Preparation of polydopamine-modified zeolitic imidazolate framework-8 functionalized electrospun fibers for efficient removal of tetracycline

Journal of Colloid and Interface Science 552 (2019) 506–516

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Regular Article

Preparation of polydopamine-modified zeolitic imidazolate framework8 functionalized electrospun fibers for efficient removal of tetracycline Shen Chao, Xiang Li ⇑, Yanzi Li, Yuannan Wang, Ce Wang Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun 130012, PR China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 18 February 2019 Revised 21 May 2019 Accepted 24 May 2019 Available online 25 May 2019 Keywords: Metal
organic frameworks Electrospinning Adsorption Tetracycline

⇑ Corresponding author. E-mail address: [email protected] (X. Li). https://doi.org/10.1016/j.jcis.2019.05.078 0021-9797/Ó 2019 Published by Elsevier Inc.

a b s t r a c t This work shows a simple and environmental friendly methodology to obtain a kind of polydopamine coating assisted preparation of zeolitic imidazolate framework-8 (ZIF-8) functionalized composite electrospun fiber (ZIF-8/PDA/PAN fibers) adsorbent. Characterization of the composite electrospun fiber was carried out and the tetracycline (TC) adsorption properties from water were also studied in detail. At the same time, principle adsorption mechanisms were thoroughly studied. The results show that the pseudo-second-order model can simulate sorption kinetics well, while sorption isotherms are able to significantly conform to the Freundlich model, and the adsorption capacity of the fibers can reach 478.18 mg/g at 298 K. In addition, the Weber–Morris model indicates that the processes of adsorption of ZIF-8/PDA/PAN fibers for TC involve surface adsorption as well as intraparticle diffusion, and the limit rate step is not only the intraparticle diffusion but also the binding of the sorbate to the sorbent. Moreover, the adsorption efficiency toward TC by ZIF-8/PDA/PAN fibers still reached over 85% of its initial adsorption capacity after five adsorption/desorption cycles, which signified that the adsorbents is stable and recyclable. This work indicates that the obtained ZIF-8/PDA/PAN fibers have practical application prospects in the field of antibiotic adsorption. Ó 2019 Published by Elsevier Inc.

S. Chao et al. / Journal of Colloid and Interface Science 552 (2019) 506–516

1. Introduction Antibiotics play an essential role in livestock, agricultural, and pharmaceutical industries. However, a lot of antibiotics are released into the environment, resulting in their accumulation inside the surface water, ground water and even sediment. Thus, as a result of their potential long-term adverse, antibiotic pollution has turned into a vital factor destroying the ecological balance and threatening human health [1]. Because of its broad-spectrum antibacterial activity, tetracycline (TC) makes up approximately one third of the production and consumption of all the antibiotics, ranking second in usage amount throughout the world among all of the antibiotics [2]. According to reports that 21 antibiotics with concentrations of up to 5800 ng/g were found from two wastewater treatment plants in the sewage sludge of Guangdong Province in China, and among them, ofloxacin, norfloxacin, oxytetracycline and tetracycline are the primary ones [3]. Therefore, there is an urgent need to reduce the total amount of antibiotics, especially TC, from the natural environment, which is an imperative public health affair faced by researchers. Until now, the reported techniques of antibiotic contaminated water treatment mainly include adsorption [4], photocatalytic [5], chemical oxidation [6] and bioremediation [7]. Metal-organic frameworks (MOFs) material is a new kind of nano-porous crystalline material, which is self-assembled from organic ligands and metal clusters, and possesses periodic network structure. As a new class of nanoporous organic/inorganic hybrid materials, MOFs has attracted increasing attention owing to their advantages over other porous materials (such as zeolites and active carbons), containing fine-tuned and designed networks, higher surface areas and pore volumes, uniform pore sizes and good compatibility with polymers [8]. MOFs are expected to be used in a variety of applications, including CO2 capture [9], gas separation and purification [10], hydrogen storage [11], catalysis [12], water purification [13], and other applications, according to the reports. As a type of novel adsorbents, MOFs materials are extremely attractive for water purification applications, such as heavy metal ions [14], dyes [15], sulfur-containing compounds [16], and other organic pollutants [17]. Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs, having outstanding chemical and thermal stability and hopeful adsorption ability for the removal of organic pollutants from aqueous solutions, which is mainly owing to the strong interactions between the molecules of pollutants adsorbed onto ZIFs surfaces [18]. Although MOFs materials have good adsorption capacity, they are usually in the form of powder, making it difficult to be recycled and reused, and subsequently can cause secondary pollution to the water bodies and reduce their practical application capacity. In the past several years, electrospinning has turned into an effective, facile and low-cost method for manufacturing onedimensional (1D) polymer fiber membranes [19,20]. Because of the miniaturization of the size of the electrospun nanofibers, the obtained fibers possess many remarkable properties, including high porosity, high pore interconnectivity, high specific surface area, flexibility, easy of preparation and recyclability. In addition, electrospun nanofibers are prone to functionalized or acquire specific favorable structures to further improve their adsorption capacity, making them ideal for candidate of adsorbents to remove diverse pollutants, including heavy metals [21], organic compounds [22], bacteria [23] and fine particles [24]. In the past few days, the researchers used electrospun fibers as tetracycline adsorbents in aqueous solution [25,26], but their adsorption capacities are weak. Therefore, if the MOFs materials are combined with the electrospinning technology, the adsorption capacity of the adsorbent will be greatly improved.

