In situ modification of ZIF-67 with multi-sulfonated dyes for great enhanced methylene blue adsorption via 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 –SO3 groups was easily achieved by the efficient adsorption of multi-sulfonated dyes due to the coordinative interaction between the unoccupied Co(II) of ZIF-67 and –SO3 of the dyes.

Available online 5 May 2020

1387-1811/© 2020 Elsevier Inc All rights reserved

In situ modification of ZIF-67 with multi-sulfonated dyes for great

enhanced methylene blue adsorption via synergistic effect

Yanfeng Liua, Duoyu Lina, Weiting Yanga,*, Xueying Ana, Ahui Suna, Xiaolei Fanb,**,

Qinhe Pana,***

aKey Laboratory of Advanced Materials of Tropical Island Resources, Ministry of Education, School of Science, Hainan University, Haikou, 570228, PR China

bDepartment 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

Keywords:

Metal-organic frameworks (MOFs)

ZIF-67

Multi-sulfonated dyes

Methylene blue

Synergistic effect

A B S T R A C T

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 –SO3groups 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 –SO3of the dyes Interestingly, highly efficient synergistic absorption of multi-sulfonated dyes to-wards methylene blue (MBþ

) upon ZIF-67 was discovered for the first time The improved adsorption capacity of ZIF-67 for MBþin presence of cotton blue (CB ) 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 –SO3of the dyes and the unoccupied Co(II) of ZIF-67, the available –SO3groups of multi-sulfonated dyes can interact with –Nþ(CH3)2 in MBþand hence greatly improving the adsorption capacity of MBþ

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 interacinterac-tion, Lewis acid-base interacinterac-tion, 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 –SO3groups [26] Moreover, MOFs modified with –SO3could improve the cationic

* Corresponding author

** Corresponding author

*** Corresponding author

E-mail addresses: yangwt@hainanu.edu.cn (W Yang), xiaolei.fan@manchester.ac.uk (X Fan), panqinhe@163.com (Q Pan)

Contents lists available at ScienceDirect Microporous and Mesoporous Materials

journal homepage: http://www.elsevier.com/locate/micromeso

https://doi.org/10.1016/j.micromeso.2020.110304

Received 16 February 2020; Received in revised form 22 April 2020; Accepted 2 May 2020

with general functionalization

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 cm 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

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 –Nþ(CH3)2 or –Nþ(CH2CH3)2 groups, methylene blue

(MBþ) and rhodamine B (RhBþ); 2) anionic type with –SO3 groups,

methyl orange (MO ), acid chrome blue K (ACBK ), fuchsin acid (FA ),

cotton blue (CB ), congo red (CR ), coomassie brililiant blue R-250

(CBBR ), eriochrome blue black R (EBBR ) The details of the 9 organic

dyes were displayed in Table S1 Synergistic adsorption performance of

several sulfonated dyes towards MBþupon ZIF-67 was conducted as

follows Take CB for an example, 4.0 mg ZIF-67 was first added into the

solution of CB (125 mg/L) and stirred for 0.5 h to obtain CB @ZIF-67,

subsequently 5 mg MBþ(125 mg/L) was added to allow the synergetic

dye adsorption The trinary-component dye adsorptive experiments

were carried out similarly with MBþ/MO mixture being added instead

The concentration of dyes was monitored by UV–vis spectroscopy

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 ZIF-

67 in the literature (Fig S1) The average particle size of ZIF-67 was

in order of MB < MO < ACBK <FA < CB < CR < RhB <CBBR

<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 MO ) to 1,250.0 mg/g (for CR ), meanwhile the adsorption amount of the dyes possessing multi–SO3

groups is significantly greater than that of MO with single –SO3group Additionally, in situ modification of ZIF-67 with –SO3 groups using multi-sulfonated dyes was also simultaneously achieved by the adsorp-tion due to the coordinative interacadsorp-tion, 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

3.2 The synergistic adsorption behavior

Considering the electrostatic interaction between the –SO3 and –Nþ(CH3)2 groups, 7 dyes with –SO3groups were selected to study the synergistic adsorption performance towards MBþ upon ZIF-67 in aqueous media As shown in Fig 2, the capability of the dyes under study for the synergistic adsorption towards MBþcan be ranked as:

ACBK > CB > FA > EBBR > CBBR > CR All of the in situ

modification of the 6 dyes on ZIF-67 resulted in the greatly enhanced absorption capacity of MBþ, and the highest adsorbed quantity of MBþ

was measured at ~1,150.8 mg/g in the presence of ACBK In com-parison, the adsorption capacity of ZIF-67 for MBþis only ~103.4 mg/g

in the single-component dye adsorption experiment Therefore, it is plausible that the greater enhanced MBþadsorption with ACBK , CB , and FA dyes than that with CBBR , EBBR and CR dyes might be due

to presence of the additional –SO3group, leading to the synergistic dye- dye interaction Additionally, such synergetic effect was not measured for MBþwith MO which only possesses a single –SO3 group These results demonstrate that the adsorption performance towards MBþcan

be enhanced via the synergistic dye-dye adsorption effect by the intro-duction of multi–SO3groups-featured dyes, and the effect is a function

of the number of –SO3groups in dyes

3.3 Adsorption performance of CB upon ZIF-67

In all of the 6 above multi-sulfonated dyes, CB was selected as the model to perform the detailed study of the synergistic adsorption of MBþ

upon ZIF-67 due to the following considerations: 1) it features three –SO3groups, 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 CB and ZIF-67 as the rate-limiting step (Fig S6 & Table S2));

3) the high uptake on ZIF-67 The correlation coefficient R 2 of the Langmuir and Freundlich adsorption models are 0.997 and 0.994, respectively (Fig S7 & Table S3) This result illustrated the experimental

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 CB and MBþ

were conducted in the following work

3.4 Synergistic adsorption of MBþin presence of CB

In binary-component dyes adsorption experiments, ZIF-67 was

added in CB aqueous solution, after being stirred for 10 min, MBþwas

added into each aqueous solution with the same concentration of CB 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 MBþ(Fig 3 & Table 1) The result indicates that the pre-adsorbed CB has highly effective synergism for binding MBþ The adsorption capacity of MBþincreased significantly

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

Fig 1 Adsorption performance of ZIF-67 towards 9 dyes

Fig 2 Synergistic adsorption performance of several sulfonated dyes towards MBþupon ZIF-67 in aqueous solution (125 mg/L of single-component for MBþ 125 mg/L of binary-component for MO , CR , CBBR , EBBR , FA , ACBK and CB respectively mixed with MBþafter being stirred with ZIF-67 for 0.5 h)

amounts of both dyes, the molar ratio of adsorbed CB to MBþ is

calculated to be about 1:2 The results demonstrate that after the

coor-dinative interaction between the Co(II) in ZIF-67 and –SO3 in CB , the

electrostatic interaction occurs between the remaining two –SO3groups

in CB and –Nþ(CH3)2 in MBþwith the molar ratio of both functions

being of 1:1

3.5 Recycling and reusability

Recycling and reusability of absorbents are the important factors in

the dye adsorption properties Therefore, the release experiments were

conducted by eluting the MBþ/CB @ZIF-67 sample using CH3OH As

shown in Fig 4a, MBþcan be released into CH3OH solution from the

saturated samples quickly in 60 min In order to confirm the durability

and reusability of CB @ZIF-67 in the adsorption process, the

adsorp-tion desorpadsorp-tion experiments were performed alternatively for 5 runs

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

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

To confirm the generic feature of such synergistic adsorption be-tween –Nþ(CH3)2 and –SO3, the sequential binary-component adsorp-tion of CB with the dyes possessing the funcadsorp-tional groups of –N(CH3)2, –Nþ(CH2CH3)2 and –Nþ(CH3)2 on ZIF-67 was investigated, and MO and RhBþwere selected, respectively The synergistic effect of CB /

MO and CB /RhBþsystems was barely measured (Fig 5a and b) While the CB /MBþsystem showed a significant improvement in adsorption performance Based on the results above, in situ modification of ZIF-67 with CB can promote the adsorption capacity of ZIF-67 for MBþwith –Nþ(CH3)2 groups Conversely, such synergistic effect for dyes with –Nþ(CH2CH3)2 and –N(CH3)2 was notobserved Thus, the electrostatic interaction between the –SO3in dyes and –Nþ(CH3)2 in MBþmight be responsible for the synergistic phenomenon While the steric hindrance

of –Nþ(CH2CH3)2 and the lack of charge of –N(CH3)2 result in no such effect

The selectivity of MBþover MO /RhBþassociated with CB @ZIF-67 was further determined by the trinary-component adsorption experi-ments The selective absorption performance of MBþby CB @ZIF-67 exhibited a similar trend The removal efficiency for MBþis 90.7% and 92.9%, respectively, under the conditions used, while only 6.3% for

