Liquid membrane based separations of actinides and fission

14 Pages·2012·3.52 MB·English

Indian Journal of Chemistry
Vol. 42A, December 2003, pp. 2925-2938
Liquid membrane based separations of actinides and fission products
P K Mohapatra & V K Manchanda+
Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India
Email: [email protected]
Received 20 Jalluary 2003
Separation of actinides and fission products is a challenging task for the chemists working at various stages of the
nuclear fuel cycle, the most important being the reprocessing and waste management processes involved at the tail end.
Amongst the various separation techniques, liquid membrane based separation methods are getting increasingly popular due
to factors sllch as ligand economy, high efficiency, low power consumption and easy scale-up due to a compact design etc.
Techniques such as emulsion liquid membrane and hollow fibre based non-dispersive solvent extraction methods are
reported to be more efficient than the solvent extraction based separation methods which are limited by emulsion/third phase
or crud formation. The present review is a compilation of literature reports on liquid membrane based separation studies
involving mainly the actinides and fission products.
Liquid membrane (LM) based separation methods are membrane (BLM) is a stirred organic phase of lower
popular in industries for effluent treatment, density than the aqueous phase positioned under it or
desalination and gas purification, yet their application vice versa. In emulsion liquid membrane (ELM), the
for separation of actinides and fission products receiver aqueous phase containing oil droplets are
relevant to nuclear industry is still in infancy'. The dispersed into the feed aqueous phase. The volume of
past few decades saw the use of exotic reagents such the receiving phase inside the oil droplets is at least
as crown ethers, calixarenes for the selective ten times smaller than that of the source phase. The
complexation and removal of actinides from solutions thickness of the membrane (organic film) is very
containing complex mixture of metal ions2-5. Specially small, while the surface area is enormous resulting in
designed uranophiles have been synthesized for the very fast separations. However, ELMs are quite
concentration of ppb level uranium from sea water6. A difficult to prepare and after transport, the oil droplets
crown ether was employed for the separation of Pu have to be separated and broken up to recover the
from a synthetic spent fuel solution 7. The two major receiving phase. Compared to the ELM, the BLMs are
drawbacks of the solvent extraction methods viz. third easier to operate. The supported liquid membranes
phase formation and prolonged phase disengagement (SLM) are categorized into two types, viz. a flat sheet
time can be overcome with the LM based separation (FSSLM) or a hollow fibre (HFSLM). Here a
methods as they involve non-dispersive mass transfer. polymeric filter with its pores filled with the organic
Hollow Fibre Supported Liquid Membrane (HFSLM) phase act as membrane. Figure 1 shows four different
separations are particularly attractive due to large types of liquid membranes.
surface to volume ratio, faster mass transfer rates and The principle involved in supported liquid
conti nuous flow 8-10. membrane transport is as follows: two homogeneous,
completely miscible liquids, which may be referred to
Types of liquid membranes as feed or source phase (F or s.p.) and receiver phase
Liquid membrane usually consists of a water (R or r.p.) solution are spatially separated by a third
immiscible (organic) layer separating a source liquid (M) which is practically insoluble in F and R
aqueous phase consisting of a mixture of metal ions and is immobilized in a porous hydrophobic
(feed) and receiving aqueous phase where the metal polymeric support. Due to the favourable thermo
ion of interest gets concentrated (recei ver) dynamic conditions created at the F/M interface, some
preferentially. Broadly speaking, there are four components are selectively extracted from the feed
different types of liquid membranes. Bulk liquid and transported into the membrane liquid. Simul-
2926 INDlAN J CHEM. SEC. A, DECEMBER 2003
r.p .
•.p .
membr:me phasl! !lI\!l1lbrane phase
c=:> o
a. Bulk-lil{uid n\emhrane b. Emulsion lil{uid memhrane
HFl't1
c. Oatsheet sUlr d. Hollow fibre sUlr
/
Fig. l--Schematic representation of some liquid membrane
systems.
taneously, at the M/R interface, conditions are such
that the reverse transport is favoured. Thus the metal Fig. 2-A schematic representation of a dual module hollow fibre
extractant chemistry in case of SLM is identical to set-up.
that found in the solvent extraction system but the
overall process is governed by diffusion kinetic rather or from waste streams of radiochemical laboratories
than by chemical equilibrium parameters. Various engaged in analytical and research activities. A
factors that could affect the transport of a metal ion schematic representation of a hollow fibre set-up is
through the LM are: (a) the (transport) resistance shown in Fig. 2.
