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Article

Removal of Hexavalent Chromium by Electrospun Silicon Dioxide Nanofibers Embedded with Copper-Based Organic Frameworks

by
Shanshan Feng
1,2,*,
Jie Ni
1,
Shouzhu Li
3,
Xun Cao
1,
Jingshuai Gao
1,
Wenyang Zhang
1,
Feng Chen
1,
Rouxue Huang
1,
Yao Zhang
1,* and
Sheng Feng
1
1
School of Environmental Science and Engineering, Changzhou University, Changzhou 213164, China
2
Jiangsu Petrochemical Safety and Environmental Protection Engineering Research Center, Changzhou 213164, China
3
Laboratory of Nanofiber Membrane Materials and Devices, Xinjiang Institute of Technology, Akesu 843100, China
*
Authors to whom correspondence should be addressed.
Sustainability 2022, 14(21), 13780; https://doi.org/10.3390/su142113780
Submission received: 21 September 2022 / Revised: 17 October 2022 / Accepted: 20 October 2022 / Published: 24 October 2022
Browse Figures
Versions Notes

Abstract

:
A novel adsorbent copper-based organic skeleton/silicon dioxide (HKUST-1/SiO2) composite nanofiber, which can effectively remove Cr (VI) under synergistic action, has been prepared by embedding growth technique. This adsorbent was characterized by embedded growth of HKUST-1 on inorganic SiO2 electrospun nanofibers, which can remove Cr (VI) in water with the help of adsorption and membrane separation under synergistic action. The results revealed that HKUST-1 was successfully embedded between the pores of SiO2 electrospun nanofibers. The factors affecting the adsorption performance of the composite nanofibers were studied, and the result displayed that the concentration of Cr (VI) solution was 120 mg/L, the best range for pH was 3~7, the adsorption equilibrium was about 45 min, and the maximum adsorption amount was 62.38 mg/g. Compared with the SiO2 fiber without HKUST-1 growth, the adsorptive property of the composite fiber was significantly increased by 15 mg/g. The adsorption process was spontaneous and belonged to the heat absorption reaction, which was consistent with Langmuir adsorption and the pseudo-second-order kinetic model. In addition, HKUST-1/SiO2 NFs can be used for the recovery of chromium resources because the HKUST-1/SiO2 NFs captured Cr (VI) can be calcined and recovered in the later stage, which reduces the consumption of desorption liquid, simplifies the recovery steps, and is conducive to energy saving and emission reduction. Therefore, HKUST-1/SiO2 NFs are expected to be applied in the field of hexavalent chromium wastewater purification and resource recovery.

1. Introduction

The Chromium (Cr) is an indispensable metal material in the industrial field, which was used to make alloy, chrome plating for rust prevention, pigment and dye production. If the "three wastes" were improperly treated, it will caused great harm to the balance of ecological environment and human health [1]. Cr mainly existed in trivalent and hexavalvalent form in water environment, and Cr (III) has lower toxic than Cr (VI) [2]. Recently, more and more people are studying the efficient treatment of Cr (VI) in wastewater. At present, there are two main treatment methods to remove hexavalent chromium. One method is to convert Cr (VI) to low-toxic Cr (III) by reduction technology and remove it by chemical precipitation, including the electrolytic reduction method [3], chemical reduction method [4], and so on. Another approach is to use resource recovery routes such as ion exchange [5], adsorption [6], and reverse osmosis [7].
Until recently, the adsorption method has been widely used in the chemical industry, environmental remediation, medicine, and health. Due to this method, biological engineering has simple operation technology, high processing efficiency, and low economic cost. The common adsorbents mainly include biochar and industrial waste-modified materials [8,9], nanomaterials, and metal-organic frameworks (MOFs). Compared with many properties of adsorbent materials, MOF has the advantages of a large specific surface area, perfect porous structure, and high adsorption capacity. In the adsorption and separation technology of water-soluble heavy metal ions, MOFs have attracted much attention. Among them, HKUST-1 (Hong Kong University of Science and Technology-1) is one typical representative of MOFs, with the advantages of the relative ease of synthesis, super-high specific surface area, tunable pores, and rich unsaturated metal positions [10]. Ratna Ediati [11] used the solvent heat method to successfully synthesize HKUST-1 in a mixed solvent of water, ethanol, and N, N-dimethylformamide (DMF), and its mother liquid was successfully repeated as a solvent four times. The studies revealed that HKUST-1 was confirmed to remove hexavalent chromium through pore surface filling, electrostatic interaction, and ion exchange. However, with further research, it has been reported in the literature that HKUST-1 has some shortcomings, such as poor water stability, mainly because its metal sites are easily occupied by water molecules, leading to the collapse of the frame structure, which limits the popularization and application of HKUST-1 [12,13,14,15,16,17]. Besides, powdered HKUST-1 is easy to aggregate, difficult to separate after use, and easy to produce secondary pollution.
Therefore, a suitable support material can be selected to make HKUST-1 load efficiently, avoid agglomeration, and make the material have excellent adsorption performance and convenient recovery performance. Significantly, electrospinning fibers have obvious advantages such as high width ratio, relatively large surface area, open pore structure, and are lightweight, so they have been widely applied in many fields. Zhou Meimei successfully deposited UIO-66-NH2 on STPAN NFs by using thermally stable electrospun PAN nanofibers as a matrix and a simple and effective in-situ growth method. Cr (VI) was reduced by photocatalysis, and its photocatalytic reduction rate reached 93% within 180 min. However, the photocatalytic degradation of heavy metals in wastewater is still in the experimental research stage, which is difficult to achieve in practical applications [18]. Besides, UIO usually uses an adsorption method to remove Pd2+ and Pt4+ from water [19]. Xu Yang adopted a method of in-situ growth of ZIF-8 on ZIF-8/polyacrylonitrile (PAN) co-electrospun nanofibers, which made the load of ZIF-8 reach 82.9%. Meanwhile, the ZIF-8 crystal in the ZIF-8/PAN nanofibers can be used as the seed for subsequent growth. The surface area of the modified adsorbent is 871.0 m2/g, and the adsorption capacity of Cr (VI) is 39.68 mg/g [20].
Among common fiber membranes, such as polypropylene cyanide fiber, polyvinyl alcohol fiber, and silicon dioxide fiber, silicon dioxide has been widely used in membrane separation because of its unique firmness, excellent mesoporous structure, high relative specific surface area, excellent thermal and mechanical stability, and highly regular pore distribution and modification ability. It is worth noting that silica nanofiber (SiO2 NFs) is a white fiber structure with a flocculation network structure, which belongs to inorganic environmental protection materials and is very easy to prepare. However, as a highly efficient adsorbent, it is required to have a specific binding site, but SiO2 NFs themselves do not possess such surface characteristics. Therefore, it is necessary to modify SiO2 NFs with the help of its surface containing hydroxyl, combined with other functional sites, to prepare a new adsorbent to expand its application in selective adsorption separation.
In view of the above problems, in this work, HKUST-1/amino modified silica composite nanofibers (HKUST-1/SiO2 NFs) were obtained by embedding HKUST-1 in the pores of amino-modified SiO2 fibers, which was used as a new adsorbent to treat Cr (VI) in wastewater under the synergistic action of adsorption and separation (Figure 1). Firstly, 3-aminopropyl tetraethoxysilane was used to modify the SiO2 NFs prepared by electrospinning technology to improve the active functional sites of SiO2 NFs. Then, the HKUST-1 anchored on the surface of SiO2 NFs was modified with amino acids to improve the stability of HKUST-1 in an aqueous solution, and finally, HKUST-1/SiO2 NFs was obtained for the efficient removal of chromium ions in water. In addition, the characteristics of HKUST-1/SiO2 NFs were studied with the help of scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Besides, the effects of different influencing factors (initial concentration, time, temperature, and pH) on the adsorption properties of the composite fibers were studied by comparing them with SiO2 NFs without HKUST-1 growth. Meantime, the adsorption mechanism of hexavalent chromium was analyzed from the aspects of adsorption kinetics, adsorption isotherm, and an adsorption thermodynamic model. In addition, compared with literature reports, this method has remarkable characteristics [11,20,21,22]: (1) HKUST-1/SiO2 NFs can be mass-produced without a complex preparation process and has excellent application potential; (2) HKUST-1/SiO2 NFs not only has significant separation characteristics, but also the water stability of HKUST-1 in the adsorbent is improved and has a high adsorption capacity for chromium ion; (3) the preparation method of HUST-1/SiO2 composite nanofibers was relatively simple, and the raw material price was relatively economical. In addition, HKUST-1 is grown on SiO2 NFs in a fixed manner, thus avoiding the secondary contamination problem of HKUST-1. Therefore, this HKUST-1/SiO2 NFs has a broad prospect in the purification of chromium-contaminated wastewater.

