Journal of Nuclear and Radiochemical Sciences, Vol. 7, No. 2, pp. 37-43, 2006

Solid Phase Extractive Preconcentration of Some Actinide Elements Using Impregnated Carbon
H. H. Someda and R. R. Sheha*
Nuclear Chemistry Department, Hot Laboratories Center, Atomic Energy Authority, P.O. 13759, Cairo, Egypt.
Received: April 12, 2006; In Final Form: November 17, 2006
Apricot stone shells were carbonized under certain chemical and thermal conditions. The produced sorbent had been
impregnated with different concentrations of oxalic and succinic acids. They were physically characterized and used
for preconcentration of U(VI) and Th(IV) from acidified aqueous solutions. Batch equilibrium studies revealed thata quantitative preconcentration could be achieved at pH ~2 for U(VI) and ~1.3 for Th(IV). The equilibrium time was
extended up to 24 h for U(VI) and 40 min for Th(IV). The modified sorbent showed a superior extractability for
U(VI) and Th(IV) with distribution coefficient mean values of 2084 and 7808 mL g-1, respectively. The sorption
capacity values are 2.39 and 8.47 mg g-1 for U(VI) and Th(IV) sorption onto apricot stone anchored with oxalic acidand 2.3 and 9.18 mg g-1 for sorbent anchored with succinic acid. Desorption of loaded uranium ions was effectively
achieved using 1 M HNO3 while only 0.5 M HNO3 was recommended to release retained thorium ions.

1. Introduction
Trace analysis of metal ions poses unique problems to analysts, as it involves rigorous requirements of versatility, specificity, sensitivity, and accuracy during chemical analysis.
These factors are crucial mainly in separation of ultra trace
amounts of inorganic species, decontamination of toxic metals
and nuclear spent fuel reprocessing processes. In such cases,
metal quantification at trace and macro quantities is ofimmense value. To face these problems, one needs to look formethods that are both efficient and economical for such separation processes. Solvent extraction technique using organo
phosphorous extractants was considered for this purpose.1–3
But owing to their tendency to undergo faster radiolytic degradation, formation of large secondary waste volumes, a better
alternative to these extractants was explored. Recently, solid
phase extraction (SPE) has been increasingly used for preconcentration/separation of trace and ultra trace amounts of inorganic and organic species from complex matrices.4–7 Various
researchers have highlighted the advantages of SPE over otherpreconcentration techniques and in particular over liquid–liquid extraction.6–8 Chelating resins have been frequently used
SPE’s as they provide good stability, high sorption capacity formetal ions, and good f lexibility in working conditions.
Iminodiacetate resins9, 10 and others including Amberlite XAD
series are widely used for this purpose.11-13 One of the newdevelopments in recent years is the usage of polymer fibers to
extract some actinide elements and remove heavy metal ionsfrom aqueous solutions.14-16 Also various natural compounds
as clay minerals,17-18 zeolites,19 and meal wood20 were successfully used for preconcentration of some metal ions. In particular, carbonized apricot stone (CAS) is reported to be one of thepromising natural and cheap materials used in preconcentration and decontamination processes.21 It has high surface areaand porosity, good thermal and radiolytic stability, and highretention ability for metal ions. 22 To improve the sorption
selectivity of this new sorbent toward some actinides elements,
which is an important task in nuclear reprocessing and decontamination processes, it was the thought to couple it with somecarboxylic acids. The applied acids are oxalic and succinicacids, ligands widely used in liquid-liquid extraction for sepa*Corresponding author. E-mail: rsheha68@yahoo.com.

