English

• 0.   Introduction

  Nowadays, as the industry is leaping forward, organic dyes have become one of several major water pollutants, and the removal of the organic contaminants before discharged into the environment takes on a critical significance^[1-4]. Among several methods (e.g., electrochemistry, photochemistry, biodegradation, and adsorption) ^[5-8], adsorption is one of the most attractive methods for its simplicity and efficiency^[9-10]. Numerous traditional adsorbents (e. g., activated carbon, zeolites, clay, cellulosic materials, and graphene composites) have been extensively used to remove organic dyes from industrial wastewater over the past few years^[11-13]. However, some of the above adsorbents face some challenges (e. g., low adsorption capacity, poor selectivity, low surface areas, and poor stability). Thus, to enhance the adsorption performance, efficient and high-capacity adsorbents of dye wastewater treatment should be explored in depth.

  Metal -organic frameworks (MOFs) have widely served as adsorbents for plentiful pollutants from waste-water for their good micropore structure and sufficient-ly high physical stability^[14-18]. Typical water -stable MOF (e.g., UiO-66, MIL^-125, ZIF-7, and ZIF-8) exhibits excellent dye adsorption properties^[19-23]. However, some defects such as powdered morphology and fragility restrict their practical applications. To overcome the above defects, the integration of MOF powder into a functional support substrate is a promising approach, which makes it easy to recycle and reuse in dye adsorption^[24-27].

  The electrospun nanofibers have recently acted as outstanding substrates for loading MOF crystals for their high surface area, high porosity, and large surface area to volume ratio^[28-31]. Moreover, the surface of electrospun nanofibers can be easily modified by functional groups, thus assisting the initial coordination of metal irons to the surface of the substrate and facilitating the uniform growth of MOF particles^[32-34].

  Based on the above discussion, zeolitic imidazolate framework-8 (ZIF-8) crystals were loaded onto the carboxymethylated polyacrylonitrile electrospun nanofibers (PAN-COOH NFs) using an in-situ growth method. To examine the adsorption performance of ZIF-8/PAN-COOH NFs, malachite green (MG) was selected as the model organic dye for the batch adsorption experiments. Furthermore, the kinetic and isothermal models were studied by the experimental data, and the reusability of the ZIF-8/PAN-COOH NFs was evaluated through the cycling test.

  1.   Experimental

  1.1   Material

  Polyacrylonitrile (PAN, M_w=150 000) and N, N -dimethylformamide (DMF) were offered by Sigma-Aldrich Co., Ltd. (Shanghai, China). 2-Methylimidazole (2 -MIM) was purchased from Shanghai Macklin Co., Ltd. (Shanghai, China). Zinc nitrate hexahydrate (Zn(NO₃) ₂·6H₂O) was provided by Nanjing Chemical Reagent Co., Ltd. (Nanjing, China). Malachite green (MG) was supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

  1.2   Synthesis of PAN-COOH NFs

  1.5 g of PAN was dissolved in 13.5 g of DMF to prepare the electrospinning solution. Subsequently, the electrospinning solution was added to a syringe with a 21G needle at an applied voltage of 20 kV. The distance between the needle tip and the collector was 15 cm. After electrospinning, the obtained nanofibers were dried in a vacuum oven at 60 ℃ for 12 h. The obtained PAN NFs were hydrolyzed in a 1.0 mol·L^-1 NaOH solution at 50 ℃ for 1 h to convert surface nitrile groups into carboxylic groups, and marked as PAN-COOH NFs. The obtained PAN-COOH NFs were dried in a vacuum oven at 55 ℃ for 12 h.

  1.3   Preparation of ZIF -8/PAN -COOH NFs and ZIF-8/PAN NFs

  0.02 g of PAN-COOH NFs was mixed with 0.587 g Zn(NO₃)₂·6H₂O in 40 mL methanol for 24 h. Subsequently, 2 -MIM methanol solution (1.298 g, 40 mL^-1) was slowly added, and the mixture was oscillated at ambient temperature for 24 h. The samples obtained were washed with methanol for several times and then dried at 50 ℃ for 6 h. Afterward, the above procedure was repeated twice, and the nanofibers were termed ZIF-8/ PAN-COOH NFs. ZIF-8/PAN NFs were prepared using the same method, by substituting PAN NFs for PAN -COOH NFs.

  1.4   Batch adsorption experiment

  The batch adsorption experiments were performed with 20 mL of MG solution by mixing 5 mg of PAN -COOH NFs and ZIF-8/PAN-COOH NFs, respectively. The pH of the original MG solution was adjusted with 0.1 mol·L^-1 NaOH solution and 0.1 mol·L^-1 HCl solu-tion. The batch adsorption experiments were performed to investigate the factors for the initial pH of MG solu-tion (2 -6), contact time (10 -420 min), and initial MG concentration (50 -1 500 mg·L^-1) in terms of the adsorp-tion of MG onto the ZIF -8/PAN-COOH NFs. All the batch adsorption experiments were performed at 298 K. The residual concentration of MG in the solutions was confirmed with a UV -Vis spectrophotometer at λ_max=616 nm. The adsorption capacity at equilibrium (q_e, mg·g^-1), the adsorption capacity at each time (q_t, mg·g^-1), and the removal efficiency (R) of the MG were determined using the formula below^[35]:

  $ q_e=\frac{\left(\rho_0-\rho_e\right) V}{m} $

  (1)

  $ q_t=\frac{\left(\rho_0-\rho_t\right) V}{m} $

  (2)

  $ R=\frac{\rho_0-\rho_{\mathrm{e}}}{\rho_0} \times 100 \% $

  (3)

  where ρ₀, ρ_e, and ρ_t (mg·L^-1) represent initial concentration, concentration at t time, and equilibrium concentration of MG, respectively. Moreover, V (L) indicates the volume of the MG solution, and m (g) denotes the mass of the adsorbent.

  1.5   Desorption and regeneration experiment

  A certain quality of adsorbed ZIF -8/PAN -COOH NFs was immersed in a proportional amount of ethanol solution. Subsequently, the solution was placed in a shaker at 25 ℃ for 12 h. The adsorbed dye (MG) was released into the ethanol solution. Afterward, the nanofibers were adequately washed with deionized water and then dried in the vacuum oven at 60 ℃. The adsorption-desorption recycling experiment was repeated nine times.

  1.6   Characterization

  The chemical structure of the samples was confirmed using Fourier transform infrared spectra (FT-IR, Nicolet, PerkinElmer, USA). The crystal structure of the obtained materials was characterized by X -ray diffractometer (XRD, Rigaku, Japan) equipped with Cu Kα radiation (λ =0.154 056 nm, 45 kV, 40 mA), and the scanning of XRD was running from 5° to 60° at the speed of 5 (°)·min^-1. The surface morphologies and microstructure of the samples were observed under field-emission scanning electron microscopy (SEM, Hitachi, Japan) with an acceleration voltage of 30 kV, and the elemental compositions of the composite nanofibers were examined using energy dispersive spectroscopy (EDS) apparatus based on the FE -SEM. The specific surface area was measured using an N₂-sorption instrument (Quantachrome Autosorb -iQASIQ). Furthermore, the specific surface area was measured based on the Brunauer-Emmett-Teller (BET) method.

  2.   Result and discussion

  2.1   Micro-structure of ZIF-8/PAN-COOH NFs The morphology and structure of the fabricated

  nanofibers were examined using SEM. As depicted in Fig. 1a and 1d, PAN NFs were intact, smooth, and uniform with an average diameter of about 200 nm. After alkaline hydrolysis, the surface roughness of PAN -COOH NFs slightly increased (Fig. 1b and 1e). Fig. 1c and 1f illustrate the morphology of ZIF -8/PAN-COOH NFs. As depicted in these figures, the ZIF -8 particles with rhombic dodecahedral shapes successfully grew on the surface of PAN-COOH NFs. The EDS mapping was further examined (Fig. 2a-2c), thus indicating that ZIF -8/PAN -COOH NFs had a high content of Zn elements (mass fraction of 9.1%) and distributed evenly on the surfaces of the nanofibers. The above results confirmed that the ZIF -8 particles were successfully loaded onto the nanofiber surfaces.

  Figure 1

  

  Figure 1.  SEM images of the nanofibers: PAN NFs (a, d); PAN-COOH NFs (b, e); ZIF-8/PAN NFs (c, f)

  下载: 全尺寸图片 幻灯片

  Figure 2

  

  Figure 2.  SEM image (a), zinc mapping image (b), and EDS element analysis diagram (c) of ZIF-8/PAN-COOH NFs

  Inset in c: corresponding element content.

  下载: 全尺寸图片 幻灯片

  N₂ adsorption-desorption measurements were performed to characterize the pore structures of the samples. Fig. 3a illustrates N₂ adsorption -desorption isotherms of PAN NFs and ZIF -8/PAN -COOH NFs. The specific surfaces of them were calculated using the BET method. The specific surface area of PAN NFs was measured as 42 m²·g^-1, while the specific surface area of the nanofibers coated with the ZIF -8 particles was obtained as 256 m² ·g^-1 (Fig. 3b). The significantly increased specific area of ZIF -8/PAN -COOH NFs would create more active sites for adsorption.

  Figure 3

  

  Figure 3.  N₂ adsorption isotherms (a) and pore size distributions (b) of PAN NFs and ZIF-8/PAN-COOH NFs

  下载: 全尺寸图片 幻灯片

  2.2   Material properties analysis

  The crystalline phases of the synthesized nanofibers were examined through XRD (Fig. 4). PAN NFs and PAN -COOH NFs indicated a strong diffraction peak at 16.9°, corresponding to PAN crystals. The XRD patterns of ZIF -8 and ZIF -8/PAN -COOH NFs showed intense diffraction peaks at 7.12°, 10.22°, 12.56°, 14, 54°, 15.18°, 16.28°, and 17.86°, respectively, corresponding to the (011), (002), (112), (022), (013), and (222) planes of the ZIF -8, which revealed the growth of ZIF-8 particles in the ZIF-8/PAN-COOH NFs.

