Mono and co-immobilization of imidazolium ionic liquids on silica: Effects of the substituted groups on the adsorption behavior of 2,4-dinitrophenol

Article abstract of DOI:10.1039/c9ra07635b

Ionic liquid modified silicas with high adsorption capacity for phenols prompt us to deeply explore the contribution of interactions between the adsorbent and adsorbate, with a particular focus on hydrophobicity, π-π, electrostatic and acid-base interactions. Herein, by introducing a series of typical substituent groups including N,N-dimethylaminopropyl (A), benzyl (B), dodecyl (D) and naphthylmethyl (N) in an imidazole ring (Im), three mono-immobilized and two co-immobilized imidazolium ionic liquid modified silicas, namely SilprAImCl, SilprBImCl, SilprNImCl, SilprDBImCl and SilprDAImCl, werre synthesized for removal and recovery of 2,4-dinitrophenol (2,4-DNP) from aqueous solutions. Adsorption kinetics, isotherms, thermodynamic analysis and desorption experiments have been carried out. The experimental results reveal that the substituent groups such as N,N-dimethylaminopropyl, benzyl and naphthylmethyl on the imidazole ring can significantly enhance the adsorption of 2,4-DNP via the acid-base interaction or π-π interaction and the adsorption capacity of 2,4-DNP follows the order: SilprNImCl > SilprAImCl > SilprBImCl. Furthermore, SilprDBImCl exhibits the largest adsorption capacity and SilprDAImCl has the lowest among the five adsorbents. These interesting finds indicate that the combination of hydrophobicity and π-π interactions lead to enhanced adsorption performance towards 2,4-DNP, while the combination of the hydrophobicity and acid-base interactions can restrain greatly adsorption of 2,4-DNP from aqueous medium. Adsorption mechanisms of 2,4-DNP on the five adsorbents have been clarified. These results will provide a deeper insight for efficient removal of phenols from water environments.

Full text of DOI:10.1039/c9ra07635b

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PAPER
Mono and co-immobilization of imidazolium ionic
liquids on silica: effects of the substituted groups
on the adsorption behavior of 2,4-dinitrophenol†
Cite this: RSC Adv., 2019, 9, 32425
ab
a
b
b
Honglian Ge, Xueyuan Wang, Cunling Ye and Shunli Fana
*
*
Zhike Wang,
Ionic liquid modified silicas with high adsorption capacity for phenols prompt us to deeply explore the
contribution of interactions between the adsorbent and adsorbate, with particular focus on
a
hydrophobicity, p–p, electrostatic and acid–base interactions. Herein, by introducing a series of typical
substituent groups including N,N-dimethylaminopropyl (A), benzyl (B), dodecyl (D) and naphthylmethyl
(N) in an imidazole ring (Im), three mono-immobilized and two co-immobilized imidazolium ionic liquid
modified silicas, namely SilprAImCl, SilprBImCl, SilprNImCl, SilprDBImCl and SilprDAImCl, werre
synthesized for removal and recovery of 2,4-dinitrophenol (2,4-DNP) from aqueous solutions.
Adsorption kinetics, isotherms, thermodynamic analysis and desorption experiments have been carried
out. The experimental results reveal that the substituent groups such as N,N-dimethylaminopropyl,
benzyl and naphthylmethyl on the imidazole ring can significantly enhance the adsorption of 2,4-DNP
via the acid–base interaction or p–p interaction and the adsorption capacity of 2,4-DNP follows the
order: SilprNImCl > SilprAImCl > SilprBImCl. Furthermore, SilprDBImCl exhibits the largest adsorption
capacity and SilprDAImCl has the lowest among the five adsorbents. These interesting finds indicate that
the combination of hydrophobicity and p–p interactions lead to enhanced adsorption performance
towards 2,4-DNP, while the combination of the hydrophobicity and acid–base interactions can restrain
greatly adsorption of 2,4-DNP from aqueous medium. Adsorption mechanisms of 2,4-DNP on the five
adsorbents have been clarified. These results will provide a deeper insight for efficient removal of
phenols from water environments.