ration of inorganic species.23
The current paper focuses on applying a new solid phaseextraction method using CAS anchored with oxalic or succinicacids for the extractive preconcentration of uranium and thorium elements from slightly acidic media. The extractive preconcentration behavior of the two actinide elements on theapplied solid supported matrix was studied under varying
experimental conditions using batch operation mode.
2. Experimental
2.1. Reagents. All chemicals and reagents used in this work
were of analytical grade purity and used without further purification. Apricot was obtained, as a natural fruit, from El-Amaar
region, Banha city. This region locates in Kalubia governorate
and is about 50 km from Cairo. The individual stock solutionsof uranium (VI) and thorium (IV) were prepared by dissolving
appropriate amounts of UO2 (NO3)2·6H2O and Th(NO3) 4 ·5H2O,(Aldrich, USA) in bi-distilled water. Five mL of conc. HNO3
was added to each 100 mL of stock solution to suppress hydrolysis. Arsenazo-III (Aldrich, USA) solution was prepared bydissolving 0.1 g of the reagent in 100 mL of bi-distilled water.
2.2. Instrumentation. A Pore Size Micrometric–9320,
USA equipment and a FT BOMEN–Michelson IR Spectrometer
were employed for structural characterization of the anchoredsorbent. The determination of metal ion concentrations was
per for med using UV-Vis spectrophotometer UV-2401(Shimadzu, Japan). MA-235 digital pH meter (Mettler Toledo,UK) was used for pH measurements. A SBS-30 thermostatedwater bath shaker (Stuart Scientific, UK) was used for batch
equilibration experiments.
2.3. Sorbent preparation. The apricot stone shells werecrushed to a fine form. The powdered shells were immersedwith H3PO4 and then carbonized at 250 ˚C. The produced carbon was washed with bi-distilled water, dried and sieved into
various particle size ranges. Impregnation with oxalic or succinic acids was carried out by conducting, in series, 10 g of
CAS particles with 100 mL oxalic or succinic acids aqueous
solutions in tightly sealed glass bottles. The initial concentration of the impregnants solutions are 10 -1, 5×10 -2, 10 -2, and 10 -3
M. The samples were shaken for a sufficient time to reachequilibrium. The progress of impregnation, after equilibrium,
was verified by taking certain volumes of both adsorbate solutions and subjecting to spectrophotometric analysis at λ = 285© 2006 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2006

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SomedaJ. Nucl. Radiochem. Sci., Vol. 7, No. 2, 2006

nm to determine oxalic acid concentration while succinic acidconcentration was determined by titration against NaOH inpresence of phenolphthalein. Then the impregnated carbon
was dried at 35 ˚C for 24 h and kept in a desiccator, to prevent
water vapour sorption, for further usage.
2.4. Recommended procedure. The experiments performedin this study were carried out in duplicates in most cases andtriplicates whenever needed. The extractive preconcentration
was carried out by equilibrating 0.1 g of apricot stone impregnated with oxalic or succinic acids with 10 mL metal ion solution of 10 -5 M U(VI) or 2.5×10 -5 M Th(IV) initial concentrations
in sealed glass bottles. The samples were shaken with 200 rpm
shaking rate at 26 ˚C. Samples were taken at appropriate timeintervals as necessary and centrifuged, and metal ion concentration was determined spectrophotometrically using ArsenazoIII reagent. The solution pH value was adjusted at ~ 2-3 forU(VI) complexation while 3 M HCl medium was applied forTh(IV)-Arsenazo complexation. The absorbance of ArsenazoIII complexes was measured at 656 nm. The sorbed amount (%)
of each metal ion and the distribution coefficient (Kd) were calculated using the relationships:
Sorbed amount (%) = Ci − Cf × 100,Ci

TABLE 1: The physical characteristics of carbonizedapricot stone (CAS)Property

Value- BET surface area

85.8 m2 g –1

- Median pore volume

5.24 µm3- Average pore diameter

0.089 µm

- Apparent density

0.858 g cm –3- Solid density

0.636 g cm –3- Porosity

25.85%- Grain size

0.125-0.09 mm

CAS
CAS-Ox(1)
CAS-SucV
Kd = Ci − Cf ×
,
m
Cf

(2)
Wave number /cm-1

where Ci and Cf are the initial and final concentrations of metal
ion in aqueous solution and V/m is the ratio of aqueous volume
to the sorbent weight. The metal capacity of the sorbents wasdetermined as follows: One g of impregnated CAS is added toa vessel containing 100 mL volume of U(VI) or Th(IV) solutions of known initial concentrations and shook till equilibrium
under optimum conditions. The aqueous phase was separated
and analyzed periodically with time for cation concentrations
and then replaced with a fresh solution of the same cation.
These procedures were repeated until no further uptake isobserved. The sorption capacity was calculated as:
Sorption capacity =V
Sorbed amount (%)
Ci Z
,
m
100

