Diglycolamide-Functionalized Calix[4]arene for Am(III)Recovery from Radioactive Wastes:Liquid Membrane Studies Using a Hollow Fiber Contactor Seraj A.Ansari,?Prasanta K.Mohapatra,*,?Pankaj Kandwal,?,#and Willem Verboom §
?Radiochemistry Division,Bhabha Atomic Research Centre,Trombay,Mumbai ?400085,India
?Department of Sciences &Humanities,NIT Uttarakhand,Temporary campus ?Polytechnic Institute,Srinagar (Garhwal)?246174,Uttarakhand,India
§Laboratory of Molecular Nanofabrication,MESA +Institute for Nanotechnology,University of Twente,P.O.Box 217,7500AE Enschede,The Netherlands
1.INTRODUCTION Diglycolamides (DGAs),a new class of diamide extractants,have been extensively studied in research related to “actinide partitioning ”,a key step in radioactive waste management.1?5The DGA ligands such as TODGA (N ,N ,N ′,N ′-tetra-n -octyl diglycolamide)display anomalous metal ion selectivities as trivalent actinide ions are better extracted as compared to the tetravalent or hexavalent actinide ions,which is an opposite trend expected from ionic potential considerations seen with analogous extractants such as CMPO (carbamoylmethyl-phosphine oxide)and malonamides (extraction of trivalent metal ions are expected to be less than tetravalent and hexavalent metal ions).6Attempts to understand this anomalous selectivities of DGA ligands with f -elements revealed that these ligands form reverse micelles in para ?nic solvents,such as n -dodecane,which apparently facilitate Am(III)extraction to a much greater extent than that of Pu(IV)and U(VI).7?9In view of the reverse micelle formation tendencies of the DGA ligands,attempts were made to synthesize multiple-DGA-functionalized ligands,where DGA moieties are appended to a pivot atom to form a tripodal platform 10?14or to a calix[4]arene 15or a pillar[5]arene skeleton 16to mimic the aggregate structure of DGA.During the last couple of years,DGA-functionalized calix[4]arene (C4DGA)ligands have been extensively studied for their complexation and separation behavior toward trivalent actinides from acidic feed solu-tions.17?20Solvent extraction studies indicated that these
C4DGA ligands show several orders of magnitude higher extraction of Am(III)than the bare DGA ligands such TODGA.20The results of solvent extraction studies with these ligands are very promising and encourage their use in supported liquid membranes,with signi ?cantly lower ligand inventory.
Though solvent extraction has been widely used for the separation of metal ions in the nuclear industry,generation of large volumes of spent extractants as the secondary waste,particularly for the treatment of dilute waste solutions,encourages the use of other techniques of which the waste generation is several orders of magnitude lower.One such alternative technique is the liquid membrane,which is a combination of solvent extraction and membrane di ?usion,which o ?ers promising alternative for metal ion separations from dilute feed solutions.21?23Key features of liquid membrane based separations are (i)simultaneous extraction and stripping of the solute (in this case,metal ions),(ii)transport rates are controlled by the di ?usion of the solute,and (iii)low secondary waste generation.Out of the liquid
membranes,supported liquid membranes are particularly attractive and there are numerous studies,involving FSSLM (
?at sheet supported liquid membrane)
and HFSLM (hollow
?ber supported liquid membrane),reported for the treatment of nuclear waste.21?23Recently,we have demonstrated the feasibility of using the C4DGA ligand in FSSLM for the separation of Am(III)from nitric acidic feeds.24The study was performed at 20mL scale Received:November 2,2015
Revised:January 14,2016
Accepted:January 25,2016
Published:January 25,2016
