An asbestos tailings-based material for hexavalent chromium reduction and a method of preparing the same

Abstract

The application provides a kind of asbestos tailings-based material for hexavalent chromium reduction and a preparation method thereof.The material uses asbestos tailings as raw material, is solidified in alkaline silicate system and is foamed to form a porous magnesium silicate skeleton;By vacuum and pressure impregnation, MXene-oxalic acid intercalation complex is loaded and confined in the internal pores of the porous skeleton, and the efficient reduction of hexavalent chromium to trivalent chromium is realized by using the redox activity of MXene, and the reduction product is stabilized and complexed by the intercalated oxalic acid;Further, an antioxidant functional layer is constructed in situ on the surface of the material to inhibit the deactivation and reoxidation of the active components in the oxygen-containing water environment, thereby improving the removal efficiency of the material for hexavalent chromium, the product fixing capacity and the long-term use stability.

Description

Technical Field

[0001] This invention relates to the intersection of solid waste resource utilization and heavy metal wastewater treatment, specifically to an asbestos tailings-based material for hexavalent chromium reduction and its preparation method. Background Technology

[0002] Industrial processes such as electroplating, leather tanning, and mineral processing generate chromium-containing wastewater, which is a typical high-risk pollutant. Hexavalent chromium (Cr(VI)) is a strong oxidizing agent, highly water-soluble, and highly mobile, easily spreading in the aquatic environment and posing a threat to ecosystems and human health. Industrial treatment methods for chromium-containing wastewater include chemical reduction precipitation, adsorption, ion exchange, membrane separation, and electrochemical treatment. The core idea of ​​these methods is to reduce hexavalent chromium to trivalent chromium, which has significantly reduced toxicity and is easily precipitated and fixed.

[0003] Existing reduction and adsorption materials still have many shortcomings when applied to actual oxygenated water systems: the reducing active components are easily detached or lost under the scouring action of water flow, resulting in a shortened material lifespan and an increased risk of secondary pollution; if the trivalent chromium generated by the reduction of hexavalent chromium is fixed only by electrostatic adsorption or weak complexation, redissolution may still occur; the materials are prone to surface re-oxidation or deactivation of active sites in oxygenated environments, thereby weakening their continuous reduction capacity and long-term stability of hexavalent chromium.

[0004] Asbestos tailings are solid waste generated during asbestos mining and processing. Due to their fibrous structure, asbestos tailings are classified as a Group 1 carcinogen. Currently, the main methods for treating asbestos tailings are safe landfilling and solidification stabilization, resulting in extremely low resource utilization rates and failure to effectively recover and utilize elements such as magnesium and silicon.

[0005] In summary, developing a multifunctional integrated material that can simultaneously achieve the safe resource utilization of high-risk solid waste, namely asbestos tailings, construct an efficient and stable reduction system, and effectively inhibit the re-oxidation of reduction products is of significant theoretical and practical value for solving the dual challenges of treating hexavalent chromium-containing wastewater and disposing of asbestos tailings. Summary of the Invention

[0006] The technical problems to be solved are: the easy loss and detachment of active components in traditional reduction or adsorption materials, the weak fixation and easy redissolution of trivalent chromium reduction products, and the easy deactivation and re-oxidation of materials in oxygen-containing water environments; and the low resource utilization rate and high disposal difficulty of high-risk solid waste such as asbestos tailings. The purpose of this invention is to provide an asbestos tailings-based material for hexavalent chromium reduction, achieving the dual goals of solid waste resource utilization and heavy metal pollution control.

[0007] Technical solution: An asbestos tailings-based material for hexavalent chromium reduction, the asbestos tailings-based material comprising a porous magnesium silicate skeleton formed by solidification and foaming of asbestos tailings in an alkaline silicate system, an MXene-oxalic acid intercalation composite loaded in the internal pores of the porous magnesium silicate skeleton, and an antioxidant functional layer on the surface of the porous magnesium silicate skeleton.

[0008] The above-mentioned method for preparing asbestos tailings-based materials for hexavalent chromium reduction includes the following steps: S1. Add water to asbestos tailings to make a slurry, add aluminum sol, stir to obtain a mixed dispersion, pulverize the mixed dispersion by wet ball milling, and then dry to obtain pretreated asbestos tailings; S2. The pretreated asbestos tailings are mixed with an alkaline silicate solution, aluminum powder is added, and the mixture is stirred and foamed. Then, it is placed in a mold and cured to obtain an asbestos tailings-based foamed porous material. S3. Disperse the MXene-oxalic acid intercalation complex in ethanol to obtain a dispersion, and prepare an aqueous solution of sodium alginate and calcium chloride; S4. The asbestos tailings-based foamed porous material was impregnated in a dispersion under vacuum. After impregnation, it was taken out and transferred to a sodium alginate aqueous solution and treated under pressure. Finally, it was cross-linked in a calcium chloride aqueous solution. After being taken out, it was cured under medium and low temperature conditions to obtain a porous material with a supported composite. S5. The porous material of the supported composite is immersed in an aqueous solution containing magnesium source, aluminum source and urea, vacuum treated, taken out and transferred to a reaction vessel for hydrothermal reaction, and after washing and drying, asbestos tailings-based material for hexavalent chromium reduction is obtained.