507

Based on the aforementioned ideas, we prepared a PDAmodified ZIF-8 functionalized nanofiber membrane and applied it to TC adsorption in this study. According to our previous studies [21,27], polyacrylonitrile (PAN), a commonly used polymer, acted as a template for electrospun fiber because of its extraordinary physical properties, such as good flexibility, excellent durability, as well as outstanding chemical properties. In order to overcome the puzzle of poor hydrophilicity of polyacrylonitrile, we made use of the self-polymerization behavior of dopamine to coat the polydopamine (PDA) onto the surface of PAN electrospun nanofibers, which increased the hydrophilicity of electrospun fibers and enhanced the synthesis of ZIF-8 nanocrystals onto the fibers surface. We characterized the properties of the obtained ZIF-8/PDA/ PAN composite fibers in detail. At the same time, the TC adsorption properties from water, containing pH effect, adsorption kinetics, adsorption isotherm and reusability, were also investigated. Moreover, the preparation technology is simple and the cost is low, and the raw materials are easy to acquire; the electrospun nanofibers obtained have good film-forming property and flexibility, and they can be easily got out from the adsorbed liquid and taken off after the adsorption is accomplished. Therefore, it is an environmentally friendly adsorbent material. Although some results about fiberbased adsorbents for TC adsorption by electrospinning have been reported, as far as we know, this is the first time that MOFs have been combined with electrospinning to prepare adsorbents to remove antibiotics from water. 2. Materials and methods 2.1. Materials Polyacrylonitrile (PAN, Mw = 80,000) was obtained from Jilin Carbon Group. N,N-dimethylformamide (DMF) was bought from Tiantai Chemical Corporation. Tris (hydroxymethyl) aminomethane hydrochloride (Tris) was obtained from Energy Chemical. Dopamine hydrochloride (DA), 2-methylimidazole and tetracycline hydrochloride (TC) were purchased from Aladdin. Zn(NO3)26H2O and absolute ethanol were purchased from Beijing Chemical Works. DI water was used in all of the experiments; and all of the reagents were used directly without further purification. 2.2. Preparation of polydopamine coated PAN fibers (PDA/PAN fibers) The pure electrospun polyacrylonitrile (PAN) fiber membrane was manufactured on the basis of our previous report [21]. In order to achieve the dopamine self polymerization, 120 mg of PAN was wetted in the ethanol solution because of its poor hydrophobicity. After that, the wetted membrane was immersed within 80 mL of 7.88 mg/mL dopamine Tris solution (pH 8.5, 50 mM Tris-HCl buffer solution). Under 318 K, the solution was gently stirred for 24 h, and then the PDA-coated PAN fibers (PDA/PAN fibers) were fetched from solution carefully and washed by DI water to clear away the excess Tris-HCl solution, and the fibers were dried at 333 K in a vacuum oven overnight. 2.3. Surface modification of PDA/PAN fibers by ZIF-8 functionalization (ZIF-8/PDA/PAN fibers) The acquired PDA/PAN fibers of 120 mg were immersed in 20 mL Zn(NO3)2 solution (18.4 mM) and stirred for 1 h at room temperature (298 K) after ultrasonication for 10 min, for the purpose of achieving sufficient mixing between the PDA/PAN fibers and Zn(NO3)2 solution. Then, 20 mL 2-methylimidazole solution (1.38 M) was added into the above solution, and the mixture was

