Relationship between Si/AL ratio and the sorption of CD(II) by natural and modified clinoptilolite-rich tuff with sulfuric acid

Abstract

In this paper, the differences in the adsorption mechanisms and adsorption capacities of clinoptilolite materials with different Si/Al ratio (SAR) for Cd ²⁺ ions from aqueous solutions are discussed. The adsorbents were characterized with respect to their phase composition, morphology, specific surface area, cation exchange capacity and point of zero charge. Batch adsorption experiments were performed considering the Cd initial ion concentration, contact time, adsorbent dose, SAR, and pH. The regression coefficient value revealed that the experimental data best fit to Pseudo-second-order model, whilst the kinetic rate constant k 2 (g/mg min ⁻¹ ) showed an exponential behavior as a function of adsorbent mass for all clinoptilolite materials. The equilibrium adsorption data were best described by the Langmuir adsorption isotherms with highest q m = 5.974 mg/g for Nat-CLI, whereas K L parameter was found increased with increasing SAR. The adsorption capacities of the acid-modified clinoptilolite (high SAR) were lower than that of natural zeolite because of the dealumination of the zeolitic material and consequently the loss of the ion exchange sites.

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1944-3994/1944-3986 © 2019 Desalination Publications. All rights reserved.
Desalination and Water Treatment
www.deswater.com
doi: 10.5004/dwt.2019.23792
* Corresponding author.
150 (2019) 157–165
May
Relationship between Si/Al ratio and the sorption of Cd(II) by natural
and modied clinoptilolite-rich tu with sulfuric acid
Y. Abdellaouia, M.T. Olguinb, M. Abatalc,*, A. Bassama, G. Giácoman-Vallejoa
aFacultad de Ingeniería, Universidad Autónoma de Yucatán, Av. Industrias no Contaminantes por Periférico Norte,
Cordemex, 150 Mérida, YUC, México
bDepartamento de Química, Instituto Nacional de Investigaciones Nucleares, A.P. 18–1027, C.P. 11801, Ciudad de México, México
cFacultad de Ingeniería, Universidad Autónoma del Carmen, C.P. 24180, Ciudad del Carmen, Campeche, México,
email: mabatal@pampano.unacar.mx
Received 30 September 2018; Accepted 10 January 2019
abstract
In this paper, the differences in the adsorption mechanisms and adsorption capacities of clinoptilolite
materials with different Si/Al ratio (SAR) for Cd2+ ions from aqueous solutions are discussed. The
adsorbents were characterized with respect to their phase composition, morphology, specific
surface area, cation exchange capacity and point of zero charge. Batch adsorption experiments were
performed considering the Cd initial ion concentration, contact time, adsorbent dose, SAR, and pH.
The regression coefficient value revealed that the experimental data best fit to Pseudo-second-order
model, whilst the kinetic rate constant k2 (g/mg min–1) showed an exponential behavior as a function of
adsorbent mass for all clinoptilolite materials. The equilibrium adsorption data were best described by
the Langmuir adsorption isotherms with highest qm = 5.974 mg/g for Nat-CLI, whereas KL parameter
was found increased with increasing SAR. The adsorption capacities of the acid-modified clinoptilolite
(high SAR) were lower than that of natural zeolite because of the dealumination of the zeolitic material
and consequently the loss of the ion exchange sites.
Keywords: Cadmium; Sorption; Clinoptilolite; Sulfuric acid
1. Introduction
Cadmium is among the most toxic heavy metals to
plants, animals, and human beings in the environment, even
at very low concentrations [1]. In fact, it is classified by the
International Agency for Research on Cancer (IARC) and
the Environmental Protection Agency (EPA) as a priority
pollutant due to the high degree of toxicity and as a “known”
or “probable” human carcinogen [2]. While, the World Health
Organization (WHO) established a limit value of 3 μg/L for
Cd in drinking water [3]. This stringent limit of cadmium in
potable water is due to its severe toxicity to the human body,
and indeed, the accumulation of this metal in organisms
tends to cause numerous health diseases and disorders [4–6].
