovered by depressurizing to atmospheric pressure. The
process does not involve any aqueous solution to dissolve the uranium oxides and leads to minimum generation of the wastes; therefore, it is an attractive process
Received December 26, 2006; accepted January 28, 2007
doi: 10.1007/s11426-007-0081-1
†
Corresponding author (email: [email protected])
Supported by the National Natural Science Foundation of China (Grant No.
20506014)
Sci China Ser B-Chem | Dec. 2007 | vol. 50 | no. 6 | 759-763
for the spent nuclear fuels reprocessing and decontamination of uranium-contaminated solid waste[10,11].
Based on the above facts, it seems reasonable that a
TRPO-HNO3 complex can also be used to direct dissolution of lanthanide and actinide oxides in SCF-CO2. In
order to understand the nature and mechanisms of lanthanide and actinide oxides dissolution in SCF-CO2 containing TRPO-HNO3 complex, the properties of TRPOHNO3 complex prepared under different initial HNO3/
TRPO volume ratios were firstly investigated.
1 Experimental method
1.1 Materials
TRPO (commercial name Cyanex 923) was purchased
from American Cyanamid Company. According to the
producer, TRPO extractant comprises a mixture of four
trialkylphosphine oxides, with the general formula R3PO
(14%), R2R′PO (42%), RR′2PO (31%) and R′3PO (8%),
in which R and R′ denote n-octyl and n-hexyl group respectively[5]. In order to decrease the content of H2O in
TRPO (about 4%), TRPO was further purified by vacuum distillation. Through purification, the content of
H2O in TRPO was about 0.2%. The main physical properties of TRPO are shown in Table 1.
Table 1 Properties of TRPO
Properties
Average molecular mass
Color
Density (25℃, g·cm−3)
Viscosity (25℃, Pa⋅s)
Surface tension (25℃, N·m−1)
Water content (%)
Flash point (℃)
Freezing point (℃)
Boiling point (℃)
Solubility of water in TRPO (mg·L−1)
Solubility of TRPO in water (mg·L−1)
Value
348
colorless
0.879
0.0328
27.5×10−3
0.2
182
−5
180－225
8
10
HNO3 (analytical grade, 15.5 mol/L) was obtained
from Beijing Chemical Plant, China. NaOH (analytical
grade) was obtained from Tainjin Unionlab Chemical
Reagent Ltd., China.
1.2 Procedures
The TRPO-HNO3 complex was prepared by vigorously
mixing TRPO with 15.5 mol/L HNO3 at a chosen initial
volume ratio in a stoppered glass test tube for 30 min,
followed by centrifuging for 1 h. After phase separation,
760
both the TRPO and the aqueous phase were sampled for
property measurements. The initial HNO3/TRPO volume
ratio ranged from 1:7 to 5:1. When the initial HNO3/
TRPO volume ratio was lower than 1:7, all the aqueous
phase dissolved in the organic phase, and the initial
HNO3/TRPO volume ratio lower than 1:7 was excluded.
1.3 Analytical methods
The concentration of HNO3 in the aqueous phase was
determined by NaOH titration. The concentration of
HNO3 in the TRPO-HNO3 complex was measured with
0.1 mol/L NaOH solution after an excessive amount of
deionized water was added to the organic phase. The
content of H2O in the TRPO-HNO3 complex was determined by Karl Fischer titration using a 787 KF Titrino
instrument (Metrohm Ltd., Switzerland). The density of
the TRPO-HNO3 complex was calculated by weighing a
known volume of sample in triplicated runs with Mettler
AE 200 balance (Mettler Toledo, Switzerland). Thus, the
concentration of TRPO in the complex was calculated by
combining the density and the concentrations of both
H2O and HNO3. The viscosity and the surface tension of
the TRPO-HNO3 complex were measured by Visco Basic Plus viscometer (Fungilab, Spain) and by Model
BZY surface tensiometer (Shanghai Henping Instrument
& Meter Factory, China), respectively. A 300 MHz
NMR spectrometer (JOEL JNM-ECA300) was used for
proton NMR measurements of the TRPO-HNO3 complex.
