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Impact of environmental conditions on the removal of Ni(II)from aqueous solution to bentonite/iron oxide magneticcomposites
Liang Chen • Shaoming Yu • Lingli Huang •
Gang Wang
Received: 29 November 2011 / Published online: 29 February 2012
� Akademiai Kiado, Budapest, Hungary 2012
Abstract The sorption of radionuclide 63Ni(II) on benton-
ite/iron oxide magnetic composites was investigated by batch
technique under ambient conditions. The effect of contact
time, solid content, pH, coexistent electrolyte ions, fulvic
acid, and temperature on Ni(II) sorption to bentonite/iron
oxide magnetic composites was examined. The results dem-
onstrated that the sorption of Ni(II) was strongly dependent on
pH and ionic strength at pH\8.0, and was independent of pH
and ionic strength at high pH values. The sorption of Ni(II)
was dominated by outer-sphere surface complexation or ion
exchange at low pH, whereas inner-sphere surface complex-
ation was the main sorption mechanism at high pH. The
experimental data were well fitted by Langmuir model. The
thermodynamic parameters (DG�, DS�, DH�) calculated from
the temperature-dependent sorption isotherms indicated that
the sorption of Ni(II) on bentonite/iron oxide magnetic com-
posites was an endothermic and spontaneous processes. The
results show that bentonite/iron oxide magnetic composites
are promising magnetic materials for the preconcentration
and separation of radionickel from aqueous solutions in
environmental pollution.
Keywords Bentonite/iron oxide magnetic composites �Magnetic separation � Nickel
Introduction
The increasing levels of toxic heavy metals or radionuc-
lides, which were discharged into the environment as
industrial wastes, pose a serious threat to public health,
living resources and ecological systems. Among the
potential pollutants, nickel is a non-essential and highly
toxic heavy metal ion that is released into the environ-
ment from metallurgical, pesticides, electrolysis, electro-
plating, nuclear power plant and mining operations [1].63Ni (T1/2 = 96 a) is an important product of the neutron
activation of the reactor materials, which is also widely
used in research and medical applications. According to
the WHO drinking-water quality standards, the permissi-
ble limits of nickel in the drinking water is 0.02 mg/L.
Above the permissible limit, it can cause nickel poisoning
and produce adverse effects such as anemia, diarrhea,
encephalopathy, hepatitis and the dysfunction of central
nervous system [2]. Up to now, various technologies such
as electrodialysis, chemical precipitation, ion exchange,
reverse osmosis, solvent extraction, coagulation and
sorption are used for removing metal ions from aqueous
solutions [3–5]. Although all these techniques afford
moderate to efficient metal removal, sorption is consid-
ered one of the best techniques due to its sludge free
clean operation, economical, availability of wide range of
adsorbents and complete removal of metals from waste-
waters. Sorption of Ni(II) on different materials has been
studied extensively by using batch [6–8], X-ray photo-
electron spectroscopy (XPS) [9, 10] and extended X-ray
adsorption fine structure [11–13] techniques, and the
results indicate that the sorption of Ni(II) is strongly
dependent on pH and ionic strength at low pH and
independent of ionic strength at high pH [14–17]. How-
ever, the sorption of Ni(II) on bentonite/iron oxide mag-
netic composites is still not available.
Bentonite has attracted great interest in environmental
pollution treatment due to its outstanding properties such
as low-cost, high swelling ability and cation exchange
L. Chen � S. Yu (&) � L. Huang � G. Wang
School of Chemical Engineering, Hefei University
of Technology, Hefei 230009, Anhui,
People’s Republic of China
e-mail: [email protected]
123
J Radioanal Nucl Chem (2012) 292:1181–1191
DOI 10.1007/s10967-012-1687-0
capacity. As is well known, bentonite is essentially com-
posed of montmorillonite, is characterized by one alumi-
num octahedral sheet placed between two silicon
tetrahedral sheets [18, 19]. Compared with other clays, it
has excellent retention ability and dominates sorption sites
available within its interlayer space as well as on the outer
surface and edges. However, it is difficult to separate ben-
tonite from aqueous solutions after the sorption is carried
out because of its small size. The application of magnetic
particle technology to solve environmental problems has
received considerable attention in recent years [20]. Mag-
netic particles can be used to adsorb contaminants from
aqueous solution and the adsorbent can be easily separated
from the medium and recovered by a simple magnetic force.
Some examples of this technology have been used for the
removal of oil [21], dyes [22], and heavy metals [23–25].
So far, few reports are available on the fabrication of
bentonite/iron oxide magnetic composites and their appli-
cation for nickel removal, especially the detailed study on
the effect of pH, ionic strength, coexistent electrolyte ions
and temperature. The basic objectives of this work are: (1)
to investigate the effect of contact time, adsorbent content,
pH, ionic strength, and fulvic acid (FA) on Ni(II) sorption
on bentonite/iron oxide magnetic composites; (2) to mea-
sure the sorption isotherms of Ni(II) on bentonite/iron
oxide magnetic composites at different temperatures and to
calculate the thermodynamic parameters; and (3) to pre-
sume the sorption mechanism of Ni(II) on bentonite/iron
oxide magnetic composites.