The wastewaters of heavy metals including cadmium are
generated by different activities, among them batteries man-
ufacturing, painting, and mining. Therefore, a necessary
treatment is required before the disposal of polluted effluent
into the ecosystem. Thus various techniques have been used
for the removal of heavy metals which include chemical
precipitation [7], chemical coagulation [8], electro-coagulation
treatment [9], bioflocculation [10], membrane technolo-
gies (reverse osmoses) [11], emulsion liquid membrane
[12], nanofiltration membranes [13], complexation-assisted
ultrafiltration [14], photocatalysis [15], and ion exchangers
as nylon 6,6 Zr(IV) phosphate, Ti(IV) iodovanadate and ace-
tonitrile stannic(IV) selenite composite [16–18]. In general,
these methods are expensive and insufficient particularly

Y. Abdellaoui et al. / Desalination and Water Treatment 150 (2019) 157–165158
when the toxic metals are present in the wastewater at low
concentrations.
Many investigations suggest that zeolites are among the
best adsorbents and ion-exchanger for the removal of cad-
mium according to their microporous structures made from
the interlinked tetrahedra of alumina (AlO4) and silica (SiO4)
moieties [19].
Clinoptilolite is the most abundant; it is commonly used
in environmental applications by its high affinity for some
heavy metals, especially for cadmium and lead elements
[20]. Thermal treatment, inorganic salts treatment, and acid
leaching are the most common modification methods of nat-
ural zeolites, which have a great influence on their practical
applications [21–23]. The treatment of natural clinoptilolite
with mineral acids, such as HCl and H2SO4, causes a destruc-
tion of impurities that block the pores and then creates an
extra-porosity modifying their morphology and chemical
compositions [24,25]. However, the effectiveness of acid
treatment depends on several factors including the chemical
composition, mineral purity and treatment condition [26].
The SiO2/Al2O3 ratio (SAR) is one of the important fac-
tors that influence on the performance of the zeolites. The
SAR indicates the amount of aluminum present in the zeolite
framework which introduces the creation of negative charges
in the zeolite structure. Thereunto, a low SAR zeolite pro-
vides more binding sites and extra framework cations in its
structure. Theoretically, low SAR of the zeolite has a high
sorption capacity of heavy metals compared with the high
SAR ones. Leinonen and Lehto studied the removal of Ni,
Zn, Cd, Cu, Cr, and Co from wastewater using some types of
zeolite with different SAR value [27]. The results showed that
the best uptake of heavy metals was achieved with the low
SAR zeolite. Furthermore, a previous study [28] reported that
Cd(II) adsorption capacity is highly dependent on the mineral
characteristic. However, to the best of our knowledge an inte-
grated research about the removal of Cd ions from aqueous
solution with different Si/Al ratio of clinoptilolite modified
by sulfuric acid has not been published elsewhere. Therefore,
the aim of this work was to describe the removal behavior
of Cd(II) from aqueous media by natural and acid-modified
clinoptilolite considering different parameters among them
Cd initial ion concentration, contact time, adsorbent dose and
pH. It is important to mention that the novelty of this paper
was to know about the influence of Si/Al ratio, on the sorp-
tion properties of the acid-modified zeolitic materials for the
sorption of Cd2+ from aqueous solutions.
2. Materials and methods
2.1. Materials
The natural zeolite used in the present work was col-
lected from “Villa de los Reyes” deposit located in the San
Luis Potosí State, Mexico. The samples were sieved to obtain
a grain size of 40 mesh, and washed with deionized water
several times to remove water-soluble impurities. The clinop-
tilolite samples were then dried for the overnight at 80°C
(labeled as Nat-CLI) and used to prepare the acid-form of
natural clinoptilolite with sulfuric acid. This modification
was carried on using 0.1, 0.2 ,0.5 ,and 1.0 M H2SO4 solutions
in contact with the zeolitic material at the ratio of 1:20 W/V
under reflux condition by 4 h. The samples were washed
with excess deionized water until the pH of the washing
solution reached approximately 6, and then dried overnight
at 80°C. The samples were labeled as follows: HCLI-0.1M,
HCLI-0.2M, HCLI-0.5M, and HCLI-1M.
