CN109621910B - Preparation method and application of nano-zero-valent iron-metal organic framework core-shell material - Google Patents

Abstract

The invention discloses a preparation method and application of a nano zero-valent iron-metal organic framework core-shell material, wherein the method comprises the following steps: dissolving ferric salt and polyvinylpyrrolidone in an ethanol solution to obtain an oxygen-free mixed solution; dissolving sodium borohydride in deionized water, dropwise adding the sodium borohydride into the oxygen-free mixed solution to obtain a crude NZVI product, washing, separating and collecting, and drying in vacuum to obtain NZVI; (2) dissolving cobalt salt in methanol, adding NZVI, and dispersing to obtain an NZVI/Co mixed solution; weighing organic ligand, dissolving in methanol, adding into NZVI/Co mixed solution, standing to obtain a crude NZVI @ ZIF-67 product, washing, separating and collecting, vacuum drying, and calcining to obtain the nano zero-valent iron-metal organic framework core-shell material. The material prepared by the invention can be applied to the treatment of wastewater containing Cr (VI), and can effectively perform synchronous adsorption and reduction on Cr (VI).

Description

Preparation method and application of nano-zero-valent iron-metal organic framework core-shell material

technical field

The invention belongs to the technical field of material preparation and water treatment, and relates to a preparation method and application of a core-shell material NZVI@ZD, and uses NZVI to enhance the removal of heavy metal Cr(VI) from water pollution by using NZVI@ZD, a metal-organic framework (MOF) derivative material.

Background technique

Hexavalent chromium (Cr(VI) is a highly toxic agent and is considered to be a carcinogen, mutagen and teratogen in biological systems. It is widely used in chrome plating, leather tanning, cooling towers, wood preservation, textiles , pigments and other fields, the concentration in the environment is relatively high, and the concentration in industrial wastewater is 0.5 ~ 270mg L ^-1 . The World Health Organization recommends that the concentration of Cr(VI) in surface water be reduced to 0.05mg L ^-1 . Therefore, in the Cr(VI) must be removed from wastewater before it is discharged into the water environment system. The selection of materials with high removal efficiency for Cr(VI) has become a major scientific problem to be solved in the field of water pollution remediation.

Metal-organic frameworks (MOFs) are a class of porous materials with crystalline structures synthesized by self-assembly of central metal ions or metal clusters with organic ligands in suitable solvents. Due to the combination of organic and inorganic materials, MOFs have unique properties such as large specific surface area, large pore volume, and tailorable chemical structure. These inherent properties make MOFs highly potential for various applications, such as gas adsorption, separation, catalysis, drug release, optics, and sensing. As adsorbents, the high specific surface area and porosity of MOFs facilitate the adsorption and diffusion of pollutants through the framework, making MOFs promising for heavy metal adsorption. In recent years, studies related to the controllable integration of nanoparticles (NPs) into MOFs to enhance material performance by pooling the unique properties of the two components have attracted increasing attention. The core-shell NPs@MOFs structure, as a common form of various NPs/MOFs composite structures, is considered to be one of the most convenient and efficient ways to realize the synergistic effect of the performance of inorganic NPs and multifunctional MOFs. The core-shell NPs@MOFs structure has two significant advantages: (1) The migration and aggregation of NPs cores with small particle size and high surface energy are greatly restricted due to the coating of the MOFs shell, thus preserving the NPs The stability and chemical activity of the core; (2) the respective properties of NPs and MOFs can be effectively combined. The adsorption performance of MOFs also mainly depends on their pore size. However, most of the reported MOFs are microporous (pore size <2 nm), which hinders the diffusion of macromolecules and limits their interactions with the active centers within the MOFs structure.

Nano-zero valent iron (NZVI) has the advantages of large specific surface area, strong reducing ability, strong adsorption performance, and easy separation, and has been used in the removal of Cr(VI). However, NZVI technology has shortcomings such as easy aggregation, easy oxidation and poor fluidity, which hinder the wide application of NZVI technology. Therefore, a suitable carrier material is usually selected to be composited with NZVI, and MOFs have been used as the carrier material for composite with NZVI in recent years. Zhou et al. (J. Chromatogr. A, 1487(2017) 22-29) first merged MOFs and NZVI (Fe@MIL-101(Cr)) in 2017. Li et al. (J. Solid State Chem., 269 (2019) 16-23) used Zn-MOF-74 as a support material to support NZVI (nZVI@Zn-MOF-74). However, both Fe@MIL-101(Cr) and nZVI@Zn-MOF-74 were prepared by co-precipitation method, and NZVI is most likely only attached to the surface of the MOF material, and cannot be stably coated on the composite material through the narrow gap of the MOF. in the material.

