Composite defluorination agent for deep defluorination of water body and preparation method thereof

Abstract

This invention discloses a composite defluoridating agent for deep defluorination of water and its preparation method. The defluoridating agent is an aqueous solution of a metal salt or an aqueous solution of a metal salt dispersed with nanoparticles. The active ingredient of the defluoridating agent contains at least three metal ions: lanthanum, aluminum, and zinc, and the molar ratio of La, Al, and Zn is 1~4:3~5:1~3. The pH of the aqueous solution is 6.0~6.5. Compared with traditional PAC, which requires more than 5 mmol / L to reduce fluoride ions to 1 mg / L, the composite defluoridating agent of this invention only requires 1.2 mmol / L (based on the total concentration of active ingredients) to reduce fluoride ions to below 1.0 mg / L, significantly reducing treatment costs and sludge production. In addition, this invention has a fast settling speed, achieving solid-liquid separation within 10 minutes. In the preferred scheme containing nanoparticles, magnetic adsorption settling can be completed within 5 minutes, greatly shortening the water treatment process cycle.

Description

Technical Field

[0001] This invention relates to a composite defluorinating agent for deep defluorination of water, and also to a method for preparing the above-mentioned composite defluorinating agent. Background Technology

[0002] Fluorine is an essential trace element for the human body, but long-term excessive intake of fluoride ions (F) can lead to health problems. - Fluoride can lead to serious health problems such as dental fluorosis and skeletal fluorosis. Therefore, it is necessary to strictly limit the concentration of fluoride ions in drinking water (usually 0.5~1.5 mg / L). However, in many areas, groundwater and industrial wastewater (such as wastewater from the metallurgical, electronics, glass, and fertilizer industries) commonly have excessive levels of fluoride.

[0003] Currently, common defluorination technologies include adsorption, membrane separation, precipitation, and coagulation-precipitation. Among these, coagulation-precipitation is widely used due to its simple operation, low cost, and large processing capacity; however, its core bottleneck lies in the development of highly efficient defluorination agents. Traditional single-metal salt defluorination agents include: 1. Aluminum salt series (such as aluminum sulfate, polyaluminum chloride PAC), which mainly rely on charge neutralization adsorption and hydroxyl bridging to remove fluoride. However, its defluorination process is similar to that of fluoride-containing chemicals. - 1. **Liquid:** Limited binding capacity makes it difficult to reduce fluoride concentration below 1.0 mg / L, and its applicable pH range is narrow (optimal pH is typically 5.5-6.5). High dosage is required when treating complex water qualities, resulting in large amounts of sludge and high subsequent treatment costs. 2. **Calcium Salts (e.g., lime):** Removing fluoride through CaF2 precipitation. However, CaF2 has a certain solubility in water (approximately 8 mg / L), meaning the theoretical fluoride removal limit cannot meet stringent wastewater discharge standards. Furthermore, the large dosage leads to sludge expansion, causing serious solid waste disposal problems. 3. **Rare Earth Salts (e.g., lanthanum salts):** La(III) and F... - It can form a LaF3 precipitate with extremely low solubility (Ksp≈10). -24 Lanthanum salts possess theoretical potential for deep fluoride removal. However, pure lanthanum salts are expensive, and the resulting LaF3 flocs are extremely small, loose, and settle slowly, making solid-liquid separation difficult and greatly limiting their large-scale engineering applications.

[0004] Existing technologies (such as CN112125344A and CN110127775A) also disclose some rare earth composite defluorinating agents, such as La-Al or La-Ce binary systems. However, the above-mentioned binary composite defluorinating agents still cannot meet the goal of achieving deep defluorination and rapid sedimentation at low dosage. Summary of the Invention

[0005] Objective of the invention: The objective of this invention is to provide a composite defluorinating agent that can achieve deep defluorination while also enabling rapid sedimentation at low dosage; another objective of this invention is to provide a method for preparing the above-mentioned composite defluorinating agent.

