Two novel extraction chromatography resins (ECRs) containing two diglycolamide (DGA) -functionalized calix[4]arenes with n-propyl and isopentyl substituents at the amide nitrogen atom, termed as ECR-1 and ECR-2, respectively, were evaluated for the uptake of Th(IV) from nitric acid feed solutions.
Trang 1Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/chroma
Rajesh B Gujara, Prasanta K Mohapatraa, Mudassir Iqbalb, Jurriaan Huskensb,
Willem Verboomb, ∗
a Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
b Laboratory of Molecular Nanofabrication, MESA + Institute for Nanotechnology, University of Twente, P O Box 217, 7500 AE Enschede, the Netherlands
a r t i c l e i n f o
Article history:
Received 19 March 2021
Revised 1 July 2021
Accepted 3 July 2021
Available online 9 July 2021
Keywords:
Thorium(IV)
Extraction chromatography
Diglycolamide
Calix[4]arene
a b s t r a c t
Two novel extraction chromatography resins (ECRs) containing two diglycolamide (DGA) -functionalized calix[4]arenes with n-propyl and isopentyl substituents at the amide nitrogen atom, termed as ECR-1 and ECR-2, respectively, were evaluated for the uptake of Th(IV) from nitric acid feed solutions While both the resins were having a quite high Th(IV) uptake ability ( Kd>30 0 0 at 3 M HNO 3), the uptake was relatively lower with the resin containing the isopentyl DGA, which appeared magnified at lower nitric acid concentrations Kinetic modeling of the sorption data suggested fitting to the pseudo-second order model pointing to a chemical reaction during the uptake of the metal ion Sorption isotherm studies were carried out showing a good fitting to the Langmuir and D-R isotherm models, suggesting the uptake con- forming to monolayer sorption and a chemisorption model Glass columns with a bed volume of ca 2.5
mL containing ca 0.5 g lots of the ECRs were used for studies to assess the possibility of actual applica- tions of the ECRs Breakthrough profiles obtained with feed containing 0.7 g/L Th(NO 3) 3solution resulted
in breakthrough volumes of 8 and 5 mL, respectively, for the ECR-1 and ECR-2 resins Near quantitative elution of the loaded metal ion was possible using a solution of oxalic acid and nitric acid A method for the separation of Th-234 from natural uranium was demonstrated for the possible application of ECR-1
© 2021 The Author(s) Published by Elsevier B.V This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Solid phase extraction is considered as an alternative separa-
tion method to solvent extraction and can alleviate some of the
issues faced with the latter such as third phase formation, phase
entrainment, phase disengagement limitations, etc Though solid
phase extraction (SPE) involves sorbents for neutral molecules or
ion exchange resins, there is a type of sorbents based on extraction
chromatography (EC) where, the organic extractant is impregnated
into an inert solid support material, is fast emerging as an efficient
SPE technique with highly promising results [1–4] Furthermore,
the extractant inventory can be very low in case of EC and hence,
the cost of the separation method can be quite low This suggests
that exotic extractants can be easily used in an EC method with-
∗ Corresponding author
E-mail address: w.verboom@utwente.nl (W Verboom)
out any significant cost consideration A variety of solid support materials have been used viz XAD-4 [5], XAD-7 [5], voltalef [6], Amberchrom – CG 71[7], Chromosorb 102 [8], Chromosorb W [8], etc Out of these, Chromosorb W (dimethyl dichlorosilane treated acid washed celite diatomaceous silica) is quite interesting since the major constituent of this material is silica, which has good ra- diation stability and hence, can be easily used for radioactive feeds For the separation of actinide ions from acidic feeds, many extractants have been used [9] However, diglycolamide (DGA) extractants such as TODGA ( N,N,N’,N’-tetra- n-octyl diglycolamide) are quite promising [10] TODGA-based extraction chromatogra- phy resins (ECR) have been prepared by many researchers and used for the separation of trivalent actinide ions from acidic feeds [ 7, 11, 12] It has been reported that multiple DGA extractants such
