CN115055679B - Zero-valent iron reducing agent and preparation method and application thereof - Google Patents

Abstract

The present invention relates to the field of industrial wastewater treatment, and discloses a zero-valent iron reducing agent and a preparation method and application thereof, wherein the preparation method comprises the following steps: A1, soaking a source reducing agent with a sodium persulfate solution, drying it, and then using a chemical vapor deposition method to deposit aniline monomer on the dried source reducing agent to obtain an intermediate; A2, pyrolyzing the intermediate at high temperature under a protective gas atmosphere to form a nitrogen-doped carbon coating layer (CN) on the surface of the zero-valent iron, that is, obtaining a zero-valent iron reducing agent with a coating structure; wherein the source reducing agent is nano zero-valent iron or supported zero-valent iron. In the present invention, by adopting a method of in-situ forming a CN coating layer to activate zero-valent iron using a vapor deposition-pyrolysis strategy, the obtained zero-valent iron reducing agent has high reduction activity; the CN coating layer coated on the surface of the zero-valent iron nanoparticles can prevent the loss of iron species, improve the utilization rate and stability of zero-valent iron, and thus achieve continuous utilization of the reducing agent.

Description

Zero-valent iron reducing agent and preparation method and application thereof

Technical Field

The present invention relates to the field of industrial wastewater treatment, and in particular to a zero-valent iron reducing agent and a preparation method and application thereof.

Background technique

Chromium is widely present in nature, and in the aquatic environment it mainly exists in the form of hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)). Among them, Cr(VI) is often produced in industrial production processes, such as electroplating, smelting, leather tanning, etc., and has the characteristics of large emissions and high emission concentrations. As a typical heavy metal, Cr(VI) is 100 times more toxic than Cr(III) and is more easily absorbed by the human body, causing serious damage to the human body. Cr(VI) is highly cumulative. If it is absorbed by the human body for a long time, it will accumulate in the human body and poison various organs of the human body, such as the kidneys, liver and stomach; high concentrations of Cr(VI) are also carcinogenic and mutagenic.

At present, the commonly used Cr(VI) treatment methods mainly include adsorption-reduction method, chemical reduction method, ion exchange method, and photocatalytic reduction method. These technologies inevitably have negative effects such as high energy consumption, low efficiency, and secondary pollution in application. For example, when using activated carbon adsorption to treat Cr(VI), the adsorbent needs to be regenerated and replaced after reaching saturation, which is costly. The chemical precipitation method refers to the use of a reducing agent to reduce Cr(VI) to Cr(III), and then adding lime or sodium hydroxide to generate a precipitate and remove it. This method requires the input of excessive reducing agents and precipitants, which is easy to cause secondary pollution. The ion exchange method for treating Cr(VI) in water bodies mainly uses ion exchange resins to exchange with Cr(VI) ions. The disadvantage is that the resin used in this method is easily contaminated and ineffective, and the impurity ions such as sodium and iron in the regenerated waste liquid cannot be directly reused, and discharge into the environment will cause secondary pollution. The photo/electrocatalytic reduction method is based on the catalyst to efficiently reduce Cr(VI) under the condition of input energy, but it consumes a lot of energy and has low economic benefits.

Nano-zero-valent iron (Fe ⁰ ) is widely used in Cr(VI) reduction due to its advantages of low cost, environmental friendliness, and low Fe ^II /Fe ⁰ electrode potential (-0.44 V vs. SHE). Its mechanism is that the strongly oxidizing Cr(VI) is directly reduced by Fe ⁰ , and Fe is oxidized. However, the reduction efficiency of existing nano-zero-valent iron reducing agents is low, and the recycling of reducing agents is difficult to achieve.

Summary of the invention

The purpose of the present invention is to overcome the problems of low reduction efficiency of zero-valent iron reducing agents in the prior art and difficulty in recycling and reusing the reducing agents, and to provide a zero-valent iron reducing agent and a preparation method and application thereof.

The inventors have found that the existing nano zero-valent iron reducing agents have low reduction efficiency and are difficult to recycle and reuse. The main reasons are as follows: (1) the inert iron oxide film (such as _FeOx and FeOOH) generated by the contact between ^Fe0 and air hinders the reaction and inhibits activity; (2) the large-sized block structure makes it difficult to utilize Fe in the bulk phase, resulting in a decrease in the utilization rate of Fe; (3) the dissolution and loss of Fe species cause waste of Fe resources and secondary pollution.

The inventors further discovered that by using vapor deposition-pyrolysis technology to activate the source reducing agent (zero-valent iron), zero-valent iron nanoparticles with a carbon-doped nitrogen (CN) coating layer, namely, a zero-valent iron reducing agent, can be produced. The zero-valent iron reducing agent has excellent reduction efficiency and stability, and its high stability enables the recycling, regeneration and reuse of the reducing agent.

