Catalyst for promoting the formation of dense and stable active catalytic layer on the surface of iron filings by citric acid, modification method thereof and application thereof

Abstract

The present invention relates to a catalyst for promoting the formation of a dense and stable active catalytic layer on the surface of iron filings by citric acid, a modification method thereof, and an application thereof. The catalyst comprises the following steps: placing pretreated iron filings in a modification solution and soaking them for a reaction period of 1.0 to 2.0 hours to obtain a surface-modified iron filings-based catalyst, wherein the modification solution contains hydrogen peroxide and citric acid in a mass ratio of (0.1-1):(0.05-0.5). Compared with the prior art, the present invention has the advantages of being environmentally friendly in its modification solution composition, being simple in its preparation method, being low in cost, having strong catalytic activity after modification, and having good resistance to wastewater corrosion.

Description

Technical Field

[0001] The present invention relates to the field of water pollution control, in particular to the use of surface-modified iron filings for catalytic ozonation wastewater treatment, and specifically to a catalyst using citric acid to promote the formation of a dense, stable, active catalytic layer on the surface of the iron filings, as well as a modification method and application thereof. Background Art

[0002] Catalytic ozonation technology has been widely used in the field of water pollution control. Under the action of catalysts, O3 is continuously converted into ·OH. The strong oxidizing property of ·OH can completely oxidize organic pollutants into CO2 without selection, thus meeting emission standards. O3 is relatively expensive. In order to further improve the advantages of catalytic ozonation technology in the field of water pollution control, it is necessary to develop catalysts that are cheap, easy to obtain, have good catalytic performance, are clean and green, and are easy to promote in engineering. Iron-based catalysts have been proven to have excellent O3 catalytic activity. The large amount of iron in nature also ensures that the source of raw materials is sufficient. There are three main ways to prepare iron-based catalysts: (1) iron salt solution synthesis method; (2) loaded impregnation sintering method; (3) iron chip surface modification method. The catalyst synthesized from iron salt solution is in powder form, has a large specific surface area, and good catalytic performance. However, powder catalysts have problems of agglomeration and loss in actual application, with large daily losses, difficulty in recycling, and high cost of using solution; the impregnation sintering method consumes a large amount of chemical agents and is complex to operate; the iron chip modification method is simple to operate, the raw materials are cheap and easy to obtain, and the modified iron chips are easy to machine and shape for engineering practice, and its advantages are becoming increasingly prominent.

[0003] CN110152667A discloses a method for modifying the surface of iron filings using H2O2. While H2O2 is oxidizing the iron filings, O3 is introduced to control the redox potential of the solution. An alkaline solution is used to adjust the pH to a neutral or slightly alkaline environment to generate γ-FeOOH. The scraps are then immersed in the modified solution for 1.0 to 3.0 hours and then naturally air-dried to obtain a modified iron filing catalyst with a dense γ-FeOOH oxide layer covering the surface. This method maintains a relatively stable oxidizing environment through continuous O3 exposure, but monitoring the redox potential is complex. Furthermore, the H2O2 used has limited oxidizing capacity, resulting in a relatively thin γ-FeOOH oxide layer deposited on the surface. This can lead to surface catalytic layer loss during wastewater treatment, and its service life needs to be improved.

[0004] CN113106433B discloses a method for using iron ions to rapidly generate γ-FeOOH on the surface of iron filings. The method involves pre-adding a ferrous sulfate solution to achieve a supersaturated state of iron ions near the filings, accelerating the deposition of iron oxides on the surface. Hydrogen peroxide solution is then added stepwise to maintain the H₂O₂ concentration in the modified system between 0.05% and 0.45%, accelerating the formation of γ-FeOOH on the surface. This method can accelerate the deposition rate on the filings' surface, but the resulting deposited product under these reaction conditions is primarily FeOOH, an iron oxide that is less dense than a chromium-containing oxide film. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a catalyst and modification method and application thereof that promote the formation of a dense and stable active catalytic layer on the surface of iron filings using citric acid. The catalyst can improve the problems of low active catalytic components after surface modification of iron filings, thin γ-FeOOH catalytic layer generated, and easy loss during use. Citric acid and hydrogen peroxide are used to modify the iron filings. The complexing properties of citric acid and iron are utilized to promote the dissolution of iron at the zero-valent iron reaction interface, increase the modification speed, and increase the fineness of the surface deposited particles and the density of the catalytic layer after modification, thereby improving the catalytic performance and stability and extending the service life.