Received 20th September 2019
Accepted 2nd October 2019
DOI: 10.1039/c9ra07635b
rsc.li/rsc-advances
glycidyl methacrylate graed cotton cellulose,20 yellow
bentonite,21 XAD-4 resin,22 activated carbon bers,23 layered
double hydroxides and their calcined products,24 carbon
1. Introduction
2,4-Dinitrophenol (2,4-DNP), a priority pollutant, is widely
acknowledged to be detrimental to human health and the
environment. The water pollution resulting from 2,4-DNP is
considered as a general problem. Many methods have been
proposed to remove the 2,4-DNP from aqueous solutions
including advanced oxidation,1 electrochemical oxidation,2–4
nanospheres,25 date seeds26 and active carbon.27
In recent years, ionic liquid-modied silicas (IL-silicas) as
adsorbents have been widely used to remove Cu(II), Fe(III), Mn(II)
and Ni(II),28 Pb(II),29–31 Cr(III,VI),32–34 lanthanides and scandium
ions,35 oxymatrine,36 bovine serum albumin,37 racemic amino
acids,28 thiophenic sulfur compounds,38 anionic dye,39,40
dibenzothiophene41 and phenolic compounds (4-chlor-
ophenol,42 polyphenols,43 phenolic acids,44 naphthols,45
bisphenol A46 and 2,4-DNP47–49). To elucidate the relation
between the structure of IL-silicas and their adsorption perfor-
mance for phenolic compounds, a series of IL-silicas were
designed by varying substituents, the functional groups of the
imidazolium ring, counter ions and silane-coupling agents. For
the adsorption of 2,4-DNP, 10 imidazolium-modied silicas
were prepared in our group. Methyl group, the substituent of
imidazolium ring, affected signicantly the adsorption perfor-
mance.47 The adsorption ability of 4-methylimidazolium-
modied silicas was enhanced with the increase of the hydro-
biodegradation,5–7
ultrasonic
degradation,8–11
photo-
degradation,12–15 electro/AIS/PS process,16 and adsorption.17–27
The adsorption process has attracted worldwide attention due
to its simple implementation and low operation cost. The re-
ported adsorbents include carbons activated by KOH, HNO3
H3PO4 and ZnCl2,17 olive wood biosorbents,18 UiO-66-NH2,19
aSchool of Environment, Henan Normal University, Xinxiang 453007, China. E-mail:
wzk@htu.cn
bSchool of Chemistry and Chemical Engineering, Henan Key Laboratory for
Environmental Pollution Control, Key Laboratory for Yellow River and Huai River
Water Environment and Pollution Control, Ministry of Education, Henan Normal
University, Xinxiang 453007, China
ꢀ
philicity of counter anions (BF4ꢀ. PF6 , Tf2Nꢀ).48 Amino group
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra07635b
also increased signicantly the adsorption performance of
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imidazole-modied silicas. However, the adsorption ability of
the adsorbent prepared using 3-mercaptopropyltrimethox-
ysilane as the coupling agent decreased signicantly.49 In order
to overcome the negative effects of coexisting anions on the
adsorptive removals of naphthols, a silica adsorbent was
prepared through the co-modication of 1-vinyl-3-
octadecylimidazolium cations and p-styrenesulphonate anions
on silica via click chemistry reaction.45 In addition, imidazole,
phenyl and hexadecyl were further co-immobilized onto silica
via silane coupling reaction or thiol–ene click chemistry for
bisphenol A adsorption from high salt aqueous solutions.46 The
review of literatures showed that one imidazolium-based ionic
liquid was oen used to prepare the IL-silicas via silane
coupling reaction. To the best of our knowledge, two kinds of
ionic liquids with imidazole ring containing different substit-
uents were seldom used to simultaneously modify the silica.
In this work, by introducing a series of typical substituent
groups including N,N-dimethylaminopropyl (A), benzyl (B),
dodecyl (D) and naphthylmethyl (N) of imidazole ring (Im), the
prepared SilprAImCl, SilprBImCl, SilprNImCl, SilprDBImCl and
SilprDAImCl (Fig. 1) were applied to further investigate the
contribution of interactions between these adsorbents and 2,4-
DNP with a particular focus on hydrophobicity interaction, p–p
interaction, electrostatic interaction and acid–basic interaction.
Our objective is to build the relationship between adsorbent
structure and their adsorption performance for 2,4-DNP, and
2. Experimental
2.1 Materials
Imidazole (99%), 3-chloropropyltriethoxysilane (98%), 1-
bromododecane (98%) and sodium hydride (60%) were
purchased from Aladdin Chemistry Co. Ltd. 2,4-DNP, silica
gel (200–300 mesh), 1-benzylimidazole (98%), 1,4-dioxane,
triethylamine, ethanol, methanol, magnesium (99.0%) and
calcium chloride were obtained from Sinopharm Chemical
Reagent Co., Ltd. 1-Chloromethylnaphthalene (97%) and
dimethylaminopropylchloride hydrochloride (96%) were ob-
tained from Sigma-Aldrich. Dry toluene and activated silica
were obtained using the same procedure reported in the
literature.46 Dry 1,4-dioxane was obtained using the similar
method for dry toluene. Oil-free sodium hydride was ob-
tained by washing sodium hydride (60%) thoroughly with
petroleum ether. Dry ethanol was obtained through the
following method: 1.5 g magnesium, 0.15 g iodine and 15 mL
ethanol were added into a round bottom ask and heated
until the iodine was dissolved. Then, 250 mL ethanol was
added. Aer the mixture was reuxed for 4–6 h, dry ethanol
was obtained by distilment.
2.2 Preparation of SilprBImCl, SilprNImCl, SilprAImCl,
SilprDBImCl and SilprDAImCl
thus guide for designing high efficient adsorbents for removal The synthesis procedures of SilprBImCl, SilprNImCl and Sil-
of phenols from water environment.
prAImCl were shown in Fig. S1 (ESI†), respectively.
2.2.1 Preparation of SilprBImCl. Briey, 4.0 g 1-benzyli-
midazole and 6.7 g 3-chloropropyltriethoxysilane were mixed
at 80 ꢁC for 24 h, (3-(1-benzyl imidazolyl)propyl)triethox-
ysilane (product (1)) was obtained. Then, 4.0 g product (1),
5.0 g activated silica and 60 mL dry toluene were mixed at
ꢁ
120 C for another 24 h. Aer cooling to room temperature,
the mixture was ltrated and washed in turn with toluene,
ethꢁanol and deionized water. The SilprBImCl were dried at
60 C for 10 h.
2.2.2 Preparation of SilprNImCl. 1-(1-Naphthylmethyl)
imidazole was synthesized according to the literature.50 2.119 g
imidazole dissolved in 50 mL dry 1,4-dioxane was added to
100 mL dry 1,4-dioxane containing 1.364 g oil-free sodium
hydride. Aer stirring at 90 ꢁC for 1 h, a solution of 5.0 g 1-
chloromethylnaphthalene in 50 mL dry 1,4-dioxane was added
ꢁ
dropwise to the above mixtures and stirred at 90 C for 22 h.
