Adsorption and possibility of separation of heavy metal cations by strong cation exchange resin

Abstract

This work is devoted to the study of the adsorption of strontium, yttrium, and zirconium ions by the strong acid cation exchange resin Dowex HCR-s/s. The adsorption of strontium, yttrium, and zirconium ions in batch and dynamic conditions from individual solutions of the studied elements, as well as from their mixture, was studied. It was shown that the dependences of the adsorption of strontium, yttrium, and zirconium ions on agitation time fit well with the pseudo-first-order and Elovich equations. Equilibrium adsorption of all three cations can be described with Langmuir's theory. Column adsorption has shown that Dowex HCR-s/s adsorb all three investigated cations from the acidic medium. The inductively coupled plasma mass spectrometry (ICP-MS) proves that separation of these cations using Dowex HCR-s/s is possible only in highly dilute solutions at a concentration of 10 ng/ml of each element. The basis of separation will be a higher rate of adsorption of tetravalent zirconium, compared with strontium and yttrium. As the concentration of the investigated elements increases to 100 ng/ml, all three cations are adsorbed by the resin in equal amounts and can be separated only using special eluents, such as Ca-EDTA.

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Chemical Physics Impact 3 (2021) 100056
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Adsorption and possibility of separation of heavy metal cations by strong
cation exchange resin
Hanna Vasylyeva
a
,
*
, Ivan Mironyuk
b
, Mykola Strilchuk
c
, Igor Maliuk
c
, Khrystyna Savka
b
,
Oleksandr Vasyliev
d
a
Department of Theoretical Physics, Uzhgorod National University, 14 Universytets’ka Street, Uzhgorod 88000, Ukraine
b
Department of Chemistry, Vasyl Stefanyk Precarpathian National University, 57 Shevchenko Street, Ivano-Frankivsk 76018, Ukraine
c
NAS of Ukraine Institute for Nuclear Research, Laboratory of Nuclear Forensics, Kyiv, Ukraine
d
NAS of Ukraine Institute of Electron Physics, 21 Universytetska Str., Uzhgorod 88017, Ukraine
ARTICLE INFO
Keywords:
Adsorption
Separation
Ion exchange resin
Strontium
Yttrium
Zirconium
ICP-MS
ABSTRACT
This work is devoted to the study of the adsorption of strontium, yttrium, and zirconium ions by the strong acid
cation exchange resin Dowex HCR-s/s. The adsorption of strontium, yttrium, and zirconium ions in batch and
dynamic conditions from individual solutions of the studied elements, as well as from their mixture, was studied.
It was shown that the dependences of the adsorption of strontium, yttrium, and zirconium ions on agitation time
t well with the pseudo-rst-order and Elovich equations. Equilibrium adsorption of all three cations can be
described with Langmuir’s theory. Column adsorption has shown that Dowex HCR-s/s adsorb all three investi-
gated cations from the acidic medium. The inductively coupled plasma mass spectrometry (ICP-MS) proves that
separation of these cations using Dowex HCR-s/s is possible only in highly dilute solutions at a concentration of
10 ng/ml of each element. The basis of separation will be a higher rate of adsorption of tetravalent zirconium,
compared with strontium and yttrium. As the concentration of the investigated elements increases to 100 ng/ml,
all three cations are adsorbed by the resin in equal amounts and can be separated only using special eluents, such
as Ca-EDTA.
Introduction
Adsorption and separation of heavy metals or radionuclides by
adsorption can be relevant for many reasons. First, the relevance of the
study of adsorption of heavy metal ions is due to environmental reasons,
namely the preservation of the purity of aquatic ecosystems, decon-
tamination of wastewater, etc. [1]. The need to separate radionuclides
may also be related to environmental monitoring. For example, during
determining
90
Sr in low-background samples by mass spectrometry,
there may be spectroscopic interference due to its daughter
90
Y and
90
Zr
[2]. In addition, the separation of radionuclides can be used in the
method of radio chronometry (archeological dating). Thus, methods of
radiological dating are used in nuclear forensics to determine the age of
the unknown radioactive source [3]. On the other hand, during rapid
high-temperature events, like a terrorist attack with a radiological
dispersal device, radiological material will be released into the envi-
ronment. In these scenarios, the adsorption can be used both: for the
adsorption of radioactive contamination and for the separation of parent
and daughter nuclides to determine their quantity for nuclear forensic
investigations [4].
Many different adsorbents are proposed for the adsorption of heavy
metal cations and radionuclides, for example, zeolites [5], salts of
polyvalent metals [6], mesoporous TiO
2
, or TiO
2
with chemisorbed
functional groups [7–9]. An important place among the adsorbents is
occupied by ion exchange resins. As usual, ion exchange resin is
cross-linked polystyrene with different types of active groups. According
to [10] different functional groups provide different properties of ion
exchange resin. Sulfonic groups –SO
2
OH giving strongly acid cation
exchangers. Carboxylic groups –COOH giving weakly acid cation ex-
changers. Quaternary ammonium groups –NR
3+
giving strongly basic
cation exchangers. Amine groups – NR
2
, -NH
2
, -NHR giving weakly basic
cation exchangers. Ion exchange resins possess advantages and disad-
vantages, like any other adsorbents. The advantage of ion exchange
resins is their granular form, and the simplicity of dynamic adsorption,
which allows ltering large volumes of solution through a column lled
with resin [11]. The disadvantages of ion exchange resins are low
* Corresponding author.
