Table Of ContentLA-1U91
UC-10
Issued: February 1988
Comparative Evaluation of DHDECMP and
CMPO as Extractants for Recovering
Actinides from Nitric Acid
Waste Streams
LA—11191
S. Fredric Marsh
Stephen 1. Yarbro DE88 °07154
'hose
of
the
Los Alamos National Laboratory
i Los Alamos,New Mexico 87545
COMPARATIVE EVALUATION OF DHDECMP AND CMPO AS EXTRACTANTS
FOR RECOVERING ACTINIDES FROM NITRIC ACID WASTE STREAMS
by
S. Fredric Marsh and Stephen L. Yarbro
ABSTRACT
Certain neutral, bifunctional organophosphorus compounds are of
special value to the nuclear industry. Dihexyl-N,N-diethylcarbamoyl-
methylphosphonate (DHDECMP) and octylphenyl-N,N-diisobutylcar-
bamoylmethylphosphine oxide (CMPO) are highly selective extractants
for removing actinide and lanthanide elements from nitric acid. We ob-
tained these two extractants from newly available commercial sources
and evaluated them for recovering Am(III), Pu(IV), and U(VI) from
nitric acid waste streams of plutonium processing operations. Vari-
ables included the extractant (DHDECMP or CMPO), extractant/tri-
butylphosphate ratio, diluent, nitrate concentration, nitrate salt/nitric
acid ratio, fluoride concentration, and contact time. Based on these ex-
perimental data, we selected DHDECMP as the preferred extractant for
this application.
INTRODUCTION Other investigators13 developed a reliable but te-
dious laboratory-scale procedure suitable for prepar-
Numerous investigators have studied carbamoyl- ing small quantities of pure DHDECMP for analyti-
methylphosphoryl derivative extractants during the cal applications.14 However, repurification of the large
past, decade.1"6 Of these compounds, carbamoyl- quantities of extractant required for plant-scale appli-
methylphosphonates and carbamoylmethylphosphine cations is impractical. The implementation of a full-
oxides appear to be the most selective structures for scale nuclear materials process based on DHDECMP
extracting actinido and lanthanide elements from ni- has therefore had to await a reliable source of pure
tric acid. Of particular significance is the high ex- extractant.
tractability of actinides in their (III). (IV). and (VI) High-purity DHDECMP recently became available
oxidation states, a fact that often eliminates the usual from a new commercial supplier who uses a patented
need for prior oxidation state adjustment. phase-transfer catalysis process15 to synthesize this
Previous published studies have demonstrated the compound. The advent of this new commercial
potential value of dihexyl-N.N-diethylearbamoyl- source prompted us to reevaluate DHDECMP for
methylphosphonate (DHDECMP) for full-scale nu- removing actinides from nitric acid waste streams
clear materials processes. Because no commercial sup- at Los Alamos. We also elected to evaluate the
pliers of pure DHDECMP existed until very recently, performance of oct ylphenyl-N.N- diisohutylcarbamoyl-
these studies used impure or purified DHDECMP in methylphosphinr oxide (CMPO). an extractant se-
con vent ional .solvent extraction7"11 and process-scale lected for the proposed TRUEX process.16 Compar-
extraction chromatography12 systems. ative performance data for both extractants. whose
structures are shown in Fig. 1, were determined over EXPERIMENTAL
a wide range of variables that encompass the compo-
sition of typical process feed streams. General
We measured all distribution coefficients for the ini-
REAGENTS AND APPARATUS tial contact of an aqueous solution and organic solu-
tion of the specified compositions. Neither the aqueous
DHDECMP, obtained from Occidental Chemical nor organic solution was pre-equilibrated. Thus, the
Corporation, Specialty Products Division, Niagara measured distribution coefficients represent the initial
Falls, New York, was used without additional purifi- solvent extraction stage rather than a later stage at
cation. which each phase has become equilibrated with the
CMPO, obtained from M&T Chemicals, Inc., Rah- other. This distinction is emphasized because some
way, New Jersey, was purified by contact with cation published distribution coefficients are based on mea-
exchange resin, anion exchange resin, and carbon- surements using pre-equilibrated solutions.