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heated at 318 K for 40 min to enable the synthesis of ZIF-8 crystals onto the surface of fibers. Finally, the membrane, named as ZIF-8/ PDA/PAN fibers, was taken from solution, washed for five times with DI water, and dried overnight in a vacuum oven at 333 K. 2.4. Characterization experiments The morphology and microstructure of the fibers were inspected using a field emission scanning electron microscope (SEM Shimadzu SSX-550) and a transmission electron microscope (TEM; JEOL JEM-3010). X-ray diffraction (XRD) measurement was implemented out using a Rigaku D/Max 2500 diffractometer with Cu Ka radiation (k = 1.54 Å) at a generator voltage of 40 kV and a generator current of 40 mA. Thermogravimetric analysis (TGA) was conducted on a TGA Instruments Q50 thermal analyzer at a heating rate of 5 °C/min. The specific surface area of the fiber sample was derived from N2 adsorption-desorption isotherms, which were obtained at
196 °C on a Micromeritics 2420 sorptometer. The TC concentrations were measured using a SHIMADZU UV2501 UV–vis spectrophotometer at 275 nm. 2.5. TC batch adsorption and desorption experiments The prepared ZIF-8/PDA/PAN fibers were used to remove TC as adsorbents from water. All the adsorption experiments were carried out at rest under room temperature (298 K), and all solutions were wrapped with aluminum foil to avoid photodegradation of TC. The influences of the pH of the solution to the TC adsorption were studied within pH range of 39, and the solution pH values were regulated by 0.1 M hydrochloric acid solution and 0.1 M sodium hydroxide solution. The adsorption isotherm of TC was obtained by initial concentrations from the range of 50 to 450 mg/L and each solution was added with approximately 8.7 mg adsorbent. The adsorption kinetics were investigated with an initial concentration of 50 mg/L and lasted for 72 h. The adsorption capacity (q) of TC adsorbed into the adsorbents was calculated on the foundation of the Eq. (1):

qðmg=gÞ ¼

ðC0
Ce ÞV W

ð1Þ

where C0 and Ce (mg/L) are the initial concentration and the equilibrium concentration of adsorbate in the measured solution, respectively, V (L) is the volume of the measured solution, and W (g) is the weight of the added adsorbent. To recycle the utilized adsorbent, the fibers were washed six times with absolute ethanol, and dried at 333 K overnight. The adsorption/desorption cycle was iterated for five times. 3. Results and discussion 3.1. Preparation and morphology The preparation route of ZIF-8/PDA/PAN composite fibers is schematically illustrated in Scheme 1. To synthesize ZIF-8/PDA/ PAN composite fibers, step one is the preparation of PAN fibers membrane utilizing electrospinning technology. Then, the selfpoly-merization of dopamine was placed on the surface of PAN fibers in a 7.88 mg/mL tris buffer solution at pH value of 8.5. Finally, the PDA layer works as a linker [28] to effectively induce the growth of ZIF-8 nanocrystals, generating ZIF-8/PDA/PAN composite fibers. The following sections show more evidence for determining the substances formed at each stage. Electrospun fibers have a lot of superiorities, including small diameter, huge specific surface area, high porosity of fibers membrane, favorable absorbability, etc. The fibrous morphology is the

essential factor of these characteristics. The morphology and microstructure of the ZIF-8/PDA/PAN samples were studied by SEM and TEM images (Fig. 1). Fig. 1a showed the typical interlaced structure of the PAN nanofiber membrane, whose average diameter of 219 nm (Fig. S1a). After surface modification by polydopamine coating, the fibers surface became much rougher (Fig. 1b) and its diameter increased to 256 nm (Fig. S1b). It can be observed from Fig. 1c that the electrospun ZIF-8/PDA/PAN fibers surface underwent the evident transform of being covered by a great quantity of the ZIF-8 crystals, and the ZIF-8 crystals had grew and distributed on the surface of fibers successfully and uniformly. Due to ZIF-8 coating, the average diameter of the obtained fibers also increased up to 349 nm (Fig. S1c). The high-magnification SEM image (Fig. 1d) showed comparative organized cubic shape of the ZIF-8 nanocrystals, which provided the corroborating evidence that ZIF-8 crystals has grew on the PDA/PAN fibers surface uniformly. According to the previous reports [29,30], the surface hydrophilicity can be enhanced by using polydopamine coating strategy, which is believed to be beneficial to the growth of nanoparticles on the external of nanofiber. Therefore, the PDA coating made ZIF-8 nanocrystals more stably fixed on the top of fibers, thereby taking shape a highly integrated and integrated electrospun membrane with distinctive structure for outstanding properties. As shown in the TEM micrographs (Fig. 1e and f), ZIF8/PDA/PAN fibers demonstrate the noticeable growing layer of ZIF-8 nanocrystals. Thus, the observed results of TEM are identical with the results of SEM. In Fig. S2, the digital photographs of ZIF-8/ PDA/PAN fibers displayed the colors variations of the electrospun fiber membranes, which also confirmed the coating of PDA and the growth of ZIF-8. 3.2. Composition characterization FT-IR spectra of the ZIF-8/PDA/PAN fibers are significantly different from those of PAN fibers and PDA/PAN fibers as shown in Fig. 2, which also confirmed the reaction and the chemical structure of the fibers. The peaks at 2242 cm
1 and 1733 cm
1 belong to the stretching vibrations of the nitrile group and the carbonyl group of the ester of the methylacrylate comonomer [21], respectively; And the peaks at 1453 cm
1 and 1360 cm
1 attribute to CAH blending and CH3 symmetric blending, respectively; In addition, the peaks at 2950 cm
1 and 2860 cm
1 are owing to the stretching vibration of CAH. These peaks above proved the existence of PAN [21]. In Fig. 2b, a newly appeared peak at 1601 cm
1 is classified to the aromatic ring C@C stretching vibration. The peak at 1507 cm
1 belongs to the shearing vibration of the amide group, while the adsorption peaks at 1287 cm
1 can be ascribed to the CAOH stretching vibration [31]. Therefore, these results can prove that the PDA has been successfully applied onto the surface of PAN fibers. After ZIF-8 growth, a strong peak at 993 cm
1 and 1147 cm
1, owing to the CAN stretching vibration, appeared. The peaks at 3137 cm
1 and 2928 cm
1 belong to the CAH bonds stretching vibrations in the imidazole ring and the methyl group [32]. These peaks indicate that the ZIF-8 has been prepared successfully. 3.3. XRD, TGA and specific surface area results The crystalline structure of ZIF-8/PDA/PAN samples was characterized by XRD analysis (Fig. 3). The XRD patterns of the ZIF-8/PDA/ PAN fibers are shown in Fig. 3, comparing with the standard pattern of the simulated ZIF-8 from the No. 602,542 crystallographic data of Cambridge Crystallographic Data Centre (CCDC). It can be noticed from the figure that the key peaks of ZIF-8/PDA/PAN fibers almost have the identical intensity and distribution compared with the standard ZIF-8 patterns, where the characteristic diffraction

