Preparation of controllable-etching binary metal hydrotalcite and application of controllable-etching binary metal hydrotalcite to synergistic removal of arsenic-cadmium co-pollution

Abstract

The invention belongs to the technical field of adsorption materials, and discloses preparation of controllable etching binary metal hydrotalcite and a method for synergistically removing arsenic-cadmium co-pollution by the controllable etching binary metal hydrotalcite, which comprises the following steps: uniformly mixing hydrotalcite obtained under different conditions with an arsenic-cadmium solution, with the water consumption of 100-200mL and the catalyst mass of 100mg, and stirring for 30-60 minutes; and adsorbing under the condition of stirring at normal temperature to obtain an aqueous solution with relatively high arsenic and cadmium removal rate. According to the method, an ultra-stable mineralization structure of the hydrotalcite light material is improved by adopting an etching and calcining method, an independent AsIII solution is quickly adsorbed in the first 60 minutes and tends to be balanced subsequently, the adsorption rate in 6 hours is close to 100%, the adsorption capacity qmax is approximately equal to 263.20 mg / g, the AsIII adsorption capacity qmax is obviously improved to be approximately equal to 577.06 mg / g and the Cd (II) adsorption capacity qmax is approximately equal to 426.36 mg / g in the synergistic adsorption of an AsIII / Cd < 2 + > mixed solution. The method is simple to operate, mild in reaction condition and high in AsIII / Cd < 2 + > removal rate.

Description

Technical Field

[0001] The invention relates to a preparation method of controllably etched binary metal hydrotalcite and a method for collaboratively removing arsenic and cadmium co-pollution thereof, belonging to the technical field of adsorption materials. Background Art

[0002] With the accelerated pace of industrialization, human activities such as non-ferrous metal smelting, electroplating, electronics manufacturing, and pesticide application continue to discharge wastewater containing heavy metals and other pollutants. This has led to the widespread accumulation of heavy metals such as As and Cd in water bodies, posing a threat to public health. Heavy metals are highly toxic and bioavailable, easily accumulating in the food chain. Long-term exposure can cause skin lesions, internal organ damage, and even cancer. Therefore, the development of efficient, economical, and recyclable heavy metal removal materials is of great significance.

[0003] Layered double hydroxides (LDH), also known as hydrotalcite, have been widely studied in recent years for the adsorption and fixation of heavy metal ions due to their unique "memory effect", adjustable interlayer cation / anion composition and good chemical stability. The typical LDH formula is [M 2+ 1–x M 2+ x (OH)2] x+ ·(A n– ) x / n mH2O, in which divalent metals provide a layered framework, trivalent metals regulate the interlayer charge, and anions can be intercalated through ion exchange, enabling the selective capture of target pollutants. Compared to traditional adsorbents such as activated carbon and biochar, LDH exhibits higher ion exchange capacity and regenerative properties. However, conventional LDH still faces the problem of low adsorption capacity for heavy metals during the adsorption process.

[0004] In order to overcome the above shortcomings, scholars have introduced a synergistic strategy of etching and calcination in recent years. Etching treatment can significantly increase surface defects and specific surface area by selectively removing some metal hydroxyls or oxygen to form vacancies; calcination regeneration allows LDH to restore the layered structure during the aqueous phase reconstruction process, while forming an oxide / hydroxide mixed active phase, thereby enhancing the adsorption capacity and kinetic performance of heavy metals. Studies have shown that acidic or alkaline etching can produce metal vacancies and oxygen vacancies in LDH, greatly increasing the density of active sites; moderate calcination can rebuild the layered structure in the aqueous phase, which can achieve the adsorption of Pb 2+ Cr 6+ 、Cu 2+ Efficient capture of various metals.

[0005] Based on this, this study took MgFe-LDH as the object and proposed a new strategy for the removal of heavy metal ions. LDH was etched and calcined and applied to the process of ultra-stable mineralization to remove heavy metal ions. The removal process of heavy metal ions by the material in this process and the removal performance of the material for specific heavy metal ions under different conditions were explored, providing new ideas and methods for the conversion of heavy metal ions to hydrotalcite. Summary of the Invention

[0006] In view of the shortcomings and deficiencies of the above prior art, the primary purpose of the present invention is to provide a method for preparing a magnesium-iron layered bimetallic oxide composite material with metal defects and oxygen vacancy defects.

[0007] The second purpose of the present invention is to use the magnesium-iron layered bimetallic oxide composite material prepared by the method to perform synergistic adsorption on As / Cd solution.