In order to achieve the above object, the present invention provides a method for preparing a zero-valent iron reducing agent, the method comprising the following steps:

A1, soaking the source reducing agent with sodium persulfate solution, drying, and then polymerizing and depositing aniline monomer on the dried source reducing agent by chemical vapor deposition to obtain an intermediate;

A2, pyrolyzing the intermediate at high temperature under a protective gas atmosphere to form a nitrogen-doped carbon coating layer on the surface of the zero-valent iron, thereby obtaining a zero-valent iron reducing agent having a coating structure;

Wherein, the source reducing agent is nano zero-valent iron or supported zero-valent iron.

Preferably, in step A1, the deposition process comprises: placing the dried source reducing agent and aniline monomer on two sides of a tube furnace respectively and sealing them for deposition.

Preferably, the mass ratio of the source reducing agent to the aniline monomer is 1:2-1:10.

Preferably, the concentration of the sodium persulfate solution is 0.15-0.25 g/mL.

Preferably, the chemical vapor deposition conditions include: a deposition temperature of 40-80° C. and a deposition time of 8-12 hours.

Preferably, in step A2, the protective gas is nitrogen or argon.

Preferably, the operating conditions of the high-temperature pyrolysis include: a protective gas flow rate of 100-200 mL/min, a temperature of 700-1000° C., and a time of 5-8 h.

Preferably, the supported zero-valent iron is porous carbon-supported zero-valent iron or nitrogen-doped porous carbon-supported zero-valent iron, preferably nitrogen-doped porous carbon-supported zero-valent iron.

Preferably, the method further comprises preparing the nitrogen-doped porous carbon-supported zero-valent iron according to the following steps:

B1, mixing ferric chloride hexahydrate, 2-aminoterephthalic acid and N,N-dimethylformamide, performing a hydrothermal reaction, and then washing, centrifuging and drying to obtain a mixture;

B2. Carbonizing the mixture at high temperature under a protective atmosphere to obtain nitrogen-doped porous carbon loaded with zero-valent iron.

Preferably, in step B1, the mass ratio of ferric chloride hexahydrate to 2-aminoterephthalic acid is 1:3 to 3:1.

Preferably, the temperature of the hydrothermal reaction is 100-200° C., and the reaction time is 18-24 h.

Preferably, the operating conditions of the high temperature carbonization include: a protective gas flow rate of 100-200 mL/min, a temperature of 700-1000° C., and a time of 5-8 h.

A second aspect of the present invention provides a zero-valent iron reducing agent, which is prepared by the preparation method of the zero-valent iron reducing agent as described above.

A third aspect of the present invention provides an application of a zero-valent iron reducing agent in treating oxidative pollutants in water, wherein the zero-valent iron reducing agent is prepared by the method described above.

Preferably, the oxidative pollutant is bromate, dichromate or selenate.

A fourth aspect of the present invention provides a method for treating hexavalent chromium in water, wherein the method uses a zero-valent iron reducing agent to treat water containing hexavalent chromium pollutants, wherein the zero-valent iron reducing agent is prepared by the method described above.

Preferably, the method comprises the following steps: adding a zero-valent iron reducing agent to a water body containing hexavalent chromium pollutants, adjusting the pH of the water body to 1.5-3, and performing a reduction reaction to remove the hexavalent chromium in the water body.

The advantages and beneficial effects of the present invention are:

(1) In the present invention, a method of activating zero-valent iron by in-situ forming a CN coating layer using a vapor deposition-pyrolysis strategy is used to improve the reduction activity of zero-valent iron, so that the reduction activity of the obtained zero-valent iron reducing agent is high, thereby overcoming the disadvantage of traditional coating technology of sacrificing activity in exchange for stability;

(2) The CN coating on the surface of zero-valent iron nanoparticles can prevent the loss of iron species and improve the utilization rate and stability of zero-valent iron. In this way, the continuous utilization of the reducing agent can be achieved by using carbothermal reaction and carbon addition regeneration;

(3) The zero-valent iron reducing agent synthesized by the present invention is used to reduce Cr(VI) in water, and can effectively remove the toxicity of Cr(VI); and during use, no other special equipment conditions are required, and it can be carried out at room temperature and pressure, which is simple to operate and has a wide range of applications;

(4) Each step in the preparation method of the zero-valent iron reducing agent provided by the present invention is a basic chemical process and is easy to operate; the source reducing agent used only needs to contain iron elements, the materials are easy to obtain, and it is technically feasible;

(5) The chemical vapor deposition-pyrolysis strategy relied upon by the present invention can be applied to source reducing agents such as commercial nano-zero-valent iron and supported zero-valent iron, which can effectively improve their reduction performance and has universal applicability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a transmission electron microscope image of the products obtained in each step of Example 1;

FIG2 is an atomic force microscope image of the CN coating layer and the zero-valent iron reducing agent in Example 1;

3 is an XRD spectrum of the source reducing agent, the zero-valent iron reducing agent, and the product after the zero-valent iron reducing agent is treated with acid in Example 1;

4 is a Raman spectrum of the source reducing agent in Example 1, the zero-valent iron reducing agent, the product of the zero-valent iron reducing agent prepared in Example 1 after acid treatment, and the zero-valent iron reducing agent prepared in Example 2;