as the ones with a calix[4]arene scaffold display a much higher separation efficiency than TODGA [13–15] The DGA-functionalized calix[4]arenes (termed as C4DGAs) with a branched alkyl chain of- fer a better selectivity, albeit a lower extraction than a linear alkyl
https://doi.org/10.1016/j.chroma.2021.462401
0021-9673/© 2021 The Author(s) Published by Elsevier B.V This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )
Trang 2Fig 1 Structure of the C4DGA ligands L I and L II
chain substituted ligand [15] The uptake of trivalent actinide / lan-
thanide ions by a C4DGA-based ECR is much superior to that of
the corresponding TODGA-based resin [16] Recently, we reported
the extraction behavior of tetravalent actinide ions such as Np(IV)
and Pu(IV) with linear and branched chain alkyl group containing
C4DGA ligands [15] However, the extraction behavior of Th(IV) is
not studied so far It is of interest from the point of view of U(VI)
/ Th(IV) separation, which is relevant for AHWR (advanced heavy
water reactor) applications [17]
Therefore, we embarked on a study to the uptake behavior of
Th(IV) and U(VI) with two novel extraction chromatography resin
(ECR) materials termed as ECR-1 and ECR-2 containing the C4DGA
ligands L I and L II, respectively ( Fig.1) impregnated into the pores
of the solid support material Chromosorb W The separation be-
havior was studied by batch studies and subsequently, separation
was demonstrated using a mixture of U and Th Kinetic model-
ing and sorption isotherm studies were also carried out To our
mind, this is the first ever report on the separation of Th(IV) from
U(VI) using an ECR, impregnated with calix[4]arene-based DGA
ligands
2 Experimental
2.1 Chemicals
The DGA-functionalized calix[4]arene (C4DGA) ligands L Iand L II
( Fig.1) have been described before [13] The same batch was used,
again characterized by comparison of both the 1H NMR spectra
(Fig S1) with the previous data and of the distribution coefficients
for the extraction of Am(III) and Eu(III) using 1 mM ligand solu-
tions of 5% isodecanol and 95% n-dodecane at 3 M HNO 3 For lig-
and L I DAm and DEu values were obtained of 78 ±3.1 and 242 ±12
versus 79.5 and 225, respectively, in ref [13] For ligand L IIthe DAm
and DEu values were 314 ±10 and 357 ±14 versus 325 and 370, re-
spectively, in ref [13] Chromosorb-W (mesh size 60-80) was pur-
chased from John Manville, USA and was cleaned and dried prior
to use by rinsing with methanol and air drying at 70 °C Supra-
pur nitric acid (Merck) was used for the preparation of dilute ni-
tric acid solutions for the uptake studies Thorium nitrate was pur-
chased from Indian Rare Earths limited, while uranyl nitrate hex-
ahydrate was obtained from Uranium Extraction Division, BARC All
other chemicals were of AR grade
2.2 Radiotracers
233U was used from the laboratory stock after carrying out
a fresh purification from its daughter products using a reported method [18] Carrier free 234Th was obtained by freshly separat- ing the radiotracer from an ‘aged’ solution of natural uranium in
6 M HNO 3 by extractive separation of the latter into a solution
of Aliquat 336 in chloroform following a reported method [19] The aqueous phase contained predominantly the carrier free 234Th tracer with <0.1% U In all studies involving the 234Th tracer, the concentration of the metal ion was ca 10 −12– 10 −13M, while the concentration of U in the studies involving 233U tracer was ca 10 −5
M
2.3 Radiometric assay
Both 233U and 234Th were assayed radiometrically While the former was assayed by liquid scintillation counting using an in- strument obtained from Hidex, Finland, 234Th was assayed using a well type NaI(Tl) scintillation counter (Para Electronics), which was interphased with a multichannel analyzer (ECIL, India) The Ultima Gold liquid scintillation cocktail was purchased from Perkin Elmer, USA
2.4 Preparation of the EC resins
