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A Tunable Template-Assisted Hydrothermal Synthesis of Hydroxysodalite
Zeolite Nanoparticles Using Various Aliphatic Organic Acids for the Removal
of Zinc(II) Ions from Aqueous Medi...
Article in Journal of Inorganic and Organometallic Polymers and Materials · September 2018
DOI: 10.1007/s10904-018-0982-9
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Journal of Inorganic and Organometallic Polymers and Materials
https://doi.org/10.1007/s10904-018-0982-9
A Tunable Template-Assisted Hydrothermal Synthesis
of Hydroxysodalite Zeolite Nanoparticles Using Various Aliphatic
Organic Acids for the Removal of Zinc(II) Ions from Aqueous Media
Ehab A. Abdelrahman1 · Dina A. Tolan2 · Mostafa Y. Nassar1
Received: 11 August 2018 / Accepted: 24 September 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Hydroxysodalite zeolite nanoparticles with different crystallite sizes (37.61–64.88 nm) were synthesized using a hydrothermal
method in the absence and presence of low-cost aliphatic organic acids as templates. The templates used were oxalic acid
dihydrate, tartaric acid, citric acid monohydrate, succinic acid, maleic acid, and ethylenediaminetetraacetic acid (EDTA). The
synthesized nanoparticles were characterized using HR-TEM, FE-SEM, FT-IR, and XRD techniques. The hydroxysodalite
zeolite synthesized using ethylenediaminetetraacetic acid (EDTA) has the smallest crystallite size (37.61 nm) whereas the
template free one has the largest crystallite size (64.88 nm). The synthesized nanoparticles could be effectively applied to
purify polluted water from the zinc(II) ions, and the maximum adsorption capacity was 8.53 mg/g. Kinetic study displayed
that the adsorption process of zinc(II) ions obeyed pseudo-second-order, intra-particle diffusion, liquid film diffusion, and
pore diffusion models whereas the rate determining step of the adsorption is only controlled by the pore diffusion model. In
addition, equilibrium study showed that the adsorption process fitted well with the Langmuir isotherm model compared to
the Freundlich isotherm model. Besides, thermodynamic study showed that the adsorption process is exothermic, spontaneous, and chemisorption. Moreover, desorption and reusability study revealed that there is a slight decrease in both of the
% removal and adsorption capacity of the hydroxysodalite adsorbent with progressing five cycles. Hence, we can infer that
this new hydroxysodalite adsorbent can possibly be utilized repeatedly without sacrificing its adsorption capacity towards
zinc(II) ions.
Keywords Hydrothermal method · Hydroxysodalite nanoparticles · Organic templates · Zinc(II) ions · Adsorption
1 Introduction
Most countries in the world have been concerned about water
pollution induced by metal ions such as zinc due to their direct
impact on humans via causing many serious diseases [1–3].
Zinc is one of the most dangerous toxins that are widespread
and exist in most vital processes in small amounts and it has
many industrial applications. Zinc contaminants have serious
health effects on humans as zinc can affect the digestive system, kidney, liver and cause many diseases such as lethargy,
* Ehab A. Abdelrahman
ehab.abdelrahman@fsc.bu.edu.eg; dr.ehabsaleh@yahoo.com
1
Chemistry Department, Faculty of Science, Benha
University, Benha 13518, Egypt
2
Department of Chemistry, Faculty of Science, Menoufia
University, Shebin El‑Kom 32512, Egypt
diarrhea, depression, nausea, etc. [4–6]. It is necessary to
remove such contaminants from polluted water to preserve our
environment. Various adsorbents have been proposed for the
removal of zinc contaminants from water such as bicopolymer
membranes [7], graphene oxide nanosheets decorated with
highly crystalline polyaniline nanofibers [8], Purolite C-100
MH resin [9], Amberlite IR-120 resin [10], and sodium dodecyl sulfate coated magnetite nanoparticles [11]. The most common disadvantage of these previous adsorbents that limit their
usage is that they are highly expensive. On the other hand,
the adsorption method was widely spread compared to other
methods such as extraction, membrane techniques, electrodialysis, and chemical precipitation. This can be explained on
the basis that the majority of those methods involve expensive
processes, complex methodology, and/or in some cases entail
the irreversible use of chemicals, which consequently cause
secondary pollution [12]. Moreover, disposal problem, which
results from the large capital requirement for electricity and the
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Journal of Inorganic and Organometallic Polymers and Materials
production of large amounts of sludge, is considered as one of
the major limitations of these common methods. In addition,
adsorption is more effective because of its simplicity, high
efficiency, fastness, regeneration, low concentrations removal,
economic, and environmental friendliness [13]. So, finding
new safe and cheap adsorbents for metals ions decontamination is still a critical demand. Therefore, many researchers have
been focused on the production of zeolite nano-adsorbents,
such as mordenite, analcime, and hydroxysodalite owing to
their excellent porosity, high surface area, and great removing ability toward inorganic and organic pollutants [14–24].
Zeolites have a three-dimensional structure comprising of tetrahedral units (­ Al2O3 and S
­ iO2) connected by shared oxygen
atoms. In zeolites, some of ­Si4+ are replaced by ­Al3+ results in
a net negative charge which is remunerated by alkali and alkaline-earth metal cations such as ­Na+ within the structure [14,
15]. Hence, zeolites possess great ion exchange properties with
metals. In addition, the efficiency of the adsorption process
can be controlled by studying different issues such as kinetics, reusability, equilibrium, pH, and thermodynamic [25–29].
