Hierarchically porous NiO-Al2O3 nanocomposite with enhanced Congo red adsorption in water

Article abstract of DOI:10.1039/c5ra23545f

Congo red (CR) has been widely used in the textile industry. However, the discharge of wastewater containing CR is a subject of great concern with regard to environmental protection. Herein, NiO, Al₂O₃, and NiO-Al₂O₃ nanocomposite adsorbents with hierarchical porous structures were prepared by a simple solvothermal method. Adsorption removal of CR dye from aqueous solutions was investigated using the prepared samples as adsorbent, which had hierarchical porous structures composed of mesopores (2-50 nm) and macropores (>50 nm). The equilibrium adsorption data of CR on the NiO-Al₂O₃ samples were well fitted by the Langmuir model and yielded a maximum adsorption amount of 357 mg g^-1, which was higher than that of NiO and Al₂O₃ samples. The high adsorption of the NiO-Al₂O₃ nanocomposite sample was caused by the synergic effect of its hierarchical porous structures, high specific surface area, and positive surface charge at pH 7. Adsorption kinetic data could be well fitted by the pseudo-second-order kinetic equation, suggesting that pseudo-second-order kinetics could well represent the adsorption kinetics of the NiO-Al₂O₃ samples. The calculated activation energy needed by NiO-Al₂O₃ samples to adsorb CR indicated that the adsorption of CR molecules on NiO-Al₂O₃ sample was facilitated by physical adsorption process.

Full text of DOI:10.1039/c5ra23545f

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PAPER
Hierarchically porous NiO–Al O nanocomposite
2
3
with enhanced Congo red adsorption in water
Cite this: RSC Adv., 2016, 6, 10272
ab
b
a
a
ad
Chunsheng Lei, Xiaofeng Zhu, Yao Le, Bicheng Zhu, Jiaguo Yu*
c
and Wingkei Ho*
Congo red (CR) has been widely used in the textile industry. However, the discharge of wastewater
containing CR is a subject of great concern with regard to environmental protection. Herein, NiO, Al
and NiO–Al nanocomposite adsorbents with hierarchical porous structures were prepared by
O ,
2 3
2 3
O
a simple solvothermal method. Adsorption removal of CR dye from aqueous solutions was investigated
using the prepared samples as adsorbent, which had hierarchical porous structures composed of
mesopores (2–50 nm) and macropores (>50 nm). The equilibrium adsorption data of CR on the NiO–
Al
57 mg g , which was higher than that of NiO and Al
Al nanocomposite sample was caused by the synergic effect of its hierarchical porous structures,
2 3
O samples were well fitted by the Langmuir model and yielded a maximum adsorption amount of
ꢀ1
3
2 3
O samples. The high adsorption of the NiO–
2 3
O
high specific surface area, and positive surface charge at pH 7. Adsorption kinetic data could be well
fitted by the pseudo-second-order kinetic equation, suggesting that pseudo-second-order kinetics
Received 8th November 2015
Accepted 18th January 2016
could well represent the adsorption kinetics of the NiO–Al
needed by NiO–Al samples to adsorb CR indicated that the adsorption of CR molecules on NiO–
Al sample was facilitated by physical adsorption process.
2 3
O samples. The calculated activation energy
DOI: 10.1039/c5ra23545f
2 3
O
www.rsc.org/advances
2 3
O
reduce the CR concentration below the permissible limit has
become a subject of priority, thereby protecting the environment
1
. Introduction
In recent years, various dyes have been detected in the effluents and preserving habitats. Various methods have been developed
of some industries, including textiles, leather, paper, dyes, and and used for the removal of CR from dye wastewater, including
plastics. Every year, more than 100 000 known commercial dyes physical adsorption, coagulation/occulation, advanced oxida-
are produced, and about 100 tons per year are discharged into tion processes, membrane ltration, photocatalysis, ozonation,
1
–5
13–16
rivers and lakes. Therefore, large amounts of dye wastewater and biological treatment.