[0006] The object of the present invention can be achieved by the following technical scheme: a method for promoting the formation of a dense, stable and active catalytic layer on the surface of iron filings by using citric acid, comprising the following steps: placing pretreated iron filings in a modifying liquid and allowing them to react for 1.0 to 2.0 hours to obtain a surface-modified iron filings-based catalyst, wherein the modifying liquid contains hydrogen peroxide and citric acid in a mass ratio of (0.1-1): (0.05-0.5).

[0007] The modified liquid contains 0.1wt%-1.0wt% of hydrogen peroxide, 0.05wt%-0.5wt% of citric acid, and the balance is water.

[0008] The modification reaction of the modified liquid is divided into two stages

[0009] (1) The early stage is the dissolution of iron: citric acid ionizes H + Oxidation of elemental iron to Fe 2+ , oxidized to Fe under the action of H2O2 3 + , Fe 3+ It preferentially complexes with citrate to achieve Cr enrichment.

[0010] (2) The later stage is the deposition of the active catalytic layer: As the reaction proceeds, the amount of Fe(III) in the solution increases, the pH rises, the precipitation reaction competes with the complexation reaction, and Fe(III) is deposited to form a composite metal oxide with the Cr enriched in the early stage, thereby improving the density of the deposited product.

[0011] Furthermore, after the citric acid oxidizes the iron element, the pH of the modified system is maintained between 2.3 and 4.5 (preferably 2.3 to 4.2).

[0012] The citric acid contains three carboxyl groups, which can form complexes with metal ions. The complex stability constant of citrate ion and iron ion is lgK=14.5, which is significantly higher than other transition metal ions in iron filings. Therefore, in the process of citric acid promoting iron dissolution, citrate ion preferentially complexes Fe 3+ , to achieve the enrichment of Cr on the surface of iron chips.

[0013] As the modification reaction proceeds, the amount of Fe(Ⅲ) in the solution increases. When pH>3, the amount of Fe in the reaction system increases. 3+ The precipitation reaction is dominant, generating FeOOH-Cr(OH)3 deposition products, which are eventually converted into (Cr,Fe)2O3 composite metal oxides, which are significantly more compact than the previous iron oxides.

[0014] The pretreatment of the iron filings is as follows: first, use detergent to wash away the obvious oil stains on the surface of the iron filings, rinse with tap water, then soak with alkali solution to remove the oil stains tightly adsorbed on the surface, and finally wash with deionized water to remove the residual alkali solution on the surface, so that the surface of the iron filings is smooth and free of impurities.

[0015] The alkali solution comprises one or more of NaOH, KOH or sodium ethoxide solution with a concentration of 0.5-2.0 mol / L.

[0016] The alkali solution soaking time is 1.0 to 2.0 hours.

[0017] The iron filings are 38CrMoAl steel, waste materials generated during metal processing. Another objective of the present invention is to utilize the Cr contained in the 38CrMoAl alloy steel to simultaneously oxidize with Fe during the modification process to form a (Cr,Fe)2O3 composite metal oxide, significantly improving the structural properties of the modified catalytic layer and reducing wear and tear during use.

[0018] After the modification is completed, the iron filings are taken out from the modification solution and naturally air-dried at room temperature to obtain a surface-modified iron filings-based catalyst.

[0019] The amount of iron filings added to the modified solution is 50g / L.