Furthermore, various studies reported that morphology and
crystallinity of the zeolites play an important role in the efficiency of zeolites in adsorption, separation, and ion exchange.
Therefore, controlling the synthetic parameters is important
in nano-zeolite synthesis for both industrial applications and
fundamental study [14–24].
There are several strategies to synthesize zeolites with
different morphologies including use of organic compounds
as templates during the synthesis process. In this light, the
organic molecule templates can act as capping agents to prevent aggregation of particles during synthesis which control both of crystallite size and morphology of the produced
zeolites [30]. However, most of the reported organic templates were expensive and/or have complex structures such
as tetrapropylammonium hydroxide, tetrapropylammonium
bromide, cetyl trimethylammonium bromide, etc. Moreover,
the produced products required high temperatures in order to
remove the applied organic templates to get porous and pure
zeolite products [12, 31]. Therefore, in the current project, the
effect of low-cost aliphatic organic acids as templates such as
oxalic acid dihydrate, tartaric acid, citric acid monohydrate,
succinic acid, maleic acid, and ethylenediaminetetraacetic acid
Fig. 1 XRD patterns of the samples synthesized after 6 h (A), 12 h (B), 18 h (C), and 24 h (D)
13
Journal of Inorganic and Organometallic Polymers and Materials
(EDTA) on both crystallinity and morphology of hydroxysodalite zeolite products was studied. In addition, the synthesized
hydroxysodalite using EDTA was used as a nano-adsorbent
for the removal of zinc(II) ions from water. Moreover, various
factors influencing the adsorption process have been explored.
2 Experimental
2.1 Chemicals
The used chemicals are aluminum isopropoxide
(Al[OCH(CH 3) 2] 3), sodium metasilicate pentahydrate
­(Na2SiO3·5H2O), sodium hydroxide (NaOH), oxalic acid
dihydrate ­(HO2CCO2H·2H2O), tartaric acid ­(C4H6O6), citric
acid monohydrate ­(C6H8O7⋅H2O), succinic acid ­(C4H6O4),
maleic acid ­(C 4H 4O 4), ethylenediaminetetraacetic acid
(EDTA) ­(C10H16N2O8), hydrochloric acid (HCl), potassium
chloride (KCl), ethylenediaminetetraacetic acid tetrasodium salt dihydrate (­ Na4–EDTA) ((NaOOCCH2)2NCH2C
H2N(CH2COONa)2·2H2O), and zinc sulfate heptahydrate
­(ZnSO4·7H2O). All the chemicals were purchased from
Sigma-Aldrich and used as received.
2.2 Synthesis of Hydroxysodalite Nanoparticles
10.00 g of sodium metasilicate pentahydrate (47.14 mmol)
was dissolved in 25 mL distilled water (solution A).
5.00 g of aluminum isopropoxide (24.48 mmol) was dissolved in 25 mL of 5 M sodium hydroxide solution (5.00 g,
125.00 mmol) (solution B). Then, solution B was added
dropwise to solution A with continuous stirring at 650 rpm
for 1 h. After that, 25 mL of 2.50 mmol organic acid solution
in 1.5 M sodium hydroxide solution (1.50 g, 37.50 mmol)
[oxalic acid dihydrate (0.32 g), tartaric acid (0.38 g), citric
acid monohydrate (0.53 g), succinic acid (0.30 g), maleic
acid (0.29 g), or EDTA (0.73 g)] was dropwise added to the
previous mixture with continuous stirring at 650 rpm for 2 h.
For more understanding of the effect of the selected organic
molecules as template, template free zeolite nanoparticles
Fig. 2 XRD patterns of the samples synthesized in the absence of organic acids (A) and in the presence of oxalic acid dihydrate (B), tartaric acid
(C), or maleic acid (D)
13
Journal of Inorganic and Organometallic Polymers and Materials
Fig. 3 XRD patterns of the samples synthesized in the presence of citric acid monohydrate (A), succinic acid (B), or EDTA (C)
Table 1 Effect of different
organic acids on the produced
phase and average crystallite
size
Organic acid
Produced phases
Chemical formula
Without template
Oxalic acid
Hydroxysodalite
Hydroxysodalite
Aluminum silicate
Hydroxysodalite
Hydroxysodalite
Hydroxysodalite
Sodium aluminum
silicate hydrate
Hydroxysodalite
Hydroxysodalite
Na4Al3Si3O12 (OH)
Na4Al3Si3O12 (OH)
Al2 ­(SiO4) O
Na4Al3Si3O12 (OH)
Na4Al3Si3O12 (OH)
Na4Al3Si3O12 (OH)
(Na2O)1.31Al2O3
­(SiO2)2.01(H2O) 1.65
Na4Al3Si3O12 (OH)
Na4Al3Si3O12 (OH)
Tartaric acid
Citric acid
Succinic acid
EDTA
Maleic acid
were synthesized using a similar procedure for comparison
studies.. The produced gel was transferred to 100 mL Teflon lined stainless autoclave then hydrothermally treated
at 150 °C for 18 h. Moreover, the precipitate was filtered,
13
JCPDS card
Average
crystal size
(nm)
00-011-0401
00-011-0401
01-089-0890
00-011-0401
00-011-0401
00-011-0401
01-075-2318
64.88
40.75
00-011-0401
00-011-0401
37.61
41.85
47.55
44.77
43.31
washed thoroughly several times with distilled water, dried
at 60 °C for 24 h, and calcined at 550 °C for 2 h to remove
organic templates.
