Microporous and Mesoporous Materials 303 (2020) 110304

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Microporous and Mesoporous Materials
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In situ modification of ZIF-67 with multi-sulfonated dyes for great
enhanced methylene blue adsorption via synergistic effect
Yanfeng Liu a, Duoyu Lin a, Weiting Yang a, *, Xueying An a, Ahui Sun a, Xiaolei Fan b, **,
Qinhe Pan a, ***
a
b

Key Laboratory of Advanced Materials of Tropical Island Resources, Ministry of Education, School of Science, Hainan University, Haikou, 570228, PR China
Department of Chemical Engineering and Analytical Science, School of Engineering, The University of Manchester, M13 9PL, UK

A R T I C L E I N F O

A B S T R A C T

Keywords:
Metal-organic frameworks (MOFs)
ZIF-67
Multi-sulfonated dyes
Methylene blue
Synergistic effect

It is essential mean to adsorptive remove organic pollutants such dyestuff for water remediation. Herein in situ
modification of the classic metal-organic framework ZIF-67 with –SOÀ3 groups was easily achieved by the effi­
cient adsorption of multi-sulfonated dyes due to the coordinative interaction between the unoccupied Co(II) of
ZIF-67 and –SOÀ3 of the dyes. Interestingly, highly efficient synergistic absorption of multi-sulfonated dyes to­

wards methylene blue (MBỵ) upon ZIF-67 was discovered for the first time. The improved adsorption capacity of
ZIF-67 for MBỵ in presence of cotton blue (CBÀ ) was measured with a record-high value of 5,857.9 mg/g. The
underlying mechanism of the synergistic adsorption was probed, showing that, after the initial coordination
between the –SOÀ3 of the dyes and the unoccupied Co(II) of ZIF-67, the available SO3 groups of multi-sulfonated
dyes can interact with Nỵ(CH3)2 in MBỵ and hence greatly improving the adsorption capacity of MBỵ.

1. Introduction
The dyes are generally applied in many chemical industries such as
textiles, plastic, prints, paper, cosmetics, etc [1]. Most dyes are
non-biodegradable, poisonous, as well as being carcinogenic. The dyes
are occasionally discharged into the environment as untreated waste,
which affects the security of living species severely [2]. As the essential
demand of environmental conservation and ecological protection, it is
extremely necessary to trap and separate the organic pollutants from
wastewater effectively [3,4]. Metal-organic frameworks (MOFs) have
demonstrated much superiority in guest uptake/separation from exem­
plar mixtures due to the tunable host-guest interactions, including
hydrogen bonds, Vander Waals interaction, ion exchange, π-π interac­
tion, electrostatic interaction, Lewis acid-base interaction, etc [5–11].
Classical MOFs including MOF-5 [12], MIL-100 [13], Fe-MOF-235 [14],
Co-ZIF-8 [15], Ni-MOF-199 [16], Cr-MIL-101 [17], and Ti-UiO-66 [18],
have been revealed successfully in the adsorptive removal of various
organic dyes from dye-containing aqueous systems. Interestingly, MOFs
functionalized with particular groups were discovered favorable in
improving the adsorption of organic dyes. For example, MOFs

functionalized with amino group such as MIL-125-NH2 [19],
MIL-101-NH2 [20], and UiO-66-NH2 [21], demonstrated the adsorption
capacity improved for cationic dyes. Compared with the pristine
MIL-101(Cr), MIL-101(Cr)–SO3H was beneficial to trap the cationic dyes

due to the presence of –SO3H groups [22,23]. Considering the modifi­
cation of MOFs in situ using the functional groups in dyes (due to the
coordinative interaction) after the initial adsorption, such synergy may
be true as well for the subsequent adsorption of other dyes by the similar
chemical and/or physical interactions. So far various pristine and
functionalized MOFs as well as MOF-based composite materials were
used for the purification of dye-containing aqueous systems [5], how­
ever, the attempt of modification of MOFs using the dyes with particular
functional groups for the synergistic adsorption of another dye was
never reported.
In this work, ZIF-67 was selected as the candidate adsorbent for
investigating the synergistic adsorption of the dyes in aqueous systems,
due to its good stability, high specific surface area, large pore volume
and the presence of unoccupied metal active sites [24,25]. Additionally,
the Co(II) centers in ZIF-67 can bind the organic dyes with –SOÀ3 groups
[26]. Moreover, MOFs modified with –SOÀ3 could improve the cationic

* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: (W. Yang), (X. Fan), (Q. Pan).
/>Received 16 February 2020; Received in revised form 22 April 2020; Accepted 2 May 2020
Available online 5 May 2020
1387-1811/© 2020 Elsevier Inc. All rights reserved.


Y. Liu et al.

Microporous and Mesoporous Materials 303 (2020) 110304


dyes adsorption via electrostatic attraction [22,23],which was poten­
tially beneficial to the possible synergistic absorption. Accordingly,
based on the considerations discussed above, MOFs after the in situ
modification of the multi-sulfonated compounds may exhibit enhanced
synergistic adsorption capacity especially for cationic dyes. Thus, effi­
cient synergistic absorption towards cationic dyes with Nỵ(CH3)2
groups was studied by introducing multi-sulfonated dyes into ZIF-67.
The experimental results showed that synergistic absorption of the
multi-sulfonated dyes towards MBỵ upon ZIF-67 was true, yet very
efficient. Mechanistic analysis suggested that the electrostatic interac­
tion between the respective SO3 and Nỵ(CH3)2 groups in the two dyes
lead to the synergistic interaction. This work demonstrates an efficient
method to enhance the adsorption and separation performance of MOFs
for dyes in aqueous media. Meanwhile, it is innovative and easy to in situ
modify MOFs with –SOÀ3 groups by coordination with dyes, compared
with general functionalization.

about 0.9 μm as analyzed by SEM (Fig. S2). The synthesized ZIF-67 was
further characterized by FTIR spectroscopy, as seen in Fig. S3, the IR
bands at 3,133.9, 2,925.9, 1,171.9, and 425.4 cmÀ 1 matched well with
the previously reported data [28,29]. The Brunauer–Emmett–Teller
(BET) surface area via nitrogen adsorption was calculated to be 1,445
m2/g (Fig. S4).
3.1. Performance of ZIF-67 for single component dye adsorption
As illustrated in Fig. 1, all the selected cationic and anionic dyes
could be absorbed by ZIF-67, and the adsorption capacity ranged from
94.3 mg/g (for RhBỵ) to 1,250.0 mg/g (for CR ). Judging from the
above adsorption behavior, the positive and negative charges of the 9
dyes are not the critical factor for the quantity adsorbed. In addition, as
shown in Table S1, the calculated molecular sizes of the 9 dyes are listed

in order of MBỵ < MO < ACBK < EBBR , which are inconsistent with the order of adsorbed quality
either. In view of the pore opening (0.34 nm) and pore diameter (1.1
nm) of ZIF-67 [24], the molecular size of the 9 dyes did not directly
determine their adsorption capacity. Interestingly, it was revealed that
ZIF-67 preferred to absorb the sulfonated dyes, and the adsorption
abilities ranged from 271.1 mg/g (for MOÀ ) to 1,250.0 mg/g (for CRÀ ),
meanwhile the adsorption amount of the dyes possessing multi–SOÀ3
groups is significantly greater than that of MOÀ with single –SOÀ3 group.
Additionally, in situ modification of ZIF-67 with –SOÀ3 groups using
multi-sulfonated dyes was also simultaneously achieved by the adsorp­
tion due to the coordinative interaction, which was confirmed by the
desorption experiments, i.e. the absorbed multi-sulfonated dyes could
not be eluted by CH3CN, CH3OH or saturated aqueous NaCl solution.