[0020] Citric acid is an organic acid that is used in the field of metal corrosion protection to dissolve oxides on the surface of steel. Compared with inorganic acids such as hydrochloric acid and sulfuric acid, it is safe, non-toxic, and easily biodegradable. As an organic acid, citric acid can form complexes with metal ions. For alloy steels containing metal elements such as Cr and Fe, citric acid preferentially reacts with Fe and Fe oxides to dissolve them, and at the same time, Cr elements are enriched on the surface. Increasing the Cr content in the surface composition can enhance the density and stability of the material. Therefore, when passivating the metal, the surface Fe is dissolved as much as possible to increase the content of Cr(OH)3 and Cr2O3 in the surface layer. Chromium oxide has excellent anti-corrosion properties and also has certain catalytic ozone properties.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) Good modification effect and uniform film formation. Citric acid is conducive to the formation of a thicker active catalytic layer. As a strong organic acid, the H + Able to oxidize iron into Fe 2+ ; converted into Fe under the action of H2O2 3+ The iron dissolution is facilitated by complexation with citrate ions. The iron dissolved in this process provides conditions for subsequent Fe(III) deposition, contributing to the formation of a thicker active catalytic layer in the later stage. The concentration of citric acid can be adjusted according to the properties of the iron filings matrix. The iron filings described in the present invention are of the type 38CrMoAl. In addition, citric acid partially ionizes in solution and has a strong buffering capacity, which can maintain a relatively stable pH value in the modified system, facilitating the stable growth of precipitated particles and forming a more uniform film.

[0023] (2) The preparation method is green and safe, and the reagents used are environmentally friendly. Citric acid and hydrogen peroxide are both green, safe and non-toxic reagents, and citric acid is easily biodegradable, which brings great convenience to engineering applications; the modified system composed of the two creates favorable conditions for the generation of γ-FeOOH. Citric acid oxidizes iron into Fe 2+ After that, the pH of the modified system can be maintained between 2.3 and 4.5, which is conducive to the conversion of Fe 2+ It is converted into γ-FeOOH and deposited on the surface of iron chips to form a film. γ-FeOOH has been proven to have good adsorption properties and sufficient active sites, making it a high-quality catalyst in the field of catalytic ozone.

[0024] (3) The modified product is dense and stable. The iron filings used are 38CrMoAl alloy steel, and the Cr element contained in them will also be oxidized during the modification process. By adjusting the H2O2 concentration, citric acid concentration, modification time, and pH value during the modification process, the Cr in the iron filings and the dissolved Fe ions are promoted to simultaneously precipitate to form (Cr,Fe)2O3 composite metal oxides, significantly improving the density of the surface active catalytic layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The following is a comparison of the appearance of iron chips before and after modification.

[0026] Figure 2 The TOC removal rate of the modified iron chip catalyst used in actual wastewater treatment.

[0027] Figure 3 is the SEM surface morphology of the modified product. Figure 3 (a) is comparative example 1 (0.3% H2O2 modification alone), Figure 3 (b) is Example 8 (0.3% H2O2 + 0.2% citric acid modification).

[0028] Figure 4 The results obtained from XRD analysis.

[0029] Figure 5 This is the EDS element content distribution corresponding to Example 8. DETAILED DESCRIPTION

[0030] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process. However, the protection scope of the present invention is not limited to the following embodiment.

[0031] The invention includes the following ideas: (1) accelerating the dissolution of surface iron by controlling the concentration of citric acid; (2) utilizing the buffering capacity of citric acid in the pH range of 3.0 to 5.0 to adjust the pH value during the modification process, thereby forming a relatively stable modification environment that is conducive to the formation of a uniform and dense catalytic film layer; (3) shortening the modification time by controlling the concentration of H2O2, thereby providing a catalyst that is conducive to the dissolution of surface iron; (4) accelerating the dissolution of surface iron by controlling the concentration of citric acid; (5) adjusting the pH value during the modification process by using the buffering capacity of citric acid in the pH range of 3.0 to 5.0 to form a relatively stable modification environment that is conducive to the formation of a uniform and dense catalytic film layer; (6) shortening the modification time by controlling the concentration of H2O2, thereby providing a catalyst that is conducive to the dissolution of surface iron by controlling the concentration of citric acid; (7) adjusting the pH value during 2+ The favorable conditions for oxidation to generate γ-FeOOH were created and deposited on the surface to form a film structure; (4) iron chips of model 38CrMoAl were selected and the Cr element contained in the iron chips was used for synchronous oxidation to improve the density of the modified iron chips; (5) degradation experiments and corrosion tests were used to verify the catalytic effect and stability of the modified iron chips, and the component concentration of the modified liquid was adjusted to achieve the optimization of catalytic activity and stability.

