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Journal of Membrane Science 403– 404 (2012) 71– 77

Contents lists available at SciVerse ScienceDirect

Journal of Membrane Science

jo u rn al hom epa ge: www.elsev ier .com/ locate /memsci

ole of organic diluents on Am(III) extraction and transport behaviour using,N,N′,N′-tetraoctyl-3-oxapentanediamide as the extractant

. Panjaa, P.K. Mohapatrab,∗, S.C. Tripathia, P.M. Gandhia, P. Janardana

Fuel Reprocessing Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, IndiaRadiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

r t i c l e i n f o

rticle history:eceived 22 September 2011eceived in revised form 6 February 2012ccepted 10 February 2012vailable online 19 February 2012

a b s t r a c t

Diluent effect on the solvent extraction and membrane transport properties of Am(III) using TODGA(N,N,N′,N′-tetraoctyl-3-oxapentanediamide) as extractant/carrier have been investigated in detail. Thediluents studied were n-dodecane, 1-octanol, tert-butyl benzene, kerosene, toluene, carbon tetra chlo-ride, chloroform, 1,2-dichloroethane, and nitrobenzene. The distribution ratio values of Am(III) withTODGA as the extractant were correlated with various diluent parameters. The nature of the extracted

eywords:iluentODGAmericiumupported liquid membraneiffusion coefficient

species formed was also determined for the different diluents. Supported liquid membrane (SLM)transport studies were carried out and the ease of Am transport followed the trend: toluene > n-dodecane > kerosene > 1-octanol > tert-butyl benzene ∼ CCl4 > CHCl3. Effect of membrane pore size onAm(III) transport for each of the diluents have been evaluated. Membrane diffusion coefficient valueshave been calculated using Wilke–Chang equation and compared with those determined experimentallyusing a tlag method.

. Introduction

Solvent extraction (SX) is the work horse of nuclear industryoth at the front and the back ends of fuel cycle. SX based sep-ration methods are popular due to their continuous nature andast mass transfer rates. However, the major disadvantages of SX

ethods include third phase formation, phase entrainment, phaseisengagement limitations, etc. Moreover, due to growing concernsor the environment, large VOC (volatile organic compound) inven-ory make the SX methods unsustainable in the long run. Moreover,here are issues concerning the generation of large volumes of sec-ndary wastes arising from the hydrolytic/radiolytic degradation of

he solvent. In view of these, alternative separation methods withow solvent inventory are required to be developed and evaluatedor separations involved at various stages of the nuclear fuel cycle.

Abbreviations: CMPO, carbamoylmethylphosphine oxide; DCE, 1,2-ichloroethane; DIAMEX, diamide extraction; DIDPA, di-iso-decyl phosphinexide; DMDBTDMA, N,N′-dimethyl-N,N′-dibutyl tetradecyl malonamide; DMDO-EMA, N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide; ESEM, environmental

canning electron microscope; HLW, high level waste; LFER, linear free energy rela-ionships; LSER, linear solvation energy relationships; MACS, magnetically assistedhemical separation; NB, nitrobenzene; PTFE, polytetrafluoroethylene; SLM, sup-orted liquid membrane; SX, solvent extraction; TBB, tert-butyl benzene; TODGA,,N,N′ ,N′-tetraoctyl-3-oxapentane-diamide; TRPO, tri-n-alkylphosphineoxide;OC, volatile organic compounds.∗ Corresponding author. Tel.: +91 22 25594576; fax: +91 22 25505151.

E-mail address: mpatra@barc.gov.in (P.K. Mohapatra).

376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2012.02.022

© 2012 Elsevier B.V. All rights reserved.

Of late, membrane based techniques have been evaluated for vari-ous possible applications in the nuclear fuel cycle [1–3]. Out of themembrane based separation methods, supported liquid membrane(SLM) based separation methods are particularly attractive due tomany advantages such as low ligand inventory, lesser amount ofsecondary waste, selective transport of target metal ions from amixture of contaminants, simultaneous extraction and strippingpossibility, no third phase limitations, ease of operation, easy scaleup, etc. [4].