[0009] The preparation method of the MXene-oxalic acid intercalation complex includes the following steps: S11. Disperse the multilayer MXene powder in ethanol to obtain an MXene dispersion; S12. The oxalic acid solution and MXene dispersion were mixed and sonicated, and a hydrothermal reaction was carried out under nitrogen protection to obtain the MXene-oxalic acid intercalation complex dispersion. S13. The obtained composite is separated, washed and dried to obtain MXene-oxalic acid intercalated composite powder.

[0010] Preferably, in step S1, the solid content of the aluminum sol is 10-30 wt%, the ratio of asbestos tailings to water in the slurry is 1 g:(5-15) mL, the volume ratio of slurry to aluminum sol in the mixed dispersion is 1:(0.1-0.3), the ball-to-material ratio of the wet ball mill is (5-20):1, the rotation speed is 200-500 rpm, the ball milling time is 2-8 h, and the drying is carried out at 80-120℃ to constant weight.

[0011] Preferably, in step S2, the alkaline silicate solution is a mixed solution of sodium silicate and sodium hydroxide, the mass ratio of the added alkaline silicate to the asbestos tailings is (0.8~1.5):1, the solidification temperature is 40~80℃, and the time is 12~24h.

[0012] Preferably, in step S3, the concentration of MXene-oxalic acid intercalated complex in the dispersion is 1~10 mg / mL, the concentration of sodium alginate aqueous solution is 1~5 wt%, and the concentration of calcium chloride aqueous solution is 2~10 wt%.

[0013] Preferably, in step S4, the vacuum level of the vacuum environment is 0.01~0.08MPa, the impregnation time is 10~60min, the pressure is 0.1~0.5MPa, the time is 10~60min, the crosslinking time is 30~60min, the curing temperature is 50~80℃, and the time is 2~12h.

[0014] Preferably, in step S5, the molar ratio of magnesium source, aluminum source and urea in the aqueous solution is (2-4):1:(8-20), the magnesium source is magnesium nitrate or magnesium chloride, the aluminum source is aluminum nitrate or aluminum chloride, the vacuum treatment pressure is 0.01~0.05MPa, the time is 10~30min, the hydrothermal reaction temperature is 90~120℃, and the time is 6~10h.

[0015] Preferably, the concentration of the MXene dispersion in step S11 is 0.5~5 mg / mL.

[0016] Preferably, in step S12, the concentration of the oxalic acid solution is 0.05~0.5mol / L, the volume ratio of the oxalic acid solution to the MXene dispersion is 1:(0.5~2), the power of the mixing ultrasound is 100~500W, the temperature of the hydrothermal reaction is 120~180℃, and the time is 6~12h.

[0017] Beneficial effects: The asbestos tailings-based material of the present invention used for hexavalent chromium reduction has the following advantages: This invention first physically disrupts the original fibrous structure of asbestos through wet ball milling, followed by pre-coating with aluminum sol to effectively suppress the escape of harmful fiber dust. Subsequently, in a strongly alkaline silicate system, the pulverized and activated tailings particles undergo a dissolution-repolymerization reaction, generating a continuous three-dimensional magnesium silicate gel network. Simultaneously, aluminum powder foaming forms interconnected hierarchical channels in situ. This process achieves the safety and high-value resource utilization of asbestos tailings, constructs a stable and efficient porous carrier, provides a high specific surface area loading platform for functional materials, and offers stable mechanical support, enabling efficient reduction and fixation of pollutants.

[0018] This invention prepares MXene-oxalic acid complex by hydrothermal intercalation. MXene, with its abundant Ti–O groups and reducing Ti atoms on its surface, provides reduction electrons during electron transfer, which can efficiently reduce Cr(VI) to Cr(III). The oxalic acid molecules intercalated between the MXene layers can coordinate with the newly generated Cr(III) ions to form a stable oxalic acid-chromium(III)-titanium oxide ternary complex, thereby firmly fixing the reduction product inside the material and effectively preventing its secondary dissolution.

[0019] The MXene-oxalic acid complex is first impregnated and attached to the inner wall of the macropores of a porous framework. Subsequently, it is permeated and encapsulated by sodium alginate solution under pressure, forming a calcium alginate gel network through calcium ion cross-linking. This gel network anchors the MXene-oxalic acid complex in the pores, preventing it from being washed away by water flow during use. At the same time, its structure allows water molecules and Cr(VI) ions to diffuse freely to the internal reaction sites, ensuring efficient mass transfer and reaction.