Experimental
Materials
The sample of bentonite was obtained from Huangshan
county (Anhui, China). The stock solution of nickel was
prepared by dissolving Ni(NO3)2�6H2O in Milli-Q water
and then diluted to 60 mg/L. Radiotracer 63Ni–NiCl was
achieved from China Isotope Corporation. FA was
extracted from the soil sample of Hua-Jia county of Gansu
province and had been characterized in detail [26, 27]. All
chemicals used in the experiments were purchased in
analytical purity and used in the experiments without any
further purification. All the reagents were prepared with
Milli-Q water.
Synthesis and characterization of bentonite/iron oxide
magnetic composites
The magnetic composites were synthesized from a suspension
of 3.0 g bentonite in 300 mL solution of 5.9 g FeCl3�6H2O
and 3.0 g FeSO4�7H2O at 70 �C under N2 conditions. NaOH
solution (60 mL, 0.05 mol/L) was added dropwise to pre-
cipitate the iron oxides. After the addition of NaOH solution,
the mixture was adjusted to pH 11.0 and stirred for 1 h. To
promote the complete growth of the composite crystals, the
reaction was carried out at 70 �C for 4 h under constant
magnetic stirring. Finally, the mixture was filtered and
washed six times with Milli-Q water. Then the composites
were dried at 70 �C and sieved by 53 lm sieve.
The bentonite/iron oxide magnetic composites were
characterized by XRD and SEM. The XRD patterns were
performed by a MAC Science Co. M18XHF diffractometer
with Cu Ka radiation (k = 0.15406 nm). The diffracted
intensities were recorded from 3 to 70 2h-angles. The
2h-scanning rate was 2 min-1. Patterns were identified by
comparison to the JCPD standards. The morphology of
pure bentonite and bentonite/iron oxide magnetic com-
posites were characterized by a field emission SEM (FE-
SEM, JEOL JSM-6700, Tokyo, Japan).
Batch sorption studies
All the experiments were carried out in polyethylene cen-
trifuge tubes using batch technique under ambient condi-
tions. The stock suspensions of bentonite/iron oxide
magnetic composites and NaNO3 were added in the poly-
ethylene test tubes, then Ni(II) solution was added in the
suspension to achieve the desired concentrations. The pH
values were adjusted by adding negligible volumes of 0.1
or 0.01 M HNO3 or NaOH. After the suspensions were
shaken for 24 h, the solid and liquid phases were separated
by magnetic process using a permanent magnet. The con-
centration of 63Ni(II) was analyzed by liquid scintillation
counting using a Packard 3100 TR/AB Liquid Scintillation
Analyzer (PerkinElmer). The scintillation cocktail was
ULTIMA GOLD ABTM
(Packard). The amount of Ni(II)
adsorbed on bentonite/iron oxide magnetic composites was
calculated from the difference between the initial concen-
tration (C0) and the final one (Ce) in supernatant after
magnetic separation. The sorption percentage and the dis-
tribution coefficient (Kd) were derived from the following
equations:
Sorption % ¼ C0 � Ce
C0
� 100% ð1Þ
Kd ¼C0 � Ce
Ce
� v
mð2Þ
where m (g) is the mass of bentonite/iron oxide magnetic
composites and V (mL) is the volume of the suspension.
All the experimental data were the averages of duplicate
or triplicate determinations. The relative errors of the data
were about 5%.
1182 L. Chen et al.
123
Results and discussion
Characterization of magnetic composites
Figure 1 presents the XRD patterns of pure bentonite and
bentonite/iron oxide magnetic composites. As can be seen
from Fig. 1, the diffraction peaks assigned to the structure
of bentonite can be clearly seen in the XRD pattern of the
bentonite/iron oxide magnetic composites, which indicates
that the montmorillonite structure is not destroyed after the
chemical coprecipitation of iron oxides. The XRD pattern
of the bentonite/iron oxide magnetic composites displays
the main diffraction peaks at 2h = 30.20�, 35.61�, 43.40�,
53.48�, 57.15�, and 62.55�, which are related to the pres-
ence of maghemite (c-Fe2O3) or magnetite (Fe3O4) [20].
Other peaks (2h = 20.82�, 41.84�, and 59.81�) might be
related to the presence of goethite (a-Fe2O3).
Figure 2 shows the micrographs obtained from benton-
ite/iron oxide magnetic composites and pure bentonite. The
micrography of the bentonite/iron oxide magnetic com-
posites (Fig. 2a) suggests that the presence of iron oxide
particles on the surface of bentonite. Figure 2b shows the
pure bentonite with a typical sheet structure.