2. Experiment
2.1. Materials and physical measurements
Co(NO3)2⋅6H2O and methanol were obtained from Guangzhou
chemical reagent factory. Dyes and 2-methylimidazole were purchased
from Macklin (Shanghai, China). All the chemicals were used directly
without further purification. Powder X-ray diffraction (PXRD) patterns
were obtained on the X-Ray Diffractometer (Rigaku MiniFlex600,
Japan) operating at 15 mA and 40 kV producing Cu Kα with λ ¼ 1.54056
Å. The morphology of ZIF-67 was performed by Scanning electron mi­
croscopy (SEM) (Hitachi, S4800, Japan) operating at 3.0 kV. Surface
areas and pore sizes were assessed by nitrogen physisorption analysis on
an ASAP2460 instrument (MICROMERITICS, USA). UV–vis spectrum
was recorded on a Lambda 750s spectrophotometer in the range of
300–750 nm. Infrared (IR) spectrum was analyzed on a Bruker
TENSOR27 spectrophotometer in the range of 4,000–400 cmÀ 1 using

KBr pellet. The surface element states of ZIF-67 and ZIF-67 loaded with
dyes were tested by X-ray photoelectron spectroscopy (XPS), being
collected at a monochromatic Al Kα (λ ¼ 1,486.6 eV), and charge was
corrected by using the C 1s (284.8 eV) line in all the spectra.

3.2. The synergistic adsorption behavior
Considering the electrostatic interaction between the SO3 and
Nỵ(CH3)2 groups, 7 dyes with –SOÀ3 groups were selected to study the
synergistic adsorption performance towards MBỵ upon ZIF-67 in
aqueous media. As shown in Fig. 2, the capability of the dyes under
study for the synergistic adsorption towards MBỵ can be ranked as:
ACBK > CB > FA > EBBRÀ > CBBRÀ > CRÀ . All of the in situ
modification of the 6 dyes on ZIF-67 resulted in the greatly enhanced
absorption capacity of MBỵ, and the highest adsorbed quantity of MBỵ
was measured at ~1,150.8 mg/g in the presence of ACBKÀ . In com­
parison, the adsorption capacity of ZIF-67 for MBỵ is only ~103.4 mg/g
in the single-component dye adsorption experiment. Therefore, it is
plausible that the greater enhanced MBỵ adsorption with ACBKÀ , CBÀ ,
and FAÀ dyes than that with CBBRÀ , EBBRÀ and CRÀ dyes might be due
to presence of the additional –SOÀ3 group, leading to the synergistic dyedye interaction. Additionally, such synergetic effect was not measured
for MBỵ with MOÀ which only possesses a single –SOÀ3 group. These
results demonstrate that the adsorption performance towards MBỵ can
be enhanced via the synergistic dye-dye adsorption effect by the intro­
duction of multi–SOÀ3 groups-featured dyes, and the effect is a function
of the number of –SOÀ3 groups in dyes.

2.2. Dyes adsorption and modification of ZIF-67 with multi-sulfonated
dyes
ZIF-67 was synthesized according to a reported procedure [27]. In
order to achieve the adsorption performance of pristine ZIF-67 towards

different organic dyes, 4.0 mg ZIF-67 with average particle size of ~0.9
μm was added into the aqueous solution of 9 dyes (40 mL, 125 mg/L),
respectively. The 9 dyes with different sizes and charges include, 1)
cationic type with Nỵ(CH3)2 or Nỵ(CH2CH3)2 groups, methylene blue
(MBỵ) and rhodamine B (RhBỵ); 2) anionic type with –SOÀ3 groups,
methyl orange (MOÀ ), acid chrome blue K (ACBKÀ ), fuchsin acid (FAÀ ),
cotton blue (CBÀ ), congo red (CRÀ ), coomassie brililiant blue R-250
(CBBRÀ ), eriochrome blue black R (EBBRÀ ). The details of the 9 organic
dyes were displayed in Table S1. Synergistic adsorption performance of
several sulfonated dyes towards MBỵ upon ZIF-67 was conducted as
follows. Take CB for an example, 4.0 mg ZIF-67 was first added into the
solution of CBÀ (125 mg/L) and stirred for 0.5 h to obtain CB @ZIF-67,
subsequently 5 mg MBỵ (125 mg/L) was added to allow the synergetic
dye adsorption. The trinary-component dye adsorptive experiments
were carried out similarly with MBỵ/MO mixture being added instead.
The concentration of dyes was monitored by UV–vis spectroscopy.

