A method for reducing hexavalent chromium

Abstract

The present invention relates to the field of environmental remediation technology, and in particular to a method for reducing hexavalent chromium. The reduction method comprises the following steps: preparing nanoscale magnetic Fe3O4 particles and loading them with polybenzidine to construct a heterojunction photocatalyst of the magnetic Fe3O4 particles and polybenzidine (Fe3O4@Pind); mixing the synthesized Fe3O4@Pind with hexavalent chromium to form a mixed solution; and irradiating the resulting mixed solution with light to cause the Fe3O4@Pind to generate photogenerated electrons that reduce the hexavalent chromium. The reduction method of the present invention transfers the photogenerated electrons generated by the Fe3O4 to the polybenzidine, while the holes generated by the polybenzidine can be transferred to the Fe3O4. Thus, the heterojunction interface created promotes the separation of electron-hole pairs. This method achieves the goal of continuously generating photogenerated electrons and effectively reducing hexavalent chromium under ultraviolet light, which is of great significance for addressing the problem of hexavalent chromium pollution.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental governance, and in particular to a method for reducing hexavalent chromium. Background Art

[0002] Chromium exists primarily as hexavalent chromium oxyanions and trivalent chromium cations. Hexavalent chromium is highly toxic to organisms, but trivalent chromium is an essential trace element in the human body. Currently, hexavalent chromium treatment technologies primarily include adsorption, membrane separation, ion exchange, electrochemical methods, and chemical precipitation. Chemical precipitation, a reduction method that uses a reducing agent to reduce heavy metal ions to elemental metals or lower-valence metal ions, holds great potential for heavy metal recovery. Polybenzole (Pind) can be photoionized by ultraviolet light to produce photogenerated electrons with strong reducing properties, which can reduce coexisting hexavalent chromium to less toxic trivalent chromium. Its stable structure makes it reusable in the reaction. Magnetic materials, such as ferroferric oxide, possess excellent magnetic properties, low toxicity, and a high specific surface area, making them promising recyclable carriers in environmental applications. Theoretically, the energy band positions of Fe₃O₄ and polybenzole (Pind) are misaligned, allowing for the formation of a Z-type heterojunction, which can prolong charge separation time and enhance interfacial charge transfer. Therefore, the present invention combines magnetic Fe3O4 particles with Pind to construct a heterojunction photocatalyst (Fe3O4@Pind), which efficiently adsorbs and reduces hexavalent chromium, thereby achieving the removal of hexavalent chromium and the resource recovery of chromium, which is of great significance for addressing chromium pollution. Summary of the Invention

[0003] The present invention provides a hexavalent chromium reduction method, which is used to solve the problems of low reduction efficiency and difficulty in recovery in existing hexavalent chromium reduction methods.

[0004] According to a first aspect of the present invention, the present invention provides a method for reducing hexavalent chromium, comprising the following steps:

[0005] Preparation of nano-sized magnetic Fe3O4 particles and loading of polybenzazole;

[0006] The synthesized nano-scale magnetic Fe3O4 particles loaded with polybenzazole are mixed with hexavalent chromium to form a mixed solution; the formed mixed solution is irradiated with light so that the nano-scale magnetic Fe3O4 particles are loaded with polybenzazole to construct a heterojunction photocatalyst to promote the reduction of hexavalent chromium.

[0007] The principle of the present invention is:

[0008] Photogenerated electrons have strong reducing properties and a high reduction rate for hexavalent chromium, showing promising application prospects. The reduction method of the present invention transfers the photogenerated electrons generated by Fe₃O₄ to polybenzazole, while the holes generated by polybenzazole can be transferred to Fe₃O₄. Thus, the heterogeneous interface created promotes the separation of electron-hole pairs. This achieves the goal of continuously generating photogenerated electrons and effectively reducing hexavalent chromium under ultraviolet light, which is of great significance for addressing the environmental pollution problem caused by hexavalent chromium.