2.2. Characterization methods
The clinoptilolite samples were characterized using XRD
APD 2000 PRO X-Ray diffractometer (35 Kv and 25 mA; angu-
lar scanning range 2°–60°, and angular speed 0.025 deg/s;
step time = 10 s). The obtained crystalline phases of the sam-
ples were identified by comparison with JCPDS cards. The
morphology and elemental composition were examined on
a SEM HITACHI S-3400N fitted with an electron dispersive
X100 ray (10 Kv and 30 pA; image magnification 1,500X
and the work distance of 10.5 mm). Infrared absorption
measurements were carried out using a Fourier transform
infrared (FTIR) spectrometer (Nicolet Nexus 670 FT-IR)
within a range from 4,000 to 400 cm–1 with a resolution of
4 cm–1 in a KBr water.
The pH of the point of zero charge (pHPZC) of natural
and modified clinoptilolite was determined by introducing
0.10 g of each adsorbent with 50 mL of 0.01 M NaCl adjusted
to different initial pH values (pH = 2, 4, 5, 6, 8, 10, and 12).
The suspensions were allowed to equilibrate for 24 h under
agitation at 25°C, decanted and the final pH values of each
remaining solution were measured using the pH-meter
Thermo Scientific (ORNION 3star pH Benchtop). The plot
of pHinitial vs. pHfinal was constructed which the intersection of
these curves determines the pHpzc.
2.3. Sorption
The experiment was performed using batch the technique
to determine the kinetics of the sorption of Cd(II) by Nat-CLI,
HCLI-0.1M, HCLI-0.2M, HCLI-0.5M, and HCLI-1M zeolitic
samples. For this purpose, 0.10 g of each adsorbent was
added to 10 mL of 10–100 mg L–1 of Cd(II) solutions at pH = 2.
The mixtures were placed in centrifuge tubes and shaken for
15, 30, 60, 120, 180, 240, 300, 360, 720, and 1,440 min. At the
end of the given contact time, the tubes were centrifuged at
4,500 rpm for 2 min, and adsorbent was removed by filtra-
tion, while the final Cd(II) concentration was determined by
atomic absorption spectroscopy (Thermo Scientific iCE 3000
Series) at λ of 213.9 nm. The amount of Cd(II) sorbed on natu-
ral and modified clinoptilolite was calculated using the mass
balance expression:
qCCV
W
t
=−
()
0 (1)
where q is the amount of Cd(II) sorbed in the natural and
modified zeolites (mg/g), V is the solution volume (mL), W is
the amount of sorbent (g), Co and Ct are the initial and final
metal concentration (mg L–1), at time t (min).
The effects of (i) Cd(II) initial concentration, (ii), initial
pH, (iii) adsorbent dosage and (iv) time, on the sorption
by clinoptilolite samples were also performed by a similar
procedure described above considering the experimental

159Y. Abdellaoui et al. / Desalination and Water Treatment 150 (2019) 157–165
conditions presented in Table 1. All the experiments were
conducted in duplicate to ensure reproducibility of the col-
lected data and the results are expressed as average values.
3. Results and discussions
3.1. Materials characterization
3.1.1. X-ray diffraction
The X-ray diffraction (XRD) results of natural and acid
treated clinoptilolite showed the presence of the crystalline
structure reported in the literature by main characteristic
peaks at 2θ = 9.85o, 11.19, 22.21o; 22.34o; 25.96o, and 28.09o in
accordance with the JCPDS card 25–1,349 [29]. Moreover,
slight decrease was observed in the relative intensity of the
diffractions peaks of modified clinoptilolite with the increase
of the concentration of sulfuric acid in the solution from
0.1 to 1 M. These results suggest the successful replacing of
the exchangeable cations by H+ ions with the treatment by
H2SO4 causing a decrease in the crystallinity and an increase
of the porosity. It is important to mention that the structural
changes could be associated also to the dealumination of the
natural zeolite after the acid treatment as will be discussed
later.