SX based separation methods have been proposed for the recov-ery of minor actinides from high level waste (HLW) which havebeen one of the proposed strategies of safe management of HLW.This strategy, also known as ‘Actinide Partitioning’ makes use ofreagents capable of extracting minor actinides such as Am, Cmand Np from acidic feed conditions [5]. Though several phospho-rous based reagents such as CMPO (carbamoyl methyl phosphineoxide), TRPO (tri-alkyl phosphine oxide) and DIDPA (di-iso-decylphosphoric acid) have shown promise for actinide partition-ing, generation of large volumes of secondary phosphate wastesrestricts their acceptability [6–9]. On the other hand, tetraalkylmalonamides such as DMDBTDMA (N,N′-dimethyl-N,N′-dibutyltetradecyl malonamide) and DMDOHEMA (N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide) have been found promising and

have been proposed in the DIAMEX (diamide extraction) pro-cess [10]. Sasaki et al., on the other hand, have reportedexceptionally high extraction ability of N,N,N′,N′-tetraoctyl-3-oxapentanediamide (TODGA) towards the minor actinides which

7 brane

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tbpnemberTpSeoSebarb[p[

Aesc

2

2

Hoba(5tbnfwdag

2

e0wi

2 S. Panja et al. / Journal of Mem

ed to large amount of research work carried out using this reagent11–13]. Though extensive SX studies were carried out usingODGA and mixer-settler runs have been reported [14–16], simul-aneous SLM studies have also been undertaken involving theransport of actinides and lanthanides [17–20]. Recent studies withollow fibre supported liquid membranes containing TODGA as thearrier extractant have shown promise for possible application toadioactive waste remediation [21].

Various properties of organic liquids that are responsible forheir chemical behaviour (including metal ion extraction), haveeen described in detail by Marcus [22]. These properties areolarity, hydrogen bonding ability, basicity, acidity and structured-ess etc. Linear free energy relationships (LFER) or linear solvationnergy relationships (LSER) have been proposed to relate the aboveentioned properties to various processes in solution such as: solu-

ility, distribution between two liquids, free energy and enthalpy ofquilibria, rates of reactions, etc. Sasaki et al., have also studied theole of several organic diluents on the extraction of actinides usingODGA as the extractant [13]. They have indicated that the diluentroperties seriously influence the extractability of the metal ions.ubsequently, Ansari et al., have also investigated the role of dilu-nts, though no specific conclusions were drawn [23]. However, tour knowledge, there is not any report on the role of diluent on theLM transport behaviour of metal ions using TODGA as the carrierxtractant. The role of diluent has been systematically investigatedy Sriram and Manchanda for Am(III) transport using DMDBTDMAs the extractant [24]. We on the other hand, had investigated theole of diluents on the transport of Sr(II) transport using di-tert-utyl-dicyclohexano 18-crown-6 (DTBCH18C6) as the extractant25]. Bartsch et al., have investigated the effect of solvent in com-etitive alkali metal cation transport across a bulk liquid membrane26].

The present work deals with the transport studies involvingm(III) pertraction in SLM studies using TODGA as the carrierxtractant in the presence of a variety of diluents. Effect of poreize for various diluents has been investigated in detail. Diffusionoefficients have been calculated for each diluent.

. Experimental

.1. Materials

TODGA, obtained from Thermax, India, was characterized byPLC, NMR, IR, and elemental analysis. PTFE membranes werebtained from Sartorius, Germany. Membrane thickness and mem-rane porosities were measured using Mitutoyo Digital micrometernd Electroscan 2020 environmental scanning electron microscopeESEM) respectively. Porosities of the PTFE membrane filters were1, 64, 74 and 84% for 0.2, 0.45, 1.2 and 5.0 �m. AR grade 1-octanol,oluene, kerosene were procured from BDH (Mumbai) while tert-utyl benzene (TBB), chloroform, carbon tetrachloride, n-dodecane,itrobenzene (NB) and 1,2-dichloroethane (DCE) were procured

rom Merck (India) at a purity level of 99% or above and were usedithout further purification. 241Am was purified using a procedureescribed elsewhere [27] and its purity was checked using alphas well as gamma ray spectrometry. All other reagents were of ARrade and were used as such if not mentioned otherwise.

.2. Solvent extraction studies

Distribution experiments were carried out by equilibrating

qual volumes (usually 1 mL) of the extractant solution (usually.1 M TODGA) in various diluents (pre-equilibrated with 1 M HNO3)ith the aqueous phase of 1 M HNO3 spiked with 241Am tracer

n stoppered Pyrex glass tubes in a thermostated water bath for

Science 403– 404 (2012) 71– 77

1 h at a temperature of 25 ± 0.1 ◦C. 100 Microlitre aliquots werewithdrawn from each phase after centrifugation of the tubes and241Am activity in both phases were measured by gamma count-ing using a well type NaI(Tl) detector. Distribution ratios (Kd) weremeasured by the ratio of activity in the organic phase to that ofaqueous phase. Each experiment was carried out in triplicate andthe mass balance of the accepted data points were within errorlimits of ±5%.