The EC resins were prepared by a previously reported method [20]which required vortex mixing of known quantities of the lig- ands L I and L II and Chromosorb W taken in a stoppered 100 mL Erlenmeyer containing 20 mL acetone, for 24 h Afterwards, the slurry was settled, and the acetone was removed by careful flush- ing of N 2 gas through a jet After the evaporation of >99% of the solvent, the ECRs were kept in a vacuum desiccator for dry- ing overnight to yield free flowing materials for subsequent uptake studies The weight difference between the ECRs and the support material (Chromosorb W) suggested about 8% (w/w) ligand loading for both the resins
2.5 Characterization of the SPE resins
The ECRs were characterized by thermogravimetry (TGA) for ligand loading and by SEM for the surface morphology; the re- sults are given in a previous paper [21] The presence of the ex- tractants inside the resin pores was characterized by FTIR recorded
on a Bruker Alpha II ATR-FTIR spectrometer
2.6 Batch uptake studies
The batch uptake studies involved acidic feeds containing Th(IV) and UO 22 + ions For the batch studies, ca 10 mg of the ECR was taken in a stoppered Pyrex glass tube (10 mL capacity) containing the radiotracer spiked dilute nitric acid solution The tubes were agitated in a thermostated water bath at 25 ± 0.1 °C for about 1 hour The time taken to attain equilibrium metal ion uptake values was much lower than the employed time which was done to en- sure attainment of equilibrium The tubes were then rested for 5 minutes and centrifuged at 30 0 0 rpm before removal of the sam- ples for radiometric assay Usually, 100 μL aliquots were removed for assaying purpose using Eppendorf fixed volume micropipettes The weight distribution coefficient ( K d) values of the metal ions were calculated using the following equation:
K d= (C o − C)
C
· V
mL / g
(1)
where C o and C are the initial and equilibrium concentrations of the metal ions, respectively, V is the aqueous phase volume (in
2
Trang 3Table 1
Various parameters for the column studies using the ECR-1 and ECR-2 resins
Fig 2 FTIR spectra of the ECRs Top: Pristine Chromosorb W (green); Freshly made
ECR-1 (black); Freshly made ECR-2 (red) Bottom: Th(IV) loaded ECR-1 (black);
Th(IV) loaded ECR-2 (red) (For interpretation of the references to color in this fig-
ure legend, the reader is referred to the web version of this article.)
mL), and W the weight of the resin (in g) As radioactive solu-
tions were used in the present study, the concentrations are con-
veniently expressed in terms of counts per unit time per unit vol-
ume All batch studies were carried out in duplicate and the mass
balance was found to be within 5%
2.7 Column studies
The column studies were carried out using small glass columns
of the dimension: 30 cm (length) × 0.4 cm (dia); bed volume ca
2.5 mL The columns were filled with a slurry containing the resins
in distilled water and subsequently allowed to settle to get resin
beds without any air bubble The top of the columns was fitted
with a glass cup (10 mL), while the bottom part was fitted with
flexible rubber tubing and pinch cocks for maintaining a constant
flow rate The detailed column parameters are given in Table1
The columns were conditioned prior to the loading of the metal
ions by passing 10 mL of HNO 3 For the breakthrough studies, the
column was loaded with a feed containing 0.7 g/L Th solution in
3 M HNO 3 The feed was spiked with 234Th tracer for easy col-
umn data analysis After loading of the Th solution, the column
was washed with 3 M HNO 3 prior to the elution of the loaded Th
using a mixture of 0.5 M oxalic acid and 0.5 M HNO 3 The loading
of the uranium solution was performed in an identical manner, but
as mentioned below the effect of the loading was quite insignifi-
cant
3 Results and discussion
3.1 Characterization of the extraction chromatography resins
The FTIR spectra of the resins were recorded (Fig S2) How-
ever, for the sake of checking C4DGA loading in the resins, the FTIR
bands in the range of 1700 – 1550 cm −1 are magnified and given
in Fig.2 Pristine Chromosorn W has no bands in this region On
Fig 3 Th(IV) uptake from 3 M HNO 3 as a function of equilibration time using the two C4DGA containing resins ECR-1 and ECR-2
