MnO2 NPs -AgX Zeolite Composite as Adsorbent for Removal of [602483]

Materials Chemistry and Physics
Manuscript Draft

Manuscript Number: MATCHEMPHYS -D-15-02691

Title: MnO2 NPs -AgX Zeolite Composite as Adsorbent for Removal of
Strontium -90 (90Sr ) from Water samples: Kinetics and Thermodynamic
Reactions Study

Article Type: Full Length Article

Keywords: 90Sr, Drinking water, MnO2NPs -AgX zeolite composite, Removal,
Kinetic, Thermodynamic.

Abstract: In this scientific research, MnO2NPs -AgX zeolite composite were
synthesized and identified via Scanning electron microscopy (SEM), Atomic
adsorption spectrometry (AAS), X -ray diffraction (XRD) and Fourier
transform infrared (FTIR) techniques. The removal of radioactive
strontium -90 (90Sr) from d rinking water sample by the synthesized
MnO2NPs-AgX zeolite composite were investigated in tap water of Ramsar
city. A single radiometric technique, namely Ultra Low -Level Liquid
Scintillation Counting (LSC), was applied for monitoring of the reaction
progress. The obtained results revealed that 90Sr was completely removed
by MnO2NPs -AgX zeolite composite after 8 h. The reaction kinetic
information was studied by utilizing pseudo first and second order
kinetic Elovich and Intra particle diffusion kinetic mo dels. The
adsorption kinetics of 90Sr was matched appropriately with the pseudo
second order kinetic model. Further the evaluation of the thermodynamic
parameters such as ∆G0, ∆H0 and ∆S0, denoted that adsorption process of
90Sr was spontaneous and illustr ates a physical adsorption properties and
exothermic nature of the adsorption. It was emphasized that MnO2NPs -AgX
zeolite as a novel composite has a high capacity and potential for the
removal of radioactive 90Sr from drinking water.

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65 1
MnO 2 NPs-AgX Zeolite Composite as Adsorbent for Removal of Strontium -90 (90Sr)
from Water samples : Kinetics and Thermodynamic Reactions Study
Meysam Sadeghi1, Sina Yekta2, Hamed Ghaedi3*, Esmaeil Babanezhad2
1Young Researchers and Elite Club, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran
2Department of Chemistry, Faculty of Basic Sciences, Islamic Azad University, Qaemshahr
Branch, Qaemshahr, Iran
3 Faculty of Engineering , Islamic Azad University, Bushehr Branch, Bushehr, Iran

_______________________________________________
*Corresponding author. Tel./Fax: +98 9173234855
E.mail address: [anonimizat] (H . Ghaedi) *Manuscript
Click here to view linked References

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Abstract
In this scientific research, MnO 2NPs-AgX zeolite composite were synthesized and identified
via Scanning electron microscopy (SEM), Atomic adsorption spectrometry (AAS), X -ray
diffraction (XRD) and Fourier transform infrared (FTIR) techniques. The removal of
radioactive strontium -90 (90Sr) from drinking w ater sample by the synthesized MnO 2NPs-
AgX zeolite composite were investigated in tap water of Ramsar city. A single radiometric
technique, namely Ultra Low -Level Liquid Scintillation Counting (LSC), was applied for
monitoring of the reaction progress. The obtained results revealed that 90Sr was completely
removed by MnO 2NPs-AgX zeolite composite after 8 h. The reaction kinetic information was
studied by utilizing pseudo first and second order kinetic Elovich and Intra particle diffusion
kinetic models. The adsorption kinetics of 90Sr was matched appropriately with the pseudo
second order kinetic model. Further the evaluation of the thermodynamic parameters such as
G0, H0 and S0, denoted that adsorption process of 90Sr was spontaneous and illus trates a
physical adsorption properties and exothermic nature of the adsorption . It was emphasized
that MnO 2NPs-AgX zeolite as a novel composite has a high capacity and potential for the
removal of radioactive 90Sr from drinking water .

Keywords : 90Sr, Drinking water, MnO 2NPs-AgX zeolite composite, Removal, Kinetic,
Thermodynamic.