[0020] This invention grows a magnesium-aluminum hydrotalcite nanosheet composite coating on the outermost layer of the material. The continuous physical barrier can effectively delay the diffusion of oxygen from the external environment into the material, providing a protective layer for the Cr(III) that has been reduced and fixed inside, thereby improving the long-term stability and reliability of the material in oxygen-containing water environments. Attached Figure Description

[0021] Figure 1 The graphs show the removal rate of Cr(VI) over 120 minutes for Examples 3, 4, and each comparative example. Figure 2 The graph shows the trend of Cr(VI) removal rate in Example 3, Comparative Example 3, Comparative Example 4 and Comparative Example 6 during 5 cycles of use. Detailed Implementation

[0022] The present invention will be further described below with reference to embodiments. These embodiments are illustrative of the present invention, but the present invention is not limited to these embodiments: The multilayer MXene powder used in this invention was purchased from Beike Nanotechnology Co., Ltd., CAS No. 12363-89-2, item number BK2020011814-03.

[0023] Example 1

[0024] A method for preparing asbestos tailings-based materials for hexavalent chromium reduction includes the following steps: S1. Add 10g of asbestos tailings to 100mL of water to make a slurry, add 15mL of aluminum sol with a solid content of 10wt%, stir evenly to obtain a mixed dispersion, and pulverize the mixed dispersion by wet ball milling for 4h. The ball-to-material ratio of the ball milling is 5:1 and the rotation speed is 200rpm. Then dry at 80℃ to constant weight to obtain pretreated asbestos tailings. S2. The pretreated asbestos tailings were mixed with 8g of sodium silicate and sodium hydroxide solution, 0.05g of aluminum powder was added, and the mixture was stirred and foamed. Then it was placed in a mold and cured at 40℃ for 12h to obtain asbestos tailings-based foamed porous material. S3. Disperse the MXene-oxalic acid intercalation complex in ethanol to obtain a dispersion, prepare a uniform dispersion with a concentration of 1 mg / mL, and prepare a sodium alginate aqueous solution with a concentration of 1 wt% and a calcium chloride aqueous solution with a concentration of 2 wt%. S4. The foamed porous material was immersed in the MXene-oxalic acid intercalation composite dispersion under a vacuum of 0.01 MPa for 10 min. After being removed under normal pressure, it was transferred to an aqueous sodium alginate solution and kept under a pressure of 0.1 MPa for 10 min. Finally, it was placed in an aqueous calcium chloride solution for crosslinking for 30 min. After being removed, it was cured at 50°C for 2 h.

[0025] S5. The porous material of the supported composite is immersed in an aqueous solution of magnesium nitrate, aluminum nitrate and urea in a molar ratio of 2:1:8, and treated under vacuum at 0.01 MPa for 10 min. It is then removed and transferred to a reaction vessel and hydrothermally reacted at 90 °C for 6 h. After washing and drying, asbestos tailings-based material for hexavalent chromium reduction is obtained.

[0026] The preparation method of the MXene-oxalic acid intercalation complex includes the following steps: S11. Disperse multilayer MXene powder in ethanol to obtain an MXene dispersion with a concentration of 0.5 mg / mL; S12. Mix 0.05 mol / L oxalic acid solution and MXene dispersion at a volume ratio of 1:2 and sonicate. Under nitrogen protection, carry out hydrothermal reaction at 120℃ for 6 h to obtain MXene-oxalic acid intercalation complex dispersion. S13. The obtained composite is centrifuged, washed and dried to obtain MXene-oxalic acid intercalated composite powder.

[0027] Example 2

[0028] A method for preparing asbestos tailings-based materials for hexavalent chromium reduction includes the following steps: S1. Add 10g of asbestos tailings to 150mL of water to make a slurry, add 30mL of aluminum sol with a solid content of 30wt%, stir evenly to obtain a mixed dispersion, and pulverize the mixed dispersion by wet ball milling for 8h. The ball-to-material ratio of the ball milling is 20:1 and the speed is 500rpm. Then dry at 120℃ to constant weight to obtain pretreated asbestos tailings. S2. The pretreated asbestos tailings were mixed with 12g of sodium silicate and sodium hydroxide solution, 0.2g of aluminum powder was added, and the mixture was stirred and foamed. Then it was placed in a mold and cured at 80℃ for 24h to obtain asbestos tailings-based foamed porous material. S3. Disperse the MXene-oxalic acid intercalation complex in ethanol to obtain a dispersion, prepare a uniform dispersion with a concentration of 10 mg / mL, and prepare a sodium alginate aqueous solution with a concentration of 5 wt% and a calcium chloride aqueous solution with a concentration of 10 wt%. S4. The foamed porous material was immersed in MXene-oxalic acid intercalation composite dispersion under a vacuum of 0.08 MPa for 60 min. After being removed under normal pressure, it was transferred to sodium alginate aqueous solution and kept under pressure of 0.5 MPa for 60 min. Finally, it was placed in calcium chloride aqueous solution for crosslinking for 60 min. After being removed, it was cured at 80℃ for 12 h.

[0029] S5. The porous material of the supported composite is immersed in an aqueous solution of magnesium nitrate, aluminum nitrate and urea in a molar ratio of 4:1:20, and treated under vacuum at 0.05 MPa for 30 min. It is then removed and transferred to a reaction vessel and hydrothermally reacted at 120 °C for 10 h. After washing and drying, asbestos tailings-based material for hexavalent chromium reduction is obtained.