ate washing, the MIX procedure of Horwitz and
Gatrone.17
Tetrachloroethylene (TCE), obtained from J. T. Extractions
Baker Chemical Co., Phillipsburg, New Jersey, was
used as received. We combined 5-milliliter portions of each combi-
Isopar H, a highly purified mixture of Cg to C12 nation of extractant solution and aqueous mixture
isoalkanes, was used as received from the manufac- in individual 1-ounce polyethylene bottles. A 0.100-
turer. Exxon Chemical Company, Houston, Texas. milliliter portion of a Pu(IV) stock solution in 7 M ni-
Reagent-grade nitric acid, sodium nitrate, and hy- tric acid was added to each bottle immediately before
drofluoric acid were used as received. extraction. (All aqueous solution compositions were
Extraction bottles, 1-ounce, high-density polyethy- calculated and prepared to provide the desired acid
lene with 18-millimeter caps (catalog number 16054- concentrations after addition of the 0.7 millimole of
010, were obtained from VWR Scientific, San Fran- nitric acid in the plutonium stock solution.)
cisco. California. The addition of 0.100 milliliter of stock solution
Dispensing spouts (catalog number F12638-0018) to to each aqueous solution provided a known, constant
fit the 1-ounce bottles with 18-millimeter caps were ob- amount of plutonium plus its 237U and 241Am daugh-
tained from Bel-Art Products, Pequannock, New Jer- ters. The plutonium content of the extracted solu-
sey. tions was always about 4 grams per liter; americium
A Burreli model 75 wrist-action shaker (catalog content was about 15 micrograms per liter, and ura-
number 57040-049) was obtained from VWR Scien- nium was present only at trace concentrations. The
tific, San Francisco. California. oxidation states of these three actinides immediately
9 9 before extraction were Am(IIl), Pu(IV), and U(VI).
Any significant change in oxidation state during the
CsH-17> P-CH -C-N < brief extraction period was unlikely.
2
CH CH(CH ) We sealed the polyethylene extraction bottles that
2 3 2
contained each aqueous/organic combination with
solid Polyseal caps, and the contents were dynami-
cally contacted for a specified time using a wrist-action
CMPO
shaker. Each bottle and its contents then were allowed
to stand for at least 5 minutes to allow the phases to
o o separate. Following phase separation, we removed the
solid cap and replaced it with a dispensing spout cap,
C H shown in Fig. 2.
2 5
C H O |J_CH -C-N C H
6 13 2 2 5
DHDECMP
Fig. 1. Chemical structures of ortylphnnyl-N.N-diisobutylcar-
bamoylmpthylphosphinp oxide (CMPO) and dihrxyl-N.N-
diethylcarbamoylmethylphosphonate (DHDEC'MP).
applying gentle pressure until the phase boundary
approached the end of the dispensing tip.
5. The few drops that contained the phase boundary
were discarded into a waste container along with
the upper phase (if it was organic).
6. If the upper phase was aqueous, it was dispensed
into a vial for assay.
Gamma Assay Technique
We assayed the extracted aqueous portions in all
cases using gamma spectrometry and compared them
with an identical unextracted aqueous portion. The
difference between these two measurements, for each
actinide, represented the quantity extracted. Gamma
spectrometric assays were based on the 59.5- keV, 129-
keV, and 208-keV gamma-ray peaks of 241 Am, 239Pu,
and 237U, respectively.
Computation of Distribution Coefficient (Kd)
Fig. 2. Polyethylene bottle, with a solid cap for extraction
Values
and with dispensing spout cap as a separatory funnel, used to
separate the two phases.
We computed Kds for each extraction using the re-
The lower, aqueous phase was transferred from the lationship
extraction bottle to an appropriate assay container as
described in the following steps: concentration of actinide in organic phase
Kd =
concentration of actinide in aqueous phase
1. The dispensing tip of the upright extraction bottle
was inserted into an inverted liquid scintillation vial Using gamma spectrometry, we indirectly deter-
(the assay container). mined the concentration of each actinide extracted into
the organic phase by measuring the difference in its
concentration before and after extraction in otherwise
2. While still being held upright, the extraction bottle identical aqueous portions.
was squeezed slightly to expel a few milliliters of
air.