  Figure 4

  

  Figure 4.  XRD patterns of PAN NFs, PAN-COOH NFs, and ZIF-8/PAN-COOH NFs

  下载: 全尺寸图片 幻灯片

  The interaction between ZIF-8 particles and PAN-COOH NFs was characterized through the FTIR analysis. As depicted in Fig. 5, the comparison of the spectrum of original PAN NFs revealed that PAN -COOH NFs exhibited two different absorption peaks at 1 650 and 3 420 cm^-1, corresponding to the stretching vibration of C=O and O—H in carboxyl functional groups. The peaks at 954, 1 175, and 1 309 cm^-1 belong to the stretching vibration of the imidazole ring. The typical peaks at 693 and 759 cm^-1 belong to the Zn—N and Zn—O stretching vibration, respectively. The band at 1 453 cm^-1 belong to CH₂ rocking and bending vibra-tions. The adsorption band at 2 243 cm^-1 belong to the —CN vibration. As revealed by the emergence of the new characteristic bands of imidazole ring and ZIF-8 in the spectra of the ZIF -8/PAN -COOH NFs, the above substances existed in the sample.

  Figure 5

  

  Figure 5.  FTIR spectra for PAN NFs, PAN-COOH NFs, and ZIF-8/PAN-COOH NFs

  下载: 全尺寸图片 幻灯片

  2.3   Adsorption evaluation for ZIF-8/PAN-COOH NFs

  The cationic dye (MG) was employed as the adsorption object to optimize the adsorption capacity of ZIF-8/PAN-COOH NFs. Batch adsorption experiments were conducted to assess the adsorption behavior.

  2.3.1   Effect of pH

  The pH of the solution is an essential parameter influencing the adsorption process of dye molecules. Since the color of the MG solution changed under the alkaline condition, the pH values tested in the experiment varied in the range of 2-6. As depicted in Fig. 6, the removal efficiency of MG by ZIF -8/PAN -COOH NFs increased with the increase of pH from 2 to 4 and leveled off in the scope of 5-6. Under high acidic conditions, ZIF-8 on the surface of ZIF-8/PAN-COOH NFs was dissolved, thus resulting in low removal efficiency. With the increase in pH, the surface charge of ZIF -8/ PAN-COOH NFs became more negative, and the electrostatic interaction between MG and the nanofibers also became stronger^[36]. As revealed by the above results, the optimal pH value of ZIF -8/PAN -COOH NFs for MG adsorption was examined as 5.

  Figure 6

  

  Figure 6.  Effect of pH on the adsorption of MG onto ZIF-8/PAN-COOH NFs

  Conditions: 100 mg·L^-1 MG, 0.25 g·L^-1 ZIF-8/PAN-COOH NFs, 25 ℃, 12 h.

  下载: 全尺寸图片 幻灯片

  2.3.2   Adsorption kinetics

  The adsorption kinetics was studied to gain more insights into the adsorption mechanism and adsorption rate of PAN -COOH NFs and ZIF -8/PAN -COOH NFs for MG. Fig. 7a and 7d illustrate the effect of contact time on the MG adsorption by PAN -COOH NFs and ZIF -8/PAN -COOH NFs, respectively. The adsorption capacity increased with the increase in the contact time. As revealed by the results, the adsorption capacity of PAN -COOH NFs reached equilibrium after 400 min, while that of ZIF -8/PAN -COOH NFs reached equilibrium after 300 min.

  Figure 7

  

  Figure 7.  Adsorption kinetic curves of MG on PAN-COOH NFs and ZIF-8/PAN-COOH NFs (a, d), and corresponding pseudo-first-order kinetic model (b, e) and pseudo-second-order kinetic model (c, f)

  Conditions: pH=5, 100 mg·L^-1 MG, 0.25 g·L^-1 ZIF-8/PAN-COOH NFs, 25℃, 12 h.

  下载: 全尺寸图片 幻灯片

  To analyze the adsorption kinetics in -depth, the kinetic data were fitted by the pseudo -first -order and pseudo -second -order equations. The equations of the two dynamical models are generally expressed as^[37-38]:

  $ \ln \left(q_{\mathrm{e}}-q_t\right)=\ln q_{\mathrm{e}}-k_1 t $

  (4)

  $ t / q_t=1 /\left(k_2 q_{\mathrm{e}}^2\right)+t / q_{\mathrm{e}} $

  (5)

  where q_t (mg·g^-1) denotes the adsorption capacity of MG at any contact time t (min); q_e (mg·g^-1) represents the adsorption capacity of MG adsorbed at the equilibrium time; k₁ (min^-1) and k₂ (g·mg^-1·min^-1) express the equilibrium rate constant of the pseudo-first-order and pseudo-second -order kinetic, respectively. Table 1 lists the adsorption kinetics parameters of MG dye molecules using PAN -COOH NFs and ZIF -8/PAN -COOH NFs, and Fig. 7b, 7c, 7e, and 7f present the corresponding graphs.

