Fast adsorption of methyl blue on zeolitic imidazolate framework-8 and its adsorption mechanism

Yi Fenga, Yu Lia, Minying Xua, Shichang Liua and Jianfeng Yao*ab
aCollege of Chemical Engineering, Nanjing Forestry University, Nanjing, Jiangsu 210037, China. E-mail: jfyao@njfu.edu.cn
bJiangsu Key Lab of Biomass-based Green Fuels and Chemicals, Nanjing, 210037, China

Received 26th September 2016 , Accepted 7th November 2016

First published on 7th November 2016


Abstract

Zeolitic imidazolate framework-8 (ZIF-8) is synthesized and the adsorption behavior and mechanism of methyl blue (MB) are studied in detail. Results show that ZIF-8 has a higher MB adsorption capacity than most of adsorbents reported so far (the maximum adsorption capacity of ZIF-8 is 2500 mg g−1) and a fast adsorption rate (sorption saturation can be achieved within 30 min). The adsorption kinetics and isotherms were studied in detail. The mechanism study shows that the existence of Zn2+ exposed on the surface of ZIF-8 nanoparticles plays an important role in the high adsorption of MB and the ionic bonding between Zn2+ in ZIF and –SO3 in MB is formed. ZIF-8 nanoparticles also show high adsorption selectivity with negligible adsorption for other dyes such as rhodamine B, methylene blue and methyl orange.


1. Introduction

Industrial dye effluents, such as paper, plastics, leather, pharmaceutical, food, textiles and cosmetics, have always been a serious environmental problem.1–4 Most of the dyes are toxic and may cause direct destruction to aquatic communities. Therefore, before discharging wastewaters to aquatic ecosystems, it is necessary to reduce dye concentration and remove them from water. Compared to other methods, an adsorption process is known to be a promising technique due to the ease of operation, high adsorption capacity and comparable low cost of application and operation.5–7 Highly functional porous materials with high surface area are generally used as suitable adsorbents to enhance adsorption efficiency for removing of dyes.

Metal–organic frameworks (MOFs) are crystalline porous materials that are well known for their various application.8–10 The particular interest in MOF materials is due to the easy tunability of their pore size and their high surface area. Therefore, MOFs are widely used in the field of adsorption, separation and storage of gases and vapors. Recently, several MOF materials such as UiO-66, MIL-101 and MIL-53 have been tried as adsorbents to remove dyes from water and moderate adsorbing capacity was achieved.11,12

Zeolitic imidazolate frameworks (ZIFs) is a subclass of MOFs.13 In particular, ZIF-8 is one of the most studied prototypical ZIF compounds, which are porous crystals with extended three-dimensional structures constructed from tetrahedral zinc ions bridged by imidazolate. Based on previous studies, zinc oxide (ZnO) show high and fast adsorption for several kinds of dyes such as methyl blue and red 23, because of the bonding formed between ZnO and dyes.14 Therefore, ZIF-8 is very promising to be used as high-efficient adsorbents to remove certain kind of dyes from water considering the well-dispersed zinc ions inside the pores and the high surface area (>1000 m3 g−1). In this study, the adsorption behavior of MB adsorbing on ZIF-8 was studied via adsorption kinetics and isotherms and the mechanism of why ZIF-8 show high adsorption selectivity for MB over other dyes is discussed in details. Through this study, we expanded the application of ZIF-8 as promising adsorbents for MB and offered some valuable design knowledge to the synthesis of high-efficient adsorbents for dye removal.

2. Materials and methods

2.1 Chemicals

Chemicals were used as received without a further purification process. They include zinc nitrate hexahydrate (≥99%, Tianjin Kemiou Chemical Reagent, China), 2-methylimidazole (Sinopharm chemical reagent Co. Ltd., China), ammonium hydroxide solution (NH3, 25–28%, Nanjing Chemical Reagent Co. Ltd., China), anhydrous ethanol (≥99.7%, Sinopharm chemical reagent Co. Ltd., China), 4A zeolites (Tianjin Nanhua Catalysis Co. Ltd., China) and ZSM-5 (Tianjin Nanhua Catalysis Co. Ltd., China).

2.2 Sample preparation

ZIF-8 was synthesized according to previously reported studies.15–18 In a typical synthesis, 2.97 g of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) was dissolved in 3 g of deionized water; 1.64 g of Hmim was added in 20.75 mL ammonium hydroxide solution; after that zinc nitrate and Hmim solutions were mixed together. The solution immediately turned into milk-like suspension, and stirred for 10 min at room temperature to complete the crystallization. The sample was collected by centrifugation and washed with deionized water three times until the final product had pH value of ∼7, then dried at 60 °C overnight.