The separation of bentonite/iron oxide magnetic com-
posites from aqueous solution by using magnetic separation
method is displayed in Fig. 3. It is observed that magnetic
composites can be easily separated from the aqueous
solution within several minutes by placing a magnet, and
10 20 30 40 50 60 700
300
600
900
12000
800
1600
2400
3200
4000
M/Mh/MnMh/Mn
M/Mh/Mn
Mh/MnInte
nsity
(au
.)
2 Theta/(degree)
M
MM
G
Q
M
G
QG
B
Inte
nsity
(au
.)
M
Q
M
M
M MM C CQ
MM
A
Fig. 1 XRD for a pure bentonite and b bentonite/iron oxide magnetic
composites. M montmorillonite, Q quartz, C Cal-Fe(Ca)CO3, Mnmagnetite, Mh maghemite, G goethite
Fig. 2 SEM micrographs of a bentonite/iron oxide magnetic composites and b pure bentonite
Fig. 3 Photographs of magnetic separation of a bentonite/iron oxide
magnetic composites and b pure bentonite from aqueous solutions
Impact of environmental conditions on the removal of Ni(II) 1183
123
then the clear solution can be easily removed by pipet or
decant off. This simple magnetic separation experiment
indicates the bentonite/iron oxide magnetic composites are
magnetic. Therefore, it can be used as a magnetic adsorbent
to remove pollutants from aqueous solutions. Moreover,
the most important is that this separation method can be
applied simply and conveniently in actual application,
which will be discussed in the following sections.
Sorption kinetics
Kinetics of sorption is an important characteristic in
defining the efficiency of sorption. Figure 4 shows the
effect of contact time on Ni(II) sorption onto bentonite/iron
oxide magnetic composites. As can be seen from Fig. 4, the
sorption of Ni(II) on bentonite/iron oxide magnetic com-
posites increases rapidly at the initial contact time of 5 h,
then the sorption maintains high level with increasing
contact time. In the following experiments, 24 h was
selected to assure the sorption equilibrium. The fast sorp-
tion indicates that chemical sorption rather than physical
sorption contributes mainly to Ni(II) sorption on bentonite/
iron oxide magnetic composites [28, 29]. The rapid uptake
of Ni(II) ions by bentonite/iron oxide magnetic compos-
ites is one of the parameters that can be taken into con-
sideration for economical wastewater treatment plant
applications.
To study the kinetics of Ni(II) sorption on bentonite/iron
oxide magnetic composites, a pseudo-second-order rate
equation was used to simulate the kinetic sorption [30]:
t
qt¼ 1
k0q2e
þ 1
qe
t ð3Þ
where qt (mg/g) is the amount of Ni(II) ions adsorbed on
the surface of bentonite/iron oxide magnetic composites at
time t (h), and qe (mg/g) is the equilibrium sorption
capacity. k0 (g/(mg h)) is the rate constant of pseudo-sec-
ond-order kinetics. The k0 and qe values calculated from the
slope and intercept of the linear plots of t/qt versus t are
0.123 g/(mg h) and 16.42 mg/g, respectively. The straight-
line plot of t/qt versus t (insert in Fig. 4) indicates that the
kinetic sorption of Ni(II) onto bentonite/iron oxide mag-
netic composites can be well described by the pseudo-
second-order rate equation. In addition, the value of k0 also
demonstrates that the sorption achieves equilibrium very
quickly.
Effect of solid content
For the removal of Ni(II) from aqueous solutions, the amount
of adsorbent is crucial for the economic application. Under
the effective removal percentage uncertainties, the less
amount of adsorbent that is used, the lower cost is applied.
Figure 5 shows the dependence of Ni(II) sorption on ben-
tonite/iron oxide magnetic composites at different solid
contents. As the adsorbent content increases from 0.1 to
1.3 g/L, the Ni(II) sorption increases from 24.6 to 93.1%.
This trend is expected because the sorption sites at the
adsorbent surfaces increase with increasing solid content,
and more exchangeable surface sites are available to form
complexes with Ni(II) at adsorbent surfaces [31]. However,
the equilibrium sorption capacity, qe, decreased from 24.6 to
6.9 mg/g with increasing adsorbent content from 0.1 to
1.3 g/L. The reason is mainly due to the content of Ni(II)
adsorbed onto unit weight of adsorbent decreases with the
increasing adsorbent content. Evidently, the sorption per-
centage and equilibrium adsorption capacity are sensitive to
the variation of adsorbent content. A 0.33 g/L of adsorbent
0 5 10 15 20 25
4
6
8
10
12
14
16
18
0 5 10 15 20 250.0
0.3
0.6
0.9
1.2
1.5
q e (m
g/g)
time (h)
t/q t (
h·g/
mg)
t (h)
Fig. 4 Effect of contact time on the sorption of Ni(II) onto bentonite/
iron oxide magnetic composites. C0[Ni(II)] = 10 mg/L, m/V = 0.33 g/
L, I = 0.01 M NaNO3, pH 6.7 ± 0.1, T = 298.15 K
0.2 0.4 0.6 0.8 1.0 1.2 1.4
6
9
12
15
18
21
24
m/v (g/L)
q e (
mg/
g)
20
30
40
50
60
70
80
90
100
Sorp
tion
(%)
Fig. 5 Effect of solid content on the sorption of Ni(II) on bentonite/
iron oxide magnetic composites. C0[Ni(II)] = 10 mg/L, I = 0.01 M
NaNO3, pH 6.7 ± 0.1, T = 298.15 K
1184 L. Chen et al.
123
was selected as adsorbent content for all further experiments
in this study because of high sorption efficiency and
acceptable sorption capacity.