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Scheme 1. Schematic diagram of the whole preparation process for ZIF-8/PDA/PAN composite fibers.

Fig. 1. SEM images of PAN fibers (a), PDA/PAN fibers (b), ZIF-8/PDA/PAN fibers (c, d) and TEM images of ZIF-8/PDA/PAN fibers (e, f).

peaks at 2h values of 7.49, 10.55, 12.90, 14.88, 16.61, and 18.21 are relevant to the crystal planes of the ZIF-8 crystals [33]. Therefore, the XRD results confirmed that the ZIF-8 nanocrystals successfully formed and fixed onto the surface of the fiber.

For the loading amount of ZIF-8 on ZIF-8/PDA/PAN fibers, the thermal behavior of the ZIF-8/PDA/PAN fibers, displayed in Fig. S3, were tested by TGA in the atmosphere of air from 50 °C up to 850 °C. It is not difficult to see that in the range from

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other electrospun nanofibrous membranes [35,36], and this data is also comparable to the materials with similar structures [37]. The extremely high BET surface area strongly supports the fact that the ZIF-8/PDA/PAN fibers have a nanoporous structure. The high surface area of ZIF-8/PDA/PAN fibers is mainly owing to the introduction of ZIF-8 nanocrystals, which is favorable to the improvement of adsorption ability as well as the increase of adsorption sites. 3.4. Determination of pHPZC

Fig. 2. FT-IR spectra of PAN fibers (a), PDA/PAN fibers (b) and ZIF-8/PDA/PAN fibers (c).

The pHPZC value was identified by salt addition method [38]. A solution of 0.01 M NaCl was used as the background electrolyte. Then 40 mL of this solution were added into 6 flasks and the pH was maintained at 49 by using 0.1 M NaOH or 0.1 M HCl solution where appropriate. Subsequently 3 ± 0.1 mg of the ZIF-8/PDA/PAN fibers were added into all the flasks and agitated on a horizontal shaker (Kylin-Bell Lab Instruments Co., Ltd., China) for 24 h at 25 °C. Then the final pH values were recorded. The difference between initial and final pH values (D pH) was plotted against initial pH values. PZC value was identified at the pH when D pH is zero, i.e., initial pH equals the final pH. The pHPZC of ZIF-8/PDA/ PAN fibers according to the results of pHPZC determination (Fig. 4) is approximately 8.12. 3.5. Adsorption experiment In this study, ZIF-8/PDA/PAN fibers were manufactured to get a novel kind of electrospun fiber to remove TC from water. All experiments were performed at rest at room temperature (298 K) and all the TC solutions were covered with several layers of aluminum foils to avoid possible photo degradation of TC. In addition, the effect of the dosage of adsorbent was also considered. The experiments were carried out by changing the ratio of adsorbent mass/TC solution volume in the range of 0.2–1.4 g/L, while the initial pH and the TC concentration were 5 and 50 mg/L, respectively. The results are shown in Fig. S5.

Fig. 3. XRD patterns of the prepared fiber samples.