with a 3.14cm 2membrane surface area,and the results were highly encouraging.However,the small surface area in FSSLM does not allow any practical application of the method.In this context,use of the HFSLM technique,with signi ?cantly large surface area,will be a more e ?cient way for the recovery of Am(III)from radioactive waste solutions which could result in large throughput values.In the present paper,therefore,the separation behavior of Am(III)was explored using a C4DGA-based HFSLM.The e ?ect of feed acidity and Nd(III)concentration on the Am(III)transport was studied.The permeability coe ?cient,mass transfer coe ?cient,and various di ?usional parameters of the metal ?ligand complex were calculated to understand the permeation behavior of Am(III).A mathematical model developed to predict Am(III)permeation rates was found to conform well to the experimental data.2.EXPERIMENTAL SECTION 2.1.Materials.C4DGA (Figure 1)was synthesized by
acylation of tetrakis(aminopropoxy)calix[4]arene with propion-yl chloride,with subsequent reduction of the product with LiAlH 4.Subsequently,the N -alkylated tetrakis(aminopropoxy)-calix[4]arene was reacted with p -nitrophenol activated diglycolamide (DGA)in the presence of triethylamine to give the desired product,C4DGA.The detailed synthesis procedure has been described elsewhere.15Characterization of the C4DGA was done by 1H NMR and HR-MS as well as IR spectroscopy.The radiotracer 241Am (from laboratory stock solution)was freshly puri ?ed before use and its radiochemical purity was ascertained by gamma ray spectroscopy using an HPGe detector (Note :Extreme precautions must be taken while handling 241Am because of the high radiation hazards from alpha radioactivity ).Nd 2O 3(99.99%,Alpha Biochem)was dissolved in nitric acid to yield the stock solution of Nd(III).Suprapur nitric acid (Merck)and Milli-Q water (Millipore)were used to prepare the nitric acid feed solutions which were subsequently standardized by acid ?base titrations using phenolphthalein indicator (Fluka).Standardization of the Nd(III)stock solution was done by complexometric titration using EDTA (complex-ing agent)and methyl thymol blue (indicator).25All the other reagents were of AR grade and were used as received from the suppliers.2.2.Hollow Fiber Supported Liquid Membrane Studies.A LiquiCel hollow ?ber membrane module (Alting,France)whose speci ?cations are given in Table 1was used in the present study.The HFSLM was made by passing the extractant (C4DGA)solution through the lumen side of the module.A pressure of about 20kPa was applied at the other end of the module to enable the C4DGA solution to percolate from the lumen side to the shell side.After soaking the
hollow ?ber
membrane,the C4DGA solution sticking to the hollow ?ber surface was washed out completely by passing distilled water from both the shell side and the tube side.A schematic diagram of the hollow ?ber membrane setup is shown in Figure 2.This technique of preparing HFSLM was found to be
reproducible with respect to Am(III)transport as the percent transport data were reproducible within an error limit of 5%.The stability of HFSLM membranes containing neutral ligands dissolved in para ?nic solvent such as n -dodecane,a case similar to the present work,were found to be excellent over several weeks of continuous operation.21The HFSLM experiments were performed by passing the feed and the strip solutions through the tube and the shell sides,respectively in countercurrent direction.Gear pumps with very precise ?ow controllers (Cole Parmer)were used to control the ?ow rates of the feed and strip solutions at 200mL/min.The volume of feed and strip solutions was 300mL each.Samples (1?2μL)from the feed and the receiver compartments were taken out at regular intervals and assayed for subsequent use to calculate the Am(III)permeability.241Am was assayed radiometrically
by
Figure 1.Structural formula of the diglycolamide-functionalized calix[4]arene (C4DGA).
Table 1.Speci ?cations of Hollow Fiber Membrane Contactor (LiquiCel Mini Module G543)Used in the Present Work
parameter speci ?cation ?ber material polypropylene number of ?bers 2200
?ber internal diameter (μm)240
?ber outer diameter (μm)300
?ber wall thickness (μm)30
e ?ective pore size (μm)0.03
porosity (%)40
tortuosity 2.5
e ?ective ?ber length (cm)11.8
e ?ective surface area (m 2)
0.18
Figure 2.Schematic diagram of the hollow ?ber contactor setup used in the present study.The aqueous feed passed through the tube side and
the aqueous strip phase passed through the shell side of the module.