(3)

where Z represents the ion valency.
Similar batch studies were performed by conducting the sorbents, loaded with metal ions, with fresh solutions of different
HNO3 concentrations or bi-distilled water. The mixtures wereshaken for different time intervals and centrifuged, and theconcentrations of metal ions desorbed in solution were analysed. The desorbed amount was calculated using the relation:
Desorbed amount (%) = Cd −xCf × 100,
C i − Cf(4)

where Cd is the total desorbed concentrations of metal ion after
time t, and x is the volume fraction of aqueous solution left initially with sorbent (mL).
3. Result and Discussions
3.1. Characterization studies. The surface characteristics
of carbonized apricot stone (CAS) is given in Table 1. The data
reveal a high surface area and provide porous surface structure.
The IR spectra of free and impregnated CAS are given in
Figure 1. The spectra show broad band at 3433 cm-1 that may
be attributed to OH stretching vibration. The bands at 2856
and 2928 cm-1 could be assigned to C=C vibration while bands
at 1702 and 1614 cm-1 can be ascribed to C=O and /or COO stretching frequency. The bands imposed at 1171 and 991 cm-1
are indicative to the presence of oxygenated inorganic constituents (M-O vibration interactions). After impregnation withoxalic acid the bands at 2856 and 1614 cm-1 disappeared while

Figure 1. FT-IR spectra for carbonized apricot stone (CAS) impregnated with oxalic and succinic acids.

1702 cm-1 band was shifted to 1712 cm-1 and new bands at
1244, 718, and 481 cm-1 were obtained. Also, impregnation
with succinic acid caused the peaks at 3433, 2928, and 2856
cm-1 to disappear and two new peaks appeared at 1419 and1305 cm-1. This indicates that the molecules of oxalic and succinic acids were attached to apricot stone surface throughchemical interactions. These interactions result in certain
chemical bonds caused a change in the vibrational frequenciesof the chemical groups in apricot stone surface. On the light ofthis investigation, it can be assumed that the applied impregnants are trapped to CAS surface through crosslinking interactions between the carbonyl sites on the sorbent surface and theoxygenated groups of the impregnant structure.
3.2. Impregnation tolerance limits. Different concentrations of oxalic and succinic acids ranged from 10-3 –10 -1 M wereapplied individually to impregnate our new sorbent (CAS).
The obtained data show that both oxalic and succinic acids
were completely sorbed on CAS surface with 100% uptake.
This behavior is coincident with the sorption of other organic
acids on CAS.22 The release of these impregnants from loaded
CAS using dilute solutions of HNO3 have pH range 1.5–5 does
not exceed 1.9% and 2.6% for oxalic and succinic acids, whilebi-distilled water was found to have no effect. This negligible
release illustrates the stability of the impregnated sorbents
under the applied experimental conditions. The sorption of
U(VI) and Th(IV) from nitric acid solution using the prepared
impregnated sorbents is clarified in Figures 2 and 3. It is evident from Figure 2 that, CAS impregnated with 10 -3 M oxalic
acid records a U(VI) sorbed amount of 42% decreased to 31%
by increasing oxalic acid concentration up to 10 -1 M. The sorption extent on free CAS was 34% and this value is very similar
to that obtained by Starvin et al24 on studying the sorption of
U(VI) by activated carbon. This means that impregnation with
10 -3 M oxalic acid enhances the sorption process but higher
concentrations of this impregnant have no effect. Impregnationof CAS with succinic acid increases the sorbed amount of
U(VI) and the sorption extent enhances regularly with increasing the impregnant concentration.