Journal of Inorganic and Organometallic Polymers and Materials
2.3 Adsorption of Zinc(II) Ions from Aqueous Media
0.20 g of adsorbent (hydroxysodalite zeolite synthesized
using EDTA template) was magnetically stirred at 650 rpm
with 50 mL of Zn(II) solution (50 mg/L), in 250 mL conical flasks. Small volume portions were taken out of the
conical flasks at various times (5–100 min), centrifuged at
2500 rpm, and measured for Zn(II) ions using the atomic
absorption spectrophotometer. Besides, the influence of pH
(2–8), which was adjusted using 0.1 M HCl or NaOH, on
the removal of Zn(II) ions was investigated with 50 mL of
Zn(II) solution (50 mg/L) at the acquired equilibrium time
(40 min). In addition, the influence of initial Zn(II) ions concentration (25–200 mg/L) was investigated at the acquired
equilibrium time (40 min) and optimum pH 8. Besides, the
influence of temperature (298–323 K) was also studied using
50 mL of Zn(II) solution (50 mg/L) at the acquired equilibrium time (40 min) and optimum pH 8. In order to determine the point of zero charge ­(pHpzc) of the hydroxysodalite adsorbent; 0.50 g adsorbent was mixed with 50 mL KCl
solutions (0.1 M) of different initial pH values (­ pHi) (2–11)
then the mixtures were magnetically stirred at 650 rpm for
24 h. After that, the liquid phases were separated by centrifugation at 2500 rpm and the final pH values (­ pHf) of the
supernatants were determined using pH meter. Finally, p­ Hf
values are presented as a function of p­ Hi values. The p­ Hpzc
is the ­pHf level where a common plateau was obtained [32].
The reusability of the hydroxysodalite adsorbent was tested
five times as the following; Zn(II)-loaded adsorbent sample was added to 50 mL N
­ a4–EDTA (0.2 M) as a desorption medium then magnetically stirred at 650 rpm for 3 h at
25 °C. After that, the separated adsorbent was magnetically
stirred at 650 rpm with 50 mL of Zn(II) solution (50 mg/L).
Then, the aqueous phase was separated from the adsorbent
by centrifugation the mixture at 2500 rpm and the concentration of Zn(II) ions in the liquid phase was determined (first
cycle). The previous reusability steps were repeated four
times (second-fifth cycle).
The adsorption capacity ­(Qt, mg/g) of the hydroxysodalite
adsorbent at time t was measured using Eq. (1).
(
)
Qt = Ci − Ct V∕m
(1)
Also, the % removal of Zn(II) ions (% removal) at time t
was estimated using Eq. (2).
(
)
% Removal = Ci − Ct 100∕Ci
(2)
where m (g) is the mass of hydroxysodalite adsorbent,
V (L) is the volume of Zn(II) solution, ­Ct is the residual
Fig. 4 EDS spectra of the samples synthesized in the absence of organic acids (A) and in the presence of oxalic acid dihydrate (B), tartaric
acid (C), or maleic acid (D)
13
Journal of Inorganic and Organometallic Polymers and Materials
concentration of Zn(II) at time t, and C
­ i (mg/L) is the initial
concentration of Zn(II).
In addition, the adsorption capacity of the hydroxysodalite adsorbent at equilibrium ­(Qe, mg/g) was measured
using Eq. (3).
(
)
Qe = Ci − Ce V∕m
(3)
Moreover, the % removal of Zn(II) ions (% removal) at
equilibrium was calculated using Eq. (4)
(
)
% Removal = Ci − Ce 100∕Ci
(4)
where C e (mg/L) is the concentration of Zn(II) at
equilibrium.
2.4 Physicochemical Measurements
XRD patterns of the synthesized hydroxysodalite samples
were performed using an 18 kW diffractometer (Bruker;
model D8 Advance) equipped with monochromated Cu K
­ α
radiation (λ) 1.54178 Å. FT-IR spectra of the synthesized
hydroxysodalite samples were recorded using KBr disk on a
Nicolet iSio FT-IR spectrophotometer in the 4000–400 cm−1
region. The elemental analysis and the morphology of
hydroxysodalite samples were investigated with an ultra-high
Table 2 EDS spectra of the synthesized hydroxysodalite nanoparticles
Organic acid
Without template
Oxalic acid
Tartaric acid
Citric acid
Succinic acid
EDTA
Maleic acid
Weight % of the elements
Si%
Al%
Na%
O%
17.99
18.34
17.23
19.42
18.49
26.49
18.91
15.86
15.99
15.31
16.57
16.72
22.80
16.15
16.23
16.27
16.54
16.18
16.00
13.95
16.47
49.92
49.40
50.92
47.82
48.80
36.76
48.47
resolution FE-SEM SU8020 microscope equipped with an
energy-dispersive X-ray spectrometer SEM-EDX (Hitachi,
Tokyo, Japan). The HR-TEM images of the hydroxysodalite
samples were collected employing a transmission electron
microscope (TEM-2100) at a speeding voltage of 200 kV.