counting the 60Mev γrays using a well type NaI(Tl)scintillation detector (Para Electronics)coupled to a multi-channel analyzer (ECIL,India).The concentration of Am(III)in the feed solution was at tracer level,which is about 10?7
mol/L.The Nd(III)in the feed was used as surrogate of Am(III)as handling of mg level of Am(III)under our experimental setup was not possible.In view of comparable chemistry of the trivalent actinide and lanthanides,the transport of 241Am was taken as same as that of Nd.An identical (within experimental error)extraction behavior of Nd(III)and Am(III)was con ?rmed independently by measuring their distribution coe ?cient values in solvent extraction.Cotransport of nitric acid was studied by the estimation of the acid in the receiver compartment by standard acid ?base titrations using phenolphthalein indicator.The transport data were reproducible within ±3%.3.RESULTS AND DISCUSSION 3.1.Metal ?Ligand Complexation Equilibria.The equilibrium reaction between Am(III)and the C4DGA ligand,taking place at the feed ?membrane and the membrane ?receiver interfaces,is described as follows:24++?·+?
x x Am 3NO E Am(NO )E (aq)33(aq)(mem)33(mem)(1)where,the term E stands for the extractant,C4DGA,while the species with the subscripts (aq)and (mem)indicate those present in the aqueous and the membrane phases,respectively.The species,Am(NO 3)3·x E,represents the extracted complex in the membrane phase (or organic phase supported inside the pores of the membrane support).The term “x ”is the number of C4DGA molecules present in the metal bearing complex,which has been con ?rmed to be 1in the earlier solvent extraction studies.24The two-phase extraction constant (K ex )for eq 1is given as =·K Am(NO )E [Am ][NO ][E]ex 33(mem)
(aq)3(aq)(mem)(2)Similarly,the distribution coe ?cient (K d )of the metal ion in equilibrium reaction 1is represented as =·K Am(NO
)E [Am ]d 33(mem)
(aq)(3)The K
ex value of Am(III)with C4DGA was calculated from the measured K d values under the given experimental conditions.
The transport of Am(III)across the liquid membrane can be best explained with the help of the extraction equilibrium given by eq 1.By proper adjustment of the feed condition one can ensure the e ?ective transfer of the metal bearing complex,Am(NO 3)
3·E,to the membrane phase.The complex (Am-(NO 3)3·E)is subsequently di ?used inside the membrane to the membrane-receiver interface as a consequence of the di ?erence in the concentrations of the complex.Similarly,the receiver phase condition shifts the equilibrium (eq 1)in the reverse direction ensuring the back extraction of the metal ion.This
results in the dissociation of the Am(NO
3)3·E complex and release of the ligand (C4DGA),for subsequent transportation of the metal ion in the membrane phase,and Am 3+and an equivalent number of nitrate ions in the receiver phase.The free ligand molecules then move toward the feed-membrane interface by di ?usion due to the negative concentration gradient to complete the transport cycle.26
3.2.Permeation of Am(III)from Nitric Acid.The
permeation of Am(III)at tracer concentrations (ca.10?7M)
was investigated from a feed solution containing 3M HNO 3to the receiver phase containing distilled water.As shown in Figure 3,near quantitative Am(III)transport was possible in <20min at 300mL feed scale.It is worth mentioning that under identical feed and receiver compositions in studies involving FSSLM (20mL feed and receiver phase volumes),>4h were needed for near quantitative Am(III)transport.24The higher transport e ?ciency in the present case is obviously due
to the signi ?cantly large e ?ective surface area (1800cm 2)
provided by the hollow ?ber contactor as compared to the FSSLM (3.14cm 2).In order to assess the e ?ect of the concentration of the metal ion in the feed (containing 241Am radiotracer)on the transport e ?ciency,macro concentrations of Nd (used as a representative of trivalent lanthanides and a surrogate of Am)were added to the feed.It should be noted that high level waste may contain 0.5?2g/L total lanthanides.1,2In view of very similar chemistry of
Am(III)
Figure 3.E ?ect of Nd concentration on the transport of Am(III)by C4DGA-HFSLM:carrier 1mmol/L C4DGA in 5%isodecanol/n -dodecane;
feed 3M HNO 3(300mL);strippant distilled water (300mL);?ow rate 200mL/min.(a)Transport pro ?le of Am(III)with increasing Nd concentration in the feed.(b)Plot of ln(C t /C o )vs time to calculate the P values.