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1. Introduction
For decades it had been a great challenge to protect the environment against hazardous
radioactive materials likely to release from nuclear activities or incidents. After the explosion
took place in Chernobyl in Russia and Fukushima in Japan power plants accident, the
enormous amounts of radioactive hazardous elements found their way to the natural
environment [1]. Of the most anthropogenic radionuclides, is Strontium -90 (90Sr) due its
existence in worldwide fall out. Strontium isotopes can be classified into two various and
separated groups containing stable isotopes (84Sr, 86Sr, 87Sr, and 88Sr) and radioactive isotopes
including (e.g., 82Sr, 83Sr, 85Sr, 89Sr, 90Sr). Radioactive strontium can be replaced instead of
calcium in biosphere and it can also transfer to human body through food chain in which it
has long retention time. 90Sr is taken up via gastrointestinal system and aggregate in the body
turning to a part of the bone marrow tissue and hurting blood – producin g cells [2]. Also it can
be cause of leukemia or skeletal cancer. This is because of its chemical propinquity and
alkaline earth metallic characteristics.
The 90Sr is a substantial component of superabundant nuclear wastes and also a high
yield fission product of 235U [3] it undergoes β− decay, emitting electrons with energy of
0.546 MeV and a half -life of 28.8 years. yttrium -90 isotope (90Y) is its decay product which
is β− emitter with half -life of 64 hours and decay energy of 2.28 MeV distributed to an
electron, an antineutrino and zirconium -90 (90Zr) which is stable [4]. In Scheme 1, the 90Sr
decay equation has been shown. Various radiometric methods such as ga s flow GM ( Geiger –
Müller) counting and liquid scintillation counting are usually used for direct measurement of
90Sr and its daughter 90Y subsequently [5]. Ultra low level liquid scintillation counting (LSC)
can be successfully utilized for counting alpha and beta ac tivity derived from alpha, beta
emitters to monitor the natural radioactivity, contamination related to nuclear fallouts,
contaminants branched from nuclear power stations or fuel reprocessing plants [5]. Reduced

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equipment requirements and relative readine ss of radiochemical procedures make LSC an
attractive technique which can be applied also by laboratories lacking specific radiochemistry
facilities and experience. The determination of radiostrontium by this technique is based on
the high counting efficie ncy for high -energy β-particles in aqueous solutions. 90Sr might enter
the water from naturally occurring deposits or human activities. So, the removal of 90Sr from
drinking water is a serious issue.
There are some available studies on application of inorg anic materials and their ion
exchange properties. Zeolites and metal oxide nanoparticles are of the most important
inorganic materials for the removal of 90Sr from nuclear waste [6-10]. These compounds are
extremely valuable for their vast application as catalysts and adsorbents. Zeolites constitute
an important class of aluminosilicate crystalline micro porous materials comprising natural
and synthetic species. Zeolites represent speci al physicochemical properties because of their
singular structure and have been widely used as molecular sieves, ion -exchangers,
absorbents, catalysts, and so on [11-14]. The combination of zeolites and metal oxide
nanoparticles renders solid catalysts in which the high surface area of nanoparticles and the
absorbent capacity provided by zeolites cooperate to increase the efficiency of the catalytic
process [15]. The methods for modifying zeolites are usually by impregnation [16] and ion –
exchange [17]. In t his research, we have utilized the combination of AgX zeolite as host and
MnO 2 nanoparticles as guest materials to synthesize an adsorbent catalyst in which the high
surface area of nanoparticles and the absorbent capacity provided by the zeolite cooperati on
to increase the efficiency of the removal process of 90Sr from drinking water. However to the
best of our knowledge, such study there has not been reported in any previous work.

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2. Experimental
2.1. Materials and Reagents
Sodium aluminate, sodium silicate, aluminum sulfate, tetramethyl ammonium chloride, silver
nitrate (AgNO 3), manganese nitrate hexahydrate (Mn(NO 3)2.6H 2O), potassium permanganate
(KMnO 4) were purchased from Merck (Merck, Darmstadt, Germany). The high -capaci ty
cocktail OptiPhase HiSafe -3 (Wallac Oy, Turku, Finland) and double -distilled water were
used throughout the work.

2.2. Instrumentation
The morphology and size of the prepared adsorbent catalyst were performed via SEM
micrographs using a scanning elec tron microscope (SEM, LEO -1530VP). Weight
percentages of the elements (silver and manganese) were measured by atomic adsorption
spectrometry (AAS, PerkinElmer, USA) coupled to a HGA 400 programmer hybrid system
and equipped with a hollow cathode lamp at re spective wavelength using an acetylene -air
flame. The powder X -ray diffraction (XRD) patterns were recorded at room temperature
using a Philips X’pert Pro diffractometer equipped with CuKα radiation and a wavelength of
1.54056 Å (30 mA and 40 kV). Data wer e collected over the range 4 –90° in 2θ with a
scanning speed of 2° min-1. The IR spectra were scanned on a PerkinElmer model 2000 FT –
IR spectrometer (USA) in the wavelength range of 400 to 4000 cm-1 using KBr pellets. An
ultra-low level Quantulus 1220 liqu id scintillation counter has been used for all
measurements. A shaker Heidolph Vibramax 100 (Heidolph Co., Schwabach, Germany) was
utilized for mixing of cocktail and sample. Samples and cocktail were mixed in 20 mL
polyethylene vials, Polyvial (Zinsser An alytik Co., Frankfurt, Germany).