the other hand, the C4DGA loaded resins show the >C = O bands
at 1650 cm −1 indicating the presence of the carbonyl groups of the C4DGA ligands present in the pores of the ECRs For compar- ison purpose, the two ECRs were contacted with Th(IV) solution and the FTIR spectra of the Th(IV) loaded resins are also presented
in Fig.2 A clear shift in the >C =O stretching frequency to lower values (to ca 1610 cm −1) suggested binding of Th(IV) ions to the carbonyl groups present in the resins
3.2 Batch uptake studies
The uptake of the metal ions was negligible ( Kd values being
2 ± 0.2 and 5 ± 0.4, respectively, for U(VI) and Th(IV) at 3 M HNO 3) when pristine Chromosorb W (without any ligand loaded onto it) was used as the sorbent On the other hand, a very large increase in the Kdvalues was observed, especially for Th(IV), when the two ECRs were used at 3 M HNO 3 ( vide infra) The Kd values
of U(VI) were extremely low and were very close to the value ob- tained with bare Chromosorb W as the sorbent ( vide infra), sug- gesting weak binding of the metal ion with the C4DGA ligands This is in line with the results obtained from the solvent extrac- tion studies reported before [14]
3.2.1 Time of equilibration
For the column operations discussed below the time taken to attain equilibrium was determined Fig.3 gives the plots of Kd vs
time for both the resins for Th(IV), showing an equilibrium time
of about 30 minutes U(IV) data are of less significance as the Kd
values obtained were very low (in the range of 5 to 30, which are about three orders of magnitude lower than the Kdvalues obtained with Th(IV)) The equilibrium Kd values for U(IV) were attained in
ca 20 minutes but are not included in the plot
3.2.2 Effect of feed acid concentration
It is well known from the solvent extraction data reported by Zhu et al [22] that the extraction of Th(IV) is highly favourable with DGA-based extractants such as TODGA The extraction of the metal ion increases sharply with the feed acid concentration as per the following equation:
Th4 +aq +nTODGAorg +4NO3 −aq=Th(NO3)4•nTODGAorg (2)
where the subscripts ‘aq’ and ‘org’ refer to the species present in the aqueous and the organic phases, respectively, and n is 3.39 for TODGA On the other hand, the extractant dependence for the C4DGA ligands L I and L II is not known Since there are four DGA
3
Trang 4Fig 4 Th(IV) and U(VI) uptake profiles as a function of aqueous feed nitric acid concentration using the C4DGA-containing resins ECR-1 and ECR-2 The separation factor
(SF) data are also presented
moieties present in the C4DGA ligands, the value of n could be 1
Consequently, Eq (1) can be extended to the following equation
when the ligand is impregnated in the ECR
Th4 +aq+C4DGAR +4NO3 −aq=Th(NO3)4.C4DGAR (3)
where the subscript ‘R’ represents species in the resin phase It is
clear from the above equations that the extraction of Th(IV) ion
has a positive dependence with the acid concentration of the feed
and hence, should increase with the feed acid concentration The
Th(IV) uptake data (quantified in terms of Kd) as a function of the
feed HNO 3 concentration (in the range of 0.5 M – 6 M) are pre-
sented in Fig.4 The Kd values for U(VI), obtained under identical
experimental conditions, are also plotted in the same figure Up-
take data were not obtained at nitric acid concentrations < 0.5 M
due to the possibility of hydrolysis of the tetravalent metal ion at
these conditions
As shown in Fig.4, the Kd values for Th(IV) increase steeply at
lower acid concentrations and reach a plateau at nitric acid con-
centrations > 3 M On the other hand, U(VI) uptake shows a steady
increase in the entire range of nitric acid concentrations The sepa-
ration factor (SF =Kd,Th/ Kd,U) values were calculated and plotted in
the same figure for a better impression of the results The SF val-
ues exhibit a rapid rise at lower acid concentrations and a sharp
decline at higher acidities However, at 3 M HNO 3, which is the
acid concentration of interest in most radioactive wastes, SF values