[0006] Technical Solution: The present invention relates to a composite defluoridating agent for deep defluorination of water. The defluoridating agent is an aqueous solution of a metal salt or an aqueous solution of a metal salt containing dispersed nanoparticles. The active ingredient of the defluoridating agent contains at least three metal ions: lanthanum, aluminum, and zinc, and the molar ratio of La, Al, and Zn is 1~4:3~5:1~3. The pH of the aqueous solution is 6.0~6.5. Within this pH range, the metal ions undergo controllable partial hydrolysis, generating highly reactive hydroxyl complexes or polynuclear hydroxyl-bridged polymers, such as [La(OH)]. 2+ [Al(OH)] 2+ [Al6(OH)] 15 ] 3+ [Zn(OH)] + The -OH groups on the surface of these hydroxyl complexes are fluoride ions (F... - The active site for ligand exchange reactions can efficiently transfer F - They are replaced into the metal ion coordination layer, forming stable MF bonds.

[0007] When the defluorinating agent is an aqueous solution of a metal salt dispersed with nanoparticles, the nanoparticles are ZnFe2O4 nanoparticles with a particle size of ~50nm. The ZnFe2O4 nanoparticles are prepared by the following method: 1 mmol of zinc sulfate heptahydrate and 2 mmol of ferric nitrate nonahydrate are dissolved in 30 mL of deionized water and stirred until clear; 3 mmol of sodium citrate is added to the above solution and stirred continuously until completely dissolved; under vigorous stirring, 1.0 M sodium hydroxide solution is slowly added dropwise until the pH of the mixed solution is adjusted to 11.0, at which point the solution becomes a deep red transparent sol; the sol is transferred to a high-pressure reactor lined with polytetrafluoroethylene, sealed, and placed in an oven for reaction at 160°C for 10 hours; after the reaction, it is naturally cooled to room temperature, and the solid is collected by high-speed centrifugation. The product is washed 3-4 times alternately with deionized water and anhydrous ethanol, and the washed product is dried in a vacuum drying oven at 60°C for 6 hours to obtain the final product. This invention simultaneously achieves crystal nucleation and crystal growth at a relatively low temperature (160°C). During crystal growth, citrate ions act as surface capping agents, adsorbing onto the surface of ZnFe2O4 particles and guiding uniform anisotropic growth. This results in ZnFe2O4 particles with small and uniform diameters (approximately 50 nm). The smaller ZnFe2O4 particles have a larger specific surface area, thus exposing more active adsorption sites. Simultaneously, the citrate ions adsorbed on the particle surface effectively provide steric hindrance during subsequent drying, preventing hard agglomeration of the nanoparticles and ensuring excellent dispersibility in aqueous solution. The amount of ZnFe2O4 nanoparticles added is 1 / 6 to 1 / 5 of the total molar amount of Zn in the system. Excessive addition will negatively impact the overall defluorination performance of the defluorinating agent. In this case, the molar ratio of the metal elements in the defluorinating agent is La:Al:Zn = 1:4:3.

[0008] When the defluorinating agent is an aqueous solution of a metal salt, the molar ratio of metal ions in the aqueous solution is La:Al:Zn = 4:3:1; or the molar ratio of the metal ions is La:Al:Zn = 2:5:1.

[0009] When the defluorinating agent is an aqueous solution of a metal salt, the concentration of the active ingredient (total concentration of metal ions) is 0.075~0.08 mol / L; when the defluorinating agent is an aqueous solution of a metal salt with dispersed nanoparticles, the concentration of the active ingredient (total concentration of metal ions + concentration of nanoparticles) is 0.075~0.08 mol / L.

[0010] When the defluorinating agent is an aqueous solution of a metal salt, the preparation method of the above-mentioned composite defluorinating agent is as follows: lanthanum salt, aluminum salt and zinc salt are added to deionized water, dissolved, and then an alkaline solution is added to adjust the pH of the solution to 6~6.5 to obtain the composite defluorinating agent.

[0011] When the defluorinating agent is an aqueous solution of a metal salt with dispersed nanoparticles, the preparation method of the above-mentioned composite defluorinating agent includes the following steps:

[0012] (1) Add lanthanum salt, aluminum salt and zinc salt to deionized water, dissolve them and obtain a salt solution;

[0013] (2) Disperse ZnFe2O4 nanoparticles in deionized water to obtain a nanoparticle dispersion;

[0014] (3) Add the salt solution to the dispersion, add alkali solution to adjust the pH of the solution to 6~6.5, and stir magnetically to obtain the composite defluorinating agent.