E-mail address: h.v.vasylyeva@hotmail.com (H. Vasylyeva).
Contents lists available at ScienceDirect
Chemical Physics Impact
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Received 5 June 2021; Received in revised form 3 November 2021; Accepted 18 November 2021

Chemical Physics Impact 3 (2021) 100056
2
selectivity or instability to acids or ionizing radiation. Sometimes ion
exchange resins show high selectivity, although they tend to adsorb
together with the analyte ions a large number of interferences [12].
Some ion exchange resins, such as tetraalkyldiglycolamide (DGA) or
chromatographic resins containing tetra-n-octyldiglycolamide
(TODGA), adsorb well microquantities of the studied elements, such as
lanthanides [12] although the maximum adsorption capacity of such
adsorbents is not very high. Therefore, a combination of several ion
exchange resins [13], or a combination of ion exchange resins with other
methods of element separation, such as precipitation [14], is often used.
The author of the publication [13] used two ion exchange resins DGA
and Dowex 50WX8. It was concluded, that the use of DGA - resin, which
selectively adsorbs yttrium, greatly simplies the oxalate method for the
determination of
90
Sr in seawater. The authors of [14] use a combination
of methods of precipitation by carbonates and ion exchange resins to
determine the radionuclides
90
Sr and
137
Cs in samples of grass and
vegetables. Sr- Resin cation exchanger is widely used for adsorption of
90
Sr, as well as for determining the age of unknown strontium-yttrium
sources [15–18]. Author Kołaci´
nska et al. [15] uses a Sr-Resin cation
exchanger for monitoring the micro amount of
90
Sr in NPP cooling
water. The method detection limit (MDL) value was evaluated as 2.9 ppq
(14.5 BqL
−1
) for 1 L initial sample volume. Alicia Surrao et al. [16] used
the Sr-Resin, which efciently adsorbs strontium and barium cations,
together with their parent and daughter nuclides. Then, the parent and
daughter radionuclides can be separated using different eluents, such as
distilled water; 0.01 M HNO
3
, 1% acetic acid, or ethylene diamine tet-
raacetic acid at pH =7 and pH =9. Ion exchange resin TEVA was used in
publication [19] for plutonium age dating (production date measure-
ment) by inductively coupled plasma mass spectrometry. According to
publication [20], an inexpensive Dowex HCR-s/s cation exchanger is an
effective adsorbent for
90
Sr from various media. The high adsorption
capacity reported by the authors of the publication [20], the ability to
adsorb strontium from different media, prompted us to investigate this
ion exchange resin for adsorption and the possibility of separation of
strontium yttrium and zirconium ions.
This work aims to investigate the adsorption of strontium, yttrium,
and zirconium ions by Dowex HCR-s/s cation exchange resin, and to
identify the basic patterns of this process. Studies of the adsorption of
strontium, yttrium, and zirconium ions in column conditions, also were
conducted, and were proposed a method of their separation using
Dowex HCR-s/s. These investigations are highly valuable for conclu-
sions about the possibility of application of Dowex HCR-s/s for separa-
tion
90
Sr and
90
Zr in real
90
Sr containing compounds.
2. Experimental technique
2.1. Ion exchange resin Dowex HCR-s/s
Ion exchange resin, Dowex HCR-s/s (DOW Chemical, USA) is a
strongly acidic cation exchange resin, with sulfonic acid as an active
surface group. The appearance of the adsorbent is amber granules
(Fig. 1). Dowex HCR-s/s resin is usually used in Na
+
form, ion exchange
capacity 1.9 mmol / g (167.2 mg / g for Sr
2+
). The size of granules is
300-1200
μ
m, bulk density is 0.8 kg /L.
The active center of adsorption in this cation exchange resin is a
sulfo- group -SO
3
H, HS(=O)
2
OH [18,20,21] (Fig. 1), which has a
tetrahedral conguration. The lengths of the oxygen-sulfur bonds in the
sulfo- group are the same and equal to 0.142 nm. The angles O-S-O are
108–110◦In general, sulfonic acids are strong acids that are as soluble in
water as their salts. They can react with bases, with the formation of
salts, such as sodium benzenesulfonate C
6
H
5
SO
3
Na. If this cation
exchanger is in sodium form, sodium is an exchangeable cation. In
electrophilic substitution reactions in aromatic compounds, the sulfo-
group is electron-accepting and directs the substitution to the target
position.
Sulfonic groups –SO
2
OH giving strongly acid cation exchangers in Na
– form:
C−ResinH +NaCl =C−ResinNa +HCl
Strongly acid cation exchangers in the sodium form will exchange
the sodium ions for other cations:
nC−ResinNa +Mn+=nC−ResinM +nNa+
These reactions are equilibrium reactions and the afnity of the resin
for the cation in solution depends on the charge, the size of the ion, and
the concentration. In dilute solution, the series Me
4+
>Me
3+
>Me
2+
has
been established. However, in concentrated solutions, the effect of
valency is reversed and univalent ions are favored over multivalent ones
[10].