2. Materials and Methods

2.1. Materials

The manufacturer of H3BTC (98%), HCl (36%–38%), and CH3CH2OH (>99.5%) were Sinopharm Chemical Reagent Co., Ltd.; Cu (NO3)2·3H2O (99.5%), N, n-dimethylformamide (DMF, >99.5%) were purchased from Shanghai Xinbao Fine Chemical Plant and Jiangsu Qiangsheng Chemical Company, and LTD. 3-Aminopropyl triethyl silane (APTES, >99.5%), purchased from Gold Wheat Company. K2Cr2O7 (99.8%) was purchased from Tianjin Tianli Chemical Reagent Co., LTD. All reagents in this experiment were analytically pure in grade, and no further purification was required. The experimental water was deionized water.

2.2. Preparation of HKUST-1/SiO2 Composite Nanofibers

2.2.1. The Synthesis of HKUST-1

Cu (NO3)2·3H2O (6 g) and H3BTC (3 g) were devoted to DMF solution (150 mL) and stirred evenly by ultrasonic machine. Then, the mixed solution after the reaction was transferred to a Teflon steel high-pressure reactor (200 mL) and placed in a convection oven at 348 K for 24 h. After the reaction, the solution was collected and washed three times by centrifugation (5000× g, 5 min) to obtain HKUST suspension. The final HKUST-1 was obtained by freeze-dried at −50 °C for 12 h.

2.2.2. Preparation of SiO2 Nanofibers

The SiO2 nanofibers were prepared by the electrostatic spinning method, and the solution ratio was TEOS: H2O: CH3CH2OH: HCl = 1: 3.5: 2: 0.01. The prepared solution was hydrolyzed at a water bath temperature of 80 °C for 2 h. The stirring speed was 250 r/min. Then the milky SiO2 spinning solution was collected and put into the spinning device (V: 19 kV, L: 20 cm, and T: 120 min). In other words, the SiO2 nanofibers were obtained until the aluminum foil surface was covered with white fibers.

2.2.3. Preparation of HKUST-1/SiO2 Composite Nanofibers

Amino-functionalized SiO2 nanofibers (M-SiO2): the mixture of CH3CH2OH (95% wt.), HCl (37%), and APTES was prepared in proportion (30:1:1, V/V/V), and hydrolyzed for one h to obtain the silane sol solution. Then, we put the SiO2 nanofibers (2 cm × 2 cm) into the mixed sol solution and soaked them for 60 min. The aminopropyl silane sol was absorbed into the pores of SiO2 nanofibers through capillary action, and after drying overnight, the sol condensed to form a Si-O-Si network, and the amino-functionalized SiO2 nanofibers were obtained. The nanofibers were first washed three times with ethanol and then dried for five h at room temperature to obtain M-SiO2.
HKUST-1/SiO2 composite nanofibers were prepared with M-SiO2 nanofibers. The M-SiO2 nanofibers (2 cm × 2 cm) were immersed in 0.5 mol/L Cu(NO3)2·3H2O solution, left for four h, and dried in a cool place to obtain Cu-M-SiO2 nanofibers. Then Cu (NO3)2·3H2O (6 g) and H3BTC (3 g) were placed into the DMF (150 mL) and stirred evenly with an ultrasonic machine to obtain the reaction stock solution. The Cu-M-SiO2 nanofibers were transferred from the stock solution to a high-pressure reactor (200 mL) and kept in a convection oven heated at 348 K for 24 h. Finally, the composite nanofibers were removed from the reactor and dried in a convection oven at 35 °C to obtain HKUST-1/SiO2 composite nanofibers.

2.3. Characterization of the Samples

Next, the morphology of HKUST-1/SiO2 nanofibers during fabrication was studied using a JEOL 2010 scanning electron microscope (SEM) of Nippon Electronics Corporation. An X-ray diffractive diffractometer of */D/MAX2500 X-ray diffractometer of Nippon Electronics Co., Ltd. was used to produce X-ray diffraction of HKUST-1/SiO2 nanofibers during fabrication, analyze the diffraction map of materials, and obtain information such as composition, internal atoms, and molecular structure. The thermal stability of the fiber was analyzed with the 209F3/F3 thermogravimeter of Germany NETZSCH, and */IS50 was used for the material analysis of samples during fabrication by Thermo Fisher & Fisher Fourier transform infrared. The diameters of nanofibers were analyzed by Nano Measurer 1.2 software.

2.4. Adsorption Experiment

We placed the hexavalent chromium stock solution with the concentration of 1000 mg/L into a 1000 mL volumetric flask and prepared the hexavalent chromium solution. The dosage of SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers is 0.0200 g. We placed 50 mL of hexavalent chromium solution into a 50 mL triangular beaker. The reaction was shaken in a shaker at 150 r/min for 12 h. After reaction for 12 h, we removed it and let it stand. We filtered the supernatant three times and diluted the solution concentration after the reaction to the standard curve range. The concentration after dilution was filtered with a 0.22 μm filter and the filtrate was determined by AA 300 atomic absorption spectrophotometer. Meantime, the influence of multiple factors (pH value, time, initial concentration, and temperature) on the adsorption effect was investigated. The calculation formula of adsorption amounts Q is Formula (1). Then the calculation formula of removal rate R% is Equation (2).
Q = C 0 − C e m × V
R % = C 0 − C e C 0 × 100 %
where C0 (mg/L) is the concentration of reaction before the test, Ce (mg/L) is the concentration after the test, m (g) is the weight of the adsorption material, V (L) is the volume of hexavalent chromium, and Q is the amount at adsorption equilibrium (mg/g).