and Nd(III),their transport e ?ciencies are expected to be same.However,in the presence of macro concentrations of Nd in the feed,it was expected that the percent metal ion transport would be slower due to loading of the metal ions by the carrier ligand present in the membrane phase.The results indicated quantitative transport of Am(III)(Figure 3a)in 5h when the feed solution contained 0.05?0.1g/L Nd.On the other hand,the permeation of Am(III)was signi ?cantly lower even after 5h of operation when the feed Nd concentration increased to 0.5g/L.The Am(III)transport in 5h for di ?erent feed solute concentrations was:>99.9%,99.5%,97.2%,84.6%,67.2%,and 31.1%for 0.05,0.1,0.2,0.3,0.4,and 0.5g/L Nd,respectively.As shown in Figure 3b,the slope of the curve decreased with increased Nd concentration in the feed solution indicating a decrease in the permeation rate.3.2.1.Calculation of Permeability Coe ?cient.In HFSLM,the permeation of metal ions from the feed side to the receiver side takes place in the following three steps:(i)extraction of the metal ions by the ligand present in the liquid membrane,(ii)di ?usion of the extracted complex from the feed side of the SLM to its receiver side,and (iii)back-extraction of the metal ion at the membrane-receiver interface.Considering (i)linear concentration gradients of the di ?using species inside the membrane,(ii)fast interfacial reactions at the membrane ?feed or membrane ?receiver interfaces,and (iii)quite fast back-extraction at the receiver ?membrane interface,the ?nal equation for the permeability of a metal ion can be obtained as described:27?29???=+??????
??????????
??C C P A V t ln 1t o (4)where,P and V are the overall permeability coe ?cient and feed volume (mL)while C t and C o represent Nd concentrations in the feed solution after the time “t ”(min)and at the start of the experiment,respectively.The parameter “A ”in eq 4stands for the total e ?ective surface area of the hollow ?ber (cm 2),given by the following equation:πε=A r 2ln i (5)where the internal radius and length of the hollow ?ber capillary are noted as r i and L (both in cm),respectively,while εis the standard notation for the porosity.The parameter “?”in eq 4is expressed as ?πε=Q P r ln i T (6)where,N is the number of ?bers and Q T is the volumetric ?ow rate of the feed solution (mL/min).Speci ?cations of various parameters of the hollow ?ber contactor used are given in Table 1.Linear graphs are expected from eq 4when one plots ln(C t /C o )vs t .The P value for the given system can be obtained from the slope using eqs 5and 6.The P values calculated from the ?tted slope of Figure 3b are listed in Table 2.The lower P values at higher feed Nd concentrations are due to loading e ?ect in the liquid membrane similar to that seen in case of solvent extraction studies,which is re ?ected in the decreased K d values of Am(III)under loading conditions.It is important to note that the cotransport of nitric acid was very low,<5%of nitric acid was transported in 5h from the feed to the receiver phase.This has signi ?cance as cotransport of nitric acid in certain transport systems were reported to be quite high under analogous experimental conditions as a consequence of competing extraction reaction such as 30
++?·+?
y y H NO E HNO E (aq)3(aq)(mem)3(mem)(7)
3.2.2.E ?ect of Feed Acidity on Permeation of Am(III).This was investigated from a feed solution containing 0.1g/L Nd,
and the results are shown in Figure 4.The Nd(III)in the feed was used as surrogate of Am(III)as handling of mg level of Am(III)under our experimental conditions was di ?cult.The
results revealed an increase in Am(III)transport rate with increasing feed acidity from 1to 4M nitric acid (Table 2).
From the results,it is clear that the K
d values of Am(III)ar
e dependent on the feed acidity,which is correlated with the P
values of the transporting species.The e ?ect can be best explained with the help of eq 3.An increase in metal ion extraction is expected both in view of higher nitrate ion concentration at higher HNO 3concentrations (on the basis of eq 3)and due to the salting out e ?ect,to a lesser extent.This e ?ect will increase the formation of the Am(NO
3)3·E complex at the feed ?membrane interface,thereby increasing the permeation rate.Investigation of the strip phase acidity con ?rmed that the transport of nitric acid at all the feed acidities was insigni ?cant (<5%)for imparting any adverse e ?ect on the back extraction taking place at the membrane ?receiver interface.
