Investigation of Mesoporous Graphitic Carbon Nitride as the Adsorbent to Remove Ni (II) Ions

Abstract

The mesoporous graphitic carbon nitride (mpg-C3N4/r, r was defined as the initial silica/dicyandiamide mass ratio) was successfully synthesized by heating the mixture of silica and dicyandiamide in a nitrogen atmosphere. The morphology and structure of mpg-C3N4/r were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer- Emmett-Teller surface area measurement (BET), X-ray powder diffraction (XRD), and Fourier Transform Infrared spectroscopy (FTIR). The adsorption performances of Ni (II) ions by mpg-C3N4/r were investigated. With increasing of r value, the BET specific surface area of the synthesized mpg-C3N4/r increased; the highest specific surface area of mpg-C3N4/1.5 increased up to 169.3 m2/g. This work shows that mpg-C3N4/1.5 is a promising, high-efficiency adsorbent that can be used to purify the water of a low Ni (II) ions concentration. The maximum adsorption capacity of Ni(II) ions by mpg-C3N4/1.5 was 15.26 mg/g. The adsorption properties of Ni (II) ions by mpg-C3N4/r complied well with pseudo-second-order kinetics and Langmuir isotherm model.

Full text

Investigation of Mesoporous Graphitic Carbon
Nitride as the Adsorbent to Remove Ni (II) Ions
Gang Xin
1
*, Yuanjiao Xia
1
, Yuhua Lv
1
, Luman Liu
1
, Bei Yu
1
ABSTRACT: The mesoporous graphitic carbon nitride (mpg-C
3
N
4/r
,
r was defined as the initial silica/dicyandiamide mass ratio) was
successfully synthesized by heating the mixture of silica and
dicyandiamide in a nitrogen atmosphere. The morphology and
structure of mpg-C
3
N
4/r
were characterized by scanning electron
microscopy (SEM), transmission electron microscopy (TEM), Bruna-
uer-Emmett-Teller surface area measurement (BET), X-ray powder
diffraction (XRD), and Fourier Transform Infrared spectroscopy (FT-
IR). The adsorption performances of Ni (II) ions by mpg-C
3
N
4/r
were
investigated. With increasing of r value, the BETspecific surface area of
the synthesized mpg-C
3
N
4/r
increased; the highest specific surface area
of mpg-C
3
N
4/1.5
increased up to 169.3 m
2
/g. This work shows that
mpg-C
3
N
4/1.5
is a promising, high-efficiency adsorbent that can be used
to purify the water of a low Ni (II) ions concentration. The maximum
adsorption capacity of Ni(II) ions by mpg-C
3
N
4/1.5
was 15.26 mg/g. The
adsorption properties of Ni (II) ions by mpg-C
3
N
4/r
complied well with
pseudo-second-order kinetics and Langmuir isotherm model. Water
Environ. Res.,88, 318 (2016).
KEYWORDS: mesoporous materials, carbon nitride, adsorption,
nickel, mechanisms.
doi:10.2175/106143015X14212658613398
Introduction
Graphitic carbon nitride (g-C
3
N
4
), a covalent polymeric
compound, has attracted much attention because of its
combination of properties such as semiconductivity, photo-
electronics, basicity, and energy-storage capacity (Kawaguchi et
al., 2004; Khabasheske et al., 2000; Huynh et al., 2005; Pevida et
al., 2008; Chen et al., 2009). These properties are useful for
energy storage and transformation. Compared to g-C
3
N
4
,
mesoporous graphitic carbon nitride (mpg-C
3
N
4
) provides
improved performance. The higher specific surface area of
mpg-C
3
N
4
is as great as 830 m
2
/g and porosities are 1.25 m
3
/g
(Vinu, 2008). Consequently, many active sites are exposed on the
surface of mpg-C
3
N
4
, which results in greater size or shape-
selectivity (Li et al., 2010). Goettmann et al. in 2007 reported
that mpg-C
3
N
4
could function as an effective, metal-free catalyst
for the cyclisation of functional nitriles and alkynes (Goettmann
et al., 2007). Also, mpg-C
3
N
4
could be used for hydrogen
evolution in photocatalytic splitting water under visible light
irradiation. This application has shown improvement in
hydrogen evolution reaction compared to the g-C
3
N
4
(Wang
et al., 2009). Based on these properties, mpg-C
3
N
4
is considered
a promising catalyst.
Adsorption is a valuable process for removal of metal ions.
The mpg-C
3
N
4
had mesoporous multilayered structure (Wang
et al., 2009), which might be an effective adsorbent for the
removal of the toxic metal ions of a low concentration. As a
pollution source, Ni (II) ions are carcinogenic (Sivulka et al.,
2005) and could lead to serious system dysfunction in human
beings (Ferreira et al., 2011).
Existing research on adsorption of mpg-C
3
N
4
and the
interaction between metals and mpg-C
3
N
4
is limited. In this
study, mpg-C
3
N
4/r
was successfully prepared using silica
nanoparticles as a hard template and characterized by scanning
electron microscopy (SEM), transmission electron microscopy
(TEM), BrunauerEmmettTeller surface area measurement
(BET), X-ray powder diffraction (XRD), and Fourier Transform
Infrared spectroscopy (FT-IR).. The adsorption performances of
Ni (II) ions by mpg-C
3
N
4/r
also were investigated. The kinetic
parameters of the Ni (II) ions adsorption process were calculated
to examine the mechanisms of removing Ni (II) ions in aqueous
solution. Compared to other adsorbents, mpg-C
3
N
4/r
is an
environmentally friendly adsorbent that has high efficiency for
removing Ni (II) ions. This work shows the potential for
removing Ni (II) ions in aqueous solution using mpg-C
3
N
4/r
as
an adsorbent.