Therefore, how to choose MOFs with high adsorption capacity for Cr(VI) as shell material, what technology to use to expand the pores of the shell material, and how to effectively encapsulate NZVI inside the shell material to make the composite material superior and improve its performance The adsorption performance of environmental pollutants, especially heavy metals, is a technical problem worthy of study.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art. In view of the deficiencies existing in the use of MOF or NZVI technology to treat heavy metal polluted water bodies, a metal organic framework (MOF) with nano-zero valent iron (NZVI) as the core is provided. ) preparation method of a novel core-shell material NZVI@ZD with shell derived material and its application to remove Cr(VI)-containing wastewater quickly and efficiently. The invention utilizes carbonization to increase the pore size of the ZIF-67 shell, encapsulates the NZVI in the porous shell, and enhances the effect of NZVI@ZD in removing Cr(VI) in water.

In order to solve the above-mentioned technical problems, the present invention adopts the following technical solutions:

The preparation method of core-shell material NZVI@ZD includes the following steps:

(1) Weigh iron salt and polyvinylpyrrolidone (PVP) and dissolve them in an ethanol solution whose volume fraction is 60%, and continue mechanical stirring under nitrogen conditions to obtain an oxygen-free mixed solution; Weigh sodium borohydride and dissolve it in deionized water , and add it drop by drop into the above-mentioned oxygen-free mixed solution, the whole process maintains nitrogen and mechanical stirring conditions to obtain NZVI crude product, wash the NZVI crude product with absolute ethanol, and vacuum dry after magnet separation and collection to obtain NZVI ;

(2) Weigh the cobalt salt and dissolve it in methanol, after complete dissolution, disperse NZVI in the cobalt salt solution to obtain a NZVI/Co mixed solution; Weigh the organic ligand and dissolve it in methanol, and then add it to the NZVI/Co mixed solution , stand still to obtain the crude product of NZVI@ZIF-67, wash with methanol, separate and collect by magnet, and then vacuum dry to obtain NZVI@ZIF-67;

(3) The NZVI@ZIF-67 material was calcined in a tube furnace with nitrogen to obtain NZVI@ZD.

In the preparation method of the above-mentioned core-shell material NZVI@ZD, preferably, in the step (1), the iron salt is ferric chloride hydrate (FeCl ₃ ·6H ₂ O), and the mechanical stirring is carried out at room temperature, so that the The stirring time is 30min～35min, the vacuum drying temperature is 59～61℃, the vacuum drying time is 7.5～8.5h, the continuous stirring time is 30min～35min, and the ethanol washing times is 3～4 Second-rate.

In the preparation method of the above-mentioned core-shell material NZVI@ZD, preferably, in the step (2), the cobalt salt is hydrated cobalt nitrate (Co(NO ₃ ) ₃ ·6H ₂ O), and the standing temperature is room temperature, the standing time is 23-25 h, the temperature of the vacuum drying is 59-61° C., the vacuum drying time is 7.5-8.5 h, and the number of washings with methanol is 3-4 times.

In the above-mentioned preparation method of the core-shell material NZVI@ZD, preferably, in the step (3), the calcination temperature is 795-805° C., the heating rate is 5-6° C. min ⁻¹ , and the calcination time is 120～805° C. 130min.

As a general technical concept, the present invention also provides an application of the NZVI@ZD prepared by the above-mentioned preparation method of the core-shell material NZVI@ZD in the treatment of heavy metal sewage.

In the above application, preferably, in the application, the heavy metal sewage is Cr(VI)-containing sewage.

Compared with the prior art, the advantages of the present invention are:

(1) The present invention provides a preparation method of a core-shell material NZVI@ZD. By adding PVP-modified NZVI during the self-assembly process of ZIF-67 raw materials, the NZVI core is encapsulated inside the ZIF-67 shell, and the pore size of the shell is enlarged through the subsequent carbonization process. The whole preparation process is simple.

(2) The NZVI@ZD produced by the preparation method of the core-shell material NZVI@ZD of the present invention has a very obvious improvement in the adsorption performance of Cr(VI).