Among these treatment methods,
6–8
have caused serious environmental pollution problems. Congo adsorption has been recognized as one of the most important
red (CR) is one of the most typical synthetic dyes and has been methods owing to the advantages of high efficiency, low cost,
widely used in the textile industry. CR is a sodium salt of simple operation, and easy recovery. Moreover, adsorption does
17–21
benzidinediazobis-1-naphthylarnine-4-sulfonic acid, and the not produce harmful by-products.
molecular structure of CR is shown in Fig. 1. CR is considered to
In recent years, many adsorbents for the removal of pollut-
be hazardous and toxic to many organisms. Specically, the ants from wastewater have been intensively studied, including
25–27
22–24
degraded by-products of CR usually contain toxic aromatic amine activated carbons from cheap carbon sources,
y ash,
28–30
compounds, which are carcinogenic and harmful to the skin, and clay minerals. Usually, adsorbents with large surface
eyes, blood, and reproductive cells of the human body. Despite areas and hierarchical porous structures are required.
31,32
In
being banned in many countries because of its health hazards addition, wastewater with anionic dyes such as CR oen
and adverse environmental effects, CR is still used in some
9–12
countries.
Thus, treatment of dye wastewater discharge to
a
State Key Laboratory of Advanced Technology for Material Synthesis and Processing,
Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070, PR China
b
College of Environmental and Security Engineering, Changzhou University, Gehu
Road 1#, Changzhou 213164, PR China
c
Department of Science and Environmental Studies and Centre for Education in
Environmental Sustainability, The Hong Kong Institute of Education, Tai Po, N.T.,
Hong Kong, P.R. China. E-mail: keithho@ied.edu.hk
Fig. 1 Molecular structure of Congo red molecule.
d
Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
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contains other cationic and neutral dye pollutants. The pres- Table 1 Preparation parameters of the NA, N, and A samples
ence of these other pollutants in wastewater sometimes reduces
2
+
3+
Ni(NO
(mol)
3
)
2
Al(NO
(mol)
3
)
3
[Ni ] : [Al ]
33
the efficiency of adsorbents for anionic dye removal. There-
fore, the selectivity of adsorbents needs to be identied for
efficient wastewater treatment.
NiO is an important functional material widely used in
catalysis, battery, electrochromic lms, and fuel cell electrodes.
Samples
Composition
molar ratio
NA
N
A
NiO–Al O3
2
0.009
0.009
0.000
0.003
0.000
0.003
3 : 1
9 : 0
0 : 3
NiO
Al O
2 3
2 3
Al O has also been widely used as catalyst, catalytic support,
adsorbent, and wear-resistant coating. Compared with the
and composition of the samples. Scanning electron microscopy
individual performances of NiO and Al O , NiO–Al O nano-
2
3
2 3
(
(
SEM) observation was performed using S-4800 eld emission
Hitachi, Japan) and JSM-6510 (JEOL, Japan) scanning electron
composites are usually more efficient catalysts and have been
used in industrial processes, including hydrogenation, dehy-
microscope. The elemental analysis of the prepared sample was
conducted by an energy-dispersive X-ray spectrometer (EDS)
attached to the FE-SEM. The Brunauer–Emmett–Teller (BET)
specic surface area (SBET) and pore structures of the powder
samples were analyzed by nitrogen adsorption in a Micro-
meritics ASAP 2020 nitrogen adsorption apparatus (Micro-
meritics, USA). A desorption isotherm was used to determine
the pore-size distribution through the Barrett–Joyner–Halenda
drogenation, petroleum rening, methanation, CO
and fuel cell operations.
2
reduction,
Therefore, many studies have re-
ported the synthesis and application of NiO–Al nano-
For example, Crisan et al. reported the
34–36
2 3
O
37–44
structures.
preparation of NiO/Al
whereas Zhang et al. prepared NiO/g-Al O catalysts by
35
2 3
O catalysts using sol–gel method,
2
3
37
combining sol–gel and impregnation methods. However, the
preparation of hierarchically porous NiO–Al O3 nano-
composites and their application as adsorbent in water treat-
ment have been rarely studied. Herein, we provide the rst
2
45
method assuming a cylindrical pore model. The nitrogen
adsorption volume at a relative pressure (P/P ) of 0.994 was used
0
to determine the pore volume and average pore size. The
Fourier transform infrared (FTIR) spectra of the samples before
and aer adsorption were recorded on a Shimadzu IR Affinity-1
spectrometer using KBr pellets as support in the wavenumber
range of 4000–500 cm . The isoelectric points (IEP) of the
samples were measured on Malvern Zetasizer Nano-ZS90 (Mal-
vern, UK).