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2.3. Synthesis of NaX Zeolite by Hydrothermal Method
20 g of sodium aluminate was dissolved in 30 ml deionized water and slowly heated up to 80
°C under vigorous stirring and maintained at this temperature. 55 g of sodium silicate was
slowly added to the sodium aluminate solution and together stirred for 2 h . After that, the
heating stirrer was turned off and the mixture aged for 48 h at 25 °C (solution A). 78 g of
sodium silicate was dissolved in 120 ml deionized water, 16 g of aluminum sulfate added and
the mixture stirred continuously for 2 h (solution B). 10 g of sodium aluminate was diluted
with 10 ml deionized water, then added drop -wise and mixed with a sodium silicate and
aluminate precursors under continuous stirring until the complete grain growth was achieved
(seed). Subsequently, 2.5 g of nucleatio n seed was added to solutions A and B. lastly, 8 g of
tetramethyl ammonium chloride was dissolved in 10 ml deionized water and added drop wise
to the above components. The final mixture was placed in a Teflon -lined stainless steel
autoclave, slowly heated to 100 °C and kept for 72 h until a gray gel was derived. Then, the
obtained gel was filtered in a Buckner funnel and washed with double distilled water until pH
of the filtrate was equal to 9. Finally, the residue was dried at 110 °C for more than a day
[18].

2.4. Preparation of AgX Zeolite by Ion Exchange Method
For the preparation of AgX zeolite, 2.2 g of the synthesized NaX zeolite calcined at 400 °C
for 3 h in a furnace. The calcined NaX zeolite was then added to a 50 mL of 0.15 M silver
nitrate (AgNO 3) solution and the mixture was stirred magnetically at 60 °C for 5 h to allow
Ag+ ions replace Na+ ions and perform ion exchange process. The resulting zeolite was
filtered and washed with deionized water to remove the excess salt ions, then dried at 110 °C
for 16 h. Finally, the clean and dry zeolite was calcined for 4 h at 400 °C [18].

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2.5. Prepara tion of MnO 2NPs-AgX Zeolite Composite by Impregnation Method
The incorporation of MnO 2NPs loaded into AgX zeolite was accomplished by the
impregnation method. 1.5 g of AgX zeolite was poured into a 20 mL of 1 M Mn(NO 3)2
aqueous solution and stirred for 5 h. Under continuous stirring, 50 mL of a 0.2 M KMnO 4
solution was added suddenly. KMnO 4 has been known among the strong oxidizing agents
[19], so that, the color of the solution immediately turned to dark brown, indicati ng the
formation and precipitation of MnO 2NPs through oxidation with KMnO 4. The obtained
sample was then dried at 100 °C for more than a day. Lastly, the calcination of the product
was performed for 4 h at 550 °C. The ionic equation of the reaction is as f ollows (1) [18]:

3Mn2+ + 2MnO 4- + 2 H 2O → 5MnO 2 + 4H+ (1)

2.6. Removal of 90Sr from tap water by MnO 2NPs-AgX Zeolite Composite
For 90Sr radionuclide removal and adsorption study, 0.5 -3 g of the MnO 2NPs-AgX zeolite
composite was added to 500 ml of the drinking water and the mixture was stirred for 2, 4, 6, 8
and 12 h via varying the adsorption temperature at 298 K, 308 K and 318 K, respectively .
After filtration of the mixture, 5 mL of supernatant solutions were analyzed by liquid
scintill ation spectrometry (LSC) (1220 Wallace Quantulus). In LSC, an aliquot of the sample
is put into a vial and mixed homogeneously with 15 mL of scintillation cocktail. A shaker
was utilized for mixing of cocktail and sample. Samples and cocktail were mixed in 20 mL
polyethylene vials, Poly vial. The outside of the vials was cleaned with acetone. All of the
polyethylene vials were stored in a cool, dark shield (about 7 °C) for 2 h to eliminate the
scintillation cocktail fluorescence. All samples were counted by LSC for 5 h. The initial

