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ARTICLE IN PRESS
hyschem
Radiation Physics and Chemistry 75 (2006) 301–308
www.elsevier.com/locate/radp
Abstr
1. In
0969-
doi:10
fax: +
E-
i
Cerium (IV) molybdate cation exchanger:Synthesis, properties and ion separation capabilities
A. Nilchi�, B. Maalek, A. Khanchi, M. Ghanadi Maragheh, A. Bagher
Jaber Ibn Hayan Research Laboratories, Atomic Energy Organization of Iran, P.O. Box 11365/8486, Tehran, Iran
Received 29 December 2004; accepted 23 July 2005
act
ared at
ysis, IR
H)2(H-
alue of
o and
V) were
Mg(II)
orption
Nuclear
A description of the preparation and properties of an H+ form of cerium (IV) molybdate ion exchanger prep
different pH values is given. The material was characterized by inductively coupled plasma (ICP) elemental anal
spectrometry and thermo-gravimetry and titration curve analysis; its composition was found to be Ce(O
MoO4)2?nH2O. It was also observed that the exchange capacity of the ion exchanger depends upon the pH v
the ceric solutions used and its value was lowered after gamma irradiation. Furthermore, the absorption of 99M
the distribution coefficient of this ion exchanger for Mg(II), Ba(II), Ca(II), Ni(II), Mo(VI), Cr(VI) and Th(I
investigated. It was found that cerium (IV) molybdate had the highest absorption for Th(IV) and Ba(II), while
and Ca(II) showed the lowest absorption, respectively. Finally, gamma irradiation, in general, lowered the abs
of the cations being investigated in this study.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Inorganic ion exchangers; Distribution coefficient; Ion-exchange capacity; Radiation stability; Elution curves;
technology
troduction
nic
at th
oura
ds h
e alt
diffic
(Nil
of
olut
elut
eatm
of wastes and in processes for recovery of metals from
en used
plating
als are
conven-
ving in
alt type
, tung-
rsenates
tanium,
, 2004;
va and
ies on
ported.
The resurgence of interest in synthetic inorga
exchangers has largely stemmed from the fact th
materials can be used under conditions unfav
towards organic resins. Their resistance towar
and ionizing radiations makes them attractiv
natives towards certain ions, a number of
separations can conveniently be carried out
et al., 1999, 2000, 2002, 2003). The ability
exchange materials to remove trace ions from s
and the concentration which may be achieved on
with suitable solutions have been used in the tr
�Corresponding author. Tel.: +9821 61384341;
ws ioncreases
806X/$ - see front matter r 2005 Elsevier Ltd. All rig
.1016/j.radphyschem.2005.07.003
98 21 638749.
mail address: [email protected] (A. Nilchi).
ion
ese
ble
eat
er-
ult
chi
ion
ion
ion
ent
very dilute solutions. Ion exchangers have be
extensively in treating rinse water wastes in
industry for example, where valuable met
recovered at costs comparable to or less than
tional chemical treatment, with appreciable sa
space for treatment plant.
Recently, a number of ion exchangers of the s
have been reported, these include phosphates
states, molybdates, antimonates, vanadates, a
and similar salts of thorium, zirconium, ti
cerium, tin and other metals (Nilchi et al.
Amphlett, 1985; Gal and Gal, 1985; Chekomo
Cherednichenko, 1998). A number of stud
molybdic-acid-based ion exchangers have been re
Molybdic acid (Clearfield et al., 1973) sho
exchange properties and the uptake of cations in
hts reserved.
with increasing pH. Although the exchange behaviour of
e be
ly be
cal a
istrib
cert
Ba(
washed with deionized water and air-dried. The product
h size.
s deter-
0mL of
table in
h metal
olution
ARTICLE IN PRESSA. Nilchi et al. / Radiation Physics and Chemistry 75 (2006) 301–308302
the molybdates of some metal cations hav
explored, cerium (IV) molybdate has relative
untouched. In this paper, the synthesis, chemi
thermal stability, gamma irradiation, and the d
tion coefficient of the ion exchanger towards
ions, namely Mo(VI), Cr(VI), Th(IV), Ni(II),
Mg(II) and Ca(II) are examined.