Effect of pH and ionic strength
Figure 6 shows the pH dependent Ni(II) sorption on the
bentonite/iron oxide magnetic composites in 0.001, 0.01
and 0.1 M NaNO3 solutions, respectively. The sorption
curve of Ni(II) on adsorbent can be divided into three
regions: (1) In region I, the sorption of Ni(II) in 0.01 M
NaNO3 solution increases gradually from 10 to 35% when
pH increases from 3.5 to 6.0; (2) The sorption of Ni(II) on
bentonite/iron oxide magnetic composites increases sharply
from 35% to a maximum value of 99% at pH 6.0–8.0
(region II); and (3) Above pH 8.0, the removal of Ni(II)
maintains the high level at 99% (region III). The sorption
edges spread over three pH units indicates the formation of
various surface complexes on the adsorbent surface and
represents different sorption mechanisms [32]. Comparing
to Ni(II) sorption on hematite [1], on titanate nanotubes [2],
on goethite [14] and on NKF-6 zeolite [15], the results of
this work are consistent with the results of the references.
The relative distribution of Ni(II) species calculated
from the overall hydrolysis constants Log K1 = -9.9,
Log K2 = -19, Log K3 = -30 and Log K4 = -27.7 are
shown in Fig. 7 [2]. It can be clearly found from Fig. 7 that
Ni(II) presents in the forms of Ni2?, Ni(OH)?, Ni(OH)20,
Ni(OH)3- and Ni(OH)4
2- species at different pH values.
At low pH values, Ni2? and Ni(OH)? are the main species
in aqueous solution and the solid surface is positively
charged due to the protonation reaction (i.e., � SOHþ Hþ
$� SOH2þ) occurred on bentonite/iron oxide magnetic
composites. The electrostatic repulsion between dissolved
metal ions and adsorbent surfaces leads to the low sorption
efficiency of Ni(II). Meanwhile, sorption of protons has
obvious preponderance over that of Ni(II) ions on the
sorption sites. Therefore, the low Ni(II) sorption that takes
place at pH \6.0 (region I) can be attributed partly to the
competition between H?/Na? and Ni2? on the surface
sites. However, at high pH values, the surface of bentonite/
iron oxide magnetic composites becomes negatively
charged as a result of the deprotonation reaction (i.e.,
� SOH$� SO� þ Hþ) and electrostatic repulsion
decreases for the reduction of positive charge density on
the sorption edges, which can enhance the sorption of the
positively charged Ni(II) through electrostatic force of
attraction, thus resulting in sharp increase of Ni(II) sorption
at pH 6.0–8.0. It is necessary to make sure whether the
formation of Ni(OH)2(s) precipitation contributes to the
drastic increasing in the uptake of Ni(II) on bentonite/iron
oxide magnetic composites at pH 6.0–8.0 (region II).
The hydrolysis constant of Ni(OH)2 is 2.0 9 10-15 [33].
Herein, the initial concentration of Ni(II) is 10 mg/L, Ni(II)
begins to form precipitation at pH about 8.5. However,
more than 99% Ni(II) is adsorbed on bentonite/iron oxide
magnetic composites at pH \8.5, thereby the high Ni(II)
sorption at pH below 8.0 is not attributed to surface pre-
cipitation of Ni(OH)2. However, the formation of Ni(II)
species with OH- plays an important role in the increase of
Ni(II) removal at higher pH values (region III). Depending
on pH and metal concentration, the hydrolytic actions of
Ni(II) may generate various complexes such as Ni(OH)2,
Ni(OH)3- and Ni(OH)4
2- at high pH values, which par-
ticipates in the sorption and precipitation onto the adsor-
bent surfaces.
It can also be seen from Fig. 6 that the sorption of Ni(II)
is obviously affected by NaNO3 concentration at pH\8.0.