X-ray electron spectroscopy was carried out using a X-ray
phototoelectron spectrometer (ESCA-5600, Japan) with a
200 W Mg K (Al K) radiation source; the diameter of X-ray
beam was 0.8 mm while the analysis diameter was 1 mm.
The samples were adhered on an indium sheet. The Zn(II)
Fig. 5 EDS spectra of the samples synthesized in the presence of citric acid monohydrate (A), succinic acid (B), or EDTA (C)
13
Journal of Inorganic and Organometallic Polymers and Materials
removal investigation was achieved utilizing atomic absorption spectrophotometer (Shimadzu, model AA-7000F).
3 Results and Discussion
3.1 XRD, EDS, and XPS Studies
Firstly, the effect of hydrothermal time (6, 12, 18, and 24 h)
was studied as shown in Fig. 1A–D, respectively. The results
showed that after 6 or 12 h a mixture of zeolite 4A (Cubic;
JCPDS No. 01-071-1557; space group: Pm-3m) and hydroxysodalite (Cubic; JCPDS No. 00-011-0401; space group:
P-43n) was formed. Also, after 18 or 24 h only hydroxysodalite was formed. It was noticed that when reaction time
increased from 18 to 24 h, the crystallite size increased from
64.88 to 72.57 nm, respectively. Therefore, 18 h was chosen as the optimum time for the subsequent hydrothermal
experiments. The characteristic peaks of hydroxysodalite at
2θ = 14, 20, 22.38, 24.50, 28.50, 31.90, 34.90, 37.90, 43.20,
48.16, 52.50, 56.67, 60.77, 62.50, 64.50, and 68.37 can be
attributed to lattice plans of (110), (200), (210), (211), (220),
(310), (222), (321), (411), (332), (510), (521), (530), (600),
(611), and (541), respectively. It is worth mentioning that the
peaks of zeolite 4A were marked by an asterisk as shown in
Fig. 1A, B. The characteristic peaks of zeolite 4A at 2θ =
7.20, 10.19, 12.49, 16.15, 21.72, 26.18, 27.19, 30.02, 40.25,
44.28, 50.43, 54.43, and 58.76 can be attributed to lattice
plans of (100), (110), (111), (210), (300), (320), (321),
(410), (521), (600), (631), (641), and (650), respectively
[33, 34].
Thereafter, the influence of using different organic templates on the hydrothermal treatment of interest was investigated. Figure 2A–D presents XRD patterns of the template-free zeolite sample (A) as well as organic template
zeolite samples (B–D) in the presence of oxalic acid dihydrate, tartaric, and maleic acid, respectively. Figure 3A–C
displays the XRD patterns of the samples synthesized in
Fig. 6 FT-IR spectra of the samples synthesized in the absence of organic acids (A) and in the presence of oxalic acid dihydrate (B), tartaric acid
(C), or maleic acid (D)
13
Journal of Inorganic and Organometallic Polymers and Materials
Fig. 7 FT-IR spectra of the samples synthesized in the presence of citric acid monohydrate (A), succinic acid (B), or EDTA (C)
Table 3 FT-IR spectra of the synthesized hydroxysodalite nanoparticles
Organic acid
T–O–T
bending
­(cm−1)
Peak no 1
Internal symmetric stretching
of T–O (­ cm−1)
Peak no 2
External symmetric stretching
of T-O ­(cm− 1)
Peak no 3
Internal asymmetric stretching
of T–O (­ cm−1)
Peak no 4
External asymmetric stretching
of T–O (­ cm−1)
Peak no 5
Bending vibration of adsorbed
water(cm−1)
Peak no 6
Stretching vibration of adsorbed
water ­(cm−1)
Peak no 7
Without organic
template
Oxalic acid
Tartaric acid
Citric acid
Succinic acid
EDTA
Maleic acid
442
629
710
990
1476
1650
3464
440
440
445
443
434
434
627
628
627
629
630
629
695
709
715
705
708
711
993
989
993
992
988
987
1474
1474
1473
1479
1483
1479
1635
1645
1637
1636
1655
1654
3440
3446
3431
3430
3525
3446
the presence of citric acid monohydrate, succinic acid, and
EDTA, respectively. The XRD results showed that the samples which were synthesized in the absence of organic acids
and in the presence of tartaric, citric acid monohydrate,
maleic acid, and EDTA consist of hydroxysodalite (Cubic;
JCPDS No. 00-011-0401; space group: P-43n) as shown in
13
Table 1 [35, 36]. On the other hand, when oxalic acid was
used, the obtained product was a composite of hydroxysodalite (Cubic; JCPDS No. 00-011-0401; space group: P-43n)
and aluminum silicate (Orthorhombic; JCPDS No. 01-0890890; space group: Cmcm) as shown in Table 1 [36]. It is
worth mentioning that the peaks of aluminum silicate were
Journal of Inorganic and Organometallic Polymers and Materials
marked by an asterisk as shown in Fig. 2B. The characteristic peaks of aluminum silicate at 2θ = 19, 27.9, 33.98,
34.86, 40.1, and 51 can be attributed to lattice plans of (020),
(110), (112), (023), (131), and (170), respectively. Besides,
when succinic acid was used, the product was a composite of hydroxysodalite (Cubic; JCPDS No. 00-011-0401;
space group: P-43n) and sodium aluminum silicate hydrate
(Hexagonal; JCPDS No. 01-075-2318; space group: P63)
as shown in Table 1 [37]. It is noteworthy that an asterisk as shown in Fig. 3B marked the peaks of sodium aluminum silicate hydrate. The characteristic peaks of sodium
aluminum silicate hydrate at 2θ = 19, 27.8, 34, and 41.07
can be attributed to lattice plans of (101), (121), (131), and
(122), respectively.