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source activity was 736.5 Bq/ml that 144.6 µL amount of it (equal to 106.5 Bq) as optimized
activity was added to solution samples.
3. Results and discussion
3.1. SEM Analysis
The characteristics like crystalline size and morphology of the as -synthesized NaX, AgX
zeolites and 18.4 wt % MnO 2NPs-AgX zeolite composite were analyzed through
magnification by SEM images as depicted in Fig. 1. The SEM images explain homogenous
morphology of the structures, approximately cubic form of NaX and AgX zeolites and quasi –
spherical MnO 2 nanoparticles dispersed and deposited on the external surface of AgX zeolite
and also specify that these morphologies and the crystallinity of the structures are maintained
with silver ion exchange and MnO 2NPs loading processes which are indicated by SEM
images in Figs. 1c. The presence of some larger particles in the micrographs is attributed to
the aggregation or overlapping of some smaller particles during composite preparation. The
SEM images also represent that MnO 2 nanoparticles have been loaded onto AgX zeolite. The
average crystalline size of MnO 2NPs was illustrated to have nanometric dimensions (less than
100 nm).

3.2. AAS Analysis
The amounts of silver and manganese elements in the adsorbent catalyst were determined
through elemental analysi s by atomic absorption spectrometry (AAS). The results revealed
that the amounts of silver and manganese were 10.3 wt % and 18.4 wt %, respectively.

3.3. X -ray Diffraction (XRD) Patterns
XRD patterns of the NaX and AgX zeolites and the 18.4 wt % MnO 2NPs-AgX zeolite
composite are displayed in Fig. 2, respectively. As can be seen from the patterns, the sharp

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peaks referring to NaX zeolite occurred at scattering angles (2θ) of 5.940° -48.685°
corresponded to miller indexes of 111 -955 respectively (Fig. 2a ) that have been crystallized
in the cubic system (Fd –3m with lattice size of 24.9600 Å and are in good agreement with
those of the NaX zeolite with molecular formula of C 5H4O2·Na 2O·Al 2O3·3.3SiO 2·7H 2O,
Reference code: (00 -041-0118).

Fig. 1. SEM images of the synthesized samples: (a) NaX, (b) AgX and (c) MnO 2NPs-
AgX

NaX zeolite structure was retained even after silver cation exchange in AgX zeolite.
Meanwhile, synthesized MnO 2NPs (as guest material) loaded as a 18.4 wt % of unit onto
AgX zeolite as the host material, possesses a series of new peaks which were obtained at 2 θ
of 35.752° -64.565° corresponded to miller indexes of 131 -421, respectively (Fig. 2c),

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respectively. No ch aracteristic peaks related to the presence of impurities were observed in
the patterns during manganese (IV) oxide loading. These peaks which are illustrated as red
points in Figure 2c reveal that MnO 2NPs have been dispersed and deposited onto AgX zeolite
and also indicate a host -guest interaction between AgX framework and MnO 2NPs. A definite
line broadening of the scattering pattern in Fig. 2c is a demonstration upon which the
synthesized MnO 2 particles are in nanoscale range. However, a small loss of crys tallinity is
observed in Figs. 2b and 2c associated with the lower intensity of the peaks at 2 θ of 9.890°,
11.630°, 18.318°, and 19.975°. This may be because of the dealumination process of AgX
zeolite and MnO 2NPs-AgX zeolite composite and associated with the location of substituted
silver and impregnated manganese cations. The Mn4+ ions within the zeolite framework can
interact with the aluminate sites more strongly strongly than that of Na+ or Ag+ ions. Totally,
it can be concluded that with silver ion ex change in NaX zeolite and subsequent loading of
MnO 2NPs onto AgX zeolite, the structure of the zeolites did not changed. On the other hand,
the capacity of the X -type zeolite to keep the guest species is limited. Consequently, the
adsorption of the host ca tions (Si, Al and Na) will stop if the capacity is filled. In contrast, the
amount of the host species in the AgX zeolite increases with increasing the manganese
dioxide content. The introduced MnO 2NPs were dispersed and deposited on the external
surface o f AgX zeolite, however, due to the relative aggregation during processing of the
composite, some particles are too large to perch inside the structure. Hence, high MnO 2NPs
loading will cause structural damage to the zeolite. The size of the prepared MnO 2NPs
deposited onto AgX zeolite was also investigated via XRD measurement and line broadening
of the peak at 2 θ=4°-90° using Debye -Scherrer equation (2 )[20] :
(2)
Where d is the crystal size, λ is the wavelength of X -ray source, β is the full width at half
maximum (FWHM) and θ is Bragg diffraction angle. Using this equation, the average

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particle size is estimated to be 13.2 nm. The particle size obtained from XRD measurement is
consistent with t he results from the SEM study.