2 3
report on the synthesis of hierarchical porous NiO–Al O
nanocomposites as adsorbents and their application in the
removal of CR from water. This study will provide new insights
into the design and fabrication of high-performance adsorption
materials to perform Congo red adsorption from waste water.
ꢀ
1
2. Experimental
2.1. Material synthesis
2.3. Adsorption equilibrium and kinetic studies
All reagents such as Al(NO ) $9H O, Ni(NO ) $6H O, urea, and Adsorption isotherm studies were conducted by adding 10 mg
3
3
2
3 2
2
ethanol were of analytical grade (purchased from Shanghai of the prepared NA, N, or A samples to a series of 250 mL conical
Chemical Industrial Company) and were used without further asks with 50 mL CR solutions at various concentrations (10–
ꢀ1
purication. Distilled water was used for all synthesis and 100 mg L ) at pH ca. 7. The conical asks were sealed and
ꢁ
treatment processes.
NiO–Al
solvothermal method. In a typical preparation, 0.009 mol and the nal CR concentration in the supernatant solution was
Ni(NO $6H O and 0.003 mol Al(NO $9H
O were dissolved in measured according to the maximum adsorption peak of CR
0 mL ethanol solution containing 0.2 mol urea. Aer stirring at (497 nm) using a UV/Vis spectrophotometer (Shimadzu UV/Vis
room temperature for 60 min, the mixed solution was trans- 1240 spectrophotometer, Japan). The amount of CR adsorbed
stirred in a thermostatic shaker at 150 rpm for 12 h at 30 C.
2 3
O
nanocomposites were synthesized by a simple Aer adsorption, the conical asks were taken from the shaker
3
)
2
2
3
)
3
2
8
ꢀ
1
ferred to a 100 mL Teon-lined stainless steel autoclave, which on the adsorbent sample at equilibrium q
was heated to 140 C and kept at the same temperature for 14 h. calculated using the following equation:
e
(mg g ) was
ꢁ
ꢁ
Then, the autoclave was cooled to 25 C. The produced precip-
ðC
0
ꢀ C ÞV
e
q
e
¼
(1)
itate was alternately washed ve times with water and ethanol
and dried at 80 C for 6 h. The air-dried precipitate was heated
to 500 C at a rate of 2 C min and kept at 500 C for 2 h. The
calcined sample was labeled NA. For comparison, the same
experimental procedure was repeated for the preparation of
single NiO and Al O , which were labeled N and A, respectively
W
ꢁ
ꢀ
1
ꢁ
ꢁ
ꢀ1
ꢁ
where C and C (mg L ) represent the beginning and equi-
0 e
librium concentrations of CR in aqueous solution, respectively;
and W and V are the mass (g) of the adsorbent used and the
volume (L) of the solution, respectively.
Adsorption kinetic experiments were performed to investi-
gate the effect of adsorption time on adsorption rate. The initial
2
3
(Table 1).
ꢀ
1
concentration of CR was xed at 50 mg L . Then, the aqueous
samples were taken at pre-set time intervals and the CR
2.2. Characterization
The X-ray diffraction (XRD) patterns were obtained using an X- concentrations were measured as previously described. The
ꢀ1
ray diffractometer (type HZG41B-PC) with Cu Ka radiation (l ¼ amount of adsorption at t time (q
t
[mg g ]) was calculated
˚
1
.5406 A), which were used to investigate the phase structure using the following equation:
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ðC
0
ꢀ C
t
ÞV
3.2. SEM-EDS
The morphology and surface microstructure of the samples
(mg L ) represent the beginning and t time were observed by FE-SEM (Fig. 3). The SEM image of the NA
q
t
¼
(2)
W
ꢀ
1
where C
0
and C
t
concentrations of CR in the solution, respectively; W is the mass sample (Fig. 3a) indicates that the NiO–Al
2
O
3
nanocomposites
(
g) of adsorbent samples; and V is the volume (L) of the CR have highly porous textures composed of ca. 5.0 mm sized
solution.