100 °C to 250 °C, PAN fibers had some weight loss of about 4%, which is related to the evaporation of solvent and water that adsorbed in fibers [21]. A rapid decline in mass took place because of PAN thermo oxidation at the temperature of about 270 °C and the removal of residual solvents or reactant molecules trapped in the nanocrystals during synthesis and post-treatment, following by a slow decrease due to PAN chains degradation [34], and another decrease at ca. 410 °C is related to the structural collapse, which indicates that the structural integrity of the crystal is destroyed. The PAN fibers finally burned down at the temperature of over 650 °C, where the weight of fibers begins to stabilize. Due to the structural collapse of ZIF-8, the final residue should be ZnO, therefore the quality of loaded ZIF-8 on the surface of ZIF-8/ PDA/PAN fibers was calculated to be approximately 50.0% by the final quality of residue. Furthermore, significant physical properties, i.e. porous properties and the specific surface area of the ZIF-8/PDA/PAN fibers were characterized by N2 adsorption/desorption isotherms, and the result is shown in Fig. S4. The Brunauer-Emmett-Teller (BET) specific surface area of ZIF-8/PDA/PAN fibers was calculated from N2 adsorption-desorption isotherms at
196 °C, and was found to be as much as 319.38 m2/g, which is much higher than most of the

3.5.1. Initial pH effect toward the TC adsorption For the TC adsorption, the initial solution pH is a vital factor because it can influence not only adsorbent but also adsorbate. On the basis of the previous literature, we know that the pH of solution influence TC adsorption highly [26,39], because the TC speciation and the basic sites of the surface of ZIF-8 are affected

Fig. 4. Difference between initial and final pH values (D pH) plot against initial pH values.

S. Chao et al. / Journal of Colloid and Interface Science 552 (2019) 506–516

511

by the transformation of pH. Thus, the initial pH effect toward the TC adsorption was investigated at pH values from 3 to 9 with the 50 mg/L TC solution, and the final results are shown in Fig. 5. It is clear that the adsorption capacity of fibers toward TC improves with the pH value of solution ranging from 3 to 5. The natural pH of the 50 mg/L TC solution is about 4.5 at room temperature. Thus, at lower pH values, adsorption ability is weak because of the competition between H+ and TC to occupy the limited sorption sites on the surface of the adsorbent, which impairs the effectiveness of TC adsorption. Because the change of pH values can affect speciation of TC, when the pH value of solution continues to increase from 6 to 9, the main species of TC in water become TCH
and TC2
[26]. Therefore these lead to a more powerful electrostatic repulsion because of the basic sites on the surface of ZIF-8 nanosrystals, which leads to the decrease of TC adsorption capacity. Drawing from the conclusion of the aforementioned study, the following experiments were conducted with the initial pH fixed at 5. Fig. 6. The comparative adsorption by different adsorbents.

3.5.2. Comparative adsorption In order to prove the influence of PDA and ZIF-8 toward the adsorption effect of TC, the adsorption amounts of PAN fibers, PDA/PAN fibers, ZIF-8/PAN fibers and ZIF-8/PDA/PAN fibers for TC were investigated by using 50 mg/L TC solution. It can be seen in Fig. 6 that PAN fibers possess extremely poor adsorption ability for TC. After coating by PDA, adsorption capacity of the fibers had almost no change, which proves that PDA has no effect on adsorption capacity toward TC. But, the adsorption ability of ZIF8/PAN fibers is evidently greater than that of PDA/PAN fibers as well as ZIF-8/PAN fibers, which indicates that ZIF-8 plays an essential role in adsorption capacity toward TC. According to our experience, it is not easy to make ZIF-8 nanocrystals load onto the PAN fibers surface due to the poor hydrophilicity of PAN fibers. Thus, for ZIF-8/PDA/PAN fibers, PDA works as the linker to make ZIF-8 load on the surface of fibers easier [28], and PDA improves the hydrophilicity of the fibers at the same time [29,30]. Through the above comparison, it is obvious that the ZIF-8/PDA/PAN fibers possess the greatest adsorption ability for TC. 3.5.3. Adsorption kinetics In order to study the adsorption performance of ZIF-8/PDA/PAN fibers, the adsorption kinetics of TC on ZIF-8/PDA/PAN fibers, which is vital to explore the process of TC adsorption, was investigated. The adsorption kinetics of the obtained fibers was

Fig. 5. Initial solution pH effect on the TC adsorption.