[0008] The preparation method of the magnesium-iron layered bimetallic oxide composite material having metal defects and oxygen vacancy defects is as follows:

[0009] a. Preparation of Mg3Fe-LDH: 0.01 mol of FeCl3·6H2O and n(Fe 3+ ) were added into 200 mL of deionized water and stirred at room temperature for 10 min to dissolve all FeCl3·6H2O and MgCl2·6H2O. The solution was named solution A. 3+ )+n(Mg 2+ )]*1.6 amount of NaOH was added to 200mL of deionized water, stirred at room temperature for 1min to completely dissolve the NaOH, and named as solution B. The above solution A and solution B were added to the colloid mill at the same speed and stirred for 3min. After stirring, the solution was transferred to a beaker to obtain a brown-yellow suspension. The suspension was continued to be placed on a constant temperature magnetic stirrer, heated to 60℃ and stirred for 6h to age the LDH crystals. After stirring, the precipitate was centrifuged and washed three times with deionized water and ethanol respectively, and then placed in a constant temperature drying oven and dried at 60℃ for 24h.

[0010] b. Etching of MgFe-LDH: Place different masses (e.g., 5g, 10g, or 20g) of NH4Cl in 100mL of deionized water and stir to completely dissolve the NH4Cl. This solution is named "etching solution." Grind the completely dried MgFe-LDH precursor solid into a powder and add it to the etching solution, stirring for 6 hours to etch the LDH. After stirring, centrifuge the precipitate, wash it three times with deionized water and ethanol, respectively, and place it in a constant temperature drying oven at 60°C for 24 hours.

[0011] c. Preparation of MgFe-LDO: The completely dried MgFe-LDH precursor was ground into powder and calcined in a muffle furnace at different temperatures for 12 h to obtain MgFe-LDO solid.

[0012] d. Preparation of MgFe-LDO after etching: The completely dried etched MgFe-LDH solid was ground into powder and placed in a muffle furnace at different temperatures to obtain MgFe-LDO solid.

[0013] The present invention also provides an application of the MgFe-LDO material prepared by the method described above in the field of ultrastable mineralization, wherein the method is:

[0014] a. As attached Figure 6 Using the adsorption properties of the ultrastable mineralized material, the effects of different metal ratios, etching with different NH4Cl solution concentrations, and calcination temperatures on the adsorption properties were explored. Ultimately, the optimal conditions were determined: Mg:Fe = 5:1, etching with 200g / L NH4Cl solution, and etching at 350°C.

[0015] b. The material of the present invention is a composite material based on magnesium-iron layered bimetallic oxide with high fixation performance for arsenic and cadmium. The composite material has the characteristics of high adsorption efficiency, fast adsorption speed and stability.

[0016] Beneficial Effects: The magnesium iron salts used in the present invention contain naturally occurring elements, are inexpensive, widely available, and environmentally friendly. The resulting reduced hydrotalcite exhibits a high specific surface area and surface charge density, facilitating the simultaneous removal of the heavy metals arsenic and cadmium. The oxidizing divalent iron effectively oxidizes trivalent arsenic to pentavalent chromium, while surface defects in the hydrotalcite mineralize the pentavalent arsenic. More importantly, the hydrotalcite exhibits a synergistic adsorption effect on solutions containing both arsenic and cadmium, resulting in efficient treatment of arsenic-chromium wastewater. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Figure 3 shows the XRD patterns of MgFe-LDH precursors synthesized under different Mg / Fe ratios under the conditions of Example 1. The diffraction peaks at 15°, 30°, and 45° in the XRD pattern are typical of the layered structure of hydrotalcite, primarily corresponding to: 1) the 003 plane; 2) the 006 plane; and 3) the 009 plane. The presence of these peaks indicates that the material possesses a typical LDH structure.

[0018] Figure 2The XRD patterns of MgFe-LDH after etching with NH4Cl solutions of different concentrations under the conditions of Example 1 of the present invention are shown. As the NH4Cl concentration increases, the diffraction peaks change significantly. The information in the figure shows that the crystallinity of LDH decreases with increasing NH4Cl concentration. At 50 g / L and 100 g / L, the diffraction peaks are distinct and sharp, indicating that the material retains a relatively good layered crystalline structure. At 200 g / L, the diffraction peaks begin to broaden and weaken, indicating that the layered structure is partially destroyed and the crystallinity is reduced. At 300 g / L, the diffraction peaks almost disappear, indicating that the layered structure is severely damaged or highly disordered, and the material has essentially transformed into an amorphous or colloidal state. High concentrations of NH4Cl solutions (such as 300 g / L) significantly damage the layered structure, but also indicate that NH4Cl can etch away the Mg layer in MgFe-LDH, thereby causing crystal defects.