3.1.2. Scanning electron microscopy and energy-dispersive
X-ray spectroscopy
SEM images indicate that the surface of the natural and
acid-treated clinoptilolite shows a similar morphology and
typical tabular and coffin shapes of heulandite/clinoptilolite
crystals [30]. This result was also confirmed by XRD analysis.
The surface of natural and acid-modified clinoptilolite
was analyzed by Energy Dispersive Spectroscopy technique.
Chemical composition obtained by EDS indicated that the
treatment of the Nat-CLI with sulfuric acid promotes the
total elimination of Na+ and Mg2+ and notably decreases
the percentage of K+ and Ca2+ ions. The decrease of alumi-
num cations in the zeolite framework leads a decrease of
exchangeable cations (K+ and Ca2+) percentage and a total
elimination of Na+ and Mg2+ when acid concentration is 0.1
and 0.2M, respectively.
3.1.3. Infrared spectroscopy
The FTIR spectra of clinoptilolite samples showed a
broader band corresponding to symmetric and asymmetric
stretching vibration of O–H at 3,446 cm–1. This band became
more intensive and is broadened as the acid concentration
increased from 0.1 to 1.0 M. The bands related to the Si–O
and O–Si–O vibrations respectively at 1,079 and 790 cm–1
appeared more intensive after acid modification. The data
obtained shows a progressive extraction of aluminum atoms
from zeolite framework and consequently the formation of
silanol nests.
3.1.4. pH of the point of zero charge
The pHpzc for Nat-CLI was found at Ph = 6.00 ± 0.01,
while it was 3.00 ± 0.01 for HCLI-0.1M, HCLI-0.2M, HCLI-
0.5M, and HCLI-1M. These results can be explained by the
dealumination of the zeolitic material, which promotes the
decrease of negative charges, the increase of SAR, and con-
sequently diminishes the number of cations and the average
electrostatic field generating very strong Lewis acid site in
the surface.
3.2. Sorption process
3.2.1. Kinetics
The sorption equilibrium for Cd(II) at pH = 2 by clinopti-
lolite materials was studied with different SARs at a variable
concentration from 10–100 mg/L. Fig. 1 shows the results of
sorption capacity of Nat-CLI as a function of the time and the
initial concentration Cd(II).
It can be observed that the amount of Cd(II) uptake
by Nat-CLI increases with increasing of metal initial
concentration (10–100 mg/L). In addition, it can be noted that
the adsorption process consisted of two main reaction; initial
fast adsorption process within 180 min followed by a slow
continuous sorption reaction.
The rapid process can be explained by the presence of
large number of vacant active binding sites, and as time
increased, the accumulation of Cd(II) on the vacant sites
become limited and the access to vacant surface sites by
metal ions would be difficult due to repulsive effects.
Pseudo-first-order and Pseudo-second-order models
were applied to check the adsorption kinetics for Cd(II) on
Table 1
Parameters considered on the adsorption processes of Cd by
clinoptilolite samples
Parameters Time
(min)
Ci
(mg/L)
pHiAdsorbent
dosage (g)
Cd(II) initial
concentration (Ci)
1,440 10–500 2 0.1
Initial pH (pHi)1,440 100 2–10 0.1
Adsorbent dosage 15–360 100 2 0.1–1.0
Time 15–1,440 100 2 0.1
Fig. 1. Cd(II) sorption capacities (qe) by Nat-CLI as a function of
time (Ci = 10–100 mgCd/L).