researched by adding 81.9 mg ZIF-8/PDA/PAN fibers membrane in 200 mL TC solution with a concentration of 50 mg/L. The experiments were carried out at rest under room temperature (298 K). The kinetic adsorption performance for ZIF-8/PDA/PAN fibers to TC is shown in Fig. 7. The TC quantity of adsorption had a rapid increase within the first 10 h, achieving more than 50% of the equilibrium adsorption quantity. Following that, the increase rate of adsorption became slow; finally the TC adsorption process achieved the equilibrium state within about 72 h. For the purpose of better understanding of adsorption process, the adsorption kinetics of ZIF-8/PDA/PAN fibers were further explored by three kinds of kinetics models. Although some literature [40] revealed the shortcomings of using linear fitting of nonlinear pseudo-firstorder and pseudo-second-order models as the model selection criteria, these are still good models available for theoretical calculations, thus we adopted these methods as references. The pseudofirst-order kinetics model, pseudo-second-order kinetics model [26] and Elovich kinetics model [25] are shown as follows:

logðqe
qt Þ ¼ logqe


k1 t 2:303

t 1 t ¼ þ qt k2 q2e qe

Fig. 7. Adsorption kinetics of TC by ZIF-8/PDA/PAN fibers.

ð2Þ ð3Þ

512

qt ¼

S. Chao et al. / Journal of Colloid and Interface Science 552 (2019) 506–516

1 1 lnðabÞ þ ln t b b

ð4Þ

where qt (mgg
1) stands for the adsorption capacity at time t (h), qe (mgg
1) stands for the equilibrium adsorption capacity, k1 (h
1) is the pseudo-first-order rate constant, k2 (gh
1mg
1) stands for the pseudo-second-order rate constant, while a is the initial adsorption rate constant, and b is the Elovich adsorption constant. The pseudo-first-order kinetic linear plot log(qe
qt) versus t, the pseudo-second-order kinetic linear plot t/qt versus t and the Elovich kinetic linear plot qt versus ln t are illustrated in Fig. 8a– c, respectively. The values of k1, k2, qe, a and b can be worked out by using Eqs. (2)–(4), which are expressed in Table 1 by using the values of linear regression. As listed in Table 1, the pseudosecond-order model, which has been widely used in the field of adsorption of pollutants in water in the past several years [41], can fit better (R2  0.99) to the data of adsorption kinetics, indicating that the rate-limiting step of the process of TC adsorption involves chemisorptions. Furthermore, this equilibrium adsorption capacity (qe) that worked out by pseudo-second-order kinetics model is also compared with the values for other reported adsorbents. As listed in Table 2, regardless of the differences in preparation method and test condition, the obtained qe of 76.27 mg/g is higher than that of most of the previously reported adsorbents. In addition, the Weber–Morris model is employed to further explore rate-controlling steps that influence the process of adsorption (Fig. 8d). The Weber–Morris model indicates that if the ratelimiting factor in the process of adsorption is intraparticle diffusion, uptake of the adsorbate will vary with the time’s square root

Table 1 The kinetics parameters of pseudo-first-order, pseudo-second-order and Elovich models for TC adsorption onto ZIF-8/PDA/PAN fibers. Experimental qexp (mg/g)

62.59 qe (mg/g)

k1 (h
1)

R2

Pseudo-first-order model

64.50

0.0637

0.9663

Pseudo-second-order model

qe (mg/g) 76.27

k2 (g/mgh) 0.0002

R2 0.9900

a (mg/g/min)

b (g/mg) 0.0676

R2 0.9570

Elovich model

13.69

Table 2 Comparison of adsorption capacities with other reported adsorbents. Absorbent

Initial concentration (mg/L)

qe (mg/ g)

Ref.

Fe3O4/PAN composite NFs Fe-nanofiber mat MSW-BC MSW-MMT MSW-RE magnetic PI@LDO composites SA-Cu beads ZIF-8 ZIF-8/PDA/PAN fibers

22 44 20 20 20 10

37.17 54.88 3.921 8.386 4.156 49.64

[26] [42] [43] [43] [43] [44]

– 50 50

18.620 90.9 76.27

[45] [46] This work

Fig. 8. The fitting curves of the pseudo-first-order model (a), the pseudo-second-order model (b), the Elovich model (c) and Weber-Morris kinetics model (d) of TC adsorption.

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[21], and the equation of Weber–Morris model is presented as follows:

qt ¼ Kd t0:5 þ L

ð5Þ

where Kd stands for the intraparticle diffusion rate constant, while L stands for the boundary layer’s thickness. It can be seen clearly from the results that the processes of adsorption can be separated into the following three consecutive stages: surface adsorption, intraparticle diffusion and final equilibrium. The Weber–Morris model is used for all the stages, and the constants of kinetic are listed in Table 3. The initial stage is owing to the TC diffusion to the fibers’ surface through the solution, which can be considered as the external diffusion; the secondary stage is attributed to the gradual adsorption, in which the rate of adsorption is limited by intraparticle diffusion [26]; and in the final stage, the adsorption gradually reaches equilibrium. In addition, in Weber–Morris model if L = 0, intraparticle diffusion is considered as the rate limiting step, while, at L > 0 both film and intraparticle diffusion are considered as rate limiting steps [47]. Thus, the processes of adsorption of ZIF-8/PDA/PAN fibers for TC involve surface

