Tuned surface area and mesopore diameter of ordered mesoporous carbon: Ultrahigh decontamination of di(2-ethylhexyl)phthalate

Article abstract of DOI:10.1039/c4ra01570c

Synthesis of carbon materials with enhanced surface areas, and regular and tuned pore diameters is always a great challenge. In this report, ordered mesoporous carbons (OMCs) were synthesized by the one-step assembly of tri-constituents, and the OMCs were applied as an adsorbent for the removal of the highly hazardous water pollutant di(2-ethylhexyl)phthalate (DEHP). Phloroglucinol-formaldehyde based carbon precursor was in situ prepared during the assembly of the tri-constituents, and the surface area and mesopore diameter of the OMCs were tuned by variation of the molar ratio of formaldehyde to phloroglucinol. Small angle X-ray diffraction patterns revealed that the obtained carbons are highly ordered, which is in agreement with the measuring results of transmission electron microscopy at low and high resolution. Scanning electron microscopy images demonstrate that OMC-F_2.0 has a hierarchical morphology. Nitrogen adsorption-desorption measurements revealed that the surface area of the OMCs (956-1801 m² g^-1) was dependent on the molar ratio of the carbon precursor constituents (formaldehyde to phloroglucinol). By varying the molar ratio of formaldehyde to phloroglucinol from 1.0 to 4.0, the mesopore diameter of the OMCs was shifted to the higher side, from 2.1 to 3.1 nm. DEHP was efficiently removed from a model water pollutant by the OMCs. OMC-F_2.0 achieved the highest adsorption capacity of 364 mg g^-1 for the removal of DEHP. The adsorption equilibrium data were treated with the two mathematical models of Langmuir and Freundlich, and the results revealed that decontamination was more favorable with the Langmuir model. This concludes that the removal of DEHP by OMCs depends on the surface area, and the DEHP molecules occupied the porous space of the OMCs in a monolayer manner.

Full text of DOI:10.1039/c4ra01570c

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Tuned surface area and mesopore diameter of
ordered mesoporous carbon: ultrahigh
Cite this: RSC Adv., 2014, 4, 23853
decontamination of di(2-ethylhexyl)phthalate
Pranav K. Tripathi, Mingxian Liu* and Lihua Gan*
Synthesis of carbon materials with enhanced surface areas, and regular and tuned pore diameters is always a
great challenge. In this report, ordered mesoporous carbons (OMCs) were synthesized by the one-step
assembly of tri-constituents, and the OMCs were applied as an adsorbent for the removal of the highly
hazardous water pollutant di(2-ethylhexyl)phthalate (DEHP). Phloroglucinol–formaldehyde based carbon
precursor was in situ prepared during the assembly of the tri-constituents, and the surface area and
mesopore diameter of the OMCs were tuned by variation of the molar ratio of formaldehyde to
phloroglucinol. Small angle X-ray diffraction patterns revealed that the obtained carbons are highly
ordered, which is in agreement with the measuring results of transmission electron microscopy at low and
high resolution. Scanning electron microscopy images demonstrate that OMC-F2.0 has a hierarchical
morphology. Nitrogen adsorption–desorption measurements revealed that the surface area of the OMCs
2
ꢀ1
(
956–1801 m g ) was dependent on the molar ratio of the carbon precursor constituents (formaldehyde
to phloroglucinol). By varying the molar ratio of formaldehyde to phloroglucinol from 1.0 to 4.0, the
mesopore diameter of the OMCs was shifted to the higher side, from 2.1 to 3.1 nm. DEHP was efficiently
removed from a model water pollutant by the OMCs. OMC-F2.0 achieved the highest adsorption capacity
Received 23rd February 2014
Accepted 11th April 2014
ꢀ1
of 364 mg g for the removal of DEHP. The adsorption equilibrium data were treated with the two
mathematical models of Langmuir and Freundlich, and the results revealed that decontamination was more
favorable with the Langmuir model. This concludes that the removal of DEHP by OMCs depends on the
surface area, and the DEHP molecules occupied the porous space of the OMCs in a monolayer manner.
DOI: 10.1039/c4ra01570c
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feasible for the removal of DEHP from contaminated water and
wastewater. Therefore it is very important to remove DEHP
1
. Introduction
7,8
Water is essential for survival of life; but on the contrary it has from polluted water and wastewater by using effective treatment
become a major source of disease because the water stream is technology.