[0030] The preparation method of the MXene-oxalic acid intercalation complex includes the following steps: S11. Disperse multilayer MXene powder in ethanol to obtain an MXene dispersion with a concentration of 5 mg / mL; S12. Mix 0.5 mol / L oxalic acid solution and MXene dispersion at a volume ratio of 1:0.5 and sonicate. Under nitrogen protection, carry out hydrothermal reaction at 180℃ for 12 h to obtain MXene-oxalic acid intercalation complex dispersion. S13. The obtained composite is centrifuged, washed and dried to obtain MXene-oxalic acid intercalated composite powder.

[0031] Example 3

[0032] A method for preparing asbestos tailings-based materials for hexavalent chromium reduction includes the following steps: S1. Add 10g of asbestos tailings to 100mL of water to make a slurry, add 20mL of aluminum sol with a solid content of 20wt%, stir evenly to obtain a mixed dispersion, and pulverize the mixed dispersion by wet ball milling for 6h. The ball-to-material ratio of the ball milling is 15:1 and the speed is 400rpm. Then dry at 100℃ to constant weight to obtain pretreated asbestos tailings. S2. The pretreated asbestos tailings were mixed with 12g of sodium silicate and sodium hydroxide solution, 0.15g of aluminum powder was added, and the mixture was stirred and foamed. Then it was placed in a mold and cured at 60℃ for 18h to obtain asbestos tailings-based foamed porous material. S3. Disperse the MXene-oxalic acid intercalation complex in ethanol to obtain a dispersion, prepare a uniform dispersion with a concentration of 5 mg / mL, and prepare a sodium alginate aqueous solution with a concentration of 3 wt% and a calcium chloride aqueous solution with a concentration of 6 wt%. S4. The foamed porous material was immersed in the MXene-oxalic acid intercalation composite dispersion under a vacuum of 0.05 MPa for 40 min. After being removed under normal pressure, it was transferred to an aqueous sodium alginate solution and kept under a pressure of 0.3 MPa for 40 min. Finally, it was placed in an aqueous calcium chloride solution for crosslinking for 45 min. After being removed, it was cured at 70°C for 8 h.

[0033] S5. The porous material of the supported composite was immersed in an aqueous solution of magnesium nitrate, aluminum nitrate and urea in a molar ratio of 3:1:15, and treated under vacuum at 0.01 MPa for 20 min. It was then removed and transferred to a reaction vessel and hydrothermally reacted at 110 °C for 8 h. After washing and drying, asbestos tailings-based material for hexavalent chromium reduction was obtained.

[0034] The preparation method of the MXene-oxalic acid intercalation complex includes the following steps: S11. Disperse multilayer MXene powder in ethanol to obtain an MXene dispersion with a concentration of 2 mg / mL; S12. Mix 0.2 mol / L oxalic acid solution and MXene dispersion at a volume ratio of 1:1 and sonicate. Under nitrogen protection, carry out hydrothermal reaction at 150℃ for 9 h to obtain MXene-oxalic acid intercalation complex dispersion. S13. The obtained composite is centrifuged, washed and dried to obtain MXene-oxalic acid intercalated composite powder.

[0035] Example 4

[0036] A method for preparing asbestos tailings-based materials for hexavalent chromium reduction includes the following steps: S1. Add 10g of asbestos tailings to 50mL of water to make a slurry, add 10mL of aluminum sol with a solid content of 10wt%, stir evenly to obtain a mixed dispersion, and pulverize the mixed dispersion by wet ball milling for 2h. The ball-to-material ratio of the ball milling is 5:1 and the rotation speed is 200rpm. Then dry at 80℃ to constant weight to obtain pretreated asbestos tailings. S2. The pretreated asbestos tailings were mixed with 12g of sodium silicate and sodium hydroxide solution, 0.05g of aluminum powder was added, and the mixture was stirred and foamed. Then it was placed in a mold and cured at 60℃ for 12h to obtain asbestos tailings-based foamed porous material. S3. Disperse the MXene-oxalic acid intercalation complex in ethanol to obtain a dispersion, prepare a uniform dispersion with a concentration of 1 mg / mL, and prepare a sodium alginate aqueous solution with a concentration of 1 wt% and a calcium chloride aqueous solution with a concentration of 2 wt%. S4. The foamed porous material was immersed in the MXene-oxalic acid intercalation composite dispersion under a vacuum of 0.01 MPa for 10 min. After being removed under normal pressure, it was transferred to an aqueous sodium alginate solution and kept under a pressure of 0.1 MPa for 10 min. Finally, it was placed in an aqueous calcium chloride solution for crosslinking for 30 min. After being removed, it was cured at 50°C for 2 h.