Then, the solvent was removed and 100 mL water was added to
the residue. Finally, the obtained solution was extracted by
dichloromethane and the organic layer was dried with anhy-
drous magnesium sulfate. 1-(1-Naphthylmethyl)imidazole was
obtained aer removal of dichloromethane. (3-(1-(1-Naph-
thylmethyl)imidazolyl)propyl)triethoxysilane (product (2)) was
obtained through the reaction of 3-chloropropyltriethoxysilane
and 1-(1-naphthylmethyl)imidazole (molar ratio: 1 : 1.2) at 80 ꢁC
for 24 h. Then, 4.2 g product (2), 5 g activated silica and 60 mL
dry toluene were mixed at 120 ꢁC for 24 h. Aer cooling to room
temperature, the mixture was ltrated and washed in turn with
toluene, dichloromethane, ethanol and deionized water.
Fig. 1 Structures of the prepared SilprAImCl, SilprBImCl, SilprNImCl,
SilprDBImCl and SilprDAImCl.
ꢁ
Finally, the SilprNImCl were dried at 60 C for 24 h.
32426 | RSC Adv., 2019, 9, 32425–32434
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2.2.3 Preparation of SilprAImCl. SilprAImCl was synthe-
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temperature, the reactant mixture was ltrated and washed
in turn with ethanol and deionized water. Then, 1-dodecy-
limidazolium and 1-(N,N-dimethylaminopropyl) imidazo-
lium co-modied silicas, SilprDAImCl, were dried at 60 ꢁC
for 10 h.
sized according to the literature with some modications.51 5 g
SilprImCl prepared previously in our group,47 3 g N,N-dime-
thylaminopropylchloride hydrochloride and 100 mL ethanol
were stirred at 90 ꢁC for 24 h. Aer cooling to room temperature,
the mixture was ltrated and washed with ethanol. Then, the
ꢁ
SilprAImCl were dried at 60 C for 10 h.
2.3 Characterization
The synthesis procedures of SilprDBImCl and SilprDAImCl
were shown in (A) and (B) of Fig. S2 (ESI†), respectively.
Elemental analysis and Fourier transform infrared (FT-IR)
spectra were performed on a Vario EL elemental analyzer
(Germany) and on a Perkin-Elmer-983 IR spectrophotometer,
respectively. Thermogravimetric analysis (STA449C, Netzsch,
Germany) was performed with a heating rate of 10 ꢁC min under
nitrogen. N2 adsorption–desorption experiments were carried
out on a Quantachrome NOVA 2000e sorption analyzer at liquid
nitrogen temperature (77 K). The specic surface area (SBET) was
estimated by the linear part of the Brunauer–Emmett–Teller
(BET) equation,53 and the pore size distribution was calculated
by the Barrett–Joyner–Halenda (BJH) method.54
2.2.4 Preparation of (3-(1-dodecylimidazolyl) propyl) trie-
thoxysilane (product (3)). 1-Dodecylimidazole was prepared
according to the literature.52 In brief, 5.06 g triethylamine,
12.46 g 1-bromododecane and 50 mL dry toluene were added
into a dry round bottom ask containing a PTFE-coated
magnetic stir bar. Aer the mixture was stirred at room
temperature for 15 min, 3.4 g imidazole was added over 10 min
ꢁ
while stirring. Then, the mixture was stirred at 120 C for 2 h.
Aer cooling to room temperature, the mixture was ltrated and
the ltrate was concentrated under reduced pressure to give 1-
dodecylimidazole. Aer that, 3-chloropropyltriethoxysilane and
2.4 Adsorption and desorption experiments
ꢁ
1-dodecylimidazole (molar ratio: 1 : 1) was stirred at 80 C for
The stock solutions of 2,4-DNP were prepared by dissolving
2,4-DNP in deionized water. 0.1 M HCl or 0.1 M NaOH were
used to adjust the pH values of the 2,4-DNP solutions. A UV/
visible spectrophotometer (UV-5100, Shanghai Metash
Instruments Co. Ltd., China) was used to determine the
concentration of 2,4-DNP. All adsorption and desorption
experiments were conducted using the same method in our
previous reports.47–49
24 h, product (3) was obtained.
2.2.5 Preparation of ((3-(1-(N,N-dimethylaminopropyl))
imidazolyl)propyl)triethoxysilane (product (4)). Firstly, 2.04 g
imidazole, 4.74 g N,N-dimethylaminopropylchloride hydro-
chloride and 30 mL dry ethanol were added into a dry round
bottom ask and stirred at 80 ꢁC for 24 h under nitrogen
atmosphere. Aer cooling to room temperature, any remaining
volatiles were removed by vacuum rotary evaporation. Then,
some water was added and the pH value of the system was
adjusted to 8–9 by the addition of 0.1 M NaOH. Aer that, the
mixture was concentrated under reduced pressure and the
white residue was dissolved in ethanol. The solution was l-
trated and concentrated under reduced pressure. Then, 1-(N,N-
dimethylaminopropyl)imidazole was dried at 80 ꢁC for 24 h.
Secondly, 3.6 g 1-(N,N-dimethylaminopropyl)imidazole, 4.58 g
3-chloropropyltriethoxysilane and 15 mL dry ethanol were
added into a dry round bottom ask and stirred at 80 ꢁC for
24 h. Product (4) was obtained aer removal of any remaining
volatiles by vacuum rotary evaporation.
3. Results and discussion
3.1 Characterization
The FT-IR spectra in Fig. 2 showed that the typical peaks of Si–
O–Si were observed around 465, 796 and 1090 cmꢀ1
.
The
29
characteristic absorption peaks of imidazole ring (1565 cmꢀ1
)
were observed.33,55 The peaks at 1391 and 1448 cmꢀ1 were due to
the naphthalene ring characteristic vibration.56 The peaks at
1459 and 1489 cmꢀ1 were assigned to the benzene ring char-
acteristic vibration.57 The peak at 713 cmꢀ1 was assigned to C–H
vibration, the peak at 2892 cmꢀ1 was assigned to the presence of
methyl groups. The peaks at 2855 and 2931 cmꢀ1 were due to
the C–H stretching of the tetrahedral carbon.