1
becoming exponentially contaminated. Increasing human
Adsorption is one of the potential methods for the removal of
1,9
dependency on plastic materials resulted in the use of di(2- soluble and insoluble water pollutants. Activated carbon (AC)
ethylhexyl)phthalate (DEHP) as a preferred plasticizer. DEHP is is widely used as an adsorbent for water and wastewater treat-
a water contaminate that is highly threatening to the health of ment because of its high surface area and chemically inert
living beings, even at very low concentration, and is considered nature. However, AC has small and irregular micropores (<2
2
as an endocrine disruptive chemical. Prolonged contact with nm), which dramatically limits its capability for the removal of
DEHP contaminated water can lead to damage of reproductive large molecules such as DEHP (Table 1). Several studies
3
capability, memory loss and many others diseases. The Inter- revealed that the abundant micropores of AC limit the access of
1
0–12
national Agency for Research on Cancer considered DEHP as big molecules to the large internal porous area.
In this
4
possibly carcinogenic to humans (Group 2B). Sources of water regards, the development of adsorbents with large and regular
contamination by DEHP are discharge wastewater from pore diameters has evoked great interest. Ordered mesoporous
manufacturing units of DEHP, tubes, tires, so plastics, and carbons (OMCs) have been extensively investigated as modern
leaching from landll sites containing plastic materials. DEHP porous materials and considered as promising adsorbents due
3,5,6
is increasingly being detected in water bodies.
DEHP has a to their high surface area, large pore volume and uniform pore
13
persistent nature, thus conventional treatment methods are not diameter. These features of OMCs allow the free movement of
atoms, ions and molecules throughout the material and allow
9
,14
them to occupy the large available internal area.
With regards to adsorption, surface area has always been
Department of Chemistry, Tongji University, 1239 Siping Road, Shanghai 200092,
China. E-mail: liumx@tongji.edu.cn; ganlh@tongji.edu.cn; Fax: +86 21 65981097;
Tel: +86 21 65982654-8430
1,15
the main attractant.
Under this consideration, several
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Table 1 Chemical structure and physical properties of DEHP
Pollutant IUPAC
name
Molecular weight
Molecular width
(nm)
Molecular length
(nm)
ꢀ
1
Chemical structure
(g mol
)
Ref.
DEHP
390.56
0.525
1.658
27
techniques have been applied via a so template approach to dissolved in an acidic ethanol solution, which was prepared by
11,16
tune the surface area of OMCs.
Generally, so template using 20.0 g ethanol and 1.0 g hydrochloric acid of 0.1 M. This
based methods involve a two step assembly process of the was followed by addition of 2.0 g of the inorganic precursor
constituents, and have multistep processing aws, like hard tetraethyl orthosilicate (TEOS). Further, 4 mmol phloroglucinol
17
templates. For example, Zhao’s group has always used the pre- and 1–4 mmol formaldehyde were added and the mixture was
polymerized carbon precursor ‘resol’ in a tri-constituent stirred for 60 min at room temperature. All three constituents
1
6,18–21
system,
precursor.
and many other groups also used resol as a carbon were assembled in a one-step process. Aer 60 min of stirring,
The groups of Dai and Yuan developed a single the assembled mixture was transferred into a dish and placed at
22,23
step synthesis process of OMCs, but failed to tune the surface room temperature for ꢁ5 h to evaporate the ethanol. Aer the
2
4,25
area.
Herein we attempted a one-step approach to assemble evaporation of ethanol, thermal polymerization was conducted
ꢂ
the tri-constituents via evaporation induced self assembly at 100 C for 24 h in an electric oven, and a thin layer membrane
(
EISA) for the tuning of the surface area and pore diameter of was obtained. The membrane was scratched from the dish and
ꢂ
the OMCs. During the one-step assembly of the tri-constituents, was carbonized in a N
the carbon precursor phloroglucinol–formaldehyde was in situ 800 C for 8 h with the following temperature program: 1 C
2
ow tubular furnace at 350 C for 3 h and
ꢂ
ꢂ
ꢀ
1
ꢂ
ꢂ
ꢀ1
ꢂ
26
prepared during the assembly of the tri-block copolymer, inor- min below 600 C and 5 C min above 600 C. Aer
ganic precursor and carbon precursor. The surface area and carbonization, a natural cooling process was performed. The
pore diameter of the OMCs were tuned by varying the molar resultant carbon–silica composites were immersed in 10% HF
ratio of formaldehyde to phloroglucinol. The resultant OMCs solution for 24 h to etch the silica, followed by water washing
were used as environmental adsorbents for the removal of until a neutral pH was reached. The obtained wet carbon was
ꢂ
DEHP from water. The molecular distribution of DEHP in the placed in oven at 100 C overnight for drying. The resultant
pore channels of the OMCs was studied by using two mathe- carbon was denoted as OMC-F
x
, where x stands for the molar
matical models, i.e., Langmuir and Freundlich. ratio of formaldehyde to phloroglucinol.