The crystallite size (D, nm) of the synthesized samples
was calculated utilizing Scherrer equation (Eq. 5):
(5)
where β, θB, and λ are the full width at half maximum
(FWHM) of the XRD diffraction peaks, the diffraction angle
according to Bragg formula, and wavelength of the X-ray
radiation, respectively [12, 19, 20]. The results showed that
the crystallite size of the samples which were synthesized
in the absence of organic acids and the presence of oxalic
acid dihydrate, tartaric acid, citric acid monohydrate, succinic acid, maleic acid, and EDTA were 64.88, 40.75, 47.55,
44.77, 43.31, 41.85 and 37.61 nm, respectively, as shown in
Table 1. Hence, the organic acids have a significant effect
D = 0.9𝜆∕β cos θB
on the produced hydroxysodalite nanoparticles through
controlling their crystallite sizes. This controlling behavior
because the organic acid templates work as capping agents
preventing the aggregation of the nanoparticles which leads
to a decrease in crystallite size [30]. Figure 4A–D presents
EDS patterns of the samples synthesized in the absence of
organic acids and in the presence of oxalic acid dihydrate,
tartaric, and maleic acid, respectively. Figure 5A–C shows
EDS patterns of the samples synthesized in the presence of
citric acid monohydrate, succinic acid, and EDTA, respectively. The results revealed that all the samples are composed
of only Si, Al, Na, and O elements, as shown in Table 2. In
addition, XPS of the sample synthesized using EDTA as an
illustrative example confirmed that the sample is composed
of only Si, Al, Na, and O elements, as shown in Fig. 9D.
This supports the high purity of the prepared zeolite products using the proposed method.
3.2 FT‑IR Study
Figure 6A–D presents FT-IR spectra of the synthesized
samples: the template-free zeolite sample (A) as well as
organic template zeolite samples (B–D) in the presence
of oxalic acid dihydrate, tartaric, and maleic acid, respectively. Figure 7A–C depicts the FT-IR spectra of the samples which were synthesized in the presence of citric acid
monohydrate, succinic acid, and EDTA, respectively. The
Fig. 8 FE-SEM images of the
samples synthesized in the
absence of organic acids (A)
and in the presence of oxalic
acid dihydrate (B), tartaric
acid (C), or maleic acid (D)
13
Journal of Inorganic and Organometallic Polymers and Materials
results exhibited 7 characteristic bands for each prepared
sample, as outlined in Table 3. The bands appeared at ca.
434–445 cm−1 is probably due to T–O–T bending, T=Si
and/or Al [12, 19, 20]. Besides, the bands appeared at
about 627–630 cm−1 can assigned to an internal symmetric
stretching of T–O bond [12, 19, 20]. In addition, the bands
observed in the range of 695–715 cm−1 can be attributed to
an external symmetric stretching of T–O bond [12, 19, 20].
Moreover, the bands observed at ca. 987–993 cm−1 can be
due to an internal asymmetric stretching of T–O bond [12,
19, 20]. Besides, the bands noticed at ca. 1473–1483 cm−1
can be assigned to an external asymmetric stretching of
T–O bond [12, 19, 20]. The bands appeared in the range
of 3430–3525 and 1635–1655 cm −1 can be attributed to
the stretching and bending vibration of H–O–H molecules
adsorbed on the surface of samples, respectively [12, 19,
20, 38–47].
3.3 FE‑SEM and HR‑TEM Studies
Figure 8A–D exhibited FE-SEM images of the prepared
samples: the template-free zeolite sample (A) as well as
organic template zeolite samples (B–D) in the presence
of oxalic acid dihydrate, tartaric acid, and maleic acid,
respectively. Figure 9A–C revealed the FE-SEM images
of the samples synthesized in the presence of citric acid
monohydrate, succinic acid, and EDTA, respectively. The
results exhibit that the samples which were synthesized in
the absence of organic acids and the presence of oxalic acid
dihydrate, tartaric, maleic acid, citric acid monohydrate, succinic acid, and EDTA are composed of spherical shapes with
an average size of ca. 2.5, 3.38, 3.36, 4.06, 3.72, 2.91, and
3.76 µm, respectively.
Figure 10A–D represents HR-TEM images of the synthesized samples: the template-free zeolite sample (A) as well
as organic template zeolite samples (B–D) in the presence of
oxalic acid dihydrate, tartaric acid, and maleic acid, respectively. Figure 11A–C displayed the HR-TEM images of the
samples synthesized in the presence of citric acid monohydrate, succinic acid, and EDTA, respectively. The results
showed that the prepared samples are composed of spherical
and irregular shaped particles with an average diameter of
ca. 68.85, 43.24, 49.35, 41.97, 46.65, 41.82, and 33.20 nm,
respectively, which coincides with the obtained XRD data.