encountered by the metal ion in the F and R phases;
(b) the physico-chemical properties of the carrier and Mechan.ism of metal ion tran.sport
diluent and (c) the nature of membrane support like its The permeation of metal species through SLMs can
pore size, porosity, tortuosity, hydrophilicity, surface be formally described as the simultaneous process in a
tension and surface area to volume ratio. This is an single stage of extraction and stripping operations,
economical separation technique because of the occurring under non-equilibrium conditions. A
relatively small inventory of extractant, low energy detailed knowledge of the solvent extraction
consumption, low capital and operating costs and the equilibria and mass transfer kinetics is therefore
possibility of achieving high separation factors. required to understand and quantitatively describe the
FSSLM represents a convenient means for the rate laws which control the permeation of metal
systematic study of the effect of various parameters species through SLMs and to exploit them for
on flux and cation selectivity in liquid membranes. separative processes II. Tn SLM studies two terms are
Furthermore, studies on FSSLM enable one to predict used viz. facilitated transport and coupled transport.
whether the hollow fibre system can be used to carry Facilitated transport is generally referred to as the
out similar separations in a continuous flow operation. case where the transport mechanism is independent of
Hollow fibre supported liquid membrane (HFSLM) any other ion while in case of coupled transport the
essentially employs the same membrane material as permeation rate of a particular ion is dependent on the
the medium of separation of feed (F) and receiver (R) concentration of another ion. The mechanism of
but with a different geometry. It has the potential to coupled transport through SLMs is shown
be employed on industrial scale separations, including schematically in Fig. 3 (a & b). The SLM is inter
different stages of nuclear fuel cycle viz. recovery of posed between two aqueous solutions. The distri
metals from hydrometallurgical leach solutions, bution ratio (Kd) between the organic phase absorbed
treatment and concentration of low level aqueous in the membrane pores and -aqueous feed solution
waste from reprocessing plants, actinide partitioning (source phase) of metal species permeating the SLM
from aqueous raffinate waste streams generated after is high enough to favour metal extraction into the
plutonium recovery operations in the PUREX process membrane phase. The K" between the organic phase
MOHAPATRA el at.: LIQUID MEMBRANE SEPARATIONS OF ACHNIDES & FISSION PRODUCTS 2927
COUNTER-TRANSPORT CO-TRANSPORT
FEED SLM STRIP FEED SLM STRIP
Low [H+j membrane High [H+] High [X-] membrane Low [X -]
High Kd LowKd ___ ~~~~~L_ LowKd
M
X
2
2
M+ 3
3 H+
4 E
4 HX -========::"
FLUX H"i-;-io========:!:" FLUX M+ - • FFLLUUXX XM -+
(a) (b)
Fig. 3--Mechanism of (a) counter and (b) co-transport in supported liquid membranes.
and the strip solution is made as low as possible in will continue until all the metal species which can
order to favour complete back extraction of the metal permeate the SLM have been transferred from the
species from the liquid membrane. If the metal calTier feed to the strip side, provided the driving force of the
is an acidic extractant, HX, the difference in Kd process is maintained. This situation often occurs in
between the feed and strip sides of the SLM is practice when very dilute solutions of metal species
generally achieved by a pH gradient. Tn this case, we are involved or when, the concentration of the
deal with a counter-transport phenomenon (Fig. 3a) chemicals responsible for the driving force is
and the chemical reaction which is responsible for the continuously adjusted to keep it constant. It follows
coupled transport can be schematised as: that in a SLM permeation process very high
concentration factors can be obtained by using a
feed !>ide
M+ + HX (membrane) ~ MX (membrane) + W volume of the strip solution that is much lower than
!i.tripside
that of the feed solution. Moreover, by using catTier
If the metal can·ier is a neutral or a basic extractant molecules, HX or E, which are very selective to given
(i.e. a long-chain amine), E, the difference in DM metal species, very clean separation processes, can be
between feed and strip is generally obtained by a performed. Since during the permeation, the calTier
concentration gradient of the counterion, X-, which is acts as a shuttle, moving metal species from the feed
accompanying the metal cation into the membrane to the strip solution and then diffusing back (being
(Fig. 3b) and the chemical reaction which is continuously regenerated during the process), very
responsible for the coupled transport can be small amounts of carrier is used in SLM separation.
schematised as: Other potential advantages of SLM separations over
separations performed by traditional solvent
feed side
M+ + X- + E (membrane) ~ MXE (membrane) extraction technique are the lack of solvent entrain
l>lripsidc
ment phenomena, the simplicity of the equipment
The aqueous phase pH and counterion concen involved, and the low energy consumption of the
tration gradients are often used as driving forces. processl2. Moreover, with respect to separation
However, any other expedient that assures a large processes performed with solid membranes, SLMs
chemical potential gradient between the two opposite offer the additional advantage of higher flux since
sides of the membrane can be used as long as coupled diffusion in liquids is much faster than in solids.