3.3. Adsorption performance of CBÀ upon ZIF-67
In all of the 6 above multi-sulfonated dyes, CBÀ was selected as the
model to perform the detailed study of the synergistic adsorption of MBỵ
upon ZIF-67 due to the following considerations: 1) it features three
–SOÀ3 groups, which would be favorable for the synergistic effect; 2) the
fast adsorption kinetics; (5 min to reach the adsorption equilibrium,
pseudo-second-order adsorption rate, indicating chemical adsorption
involving valence forces through sharing or exchanging of electrons
between CBÀ and ZIF-67 as the rate-limiting step (Fig. S6 & Table S2));
3) the high uptake on ZIF-67. The correlation coefficient R2 of the
Langmuir and Freundlich adsorption models are 0.997 and 0.994,
respectively (Fig. S7 & Table S3). This result illustrated the experimental

3. Results and discussion
The phase purity and crystallinity of the harvested ZIF-67 was veri­
fied by comparing the diffraction peaks to the simulated patterns of ZIF67 in the literature (Fig. S1). The average particle size of ZIF-67 was
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Fig. 1. Adsorption performance of ZIF-67 towards 9 dyes.

Fig. 2. Synergistic adsorption performance of several sulfonated dyes towards MBỵ upon ZIF-67 in aqueous solution. (125 mg/L of single-component for MBỵ. 125
mg/L of binary-component for MOÀ , CRÀ , CBBRÀ , EBBRÀ , FA , ACBK and CB respectively mixed with MBỵ after being stirred with ZIF-67 for 0.5 h)

data are better fitted with Langmuir model. The maximum adsorption
capacity of 6,004.94 mg/g calculated by the Langmuir model matches
well with experimental data (5,860.1 mg/g). All the above results
indicate that monolayer adsorption of ZIF-67 adsorbent is common. As a
result, binary-component adsorption investigations of CBÀ and MBỵ
were conducted in the following work.

added into each aqueous solution with the same concentration of CBÀ .
The color of the aqueous solution after adsorption became obviously
lighter in sequential binary-component adsorption process, as shown in
Fig. S8. The adsorption capacity increased significantly with a record
high capacity value of 5,857.9 mg/g for MBỵ (Fig. 3 & Table 1). The
result indicates that the pre-adsorbed CB has highly effective synergism
for binding MBỵ. The adsorption capacity of MBỵ increased significantly
in the experiments with the initial concentration of CB and MBỵ ranged

from 100 to 700 mg/L. Nevertheless, the adsorption capacity of CBÀ in
binary-component adsorption did not decrease compared with that in
single-component experiments. Therefore, based on the adsorption

3.4. Synergistic adsorption of MBỵ in presence of CB
In binary-component dyes adsorption experiments, ZIF-67 was
added in CBÀ aqueous solution, after being stirred for 10 min, MBỵ was
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Fig. 3. Single-component adsorption experiments of MBỵ or CB , and the synergistic adsorption performance of CB towards MBỵ upon ZIF-67.

(Fig. 4b). And the crystallinity of CBÀ @ZIF-67 was preserved well as
evidenced by PXRD analysis (Fig. S9).

Table 1
Comparison of the MBỵ adsorption capacity of CBÀ @ZIF-67 with other
adsorbents.
Adsorbents

qmax (mg/
g)

Ref.