of >100 suggest that the resins can be employed for the separation
of Th(IV) from U(VI), which has relevance in THOREX type feeds
[23] for application in AHWR Finally, comparing the efficiency of
the two resins, ECR-1 is superior to ECR-2, the reason being a less
efficient extraction of the metal ions with C4DGA ligand L II con-
taining branched alkyl chains, as discussed before [15]
3.2.3 Back extraction and reusability
The back extraction (stripping) of the loaded Th(IV) was at-
tempted by three stripping solutions, viz a) 0.5 M oxalic acid; b)
0.5 M oxalic acid + 0.5 M nitric acid; c) 0.05 M HEDTA (N-(2-
hydroxyethyl)ethylenediamine triacetic acid) + 0.05 M nitric acid
The back extraction of the loaded metal ion was done by first load-
ing the resin (ca 20 mg) with 234Th radiotracer from 3 M HNO 3
followed by careful removal of the aqueous phase completely from
the tube After this, the stripping solution was added (0.5 mL) to
the tube and aqueous phase samples were removed after 1 hour
of equilibration The results of the back extraction are given in
Table2
Table 2
Bach extraction of Th(IV) using various stripping solutions from ECR-1 and ECR-2 resins
Stripping solution
% Stripping (ECR-1)
% Stripping (ECR-2)
0.5 M oxalic acid + 0.5 M nitric acid 86.5 86.1 0.05 M HEDTA + 0.05 M nitric acid 87.1 86.3
Fig 5 Reusability of the ECRs based on the Th(IV) uptake data after regeneration
using stripping conditions of 0.5 M HNO 3 and 0.5 M oxalic acid
Though HEDTA was marginally better as a stripping agent as compared to the mixture of oxalic acid and nitric acid, we have used the latter in view of some previous reports where the strip- ping mixture has been successfully used In our previous stud- ies involving tetravalent ions such as Np(IV) and Th(IV), strip- ping was achieved using oxalic acid or a mixture of it with ni- tric acid [ 24, 25] This is due to the rather strong complexation of these tetravalent ions with oxalate anion [26] Subsequently, the reusability of the resin was studied by carrying out repeated up- take and stripping for five cycles ( Fig 5) ECR-1 resin showed al- most no change in the Kd values even after about four cycles, though a clear deterioration was seen after the fourth cycle Even the Kd value obtained after four cycles of uptake and stripping was
4
Trang 5Fig 6 Kinetic modeling of the Th(IV) uptake data as fitted to (a) pseudo-first and (b) pseudo-second order models
ca 20 0 0, which is quite high for metal ion uptake On the other
hand, the ECR-2 resin appeared stable (based on the Kd values)
only up to two cycles of uptake and stripping runs The Kd val-
ues with ECR-2 fall sharply from ca 2400 in the second cycle to
ca 750 in the fifth cycle, suggesting clear degradation which is at-
tributed to the leaching of the extractant from the resin pores
3.3 Kinetic modeling of the uptake data
To understand the mechanism of sorption, kinetic modeling of
the Th 4 + ion uptake data was done Uptake studies were carried
out at different time intervals (for 3-4 hours) using the two extrac-
tion chromatographic resins, where the feed contained 700 mg/L
Th in 3 M HNO 3and 234Th radiotracer for easy assaying by radiom-
etry ( vide supra) The uptake data were fitted using the following
equations:
Pseudo first-order kinetic model:
Pseudo second-order kinetic model:
t / q t=1/
k2q e2
where qe and qt are the mass of the metal ion retained per unit
mass of the resin at equilibrium and at time ‘t’, respectively The
pseudo-first order and pseudo-second order rate constants are de-
noted as k1and k2, respectively
The pseudo first-order kinetic model or the Lagergren model
[27] is operating if the Ln ( qe qt) vs t plot should conform to
straight-line fits with negative slopes to give the pseudo-first or-
der rate constant ( Fig.6a) On the other hand, when t/ qt vs t plots
conform to straight-line fits, it is presumed that the data obey the
Ho’s pseudo second-order model [28] From Fig.6it is clear that
the fit to the Lagergren model is very poor (R 2 = 0.72 and 0.70,
for ECR-1 and ECR-2, respectively), while near perfect straight-line