[0019] Figure 3 XRD patterns of MgFe-LDO calcined at different temperatures under the conditions of Example 1. As the calcination temperature increases to 350°C, the layered structure gradually collapses, and the diffraction peaks weaken. Upon further heating to 450–550°C, a series of new, high-intensity diffraction peaks emerge, indicating a transition from layered double hydroxide to an oxide phase, generating crystalline phases such as MgO and Fe2O3.

[0020] Figure 4 XRD pattern of MgFe-LDO after etching and calcination under the conditions of Example 1 of the present invention.

[0021] Figure 5 SEM image of MgFe-LDO after etching and calcination under the conditions of Example 1 of the present invention.

[0022] Figure 6 Effect of different Mg / Fe ratios on the synthesis of MgFe-LDH precursors under the conditions of Example 1 of the present invention on As Ⅲ The results showed that the adsorption performance of 5:1 MgFe-LDH on As Ⅲ The removal performance of As was the best after 6 hours. Ⅲ The removal rate was 79.58%, indicating that the layered structure, surface active sites and cation exchange capacity of the material worked together best at this ratio.

[0023] Figure 7 The MgFe-LDH etched by NH4Cl solution with different concentrations under the conditions of Example 1 of the present invention is Ⅲ The results showed that the adsorption performance of As by MgFe-LDH after etching with 200g / L NH4Cl solution was Ⅲ The removal performance of As was the best after 6 hours. ⅢThe removal rate is 81.48%. Comparing MgFe-LDH etched by four different NH4Cl solutions with different mass concentrations, with the increase of NH4Cl concentration, the removal rate of As is increased after the same time. Ⅲ The amount of MgO showed a trend of first increasing and then decreasing, indicating that excessive etching weakened the structural stability and adsorption capacity of the material.

[0024] Figure 8 Effect of MgFe-LDO calcined at different temperatures on As under the conditions of Example 1 of the present invention Ⅲ The results showed that the adsorption performance of As Ⅲ The removal performance of As was the best after 6 hours. Ⅲ The removal rate of As is 93.96%. Comparing the MgFe-LDO calcined under five different temperature conditions, with the increase of temperature, the As removal rate is increased after the same time. Ⅲ The amount of As shows a trend of increasing first and then decreasing. This shows that the active sites of the material are insufficient at lower temperatures, while the structure of the material collapses and the grains grow at higher temperatures, resulting in a decrease in adsorption performance. Therefore, a moderate calcination temperature helps to optimize the specific surface area and active site density of the material, and improve the As Ⅲ removal efficiency.

[0025] Figure 9 The MgFe-LDO after etching and calcination under the conditions of Example 1 of the present invention has a significant effect on As Ⅲ Adsorption performance of As Ⅲ The optimal performance removal rate is 96.94%.

[0026] Figure 10 Effect of MgFe-LDH precursor, etched MgFe-LDH, calcined MgFe-LDO and etched and calcined MgFe-LDO on As under the conditions of Example 1 of the present invention Ⅲ Adsorption capacity curve. The results show that the adsorption capacity of the etched and calcined MgFe-LDO is the highest at 263.20 mg / g, indicating that the performance of MgFe-LDH is greatly improved by the simultaneous etching and calcination treatment.

[0027] Figure 11 MgFe-LDO etched and calcined under the conditions of Example 1 of the present invention is Ⅲ / Cd 2+ =1:1 coexistence system for As Ⅲ The adsorption capacity curve of As Ⅲ In a separate environment, Ⅲ Comparison of adsorption capacity curves of As Ⅲ For example, the adsorption of As alone ⅢThe adsorption capacity is 263.20 mg / g, and when co-adsorbed with divalent cadmium, As Ⅲ The adsorption capacity was changed to 577.06 mg / g, which was greatly improved. This is different from the situation of competitive inhibition in general co-adsorption, indicating that Cd 2+ The presence of may change the surface properties of hydrotalcite or provide additional adsorption sites, promoting the As Ⅲ adsorption.