3.3.Di ?usional Parameters.The transport of the metal ?C4DGA complex across the membrane takes place in three steps as mentioned above (section 3.2.1).Therefore,the permeability of a transporting species in HFSLM
depends upon the following three mass transfer resistances:31,32(i)resistance due to the ?owing feed solution inside the hollow ?ber lumen (k f ),(ii)resistance due to di ?usion of the metal ?C4DGA complex across the carrier solution inside the pores of the hollow ?ber (k
m ),and (iii)resistance due to the ?owing receiver solution in the shell side (k
r ).The overall permeability coe ?cient (P )is related to the three resistances by the following equation:31
=++P k r r P r
r k 11
f i
lm m i
o r (8)
where r i and r o are the inner and outer radii,respectively,of a single ?ber and r lm is its log-mean radius.P m is termed as the permeability of the complex inside the membrane while k f and
Table 2.Permeability Coe ?cient and Distribution Behaviour of Am(III)by C4DGA in HFSLM a
[Nd],g/L b [HNO 3],M P ×10?3(cm/min)K d ,Am(III)0.0c 369.5±0.1180±3.80.053 5.49±0.05 3.2±0.420.13 4.96±0.01 1.1±0.080.23 3.76±0.080.47±0.130.33 2.49±0.050.30±0.050.43 1.42±0.030.22±0.020.530.69±0.020.18±0.010.11 1.00±0.080.59±0.020.12 2.72±0.050.84±0.050.13 4.96±0.01 1.1±0.080.14 5.42±0.04 1.3±0.18
a [C4DGA]:1mmol/L in 5%iso -decanol/n -dodecane.b
All the feeds containing
Nd are spiked with 241Am tracer.c Only 241Am tracer was used (10?7M).
k r are the mass transfer coe ?cients of the feed and receiver phases,respectively.P m is related to the K d by the following equation:=P K k m d m (9)Where,k m represents mass transfer coe ?cient inside the membrane.Since,K d is correlated with the extraction constant,K d =K ex [NO 3?]3·[E](eqs 2and 3),the following relation can
be easily derived:=?P K k [NO ][E]m ex 33m (10)
Therefore,eq 8becomes
=++
P k r r k K r r k 1
1
[NO ][E]f i lm m ex 3i
o r (11)
Assuming that the back-extraction of the metal ion in the receiver phase is instantaneous,the term “k r ”is considered
insigni ?cant.Thus,eq 11can be reduced to =+P k r r k K 1
1
[NO ][E]f i
lm m ex 3(12)
The rate-controlling step for the Am(III)transport can be predicted from the above equation.The value of K d controls
the contribution of k f to the overall permeability coe ?cient,P .That is to say that the contribution of k f to P is negligible for a less e ?cient ligand and an opposite trend is expected with an e ?cient extractant like C4DGA,where k f will have a signi ?cant role in the determination of the rate-controlling step.It is important to mention that for the calculation of various di ?usional parameters,the concentration of metal ions on the feed side was used for two reasons:(i)The geometry of the hollow ?bers are more precisely known when obtained from the tube side,for example,length,inner diameter,etc.These parameters are important for calculation of di ?usional parameters.(ii)Most of the calculations are performed assuming laminar ?ow of the liquid inside the tube which is perfectly right when the feed solution is passed through the tube side.The laminar ?ow of the liquid is not maintained due to presence of ba ?e on the shell side which ensures maximum contact of solution with the ?bers.The k f was obtained from the intercept of the straight line plots of 1/P vs 1/K ex [E][NO 3]3(eq 12).Similarly,the value of the membrane mass transfer coe ?cient (k m )was obtained from
the slope of the same plot (i.e.,slope =r i /r lm k m )from the known values of r i and r lm .Here,the shell side mass transfer
coe ?cient (k r )is not included in eq 12,as mentioned above.Figure 5shows the plot of 1/P vs 1/K
ex [NO 3]3[E].