microspheres assembled from building blocks of 2D nano-
sheets. The morphology of the N sample is highly similar to that
of NA, which is also composed of 2D nanosheets (shown in
Fig. 3b) and also has an obviously layered structure. The SEM
image of the A sample is different from that of the NA and N
samples, containing irregularly thick plates and uniform
spherical particles and exhibiting an irregular and ill-dened
morphology (Fig. 3c). The EDS analysis of the NA sample
3. Results and discussion
3.1. XRD
The phase structures of the prepared samples were investigated
by XRD. Fig. 2 presents the comparison of XRD patterns from
the NA, N, and A samples. The XRD pattern of the NA sample
shows NiO peaks (JCPDS data card no. 65-2901) and NiAl O
(Fig. 3d) indicates the presence of Ni, Al, and O elements.
2
4
peaks (JCPDS data card no. 10-0339), indicating that the NA
sample contains NiO and NiAl O in two phases. The XRD
3
.3. Nitrogen adsorption
2
4
The nitrogen adsorption–desorption isotherms and the corre-
sponding pore-size distribution curves of the prepared NA, N,
and A samples are presented in Fig. 4a and b. The three samples
are type IV isotherms according to the Brunauer–Deming–
patterns of the N and A samples show that they mainly consist
of NiO and Al (JCPDS data card no. 50-0741), respectively.
2 3
O
45
Deming–Teller (BDDT) classication. The hysteresis loops of
the NA and N samples are relatively smaller than those of the A
sample because of their better crystallization (Fig. 2) and lower
pore volume. The hysteresis loops of the NA and N samples are of
typical H3 type at high relative pressure (approaching 1), indi-
cating the presence of slit-shaped mesopores and macropores
from the aggregation of 2D nanosheets within the microsphere
(Fig. 3a and b). However, the A sample has higher nitrogen
Fig. 2 XRD patterns of the NA, N, and A samples.
Fig. 3 SEM images of the NA (a), N (b), and A (c) samples; and the EDS Fig. 4 Nitrogen adsorption–desorption isotherms (a) and corre-
spectrum of the NA sample (d).
sponding pore-size distribution curves (b) of the NA, N, and A samples.
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ꢀ
1
Table 2 Specific surface areas and pore parameters of the NA, N, and sample. The bands at 866 and 835 cm are from the vibrational
a
A samples
50,51
modes of the surface Al–O bond,
whereas the bands at 572
ꢀ1
52
3
ꢀ1
2
ꢀ1
and 534 cm are due to Ni–O stretching vibration. The char-
Samples
Composition
APS (nm)
PV (cm g
)
S
BET (m g )
ꢀ
1
acteristic band observed at 1040 cm belongs to the symmetric
53
NA
N
A
NiO–Al
NiO
2
O
3
4.7
7.6
6.5
0.18
0.09
0.31
157
52
188
and asymmetric stretching vibrations of S]O in CR molecules.
FTIR results indicate that CR molecules indeed adsorb on the NA
sample surface.
Al
2
O
3
a
APS: average pore size; PV: pore volume.
3.5. Adsorption kinetics
Adsorption kinetics is crucial in adsorption investigation
because it can predict the adsorption removal rate of a pollutant
from aqueous solutions. It can also enhance the understanding
on adsorption mechanism. Because of the novel hierarchical
porous structure and large specic surface area, the NA sample
should have good adsorption affinity toward dye molecules in
water. Thus, the CR dye adsorption activity of the NA sample in
water was studied using batch adsorption kinetic experiments.