CBÀ @ZIF-67

5,857.9

Calcium alginate membrane
Carboxy methyl cellulose/poly(methyl acrylate) hydrogels
SA nanofiber membranes
Aminocarboxylate/maleic acid resin
Lignocellulose-g-poly(acrylic acid)/montmorillonite
hydrogel composites
NaAlg-g-p(AA-co-St)/organo-I/S
Poly(N-vinyl caprolactam-co-maleic acid)
Fe3O4/HKUST-1
MIL-100 (Fe)
GO/lignosulfonate aerogel
NH2-MIL-125(Ti)
Calcium alginate–bentonite–activated carbon composite
beads
Core@double-shell structured HNTs/Fe3O4/poly(DA þ
KH550) nano-hybrids
MOF-235

3,506.4
2,370
2,357.9
2,101
1,994.4

this
work
[30]

[31]
[32]
[33]
[34]

1,843.5
1,441
1,277
1,105
1,023.9
862
756.9

[35]
[36]
[37]
[38]
[39]
[20]
[40]

714.3

[41]

477

[14]

3.6. Synergistic and selective adsorption performance of dyes upon ZIF-67

To confirm the generic feature of such synergistic adsorption be­
tween Nỵ(CH3)2 and SO3 , the sequential binary-component adsorpư
tion of CB with the dyes possessing the functional groups of N(CH3)2,
Nỵ(CH2CH3)2 and Nỵ(CH3)2 on ZIF-67 was investigated, and MO
and RhBỵ were selected, respectively. The synergistic effect of CB /
MO and CB /RhBỵ systems was barely measured (Fig. 5a and b). While
the CBÀ /MBỵ system showed a significant improvement in adsorption
performance. Based on the results above, in situ modification of ZIF-67
with CBÀ can promote the adsorption capacity of ZIF-67 for MBỵ with
Nỵ(CH3)2 groups. Conversely, such synergistic effect for dyes with
Nỵ(CH2CH3)2 and N(CH3)2 was notobserved. Thus, the electrostatic
interaction between the –SOÀ3 in dyes and Nỵ(CH3)2 in MBỵ might be
responsible for the synergistic phenomenon. While the steric hindrance
of Nỵ(CH2CH3)2 and the lack of charge of N(CH3)2 result in no such
effect.
The selectivity of MBỵ over MO /RhBỵ associated with CB @ZIF-67
was further determined by the trinary-component adsorption experiư
ments. The selective absorption performance of MBỵ by CB @ZIF-67
exhibited a similar trend. The removal efficiency for MBỵ is 90.7% and
92.9%, respectively, under the conditions used, while only 6.3% for
MO and 2.9% for RhBỵ were measured (Fig. 5d). Therefore, the find­
ings suggest that CBÀ @ZIF-67 has comparatively good selectivity to
MBỵ due to the synergistic adsorption.

amounts of both dyes, the molar ratio of adsorbed CB to MBỵ is
calculated to be about 1:2. The results demonstrate that after the coor­
dinative interaction between the Co(II) in ZIF-67 and –SOÀ3 in CBÀ , the
electrostatic interaction occurs between the remaining two –SOÀ3 groups
in CBÀ and Nỵ(CH3)2 in MBỵ with the molar ratio of both functions
being of 1:1.

3.7. Mechanistic study

3.5. Recycling and reusability

PXRD patterns of ZIF-67 before and after dye adsorption are illus­
trated in Fig. S10. All the diffraction peaks of ZIF-67 (as-synthesized),
CB @ZIF-67, MBỵ@ZIF-67, and MBỵ/CB @ZIF-67 are agreed with the
reported patterns in literature [24]. Thus, the crystallinity of ZIF-67 was
unchanged after the adsorption of CB and MBỵ.
To explore the interaction mechanism between ZIF-67 and the dyes
under study, FTIR spectra of ZIF-67, CBÀ , MBỵ and dyes@ZIF-67 were
compared together (Fig. 6). In the spectrum of CBÀ (Fig. 6a), the peak at

Recycling and reusability of absorbents are the important factors in
the dye adsorption properties. Therefore, the release experiments were
conducted by eluting the MBỵ/CB @ZIF-67 sample using CH3OH. As
shown in Fig. 4a, MBỵ can be released into CH3OH solution from the
saturated samples quickly in 60 min. In order to confirm the durability
and reusability of CBÀ @ZIF-67 in the adsorption process, the adsorp­
tionÀ desorption experiments were performed alternatively for 5 runs
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Fig. 4. a) Release experiments of MBỵ from the corresponding CB @ZIF-67 adsorbed sample in the solution of CH3OH; b) the reusability of CB @ZIF-67 for MBỵ
adsorption for 5 times.