[0032] The present invention can be implemented through the following solutions.

[0033] Prepare a modifying liquid with the following specific composition: 0.05%-0.5% citric acid and 0.1%-1.0% hydrogen peroxide.

[0034] The function of citric acid is to dissolve the oxides on the surface of iron filings, accelerate the release of elemental iron, and promote the dissolution of elemental iron at the reaction interface by the complexation between citrate and iron ions.

[0035] Hydrogen peroxide, acting as an oxidant, promptly oxidizes the ferrous ions produced by the oxidation of elemental iron with citric acid, generating γ-FeOOH that deposits on the surface of the iron filings. Simultaneously, other metallic elements within the iron filings, such as chromium, are also oxidized, forming a (Cr,Fe)2O3 composite metal oxide on the surface. This co-deposition of chromium oxide significantly improves the density and stability of the modified product.

[0036] The modification operation mainly includes the following steps.

[0037] (1) Select scrap iron filings, pre-treat them, and clean the surface stains. Pre-treatment specifically refers to: first use detergent to wash away the obvious oil stains on the surface of the iron filings, rinse with tap water, and then soak them in alkaline solution to remove the oil stains that are tightly adsorbed on the surface. Finally, use deionized water to wash away the residual alkaline solution on the surface, so that the surface of the iron filings is smooth and free of impurities. The alkaline solution includes one or more of NaOH, KOH, or sodium ethoxide solutions with a concentration of 0.5 to 2.0 mol / L. The soaking time in the alkaline solution is 1.0 to 2.0 hours.

[0038] (2) Surface modification of the cleaned iron filings is performed by preparing a modification solution of a specific composition, immersing the cleaned iron filings in the modification solution to accelerate the dissolution of surface oxides and the dissolution and simultaneous oxidation of elemental iron, and allowing the reaction to proceed for 1.0 to 2.0 hours. The modification solution comprises: H2O2 at a mass concentration of 0.1% to 1.0% and citric acid at a mass concentration of 0.05% to 0.5%. The immersion time for the operation is 1.0 to 2.0 hours.

[0039] (3) After the modification is completed, the iron filings are taken out from the modification solution and naturally air-dried at room temperature to obtain a surface-modified iron filings-based catalyst.

[0040] Examples 1-10 and Comparative Examples 1-2

[0041] The results of catalytic ozonation of wastewater using iron filings with different modification conditions were analyzed, and the effluent TOC and total iron content were measured. The wastewater used was a 100 mg / L sodium benzoate solution.

[0042] The surface modification method of iron chips according to the present invention is used to modify the iron chips, and the specific steps are as follows:

[0043] (1) Iron filings pretreatment: The selected 38CrMoAl iron filings were cleaned with detergent to remove the outer oil stains; then they were soaked in 1 mol / L NaOH solution for 1.0 h to remove the oil stains tightly adhering to the surface; finally, the residual alkali solution on the surface was rinsed with clean water.

[0044] (2) Modification operation: Prepare the iron filings surface modification liquid (the formulation of each component is described in Table 1). Divide the iron filings treated in step 1 into several equal portions and immerse them in the series of modification liquids described in Table 1. Allow them to react for 1.0 to 2.0 hours and measure the pH before and after the reaction. After the reaction is completed, remove the iron filings and air dry them. After the surface is dry, rinse with clean water to remove any residual modifier on the surface. The modification is complete, and modified iron filings are obtained.

[0045] (3) Using the modified iron filings obtained in step (2) as a catalyst, a catalytic ozonation experiment was conducted. Experimental conditions: 50 g / L modified iron filings, 0.5 L wastewater, pH 6.8, ozone concentration 10 mg / min, reaction time 2.0 h.

[0046] Wherein comparative example 1 and comparative example 2 are used as contrast:

[0047] In Comparative Example 1, the modified liquid composition is 0.3% H2O2 alone, and the other treatments are the same as the above steps (1) to (3); the comparison of the results of Comparative Example 1 with those of Example 8 can highlight the effect of citric acid.

[0048] Comparative Example 2 was used as a blank control. 38CrMoAl iron filings were selected and directly subjected to the catalytic ozonation experiment of step (3) without any treatment. Comparison of the results of Comparative Example 2 with those of Examples 1-10 showed the optimization effect of the modification system used in the present invention.