Solid Phase Extractive Preconcentration of Some Actinide Elements

J. Nucl. Radiochem. Sci., Vol. 7, No. 2, 2006

42

40

39

Sorbed amount %Sorbed amount %Sorbents:CAS + Ox
CAS + Suc

36

39

Sorbents:Apricot stone-3
CAS + 10 M Ox.
-1
CAS + 10 M Suc.

30

20
33

30
0.00

0.03

0.06

0.0

0.09

0.4

Figure 2. Sorption of UO22+ from HNO3 aqueous solution on CAS
impregnated with different concentrations of oxalic and succinicacids (V/m 100, pH=~2, time=48 h, temp.= 26 ± 2 ˚C).
100

100

1.2

1.6

2.0

Figure 4. Effect of pH on sorption of UO22+ from HNO3 aqueous
solution on different apricot stone sorbents (V/m=100, time= 48 h,
temp.=26 ± 2 ˚C).
100

96

Sorbed amount %92

60

Sorbents:CAS + Ox
CAS + Suc

88

40

80

Sorbed amount %80

Sorbed amount %

0.8

pH

Impregnation conc., M

Sorbents:Apricot stone-3
CAS + 10 M Ox.
-1
CAS + 10 M Suc.

60

40

84

20

20

80
0.00

0.02

0.04

0.06

0.08

0.10

0.0

0.3

0.6

0.9

1.2

1.5

Impregnation conc., M

pH

Figure 3. Sorption of Th4+ from HNO3 aqueous solution on CAS
impregnated with different concentrations of oxalic and succinicacids (V/m=100, pH~1.5, time=2 h, temp.=26 ± 2 ˚C).

Figure 5. Effect of pH on sorption of Th4+ from HNO3 aqueous solution on different apricot stone sorbents (V/m =100, time =2 h,
temp.=26 ± 2 ˚C).

Figure 3 shows the sorption of Th(IV) onto CAS anchoredwith different concentrations of oxalic and succinic acids. The
plots imply that the sorbed amount of Th(IV) decreased from
95% to 11% by increasing the concentration of oxalic acid, as
an impregnant, from 10 -3 to 10 -1 M. This sorption reduction
clarifies that increasing the impregnant concentration decreases
the sorption ability of CAS towards thorium species. Thisbehavior can be referred to the formation of thorium-oxalate
complex species having the composition Th(C 2 O 4 ) 4 - 4 . 25, 26
These highly anionic species increase the solubility of thorium
ions in aqueous phase and consequently decrease their retention on carbonized apricot stone surface. On other hand,
impregnation with 10 -3 M succinic acid slightly increases thesorbed amount percent of thorium ions on CAS and thisenhancement was improved by increasing succinic acid concentrations up to 10 -1 M. Therefore, CAS impregnated with

10 -3 M oxalic acid and 10 -1 M succinic acid was subjected into
the further investigations.
3.3. Sorption studies
3.3.1. pH effect. Sorption of U(VI) and Th(IV) from aqueous solution as a function of solution pH was studied using thepreviously described anchored sorbents and the revealed data
are illustrated in Figures 4 and 5. The results clarify that thesorption of both analytes generally increases with increasing
the pH value. The average sorbed amount was increased from
19–24% to 33.6 – 41.6% as pH value increased from 0.2 up to 2
for U(VI) and increased form 5 – 6.9% to 94 –99.6% by rising
pH value from 0.05 up to 1.3 for Th(IV). This behavior can be
discussed on the light of the exchangeable properties of hydrogen ions and their effect on the solubility equilibria. At low pH
value, hydrogen ions compete for the available active sites onsorbent surface protonating their function groups and this neg-

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SomedaJ. Nucl. Radiochem. Sci., Vol. 7, No. 2, 2006
45

4.5
40

4.0
35

Sorbed amount %log (K d /mL g-1)

3.5
3.0

Metal ions
4+
Th
2+
UO2

2.5
2.0

30
25
20
15

1.5

10

1.0

5

0.5
0.0

0.4

0.8

1.2

1.6

Sorbents:Apricot stone-3
CAS + 10 M Ox
-1
CAS + 10 M Suc
0

2.0

1

2

3

Figure 6. Variation of log Kd versus pH for sorption of UO22+ andTh4+ from aqueous solution on CAS impregnated with succinic acid.