2
. Experimental
2.3. Characterization
2.1. Chemicals
Nitrogen adsorption and desorption isotherms were measured
Pluronic F127 (M
w
¼
12600, PEO106PPO70PEO106) was
ꢂ
at ꢀ196 C using a Micromeritics Tristar 3000 gas adsorption
purchased from Aldrich. Tetraethyl orthosilicate (TEOS) and
phloroglucinol were purchased from Aladdin. Formaldehyde,
ethanol, hydrochloric acid and DEHP were purchased from
Sinopharm Chemical Co., Ltd. All chemicals were used as
received without any further purication.
analyzer. Before measurement, the samples were degassed in a
ꢂ
vacuum at 200 C for at least 4 h. The Brunauer–Emmett–Teller
(
(
0
BET) method was utilized to calculate the specic surface area
BET) by using adsorption data in a relative pressure range from
t
.05 to 0.25. Mesopore diameters and total pore volumes (V )
S
were calculated using the Barret–Joyner–Halenda (BJH) model
and the micropore volumes (Vmic) were calculated from t-plots.
2.2. Synthesis of OMCs
An evaporation induced self assembly (EISA) method was Powder small angle X-ray diffraction (SAXRD) was carried out by
applied to assemble the tri-constituents in a one-step process. using an X-ray diffractometer (Bruker AXS, D8 Advance). The
The structure directing agent Pluronic F127 (1.6 g) was structural morphologies of the resultant OMCs were observed
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29
by using transmission electron microscopy (TEM, JEM-2100) Pluronic F127 forms the core of the assembled constituents.
and scanning electron microscopy (SEM, Hitachi, S-4800).
The evaporation of ethanol and thermal polymerization lead to
formation of a 3D hexagonal array of all three assembled
16
2.4. Adsorption studies
constituents. The carbon precursor of phloroglucinol–form-
aldehyde was in situ prepared, instead of using pre-polymerized
carbon precursor. By varying the molar ratio of formaldehyde to
phloroglucinol, it was possible to tune the surface area of the
OMCs. A carbon–silica composite was obtained aer the
carbonization of the assembled hexagonal polymeric array of
tri-constituents at a higher temperature in a nitrogen ow
atmosphere. The inorganic precursor (TEOS) was used to
The batch adsorption experiments of the OMCs were performed
using DEHP (the chemical structure and physical properties of
DEHP are shown in Table 1) as a water pollutant. Since DEHP is
poorly dissolved in water, the stock solution of DEHP was
prepared by dissolving 1.0 g of DEHP in 1000 mL of ethanol.
Further, different concentrations of water soluble model
ꢀ
1
pollutants of DEHP were prepared from 20 to 285 mg L by
dissolving the stock solution in distilled water. The adsorption
capacities of the OMCs were measured by varying the initial
DEHP concentration. In a typical experiment, 0.010 g of
adsorbent was mixed with 25 mL of the aqueous solutions with
16
prevent shrinkage during the carbonization process and also
for the generation of micropores in the mesopore walls of
the OMCs aer the etching of silica from the carbon–silica
composites. The etching of silica from the carbon–silica
composite formed OMCs with attractive textural features.