At pH[8.0, no obvious influences of NaNO3 concentration
on Ni(II) sorption is observed. The NaNO3 concentration
can influence the double electrode layer thickness and
interface potential, thereby can affect the binding of the
3 4 5 6 7 8 9 10 11
0
20
40
60
80
1000.001MNaNO3
0.01MNaNO3
0.1MNaNO3
pH
Sor
ptio
n (%
)
0.0
5.0x10-16
1.0x10-15
1.5x10-15
2.0x10-15
2.5x10-15
3.0x10-15
3.5x10-15
4.0x10-15
[Ni2+
][O
H- ]2
Fig. 6 Effect of pH and ionic strength on the sorption of Ni(II) on
bentonite/iron oxide magnetic composites. C0[Ni(II)] = 10 mg/L, m/
V = 0.33 g/L, T = 298.15 K
4 6 8 10 12 14
0.0
0.2
0.4
0.6
0.8
1.0
Ni2+
Rel
ativ
e pr
opor
tion
of n
icke
l spe
cies
pH
Fig. 7 Relative proportion of 63Ni(II) species as a function of pH
Impact of environmental conditions on the removal of Ni(II) 1185
123
adsorbed species [34]. Outer-sphere surface complexes are
expected to be more impressionable to ionic strength
variations than inner-sphere surface complexes as the
background electrolyte ions are placed in the same plane
for outer-sphere surface complexes [35]. The ionic strength
dependent sorption demonstrates that ion exchange or
outer-sphere surface complexation mainly contributes to
Ni(II) sorption on bentonite/iron oxide magnetic compos-
ites at pH \8.0, while the ionic strength independent
sorption at pH [8.0 indicates that inner-sphere surface
complexation is the main sorption mechanism of Ni(II) on
bentonite/iron oxide magnetic composites at high pH val-
ues [36].
Effect of foreign ions
Figure 8a shows the effect of foreign ions on the removal
of Ni(II) from aqueous solutions to bentonite/iron oxide
magnetic composites in 0.01 M LiNO3, KNO3, and NaNO3
solutions, respectively, as a function of pH. The removal of
Ni(II) to bentonite/iron oxide magnetic composites is
strongly affected by cations at pH \8.0. One can see that
the removal percent of Ni(II) under the same pH values is
in the following order: Li? [ Na? [ K?, which indicates
that cations can change the surface properties of adsorbent
and, therefore, influences the sorption of Ni(II) onto ben-
tonite/iron oxide magnetic composites. The results can be
considered as a competition of Ni(II) with different cation
ions for the binding sites on bentonite/iron oxide magnetic
composite surfaces. From this competition principle, the
order of Ni(II) uptake under the same pH value is found to
be the lowest for K? and the highest for Li?, which is the
order of their radii of hydration: Li? = 3.4 A, Na? = 2.76
A and K? = 2.32 A [14]. The radius of Li? is larger than
those of the other two cations and thus the influence of Li?
on Ni(II) removal is smaller than Na? and K? removal.
One can also see that no obvious difference of Ni(II)
sorption on bentonite/iron oxide magnetic composites in
LiNO3, NaNO3 and KNO3 solutions at pH [8.0, which
may be attributed to the inner-sphere surface complexation
at high pH values as mentioned above. Sheng et al. [2]
investigated the effect of Li?, Na? and K? on the sorption
of Ni(II) on titanate nanotubes and similar results were also
found.
As can be seen from Fig. 8b, the removal of Ni(II) from
aqueous solutions to bentonite/iron oxide magnetic com-
posites is not influenced by the background electrolyte
foreign anions. The inorganic acid radical radius order is
Cl- \ NO3- \ ClO4
-, and such smaller radius and nega-
tive charged inorganic acid radicals may form complexes
with the oxygen-containing functional groups on the sur-
faces of bentonite/iron oxide magnetic composites and
thereby leads to the decrease of Ni(II) sorption. However,
the effects of Cl-, NO3- and ClO4
- on Ni(II) sorption to
bentonite/iron oxide magnetic composites are very weak,
which suggests that surface complexes are formed on
bentonite/iron oxide magnetic composites. The effect of
foreign anions on Ni(II) sorption from solution to benton-
ite/iron oxide magnetic composites can be negligible. This
result is consistent with the sorption of Ni(II) on oxidized
multiwalled carbon nanotubes [37]. However, Sheng et al.
[2] reported that the sorption of Ni2? on titanate nanotubes
was influenced by foreign anions. The differences may be
due to the surface properties of adsorbent, the physico-
chemical properties of metal ions and other environmental
parameters such as pH and ionic strength.
Effect of FA
Figure 9 illustrates the pH dependent of Ni(II) sorption onto
bentonite/iron oxide magnetic composites in the absence and
presence of FA. As can be seen from Fig. 9, a positive effect
of FA on Ni(II) sorption onto bentonite/iron oxide magnetic
composites is observed at low pH values, while a negative
effect of FA on Ni(II) sorption to adsorbent is observed at
high pH values. FA has a macromolecular structure, only a
small fraction of the adsorbed groups is free to interact with
metal ions [38]. The complexation between Ni(II) and FA is
0
20
40
60
80
100
3 4 5 6 7 8 9 100
20
40
60
80
100
Sorp
tion
(%)
0.01M LiNO3
0.01M NaNO3
0.01M KNO3
A
0.01MNaNO3
0.01MNaCl
0.01MNaClO4
Sorp
tion
(%)
