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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
doi: 10.5004/dwt.2019.23792
* Corresponding author.
150 (2019) 157–165
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,
Received 30 September 2018; Accepted 10 January 2019
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
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:
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
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
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
et e
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
eF e
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
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
Adsorbents Langmuir Freundlich
(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.
The authors gratefully acknowledge the financial support
of the National Council of Science and Technology of Mexico
(Projects 254665).
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... Hence, various treatments have been used to remove Cd 2+ from contaminated waters, including membrane ltration, 4 chemical electrodeposition, 5 precipitation, 6 ion exchange and adsorption. 7,8 Adsorption, which has gained a great deal of interest, is preferred because of its easy operation, high efficiency, fast pollutant removal rate, eco-friendly nature, low cost, and reversibility. [9][10][11][12][13][14] The commonly reported adsorbents materials are activated carbon, 15,16 zeolite, 17,18 clay, 19 apatite. ...
... Isotherm models are frequently applied in batch experiments to determine the most suitable adsorption equilibrium behavior, and to evaluate the adsorbent performance under different experimental conditions. In this study, three isotherm models, namely Langmuir (eqn (7)), Freundlich (eqn (8)) and Temkin (eqn (9)) were tted to the experimental data and the isotherm parameters obtained from these equations are presented in Table 2. ...
Full-text available
In search for more effective and eco-friendly adsorbent materials, this study comprehensively investigated Cd2+ adsorption onto phosphorylated cellulose paper (PCP). For this, cellulose microfibers (CMF) was extracted from Alfa fibers and phosphorylated using the solid-state phosphorylation approach. Then, the prepared PCP samples were characterized by SEM, EDX, XRD, FTIR, TGA, conductometric titration and zeta potential measurement. The adsorption of cadmium ions, the effect of time, pH and Cd2+ initial concentration were systematically studied in batch experiments. Based on the results, the highest adsorption capacity achieved was 479 mg of Cd2+ per g of PCP, which was remarkable compared to other modified cellulose capacities cited in the literature. Furthermore, the Cd2+ removal mechanism was investigated based on characterization results before and after adsorption and also based on the kinetics results. It was concluded that cation exchange and electrostatic attraction between phosphorylated cellulose and the cadmium ion mainly dominated the adsorption process. These findings highlighted that the phosphorylated cellulose paper has a broad application prospect in removal of divalent metal from aquatic solution.
... Previous studies have documented some modifications enhanced the adsorption performance of zeolite. Abdellaoui et al. [24] indicated that the removal rate of Cd was improved from 13.13 to 35.65% via the application of sulfuric acid-modified zeolite. Wajima et al. [25] also reported that the addition of diatomite to NaOH solution increased the Si content to synthesize zeolites, and the ion exchange capacity was increased from 50 to 130 cmol/kg, performed at a high cation exchange capacity. ...
Full-text available
Water cadmium (Cd) pollution has widely aroused concerns due to high Cd toxicity in water bodies and its serious health risks to humans. Adsorption has been identified as an effective and widely utilized technology for water purification with heavy metal pollution. To develop a newly identified adsorbent of modified zeolite that can easily and effectively purify Cd-polluted water, NaOH modification (JZ), high-temperature modification (HZ), humic acid modification (FZ), Na2S modification (SZ), and ultrasonic modification (CZ) zeolites were developed, and their appearances and adsorption and desorption characteristics were investigated. The results showed that the adsorption capacity of Cd by JZ and SZ were improved by 68.87% and 32.06%, respectively, relative to that by natural zeolite (NZ); however, HZ, FZ, and CZ decreased the adsorption capacity. JZ had a higher adsorption capacity than SZ and could remove 99.90% Cd at an initial concentration of 100 mg/L. The dominant adsorption mechanism of Cd by JZ was the chemisorption of the monolayer. The preferred temperature and pH that enhanced Cd adsorption by JZ were 25–35 °C and 4–8, respectively. With an equilibrium adsorption capacity of 9.37–9.74 mg/g at an initial concentration of 280 mg/L, JZ reached its maximum saturated adsorption capacity; compared with SZ and NZ, the adsorption capacity increase was 27.83%–68.81%. The R2 fitted by JZ's Langmuir model and quasi-second-order dynamics model were both above 0.93. In summary, JZ was recognized as a novel absorbent for Cd-polluted water purification.