transport of metal ions and some other chemical The various steps that characterise the transport of
species occur through the SLM. metal species through SLMs can be described with
From this schematic description of coupled the help of Fig. 3(a&b). Step 1: The metal species,
transport, it follows that metal species can be after diffusing to the feed solution-SLM interface,
transported across the membrane against their react with the metal CaITler, H+ ions are
concentration gradient. This type of "uphill" transport simultaneously released into the feed solution
2928 INDIAN J CHEM. SEC. A. DECEMBER 2003
(counter-transport, acidic carrier) or X- ions Tri-n-butyl phosphate (TBP) and di-2-ethyl hexyl
accompany the metal ions into the membrane (co phosphoric acid (ROEHP) have been used as
transport, neutral or basic carriers); Step 2: The metal extractants in several hydrometallurgical operations
carrier complex diffuses across the membrane (Fig. 4). Selective transport of Pu4+ over other long
because its concentration gradient is negative; Step 3: lived fission product contaminants was accomplished
At the SLM-strip solution interface the metal-carrier from aqueous acidic solutions through an organic
complex releases metal ions into the aqueous phase, BLM containing TBP as the mobile caITier and
H+ ions replace the M+ ions into the membrane dodecane as the membrane solvent and O.SM ascorbic
(counter transport) or X- ions are simultaneously acid as the strippant. With increase in carrier
released together with M+ ions into the strip solution concentration in the organic membrane, the amount of
(co-transport); Step 4: The uncomplexed carrier plutonium that could be extracted into the membrane
diffuses back across the membrane. as well as the viscosity of the organic solution
A large number of reviews have appeared during increased. These opposing effects resulted in
the last few years that underline the importance of this maximum plutonium permeation with about 30%
upcoming area 13. An important parameter that TBP in dodecanel4.
characterizes the transport of metal ions across SLM BLM comprising of di-(2-ethylhexyl) phosphoric
is the permeability coefficient (P) that can be obtained acid (HOEHP) in chloroform was successfully used as
using the following equation: a carrier for the selective and efficient transport of Th.
= It was observed that 99% of Th selecti vely permeated
Ln([M]/[M]o) -(Q/V)Pt
across the. membrane in less than three hours while
where [M], and [M]o are the concentration of metal the transport of other cations present was less than
ion in aqueous feed at time t and initial metal ion three percent during the same tim~I.,). In a study
concentration (at t = 0) respectively, Q is A£ where A involving Eu(III), its coupled transport through a
is the geometrical surface area and £ is the porosity, V BLM containing mono-2-ethylhexyl 2-ethylhexyl
is the aqueous feed volume (in mL). The above phosphonate [REH(ERP)] in kerosene revealed that
equation is however valid only when caITier is not there was a counter transport of protons 16.
saturated by the metal ion.
Hollow fibre supported liquid membrane studies Emulsion liquid membranes
are carried out usually with cylindrical modules
containing lumens made from macroporous polymeric Many literature reports on the use of ROEHP for
material. The organic carrier solution is held within metal extraction involve emulsion liquid membranes.
the pores of the membrane wall. The source phase is Comparative studies between column and batch liquid
passed usually through the shell side while the emulsion membrane techniques based on
receiver phase is passed through the lumen side of the HOERP/HCI system were carried out to develop a
membrane. system for isolation of Th-234 from natural uranium
In all the versions of membrane based transport which showed that, kinetically, the equilibrium for
described above, organic phase acting as liquid thorium separation using batch technique is faster
membrane consists of a judiciously chosen mixture of than the continuous column systeml?
extractant and diluent. With the discovery of novel Uranium extraction by liquid surfactant membranes
metal ion specific reagents (Fig. 4), separation of a containing HOEHP as a mobile carrier has been
particular metal ion from complex matrices including studied by Akiba et af. in connection with the stability
those from nuclear industry, have become much of water-in-oil (W/O) emulsion 18. The apparent rate
simpler. constant (kobs) increased with the carrier concentration
and external pH, while it varied slightly with internal
Bulk liquid membranes acid concentrations. The emulsion globules were
In case of bulk ljquid membranes, the two aqueous deemulsified by a high voltage electrostatic method in
phases viz. the feed and the receiver, are separated by order to recover the extracted uranium. The extracted
an immiscible organic carrier phase. As the dri ving uranium was efficiently recovered leaving little
force in the transport is through the bulk organic remnants in the liquid membrane phase. The extrac
phase, it often provides the resistance to the transport tion of uranium as uranyl ni~rate complex in the nitric
of the metal ion of interest. acid solution was also carried out by Suripto et af. 19,
MOHAPATRA ef al.: LIQUID MEMBRANE SEPARATIONS OF ACTINIDES & FISSION PRODUCTS 2929
TBP TOPO
Ph
Ph
o
OH
ALIQUAT 336
HPBI CYANEX 301
CMPO Diamide
t-8u
t-8u
DC 18crown6 t-8u
Calixarene
Fig. 4--Structures of some commonly used extractants in the LM studies.