pH
B
Fig. 8 Influence of foreign cations (a) and anions (b) on the sorption
of Ni(II) on bentonite/iron oxide magnetic composites. C0[Ni(II)] =
10 mg/L, m/V = 0.33 g/L, T = 298.15 K
1186 L. Chen et al.
123
much stronger than that between Ni(II) and bentonite/iron
oxide magnetic composites. The free energy of the formation
of FA–Ni(II) complex is smaller than that of adsorbent-
Ni(II). In addition, the point of zero charge of FA is about pH
2.0 [39]. The increase of Ni(II) sorption on FA-bentonite/
iron oxide magnetic composite hybrids at low pH may be due
to a reduction in positive surface charge caused by the
sorption of negatively charged FA at solid surfaces, which
results in a more favorable electrostatic environment for
Ni(II) sorption and enhances the formation of ternary Ni–
FA-adsorbent surface complexes. However, the negative
effect of FA at pH [6.5 is attributed to the strong soluble
FA–Ni complexes in solution. At pH [6.5, the surface
charge of bentonite/iron oxide magnetic composites is neg-
ative and the sorption of negatively charged FA on the
negatively charged bentonite/iron oxide magnetic compos-
ites decreases with increasing pH due to electrostatic
repulsion. This causes the formation of soluble complexes of
FA–Ni in solution, and the strong complexation ability of FA
with Ni(II) results in the decrease of Ni(II) sorption on
adsorbent at high pH values [40].
Sorption isotherms and thermodynamic data
It is well known that temperature is one of important
parameters which can affect the physicochemical behavior
of metal ions in the environment. The sorption isotherms of
Ni(II) at 298.15, 323.15 and 348.15 K are shown in Fig. 10.
It is clear that the sorption isotherm is the highest at
T = 348.15 K and is the lowest at T = 298.15 K, suggesting
that the sorption process of Ni(II) on bentonite/iron oxide
magnetic composites is favored at high temperature and is
blocked at low temperature. Hence the sorption reaction is an
endothermic process [41, 42]. The endothermic process
during the sorption of Ni(II) onto various adsorbents has also
been reported [33, 43, 44]. Two different models, viz.
Langmuir and Freundlich isotherm equations, are adopted to
simulate the sorption isotherms.
The Langmuir isotherm model is a theoretical model to
describe monolayer sorption process onto a surface. Its
form can be expressed by the following equation [45]:
qe ¼bqmaxCe
1þ bCe
ð4Þ
Equation 4 can be expressed in linear form:
Ce
qe
¼ 1
bqmax
þ Ce
qmax
ð5Þ
where qmax is the maximum sorption capacity corresponding
to the amount of adsorbate at complete monolayer coverage
(mg/g), and b (L/mg) is the equilibrium constant.
The Freundlich isotherm model allows for several kinds
of sorption sites on the solid and represents properly the
sorption data at low and intermediate concentrations on
heterogeneous surfaces [46]. The model can be represented
by the following equation:
qe ¼ KFCne ð6Þ
Equation 6 can be expressed in linear form:
log qe ¼ log KF þ n log Ce ð7Þ
where KF (mg1-n Ln/g) and n are Freundlich constants,
associated with sorption capacity when metal ion equilib-
rium concentration equals to 1 and the degree of dependence
of sorption with equilibrium concentration, respectively.
The experimental data of Ni(II) sorption (Fig. 10) were
regressively analyzed with the two models and the results
3 4 5 6 7 8 9 100
15
30
45
60
75
90
105So
rptio
n (%
)
pH
No FA 10mg/L FA
Fig. 9 Effect of pH on Ni(II) sorption on bentonite/iron oxide
magnetic composites in the presence and absence of FA. C0[Ni(II)] =
10 mg/L, m/V = 0.33 g/L, I = 0.01 M NaNO3, T = 298.15 K
0 5 10 15 20 25 30
5
10
15
20
25
30
35
40 298.15K323.15K348.15K
q e(m
g/g)
Ce (mg/L)
Fig. 10 Sorption isotherms of Ni(II) on bentonite/iron oxide mag-
netic composites at three different temperatures. m/V = 0.33 g/L,
I = 0.01 M NaNO3, pH 6.7 ± 0.1
Impact of environmental conditions on the removal of Ni(II) 1187
123
were given in Fig. 11. The relative parameters calculated
from the two models are listed in Table 1. As can be seen
from Fig. 11, the two models fit the sorption isotherms
well. However, from the correlation coefficients it can be
concluded that Langmuir model simulates the experimental
data better than Freundlich model. The fact that the sorp-
tion of Ni(II) according with Langmuir model indicates
that the binding energy on the whole surface of bentonite/
iron oxide magnetic composites is uniform. In other words,
the whole surface has identical sorption activity and hence
the adsorbed Ni(II) ions do not interact or compete with
each other, and they are adsorbed by forming an almost
complete monolayer coverage of the magnetic particles. In
addition, magnetic composites has a finite specific surface
and sorption capacity, therefore, the sorption could be
better described by Langmuir model rather than by
Freundlich model, as an exponentially increasing sorption
was assumed in the Freundlich model.