adsorption as well as intraparticle diffusion, and the limit rate step is not only the intraparticle diffusion but also the binding of the sorbate to the sorbent. 3.5.4. Adsorption isotherm The experiment of adsorption isotherm was carried out at the initial concentration of TC from 50 mg/L to 450 mg/L in order to determine maximum adsorption capacity and adsorption equilibrium data of the obtained fibers, and the consequents are presented in Fig. 9a. It is easy to find that the adsorption capacity of the fibers for TC grows gradually with the increase of initial solution concentration. In addition, Langmuir isotherm model, Freundlich isotherm model and Temkin isotherm model are all used to explore the process of adsorption of the fibers for TC [48–50], which are the most universal isotherm models that applied to characterize the solid-liquid adsorption system. Their linear equations are the following ones: Langmuir isotherm (homogeneous and monolayer adsorption):

1 1 1 ¼ þ qe qm bqm Ce

ð6Þ

Table 3 Parameters of Weber-Morris model of TC onto ZIF-8/PDA/PAN fibers. Weber-Morris model

Parameters

Kd1 (mgg
1h
0.5)

R2

Kd2 (mgg
1h
0.5)

R2

Kd3 (mgg
1h
0.5)

R2

12.35

0.9694

6.05

0.9723

3.02

0.9243

Fig. 9. (a) Adsorption isotherms of TC onto ZIF-8/PDA/PAN fibers. (b) Linear Langmuir model. (c) Linear Freundlich model. (d) Linear Temkin model.

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Table 4 Langmuir, Freundlich and Temkin isotherm model constants and correlation coefficients for TC adsorption onto ZIF-8/PDA/PAN fibers. Absorbent

Langmuir isotherm

Freundlich isotherm

qmax (mg/g) b (L/mg) ZIF-8/PDA/PAN

Freundlich adsorption):

671.14

isotherm

1 logqe ¼ logKF þ logCe n

R
3

8.25  10

(heterogeneous

2

0.8850

and

multilayer

KF

n

R

15.77

1.60

0.9855

KT

f

R2

130.89

0.10

0.9060

research will be a superb candidate to remove TC from real water body contaminated with antibiotics due to the excellent regeneration and the ability to remove TC from water.

ð7Þ

Temkin isotherm (the heat of adsorption decreases linearly with coverage of adsorbent surface [51]):

qe ¼ K T lnC e þ K T lnf

Temkin isotherm 2

ð8Þ

where qe stands for the equilibrium adsorption capacity (mg/g), Ce stands for the equilibrium concentration (mg/L), while qm and b respectively stand for Langmuir constants that concerned about maximum adsorption capacity as well as binding energy; KF and n respectively stand for the Freundlich constant that related to empirical constants (L/mg) and heterogeneity factor; KT stands for the Temkin constant (L/mg) related to the heat of adsorption, f stands for the maximum binding energy constant. The parameter values obtained from the plots presented in Fig. 9b–d are listed in Table 4. It is obvious that, in the range of tested concentration, the Freundlich isotherm model (R2 = 0.9855) fits the adsorption data better than Langmuir isotherm model (R2 = 0.8850) and Temkin isotherm model (R2 = 0.9060), demonstrating a probable heterogeneous and multilayer adsorption on the surfaces of the ZIF-8/PDA/PAN fibers for TC molecules. 3.5.5. Reusability of ZIF-8/PDA/PAN fibers One of the most vital items is the adsorbent’s reusability, which can be used to assess the feasibility in a large scale of application. Therefore, the adsorption-desorption experiments were conducted to investigate the reusability of the obtained fibers. Absolute ethanol was used for the regeneration in this study and the result is shown in Fig. 10. It can be seen that compared to the original fibers, the fibers after regenerating displayed outstanding reusability and consistency in terms of adsorption efficiency after five cycles, and adsorption efficiency of the fibers for TC was still kept not less than 85% of the initial adsorption capacity of the fibers after five cycles. Consequently, the as-prepared ZIF-8/PDA/PAN fibers in the current

Fig. 10. Adsorption–desorption cycles of TC adsorption by ZIF-8/PDA/PAN fibers.