2.3. Transport studies

A two-compartment Pyrex glass transport cell having 30 mLfeed phase and receiver phase capacities was used for the trans-port experiments. The PTFE membrane filters were soaked for about30 min in the carrier solution (0.1 M TODGA in different diluents)prior to being mounted in between the flanges of the transportcell. During all the experiments, the feed compartment contained1 M HNO3 while the receiver phase contained 0.1 M HNO3. Thefeed and the receiver compartment solutions were mixed by syn-chronous motors stirred at 200 rpm as standardized in a previousstudy [28]. Aliquots were removed from feed as well as receivingphase after regular intervals and the 241Am activity in the sam-ples was assayed as mentioned above. The transport studies werecarried out at ambient temperature (24 ± 1 ◦C) and the materialbalance in these studies was found to be within ±5%.

2.4. Transport equations

Transport process in the supported liquid membranes mainlyinvolves three steps, viz. extraction at the feed–membraneinterface, diffusion inside the membrane and stripping at themembrane–receiver interface. The transport experiments arecarried out under the conditions that the distribution ratio (sym-bolized as Kd) is much larger at the feed–membrane interface ascompared to the membrane–receiver interface. Under steady statecondition, by ignoring the concentration of the metal ion in thereceiver phase one can get the flux (J) from the following equation[29]:

J = Pf Cf (1)

where Pf is the feed side permeability coefficient while Cf is theconcentration of the metal ion at the feed side. The flux can also beexpressed by the following equation:

J = − 1Q

· dVfCf

dt(2)

where Vf is the feed volume and Q is the exposed area of the mem-brane. Combining Eqs. (1) and (2), and integrating one obtains,

ln

(Vf,0Cf,0

Vf,tCf,t

)= QPt

Vf(3)

where Vf,0, Cf,0, Vf,t and Cf,t represent the volume and concentra-tion of feed at the beginning of the transport study and after time‘t’, respectively. If the volume of the feed does not change signifi-cantly during the transport study, then the following expression isobtained:

ln

(Cf,t

Cf,0

)= − Q

VfPt (4)

Q can be expressed as the product of the geometrical surface area

(A) and the porosity of the membrane (ε). The permeability coeffi-cient (P) values were calculated using Eq. (4). The geometric surfacearea of the membrane was 4.94 cm2 while the effective surface areafor the 0.2 �m PTFE membrane was measured as 2.51 cm2. The

rane Science 403– 404 (2012) 71– 77 73

cb

%

Ew

3

3

iefi(s[tooeCz1ntNodwhWwertta

ttqtiwRdrdcsToraieTeTe

2322212019181716

0

50

100

150

200

Toluene

CCl4

n-dodecane

Chloroform

DCE

1-octanol

NB

Hansen's cohesion parameter

Kd

,Am

They have observed that 3 molecules of dimethyldibutyltetradecyl-1,3-malonamide (DMDBTDMA) attach with Am(III) in varyingdiluents. So both the extractability and nature of species of Am(III)were bound to be dependent on the diluent used for TODGA as

0

50

100

150

200

250

Toluene

CCl4 DCE

Chloroform

1-Octanol

Nitrobenzene

Kd

,Am

S. Panja et al. / Journal of Memb

umulative percent transport (%T) at a given time is determinedy the following equation,

T = Cf,0 − Cf,t

Cf,0× 100 (5)

The %T as well as permeability coefficient data calculated usingqs (4) and (5), respectively. The material balance in these studiesas found to be within ±5%.