Material and Methods
Ludox (10-20 nm, 30 wt% dispersion of silicon dioxide
particles) was obtained from Snow Chemical Co., Ltd. Nickel
sulfate, supplied by Sinopharm Chemical Reagent Co. Ltd, was
used to prepare the nickel ions solution in adsorption
experiments. Dicyandiamide, methanol, ethanol, and ammoni-
um bifluoride were purchased from Sinopharm Chemical
Reagent Co. Ltd. All chemicals were reagent grade and used
without further purification. Deionized water was used in all
experiments.
The mesoporous graphitic carbon nitride (mpg-C
3
N
4/r
, r was
defined as the initial silica/dicyandiamide mass ratio) was
synthesized using a hard template method (Wang et al., 2009;
Gao et al., 2008). Then 3 g dicyandiamide was dissolved in a
mixture of ludox and 50 mL methanol with stirring at 353 K,
until the methanol and water vaporized completely. The mass of
ludox was 2.0 g, 5.0 g, 10.0 g, and 15.0 g, namely the initial silica/
dicyandiamide mass ratio r¼0.2, 0.5, 1.0, 1.5. Then the mixture
was heated at a rate of 20 K/min to reach the given temperature
in a nitrogen atmosphere and maintained at corresponding
temperature for another four hours. Subsequently, the powder
1
Faculty of Chemical, Environmental and Biological Science and
Tech no l o g y .
*
Faculty of Chemical, Environmental and Biological Science and
Technology, Dalian University of Technology, Dalian, 116024 China;
e-mail: gxin@dlut.edu.cn.
318 Water Environment Research, Volume 88, Number 4

was immersed in a 4 M NH
4
HF
2
solution for 24 hours to remove
the silica. The powders were then centrifuged and washed three
times with distilled water and twice with ethanol. Finally the
powders were dried at 353 K for 8 hours. The resulting samples
were denoted as mpg-C
3
N
4/r
-T (T was the given temperature).
For comparison, bulk g-C
3
N
4
was synthesized in the same
process without the silica template.
The morphology was observed by scanning electron micros-
copy (SEM, JEOL JSM-5600LV) and transmission electron
microscopy (TEM, JEM-2010). The specific surface areas of
the samples were determined by Brunauer-Emmet-Teller
analyzer (ASAP-2010). The crystalline phases and crystallinity
were identified by X-ray powder diffraction (DX-2000). Each
sample was scanned through a 2 hrange from 58to 808using Cu
Karadiation. The structure of the mpg-C
3
N
4/1.5
was examined
by FT-IR (Bruker TENSOR 27 FT-IR).
Next, NiSO
4
6H
2
O was selected as the adsorbate to examine
removal performance of Ni (II) ions from aqueous solution by
bulk g-C
3
N
4
and mpg-C
3
N
4/r
. In a typical experiment, nickel
stock solution (1000 mg/L) was prepared by dissolving
NiSO
4
6H
2
O in distilled water. The solution was further
diluted to the required concentrations before adsorption. All
adsorption experiments were performed at 293K in covered
beakers. To study the equilibrium adsorption capacity of Ni (II)
ions, 30 mg mpg-C
3
N
4/r
was added to 30 mL Ni (II) ions
solution at a concentration of 20.0 mg/L and maintained for 24
hours. Adsorption isotherms were obtained by adding 30 mg
mpg-C
3
N
4/1.5
to 30 mL Ni (II) ions solutions at initial
concentrations of 10.0 mg/L, 20.0 mg/L, 50.0 mg/L, 80.0 mg/
L, and 100.0 mg/L. For the kinetics experiments, the Ni (II)
ions removal amounts were determined by analyzing the
solution at appropriate time intervals, and the concentrations
of Ni (II) ions were determined with UV-Vis diffuse reflectance
spectroscopy (JASCA, UV-550).
The adsorption capacity (mg/g) was determined by the
equation:
Qe¼ðc0ceÞ3V
mð1Þ
Where
Q
e
¼removal amounts of Ni (II) ions per mass of the absorbent
at equilibrium (mg/g),
c
0
¼initial concentration of Ni (II) ions (mg/L),
c
e
¼concentration of Ni (II) ions at equilibrium (mg/L),
m¼mass of mpg-C
3
N
4/r
added (g), and
V¼the volume of solution (L).
Results and Discussion
Morphological, Structural, and Textural Features. Figure 1
illustrates the typical SEM and TEM images of the prepared
samples. Figure 1(A) shows that the bulk g-C
3
N
4
(r ¼0) was
composed of thick layers with a stratified structure and had
little mesoporous structure. In Figure 1(B), the mpg-C
3
N
4/0.2
sample consisted of interconnected thin layers with some
pores.Inthecaseofthempg-C
3
N
4/1.5
sample, as shown in
Figure 1(E), several small thin layers with abundant pores could
be observed. Figure 1(B–E) showed that the alveolate
architecture was well replicated on the mpg-C
3
N
4/r
(r¼0.2,
0.5, 1.0, 1.5) synthesized at 823 K. The removing of the silica
template played a key role in the formation of porous structure.