... The challenge of removing contaminants, especially heavy metals, from wastewater has become a fundamental concern for the environment; as industrial activities grow, it is becoming more critical [1][2][3]. For decades, heavy metals have endangered human health because of their toxicity even at low concentrations, non-biodegradability, accumulation in the food chain and organisms, and carcinogenicity [4,5]. Nickel represents the most common contaminant found in contaminated surface water and industrial wastewater from mineral processing, electroplating, steel production and battery manufacturing [6,7]. ...
This paper thoroughly investigated the adsorption mechanism of nickel ions (Ni²⁺) on fluorapatite-based natural phosphate (AFap) by Rietveld refinement combined with Monte Carlo simulations. The experimental results indicated that the cation exchange was the predominant mechanism in removing Ni²⁺ ions and the Pseudo first order described better the kinetic data suggesting that the adsorption rate depended on the concentration. The application of Rietveld structure refinement confirmed the preferable cation-exchange sites for Ni²⁺ in the AFap structure at low and high metal concentrations. Accordingly, Ni²⁺ occupancies within the AFap structure were estimated, and a new chemical formula was proposed. Additionally, Monte Carlo simulations gave more insights into the pH effect on the Ni²⁺ uptake behavior onto AFap surface through the energetic analysis and system configuration. The results highlight the noticeable ability of the phosphate surface to adsorb Ni²⁺ in the near-neutral environment compared to the acidic one.
... In recent years, several technologies have been applied to treat wastewater effluent including, ion-exchange, distillation, reverse osmosis, coagulation, oxidation, solvent extraction, electrolysis, adsorption, etc. [15][16][17][18][19][20]. The cost of water treatment for these methods varies between US$ 10-450 per cubic meter of treated water, apart from adsorption [21]. ...
Enormous interest in using marine biomass as a sustainable resource for water treatment has been manifested over the past few decades. Herein, the objective was to investigate the possible use of green macroalgae (Codium tomentosum) for cellulose-based foam production through a versatile and convenient process. Macroporous cellulose monolith was prepared from cellulose hydrogel using freeze-drying process, resulting in a mechanically rigid monolith with a high swelling ratio. The as-produced spongy-like porous cellulosic material was used as bio-sorbent for wastewater treatment, particularly for removing methylene blue (MB) dye from concentrated aqueous solution. The adsorption capacity of MB was subsequently studied, and the effect of adsorption process parameters was determined in a controlled batch system. From the kinetic studies, it was found that the adsorption equilibrium was reached within 660 min. Furthermore, the analysis of the adsorption kinetics reveals that the data could be fitted by a pseudo-second order model, while the adsorption isotherm could be described by Langmuir isotherm model. The maximum adsorption capacity was found to be 454 mg/g. The findings suggested that the produced cellulose monolith could be used as a sustainable adsorbent for water treatment.
... Many researchers used different clinoptilolite modified forms in order to enhance its sorption properties for cadmium uptake: incorporation of magnetite nanoparticles and surface modification with cysteine [28]; sodium-and acid-modified forms [29]; acidmodified form [30]; Na-modified synthetic clinoptilolite [31]; pentetic acid-clinoptilolite nanoparticles adsorbent [32]; sonochemically modified clinoptilolite [33]. ...