2. Experimental
alyti
rad
Th(I
rmin
Nucl
f Ira
tt p
ecord
ogra
t mo
btain
oup
am
oni
repa
sam
ate a
e pH
SO4
ith
It w
d wa
ogen
as th
tration
affected
cerium
metal
Ba(II),
er were
in a
omplete
of each
4mL of
100mL
ach ion
Plasma
mn that
s bottle
low the
lectrode
H was
out, by
f about
.
ed with
All the reagents and chemicals used were of an
grade obtained from Merck or Aldrich. 99Mo
isotope and ionic solutions of Mo(VI), Cr(VI),
Ni(II), Ba(II), Mg(II) and Ca(II) used for dete
distribution coefficient values were supplied by
Research Centre, Atomic Energy Organization o
The pH values were measured using a Scho
meter, model CG841; the infrared spectra were r
using a Perkin-Elmer spectrophotometer; therm
metric analyses were carried out using a DuPon
951 thermobalance, the elemental analysis was o
using a Perkin-Elmer model 5500 Inductively C
Plasma Spectrophotometer and finally an Ortec G
Spectrometer was used.
2.1. Preparation of cerium (IV) molybdate
The effective concentration of ceric amm
nitrate and ammonium molybdate used in the p
tion of seven samples are given in Table 1. The
was prepared by mixing ceric ammonium nitr
ammonium molybdate at room temperature. Th
the ceric solution was adjusted with 0.1M H2
0.80, 0.50 and 0.15. The precipitate in contact w
liquid was left overnight at room temperature.
then filtered, washed with 0.1M H2SO4, deionize
and air-dried. The sample was converted to hydr
immersing it in 1M HNO3 for 48 h, and w
a dose
stigated
) of the
ibution
changer
values
A 0.2 g
lutions
vessel.
Table 1
The preparation conditions used
Sample no. pH of Ce4+
solution
Concentration (M) Mixi
ratio
Ce/MCe4+ Mo6+
1 0.80 0.05 0.10 0.50
2 0.50 0.05 0.10 0.50
3 0.15 0.05 0.10 0.50
4 0.15 0.05 0.2 0.25
5 0.15 0.05 0.15 0.33
6 0.15 0.25 0.25 1.00
7 0.15 0.10 0.05 2.00
en
en
nd
u-
ain
II),
cal
io-
V),
ing
ear
n.
H-
ed
ve-
del
ed
led
ma
um
ra-
ple
nd
of
to
the
as
ter
by
en
was then ground and sieved to appropriate mes
2.2. Chemical stability
Chemical stability of the ion exchanger wa
mined by dissolving 0.1 g of the exchanger in 5
different solvents for 24 h. The exchanger is s
water and in solution of alkali and alkaline eart
salts. On refluxing with water, a milky colloidal s
is obtained. It completely dissolved in 4M concen
of acids, while in alkalic solutions it remained un
up to a concentration of 0.1M.
2.3. Distribution coefficient, Kd
For distribution coefficient studies, 0.2 g of the
molybdate ion exchanger and 0.5mL of 0.005M
ion solutions of Mo(VI), Cr(VI), Th(IV), Ni(II),
Mg(II) and Ca(II) and 19.5mL of deionized wat
added in different vessels and were placed
thermostated shaker at 25 1C for 5 h to attain c
equilibrium. The mixture was filtered and 10mL
solution was poured into separate bottles and
HNO3 concentration was added and diluted to
with deionized water. The concentration of e
metal was determined by Inductively Coupled
analysis.
2.4. Titration of ion exchanger
Cerium molybdate (0.2 g) was placed in a colu
was fitted with glass wool at its bottom. A glas
containing 20mL of 0.001M HCl was placed be
column, and for determination of pH, a glass e
was placed in the solution, then 70mL of NaO
poured into the column. Titration was carried
passing the NaOH solution at a drop rate o
0.3mL/min, and continued to a pH of about 13
2.5. Radiation stability
Cerium molybdate in H+ form was irradiat60Co source for the total of 50 and 200 kGy at
rate of 0.484Gy/s. Radiation stability was inve
by determining the ion exchange capacity (IEC
solid before and after irradiation.