2.5. Adsorbent Regeneration Experiment

To explore the regeneration performance of HKUST-1/SiO2 composite nanofibers in practical application, 0.02 g composite fiber was placed into 100 mg/L Cr (VI) solution for 24 h, and elution with 0.1 M HCl was used to evaluate the recycling performance of the composite fiber. The composite fiber was dried at room temperature for 24 h, and fibers were again used for adsorption experiments. Then the composite fiber was recycled five times to test the removal effect of the composite fiber on hexavalent chromium.

2.6. Real Sample Analysis

The actual water sample was taken from Ge Lake, Changzhou city, Jiangsu Province, China, in Spring 2022. The sample was filtered with a 0.22 μm filter membrane to remove impurities before use. Tap water samples were from Mingxing Building, Science and Education Town, Changzhou University, Changzhou city, Jiangsu Province. At 293 K and pH 5.5, different water samples (with an initial concentration of chromium hexavalent of 30 mg/L) were added to the experiment, and then the residual concentration of hexavalent chromium ion was determined after 24 h of composite fibers reaction. The change of hexavalent chromium ion concentration before and after adsorption was detected, and the removal rate of hexavalent chromium ion by composite fiber was calculated according to Formula (2) above.

3. Results and Discussion

3.1. Synthesis and Characterization of HKUST-1/SiO2 Composite Nanofibers

3.1.1. Morphological Characteristics Analysis of Composite Fibers

The structure of HKUST-1, SiO2 nanofibers, M-SiO2 nanofibers, and HKUST-1/SiO2 composite nanofibers can be seen clearly by SEM. As shown in Figure 2a, when the reaction condition was 75 °C, HKUST-1 was an eight-sided cube structure with a sharp shape [11,23], and its diameter was about 10.09 μm ± 0.23 by Image J software test in Figure 2f. However, as the temperature rose, the crystals of Hkust-1 became irregular in shape, with a globular crystal structure and a small crystal size. It was found that this might be caused by excessive reactants in the solution for the synthesis of HKUST-1. As shown in Figure 2b, SiO2 nanofibers were scattered and uniform in thickness, with a diameter of about 0.67 μm ± 0.22 by Image J software test in Figure 2g, without fracture and adhesion. Meantime, TEOS in the synthetic material has amorphous properties, resulting in a smooth surface of SiO2 nanofibers. Furthermore, SiO2 fibers were randomly oriented and belonged to typical electrostatic spinning fibers. After modification with APTES, the diameter range of M-SiO2 fiber was significantly increased to about 1~3 μm in Figure 2c. The diameter of M-SiO2 fiber was about 2.44 μm ± 0.05, calculated by the software in Figure 2h. This was mainly attributed to the hydrolysis of 3-aminopropyl triethoxylsilane to form 3-aminopropyl triethoxylsilanol, and the polycondensation reaction to form a silica gel system under the action of acid. The exposed hydroxyl groups in the system organically combine with the siloxane gel itself and the hydroxyl complex on the surface of SiO2 fibers to form aminated silica gel composite film. Finally, Figure 2d showed that HKUST-1 particles were grown on the surface of aminoated SiO2 nanofibers by in situ anchoring growth, resulting in the fiber surface changing from smooth to rough. The diameter of HKUST-1/SiO2 fiber was about 4.27 μm ± 0.34, calculated by the software in Figure 2i. We also photographed and recorded the fiber state at different stages during the experiment (Supporting Information, Figure S1).
The principle of the above reaction was mainly attributed to Cu2+ fixed on the surface of M-SiO2 fibers by co-precipitation, and then HKUST-1/SiO2 composite fiber was grown by embedding HKUST-1 in the pore of M-SiO2 fiber by hydrothermal method. In addition, HKUST/M-SiO2 nanofibers were arranged in a chaotic manner and were highly intertwined with each other, forming a three-dimensional open non-woven fiber membrane. The formation of this structure enables the fiber membrane to have a high flux, which is conducive to rapid air circulation and selective adsorption of hexavalent chromium ions. Therefore, HKUST-1 was successfully embedded and grown in many pores of M-SiO2 nanofibers. At present, the HKUST/M-SiO2 nanofibers are of 0.1434 mL/g pore volume, 17.4923 nm pore size, and 32.799 m2/g surface area (Figure S2 and Table S1).
On the other hand, in order to further characterize the structural characteristics of composite fibers, element distribution tests were carried out. Firstly, Figure 2e shows the elemental mapping for HKUST-1/SiO2 composite nanofibers. We found that the elements in HKUST-1/SiO2 composite nanofibers were O (47%), Cu (25%), Si (19%), and N (7%), and these elements were evenly distributed on the HKUST-1/SiO2 composite nanofibers. Then, Figure 3a shows the element content of the three samples based on the element mapping. The experimental results revealed that the increased content of N (5%) indicated that the SiO2 nanofibers were successfully functionalized by amino groups. In addition, the contents of O and Cu in HKUST-1/M-SiO2 composite nanofibers increased while the contents of Si decreased, indicating that a large amount of HKUST-1 nanomaterials were absorbed in the composite nanofibers. Besides, to further determine the distribution of elements in HKUST-1, SiO2, and M-SiO2, the EDS element surface scanning was arranged (Supporting Information, Figure S3). In conclusion, the embedded growth of HKUST-1 in the pores of aminoacylated SiO2 nanofibers greatly increases the active site of nanofibers, improves the dispersion of HKUST-1, improves the dispersion of HKUST-1, and thus effectively adsorbs hexavalent chromium ions.

3.1.2. Analysis of Crystal Structure Characteristics of Composite Fibers

XRD was used to determine the crystal structure of the samples. Figure 3b shows the XRD patterns of SiO2 nanofibers, M-SiO2 nanofibers, HKUST-1, and HKUST-1/SiO2 composite nanofibers. Firstly, Figure 3b shows that the diffraction peak of the sample was sharp, and the crystallinity was good, indicating the high purity of the synthetic product. Then, the SiO2 fiber had a wide and weak diffraction peak at 20°~30°, indicating that the structure of SiO2 was amorphous. In addition, the diffraction peak of M-SiO2 at 20°~30° was wider, and the diffraction peak at 22.2° was the strongest. At the same time, it was found that the relative intensity of the diffraction peak on the SiO2 crystal plane modified by APTES increased, and the corresponding peak shape was sharper, indicating that the modified SiO2 had a more complete crystal shape, which was consistent with the literature reports [24]. In addition, the main characteristic peaks of HKUST-1 were 6.66°, 9.46°, 11.6°, 13.40°, 17.46°, 19.02° and 25.96° which were basically consistent with the spectra recorded previously [25,26]. Because of this, HKUST-1 didn’t produce other copper compound impurities. Besides, the diffraction peak position of the composite nanofibers basically corresponded with that of HKUST-1, which proved that HKUST-1 had been attached to the surface of SiO2 nanofibers. However, the diffraction peak intensity of the HKUST-1/SiO2 composite nanofibers was lower than that of HKUST-1, which was mainly caused by the dehydration of the composite nanofibers. For the same mass, HKUST-1 absorbed more water than composite nanofibers. Besides, XRD patterns of the SiO2 nanofibers and the M-SiO2 nanofibers showed wide peaks, indicating that SiO2 and M-SiO2 nanofibers were amorphous.