Y. Abdellaoui et al. / Desalination and Water Treatment 150 (2019) 157–165160
natural and modified clinoptilolites. The linear forms of the
Pseudo-first-order and Pseudo-second-order equations are
respectively expressed by Eqs. (2) and (3),
ln lnqq
qk
t
et e
−
()
=−
1 (2)
t
qkq
t
q
tee
=+
1
2
2 (3)
where qe and qt (mg/g) are respectively the amounts of Cd(II)
adsorbed at the equilibrium and at a time t. k1 (min–1) and
k2 (mg–1 g min–1) are respectively the rate constants of the
pseudo-first and the Pseudo-second-order sorption.
The kinetic models mentioned earlier have been
considered by other researchers where they described the
sorption of heavy metals by synthetic ion exchangers [31–33].
The slopes and intercepts of these curves were used to
determine the values of k1 and k2, as well as the equilibrium
capacity (qe). The plot of the experimental data according
Eqs. (2) and (3) showed that the pseudo second-order kinetics
models gave considerably good fit to the data. The calculated
values of qe,cal from the pseudo-second-order kinetics model
first-order kinetics model was very close to the experimental
values (q,exp) (Table 2). The linearized pseudo-second-order
kinetics model, model provided much better R2 values
(0.994–0.999) than those for the first-order model (0.686–0.901).
Fig. 2 shows the effect of the acid modification of natural
clinoptilolite on the sorption capacity (qe). It can be observed
that the adsorption efficiency of the sorbents decrease
with the increase of the SAR. This can be attributed to low
accessibility of the adsorbent active site on the surface,
which could be attributed to the following reasons: (a) The
high competition between Cd2+ ions and H+ ions at low pH
(pH = 2), (b) the high hydration of Cd2+ ions, that makes
more difficult entering the clinoptilolite channels than the
small hydrogen ions (H+). An explanation on the basis of
surface charge could be added regarding the low pH of zero
charge recorded for the acid-modified materials with pH
value of 3, therein the surface is positively charged and then
decrease the electrostatic interaction between the zeolite and
cadmium ions; however, this point will be widely discussed
in upcoming part of the pH effect and principally, the effect
of the dealumination of each acid-modified natural zeolites
on the ion exchange capacity for Cd2+.
Furthermore, from the results obtained by the
pseudo-second-order kinetic model, it can be noted that the
rate constant (k2) decreases with the increase of Si/Al ratio
and this behavior is similar at Cd concentration from 10 to
100 mg/g (Fig. 3). This could be explained considering that at
Table 2
Kinetic parameters for the sorption of Cd(II) on the Nat-CLI, CLI-0.1M, CLI-0.2M, CLI-0.5M, and CLI-1.0
Zeolitic material Pseudo-second-order Pseudo-first-order
qe,cal (mg/g) qe,exp (mg/g) k2 (*10–2)R2qe,cal (mg/g) k1 (*10–2)R2
Nat-CLI 2.412 2.396 1.262 0.995 0.582 0.21 0.889
CLI-0.1M 2.083 2.025 1.174 0.999 0.723 0.22 0.735
CLI-0.2M 1.691 1.633 0.996 0.994 0.722 0.21 0.852
CLI-0.5M 1.675 1.591 0.962 0.998 0.728 0.20 0.686
CLI-1.0M 1.472 1.419 1.021 0.996 0.707 0.20 0.901
Fig. 2. Cd(II) sorption capacities (qe) by non- and acid-modified
natural zeolites with different SAR (Ci=10–100 mgCd/L).
Fig. 3. Second order rate constant (k2) as a function of SAR of
non- and acid-modified natural zeolite (Ci = 10–100 mgCd/L).

161Y. Abdellaoui et al. / Desalination and Water Treatment 150 (2019) 157–165
lower concentration, we have lower competition in the sorp-
tion surface sites. Whereas, at higher concentration, the com-
petition for the surface active sites is high and consequently
lower sorption rates are observed.
3.2.2. Mass effect
The adsorbent dosage is another important parameter,
which influences on the metal uptake from the solution.