Experimental data show a good ?tting to eq 12with a regression coe ?cient (R 2)of 0.988.Though the number of points present in the Figure 5is less,this is the best ?gure which could be obtained.The experiments have been carried out with nitrate ion concentrations:1,2,3,and 4mol/L.However,the calculation of the values {K ex [NO 3]3[L]}on x axis yielded very close numbers,and therefore,the ?rst three points in the ?gure are very close.It is di ?cult to obtain ?gure having points spread over the line between the ?rst and last point,as most of the calculated data comes out to be very close to each other.The k f and k m for the present HFSLM transport system were calculated as 5.17×10?4and 6.67×10?5cm/s,respectively.The value of k f is an order of magnitude higher
than that of k m ,suggesting that the membrane mass transfer is the rate-controlling step.The membrane di ?usion coe ?cient (D mem )for the Am(NO 3)3·E complex (where,E =C4DGA)
is
Figure 4.
E ?
ect of
feed acidity on the permeation of Am(III)by C4DGA-HFSLM:carrier 1mmol/L C4DGA in 5%iso -decanol/n -dodecane;feed 0.1g/L Nd (300mL);strippant distilled water (300mL);?ow rate 200mL/min.(a)Transport pro ?le of Am(III)with increased feed acidity.(b)Plot of ln(C t /C o )vs time to calculate the P
values.
Figure
5.Plot of 1/P vs 1/K ex [E][NO 3?
]3for the calculation of mass transfer coe ?cients.[E]in the x axis refers to the concentration of
C4DGA in the membrane phase.
known to be related to the inherent properties of the polymeric ?ber (for example,thickness and tortuosity)and is given by the following equation:31τ=k D d m mem
o (13)where d o is the hollow ?ber wall thickness (3×10?3cm)and τis the tortuosity factor (2.5).The D mem value for the present system was calculated to be 5.01×10?7cm 2/s.The D mem values obtained from the Wilke ?Chang equation 33and those experimentally obtained by a lag-time experiment 34were 5.42×10?6and 1.93×10?7cm 2/s,respectively,as reported in a previous report where PTFE ?at sheets were used as the membrane support.24The D mem values are in variance with those reported previously,which is due to the di ?erent polymeric material used in these studies and also to the di ?erence in the porosity and tortuosity factors.After obtaining the k f value experimentally by plotting eq 12,an attempt was made to calculate it theoretically using the following reported correlation:35=???????????????
?k r D m r v LD n
f i aq i 2aq (14)where,v is the linear ?ow velocity (cm/s)of the feed solution through the tube of the hollow ?ber,and D aq is the aqueous di ?usion coe ?cient of the metal ion.The terms on the left-hand side and the right-hand side within parentheses are termed as the Sherwood number (Sh )and Graetz number (Gz )as follows:=Sh mGz n (15)Here,the coe ?cient m is linked to the packin
g density,and n is a characteristic of the hydrodynamic conditions prevalent in the HFSLM transport study.36Theoretically,for a di ?usion process wit
h a fast chemical reaction,the values of m and n were determined as 1.62and 0.33,respectively,according to the Leveque approximation.37Many authors have also reported similar values from mass transfer experiments.38,39Therefore,we used the relation,Sh =1.62Gz 0.33,to calculate the k f values.For calculating the Sherwood and Graetz numbers,the D aq value for Am(III)was taken as 6×10?6cm 2/s as used earlier.40
The calculated k f value (1.52×10?4cm/s)is in reasonably good agreement with the experimentally obtained value (5.17×10?4cm/s).
3.4.Mathematical Modeling.Development of mathe-matical model to predict the transport behavior of the metal ion through a HFSLM is essential for scale up applications.In view of this,an attempt was made for model development taking
into consideration the phenomena of di ?usional mass transport across the pores containing the carrier extractant solution.41The model was developed under the assumptions of (i)laminar aqueous ?ow pattern inside the hollow ?ber contactor,(ii)linear concentration gradients across the path used by the di ?using species,(iii)di ?usion rates of the metal ?carrier complex are signi ?cantly slower than the extraction and back extraction reactions at the aqueous-membrane interfaces,and (iv)negligible mass accumulation inside the membrane phase (pseudo steady state approximation).Applying above assump-tions,the following model was obtained by considering the mass balance in the feed tank and inside the membrane phase.41ε=?Δ+Δ??????