Manufacturers offer to use a solution of 10–12% NaCl for regenera-
tion of cation exchange resin. Dowex HCR s/s does not lose its adsorp-
tion capacity toward bivalent cations even after 10 cycles of adsorption-
desorption, during the regeneration of the resin by 8–12% NaCl.
2.2. Adsorption in batch conditions
The adsorption capacity of the ion exchange resin toward strontium,
yttrium, and zirconium cations was investigated under batch conditions
from individual solutions of salts of the corresponding element. The
weight of the resin was 0.1 g; the volume of solution was 5 ml (L: S =
50), the acidity of solution pH =7. The duration of the interaction of the
solution with the ion exchange resin was from 10 to 80-90 min, to study
the adsorption kinetics. The equilibrium concentrations of the adsorbate
were varied to study the equilibrium adsorption. The dependence of the
adsorption of strontium, yttrium, and zirconium ions by Dowex HCR-s/s
on the acidity of the solution was also investigated. The pH of the so-
lution was changed by adding an appropriate amount of nitric acid or
ammonia to the solution and the resulting pH was measured on a pH
meter "Belarus". The ratio of liquid: solid phase was 100 (L: S =100)
because the volume of the solution was 10 ml (5 ml of the test solution of
the corresponding element +5 ml of medium). The initial and equilib-
rium concentration of Sr
2+
and Y
3+
cations, as well as Ba
2+
, Zn
2+
, Ca
2+
,
and Mg
2+
was controlled by complexometric titration with indicators of
Eriochrome Black T (PubChem 135,465,089). The Zr
4+
concentration
(and Co
2+
in test investigations) is determined by the same method with
the Xylenol orange as an indicator (PubChem 73,041) in a strong acid
medium. Adsorption values were calculated by the formula (1):
qe=[(Co−Ce)V]
m(1)
Fig. 1. The active site of Dowex HCR-s/s (a). Digital images of Dowex HCR-s/s
and separate resin granules (b).
H. Vasylyeva et al.

Chemical Physics Impact 3 (2021) 100056
3
Where q
e
is the amount of up-taken adsorbate, mg/g; C
o
and C
e
are
the initial and residual concentrations of adsorbate, mg/L; V is the so-
lution volume, L; and m is the mass of adsorbent, g.
The percentage of the element, which was taken up, was calculated
by the formula (2):
%90Zr =Cinitial −Cresidual
Cinitial
×100 (2)
Where % 90 Zr−is the percentage of
90
Zr which was taken up from
the mixture with
88
Sr and
89
Y; Cinitial,and Cresidual are initial and residual
concentrations of
90
Zr, ng
Pseudo-rst and pseudo-second-order equations from Lagergren ki-
netic models were applied to the experimental results of the adsorption
kinetics of strontium, yttrium, and zirconium ions. The equations of
these models (3) and (4) are given below.
log(q0−qt) = logq0−k1t/2.303 (3)
t
qt =1k2q2
0+tqo(4)
q
o
and q
t
(mg/g) - adsorption capacity at equilibrium and time t,
respectively; k
1
(min
−1
), k
2
(g·mg
−1
min
−1
) rate coefcients of pseudo-
rst-order equation and pseudo-second-order equation.
Elovich’s model (Eq. (5)) and the intra-particle diffusion kinetic
model (Eq. (6)) were also used [22–25].
qt=1
bln(ab) + 1
bln(t)(5)
β (mg/g)- desorption constant,
α
(mg/g min) – rate coefcients of
Elovich equation.
qt=Dipd ×t1/2+k0(6)
D
ipd
(mg/g min
0,5
) – coefcient of intra-particle diffusion;
The equilibrium adsorption studies were performed under agitation
times not less than 120 min. Nonlinear approximation of the experi-
mental results was carried out by the Langmuir and Freundlich theories
using the "Solver add-in" application to Microsoft Excel ofce program,
according to [25]. The equations for both theories are given below:
qe=A∞KLCe
1+KLCe
(7)
where, А
∞
– maximal adsorption value, which corresponds of ll in the
whole adsorption centers, mg/g; К
L
– Langmuir equation’s constant, L/
mg; С
е
– adsorbate equilibrium
qe=Kf×Cn
e(8)
where q
e
(mg/g) – quantities of adsorbate which was taken up at equi-
librium; K
f
– Freundlich constant, mg/g
mg
Ln
; n- Freundlich intencity
parameter.
For a nonlinear approximation, the authors [25] recommend calcu-
lating together with R
2
the value of
χ
2
– the divergence between the
experimental values of adsorption and those calculated according to
one of the theories.
R2=1−qe,exp,−qe,calc 2
qe,exp −qe,mean2(9)
χ
2=qe,exp −qe,calc2
qe,calc
(10)
These values were calculated, using the Eqs. (9) and (10) above.
2.3. ICP-MS
The adsorption of micro quantities of elements and the possibility of
direct one-step separation of a mixture of strontium, yttrium, and zir-
conium cations using Dowex HCR-s/s ion exchange resin was performed
in batch conditions. Analysis of nanograms of strontium, yttrium, and
zirconium by their isotopes
88
Sr,
89
Y,
90
Zr,
91
Zr (High Purity Standards,
USA) was performed using the method of inductively coupled plasma
mass spectrometry (ICP-MS). The weight of the adsorbent was 100 mg,
the volume of the mixture was 10 ml. Duration of interaction 60 min.