The effect of sorbent dosages on the percentage removal of
Cd(II) is shown in Fig. 4. It can be clearly seen that the percent
removal of metal ions increases with increasing the amount
of clinoptilolite adsorbents from 0.1 to 1.0 g. This increment
in adsorption capacity is attributed to the availability of
larger surface area and larger number of adsorption sites. As
shown in Fig. 4, it can be noted that for Cd(II) (Ci = 100 mg/L)
ions, the removal uptake increased approximately from
15% with 0.1 g up to 52% with 1.0 g of the non-modfied and
acid-modified zeolitic materials, respectively. On the other
hand, the increase of SAR affected the removal efficiency of
Cd (II) ions; however, this effect could be clearly seen for the
high dosage of the adsorbent materials.
The results calculated by the pseudo-second-order model
shows that the variation of kinetic rate constant k2 (g/mg min)
as a function of the adsorbent mass presents an exponential
behavior for each SAR clinoptilolite materials. Furthermore,
strong dependence on SAR and the kinetic rate constant of
the removed metal ions are viewed. Thus, as shown in Fig. 5,
k2 of the sorbent with high SAR has shown the highest value.
3.2.3. pH Effect
The adsorption of metal ions from effluent as a function
of the initial pH of the solution is illustrated in Fig. 6. At pH
values greater than the pHpzc, the surface of the adsorbent
is negatively charged, favoring the adsorption of positively
charged metal ions, while at lower pH the surface is posi-
tively charged specially for the acidified clinoptilolite, their
surface area contain almost positive charge H+ which affects
the exchange ions capacity of the adsorbent in this case.
Fig. 4. Cd(II) removal from solutions as a function of SAR of
non- and acid-modified natural zeolites (dosage between 0.1
and 1.0 g). Fig. 6. Cd(II) removal from solutions as a function of initial pH
for non- and acid-modified natural zeolites with different SAR.
Fig. 5. k2 as a function of dosage of SAR of non- and acid-modified
natural zeolites.

Y. Abdellaoui et al. / Desalination and Water Treatment 150 (2019) 157–165162
Insignificant adsorption was therefore recorded at lower pH.
Furthermore, at low pH, the excess H+ ions in solution com-
pete with the metals for the active sites on the clinoptilolite,
leading to decreased metal uptake with the increase of acid
treatment intensity. As the pH increases, the number of H+
ions in solution decreases, thereby reducing the competition
with metal ions and leading to greater adsorption.
At pH > 4.0, the hydrolysis of hydrated Cd(II) complexes
started forming highly charged metal complexes (e.g. CdOH+
and Cd2OH3+ at pH values of 4.2 and 6.5, respectively) which
promote the adsorption capacity of clinoptilolite materials
because of small radius of Cd(II) in these complexes rather
than the hydrated ones [20]; in addition, after the pH reached
the value of 8, the metal precipitation started resulting an
increment of Cd(II) removal. The behavior of the materials
changes with the value of SAR. As a result, the sorption onto
clinoptilolite with high SAR increases with increasing pH
whereas the low SAR values of the sorption reached a pla-
teau at pH value of 6.
From Fig. 7 it can observed the influence of SAR on the
Cd(II) removal efficiency. At pH value lower than 4, the H+
should be considered as competitive ions in the exchange
process. When pH of the Cd solutions increases, the ion-ex-
change between cation-containing adsorbent materials (Na+
and Ca2+ for Nat-CLI and H+ for HCLI), and Cd2+is favored.
3.3. Isotherm
The equilibrium data can be evaluated using well known
adsorption isotherms providing the basis for the design of
adsorption systems. Langmuir and Freundlich equations are
the most widely used for modeling the experimental data
[31–33], which determine whether the sorption is of mono-
layer or multilayer nature, in order to predict the type of
adsorption mechanism involved.
Langmuir isotherm model assumes that the adsorption
takes place at specific homogeneous sites of the adsorbent.