2.6. Absorption of 99Mo on cerium molybdate
In order to determine the absorption and distr
coefficients of 99Mo on cerium molybdate ion ex
at different media, solutions with different pH
were prepared and placed into different vessels.
of cerium molybdate and 19.5mL of various so
with different pH values were added to each
ng
o
A 0.25mL of 99Mo was added to each vessel, and then
h. T
olut
le w
bands at 365 and 890 cm�1 show the presence of
can be
e–O–H
ge was
dose of
urve of
ARTICLE IN PRESSA. Nilchi et al. / Radiation Physics and Chemistry 75 (2006) 301–308 303
placed in a thermostated shaker at 25 1C for 3
solutions were then filtered and 5mL of each s
was removed and the activity of each samp
counted by gamma spectrometer.
3. Results and discussion
n cu
ysis,
s be
ed a
infra
ion
00 cm�1. T
istics
ows
00 cm
ows
bserv
due
on a
ally
molyb-
changer
al water
300 1C,
ps take
nic ion
eight is
e curve
e up to
nted by
loss of
alue of
ated by
nger by
olecular
ives the
On the basis of infrared spectrometry, titratio
analysis, thermal studies and ICP elemental anal
molecular formula of cerium molybdate ha
determined.
Fig. 1 shows the IR spectrum of unirradiat
irradiated samples of the ion exchanger. The
spectrum shows a broad band in the reg
3000–3650 cm�1, sharp peaks at 1610 and 11
and a weak broad band in the region 500–800 cm
peaks at 3400–3650 and 1610 cm�1 are character
O–H stretching and bending mode, and hence sh
presence of lattice water while the peak at 11
corresponds to M–O–H bending mode which sh
presence of Ce–O–H. As such, the broad peaks o
in this compound at 550, 610 and 840 cm�1 are
the overlapping of the bands of molybdate i
Ce(OH)2+ in the regions of 500–800 cm�1 and fin
Fig. 1. IR spectra of (a) unirradiated and irradiated samp
50 kGy. (Curves b and c are shifted up, as related to transm
he
ion
as
rve
the
en
nd
red
of�1
he
of
the�1
the
ed
to
nd
the
molybdate ion. Hence, from the IR analysis, it
deduced that H2O, �OH, molybdate ion and C
are present. Furthermore, no significant chan
observed in the irradiated samples up to a total
200 kGy.
Figs. 2 and 3 show the thermogram and DCS c
cerium molybdate. The thermogram of cerium
date suggests that the weight loss of the ion ex
up to 150 1C is due to the removal of free extern
molecules. At higher temperatures up to
condensation of exchangeable hydroxyl grou
place, which is characteristic of synthetic inorga
exchangers. Above 300 1C the gradual loss in w
due to the removal of structural water. Th
pattern suggests that the ion exchanger is stabl
800 1C. The 11.2% weight loss of mass represe
the TGA curve at 100 1C must be due to the
nH2O from the ion exchanger structure. The v
‘‘n’’, the external water molecules, can be calcul
18n ¼X ðM þ 18nÞ
100,
where X is the percent weight loss in the excha
heating up to 100 1C, and (M þ 18n) is the m
weight loss of the material. The calculation g
les of cerium (IV) molybdate. Total dose absorbed (b) 200 kGy and (c)
ittance).
value of 4 for the external water molecules (n) per
Mo
s m
ratio
e abo
4)2?
ceri
nts
hrou
ows
n. T
po
indicating that the ceric molybdate behaves as mono-
. 5) and
of this
trations
n. As a
tandard
on and
curves
tion of
Cl. The
slowly,
reciable
ed with
m gives
ARTICLE IN PRESSA. Nilchi et al. / Radiation Physics and Chemistry 75 (2006) 301–308304
molecule of the exchanger.
The concentration of two elements of Ce and
2� 10�2 g/L of cerium molybdate solution wa
sured by ICP elemental analysis, and the molar
Ce/Mo was found to be 0.44. As a result of th
findings, a tentative formula of Ce(OH)2 (HMoO
H2O has been assigned to the exchanger.
Fig. 4 shows the pH-titration curve of the
molybdate. In this figure, the X-axis represe
number of millimoles of 0.01M NaOH passed t
per gram of cerium molybdate; and the Y-axis sh
pH value of the effluent passed through the colum
pH titration curve shows only one inflexion
Fig. 2. Thermogram of cerium (IV) molybdate.
Fig. 3. DSC cur
in
ea-
of
ve
4-
um
the
gh
the
he
int
functional.