Extractants Tested
3. With the spout tip of the extraction bottle still in
the vial, both bottles were inverted to restore the The value of DHDECMP for extracting actinides
vial to an upright position while pressure was simul- has been recognized for many years. More recently, re-
taneously released on the extraction bottle. (The searchers have developed and proposed CMPO as the
incoming air removed any of the upper phase that extractant for the TRUEX process.16 CMPO is un-
otherwise could remain in the spout tip as the ex- questionably a stronger extractant than DHDECMP;
traction bottle is inverted.) however, the higher extractability of actinides makes
back-extraction from CMPO more difficult. CMPO
is expected to offer superior chemical and radiolytic
4. While the extraction bottle was in the inverted po- stability and also lower solubility in aqueous nitrate
sition, the lower phase was expelled from the ex- solutions. DHDECMP. on the other hand, is more
traction bottle into the assay vial (if it was aque-
selective in rejecting common impurity elements, as
ous) or into a waste container (if it was organic) by
shown in Fig. 3.
3
H
• RETAINED
Li Be O ELUTED B C N O F
SL 0
• PARTIAL O
Na M Al Si P S Cl
g
O O
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br
O O
O O • • O O O O O O
Rb Sr Y Zr Nb Mo Tc Ru Rb Pd Cd In Sn Sb Te I
O
O • _•_! wO • • • O O
Cs Ba La Hf Ta Re Os Ir Pt Au Hg Tl Pb Bi Po At
o
O • • O O • O O •
Fr Ra Ac
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
• • • •
Fig. 3. Distribution of many elements on DHDECMP extraction chromatography column from 5.5 M nitric acid.14
Because each extractant appears to offer unique record of safe operation with such kerosene-type dilu-
advantages and disadvantages, we elected to evalu- ents within the nuclear industry attests to the ade-
ate both under identical conditions. The only signifi- quacy of this safety margin. Moreover, Isopar H is an
cant difference was that CMPO solutions were 0.25 M, order of magnitude less volatile than TCE under iden-
whereas DHDECMP solutions were 0.75 M. Despite tical conditions. It provides rapid and complete phase
the threefold-higher concentration of DHDECMP, the separation after extraction and is nontoxic. This com-
current threofold-lower price of DHDECMP made the bination of favorable properties prompted us to also
overall costs of the two extractants essentially equal. evaluate Isopar H as a diluent.
The present prices of both oxtractants reflect the small
current market; however, the cost of either extractant
would be expected to drop substantially as a larger Other Variables Tested
market develops.
We evaluated total nitrate concentrations of 0.45
M, 1.5 M, 4.5 M, and 7.5 M. Each nitrate level was
Diluents Tested evaluated as 100% nitric acid, as 2/3 nitric acid and
1/3 sodium nitrate, and as 1/3 nitric acid and 2/3
We evaluated tetrachloroethylene (TCE) as a dilu- sodium nitrate. Each of the 12 combinations of total
ent because researchers have proposed it an the dilu- nitrate and nitric acid/nitrate salt ratio was evaluated
ent for the TRUEX process, primarily because TCE is at fluoride concentrations of zero. 0.02 M, and 0.05 M.
noniiammable. The disadvantages of TCE include its
Evaluation of the 36 different aqueous compositions
propensity for forming persistent organic dispersions
with each of the 4 extractant/dilueut combinations
in the aqueous phase, its high volatility, its classifica-
produced Kd values of Am(III), Pu(IV). and U(VI)
tion as a carcinogen, and its ability to form radiolysis
for each of the 144 unique aqueous/organic combina-
products that are corrosive to stainless steel process
tions.
environments.