2.2.6 Preparation of SilprDBImCl. 6 g activated silica and
40 mL dry toluene were added into a dry round bottom ask
containing a PTFE-coated magnetic stir bar. Then, 20 mL dry
toluene solutions containing 5.99 g product (1) and 7.16 g
product (3) were added dropwise to the above mixture. The
mixture was stirred at 120 ꢁC for 24 h. Aer cooling to room
temperature, the reactant mixture was ltrated and washed in
turn with toluene, ethanol and deionized water. Then, 1-dode-
cylimidazolium and 1-benzylimidazolium co-modied silicas,
The ve adsorbents were further analyzed by N2 adsorp-
tion–desorption isotherm at 77 K and the corresponding
physical properties including BET surface area, pore size and
pore volume were listed in Table 1. It can be seen from Fig. 3
that the ve adsorbents showed Type IV N2 adsorption–
desorption isotherms with a hysteresis loop according to the
IUPAC classication58 and the pore size distributions were
uniform.
The carbon, hydrogen and nitrogen contents of the ve
adsorbents by Elemental analysis were presented in Table 2.
The thermogravimetric curves (Fig. 4) further provided the
content of functional groups and the thermal stability of the ve
adsorbents. It can be seen from this gure that and the weight
losses of SilprBImCl, SilprNImCl, SilprAImCl, SilprDBImCl and
ꢁ
SilprDBImCl, were dried at 60 C for 10 h.
2.2.7 Preparation of SilprDAImCl. 6 g activated silica and
40 mL dry ethanol were added into a dry round bottom ask
containing a PTFE-coated magnetic stir bar. Then, 20 mL dry
ethanol solutions containing 7.16 g product (3) and 5.96 g
product (4) were added dropwise to the above mixture. The
ꢁ
mixture was stirred at 90 C for 24 h. Aer cooling to room
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Table 1 Structural parameters of the five adsorbents
BET surface area
Total pore volume
Average pore
width (nm)
Samples
(m2 gꢀ1
)
(cm3 gꢀ1
)
SilprBImCl
SilprNImCl
SilprAImCl
SilprDBImCl
SilprDAImCl
48
218
219
225
291
0.04
0.40
0.46
0.43
0.61
3.61
7.26
8.46
7.71
8.44
SilprDAImCl between 200 and 600 ꢁC were 17.9%, 16.5%,
12.2%, 17.3% and 9.6%, respectively.
3.2 Adsorption kinetics, isotherms and thermodynamic
analysis
To investigate the inuence of shaking time, 25 mg SilprBImCl,
SilprNImCl, SilprAImCl, SilprDBImCl and SilprDAImCl were
added to 50 mL of 2,4-DNP solution with an initial concentra-
tion of 50 mg Lꢀ1 at pH 4.0, 160 rpm and 30 C, respectively
ꢁ
(Fig. 5). It is evident that the adsorption processes reached
equilibrium aer 780 min for SilprBImCl and 40 min for the
other four adsorbents. The SilprBImCl had the longest
adsorption equilibrium time among the ve prepared adsor-
bents. This is attributed to its lowest values of total pore volume
and average pore width (Table 1), which can inhibit the diffu-
sion process of 2,4-DNP between solid–liquid interface. As
shown in Fig. 5, for the three mono immobilization of imida-
zolium ionic liquids modied silicas, the adsorption capacity of
2,4-DNP increased in order: SilprNImCl > SilprAImCl > Silpr-
BImCl, demonstrating that the naphthylmethyl of imidazole
ring on the contributions of the adsorbate–adsorbent interac-
tions was the highest, followed by N,N-dimethylaminopropyl
and benzyl. For the two co-immobilization of imidazolium ionic
liquids modied silicas, the SilprDBImCl had the largest
adsorption capacity of 2,4-DNP and the SilprDAImCl had the
lowest among the ve adsorbents. It can be said that the
combination of dodecyl and N,N-dimethylaminopropyl of
SilprDAImCl resulted in a negative effect for the adsorption of
2,4-DNP, while the combination of dodecyl and benzyl of
SilprDBImCl enhanced signicantly the adsorption capacity of
2,4-DNP. The data in Fig. 5 were further analyzed using the
pseudo-rst-order (eqn (1)), the pseudo-second-order (eqn (2))
and intraparticle diffusion kinetic models (eqn (3)),
respectively.20,29
Qt ¼ Qe(1 ꢀ exp(ꢀk1t))
(1)
(2)
t
Qt ¼
1
t
þ
2
Qe
k2Qe
pffiffi
Fig. 2 FTIR spectra of SilprBImCl, SilprNImCl, SilprAImCl, SilprDBImCl
and SilprDAImCl.
Qt ¼ k3 t þ C
(3)
where Qe (mg gꢀ1) and Qt (mg gꢀ1) represent the adsorption
capacity of 2,4-DNP at equilibrium and any time t, respectively
(mg gꢀ1); k1 (minꢀ1), k2 (g mgꢀ1 minꢀ1) and k3 (mg gꢀ1 minꢀ1/2
)
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Table 2 Elemental analysis of the five adsorbents
Samples
C (%)
H (%)
N (%)
SilprBImCl
SilprNImCl
SilprAImCl
SilprDBImCl
SilprDAImCl
14.3
12.4
7.7
11.9
6.4
2.2
1.8
1.7
2.0
1.5
2.3
1.7
2.5
1.9
0.9
Fig. 4 Thermogravimetric curves of SilprBImCl (a), SilprNImCl (b),
SilprAImCl (c), SilprDBImCl (d) and SilprDAImCl (e).
are the rate constant of the pseudo-rst-order model, the
pseudo-second-order model and the intraparticle diffusion
model, respectively. C (mg gꢀ1) is a constant.