atively affects the sorption of both U(VI) and Th(IV) where
the electrostatic interactions would be electrically unfavorable.
Also, acidification with HNO3 forms the neutral nitrate salts
UO2 (NO3) 2 and Th(NO3) 4 that expected to be the likely predominating species in this acidic range. The absence of electrical charge at the surface of these neutral species and hence thesurface inactivity may be a reasonable proof for the observed
decrease in sorption extent at low pH range. The loose contactof both analytes on the sorbent surface was probably an acceptable reason for the effective regeneration of these analytes
using acidic eluents. As pH increase, hydrolysis was occurredand different proportions of several hydrolyzed species areformed changing the net charge at the interface between theworking solution and the applied sorbents surfaces. It was
identified that the hydrolyzed ionic species of U(VI) have theforms UO2 (OH) +, [(UO2)2 (OH)2] 2+, and [(UO2)3(OH)5]+.27 The
species [Th(OH)]3+, [Th(OH)2] 2+, [Th2 (OH)2] 6+, [Th4 (OH) 8] 8+,
and finally [Th6 (OH)15] 9+ were reported as hydrolyzed thorium
species.28 These hydrolyzed species exhibit an overall positive
charge, therefore their surface interactions with CAS would beelectrically attractive. This may explain the gradual increasein sorption of these cationic species with increasing pH up to 2
and 1.3 for U(VI) and Th(IV), respectively.
The interaction of the hydrolyzed species of uranyl and thorium ions with CAS sorbent is more complex where the different species interact with the sorbent surface through different
mechanisms and are substantially different in their affinities.
The mechanism considered to predominate the sorption behavior of the studied metal ions can be explained on the light ofion exchange process between metal ion and the weakly acidic
functional groups in the sorbent surface. It is likely that carboxylic groups are largely responsible for such ion exchangeinteractions. Therefore, the cation exchange process between ametal ion Mn+ and a proton in a dilute aqueous solution, activity coefficient ~1, could be described by the following equilibrium reaction:+→
M n+
aq + nHR ← nH aq + MR n ,4

5

20 30 40 50 60 70 80

Time /hpH(5)where M n+
aq is the cation exchanged in solution and the superscript bar denotes the exchanger phase. The equilibrium constant for such reactions is given by the following equation:

Figure 7. Effect of contact time on sorption of UO22+ from HNO3
solution on different apricot stone sorbents (V/m =100, pH=~2,
temp.=26 ± 2 ˚C).

[
[H+aq ] n⎯MR
n]
,
(6)
n+
n
[
⎯HR]
[M aq ]n[are the concentration of M n+ and H +where ⎯MR[n ] and ⎯HR]
ions in sorbent surface, [H+aq ] n and [M n+aq ] are their concentrations in aqueous solution. At equilibrium, the distribution
coefficient (Kd) is given by the equation:
KMH =log Kd = C + n pH.

(7)

Plotting log Kd values of n+ valent metal ion against pH valuesof aqueous solution should give a straight line of slope (n)which equals to the charge on the metal ion, if the sorption is apure ion exchange process. The variation of log Kd values for
4+ions against pH of the aqueous solution is clariUO 2+
2 and Th
fied in Figure 6. The plots show linear relations with slopes
equal to 0.14 and 2.36 for sorption of U(VI) and Th(IV) on
CAS anchored with succinic acid. The slope values 0.13 and2.12 were obtained for sorption of U(VI) and Th(IV) on CAS
anchored with oxalic acid, respectively. This behavior represents a deviation form ideal sorption and thus the mechanism is
not pure ion exchange. The non-ideality may be due to thepresence of other mechanisms besides the ion exchange mechanism such as specific complexation and/or columbic interactions. This is due to the complexity of systems involving
sorption onto natural sorbents where there is a possibility ofsharing of more than one mechanism in the sorption process.
3.3.2. Contact time effect. The changes in sorption of U(VI)and Th(IV) onto free and impregnated CAS as a function oftime at a solution pH about 2 and 1.3 for U(VI) and Th(IV) arerepresented in Figures 7 and 8. The figures show that the sorption percentage generally increases with elapsed time till reaching equilibrium where the sorbed amount of U(VI) attains
values ranged from 34% up to 42% after elapsing 24 h while
the sorption equilibrium was reached very fast for Th(IV) andtook only about 40 min to achieve values of the range 92–
100%. On the light of these data, the sorbed amount of tetravalent thorium is greatly high comparing to that of uranyl ions,and this can be explained on the basis of increasing the electrostatic interactions between the analyte species and impregnant
groups with increasing the cationic charge of the analyte.29 In
addition, the steric hindrance of the oxygenated U(VI) ions has
a negative contribution to their sorption extent.