Initial concentration of ions were at the nanogram level of each element:
Sr
2+
9.9 ng; Y
3+
10.03 ng; Zr
4+
9.9 ng. The initial concentrations of ions
were Sr
2+
101.6 ng; Y
3+
101.9 ng; Zr
4+
101.8 ng in the second series of
measurements. The weight of the adsorbent and the volume of solutions
were the same. The inductively coupled plasma mass spectrometry was
performed using mass- spectrometer “Element-2′′ (Fig. 7). It has fol-
lowed characteristics: dual-mode secondary electron multiplier (SEM);
low dark noise: <0.2 [Cps]; sample times of down to 100
μ
s; large SEM
plateau range ≈300 V; dynamic range 0.2 [Cps] - 5·10
8
[Cps]. The
mixture of investigated elements was ionized in argon plasma and, after
transition through the ion optics system, ions arrive at the mass
analyzer. The signal measured by the ICP-MS detector is in units of
counts per second [Cps]. External calibration of ICP-MS was performed
using calibration standard A, containing known concentrations of the
elements [26]. According to [26] there are such types of spectroscopic
interference in ICP-MS analysis, such as polyatomic ions and tailoring
interference. The formation of polyatomic ions was conducted by
measuring
88
Sr
16
O,
89
Y
16
O, and
90
Zr
16
O, as well as the formation of
89
Y
1
H. It must be noted, that
90
Y and
90
Zr ions are the daughter radio-
nuclides of
90
Sr, therefore they are spectroscopic interferences during
ICP-MS analysis and therefore we try to separate them. One of the aims
of this work is to decrease spectroscopic interferences, during
90
Sr
ICP-MS analysis.
2.4. Studies of column adsorption and adsorbent regeneration
Adsorption under dynamic conditions was investigated using a 5 ml
plastic column in which 2 ml (1.6 g) of Dowex HCR-s/s ion exchange
resin was added. An individual aqueous solution of the salt of the test
element or a mixture of salts of the test elements with a volume of 5 ml
was fed to this column by a peristaltic pump. A mixture of salts of
strontium, yttrium, and zirconium, contained 10 ml of 0.005 M SrCl
2
,
10 ml of 0.005 M YCl
3
; 15 ml of 0.005 M ZrOCl
2
. 5 ml of this mixture was
taken for column adsorption. The ow rate of the solution was 0.05 ml
/1 s or 1 drop per second. The process took place at pH =2. After passing
through the column, the solution was taken in vials. The content of the
testing element, for example, strontium was performed. The chemical
composition of resin before and after adsorption also was conrmed by
uorescent X-ray analysis using S2Ranger ©2010 Bruker AXS (Karls-
ruhe, Germany). The XRF analysis was provided with voltage 50 kV;
tube current 1000
μ
A; pressure 1000 mBar; lter 250 mm Cu. The
content of corresponding elements, was determined by KA1 lines with
energy of 1.041 keV (Na); 2.307 keV (S); 3.69 keV (Ca); 14.95 keV (Y);
14.166 keV (Sr); 15.6 keV (Zr). Regeneration (leaching) of the adsorbed
elements from the column was carried out with 1 M NaCl solution, as
recommended by the manufacturers of ion exchange resin.
2.5. Study of the rate of displacement of calcium ions by zirconium ions
from the Ca EDTA complex
The rate of displacement of calcium ions by zirconium ions was
investigated as follows: a mixture of ZrOCl
2
and Xylenol Orange indi-
cator (pH =2) was added to the Ca-EDTA complex with a concentration
of 0.01 M (with a small excess of calcium above the equivalent amount).
The formation of the Zr-EDTA complex was accompanied by a change in
the color of the indicator from red to yellow (Fig. 2). Since there was an
H. Vasylyeva et al.

Chemical Physics Impact 3 (2021) 100056
4
excess of calcium in the initial reaction mixture, the color transition was
due to the displacement of calcium from the Ca-EDTA complex. The
average duration of the process is approximately 1 min 09 s or 69 s. A
schematic representation of the process is illustrated in Fig. 2.
3. Results
3.1. Adsorption of strontium, yttrium, and zirconium ions in batch
conditions
It is shown that the strong acid cation exchange resin Dowex HCR-s/s
intensively adsorbs strontium as well as yttrium even in a neutral me-
dium. Results are shown in Fig. 3 (a, b). These results are in good
agreement with the results described in [20]. Adsorption of zirconium
ions is not so high in a neutral medium (Fig. 3(b)), which is due to the
high degree of hydrolysis of zirconium ions. Dependence of adsorption
values on agitation time indicates that Dowex HCR-s/s cannot "instant
adsorption", which was observed, for example, for TiO
2
and was
described in [27–29].