Fig. 3 N2 adsorption–desorption isotherms and pore size distribu-
tions of SilprBImCl (a), SilprNImCl (b), SilprAImCl (c), SilprDBImCl (d)
and SilprDAImCl (e).
Fig. 5 Effect of shaking time on the adsorption of 2,4-DNP onto the
five adsorbents.
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Table 3 Parameters of pseudo-second order kinetic model for the adsorption of 2,4-DNP
Adsorbents
Parameters
SilprBImCl
SilprNImCl
SilprAImCl
SilprDBImCl
SilprDAImCl
Qe(exp) (mg gꢀ1
)
61.9
0.00026
64.7
68.8
0.0222
68.8
65.2
0.1229
64.9
83.6
0.02722
83.7
42.7
0.0531
42.3
k2 (g mgꢀ1 minꢀ1
)
Qe(cal) (mg gꢀ1
)
R2
0.9927
0.9998
0.9999
0.9999
0.9998
Table 3 showed that the values of Qe(cal) calculated by eqn (2) where Qm is the monolayer adsorption capacity (mg gꢀ1); b is
were in agreement with those Qe(exp) and the correlation coef- the Langmuir constant; KF and 1/n are the constant indicative of
cients (R2) were over 0.99, which highlighted the applicability the relative adsorption capacity of the adsorbent and the
of the pseudo-second-order adsorption kinetic model for the constant indicative of the heterogeneity factor of Freundlich
adsorption of 2,4-DNP onto the ve adsorbents.
adsorption isotherm, respectively.
The adsorption equilibrium of 2,4-DNP was investigated over
The Langmuir and Freundlich parameters were calculated
a range of concentration of 5–100 mg Lꢀ1 at 30 C by plotting using the nonlinear optimization method (Table 4). The equi-
the equilibrium adsorption capacity of 2,4-DNP onto each librium adsorption data were well described by Langmuir
adsorbent (Qe, mg gꢀ1) versus the equilibrium concentration of according to the value of correlation coefficient (R2). The
2,4-DNP. The results were shown in Fig. 6. The equilibrium Langmuir maximum adsorption capacity for the ve adsorbents
adsorption data were further analyzed by the most common followed the order: SilprAImCl > SilprDBImCl > SilprNImCl >
Langmuir (eqn (4).) and Freundlich (eqn (5)) models, SilprBImCl > SilprDAImCl. Compared with other adsorbents
ꢁ
respectively.55
such as granular activated carbons impregnated with phos-
phoric acid (299.6 mg gꢀ1),17 UiO-66-NH2 (29.6 mg gꢀ1),19
carbon nanospheres (32.9 mg gꢀ1),25 SilprImCl (33.3 mg gꢀ1),
SilprM1ImCl (33.9 mg gꢀ1), SilprM2ImCl (35.9 mg gꢀ1),
SilprM4ImCl (37 mg gꢀ1), SilprM1M2ImCl (35.3 mg gꢀ1),47
SilprP3NImBr (91.74 mg gꢀ1) and SilprSP3NImBr (51.81 mg
gꢀ1),49 the ve adsorbents in this study possessed a high 2,4-
DNP adsorption capacity.
QmbCe
Qe ¼
(4)
(5)
1 þ bCe
1=n
Qe ¼ KFCe
The thermodynamic properties of the ve adsorbents for the
adsorption of 2,4-DNP were explored at 25, 30, 35, 40, 45 and
ꢁ
50 C (Fig. 7). The related thermodynamic parameters such as
Gibbs free energy change (DG), enthalpy change (DH) and
entropy change (DS) were further calculated by the following
equations.20,23
Qe
K ¼
(6)
Ce
DG ¼ ꢀRT ln K
DS DH
(7)
(8)
ln K ¼
ꢀ
R
RT
Fig. 6 Adsorption isotherms of 2,4-DNP onto the five adsorbents.
Table 4 Langmuir and Freundlich isotherm constants
Adsorbents
Models
Parameters
SilprBImCl
SilprNImCl
SilprAImCl
SilprDBImCl
SilprDAImCl
Langmuir
Qm (mg gꢀ1
)
67.5
81.2
116.3
101.6
57.9
b (L mgꢀ1
)
1.0094
0.9784
32.1
0.3096
0.9922
26.8
0.0719
0.9989
15.4
0.4929
0.9836
39.0
0.1229
0.9889
13.1
R2
Freundlich
KF (mg gꢀ1) (mg Lꢀ1 n
)
1/n
0.2070
0.8511
0.2877
0.8989
0.4682
0.9618
0.2623
0.8107
0.3442
0.9583
R2
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polymeric adsorbents61 and the adsorption of 2,4-dichlor-
ophenol onto cyclodextrin-ionic liquid polymer.62
3.3 Adsorption mechanism
To further clarify the adsorption process of 2,4-DNP onto the
ve adsorbents, the effects of competitive anions were studied
by adding NaCl or NaNO3 solutions with different concentra-
tions from 0.005 to 0.1 mol Lꢀ1 into the corresponding
adsorption system. As shown in Fig. 8, the adsorption capacity
of 2,4-DNP onto the ve adsorbents decreased signicantly with
the increase of NaCl or NaNO3 concentrations, which indicated
the occurrence of the competition adsorptions between 2,4-DNP
and anions. These interesting results conrmed that the ve
adsorbents kept electrostatic nature.