FIG5 is a UPS curve of the source reducing agent and the zero-valent iron reducing agent in Example 1, and the calculated work function;

FIG6 is a Nyquist plot of the source reductant and the zero-valent iron reductant in Example 1;

7 is a diagram showing the magnetic saturation and magnetic separation effect of the source reducing agent and the zero-valent iron reducing agent prepared in Example 1, and the zero-valent iron reducing agent after being used in Example 1;

FIG8 shows the treatment effects on Cr(VI) in Application Example 1 and Comparative Examples 1-3, as well as the reduction performance test results of various reducing agents;

FIG9 is a test result of the cycle performance and regeneration effect of Application Example 1, and a test result of the reducing performance of the reducing agent;

FIG10 is a diagram showing the treatment effect of Cr(VI) in water using Example 1-4;

FIG11 is a diagram showing the treatment effects of Application Example 1 and Application Examples 5-6 on Cr(VI) in water;

FIG12 is a diagram showing the treatment effects of Example 7 and Comparative Example 4 on Cr(VI) in water;

FIG13 is a diagram showing the treatment effects of Example 8 and Comparative Example 5 on Cr(VI) in water;

FIG. 14 is a diagram showing the treatment effect of Comparative Example 6 on Cr(VI) in water.

Detailed ways

The specific implementation of the present invention is described in detail below in conjunction with the accompanying drawings. It should be understood that the specific implementation described herein is only used to illustrate and explain the present invention, and is not used to limit the present invention.

The endpoints and any values of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, the endpoint values of each range and the individual point values, and the individual point values can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

The present invention provides a method for preparing a zero-valent iron reducing agent, the method comprising the following steps:

Step A1, soaking the source reducing agent with a sodium persulfate solution, drying it, and then polymerizing and depositing the aniline monomer on the dried source reducing agent using a chemical vapor deposition method to obtain an intermediate;

Step A2, pyrolyzing the intermediate at high temperature under a protective gas atmosphere to form a nitrogen-doped carbon coating layer on the surface of the zero-valent iron, thereby obtaining a zero-valent iron reducing agent with a coating structure;

Wherein, the source reducing agent is nano zero-valent iron or supported zero-valent iron.

It should be noted that, when the source reducing agent is supported zero-valent iron, before step A1, the process further includes: grinding the source reducing agent to form uniform powder for easy uniform deposition.

In the present invention, the nitrogen-doped carbon coating layer (CN coating layer) is obtained by chemical vapor deposition-pyrolysis of the precursor aniline monomer, and the zero-valent iron precursor is obtained by carbonization of the Fe-based metal organic framework. In addition, the iron content in the zero-valent iron reducing agent is 20-80%.

In a specific embodiment, in step A1, the deposition process includes: placing the dried source reducing agent and aniline monomer on two sides of a tube furnace respectively and sealing them, and performing deposition to obtain an intermediate.

In a specific embodiment, the mass ratio of the source reducing agent to the aniline monomer is 1:2-1:10, specifically, it can be 1:2, 1:4, 1:5, 1:7 or 1:10, preferably 1:4.

In a specific embodiment, the concentration of the sodium persulfate solution can be 0.15 g/mL, 0.18 g/mL, 0.20 g/mL or 0.25 g/mL.

In a preferred embodiment, the chemical vapor deposition temperature may be 40° C., 45° C., 50° C., 54° C. or 60° C., and the time may be 8 h, 10 h, 11 h or 12 h.

In a preferred embodiment, in step A2, the protective gas is nitrogen or argon.

In a preferred embodiment, in step A2, the operating conditions of the high-temperature pyrolysis include: a protective gas flow rate of 100-200 mL/min, a temperature of 700-1000°C, and a time of 5-8 h. During the high-temperature pyrolysis process, the inert oxide film on the surface of the zero-valent iron particles can be removed, thereby increasing the activity of the zero-valent iron.

The supported zero-valent iron is porous carbon-supported zero-valent iron (Fe ⁰ /C) or nitrogen-doped porous carbon-supported zero-valent iron (Fe ⁰ /CN), preferably nitrogen-doped porous carbon-supported zero-valent iron.

The present invention does not limit the specific preparation method of nitrogen-doped porous carbon-supported zero-valent iron, which can be purchased or prepared in a conventional manner in the art. Preferably, the nitrogen-doped porous carbon-supported zero-valent iron is prepared according to the following steps:

Step B1, mixing ferric chloride hexahydrate, 2-aminoterephthalic acid and N,N-dimethylformamide, performing a hydrothermal reaction, and then washing, centrifuging and drying to obtain a mixture,

Step B2: carbonizing the mixture at high temperature under a protective atmosphere to obtain nitrogen-doped porous carbon loaded with zero-valent iron.

In a specific embodiment, in step B1, the mass ratio of ferric chloride hexahydrate to 2-aminoterephthalic acid is 1:3-3:1, that is, it can be 1:3, 1.5:3, 3:3, 3:2 or 3:1, preferably 3:1.