Solid Phase Extractive Preconcentration of Some Actinide Elements

41

J. Nucl. Radiochem. Sci., Vol. 7, No. 2, 2006

100

2.7×10-5

2.4×10-5

2.1×10-5

Co - Ce /M

Sorbed amount %

80

60

40

1.8×10-5

Metal ions
4+
Th
2+
UO 2

6.0×10-6

4.0×10-6

Sorbents:Apricot stone-3
CAS + 10 M Ox
-1
CAS + 10 M Suc

20

0

20

40

60

80

2.0×10-6

120 180 240 300 360

0Time /min

5

10

15

20

30

45

60

75

Time (min)Figure 8. Effect of contact time on sorption of Th4+ from HNO3
aqueous solution on different apricot stone sorbents (V/m =100,
pH=~1.3, temp.=26 ± 2 ˚C).

Figure 9. The rate of UO22+ and Th4+ ions sorption from solution on
CAS impregnated with succinic acid.

3.3.3. Reproducibility. The distribution coefficient (Kd) for
sorption of Th(IV) on CAS, oxalic anchored CAS, and succinic
anchored CAS achieves the values 1927, 3550, and 12066 mL g-1,
respectively while U(VI) ions attain the values 1007, 2718, and
1450 mL g-1 for their sorption onto the same sorbents. The
reproducibility of the distribution of U(VI) and Th(IV) onto
CAS anchored with oxalic and succinic acids was clarified by
performing four parallel series of batch experiments. The standard deviation for a single distribution coefficient determination (∆Kd) was calculated using the following relation.30

ones and the values of deviation between the repeated experiments, under the same conditions, confirm the reproducibilityof the revealed results. Hence, the change in behavior of thedifferent impregnated CAS could not be attributed to the intrinsic irreproducibility of these row materials but may be implied
to the role played with the applied impregnants.
3.4. Sorption kinetics. The process of mass transfer associated with sorption of metal ions from aqueous solution byporous sorbents occurs via three consecutive steps of bulk,
film, and intraparticle transport and the predominant rate ofsorption is determined by one of these steps. To determine
which of these processes is the rate controlling step, the sorption data were plotted as a function of the square root of contact
time, Figure 9. The plots clarify that the initial sorption step isrelatively high for Th(IV) ions where more than 85% arealready sorbed within 10 min, while the rate of U(VI) sorption
is relatively low. Also, the figure implies two distinguishable
sorption stages. The first one may be related to the sorption of
metal ions, at phase boundaries, to sorbent surface while thatremaining in bulk solution are directed to sorbent-liquid interface under the influence of concentration gradient forces.31 At
the second stage, the sorbed molecules diffuse into the pores ofsorbent surface.
From a diffusion point of view, these multistage sorption
processes reflect that the rate of sorption of both ions is not
purely controlled by a certain one diffusion mechanism where
external mass transfer and intraparticle diffusions are sharing
in the rate controlling step. The relative sorption rate constant(Ksrc, mg g-1min-0.5), that is characteristic to the rate of sorption
under the working conditions, was graphically determinedfrom the plots given in Figure 9 and found to have the values
8.03 ×10 -6 and 4.67×10 -8 mg g-1min-0.5 for Th(IV) and U(VI),respectively. This significant difference in the rate values indicates that different interaction mechanisms are taking place
between the two ions and the sorbent surface during sorption
process. The rapid rate of sorption of Th(IV) may be ascribedto that the anchored sorbents have high retainability to Th(IV)ions that easily sorbed and quickly diffused into the pores ofsorbent, and thus sorption of Th(IV) is suggested to be controlled by their transport from bulk solution to sorbent surface.
On other hand, the low rate of U(VI) sorption can be considered as an indication that intraparticle diffusion, i.e. transport∆K d =