MO and 2.9% for RhBþwere measured (Fig 5d) Therefore, the find-ings suggest that CB @ZIF-67 has comparatively good selectivity to

MBþdue to the synergistic adsorption

3.7 Mechanistic study

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, MBþ@ZIF-67, and MBþ/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 MBþ

To explore the interaction mechanism between ZIF-67 and the dyes under study, FTIR spectra of ZIF-67, CB , MBþand dyes@ZIF-67 were compared together (Fig 6) In the spectrum of CB (Fig 6a), the peak at

Fig 3 Single-component adsorption experiments of MBþor CB , and the synergistic adsorption performance of CB towards MBþupon ZIF-67

Table 1

Comparison of the MBþ adsorption capacity of CB @ZIF-67 with other

adsorbents

work

Calcium alginate membrane 3,506.4 [ 30 ]

Carboxy methyl cellulose/poly(methyl acrylate) hydrogels 2,370 [ 31 ]

SA nanofiber membranes 2,357.9 [ 32 ]

Aminocarboxylate/maleic acid resin 2,101 [ 33 ]

Lignocellulose-g-poly(acrylic acid)/montmorillonite

NaAlg-g-p(AA-co-St)/organo-I/S 1,843.5 [ 35 ]

Poly(N-vinyl caprolactam-co-maleic acid) 1,441 [ 36 ]

GO/lignosulfonate aerogel 1,023.9 [ 39 ]

Calcium alginate–bentonite–activated carbon composite

Core@double-shell structured HNTs/Fe3O4/poly(DA þ

1,337.2 cm 1 is ascribed to asymmetric S–O stretching vibrations, and

the peak at 1,169.1 cm 1 is associated with aromatic C–N stretching

vibrations [42] In the spectrum of MBþ(Fig 6b), a band appears at 3,

425.6 cm 1, attributable to the O–H stretching vibration The

charac-teristic bands of MBþat 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 –Nþ(CH3)2, respectively According to Fig 6c, a characteristic band of ZIF-67 at 425.4 cm 1 is attributed to the Co–N stretching vibration [43,

44] The stretching vibration peaks of C–N at 1171.9 cm 1 and C––N at 1579.1 cm 1 are also observed [45,46]

Fig 4 a) Release experiments of MBþfrom the corresponding CB @ZIF-67 adsorbed sample in the solution of CH3OH; b) the reusability of CB @ZIF-67 for MBþ

adsorption for 5 times

Fig 5 UV–Vis spectra of several binary-component dyes during the adsorption process a) MO /CB , b) RhBþ/CB , c) MBþ/CB (The initial concentrations of CB ,

MO , RhBþand MBþwere 120, 50, 60, and 25 mg/L, respectively.) d) Selectivity of MBþover MO /RhBþin trinary-component dyes adsorption (CB @ZIF-67 aqueous solution obtained after 120 mg/L CB mixed with ZIF-67 stirred for 10 min, and then MBþ/MO or MBþ/RhBþwere added with each single component concentration being 50 mg/L)

As observed in Fig 6d, differences are observed obviously in the

spectra of ZIF-67 and CB @ZIF-67 The sharp peak of Co–N stretching

vibrations at 425.4 cm 1 in ZIF-67 is shifted to 424.7 cm 1 in CB @ZIF-

67, meanwhile, the vibration frequencies of the bands at 1,579.1 cm 1

of C––N and 1,171.8 cm 1 of C–N are shifted to 1,593.3 and 1,172.8

cm 1, respectively, which might be due to the interaction of –S(O2)–O 1

from CB 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

bonding electron and the stretching vibration frequencies of C––N and

C–N increases In the similar way, the interaction of –S(O2)–O 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 cm 1 for asymmetric S–O stretching vibrations in CB is shifted to