[0009] Furthermore, the preparation of Fe3O4@Pind specifically includes the following steps:

[0010] Weigh ferric chloride, sodium acetate, and trisodium citrate and dissolve them in ethylene glycol and diethylene glycol solutions to form a mixed solution;

[0011] The mixed solution is shaken, subjected to a high-temperature solvent thermal reaction, and then cooled to room temperature to form a nano-scale magnetic Fe3O4 solution;

[0012] Weigh indole and dissolve it in nano-magnetic Fe3O4 solution to form an indole mixed solution; weigh ammonium persulfate and dissolve it in pure water to form an ammonium persulfate solution;

[0013] The ammonium persulfate solution is added dropwise to the indole mixed solution, and then shaken, and then filtered to obtain nanoscale magnetic Fe3O4 particles loaded with polybenzazole; the nanoscale magnetic Fe3O4 particles loaded with polybenzazole are repeatedly washed with pure water to remove impurities, and then vacuum dried and sieved to obtain nanoscale magnetic Fe3O4 particles loaded with polybenzazole.

[0014] In the above scheme, after synthesizing nanoscale magnetic Fe3O4 particles, a chemical oxidation method is used to oxidatively polymerize indole monomers under the action of ammonium persulfate oxidant to generate polyindole, thereby obtaining nanoscale magnetic Fe3O4 particles loaded with polyindole, and successfully constructing Fe3O4@Pind, in order to retain the structure and magnetic properties of ferrosoferric oxide while also having the reducing properties of polyindole, thereby continuously and stably generating hydrated electrons.

[0015] Furthermore, the ratio of ethylene glycol to diethylene glycol in the mixed solution is (2-10): (1-5), preferably 2:1; the concentration of ferric chloride in the mixed solution is 5-15, preferably 9.9-10.1 g / L; the concentration of sodium acetate in the mixed solution is 30-75, preferably 49.9-50.1 g / L; the concentration of trisodium citrate in the mixed solution is 10-20, preferably 14.9-15.1 g / L.

[0016] In the above scheme, by limiting the concentration of each substance in the mixed solution to a reasonable range, the efficiency of the polymerization reaction is improved.

[0017] Furthermore, the shaking speed is 400-600 rpm, and the time is 25-35 minutes; preferably, the shaking speed is 500 rpm, and the time is 30 minutes.

[0018] In the above scheme, by limiting the shaking speed and time within a reasonable range, it is beneficial to fully carry out the polymerization reaction.

[0019] Furthermore, the temperature of the high-temperature solvent thermal treatment is 190-210° C., and the time is 9-11 hours; preferably, the temperature of the high-temperature solvent thermal treatment is 200° C., and the time is 10 hours.

[0020] In the above scheme, by limiting the temperature and time of the high-temperature solvent heating to a reasonable range, it is beneficial to fully carry out the polymerization reaction.

[0021] Furthermore, the concentration of indole in the indole mixed solution is 1.6-1.7 g / L.

[0022] In the above scheme, by limiting the concentration of indole in the indole mixed solution to a reasonable range, the efficiency of the polymerization reaction is improved.

[0023] Furthermore, the pure water is pure water at room temperature; and / or the vacuum drying is vacuum drying at 60° C. for 12 hours.

[0024] In the above scheme, by selecting a suitable type of washing solvent, impurities on the nano-scale magnetic Fe3O4 particles loaded with polybenzazole can be fully washed away, thereby improving the purity of the product. By limiting the conditions of vacuum drying, the purpose of high-quality drying can be achieved.

[0025] Furthermore, the synthesized Fe3O4@Pind is mixed with hexavalent chromium to form a mixed solution; the formed mixed solution is irradiated with light so that the constructed Fe3O4@Pind nanoscale magnetic Fe3O4 particles are loaded with polybenzazole to construct a heterojunction to promote the reduction of hexavalent chromium, which specifically includes the following steps:

[0026] Prepare hexavalent chromium aqueous solution;

[0027] The synthesized Fe3O4@Pind is dispersed in the prepared hexavalent chromium aqueous solution, and the pH value is adjusted to 3.0-4.0, and then stirred to form a mixed solution; a mercury lamp is used as a light source for illumination, and the light is turned on to carry out a reduction reaction.

[0028] In the above scheme, by functionalizing polybenzazole nanoparticles on the surface of nanoscale magnetic Fe3O4 particles to construct a heterojunction, the electron-hole separation efficiency can be effectively improved and the electron lifetime can be extended, thereby promoting the reduction of hexavalent chromium.

[0029] Furthermore, the reduction reaction system is an open system and is not isolated from air.

[0030] In the above scheme, the reduction reaction system is an open system and is not isolated from the air, which makes the degradation conditions simple and not harsh.