Σ(Kd' –Kdi)2 ,
n –1

(8)

where Kdi is the distribution coefficient for an experiment, Kd'
is the mean value of distribution coefficients for all experiments.
Kd' =Kd1 + Kd2 + Kd3 + ······
,
n

(9)

where n is the number of experiments carried out under thesame experimental conditions.
The sorption of U(VI) and Th(IV) onto CAS impregnated withoxalic and succinic acids was classified into four categories ofexperiments. Each set was performed four times and one ofthe data revealed is represented in Table 2. The standard deviation for a single distribution coefficient determination (∆Kd) for
sorption of U(VI) on CAS anchored with oxalic and succinicacids are 0.56 and 3.7% while that for sorption of Th(IV) are
1.98 and 0.85%, respectively. The obtained data show that ∆Kddoes not exceed 3.7% for any working system of the studied
TABLE 2: The standard deviation for a single distribution
coefficient determination (∆Kd) for sorption of U(VI) on
CAS anchored with oxalic acid
Experiment
No.

1
2
3
4
Kd' = 2718

Kdi

Kd' - Kdi

(Kd' - Kdi) 2

∆Kd

2733
2698
2715
2726

-15
20
3
-8

225
400
9
64

15.25
±0.56%

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SomedaJ. Nucl. Radiochem. Sci., Vol. 7, No. 2, 2006
140

TABLE 3: Sorption capacities of impregnated CAS sorbents for U(VI) and Th (IV)-1

Sorption capacity /mg g

Succinic acidimpregnated CAS

U(VI)

2.39

2.30

Th (IV)

8.47

9.18

of metal ions from the exterior surface of sorbent to the interiorsites is possibly the sorption rate controlling step.
3.5. Sorption capacity. The metal sorption capacity ofanchored CAS sorbent for the analytes of interest was individually determine via batch operation mode and data are given in
Table 3. The data clarifies that impregnated CAS exhibited ahigher sorption affinity to Th(IV) rather for U(VI). Comparing
with other reported data, the capacity of carboxylic acids
impregnated CAS is higher than that of 5,7-dichloroquinoline8-ol supported on naphthalene (1.88 mg g-1)32 and similar to thatof benzophenone supported on poly acrylonitrile (2.34 mg g-1).33
Other compounds as diarylazobisphenol modified activatedcarbon 24 and Amberlite XAD4 functionalized with succinic
acid12 were reported to have higher sorption capacity values.
The high sorption capacities of such compounds for U(VI) may
be referred to the technique applied to impregnate the solid
support with the chelate.
One of the promising findings noticed in the successive
sorption of uranyl ions on the applied anchored sorbents is thegreat enhancement in sorption extent during reloading processes. It was found out that, uranyl ions were weakly sorbed
in the first run where their sorbed amount reached only about
39%. By subjection in consecutive runs, the sorbed amount
was highly promoted to 96% and the sorbent retainability still
constant up to five runs, Figure 10. This promotion, in sorption
extent, may be referred to that the firstly sorbed uranyl ions
may serve as a surface moderator for the further sorption runs.
This modification can be highlighted through the following
proposed sorption scheme.