The results of Cd(II) adsorption on the clinoptilolite materials
were analyzed using the Langmuir model represented by the
following equation:
11 1
qq KCq
em Lem
=+ (4)
where qe is the amount adsorbed at equilibrium (mg/g), Ce
is the equilibrium concentration (mg/L), KL is the Langmuir
constant related to the affinity of the binding site (L/mg) and
qm is the maximum amount of solute adsorbed (mg/g).
Freundlich isotherm assumes that the adsorbent is het-
erogeneous and that the adsorption is multilayered. It can be
expressed by the following equation:
ln ln lnqqK
n
C
eF e
=+
1
(5)
where KF (mg/g)(L/g)1/n is the adsorption coefficient, n an
empirical constant, qe is the amount adsorbed at equilibrium
and Ce is the equilibrium concentration (mg/L).
Fig. 8 shows the isotherms of Cd(II) for the zeolitic mate-
rials Nat-CLI, HCLI-0.1M, HCLI-0.2M, HCLI-0.5M, and
HCLI-1M. It was found that the experimental data were well
fitted to Langmuir model to describe the sorption mechanism
involved with the highest determination coefficient values
(R2 > 0.992, 0.994 and 0.990).
The theoretical parameters of Langmuir and Freundlich
models are listed in Table 3, the highest Cd(II) adsorp-
tion capacity was for Nat-CLI (with lowest SAR) where
the qm,Nat-CLI value is 2 and 2.5 times higher than HCLI-0.5M
and HCLI-1M values, respectively. The affinity of the bind-
ing site constant (KL) was found lowest for Nat-CLI, whilst
it increases with increasing SAR after acid modification,
this indicated the higher affinity of cadmium ions toward
acid-modified materials.
The maximum cadmium sorption capacity of Nat-CLI
is comparable with that observed by banana peels (around
5 mg/g) [34]. However, the synthetic ion exchangers present
capacities 50 times higher than that for the natural zeolite
considered in the present work, for example the curcumin
formaldehyde resin [35].
Fig. 7. Cd(II) removal as a function of SAR of non- and
acid-modified natural zeolites. Fig. 8. Cd(II)sorption isotherms for non- and acid-modified
zeolitic materials.

163Y. Abdellaoui et al. / Desalination and Water Treatment 150 (2019) 157–165
4. Proposal mechanism for cadmium sorption
The clinoptilolite framework has two-dimensional sys-
tem formed by 10-ring and 8-ring channels parallel to [001]
(called channels A and B, respectively) which are intercon-
nected to 8-ring channels parallel to [100] (channel C) [36,37].
The Ca2+ and Na+ occupy two sites in the framework, M1
and M2 in channels A and B, respectively. There are two
sites, M3 occupied by K+ in channel C, and M4 in channel
A occupied by Mg2+ [38]. M3 is in a more confined position
within clinoptilolite structure. Therefore, K+ is less probable
to be exchanged [37]. According to the zeolite framework
characteristics and the previously performed elemental
composition (Table 2), a decrement of the Na+, Mg2+, K+ and
Ca2+ content from clinoptilolite after the modification with
sulfuric acid is observed and consequently the zeolite par-
tially collapsed diminishing the ion exchange sites, into the
clinoptilolite structure. Furthermore, the maximum capacity
of the acid modified zeolitic materials for Cd2+ diminished as
well with base on these results. It is reasonable to propose
that the M1 and M2 sites are the most probable positions for
Cd2+ in clinoptilolite. Garcia-Basabe et al. [39] pointed that
the extra-framework cations in channels A and B are the most
exchangeable species for this zeolite type.
5. Sensitivity analysis
A sensitivity analysis was conducted in order to quan-
tify the relative influence of each parameter studied on the
behavior of removal efficiency. In the current work, the
ReliefF [40] method was used to carry out the analysis; it
detects the conditional dependencies between attributes and
provides a unified view of the relevancy based on proba-
bility and information theories [41]. This method assigned
an importance weight value to each parameter, where the
parameter with higher weight indicates a greater influence.
In order to adequately represent all possible interactions that
affect the removal efficiency, the analysis considered all the
experimental results reported in the previous sections.