[0015] In step (3), the magnetic stirring time is 1 to 2 hours.

[0016] The lanthanum salt, aluminum salt, and zinc salt are one of the corresponding nitrates, chlorides, or sulfates, with nitrates being preferred.

[0017] ZnFe2O4 on F - It exhibits excellent adsorption properties. After the addition of the defluorination agent, ZnFe2O4 nanoparticles act as "magnetic nuclei," encapsulating LaF3, Al(OH)3, and Zn(OH)2 flocs during the flocculation process, giving the entire floc superparamagnetic composition. After flocculation, under the influence of an external magnetic field, the magnetic flocs are rapidly magnetized and migrate directionally, achieving rapid sedimentation and enrichment within 5 minutes.

[0018] Fluorine-rich magnetic sludge is treated with dilute acid (such as dilute nitric acid). Under acidic conditions, the floc structure of LaF3, Al(OH)3, and Zn(OH)2 is destroyed, and F... - Released into the acid solution, the desorbed ZnFe₂O₄ magnetic nuclei are rapidly separated from the acid washing solution using a magnetic field. The separated ZnFe₂O₄ magnetic nuclei, after washing, can be directly reused in the preparation of a new round of defluorination agents, achieving the recycling of magnetic components. Meanwhile, the La-rich... 3+ Al 3+ F - The pickling solution can be used to selectively precipitate and recover valuable metals by adjusting the pH.

[0019] Beneficial Effects: Compared with existing technologies, this invention has the following significant advantages: Compared with traditional PAC, which requires a dosage of more than 5 mmol / L to reduce fluoride ions to 1 mg / L, the composite defluorinating agent of this invention only requires a dosage of 1.2 mmol / L (based on the total concentration of active ingredients) to reduce fluoride ions to below 1.0 mg / L (if PAC is used, the fluoride ion concentration is reduced to 8.5 mg / L at this dosage), thereby significantly reducing treatment costs and sludge production; In addition, this invention can achieve rapid magnetic sedimentation within 5 minutes, and even under conventional gravity sedimentation conditions, the sedimentation time is only 8-10 minutes. In contrast, existing single aluminum salt or iron salt defluorinating agents (such as PAC, ferric chloride) or their simple binary complexes (such as aluminum-iron composite defluorinating agents) typically require more than 30 minutes for complete sedimentation and separation due to the loose floc structure and low density of the formed flocs. Therefore, this invention can greatly shorten the cycle of water treatment processes. Attached Figure Description

[0020] Figure 1 The graph shows a comparison of the defluorination effects of Examples 1-3 and Comparative Example 1 (conventional PAC) at different dosages.

[0021] Figure 2 This is a schematic diagram of the settling process of flocs in Example 3 (LAZ-3 magnetic type) after 5 minutes of static settling and under an applied magnetic field. Detailed Implementation

[0022] Example 1

[0023] The preparation method of the composite defluorinating agent of the present invention is as follows: 17.32g of La(NO3)3·6H2O (0.04mol), 11.25g of Al(NO3)3·9H2O (0.03mol), and 2.96g of Zn(NO3)2·6H2O (0.01mol) are weighed according to the metal ion molar ratio La:Al:Zn=4:3:1 and dissolved in deionized water. Water is added to make up the volume of the solution to 1L, and the pH of the solution is adjusted to 6.5 with NaOH to obtain the defluorinating agent LAZ-1.

[0024] Take 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4 with a concentration of 50 mg / L), and adjust the pH of the wastewater to 7.0. Add different amounts of defluorinating agent LAZ-1 to eight groups of identical wastewater (the amount of defluorinating agent LAZ-1 added is based on the total molar amount of metal ions (La, Al, Zn) in the wastewater). The amounts of defluorinating agent LAZ-1 added to the eight groups of wastewater are 0.4, 0.8, 1.2, 1.6, 2.0, 2.4, 2.8, and 3.2 mmol / L, respectively. After adding the defluorinating agent, stir rapidly at 200 rpm for 5 minutes, then stir slowly at 40 rpm for 5 minutes. After settling for 10 minutes, take the supernatant and filter it through a 0.45 μm filter membrane to determine the residual fluoride ion concentration (using the fluoride ion selective electrode method).