Fig. 5. UV–Vis spectra of several binary-component dyes during the adsorption process. a) MO /CB , b) RhBỵ/CB , c) MBỵ/CB . (The initial concentrations of CB ,
MO , RhBỵ and MBỵ were 120, 50, 60, and 25 mg/L, respectively.) d) Selectivity of MBỵ over MO /RhBỵ in trinary-component dyes adsorption. (CB @ZIF-67
aqueous solution obtained after 120 mg/L CBÀ mixed with ZIF-67 stirred for 10 min, and then MBỵ/MO or MBỵ/RhBỵ were added with each single component
concentration being 50 mg/L).

1,337.2 cmÀ 1 is ascribed to asymmetric S–O stretching vibrations, and
the peak at 1,169.1 cmÀ 1 is associated with aromatic C–N stretching
vibrations [42]. In the spectrum of MBỵ (Fig. 6b), a band appears at 3,
425.6 cmÀ 1, attributable to the O–H stretching vibration. The characư
teristic bands of MBỵ at 1,354.2 and 1,184.3 cm 1 are attributed to the

stretching mode of C–N from the aromatic ring and the aliphatic chain of
Nỵ(CH3)2, respectively. According to Fig. 6c, a characteristic band of
ZIF-67 at 425.4 cmÀ 1 is attributed to the Co–N stretching vibration [43,
– N at
44]. The stretching vibration peaks of C–N at 1171.9 cmÀ 1 and C–
1579.1 cmÀ 1 are also observed [45,46].
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Fig. 6. FTIR spectra of a) CB , b) MBỵ, c) ZIF-67, as well as that of d) CBÀ @ZIF-67, e) MBỵ@ZIF-67, and f) MBỵ/CB @ZIF-67 after adsorption of CB , MBỵ, or the
mixture of CB and MBỵ, respectively.

As observed in Fig. 6d, differences are observed obviously in the
spectra of ZIF-67 and CBÀ @ZIF-67. The sharp peak of Co–N stretching

vibrations at 425.4 cmÀ 1 in ZIF-67 is shifted to 424.7 cmÀ 1 in CBÀ @ZIF67, meanwhile, the vibration frequencies of the bands at 1,579.1 cmÀ 1
– N and 1,171.8 cmÀ 1 of C–N are shifted to 1,593.3 and 1,172.8
of C–
cmÀ 1, respectively, which might be due to the interaction of –S(O2)–OÀ 1
from CBÀ with Co(II) centers of ZIF-67 [26]. Moreover, the bonding
electron cloud of Co–N bond is far away from N core, the density of
electron cloud around N core decreases, as well as the attraction of
– N and
bonding electron and the stretching vibration frequencies of C–
C–N increases. In the similar way, the interaction of –S(O2)–OÀ 1 with Co
(II) increases the donating electronic activity of negative oxygen ion,
and the strength of S–O bond is weakened. Therefore, the peak at 1,
337.2 cmÀ 1 for asymmetric S–O stretching vibrations in CBÀ is shifted to
1,335.5 cmÀ 1 in CBÀ @ZIF-67. IR analysis reveals the binding of CBÀ to
the framework of ZIF-67 via chemisorption of –SOÀ3 group on Co(II). The
open Co(II) centers in ZIF-67 are occupied by –OH, and the –OH group
could be replaced by some stronger Lewis bases. Consequently, the
interaction between Lewis acidity of Co(II) in ZIF-67 and the Lewis
basicity of –SOÀ3 group in CBÀ could occur by the replacement of –OH by
–SOÀ3 [47,48]. This illustrates that the chemisorption is dominant in the
adsorption of CBÀ on ZIF-67.
In spectra of MBỵ and MBỵ@ZIF-67 (Fig. 6b and e), O–H and Co–N
bands from 3,425.6 cmÀ 1 and 425.4 cm 1 in MBỵ shifted to 3,447.5
cm 1 and 424.5 cm 1 in MBỵ@ZIF-67, respectively. The relevant
wavelength shift might be caused by the formation of hydrogen-bonding
and π-π stacking interactions between MBỵ and ZIF-67 [49,50]. The
CoN peak at 424.7 cm 1 was unchanged after the adsorption of MBỵ on
CB @ZIF-67. The bands at 1,335.5 cmÀ 1 (S–O group), 1,593.3 cmÀ 1
– N group) and 1,172.8 cmÀ 1 (C–N group) in CBÀ @ZIF-67 are shifted
(C–