[0028] Figure 12 MgFe-LDO etched and calcined under the conditions of Example 1 of the present invention is Ⅲ / Cd 2+ =1:1 coexistence system for Cd 2+ The adsorption capacity curve of Cd 2+ Cd in a single environment 2+ Comparison of adsorption capacity curves of Cd 2+ For example, the adsorption of Cd alone 2+ The adsorption capacity is 356.89 mg / g. When co-adsorbed with trivalent arsenic, Cd 2+ The adsorption capacity is 426.36 mg / g. 2+ The adsorption amount of As at each concentration was significantly higher than that of the single environment, especially in the low concentration area, where the upward trend was more obvious. Ⅲ The adsorption capacity increases in the coexistence system, and it is speculated that Cd 2+ There may not be strong competition Ⅲ adsorption sites, and As Ⅲ The presence of Cd 2+ Adsorption. DETAILED DESCRIPTION

[0029] Example 1

[0030] Preparation of a MgFe-LDH:

[0031] a. Mix 0.01 mol of FeCl3·6H2O and n(Fe 3+ ) were added into 200 mL of deionized water and stirred at room temperature for 10 min to dissolve all FeCl3·6H2O and MgCl2·6H2O. The solution was named solution A.

[0032] b. Change [n(Fe 3+ )+n(Mg 2+ )]*1.6 amount of NaOH was added to 200 mL of deionized water and stirred at room temperature for 1 min to dissolve all the NaOH. The solution was named solution B.

[0033] c. Add Solution A and Solution B simultaneously and at the same speed to the colloid mill and stir for 3 minutes. After stirring, transfer the solution to a beaker to obtain a brown-yellow suspension. Place the suspension on a thermostatic magnetic stirrer, heat to 60°C, and stir for 6 hours to age the LDH crystals. After stirring, centrifuge and separate the precipitate. Wash three times with deionized water and three times with ethanol, then dry in a thermostatic drying oven at 60°C for 24 hours.

[0034] Example 2

[0035] Preparation of partially etched MgFe-LDH: Completely dried MgFe-LDH precursor solid was ground into a powder and added to an NH4Cl etching solution with stirring for 6 hours to etch the LDH. After stirring, the precipitate was separated by centrifugation, washed three times with deionized water and three times with ethanol, and then dried in a constant temperature drying oven at 60°C for 24 hours.

[0036] Example 3

[0037] A preparation method of calcined MgFe-LDH: a completely dried MgFe-LDH precursor is ground into powder, and the powder is placed in a muffle furnace and calcined at different temperatures for 12 hours to obtain MgFe-LDO solid.

[0038] Example 4

[0039] A preparation method of MgFe-LDH after etching and calcination: the completely dried etched MgFe-LDH solid is ground into powder, and the powder is placed in a muffle furnace and calcined at different temperatures for 12 hours to obtain MgFe-LDO solid.

Claims

1. A method for preparing and applying a partially etched ultrastable mineralized structure of binary metal hydrotalcite, characterized in that: The steps include: (1) Add FeCl3·6H2O and MgCl2·6H2O to deionized water and stir at room temperature for 10 min to completely dissolve FeCl3·6H2O and MgCl2·6H2O. This solution is named solution A. (2) Add NaOH to deionized water and stir at room temperature for 1 min to dissolve all the NaOH. This solution is named Solution B. (3) Add Solution A and Solution B to the colloid mill simultaneously and at the same speed and stir for 3 minutes. After stirring, transfer the solution to a beaker to obtain a brown-yellow suspension. Place the suspension on a constant temperature magnetic stirrer and heat and stir to age the LDH crystals. Stir, centrifuge, and dry. (4) Grind the completely dried MgFe-LDH precursor solid into powder, add it into NH4Cl etching solution and stir, then centrifuge and dry it. (5) The completely dried etched MgFe-LDH solid was ground into powder and calcined in a muffle furnace at different temperatures to obtain MgFe-LDO solid.

2. The method according to claim 1, characterized in that The molar ratio of MgCl2·6H2O to Al(NO3)3·9H2O in step (1) is 3:

1.

3. The method according to claim 1, characterized in that The pH of NaOH in step (2) is 8–9.

4. The method according to claim 1, wherein The stirring speed in the reactor in step (3) is 2500-3500 r / min.

5. The method according to claim 1, wherein The temperature of the heating and stirring in step (3) is 60°C.

6. The method according to claim 1, characterized in that The aging time in step (3) is 6–8 h.

7. The method according to claim 1, characterized in that The concentration of the NH4Cl etching solution in step (4) is 200 g / L.

8. The method according to claim 1, characterized in that The stirring time in step (4) is 24–48 h.

9. The method according to claim 1, characterized in that The calcination temperature in step (5) is 350°C.

10. The method according to claim 1, characterized in that The calcination time in step (5) is 4 hours.

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