[0025] When the dosage of the defluorinating agent is 1.2 mmol / L (based on the total concentration of metal ions in the wastewater), the residual fluoride ion concentration drops to 0.65 mg / L, and the removal rate reaches 96.8%.

[0026] Example 2

[0027] The preparation method of the composite defluorinating agent of the present invention is as follows: 8.66g of La(NO3)3·6H2O (0.02mol), 18.75g of Al(NO3)3·9H2O (0.05mol), and 2.96g of Zn(NO3)2·6H2O (0.01mol) are weighed according to the metal ion molar ratio La:Al:Zn=2:5:1 and dissolved in deionized water. Water is added to make up the volume of the solution to 1L. The pH of the solution is adjusted to 6.0 with HNO3 to obtain the defluorinating agent LAZ-2.

[0028] Take 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4 with a concentration of 50 mg / L), and adjust the pH of the wastewater to 7.0. Add different amounts of defluorinating agent LAZ-2 to eight groups of identical wastewater (the amount of defluorinating agent LAZ-2 added is based on the total molar amount of metal ions (La, Al, Zn) in the wastewater). The amounts of defluorinating agent LAZ-2 added to the eight groups of wastewater are 0.4, 0.8, 1.2, 1.6, 2.0, 2.4, 2.8, and 3.2 mmol / L, respectively. After adding the defluorinating agent, stir rapidly at 200 rpm for 5 minutes, then stir slowly at 40 rpm for 5 minutes. After settling for 8 minutes, take the supernatant and filter it through a 0.45 μm filter membrane to determine the residual fluoride ion concentration (using the fluoride ion selective electrode method).

[0029] When the dosage of the defluorinating agent is 1.2 mmol / L (based on the total concentration of active ingredients), the residual fluoride ion concentration drops to 0.85 mg / L, and the removal rate reaches 95.8%.

[0030] At the same dosage, LAZ-2 sedimentation took only 5 minutes, and its supernatant turbidity dropped to below 5 NTU, with a residual fluoride ion concentration of 0.92 mg / L. Under the same conditions, LAZ-1 supernatant turbidity was 8 NTU after 5 minutes of settling. This indicates that LAZ-2 can form larger and denser flocs, effectively shortening the sedimentation separation time and greatly improving solid-liquid separation efficiency.

[0031] Example 3

[0032] The preparation method of the composite defluorinating agent of the present invention is as follows:

[0033] (1) Weigh 4.33g La(NO3)3·6H2O (0.01mol), 15g Al(NO3)3·9H2O (0.04mol) and 7.4g Zn(NO3)2·6H2O (0.025mol) and dissolve them in deionized water to obtain a salt solution;

[0034] (2) Weigh 1.205g of ZnFe2O4 nanopowder (0.005mol, particle size ~50nm) and disperse it in 100mL of deionized water. Sonicate the dispersion for 30 minutes to obtain a dispersion.

[0035] (3) Add the salt solution from step (1) to the dispersion from step (2), bring the volume to 1L, adjust the pH to 6.5, and then stir magnetically for 1 hour to obtain the magnetic defluorinating agent LAZ-3.

[0036] The ZnFe2O4 nanoparticles in this embodiment were prepared by the following method: 1 mmol of zinc sulfate heptahydrate and 2 mmol of ferric nitrate nonahydrate were dissolved in 30 mL of deionized water and stirred until clear; 3 mmol of sodium citrate was added to the above solution and stirred continuously until completely dissolved; under vigorous stirring, 1.0 M sodium hydroxide solution was slowly added dropwise until the pH of the mixed solution was adjusted to 11.0, at which point the solution became a deep red transparent sol; the sol was transferred to a polytetrafluoroethylene-lined high-pressure reactor, sealed, and placed in an oven at 160°C for 10 hours; after the reaction, the mixture was naturally cooled to room temperature, and the solid was collected by high-speed centrifugation. The product was washed 3-4 times alternately with deionized water and anhydrous ethanol, and the washed product was dried in a vacuum drying oven at 60°C for 6 hours to obtain the final product.