3.1.3. Thermal Stability Analysis of Composite Fibers

TGA tests were conducted on the different stages of fiber preparation to study the thermal stability of HKUST-1/SiO2 nanofibers, and the TG curve is shown in Figure 3c. At the initial weightlessness stage (42.5 °C to 59.13 °C), the weight loss rate of HKUST-1 was about 12.61% because of the absorption of water and the elimination of free carboxyl groups, while the weight loss rate of the composite nanofibers was only 5.95%. At 150 °C to 350 °C, the weight of SiO2 nanofibers changes little, and the HKUST-1 decreases sharply. The weight loss was mainly due to the decomposition of HKUST-1 and the release of Cu-bound molecular water [27,28,29]. The coordination bond between Cu and water can only be broken at temperatures between 120 °C and 300 °C. When the temperature reaches 320 °C, the coordination bonds between Cu begin to break [30]. At 350 °C at the end of weight loss, the composite nanofibers gradually reduced their weight and tended to be stable. The chemical composition after 400 °C includes SiO2, Cu, Cu2O, and CuO. The irregular weight loss of the composite nanofibers in this study may be caused by the impurity of the composite film material formed by the M-SiO2 nanofibers attached to HKUST-1.

3.1.4. Structural Analysis of Functional Groups of Composite Fibers

Characterization of functional groups of nanofiber preparation by infrared spectroscopy is of significance to reveal the adsorption behavior of chromium hexavalent and verify whether the HKUST-1/SiO2 composite nanofibers are successfully constructed. Firstly, the infrared spectrum of SiO2 nanofibers showed the characteristic band of silicon-oxygen in Figure 3d, Si-O-Si asymmetric stretching vibration appeared at 1047 cm−1, Si-O-Si asymmetric stretching vibration peak appears at 1047 cm−1 (1233 cm−1), Si-OH stretching vibration characteristic band appears at 968 cm−1. The bending vibration peak of -Si-OH was 803 cm−1, and the characteristic peak of δ Si-O-Si was 479 cm−1. Generally speaking, one Si-OH of APTES reacts with the hydroxyl group on the surface of SiO2 nanofibers to form a covalent bond, and the other two Si-OH react or condensation with Si-OH [31]. Other APTES molecules may be in a free state. A new adsorption film was attached to the surface near 3414 cm−1 to attenuate the vibration of OH bands. In addition, SiO2-APTES showed -NH2 near 1581 cm−1 and -CH2 peak near 2924 cm−1, indicating that APTES with nitrogen functional groups and methylene have been successfully grafted onto the surface of SiO2 nanofibers. Besides, it can be clearly seen from Figure 3d that there were five specific absorption bands in the spectrum of HKUST-1. The characteristic peak of HKUST-1 at 730 cm−1 was Cu-O tensile vibration [32]. The bands at 1374 cm−1, 1448 cm−1, and 1643 cm−1 were attributed to the stretching vibration of the C-O group, C-C aromatic group, and C-O group of H3BTC [33]. The wide peak formed around 3448 cm−1 indicated the presence of -OH group vibrations, namely the presence of water molecules in HKUST-1. The band of the HKUST-1/SiO2 composite nanofibers in both 1047 cm−1 and 1101 cm−1 can be attributed to the Si-O bond of the composite nanofibers. For the HKUST-1/SiO2 composite nanofibers, the FTIR absorption band characteristics of SiO2 and HKUST-1 can be clearly observed, further indicating that HKUST-1 was successfully loaded on the surface of M-SiO2 nanofibers.
In addition, HKUST-1 has excellent Cr (VI) ion capturability but poor water stability. The water stability experiment of HKUST-1 was carried out and compared with composite fiber. HKUST-1 was first dispersed in deionized water (1000 mg L−1) for six h. The mixture was then removed from the suspension and filtered through a 0.22 μm syringe filter. The filtered solution was subjected to AAS analysis to determine the release of copper ions from HKUST-1 [34,35,36,37]. The copper release of HKUST-1 and HKUST/SiO2 detected by AAS was 14.24 mg/L and 7.735 mg/L, respectively. In other words, the amount of copper ions released from HKUST-1 is twice as much as that released from HKUST-1/SiO2. The experimental results show that the structure of HKUST is damaged in the two materials, but the structure of HKUST-1 is more seriously damaged, while the structure of HKUST-1 is relatively intact after fixation, which indicates that the structure of HKUST-1 is protected by fixing it on the surface of SiO2 fiber.

3.2. Adsorption Performance of Composite Fibers

3.2.1. Effect of pH on Cr (VI) Adsorption

The influence of SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers on the adsorption amount of Cr (VI) was studied at a pH value of 3.0~8.0 in Figure 4a. The adsorption quantity of HKUST-1/SiO2 composite nanofibers decreased with the increase in pH value, and the maximum value was 38.76 mg/g at pH = 3. In this period, the adsorption mechanism of Cr (VI) was caused by electrostatic interaction. For one thing, pH value affected the protonation of the composite nanofibers [38]. When the pH was 3~7, the main metal element of HKUST-1 was Cu, which was easily protonated to Cu2+, while the main forms of Cr(VI) were HCrO4− and Cr2O72− [39]. Electrostatic attraction existed between Cu2+ and Cr (VI), which led to an increase in the adsorption quantity of the composite nanofibers for Cr (VI). Besides, the acidic solution contains more H+ than the alkaline solution, which was more likely to have an electrostatic attraction with Cr (VI) in the system, thus increasing the adsorption capacity of the material. When pH was 7~8, the amount of OH− in the solution increased, and Cr (VI) was reflected in the form of CrO4−. Meantime, OH− competed with CrO4− for Cu2+, thus affecting the adsorption amount of the HKUST-1/SiO2 nanofibers for Cr (VI). However, the unmodified SiO2 fiber only maintained a very small adsorption quantity of 28 mg/g at pH 3. In controlled experiments, the adsorption quantity of SiO2 fibers was much lower than that of HKUST-1/SiO2 composite nanofibers sorbent, confirming that the efficient removal of Cr(VI) by HKUST-1/SiO2 nanofibers was attributed to immobilized HKUST.

3.2.2. Effect of Contact Time on Cr (VI) Adsorption

One of the key parameters for evaluating the efficiency and feasibility of the HKUST-1/SiO2 nanofibers in controlling water pollution is the reaction time. Figure 4b displayed the influence of time on the chromium adsorption amount of nanofibers. The experimental results confirmed that the reaction process consists of two time phases. The first period is the initial reaction period (0~60 min), when the active site of the nanofiber is completely exposed and binds to Cr (VI), resulting in a rapid increase in the adsorption amount of Cr(VI). The adsorption trend of the two adsorbents for hexavalent chromium is consistent. That is to say, the adsorption capacity of the two adsorbents increased sharply within one h, indicating that there was a rapid interaction between Cr (VI) and the surface-active sites of the nanofiber. Significantly, the action time of SiO2 nanofibers was balanced at about 30 min, and the adsorption capacity gradually stabilized at about 24 mg/g. The adsorption amount of HKUST-1/SiO2 composite nanofibers was gradually stabilized at about 35 mg/g after about 45 min. At this time, after the adsorption gradually approached the equilibrium state, the active sites on the surface of the nanofiber were occupied by Cr (VI) and reached saturation. In the second period (60~120 min), the surface of the nanofiber was completely occupied by Cr(VI), and no more active sites could be given, resulting in no increase in the adsorption amount of the nanofiber, and the reaction reached equilibrium after 120 min. The above data confirmed that the equilibrium time and adsorption amount of HKUST-1/SiO2 composite nanofibers was obviously better than that of SiO2 nanofibers, which was mainly because of the fact that a large amount of HKUST-1 with a high specific surface area was attached to the composite nanofibers, and the hexavalent chromium ion diffuses to the surface of the composite nanofibers first and then reached the interior of the HKUST-1/SiO2 composite nanofibers.