ꢀ
1
different concentrations of DEHP (20 to 285 mg L ). The
mixture of pollutant and adsorbent was agitated at 150 ꢃ 5 rpm
until an equilibrium was reached, at a temperature of 25 ꢃ
3.2. Evaluation of surface area, pore diameter and pore
ꢂ
0
.1 C. Prior to analysis, the suspension was separated using a
volume of OMCs
0.45 mm membrane lter. The concentration of the DEHP
The surface area, pore volume and pore diameter of the OMCs
were evaluated by nitrogen adsorption–desorption analysis. The
solution was determined using a calibration curve obtained
using a high-performance liquid chromatography (HPLC, Agi-
lent 1260 Innity) system with an Agilent ZORBAX SB-C18,
N adsorption–desorption isotherms of the OMCs are presented
2
in Fig. 2a. All the OMCs are assigned to type IV with a H1
hysteresis loop according to the International Union of Pure
5
mm, 4.6 ꢄ 250 mm column and a UV absorbance detector
(G1314B) operated at 235 nm. The ow of the mobile phase was
30,31
ꢀ
1
and Applied Chemistry (IUPAC) classication.
A type IV
1.0 mL min of 90% acetonitrile and 10% methanol (HPLC
ꢀ1
curve and H1 hysteresis indicate the presence of high level and
e
grade). The amount of adsorbed DEHP, Q (mg g ), was
30
uniform mesoporosity in the OMCs. The steep increase at low
relative pressures (P/P ) of 0.008–0.025 could be related to
calculated by
0
ðC
0
ꢀ C ÞV
e
nitrogen molecular adsorption in the micropores of the OMCs.
This initial process has been related to monolayer adsorption
Q
e
¼
(1)
W
where C and C are the initial and equilibrium concentrations on each micropore wall, therefore a single layer or double layers
0
e
ꢀ1
32
(
mg L ), respectively, V is the volume of DEHP aqueous solu- can be formed between two walls. On the other hand, the well
tion (L), and W is the weight (g) of the OMC adsorbent.
dene H1 loop at high relative pressure (P/P 0.5 to 0.85) is
0
attributed to condensation in the mesopores of the OMCs. All
hysteresis ts to H1 with little difference, for example, OMC-F2.0
has parallel steep adsorption–desorption isotherms, which
could be assigned to a uniform mesopore diameter throughout
3
. Results and discussion
3.1. Synthesis of OMCs
OMCs were synthesized by the one-step self assembly of tri- the material. The uniformity of the pore diameter of OMC-F2.0
constituents via an EISA method. As it is shown in Fig. 1, the can be seen in the pore size distribution curve in Fig. 2b. By
structure directing agent (Pluronic F127), organic precursor gradually increasing the molar ratio of formaldehyde to phlor-
(phloroglucinol–formaldehyde) and inorganic precursor (TEOS) oglucinol, the hysteresis shied to the higher relative pressure
are self assembled via hydrogen bonding in acidic ethanol side, which could be attributed to an increase in the pore size,
solution. Formaldehyde acted as a bridging agent between two and the pore size distribution curve was in agreement with the
phloroglucinol molecules via electrophilic substitution and hysteresis shi towards the higher side of Fig. 2b. The pore
28
elimination of H O. In the competitive cooperative self- diameter was increased from 2.1 to 3.1, as shown in Table 2.
2
assembly of organic–organic (phloroglucinol–formaldehyde– While up to a molar ratio of 2.0, there was no major increment
Pluronic F127) and organic–inorganic (phloroglucinol–formal- noticed. In our previous study, the microporosity of ordered
dehyde–TEOS and Pluronic F127–TEOS), the hydrogen bonding carbon derived from tri-constituent assembly enormously
played a vital role. On the one hand, phloroglucinol has three depended on the removal of silica from the carbon–silica
13
meta hydroxyl groups (positions 1, 3, and 5), which are composite. In this work, silica also played a similar role, and
26
considered as favorable towards hydrogen bonding. On the its removal from the carbon–silica composites aer the
other hand, Pluronic F127 has a long PEO domain, which could carbonization of the assembled tri-constituents results in the
allow formation of the organic–organic self assembly via high microporosity of the OMCs. The microporosity was also
24
enhanced hydrogen bonding (as shown in Fig. 1), and it affected by the molar ratio of formaldehyde to phloroglucinol,
favours the organization of ordered nanocomposite meso- as shown in Table 2 and Fig. 2b. Beyond the optimum molar
16
structures. In the self assembly process, the PPO domain of ratio (2.0), the micropore volume was decreased from 0.8 to 0.2
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x
Fig. 1 Schematic representation of OMC-F synthesis.