[0037] S5. The porous material of the supported composite is immersed in an aqueous solution of magnesium nitrate, aluminum nitrate and urea in a molar ratio of 2:1:8, and treated under vacuum at 0.01 MPa for 10 min. It is then removed and transferred to a reaction vessel and hydrothermally reacted at 90 °C for 8 h. After washing and drying, asbestos tailings-based material for hexavalent chromium reduction is obtained.

[0038] The preparation method of the MXene-oxalic acid intercalation complex includes the following steps: S11. Disperse multilayer MXene powder in ethanol to obtain an MXene dispersion with a concentration of 0.5 mg / mL; S12. Mix 0.05 mol / L oxalic acid solution and MXene dispersion at a volume ratio of 1:2 and sonicate. Under nitrogen protection, carry out hydrothermal reaction at 120℃ for 6 h to obtain MXene-oxalic acid intercalation complex dispersion. S13. The obtained composite is centrifuged, washed and dried to obtain MXene-oxalic acid intercalated composite powder.

[0039] Example 5

[0040] A method for preparing asbestos tailings-based materials for hexavalent chromium reduction includes the following steps: S1. Add 10g of asbestos tailings to 120mL of water to make a slurry, add 20mL of aluminum sol with a solid content of 25wt%, stir evenly to obtain a mixed dispersion, and pulverize the mixed dispersion by wet ball milling for 5h. The ball-to-material ratio of the ball milling is 12:1 and the speed is 350rpm. Then dry at 110℃ to constant weight to obtain pretreated asbestos tailings. S2. The pretreated asbestos tailings were mixed with 12g of sodium silicate and sodium hydroxide solution, 0.12g of aluminum powder was added, and the mixture was stirred and foamed. Then it was placed in a mold and cured at 70℃ for 20h to obtain asbestos tailings-based foamed porous material. S3. Disperse the MXene-oxalic acid intercalation complex in ethanol to obtain a dispersion, prepare a uniform dispersion with a concentration of 7 mg / mL, and prepare a sodium alginate aqueous solution with a concentration of 4 wt% and a calcium chloride aqueous solution with a concentration of 8 wt%. S4. The foamed porous material was immersed in the MXene-oxalic acid intercalation composite dispersion under a vacuum of 0.06 MPa for 50 min. After being removed under normal pressure, it was transferred to an aqueous sodium alginate solution and kept under a pressure of 0.4 MPa for 50 min. Finally, it was placed in an aqueous calcium chloride solution for crosslinking for 40 min. After being removed, it was cured at 70°C for 8 h.

[0041] S5. The porous material of the supported composite is immersed in an aqueous solution of magnesium nitrate, aluminum nitrate and urea in a molar ratio of 4:1:10, and treated under vacuum at 0.01 MPa for 20 min. It is then removed and transferred to a reaction vessel and hydrothermally reacted at 115 °C for 8 h. After washing and drying, asbestos tailings-based material for hexavalent chromium reduction is obtained.

[0042] The preparation method of the MXene-oxalic acid intercalation complex includes the following steps: S11. Disperse multilayer MXene powder in ethanol to obtain an MXene dispersion with a concentration of 3 mg / mL; S12. Mix 0.3 mol / L oxalic acid solution and MXene dispersion at a volume ratio of 1:1 and sonicate. Under nitrogen protection, carry out hydrothermal reaction at 120℃ for 10 h to obtain MXene-oxalic acid intercalation complex dispersion. S13. The obtained composite is centrifuged, washed and dried to obtain MXene-oxalic acid intercalated composite powder.

[0043] Comparative Example 1 The difference between Comparative Example 1 and Example 3 is that Comparative Example 1 is only an asbestos tailings-based foamed porous material, without the loading of MXene-oxalic acid intercalation complex, and without the formation of an antioxidant functional layer.

[0044] Comparative Example 2 The difference between Comparative Example 2 and Example 3 is that: in Comparative Example 2, an equal amount of MXene was dispersed in ethanol, impregnated in a porous framework under vacuum, then transferred to an aqueous sodium alginate solution, and finally crosslinked in an aqueous calcium chloride solution. Subsequently, an antioxidant functional layer treatment was performed, i.e., MXene was not subjected to oxalic acid intercalation treatment. The remaining steps were the same as in Example 3.

[0045] Comparative Example 3 The difference between Comparative Example 3 and Example 3 is that in Comparative Example 3, the MXene dispersion was directly mixed with the oxalic acid solution without hydrothermal intercalation treatment, and then introduced into the asbestos tailings-based foamed porous material according to the loading method in the example. The remaining steps are the same as in Example 3.

[0046] Comparative Example 4 The difference between Comparative Example 4 and Example 3 is that Comparative Example 4 loads the MXene-oxalic acid intercalated composite into a porous framework, but does not include the sodium alginate solution impregnation and calcium chloride crosslinking steps in step S4. The remaining steps are consistent with those in Example 3.

[0047] Comparative Example 5 The difference between Comparative Example 5 and Example 3 is that in Comparative Example 5, pretreated asbestos tailings were mixed with sodium silicate and sodium hydroxide solution without adding aluminum powder, and the mixture was directly solidified to obtain a dense block. The remaining steps were the same as in Example 3.