Fig. 2 . XRD patterns of the synthesized samples: (a) NaX, (b) AgX and (c) MnO 2NPs-AgX

3.4. FTIR Study
The characterization of the prepared adsorbent catalyst along with the X -type zeolite
precursors was further surveyed by FT -IR spectra as plotted in Fig. 3. Peak positions are
nearly identical for three samples. All of the three as -synthesized typical samp les, namely
NaX zeolite, AgX zeolite and 18.4 wt % MnO 2NPs-AgX zeolite composite have peaks
around 456 cm-1 and 562 cm-1 which are assigned to the bending vibrations of the insensitive
internal TO 4 (T=Si or Al) tetrahedral units and double six rings (D6R) external linkage within
the X -type zeolite structure, respectively. The peaks around 674 cm-1 and 754 cm-1 are
attributed to the external linkage and internal tetrahedral symmetrical stretching vib rations,
respectively. Furthermore, the peaks around 984 cm-1 are corresponded to the external
linkage and internal tetrahedral asymmetrical stretching vibrations, and the peaks around

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1643 cm-1 and 3459 cm-1 are assigned to H –O–H bending and O –H bonding ( hydroxyl
groups) vibrations of the X -type zeolite structure, respectively. Surveying Figs. 3a and 3b
confirms that no changes has occurred in the bands of AgX zeolite compared with the
original NaX zeolite, which tends to lend further support to the idea t hat the ion exchange
modification of NaX zeolite by silver ion has a very little influence on the chemical structure
of the zeolite framework. On the other hand, Fig. 3c illustrates three new peaks related to the
synthesized loaded MnO 2NPs. The absorption peak at 577 cm-1 is corresponded to Mn –O
bond. The peaks around 1474 cm-1 and 3347 cm-1 are attributed to H –O–H bending and O –H
bonding (hydroxyl groups) vibrations of the nanoparticles, respectively.

Fig. 3 . FTIR spectra of the synthesized samples: (a) NaX, (b) AgX and (c) MnO 2NPs-AgX

3.5. Removal and Adsorptive Properties study

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In order to study the removal and adsorption of 90Sr as a radioactive element, the adsorbent
performance of MnO 2NPs-AgX zeolite composite was evaluated and those progresses were
monitored by Ultra low -level liquid scintillation spectroscopy. The effects of several
operational parameters such as pH, amount of adsorbent and contact time, and kinetics and
thermodynamic reactions were investigated.

3.6. Effect of pH
One of the most significant factors affecting the sorption of metal ions is the pH of the
applied solution. The effect of pH on the adsorption capacity of MnO 2NPs-AgX zeolite
composite was accomplished utilizing 90Sr solution of 109 Bq at optimized temperature
(25±1 °C) for 8 h. As depicted in Fig. 4, the adsorption characteristic of 90Sr was investigated
at pH ranges of 2.5 -12.5 on the removal of 90Sr by composite adsorbent. To gain the best
selectivity and removal efficiency, pH of 8.5 was selected for the further modifications and
100% adsorption yield. The solution pH was adjusted by 1M solutions of NaOH and HNO 3.
Afterwards, the sorption equilibr ium was reached, the supernatant solution of 90Sr were
brought out and introduced to the Ultra Low -Level Liquid Scintillation Counter (LSC).
Subsequently, the adsorption percentage or in the other word the removal value of 90Sr by
composite adsorbent was c alculated. The interaction of hydrogen ions with an oxygen radical
of the zeolite body generates hydroxyl groups and lowers the charge of the matrix, which is
accompanied by a decrease in the sorption ability of zeolites in relation to 90Sr. Besides, a
higher sorption of the radionuclide due to increasing pH shows that in the solution they are in
an ionic state.

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Fig. 4 . The effect of pH on the removal efficiency of 90Sr by MnO 2NPs-AgX zeolite
composite

3.7. Effect of amount of adsorbent
Selecting the appropriate amounts of adsorbent is a key factor which affects the whole
removal process. The adsorption characteristic of 90Sr was investigated at ranges of 0.5 -3 g of
composite adsorbent to select the best and optimized amount of adsorbent for 90Sr removal.
As depicted in Fig. 5, with increasing of adsorbent amount, the removal efficiency increases,
until the point after which no more significant variations is seen and the curve slope tend to a
linear form which means constant values. Hence , 1.5 g was chosen as the appropriate mass
for MnO 2NPs-AgX zeolite composite to fulfill high yield adsorption.