The application of kinetic models to experimental adsorption results
indicates that the adsorption of the studied cations t well with the
Lagergren kinetic model, based on the pseudo-rst-order equation and
the Elovich kinetic model. The Lagergren kinetic model based on the
pseudo-second-order equation gives a coefcient of linear approxima-
tion close to the unit (R
2
=0.8974) only for the application of this model
to the adsorption of strontium cations. Results can be seen in Fig. 4 (a–d)
and Table 1.
It is known, that the kinetic model based on the pseudo-rst-order
equation describes the second-order chemical reactions in which one
of the reagents is in excess. In this case, functional groups of the ion
exchange resin are in excess. The mechanisms of adsorption can be
described by the following reactions (according to the literature) [28,30,
31]:
nC−Resin…S−O−−Na++Mn+=nC−ResinM +nNa+
2nC−Resin…S−O−−Na++Sr2+=2(C−Resin…S−O)− Sr +2Na+
C−Resin…S−O−−Na++Y+(OH)2
=C−Resin…S−O−Y− (OH)2+Na+
C−Resin…S−O−−Na++ZrO2+(OH)2
=C−Resin…S−O−ZrO+−OH +Na+
It can be assumed, that in a neutral medium only strontium ion exists
as a divalent cation, based on the application of kinetic models to the
Fig. 2. Schematic representation of the process of replacement of Ca
2+
ions by Zr
4+
ions in the complex with Ca-[EDTA]. The color of the mixture changes from red
to yellow during the transition of replacement reaction from stage 1- 2 to stage 3.
Fig. 3. Dependence of strontium ion adsorption by Dowex HCR-S/S from agitation time (a), and (b) dependence of yttrium and zirconium ions adsorption by Dowex
HCR-S/S from agitation time.
H. Vasylyeva et al.

Chemical Physics Impact 3 (2021) 100056
5
experimental results of adsorption of the studied ions with Dowex HCR
s/s resin. Yttrium ions are partially hydrolyzed cations, and zirconium
ions are strongly hydrolyzed. Literature sources [32,33] also conrm
this assumption.
3.2. Equilibrium adsorption of strontium, yttrium, and zirconium ions
Results of studies of the equilibrium adsorption of strontium,
yttrium, and zirconium ions are shown in Fig. 5 (a–d) and Table 2.
Analysis of the experimental data indicates that the process of
adsorption of the studied ions by Dowex HCR s/s t well with Lang-
muir’s theory. The values of maximal adsorption for strontium cations A
max
=166.98 mg / g calculated by Langmuir’s theory agree very well
with the value of the maximal ion exchange capacity of the resin (1.9
mmol / g or 167.2 mg / g for Sr
2+
) reported by the manufacturer. High
correlation coefcients and low values of
χ
2
under applying Langmuir’s
theory, conrm, that the studied cations are adsorbed at adsorption
centers and the adsorption process is limited to the formation of a
monolayer.
The dependence of strontium, yttrium, and zirconium ions adsorp-
tion by Dowex HCR-s/s on pH is shown in Fig. 6. Adsorption values of all
three cations are higher in an acidic environment. Starting from pH=4
(from pH =6 for zirconium ions), the values of adsorption of these
cations by Dowex HCR - s/s ion exchange resin remain constant to the
pH equal 11 and are practically independent of solution acidity.
3.3. Adsorption of Sr
2+
, Y
3+
, and Zr
4+
from their mixture. analysis by
ICP-MS
The digital image of mass spectrometer “Element-2′′ loaded in NF
Laboratory, Kyiv Institute of nuclear research (KINR) is shown in Fig. 7.
Results of analysis of Sr
2+
, Y
3+
, Zr
4+
adsorption from their mixture by
Dowex HCR-S/S using ICP-MS are shown in Table 3 and Fig. 8–10. The
formation of polyatomic ions was conducted by measuring
88
Sr
16
O,
89
Y
16
O, and
90
Zr
16
O, as well as the formation of
89
Y
1
H. The amounts of
polyatomic ions were quite low:
88
Sr
16
O/
88
Sr =0.03%;
89
Y
16
O/
89
Y=0.9% and
90
Zr
16
O/
90
Zr =1.6%;
89
Y
1
H/
89
Y =0.001%. The
Fig. 4. Dependence of Sr
2+
, Y
3+
and Zr
4+
ions adsorption by Dowex HCR-s/s from time interaction. Linear approximation by kinetic models based on (a) pseudo-
rst-order equation and (b) pseudo-second-order equation; (c) Intra-particle diffusion kinetic model, and Elovich kinetic model (d).
Table 1
Linear analytical equations of kinetic models and R
2
values for Sr
2+
, Y
3+
and
Zr
4+
adsorption by Dowex HCR s/s.
Kinetic model cation equation R
2
Elovih Sr
2+
q
t
=7.015lnt-7.45 0.9313
Y
3+
q
t
=32.08lnt-44.14 0.9632
Zr
4+
q
t
=6.384lnt-9.58 0.9229
Diffusion Sr
2+
q
t
=3.5t
0,5
– 3.88 0.9802
Y
3+
q
t
=16.09 t
0,5
–25.98 0.974
Zr
4+
q
t
=3.07 t
0,5
–5.786 0.8994
Pseudo-rst-order Sr
2+
Log(q
o
-q
t
) = − 0.024t +1.41 0.8929
Y
3+
Log(q
o
-q
t
) =− 0.0215t +2.02 0.9807
Zr
4+
Log(q
o
-q
t
) =− 0.06t +1.59 0.9489
Pseudo-second-order Sr
2+
t/q
t
=0.025t +1.5 0.8974
Y
3+
– 0.1689
Zr
4+
– 0.3622
H. Vasylyeva et al.

Chemical Physics Impact 3 (2021) 100056
6
corresponding gures are given in research data related to this article, as
well as Dowex HCR s/s surface contaminates.