Fig. 9 FE-SEM images of the samples synthesized in the presence of citric acid monohydrate (A), succinic acid (B), or EDTA (C)
13
Journal of Inorganic and Organometallic Polymers and Materials
3.4 Adsorption of Zinc(II) Ions from Aqueous Media
Using Hydroxysodalite Nanostructure
The adsorption of zinc(II) ions on the hydroxysodalite sample which was synthesized using EDTA has been investigated. This sample was chosen over the other hydroxysodalite samples because it has the smallest crystallite size
and the largest surface area. Different effects such as time
(kinetic study), pH, initial zinc(II) concentration (equilibrium study), temperature (thermodynamic study), and desorption and reusability have been studied.
3.4.1 Kinetics Study
Figure 12A and B represents the influence of contact time
on the % removal of zinc(II) ions (% removal) and adsorption capacity of the adsorbent ­(Qt), respectively. The results
clarified that both % removal and Q
­ t increased significantly
until it achieved ca. 58.70% and 7.34 mg/g after 40 min,
respectively. These values did not change markedly when
time increased because of saturation of the active sites of the
adsorbent i.e. the equilibrium state. Thus, the optimum equilibrium time for zinc(II) removal was chosen to be 40 min.
Based on the results, some kinetic models [12, 19, 20] were
applied such as pseudo-first-order (Eq. 6), pseudo-secondorder (Eq. 7), and intra-particle diffusion (Eq. 8).
(
)
log Qe − Qt = log Qe − K1 t∕2.303
(6)
(
) (
)
t∕Qt = 1∕K2 Qe 2 + 1∕Qe t
(7)
(8)
where ­k1 (1/min) is the pseudo-first-order rate constant
of the adsorption process, ­k2 (g/mg min) is the pseudosecond-order rate constant of the adsorption process, ­kint
(mg/(g min0.5)) is internal diffusion constant, Q
­ t (mg/g) is
the quantity of the adsorbed zinc(II) ions at time t (min),
­Qe (mg/g) is the quantity of the adsorbed zinc(II) ions at
Qt = Kint t0.5 + C
Fig. 10 HR-TEM images of
the samples synthesized in the
absence of organic acids (A)
and in the presence of oxalic
acid dihydrate (B), tartaric
acid (C), or maleic acid (D)
13
Journal of Inorganic and Organometallic Polymers and Materials
Fig. 11 HR-TEM images of the
samples synthesized in the presence of citric acid monohydrate
(A), succinic acid (B), or EDTA
(C)
Fig. 12 The influence of contact time on the % removal of zinc(II) ions (A) and on the adsorption capacity of the adsorbent (B)
13
Journal of Inorganic and Organometallic Polymers and Materials
Fig. 13 Pseudo-first-order (A), pseudo-second-order (B), and intra-particle diffusion (C) models
Table 4 Parameters of the
applied kinetic models of the
adsorption process of zinc(II)
ions on the hydroxysodalite
adsorbent
Kinetic model
Constants
Pseudo first order
Qe (mg/g)
12.78
Qe (mg/g)
10.59
C (mg/g)
0.171
Co (mg/g)
42.66
Kext (1/min)
0.119
α (mg/g min)
0.542
Pseudo second order
Intra particle diffusion
Spahn and Schlunder
Iqbal et al.
Bangham
K1 (1/min)
0.119
K2 (g/mg min)
5.4E−3
Kint (mg/(g min0.5))
1.234
Kext (1/min)
0.019
R2
0.907
Ko (g/mg min)
1.06E−3
R2
0.907
R2
0.991
R2
0.982
R2
0.965
R2
0.989
13
Journal of Inorganic and Organometallic Polymers and Materials
Fig. 14 Spahn and Schlunder (A), Iqbal et al. (B), and Bangham’s (C) models
equilibrium, and C (mg/g) is the thickness of boundary layer.
Figure 13A–C presents the pseudo-first-order, pseudo-second-order, and intra-particle diffusion models, respectively.
In addition, all constants for the three models were calculated and presented in Table 4. The results showed that the
adsorption process complies with the pseudo-second-order
model. This is because of the closer of the value of the
correlation coefficient of pseudo-second-order model (­ R2
= 0.991) compared to that of the pseudo-first-order (­ R2 =
0.907). The intra-particle diffusion model (i.e. inner diffusion process) expects that the main rate-controlling process
is diffusion inside the particles. Besides, the zero intercepts
of the plot of ­Qt versus t­0.5 demonstrates the legitimacy of
this model. In practice, the plot of Q
­ t versus ­t0.5 gave linear
2
fitting ­(R = 0.994) which did not pass through the origin.
This demonstrated that intra-particle diffusion isn’t the main
13
rate-controlling mechanism of zinc(II) ions adsorption onto
the hydroxysodalite sample but it is also controlled by other
mechanisms including film diffusion (diffusion process of
adsorbate to the surface of adsorbent from the liquid solution
i.e. outer diffusion process) and pore diffusion [12, 19, 20].