  Table 1

  

  Table 1.  Kinetic parameters for the adsorption of MG onto PAN-COOH NFs and ZIF-8/PAN-COOH NFs

  下载: 导出CSV

  Sample	ρe / (mg·L-1)	qe, exp / (mg·g-1)	Pseudo-first-order kinetic				Pseudo-second-order kinetic
  			qe, cal / (mg·g-1)	k1 / min-1	R2		qe, cal / (mg·g-1)	k2 / (g·mg-1·min-1)	R2
  PAN-COOH NFs	100	85.3		0.007 2	0.95			0.011	0.999
  ZIF-8/PAN-COOH NFs	100	381.7	386	0.012	0.98		386.1	0.000 34	0.999

  The result suggested that the coefficients of correlation (R²) were found to be higher for the pseudo-second-order kinetics than those of the pseudo-first-order kinetic model, establishing that the adsorption to MG conformed to the pseudo -second -order kinetic equation. The above results confirmed that the adsorption process of PAN -COOH NFs and ZIF -8/PAN -COOH NFs arose from the chemical interactions between the polar functional groups on the surface of nanofibers and MG molecules.

  2.3.3   Adsorption isotherm

  Adsorption isotherms take on great significance in describing the interaction between the MG molecules and ZIF-8/PAN-COOH NFs. The adsorption equilibrium isotherm was studied for the Langmuir isotherm and Freundlich isotherm models based on the above obtained experimental data. The Langmuir isotherm model was adopted to assume that the adsorption of adsorbate onto the adsorbent was monolayer adsorption. The Freundlich isotherm model was employed to express that the adsorption of the adsorbate onto the adsorbent was multilayer adsorption. The linear forms of Freundlich isotherm and Langmuir isotherm are written as^[39-40]:

  $ q_{\mathrm{e}}=K_{\mathrm{F}} \rho_{\mathrm{e}}^{1 / n} $

  (6)

  $ q_{\mathrm{e}}=\frac{q_{\mathrm{m}} K_{\mathrm{L}} \rho_{\mathrm{e}}}{1+K_{\mathrm{L}} \rho_{\mathrm{e}}} $

  (7)

  where q_m (mg·g^-1) denotes the maximum adsorption capacity of MG; q_e (mg·g^-1) represents the adsorption capacity of MG at the equilibrium time; ρ_e (mg·L^-1) expresses the concentration MG solution at equilibrium; K_L (L·mg^-1) represents the Langmuir constant, correlat-ed with the adsorption bonding energy; K_F (mg^1-1/n·L^1/n· g^-1) is the Freundlich constant; n is the adsorption strength; 1/n is the heterogeneity factor of the adsorbent.

  Table 2 lists the isotherm obtained constants for the Langmuir isotherm model and Freundlich isotherm model fitting, and the corresponding curves are presented in Fig. 8. As depicted in the figure, the linear correlation coefficient (R²) in the Langmuir isotherm model was higher than the Freundlich isotherm model, thus suggesting that it was more suitable for the Langmuir model. It can be inferred that the adsorption of MG by the ZIF -8/PAN -COOH NFs tend to be mono -layer adsorption. The maximum adsorption capacity of the ZIF -8/PAN -COOH NFs for the adsorption of MG was obtained as 3 604 mg·g^-1 for MG. Table 3 presents different adsorbents for removing dyes from aqueous media.

  Table 2

  

  Table 2.  Langmuir and Freundlich isotherm parameters using ZIF-8/PAN-COOH NFs for the adsorption of MG

  下载: 导出CSV

  Langmuir				Freundlich
  qm / (mg·g-1)	KL / (L·mg-1)	R2		KF / (mg1-1/n·L1/n·g-1)	n	R2
  3 604	0.08	0.94		762	3.78	0.76

  Figure 8

  

  Figure 8.  Adsorption isotherm of MG on ZIF-8/PAN-COOH NFs at 25 ℃ (a); Corresponding Langmuir and Freundlich isotherm models (b)

  Conditions: pH=5, 0.25 g·L^-1 ZIF-8/PAN-COOH NFs, 12 h.

  下载: 全尺寸图片 幻灯片

  Table 3

  

  Table 3.  Comparison of dye adsorption capacities of numerous ZIF composites as adsorbents

  下载: 导出CSV

  Material	Adsorption capacity / (mg·g-1)	Reference
  ZIF-@GO	3 325	[35]
  14-ZIF-8/C	3 056.84	[41]
  EW@ZIF-8@PAA	2 764.9	[42]
  ZIF-8/CoFe2O4/GO	2 610	[43]
  QD@ZIF-8	2 500	[44]
  ZIF-8/PAN-COOH NFs	3 604	This work

  2.3.4   Recyclability experiments

  The recyclability of adsorbents has been found as the key to their practical application. Accordingly, the renewability of the ZIF-8/PAN -COOH NFs was investigated for MG adsorption. The ethanol washing method was employed to desorb MG for its simplicity and maneuverability. As depicted in Fig. 9, after nine-cycle regeneration, ZIF -8/PAN -COOH NFs still exhibited 96% of the first adsorption capacity, thus revealing that ZIF -8/PAN -COOH NFs could be easily regenerated, and the adsorption capacity was nearly constant.