The effect of the concentration of NaCl (Fig
equilibration time (Fig. 6) on exchange capacity
compound shows a constant capacity at concen
greater than 0.6M and after 48 h of equilibratio
result the exchange capacity was determined by s
method (Samuelson, 1965) in 1M salt soluti
samples are equilibrated for 48 h. The elution
(Fig. 7) show that practically complete elu
hydrogen ions take place in 100mL of 1M Na
elution is fat at the beginning and then decreases
becoming insignificant after 100mL. No app
change in the capacity of the sample is observ
increasing pH (2–10). This exchanger in acid for
a buffer system of pH 2–3.
ve of cerium (IV) molybdate.
0
2
4
6
8
10
12
14
0 5 10 15 20 25 30 35No. mmoles of 0.01M NaOH
pH
Fig. 4. The pH-titration curve of cerium (IV) molybdate with
0.1M NaOH.
The ion exchange capacity of seven samples of ceri
red
apac
ns. T
ampl
precipitated at pH ¼ 0.15 has higher capacity than
er, the
e effect
mixing
change
:1 ratio.
ingoing
shown
s. It can
bivalent
ith the
us ions
repared
he Kd is
tion, Fc
passing
ed (mL)
ARTICLE IN PRESS
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1 1.2Molarity of NaCl
Exc
hang
e ca
paci
ty,
meq
/g
Fig. 5. Exchange capacity as a function of NaCl concentration.
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40 50 60 70 80
Equilibration time, hr
Exc
hang
e ca
paci
ty,
meq
/g
Fig. 6. Effect of equilibration time on exchange capacity.
0
1
2
3
4
5
6
7
8
9
0 20 40 60 80 100 120 1Effluent, mL
Elu
tion
of [
H+
] (v
olum
e of
0.0
1 N
NaO
H)
eluant 1.0 M NaCl
Eluant 0.1 M NaCl
Fig. 7. Elution curve of hydrogen ions for ceric molybdat
Table 2
The ion exchange capacity (IEC) of cerium molybdate ion
exchanger precipitated at different pH values with 0.1M NaCl
Sample no. pH IEC (meq/g) Mixing ratio
1 0.15 5.27 0.50
2 0.15 3.30 0.33
3 0.15 2.40 0.25
4 0.15 1.70 1.00
5 0.15 2.90 2.00
6 0.50 4.24 0.50
7 0.80 3.67 0.50
Table 3
Effect of size and charge of the exchanged ion on the exchange
capacity
Exchanged ion Hydrated ionic
radius (pm)
Exchange capacity
(mequiv/g; dry mass)
Li(I) 340 0.35
Na(I) 276 0.60
K(I) 232 0.75
Mg(II) 700 0.43
Ca(II) 630 0.53
Sr(II) 610 0.65
Ba(II) 590 0.73
A. Nilchi et al. / Radiation Physics and Chemistry 75 (2006) 301–308 305
molybdate ion exchanger which were prepa
different pH values is given in Table 2.
The results show that the ion exchange c
depends on the pH value of the ceric solutio
capacity increases with decrease in pH values; s
um
at
ity
he
e 1
sample 7 precipitated at pH ¼ 0.80. Howev
concentration of the solutions mixed have littl
on the exchange capacity. Sample 1 obtained by
ceric and molybdate in 1:2 ratio has a higher ex
capacity than sample 5 obtained by mixing in 2
The effect of the size and charge of the
hydrated ion on the capacity of the exchanger is
in Table 3 for alkali and alkaline earth metal ion
be seen clearly that for univalent as well as
cations, the exchange capacity increases w
decrease in hydrated ionic radii.
The value of Kd measurements for vario
studied using cerium molybdate ion exchanger p
at different pH values are given in Tables 4–6. T
defined by
KdI c � F c
F c�
V
MðmL=gÞ,
where Ic is the initial concentration of ion in solu
the final concentration of ion in solution after
from ion exchanger, V the volume of solution us
40
e.
and M the weight of exchanger in contact with VmL
ate ion
to have
studied
he ion
V) and
I). The
h(IV)4II).
ibution
ellings,
l bond,
solvent distribution and nature of the ion exchanger.