With the increasing of rvalue, the thickness of the sample was
significantly reduced and porous structure was generated
simultaneously. Therefore, enhancing the weight of the silica
template could make the resulted mpg-C
3
N
4/r
samples possess
small size, thin layers, and porous structure. The microstruc-
ture was further investigated by TEM. Figure 1(F) shows that
the mpg-C
3
N
4/1.5
sample had porous structure and possessed
many pores, which was beneficial for removing Ni (II) ions in
aqueous solution.
The textural properties of the samples were analyzed by
nitrogen adsorptiondesorption measurement. Figure 2 (a)
and (b) showed that the samples exhibited mesoporous
structure. With the increasing of rvalue, the surface area
and pores of mpg-C
3
N
4/r
increased. The curves of bulk g-
C
3
N
4
and mpg-C
3
N
4/r
samples were type IV with a hysteresis
loop at high relative pressure between 0.5 and 1.0, suggesting
the presence of mesopores (2-50 nm) and macropores (.50
nm) (Dong et al., 2014). The capillary condensation step of the
synthesized samples occurred at relative pressure of approx-
imately 0.45, suggesting pore size smaller than 5 nm (Sing et
al., 1985). Figure 2(b) shows the pore-size distribution curves
estimated using the desorption branch of the isotherm by the
Barrett-Joyner-Halenda (BJH) method. The curves in Figure
2(b) illustrate that bulk g-C
3
N
4
and mpg-C
3
N
4/r
exhibited
narrow pore-size distribution. The pore-size distribution
curves of bulk g-C
3
N
4
and mpg-C
3
N
4/r
display a sharp peak
at around 3.42 nm and a weak broad peak that gradually
declines, which indicates that there were many mesoporous
with different sizes in the samples (Sing et al., 1985). Figure
2(c) shows the BET specific surface area increases gradually
with the increasing of rvalue. The textural properties of all
samples are presented in Table 1. The mpg-C
3
N
4/1.5
exhibited
thehighestsurfaceareaamongallsamples,uptoas169.31
m
2
/g. Moreover, the pore size and total pore volume of the
mpg-C
3
N
4/1.5
(3.61 nm, 0.243 cm
3
/g) were larger than that of
bulk g-C
3
N
4
(3.41 nm, 0.033 cm
3
/g). The mpg-C
3
N
4/1.5
had
Figure 1—Scanning electron microscope (SEM) images of bulk
g-C
3
N
4
(A); mpg-C
3
N
4/0.2
(B); mpg-C
3
N
4/0.5
(C); mpg-C
3
N
4/1.0
(D);
and mpg-C
3
N
4/1.5
(E) synthesized at 823K. Transmission electron
microscopy (TEM) image of mpg-C
3
N
4/1.5
(F) synthesized at 823K.
Xin et al.
April 2016 319

the largest pore size and highest pore volume, which could be
used as a preferable absorbent to removing Ni (II) ions in
aqueous solution. According to the literature (Vinu, 2008), the
surface area of mpg-C
3
N
4
could reach 830 m
2
/g, which was
greater than the data of mpg-C
3
N
4/1.5
. The reason for this is
that a different template was used. The experiment in the
literature used SBA-15 materials with different pore diameters
as templates (Vinu, 2008), and this experiment used Ludox
(10-20 nm, 30 wt% dispersion of silicon dioxide particles) as
template. In this study, it was predicted that mpg-C
3
N
4
with a
higher surface area would have better adsorption capacity. But
many reactive amino groups are found on the surface of mpg-
C
3
N
4/1.5
because of its mesoporous multilayered structure,
which was beneficial for removing Ni (II) ions in aqueous
solution of a low concentration. The adsorption free energy
analysis (E ¼8.45 kJ/mol) also proved that the adsorption
process of Ni (II) ions onto mpg-C
3
N
4/1.5
took place via a
chemical adsorption mechanism. The surface area had a
strong effect on physical absorption and little effect on
chemical absorption. In our experiment, the mpg-C
3
N
4/1.5
already has presented a high removal capacity of Ni (II) ions
reaching a value of 15.26 mg in 20 mg/L nickel solution.
Figure 3 shows the XRD pattern of the bulk g-C
3
N
4
and
mpg-C
3
N
4/r
synthesized at the temperature of 823 K. All of the
samples had similar diffraction patterns as those seen in Figure
3. A typical peak of approximately 27.58was indexed as (002)
crystal face of C
3
N
4
(JCPDS Card No. 87-1526), which
indicates the graphite-like stacking of the conjugated aromatic
units of CN with an interlayer distance of 0.33 nm (Goettmann
et al., 2006). The small additional peak at around 13.28,
corresponding to interplanar distance of 0.670 nm, was
indexed as (100) crystal face, which was associated with
interlayer stacking (Goettmann et al., 2006). Figure 3 also
illustrates that the diffraction peak intensity weakens with
increasing of rvalue. The decrease of the diffraction peak
intensity implies that the crystallinity of the samples had been
reduced. In addition, there is no typical diffraction peak of
silica in Figure 3, which indicates that the silica had been
removed completely.
The FT-IR adsorption spectrum of mpg-C
3
N
4/1.5
is displayed
in Figure 4, which shows the characteristic vibrational peaks
related to the bonds between carbon and nitrogen. The
absorption peak at 810 cm
1
corresponds to the characteristic
breathing mode of the triazine units. Several strong bands in
the region of 1240-1590 cm
1
could be attributed to the
stretching modes of C-N heterocyclics (Tang et al., 2013;
Figure 2—N
2
adsorption-desorption isothermal (a), the
corresponding pore-size distribution curves calculated by
Barrett-Joyner-Halenda (BJH) method (b) and Brunauer-
Emmett-Teller (BET) surface area (c) of bulk g-C
3
N
4
(r ¼0) and
mpg-C
3
N
4/r
(r ¼0.2, 0.5, 1.0, 1.5) sample. The pore-size
distribution was determined from the desorption branch of the
isothermal. The samples were synthesized at the temperature of
823K.