Fig. 2
2
N adsorption–desorption isotherm of OMC-F1.0–4.0 (a) and the corresponding pore size distribution curves (b).
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Table 2 Surface area, pore size/volume and adsorption capacities of the OMCs
Micropore area External surface
2
ꢀ1
2
ꢀ1
2
ꢀ1
3
ꢀ1
3
ꢀ1
3
ꢀ1
ꢀ1
Adsorbent
S
BET (m g
)
(m g
)
area (m g
)
V
Total (cm g
)
V
meso (cm g
)
V
micro (cm g
)
D (nm)
Q
e
(mg g
)
OMC-F1.0
OMC-F2.0
OMC-F3.0
OMC-F4.0
1660
1801
1644
956
1101
1065
684
559
736
960
846
1.7
2.0
2.1
1.5
0.9
1.2
1.4
1.3
0.8
0.8
0.7
0.2
2.1
2.3
2.9
3.1
276
364
241
187
110
3
ꢀ1
cm g . This could be due to the collapse of the open frame- formaldehyde to phloroglucinol in the one-step assembly
work of the formaldehyde–phloroglucinol at high ratios. On the process of the tri-constituents.
other hand, no major increment in mesoporosity was observed.
The surface morphology and order structure of OMC-F2.0 are
ꢀ3
ꢀ1
However, the maximum total pore volume (2.1 cm
g
) was presented in Fig. 4. Fig. 4a reveals that the large domain of
ꢀ
3
ꢀ1
observed in OMC-F3.0, and was a little higher (0.1 cm
g
)
OMC-F2.0 has uniform mesopore channels. Fig. 4b reveals that
than that of OMC-F2.0 prepared at the optimum molar ratio. some of the mesopores have collapsed with each other and
Importantly, the surface area was also dependent on the molar formed wide mesopores, which could be attributed to the
ratio of formaldehyde to phloroglucinol in the one-step etching of silica from the mesopore walls. It was noted that
assembly process of the tri-constituents, as were other texture there were no changes observed in the structure due to the
features. The surface area was calculated by using the BET etching of silica, as it is shown in Fig. 3. Furthermore, the
0
method at relative pressures, P/P , from 0.07 to 0.20, and in this obtained mesopore diameter of OMC-F2.0 from the BJH method
range all the OMCs followed a straight line and the reported was also in correlation with the FESEM images (Fig. 4b). The
correlation coefficient was 1. As the molar ratio of formaldehyde order morphology of OMC-F2.0 can be seen in the TEM image
to phloroglucinol was increased to higher values (form 1.0 to (Fig. 4c). This reveals that large segments are ordered and it is in
2
4
.0) the uptake volume of N was also increased (form 344 and agreement with the SAXRD patterns. The irregularity in the
2
45 cm g to 364 and 476 cm g ), which contributed to the ordered channel can be seen in the HRTEM image (Fig. 4d),
3
ꢀ1
3
ꢀ1
high surface area of the OMCs. However, increasing the molar indicating the etching of silica from the wall of the carbon–
ratio of formaldehyde to phloroglucinol aer the optimum silica composites. This conrms that the etching process was
molar ratio of 2.0 failed to enhance the surface area. This is responsible for enhancing the surface area without destroying
mainly based on bulk porosity, and it was assumed that the the ordered morphology.
optimum molar ratio of formaldehyde to phloroglucinol in the
one-step assembly process of the tri-constituents made open
and smaller size hydrocarbon networks, which generated high 3.4. Adsorption capacity evolution of OMCs and surface area
33
porosity and contributed to the high surface area. Dai and effect on adsorptive removal of DEHP
Yuan used phloroglucinol–formaldehyde based precursor to
The adsorption behavior of adsorbents depends on many
aspects, such as surface area, pore volume, pore diameter, size
of guest adsorbate and surface chemical properties. Here we
mainly looked at the effect of the surface area of the OMCs
towards the adsorptive removal of a DEHP model water
24,25
synthesise OMCs
but in their work, the surface area was not
high enough to use as adsorbents. The above assumption is
supported by the micropore area and external surface area, as
shown in Table 2.