Full-text available
Cadmium exchange on clinoptilolite is performed and structurally studied for different durations of the ion exchange process (2 h, 24 h, 72 h, 168 h, 12 days, 22 days) at room temperature and 90 °C. The distribution of Cd2+ ions in all samples is elucidated after exchange on clinoptilolite using powder XRD data processed by Rietveld structural software. Clinoptilolite is not selective for cadmium cations, but at 90 °C the exchange is ~2.5 cations per unit cell. At RT it reaches ~1.25 cations per unit cell being twice as low. The obtained maximum exchanged sample for 22 days 90 °C was structurally refined in order to find the cadmium positions in the clinoptilolite voids. The structural refinements of the occupations of the incoming and outgoing cations give an idea of how the intracrystalline diffusion is processed. A good correlation between results obtained by structural refinement of the Cd-exchanged samples and the data of the EDS measurements was achieved.
... Magnetization with iron ions led to a decrease in crystallinity and an increase in porosity. It might be resulted from the partial removal of the exchangeable cations on structure of the natural zeolite [35]. Sharp peaks and negligible changes after magnetic modification indicated that the adsorbents possess a high degree of crystallinity and structural stability. ...
A new magnetic zeolite was prepared by the chemical co-precipitation of Fe²⁺ and Fe³⁺ in the presence of natural Manisa- Gördes clinoptilolite. The adsorption of Basic Blue 41 (BB41) from aqueous solutions on natural and magnetically modified zeolite were studied at 298-323 K in a batch system. Natural and magnetic zeolites were characterized by N2 adsorption-desorption, XRD, FTIR, SEM-EDX and VSM analyses. Compared to natural zeolite, SBET-specific surface area of magnetic zeolite increased by modification process. XRD pattern and FTIR spectra of magnetic zeolite showed the characteristic Fe3O4 peaks. Optimum parameters were determined based on the experimental data by investigating the various parameters such as pH, initial dye concentration, adsorbent dosage, adsorbent particle size, contact time, stirring speed and temperature. Adsorption isotherms and kinetics were analyzed using Langmuir, Freundlich, Dubinin-Radushkevich (D-R), Temkin, Pseudo first order and Pseudo second order models. Adsorption of BB41 on natural and magnetic zeolites well fitted to Langmuir isotherm and pseudo second order kinetic models. The maximum adsorption capacities of natural and magnetic zeolites were determined as 149.25 mg/g and 370.37 mg/g at 323 K, respectively. Thermodynamic analysis revealed a spontaneous and endothermic nature of natural and magnetic zeolites.
... In recent years, environmental degradation caused by industrialization and the use of toxic chemicals became a serious global issue [1]. This degradation manifests in many forms, such as pollution and especially the contamination of wastewater with heavy metals like arsenic (As), cadmium (Cd), copper (Cu), etc. that are of major concern [2,3]. However, copper is one of the most valuable and prevalent metals used in the industry and is usually found at different concentrations in wastewater. ...
Water pollution with copper (Cu) has a significant impact on the environment and human health. Withinthis context, the present study aims to synthesize a composite bead based on Moroccan natural clay fromChaouia (CH) and sodium alginate (Na-AL) as adsorbents to remove copper ions from aqueous solution.The adsorbents were characterized using X-Ray Diffraction (XRD), Scanning Electron Microscopy coupledwith Energy Dispersive X-ray (SEM-EDX) and Thermogravimetric Analysis (TGA). The effect of adsorbentdose, kinetic time, initial concentration of copper ions and temperature were investigated. Nonlinearregression was applied to fit the isothermal and kinetic models. The results showed that the clay is com-posed of kaolinite and quartz and the SEM confirmed the successful incorporation of sodium alginate intothe clay. The adsorption process follows the Langmuir isotherm with a maximum adsorption capacity of48.24 and 92.44 mg.g�1for the clay (CH) and the beads composite designated as (CH@AL), respectively.Pseudo-second-order model best described the kinetic data. Moreover, the thermodynamics of theadsorption was spontaneous and endothermic. All of these results proved that the used adsorbents areeco-friendly and efficient for the adsorption of Cu(II) ions.