value of
stigated,
hemical
o(VI),
I) from
(II) and
of this
s which
cerium
ric and
xchange
Gy are
hich are
radiated
cient of
ate ion
that the
e lower
studied
is also
ve the
I)4Cr
ame as
ARTICLE IN PRESS
Table 5
Distribution coefficient measurement of various catio
pH ¼ 0.5
Cations Kd (mL/g)
Mg(II) 4.0
Ca(II) 63.0
Ba(II) 1910.0
Ni(II) 34.5
Cr(VI) 74.0
Mo(VI) 22.1
Th(IV) 4399.5
Table 4
Distribution coefficient measurement of various catio
pH ¼ 0.8
Cations Kd (mL/g)
Mg(II) 1.3
Ca(II) 36.8
Ba(II) 1204.2
Ni(II) 22.7
Cr(VI) 7.0
Mo(VI) 20.6
Th(IV) 4237.2
Table 6
Distribution coefficient measurement of various cations,
pH ¼ 0.15
Cations Kd (mL/g)
Mg(II) 17.0
Ca(II) 65.0
Ba(II) 6341.0
Ni(II) 40.0
Cr(VI) 81.0
Mo(VI) 99.6
Th(IV) 11 722
0 20 40 60 80 1000123456789
Volume of effluent (mL)
Rel
ativ
e im
puls
es
(a) (b)
Fig. 9. Elution curve of separation of Th(IV)–Mg(II): (a)
deionised water, 0.1mL/min; (b) 0.5M HCl, 0.4mL/min; 2.5 g
of cerium molybdate.
0123456789
0 20 40 60 80 100Volume of effluent (mL)
Rel
ativ
e im
puls
es
(a) (b)
Fig. 8. Elution curve of separation of Th(IV)–Mo(VI): (a)
deionised water, 0.1mL/min; (b) 0.5M HCl, 0.4mL/min; 2.5 g
of cerium molybdate.
A. Nilchi et al. / Radiation Physics and Chemistry 75 (2006) 301–308306
solution/g
Tables 4–6 show that the cerium molybd
exchanger prepared at lower pH value tends
higher distribution coefficient for various cations
in comparison with others. Furthermore, t
exchanger shows a greater selectivity for Th(I
Ba(II) and low absorption for Mg(II) and Ni(I
absorption of cations follows the order T
Ba(II)4Mo(VI)4Cr(VI)4Ca(II)4Ni(II)4Mg(
Some of the factors which affect the distr
coefficient of cations are the charge, size, sw
formation of complexes, nature of the chemica
Cerium molybdate shows a great difference in the
its distribution coefficient for the ions being inve
therefore numerous separation of analytical and c
interest for example, that of Th(IV) from Ba(II), M
Cr(VI); Ba(II) from Ca(II), Ni(II), Mg(II); Ca(I
Ni(II) and Mg(II); Cr(VI) fromMo(VI), Ni(II), Ca
Mg(II) can be performed on the columns
exchanger. Figs. 8–10 show some of the separation
have actually been carried out in this work.
In order to investigate the radiation stability of
molybdate; sample 1 obtained by mixing ce
molybdate in 1:2 ratio was irradiated. The ion e
capacities of irradiated sample at 50 and 200k
found to be 2.67 and 2.27meq/g, respectively, w
much lower than the value obtained for unir
sample.
Tables 7 and 8 show the distribution coeffi
various cations, using irradiated cerium molybd
exchangers at 50 and 200 kGy. The results show
irradiated samples of the ion exchanger hav
distribution coefficient values for various cations
in comparison with unirradiated sample. It
observed that the absorption of cations ha
following sequence: Th(IV)4Ba(II)4Mo(V
(VI)4Ca(II)4Ni(II)4Mg(II), which is the s
ns,
ns,
the one observed for the ion exchanger before gamma
ceri
be
owa
lues
t can
hig
absorption of 99Mo in acidic media in comparison with
tion of
the pH
4. Conclusion
ptimum
aracter-
studies
to have
a Ce/
ts little
spectra
towards
the ion
s found
g ratio
rder to
lly, the
the ion
cations.