Description of drawings

Fig. 1 is a graph showing the adsorption effect of the adsorbents synthesized in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention on Cr(VI) in solution.

FIG. 2 is the isotherm adsorption fitting curve of the adsorption performance of the adsorbents synthesized in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention to Cr(VI).

FIG. 3 is the isotherm adsorption fitting parameters of the adsorption performance of the adsorbents synthesized in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention to Cr(VI).

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but the protection scope of the present invention is not limited thereby.

The materials and instruments used in the following examples are commercially available.

Example 1:

A preparation method of a core-shell material NZVI@ZD of the present invention, comprising the following steps:

(1) Dissolve 1.45g FeCl ₃ ·6H ₂ O and 50mg PVP in 60mL of 70% (v/v) ethanol solution, and stir with nitrogen. NaBH ₄ (0.7 g+20 mL of water) was added dropwise to the solution, and the stirring was continued for 30 min with nitrogen flow after the dropwise addition. Washed with alcohol three times, centrifuged, and dried under vacuum at 60 °C for 12 h. The resulting product is NZVI.

(2) Dissolve 0.87g Co(NO ₃ ) ₃ ·6H ₂ O and 0.0168g NZVI in 30mL anhydrous methanol, ultrasonicate for 5min to obtain a homogeneous mixture; weigh 0.98g dimethylimidazole MeIm and dissolve it in 10mL In water and methanol, it was slowly added to the above mixed solution, and the resulting solution was allowed to stand at room temperature for 24 hours to obtain a crude product of NZVI@ZIF-67, which was washed with methanol, collected by magnet separation, and then vacuum-dried at 60 °C for 8 hours to obtain NZVI@ZIF -67.

(3) Transfer the NZVI@ZIF-67 obtained in step (2) to a tube furnace, continue to pass nitrogen, heat up to 800°C at a rate of 5°C/min, and calcine at this temperature for 120min, and naturally cool down to room temperature, that is, Get NZVI@ZD.

Comparative Example 1:

A method for preparing ZIF-67 monomer, comprising the following steps:

(1) Dissolve 0.87g Co(NO ₃ ) ₃ ·6H ₂ O and 0.0168g NZVI in 30 mL anhydrous methanol to obtain Co(NO ₃ ) ₃ ·6H ₂ O solution.

(2) 0.98 g of dimethylimidazole MeIm was weighed and dissolved in 10 mL of anhydrous methanol to obtain a MeIm solution.

(3) The MeIm solution was slowly added to the Co(NO ₃ ) ₃ ·6H ₂ O solution, and allowed to stand at room temperature for 24 hours to obtain a crude ZIF-67 product, which was washed with methanol, collected by centrifugation, and then vacuum-dried at 60°C for 8 hours to obtain ZIF-67. 67 monomers.

Comparative Example 2:

A method for preparing a core-shell material NZVI@ZIF-67 with NZVI as the core and ZIF-67 as the shell, comprising the following steps:

(1) Dissolve 1.45g FeCl ₃ ·6H ₂ O and 50mg PVP in 60mL of 70% (v/v) ethanol solution, and stir with nitrogen. NaBH ₄ (0.7 g+20 mL of water) was added dropwise to the solution, and the stirring was continued for 30 min with nitrogen flow after the dropwise addition. Washed with alcohol three times, centrifuged, and dried under vacuum at 60 °C for 12 h. The resulting product is NZVI.

(2) Dissolve 0.87g Co(NO ₃ ) ₃ ·6H ₂ O and 0.0168g NZVI in 30mL anhydrous methanol, ultrasonicate for 5min to obtain a homogeneous mixture; weigh 0.98g dimethylimidazole MeIm and dissolve it in 10mL In water and methanol, it was slowly added to the above mixed solution, and the resulting solution was allowed to stand at room temperature for 24 hours to obtain a crude product of NZVI@ZIF-67, which was washed with methanol, collected by magnet separation, and then vacuum-dried at 60 °C for 8 hours to obtain NZVI@ZIF -67.