  Figure 9

  

  Figure 9.  Recycling efficiency of ZIF-8/PAN-COOH NFs for MG removal

  Conditions: pH=5, 100 mg·L^-1 MG, 0.25 g·L^-1 ZIF-8/PAN-COOH NFs, 25 ℃, 12 h.

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  In this study, ZIF -8 particles were successfully immobilized on electrospun nanofibers compactly and uniformly using the in -situ growth method. Furthermore, the in -situ growth of ZIF-8 particles on the surface of the carboxymethylated PAN NFs led to an increase in the specific surface area of the original nanofibers. The ZIF -8/PAN-COOH NFs exhibited high adsorption capacity for MG (3 604 mg·g^-1). As revealed by the results, the equilibrium data matched the Langmuir isothermal model, and the kinetic equilibrium data were consistent with the pseudo-second-order kinetic equation. Furthermore, the ZIF -8/PAN -COOH NFs exhibited high reusability. The above features enable the synthetic sample to be an effective and promising adsorbent in the removal of MG from waste-water.

  
  Acknowledgements: This investigation was supported by the Jiangsu Province Natural Science Foundation (Grant No. BK20130969).
• 1. [1]

     Mishra S, Cheng L, Maiti A. The utilization of agro-biomass/byproducts for effective bio-removal of dyes from dyeing wastewater: A comprehensive review[J]. J. Environ. Chem. Eng., 2021, 9(1):  104901. doi: 10.1016/j.jece.2020.104901

  2. [2]

     Uddin F. Environmental hazard in textile dyeing wastewater from local textile industry[J]. Cellulose, 2021, 28(17):  10715-10739. doi: 10.1007/s10570-021-04228-4

  3. [3]

     Donkadokula N Y, Kola A K, Naz I, Saroj D. A review on advanced physico-chemical and biological textile dye wastewater treatment techniques[J]. Rev. Environ. Sci. Bio/Technol., 2020, 19(3):  543-560. doi: 10.1007/s11157-020-09543-z

  4. [4]

     黄婧, 陈慧君, 冯晓苗. Ni-Mn双金属氧化物微马达用于高效染料吸附[J]. 无机化学学报, 2022,38,1411-1420. HUANG J, CHEN H J, FENG X M. Micromotors based on Ni-Mn binary oxide and its application for effective dye adsorption[J]. Chinese J. Inorg. Chem., 2022, 38:  1411-1420.

  5. [5]

     Osugi M E, Umbuzeiro G A, Anderson M A, Zanoni M V B. Degradation of metallophtalocyanine dye by combined processes of electro-chemistry and photoelectrochemistry[J]. Electrochim. Acta, 2005, 50(25/26):  5261-5269.

  6. [6]

     Jin Z K, Dong W J, Yang M, Wang J J, Gao H Y, Wang G. One-pot preparation of hierarchical nanosheet-constructed Fe₃O₄/MIL-88B(Fe) magnetic microspheres with high efficiency photocatalytic degradation of dye[J]. ChemCatChem, 2016, 8(22):  3510-3517. doi: 10.1002/cctc.201600952

  7. [7]

     Oelgemöller M, Healy N, de Oliveira L, Jung C, Mattay J. Green photochemistry: Solar-chemical synthesis of Juglone with medium concentrated sunlight[J]. Green Chem., 2006, 8(9):  831-834. doi: 10.1039/B605906F

  8. [8]

     Wang Z Y, Ju C J, Zhang R, Hua J Q, Chen R P, Liu G X, Yin K, Yu L. Acceleration of the bio-reduction of methyl orange by a magnetic and extracellular polymeric substance nanocomposite[J]. J. Hazard. Mater., 2021, 420:  126576. doi: 10.1016/j.jhazmat.2021.126576

  9. [9]

     Fan J X, Chen D Y, Li N J, Xu Q F, Li H, He J H, Lu J M. Adsorption and biodegradation of dye in wastewater with Fe₃O₄@MIL-100(Fe) core-shell bio-nanocomposites[J]. Chemosphere, 2018, 191:  315-323. doi: 10.1016/j.chemosphere.2017.10.042

  10. [10]

      Wang X H, Jiang C L, Hou B X, Wang Y Y, Hao C, Wu J B. Carbon composite lignin-based adsorbents for the adsorption of dyes[J]. Chemosphere, 2018, 206:  587-596. doi: 10.1016/j.chemosphere.2018.04.183

  11. [11]

      Gohr M S, Abd- Elhamid A I, El- Shanshory A A, Soliman H A. Adsorption of cationic dyes onto chemically modified activated carbon: Kinetics and thermodynamic study[J]. J. Mol. Liq., 2022, 346:  118227. doi: 10.1016/j.molliq.2021.118227

  12. [12]

      Atrous M, Sellaoui L, Bouzid M, Lima E C, Thue P S, Bonilla-Petriciolet A, Ben Lamine A. Adsorption of dyes acid red 1 and acid green 25 on grafted clay: Modeling and statistical physics interpretation[J]. J. Mol. Liq., 2019, 294:  111610. doi: 10.1016/j.molliq.2019.111610