The separation of strontium, yttrium, and zirconium under batch
conditions can be based on different adsorption rates of the studied
Fig. 5. (a) Experimental isotherms of strontium, yttrium, and zirconium ions adsorption by Dowex HCR-s/s (pH=7; L: S =50). (b) Nonlinear approximation of
isotherm of Sr
2+
ions by Langmuir and Freundlich theories. Nonlinear approximation of experimental isotherms of yttrium (c) and zirconium (d) ions adsorption by
Dowex HCR-s/s using Langmuir and Freundlich theories.
Table 2
Parameters of the equations of Langmuir and Freundlich’s theories applying to
the experimental results of strontium, yttrium, and zirconium ions adsorption by
Dowex HCR-s/s.
Adsorption
process
theory Parameters of
the equation of
the
corresponding
theory
calculated
q
max
, mg/g
R
2
χ
2
Sr
2+
q
e exp
=
135.2
mg/g
Langmuir K
L
=0.00095
Amax =166.98
143.17 0.9343 14.3
Freundlich K
f
=4.419 n =
0.4
148.87 0.8471 42.13
Y
3+
q
e exp
=
140 mg/g
Langmuir K
L
=0.00134
Amax =163
145.89 0.9795 9.52
Freundlich K
f
=5.06 n =
0.39
156 0.8702 45.87
Zr
4+
q
e exp
=28.5
mg/g
Langmuir K
L
=0.0014
Amax =30.88
27.758 0.9912 0.8642
Freundlich K
f
=1.047 n =
0.383
29.86 0.9641 2.267
Fig. 6. pH dependence of adsorption of strontium, yttrium and zirconium ions
by Dowex HCR s/s.
H. Vasylyeva et al.

Chemical Physics Impact 3 (2021) 100056
7
cations by Dowex HCR-s/s resin. Dependence of the adsorption of cat-
ions Sr
2+
, Y
3+
, and Zr
4+
by Dowex HCR-s/s from their ionic radius for
initial concentrations of the elements of 10 ng/ml is shown in Fig. 9. The
degree of separation of zirconium ions from strontium and yttrium is
82% percent and is quite high in the described conditions. This gure
fully conrms the direction of sorption of cations by strongly acid cation
exchange resins [10], in which cations with a higher ionic charge are
better adsorbed from dilute solutions. The dependence of the rate of
adsorption by the ion exchange resin of cations on their charge is almost
linear with a coefcient of linear approximation close to the unit (R
2
=
0.9788). The disadvantage of this method of separation is that micro-
quantities of strontium are also adsorbed. On the other hand, the Dowex
HCR s/s adsorbs yttrium micro -amounts more strongly than strontium
micro -amounts. Therefore, a large amount of yttrium can affect the
selectivity of the Dowex HCR s/s toward zirconium cations.
In more concentrated solutions, the effect of valence is reversed and
univalent ions are favored over multivalent ones [10] i.e. cations with a
lower ionic charge will be adsorbed more intensively [10]. At the initial
concentration of Sr
2+
, Y
3+
and Zr
4+
at 100 ng/ml Dowex HCR s/s loses
its selectivity toward zirconium cations and adsorb all three cations
simultaneously. Results are shown in Table 4 and Fig. 10.
4. Column adsorption
4.1. Separation of strontium, yttrium, and zirconium ions using column
adsorption (discussion)
As the concentration of the investigated elements increases to 100
ng/ml, all three cations are adsorbed by the resin in equal amounts in
batch conditions and can be separated only in column conditions using
special eluents, such as Ca-EDTA. After passing the solution of the test
elements through the column ll in the resin, strontium and yttrium
Fig. 7. ICP-MS spectrometer “Element-2” loaded in Laboratory of Nuclear Forensics, Kyiv Institute of nuclear research (KINR).
Table 3
Results of analysis of Sr
2+
, Y
3+
, Zr
4+
adsorption from their mixture by Dowex
HCR-s/s using ICP-MS.
isotope Resin
contaminations
Intensity
[Cps] Of
Initial
mixture
Intensity [Cps]
of Mixture after
separation
Percentage of
up-taken,%
88
Sr 55 373 2 101 905 ±
11 741
1 978 584 ±25
092
5.867
89
Y 2 729 2 431 139 ±
39 245
1 126 929 ±6
515
53.646
90
Zr 43 017 1 057 427 ±
14 588
190 163 ±1 683 82.016
91
Zr 9 480 227 697 ±1
768
40 985.46 ±
1089
82.00
Fig. 8. ICP-MS spectrum of
88
Sr,
89
Y,
90
Zr, and partially
91
Zr from their mixture, before and after adsorption (initial concentration of cations is 10 ng/ml).