2 Results and discussion
2.1 Concentration of HNO3 and molar ratio of
HNO3/TRPO in the TRPO-HNO3 complex
The effect of the initial HNO3/TRPO volume ratio on
the acidity of both phases is shown in Figure 1. From
Figure 1, it is shown that when the initial HNO3/TRPO
volume ratio changes from 1:7 to 5:1, the concentration
of HNO3 in the TRPO-HNO3 complex increases from
2.12 to 6.16 mol/L, and the acidity of the equilibrated
aqueous phase increases from 1.55 to 15.18 mol/L. The
high acid content of the TRPO-HNO3 complex is important for dissolving lanthanide and actinide oxides in
SCF-CO2.
From Figure 2, it is observed when the concentration
of HNO3 in the TRPO-HNO3 complex changes from
DUAN WuHua et al. Sci China Ser B-Chem | Dec. 2007 | vol. 50 | no. 6 | 759-763
Figure 1 Concentration of HNO3 in the organic and aqueous phases.
Figure 3 Content of H2O in TRPO-HNO3 complex (25℃).
2.3 Density, viscosity and surface tension of the
TRPO-HNO3 complex
Density, viscosity and surface tension of the TRPOHNO3 complex are shown in Table 2. With increase of
the concentration of HNO3 in the TRPO-HNO3 complex
from 2.12 to 6.16 mol/L, density of the TRPO-HNO3
complex increases from 0.941 to 1.051 g/cm3, viscosity
decreases from 42.3 to 12.71 mPa⋅s, and surface tension
increases from 29.5 to 32.3 mN/m.
Figure 2 Molar ratio of HNO3/TRPO in the TRPO-HNO3 complex. The
subscript ‘O’ represents the TRPO-HNO3 complex phase.
2.12 to 6.16 mol/L, the molar ratio of [HNO3]/[TRPO]
in the TRPO-HNO3 complex increases from 0.93 to
3.38.
Inorganic acids such as HNO3 which is usually insoluble in CO2 become soluble by the complexation
with a CO2-soluble Lewis base such as TRPO. The approach of Lewis acid-base complex provides a method
of dispersing various CO2-insoluble acids in the
SCF-CO2 phase for chemical reactions with metal oxides[6].
Table 2 Density, viscosity and surface tension of the TRPO-HNO3 complex (25℃)
[HNO3]O
Density
Viscosity
Surface tension
(mol·L−1)
(g·cm−3)
(mPa·s)
(mN·m−1)
6.16
1.051
12.71
32.3
6.12
1.051
13.06
32.3
5.95
1.050
13.54
32.3
5.59
1.042
15.78
32.3
5.12
1.027
18.18
32.1
4.23
0.999
24.46
31.5
3.55
0.982
29.63
31.2
3.02
0.968
32.01
30.8
2.65
0.956
34.30
30.4
2.45
0.949
36.39
29.8
2.12
0.941
42.30
29.5
2.2 Content of H2O in the TRPO-HNO3 complex
From Figure 3, it is observed the content of H2O in the
TRPO-HNO3 complex firstly decreases from 1.39% to
0.97% with increase of the concentration of HNO3 in the
TRPO-HNO3 complex from 2.12 to 2.65 mol/L, and
then increases from 0.97% to 2.70% with increase of the
concentration of HNO3 in the TRPO-HNO3 complex
from 2.65 to 6.16 mol/L. When the concentration of
HNO3 in the TRPO-HNO3 complex is low, the competition of HNO3 makes the content of H2O in the
TRPO-HNO3 complex decrease. While the concentration
of HNO3 in the TRPO-HNO3 complex is higher, complex with higher content of H2O is further formed.