In order to study the effect of surface area on adsorption capacity, ZIF-8 derived ZnO and ZnO@C were synthesized. ZIF-derived ZnO and ZnO@C were obtained via calcination or carbonization of as-synthesized ZIF-8 at 550 °C for 2 h with a heating rate of 5 °C min−1.

2.3 Characterization

X-ray diffraction (XRD) patterns of the samples were obtained on an Ultima IV diffractometer with Cu Kα radiation at a scan rate of 2° min−1 with a step size of 0.02°. Nitrogen adsorption–desorption isotherms were measured at liquid-nitrogen temperature (77 K) using a volumetric adsorption analyzer (Micromeritics ASAP 2020). Surface areas were determined by the Brunauer–Emmett–Teller (BET) method. The surface chemistry of ZIF-8 after MB adsorption was characterized using an X-ray photoelectron spectroscopy (XPS, AXIS UltraDLD, Japan).

2.4 Adsorption kinetics and isotherms of methyl blue (MB)

The adsorption capacity of MB on the adsorbent is calculated as:
Qt = V(C0Ct)/m
where V is the solution volume, C0 is the initial MB concentration, Ct is the MB concentration in the solution at a given time (t), and m is the adsorbent mass. In a typical adsorption test, 20 mg adsorbent was added in 40 mL MB solution with the initial concentration of 100, 300, 400, 500, 800, 1000 ppm at room temperature (20 °C). The concentration of MB was determined with an UV-Vis spectrometer (TU-1900, Persee Co. Ltd., China) at λ = 600 nm and then calculated using the analytical calibration curve. For MB solution with high concentration, the solution was diluted for several times before test. The adsorption kinetics was investigated using pseudo-second-order models, as described below:19,20
image file: c6ra23870j-t1.tif

h = k2Qe2
where k2 is the pseudo-second order rate constant (mg g−1 min−1), Qe is the sorption capacity at equilibrium, Qt is the sorption at the time of t and h is the initial adsorption rate (mg g−1 min−1). The h and k2 can be obtained by linear plot of t/Qt versus t.

For adsorption isotherms, Langmuir isotherm and Freundlich isotherm equation are used. A plot of Qe vs. Ce, the residual concentration in the solution, was then performed to validate the applicability of Langmuir isotherm and Freundlich isotherm. The Langmuir isotherm equation is shown below:21,22

image file: c6ra23870j-t2.tif
where Qm is the maximum amount of adsorption (mg g−1) and KL is the Langmuir constant related to the energy of adsorption. If the plot of 1/Qe vs. 1/Ce is linear, the adsorption is Langmuir isotherm.

For Freundlich isotherm, the equation is calculated below:

image file: c6ra23870j-t3.tif
where KF and n are Freundlich constants. If the plot of log[thin space (1/6-em)]Qe vs. log[thin space (1/6-em)]Ce is linear, the adsorption is Freundlich isotherm.

2.5 Desorption of MB and recyclability of adsorbents

After adsorption, the adsorbents were collected by centrifugation. The desorption was performed using ethanol to desorb the MB from adsorbent for several times and dried at 60 °C before next cycle of adsorption.

3. Results and discussion

3.1 Characterization of ZIF-8

XRD pattern of the as-synthesized sample is shown in Fig. 1a. According to the XRD pattern, the samples can be assigned to ZIF-8 and no other phase was observed in such samples.18 Fig. 1b shows the morphology of as-synthesized ZIF-8. From Fig. 1b, it can be seen that ZIF-8 is a cubic-like structure with average particle size of approximately 0.5 μm. Fig. 1c shows FTIR spectra of ZIF-8. The intense and convoluted bands at 1350–1500 cm−1 are associated with the entire ring stretching, the bands in the spectral region of 900–1350 cm−1 are for the in-plane bending of the ring, and the band at 421 cm−1 is ascribed to Zn–N stretch.23 Fig. 1d shows the nitrogen adsorption–desorption isotherm of ZIF-8. It can be seen that there is no mesoporous structure and only microporous in ZIF-8. The BET surface area and Langmuir surface area of ZIF-8 are 1007.4 and 1322.9 m3 g−1, respectively.
image file: c6ra23870j-f1.tif
Fig. 1 (a) XRD patterns of the as-synthesized ZIF-8; (b) SEM image of as-synthesized ZIF-8; (c) FTIR spectra of as-synthesized ZIF-8 and (d) N2 sorption isotherm of ZIF-8.