[0048] Comparative Example 6 The difference between Comparative Example 6 and Example 3 is that Comparative Example 6 does not have the step of constructing an antioxidant functional layer, while the remaining steps are consistent with Example 3.

[0049] Comparative Example 7 The difference between Comparative Example 7 and Example 3 is that Comparative Example 7 uses an equal amount of iron powder as a reducing agent to replace the MXene-oxalic acid intercalation complex, while the remaining steps are the same as in Example 3.

[0050] Comparative Example 8 Comparative Example 8 was prepared directly from the MXene-oxalic acid intercalation complex powder prepared in Example 3 without any carrier.

[0051] Test Example 1 A 50 mL stock solution of hexavalent chromium with an initial concentration of 200 mg / L was prepared using K₂Cr₂O₇. 25 mL of a hexavalent chromium solution diluted to an initial concentration of 50 mg / L to simulate wastewater was placed in a 100 mL Erlenmeyer flask, and 0.2 g of the material was added. The mixture was shaken at 25 °C and 150 rpm. Samples were taken at set time points (5, 15, 30, 60, 120, and 240 min), filtered through a 0.45 μm filter membrane, and the residual Cr(VI) concentration in the filtrate was determined using diphenylcarbazide spectrophotometry. The removal rate was then calculated.

[0052] Reference Figure 1 As shown, the asbestos tailings-based material for hexavalent chromium reduction prepared in this invention achieved Cr(VI) removal rates of 98.5% and 97.8% within 120 min, respectively, demonstrating excellent reduction performance and rapid reaction kinetics. This is mainly due to the following factors: The porous magnesium silicate framework formed by foaming in the alkaline silicate system provides a high specific surface area loading platform, ensuring efficient contact between reactants and products; The invention uses a hydrothermal intercalation method to firmly introduce oxalic acid molecules into the MXene interlayer. The resulting MXene-oxalic acid complex provides electrons through the abundant Ti-O groups and reducing Ti atoms on the MXene surface, achieving the chemical reduction of Cr(VI). Comparative Example 1 showed a low removal rate, indicating that the framework itself only has weak physical adsorption and lacks reduction ability. Comparative Example 2's MXene material lacked oxalic acid intercalation, indicating that the lack of oxalic acid intercalation led to weak Cr(III) fixation and partial redissolution, affecting the apparent removal rate. Comparative Example 3 showed that oxalic acid was not effectively intercalated between MXene layers during physical mixing, easily dissolving and causing instability in the reduction system. Comparative Example 5 showed that the porous structure is crucial for mass transfer and exposure of active sites. Comparative Example 7 showed that although traditional reducing agents have reduction ability, their reduction efficiency is not as high as that of the MXene-oxalic acid complex of this invention. Comparative Example 8 showed a high reduction efficiency, but because it was in powder form, it suffered from easy loss and difficulty in recovery in practical applications. In summary, this invention achieves efficient reduction of Cr(VI) through multi-level structural design and the synergistic effect of functional components.

[0053] Test Example 2 The material after the Cr(VI) reduction reaction in Test Example 1 was collected by filtration, washed three times with deionized water, and then immersed in 0.1 mol / L HCl solution. It was then desorbed and regenerated by shaking and washing at 150 rpm for 30 min at room temperature. After washing with deionized water until neutral, the material was dried at 60℃ to obtain the regenerated material. 25 mL of hexavalent chromium solution diluted to an initial concentration of 50 mg / L to simulate wastewater was placed in a 100 mL Erlenmeyer flask, and 0.2 g of the material was added. The shaking reaction process of Test Example 1 was repeated. After reacting for 240 min, a sample was taken to determine the Cr(VI) removal rate. The above regeneration and reaction constituted one cycle, and the test was repeated five times. The Cr(VI) removal rate and changes in the appearance of the material were recorded in each cycle.

[0054] Reference Figure 2As shown, the asbestos tailings-based material prepared by this invention exhibits excellent cycle stability. This is because the invention first forms a porous magnesium silicate framework with interconnected channels through aluminum powder foaming, providing a high specific surface area and stable mass transfer channels for Cr(VI) reduction. Secondly, oxalic acid in the MXene-oxalic acid intercalation complex forms a stable ternary complex with Cr(III), fixing the reduction product inside the material and preventing it from dissolving during regeneration. The calcium alginate gel network anchors the MXene-oxalic acid reduction complex within the channels, inhibiting its loss. The magnesium aluminum hydrotalcite antioxidant functional layer on the surface effectively blocks oxygen, slows down the oxidative deactivation of the MXene reduction components, and protects the fixed Cr(III) from re-oxidation. Comparative Example 3 showed significant performance degradation, indicating that the complexation and fixation of Cr(III) by oxalic acid intercalation is key to maintaining long-term reduction capacity. Comparative Example 4 showed gradual loss of active components and a decrease in removal rate under oscillation conditions, confirming the important role of calcium alginate gel in preventing the shedding of reducing components. Comparative Example 6 showed good reduction efficiency in the first cycle, but the removal rate decreased significantly with the increase of cycle number. This indicates that without the surface magnesium aluminum hydrotalcite antioxidant layer, the unprotected MXene-oxalic acid complex may lose part of its reduction capacity due to oxidation. At the same time, the fixed Cr(III) may also be re-oxidized or dissociated during the regeneration process, leading to a decrease in reduction efficiency in subsequent cycles. Although Comparative Example 8 showed high reduction efficiency in the first reaction, it was only in powder form and had problems such as low recovery rate and easy loss and agglomeration during the cycle, resulting in significant material loss after each cycle and poor cycle stability.