Isopar H. although flammable, lias a sufficiently
high flash point to make ignition unlikely. The long
RESULTS AND DISCUSSION minutes were equivalent to those obtained for a 1-
minute contact period. Based on this equivalency, the
General remainder of this study employed 1-minute dynamic
contact periods,
Experimental distribution data usually are pre-
sented in a format that illustrates the effect of a single Extract ant. CMPO consistently extracts ameri-
variable. Because this study evaluated multiple vari- cium, plutonium, and uranium more strongly than
ables, we sought a format that would allow presenta- does DHDECMP. This fact is reflected in Figs. 4-6,
tion of the individual and interactive effects of many which show the distribution coefficients of individual
variables in a single figure. A "tree" format suggested actinides as a function of the extractant/diluent com-
by Richard J. Beckman of the Los Alamos National binations from nitric acid solutions (without salt or
Laboratory Statistics Group simultaneously illustrates fluoride).
the effects of total nitrate, nitrate salt/nitric acid ra- The higher extraction of actinides by CMPO from
tio, and fluoride concentration. medium concentrations of nitric acid can be beneficial;
by contrast, high Kds from dilute nitric acid are a dis-
tinct disadvantage because they make back-extraction
Effects of Specific Variables much more difficult. DHDECMP, however, provides
adequately high Kds from medium concentrations of
nitric acid but offers Kds from dilute acid that are
Contact Time. Preliminary experiments demon- sufficiently low to allow americium and plutonium to
strated that Kds for a dynamic contact period of 10 readily be back-extracted.
Am(lll)
1000
f- O 0.25 M CMPO/1.00 M TBP/ISOPAR H
A 0.25 M CMPO/1.00 M TBP/TCE
UJ
• 0.75 M DHDECMP/1.00 M TBP/ISOPAR H
£2 100 • 0.75 M DHDECMP/1.00 M TBP/TCE
UL
LL
LU
O
O
— 10
E
0.1
2 3 4 5 6 7
NITRIC ACID MOLARITY
Fig. 4. Extraction of Am(Ill) into various extractant/diluent combinations as a function of nitric acid concentration.
Pu(IV)
1000
H
UJ
y 100
u.
u.
Ill
o
O
_ 10
CO O 0.25 M CMP0/1.00 M TBP/ISOPAR H
E A 0.25 M CMPO/1.00 M TBP/TCE
• 0.75 M DHDECMP/1.00 M TBP/ISOPAR H
• 0.75 M DHDECMP/1.00 M TBP/TCE
0.1
12 3 4 5 6 7
NITRIC ACM- MOLARITY
Fig. 5. Extraction of Pu(IV) into various extractant/diluent combinations as a function of nitric acid concentration.
U(VI)
1000
UJ
y 100
LL
(JL
HI
O
a
z
10
o
I
O 0.25 M CMP0/1.00 M TBP/ISOPAR H
A 0.25 M CMPO/1.00 M TBP/TCE
• 0.75 M DHDECMP/1.00 M TBP/ISOPAR H
0)
• 0.75 M DHDECMP/1.00 M TBP/TCE
0.1
NITRIC ACID MOLARITY
Fig. 6. Extraction of L'(VI) into various extractant/diluent combinations as a function of nitric acid concentration.
Diluent. Investigators have proposed TCE for TBP/Extractant Ratio. The PUREX pro-
use in the TRUEX process not only because it is non- cess has used TBP for many years to extract ura-
flammable but also because it is a reasonably good nium and plutonium from various nitrate solutions.
solvent for CMPO and CMPO/metal complexes. In TBP, proposed as a phase modifier with CMPO in
our studies, however, TCE formed persistent organic the TRUEX process, decreases the extraction of ac-
dispersions in the aqueous phase, a drawback that tinides from dilute acid, although it increases the ex-
made assay of the aqueous phase difficult and unre- traction of actinides from medium concentrations of
liable. These dispersions required that the extracted nitric arid when compared with extraction by only
aqueous phase be scrubbed with a fresh portion of CMPO.18 Moreover, TBP increases the solubility of
TCE to remove residual extractant before we could CMPO and CMPO/metal complexes in the organic
obtain the TCE-diluent data reported herein. Dilu- phase.