Novel shellfish waste-derived chitosan (CS) has been developed to adsorb As(V) from simulated wastewater under evaluating adsorption process parameters. The coexistence of some competing ions, like SiO3²⁻, Cl⁻, NO3⁻ and PO4³⁻ as well as the regeneration capacity of the spent adsorbent, was explored. The experimental data were modeled using several kinetics and isotherm models to understand the mechanism related to the uptake process. As(V) uptake was relatively rapid and highly dependent on pH. The Avrami-fractional-order expression supported data best, while the Liu equation described well isotherm data at pH 5.0. The maximum uptake capability (Liu) was 12.32 mg/g, and the highest removal performance (99%) was obtained at optimum pH 5.0. Molecular dynamics simulations were performed to more clearly illuminate the atomic-level interactions between arsenic species and CS surface in both acidic and basic mediums. After four adsorption-desorption cycles, CS exhibited more than 90% As(V) removal efficiency.
Iron oxides and ferrite-supported zeolites have been successfully used in the cheap and environmentally friendly removal of organic pollutants such as antibiotics. In this study, the adsorption of Oxytetracycline hydrochloride (OTC-HCL) on zeolite/Fe3O4 particles synthesized from natural Manisa-Gördes clinoptilolite by co-precipitation method was investigated in the batch system at 298–323 K. Various parameters such as pH, initial antibiotic concentration, adsorbent dosage, contact time, stirring speed and temperature were examined and optimum parameters were determined according to experimental data. Adsorption isotherms were investigated with Langmuir, Freundlich, Dubinin-Radushkevich (D-R), Temkin models. Kinetic constants were determined according to pseudo first order, pseudo second order, intraparticle diffusion and Elovich models. OTC-HCL adsorption on zeolite/Fe3O4 best fitted to the Langmuir isotherm and pseudo second order kinetic models. The maximum adsorption capacity of zeolite/Fe3O4 was determined as 83.33 mg/g at 323 K. Adsorption of OTC-HCL on zeolite/Fe3O4 occurred spontaneously and endothermic. Physicochemical characterization of zeolite/Fe3O4 was performed before and after adsorption, by N2 adsorption-desorption, XRD, FTIR, SEM-EDX, XPS, TEM analyses. Magnetic properties of zeolite/Fe3O4 were determined by VSM analysis. The BET specific surface area of zeolite/Fe3O4 decreased after adsorption of OTC-HCL. VSM and SEM-EDX results showed that zeolite/Fe3O4 had superparamagnetic property and OTC-HCL adsorbed on zeolite/Fe3O4 successfully.
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Mechanisms of Cr(VI) reduction by Fe(II) modifed zeolite (clinoptilolite/mordenite) and vermiculite were evaluated. Adsorbents were treated with Fe(SO4)·7H2O to saturate their exchange sites with Fe(II). However, this treatment decreased their CEC and pHPZC, probably due to the dealumination process. Vermiculite (V-Fe) adsorbed more Fe(II) (21.8 mg g−1) than zeolite (Z-Fe) (15.1 mg g−1). Z-Fe and V-Fe were used to remove Cr(VI) from solution in a batch test to evaluate the efect of contact time and the initial concentration of Cr(VI). The Cr(VI) was 100% reduced to Cr(III) by Z-Fe and V-Fe in solution at 18 mg L−1 Cr(VI) after 1 min. Considering that 3 mol of Fe(II) are required to reduce 1 mol of Cr(VI) (3Fe+2+ Cr+6→ 3Fe+3+ Cr+3), the iron content released from Z-Fe and V-Fe was sufcient to reduce 100% of the Cr(VI) in solutions up to 46.8 mg L−1 Cr(VI) and about 90% (V-Fe) and 95% (Z-Fe) at 95.3 mg L−1 Cr(VI). The Fe(II), Cr(III), Cr(VI), and K+ contents of the adsorbents and solutions after the batch tests indicated that the K+ ions from the K2Cr2O7 solution were the main cation adsorbed by Z-Fe, while vermiculite did not absorb any of these cations. The H+ of the acidic solution (pH around 5) may have been adsorbed by V-Fe. The release of Fe(II) from Z-Fe and V-Fe involved cation exchange between K+ and H+ ions from solution, respectively. The reduction of Cr(VI) by Fe(II) resulted in the precipitation of Cr(III) and Fe(III) and a decrease in the pH of the solution to<5. As acidity limits the precipitation of Cr(III) ions, they remained in solution and were not adsorbed by either adsorbent (since they prefer to adsorb K+ and H+). To avoid oxidation, Cr(III) can be removed by precipitation or the adsorption by untreated minerals.