[0049] Table 1. Mass ratio of each component of the iron chip surface modifier: Examples 1-10, Comparative Examples 1-2

[0050]

[0051]

[0052] The performance evaluation results of Examples 1-10 and Comparative Examples 1-2 are shown in Table 2.

[0053] Table 2. Performance evaluation results of examples and blank controls

[0054]

[0055] From Table 1-2 above, we can see that:

[0056] 1. Citric acid concentration

[0057] Citric acid dissolves oxides on the surface of iron filings and also dissolves zero-valent iron from the substrate surface. A citric acid concentration in the range of 0.05% to 0.5% produces an iron filing catalyst with both catalytic effectiveness and structural strength, with optimal performance at 0.1% to 0.3%. At lower citric acid concentrations, iron dissolution increases in tandem with citric acid concentration. Therefore, within the range of 0.05% to 0.2%, catalytic performance and compactness increase simultaneously with increasing citric acid concentration. Continuing to increase the citric acid concentration beyond 0.2% does not improve performance. Excessive citric acid not only results in waste but also causes the pH value to be too low in the later stages of the reaction, affecting the precipitation reaction. Therefore, a citric acid concentration between 0.05% and 0.5% is recommended.

[0058] 2. Hydrogen peroxide concentration

[0059] The role of H2O2 is to dissolve Fe in citric acid 2+ It is then oxidized to form γ-FeOOH, and the Cr in the alloy steel is oxidized to Cr(OH)3 and Cr2O3. The two together form a γ-FeOOH / Cr(OH)3 composite structure, ultimately transforming into (Cr,Fe)2O3, further improving the density of the catalytic membrane. According to experimental results, the optimal H2O2 concentration is 0.1% to 1.0%, with optimal performance at 0.3% to 0.5%. At lower H2O2 concentrations, the modified system's oxidizing ability is weak and cannot meet the conditions for the formation of γ-FeOOH; however, excessively high H2O2 concentrations result in waste.

[0060] 3. pH value

[0061] The initial pH value of the modified system is determined by the concentration of citric acid. During the entire modification process, the pH of the system is maintained between 2.3 and 4.5, which meets the pH requirements for iron filings dissolution and subsequent precipitation. As the modification reaction proceeds, citric acid continuously ionizes and releases H + , maintaining a relatively stable pH is conducive to a more uniform and dense surface film, and keeping the pH value of the reaction system unchanged before and after modification.

[0062] 4. Modification time

[0063] If the reaction time is too short, the γ-FeOOH film is not fully formed; if the reaction time is too long, the H2O2 is completely consumed, the oxidation capacity of the modification system is insufficient, and the thickness of the γ-FeOOH catalytic film no longer increases. Experimental results show that the optimal modification time is between 1.5h and 2.0h.

[0064] Water sample treatment effect

[0065] According to the treatment effect in Table 2, (blank control) iron filings also have a certain TOC degradation effect without modification, but the process is Fe 0During the ozone activation process, a large amount of iron ions are dissolved and the iron filings are seriously lost. The modified iron filings are added as catalysts to the ozonation wastewater treatment system, and the TOC removal rate is significantly improved. After 2 hours of reaction, the removal rate reaches more than 70% (Examples 5, 8, and 9), and the total iron in the effluent is less than 10 mg / L. The above experimental results show that the modification method described in the present invention can obtain an iron filing-based catalyst with good catalytic effect and excellent stability. The reagents used in the present invention are safe and non-toxic, citric acid is easily biodegradable, the modification operation is simple, the raw materials are cheap, and it has practical application value.

[0066] Figure 1 The following is a comparison of the appearance of iron filings before and after modification in Example 8. Before modification, the iron filings were silvery white and bright, while after modification, the surface turned brownish yellow, became rougher, and was covered with fine iron oxides.

[0067] Figure 2 This is the TOC removal rate of the iron filings catalyst used in actual wastewater treatment after the reaction in Example 8.