. Results and discussion

.1. Solvent extraction studies

The membrane carrier (extractant) and solvent (diluent) maynfluence the transport of metal ion through SLM in many differ-nt ways. The different properties of diluents that are responsibleor the extraction of a particular metal ion are polarity, polarizabil-ty, H-bonding ability, viscosity, Hildebrand’s solubility parameterı) or Hansen’s cohesion parameter (extended the Hildebrandt’solubility parameter method to polar and H-bonding systems30]), etc. It was, thus, required to carry out solvent extrac-ion studies using a variety of diluents to understand the rolef the organic diluents. These studies included the extractionf Am(III) from 1 M HNO3 using 0.1 M TODGA in various dilu-nts like n-dodecane, nitrobenzene (NB), chloroform, 1-octanol,Cl4, kerosene, 1,2-dichloroethane, toluene and tert-butyl ben-ene. The values for Kd,Am varied from 0.7 in case of CHCl3 to95 in case of NB where the feed was 1 M HNO3. Apparently,o regular trend with the diluent polarity or H-bonding interac-ions was seen. Though the highest Kd,Am values were noted withB, the trend with respect to Kd,Am value is as follows: NB > 1-ctanol > n-dodecane> tert-butyl benzene (TBB) > kerosene > 1,2-ichloroethane (DCE) > toluene > CCl4 > CHCl3 (Table 1). Studiesith TODGA have been carried out by Sasaki et al. [13], and theyave also shown the same trend as reported in the present work.hile the Kd,Am value obtained in the present study matches veryell with that obtained by Sasaki et al., for n-dodecane as the dilu-

nt, those with NB was somewhat lower and was more close to thateported by Ansari et al. [23], in another study. Similarly, the dis-ribution ratio with CHCl3 was found to be about five times higherhan that reported by Sasaki et al. [13]. The reason for the exception-lly high Kd,Am value obtained with NB is not clearly understood.

The respective Kd,Am values for different diluents along withhe Hansen’s cohesion parameter and Schmidt diluent parame-er [22] are presented in Figs. 1 and 2, respectively. Though it isuite evident that diluents have profound effect on the extrac-ion of Am(III), no specific correlation was possible with these twomportant diluent parameters (though a rough correlation was seen

ith the Schmidt’s diluent parameter with the exception of NB).oy Chowdhury and Sanyal have made different correlations withifferent diluents with TBP as the extractant [31]. They have cor-elated the Hildebrandt’s solubility parameters for the non-polariluents while for the polar diluents the polarity coefficient wasorrelated. Attempts were made to correlate the dielectric con-tant of the medium (Table 1) with the distribution ratio values.he high Kd,Am with NB can be attributed to the high polarityf the diluent system while 1-octanol with next highest polarityesulted in reasonably high Kd,Am value. On the other hand, CHCl3nd DCE resulted in low Kd,Am values due to hydrogen bondingnteractions and large enthalpy loss expected for the metal ionxtraction with these diluents. Aromatic diluents like toluene and

BB interact with the extractant making the availability of freextractant molecules (responsible for metal ion extraction) lower.his could be the reason for the low Kd,Am values with such dilu-nts. Higher Kd,Am values in case of n-dodecane was attributed to

Fig. 1. Correlation of Hansen’s cohesion parameters with Kd,Am for various diluentsused in the present work.

aggregation of the extractant leading to reverse micelle formation(vide infra).

The nature of species formed by TODGA–Am(III) in differentdiluents was not reported by Sasaki et al., while it was reportedby Ansari et al. for five different diluents. This was done by plot-ting log Kd,Am vs log [TODGA] at a fixed acidity. In the present work,the extracted species for nine different diluents were determinedand the log–log plots are presented in Fig. 3. The slope value foreach diluent with the error limits is listed in Table 2. Literaturedata by Ansari et al. [23], is also listed in the table. As evidentfrom Fig. 2 and Table 2 the number of TODGA molecules asso-ciated in the complexed species varied from 2 to 4 for varyingdiluents. Chloroform which was found to give lowest extractabil-ity had maximum four molecules of TODGA attached with Am(III).Nitrobenzene which showed maximum extractability had twomolecules of TODGA attached with Am(III). These findings donot match with the observations of Sriram and Manchanda [24].

7654321

Schmidt's diluent parameter

Fig. 2. Correlation of Schmidt’s diluent parameters with Kd,Am for various diluentsused in the present work.

74 S. Panja et al. / Journal of Membrane Science 403– 404 (2012) 71– 77

Table 1Distribution ratios of Am(III) with different diluents using 0.1 M TODGA and 1 M HNO3 as the feed solution.