In a specific embodiment, in step B1, 2-aminoterephthalic acid and N,N-dimethylformamide (DMF) are first mixed to obtain a 2-aminoterephthalic acid solution, and then ferric chloride hexahydrate is added, and then a hydrothermal reaction is performed. Preferably, the temperature of the hydrothermal reaction is 100-200 ° C, and the reaction time is 18-24 h.

In a preferred embodiment, the operating conditions of the high temperature carbonization include: a protective gas flow rate of 100-200 mL/min, a temperature of 700-1000° C., and a time of 5-8 h.

In the present invention, aniline is used as a precursor, and a source reducing agent is soaked and dried with a sodium persulfate solution, and is placed in a tube furnace together with an aniline monomer, and the volatilized aniline molecules are captured by vapor deposition, followed by oxidative polymerization, that is, a layer of polyaniline (PANI) is coated on the surface of the source reducing agent, and after high-temperature pyrolysis, a uniform CN coating layer is formed to coat the surface of zero-valent iron nanoparticles, namely, a zero-valent iron reducing agent.

The present invention also provides a zero-valent iron reducing agent, which is prepared by the preparation method of the zero-valent iron reducing agent as described above.

The present invention also provides an application of a zero-valent iron reducing agent in treating oxidative pollutants in water bodies, wherein the zero-valent iron reducing agent is prepared by the method as described above.

In a specific embodiment, the oxidized pollutant is bromate, chromate or selenate.

In addition, the present invention also provides a method for treating hexavalent chromium in water, which comprises: using a zero-valent iron reducing agent to treat the water containing hexavalent chromium pollutants, wherein the zero-valent iron reducing agent is prepared by the method described above.

In a preferred embodiment, the method comprises the following steps: adding a zero-valent iron reducing agent to a water body containing hexavalent chromium pollutants, and adjusting the pH of the water body to 1.5-3 to perform a reduction reaction. More preferably, the pH of the water body is adjusted to 2.

The present invention does not limit the usage of the zero-valent iron reducing agent, which can be determined according to the concentration of specific pollutants. In the present invention, when the initial concentration of hexavalent chromium pollutants is 5-15 mg/L, the usage of the zero-valent iron reducing agent is 30-50 mg/L. In this way, the removal rate of hexavalent chromium pollutants in the water body can be more than 80% within 50-120 minutes.

The present invention will be described in detail below through examples, but the protection scope of the present invention is not limited thereto.

Example 1

This example is used to illustrate the preparation method of the zero-valent iron reducing agent of the present invention.

(1) 2-aminoterephthalic acid and DMF are mixed, and then ferric chloride hexahydrate is added and stirred evenly, and hydrothermally reacted at 110° C. for 24 h, and then washed, centrifuged, and dried to obtain a mixture, wherein the mass ratio of ferric chloride hexahydrate to 2-aminoterephthalic acid is 3:1;

(2) The above mixture was carbonized at a N ₂ flow rate of 150 mL/min and a temperature of 900°C for 5 h to obtain a source reducing agent, which was recorded as Fe ⁰ /CN;

(3) grinding the source reducing agent, soaking it in a sodium persulfate solution with a concentration of 0.15 g/mL and drying it, placing it together with the aniline monomer in a tube furnace with both sides sealed, and depositing it at 50 °C for 12 h to obtain an intermediate, wherein the mass ratio of the source reducing agent to the aniline monomer is 1:4;

(4) The intermediate was pyrolyzed at 800 °C for 5 h under the conditions of a N ₂ flow rate of 150 mL/min to obtain a CN-coated zero-valent iron reducing agent, i.e., a zero-valent iron reducing agent, denoted as Fe ⁰ @CN.

Example 2

This example is used to illustrate the preparation method of the zero-valent iron reducing agent of the present invention.

The method described in Example 1 was followed, except that the source reducing agent in step (3) was porous carbon-supported zero-valent iron (Fe ⁰ /C), and the obtained zero-valent iron reducing agent was recorded as Fe ⁰ /C@CN, wherein the porous carbon-supported zero-valent iron was prepared according to the following process: ferric nitrate hexahydrate and trimesic acid were dissolved in deionized water, stirred for 30 min, and then transferred to a hydrothermal reactor for hydrothermal reaction at 180 °C for 12 h. After cooling to room temperature, the mixture was filtered, washed with deionized water and methanol, and dried to obtain a mixture MIL-100 (Fe); MIL-100 (Fe) was carbonized under nitrogen to obtain Fe ⁰ /C.

Example 3

This example is used to illustrate the preparation method of the zero-valent iron reducing agent of the present invention.

The method described in Example 1 is followed, except that the source reducing agent in step (3) is commercial zero-valent iron nZVI purchased from the market. It is understood that steps (1) and (2) can be adaptively deleted.