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Sorption of Cd²⁺, Ni²⁺ and Zn²⁺ ions on natural (ZPCli), sodium modified (ZPCliNa) and acid modified (ZPCliH) zeolites have been investigated in function of the contact time, pH, and metal concentration by the batch technique. The characterization of ZPCIi, ZPCliNa, and ZPCliH materials was performed using X-ray powder diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques. The surface area (BET) and the pHpzc were also determined. The pH in the point of the zero charge of ZPCli, ZPCliNa, and ZPCliH zeolites was 8.25, 8.00, and 2.05, respectively. The kinetic sorption data for ZPCli, ZPCliNa and ZPCliH were well fitted to the pseudo-second order model (R² > 0.99). The linear model described the Cd, Ni and Zn sorption isotherms for ZPCliH, while for ZPCliNa and ZPCliH it was the Freundlich model. The unmodified and modified zeolitic materials showed the highest sorption capacity for Cd²⁺, lower for Zn²⁺ and Ni²⁺.
Full Article Available Here: In this study, natural clinoptilolite was modified by cation exchange using alkali metal cations (Li+, Na+, K+, Rb+, Cs+), alkaline earth metal cations (Be2+, Mg2+, Ca2+, Sr2+, Ba2+ ), transition metal cations (Fe3+, Ni2+, Cu2+, Zn2+, Ce3+ ), and acid treatment (H+). The composition and structural properties of the modified samples were assessed using EDS and XRD analysis, and compared to the original clinoptilolite sample. Adsorption isotherms for CO2, N2, and CH4 gases were measured using a microgravimetric adsorption analyser. For all of the clinoptilolite samples, the adsorption equilibria and the diffusion rates were measured at 303K over a pressure range from 0 to 8atm. The results of this screening analysis show that the cation exchanged clinoptilolites exhibit a wide range of adsorption characteristics, which make them suitable for a variety of gas separation applications by pressure swing adsorption. The high adsorption selectivity towards CO2 and CH4 found with the Cs+ exchanged clinoptilolite makes it advantageous for possible CH4/N2 and CO2/N2 equilibrium separations. Reduced CH4 equilibrium capacity resulting from pore blocking in the Ca2+ exchanged clinoptilolite leads to high selectivity for CO2/CH4 and N2/CH4 selective adsorbent. However, the potential of this material is limited due to increased microporous diffusion resistance. While both Li+ and Ni2+ clinoptilolites exhibit low CH4/N2 ideal selectivity, they present high N2/CH4 kinetic selectivity, thereby making them suitable for possible N2/CH4 kinetic separations.