fits are obtained for the pseudo second-order kinetic model, sug-
gesting chemisorption as the rate-limiting step [29]i.e., a chemical
reaction controls the uptake mechanism This is only logical as the
uptake reaction mainly involves the extraction of the metal ion as
per Eq.(2) The rate constants obtained from the kinetic model-
ing along with the fitting parameters are listed in Table3 As indi-
cated from the square of the correlation coefficient (R 2) values, the
pseudo second-order kinetics has a better fitting and hence, should
be valid for the present case
3.4 Sorption isotherms
To understand the nature of the interaction of the metal ion
(Th(IV)) with the ligands present in the resin pores, it was imper-
ative to carry out sorption isotherm studies The linear forms of
Table 3
Parameters obtained from the straight-line plots obtained by data fitting to Eqs (4) and (5)
Pseudo-first order kinetic model
Pseudo-second order kinetic model
Table 4
Linearized form of different sorption isotherm models a Model Linearized form model equation Plot [Ref] Langmuir C e
q e = b ·q[1]max + C e
qmax
C e
q e v s · C e [31] Freundlich log q e = log K f + 1n log C e log q e v s · log C e [32] D-R ln q e = ln q max −βε2 ln q e v s · ε2 [ 33 , 34 ] Temkin q e = B T ln A T + B T ln C e q e v s · ln C e [35]
a q e is the concentration of metal ion sorbed per gram of the solid at equilibrium;
Ce is the equilibrium concentration of the metal ion in the aqueous phase; q max and K f are the maximum sorbed mass of Th(IV) at saturation and the Freundlich constant, respectively; β and εrepresent the D-R constant and Polanyi potential, respectively; B T and A T are the Temkin constants
the different isotherm models used for data analysis are given in Table4 The Th(IV) uptake studies were carried out using a 234Th tracer spiked carrier solution containing 1.2 g/L Th in 3 M HNO 3 The feed acid concentration was chosen since most of the radioac- tive waste feeds contain ca 3-4 M HNO 3 [30]
The sorption isotherm data when fitted to the Langmuir adsorp- tion isotherm equation ( Table 4) by plotting Ce/ qe vs Ce yielded straight lines ( Fig 7a) with very good correlation coefficient val- ues of >0.99 ( Table5) This indicates that the uptake of Th(IV) by both the ECRs conform to the monolayer model The qmax values for ECR-1 and ECR-2 were found to be 12.4 ± 1.3 and 5.1 ± 1.1 mg, respectively, as against the experimentally determined saturation Th(IV) uptake values of 13.2 ± 1.4 and 5.8 ± 1.2 mg, showing a reasonably good agreement The intercept of the straight-line plots yield ‘ b’, which is related to a dimensionless equilibrium constant
R L(also known as the separation factor [36]) as given by:
where a value of RL> 1 suggests an unfavourable sorption On the other hand, a favourable sorption process is considered where RL
falls in between 0 and 1, while RL = 0 points to an irreversible up- take process The experimentally obtained b values were found to
be 0.21 and 0.95, respectively, for ECR-1 and ECR-2, which when
5
Trang 6Fig 7 Th(IV) uptake data fitted to the linearized forms of (a) Langmuir, (b) Freundlich, (c) D-R, and (d) Temkin sorption isotherms
Table 5
Parameters calculated from Langmuir, Freundlich, D-R, and Temkin isotherm models
for the sorption of Th(IV) onto the extraction chromatographic resins ECR-1 and
ECR-2; Aqueous phase: 25 mg/L to 300 mg/L Th(IV) solution in 3 M HNO 3
Isotherms Parameters
Values at 25 °C
Langmuir b (mL/mg)
qmax (mg/g)
R 2
12.4 ± 1.3 (13.2 ± 1.4) a 5.1 ± 1.1 (5.8 ± 1.2) a
Freundlich K f (mg/g)
n
R 2
(mmol/g)
E (kJ/mole)
R 2
0.089 ± 0.010 0.024 ± 0.018
Temkin AT
BT
R 2
a Values in parentheses refer to the experimental values
used in Eq (6)resulted in R L values of 0.799 and 0.467, respec-
tively, and suggest a favourable uptake of the metal ion into the
resins
When the uptake data were fitted to the Freundlich isotherm
model [32] by plotting log qevs log C e, the scatter ( Fig.7b) could
not be fitted well to a linear regression line, the correlation co-
efficient (R 2) values being very poor ( Table 5) This suggests the
absence of the multilayer sorption phenomenon for the Th(IV) up-