Example 4

(1) 2-aminoterephthalic acid and DMF are mixed, and then ferric chloride hexahydrate is added and stirred evenly, and hydrothermally reacted at 100° C. for 20 h, and then washed, centrifuged, and dried to obtain a mixture, wherein the mass ratio of ferric chloride hexahydrate to 2-aminoterephthalic acid is 3:3:

(2) carbonizing the mixture at a N ₂ flow rate of 100 mL/min and a temperature of 700 °C for 8 h to obtain a source reducing agent, which is recorded as Fe ⁰ /CN;

(3) grinding the source reducing agent, soaking it in a sodium persulfate solution with a concentration of 0.2 g/mL and drying it, placing it together with the aniline monomer in a tube furnace with both sides sealed, and depositing it at 40°C for 10 h to obtain an intermediate, wherein the mass ratio of the source reducing agent to the aniline monomer is 1:10;

(4) The intermediate is pyrolyzed at a _N2 flow rate of 200 mL/min and a temperature of 1000°C for 6 h to obtain a CN-coated zero-valent iron reducing agent, i.e., a zero-valent iron reducing agent.

Example 5

(1) 2-aminoterephthalic acid and DMF are mixed, then ferric chloride hexahydrate is added and stirred evenly, and hydrothermally reacted at 200° C. for 18 h, and then washed, centrifuged, and dried to obtain a mixture, wherein the mass ratio of ferric chloride hexahydrate to 2-aminoterephthalic acid is 1:3;

(2) The above mixture was carbonized at a _N2 flow rate of 200 mL/min and a temperature of 1000 °C for 6 h to obtain a source reducing agent, which was recorded as ^Fe0 /CN;

(3) grinding the source reducing agent, soaking and drying it with a sodium persulfate solution having a concentration of 0.25 g/mL, placing it in a tube furnace with the aniline monomer and sealing it on both sides, and depositing it at 80° C. for 8 h to obtain an intermediate, wherein the mass ratio of the source reducing agent to the aniline monomer is 1:2;

(4) The intermediate is pyrolyzed at a _N2 flow rate of 100 mL/min and a temperature of 700°C for 8 h to obtain a CN-coated zero-valent iron reducing agent, i.e., a zero-valent iron reducing agent.

Comparative Example 1

The method described in Example 1 is used for implementation, except that the aniline monomer is oxidatively polymerized and coated in the liquid phase and then pyrolyzed to synthesize the zero-valent iron reducing agent. Specifically, the method includes the following steps:

(1) preparing the source reducing agent Fe ⁰ /CN by the method described in Example 1;

(2) Dissolve sodium persulfate in 1M hydrochloric acid solution, disperse ^Fe0 /CN in the solution, stir for 0.5 h, and then drop purified aniline monomer into the mixture. Continue stirring for 5 h, and the material will eventually turn dark green.

(3) The above materials were magnetically separated from the solution, filtered and washed several times, and pyrolyzed in a tube furnace under _N2 atmosphere (flow rate of 150 mL/min) and temperature of 800°C for 5 h. The final product was ^Fe0 @CN reducing agent synthesized by liquid phase deposition-pyrolysis method.

Application Example 1

This application example is used to illustrate the method for treating hexavalent chromium in water (i.e., the application of a zero-valent iron reducing agent) according to the present invention.

The zero-valent iron reducing agent prepared in Example 1 was added to a water body containing hexavalent chromium pollutants (wherein the initial concentration of Cr(VI) in the water body was 10.4 mg/L) to make the concentration of the reducing agent 40 mg/L, and the pH of the water body was adjusted to 2, and the reduction reaction was carried out at normal temperature and pressure for 2 h.

Application Examples 2~4

The method of Application Example 1 was followed, except that in Application Example 2, the initial concentration of Cr(VI) in the water was 6.2 mg/L; in Application Example 3, the initial concentration of Cr(VI) in the water was 8.2 mg/L; and in Application Example 4, the initial concentration of Cr(VI) in the water was 12.4 mg/L.

Application Examples 5-6

The method of Application Example 1 was followed, except that in Application Example 5, the concentration of the reducing agent in the water was 32 mg/L; and in Application Example 6, the concentration of the reducing agent was 51 mg/L.

Application Example 7

The method of Example 1 is used for implementation, except that the reducing agent used is the zero-valent iron reducing agent prepared in Example 2 (ie, the source reducing agent is Fe ⁰ /C).

Application Example 8

The method of Example 1 is followed, except that the reducing agent used is the zero-valent reducing agent prepared in Example 3 (ie, the source reducing agent is commercial zero-valent iron).

Application Comparative Examples 1~3

The method of Example 1 was followed, except that the reducing agent used in Example 1 was pure CN material; the reducing agent used in Example 2 was ^Fe0 /CN obtained in step (2) of Example 1; and the reducing agent used in Example 3 was the zero-valent iron reducing agent obtained in Example 1 after acid treatment (i.e., the zero-valent iron reducing agent was stirred in a 2 M hydrochloric acid solution for 3 h, washed until the washing solution was neutral, and the obtained solid was dried), which was recorded as ^Fe0 @CN-acid treatment.

Application Comparative Example 4

The method of Application Example 1 was followed, except that the reducing agent used was the source reducing agent (Fe ⁰ /C) in Example 2.

Application Comparative Example 5

The method of Application Example 1 was followed, except that the reducing agent used was the source reducing agent commercial zero-valent iron (nZVI) in Example 3.