using an emulsion that consisted of kerosene as the membrane extraction process is used for pilot plant
solvent, sorbitan mon08leate (SPAN80) as the scale extraction of uranium from wet process
surfactant, HDEHP as the carrier agent, phosphoric phosphoric acid where uranium concentration is up to
acid as the internal phase while 2 M nitric acid was 100 ppm which in principle makes it ideal for
used as the external phase. Within 5 minutes an treatment with a membrane process22. The membrane
extraction efficiel!cy of >94% was obtained though system studied was based on extraction using
the amount of extracted uranium by the ELM was HDEHP-TOPO reagents which were contained within
limited. In a mechanistic study by Zhou et ai., the the organic phase of water in oil emulsion. El-Reefy
kinetics of extraction of uranium (VI) from sulfuric et ai., have also used HDEHP based ELM containing
acid solutions with HDEHP in cyclohexane and liquid aqueous HCl to study the permeation of Eu(III) and
surfactant membranes containing HDEHP as a mobile Am(III) from aqueous solutions containing lactic acid
carrier was investigated using Lewis cell technique2o. and diethylenetriamine-N,N,N',N",N"-pentaacetic acid
El-Sherif studied the separation of Th from U and (DTPA)23. The kinetics of Am permeation was fast
extraction of Am by ELM using HDEHP in cyclo under these conditions «55% permeated within 2
hexane as the carrier, polyethylene glycol dioleate as mins); whereas the kinetics of Eu permeation was
surfactant and HCl as a stripping solution21 A liquid relatively slow (equilibrium occurred after 8 min).
.
2930 INDIAN J CHEM. SEC. A, DECEMBER 2003
This difference in permeation rates could be used to ion was found to be mainly dependent on TOPO
separate Am(lII) from Eu(III). concentration and concentration of U(VI) and HN03
The transfer kinetics of uranium (VI) through an in the external phase was of little cons.equence.
emulsion liquid membrane with di-l-(1-methyl
heptyl) phosphoric acid (DMHPA) as a mobile carrier Supported liquid membranes
was investigated by Han-Jingtian et at. 24. The experi Supported liquid membrane (SLM) comprised the
mental results showed that more than 96% of the bulk of the published literature on the transport
initial U(VI) was recovered. Extraction of uranium studies of metal ions across thin polymeric films3).
(VI) by a liquid surfactant membrane containing 5,8- The various types of extractants used as the carrier for
diethyl-7-hydroxy-6-dodecanone oxime (LIX 63) was the separation of actinides and fission products are
investigated by Akiba et ai. 25 and the apparent rate summarized here.
constants were calculated. Han et ai. 26, studied the
transfer behavior of uranium (VI) through an
Acidic extractants as the carriers
emulsion liquid membrane using N,N-dibutyloctan
Akiba et ai., investigated the caITier-mediated
amide (DBOA) as a mobile carrier and suggested that
transport of uranium (VI) by a liquid membrane of 7-
the liquid membrane process can effectively and
dodecenyl-8-quinolinol (Kelex 100)32. The apparent
rapidly concentrate uranium (VI) from nitric acid
rate constant of uranium transport was not affected
medium.
significantly with the increase in the carrier
The coupled transport of europium(IIT) ion and
concentration and the pH of the feed solution. They
hydrogen ion, via emulsion liquid membrane
had also reported that uranium can be effectively
containing PC-88A «2-ethylhexyl) phosphonic acid
recovered from sea-water through the liquid mem
mono-2-ethylhexyl ester) as mobile carrier and
brane without any preliminary treatment. Compounds
SPAN-80 as surfactant in n-dodecane, was studied by
analogous to Kelex 100, with long chain aliphatic
Lee et al.27. The extraction behavior of U(VI) by 2- linkages, have been used by this group33.
ethylhexylphosphonic acid mono-2-ethylhexylester Seguira et ai., used 2-thenoyl trifluoroacetone
(EHPA) was investigated by Nakamura et ai.28, where (HTT A) as the carrier for the transport of lanthanide
the extracted species was found to be U02 (EHPA2)2. ions34. Carrier mediated transport of Am(III) was
Uranium was transported from the feed solution into a investigated by Mohapatra et ai., using a synergistic
sulfuric acid solution (receiving phase) by a large
mixture of 3-phenyl-4-benzoyl-5-isoxazolone (HPBI)
concentration gradient of hydrogen ions across two and TOPO in dodecane as the carrier and 1M oxalic
sides of the SLM. The addition of I-decanol to the 35
acid as the strippant . Apart from enhancing the flux
membrane solution improved the U stripping and
due to synergistic extraction, TOPO also helped in the
contributed further to stabilize the EHPA-membranes
solubility of HPBI in the organic phase. It was
without any leaching loss into the aqueous phases.
possible to achieve >90% transport of Am(lIl) in
ELM recovery of Pu from aqueous nitrate solutions about 4 hours with the feed acidity as <1M HN0 and
3
using 2-ethylhexyl phosphonic acid mono-2-ethyl the carrier concentration as O.lM HPBI+0.2M TOPO.
hexyl ester in dodecane and SPAN-80 as carrier and Nakamura et ai., investigated the permeation of
surfactant respectively suggested about 90% recovery/
lanthanide ions using a SLM impregnated with
removal of Pu from the waste in perchloric acid Versatic 10 (V A 10) alone in kerosene36.37 The
medium with the concentration factor of the order of
separation factors for heavier lanthanoids were
29
16 in 20 min of contact time .