Figure 3—X-ray powder diffraction (XRD) patterns of bulk g-C
3
N
4
(r ¼0) and mpg-C
3
N
4/r
(r ¼0.2, 0.5, 1.0, 1.5) synthesized at the
temperature of 823K.
Table 1—Textural properties of bulk g-C
3
N
4
and mpg-C
3
N
4/r
(r¼
0.2, 0.5, 1.0, 1.5).
Sample S
BET
(m
2
/g)
Total pore
volume (cm
3
/g)
Peak pore
size (nm)
g-C
3
N
4
15.88 0.033 3.41
mpg-C
3
N
4/0.2
27.02 0.044 3.41
mpg-C
3
N
4/0.5
46.4 0.146 3.56
mpg-C
3
N
4/1.0
137.44 0.133 3.53
mpg-C
3
N
4/1.5
169.31 0.243 3.61
Figure 4—Fourier Transform Infrared spectroscopy (FT-IR)
spectra for mpg-C
3
N
4/1.5
powder synthesized at the temperature
of 823K diluted in a potassium bromide pellet.
Xin et al.
320 Water Environment Research, Volume 88, Number 4

Komatsu, 2001a; Komatsu, 2001b; Komatsu, 2001c; Li et al.,
2009). The adsorption peak at approximately 1336 cm
1
could
be attributed to C-N and at 1641 cm
1
comes from the C¼N
stretching mode (Akbal, 2005; Yan et al., 2009; Shi et al., 2014;
Groenewolt and Antonietti, 2005). The broad bands in the
range of the 3000-3700 cm
1
could be attributed to the
adsorbed H
2
O molecules and N-H vibration (Gao et al., 2008).
The residual hydrogen atoms bound to the edge of the C-N
sheet in the form of C-NH
2
and 2 C-NH bonds (Li et al., 2009).
The infrared characteristic peaks of silica could not be found in
Figure 4, which also indicates that the silica had been removed
completely.
Adsorptive Property for Nickel Ions. The equilibrium
adsorption capacity of mpg-C
3
N
4/r
for Ni (II) ions is shown in
Figure 5. The mpg-C
3
N
4/r
-823 presents a greater adsorption
capacity than mpg-C
3
N
4/r
-793 in Ni (II) ions solution at the
concentration of 20.0 mg/L. Moreover, the equilibrium adsorp-
tion capacity of mpg-C
3
N
4/r
-793 and mpg-C
3
N
4/r
-823 increase
rapidly with the increasing rvalue. The adsorptive capacity of
mpg-C
3
N
4/1.5
-823 (15.26 mg/g) was approximately 14-times
greater than that of bulk g-C
3
N
4
-823 (1.14 mg/g).
Comparison with Other Adsorbents. The adsorption
capacities of Ni(II) ions on mpg-C
3
N
4/1.5
were compared with
other various adsorbents reported in previous literature as
(Table 2). In some cases, mpg-C
3
N
4/1.5
had greater removal
efficiency for Ni(II) ions in this experiment compared with other
adsorbents, which could be ascribed to its relatively larger
specific surface area. It also has the advantage of simpler
synthesis and greater affordability and environmental friendli-
ness than many other adsorbents. These advantages make mpg-
C
3
N
4/1.5
more competitive to remove Ni(II) ions in terms of
potential application in wastewater treatment.
Effect of Contacting Time. The time to reach equilibrium
was a critical parameter for the economic concern in the
practical applications. The effect of contact time on the
adsorption of Ni (II) ions by mpg-C
3
N
4/1.5
is presented in
Figure 6(a). The initial concentration of Ni (II) ions was 20.0 mg/
L. More than 50 % of Ni (II) ions were absorbed within 5
minutes, and the adsorption reached equilibrium in about 60
minutes.
Kinetics Studies. Two well-known adsorption models,
pseudo-first-order and pseudo-second-order equations were
used to investigate the adsorption mechanism and kinetics of
Ni (II) ions by mpg-C
3
N
4/1.5
(Zhou et al., 2010).
The pseudo-first-order equation was presented as:
dQt
dt ¼k1ðQeQtÞð2Þ
Where
Q
e
¼removal amounts of Ni (II) ions per mass of the
absorbent at equilibrium (mg/g),
Q
t
¼removal amounts of Ni (II) ions per mass of the
absorbent at time t(mg/g), and
k
1
¼the rate constant of the pseudo-first-order model
(min
1
).
Figure 5—Equilibrium adsorption capacity for Ni (II) ions of bulk
g-C
3
N
4
(r ¼0) and mpg-C
3
N
4/r
(r ¼0.2, 0.5, 1.0, 1.5) synthesized
at the temperature of 793K and 823K.
Table 2—Adsorption capacities for Ni(II) ions onto mpg-C
3
N
4/1.5
and various adsorbents reported in the previous literature.