[0055] Test Example 3 0.10 g of the material from Test Example 1, after Cr(VI) reduction and washing with deionized water and drying, was placed in 50 mL of leaching solution with a pH of 7.0 ± 0.5. The solution was continuously shaken and leached at 25 °C and 150 rpm for 48 h to simulate long-term contact under dynamic water flow. After leaching, the mixture was centrifuged and filtered. The total chromium concentration in the supernatant was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES), and the total chromium leaching rate was calculated. Simultaneously, the presence of Cr(VI) in the leaching solution was determined using diphenylcarbazide spectrophotometry to determine whether re-oxidation of the reduction products occurred.

[0056] Table 1 Comparison of Cr dissolution rates

[0057] Referring to Table 1, the asbestos tailings-based material prepared by this invention exhibits excellent chromium fixation ability, and Cr(VI) was not detected in the leachate. This is because the material achieves efficient complexation and fixation of the reduction product Cr(III) through the MXene-oxalic acid intercalation complex. At the same time, the magnesium aluminum hydrotalcite antioxidant functional layer on the surface effectively inhibits the re-oxidation of Cr(III) in the oxygen-containing environment, thereby ensuring the long-term stable fixation of the reduction product and avoiding the risk of secondary pollution. In contrast, the MXene material of Comparative Example 2, without oxalic acid intercalation, also has reduction ability, but due to the lack of stable coordination of Cr(III) by oxalic acid molecules, the total chromium leaching... The significantly increased chromium leaching rate indicates that when relying solely on MXene reduction without effective fixation methods, the generated Cr(III) mainly exists in free or weakly adsorbed forms, making it easily leached in the environment. Comparative Example 6, lacking an antioxidant layer, had a lower total chromium leaching rate than Comparative Example 2, but still higher than Example 3, and Cr(VI) was detected in the leachate. This demonstrates that although oxalic acid intercalation provides some complexation and fixation, without a surface antioxidant protection layer, some of the fixed Cr(III) will still undergo re-oxidation and dissolve in an oxygen-containing neutral environment. This highlights the crucial role of the antioxidant functional layer in maintaining the long-term stability of the material and preventing product re-oxidation. In summary, this invention achieves efficient reduction of hexavalent chromium, robust fixation of the product, and long-term stability through the synergistic effect of MXene reduction, oxalic acid complexation and fixation, and surface hydrotalcite antioxidant protection.

[0058] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. An asbestos tailings-based material for the reduction of hexavalent chromium, characterized in that, The asbestos tailings-based material includes a porous magnesium silicate skeleton formed by solidifying and foaming asbestos tailings in an alkaline silicate system, an MXene-oxalic acid intercalation composite loaded in the internal pores of the porous magnesium silicate skeleton, and an antioxidant functional layer on the surface of the porous magnesium silicate skeleton. A method for preparing asbestos tailings-based materials for hexavalent chromium reduction includes the following steps: S1. Water is added to asbestos tailings to make a slurry, aluminum sol is added, and the mixture is stirred to obtain a mixed dispersion. The mixed dispersion is pulverized by wet ball milling and then dried to obtain pretreated asbestos tailings. S2. The pretreated asbestos tailings are mixed with an alkaline silicate solution, aluminum powder is added, and the mixture is stirred and foamed. Then, it is placed in a mold and cured to obtain an asbestos tailings-based foamed porous material. S3. Disperse the MXene-oxalic acid intercalation complex in ethanol to obtain a dispersion, and prepare an aqueous solution of sodium alginate and calcium chloride; S4. The asbestos tailings-based foamed porous material was impregnated in a dispersion under vacuum. After impregnation, it was taken out and transferred to a sodium alginate aqueous solution and treated under pressure. Finally, it was cross-linked in a calcium chloride aqueous solution. After being taken out, it was cured under medium and low temperature conditions to obtain a porous material with a supported composite. S5. The porous material of the supported composite is immersed in an aqueous solution containing magnesium source, aluminum source and urea, vacuum treated, taken out and transferred to a reaction vessel for hydrothermal reaction, and after washing and drying, asbestos tailings-based material for hexavalent chromium reduction is obtained. The preparation method of the MXene-oxalic acid intercalation complex includes the following steps: S11. Disperse the multilayer MXene powder in ethanol to obtain an MXene dispersion; S12. The oxalic acid solution and MXene dispersion were mixed and sonicated, and a hydrothermal reaction was carried out under nitrogen protection to obtain the MXene-oxalic acid intercalation complex dispersion. S13. The obtained composite is separated, washed and dried to obtain MXene-oxalic acid intercalated composite powder.