The combination of magnetic nanoparticles and metal–organic frameworks (MOFs) has demonstrated their prospective for pollutant sequestration. In this work, a magnetic metal–organic framework nanocomposite (Fe3O4@AMCA-MIL53(Al) was prepared and used for the removal of U(VI) and Th(IV) metal ions from aqueous environment. Fe3O4@AMCA-MIL53(Al) nanocomposite was characterized by TGA, FTIR, SEM-EDX, XRD, HRTEM, BET, VSM (vibrating sample magnetometry), and XPS analyses. A batch technique was applied for the removal of the aforesaid metal ions using Fe3O4@AMCA-MIL53(Al) at different operating parameters. The isotherm and kinetic data were accurately described by the Langmuir and pseudo-second-order models. The adsorption capacity was calculated to be 227.3 and 285.7 mg/g for U(VI) and Th(IV), respectively, by fitting the equilibrium data to the Langmuir model. The kinetic studies demonstrated that the equilibrium time was 90 min for each metal ion. Various thermodynamic parameters were evaluated which indicated the endothermic and spontaneous nature of adsorption. The collected outcomes showed that Fe3O4@AMCA-MIL53(Al) was a good material for the exclusion of these metal ions from aqueous medium. The adsorbed metals were easily recovered by desorption in 0.01 M HCl. The excellent adsorption capacity and the response to the magnetic field made this novel material an auspicious candidate for environmental remediation technologies.
Chloride-induced damage of coastal concrete structure leads to serious structural deterioration. Thus, chloride content in concrete is a crucial parameter for determining the corrosion state. This study aims at establishing machine learning models for chloride diffusion prediction with the utilizations of the Multi-Gene Genetic Programming (MGGP) and Multivariate Adaptive Regression Splines (MARS). MGGP and MARS are well-established methods to construct predictive modeling equations from experimental data. These modeling equations can be used to express the relationship between the chloride ion diffusion in concrete and its influencing factors. Moreover, a data set, which contains 132 cement mortar specimens, has been collected for this study to train and verify the machine learning approaches. The prediction results of MGGP and MARS are compared with those of the Artificial Neural Network and Least Squares Support Vector Regression. Notably, MARS demonstrates the best prediction performance with the Root Mean Squared Error (RMSE) = 0.70 and the coefficient of determination (R2) = 0.91.
Nickel ferrite bearing nitrogen-doped mesoporous carbon (NiFe2O4-NC) was prepared using polymer bimetal complexes and used for the removal of Hg²⁺ from aqueous medium. The nanocomposite was characterized using several analytical techniques such as SEM, TEM, FTIR, Raman, TGA/DTA, XRD, VSM, XPS and BET. The adsorption behavior of NiFe2O4-NC nanocomposites was investigated via adsorption kinetics, isotherms and thermodynamic. The adsorption isotherm could be well described with Langmuir model, with the maximum adsorption capacity of 476.2 mg g⁻¹ at 25 °C. The desorption results showed the best recovery of Hg²⁺ metal ion using 0.01 M HCl. The remarkable adsorption properties are mainly attributed to the synergetic chemical coupling effects between NiFe2O4 nanoparticles and nitrogen doped graphitized carbon. The presented cost-effective strategy is developed to prepared NiFe2O4 nanocrystals embedded in nitrogen-doped graphitized carbon matrix using a single source precursor offers prospects in developing highly effective magnetic adsorbent for removal of toxic pollutant form contaminated water.
Considering the occurrence of nitrite in drinking water sources, this paper investigated the modification of natural zeolite by acid, base, salt, organic surfactant, calcination and ultrasonication for eliminating nitrite from aqueous solutions. The structural and surface properties were analysed by X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and nitrogen adsorption-desorption isotherm, and were used to explain the nitrite adsorption capacities of the zeolites. The results show that except acid modification, the rest measures did not impose notable influences on the zeolite properties. Modification of zeolite by acids markedly increased the specific surface area and strongly protonated the material, which greatly favoured the nitrite adsorption onto the resulting zeolite. The adsorption isotherms were in good agreement with Langmuir-Freundlich models, and indicated an endothermic process. At 25 °C, the nitrite adsorption capacity of the zeolite modified by 0.75 mol L⁻¹ of H2SO4 for 12 h (ACMZ) was 54.5 mg N g⁻¹, being over 7-fold higher than that by the raw zeolite. The adsorption of nitrite onto ACMZ followed pseudo-first order kinetics. These facts suggest that the uptake of nitrite by acid modified zeolite was a chemical adsorption in nature.