Fig. 6 presents the comparison of CR adsorption kinetics on the
NA, N, and A samples, which were performed by batch
adsorption–desorption isotherms compared with the NA and N
samples, suggesting its larger specic surface area owing to its
weak crystallization (Fig. 2). Further observation indicates that
the A sample has two evident hysteresis loops. At a low relative
pressure range of 0.4–0.7, the hysteresis loop is type H2, which
indicates that the A sample contains ink-bottle pores (with
narrow necks and wide bodies) caused by the aggregation of
nanoparticles. Meanwhile, at a high relative pressure range of
0.7–1.0, the loop is type H3, which is related to the plate-like
ꢀ
1
particles that form slit-shaped pores. The pore size distribution
curves of the three samples (Fig. 4b) present a wide pore range
from 2 nm to more than 100 nm, which implies that the prepared
NA, N, and A samples contain hierarchically porous struc-
adsorption tests at a xed initial CR concentration (50 mg L
)
ꢁ
at 30 C. During the rst 25 min, the NA, N, and A samples all
presented high adsorption rates; and aer 150 min, adsorption
equilibrium was achieved. Although the same experimental
conditions were used for all three samples, the differences in
adsorption rate and capacity were clearly observed. The NA
sample had the largest adsorption capacity, followed by N
sample, and nally, the A sample. Notably, the A sample had the
46–49
tures.
The average pore size, pore volume, and BET surface
areas of the NA, N, and A samples are listed in Table 2. These
results indicate that because of its weaker crystallization, thus
small crystallite size, the A sample has larger specic surface area
47
2
ꢀ1
and pore volume than the NA and N samples have.
largest BET specic surface area (188 m g ), but it possessed
ꢀ1
the lowest adsorption capacity of about 120 mg g , indicating
that the specic surface area is not the only factor that inu-
ences adsorption. On the contrary, other factors, such as surface
3.4. FTIR spectra
To identify the functional groups of CR and determine the charge of the sample, probably have greater inuence on
interaction between the prepared samples and CR during the adsorption than specic surface area.
adsorption process, FTIR spectra of the NA sample before and
To investigate the surface charges of the NA, N, and A
aer CR adsorption are shown in Fig. 5a and b, respectively. The samples in water, their zeta potentials were analyzed by Malvern
ꢀ
1
broad absorption vibration bands at 3449 and 1376 cm are Zetasizer Nano ZS90. The variations in zeta potentials of NA, N,
attributed to the stretching vibration of the –OH group from and A samples along with the pH value of the suspensions are
4
4
water molecules (physical adsorption of molecular water),
shown in Fig. 7. For the suspension with the initial pH (at ca. 7),
ꢀ
1
whereas the band at 2930 cm is attributed to C–H stretching the zeta potentials of the NA, N, and A samples were 10.0, 16.9,
vibration. The band at 2370 cm is caused by the stretching and 2.0 mV, respectively. The zeta potentials gradually
vibration of CO from CO adsorption on the surface of the
50
ꢀ1
2
2
Fig. 6 Changes in adsorption capacities with adsorption times for CR
ꢁ
Fig. 5 FTIR spectra of the NA sample before (a) and after (b) adsorp- molecules adsorbed on the NA, N, and A samples (T ¼ 30 C, adsor-
ꢀ1
ꢀ1
tion of CR.
bent dose ¼ 200 mg L , CR concentration ¼ 50 mg L , and pH ¼ 7).
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Fig. 7 Variation in zeta potentials with pH value of the suspensions of Fig. 8 Pseudo-second-order kinetics of the NA, N, and A samples for
ꢁ
ꢀ1
ꢀ1
the NA, N, and A samples.
CR adsorption (T ¼ 30 C, adsorbent dose ¼ 200 mg L , CR
concentration ¼ 50 mg L , and pH ¼ 7).
decreased as pH value increased. In addition, the IEP of NA, N,
Table 3 Parameters of the pseudo-second-order adsorption kinetics
and A samples were 8.6, 8.9, and 7.6, respectively. Thus, the of NA, N, and A samples
oxide particle surface was positively charged at pH values below
the IEP but negatively charged above IEP. In addition, the zeta
potential of A sample suspension was less positive than that of
Pseudo-second-order model
C
0
q
e,exp
q
e,cal
k
S.D.
(%)
ꢀ1
ꢀ1
ꢀ1
ꢀ4
ꢀ1
ꢀ1
2
NA and N sample suspensions at a neutral environment.
Expectedly, among the three samples, the A sample had the
lowest adsorption capacity with negative CR molecules because
of the extremely weak electrostatic interaction between the CR
Samples (mg L ) (mg g ) (mg g ) (ꢂ10 g mg min ) R
NA
N
50
50
50
244
187
124
250
189
132
5.9
2.4
1.6
0.999 0.71
0.999 0.30
0.995 1.82
A
54
molecule and the A sample surface.