3.3. Structure and morphology features of the OMCs
SAXRD patterns of the OMCs are presented in Fig. 3. All the
OMCs had one strong and one weak peak, which can be indexed
to (100) and (110) reection, corresponding to a 2D hexagonal
34
ordered mesostructure with space group P6mm symmetry. The
effect of the molar ratio of formaldehyde to phloroglucinol was
also seen in the order structure of the OMCs. When increasing
the molar ratio from 1.0 to 2.0, the (100) and (110) indexed
peaks become narrow, suggesting that the ordered hexagonal
mesostructure was retained and becomes better. With further
increment of the molar ratio aer the optimum molar ratio
(2.0), the (100) indexed peak was shied to the le side and the
visibility of (110) indexed peak became poorer. At the molar
ratio of 4.0, the (100) indexed peak became sharper and nar-
rower and the (110) indexed peak was very poorly visible. This
35
indicated the partial collapse of mesostructure regularity,
which might have occurred due to such a high molar ratio of Fig. 3 Small angle X-ray diffraction patterns of OMC-F1.0–4.0
.
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ꢀ
1
2
ꢀ1
g
. OMC-F2.0 had the highest surface area (1801 m g ) and
ꢀ
1
exhibited the highest adsorption capacity (364 mg g ) amongst
all the four OMCs. This suggests that the adsorption capacity of
the OMCs for the removal of DEHP depended on surface area.
Importantly, OMC-F1.0 and OMC-F3.0 do not have large surface
area differences (1660 and 1644 m g , respectively) but the
adsorption capacity differences were noted to be a bit higher
2
ꢀ1
ꢀ
1
(
276 and 241 mg g ). This could be due to the difference of the
micropore area and the external surface area, as is presented in
Table 2. Since the molecular size of DEHP is in the nano range,
less than 2 nm, the uniform micropore area can also welcome
the DEHP molecule to occupy the place in the internal area
developed by the micropores. However, the micropore area
difference of OMC-F1.0 and OMC-F3.0 was very high, 1101 and
2
ꢀ1
6
84 m g , respectively, and this is not in correlation with the
adsorption capacity data. On the other hand, the external
surface area of OMC-F1.0 and OMC-F3.0 increased from 559 to
2
ꢀ1
9
60 m g , respectively, and this is also not in correlation with
the adsorption capacity. As can be seen in Table 2, a large part of
the BET surface area of OMC-F4.0 is generated by mesopore
volume and similar trends are noted in all the other three
OMCs. Therefore, the adsorption capacity for the removal of
DEHP from water was mainly dependent on the BET surface
area of the OMCs.
Fig. 4 SEM (a and b) and TEM (c and d) images of OMC-F2.0
.
pollutant. Equilibrium adsorption experiments were performed
to determine the adsorption capacity of the OMCs for the
removal of DEHP water contaminate. Fig. 5a represents the
3.5. Mathematical model tting for decontamination of
equilibrium adsorption curves of four different adsorbents for DEHP by OMCs
the removal of DEHP. It can be seen that the adsorption capacity
of OMC-F1.0, OMC-F2.0, OMC-F3.0 and OMC-F4.0 increased from
The obtained adsorption equilibrium data of the OMCs from
the decontamination of DEHP water pollutant were treated with
two mathematical models, Langmuir and Freundlich. The
Langmuir model assumes that adsorption occurs onto the
homogeneous surfaces by monolayer coverage and that there is
ꢀ
1
4
9 to 276, 364, 241 and 187 mg g , respectively, with increasing
ꢀ1
the equilibrium concentration of DEHP from 20 to 284 mg L
,
and reached saturation progressively. This could be due to the
increase in the driving force of the concentration gradient, as an
increase in the initial DEHP concentration could accelerate the
diffusion of DEHP into the porous channels of the OMC
36
no transmigration of the adsorbate in the plane of the surface.
The straight line equation is as follows:
C
e
1
1
31
adsorbents. As can be seen in Fig. 5b and Table 2, with
increasing the surface area of OMC-F and OMC-F2.0 from 1660
¼ Q
þ Q Ce
(2)
Q
e
m
K
L
m
1
.0
2
ꢀ1
to 1801 m g , the adsorption capacities were increased from where Ce is the equilibrium concentration of the adsorbate
76 to 364 mg g , respectively. Similarly, with the decrease in (mg L ), Q is the amount of adsorbate adsorbed per unit mass
surface area of OMC-F3.0 and OMC-F4.0 from 1644 to 956 m g
ꢀ
1
ꢀ1
2
e
2
ꢀ1
ꢀ1
,
L
of adsorbent (mg g ), K is the Langmuir adsorption constant
ꢀ
1
the adsorption capacities were decreased from 241 to 187 mg (L mg ) and Qm is the theoretical maximum adsorption
Fig. 5 (a) Adsorption capacity evaluation curve of OMC-F1.0–4.0 by varying the concentration gradient of model water pollutant DEHP and (b) a
comparative plot of surface area vs adsorption capacity of OMC-F1.0–4.0
.