The thermodynamic parameters (free energy change
(DG�), enthalpy change (DH�) and entropy change (DS�))
were calculated from the temperature dependent sorption
isotherms to evaluate the sorption process. The values of
enthalpy (DH�) and entropy (DS�) were calculated from the
slope and intercept of the plot of ln Kd versus 1/T (Fig. 12)
using the following equation:
ln kd ¼DS�
R� DH�
RTð8Þ
The change of Gibbs free energy (DG�) was calculated
from the equation:
DG� ¼ DH� � TDS� ð9Þ
where R is the ideal gas constant (8.314 J/(mol K)), and
T (K) is the absolute temperature in Kelvin. The values
-0.3 0.0 0.3 0.6 0.9 1.2 1.5 1.8
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0 5 10 15 20 25 30 350.0
0.2
0.4
0.6
0.8
1.0
298.15K
323.15K
348.15K
Log
qe
(mg/
g)
Log Ce (mg/L)
B
Ce/
q e(m
g/L
)
Ce (mg/L)
298.15K
323.15K
348.15K
A
Fig. 11 Langmuir (a) and Freundlich (b) model fitting isotherms of
Ni(II) on bentonite/iron oxide magnetic composites at three different
temperatures. m/V = 0.33 g/L, I = 0.01 M NaNO3, pH 6.7 ± 0.1
Table 1 Langmuir and Freundlich isotherm model parameters
Temperature (K) Adsorbent Langmuir constants Freundlich constants
qmax (mg/g) b (L/mg) R2 kF (mg1-n Ln/g) n R2
298.15 Iron oxides 7.40 0.309 0.994 2.93 0.254 0.910
298.15 Pure bentonite 45.87 0.157 0.994 7.54 0.525 0.975
298.15 Bentonite/iron oxides 35.71 0.179 0.990 6.35 0.509 0.938
323.15 Bentonite/iron oxides 39.37 0.269 0.993 8.91 0.467 0.894
348.15 Bentonite/iron oxides 41.49 0.522 0.999 13.60 0.373 0.872
2.8x10-3
2.9x10-3
3.0x10-3
3.1x10-3
3.2x10-3
3.3x10-3
3.4x10-3
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0 3mg/L 7mg/L 15mg/L20 mg/L 30mg/L 40mg/L
LnK
d (m
g/L
)
1/T ( K-1)
Fig. 12 The linear plot of ln Kd versus 1/T for Ni(II) sorption on
bentonite/iron oxide magnetic composites
1188 L. Chen et al.
123
obtained from Eqs. 8 and 9 are summarized in Table 2.
The positive value of the standard enthalpy change indi-
cates that the interaction of Ni(II) sorption on the bentonite/
iron oxide magnetic composites is endothermic. This
phenomenon can be explained that Ni(II) is dissolved well
in water, and the hydration sheath of Ni(II) have to be
destroyed before their sorption on bentonite/iron oxide
magnetic composites. This dehydration process needs
energy, so it is favored at high temperature [47]. This
energy exceeds the exothermicity of ions to attach to the
solid surface. The assumption indicates that the endo-
thermicity of the desolvation process exceeds the enthalpy
of sorption to a considerable extent. DG� is negative as
expected for a spontaneous process under the conditions
applied. The free energy changes are more negative with
the increase of temperature, indicating that the reaction is
more efficient at higher temperature. In addition, at high
temperature, metal ions are readily desolvated and hence
their sorption becomes more favorable [48]. The positive
values of entropy change (DS�) reflect the affinity of the
adsorbent towards metal ions in aqueous solutions and may
suggest the randomness increases at the solid–liquid
interface during the sorption of Ni(II)ions on the bentonite/
iron oxide magnetic composites [49]. Ni(II) ions in solution
are surrounded by a firmly bound hydration layer where
water molecules are more highly ordered. When Ni(II) ions
come into close interaction with the hydration surface of
adsorbent, the ordered water molecules in the two hydra-
tion layers are compelled and disturbed, hence lead to an
increase of the entropy of water molecules. The thermo-
dynamic data derived from temperature dependent sorption
isotherms suggests that the sorption process of Ni(II) on
bentonite/iron oxide magnetic composites is spontaneous
and endothermic [50, 51].