3.2.3. Effect of Temperature on Cr (VI) Adsorption

The influence of temperature on the activity of Cr (VI) and adsorption materials cannot be ignored. Therefore, the influence of the HKUST-1/SiO2 composite nanofibers and SiO2 nanofibers on the removal amount of hexavalent chromium at multiple temperatures was investigated in this paper. Figure 4c displayed that temperature increase promoted the adsorption amount of hexavalent chromium by nanofibers. Among them, when the solution temperature was 55 °C, the adsorption effect was the best, and the adsorption amount of the composite nanofibers reached the maximum (44.77 mg/g). This is because the activity on HKUST-1/SiO2 composite nanofibers increases with the increasing temperature, which promotes intermolecular movement and increases the adsorption amounts of Cr (VI). However, when the adsorption site was saturated, hexavalent chromium ions could not find a new adsorption site. At this time, the adsorption amount of the HKUST-1/SiO2 composite nanofibers to hexavalent chromium ions reached the maximum. The reason for this is that the temperature increases the particle movement and increases the collision frequency, which enables the nanofiber to react fully with hexavalent chromium. The results also displayed that the adsorption reaction of HKUST-1/SiO2 composite nanofibers belongs to an endothermic reaction.

3.2.4. Effect of the Initial Concentration on Cr (VI) Adsorption

The initial concentration of hexavalent chromium plays a decisive role in the adsorption effect of nanofibers. Figure 4d exhibited that the adsorption amount of the SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers for hexavalent chromium was affected by the initial concentration of Cr (VI) and increased with the increase of the initial concentration. Moreover, when the solution concentration was 120 mg/L, the adsorption amount of the composite nanofiber was 35.30 mg/g. Especially in the low concentration range of 0~80 mg/L, composite nanofibers contained a large number of functional groups and unsaturated adsorption sites, which could quickly adsorb hexavalent chromium ions [40]. However, because there is a limited content of hexavalent chromium ions in a low-concentration solution, the adsorption capacity was not large. At a concentration greater than 120 mg/L, the increase of the concentration of hexavalent chromium ions caused the ions in the liquid to transfer to the solid of the HKUST-1/SiO2 composite nanofibers, resulting in mass transfer effect. Therefore, the results confirmed that the adsorption property of HKUST-1 was combined with the loading capacity of SiO2 nanofibers, resulting in good adsorption capacity of the HKUST-1/SiO2 composite nanofibers.

3.2.5. Adsorption Kinetics of Cr (VI)

To assess the adsorption process, we described the adsorption kinetics of Cr (VI) on the SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers with the help of quasi-first-order and quasi-second-order kinetic models. The formulas were shown in Equations (3) and (4) [28].
L n ( Q e − Q t ) = L n Q e − K 1 t
t / Q t = 1 / ( K 2 × Q e 2 ) + t / Q e
where Qe (mg/g) represents the adsorption amount of nanofiber at adsorption equilibrium; Qt (mg/g) is the adsorption capacity of nanofiber at time t; In addition, K1 (1/min) in the formula is pseudo-first-order adsorption rate constant; K2 (g/(mg/min)) in the formula is the adsorption rate constant for pseudo-second-order.
As shown in Figure 5a,b and Table S2, the R2 values of the pseudo-second-order kinetic models were better than that of the pseudo-first-order kinetic models of the two adsorbents. The results revealed that the reaction process of SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers to hexavalent chromium conformed to the description of quasi-second-order kinetics, and R2 (the linear correlation coefficients) were 0.986 and 0.975, respectively. According to Table 1, when the adsorption values calculated by the second-order kinetics are compared with those calculated by the first-order kinetics, the fitted data of the second-order kinetics were near to the real experimental data. The above data analysis illustrated that the reaction process of composite nanofibers to hexavalent chromium was consistent with the quasi-second-order kinetic model. It was also further shown that the reaction process of composite nanofibers was controlled by chemisorption and hexavalent chromium shares or exchanges electrons with HKUST-1/SiO2 composite nanofibers.

3.2.6. Adsorption Isothermal of Cr (VI)

Adsorption isotherm is used to represent the relationship between the amount of the nanofibers and the remaining amount of the nanofibers to describe the reaction process. The construction of the adsorption isothermal model can intuitively judge the adsorption model, which is very helpful to the judgment of the experimental scheme and the reaction type. The adsorption isothermal model constructed in this study included Langmuir adsorption isothermal model (Equation (5)) and Freundlich adsorption isothermal model (Equation (6)).
Q e = K L Q m C e 1 + K L C e
L n Q e = 1 n L n C e + L n K F
where Ce and Qe are the equilibrium adsorption concentration (mg/L) and equilibrium adsorption capacity (mg/g) of hexavalent chromium, respectively; Qm (mg/g) is the maximum adsorption amount of nanofibers; KL (L/mg) is the constant of Langmuir model; KF [(mg/g) (mg/L)−n] is the constant of Freundlich model; 1/n is the adsorption strength, without dimension.
The isothermal adsorption line fitting model at 298K was shown in Figure 5c,d and Table S3 summarized the relevant parameters of the adsorption isotherm model for SiO2 and HKUST-1/SiO2 nanofibers. As shown in Figure 5c,d, under the same conditions, the adsorption amount of SiO2 nanofibers was significantly lower than that of HKUST 1/SiO2 nanofibers. The results indicated that HKUST-1/SiO2 nanofibers have better adsorption capacity for hexavalent chromium. At the same time, the R2 of the Langmuir model was closest to 1(R2 of SiO2 was 0.965; R2 of HKUST-1/SiO2 was 0.938), which was higher than the Freundlich model (R2 of SiO2 was 0.929; R2 of HKUST-1/SiO2 was 0.905). It was proved that the adsorption process of hexavalent chromium on the nanofibers was in line with the Langmuir model, confirming that the adsorption sites of nanofibers are evenly distributed, which was monomolecular adsorption and chemical adsorption.
Meanwhile, the Freundlich model was used to calculate the value of parameter 1/n to determine the adsorption strength of SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers. Among them, when 0.1 < 1/n < 1, the reaction is favorable for adsorption, while when 1/n > 2, the reaction is unfavorable for adsorption. In this paper, the 1/n values of SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers were 0.597 and 0.596, respectively. Their 1/n values were 0.1 < 1/n < 1, which proved that these two fibers have adsorption capacity for hexavalent chromium.
In addition, the constant KL equation in the Langmuir model was used to obtain the dimensionless constant separation factor (RL), and the adsorption affinity of SiO2 nanofibers and HUST-1/SiO2 composite nanofibers for hexavalent chromium was further studied. Then, RL can be obtained from the constant KL Equation (7).
RL = 1/(1 + KLC0)
where C0 (mg/L) is the concentration of hexavalent chromium ion before the reaction; KL (L/mg) is the constant of Langmuir adsorption isothermal model; RL in different intervals represents different adsorption types, including unfavorable (RL >1), linear (RL = 1) and favorable (0 < RL < 1), or irreversible (RL = 0). Figure S4. (a) displayed that RL values of SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers revealed that the two nanofibers have a good adsorption effect on all concentrations of hexavalent chromium ions at 293K. The RL values of SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers were 0.4068~0.8605 and 0.4425~0.8772, respectively, indicating that the adsorption of hexavalent chromium ions on SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers were a process of favorable reaction. In addition, the RL value of HKUST-1/SiO2 composite nanofibers was higher than that of SiO2 nanofibers, which indicated that the affinity of HKUST-1/SiO2 composite fibers for hexavalent chromium was enhanced after the successful loading of HKUST-1, which was consistent with the results of adsorption experiment. Therefore, the results confirmed that the data were highly consistent with the Langmuir isotherm model, revealing that the SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers have uniform reaction sites.