[0031] Furthermore, the reduction reaction temperature is controlled at 25±1° C., the reaction time is 0.5-3 h, and the light source is a 300W mercury lamp, the wavelength of the emitted light of which is mainly concentrated at 365 nm.

[0032] In the above scheme, the reduction efficiency can be improved by reasonably selecting the temperature, time and light source of the reduction reaction.

[0033] Furthermore, in the mixed solution, the concentration of Fe3O4@Pind in the mixed solution is 0.05-0.5 g / L, and the water used in the reaction is ultrapure water.

[0034] In the above scheme, by limiting the contents of Fe3O4@Pind and hexavalent chromium in the mixture, Fe3O4@Pind can more effectively reduce hexavalent chromium, thereby improving the reduction efficiency.

[0035] The present invention provides a hexavalent chromium reduction method that uses nanoscale magnetic Fe₃O₄ particles loaded with polybenzazole to create a heterogeneous interface to generate photogenerated electrons. The photogenerated electrons from the Fe₃O₄ are transferred to the polybenzazole, while the holes generated by the polybenzazole can be transferred to the Fe₃O₄. Thus, the heterogeneous interface facilitates the separation of electron-hole pairs. This method achieves the goal of continuously generating photogenerated electrons and effectively reducing hexavalent chromium under ultraviolet light, which is of great significance for addressing the environmental pollution problem caused by hexavalent chromium. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the present invention or the prior art, a brief introduction will be given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0037] Figure 1 This is the infrared spectrum of Fe3O4@Pind obtained in Example 1 of the present invention;

[0038] Figure 2 This is the XRD spectrum of Fe3O4@Pind obtained in Example 1 of the present invention;

[0039] Figure 3 This is a comparison chart of the reduction rates of hexavalent chromium by different materials in Example 1 of the present invention;

[0040] Figure 4This is a graph showing the reduction rate of hexavalent chromium at different concentrations of Fe3O4@Pind and hexavalent chromium in Example 2 of the present invention;

[0041] Figure 5 This is a graph showing the reduction rate of hexavalent chromium of Fe3O4@Pind at different pH values ​​in Example 3 of the present invention. Reaction conditions: C0(Cr): 50 mg / L;

[0042] Figure 6 This is a comparison chart of the reduction rate of hexavalent chromium of Fe3O4@Pind under different anion concentrations in Example 4 of the present invention, reaction conditions: C0(Cr): 50 mg / L. DETAILED DESCRIPTION

[0043] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention. Example 1

[0044] This embodiment provides a method for reducing hexavalent chromium, which specifically includes the following steps:

[0045] (1) Preparation and characterization of Fe3O4@Pind:

[0046] 120 mL of ethylene glycol and 60 mL of diethylene glycol were placed in a beaker and mixed thoroughly. 1.8 g of ferric chloride, 9 g of sodium acetate, and 2.7 g of trisodium citrate were then quickly added and stirred thoroughly to dissolve. The mixed solution was shaken at 500 rpm for 30 minutes, then subjected to a high-temperature solvothermal reaction at 200°C for 10 hours. The solution was then cooled to room temperature to form a nanoscale magnetic Fe₃O₄ solution. 0.3 g of indole was dissolved in the nanoscale magnetic Fe₃O₄ solution to form an indole mixed solution. Subsequently, 33 mL of an aqueous solution containing 0.067 g of ammonium persulfate was added dropwise. The mixed solution was shaken at 500 rpm for 30 minutes. The polybenzazole-loaded nanoscale magnetic Fe₃O₄ particles were repeatedly washed with pure water to remove impurities and then vacuum-dried at 60°C for 12 hours. Finally, polybenzazole was ground using an agate mortar and passed through a 100-mesh sieve to obtain nanoscale magnetic Fe3O4 particles loaded with polybenzazole, and Fe3O4@Pind was successfully prepared.

[0047] The prepared Fe3O4@Pind was characterized by infrared spectroscopy ( Figure 1 ), compared with the infrared spectrum of polybenzazole reported in the literature, at 1107 cm -1The stretching vibration of the polyindole benzene ring structure was observed, proving that polyindole was successfully loaded.