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ꢀ
1
capacity (mg g ). The essential characteristics of the Langmuir especially the values of 0.9987 and 0.8521 for OMC-F2.0. This
equation can be expressed in terms of a dimensionless sepa- indicates that adsorption occurred in a monolayer manner in
3
1
ration factor RL, which is dened as:
the high surface area OMCs. Moreover, the calculated
ꢀ1
maximum monolayer adsorption capacity (Q ) of 370 mg g
m
1
R
L
¼ 1
(3)
from the Langmuir model was near to the experimental
þ K
L 0
C
ꢀ
1
adsorption capacity of 364 mg g . This also indicates that
the adsorptive removal of DEHP by the OMCs depended on the
ꢀ
1
where K is the Langmuir isotherm constant (L mg ) and C is
the initial DEHP concentration (mg L ). The R value indicates
the type of isotherm to be either favorable (0 < R < 1), unfa-
L
0
ꢀ
1
L
surface area. The dimensionless separation factor (R ) of the
L
L
Langmuir model calculated from eqn (3) is presented in Table 3.
vorable (R
L
> 1), linear (R
L
¼ 1) or irreversible (R ¼ 0).
L
The obtained R values of all the OMCs for the Langmuir model
L
The Freundlich model assumes that adsorption occurs at the
heterogeneous surface and can allow the guest object to adsorb
in a multilayer manner. The logarithmic straight line equation
is as follows:
were between 0.008–0.287, which indicates that adsorption is
more favorable rather than irreversible. The 1/n values calcu-
lated from the Freundlich model of all the OMCs were between
0.2537–0.3376, which is below one and not very close to zero
1
indicating a high affinity to Langmuir adsorption. Therefore,
the Langmuir model is thus found to give better prediction for
the adsorptive removal of DEHP at all concentrations.
ln Q
e
¼ ln K
F
þ
ln C
e
(4)
n
ꢀ1
F
where K (L mg ) is the Freundlich constant and 1/n is the
heterogeneity factor. K is dened as an adsorption or distri-
F
bution coefficient representing the amount of adsorbate
adsorbed on an adsorbent for a unit equilibrium concentration,
4. Conclusion
while 1/n gives an indication of how favorable the adsorption We report the one-step assembly of tri-constituents via an EISA
process is. The 1/n range between 0 and 1 is a measure of method for the synthesis of high surface area OMCs. The
adsorption intensity or surface heterogeneity. If the value of 1/n organic precursor of phloroglucinol–formaldehyde was in situ
gets closer to zero, it becomes more heterogeneous. A 1/n value prepared during the one-step assembly of the tri-constituents.
below one indicates adsorption following the Langmuir model, By varying the molar ratio of formaldehyde to phloroglucinol
while
adsorption.
a
value of 1/n above one indicates cooperative from 1.0 to 4.0, it was possible to tune the surface area of the
2
ꢀ1
37
OMCs from 956 to 1801 m g . Small angle XRD patterns and
The experimental data for the removal of DEHP by OMCs TEM images demonstrated that the OMCs were highly ordered
were well tted by the Langmuir model (Fig. 6a). From Fig. 6b it materials with hexagonal space symmetry of P6mm and SEM
appears that the present adsorption process does not ideally
follow the Freundlich isotherm model and exhibits deviation
from linearity over the entire concentration range. However, if Table 3 Langmuir and Freundlich calculated parameters of DEHP
adsorption on OMCs
the total concentration range is divided into several regions,
good ts to the experimental data can be noted, especially in the
lower concentration range. Therefore, the Freundlich equation
cannot describe the adsorption process at higher concentration Adsorbent
Langmuir
Freundlich
1/n
2
2
Q
m
K
L
R
L
R
K
F
R
ranges for the removal of DEHP by OMCs. The parameters
OMC-F1.0
OMC-F2.0
286 0.13 0.027–0.287 0.9962 55 0.3376 0.9755
370 0.40 0.008–0.110 0.9987 83 0.3353 0.8521
250 0.13 0.025–0.275 0.9973 53 0.3119 0.975
189 0.14 0.023–0.260 0.9975 52 0.2537 0.9799
calculated from both models as above explained are presented
2
in Table 3. Comparatively, the correlation coefficient (R ) of the
OMC-F3.0
Langmuir model was better than that of the Freundlich model, OMC-F4.0
Fig. 6 Mathematical model fitting of the experimental data for the adsorptive removal of DEHP by OMC-F1.0–4.0: the Langmuir model (a) and the
Freundlich model (b).