  13. [13]

      Tang C Y, Yu P, Tang L S, Wang Q Y, Bao R Y, Liu Z Y, Yang M B, Yang W. Tannic acid functionalized graphene hydrogel for organic dye adsorption[J]. Ecotoxicol. Environ. Saf., 2018, 165:  299-306. doi: 10.1016/j.ecoenv.2018.09.009

  14. [14]

      Ma X L, Wang W L, Sun C G, Li H, Sun J, Liu X. Adsorption performance and kinetic study of hierarchical porous Fe-based MOFs for toluene removal[J]. Sci. Total Environ., 2021, 793:  148622. doi: 10.1016/j.scitotenv.2021.148622

  15. [15]

      Andres M A, Fontaine P, Goldmann M, Serre C, Roubeau O, Gascon I. Solvent-exchange process in MOF ultrathin films and its effect on CO₂ and methanol adsorption[J]. J. Colloid Interface Sci., 2021, 590:  72-81. doi: 10.1016/j.jcis.2021.01.030

  16. [16]

      Banerjee D, Elsaidi S K, Thallapally P K. Xe adsorption and separation properties of a series of microporous metal-organic frameworks (MOFs) with V-shaped linkers[J]. J. Mater. Chem. A, 2017, 5(32):  16611-16615. doi: 10.1039/C7TA02746J

  17. [17]

      Gao Y X, Liu K, Kang R X, Xia J, Yu G, Deng S B. A comparative study of rigid and flexible MOFs for the adsorption of pharmaceuticals: Kinetics, isotherms and mechanisms[J]. J. Hazard. Mater., 2018, 359:  248-257. doi: 10.1016/j.jhazmat.2018.07.054

  18. [18]

      Xie L H, Liu X M, He T, Li R. Metal-organic frameworks for the capture of trace aromatic volatile organic compounds[J]. Chem, 2018, 4(8):  1911-1927. doi: 10.1016/j.chempr.2018.05.017

  19. [19]

      Ahmadipouya S, Heidarian H M, Ahmadijokani F, Jarahiyan A, Molavi H, Matloubi M F, Rezakazemi M, Arjmand M. Magnetic Fe₃O₄@UiO-66 nanocomposite for rapid adsorption of organic dyes from aqueous solution[J]. J. Mol. Liq., 2021, 322:  114910. doi: 10.1016/j.molliq.2020.114910

  20. [20]

      Khosravi M J, Hosseini S M, Vatanpour V. Performance improvement of PES membrane decorated by Mil-125(Ti)/chitosan nanocomposite for removal of organic pollutants and heavy metal[J]. Chemosphere, 2022, 290:  133335. doi: 10.1016/j.chemosphere.2021.133335

  21. [21]

      Mohammad N S, Seyedpour S F, Aghapour A S, Dadashi F M, Elliott M, Tiraferri A, Sadrzadeh M, Rahimpour A. Loose nanofiltration membranes functionalized with in situ-synthesized metal organic framework for water treatment[J]. Mater. Today Chem., 2022, 24:  100909. doi: 10.1016/j.mtchem.2022.100909

  22. [22]

      Khajavian M, Salehi E, Vatanpour V. Chitosan/polyvinyl alcohol thin membrane adsorbents modified with zeolitic imidazolate framework (ZIF-8) nanostructures: Batch adsorption and optimization[J]. Sep. Purif. Technol., 2020, 241:  116759. doi: 10.1016/j.seppur.2020.116759

  23. [23]

      黄婧, 田欣雨, 杨澜, 冯晓苗. Mn-MOF衍生的Mn₂O₃微马达用于水中甲基蓝的去除[J]. 无机化学学报, 2022,38,153-160. doi: 10.11862/CJIC.2022.008HUANG J, TIAN X Y, YANG L, FENG X M. Mn-MOF derived Mn₂O₃ micromotors applied to removal of methyl blue in water[J]. Chinese J. Inorg. Chem., 2022, 38:  153-160. doi: 10.11862/CJIC.2022.008

  24. [24]

      Park J, Oh M. Construction of flexible metalorganic framework (MOF) papers through MOF growth on filter paper and their selective dye capture[J]. Nanoscale, 2017, 9(35):  12850-12854. doi: 10.1039/C7NR04113F

  25. [25]

      Xu Y, Zhai X, Wang X H, Li L L, Chen H, Fan F Q, Bai X J, Chen J Y, Fu Y. Fabrication of a robust MOF/aerogel composite via a covalent postassembly method[J]. Chem. Commun., 2021, 57(48):  5961-5964. doi: 10.1039/D1CC01613J

  26. [26]

      Maan O, Song P, Chen N X, Lu Q Y. An in situ procedure for the preparation of zeolitic imidazolate framework-8 polyacrylamide hydrogel for adsorption of aqueous pollutants[J]. Adv. Mater. Interfaces, 2019, 6(10):  1801895. doi: 10.1002/admi.201801895

  27. [27]