???????????????????C C Q V rv K L K t
ln exp 2()1t o T i d d aq m (16)
where Q T and V are the volumetric ?ow rate (mL/min)and the
volume of the feed solution (mL),respectively.The terms Δ
aq
and Δm are de ?ned as
εΔ=?r d r d D ()aq i aq
i aq aq (17)τ
Δ=d D m o
mem (18)The values of Δaq and Δm were calculated by using basic data from Tables 1and 2,while the aqueous di ?usion coe ?cient (D aq )of the Am(III)ion was taken from the literature.40Using these parameters,eq 16was used to calculate the transport of Am(III)across the C4DGA-based HFSLM.As shown in Figure 6,excellent matches between the predicted transport data
and
Figure https://www.wendangku.net/doc/fb3020487.html,parative display of the calculated and experimentally obtained Am(III)transport pro ?les by the HFSLM containing C4DGA:carrier 1mmol/L C4DGA in 5%iso-decanol/n -dodecane;feed 3M HNO 3(300mL);strippant distilled water (300mL);?ow rate 200mL/min.C t /C o refers to
the ratio of the metal concentrations in the feed solution.
the experimental data were obtained,validating the present transport model.
4.CONCLUSIONS
We demonstrated the possible application of a DGA-function-alized calix[4]arene(C4DGA)ligand,an excellent ionophore for trivalent f-elements,for the recovery of Am(III)from HNO3feeds using an HFSLM technique.The e?ciency of the recovery was excellent at moderate feed acidities of2?4M nitric acid,where quantitative transport of Am(III)could be achieved even in the presence of0.1g/L Nd.The di?usion coe?cient of the Am(NO3)3·C4DGA complex inside the hollow?ber membrane was obtained experimentally and agreed reasonably well with those obtained previously for a PTFE-based FSSLM,the variation being attributed to membrane parameters such as hydrophobicity,porosity,and tortuosity. The transport pro?le of Am(III)was predicted by a mathematical model under di?erent experimental conditions, which was validated by the experimental data.The present study gives an opportunity to use the HFSLM technique with speci?c ligands,such as C4DGA,for radioactive waste treatment since the ligand inventory is very low.It also gives the option to increase the throughput by increasing the ligand
concentration.
■AUTHOR INFORMATION
Corresponding Author
*E-mail:mpatra@https://www.wendangku.net/doc/fb3020487.html,.in.
Author Contributions
#Computations:pankaj.kandwal@nituk.ac.in.
Notes
The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS
S.A.A.and P.K.M.thank Dr.P.K.Pujari,Head Radiochemistry
Division,BARC,for his constant encouragement and support.■ABBREVIATIONS
C4DGA=diglycolamide-functionalized calix[4]arene DGA=diglycolamide
FSSLM=?at sheet supported liquid membrane
HFSLM=hollow?ber supported liquid membrane
SLM=supported liquid membrane
TODGA=N,N,N′,N′-tetra-n-octyl diglycolamide Symbols
A=e?ective surface area of the membrane
C o=initial metal ion concentration in the feed(at t=0)
C t=concentration of metal ion in the feed at time t
D aq=aqueous di?usion coe?cient
d aq=thickness of th
e aqueous di?usion layer
D mem=membrane di?usion coe?cient
d o=wall thickness hollow?ber
K d=distribution coe?cient of the metal ions
k f=aqueous feed mass transfer coe?cient
k m=membrane mass transfer coe?cient
k r=receiver phase mass transfer coe?cient
L=length of the?ber
N=number of?bers
P=overall permeability coe?cient
P m=membrane permeability coe?cient
Q T=volumetric?ow rate
r i=internal radius of the hollow?ber
r lm=log mean radius of the hollow?ber
r o=outer radius of the hollow?ber
V=volume of the feed solution
Greek Letters
v=linear?ow rate
?=porosity of the membrane
τ=tortuosity of the membrane
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