The above investigation and discussion indicate that the
difference in the adsorption affinity of the samples toward
pollutant molecules is mainly related to two key factors, namely,
zeta potential and surface area of the samples. For example, the number of the samples studied was 5. Both equilibrium
2
ꢀ1
N sample has lower specic surface area (52 m g ) but a more adsorption amount and k value follow the order NA > N > A.
positive zeta potential (16.9 mV) and still exhibits higher
Adsorption kinetics data of the NA sample were also tested at
adsorption capacity for CR molecules than the A sample. This different adsorption temperatures (25, 35, and 45 C) to calcu-
ꢁ
illustration indicates that, compared with specic surface area, late the activation energy (E
surface charge has a stronger inuence on the affinity and mated using the Arrhenius equation.
) of adsorption. The E
was esti-
a
a
56
adsorption of the sample surface toward pollutant molecules.
E
a
ln k
2
¼ ln A ꢀ
(4)
Thus, it is expected that among the three samples, the NA
sample exhibited the highest adsorption capacity toward CR
molecules because of its relatively large specic surface area
and positive zeta potential. The hierarchically porous structures
of the NA sample, which facilitated the transport and diffusion
of pollutant molecules in the adsorbent, also enhanced its
adsorption activity toward CR molecules.
RT
where E and k represent the Arrhenius activation energy (kJ
mol ) and the pseudo-second-order rate constant, respectively.
A, R, and T represent the Arrhenius factor, gas constant (8.314 J
a
2
ꢀ
1
ꢀ
1
ꢀ1
mol
ln k
was obtained (not shown here). The level of E
determine the type of adsorption, that is, chemical or physical
adsorption. Physical adsorption oen has E in the range of 0–
0 kJ mol , whereas chemical adsorption has higher E at 40–
K
), and the absolute temperature, respectively. When
versus 1/T was plotted, a straight line with the slope ꢀE /R
can be used to
2
a
a
Further investigation indicates that the adsorption kinetic
data of the NA, N, and A samples can be well tted by a pseudo-
55
a
second-order kinetic equation as follows:
ꢀ
1
4
8
a
ꢀ1
t
1
t
ꢀ1
00 kJ mol . The above calculated E is 19.8 kJ mol , indi-
¼ kq
þ
(3)
a
2
q
t
e
q
e
cating that the CR molecules were physically adsorbed on the
4
,56,57
ꢀ
1
surface of the NA sample.
where q
t e
and q (mg g ) represent the adsorption amount of CR
ꢀ1
at time t (min) and equilibrium, respectively; and k (g mg
min ) is the pseudo-second-order rate constant. The linear
relationship of t/q versus t is presented in Fig. 8, and the
ꢀ
1
3
.6. Adsorption isotherms
t
calculated kinetic parameters are listed in Table 3. The obtained The adsorption isotherms of CR on the NA, N, and A samples
2
R values were greater than 0.995 for samples NA, N, and A, and are presented in Fig. 9. These adsorption isotherms were tted
the normalized standard deviation S.D. (%) values were lower using the Langmuir and Freundlich models. The Langmuir
than 1.82 for all the three samples. For statistical analysis, the model assumes that no interaction occurs among adsorbate
molecules, and that the adsorption is
a
monolayer.
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heterogeneous surfaces. The linear equations of Langmuir and
58–62
Freundlich isotherm are expressed as follows:
C
e
C
e
1
¼ q þ K
(5)
(6)
q
e
max
L
q
max
1
ln q
e
¼ ln K
F
þ
ln C
e
n
e
where C is the equilibrium concentration of CR in the solution
ꢀ
1
(
(
mg L ), q
e
is the equilibrium capacity of CR on the adsorbent
ꢀ
1
mg g ), and qmax is the maximum adsorption capacity of the
Fig. 9 Adsorption isotherms of the NA, N, and A samples for CR (T ¼
adsorbent corresponding to a complete monolayer coverage on
ꢁ
ꢀ1
3
0 C, adsorbent dose ¼ 200 mg L , CR concentration ¼ 10–100 mg
ꢀ1
the surface (mg g ). K is the Langmuir adsorption constant (L
ꢀ1
L
L
, and pH ¼ 7).