to 1,334.0, 1,598.2 and 1,173.5 cmÀ 1 in MBỵ/CB @ZIF-67, respectively
(Fig. 6e and f). The results suggest the interaction exists between
Nỵ(CH3)2 in MBỵ and S(O2)O 1 in CB , which is responsible for
promoting the adsorption capacity of MBỵ upon ZIF-67 in the aqueous

solution of MBỵ and CB @ZIF-67.
XPS analysis of the several representative samples was investigated
to further indentify the mechanism of the measured synergistic
adsorption on ZIF-67 for CBÀ and MBỵ. As shown in Fig. 7b, the Co 2P of
ZIF-67 consists of Co 2p3/2 (781.2 eV) and Co 2p1/2 (796.7 eV)
accompanied with two satellite peaks at 786.0 and 802.2 eV implying
the presence of Co(II) phase [51]. The Co 2p binding energies of
MBỵ@ZIF-67 and ZIF-67 were similar. While the Co 2p peaks shifted
from 781.2 eV to 796.7 eV in ZIF-67 to 781.5 eV and 797.2 eV in
CBÀ @ZIF-67, respectively, which resulted from the interaction between
unoccupied Co(II) of ZIF-67 and the –SOÀ3 group in CBÀ [52,53]. The
binding energies S 2p at 167.9 and 169.0 eV in CBÀ @ZIF-67 originate
from central sulphur atoms in –SO3-Co and –SO3-Na, respectively
(Fig. 7c) [54]. The S 2p peaks in MBỵ/CB @ZIF-67 showed a slight
downshift in comparison with that in CBÀ @ZIF-67, illustrating that the S
chemical state had been changed by introducing MBỵ. In the case of
ZIF-67 and MBỵ@ZIF-67, signals related to S were not detected. As
shown in Fig. 7d, the O 1s peak at 531.5 eV in ZIF-67 indicated the
presence of surface –OH groups associated on Co(II) [55,56]. The exis­
tence of hydrogen-bond interactions between ZIF-67 and MBỵ could be
affirmed by O 1s peak shifting from 531.5 eV in pristine ZIF-67 to 531.3
eV in MBỵ@ZIF-67 [26,57]. Coordination interactions also were formed
between Co(II) and –SOÀ3 groups, which could be further confirmed by
the presence of 531.7 eV (–SO3-Co) and 532.9 eV (–SO3-Na) in
CBÀ @ZIF-67, as well as the absence of O 1s peak at 531.5 eV in pristine

ZIF-67 [54]. While the O 1s at 532.9 eV in CBÀ @ZIF-67 is shifted to
lower binding energy at 532.1 eV in MBỵ/CB @ZIF-67, which should be
caused by the interaction between –SOÀ3 in CB and Nỵ(CH3)2 in MBỵ.
Considering the analysis findings above, possible mechanism in the
synergistic adsorption process for CB and MBỵ upon ZIF-67 is proposed
as shown in Scheme 1. In single-component adsorption of MBỵ upon ZIF67, hydrogen-bonding interaction is present between Nỵ(CH3)2 in MBỵ
and OH in active sites of ZIF-67 [49], meanwhile, the interaction be­
tween the benzene rings and the imidazole rings of them causes the
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Fig. 7. XPS spectra of ZIF-67, MBỵ@ZIF-67, CB @ZIF-67, and MBỵ/CB @ZIF-67 products.
a) XPS survey spectra, b) Co 2p, c) S 1s, d) O 1s.