1,335.5 cm 1 in CB @ZIF-67 IR analysis reveals the binding of CB to

the framework of ZIF-67 via chemisorption of –SO3group 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 –SO3group in CB could occur by the replacement of –OH by

–SO3[47,48] This illustrates that the chemisorption is dominant in the

adsorption of CB on ZIF-67

In spectra of MBþand MBþ@ZIF-67 (Fig 6b and e), O–H and Co–N

bands from 3,425.6 cm 1 and 425.4 cm 1 in MBþshifted to 3,447.5

cm 1 and 424.5 cm 1 in MBþ@ZIF-67, respectively The relevant

wavelength shift might be caused by the formation of hydrogen-bonding

and π-π stacking interactions between MBþand ZIF-67 [49,50] The

Co–N peak at 424.7 cm 1 was unchanged after the adsorption of MBþon

CB @ZIF-67 The bands at 1,335.5 cm 1 (S–O group), 1,593.3 cm 1

(C––N group) and 1,172.8 cm 1 (C–N group) in CB @ZIF-67 are shifted

to 1,334.0, 1,598.2 and 1,173.5 cm 1 in MBþ/CB @ZIF-67, respectively

(Fig 6e and f) The results suggest the interaction exists between

–Nþ(CH3)2 in MBþand –S(O2)–O 1 in CB , which is responsible for

promoting the adsorption capacity of MBþupon ZIF-67 in the aqueous

solution of MBþand 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 CB and MBþ 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

MBþ@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

CB @ZIF-67, respectively, which resulted from the interaction between unoccupied Co(II) of ZIF-67 and the –SO3 group in CB [52,53] The binding energies S 2p at 167.9 and 169.0 eV in CB @ZIF-67 originate from central sulphur atoms in –SO3-Co and –SO3-Na, respectively (Fig 7c) [54] The S 2p peaks in MBþ/CB @ZIF-67 showed a slight downshift in comparison with that in CB @ZIF-67, illustrating that the S chemical state had been changed by introducing MBþ In the case of ZIF-67 and MBþ@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 MBþcould be affirmed by O 1s peak shifting from 531.5 eV in pristine ZIF-67 to 531.3

eV in MBþ@ZIF-67 [26,57] Coordination interactions also were formed between Co(II) and –SO3 groups, which could be further confirmed by the presence of 531.7 eV (–SO3-Co) and 532.9 eV (–SO3-Na) in

CB @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 CB @ZIF-67 is shifted to lower binding energy at 532.1 eV in MBþ/CB @ZIF-67, which should be caused by the interaction between –SO3in CB and –Nþ(CH3)2 in MBþ Considering the analysis findings above, possible mechanism in the synergistic adsorption process for CB and MBþupon ZIF-67 is proposed

as shown in Scheme 1 In single-component adsorption of MBþupon ZIF-

67, hydrogen-bonding interaction is present between –Nþ(CH3)2 in MBþ

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

Fig 6 FTIR spectra of a) CB , b) MBþ, c) ZIF-67, as well as that of d) CB @ZIF-67, e) MBþ@ZIF-67, and f) MBþ/CB @ZIF-67 after adsorption of CB , MBþ, or the mixture of CB and MBþ, respectively

Fig 7 XPS spectra of ZIF-67, MBþ@ZIF-67, CB @ZIF-67, and MBþ/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 MBþupon ZIF-67

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 –SO3 groups due to the coordinative interaction The

adsorption capacity of ZIF-67 towards CB was very high at 5,860.1 mg/

g Importantly; the synergistic absorption of multi-sulfonated dyes and

MBþ upon ZIF-67 was discovered for the first time Specifically, a

considerable increase in the adsorption capacity of ZIF-67 for MBþ

(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 –SO3groups which interact strongly with the –Nþ(CH3)2 group in

MBþ 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

Declaration of competing interest

There are no conflicts to declare

CRediT authorship contribution statement

Yanfeng Liu: Conceptualization, Methodology, Investigation,

Re-sources, Writing - original draft Duoyu Lin: ReRe-sources, 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

Acknowledgements

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 start-

up fund (KYQD(ZR) 1806)

Appendix A Supplementary data

Supplementary data related to this article can be found at https://doi

.org/10.1016/j.micromeso.2020.110304

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