enhanced by the addition of citrate to the feed
37
EI-Reefy et ai., observed that U(VI), Th(IV), solution . Several complexing agents have been used
Zr(IV) and Fe(III) were extracted to different extent in the receiving phase by Chitra et al. 38, during their
from 0.l-3M HN03 solution by O.IM TOPO in SLM studies on Eu with PC-88A and V A 10 as
cyclohexane solution using ELM30. Among these immobilised carriers. Under similar experimental
elements, only U(VI) and Th(lV) were found to be conditions, the effect of complexing agents on
stripped from their organic phase by O.IM sodium permeation of europium was found as: citric acid>
citrate solution. On the other hand, cerium, copper succinic acid> malonic acid. Permeability coefficient
and cadmium were not extracted by TOPO under of europium was found to be greater with PC-88A in
similar conditions. Forward permeation rate of uranyl comparison with that of V A 10.
MOHAPATRA et at.: LIQUID MEMBRANE SEPARATIONS OF ACTINIDES & FISSION PRODUCTS 2931
Mikheeva et al., investigated the transport of Am a DTMPPA liquid membrane supported on porous
under conditions of different redox potentials in polytetrafluoroethylene to a strip solution (O.lM nitric
39
various aqueous phases using HDEHP as the carrier . acid) was investigated. The selective transport of Am
The various steps leading to the transport were across a supported liquid membrane (SLM) has been
explained on the basis of physico-chemical model of investigated by using bis-(2,4,4-trimethylpentyl)di
the process including the americium oxidation in feed thiophosphinic acid (Cyanex 301) as a mobile
solution, extraction by membrane, partial reduction on carrier43. Extractants containing soft donor atoms
membrane surface, trans-membrane diffusion and re exhibit strong affinity for actinoids, giving a large
extraction to strip solution. In another study, the separation factor between trivalent Am and Eu.
coupled transport of Eu(III) ions through a supported Americium was preferentially transported across the
liquid membrane (SLM) consisting of a porous SLM and concentrated in product solution, while
polypropylene film immobilizing HDEHP solution in most of Eu remained in the feed solution.
4o
n-dodecane was investigated . The study on tervalent
elements in nitrate medium through a flat sheet
Alkyl phosphates and phosphine oxides as the carriers
membrane comprising HDEHP in dodecane indicated
The extraction of uranyl nitrate from aqueous
that the transport rates of Ce and Gd decreased with
solutions in the nuclear industry is traditionally
increasing feed acidity. The transport of all the metals
effected by solvent extraction with TBP. Supported
increased to some extent with increasing concen
liquid membrane based transport studies of uranyl
tration of HDEHP in the organic phase. Conditions
ion have been carried out from nitric acid medium
were established for separating the pairs Ce-Tm,
Ce-Yb and Ce - SC41. using TBP as the carrier. TBP.HN03 complex
was transpOlted alongwith the complex
Separation of lanthanides and actinides is a UOz(N03h.2TBp44.45. Based on the selective transport
challenging area of research in separation science.
properties of TBP and its mixture with di-2-ethyl
Impetus in this area of research has been provided by
hexyl phosphonate in benzene a uranyl ion selective
the need to develop safer and economically viable
membrane electrode was developed by Sere
methodologies of high level waste (HL W) disposal. brennikova et al.46. This membrane electrode
The TRUEX as well as the DIAMEX processes
possesses higher selectivity as compared to the
developed during the last decade for actinide
membrane electrode containing only di-2-ethylhexyl
partitioning do not distinguish amongst trivalent phosphonate in benzene47. Table 1 summarizes the
lanthanides and trivalent actinides. Either of the two
experimental conditions and observations on some
strategies being evolved for the HLW management
uranium/plutonium transport system using alkyl
viz. vitrification followed by disposal in deep
phosphates and phosphine oxides as carriers.
repositories or transmutation of long lived actinide
Changing the alkyl groups in TBP had shown
isotopes to short lived fission products using high
interesting selectivities during solvent extraction
energy high flux reactors' or accelerator driven studies48.49. Selective permeation of plutonium from
subcritical systems require the separation of Ln(IIJ)
aqueous nitrate medium was investigated using an
from An(rn). This is necessitated in view of the larger
SLM impregnated with tri-iso-amyl phosphate (TAP).
mass of lanthanides (30 times) over that of the minor
More than 90% pertraction of plutonium could be
actinides and high neutron absorption cross-section of
easily accomplished in single run employing a feed
some isotopes of lanthanides which may act as
solution consisting of about 1 mg/L Pu and 4M HN03,
neutron poisons thereby rendering the transmutation
at a carrier concentration of 0.8M T AP/dodecane
process inefficient. It is desirable therefore to develop
while the receiving phase was O.SM sodium carbonate
suitable complexing agents which can recognise or O.SM ascorbic acid50. Selective transport of U(V!)