Adsorbent
Adsorption
capacity (mg/g) Reference
Grapefruit peel 46.13 Torab-Mostaedi et al., 2013
Sericite 44.0 Kwon and Jeon., 2013
Chlorella vulgaris 29.29 Ferreira et al., 2011
Zeolite X 24.88 Quintelas et al., 2013
Arthrospira (spirulina)
platensis 20.78 Ferreira et al., 2011
Cone biomass of Thuja
orientalis 18.42 Malkoc., 2006
Activated carbon 17.24 Lata et al., 2008
CA/zeolite fiber 16.95 Ji et al., 2012
mpg-C
3
N
4/1.5
-823 15.26 Present study
Black carrot residues 12.42 G¨
uzel et al., 2008
Macroporous chitosan
membrane 10.3 Ghaee et al., 2012
mpg-C
3
N
4/1.5
-793 8.19 Present study
Multiwalled carbon
nanotubes 6.09 T. Abdel-Ghani et al., 2014
PVDF membrane 5.306 Madaeni et al., 2011
Beech sawdust 4.0 Boˇ
zi´
c et al., 2013
Kaolinite 1.669 Yavuz et al., 2003
Figure 6—Isothermal kinetics adsorption for Ni(II) ions of mpg-
C
3
N
4/1.5
at 293K (a). Plots of pseudo-first-order (b) and pseudo-
second-order (c) kinetics model for adsorption of Ni(II) ions on
mpg-C
3
N
4/1.5
at 293K.
Xin et al.
April 2016 321

The linear form of pseudo-first-order equation was expressed
as:
lnðQeQtÞ¼lnQek1tð3Þ
According to the experimental isotherm data, the correlation
coefficient (R
2
) was 0.9051 (Figure 6(b)), which suggested that
the date was not well fitted to pseudo-first-order equation.
The pseudo-second-order equation was described as:
dQt
dt ¼k2ðQeQtÞ2ð4Þ
Where
Q
e
¼removal amounts of Ni (II) ions per mass of the
absorbent at equilibrium (mg/g),
Q
t
¼removal amounts of Ni (II) ions per mass of the
absorbent at time t(mg/g), and
k
2
¼the rate constant of the pseudo-second-order model (g/
mgmin).
The linear form of pseudo-second-order equation was
expressed as:
t
Qt¼1
k2Q2
e

þt
Qeð5Þ
As shown in Figure 6(c), the linear plots of t/Q
t
against t
presented a high correlation coefficient (R
2
) of 0.9999.
Therefore, the adsorption of Ni (II) ions by mpg-C
3
N
4/1.5
followed the pseudo-second-order equation. It suggests that the
adsorption process was a chemisorption process involving
exchange or sharing of electrons between Ni (II) ions and
mpg-C
3
N
4/r
(Ncibi et al., 2007). The parameters of pseudo-first-
order and pseudo-second-order kinetics model for the adsorp-
tion of Ni (II) ions onto mpg-C
3
N
4/1.5
are presented in Table 3.
Adsorption Isotherms. The way adsorbates interacted with
adsorbents was the most important parameter for designing an
expected adsorption system, which could be successfully
described by isotherms studies. Adsorption isotherm models
could provide useful data to understand the mechanisms of the
adsorption process.
Figure 7 shows that the equilibrium adsorption capacity of
mpg-C
3
N
4/1.5
was enhanced with increasing initial concentra-
tion of Ni (II) ions. The Langmuir, Freundlich, and Dubinbin-
Radushkevich (D-R) isotherm models are widely used isotherm
models (Zhou et al., 2010; Su et al., 2010; Ge et al., 2012; Ncibi et
al., 2007; Dubinin and Radushkevich, 1947), which were applied
to quantify the adsorption capacity of Ni (II) ions onto mpg-
C
3
N
4/1.5
. The Langmuir and Freundlich isotherm plots for Ni(II)
ions of mpg-C
3
N
4/1.5
are given in Figure 7(a) and Figure 7(b),
respectively.
The Langmuir equation was represented as:
1
Qe¼1
Qmax þ1
KLQmaxCeð6Þ
Where
C
e
¼equilibrium Ni (II) ions concentration in solution
(mg/L),
Q
e
¼equilibrium Ni (II) ions concentration on the
adsorbent (mg/g),
Q
max
¼monolayer capacity of mpg-C
3
N
4/1.5
samples (mg/g),
and
K
L
¼Langmuir adsorption constant related to the free
energy of adsorption (L/mg).
The value of 1
Qeagainst 1
Ceis shown in Figure 7(c). According to
the experimental isotherm data, the correlation coefficient (R
2
)
was 0.9676.
The Freundlich equation was represented as:
lnQe¼lnKFþbFlnCeð7Þ
Where
C
e
¼equilibrium Ni (II) ions concentration in solution (mg/L),
Q
e
¼equilibrium Ni (II) ions concentration on the adsorbent
(mg/g),
b
F
¼Freundlich constants corresponding to adsorption
intensity, and
K
F
¼Freundlich constants corresponding to adsorption
capacity (mg/g (L/mg)
1/n
).
The plot of lnQ
e
versus lnC
e
was used to generate the
intercept K
F
and slope b
F
. According to the experimental
isotherm data, the linear plots of lnQ
e
versus lnC
e
in Figure 7(d)
presented a lower correlation coefficient (R
2
) of 0.9444,
indicating that the Langmuir isotherm model showed a better
Table 3—Parameters of pseudo-first-order and pseudo-second-
order model for the adsorption of Ni (II) ions onto mpg-C
3
N
4/1.5
at
293K.
Pseudo-first-order model Pseudo-second-order model
k
1
(min
1
)Q
e
(mg/g) R
2
k
2
(g/mgmin) Q
e
(mg/g)R
2
R
2
0.0181 4.2431 0.9051 0.0166 15.5763 0.9999
Figure 7—Plots of Langmuir (a) and Freundlich (b) isotherm
model for adsorption of Ni(II) ions on mpg-C
3
N
4/1.5
at 293K. The
values of 1
Qeagainst 1
Cebased on the Langmuir isotherm model (c).