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Fig. 5 . The effect of amount MnO 2NPs-AgX zeolite composite on the removal efficiency of
90Sr

3.8. Effect of Contact Time
In order to provide an exact comparison between adsorption capability of MnO 2NPs-AgX
zeolite composite adsorbent and reaction time, the effect of different contact time intervals on
the adsorption of 90Sr was fulfilled. The diversity of adsorption value (%) with shaki ng time
has been represented in Fig. 6 , which that show n the reliability of adsorption yield of 90Sr on
the composite adsorbent to the contact time. As the reaction time increases, the adsorption
will increase slightly. The adsorption time was surveyed in the range of 2 –12 h and LSC
analysis showed that the removal first increased up to 8 h and then remained constant.
Therefore, to achieve a shorter analysis time 8 h was chosen as optimum value. The obtained
results from designed experiment showed that the sorption procedure was rapid and
equilibrium gained quickly after mixing the composite adsorbent with target containing
solution. 90Sr uptake on the MnO 2NPs-AgX zeolite composite adsorbent may be the cause of
exchange of target metallic ion with the other ions presented on the adsorbent surface. The
spectra of 90Sr/90Y and those results are represented in Figs. 7 to 10 and Table 1, respectively.
The energetic window A (150 -760) includes all the 90Sr spectrum and low energy region of

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90Y spectrum. The window B (760 -940) includes the high -energy region of the 90Y spectrum.
90Sr analysis of natural water sample from the Ramsar city of Iran has been performed. A
typical spectrum of 90Sr in equilibrium with its daughter is shown in Fig. 7.
The minimum detectable activity (MDA) was evaluated using Currie formulas ( 3) and
(4) [21]:

Where ε is the detection efficiency; T is the counting time (s); Q is the sample quantity (kg);
B is the background count rate (s-1). The removal efficiency was also calculated using the
following equation ( 5):

Where A 0 and A e are the 90Sr activities in the aqueous phase before and after sorption.

Fig. 6 . The effect of contact time on the removal efficiency of 90Sr by MnO 2NPs-AgX zeolite
composite

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Fig. 7 . Liquid scintillation counting (LSC) spectra for removal of 90Sr (count versus channel);
(a) before contacting with prepared MnO 2NPs-AgX zeolite composite, (b) 2 h, (c) 4 h, (d) 6
h, (e) 8h and (f) 12h, pH=8.5, temperature (T=25°C) and adsorbent amount of 1.5 g

Fig. 8 . Liquid scintillation counting (LSC) spectra for removal of 90Sr (count versus energy);
(a) before contacting with prepared MnO 2NPs-AgX zeolite composite, (b) 2 h, (c) 4 h, (d) 6
h, (e) 8h and (f) 12h, pH=8.5, temperature (T=25°C) and adsorbent amount of 1 .5 g.

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Fig. 9 . Liquid scintillation counting (LSC) spectra for removal of 90Sr (count versus channel),
after contacting with prepared MnO 2NPs-AgX zeolite composite (8h), pH=8.5, temperature
(T=25°C) and adsorbent amount of 1.5 g, after 8 h.

Fig. 10 . Liquid scintillation counting (LSC) spectra for removal of 90Sr (count versus energy),
after contacting with prepared MnO 2NPs-AgX zeolite composite (8h), pH=8.5, temperature
(T=25°C) and adsorbent amount of 1.5 g, after 8 h.

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Table 1 . Liquid scintillation counting (LSC) results for removal of 90Sr, after contacting with
prepared MnO 2NPs-AgX zeolite composite
MDA
(mBq/Sam
ple) Count
Time
Min. Fy Unc.
%
(±1
) Activity
(Bq/Sample) CPM
(B) CPM
(A) Time
(h)
6.92 30 1.167 0.11 109.40 2552.36 7904.88 0
6.92 30 1.167 0.38 22.35 469.093 1555.62 2
6.92 30 1.167 0.72 9.49 192.395 654.28 4
6.92 30 1.167 1.30 3.26 45.88 202.98 6
6.92 30 1.167 – 0.0067 0.225 2.438 8
6.92 30 1.167 4.81 0.16 4.157 14.684 12

3.9. Kinetics of Adsorption Reaction
3.9.1 Pseudo first order and pseudo second order kinetic models
The kinetic study of the adsorption of 90Sr by MnO 2NPs-AgX zeolite composite was
investigated utilizing pseudo first order, pseudo second order, Elovich and intra particle
diffusion kinetic models. The pseudo first order Lagergren [22] model presumes that the rate
of variation of solute uptake by reaction time is certainly related to versatility in glut
concentration and solid uptake amount by reaction time ( 6).
(6)
Where qe and qt parameters are considered as the values of 90Sr which are adsorbed per mass
unit of the composite adsorbent (mg·g−1) at the equilibrium and time t, respectively. k1 is
recognized as the rate constant of the adsorption reaction (min−1). was also
plotted versus time interval (Fig 11) , a straight line should be obtained with a slope of k1, if