Comparison of different adsorbents
Figure 13 illustrates the sorption isotherms of Ni(II) on
bentonite/iron oxide magnetic composites, pure bentonite,
and iron oxides, respectively. The sorption isotherms of
Ni(II) on three different adsorbents are simulated by the
Langmuir and Freundlich models (Fig. 14), and the relative
parameters are listed in Table 1. The maximum sorption
Table 2 Relative thermodynamic parameters for Ni(II) sorption on
bentonite/iron oxide magnetic composites
C0
(mg/L)
DH�(kJ/mol)
DS�(J/(mol K))
DG� (kJ/mol)
298.15 K 323.15 K 348.15 K
3 4.84 78.80 -18.66 -20.62 -22.59
7 6.12 82.13 -18.37 -20.42 -22.47
20 6.22 81.52 -18.09 -20.12 -22.16
25 4.92 76.22 -17.80 -19.71 -21.62
30 4.26 71.40 -17.03 -18.81 -20.60
40 5.08 71.80 -16.33 -18.12 -19.92
0 5 10 15 20 25 30 35
5
10
15
20
25
30
35
40 pure bentonitebentonite/iron oxidesiron oxides
q e(m
g/g)
Ce (mg/L)
Fig. 13 Sorption isotherms of Ni(II) sorption on bentonite/iron oxide
magnetic composites, pure bentonite, and iron oxides. m/V = 0.33 g/L,
I = 0.01 M NaNO3, pH 6.7 ± 0.1, T = 298.15 K
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0 5 10 15 20 25 30 350
1
2
3
4
5
pure bentonitebentonite/iron oxidesiron oxides
Log
qe
(mg/
g)
Log Ce (mg/L)
B
pure bentonitebentonite/iron oxidesiron oxides
Ce/
q e(m
g/L
)
Ce (mg/L)
A
Fig. 14 Langmuir (a) and Freundlich (b) isotherms for the sorption
of Ni(II) on bentonite/iron oxide magnetic composites, pure benton-
ite, and iron oxides. m/V = 0.33 g/L, I = 0.01 M NaNO3, pH
6.7 ± 0.1, T = 298.15 K
Impact of environmental conditions on the removal of Ni(II) 1189
123
capacities of Ni(II) are 35.71 mg/g for bentonite/iron oxide
magnetic composites, 45.87 mg/g for pure bentonite, and
7.40 mg/g for iron oxides. It can be seen that the maximum
sorption capacity of Ni(II) is the highest for pure bentonite.
It is because of a high quantity of the iron oxide particles
covering on the bentonite, the active sites were occupied
and lead to the decrease of metal ions to contact with the
sorption sites on the bentonite surface [52]. In addition, one
can concluded from Table 1 that the presence of iron oxide
in the composite also contributes to Ni(II) sorption.
Although the sorption capacity of Ni(II) on bentonite/iron
oxide magnetic composites is a little lower than that of
pure bentonite, the magnetic composites can be separated
from solution easily by magnetic separation technique,
which is very significant for their environmental applica-
tion in the removal of pollutants from large volumes of
aqueous solutions.
Regeneration
An adsorbent not only possesses high sorption capability
but also shows good desorption property, which will sig-
nificantly reduce the overall cost for an adsorbent. The
repeated availability of bentonite/iron oxide magnetic
composites for Ni(II) removal through many cycles of
sorption–desorption cycles is quite significant for the
application of bentonite/iron oxide magnetic composites in
the removal of Ni(II) from wastewater in real work. Herein,
the recycling of bentonite/iron oxide magnetic composites
in the removal of Ni(II) was investigated. After sorption,
desorption experiment was carried out by washing out the
adsorbent with HClO4 (pH * 4) and Milli-Q water, and
then the magnetic composites were dried at 95 �C. From
Fig. 15, it is observed that there is no drastic decrease in
the sorption capacity of Ni(II) after seven times of the reuse
and the removal percentage was also satisfying, indicating
that the bentonite/iron oxide magnetic composites have a
good reusability. This result suggests that the bentonite/
iron oxide magnetic composites can be employed repeat-
edly in Ni(II) removal from aqueous solution in real work.
Conclusion
The bentonite/iron oxide magnetic composites were syn-
thesized by using the co-precipitation method. The sorption
of Ni(II) is dependent on pH and influenced by ionic strength,
foreign cations at pH\8.0. The presence of different foreign
anions has no obvious effects on the sorption of Ni(II).
Sorption isotherms are better described by Langmuir model
than by Freundlich model and the thermodynamic parame-
ters calculated from the temperature dependent sorption
isotherms indicate that the sorption process is spontaneous
and endothermic. Although the sorption capacity of Ni(II) on
bentonite/iron oxide magnetic composites is a little lower
than that of Ni(II) on pure bentonite, the magnetic compos-
ites can be separated from solution easily by magnetic sep-
aration technique. In view of above-mentioned results, one
can conclude that the bentonite/iron oxide magnetic com-
posites are very suitable materials for the preconcentration
and immobilization of heavy metals and radionuclides from
large volumes of aqueous solutions. The material can be
recovered from solution by using magnetic separation
method, which assures that the magnetic composites can be
used to remove pollutants in large scale in real work.
Acknowledgment Financial support from National Natural Science
Foundation of China (20971033) is acknowledged.
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