Scheme 1. Possible mechanism of the synergistic adsorption of multi-sulfonated dyes towards MBỵ upon ZIF-67.

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formation of π-π stacking [50]. Significant differences appear in the
adsorption of multi-sulfonated dyes upon ZIF-67, strong electrostatic
attraction exists between –SOÀ3 groups in multi-sulfonated dyes and the

Co(II) centers in ZIF-67 [58]. Additionally, MOFs modified with –SOÀ3
groups also showed relatively strong affinity towards cationic dyes (like
methylene blue and malachite green) [59]. After the pre-adsorption of
dyes with multi–SOÀ3 groups on ZIF-67, the electrostatic attraction beư
tween the available SO3 and Nỵ(CH3)2 in another dye resulted in
significant enhancement on the uptake of the second dye. Nevertheless,
findings above exhibit that ZIF-67 functionalized with multi–SOÀ3 groups
can synergistically adsorb other cationic dyes with Nỵ(CH3)2 groups in
aqueous media, which contributes to the great improvement on the
adsorption capacity of ZIF-67 adsorbent.

[3]
[4]

[5]
[6]
[7]

4. Conclusions

[8]

In summary, ZIF-67 can be used as an optional adsorbent for
removing organic dyes from aqueous media. Interestingly, ZIF-67
preferred to adsorb multi-sulfonated dyes and was easily functional­
ized with –SOÀ3 groups due to the coordinative interaction. The
adsorption capacity of ZIF-67 towards CBÀ was very high at 5,860.1 mg/
g. Importantly; the synergistic absorption of multi-sulfonated dyes and
MBỵ upon ZIF-67 was discovered for the first time. Specifically, a
considerable increase in the adsorption capacity of ZIF-67 for MBỵ

(5,857.9 mg/g) occurred by pre adsorbing CB on ZIF-67 in aqueous
media. The inherent mechanism of the synergistic adsorption of
different dyes on ZIF-67 was proposed, that is, the pre-adsorption of
multi-sulfonated dyes on ZIF-67 functionalized ZIF-67 with the addi­
tional –SOÀ3 groups which interact strongly with the Nỵ(CH3)2 group in
MBỵ. The synergistic adsorption effect of different organic dyes upon the
adsorbent reveals a novel idea for the in situ modification of MOFs
adsorbent in greatly enhancing the adsorption efficient of trapping and
separating the organic compounds from waste water.

[9]
[10]
[11]

[12]
[13]
[14]
[15]
[16]

Declaration of competing interest

[17]

There are no conflicts to declare.
CRediT authorship contribution statement

[18]

Yanfeng Liu: Conceptualization, Methodology, Investigation, Re­

sources, Writing - original draft. Duoyu Lin: Resources, Investigation.
Weiting Yang: Supervision, Resources, Writing - review & editing, Data
curation. Xueying An: Formal analysis, Visualization, Investigation.
Ahui Sun: Formal analysis. Xiaolei Fan: Supervision, Writing - review
& editing. Qinhe Pan: Project administration, Writing - review &
editing.

[19]
[20]

[21]

Acknowledgements

[22]

This work was supported by the Natural Science Foundation of
Hainan Province (218QN185 and 2019RC005), the National Natural
Science Foundation of China (21761010), and Hainan University startup fund (KYQD(ZR) 1806).

[23]

Appendix A. Supplementary data

[25]

Supplementary data related to this article can be found at https://doi
.org/10.1016/j.micromeso.2020.110304.

[26]

[24]

[27]

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