trivalent lanthanides from trivalent actinides.
from aqueous nitratf? solutions across an SLM, impre
Though membrane based separation methods for gnated with tris(2-ethyihexyl) phosphate (TEHP), was
An(IIl) and Ln(Ill) separation are relatively scarce, investigated by Dharmapurkar et al.51• The permea
some of them are discussed here. The extraction bility of U(VI) was evaluated as a function of HN0
3
behaviour of Eu(III) with di(2,4,4-trimethylpentyl) molarity in the feed phase (F), carrier concentration in
phosphinic acid (DTMPPA) in kerosene was the membrane phase and chemical composition of the
4z
investigated . The transport of Eu(III) at pH 3 across receiving phase (R). More than 90% transport of U
2932 INDIAN J CHEM. SEC. A, DECEMBER 2003
Table l--{:omparative performances of some alkyl phosphates and phosphine oxides used as carriers in SLM studies
Metal ion Carrier Feed Strip Comments Ref.
U(VI) TBP I-8M HN0 1-9M Quantitative transport (>99%) of 44-45
3
Ammonium uranyl ion is possible as acid
carbonate transport is found to be
insignificant
U(VI) TBP+ D2EHP HN0 Ion selective electrode 47
3
Pu(IV) TAP in dodecane 4MHN03 0.5M Na2C03 More than 90% transport 50
U(VI) T2EHP 4M HN0 0.5M Na2C03 More than 90% transport 51
3
U(VI) TOPO HN03 Carbonate solution 52
Pu(IV) Cyanex 923 in dodecane Oxalic acid + More than 95% Pu recovery 54
HN0
3
Pu(III) Bis-(diphenylphosphoryl)-methane lMHCl O.IMHCI Ion selective electrode 55
57
could easily be accomplished in single run employing Na2C03 or O.OIM mtnc acid . Several newly
F containing about 0.2 gil of U in 4M HN03, carrier synthesized monoamides such as di-butyloctanamide
concentration of 0.6M TEHP/dodecane while the R (DBOA), methylbutyldecanamide (MBDA), methyl
was O.SM Na2C03. Table 1 lists the comparative butyl octanamide (MBOA), di-butyl decanamide
performances of some alkyl phosphates and phos (DBDA), di-n-octyl ethylhexyl amide (DOEHA), and
phine oxides used as carriers in SLM studies. di-hexyl octanamide (DHOA) were tested by Shukla
Carrier (TO PO) mediated transport of uranium(VI) et al., as carriers across supported liquid membranes
has been studied by Akiba et al. 52, who have observed for selective transport of Pu(IV)58. Of them, MBDA
that uranium was extracted in the liquid membrane as and DBOA proved to be quite suitable as ionophores
U02(N03h.2TOPO and stripped into the carbonate from HN03 media providing >95% and >82%
solutions as U02(C03h4-. Using TOPO and HDEHP transport of Pu(IV) when operated for 7 hours.
mixture, liquid membrane technique was applied to
the recovery of uranium from Wet Process Phosphoric CMP, CMPO and diamides as the carriers
Acid (WPPA)53. Carrier-facilitated Pu(IV) pertraction Advanced fuel reprocessing incorporating actinide
through a supported liquid membrane (SLM) was partitioning is a major recent innovation in nuclear
standardized for its decontamination from oxalate industry aimed at reducing the cost of disposal and
wastes employing a commercially available Cyanex- surveillance of high level liquid waste. It will help a
54
923 (TOPO analog) in dodecane as the receptor . great deal in achieving better public understanding
More than 95% of plutonium could be easily and acceptance of nuclear energy programmes all
recovered from Pu oxalate wastes solution generated around the world. Risk perception is essentially due to
during Pu reconversion operations. the large radiological toxicity associated with alpha
Plutonium(IlI)-ion selective electrode consisting of emitters like 237Np, 241A m, 243 Am and 245Cm. Isotopes
phosphine oxides such as bis-(diphenylphosphoryl) of Pu (left unrecovered) present in HL W also con
methane as an ionophore was developed by Kitatsuji tribute towards radiological toxicity. It is perceived
et al. 55. The Pu(III)-ISE prepared exhibited Nernstian that immobilized HLW after actinide partitioning (and
3
response to the concentration of Pu + in the sample preferably after the removal of heat generating
solution. Interference of U02(VI), U(IV), Np02(V) radionuclides 137Cs;9°Sr) can be deposited in near
and Pu(lV) was investigated. Cristau et al. 56, have surface repositories.
carried out extraction as well as permeation studies of In recent years, solvent extraction processes viz.