Fig. 7 Effect of temperature on the adsorption of 2,4-DNP onto the
five adsorbents.
The effects of pH on the adsorption process were carried out
from pH 2 to 10 (Fig. 9). For the SilprNImCl, SilprDBImCl and
where K is the distribution coefficient, T (K) is the absolute
temperature and R is the ideal gas constant (8.314 J molꢀ1 Kꢀ1).
The obtained thermodynamic parameters were listed in
Table 5. Firstly, each adsorbent presented a negative value of
DG, which indicated that the adsorption reactions were spon-
taneous process and thermodynamically favourable. Secondly,
an interesting observation was that the value of DH for 2,4-DNP
adsorption onto SilprBImCl was positive while the rest were
negative for the other four adsorbents. The positive value of DH
suggested an endothermic adsorption for 2,4-DNP onto Silpr-
BImCl. As Fig. 7 showed, the adsorption capacity of 2,4-DNP
onto SilprBImCl increased with increasing temperature from 25
ꢁ
to 50 C because of its lowest values of total pore volume and
average pore width, and higher temperature was favour for the
mass transfer of 2,4-DNP. The similar results were also reported
in different adsorption systems such as the adsorption of
catechol onto montmorillonite modied with two hydroxyl-
containing Gemini surfactants59 and the adsorption of 4-
nitrotoluene-2-sulfonic acid onto two magnetic resins at higher
concentrations.60 In addition, the obtained negative values of
DH suggested an exothermic adsorption for 2,4-DNP on the
other four adsorbents, since these adsorbents had larger BET
surface area compared with SilprBImCl. 2,4-DNP could easily
approach active sites of these adsorbents and temperature was
not the main factor inuencing the adsorption of 2,4-DNP. The
observations obtained in this work agreed well with the pub-
lished results such as the adsorption of p-nitroaniline onto
Fig. 8 Effect of NaCl (a) and NaNO3 (b) on the adsorption of 2,4-DNP
onto the five adsorbents.
Table 5 Thermodynamic parameters for the adsorption of 2,4-DNP
DG (kJ molꢀ1
)
Adsorbents
DH (kJ molꢀ1
)
DS (kJ molꢀ1 Kꢀ1
)
25 ꢁC
30 ꢁ
C
35 ꢁ
C
40 ꢁ
C
45 ꢁ
C
50 ꢁ
C
SilprBImCl
SilprNImCl
SilprAImCl
SilprDBImCl
SilprDAImCl
28.8
ꢀ26.9
ꢀ18.6
ꢀ22.6
ꢀ8.8
0.1620
ꢀ0.0199
0.0068
0.0015
0.0322
ꢀ19.4
ꢀ20.7
ꢀ20.5
ꢀ22.7
ꢀ18.3
ꢀ20.3
ꢀ20.9
ꢀ20.6
ꢀ23.2
ꢀ18.6
ꢀ21.0
ꢀ21.1
ꢀ20.9
ꢀ23.3
ꢀ18.7
ꢀ22.4
ꢀ20.9
ꢀ20.8
ꢀ23.5
ꢀ19.1
ꢀ22.9
ꢀ20.6
ꢀ20.8
ꢀ22.9
ꢀ19.0
ꢀ23.3
ꢀ20.2
ꢀ20.7
ꢀ22.7
ꢀ19.0
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Fig. 10 Effect of volume of 0.5 mol Lꢀ1 HCl on the recovery of 2,4-
DNP.
Fig. 9 Effect of pH on the adsorption of 2,4-DNP onto the five
adsorbents.
were presented in Fig. 10. For the ve adsorbents, the recovery
(R) increased with increase of HCl volumes from 2 to 14 mL and
the recovery was over 96% when the volume of HCl was 14 mL. It
was interesting to nd when the volume of HCl was 2 mL, the
recovery was 25.3% and 20.8% for the SilprNImCl and SilprD-
BImCl respectively, which was lower than that of the other three
adsorbents. This may be attributed to the higher affinity of the
two adsorbents for 2,4-DNP resulting in the low recovery.
SilprDAImCl, the adsorption capacity of 2,4-DNP onto the three
adsorbents increased signicantly with the pH value from 2 to 3.95
and began to decrease signicantly with further increase of the pH
values. This can be attributed to the competition effects between
2,4-DNP and OHꢀ ions at higher pH. However, for the SilprBImCl
and SilprAImCl, the adsorption capacity for 2,4-DNP kept a plat-
form in the pH range of 4–7. It was evident that SilprDBImCl
exhibited the highest adsorption performance for 2,4-DNP in the
pH range of 4–8 among the ve adsorbent. The adsorption
capacities of 2,4-DNP onto SilprBImCl, SilprNImCl and SilprAImCl
at the original pH value (3.95) were 61.8, 66.7 and 63.4 mg gꢀ1
,
4. Conclusion
respectively. Compared with the previously synthesized imidazole-
modied silica adsorbents in our group,47 it was concluded that
benzyl, naphthylmethyl or dimethylaminopropyl enhanced the
adsorption of 2,4-DNP, attributing to the additional interactions
such as p–p interaction between the benzyl, naphthylmethyl and
2,4-DNP for SilprBImCl and SilprNImCl, and acid–base interaction
between N,N-dimethylaminopropyl and 2,4-DNP for SilprAImCl.