3.2.7. Adsorption Thermodynamics of Cr (VI)

The model of adsorption thermodynamics can intuitively understand the energy changes of the whole system during the adsorption process. It can be clearly understood that the heat absorption and heat release in the reaction process judge whether the reaction can be carried out spontaneously and the influence of different temperature conditions on the adsorption. The three main thermodynamic parameters, Gibbs free energy change (ΔG), standard enthalpy (ΔH), and standard entropy (ΔS), were obtained from Equations (8)–(10) [41].
L n K d = Δ S / R − Δ H / R T
K d = Q e / C e
Δ G = − R T L n K d
where firstly, Qe (mg/g) is the adsorption amount of solution adsorption equilibrium, and Ce (mg/L) is the adsorption equilibrium concentration at this temperature. Secondly, Kd is the partition coefficient of the adsorbent, and R is the gas constant in 8.314 J/mol·K; Finally, T (K) is the absolute temperature of the reaction; ΔH (KJ/mol) is the enthalpy change of reaction; ΔS (KJ/(mol·K)) is Gibbs free energy in adsorption.
As shown in Figure S4. (b), Ln (Kd) has a linear relationship with 1/T. Δ S and Δ H can be calculated using the equation of this line. In addition, thermodynamic parameters of adsorption of hexavalent chromium ions by the SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers at different temperature ranges (298~328 K), as shown in Table S4.
At 298, 308, 318, and 328 K temperatures, the ΔG of SiO2 nanofibers adsorption reaction to chromium hexavalent were −2.707, −2.994, −3.363, −3.605 KJ/mol, respectively. The ΔG of HKUST-1/SiO2 composite nanofibers for the adsorption of chromium hexavalent at 298, 308, 318, and 328 K were −3.687, −4.188, −4.371 and −4.868 KJ/mol, respectively. It showed that the SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers have spontaneous adsorption. And |ΔG| value was proportional to the temperature change, suggesting that the temperature increase was helpful to the active adsorption of hexavalent chromium. Then, ΔH of the SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers were 6.43 and 7.39 KJ/mol, respectively. Their ΔH > 0 indicated that the reaction between hexavalent chromium ions and nanofibers was endothermic, which corresponded to the result of the temperature adsorption reaction. More importantly, ΔS of the SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers were 0.03669 and 0.16718 KJ/(mol·K), respectively, and ΔS > 0 indicated a high degree of disorder in the adsorption process, which was helpful for the active adsorption reaction. The data displayed that HKUST-1/SiO2 composite nanofibers have an excellent thermal effect on hexavalent chromium ions. The reported adsorbents have the same endothermic properties and enthalpy change for chromium hexavalent adsorption.

3.3. Mechanism of Adsorption of Cr (VI)

To investigate the adsorption nature of HKUST-1/SiO2 composite nanofibers for Cr (VI), the XPS of HKUST-1/SiO2 composite nanofibers before and after the reaction was shown in Figure 6a~g. Firstly, the survey spectrum of HKUST-1/SiO2 after the reaction indicated that the sample was composed of Cu, Cr, O, N, C, and Si in Figure 6a. The results showed that a new characteristic peak (Cr 2p: 571.15, 575.65 eV) appeared after the adsorption of Cr on the HKUST-1/SiO2, which proved that Cr (VI) was successfully adsorbed on the surface of HKUST-1/SiO2 nanofibers. The data demonstrated that the growth of HKUST-1 nanoparticles embedded in the HKUST-1/SiO2 nanofibers was very effective.
Meanwhile, the deconvolution of typical signal peaks such as Cu 2p, Cr 2p, N 1s, C 1s, Si 2p and O 1s was analyzed in detail into different characteristic peaks, as shown in Figure 6. First of all, Figure 6b was the XPS spectra of Cr 2p, 571.5, and 575.65 eV were assigned to Cr(III). In other words, toxic Cr(VI) was reduced to non-toxic Cr(III), and redox reactions occurred during the purification and separation of Cr(VI) [42,43]. Then, the Si 2p spectrum of the HKUST-1/SiO2 nanofibers before and after the reaction was depicted in Figure 6c. The two peaks have little difference, at 102.93 and 103.13 eV. They all belonged to SiO2 nanofibers. In addition, the fitting map of C 1s in Figure 6d displayed that C-C (284.36 eV) was the main carbon functional group, and a small amount of C-N (286.06 eV) and C=O (288.14 eV) functional groups were also present [27].
Next, for the N 1s spectra in Figure 6e, three peaks were located at 399.63 eV, 401.72 eV, and 406.33 eV before absorption, which were ascribed to N-Si, -NH2, and -NO3, respectively [44]. However, only the peaks at 399.66 eV and 401.83 eV remain after adsorption. The results showed that -NO3 participated in the reaction, and -NO3 was replaced by the oxygen anion in Cr (VI), which resulted in ion exchange [21,45]. In addition, the peak intensity of amino acid is weakened because the positively charged amino acid neutralizes the negative charge of the Cr ion, and the proton on the adsorbent surface will be released into the solution during the reduction process [46].
Moreover, Figure 6f presented the O 1s spectrum of HKUST-1/SiO2 nanofibers. Before adsorption, the peaks of O 1s can be observed at 531.93 eV, and it was geared to the C=O bonds [47]. The C=O bond released some energy during the reaction. There were two peaks of O 1s after adsorption, namely 531.23 eV (C=O) and 532.41 eV (C-O).
Finally, comparing the spectrum of Cu 2p before and after adsorption in Figure 6g, it can be found that a new peak appears in 935.13 eV. It might be ascribed to Cu(OH)2. Then, two characteristic peaks closely related to Cu 2p1/2 and Cu 2p3/2 were found at 954.28 eV (954.20 eV) and 934.26 eV (934.30 eV), respectively. These also indicated that Cu in HKUST-1/SiO2 composite nanofibers mainly existed in bivalent form, and peaks located at 932.44 eV and 932.56 eV, which were ascribed to Cu-O. In addition, it also contains other shaking peaks, which belong to Cu(OH)2, mainly due to air exposure [30].