[0048] The prepared Fe3O4@Pind was characterized by XRD ( Figure 2 ), compared with the XRD patterns of prepared polybenzazole and magnetic ferroferric oxide, the Fe3O4@Pind curve showed 5 peaks, corresponding to different crystal planes of Fe3O4 (220), (311), (400), (511), and (440). Fe3O4@Pind did not change the position of its peaks, indicating that the Fe3O4 crystal structure was well preserved.

[0049] (2) Photochemical reduction reaction: Before the reduction reaction, a 50 mg / L hexavalent chromium aqueous solution was prepared. Fe3O4@Pind was then dispersed in the hexavalent chromium aqueous solution and the pH of the solution was adjusted to 3.0 using 0.1 M NaOH and HCl. The prepared reaction solution was stirred for 0.5 hours using a magnetic stirrer to form a mixed solution. The prepared mixed solution was transferred to a quartz reaction tube and stirred using a magnetic stirrer to ensure uniform reaction during the reaction. A mercury lamp was placed in the middle of the reaction device, close to the reaction solution. After 30 minutes of reaction in a dark environment, the lamp was turned on for the reduction reaction. The reaction volume was 20 mL, the reaction temperature was controlled at 25 ± 1 °C, the reaction time was 3 hours, and the light source was a 300 W mercury lamp (the wavelength of the emitted light was mainly concentrated at 365 nm). The contents of Fe3O4@Pind and hexavalent chromium in the reaction solution were 0.5 g / L and 50 mg / L, respectively. 1 mL of sample was collected every 30 minutes, and the residual hexavalent chromium content of the sample was detected by diphenylcarbazide colorimetry using a UV-Visible Spectrophotometer to calculate the hexavalent chromium reduction rate.

[0050] From the experimental results, we can see that ( Figure 3 ). Using UV irradiation alone, the reduction rate of hexavalent chromium was approximately 8% after 3 hours of reaction. With the addition of polybenzazole, the reduction rate of hexavalent chromium was approximately 84% after 3 hours of reaction; with the addition of nanoscale magnetic Fe3O4 particles, the reduction rate was approximately 100% after 3 hours of reaction. The addition of Fe3O4@Pind significantly accelerated the reduction of hexavalent chromium, achieving complete reduction after 1 hour of reaction, with a reduction rate of approximately 100%. The reaction system was open and not isolated from air. These experimental results demonstrate that under UV irradiation, Fe3O4@Pind enhances interfacial charge transport while increasing the redox potential, generating photogenerated electrons that efficiently reduce hexavalent chromium. Example 2

[0051] This example studies the reduction rate of hexavalent chromium under different concentrations of Fe3O4@Pind and hexavalent chromium ( Figure 4 ). Four concentrations of Fe3O4@Pind in the system were set, namely 0.05g / L, 0.1g / L, 0.2g / L, and 0.5g / L, wherein the hexavalent chromium concentration of the same batch remained unchanged. A total of 4 batches were set, and the hexavalent chromium concentrations were set to 10mg / L, 25mg / L, 50mg / L, and 100mg / L, respectively. The other reaction conditions were the same as those of the photochemical reduction reaction in Example 1. The results show that the reduction efficiency of hexavalent chromium is proportional to the amount of Fe3O4@Pind used. As the concentration of hexavalent chromium increases, the reduction rate of hexavalent chromium by Fe3O4@Pind decreases, but it still has certain reduction performance. Example 3

[0052] This example studies the reduction rate of hexavalent chromium in Fe3O4@Pind at different pH values ​​( Figure 5 Five pH values ​​were set: 2, 3, 4, 6, and 8. The hexavalent chromium concentration remained constant at 50 mg / L, and the Fe3O4@Pind concentration in the system remained constant at 0.5 g / L. All other reaction conditions were identical to those for the photochemical reduction reaction in Example 1. The results showed that the adsorption performance of Fe3O4@Pind for Cr(VI) at pH values ​​of 6 and 8 was much lower than that at pH values ​​of 2, 3, and 4. The optimal hexavalent chromium reduction rate was achieved at pH 3. Example 4