In order to ponder those mechanisms, Spahn and Schlunder model (Eq. 9), Iqbal et al. model (Eq. 10), and Bangham’s model (Eq. 11) were applied [12, 19, 20].
ln Ct = ln Ci − Kext t
− ln (1 − F) = Kext t
[
]
log R = log Ko m∕2.303 V + α log t
−1
(9)
(10)
(11)
where ­Kext ­(min ) is external diffusion constant, ­Ct (mg/L)
is the concentration of zinc(II) ions at time t, F is the fraction
Journal of Inorganic and Organometallic Polymers and Materials
showing that liquid film diffusion had a predominant impact
in the primary stage of the adsorption process. Furthermore,
the time required for liquid film diffusion was 40 min. Also,
good linear relationships (­ R2 = 0.965) between − ln (1 − F)
and t do not pass through the origin indicating that liquid
film diffusion had an overwhelming influence in the primary stage of the adsorption process. Besides, the plot of
log R versus logt was found to be linear with an excellent
correlation coefficient (­ R2 = 0.989) indicating that the rate
determining step is mainly governed by the pore diffusion
mechanism.
3.4.2 pH Study
We have studied the effect of pH of the zinc(II) solution on
the adsorption of this ion on the as-prepared zeolite product, due to the importance of this factor [48]. Figure 15A
and B displays the influence of pH on the % removal of
zinc(II) ions (% removal) and adsorption capacity of the
adsorbent (Q), respectively. The point of zero charge of
the adsorbent (­ pHpzc) was estimated to be 6.25, as shown
in Fig. 15C. The results showed that as the pH value
increased from 2 to 8, the % removal and Q increased
from 8.64 to 66% and from 1.08 to 8.25 mg/g, respectively.
Hence, the optimum pH value was 8. This can be clearly
explained on the basis of p­ H pzc of the hydroxysodalite
adsorbent. If pH of zinc(II) solution < p­ Hpzc, the surface
of the hydroxysodalite adsorbent is surrounded by H
­ + ions
which in turn repelled with positive zinc(II) ions leading
to a decrease in the % removal and Q values. Besides, if
pH of zinc(II) solution > ­pHpzc, the surface of the hydroxysodalite adsorbent is surrounded by ­OH− ions which in
turn attract positively charged zinc(II) ions leading to an
enhancement in the % removal and Q values [32].
3.4.3 Equilibrium Study
Fig. 15 The influence of pH on the % removal of zinc(II) ions (A)
and adsorption capacity of the adsorbent (B). The plot of p­ Hf values
versus ­pHi values (C)
attainment of equilibrium and it is calculated using Eq. (12).
(α (mg/g min) < 1) and ­Ko (g/mg min) are the Bangham
constants, and R is a parameter calculated using Eq. (13)
F = Qt ∕Qe
[ (
)]
R = log Ci ∕ Ci − Qt m
(12)
(13)
Figure 14A–C presents Spahn and Schlunder, Iqbal et al.
model, and Bangham’s models. In addition, all constants for
the three models were calculated and tabulated in Table 4.
The results showed that good linear relationships ­(R2 =
0.962) between ­lnCt and t observed in the initial 40 min
Figure 16A and B represents the influence of the initial zinc(II)
concentration on the % removal of zinc(II) ions (% removal)
and adsorption capacity of the adsorbent ­(Qe), respectively.
The results showed that as the initial zinc(II) concentration
reduced the % removal increased. In addition, as the zinc(II)
concentration increased from 25 to 200 mg/L the ­Qe increased
from 6.25 to 11.44 mg/g. Moreover, the experimental adsorption results were investigated by fitting to Langmuir (Eq. 14)
and Freundlich (Eq. 15) as shown in Fig. 16C and D, respectively [12, 19, 20].
(
) (
)
Ce ∕Qe = 1∕bQm + Ce ∕Qm
(14)
(15)
where ­Ce (mg/L) is the remained zinc(II) concentration in
the solutions at the equilibrium, ­Qe (mg/g) is the quantity
ln Qe = ln Kf + (1∕n) ln Ce
13
Journal of Inorganic and Organometallic Polymers and Materials
Fig. 16 The influence of the initial zinc(II) concentration on the % removal of zinc(II) ions (A) and on the adsorption capacity of the adsorbent
(B). Langmuir (C) and Freundlich (D) models
of the adsorbed zinc(II) ions at the equilibrium, Q
­ m (mg/g)
is the maximum sorption capacity, K
­ f (mg/g) is the FreunTable 5 Constants of the applied isotherm models of the adsorption
process of zinc(II) ions on the hydroxysodalite adsorbent
Adsorption model
Constants
Langmuir
Qm (mg/g)
8.53
Qm (mg/g)
9.63
Freundlish
b (L/mg)
1.37
Kf (mg/g)(L/mg)1 / n
5.09
R2
0.994
R2
0.982
dlich constant, b (L/mg) is the Langmuir constant, and 1/n
is the heterogeneity factor. All constants for the two models
were calculated and tabulated in Table 5. Besides, ­Qm can
be calculated from Freundlich isotherm, using Eq. (16), as
revealed by Halsey [12, 19, 20].
( )1∕n
Qm = Kf Ci
13
(16)
The results showed that the adsorption process fitted well
with the Langmuir isotherm more than Freundlich. This is
because of the value of the correlation coefficient of Langmuir
model ­(R2 = 0.994) is closer to unity compared to that of Freundlish ­(R2 = 0.982). In addition, the maximum adsorption
capacity calculated from Langmuir model was found to be
8.53 mg/g which was closer to the experimentally calculated
value.