actinides like neptunium, plutonium and americium TRUEX and DIAMEX have been proposed for the
using polyphosphine polyoxides with (O)PCP(O) partitioning of trivalent actinides from high level
linkage which exhibit very high selectivity. waste using a carbamoyl methyl phosphine oxide
(CMPO) and a pentaalkyl malonamide
Dia/kyl amides as the carriers (DMDBTDMA) as the extractants respectively. As a
Di-hexyl octanamide (DHOA) was used by Patil et logical extension, a large number of investigations
a/., for the effective transport of uranyl ion from 4M using these reagents as carriers have been carried Ollt.
nitric acid to a strip solution containing either l.OM Literature reports on CMP, which has analogolls
MOHAPATRA el at.: LIQUID MEMBRANE SEPARATIONS OF ACTINIDES & FISSION PRODUCTS 2933
extraction properties to CMPO, are also available. A membrane/NaOH solution allowed the quantitative
comparison of some such carriers is made in Table 2. removal of actinide and lanthanide ions from the feed
The transport of europium was studied by solution. Chiarizia et al.62, used CMPO loaded
Nakamura et al. 59, through a supported liquid membranes for analysis of Am in biological samples
membrane impregnated with dihexyl-N,N after leaching in nitric acid medium. Two membranes
diethylcarbamoylmethylphosphonate (CMP) which were used in the transport studies consisting of 0.25M
suggested that europium was effectively extracted octy I( phen y 1)-N, N -di i sobu ty lcarbamoy lmeth y Ip hos
from the perchlorate solution into SLM, but was phine oxide (CMPO)/0.75M TBP and primary
insufficiently stripped to a dilute acid solution. The aliphatic amine (PAA) (as a second membrane) for
addition of I-decanol improved the stripping process, the selective extraction of actinides from nitric acid
and quantitative transport of europium was achieved. solutions. In another SLM study using CMPO as the
By the combination of two SLM systems consisting calTier, Ramanujam et al., had used a mixture of citric
of diisodecylphosphoric acid and CMP, europium was acid, formic acid, and hydrazine hydrate as the
transported from the feed solution (O.lM HN03) receiving phase for partitioning of actinides from high
through the intermediate solution (lM HCI04 + 4M level waste of PUREX origin63. The studies indicated
NaCI04) to the product solution (O.IM HN03) and good transport of actinides like neptunium,
effectively concentrated by a factor of about 20 60. americium, and plutonium across the membrane from
Supported liquid membranes (SLM), consisting of nitric acid medium. A high concentration of uranium
an organic solution of n-octyl(phenyl)-N,N-diiso in the feed retards the transport of americium,
butylcarbamoylmethylphosphine oxide (CMPO) and suggesting the need for prior removal of uranium
tributyl-phosphate (TBP) in decalin were able to from the waste. The separation of actinides from
perform selective separation and concentration of uranium-lean simulated samples was found to be
actinide and lanthanide ions from aqueous nitrate feed feasible using the above technique.
solutions and synthetic nuclear wastes where the strip Danesi et ai. studied the kinetics of transport of
solution is a mixture of formic acid and hydroxyl Am(lII) from aqueous nitrate solutions to formic acid
ammonium formate (HAF)61. The effectiveness of this aqueous solutions using an SLM which consists of a
strip solution was gradually reduced by the solution of a new (carbamoylmethyl)phosphine oxide
simultaneous transfer of HN0 in the strip solution. A in diethylbenzene (DEB)64. Sriram et al. have carried
3
possible way to overcome this drawback was to make out SLM studies on the transport of Am3+ from nitric
use of a second SLM consisting of a primary amine, acid medium using dimethyl dibutyl tetradecyl
which was able to extract only HN0 from the strip malonamide (DMDBTDMA) in dodecane as the
3
solution. The use of second liquid membrane in the carrier into O.OIM nitric acid65. Significant transport
system synthetic nuclear waste/CMPO-TBP mem of acid into the strip side resulted in relatively slower
brane/HCOOH-HAF strip solution/primary amine transport rate. It was observed that a mixture of O.IM
Table 2- Comparative performances of some substituted CMP, CMPO and diamides used as carriers in SLM studies of
actinides and lanthanides
Metal ion Carrier Feed Strip Comments Ref.