Diluent Dipole moment (D)a Dielectric constanta Distribution ratio values (Kd,Am)

Present work Ref [13] Ref [23]

Nitrobenzene 4.22 35.6 195 220 202tert-Butyl benzene 0.83 2.359 3.55 – –Toluene 0.37 2.379 1.57 0.3 0.31-Octanol 1.8 10.3 65.2 81 68Chloroform 1.04 4.81 0.7 0.12 –CCl4 0 2.237 0.097 0.095 –1,2-Dichloroethane 1.8 10.42 10.4 9.9 12n-Dodecane ∼0 2.012 30.3 30 24Kerosene – – 14.1 – –

a Data taken from Ref. [33].

0.10.01

1

10

100

1,2-Dichloroethane

Tert butyl benzene

Octanol

Kerosene

n-Dodecane

Nitro benzene

CCl4 Toluene

CHCl3

[TODGA], M

Kd

,Am

Fe

eArpATa

A

woin

TSd

N

300240180120600

0

20

40

60

80

100

1-Octanol

TBB

Toluene

n-Dodecane

CCl4

Kerosene

CHCl3

% A

m t

ran

sp

ort

ed

ig. 3. Log–log plot of Kd,Am vs TODGA concentration at 1 M HNO3 in different dilu-nts.

xtractant. When we tried to correlate the extraction strength ofm(III) by TODGA in different diluents, no single parameter cor-elation was found to be valid. So it is imperative to use multiarameter approach to explain the variations in extractability ofm(III) of 0.1 M TODGA in different diluents from HNO3 medium.he equation describing the extraction equilibrium for nitrate ionssisted complexation of Am(III) is shown below:

m3+ + 3(NO3−) + nTODGA(o) � Am(NO3)3.n(TODGA)(o) (6)

here the species with the subscript ‘(o)’ indicate those in the

rganic phase and those without any subscript indicate speciesn the aqueous phase. Though the value of ‘n’ is close to 4 in-dodecane as the diluent, the structure of the complex is not

able 2lopes of log–log plot of Kd,Am as a function of TODGA concentration in variousiluents at 1 M HNO3.

Diluent Slope

Nitrobenzene 1.75 ± 0.14 (1.76 ± 0.06)Chloroform 3.80 ± 0.36CCl4 2.44 ± 0.25Toluene 3.28 ± 0.14 (3.58 ± 0.09)n-Dodecane 3.87 ± 0.04 (3.68 ± 0.12)Kerosene 2.15 ± 0.241-Octanol 2.24 ± 0.06 (2.33 ± 0.03)tert-Butyl benzene 2.99 ± 0.151,2-Dichloroethane 1.87 ± 0.11 (1.92 ± 0.07)

ote: Values inside parentheses are taken from Ref. [23].

Time (min)

Fig. 4. Transport of Am(III) from 1 M HNO3 using 0.1 M TODGA in different diluents.

reported yet. Jensen et al. [32] from their small angle neutronscattering (SANS) studies have proposed that a reverse micelle con-taining four TODGA molecules may be responsible for the metal ionextraction. However, the nature of the feed acid and its concentra-tion has a strong influence on the aggregate formation.

3.2. Transport studies

To understand the effect of diluents on the transport of Am(III),several experiments were carried out using 0.1 M TODGA as the car-rier, 1 M HNO3 as feed solutions and 0.1 M HNO3 as the receivingphase. Fig. 4 shows the corresponding transport profiles of Am(III).

Maximum transport of Am was observed for toluene as the diluentwhereas CHCl3 was found to result in minimum Am transport. Thepercentage transport data after 5 h for different diluents using 0.1 MTODGA are listed in Table 3. The trend with respect to transport

Table 3% Transport of Am(III) from 1 M HNO3 medium using 0.1 M TODGA in differentdiluents.

Diluent %T (5 h) P(×104 cm/s)

Viscosity(mPa S)a

Density(g/cc)a

1-Octanol 78.88 11.8 1.8561 0.6451TBB 73.30 9.6 2.0102 0.5723Toluene 90.93 18.8 1.7213 0.4218n-Dodecane 89.93 15.4 1.6680 0.7596CCl4 69.95 9.8 2.2010 1.5921Chloroform 49.01 5.6 2.1501 1.5021Kerosene 86.24 14.6 1.9509 0.6321

a Data refer to 0.1 M TODGA solution in the respective diluent.

rane Science 403– 404 (2012) 71– 77 75

ooabatmsstAoftaArbm

itdiAthKt(aiwawhTlrtfi

3

oacb[

P

wisfrimwwpi

543210

4

8

12

16

20

n-dodecane

1-octanol

kerosene

TBB

Px

10

4 , (c

m/s

)