take in case of both resins On the other hand, the linear fitting
of the sorption data to the D-R isotherm model [34] by plotting
ln qt vsε2gave reasonably good correlation coefficients (0.996 for
ECR-1 and 0.981 for ECR-2; Table5; Fig.7c) Other parameters of
Fig 8 Breakthrough profiles for the Th(IV) uptake columns containing ECR-1 and
ECR-2 resins Feed contained a 234 Th tracer spiked solution of 3 M HNO 3 containing 0.7 g/L Th
the fitting of the sorption data to the D-R model are also listed in Table 5 The slope gives the quantity β which can be correlated
to E (mean sorption energy), which is defined as the free energy needed to transfer one mole of the Th(IV) ions from infinity to the surface of the ECRs [35]and is given as:
E=1/
Depending on the value of E one can get a rough idea about the mechanism of sorption, such as chemisorption ( E >8 kJ/mol) or physisorption ( E <8 kJ/mol) [37] The values of E for Th(IV) uptake
6
Trang 7Fig 9 Th(IV) breakthrough data fitted to the Thomas kinetic model as given by Eq (8)
onto ECR-1 and ECR-2 were calculated to be 17.9 ± 1.7 kJ/mol and
54.2 ± 1.8 kJ/mol, respectively ( Table5), indicating that the metal
ion uptake conforms to a chemisorption process via some type of
chemical interactions
Finally, an attempt was made to fit the sorption data to the
Temkin isotherm, which considers the effects of indirect adsorbate-
adsorbent interactions on the Th(IV) ion uptake onto the two
resins ( Table 5) The main assumption of the Temkin isotherm
model is a multi-layer sorption phenomenon which takes care of
the indirect adsorbate–adsorbent interactions and where the heat
of sorption would more often decrease linearly with coverage The
Temkin sorption fit lines are presented in Fig.7d and the fitted pa-
rameters are listed in Table5
3.5 Column studies
3.5.1 Breakthrough profile of Th(IV)
For obtaining the breakthrough profiles, the Th(IV) feed solution
was loaded onto the column at 1 mL at a time and a total of 14 mL
each of the feed solutions were passed through the two columns
The breakthrough profiles are plotted in Fig 8 which show that
ECR-1 is a better sorbent than ECR-2, 8 vs 5 mL, from the Th(IV)
loading point of view The breakthrough volumes were found to
be 4 and 8 bed volumes for the ECR-1 and ECR-2 based columns,
respectively
To determine the characteristic parameter of the columns used
in this study, the data obtained from the breakthrough loading
studies were subjected to fitting with the Thomas kinetic model
[ 38, 39] given by the following equation:
C t
1+exp K T h q ads m
where Ct and C0 are the effluent and influent concentration, re-
spectively, at time ‘ t’ (minutes), KTh is the Thomas rate constant
(mL/mg.min), qads is the sorption capacity of the column (mg/g),
‘ m’ is the amount of adsorbent (g) in the column, and Q (flow rate)
is 0.05 mL/minute The plots of Ct/ C0against time for a given flow
rate were done and the data fitted using Eq.(8)and presented in
Fig 9 The value of C0 was 0.7 mg/mL and that of m was 0.5 g
Values for the Thomas constant KTh were calculated to be 0.302 ±
0.005 and 0.030 ± 0.002, respectively for ECR-1 and ECR-2
Subsequently, the loaded Th was eluted using a mixture of 0.5
M HNO 3 and 0.5 M oxalic acid; the elution curves for both resins
are presented in Fig 10 It is clearly seen from the elution pro-
files that there is near quantitative elution of the loaded metal
ions in about 7 mL of the eluent Furthermore, the elution profile
is sharper for the column containing ECR-1 as compared to that
Fig 10 Elution profiles of Th(IV) from columns containing ECR-1 and ECR-2 resins
Feed: 0.7 g/L Th in 3 M HNO 3 and spiked with 234 Th tracer Eluent: mixture of 0.5
M HNO 3 and 0.5 M oxalic acid
containing ECR-2 In view of this, it is recommended that the ECR- 1-based column can be used for actual applications to feeds con- taining Th(IV) in nitric acid
3.5.2 Separation of 234 Th(IV) from natural U