2.4 NMR spectroscopy of the TBP-HNO3 complex
The 1H NMR spectra of the TRPO-HNO3 complex at
room temperature were taken using a deuterium locked
technique[12,13]. Deuterated water (D2O) was placed in an
insert and fitted into an NMR tube containing the
TRPO-HNO3 complex solution. The purpose of the D2O
insert was to lock the NMR. The objective of the NMR
study is to investigate the chemical shift of the protons
of HNO3 and H2O in the TRPO-HNO3 complex as a
probe for the chemical environment of the system.
The typical NMR spectra of water saturated TRPO
and the TRPO-HNO3 complex with a D2O insert are
DUAN WuHua et al. Sci China Ser B-Chem | Dec. 2007 | vol. 50 | no. 6 | 759-763
761
shown in Figures 4 and 5 respectively. From Figure 4, it
is observed that the singlet peak at δ 4.22 corresponds to
the proton of H2O in water saturated TRPO. From Figure 5, it is observed that the singlet peak at δ 12.22 corresponds to the proton of HNO3 and H2O in the
TRPO-HNO3 complex. The singlet peak at δ 4.71 in
Figures 4 and 5 corresponds to the proton of H2O in D2O.
Samples prepared from different volume ratios of
TRPO/HNO3 were studied. All NMR spectra in this series of samples show a singlet peak for the protons corresponding to HNO3 and H2O. The NMR shift of the
HNO3-H2O singlet peak with respect to the concentration of HNO3 in the TRPO-HNO3 complex from 2.12 to
6.16 mol/L is shown in Figure 6. The chemical shift increases firstly with increase of the concentration of
HNO3 in the TRPO-HNO3 complex, and then decreases
slightly with increase of the concentration of HNO3 in
the TRPO-HNO3 complex; finally, the chemical shift of
this peak approaches constant when the concentration of
HNO3 in the TRPO-HNO3 complex is higher than 5.95
mol/L.
We chose to use deuterated chloroform (CDCl3) as
Figure 4
1
H NMR spectrum of water saturated TRPO with D2O insert.
Figure 5 1H NMR spectrum of the TRPO-HNO3 complex with D2O
insert. The initial HNO3/TRPO volume ratio is 1/1.
762
Figure 6 Variation of chemical shift of H2O proton peak with [HNO3]O
in the TRPO-HNO3 complex.
solvent to evaluate the antisolvent effect because chloroform has a small dielectric constant at room temperature. The NMR spectrum of the TRPO-HNO3 complex
in CDCl3 is given in Figure 7. The NMR spectrum
shows two peaks in addition to the protons in the butyl
group of TRPO. The peak at δ 6.84 in Figure 7 is the
peak belonging to the nitric acid droplets formed in the
system. The peak at δ 14.14 in Figure 7 is the peak representing the averaged protons of HNO3 and H2O in the
complex. The nitric acid droplets are not dissolved in the
CDCl3 solution. The peak was identified by a separate
NMR study using different concentrations of HNO3
mixed with CDCl3. The nitric acid peak in CDCl3 tends
to shift upfield with increasing HNO3 concentration in
the acid solution. The NMR results obtained from the
CDCl3-TRPO-HNO3 complex system, though qualitative in nature, demonstrate the formation of nitric acid
droplets when TRPO-HNO3 complex dissolves in a low
dielectric constant solvent as a result of an antisolvent
effect. The acid droplets probably start as very small
droplets and aggregate to certain sizes that would make
the solution cloudy.
Figure 7 1H NMR spectrum of the TRPO-HNO3 complex in CDCl3. The
initial HNO3/TRPO volume ratio is 1/4.
DUAN WuHua et al. Sci China Ser B-Chem | Dec. 2007 | vol. 50 | no. 6 | 759-763
3 Conclusion
Some properties of the TRPO-HNO3 complex, including
HNO3 concentration, H2O content, density, viscosity,
surface tension, and 1H NMR, have been studied. The
complex allows the dispersion of high concentration
HNO3 for dissolving lanthanide and actinide oxides in
the supercritical fluid phase. H2O in the complex may
facilitate the dissolution of the oxides. The protons of
1
Zhu Y J, Song C L. Recovery of Np, Pu, and Am from highly active
HNO3 and H2O in the complex undergo rapid exchange
to exhibit a singlet resonance peak in NMR spectra.