ents that have been used safely for many decades in Our preliminary study that compared 1.00 M
the nuclear industry include certain flammable liquids DHDECMP/TCE and 1.00 M DHDECMP/0.35 M
such as kerosene, whose high flash points provide an TBP/TCE showed that the Kds of uranium, pluto-
adequate margin of safety. Although DHDECMP and nium, and americium were not significantly affected
CMPO are only slightly soluble in kerosene, the addi- by the presence or absence of TBP. We elected to
tion of tributylphosphate (TBP) makes the three com- include TBP as a component of both organic ex-
ponents miscible. tractants studied, however, (1) to be consistent with
Initial experiments with Isopar H, a kerosene-type the recommended TRUEX composition when CMPO
diluent approved for use in the Los Alamos Pluto- was the extractant, (2) to increase the solubility
nium Facility, showed much-improved phase separa- of the DHDECMP and its metai complexes when
tion, which eliminated the need to scrub the aque- DHDECMP was the extractant, and (3) to maintain
ous phase as done with TCE. An additional benefit similar compositions where possible during the com-
of Isopar H was its ability to provide higher Kds for parative evaluation of DHDECMP and CMPO.
Am(III) using either extractant than provided by TCE A serious disadvantage of TBP is that it also ex-
(Fig. 4). The Kd enhancement with Isopar H is espe- tracts large quantities of nitric acid. The extracted
cially pronounced for Am(III) extraction by CMPO. acid subsequently back-extracts into dilute acid used
Comparative data for Pu(IV) and U(VI) extraction to back-extract the extracted actinides. The net result
into DHDECMP and CMPO, using TCE or Isopar H is a requirement for additional strip stages that serve
as the diluents, are shown in Figs. 5 and 6. no purpose other than to remove the excess acid pre-
viously extracted by TBP. Substitution of a different
Nitrate Salt/Nitric Acid Ratio. High con- organic phase- modifier that does not extract nitric
centrations of nitrate salts are common components of acid would allow the number of strip stages to be min-
nuclear p>rocess solutions. Evaluation of the effect of imized. (This possibility is being investigated.)
nitrate in the form of nitrate salts was therefore es-
sential. The general effect of replacing nitric acid with
nitrate salt is a small increase in the distribution co- Behavior of Individual Actinide Elements
efficient in all of the systems studied. This increase
is attributed to the higher activity of nitrate ion that Figures 7 30 show the results of actinide element
results from the more complete dissociation of nitrate extraction under various conditions.
salt as compared with nitric acid.
Amtricmm
DHDECMP/TBP/TCE. (Figs. 7 and 8) Aniericium extraction increases slightly with nitrate salt levels. A
small but consistent, increase results from increasing fluoride when nitrate salt is absent; however, a consistent
decrease results from increasing fluoride at the highest nitrate salt levels.
Am(lll) Am(lll)
1000 1000 F.
UJ
O O
100 .
UJ o
O o
o
o
m
E
E
to a
5 0.1
Fluofld*, Mo Fluoride, Mi
SALT:ACID " SALT.ACID ~ 1_
Total Nitrate, M 0.45 1.5 Total Nitrate, M 4.5 7.5
0.75 M DHDECMP/1.00 M TBP/TCE 0.75 M DHDECMP/1.00 M TBP/TCE
Fig. 7. Extraction of Am(III) into 0.75 M DHDECMP/ Fig. 8. Extraction of Am(IH) into 0.75 M DHDECMP/
1.00 M TBP/TCE as a function of nitrate concentra- 1.00 M TBP/TCE as a function of nitrate concentra-
tion, nitrate salt/nitric acid ratio, and fluoride con- tion, nitrate salt/nitric acid ratio, and fluoride con-
centration. centration.
DHDECMP/TBP/Isopar. (Figs. 9 and 10) Americium extraction increases significantly with nitrate salt
levels but is essentially unaffected by fluoride. Kd values with Isopar diluent are generally double to triple those
obtained with TCE diluent.