The linear dependence of lnQ
e
on lnC
e
based on the Freundlich
isotherm model (d).
Xin et al.
322 Water Environment Research, Volume 88, Number 4

fit to adsorption data than the Freundlich isotherm model for
the adsorption of Ni (II) ions onto mpg-C
3
N
4/1.5
.
The D-R isotherm model postulated having an adsorption
space close to the adsorbent surface (Valsala et al., 2010). The
model was represented as:
Qe¼Qmaxebe2ð8Þ
Where
Q
e
¼equilibrium Ni (II) ions concentration on the adsor-
bent (mg/g),
Q
max
¼monolayer capacity of mpg-C
3
N
4/1.5
samples (mg/g),
b¼the activity coefficient (mol
2
/kJ
2
), and
e¼Polanyi potential (kJ
2
/mol
2
).
ecan be calculated by the following equation:
e¼RTln 1 þ1
Ce
 ð9Þ
Where
C
e
¼equilibrium Ni (II) ions concentration in solution (mol/
m
3
),
R¼universal gas constant, 8.314 310
3
kJ/ (molK), and
T¼temperature in Kelvin (K).
The mean adsorption free energy (Valsala et al., 2010), E (kJ/
mol), provided information about the mechanisms of the
adsorption process. If E value was in the range of 8-16 kJ/mol,
then the adsorption process occurred chemically; whereas when
E,8 kJ/mol, then the mechanism of the adsorption process was
physical (Dubinin and Radushkevich, 1947).
The E value could be calculated using the following equation:
E¼1
ffiffiffiffiffiffiffiffiffi
2b
pð10Þ
The linear form of D-R isotherm model was expressed as:
lnQe¼lnQmax be2ð11Þ
The D-R model constants, Q
max
and b, could be determined
from the intercept and slope of linear plot of lnQ
e
versus e
2
,
respectively.
The parameters of Langmuir, Freundlich and D-R isotherm
models are presented in Table 4. The mean adsorption energy (E)
was calculated as 8.45 kJ/mol for the adsorption of Ni (II) ions
(Fig. 8). There were a lot of reactive amino groups on the surface
of mpg-C
3
N
4/1.5
on account of mesoporous multilayered
structure, which was beneficial for removing Ni (II) ions in
aqueous solution of a low concentration. The adsorption free
energy analysis (E ¼8.45 kJ/mol) also proved that the adsorption
process of Ni (II) ions onto mpg-C
3
N
4/1.5
took place in chemical
adsorption mechanism.
Conclusions
The mpg-C
3
N
4/r
was successfully prepared using silica
nanoparticles as a hard template, and the adsorption perfor-
mances of Ni (II) ions by mpg-C
3
N
4/1.5
were investigated. The
show that mpg-C
3
N
4/1.5
is an effective adsorbent for removal of
Ni (II) ions from aqueous solutions. The mpg-C
3
N
4/1.5
-823
presented the highest removal capacity of Ni (II) ions, reaching a
value of 15.26 mg in 20 mgL
1
nickel solution. The adsorption
kinetics and isotherm of Ni (II) ions onto mpg-C
3
N
4/1.5
showed
that the adsorption process complied with the pseudo-second-
order kinetics and Langmuir isotherm model. The adsorption
process was a chemical adsorption behavior, which could be
proved by adsorption free energy analysis (E ¼8.45 kJ/mol).
Considering the excellent adsorptive capacity for Ni (II) ions, it
was confirmed that mpg-C
3
N
4/1.5
is a promising high-efficiency
adsorbent to purify water of a low Ni (II) ions concentration.
Acknowledgment
This work was financially supported by the Fundamental
Research Founds for the Central Universities (DUT13LK43) and
the fund of Metastable Materials Science and Technology State
Key Laboratory.
Submitted for publication May 26, 2014; accepted for
publication February 3, 2015.
References
Akbal, F. (2005) Photocatalytic Degradation of Organic Dyes in the
Presence of Titanium Dioxide under UV and Solar Light: Effect of
Operational Parameters. Environ. Prog., 24, 317–322.
Boˇ
zi´
c, D.; Gorgievski, M.; Stankovi´
c, V.; ˇ
Strbac, N.; ˇ
Serbula, S.; Petrovi´
c
N. (2013) Adsorption of Heavy Metal Ions by Beech Sawdust –
Kinetics, Mechanism and Equilibrium of the Process. Ecol. Eng., 58,
202–206.
Table 4—Langmuir, Freundlich and Dubinin-Radushkevich (D-R)
Isotherm Parameters for the adsorption of Ni (II) ions onto mpg-
C
3
N
4/1.5
at 293K.
Langmuir Q
max
(mg/g) 19.39
K
L
(L/mg) 1.127
R
L2
0.9676
Freundlich K
F
(mg/g(L/mg)
1/n
) 10.24
b
F
0.1686
R
F2
0.9444
Dubinin-Radushkevich Q
max
(mg/g) 20.15
b(mol
2
/kJ
2
) 0.007
E(kJ/mol) 8.45
R
2
0.9825
Figure 8—Dubinin-Redushckevich (D-R) isotherm plots for the
adsorption of Ni(II) ions onto mpg-C
3
N
4/1.5
at 293K.
Xin et al.