Pore size( μm)

Fig. 5. Effect of membrane pore size on the P values of Am(III) for 0.1 M TODGA in

543210

0

5

10

15

20

25

30

35

40

45

Px

10

4, (c

m/s

)

toluene

CCl4

CHCl3

S. Panja et al. / Journal of Memb

f Am(III) is as follows: toluene ∼ n-dodecane > kerosene > 1-ctanol > tert-butyl benzene > CCl4 > CHCl3 (Fig. 4). Data with NBre not included as it was not possible to wet the PTFE mem-ranes with the diluent. Carrier facilitated transport of metal ioncross SLM is dependent on various parameters out of whichhree factors assume utmost significance viz., extractability of the

etal ion by the carrier at the membrane–feed interface, diffu-ion of the metal–carrier complex through the membrane phase,tripping of the metal ion from the metal–carrier complex athe membrane–strip interface. Stripping of the metal ion fromm(III)–TODGA complex was found to be independent of the naturef the diluents as 0.1 M TODGA showed Kd,Am < 10−3 at 0.1 M HNO3or all the diluents studied in the present work. Therefore, the Amransport rates are dependent on the first two factors as mentionedbove. From the trends of Kd,Am values and the transport rates ofm, it is inferred that there is no parallel between the transportate and the distribution ratios. Thus, the extractability of Am(III)y TODGA though important is not the sole deciding factor for theetal ion transport.The diffusion of the metal–carrier complex through membrane

s dependent on various factors like, nature of species, viscosity ofhe carrier, polarity of the solvent and H-bonding ability. As will beiscussed below, the size of the diffusing species and the viscos-

ty of the medium are two very important factors in this regards.s shown in Table 3, toluene showed maximum transport rate

hough the Kd,Am value with it was not very high. On the otherand, poor transport rates with CHCl3 is both due to the lowestd,Am value and extraction of a tetra-solvate species which can leado slow diffusion rates as indicated by the Wilke–Chang equationvide infra). Toluene extracts tri-solvated species and has reason-bly low viscosity (higher than only n-dodecane out of the diluentsnvestigated) which compensated for the low Kd,Am value obtained

ith the diluent. Amongst the other diluents, tetra-solvated speciesre extracted also with n-dodecane whereas di-solvates are formedith kerosene. However, the Am transport rate was found to beigher in n-dodecane as compared to that observed in kerosene.his could be due to higher distribution ratio obtained with andower viscosity of n-dodecane as compared to kerosene. Theseesults indicated that diffusion has a much more significant roleo play in the transport of the metal ion and the nature of the dif-using species and viscosity of the medium are of great relevancen the overall transport rates.

.3. Effect of membrane pore size

Membrane pore size plays an important role in the transportf a metal ion across SLM. In a diffusion controlled process across

polymeric matrix, the permeability coefficient (P) value can beonsidered directly proportional to the membrane pore size as cane seen from the d’Arcy equation [34] as proposed by Rodgers et al.35],

= εR2

�2do(7)

here R is the membrane pore size, � is the tortuosity factor, εs the membrane porosity and do is the membrane thickness. Thisuggests that with increasing pore size of the membrane the dif-using species experiences less hindrance and hence the transportate increases with it. However, it cannot increase indefinitely asncreasing pore size results in release of the organic phase from the

embrane. This can be explained on the basis of Laplace’s equation

hich suggests that the minimum trans-membrane pressure (�p)hich is required to displace the extractant from the membranehase is inversely proportional to the membrane pore radius. Hence

ncrease in pore radius results in enhanced release of the organic

varying diluents.

phase from the membrane causing the transport rate to decrease.These two factors play opposite role towards transport rate.

The effect of membrane pore size on the transport rate of Am(III)for TODGA in different diluents is presented in Figs. 5 and 6 whichshow the plot of P vs membrane pore size for different diluents. Asevident from the figures, there is no particular trend as far as varia-tion of membrane pore size is concerned. For n-dodecane, kerosene,1-octanol and tert-butyl benzene (TBB), the P values decrease withincreasing membrane pore size. On the other hand, for toluene,CHCl3 and CCl4 no particular trend was observed. While an increasein the P values was seen in case of toluene up to 1.2 �m pore size asubsequent increase in pore size resulted in decrease in the P value.For the chloro diluents, the P value decreased while increasing thepores size from 0.2 �m to 0.45 �m and further increase to 1.2 �mresulted in an increase which finally showed a decreasing trend formembranes with 5 �m pore size.