Eu(lIl) Dihexyl-N,N-diethylcarbamoylmethyl HNO)/ O.IM HNO) Concentration of Eu(IlI) is 59-60
phosphonate (CMP) HCI0 possible from dilute solutions
4
Am(lll) n-Octyl(phenyl)-N,N- HNO) Formic acid and Significant amount of acid 61
diisobutylcarbamoylmethylphosphine hydroxylammonium transport was noticed
oxide (CMPO) + TBP formate (HAF)
Am(III) 1I-0ctyl(phenyl)-N,N- HNO, HCOOH +HAF Biological sample analysis 62
di isobutylcarbamoy Im ethylphosphine done using 3 compartment
oxide (CMPO) + TBP cell
Am(IJI), CMPO HNO Citric acid, formic Am(IIJ) was recovered from 63
J
Np(IV). acid and hydrazine high level waste
Pu(IV) hydrate
Am(lIl) Dimethyl dibutyl tetradecyl HNO DTPA + hydrazine + Higher rate of transport was 66
J
malonamide HCOOH observed compared to pH 2
solution as strippant
2934 INDIAN J CHEM. SEC. A, DECEMBER 2003
DTPA, OAM hydrazine and OAM formic acid led to medium by an SLM made from Aliquat-336 in xylene
highest transport rate (>60% in 8 hours) when used as as carrier and observed that the flux values for HCl or
the stripping solution66 The effect of organic diluents for uranium were higher at lower carrier concentration
•
on the extraction and hence the transport rates of which was explained as due to the aggregation
3
Am + was quantified by an empirical correlation using behaviour of the carrier molecules in membrane phase
several diluents, viz., n-dodecane, toluene, n-hexane, at large concentrations 73. Effect of phase modifier on
cyc1ohexane, 1,2-dichloroethane, nitrobenzene, uranium transportation from hydrochloric acid
67
decal in and diethylbenzene . medium across supported liquid membrane using
Aliquat-336 as the carrier in xylene medium was
investigated to understand the effect of degree of
Amines as the carriers
74
aggregation of carrier molecules . In this study,
Chaudry et al.68 had used tri-n-octyl amine (TOA)
reduction of uranium flux was observed at carrier
dissolved in xylene as the SLM in Celgard 2400
concentration where viscosity effects were not
polypropylene filter for the transport of uranyl ions
dominant.
from nitric acid solutions to an alkaline aqueous
phase. About 10 hours were needed for the
Macrocyclic carriers in SLM
quantitative transport of uranyl ion from an aqueous
feed of 6M HN03. Several other tertiary amines Shukla et al. 75 studied the uranyl ion permeation
alongwith TOA have been used as carriers in SLM fluxes, physical stability (loss of carrier) and chemical
studies by Sonawane et al.69. The diffusion-limited stability of SLMs using crown ethers in various
and amine-facilitated Pu(IV) cation permeation in aromatic and aliphatic diluents. They have concluded
nitric acid media across a SLM was investigated to that the chlorinated hydrocarbons such as chloroform,
quantify the membrane carrier effects on its transport 1,2-dichloroethane, dichloromethane, afford much
using basic extractants viz. Primene JM-T (JMT) as poor permeability compared to aromatic diluents like
primary, Amberlite LA-2 (Amb LA-2) as secondary, toluene and o-dichlorobenzene. Plutonium transport
trilaurylamine (TLA) and tri-iso-octyl amine (TIOA) could be achieved using O.2M DC18C6/toluene as
as tertiary and Adogen-464 and Aliquat-336 as carrier and sodium carbonate as strippant. The proven
quaternary amines. Recovery of Pu steadily increased extractability and profound selectivity of dicyclo
from primary to quaternary amines as it follows the hexano-18-crown-6 (DC18C6) has been exploited by
order quaternary > tertiary > secondary > primary, selecting this crown ether as the ionophore in liquid
similar to that generally observed in liquid-liquid membrane transport. Macrocycle-facilitated transport
distribution experiments. More than 95% pertraction of Pu(lV) and U(VI) against their concentration
of Pu(lV) was easily accomplished using tertiary or gradient from aqueous nitric acid solutions across
quaternary amine as carrier employing a feed solution SLM containing DC18C6 in toluene have also been
containing about 5 mg/L Pu in 4M nitric acid solution investigated 76. In another study involving the
while the receiving phase was O.IM NH 0H.HCI (in transport of Pu(IV), Shukla et al. have investigated
2
0.3M HN03). On the other hand, plutonium the role of polymer support upon the performance and
permeation was found to plummet to about 49% on stability of an SLM using several solid supports such
employing secondary amine, Amb LA-2, as the as polytetrafluroethylene (TE-35, TE-36, TE-37),
carrier70. polypropylene (HF-PP), cellulose nitrate (BA-S-83)
Lakshmi et al. had investigated the transport of of different pore sizes and varying thickness and
uranyl ion from HCI medium using 30% Aliquat 336 DC18C6 in toluene as the mobile carrier77. Selectivity
7l
in CHCl3 as the carrier . Significant amount of acid of plutonium transport from AISSlon product
transport (about 15%) was observed which affected contaminants such as Cs-137, Ru-103 and Ce-144
the metal ion transport though >90% transport of U was found to be poor with large pore size (0045 11m)
was observed in about 3 hours. In another study support. Polytetrafluroethylene supports like TE-35,
involving Alamine 336, the same authors had shown TE-36, TE-37 and polypropylene HF-PP membranes
that aromatic solvents like toluene or t-butyl benzene did not show any marked deterioration with nitric acid
effect higher transport rates than solvents such as and organic solvents while cellulose nitrate membrane
chloroform or hexane72. Subbarao et al. had such as BA-S-83 was attacked by organic solvent. On
investigated the transportation of uranium from HCI the other hand, these membranes were found to have