3.2 Adsorption kinetics

A series of experiments was conducted to study the adsorption kinetics of MB on ZIF-8. The plots of Qt versus time (t) for various initial concentrations of MB on such adsorbents are shown in Fig. 2. The adsorption capacity of such adsorbents increases dramatically within the first 5 min, and then increases slowly until equilibrium within 30 min. The adsorption capacities at equilibrium for ZIF-8 are 199.0, 597.4, 794.6, 990.9, 1544.8, 1917.5 mg gadsorbent−1 with the initial MB content of 200, 600, 800, 1000, 1600 and 2000 mg gadsorbent−1 (Table 1).
image file: c6ra23870j-f2.tif
Fig. 2 Adsorption kinetics at different concentrations (from bottom to up: 100, 300, 400, 500, 800, 1000 ppm) of methyl blue on ZIF-8.
Table 1 The equilibrium capacities, pseudo-second order rate constant and initial adsorption rate of ZIF-8 adsorbing MBa
Sample C0 (ppm) Qe (mg g−1) K2 mg (g min)−1
a Note: 20 mg adsorbent was added into 40 mL MB solution.
ZIF-8 100 199.0 ± 1.33 0.00676
300 597.4 ± 2.13 0.00251
400 794.6 ± 4.03 0.00282
500 990.9 ± 5.17 0.00167
800 1544.8 ± 13.64 0.00038
1000 1917.5 ± 5.86 0.00042


3.3 Adsorption isotherms

The Langmuir model and Freundlich model are used to describe the adsorption process. From Fig. 3, it can be seen that Langmuir model is more applicable compared to Freundlich model because of the higher correlation of coefficient obtained in Langmuir model (the correlation of coefficient of Langmuir and Freundlich model is 0.982 and 0.888 respectively). The Langmuir model assumes that the adsorption is monolayer and is dependent on the assumption that the adsorbent surfaces consist of active sites having a uniform energy and the adsorption energy is constant.24,25 Therefore, it can be indicated that the surface of ZIF-8 has a uniform adsorption energy and the adsorption is mainly monolayer instead of multilayer.
image file: c6ra23870j-f3.tif
Fig. 3 (a) Langmuir adsorption and (b) Freundlich adsorption isotherm of MB adsorbing on ZIF-8.

Based on the Langmuir isotherm equations, the maximum adsorption capacity of ZIF-8 is 2500 mg g−1 with the Langmuir constant of 0.0889 L mg−1 (KL).

3.4 Adsorption mechanism

It has been reported that ionic bonding between the negatively charged functional groups of MB (–SO3) and positively charged center of ZnO (Zn(OH)+) can be formed. Similarly, in this study, the formation of ionic bonding between Zn2+ in ZIF-8 and –SO3 might be the main contribution to the fast and high MB adsorption on ZIF-8.

In order to verify the occurrence of such interaction between MB and ZIF-8, the surface chemical information at an atomic scale was investigated by detailed XPS analysis. The XPS of ZIF-8 before and after adsorption of MB is shown in Fig. 4. The binding energy of Zn 2p at 1023 and 1046 eV shifts to higher energy and new peaks show after adsorption of MB. The new peaks are mainly attributed to the ionic bonding between Zn2+ and O2− ions in the sulfonic groups of MB. Similarly, the binding energy of O 1s in MB also shifts to higher energy, which further proves the above-mentioned reaction.


image file: c6ra23870j-f4.tif
Fig. 4 XPS spectra of (a) Zn 2p and (b) O 1s for ZIF-8, MB and ZIF-8 after MB adsorption.

It should be noted that the pore diameter of ZIF-8 is about 3.4 Å whereas the theoretical size of methyl blue is too big to enter the pores. The dye molecules might be encapsulated in interparticular pores and/or pores of the outer surface of the particles. The intrinsic micropores might have a function for solvent transport away from the adsorbed dye molecules.

In order to determine the role of zinc ions and the pores on the adsorption capacities, ZIF-8 derived ZnO and ZnO@C, ZSM-5 and 4A zeolites were used to adsorb MB (20 mg adsorbent added into 500 ppm 40 mL MB solution at room temperature). After calcination in air, ZIF-8 was converted to ZnO whereas porous carbon/ZnO was obtained when ZIF-8 was carbonized in inert gas (e.g. N2).26,27 From Fig. 5a, it can be seen that the adsorption capacity of ZnO@C derived from ZIF-8 is similar to that of ZIF-8 and the adsorption capacity of ZnO is slightly lower compared to ZIF-8. Moreover, if other porous materials, such as ZSM-5 and 4A zeolites, were used, only less than 200 mg g−1 MB was adsorbed on such adsorbents. Therefore, from the above-mentioned discussion, high surface area itself cannot guarantee high MB adsorption whereas the high exposure of Zn2+ onto the surface is the key factor for the high adsorption of MB onto ZIF-8.


image file: c6ra23870j-f5.tif
Fig. 5 (a) MB adsorption capacity of different adsorbents and (b) adsorption capacity for different dyes on ZIF-8.