April 2016 323

Chen, X. F.; Jun, Y. S.; Takanabe, K.; Maeda, K.; Domen, K.; Fu, X. Z.;
Antonietti, M.; Wang, X. C. (2009) Ordered Mesoporous SBA-15
Type Graphitic Carbon Nitride: A Semiconductor Host Structure
for Photocatalytic Hydrogen Evolution with Visible Light. Chem.
Mater., 21, 4093–4095.
Dong, F.; Ou, M. Y.; Jiang, Y. K.; Guo, S.; Wu, Z. B. (2014) Efficient and
Durable Visible Light Photocatalytic Performance of Porous Carbon
Nitride Nanosheets for Air Purification. Ind. Eng. Chem. Res., 53,
2318–2330.
Dubinin, M. M.; Radushkevich, L. V. (1947) Equation of the Character-
istics Curve of Activated Charcoal. Chem. Zent., 1, 875.
Ferreira, L. S.; Rodrigues, M. S.; de Carvalho, J. C. M.; Lodi, A.; Finocchio,
E.; Perego, P.; Converti, A. (2011) Adsorption of Ni
2þ
,Zn
2þ
and Pb
2þ
onto Dry Biomass of Arthrospira (Spirulina) platensis and Chlorella
vulgaris. I. Single metal systems. Chem. Eng. J., 173, 326–333.
Gao, Z. M.; Bandosz, T. J.; Zhao, Z. B.; Han, M.; Liang, C. H.; Qiu, J. S.
(2008) Investigation of the Role of Surface Chemistry and
Accessibility of Cadmium Adsorption Sites on Open-Surface
Carbonaceous Materials. Langmuir,24, 11701–11710.
Ge, L.; Zuo, F.; Liu, J. K. (2012) Synthesis and Efficient Visible Light
Photocatalytic Hydrogen Evolution of Polymeric g-C
3
N
4
Coupled
with CdS Quantum Dots. J. Phys. Chem. C,116, 13708–13714.
Ghaee, A.; Shariaty-Niassar, M.; Barzin, J.; Zarghan, A. (2012) Adsorption
of Copper and Nickel Ions on Macroporous Chitosan Membrane:
Equilibrium Study. Appl. Surf. Sci., 258, 7732–7743.
Goettmann, F.; Fischer, A.; Antonietti, M.; Thomas, A. (2006) Chemical
Synthesis of Mesoporous Carbon Nitrides Using Hard Templates
and Their Use as a Metal-Free Catalyst for Friedel–Crafts Reaction
of Benzene. Angew. Chem. Int. Ed., 45, 4467–4471.
Goettmann,F.;Fischer,A.;Antonietti,M.;Thomas,A.(2007)
Mesoporous Graphitic Carbon Nitride as a Versatile, Metal-free
Catalyst for the Cyclisation of Functional Nitriles and Alkynes. New
J. Chem., 31, 1455–1460.
Groenewolt, M.; Antonietti, M. (2005) Synthesis of g-C
3
N
4
Nanoparti-
cles in Mesoporous Silica Host Matrices. Adv. Mater., 17, 1789–
1792.
G¨uzel, F.; Yakut, H.; Topal, G. (2008) Determination of Kinetic and
Equilibrium Parameters of the Batch Adsorption of Mn(II), Co(II),
Ni(II) and Cu(II) from Aqueous Solution by Black Carrot (Daucus
carota L.) residues. J. Hazard. Mater., 153, 1275–1287.
Huynh, M. H. V.; Hiskey, M. A.; Archuleta, J. G.; Roemer, E. L. (2005)
Preparation of Nitrogen-Rich Nanolayered, Nanoclustered, and
Nanodendritic Carbon Nitrides. Angew. Chem. Int. Ed., 44, 737–
739.
Ji, F.; Li, C. L.; Tang, B.; Xu, J. H.; Lu, G.; Liu P. (2012) Preparation of
Cellulose Acetate/Zeolite Composite Fiber and its Adsorption
Behavior for Heavy Metal Ions in Aqueous Solution. Chem. Eng. J.,
209, 325–333.
Kawaguchi, M.; Yagi, S.; Enomoto, H. (2004) Chemical Preparation and
Characterization of Nitrogen-rich Carbon Nitride Powders. Carbon,
42, 345–350.
Khabasheske, V. N.; Zimmerman, J. L.; Margrave, J. L. (2000) Powder
Synthesis and Characterization of Amorphous Carbon Nitride.
Chem. Mater., 12, 3264–3270.
Komatsu, T. (2001a) The First Synthesis and Characterization of
Cyameluric High Polymers. Macromol. Chem. Phys., 202, 19–25.
Komatsu, T. (2001b) Prototype Carbon Nitrides Similar to the Symmetric
Triangular form of Melon. J. Mater. Chem., 11, 802–803.
Komatsu, T. (2001c) Attempted Chemical Synthesis of Graphite-like
Carbon Nitride. J. Mater. Chem., 11, 799–801.
Kwon, T. N.; Jeon, C. (2013) Adsorption Characteristics of Sericitefor Nickel
Ions from Industrial Waste Water. J. Ind. Eng. Chem., 19, 68–72.
Lata, H.; Garg, V. K.; Gupta, R. K. (2008) Sequestration of Nickel from
Aqueous Solution onto Activated Carbon Prepared from Parthe-
nium hysterophorus L. J. Hazard. Mater., 157, 503–509.
Li, X. F.; Zhang, J.; Shen, L. H.; Ma, Y. M.; Lei, W. W.; Cui, Q. L.; Zou, G.