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the first order kinetics is credible. Ho and [23] proposed a pseudo second order model for the
adsorption of divalent metal ions onto sorbent particles that following below equation ( 7):

Where qe and qt parameters represent the amount of 90Sr (g·mg−1) at equilibrium and other
time intervals. k2 is the rate constant of the pseudo second order equation (g·mg−1 min−1).
When the second order model is a suitable expression, a pattern of against time ( t) will
gain a linear result with a slope of and an ex cise of . The adsorbed amounts ( q)
of Sr2+ were calculated using the following equation (8):

(8)
Where C 0 and C e are the initial and equilibrium concentrations of Sr2+ (g·mg−1) in the liquid
phase, respectively, V is the volume of solution (L) and also m is the mass of adsorbent (g).
The rate constant of pseudo first order and pseudo second order of the adsorption and R2 were
determined from the pattern among versus time t (Fig 11) and the pattern of t/q
versus time t (Fig 12) .

Fig. 11. Plot of pseudo first order for the adsorption of 90Sr on MnO 2 NPs-AgX zeolite
composite

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Fig. 12. Plot of pseudo second order for the adsorption of 90Sr on MnO 2 NPs-AgX zeolite
composite

3.9.2. Elovich kinetic model
The Elovich equation is represented as can be observed in below ( 9) [24]:

where and are considered as the initial sorption rate and the desorption constant both
(mg.g-1) respectively. The Elovich equation can be simplified if it is presumed that .
At the boundary conditions at , the above men tioned equation changes to (1 0)
[25] (Fig 13).

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Fig. 13. Plot of Elovich kinetics for the adsorption of 90Sr on MnO 2 NPs-AgX zeolite
composite

3.9.3. Intra particle diffusion model
Each adsorption process includes various surface diffusion follows by intra particle diffusion.
Generally, the liquid phase mass transport managed the adsorption process. Also the mass
transport rate can be imparted as a function of the square root of time (t). As explained above,
the intra particle diffusion model was demonstrated by formula in below (1 1) [25]:

At the above mentioned formula, is the amount of the adsorbed 90Sr on the MnO 2 NPs-
AgX zeolite composite. Also t and C, are time and intra particle diffusion rate constant
respectively. Plus, the amount of correlation coefficient (R2) was calculated from the slope
and intercept of the drawing of qt versus (Fig. 14). The kinetic model along with upper
corre lation coefficient R2 was considered as the most appropriate model. Table 2 represent
the kinetic factors of the 90Sr adsorption on the MnO 2NPs-AgX zeolite composite. The
obtained results were shown the 90Sr adsorption on MnO 2NPs-AgX zeolite composite is
followed via pseudo second order.

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Fig. 14. Plot of intra particle diffusion kinetics for the adsorption of 90Sr on MnO 2 NPs-AgX
zeolite composite .

Table . 2 The different kinetics model rate constants for the adsorption of 90Sr on MnO 2 NPs-
AgX zeolite composite
Type of kinetic Model R2 k1 (min-1) k2 (g.mg-1.min-1)
First order kinetic 0.7718 0.2686 –
Second order kinetic 0.9970 – 7E+07
Elovich 0.9172 – –
Intra particle diffusion 0.8252 – –

3.10. Thermodynamic of Adsorption Reaction
3.10.1. Effect of Temperature
The temperature in which the experiment fulfills is a significant factor that cannot be over
looked. In this work, the adsorption of 90Sr on the MnO 2NPs-AgX zeolite composite
adsorbent was investigated in the temperature range of 25 -45 °C under optimized conditions.