      Dong C, Yang J J, Xie L H, Cui G, Fang W H, Li J R. Catalytic ozone decomposition and adsorptive VOCs removal in bimetallic metal-organic frameworks[J]. Nat. Commun., 2022, 13(1):  4991. doi: 10.1038/s41467-022-32678-2

  28. [28]

      Mahmoodi N M, Oveisi M, Taghizadeh A, Taghizadeh M. Synthesis of pearl necklace-like ZIF-8@chitosan/PVA nanofiber with synergistic effect for recycling aqueous dye removal[J]. Carbohydr. Polym., 2020, 227:  115364. doi: 10.1016/j.carbpol.2019.115364

  29. [29]

      Huang J M, Huang D, Zeng F B, Ma L, Wang Z B. Photocatalytic MOF fibrous membranes for cyclic adsorption and degradation of dyes[J]. J. Mater. Sci., 2020, 56(4):  3127-3139.

  30. [30]

      Xie X Y, Qian X Y, Qi S C, Wu J K, Liu X Q, Sun L B. Endowing Cu-BTC with improved hydrothermal stability and catalytic activity: Hybridization with natural clay attapulgite via vapor-induced crystal-lization[J]. ACS Sustain. Chem. Eng., 2018, 6(10):  13217-13225. doi: 10.1021/acssuschemeng.8b02827

  31. [31]

      朱亚波, 唐晓彤, 段连威, 刘万英, 曹星星, 冯培忠. Mn⁴⁺掺杂Co₃O₄纳米纤维的静电纺丝法制备及其电化学性能[J]. 无机化学学报, 2018,34,317-324. ZHU Y B, TANG X T, DUAN L W, LIU W Y, CAO X X, FENG P Z. Mn⁴⁺ doped Co₃O₄ nanofibers: Preparation by electro-spinning and electrochemical performance[J]. Chinese J. Inorg. Chem., 2018, 34:  317-324.

  32. [32]

      Dou Y, Zhang W, Kaiser A. Electrospinning of metal-organic frameworks for energy and environmental applications[J]. Adv. Sci., 2020, 7(3):  1902590. doi: 10.1002/advs.201902590

  33. [33]

      Jia M, Zhang X F, Feng Y, Zhou Y, Yao J. In-situ growing ZIF-8 on cellulose nanofibers to form gas separation membrane for CO₂ separation[J]. J. Membr. Sci., 2020, 595:  117579. doi: 10.1016/j.memsci.2019.117579

  34. [34]

      Lu J, Wu J K, Jiang Y, Tan P, Zhang L, Lei Y, Liu X Q, Sun L B. Fabrication of microporous metal-organic frameworks in uninterrupted mesoporous tunnels: Hierarchical structure for efficient trypsin immobilization and stabilization[J]. Angew. Chem. Int. Ed., 2020, 59(16):  6428-6434. doi: 10.1002/anie.201915332

  35. [35]

      Abdi J, Vossoughi M, Mahmoodi N M, Alemzadeh I. Synthesis of metalorganic framework hybrid nanocomposites based on GO and CNT with high adsorption capacity for dye removal[J]. Chem. Eng. J., 2017, 326:  1145-1158. doi: 10.1016/j.cej.2017.06.054

  36. [36]

      Wang S M, Li Z H, Lu C. Polyethyleneimine as a novel desorbent for anionic organic dyes on layered double hydroxide surface[J]. J. Colloid Interface Sci., 2015, 458:  315-322. doi: 10.1016/j.jcis.2015.07.056

  37. [37]

      Zhao R, Yong X Y, Pan M, Deng J P, Pan K. Aldehyde-containing nanofibers electrospun from biomass vanillin-derived polymer and their application as adsorbent[J]. Sep. Purif. Technol., 2020, 246:  116916. doi: 10.1016/j.seppur.2020.116916

  38. [38]

      Habiba U, Siddique T A, Li Lee J J, Joo T C, Ang B C, Afifi A M. Adsorption study of methyl orange by chitosan/polyvinyl alcohol/zeolite electrospun composite nanofibrous membrane[J]. Carbohydr. Polym., 2018, 191:  79-85. doi: 10.1016/j.carbpol.2018.02.081

  39. [39]

      Gao Y, Deng S Q, Jin X, Cai S L, Zheng S R, Zhang W G. The construction of amorphous metal-organic cage-based solid for rapid dye adsorption and time-dependent dye separation from water[J]. Chem. Eng. J., 2019, 357:  129-139. doi: 10.1016/j.cej.2018.09.124

  40. [40]

      Wang J L, Guo X. Adsorption isotherm models: Classification, physical meaning, application and solving method[J]. Chemosphere, 2020, 258:  127279. doi: 10.1016/j.chemosphere.2020.127279

  41. [41]

      Li Y F, Yan X L, Hu X Y, Feng R, Zhou M, Han D Z. In situ growth of ZIF-8 onto porous carbons as an efficient adsorbent for malachite green removal[J]. J. Porous Mater., 2020, 27(4):  1109-1117. doi: 10.1007/s10934-020-00887-z

  42. [42]