Experiments on the removal of heavy metal ions Cr(VI) in water by the products obtained in the three embodiments:

Get the series of adsorbent materials prepared in Example 1, Comparative Examples 1 and 2, and carry out adsorption and removal to Cr(VI) in the aqueous solution, and the specific method is:

A Cr(VI) solution with a concentration gradient of 5, 10, 25, 50, 100, 200, 400, 600 mg L ^-1 was prepared. Take 20mL of Cr(VI) solution in 50mL conical flask respectively, adjust pH=5.0. 0.01 g of the series of adsorbent materials produced in Example 1, Comparative Example 1, and Comparative Example 2 were added respectively. Then shake at 25°C for 24h in a constant temperature oscillator with a rotational speed of 170rpm. After the adsorption was saturated, the solution was collected through a 0.22 μm filter, and the Cr(VI) concentration was measured on an ultraviolet spectrophotometer, and the absorption wavelength was set to 540 nm.

It can be seen from Fig. 1 that the NZVI@ZD prepared by Example ¹ has reached adsorption saturation compared with the adsorbents synthesized by the other two comparative schemes. However, Example 1 did not reach saturation even if the concentration increased to 600 mg L ^-1 , and the adsorption amount increased with the increase of the concentration, indicating that Example 1 has a better effect on hexavalent chromium in solution than Comparative Examples 1 and 2. Very superior adsorption performance.

Cr(VI) adsorption experiment results of a series of materials were fitted by the origin software for the L-type adsorption isotherm model. The results are shown in Figures 2 and 3, and the adsorption capacities of the three adsorbents were 226.5 mg g ^-1 (Example 1). ), 29.35 mg g ^-1 (Comparative Example 1), 36.53 mg g ^-1 (Comparative Example 2).

The above descriptions are only preferred embodiments of the present invention, and do not limit the present invention in any form. Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art, without departing from the spirit and technical solutions of the present invention, can make many possible changes and modifications to the technical solutions of the present invention by using the methods and technical contents disclosed above, or modify them to be equivalent. Variant equivalent embodiments. Therefore, any simple modification, equivalent replacement, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims (5)

1. A method for preparing a nano-zero-valent iron-metal organic framework core-shell material, comprising the following steps: (1) Weigh iron salt and polyvinylpyrrolidone and dissolve them in an ethanol solution with a volume fraction of 60%, and continue to mechanically stir under nitrogen conditions to obtain an oxygen-free mixed solution; weigh sodium borohydride and dissolve it in deionized water. It is added dropwise to the above-mentioned oxygen-free mixed solution, and the whole process maintains nitrogen and mechanical stirring conditions to obtain the NZVI crude product, and the NZVI crude product is washed with absolute ethanol, and vacuum-dried after magnet separation and collection to obtain NZVI; (2) Weigh the cobalt salt and dissolve it in methanol. After complete dissolution, disperse the NZVI in the cobalt salt solution to obtain a NZVI/Co mixed solution; weigh the organic ligand and dissolve it in methanol, and then add it to the NZVI/Co mixed solution , stand still to obtain the crude product of NZVI@ZIF-67, wash with methanol, separate and collect by magnet, and then vacuum dry to obtain NZVI@ZIF-67; (3) The NZVI@ZIF-67 material is calcined in a tube furnace with nitrogen to obtain a nano-zero-valent iron-metal organic framework core-shell material; the calcination temperature is 795~805 ℃, and the heating rate is 5~6 ℃ min ^-1 , the calcination time is 120~130min. 2. the preparation method of nano zero-valent iron-metal organic framework core-shell material according to claim 1, is characterized in that, in described step (1), described iron salt is ferric chloride hydrate, and mechanical stirring is at room temperature The time of described stirring is 30min～35min, the temperature of described vacuum drying is 59～61 ℃, the time of vacuum drying is 7.5～8.5h, the time of described continuous stirring is 30min～35min, the washing times of ethanol 3 to 4 times. 3. the preparation method of the nano zero-valent iron-metal organic framework core-shell material according to any one of claims 1～2, is characterized in that, in described step (2), described cobalt salt is hydrated cobalt nitrate , the standing temperature is room temperature, the standing time is 23～25h, the temperature of the vacuum drying is 59～61 ℃, the vacuum drying time is 7.5～8.5h, and the number of washings with methanol is 3～4 times. 4. the application of the nano-zero-valent iron-metal-organic framework core-shell material obtained by the preparation method of any one of claims 1 to 3 in the treatment of heavy metal sewage. The application according to claim 4, characterized in that, in the application, the heavy metal sewage is Cr-containing sewage.

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