2. The method for preparing asbestos tailings-based materials for hexavalent chromium reduction according to claim 1, characterized in that, Includes the following steps: S1. Water is added to asbestos tailings to make a slurry, aluminum sol is added, and the mixture is stirred to obtain a mixed dispersion. The mixed dispersion is pulverized by wet ball milling and then dried to obtain pretreated asbestos tailings. S2. The pretreated asbestos tailings are mixed with an alkaline silicate solution, aluminum powder is added, and the mixture is stirred and foamed. Then, it is placed in a mold and cured to obtain an asbestos tailings-based foamed porous material. S3. Disperse the MXene-oxalic acid intercalation complex in ethanol to obtain a dispersion, and prepare an aqueous solution of sodium alginate and calcium chloride; S4. The asbestos tailings-based foamed porous material was impregnated in a dispersion under vacuum. After impregnation, it was taken out and transferred to a sodium alginate aqueous solution and treated under pressure. Finally, it was cross-linked in a calcium chloride aqueous solution. After being taken out, it was cured under medium and low temperature conditions to obtain a porous material with a supported composite. S5. The porous material of the supported composite is immersed in an aqueous solution containing magnesium source, aluminum source and urea, vacuum treated, taken out and transferred to a reaction vessel for hydrothermal reaction, and after washing and drying, asbestos tailings-based material for hexavalent chromium reduction is obtained.

3. The method for preparing asbestos tailings-based materials for hexavalent chromium reduction according to claim 2, characterized in that, The preparation method of the MXene-oxalic acid intercalation complex includes the following steps: S11. Disperse the multilayer MXene powder in ethanol to obtain an MXene dispersion; S12. The oxalic acid solution and MXene dispersion were mixed and sonicated, and a hydrothermal reaction was carried out under nitrogen protection to obtain the MXene-oxalic acid intercalation complex dispersion. S13. The obtained composite is separated, washed and dried to obtain MXene-oxalic acid intercalated composite powder.

4. The method for preparing asbestos tailings-based materials for hexavalent chromium reduction according to claim 2, characterized in that: In step S1, the solid content of the aluminum sol is 10-30 wt%, the ratio of asbestos tailings to water in the slurry is 1 g:(5-15) mL, the volume ratio of slurry to aluminum sol in the mixed dispersion is 1:(0.1-0.3), the ball-to-material ratio of the wet ball mill is (5-20):1, the rotation speed is 200-500 rpm, the ball milling time is 2-8 h, and the drying is carried out at 80-120℃ to constant weight.

5. The method for preparing asbestos tailings-based materials for hexavalent chromium reduction according to claim 2, characterized in that: In step S2, the alkaline silicate solution is a mixed solution of sodium silicate and sodium hydroxide. The ratio of the amount of alkaline silicate added to the mass of asbestos tailings is (0.8~1.5):

1. The solidification temperature is 40~80℃ and the time is 12~24h.

6. The method for preparing asbestos tailings-based materials for hexavalent chromium reduction according to claim 2, characterized in that: In step S3, the concentration of MXene-oxalic acid intercalated complex in the dispersion is 1~10 mg / mL, the concentration of sodium alginate aqueous solution is 1~5 wt%, and the concentration of calcium chloride aqueous solution is 2~10 wt%.

7. The method for preparing asbestos tailings-based materials for hexavalent chromium reduction according to claim 2, characterized in that: In step S4, the vacuum level of the vacuum environment is 0.01~0.08MPa, the impregnation time is 10~60min, the pressure is 0.1~0.5MPa, the time is 10~60min, the crosslinking time is 30~60min, the curing temperature is 50~80℃, and the time is 2~12h.

8. The method for preparing asbestos tailings-based materials for hexavalent chromium reduction according to claim 2, characterized in that: In step S5, the molar ratio of magnesium source, aluminum source and urea in the aqueous solution is (2-4):1:(8-20). The magnesium source is magnesium nitrate or magnesium chloride, and the aluminum source is aluminum nitrate or aluminum chloride. The vacuum treatment pressure is 0.01~0.05MPa, the time is 10~30min, the hydrothermal reaction temperature is 90~120℃, and the time is 6~10h.

9. The method for preparing asbestos tailings-based materials for hexavalent chromium reduction according to claim 3, characterized in that: The concentration of the MXene dispersion in step S11 is 0.5~5 mg / mL.

10. The method for preparing asbestos tailings-based materials for hexavalent chromium reduction according to claim 3, characterized in that: In step S12, the concentration of the oxalic acid solution is 0.05~0.5mol / L, the volume ratio of the oxalic acid solution to the MXene dispersion is 1:(0.5~2), the power of the mixing ultrasound is 100~500W, the temperature of the hydrothermal reaction is 120~180℃, and the time is 6~12h.

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