3.4.4 Thermodynamic Study
Figure 17A and B depicts the influence of the temperature
on the % removal of zinc(II) ions (% removal) and adsorption capacity of the adsorbent ­(QT), respectively. The results
showed that as the temperature increased the % removal and
­Qe decreased. The % removal values at 298, 313, and 323 K
were found to be ca. 65.6, 49.2, and 30.0%, respectively. And,
­Qe values at 298, 313, and 323 K were estimated to be ca. 8.25,
6.15, and 3.75 mg/g, respectively. Thus, the results proved the
exothermic nature of the adsorption process of interest. This
can be attributed to the extra ordinary ability of zinc(II) ions
Journal of Inorganic and Organometallic Polymers and Materials
change in free energy (ΔG°) were determined using Eqs. (17)
and (18) [12, 19, 20].
lnKd = (ΔS◦ ∕R) − (ΔH◦ ∕RT)
(17)
ΔG◦ = ΔH◦ − TΔS◦
(18)
where R (KJ/mol K) is gas constant, T (K) is temperature,
and ­Kd (L/g) is distribution co-efficient which is calculated
from Eqs. (19) or (20).
[
]
Kd = % Removal∕(100 − % Removal) V∕m
(19)
(20)
Figure 17C represents the relation between ­lnKd and 1/T.
The results of thermodynamic parameters were tabulated
in Table 6. The results showed that the adsorption process
of zinc(II) ions on the hydroxysodalite sample is exothermic and spontaneous (feasible) process due to the obtained
negative ΔH° and ΔG° values, respectively. Besides, the
adsorption process is chemisorption because the calculated
ΔH° value was found to be − 46.475 kJ/mol [12, 19, 20].
Kd = Qe ∕Ce
3.4.5 Desorption and Reusability Study
Figure 18A and B presents the effect of repeated adsorption/
desorption cycle number for zinc(II) ions on the % removal
of zinc(II) ions (% removal) and adsorption capacity of the
adsorbent ­(Qe), respectively. The results showed that with
progressing cycles there is a slight decrease in both of the
% removal and adsorption capacity of the adsorbent. Hence,
this promising hydroxysodalite adsorbent can possibly be
utilized repeatedly without sacrificing its adsorption capacity towards zinc(II) ions.
4 Conclusions
Fig. 17 The influence of the temperature on the % removal of zinc(II)
ions (A) and on the adsorption capacity of the adsorbent (B). The plot
of ln ­Kd versus 1/T (C)
Table 6 Thermodynamic parameters of the adsorption of zinc(II)
ions on the hydroxysodalite adsorbent
Temperature (K)
∆G° (KJ/mol)
∆S° (KJ/molK)
∆H° (KJ/mol)
298
313
323
− 95.049
− 97.494
− 99.124
0.163
–
–
− 46.475
–
–
to be desorbed and got away to the liquid phase at higher temperatures [12, 19, 20]. Thermodynamic constants, for example,
change in the entropy (ΔS°), change in enthalpy (ΔH°), and
In synopsis, hydroxysodalite zeolite nanoparticles were
successfully synthesized by means of a hydrothermal in the
absence and presence of low-cost aliphatic organic acids as
templates: oxalic acid dihydrate, tartaric acid, citric acid
monohydrate, succinic acid, maleic acid, and ethylenediaminetetraacetic acid (EDTA). The XRD results showed that,
using the organic templates decreases the crystallite size
of the hyroxysodalite zeolite nanoparticles by 34% (average 42.64 nm) compared with template free zeolite nanoparticles (64.88 nm). In addition, the nanozeolites synthesized with EDTA template gave the smallest crystallite size
(37.61 nm) compared to the others. This indicates that the
selected organic acids templates greatly affect the crystallite size of the synthesized zeolite nanoparticles. The zeolite
nanostructure prepared using EDTA template was chosen
over the other samples for further adsorption studies because
13
Journal of Inorganic and Organometallic Polymers and Materials
Fig. 18 The effect of repeated adsorption/ desorption cycle number for zinc(II) ions on the % removal of zinc(II) ions (A) and on the adsorption
capacity of the adsorbent (B)
of its lowest crystallite size and highest surface area. This
zeolite product revealed great adsorption efficiency toward
the removal of zinc(II) ions from water (Q = 8.25 mg/g, %
removal = 66%). The kinetic study showed that the experimental adsorption data fitted well with pseudo-second-order,
intra-particle diffusion, liquid film diffusion, and pore diffusion models whereas the rate determining step was governed by the pore diffusion model. Besides, equilibrium
study indicates that the adsorption process follows Langmuir isotherm model. The maximum adsorption capacity
calculated using the Langmuir model was found to be ca.
8.53 mg/g. In addition, the adsorption process is exothermic
and spontaneous because of the obtained negative ΔH° and
ΔG° values, respectively. Moreover, the adsorption process
is chemisorption because the ΔH° value is estimated to be
ca. − 46.48 kJ/mol.
Acknowledgements The authors sincerely thank Dr “Mai Maize” for
helping them in performing some analysis such as FE-SEM, EDX, and
XPS in Japan.
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