H. Vasylyeva et al.

Chemical Physics Impact 3 (2021) 100056
8
were completely retained on the column, at the same time, part of the
zirconium (0.2 mg/ml) passed through the column. Adsorbent manu-
facturers offer to regenerate the adsorbent 8–12% NaCl. Experimental
studies have shown that the NaCl solution of this concentration very well
washes away all three adsorbed elements simultaneously. Strontium
(yttrium) and zirconium were washed out of the column completely and
simultaneously with the rst 10 ml of 1 M NaCl solution during the
regeneration (the result is shown in Fig. 11).
Separation of these elements in column conditions can be carried out
as follows: rst stage - adsorption of strontium, yttrium, and zirconium
at pH =1–2 on the column lled in Dowex HCR S/S, and then 2nd stage -
separation investigated elements using different eluents. The results
described in [23] indicate that HNO
3
is a better eluent for strontium and
zirconium than HCl. However, ion exchange resin manufacturers do not
recommend regeneration of the adsorbent with HNO
3
acid, as this de-
stroys the adsorbent. The experience described in publications [16] can
be used to separate strontium, yttrium, and zirconium under dynamic
conditions. Authors A. Surrao et al. [16] adsorbed strontium and barium
cations on the strong acid cation exchanger Sr-Resin. To separate
Ba
2+
from their parent nuclides
137
Cs, the authors use distilled water as
eluent (95.27 ±0.38%), as well as 0.01 M HNO
3
(95.05±0.75%), 1%
Acetic Acid (96.62±0.58%), 0.05 M Na
2
[EDTA H
2
] (98.64 ±0.31%),
0.05 M Na
3
[EDTA H] (pH=9, NaOH) (100.9 ±1.21%). To separate
90
Sr
from
90
Y the authors propose distilled water (95.29 ±0.41%), 0.01 M
HNO
3
(96.4 ±0.27%), 1% Acetic Acid (95.9 ±0.79%), 0.05 M Na
3
[EDTA H] (pH=9, NaOH) (100.14 ±0.53%), 0.05 M K
3
[EDTA H]
(pH=9, NaOH) (100.19±0.97%). As can be seen from their results, the
most effective eluents are ethylene-diamine-tetraacetic, or salts. How-
ever, we can use instead of EDTA solution, an aqueous solution of
Ca-EDTA complex. This method of cation desorption is partially
described in [30]. The separation, in this case, is determined by the
stability constant of the Cat
n+
-EDTA complex. The constant of stability
of the complexes of the studied elements and calcium with
Fig. 9. Dependence of the adsorption of cations Sr
2+
, Y
3+
, and Zr
4+
by Dowex
HCR-s/s from their ionic charge. Initial concentrations of the elements are 10
ng/ml.
Table 4
Adsorption of Sr
2+
, Y
3+,
and Zr
4+
from their mixture with the initial concen-
tration of each element 100 ng/ml.
isotope Initial mixture Mixture after
separation
Percentage of up-
taken,%
88
Sr 29 247 466 ±49
497
4 196 408 ±144 802 85.65
89
Y 33 700,147 ±263
321
2 753 293 ±67 245 91.83
90
Zr 15 202,047 ±44
528
1 440 507 ±34 910 90.52
Fig. 10. Adsorption of
88
Sr,
89
Y, and
90
Zr by Dowex HCR s/s from their
mixture, initial concentration of each element is 100 ng/ml.
Fig. 11. Elution prole of strontium and zirconium ions from the Dowex HCR
s/s resin (eluent is 1 M NaCl solution).
Table 5
The Constant stability of the complexes of the studied elements and calcium with
ethylene diamine tetraacetate, and adsorption values of heavy metal cations by
Dowex HCR s/s.
cation Lg K (Cat
n+
-EDTA) Adsorption values by Dowex HCR s/s for initial
concentration of cations equal 0.025M
q, mg/g q, mmol/g
Li
+
2.8
Na
+
1.7
Ag
+
7.3
Ba
2+
7.8 126.1 0.9184
Sr
2+
8.63 73.4 0.8341
Mg
2+
8.7 22.7 0.9458
Ca
2+
10.7 30.0 0.75
Co
2+
16.3 41.5 0.7034
Zn
2+
16.5 49.25 0.7576
Y
3+
18.1 90.73 1.019
Cr
3+
23 48.55 0.9519
Zr
4+
29 15.0 0.1667
H. Vasylyeva et al.

Chemical Physics Impact 3 (2021) 100056
9
ethylenediaminetetraacetic acid increases from Sr-EDTA, Ca-EDTA,
Y-EDTA, to Zr-EDTA (Table 5). Therefore, during washing the column
with the resin, on which these elements are adsorbed, strontium will not
displace calcium in the Ca-EDTA complex and will remain in the
adsorbed state on the resin column. While yttrium and zirconium form
more stable complexes with EDTA, they replace calcium in the Ca-EDTA
complex and are leached from the ion exchange resin column (according
to the scheme, which is shown in Fig. 12). The resin adsorbs free cal-
cium, after this. A study of the rate of displacement of calcium ions by
zirconium ions from the Ca-EDTA complex, described in Section 2.5 of
this work indicates that elution should be carried out rather slowly
because zirconium does not immediately replace calcium ions in the
complex.