      Wang Q Q, Lei L L, Wang F C, Chen C T, Kang X Y, Wang C, Zhao J H, Yang Q X, Chen Z J. Preparation of egg white@zeolitic imidazolate framework-8@polyacrylic acid aerogel and its adsorption properties for organic dyes[J]. J. Solid State Chem., 2020, 292:  121656. doi: 10.1016/j.jssc.2020.121656

  43. [43]

      Mahmoodi N M, Oveisi M, Bakhtiari M, Hayati B, Shekarchi A A, Bagheri A, Rahimi S. Environmentally friendly ultrasound-assisted synthesis of magnetic zeolitic imidazolate framework—Graphene oxide nanocomposites and pollutant removal from water[J]. J. Mol. Liq., 2019, 282:  115-130. doi: 10.1016/j.molliq.2019.02.139

  44. [44]

      Rabeie B, Mahkam M, Mahmoodi N M, Lan C Q. Graphene quantum dot incorporation in the zeolitic imidazolate framework with sodalite (SOD) topology: Synthesis and improving the adsorption ability in liquid phase[J]. J. Environ. Chem. Eng., 2021, 9(6):  106303. doi: 10.1016/j.jece.2021.106303

• 

  Figure 1  SEM images of the nanofibers: PAN NFs (a, d); PAN-COOH NFs (b, e); ZIF-8/PAN NFs (c, f)

  下载: 全尺寸图片 幻灯片
  

  Figure 2  SEM image (a), zinc mapping image (b), and EDS element analysis diagram (c) of ZIF-8/PAN-COOH NFs

  Inset in c: corresponding element content.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  N₂ adsorption isotherms (a) and pore size distributions (b) of PAN NFs and ZIF-8/PAN-COOH NFs

  下载: 全尺寸图片 幻灯片
  

  Figure 4  XRD patterns of PAN NFs, PAN-COOH NFs, and ZIF-8/PAN-COOH NFs

  下载: 全尺寸图片 幻灯片
  

  Figure 5  FTIR spectra for PAN NFs, PAN-COOH NFs, and ZIF-8/PAN-COOH NFs

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Effect of pH on the adsorption of MG onto ZIF-8/PAN-COOH NFs

  Conditions: 100 mg·L^-1 MG, 0.25 g·L^-1 ZIF-8/PAN-COOH NFs, 25 ℃, 12 h.

  下载: 全尺寸图片 幻灯片
  

  Figure 7  Adsorption kinetic curves of MG on PAN-COOH NFs and ZIF-8/PAN-COOH NFs (a, d), and corresponding pseudo-first-order kinetic model (b, e) and pseudo-second-order kinetic model (c, f)

  Conditions: pH=5, 100 mg·L^-1 MG, 0.25 g·L^-1 ZIF-8/PAN-COOH NFs, 25℃, 12 h.

  下载: 全尺寸图片 幻灯片
  

  Figure 8  Adsorption isotherm of MG on ZIF-8/PAN-COOH NFs at 25 ℃ (a); Corresponding Langmuir and Freundlich isotherm models (b)

  Conditions: pH=5, 0.25 g·L^-1 ZIF-8/PAN-COOH NFs, 12 h.

  下载: 全尺寸图片 幻灯片
  

  Figure 9  Recycling efficiency of ZIF-8/PAN-COOH NFs for MG removal

  Conditions: pH=5, 100 mg·L^-1 MG, 0.25 g·L^-1 ZIF-8/PAN-COOH NFs, 25 ℃, 12 h.

  下载: 全尺寸图片 幻灯片

  Table 1.  Kinetic parameters for the adsorption of MG onto PAN-COOH NFs and ZIF-8/PAN-COOH NFs

  Sample	ρe / (mg·L-1)	qe, exp / (mg·g-1)	Pseudo-first-order kinetic				Pseudo-second-order kinetic
  			qe, cal / (mg·g-1)	k1 / min-1	R2		qe, cal / (mg·g-1)	k2 / (g·mg-1·min-1)	R2
  PAN-COOH NFs	100	85.3		0.007 2	0.95			0.011	0.999
  ZIF-8/PAN-COOH NFs	100	381.7	386	0.012	0.98		386.1	0.000 34	0.999

  下载: 导出CSV

  Table 2.  Langmuir and Freundlich isotherm parameters using ZIF-8/PAN-COOH NFs for the adsorption of MG

  Langmuir				Freundlich
  qm / (mg·g-1)	KL / (L·mg-1)	R2		KF / (mg1-1/n·L1/n·g-1)	n	R2
  3 604	0.08	0.94		762	3.78	0.76

  下载: 导出CSV

  Table 3.  Comparison of dye adsorption capacities of numerous ZIF composites as adsorbents

  Material	Adsorption capacity / (mg·g-1)	Reference
  ZIF-@GO	3 325	[35]
  14-ZIF-8/C	3 056.84	[41]
  EW@ZIF-8@PAA	2 764.9	[42]
  ZIF-8/CoFe2O4/GO	2 610	[43]
  QD@ZIF-8	2 500	[44]
  ZIF-8/PAN-COOH NFs	3 604	This work

  下载: 导出CSV
•