3.4. Regeneration Property Analysis of Composite Fiber

Among them, the regeneration performance is the key factor affecting the engineering application of HKUST-1/SiO2 composite nanofibers. Therefore, this paper evaluated the regeneration performance of HKUST-1/SiO2 composite nanofibers through five cycles. As shown in Figure 6h, the removal rate of Cr (VI) from HKUST-1/SiO2 composite nanofibers did not change significantly after five cycles. The maximum adsorption capacity of the HKUST-1/SiO2 composite nanofibers to hexavalent chromium ions after the fifth cycle was 88.32 ± 4.40%. The experimental results showed that the HKUST-1/SiO2 composite nanofibers were an efficient adsorbent for membrane separation that can be used continuously in batches.

3.5. Application Analysis of Composite Fiber in Real Samples

The feasibility of removing hexavalent chromium with composite fiber membrane was investigated by using deionized water, tap water of the College of Environmental Science and Engineering, Changzhou University, and Ge Lake Combined in Changzhou city, Jiangsu Province, as the research objects. The results showed that the composite fiber could effectively remove hexavalent chromium in the three kinds of water quality. In Figure S5, when the hexavalent chromium was 30 mg/L, the hexavalent chromium in lake water by HKUST-1/SiO2 nanofibers could be removed 81.34 ± 8.32%, the hexavalent chromium in tap water by HKUST-1/SiO2 nanofibers can be removed 89.96 ± 6.99%, and the hexavalent chromium in deionized water by HKUST-1/SiO2 composite nanofibers can be removed 95.20 ± 5.12%. Although the removal rate of hexavalent chromium in actual lake water and tap water was slightly lower than that in DI water, it still showed a good effect. In addition, pollutants in the lake can bind to the HKUST-1/SiO2 composite nanofibers, which was another reason for the reduced removal capacity. These results still indicated that the HKUST-1/SiO2 composite nanofibers were promising adsorbents for the selective removal of heavy metal contaminants in real water samples.

3.6. Comparative Analysis of Composite Fiber and Other Adsorbents

The adsorption amounts of HKUST-1/SiO2 composite nanofibers for Cr (VI) ions were compared with other published Cr (VI) adsorbents in Table 1 [11,21,48,49,50,51,52,53,54,55,56] it can be found that the maximum adsorption amount of HKUST-1/SiO2 nanofibers was better than that of FCA/SiO2 membranes, PAN-CeO2 NP NFs, UiO-66-NH2, ZIF-8@CA, Cu-BTC, nylon 6,6/graphene oxide nanofiber and fibrillar magnetic carbon. This was because of the large contact area and binding site of amino-functionalized silica electrospun nanofibers, chelating copper ions, and then using hydrothermal synthesis to make a large amount of HKUST-1 grow on the surface of silica nanofibers, which effectively improves the ability of HKUST/SiO2 composite nanofibers to remove hexavalent chromium. In addition, some unreacted functional groups (-COOH, -OH, and -NH2) in the HUST-1/SiO2 nanofibers can also adsorb hexavalent chromium ions. The maximum adsorption capacity of HUST-1/SiO2 composite nanofibers was lower than that of pristine HKUST-1 and keratin/PET nanofiber. However, it has many advantages worth appreciating. On the one hand, pristine MOFs materials are easy to cause secondary pollution because their powder form is difficult to recycle. HKUST-1/SiO2 composite nanofibers were easier to separate MOFs loaded with adsorbents from wastewater. On the other hand, the preparation method of HUST-1/SiO2 composite nanofibers was relatively simple, and the raw material price was relatively economical. As a new composite material for chromium removal from aqueous solution, it has great potential. It is obvious that the adsorption advantage of HKUST-1/SiO2 composite nanofibers was slightly better than other similar composite materials.

4. Conclusions

In this study, a modified adsorbent of HKUST-1/SiO2 composite fiber was prepared through embedded growth of metal-organic skeleton HKUST-1 on the surface of amino-modified silica electrospun nanofibers to remove Cr (VI). The adsorption capacity of HUST-1/SiO2 nanofibers was compared with that of unmodified SiO2 nanofibers. The results suggested that HKUST-1/SiO2 composite nanofibers had a better adsorption effect under the condition of acidic (pH = 5.5) and 120 mg/L. The adsorption reached balance for about 45 min, and the maximum adsorption amounts were stable at 60.82 mg/g. The adsorption of Cr (VI) in the composite nanofibers followed the pseudo-second-order kinetic model and the Langmuir model. According to the influence of pH on adsorption performance and XPS analysis, the essence of Cr (VI) removal included electrostatic attraction, ion exchange, and reduction of hexavalent chromium. The preparation process of HKUST-1/SiO2 nanofibers was simple and had the advantages of a good adsorption effect and easy separation and recovery, which solved the problem of easy aggregation of nanomaterials. It is very important that the composite fiber can also be used for the extraction and recovery of chromium resources. In other words, the HKUST-1/SiO2 NFs captured chromium can be recovered by simple calcination in the later stage, which not only avoids the consumption of absorbed liquid and simplifies the recovery steps but also is conducive to energy saving and emission reduction. Therefore, HKUST-1/SiO2 NFs have a high potential for engineering application in the field of hexavalent chromium wastewater purification and resource recovery.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su142113780/s1, Figure S1. The photos of samples: SiO2 nanofibers (a), M-SiO2 nanofibers (b), and HKUST-1/SiO2 composite nanofibers (c). Figure S2 shows: (a) N2 adsorption-desorption isotherms of the products; (b) BJH pore size distribution of the products; Figure S3. (a) HKUST-1, (b) SiO2 nanofibers, and (c) M-SiO2 nanofibers EDS elemental mapping; Figure S4. (a) Distribution coefficient (RL) of SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers; (b) Adsorption thermodynamics of Cr (VI) by SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers; Figure S5. The removal rate of hexavalent chromium by HKUST-1/SiO2 composite nanofibers in real samples (C0: 30 mg/L; m: 0.01 g; T: 298 K; V: 50 mL; t: 24 h); Table S1. The specific areas, pore size distribution, and pore volumes of the materials; Table S2. Related parameters reflecting the sorption kinetics calculated using the pseudo-first-order and the pseudo-second-order kinetic models; Table S3. The parameters of Langmuir and Freundlich model; Table S4. The adsorption thermodynamic parameters.

Author Contributions

Conceptualize, S.F. (Shanshan Feng); writing—original draft preparation, S.F. (Shanshan Feng), J.N.; Writing—review & editing, S.L.; Formal analysis, X.C.; investigation, J.G.; Methodology, W.Z.; Data curation, J.N., F.C.; Data curation, R.H. and J.N.; Funding acquisition, Y.Z.;. Supervision, S.F. (Sheng Feng). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Changzhou University Students’ Extracurricular Innovation and Entrepreneurship Fund, the Natural Science Fund for Colleges and Universities in Jiangsu Province, The Science and Technology Project of Changzhou City, Tianshan Innovation Team Project of Xinjiang Uygur Autonomous Region, the Natural Science Foundation of China and National Key Research and Development Program, grant number ZMF21020079, 18KJB610001, BK20180964, CJ20210119, 2021D14013, 22075032, 2021YFC3001104 and 2021YFC3001104, respectively.