Test studies of the other heavy metal cations adsorption show, that
Dowex HCR s/s effectively adsorbs Sr
2+
and Y
3+
cations, as well as Ba
2+
,
Zn
2+
, Cr
3+
, Ca
2+
, and Mg
2+
, and less intensive zinc, cobalt, and zirco-
nium. It can be supposed, that ion exchange resin can remove calcium,
magnesium, barium, cesium, which may be present in the source,
together with strontium. However, cations of micro impurities of heavy
metals such as Zn
2+
(16.5), Co
2+
(16.3), Fe
2+
(14.3), Cr
3+
(23) will be
washed away together with yttrium and zirconium [34–36] during the
separation process.
The subsequent separation of yttrium and zirconium cations depends
on the purpose of the research. The main task is the separation of
90
Sr і
90
Zr (
90
Sr’s granddaughter) if the aim of the investigations is the age
dating of
90
Sr-
90
Y radioactive source [18]. It should be noted, that in the
isobar species of
90
Sr −
90
Y-
90
Zr – the rst two elements are β
−
-radio-
active, and
90
Zr is the stable isotope [36–38]. According to formula (11),
based on Bateman equations, the age of the radioactive source can be
calculated as follows:
T=1
λln1+NGD
NP(11)
where N
GD
– the amount of
90
Zr nuclei, N
P
- the amount of
90
Sr nuclei; λ-
90
Sr decay constant; T – time of fabrication (i.e. last chemical separation
or “age” of the
90
Sr-contain compound) [19]. Since the half-life of
90
Y is
much shorter (64 h) than the half-life of
90
Sr (28.8 years) its amount
after several
90
Y half-lives throughout the whole life of
90
Sr remains
constant [36–38]. After several half-lives, an equilibrium establishes
and the amount of
90
Y can be calculated:
NY=λ1
λ2
(NSr),or NY
T1/22
=NSr
T1/21
(12)
T1
22
,Half −lifeof90 Y;T1
21
−Half −lifeof90 Sr.
Therefore, there is no strong necessity to separate the
90
Y from
90
Zr
for the radiochronometry
90
Sr-contain device, since it can be calculated.
If the scientic problem of separation yttrium and zirconium ions relates
to other elds of scientic research, the ion exchange resin DGA can be
used, which very intensively adsorb elements of the YRE group against
the background of various impurities [34].
5. Conclusions
The adsorption of strontium yttrium and zirconium ions by Dowex
HCR-s/s cation exchange resin was investigated; the basic patterns of
this process were established. The application of kinetic models to
experimental adsorption results indicates that the adsorption of the
studied cations ts well with the Lagergren kinetic model, based on the
pseudo-rst-order equation, and the Elovich kinetic model. The
Lagergren kinetic model, based on the pseudo-second-order equation
gives the coefcient of linear approximation close to unit (R
2
=0.8974)
only for the adsorption of strontium cations. Studied cations are xed at
adsorption sites of the Dowex HCR s/s, and their adsorption is limited to
the formation of a monolayer, according to high correlation coefcients
and low values of
χ
2
under application of Langmuir’s adsorption theory
to the experimental process. The values of maximum adsorption for
Fig. 12. Scheme of leaching of zirconium cations by the Ca [EDTA] complex. Yttrium cations can be washed out according to the same scheme.
H. Vasylyeva et al.

Chemical Physics Impact 3 (2021) 100056
10
strontium cations A
max
=166.98 mg / g calculated by Langmuir’s theory
agree very well with the value of the maximal ion exchange capacity of
1.9 mmol / g (167.2 mg / g for Sr
2+
) reported by the manufacturer.
Adsorption values of all three cations are higher in an acidic environ-
ment. Study the adsorption of strontium, yttrium, and zirconium ions in
column conditions show that strontium and yttrium were completely
retained on the column, at the same time, a little amount of the zirco-
nium cations (0.2 mg/ml) passed through the column. After regenera-
tion of the ion exchange resin by 1 M NaCl, strontium (yttrium) and
zirconium were washed out of the column completely, simultaneously
with the rst 10 ml of 1 M NaCl solution. Test studies of the other heavy
metal cations adsorption show, that Dowex HCR s/s effectively adsorbs
Sr
2+
and Y
3+
cations, as well as Ba
2+
, Zn
2+
, Ca
2+
, and Mg
2+
, and less
intensive zinc, cobalt, and zirconium. The separation of these isotopes
using ion exchange resin Dowex HCR-s/s can be provided in batch
conditions at low concentrations of the elements (10 ng/ml) or in col-
umn conditions using Ca-EDTA as eluent. Dowex HCR-S/S is applicable
for separation
90
Sr and
90
Zr in real
90
Sr containing compounds.
CRediT authorship contribution statement
Hanna Vasylyeva: Methodology, Investigation, Writing – original
draft. Ivan Mironyuk: Methodology, Formal analysis, Writing – original
draft. Mykola Strilchuk: Methodology, Formal analysis. Igor Maliuk:
Methodology, Formal analysis. Khrystyna Savka: . Oleksandr Vasy-
liev: .
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
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