[0053] This example studies the reduction rate of hexavalent chromium in Fe3O4@Pind at different anion concentrations compared to the change without adding each ion ( Figure 6 ). The concentrations of four different anions in the Fe3O4@Pind system were set, namely nitrate ion, phosphate ion, sulfate ion and chloride ion. A total of 4 batches were set, and the concentrations of each anion were set to 5 mg / L, 10 mg / L, 20 mg / L and 50 mg / L respectively. The other reaction conditions were the same as those of the photochemical reduction reaction in Example 1. The results showed that, except for nitrate ion, which had almost no effect on the reaction system, the coexisting anions had a negative impact on the photoreduction of hexavalent chromium, and the order was as follows: phosphate ion > sulfate ion > chloride ion.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for reducing hexavalent chromium, characterized in that: The steps include: Nano-scale magnetic Fe3O4 particles were prepared and loaded with polybenzazole to construct a heterojunction photocatalyst of magnetic Fe3O4 particles and polybenzazole; The synthesized magnetic Fe3O4 particles and polyindole heterojunction photocatalyst are mixed with hexavalent chromium to form a mixed solution; the formed mixed solution is irradiated with light so that the magnetic Fe3O4 particles and polyindole heterojunction photocatalyst promote the reduction of hexavalent chromium.

2. The reduction method according to claim 1, characterized in that The preparation of magnetic Fe3O4 particles and polybenzazole heterojunction photocatalyst specifically includes the following steps: Weigh ferric chloride, sodium acetate, and trisodium citrate and dissolve them in ethylene glycol and diethylene glycol solutions to form a mixed solution; The mixed solution is shaken, subjected to a high-temperature solvent thermal reaction, and then cooled to room temperature to form a nano-scale magnetic Fe3O4 solution; Weigh indole and dissolve it in nano-magnetic Fe3O4 solution to form an indole mixed solution; weigh ammonium persulfate and dissolve it in pure water to form an ammonium persulfate solution; The ammonium persulfate solution was added dropwise to the indole mixed solution, and then shaken, and then filtered to obtain nanoscale magnetic Fe3O4 particles loaded with polyindole; the nanoscale magnetic Fe3O4 particles loaded with polyindole were repeatedly washed with pure water to remove impurities, and then vacuum dried and sieved to obtain nanoscale magnetic Fe3O4 particles loaded with polyindole, successfully constructing a magnetic Fe3O4 particle and polyindole heterojunction photocatalyst.

3. The reduction method according to claim 2, characterized in that The ratio of ethylene glycol to diethylene glycol in the mixed solution is (2-10): (1-5); the concentration of ferric chloride in the mixed solution is 5-15 g / L; the concentration of sodium acetate in the mixed solution is 30-75 g / L; and the concentration of trisodium citrate in the mixed solution is 10-20 g / L.

4. The reduction method according to claim 2, characterized in that The shaking speed is 400-600 rpm and the shaking time is 25-35 minutes.

5. The reduction method according to claim 2, characterized in that The temperature of the high-temperature solvent heating is 190-210° C., and the time is 9-11 hours.

6. The reduction method according to claim 2, characterized in that The concentration of indole in the indole mixed solution is 1.6-1.7 g / L.

7. The reduction method according to claim 2, characterized in that: The pure water is pure water at room temperature; and / or the vacuum drying is vacuum drying at 60° C. for 12 hours.

8. The reduction method according to claim 1, characterized in that The synthesized nano-scale magnetic Fe3O4 particles loaded with polybenzazole are mixed with hexavalent chromium to form a mixed solution; the formed mixed solution is irradiated with light so that the nano-scale magnetic Fe3O4 particles are loaded with polybenzazole to construct a heterojunction to promote the reduction of hexavalent chromium; the specific steps include: Prepare hexavalent chromium aqueous solution; The synthesized nano-scale magnetic Fe3O4 particles loaded with polybenzazole are dispersed in a prepared hexavalent chromium aqueous solution, and the pH value is adjusted to 3.0-4.0, and then stirred evenly to form a mixed solution; a mercury lamp is used as a light source for illumination, and the light is turned on to carry out a reduction reaction.

9. The reduction method according to claim 8, characterized in that The reduction reaction system is an open system and is not isolated from air; and / or The reduction reaction temperature is controlled at 25±1°C, the reaction time is 0.5-3h, and the light source is a 300W mercury lamp, the wavelength of which is mainly concentrated at 365nm; And / or in the mixed solution, the concentration of the nano-scale magnetic Fe3O4 particles loaded with polybenzazole in the mixed solution is 0.05-0.5 g / L, and the water used in the reaction is ultrapure water.

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