such as
xcellent
ractical
nuclear
References
national
, vol. 17,
irconium
ation of
himii 53,
ARTICLE IN PRESS
Table 7
Distribution coefficient values of various cations using ir
diated cerium molybdate ion exchanger at 50 kGy
Cations Kd (mL/g)
Mg(II) 0.9
Ca(II) 16.5
Ba(II) 2189.2
Ni(II) 7.3
Cr(VI) 47.2
Mo(VI) 64.7
Th(IV) 7125.3
Table 8
Distribution coefficient values of various cations using ir
diated cerium molybdate ion exchanger at 200 kGy
Cations Kd (mL
Mg(II) 0.3
Ca(II) 9.4
Ba(II) 1418.3
Ni(II) 6.3
Cr(VI) 36.5
Mo(VI) 58.4
Th(IV) 6650.2
Table 9
The absorption of 99Mo on cerium molybdate in different
media
Concentration of solution (M) pH Kd
0.001 HCl 3.0 1641.2
0.01 HCl 2.0 1335.3
0.1 HCl 1.0 1293.0
0.5 HCl 0.3 789.5
Distilled water 7.0 1267.7
0.001 NaOH 11.0 1196.6
0.01 NaOH 12.0 806.0
0.1 NaOH 13.0 539.0
0.5 NaOH 13.7 523.5
0
1
2
3
4
5
6
7
0 20 40 60 80 100Volume of effluent (mL)
Rel
ativ
e im
puls
es
(a) (b)
Fig. 10. Elution curve of separation of Ba(II)–Ca(II): (a)
deionised water, 0.1mL/min; (b) 0.5M HCl, 0.4mL/min; 2.5 g
of cerium molybdate.
A. Nilchi et al. / Radiation Physics and Chemistry 75 (2006) 301–308 307
irradiation. It is worth noting that although
molybdate exhibits little change in its sorption
viour on irradiation, it still shows good stability t
radiation damage.
Table 9 shows the distribution coefficient va99Mo on cerium molybdate in different media. I
seen from the results that cerium molybdate has a
basic ones. It can be concluded that the absorp99Mo on cerium molybdate directly depends on
of the solutions.
ra-
Cerium (IV) molybdate prepared under the o
conditions of concentration and acidity was ch
ized on the basis of chemical analysis, thermal
and infrared spectra and has been demonstrated
the formula Ce(OH)2(HMoO4)2?H2O, having
Mo ratio of 0.44. Cerium molybdate exhibi
change in its sorption behaviour and infrared
after gamma irradiation; it is relatively resistant
heat, and it is chemically stable. Furthermore,
exchange capacity of cerium (IV) molybdate wa
to directly depend on the pH of solution. Mixin
has little effect on the exchange capacity.
Although further investigations are needed in o
understand the ion exchange mechanism more fu
results obtained here give a fairly good idea of
exchange behaviour towards several inorganic
The very high selectivity toward certain cations
Th(IV), Ba(II), Cr(VI) and Mo(VI), and e
absorption of 99Mo, increases the possible p
applications of cerium (IV) molybdate in
industries.
ra-
/g)
um
ha-
rds
of
be
her
Amphlett, C.B., 1985. Proceedings of the Second Inter
Conference Peaceful Uses Atomic Energy, Geneva
p. 28.
Chekomova, L.F., Cherednichenko, N.V., 1998. Z
phosphate as an ion exchanger for the separ
samarium and neodymium. Zh. Analiticheskoj K
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Clearfield, A., Nancollas, G.H., Blessing, R.H., 1973. In:
anic
n, vo
natio
, vol.
., 19
appl
able
., 20
m ph
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s cation
applica-
heri, A.,
naturally
against
7.
ajzadeh,
proper-
sphates.
ey, New
ARTICLE IN PRESSA. Nilchi et al. / Radiation Physics and Chemistry 75 (2006) 301–308308
Marinsky, J.A., Marcus, Y. (Eds.), New Inorg
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Dekker, New York.
Gal, I.J., Gal, O.S., 1985. Proceedings of Second Inter
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p. 28.
Nilchi, A., Ghanadi Maragheh, M., Khanchi, A
Properties, ion exchange behaviour and analytical
tion of cerium phosphate cation exchangers suit
column use. Sep. Sci. Technol. 34, 1833–1843.
Nilchi, A., Ghanadi Maragheh, M., Khanchi, A
Characteristics of novel types of substituted ceriu
phates. J. Radioanal. Nucl. Chem. 245, 589–594.
Ion
l. 5.
nal
24,
99.
ica-
for
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