Sorption capacity /mg g-1

Oxalic acidimpregnated CAS

120

100

80

60

40

20

1

2

3

4

5

Run number
Figure 10. Comparison of sorption capacity of different apricotstone sorbents for UO22+ ions.
100

80

Desorbed amount %Metal ion

Apricot stone-3
CAS + 10 M Ox
-1
CAS + 10 M Suc

60

40

Sorbents:Apricot stone-3
CAS + 10 M Ox
-1
CAS + 10 M Suc

20

0

30

60

90

120

150

180

Time /min
Uranyl ions seem to behave as rather like divalent andaccordingly they readily form complexes with O-donor ligands.
Due to the presence of empty f-orbitals, the O=U=O groups
are linear in all cases and coordination of a further 4, 5, and 6
ligands is possible in equatorial plane.34 These ligands laying
the plane may be neutral molecules or bidentate anions. Since
there are empty f-orbitals and lone pairs of electrons in oxygen
atoms of uranyl molecules, these pairs can be donated to forma shared bond between the sorbed uranyl ions and other freespecies in bulk solution. Hence, this type of surface complexation or chelation interactions probable plays an important rolein increasing the sorbed amount in the consecutive runs. Also
considering the former scheme, one can expect a formation ofhydrogen bonding, between H-atom in the hydrolyzed species
and O-atom in uranyl molecules, would occur and this increase
the contribution of hydrogen bonding in enhancing the sorption extent.
3.6. Desorption studies. The recovery of U(VI) and Th(IV)from loaded sorbents was checked using distilled water anddifferent concentrations of nitric acid as eluants and dataobtained are given in Figures 11, 12, and 13. The desorption of

Figure 11. Desorption of Th4+ from different loaded apricot stonesorbents using 0.5 M HNO3.

loaded thorium ions from CAS was very fast with using 0.5 M
HNO3 as an eluant and it took only about 1 h to reach equilibrium, Figure 11. In contrast, H 2O slightly eluted Th(IV) ionsfrom the applied solid matrix, Figure 12. The description ofdesorption behavior of U(VI) ions is depicted in Figure 13. The
plots show that U(VI) is highly eluted with using 1 M nitricacid and equilibrium was achieved after 24 h of contact. These
performances reflect the potential recovery of the studied analytes using acidic eluants and verify the reusability of the proposed CAS sorbent.
4. Conclusion
Carbonized apricot stone (CAS) anchored with oxalic andsuccinic acids is effectively used as a sorbent for preconcentration of U(VI) and Th(IV) from slightly acidic aqueous solutions. The analytical data confirm that the sorption of bothanalytes increases with rising pH value of the aqueous solution

Solid Phase Extractive Preconcentration of Some Actinide Elements
20

Desorbed amount %43

References
Sorbents:Apricot stone-3
CAS + 10 M Ox
-1
CAS + 10 M Suc

16

12

8

4

0

40

80

120

160

200

Time /minFigure 12. Desorption of Th4+ from different loaded apricot stonesorbents using H2O.
100

80

Desorbed amount %J. Nucl. Radiochem. Sci., Vol. 7, No. 2, 2006

60

40

Sorbents:Apricot stone-3
CAS + 10 M Ox
-1
CAS + 10 M Suc

20

0

1

2

3

20

30

40

50

Time /hFigure 13. Desorption of UO22+ from different loaded apricot stonesorbents using 1 M HNO3.

and a quantitative preconcentration achieved at pH ~ 2 forU(VI) and ~ 1.3 for Th(IV). The values of deviation between
the repeated experiments under same conditions confirm thereproducibility of the obtained results and the change in behavior of the different impregnated CAS could not be attributed tothe intrinsic irreproducibility of these row materials. The sorption of Th(IV) is probably controlled by their transport frombulk solution to sorbent surface while intraparticle diffusion
could be considered as the rate controlling step in U(VI) sorption. The produced CAS sorbent has better metal sorption
capacity and exhibits a new dimension for actinides separationfrom acidic waste solutions. It provides a novel and economical solid matrix appears promising for a number of industrial
applications. The sorption performance highlights better reliability and reproducibility of the sorbent matrix even up fivecycles of consecutive usage. Desorption of loaded uranium
ions was effectively achieved using 1 M HNO3 while only 0.5 M
HNO3 was recommended to release retained thorium ions.

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