3.5.6. Adsorption mechanism of ZIF-8/PDA/PAN fibers for TC It can be seen from the previous test (Fig. 5), the PAN fibers and PDA coating have no effect on the TC adsorption, thus, the ZIF-8 nanocrystals play the essential role in the adsorption of TC. It is significant for effective removal of contaminants to study the probable adsorption mechanism. Normally, the MOFs’ adsorption mechanisms contain electrostatic interaction, p-p interactions, hydrogen bonding, as well as pore/size-selective adsorption, etc [48]. A previous study has reported that at pH  5, the dominant species of TC is TCH2 [26], which is a neutral molecule. Only a small amount of TCH+3 [2] can be adsorbed by lots of basic sites, including OH groups and N-moieties, which are existed on the outer surface of ZIF-8 nanocrystals [18]. Due to the pHPZC of ZIF-8/PDA/PAN fibers is approximately 8.12, according to the pHPZC determination (Fig. 4), at the test condition of pH = 5 (pH < pHPZC), the positively charged adsorbent [52] would inhibit the adsorption for TCH+3 through electrostatic repulsion. Therefore, at the test condition of pH = 5, electrostatic interaction was not the main mechanism for the high-efficient adsorption of TC toward ZIF-8/PDA/PAN fibers. In the meantime, because of the imidazole ring possessing the electron distribution and bonds which is the same as the benzene ring [53,54], the 2-methylimidazole molecules from ZIF-8 can be regarded as aromatic molecules thus are also able to possess the p-p stacking interactions. Moreover, the force of strong chemical

Scheme 2. TC adsorption mechanism by ZIF-8/PDA/PAN fibers.

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bonding between Zn (transition metal) and TC may promote the adsorption of ZIF-8/PDA/PAN fibers for TC [2]. In addition, the coordination interaction between the oxygen atoms from TC and the open metal sites (Zn2+) from ZIF-8 molecules could have a positive effect on the effective removal of TC [18]. The probable adsorption mechanism between the fibers and TC is displayed in Scheme 2. 4. Conclusion In the present work, MOFs materials were combined with electrospinning technology to prepare a kind of PDA coating assisted preparation of ZIF-8 functionalized composite electrospun fiber adsorbent. Owing to the p-p stacking interactions, coordination interaction, strong chemical bonding between transition metals and TC and other interactions, ZIF-8/PDA/PAN fibers exhibit excellent adsorption capacity toward TC. The adsorption kinetics described by pseudo-second-order model and Weber–Morris model indicate that the processes involve surface adsorption as well as intraparticle diffusion, and the limit rate steps are the intraparticle diffusion and the binding of the sorbate to the sorbent. The Freundlich model presented a better correlation for the isotherm adsorption data and the adsorption capacity of the fibers can reach 478.18 mg/g at 298 K. ZIF-8/PDA/PAN fibers possess a higher efficiency for TC removal compared with the adsorption capacities of other adsorbents [26,42–46]. More importantly, the fibers can be easily taken out after the adsorption is accomplished and reused compared with other powdered adsorbent and granular adsorbent [55–57]. In addition, the preparation technology is simple, the cost is low, and the raw materials are easy to acquire; the electrospun nanofibers obtained are provided with good film-forming property and flexibility, and they can be easily got out from the adsorbed liquid. Thus, ZIF-8/PDA/PAN fibers could be considered as an environmentally friendly and promising adsorbent applied in adsorption of antibiotic wastewaters. Although some results on fiberbased adsorbents for TC adsorption by electrospinning have been reported, as far as we know, this is the first time that MOFs have been combined with electrospinning to prepare adsorbents to remove antibiotics from water. Acknowledgements The authors thank the support by the research grants of the National Natural Science Foundation of China (Grant No. 51773082). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.05.078. References [1] W. Li, Y. Shi, L. Gao, J. Liu, Y. Cai, Occurrence, distribution and potential affecting factors of antibiotics in sewage sludge of wastewater treatment plants in China, Sci. Total Environ. 445 (2013) 306–313. [2] J. Jin, Z. Yang, W. Xiong, Y. Zhou, R. Xu, Y. Zhang, J. Cao, X. Li, C. Zhou, Cu and Co nanoparticles co-doped MIL-101 as a novel adsorbent for efficient removal of tetracycline from aqueous solutions, Sci. Total Environ. 650 (2019) 408–418. [3] L.J. Zhou, G.G. Ying, S. Liu, J.L. Zhao, B. Yang, Z.F. Chen, H.J. Lai, Occurrence and fate of eleven classes of antibiotics in two typical wastewater treatment plants in South China, Sci. Total Environ. 452 (2013) 365–376. [4] M.R. Azhar, H.R. Abid, H. Sun, V. Periasamy, M.O. Tadé, S. Wang, Excellent performance of copper based metal organic framework in adsorptive removal of toxic sulfonamide antibiotics from wastewater, J. Colloid Interf. Sci. 478 (2016) 344–352. [5] X. Wang, A. Wang, J. Ma, Visible-light-driven photocatalytic removal of antibiotics by newly designed C3N4@ MnFe2O4-graphene nanocomposites, J. Hazard. Mater. 336 (2017) 81–92.

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