It was worth noting that SilprDAImCl exhibited the smallest
adsorption performance among the ve adsorbents, which
indicated that the co-modication of silicas with 1-dodecyli-
midazolium and 1-N,N-dimethylaminopropylimidazolium was
not favorable for the adsorption process. It can be concluded
that the combination of hydrophobicity interaction and p–p
interaction favours the enhancement of adsorption perfor-
mance. However, the combination of hydrophobicity interac-
tion and acid–base interaction decreased signicantly the
adsorption performance. One possible explanation for this
phenomenon was that the 2,4-DNP adsorption achieved by
acid–base interactions may be hindered due to the direction-
ality and short range of hydrogen bonding.63 These interesting
ndings indicated that the contributions of substituent groups
of imidazole ring for the adsorption of 2,4-DNP onto ionic
liquids-modied silicas was not a simple sum of that of the
single substituent group.
Three mono-immobilization and two co-immobilization of
imidazolium ionic liquids modied silicas were successfully
fabricated and characterized with regard to the contribution of
substituent groups of imidazole ring for the adsorption of 2,4-
DNP. Benzyl, naphthylmethyl and N,N-dimethylaminopropyl of
imidazole ring displayed the improved adsorption performance
for 2,4-DNP via p–p or acid–basic interactions. And the
contribution of substituent groups of imidazole ring for the
adsorption of 2,4-DNP followed the order: naphthylmethyl >
N,N-dimethylaminopropyl > benzyl. Besides, the SilprDBImCl
had the largest adsorption capacity and the SilprDAImCl had
the lowest among the ve adsorbents. That was to say, the
contributions of substituent groups of imidazole ring for the
adsorption of 2,4-DNP onto these ionic liquids-modied silicas
were not a simple sum of that of the single substituent group.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The nancial support provided by Natural Science Foundation
of Henan province of China (No. 182300410257), the Key
Scientic Research Project of Higher Education of Henan
3.4 Desorption experiments
Dynamic column desorption experiments were carried out with Province of China (No. 18A150034) and Science and Technology
various volumes (2 to 14 mL) of 0.5 mol Lꢀ1 HCl and the results Department (No. 182102311025) are gratefully acknowledged.
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27 K. A. Krishnan, S. S. Suresh, S. Arya and K. G. Sreejalekshmi,
Desalin. Water Treat., 2015, 54, 1850–1861.
28 G. Qian, H. Song and S. Yao, J. Chromatogr. A, 2016, 1429,
127–133.
29 B. Ekka, L. Rout, M. K. S. A. Kumar, R. K. Patel and P. Dash, J.
Environ. Chem. Eng., 2015, 3, 1356–1364.
30 M. E. Mahmoud, Desalination, 2011, 266, 119–127.
31 M. E. Mahmoud and H. M. Al-Bishri, Chem. Eng. J., 2011,
166, 157–167.
Notes and references
1 Y. Ji, Y. Shi, Y. Yang, P. Yang, L. Wang, J. Lu, J. Li, L. Zhou,
C. Ferronato and J. Chovelon, J. Hazard. Mater., 2019, 361,
152–161.
˜
´
2 P. Canizares, C. Saez, J. Lobato and M. A. Rodrigo,
Electrochim. Acta, 2004, 49, 4641–4650.
3 I. M. S. Pillai and A. K. Gupta, J. Electroanal. Chem., 2015, 756,
108–117.
32 Y. Liu, L. Guo, L. Zhu, X. Sun and J. Chen, Chem. Eng. J.,
2010, 158, 108–114.
4 I. M. S. Pillai and A. K. Gupta, J. Electroanal. Chem., 2016, 762,
66–72.
33 Z. Wang, C. Ye, X. Wang and J. Li, Appl. Surf. Sci., 2013, 287,
232–241.
5 T. Tang, Z. Yue, J. Wang, T. Chen and C. Qing, J. Hazard.
Mater., 2018, 343, 176–180.
34 Y. Zhang, H. Wang, N. Sun and R. Chi, Sep. Purif. Technol.,
2018, 205, 84–93.
6 R. A. Kristanti, T. Toyama, T. Hadibarata, Y. Tanaka and
K. Mori, RSC Adv., 2014, 4, 1616–1621.
35 A. N. Turanov, V. K. Karandashev, N. S. Sukhinina,
V. M. Masalov and G. A. Emelchenko, J. Environ. Chem.
Eng., 2016, 4, 3788–3796.
7 A. Ghosh, M. Khurana, A. Chauhan, M. Takeo,
A. K. Chakraborti and R. K. Jain, Environ. Sci. Technol.,
2010, 44, 1069–1077.
36 W. Bi, M. Tian and K. H. Row, J. Chromatogr. B: Anal. Technol.
Biomed. Life Sci., 2012, 880, 108–113.
8 Z. Guo, R. Feng, J. Li, Z. Zheng and Y. Zheng, J. Hazard.
Mater., 2008, 158, 164–169.
37 H. Song, C. Yang, A. Yohannes and S. Yao, RSC Adv., 2016, 6,
107452–107462.
9 Z. Guo, Z. Zheng, S. Zheng, W. Hu and R. Feng, Ultrason.
Sonochem., 2005, 12, 461–465.
38 F. Wang, Z. Zhang, J. Yang, L. Wang, Y. Lin and Y. Wei, Fuel,
2013, 107, 394–399.
10 M. V. Bagal, B. J. Lele and P. R. Gogate, Ultrason. Sonochem.,
2013, 20, 1217–1225.
39 M. Zarezadeh-Mehrizi, A. Badiei and A. R. Mehrabadi, J. Mol.
Liq., 2013, 180, 95–100.
11 M. V. Bagal and P. R. Gogate, Ind. Eng. Chem. Res., 2013, 52,
8386–8391.
40 O. Tkachenko, A. Panteleimonov, I. Padalko, A. Korobov,
Y. Gushikem and Y. Kholin, Chem. Eng. J., 2014, 254, 324–
332.
41 J. Zhang, S. Sun, Y. Bian, W. Li, R. Liu and D. Zhao, Fuel,
2018, 220, 513–520.