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Figure 15 illustrates the effect of temperature on the adsorption of 90Sr on the composite
adsorbent surface. As can be seen, the adsorption of 90Sr on the MnO 2NPs-AgX zeolite
composite adsorbent decreases as the temperature increases gradually. The reaction
efficiency for the temperatures of 25, 35 and 45 °C were 100%, 82% and 46%, respectively
(Figs 15 and 16). This is why in high temperatures the formed bonds between 90Sr and active
sites of adsor bent will be weak and break down eventually. The behavior study of the
adsorption of 90Sr ions by MnO 2NPs-AgX zeolite composite was surveyed as a function of
temperature. Moreover, the dependence of distribution ratios on the temperature was
evaluated. The relationship between K and Gibbs free energy variation in sorption is
presented in below (1 2):

Gibbs free energy variation can also be introduced in terms of enthalpy variation, ,
entropy variation, , as is shown below (1 3):

Beside, by mixing the two above mentioned equations (1 2 and 1 3) a new exposition is
achieved as is seen in following ( Vans Hoff equation (1 4)):

The enthalpy ( ) of adsorption and the entropy ( ) of adsorption can be
determined from the slope and the intercept of the linear fits which are gained by drawing Ln
K against
respectively. Also the negative amounts show that the adsorption process is
spontaneous. The values are well under those related to chemical bond constitution, showing
the physical property of the adsorption process. Forby, the enthalpy variation following
adsorption is positive in all cas es representing the exothermic nature of adsorption that is the
removal of 90Sr is decreased as the temperature increases. The entropy variations of the

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system along with the adsorption of 90Sr ions on the MnO 2NPs-AgX zeolite composite is
positive in all cases showing that more discover is generated following adsorption. The
results were calculated in Tables 3-5.

Fig. 15. Plot of 90Sr removal% versus temperature ( °C)

Fig. 16. Plot of Vans Hoff for the adsorption of 90Sr on MnO 2 NPs-AgX zeolite composite at
different temperature (Ln K versus 1/T)

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Table . 3. Liquid scintillation counting (LSC) results for removal of 90Sr, after contacting with
prepared MnO 2NPs-AgX zeolite composite at temperature (T=35°C)
MDA
(mBq/Sam
ple) Count
Time
Min. Fy Unc.%
(±1
) Activity
(Bq/Sample) CPM
(B) CPM
(A) Time
(h)
6.92 30 1.167 0.18 100.06 2244.84 7125.26 0
6.92 30 1.167 0.29 42.27 920.36 2978.90 2
6.92 30 1.167 0.31 21.80 640.76 1731.50 4
6.92 30 1.167 0.38 22.84 458.86 1566.07 6
6.92 30 1.167 0.35 18.90 448.968 1376.63 8
6.92 30 1.167 0.28 19.09 385.815 1311.99 12

Table . 4. Liquid scintillation counting (LSC) results for removal of 90Sr, after contacting with
prepared MnO 2NPs-AgX zeolite composite at temperature (T=45°C)
MDA
(mBq/Sam
ple) Count
Time
Min. Fy Unc.%
(±1
) Activity
(Bq/Sample) CPM
(B) CPM
(A) Time
(h)
6.92 30 1.167 0.16 106.51 2302.12 7515.33 0
6.92 30 1.167 0.21 79.98 1363.02 5192.33 2
6.92 30 1.167 0.23 64.24 1088.92 4164.35 4
6.92 30 1.167 0.31 56.44 898.22 3590.60 6
6.92 30 1.167 0.23 49.65 950.93 3346.61 8
6.92 30 1.167 0.23 50.30 937.32 3360.00 12

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Table 5. Thermodynamic parameters for the adsorption of 90Sr on MnO 2 NPs-AgX zeolite
composite
T (0K) ΔG (kj/mol) ΔS(j/mol.K) ΔH(kj)
298 -11.9232
-44.5239 -25.1914 308 -11.478 0
318 -11.0327

4. Conclusions
In summary , MnO 2NPs-AgX zeolite composite was successfully developed and applied for
effective removal of radioactive 90Sr ions from drinking water of Ramsar city. The
synthesized adsorbent was characterized by SEM, AAS, XRD and FTIR techniques and the
removal process followed via Ultra Low -Level Liquid Scintillation Counting (LSC) analysis.
The different conditions such as pH, amount of adsorbent, the contact time and temperature
were investigated and optimized. The pH=8.5, amount of adsorbent (1.5 g), contact time ( 8 h)
and temperature (25°C) were optimized conditions for this process. The results denoted that
MnO 2NPs-AgX zeolite composite leads to maximum removal and adsorption of 90Sr. The
reaction kinetic information was studied by utilizing pseudo first and second order, Elovich
and Intra particle diffusion kinetic models. The adsorption kinetics of 90Sr was mat ched
nicely with the pseudo second order kinetic model. Also, thermodynamic study for the
adsorption reaction was evaluated and the results denoted that by increasing the temperature,
efficiency reaction decreased.

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Acknowledgements
The authors gratefully acknowledge the financial supports of Islamic Azad University, Ahvaz
Branch, Ahvaz, Iran and Islamic Azad University , Bushehr branch, Bushehr, Iran .

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Graphical Abstract