ꢀ
1
mg ) associated with the free energy of adsorption, which can
be obtained from the intercepts and slopes of the linear plots of
ꢀ
1
C
(
e
/q
e
versus C
e
(Fig. 10). K
F
is the Freundlich constant (mg g )
ꢀ
1 1/n
L mg
)
, and 1/n is the heterogeneity factor. The tted
2
parameters and R are listed in Table 2. Fig. 10 and 11 show that
the experimental data can be better tted by the Langmuir
model, indicating the homogeneous nature of the sample
surface and a monolayer adsorption of CR molecules on the
outer surface of the adsorbent. According to the Langmuir

tting results (see Table 4), the maximum adsorption amount
follows the order of NA > N > A. By combining the adsorption
isotherms and CR kinetics of the three samples, the NiO and
Al O composite NA sample exhibited the best adsorption
2 3
Fig. 10 Langmuir isotherms of the NA, N, and A samples for CR performance.
ꢁ
adsorption at 30 C.
The adsorption selectivity of the NA sample was further
investigated by selective adsorption of the mixed aqueous
solution of CR and methylene blue (MB). The results (not shown
here) indicated that the adsorption of CR was faster than that of
MB at pH 7. This observation was caused by the negatively
charged CR molecules preferentially adsorbed on the surface of
the positively charged NA sample. By contrast, the NA sample
had negative surface charge and exhibited better adsorption
selectivity toward MB molecules at pH 10. The above adsorption
selectivity experiments indicate that the selective adsorption of
CR and MB on the NA sample surface can be easily tuned by
simply changing the pH values of the CR and MB mixture. This
also suggests the possibility of tuning the adsorption selectivity
by simply modifying the surface charges of the samples in
Fig. 11 Freundlich isotherms of the NA, N, and A samples for CR different pH. Moreover, the adsorption capacities of the
ꢁ
adsorption at 30 C.
prepared samples for CR are compared with the previously re-
ported results and listed in Table 5, indicating that the hierar-
chical NiO–Al O nanocomposite obtained in this work
2
3
Additionally, once a site is occupied by an adsorbate molecule,
no further adsorption can occur. However, the Freundlich
adsorption model assumes that adsorption occurs on
exhibited excellent adsorption performance for CR.
Table 4 Adsorption isotherm parameters of the NA, N, and A samples obtained by fitting in Langmuir and Freundlich isotherm equations
Langmuir isotherm model Freundlich isotherm model
ꢀ1
(ꢂ10ꢀ1 L mgꢀ1
2
ꢀ1
ꢀ1 1/n
2
Samples
qmax (mg g
)
k
L
)
R
R
L
K
F
(mg g ) (L mg
)
n
R
NA
N
A
357
250
164
5.60
1.21
1.27
1.000
0.999
0.998
0.004
0.027
0.031
11.2
8.86
7.73
8.22
6.70
8.22
0.770
0.892
0.697
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Table
5
The adsorption capacities of the prepared NiO–Al
2
O
3
SKLWUT (2015-ZD-1) and the Natural Science Foundation of
Hubei Province of China (No. 2015CFA001) for Jiaguo Yu. This
research was nancially supported by the research grant of Early
Career Scheme (ECS 809813) from the Research Grant Council,
Hong Kong SAR Government, Dean's Research Fund-Early Career
Researchers (04022), and Research Equipment Grant (REG-2) for
Wingkei Ho. This work was supported by the National Natural
Science Funds (Grant No. 51208068, BY2013118) for Chunsheng
Lei.
samples for CR compared with other reported results
ꢀ1
Adsorbents
q
max (mg g
)
References
This work
Hierarchical NiO–Al
nanocomposite
Hierarchical NiO
2
O
3
357
250
164
152
83
204
60
99
60
208
37
300
189
This work
This work
Al
Hierarchical NiO nanosheets
Hierarchical Ni(OH) nanosheets
NiO–SiO hollow microspheres
Hollow hierarchical MnO
Hierarchically structured g-AlOOH
Porous ZrO hollow sphere
2
O
3
60
60
63
64
65
66
67
68
69
70
2
2
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