When the TRPO-HNO3 complex dissolves in a low dielectric constant solvent (as chloroform), small droplets
of HNO3 may appear. This phenomenon should be paid
more attention to in further studies.
The authors thank Mr. Yang Haijun for NMR analyses of TRPO-HNO3
complex.
8
of UO2 in supercritical CO2 containing a CO2-philic complexant of
Transuranium Elements: A Half Century. Washington, DC: American
tri-n-butylphosphate and nitric acid. Ind Eng Chem Res, 2002, 41(9):
Chemical Society, 1992. 318－330
2
3
Zhu Y J, Jiao R Z. Chinese experience in the removal of actinides
9
F, Wai C M. Dissolution of actinide oxides in supercritical fluid car-
Technol, 1994, 108: 361－369
bon dioxide containing various organic ligands. J Nucl Sci Technol,
Wang J C, Song C L. Hot test of trialkyl phosphine oxide (TRPO) for
10
6
Decontamination of uranium oxides from solid wastes by supercritical
Song C L, Jin G Y, Wang J C, Zhu Y J. Treatment of Chinese HLLW
CO2 fluid leaching method using HNO3-TBP complex as a reactant. J
Euzebiusz D, Jan S. Composition of CYANEX 923, CYANEX 925,
11
Supercrit Fluid, 2004, 31: 141－147 [DOI]
Shimada T, Ogumo S, Ishihara N, Kosaka Y, Mori Y. A study on the
CYANEX 921 and TOPO. Solvent Extr Ion Exc, 1998, 16(6):
technique of spent fuel reprocessing with supercritical fluid direct
1515－1525[DOI]
extraction method (Super-DIREX method). J Nucl Sci Technol, 2002,
Tomioka O, Meguro Y, Enokida Y, Yamamoto I, Yoshida Z. Dissolution behavior of uranium oxides with supercritical CO2 using
7
2002, (Suppl 3): 263－266
Meguro Y, Iso S, Yoshida Z, Tomioka O, Enokida Y, Yamamoto I.
(HLLW). Solvent Extr Ion Exc, 2001, 19(2): 231－242[DOI]
by partitioning. Tsinghua Sci Technol, 1996, 1(1): 81－85
5
2282－2286 [DOI]
Samsonov M D, Trofimov T I, Vinokurov S E, Lee S C, Myasoedov B
from highly active waste by trialkylphosphine-oxide extractant. Nucl
removing actinides from highly saline high-level liquid waste
4
Enokida Y, El-Fatah S A, Wai C M. Ultrasound-enhanced dissolution
waste: trialkylphosphine oxide extraction. In: Morss L R, Fuger J. eds.
12
(Suppl): 757－760
Enokida Y, Tomioka O, Lee S C, Rustenholtz A, Wai C M. Charac-
HNO3-TBP complex as a reactant. J Nucl Sci Technol, 2001, 38(12):
terization of a tri-n-butyl phosphate-nitric acid complex: a CO2-
1097－1102
soluble extraction for dissolution of uranium dioxide. Ind Eng Chem
Tomioka O, Enokida Y, Yamamoto I. Selective recovery of neodymium from oxides by direct extraction method with supercritical CO2
13
Res, 2003, 42(21): 5037－5041 [DOI]
Samsonov M D, Wai C M, Lee S C, Kulyako Y, Smart N G. Dissolu-
containing TBP-HNO3 complex. Sep Sci Technol, 2002, 37(5):
tion of uranium dioxide in supercritical fluid carbon dioxide. Chem
1153－1162[DOI]
Commun, 2001, 1868－1869
DUAN WuHua et al. Sci China Ser B-Chem | Dec. 2007 | vol. 50 | no. 6 | 759-763
763