Fig. 9 illustrates the results in percentage form of this
analysis for the different SAR values. According to this figure,
the pH, adsorbent dosage, and contact time are the parame-
ters with the greatest influence in most cases (SAR of 4.37,
5.10, 9.04, and 9.50). The pH is the parameter with the greatest
impact on the removal efficiency for the four values of SAR
evaluated. It can be appreciated that as SAR increases the
influence of pH on the removal capacity acquires greater rel-
evance (55.34%, 53.76%, 61.33%, 65.45%, and 70.95% at SAR
of 4.37, 5.10, 7.90, 9.04, and 9.50, respectively) until reaching
70.95% at a SAR value of 9.5. The results also indicate that
the contact time is the second parameter with the greatest
influence to a SAR of 4.37 (25.02%), followed by the adsorbent
dosage (18.65%). Nevertheless, for SAR of 5.10 (t = 10.16%
and m = 26.03%), 9.04 (t = 15.35% and m = 16.43%), and 9.5
(t = 13.15% and m = 14.84%) the relevance impact of sorbent
dosage on the adsorption capacity of Cd increases with
respect to contact time, reaching similar and almost stable
relevance in the highest SAR. Finally, the initial concentration
was the least relevant parameter for SAR of 4.37, 5.1, 9.04, and
9.5 with values of 1%, 10%, 2.7%, and 1.05%, respectively;
where according to Fig. 9 as the SAR approaches to 7.09, the
influence of the initial concentration increases.
6. Conclusions
The acid treatment of the natural zeolite (clinoptilolite-rich
tuff) causes structural changes of the natural clinoptilolite,
whereas, promotes the release of the aluminum from the
zeolitic network increasing the SAR. The pHPZC is similar for
all the acid-modified materials (pHPZC = 3), while the pHPZC of
the Nat-CLI has a value slightly acid.
The pseudo-second-order kinetic model best describes
the sorption behavior and the k2 values depend on the ini-
tial concentration of the Cd(II) and SAR of the clinoptilolite
materials.
The experimental data of the Cd(II) sorption isotherms
well fit to the Langmuir model, the maximum adsorption
capacity (qm) for the Nat-CLI is higher 2 and 2.5 times than
Table 3
Parameters obtained from Langmuir and Freundlich models that describe the Cd(II) sorption by non- and acid-modified natural
zeolites
Adsorbents Langmuir Freundlich
qm
(mg/g) KL
(L/mg) R2KF (mg/g) 1/n R2
Nat-CLI 5.974 0.66 × 10–2 0.996 0.129 1.710 0.923
HCLI-0.1M 4.907 0.68 × 10–2 0.995 0.122 1.802 0.884
HCLI-0.2M 3.128 0.93 × 10–2 0.992 0.082 1.662 0.961
HCLI-0.5M 2.590 1.11 × 10–2 0.988 0.090 1.791 0.953
HCLI-1M 2.391 1.16 × 10–2 0.997 0.100 1.948 0.908
Fig. 9. Sensitivity analysis results for each of the SAR evaluated.

Y. Abdellaoui et al. / Desalination and Water Treatment 150 (2019) 157–165164
HCLI-0.5M and HCLI-1M. The KL parameter increases with
increasing SAR after acid modification.
The increase of adsorbent dosage increases the Cd(II)
uptake from 15% with 0.1 g up to 52% with 1.0 g for Nat-
CLI. However, these adsorption values decrease significantly
after acid modification from 10% with 0.1 g up to 21% with
1.0 g for HCLI-1.0M.
The sorption of Cd(II) by non- and acid-modified natural
zeolites is carried out preferentially by ion exchange mecha-
nism. The natural clinoptilolite shows the highest adsorption
capacity owing to the high mobility of exchangeable cations
(Na+, Ca2+) presents in its framework.
Acknowledgment
The authors gratefully acknowledge the financial support
of the National Council of Science and Technology of Mexico
(Projects 254665).
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