Application Comparative Example 6

The method of Example 1 was followed, except that the reducing agent used in Comparative Example 6 was the zero-valent reducing agent prepared in Comparative Example 1 (ie, prepared by aqueous phase polymerization-thermal decomposition strategy).

Test Example 1

The mixture obtained in step (1), the source reducing agent obtained in step (2), the intermediate obtained in step (3) and the zero-valent iron reducing agent obtained in step (4) in Example 1 were characterized using a transmission electron microscope. The results are shown in FIG1 .

Among them, Figure 1 (a) is an electron microscope image of the mixture. As can be seen from Figure 1 (a), the obtained product is an iron-based MOFs with a regular octahedral structure.

Figure 1(b) is an electron microscope image of the source reducing agent. It can be seen from Figure 1(b) that the Fe particles are loaded on the porous carbon surface.

Figure 1(c) is an electron microscope image of the intermediate. It can be seen from Figure 1(c) that the outline of the Fe particles is blurred after deposition, indicating that they are effectively coated.

Figure 1 (d) is an electron microscope image of the zero-valent iron reducing agent. It can be seen from Figure 1 (d) that the coating layer and the particle state have not changed significantly after pyrolysis, indicating that high-temperature pyrolysis will not destroy the coating structure.

In addition, the iron content in the zero-valent iron reducing agent was tested to be 28.9 wt %.

Test Example 2

The CN coating layer and the zero-valent iron reducing agent prepared in Example 1 were characterized using an atomic force microscope, and the results are shown in FIG. 2 .

Among them, Figure 2 (a) is the characterization result of the CN coating layer. It can be seen from Figure 2 (a) that the thickness of the CN coating layer is about 5nm. Figure 2 (b) is the characterization result of the zero-valent iron reducing agent. It can be seen from Figure 2 that the zero-valent iron reducing agent (i.e., the Fe ⁰ nanoparticles and the CN coating layer after coating) is about 28.6 nm, so the particle size of the Fe particles is about 24 nm.

The source reducing agent (Fe ⁰ /CN), the zero-valent iron reducing agent (Fe ⁰ @CN) and the product obtained by acid treatment of the zero-valent iron reducing agent (Fe ⁰ @CN-acid treatment) prepared in Example 1 were subjected to X-ray diffraction, and the results are shown in Figure 3. It should be noted that the acid treatment step is to stir the Fe ⁰ @CN reducing agent in a 2 M hydrochloric acid solution for 3 h, wash until the washing solution is neutral, and dry the obtained solid.

The spectra corresponding to Fe ⁰ @CN and Fe ⁰ /CN in Figure 3 both show characteristic peaks of Fe with similar intensities. This is because the thickness of the carbon coating is not enough to affect the strength of Fe, and after acid treatment, the Fe particles are removed, so the characteristic peaks disappear.

Test Example 3

The source reducing agent (Fe ⁰ /CN) prepared in Example 1, the zero-valent iron reducing agent (Fe ⁰ @CN), the product obtained by acid-treating the zero-valent iron reducing agent prepared in Example 1 (Fe ⁰ @CN-acid treatment), and the zero-valent iron reducing agent prepared in Example 2 were subjected to Raman spectroscopy analysis, and the results are shown in FIG4 .

From the comparison in Figure 4, it can be seen that the graphitization degree of the zero-valent iron reducing agent obtained after coating with CN is higher than that of the source reducing agent before coating, which is conducive to the improvement of activity. The graphitization degree of the supported Fe ⁰ /C in Example 2 is lower after coating because the pure carbon carrier is not easy to graphitize.

The source reducing agent (Fe ⁰ /CN) and the zero-valent iron reducing agent (Fe ⁰ @CN) prepared in Example 1 were tested using an ultraviolet photoelectron spectrometer (UPS), and the corresponding work functions were calculated. The results are shown in FIG5 .

As can be seen from Figure 5, the work function of Fe ⁰ @CN is lower than that of Fe ⁰ /CN, which indicates a lower energy barrier and is conducive to the reaction. Similar conclusions were also verified by the Nyquist curve. The impedance of Fe ⁰ @CN is 10.03Ω, which is lower than that of Fe ⁰ @CN (15.41Ω) (as shown in Figure 6).

7 is a diagram showing the magnetic saturation (a) and magnetic separation (b) effects of the source reductant (Fe ⁰ /CN) and the zero-valent iron reductant (Fe ⁰ @CN) prepared in Example 1, and the zero-valent iron reductant after use in Example 1 (Fe ⁰ @CN-after use).

As shown in Figure 7 (a), the magnetic saturations of Fe ⁰ @CN, Fe ⁰ /CN and Fe ⁰ @CN- after use are 12 emu/g, 47 emu/g and 49 emu/g, respectively, indicating that the magnetic saturation of the reducing agent after activation and use is higher, which is conducive to magnetic separation. These magnetic saturations are sufficient to effectively separate the materials within 3 min (as shown in Figure 7 (b)).