This journal is © The Royal Society of Chemistry 2014
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RSC Advances
Paper
images revealed that OMC-F2.0 was a highly mesoporous mate- 11 X. Zhuang, Y. Wan, C. Feng, Y. Shen and D. Zhao, Chem.
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and adsorption capacity were evaluated and revealed that 201–206.
decontamination of DEHP depended on the surface area. The 13 P. K. Tripathi, M. Liu, L. Gan, J. Qian, Z. Xu, D. Zhu and
ꢀ
1
maximum adsorption capacity of 364 mg g was observed at
N. N. Rao, J. Mater. Sci., 2013, 48, 8003–8013.
2
ꢀ1
the highest surface area of 1801 m
g
of OMC-F2.0. The 14 M. E. Davis, Nature, 2002, 417, 813–821.
treatment of the experimental data with two well know mathe- 15 M. Teng, J. Qiao, F. Li and P. K. Bera, Carbon, 2012, 50, 2877–
matical adsorption models, the Langmuir and Freundlich 2886.
models, revealed that adsorption of DEHP occurred in a 16 R. Liu, Y. Shi, Y. Wan, Y. Meng, F. Zhang, D. Gu, Z. Chen, B. Tu
monolayer manner and followed the Langmuir adsorption and D. Zhao, J. Am. Chem. Soc., 2006, 128, 11652–11662.
isotherms in the high surface area mesoporous ordered 17 Z. Li, X. Wang, C. Wang and L. Yin, RSC Adv., 2013, 3, 17097–
carbons. Therefore, we believe that adsorptive decontamination 17104.
of DEHP water pollutant depended on the surface area and 18 W. Gao, Y. Wan, Y. Dou and D. Zhao, Adv. Energy Mater.,
mesoporosity and the OMCs derived by the one-step assembly 2011, 1, 115–123.
method have great future prospects for the removal of various 19 Y. Zhai, Y. Dou, X. Liu, S. S. Park, C.-S. Ha and D. Zhao,
hazardous environmental pollutants.
Carbon, 2011, 49, 545–555.
20 Y. Huang, H. Cai, T. Yu, F. Zhang, F. Zhang, Y. Meng, D. Gu,
Y. Wan, X. Sun, B. Tu and D. Zhao, Angew. Chem., Int. Ed.,
Acknowledgements
2007, 46, 1089–1093.
This work was nancially supported by the National Natural 21 J. Wang, C. Xue, Y. Lv, F. Zhang, B. Tu and D. Zhao, Carbon,
Science Foundation of China (21207099, 21273162), the Science
2011, 49, 4580–4588.
and Technology Commission of Shanghai Municipality, China 22 K. Hou, A. Zhang, M. Liu and X. Guo, RSC Adv., 2013, 3,
11nm0501000, 12ZR1451100), the Key Subject of Shanghai
25050–25057.
Municipal Education Commission (J50102) and the Funda- 23 B. You, Z. Zhang, L. Zhang, J. Yang, X. Zhu and Q. Su, RSC
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Adv., 2012, 2, 5071–5074.
2011KJ023). Pranav K. Tripathi acknowledges the Chinese 24 C. Liang and S. Dai, J. Am. Chem. Soc., 2006, 128, 5316–5317.
Scholarship Council, Govt. of China and Ministry of Higher 25 L. Liu, F. Y. Wang, G. S. Shao, T. Y. Ma and Z. Y. Yuan,
(
(
Education, Govt. of India for the research fellowship.
Carbon, 2010, 48, 2660–2664.
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