T. (2009) Preparation and Characterization of Graphitic Carbon
Nitride through Pyrolysis of Melamine. Appl. Phys. A: Mater. Sci.
Proc., 94, 387–392.
Li, Q.; Yang, J. P.; Feng, D.; Wu, Z. X.; Wu, Q. L.; Park, S. S.; Ha, C. S.;
Zhao, D. Y. (2010) Facile Synthesis of Porous Carbon Nitride
Spheres with Hierarchical Three-Dimensional Mesostructures for
CO
2
Capture. Nano Res., 3, 632–642.
Madaeni, S. S.; Zinadini, S.; Vatanpour, V. (2011) Convective Flow
Adsorption of Nickel Ions in PVDF Membrane Embedded with
Multi-walled Carbon Nanotubes and PAA Coating. Sep. Pur.
Technol., 80, 155–162.
Malkoc, E. (2006) Ni(II) Removal from Aqueous Solutions Using Cone
Biomass of Thuja Orientalis. J. Hazard. Mater. B,137, 899–908.
Ncibi, M. C.; Mahjoub, B.; Seffen, M. (2007) Kinetic and Equilibrium
Studies of Methylene Blue Biosorption by Posidonia oceanica (L.)
fibers. J. Hazard. Mater. B,139, 280–285.
Pevida, C.; Drage, T. C.; Snape, C. E. (2008) Silica-templated Melamine–
formaldehyde Resin Derived Adsorbents for CO
2
Capture. Carbon,
46, 1464–1474.
Quintelas, C.; Pereira, R.; Kaplan, E.; Tavares T. (2013) Removal of Ni(II)
from Aqueous Solutions by an Arthrobacter viscosus Biofilm
Supported on Zeolite: From Laboratory to Pilot Scale. Biores.
Technol., 142, 368–374.
Su, F. Z.; Mathew, S. C.; Lipner, G.; Fu, X. Z.; Antonietti, M. (2010) mpg-
g-C
3
N
4
-Catalyzed Selective Oxidation of Alcohols Using O
2
and
Visible Light. J. Am. Chem. Soc., 132, 16299–16301.
Shi, Y. Q.; Jiang, S. H.; Zhou, K. Q.; Wang, B. B.; Wang, B.; Gui, Z.; Hu, Y.;
Yuen, R. K. K. (2014) Facile Preparation of ZnS/g-C
3
N
4
Nano-
hybrids for Enhanced Optical Properties. RSC Adv., 4, 2609–2613.
Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.;
Rouquerol, J.; Siemieniewska, T. (1985) Reporting Physisorption
Data for Gas/Solid Systems. Pure Appl. Chem., 57, 603–619.
Sivulka, D. J. (2005) Assessment of Respiratory Carcinogenicity
Associated with Exposure to Metallic Nickel: A Review. Regul.
Toxicol. Pharm., 43, 117–133.
T. Abdel-Ghani, N.; A. El-Chaghaby, G.; S. Helal, F. (2014) Individual and
Competitive Adsorption of Phenol and Nickel onto Multiwalled
Carbon Nanotubes. J. Adv. Res., In Press.
Tang, J.; Wang, T.; Pan, X. C.; Sun, X.; Fan, X. L.; Guo, Y. X.; Xue, H. R.;
He, J. P. (2013) Synthesis and Electrochemical Characterization of
N-Doped Partially Graphitized Ordered Mesoporous Carbon-Co
Composite. J. Phys. Chem. C,117, 16896–16906.
Torab-Mostaedi, M.; Asadollahzadeh, M.; Hemmati, A.; Khosravi A.
(2013) Equilibrium, Kinetic, and Thermodynamic Studies for
Biosorption of Cadmium and Nickel on Grapefruit Peel. J. Taiwan
Inst. Chem. Eng., 44, 295–302.
Valsala, Annie Joseph, T. P.; Sonar, N. L.; Sonavane, M. S.; Shah, Kanwar
Raj, J. G.; Venugopal, V. (2010) Separation of Strontium from Low
Level Radioactive Waste Solutions Using Hydrous Manganese
Dioxide Composite Materials. J. Nucl. Mater., 404, 138–143.
Vinu, A. (2008) Two-Dimensional Hexagonally-Ordered Mesoporous
Carbon Nitrides with Tunable Pore Diameter, Surface Area and
Nitrogen Content. Adv. Funct. Mater., 18, 816–827.
Wang, X. C.; Maeda, K.; Chen, X. F.; Takanabe, K.; Domen, K.; Hou, Y. D.;
Fu, X. Z.; Antonietti, M. (2009) Polymer Semiconductors for
Artificial Photosynthesis: Hydrogen Evolution by Mesoporous
Graphitic Carbon Nitride with Visible Light. J. Am. Chem. Soc.,
131, 1680–1681.
Yan, S. C.; Li, Z. S.; Zou, Z. G. (2009) Photodegradation Performance of
g-C
3
N
4
Fabricated by Directly Heating Melamine. Langmuir,25,
10397–10401.
Yavuz, ¨
O.; Altunkaynak, Y.; G¨uzel, F. (2003) Removal of Copper, Nickel,
Cobalt and Manganese from Aqueous Solution by Kaolinite. Water
Res., 37, 948–952.
Zhou, L.; Gao, C.; Xu, W. J. (2010) Magnetic Dendritic Materials for
Highly Efficient Adsorption of Dyes and Drugs. ACS Appl. Mater.
Int., 2, 1483–1491.
Xin et al.
324 Water Environment Research, Volume 88, Number 4