Data Availability Statement

Valid data from the experiments involved in this study are listed in this paper and in the supplementary materials, and the original data are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Preparation steps of HKUST-1/SiO2 composite nanofibers and flow chart of Cr (VI) removal.
Figure 1. Preparation steps of HKUST-1/SiO2 composite nanofibers and flow chart of Cr (VI) removal.
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Figure 2. SEM images of the nanofibers at different stages: (a) HKUST-1, (b) SiO2 nanofibers; (c) M-SiO2 nanofibers, (d) HKUST-1/SiO2 composite nanofibers; (e) The elemental mapping of HKUST-1/SiO2 composite nanofibers, the scar bar is 10 μm; The diameter distribution histograms of HKUST-1(f), SiO2 nanofibers (g), M-SiO2 nanofibers (h) and HKUST-1/SiO2 composite nanofibers (i).
Figure 2. SEM images of the nanofibers at different stages: (a) HKUST-1, (b) SiO2 nanofibers; (c) M-SiO2 nanofibers, (d) HKUST-1/SiO2 composite nanofibers; (e) The elemental mapping of HKUST-1/SiO2 composite nanofibers, the scar bar is 10 μm; The diameter distribution histograms of HKUST-1(f), SiO2 nanofibers (g), M-SiO2 nanofibers (h) and HKUST-1/SiO2 composite nanofibers (i).
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Figure 3. (a) Elemental content of the samples; (b) XRD patterns of the nanofibers; (c) TGA curves of the nanofibers; (d) FT-IR of the nanofibers.
Figure 3. (a) Elemental content of the samples; (b) XRD patterns of the nanofibers; (c) TGA curves of the nanofibers; (d) FT-IR of the nanofibers.
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Figure 4. (a) Effect of pH on Cr(VI) adsorption by SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers (m: 0.01 g, V: 50 mL, C0: 120 mg/L, T: 298 K, t: 6 h); (b) Effect of time on Cr(VI) adsorption by SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers (m: 0.01 g, V: 50 mL, C0: 120 mg/L, pH: 5.5, T: 298 K); (c) Effect of contact temperature on Cr(VI) adsorption by SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers (m: 0.01 g, V: 50 mL, C0:120 mg/L, pH: 5.5, t: 6 h); (d) Effect of initial Cr(VI) concentration on Cr(VI) adsorption by SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers (m: 0.01 g, V: 50 mL, pH: 5.5, T: 298 K, t: 6 h).
Figure 4. (a) Effect of pH on Cr(VI) adsorption by SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers (m: 0.01 g, V: 50 mL, C0: 120 mg/L, T: 298 K, t: 6 h); (b) Effect of time on Cr(VI) adsorption by SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers (m: 0.01 g, V: 50 mL, C0: 120 mg/L, pH: 5.5, T: 298 K); (c) Effect of contact temperature on Cr(VI) adsorption by SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers (m: 0.01 g, V: 50 mL, C0:120 mg/L, pH: 5.5, t: 6 h); (d) Effect of initial Cr(VI) concentration on Cr(VI) adsorption by SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers (m: 0.01 g, V: 50 mL, pH: 5.5, T: 298 K, t: 6 h).
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Figure 5. (a) Pseudo-first-order kinetic of Cr(VI) on SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers; (b) Pseudo-second-order kinetics of Cr(VI) on SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers; Langmuir isotherm and Freundlich isotherm of (c) SiO2 nanofibers and (d) HKUST-1/SiO2 composite nanofibers.
Figure 5. (a) Pseudo-first-order kinetic of Cr(VI) on SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers; (b) Pseudo-second-order kinetics of Cr(VI) on SiO2 nanofibers and HKUST-1/SiO2 composite nanofibers; Langmuir isotherm and Freundlich isotherm of (c) SiO2 nanofibers and (d) HKUST-1/SiO2 composite nanofibers.
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Figure 6. XPS spectra of wide scan of HKUST-1/SiO2 composite nanofibers before and after Cr(VI) adsorption: (a) Full spectrum; (b) Cr(VI) 2p XPS spectra; (c) Si 2p XPS spectra; (d) C 1s XPS spectra; (e) N 1s XPS spectra; (f) O 1s XPS spectra; (g) Cu 2p XPS spectra. (h) Repeatability of Cr (VI) adsorption by HKUST-1/SiO2 composite nanofibers (m: 10 mg, V: 50 mL, C0: 120 mg/L, pH: 5.5, T: 298 K, t: 12 h).
Figure 6. XPS spectra of wide scan of HKUST-1/SiO2 composite nanofibers before and after Cr(VI) adsorption: (a) Full spectrum; (b) Cr(VI) 2p XPS spectra; (c) Si 2p XPS spectra; (d) C 1s XPS spectra; (e) N 1s XPS spectra; (f) O 1s XPS spectra; (g) Cu 2p XPS spectra. (h) Repeatability of Cr (VI) adsorption by HKUST-1/SiO2 composite nanofibers (m: 10 mg, V: 50 mL, C0: 120 mg/L, pH: 5.5, T: 298 K, t: 12 h).
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Table
Table 1. Comparison of the maximum adsorption amount of Cr (VI) adsorbent in recent years.
Table 1. Comparison of the maximum adsorption amount of Cr (VI) adsorbent in recent years.
AbsorbentsQm (mg/g)References
HKUST-1(S0)90.911
HKUST-1(S5)10011
ZIF-8@ZIF-8/PAN39.6820
silver-triazolate MOF37.021
FCA/SiO2 membranes19.4548
PAN-CeO2 NP NFs28.0049
Uio-66-NH232.2650
ZIF-8@CA41.80051
Cu-BTC48.00052
nylon 6,6/graphene oxide nanofiber47.1753
fibrillar magnetic carbon43.1754
keratin/PET nanofiber75.86055
Cu-DPA MOF3.2156
SiO2 nanofibers47.391This work
HKUST-1/SiO2 composite nanofibers62.380This work
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Feng, S.; Ni, J.; Li, S.; Cao, X.; Gao, J.; Zhang, W.; Chen, F.; Huang, R.; Zhang, Y.; Feng, S. Removal of Hexavalent Chromium by Electrospun Silicon Dioxide Nanofibers Embedded with Copper-Based Organic Frameworks. Sustainability 2022, 14, 13780. https://doi.org/10.3390/su142113780

AMA Style

Feng S, Ni J, Li S, Cao X, Gao J, Zhang W, Chen F, Huang R, Zhang Y, Feng S. Removal of Hexavalent Chromium by Electrospun Silicon Dioxide Nanofibers Embedded with Copper-Based Organic Frameworks. Sustainability. 2022; 14(21):13780. https://doi.org/10.3390/su142113780

Chicago/Turabian Style

Feng, Shanshan, Jie Ni, Shouzhu Li, Xun Cao, Jingshuai Gao, Wenyang Zhang, Feng Chen, Rouxue Huang, Yao Zhang, and Sheng Feng. 2022. "Removal of Hexavalent Chromium by Electrospun Silicon Dioxide Nanofibers Embedded with Copper-Based Organic Frameworks" Sustainability 14, no. 21: 13780. https://doi.org/10.3390/su142113780

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