12 H. Bashir, X. Yi, J. Yuan, K. Yin and S. Luo, J. Photochem.
Photobiol., A, 2019, 382, 111930.
13 P. Shandilyaa, D. Mittalb, A. Sudhaika, M. Sonic,
P. Raizadaa, A. K. Saini and P. Singh, Sep. Purif. Technol.,
2019, 210, 804–816.
42 H. M. Marwani and E. M. Bakhsh, Chem. Eng. J., 2017, 326,
794–802.
43 W. Zhang, X. Feng, Y. Alula and S. Yao, Food Chem., 2017,
230, 637–648.
14 Q. He, Y. Ni and S. Ye, RSC Adv., 2017, 7, 27089–27099.
15 X. Zhou, C. Lai, D. Huang, G. Zeng, L. Chen, L. Qin, P. Xu,
M. Cheng, C. Huang, C. Zhang and C. Zhou, J. Hazard.
Mater., 2018, 346, 113–123.
44 L. Nie, J. Lu, W. Zhang, A. He and S. Yao, Sep. Purif. Technol.,
2015, 155, 2–12.
16 J. Li, Y. Ren, L. Lai and B. Lai, J. Hazard. Mater., 2018, 344,
778–787.
´
45 Z. Wang, C. Ye, L. Chen and H. Wu, RSC Adv., 2017, 7, 2650–
2657.
17 A. M. Carvajal-Bernal, F. Gomez and L. Giraldo, Microporous
Mesoporous Mater., 2015, 209, 150–156.
46 Z. Wang, Y. Zhu, H. Chen, H. Wu and C. Ye, J. Taiwan Inst.
Chem. Eng., 2018, 86, 120–132.
47 Z. Wang, C. Ye, J. Li, H. Wang and H. Zhang, J. Hazard.
Mater., 2013, 260, 955–966.
18 A. H. EI-Sheikh, A. P. Newman, A. J. Said, A. M. Alzawahreh
and M. M. Abu-Helal, J. Environ. Manage., 2013, 118, 1–10.
19 Z. Xu, Y. Wen, L. Tian and G. Li, Inorg. Chem. Commun., 2017,
77, 11–13.
48 C. Ye, X. Wang, H. Wang and Z. Wang, J. Taiwan Inst. Chem.
Eng., 2014, 45, 2868–2877.
49 Z. Wang, C. Ye and H. Wang, Int. J. Environ. Sci. Technol.,
2016, 13, 113–124.
20 E. Vismara, L. Melone, G. Gastaldi, C. Cosentino and
G. Torri, J. Hazard. Mater., 2009, 170, 798–808.
21 Z. Yaneva and B. Koumanova, J. Colloid Interface Sci., 2006,
293, 303–311.
50 S. Liu, X. Wu, Q. Liu, Z. Zheng, Y. Zang, S. Ge and J. Feng,
Chin. J. Inorg. Chem., 2008, 24, 1444–1449.
51 M. Tian, W. Bi and K. H. Row, Phytochem. Anal., 2013, 24, 81–
86.
52 H. Qiu, Q. Jiang, X. Liu and S. Jiang, Chromatographia, 2008,
68, 167–171.
22 Y. Ku and K. Lee, J. Hazard. Mater., 2000, 80, 59–68.
23 Q. Liu, T. Zheng, P. Wang, J. Jiang and N. Li, Chem. Eng. J.,
2010, 157, 348–356.
24 D. Chaara, I. Pavlovic, F. Bruna, M. A. Ulibarri, K. Draoui and
C. Barriga, Appl. Clay Sci., 2010, 50, 292–298.
´
25 J. C. Lazo-Cannata, A. Nieto-Marquez, A. Jacoby,
53 M. Gao, W. Wang, H. Yang and B. Ye, Chem. Eng. J., 2020,
380, 122459.
A. L. Paredes-Doig, A. Romero, M. R. Sun-Kou and
J. L. Valverde, Sep. Purif. Technol., 2011, 80, 217–224.
26 N. Z. Al-Mutairi, Desalination, 2010, 250, 892–901.
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RSC Advances
Paper
54 Z. Wen, J. Lu, Y. Zhang, G. Cheng, S. Huang, J. Chen, R. Xu, 59 Y. Liu, M. Gao, Z. Gu, Z. Luo, Y. Ye and L. Lu, J. Hazard.
Y. Ming, Y. Wang and R. Chen, J. Hazard. Mater., 2020, 383,
121172.
55 G. Zhu, G. Cheng, T. Lu, Z. Cao, L. Wang, Q. Li and J. Fan, J.
Hazard. Mater., 2019, 373, 347–358.
56 M. Agrawal, V. Deval, A. Gupta, B. R. Sangala and
S. S. Prabhu, Spectrochim. Acta, Part A, 2016, 167, 142–156.
57 Y. Jiao, C. Cao and X. Zhao, J. Mol. Struct., 2012, 1027, 57–63.
Mater., 2014, 267, 71–80.
60 W. Wang, Y. Ma, A. Li, Q. Zhou, W. Zhou and J. Jin, J. Hazard.
Mater., 2015, 294, 158–167.
61 G. Xiao, R. Wen, D. Wei and D. Wu, J. Hazard. Mater., 2014,
280, 97–103.
62 M. Raoov, S. Mohamad and M. R. Abas, J. Hazard. Mater.,
2013, 263, 501–516.
58 A. Kara, E. Demirbel, N. Tekin, B. Osman and N. Besirli, J. 63 M. C. Xu, Y. Zhou and J. H. Huang, J. Colloid Interface Sci.,
Hazard. Mater., 2015, 286, 612–623. 2008, 327, 9–14.
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