Pore size (μm)

Fig. 6. Effect of membrane pore size on the P values of Am(III) for 0.1 M TODGA invarying diluents.

76 S. Panja et al. / Journal of Membrane

Table 4Diffusion coefficient values of Am(III) for 0.1 M TODGA in varying diluents, feed: 1 MHNO3, strip: 0.01 M HNO3, membrane pore size: 0.2 �m.

Diluent Lag time(tlag) (min)

Deff (×106 cm2/s) (lagtime method)

Do (×106 cm2/s)(Wilke–Chang) [15]

Dodecane 9 5.10 5.331-Octanol 10 4.59 4.75Toluene 9.5 4.83 5.35Kerosene 9.4 4.92 5.16tert-Butyl benzene 10.5 4.37 4.67

3

id

D

w5erpa8Tlc

D

wnttipeosemc

4

ittwttrrtmedmes

[

[

[

CHCl3 12 3.83 4.17CCl4 11 4.17 4.45

.4. Calculation of diffusion parameters

The diffusion coefficient Do can be calculated as per the empir-cal Wilke–Chang equation [36] for the bulk diffusion coefficientefined as:

o = 7.4 × 10−8 �0.5M0.5T

�V0.6m

(8)

here M, � and � are the molecular weight (M for TODGA is80 and for Am(NO3)3·nTODGA, M varies from diluent to dilu-nt), solvent association parameter and the viscosity of the solvent,espectively, Vm the molar volume of TODGA and T is the tem-erature. The association parameters for the diluents are takens unity [36]. The molar volume of TODGA was calculated to be34.06 cm3 mol−1[37]. The calculated D0 values are reported inable 4. The effective diffusion coefficient (Deff) can also be calcu-ated from lag time (tlag) measurements involving the metal carrieromplex from the following expression [38]:

eff = d2oε

6tlag(9)

here ε is the membrane porosity and do is the membrane thick-ess. For 0.1 M TODGA as the carrier and 180 �m as the membranehickness (three 60 �m membranes were put together) the lagimes for Am(III) were measured for varying diluents. Feed acid-ty was maintained at 1 M HNO3. The Deff values calculated by therocedure are also reported in Table 4 along with tlag values. Inter-stingly, the diffusion coefficient values were comparable for mostf the diluents and the experimentally determined Deff value isomewhat lower than the value calculated by the Wilke–Changquation (vide supra). This is possibly due to the fact that threeembranes of 60 �m are used instead of a single membrane of a

ontinuous thickness of 180 �m.

. Conclusions

The solvent extraction and supported liquid membrane stud-es using TODGA dissolved several organic diluents have indicatedhat diluent properties play an important role in the overall extrac-ion and transport efficiencies. Though high extraction efficiencyas noticed in polar diluents such as nitrobenzene and 1-octanol,

he unusually high extraction observed in n-dodecane is attributedo reverse micellar mechanism where an aggregate of TODGA isesponsible for Am(III) extraction. In sharp contrast to the SXesults, liquid membrane transport data indicated efficient massransfer in non-polar diluents. This suggests diffusion limited

ass transfer which is primarily dependent on the nature of thextracted species and diluent viscosity. Pore size data indicates

ecreased in permeability coefficients with increasing pore size inost of the diluents and exceptional behaviour was seen in dilu-

nts such as toluene and CCl4 where maximum mass transfer waseen for 0.45 �m pore size membranes. No significant difference in

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Science 403– 404 (2012) 71– 77

the diffusion coefficients were observed which showed good matchbetween the calculated and experimentally determined values.

Acknowledgement

P.K.M. wishes to thank Dr. A. Goswami, Head, RadiochemistryDivision for his keen interest and helpful suggestions.

Nomenclature

SymbolsA geometrical surface areaCf,t concentration of metal ion in aqueous feed at time tCf,0 initial metal ion concentration (at t = 0)Do diffusion coefficient in the membrane phaseJ overall flux in the transport processKd distribution ratio of metal at feed membrane inter-

faceKex two-phase extraction constantM molecular weightP permeability coefficientQ effective surface areaR pore radius%T percent transportVf time average aqueous feed volumeVf,0 feed volume at zero timeVf,t feed volume at a given time, tVm molar volume

Greek lettersε porosity of the membrane� viscosity of the solvent� membrane tortuosity� solvent association parameter

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