Besides methyl blue (MB), we compared the adsorption capacity of rhodamine B (RhB), methylene blue and methyl orange using ZIF-8. The adsorption effect for RhB, methylene blue and methyl orange solution are all negligible with the adsorption capacity of 40, 20, 18 mg g−1 respectively as shown in Fig. 5b (20 mg ZIF-8 added into 500 ppm, 40 mL dye solution), which indicates that the adsorption of MB by ZIF-8 is highly selective.28

From the XPS results, it can be seen that the interaction between –SO3 and Zn2+ is the major contribution to the high adsorption of ZIF-8 for methyl blue but negligible adsorption for other dyes, which has no –SO3 group. Though there is –SO3 group in MO, it should be noted that the –SO3 groups in MO usually exist as –SO3Na, and the –SO3 group in methyl blue exist as –SO3Na and –SO3H(–SO3) as shown in Fig. S1. As a result, ZIF-8 shows high adsorption for MB but negligible adsorption for MO. Schematic illustration of mechanism of MB adsorbed onto ZIF-8 particles is shown in Fig. S2.

To further demonstrate the selective adsorption capacity of ZIF-8, mixtures of MB–MO and MB–RhB were used and the selective adsorption results were shown in Fig. S4. For different dye mixture, ZIF-8 still shows high adsorption of MB, and much less adsorption of other dyes. It should be noted that compared to the adsorption when only MO was used, ZIF-8 adsorbed more MO when MB–MO mixture was used probably due to the fact that MB is cationic whereas MO is anionic; therefore, when MB was adsorbed onto ZIF-8, more MO was further adsorbed due to the charge effect.

Fig. 6 shows the recycling ability of ZIF-8 (the adsorbent loading is 0.5 mg mL−1 and the initial MB concentration is 500 and 1000 ppm respectively). The adsorption ability does not lose much after three cycles of adsorption (the adsorption capacity decrease after three cycles of adsorption is only 30 mg g−1) and most of MB can be desorbed from ZIF-8, as shown in the inserted photos in Fig. 6b. XRD and SEM were performed to test the crystal structure after ZIF-8 adsorbing MB, as shown in Fig. S4. It can be seen that ZIF-8 still maintains their crystal structure, indicating the good stability of ZIF-8 as dye adsorbents.


image file: c6ra23870j-f6.tif
Fig. 6 Cycling ability of ZIF-8 for MB adsorption: (a) 500 ppm MB and (b) 1000 ppm MB. Inserted photos: ZIF-8 after MB adsorption (left) and desorbed ZIF-8 after three cycles of adsorption (right).

Table 2 shows the MB adsorption capacity of several typical adsorbents reported in other studies. From Table 2, ZIF-8 shows a higher adsorption capacity and faster absorption rate for MB than most of adsorbents reported so far. The high adsorption capacity and fast absorption rate for MB are mainly due to the ionic interaction between Zn2+ and sulfonic groups of MB.

Table 2 Adsorption capacity of different adsorbents
Materials Adsorption capacity Contact time Reference
ZIF-8 3.126 mmol g−1 (2500 mg g−1) 30 min This work
Barium phosphate nano-flake 1.88 mmol g−1 60 min 29
Ni-MCM-41 100 mg g−1 60 min 30
Graphene oxide (GO) 4.0–4.5 mmol g−1 120 min 31
rGO 2.6–2.9 mmol g−1 120 min 31
Chitosan/GO 95.16 mg g−1 90 min 32
Ni0.5Zn0.5Fe2O4 60 mg g−1 100 min 21
Co-Based ternary nanocomposites 1200 mg g−1 10 min 33
Activated charcoal 25.25 mg g−1 30 min 34


4. Conclusions

In this study, ZIF-8 was synthesized and used as high-efficient adsorbents for MB. ZIF-8 show very high adsorption capacity (maximum adsorption capacity is 2500 mg g−1) and fast adsorption rate (adsorption equilibrium can be achieved within 1 h). The adsorption rate is pseudo-second order and the adsorption type is Langmuir adsorption. The exposure of Zn2+ onto the porous surface is the key factor for the high adsorption of MB onto ZIF-8; yet only high BET surface area itself cannot guarantee high adsorption capacity. Due to the ionic bonding between Zn2+ in ZIF-8 and –SO3 in MB, the adsorption is highly selective with high adsorption for MB, which has many –SO3 groups but negligible adsorption for other dyes.

Acknowledgements

The authors are grateful for the financial support of Jiangsu Specially-Appointed Professor Program, Natural Science Key Project of the Jiangsu Higher Education Institutions (15KJA220001) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23870j

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