Test Example 4

The treatment effects of Cr(VI) in water bodies in Example 1 and Comparative Examples 1-3 were tested, and the activity and removal amount of each reducing agent were tested. The test results are shown in Figure 8. It should be noted that in Figures 8-14, C _t /C ₀ refers to the ratio of the pollutant concentration at time t to the initial pollutant concentration, initial activity refers to the initial activity, and removal capacity refers to the removal capacity.

As can be seen from Figure 8, pure CN material cannot effectively remove Cr(VI), and the reduction of Cr(VI) occurs on the iron-containing reductant. Therefore, the reaction of this system is the reduction of Cr(VI) with the participation of Fe. Fe ⁰ @CN can completely reduce the set Cr(VI) in 70 min, while the amount of Cr(VI) removed by Fe ⁰ /CN is less than 10%, showing ultra-high reduction performance. The specific activity and removal amount comparison data are shown in Figure 8 (b). After coating, the activity (2037 mg/g _reductant ·h·L) and the removal amount of Cr(VI) (0.26 gCr/g _reductant ) are 75.4 times and 10 times that of Fe ⁰ /CN, respectively, indicating that the reduction performance of Fe 0 /CN can be greatly improved by activating the source reductant Fe ⁰ /CN using a chemical vapor deposition-pyrolysis strategy.

After repeating the steps of Example 1 for 4 times, the separated ^Fe0 @CN was subjected to carbon thermal regeneration and carbon addition treatment, wherein the carbon addition treatment was to perform the vapor deposition-pyrolysis steps ((3) and (4) in Example 1). The steps of Example 1 were repeated once again, and the treatment effect on Cr(VI), as well as the corresponding activity and removal amount of the reducing agent were tested.

As can be seen from Figure 9, under the same reaction conditions, ^Fe0 @CN can maintain high stability after carbon thermal regeneration (the same conditions as those for synthesizing the CN coating layer) and carbon addition. As shown in Figure 9 (a), the reduction performance of the reductant in the first three cycles of regeneration remains basically unchanged, while the activity decreases in the fourth cycle. This is mainly because the carbon layer in the material is not enough to reduce zero-valent iron, so the performance after carbon addition treatment returns to the initial level, as shown in Figure 9 (b).

Test Example 5

The treatment effect of Example 1-4 on Cr(VI) in water was tested, and the results are shown in FIG10 , namely the reduction curves of Cr(VI) with different initial concentrations (6.2 mg/L, 8.2 mg/L, 10.4 mg/L and 12.4 mg/L) in Fe ⁰ @CN reducing agent, the corresponding initial activity and the removal amount.

As can be seen from Figure 10, when the dosage of the reducing agent is 40 mg/L, the reducing agent can completely remove Cr(VI) within 50 minutes when the concentration of Cr(VI) in the water is lower than 10.4 mg/L (inclusive), but it cannot be completely removed even if it is extended to 120 minutes above this concentration (as shown in Figure 10 (a)). This is mainly because the Fe in the reducing agent is sufficient to reduce all Cr(VI) below 10.4 mg/L, while the Fe content above this concentration becomes a limiting factor. Regarding the kinetic analysis, this reaction is an adsorption-controlled reaction (as shown in Figure 10 (b)), so as the initial Cr(VI) concentration increases, the activity increases, and the removal amount also increases accordingly, until the reducing agent is completely reacted (as shown in Figure 10 (c)).

The treatment effects of Application Example 1 and Application Examples 5-6 on Cr(VI) in water were tested, and the results are shown in FIG11 , namely, the reduction curves of Cr(VI) at different reducing agent dosages (32 mg/L, 40 mg/L, and 51 mg/L), the corresponding initial activity, and the removal amount.

As can be seen from Figure 11, the results are similar to those in Figure 10. Under the condition of excess reducing agent, Cr(VI) can be completely reduced within 70 min, while when the amount of reducing agent is insufficient, the amount of Cr(VI) reduction is reduced accordingly. The removal amount and initial activity are shown in Figure 11 (b), which are also consistent with the results in Figure 10.

Test Example 6

The treatment effects of Example 7 (Fe ⁰ /C@CN) and Comparative Example 4 (Fe ⁰ /C) on Cr(VI) in water were tested, and the results are shown in FIG12 , i.e., the Cr(VI) reduction curves, the corresponding initial activity and the removal amount of the Fe ⁰ /C reducing agent and the corresponding Fe ⁰ /C@CN reducing agent treated by vapor deposition-pyrolysis.

As shown in Figure 12 (a), the reduction of Cr(VI) by Fe ⁰ /C was less than 10% within 120 min, while that by Fe ⁰ /C@CN was more than 80%. As shown in Figure 12 (b), the corresponding activity and removal amount of Fe ⁰ /C@CN were 17 times and 8.5 times higher than those of Fe ⁰ /C. This indicates that the reduction performance of the source reductant Fe ⁰ /C can be greatly improved by activating it with a chemical vapor deposition-pyrolysis strategy.