[0068] Figure 3 is the SEM surface morphology of the modified product. Figure 3 (a) is comparative example 1 (0.3% H2O2 modification alone), the modified product is flaky crystals with dense stacking; Figure 3 (b) is Example 8 (0.3% H₂O₂ + 0.2% citric acid modification). The modified product is spherical particles. The small spherical particles further grow and combine to form larger particles, which then connect to form an uneven shape. The stacking creates larger gaps, facilitating O₃ contact with the surface. The SEM comparison above shows that the addition of citric acid to the modifier results in finer particles and a higher porosity, improving O₃ catalytic performance.

[0069] Figure 4 The results of XRD analysis of the modified product of Example 8 show that the main substance in the modified product is γ-FeOOH, and a certain amount of (Cr, Fe)2O3 is also formed.

[0070] Figure 5 The EDS element content distribution corresponding to Example 8 shows that the Cr content is 3.23wt%, which is much higher than the 1.35%-1.65% in the original steel, proving that the modification method of the present invention achieves the enrichment of Cr in the surface layer and enhances the density of the catalytic film of the modified product; compared with the total iron dissolution results in Table 2, it further proves that the catalyst obtained after the iron filings are modified according to the present invention has less iron dissolution, better stability, and extended service life.

[0071] In summary, the present invention utilizes the complexing properties of citric acid and iron to promote the dissolution of iron at the zero-valent iron reaction interface, increase the modification rate, and increase the fineness of the surface deposited particles and the thickness of the catalytic layer after modification, thereby improving the catalytic performance and extending the service life.

Claims

1. A method for promoting the formation of a dense and stable active catalytic layer on the surface of iron filings using citric acid, characterized in that: The method comprises the following steps: placing pretreated iron filings in a modification solution and allowing the solution to react for 1.0 to 2.0 hours to obtain a surface-modified iron filings-based catalyst, wherein the modification solution contains hydrogen peroxide and citric acid in a mass ratio of (0.1-1): (0.05-0.5); after the citric acid oxidizes the iron element, the pH of the modification system is maintained between 2.3 and 4.5; the main substance in the modified product is γ-FeOOH, and also includes (Cr,Fe)2O3; The modified liquid contains 0.1wt%-1.0wt% of hydrogen peroxide, 0.05wt%-0.5wt% of citric acid, and the balance is water; The iron filings are waste materials generated during metal processing; The obtained surface-modified iron filings-based catalyst is used to catalyze ozonation wastewater treatment.

2. The method for promoting the formation of a dense and stable active catalytic layer on the surface of iron filings by using citric acid according to claim 1, characterized in that: The citric acid has a molecular formula of C6H8O7 and is a small molecule organic acid containing three carboxyl groups. In the early stage, the citric acid acts on the iron filings to promote the dissolution of iron. In the later stage, under the action of the oxidant H2O2, it is conducive to the deposition and formation of iron-chromium composite metal oxides, thereby realizing the utilization of transition metal elements in the iron filings and obtaining a catalyst with better stability.

3. The method for promoting the formation of a dense and stable active catalytic layer on the surface of iron filings by using citric acid according to claim 1, characterized in that: The pretreatment of the iron filings is as follows: first, use detergent to wash away the obvious oil stains on the surface of the iron filings, rinse with tap water, then soak with alkali solution to remove the oil stains tightly adsorbed on the surface, and finally wash with deionized water to remove the residual alkali solution on the surface, so that the surface of the iron filings is smooth and free of impurities.

4. The method for promoting the formation of a dense and stable active catalytic layer on the surface of iron filings by using citric acid according to claim 3, characterized in that: The alkali solution includes one or more of NaOH, KOH or sodium ethoxide solution with a concentration of 0.5-2.0 mol / L.

5. The method for promoting the formation of a dense and stable active catalytic layer on the surface of iron filings by using citric acid according to claim 3, characterized in that: The alkali solution soaking time is 1.0 ~ 2.0h.

6. The method of promoting the formation of a dense and stable active catalytic layer on the surface of iron filings by using citric acid according to claim 1, characterized in that: The model of the iron filings is 38CrMoAl.

7. A surface-modified iron filings-based catalyst prepared by the method according to claim 1.

8. Use of the catalyst according to claim 7, characterized in that: The obtained surface-modified iron filings-based catalyst is used to catalyze ozonation wastewater treatment.

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