Am(lll) Am(lll)
1000 F
HI UJ
o O
u.
100
u.
HoI o
o o
z
g o
3
CO m
E E
a 0 1 0.1
Fluoride. M ., Fluorldt, M o cos o oos o 0 06 o oos o oos o 005
SALTACID ~ SALT.ACID o i; ;, o ,2 ~7T
Total Nitrate, M L 0.45 1.5 Total Nitrate, M ' 4"ti5 ' ' 7^5 '
0.75 M DHDECMP/1.00 M TBP/ISOPAR H 0.75 M DHDECMP/1.00 M TBP/ISOPAR H
Fig. 0. Extraction of Am(III) into 0.75 M DHDECMP/ Fig. 10. Extraction of Am(III) into 0.75 M DHDECMP/
1.00 M TBP/Isopar H as a function of nitrate con- 1.00 A/ TBP/Isopar H as a function of nitrate con-
centration, nitrate salt/nitric acid ratio, and fluoride centration, nitrate salt/nitric acid ratio, and fluoride
concentration. concentration.
CMPO/TBP/TCE. (Figs. 11 and 12) Americium extraction increases significantly with nitrate salt levels.
Fluoride has no effect on americium extraction. Kd values are similar from 7.5 M nitrate but are approximately
30 times higher from 0.45 M nitrate when compared with the DHDECMP/ TBP/TCE system. Thus, CMPO
offers little improvement in extraction from medium concentrations of nitric acid, but back-extraction into dilute
acid is far more difficult.
Am(lll) Am(lll)
1000 1000
2 z
UE u
LL 100
in LL
o UJ
u 8
10 10 •-*-•- -•-•=«-
GO
E
V)
0.1 0.1
) 0.02O.Of
SALT:ACID ~ SALT:ACID
Total Nitrate, M 0.45 1.5 Total Nitrate, M 4.5 7.5
0.25 M CMP0/1.00 M TBP/TCE 0.25 M CMP0/1.00 M TBP/TCE
Fig. 11, Extraction of Am(III) into 0.25 M CMPO/1.00 Pig. 12. Extraction of Am(III) into 0.25 M CMPO/1.00
M TBP/TCE as a function of nitrate concentration, M TBP/TCE as a function of nitrate concentration,
nitrate salt/nitric acid ratio, and fluoride concentra- nitrate salt/nitric acid ratio, and fluoride concentra-
tion. tion.
CMPO/TBP/Isopar. (Figs. 13 and 14) A slight decrease in americium extraction is associated with increasing
fluoride levels. Kd values with Isopar diluent are approximately four times those obtained with TCE diluent.
(This difference represents the largest diluent effect of the entire study.) Kd values of this CMPO/TBP/Isopar
system, when compared with the DHDECMP/TBP/ Isopar system, nre only double to triple from 7.5 M nitrate
but fire approximately 40 times higher from 0.45 M nitrate. Again, the demonstrated increased difficulty in
back-extracting americium from CMPO is a major disadvantage.
Am(lll) Am(lll)
1000 F
UJ UJ
O o
iZ 100 £ 100
u. u.
HI
O Ui
O
o
o
o
o
m eg
£
CO
0.1
rid*,
SALTACID 5ALT:ACID ~
Total Nitrate, M 0^45" 1.5 Total Nitrate, M 4^ 7.5
0.25 M CMPO/1.00 M TBP/ISOPAR H 0.25 M CMPO/1.00 M TBP/ISOPAR H
Fig. 13. Extraction of Am(III) into 0.25 M CMPO/1.00 M Fig. 14. Extraction of Am(III) into 0.25 M CMPO/1.00 M
TBP/Isopar H as a function of nitrate concentration, TBP/Isopar H as a function of nitrate concentration,
nitrate salt/nitric acid ratio, and fluoride concentra- nitrate salt/nitric acid ratio, and fluoride concentra-
tion. tion.
Description:ables included the extractant (DHDECMP or CMPO), extractant/tri- butylphosphate ratio, diluent, nitrate concentration, nitrate salt/nitric acid ratio