The batch uptake studies show that U(VI) ion uptake is neg- ligible in view of the very low Kd values at 3 M HNO 3, while the uptake of Th(IV) is rather high ( Table6) This was used for the sep- aration of 234Th from natural uranium A feed containing 1 g/mL of natural U was first used for the 234Th as well as 235U contents as identified by their gamma ray peaks ( 234Th: 63 keV and 93 keV;
235U: 186 keV) as indicated in Fig.11a The loaded U was not held
in the column The 234Th tracer held in the column was eluted as mentioned above using a mixture of oxalic acid and nitric acid The separation of Th from U was quite efficient as indicated from the gamma ray spectra of the eluted fractions from ECR-1 ( Fig 11b) and ECR-2 ( Fig.11c) which suggested the U content in the Th frac- tions being below the detection limit of U (0.3 counts per minute) The decontamination factor (DF) values (defined as the ratio of the
Th / U ratio in the product to that in the feed) were calculated
to be 178 and 165, respectively, for the resins ECR-1 and ECR-2 Table6gives a comparison of the results reported in the literature
We have previously evaluated an Aliquat 336-based resin material for the same purpose [25] However, the present sets of resins are found to be superior to that in the previous report
7
Trang 8Table 6
Comparative presentation of Th/U separation from literature reports
Nitric acid Combination of leaching, solvent extraction,
precipitation and chromatography
99.5% pure Th-230 was obtained But the method is very complicated F − is used which is corrosive
[40]
as it is corrosive in nature
[41] Nitric acid / acetic
acid
Dowex 1 × 8 (nitrate form) Kd value of Th(IV) > 10 5 ; K d value of U(VI) ~ 10 2 Use of 90%
glacial acetic acid is needed for loading
[42]
Fig 11 Separation of U(VI) from Th(IV) as measured from their gamma ray spectra
(a) Natural uranium solution in the feed; (b) Eluted fraction from the ECR-1 column;
(c) Eluted fraction from the ECR-2 column
4 Conclusions
Uptake of Th(IV) from nitric acid was studied by two novel ex-
traction chromatography resins containing two DGA-functionalized
calix[4]arene ligands (ECR-1 and ECR-2) using 234Th radiotracer for
easy radiometric assay The Kd values for Th(IV) were found to
be significantly higher than those obtained for U(VI) in the entire
range of acid concentrations studied In view of the very low lig-
and loading into the ECRs (ca 8%), the resins are quite efficient for
Th(IV) ion uptake These resins are far superior to the known ECRs
reported in the literature A previously reported Aliquat 336-based
resin displayed much lower (about 10 to 20 times) Kd values at
3 M HNO 3 demonstrating that the present set of resins are much
more superior On the other hand, the uptake of U(VI) was negli-
gible suggesting that the resins can be used for the separation of
234Th tracer from natural U for possible applications as a radio-
tracer
Kinetic modeling of the Th(IV) uptake data pointed to pseudo-
second order kinetics, which is the possible mode for chemical in-
teractions The batch uptake data were fitted to various isotherms
and conformed both to the Langmuir isotherm, suggesting a mono-
layer adsoption mechanism, and to the D-R isotherm, indicating
chemisorption The column studies were carried out by loading
Th(IV) in a feed containing 3 M HNO 3 and the breakthrough pro-
files showed much better loading with ECR-1 than with ECR-2
Sharp elution peaks were obtained while using an eluent mixture
of 0.5 M oxalic acid and 0.5 M nitric acid The separation of 234Th
was attempted and DF (Th/U) values of 178 and 165 were obtained with the ECR-1 and ECR-2 resins, respectively
Declaration of Competing Interest
The authors have no conflict of interest to declare
Rajesh B Gujar: Formal analysis, Investigation Prasanta K Mohapatra: Conceptualization, Methodology, Writing – original draft Mudassir Iqbal: Investigation Jurriaan Huskens: Methodol- ogy, Supervision Willem Verboom: Methodology
Acknowledgement
The authors (RBG and PKM) thank Dr P.K Pujari, Head, Radio- chemistry Division, Bhabha Atomic Research Centre for his con- stant encouragement
Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.chroma.2021.462401
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