The treatment effects of Example 8 and Comparative Example 5 on Cr(VI) in water were tested, and the results are shown in FIG13 , namely, the Cr(VI) reduction curves, corresponding initial activities, and removal amounts of commercial zero-valent iron (nZVI) and the corresponding zero-valent iron reducing agent (nZVI@CN) treated by vapor deposition-pyrolysis.

As can be seen from Figure 13, after commercial zero-valent iron is activated by chemical vapor deposition-pyrolysis strategy, both the activity and removal amount are greatly improved.

The treatment effect of Comparative Example 6 on Cr(VI) in water was tested, and the results are shown in FIG14 .

As can be seen from Figure 14, the reduction activity and removal amount of the zero-valent iron reducing agent prepared by the aqueous phase polymerization-pyrolysis strategy are relatively low.

In Examples 4 and 5, hexavalent chromium in water was treated in the same manner as in Example 1, and the treatment effect was tested. The results showed that the reduction performance of Examples 4 and 5 was equivalent to that of Example 1, which will not be described in detail here.

In summary, after the source reducing agent is activated by the vapor deposition-pyrolysis strategy provided by the present invention, the activity of the source reducing agent and the amount of pollutants removed can be greatly improved.

The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims (16)

1. A method of preparing a zero-valent iron reducing agent, the method comprising the steps of:

A1, soaking a source reducing agent in a sodium persulfate solution, drying, and then adopting a chemical vapor deposition method to polymerize and deposit an aniline monomer on the dried source reducing agent to obtain an intermediate;

A2, performing high-temperature pyrolysis on the intermediate in a protective gas atmosphere to form a nitrogen-doped carbon coating layer on the surface of the zero-valent iron, thereby obtaining the zero-valent iron reducing agent with a coating structure;

wherein the source reducing agent is nano zero-valent iron or load-type zero-valent iron.

2. The method of preparing a zero-valent iron reducing agent according to claim 1, wherein in step A1, the depositing process comprises: and respectively placing the dried source reducing agent and aniline monomer on two sides of a tube furnace, and sealing for deposition.

3. The method for producing a zero-valent iron reducing agent according to claim 1 or 2, characterized in that the mass ratio of the source reducing agent to the aniline monomer is 1: 2-1: 10.

4. The method for preparing a zero-valent iron reducing agent according to claim 1, wherein the concentration of the sodium persulfate solution is 0.15-0.25 g/mL.

5. The method for preparing a zero-valent iron reducing agent according to claim 1, wherein the conditions of the chemical vapor deposition include: the deposition temperature is 40-80 ℃, and the deposition time is 8-12 h.

6. The method for producing a zero-valent iron reducing agent according to claim 1, wherein in step A2, the shielding gas is nitrogen or argon.

7. The method of preparing a zero-valent iron reducing agent of claim 1, wherein the operating conditions of the high temperature pyrolysis include: the flow rate of the shielding gas is 100-200 mL/min, the temperature is 700-1000 ℃ and the time is 5-8 h.

8. The method for preparing a zero-valent iron reducing agent according to claim 1, wherein the supported zero-valent iron is porous carbon supported zero-valent iron.

9. The method for preparing a zero-valent iron reducing agent according to claim 1, wherein the supported zero-valent iron is nitrogen-doped porous carbon supported zero-valent iron.

10. The method of preparing a zero-valent iron reducing agent of claim 9, further comprising preparing the nitrogen-doped porous carbon-loaded zero-valent iron according to the following procedure:

B1, uniformly mixing ferric trichloride hexahydrate, 2-amino terephthalic acid and N, N-dimethylformamide, performing hydrothermal reaction, and then washing, centrifuging and drying to obtain a mixture;

And B2, carbonizing the mixture at a high temperature in a protective atmosphere to obtain the nitrogen-doped porous carbon-loaded zero-valent iron.

11. The method for preparing a zero-valent iron reducing agent according to claim 10, wherein in the step B1, the mass ratio of the ferric trichloride hexahydrate to the 2-amino terephthalic acid is 1:3~3:1.

12. The method for preparing a zero-valent iron reducing agent according to claim 10, wherein in the step B1, the hydrothermal reaction is performed at a temperature of 100-200 ℃ for 18-24 hours.

13. The method for preparing a zero-valent iron reducing agent according to claim 10, wherein in step B2, the operating conditions of the high-temperature carbonization include: the flow rate of the shielding gas is 100-200 mL/min, the temperature is 700-1000 ℃ and the time is 5-8 h.

14. A zero-valent iron reducing agent, characterized by being produced by the method for producing a zero-valent iron reducing agent according to any one of claims 1 to 13.

15. A method of treating hexavalent chromium in a water body, characterized in that the water body containing hexavalent chromium contaminants is treated with a zero-valent iron reducing agent, said zero-valent iron reducing agent being produced by the method of any one of claims 1 to 13.

16. The method of treating hexavalent chromium in a water body according to claim 15, characterized in that said method comprises the steps of: adding a zero-valent iron reducing agent into a water body containing hexavalent chromium pollutants, adjusting the pH value of the water body to 1.5-3, and performing a reduction reaction to remove hexavalent chromium in the water body.