[0037] Take 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4 with a concentration of 50 mg / L), and adjust the pH of the wastewater to 7.0. Add different amounts of defluorinating agent LAZ-3 to eight groups of identical wastewater (the amount of defluorinating agent LAZ-3 added is based on the total molar amount of metal elements (La, Al, Zn) in the wastewater). The amounts of defluorinating agent LAZ-3 added to the eight groups of wastewater are 0.4, 0.8, 1.2, 1.6, 2.0, 2.4, 2.8, and 3.2 mmol / L, respectively. After adding the defluorinating agent, stir rapidly at 200 rpm for 5 minutes, then stir slowly at 40 rpm for 5 minutes. Place a magnet at the bottom of the beaker, and after magnetic adsorption and settling for 5 minutes, take the supernatant and filter it through a 0.45 μm filter membrane to determine the residual fluoride ion concentration (using the fluoride ion selective electrode method).

[0038] When the dosage of the defluorinating agent was 1.2 mmol / L, the residual fluoride concentration was 0.98 mg / L, and the removal rate reached 95.1%. The settling time in Example 3 was <5 min.

[0039] After flocculation and settling, a neodymium iron boron permanent magnet was applied to the bottom of the beaker. The flocs rapidly (<5 minutes) settled and aggregated towards the magnetic source, while the supernatant became very clear. The settling rate was much faster than under conditions without a magnetic field. Figure 2 As shown, the left beaker is in the static sedimentation state under no magnetic field conditions. After 5 minutes, the flocs are dispersed and suspended in the solution. Under no magnetic field conditions, it takes ≥8 minutes of sedimentation for the turbidity of the supernatant to drop below 6 NTU. The right beaker is in the magnetic attraction sedimentation state under an applied magnetic field conditions after 5 minutes. It can be seen that the flocs quickly gather towards the bottom magnet under the action of the magnetic field, and the supernatant is clear and transparent.

[0040] The sludge after magnetic sedimentation was collected, and 50 mL of 0.1 mol / L dilute nitric acid was added. The mixture was stirred for 30 minutes for acid washing. Subsequently, the desorbed black ZnFe₂O₄ magnetic nuclei were separated from the acid washing solution using a magnet. After washing twice with deionized water, the recovery rate was found to be over 95%. The recovered ZnFe₂O₄ nanoparticles can be used to formulate a new defluorinating agent, LAZ-3. After five cycles of use, when the dosage of the newly formulated defluorinating agent LAZ-3 was 1.2 mmol / L, the residual fluoride concentration was 1.03 mg / L, and the removal rate still reached 94.85%.

[0041] Comparative Example 1

[0042] Commercially available high-quality polyaluminum chloride (PAC, Al2O3 content ≥30%) was selected. 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L was taken (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4, with a Na2SO4 concentration of 50 mg / L), and the pH of the wastewater was adjusted to 7.0. Different amounts of PAC were added to eight identical groups of wastewater (the amount of PAC added was based on the molar concentration of Al in the wastewater). The dosages of the defluorinating agent PAC in the eight groups of wastewater were 0.4, 0.8, 1.2, 1.6, 2.0, 2.4, 2.8, and 3.2 mmol / L, respectively. After adding the defluorinating agent, the mixture was first stirred rapidly at 200 rpm for 5 minutes, then slowly stirred at 40 rpm for 5 minutes. After settling for 30 minutes, the supernatant was filtered through a 0.45 μm filter membrane, and the residual fluoride ion concentration was measured (using the fluoride ion selective electrode method).

[0043] pass Figure 1 It can be seen that, under the same conditions, the residual fluoride concentration is as high as 8.5 mg / L when 1.2 mmol / L (based on the Al element concentration in the wastewater) is added. The settling time of Comparative Example 1 is >30 min. Under the same conditions, the turbidity of the supernatant after 30 minutes of settling of PAC is still greater than 12 NTU.

[0044] Comparative Example 2

[0045] The La-Al binary defluorinating agent was prepared as follows: 4.33 g (0.01 mol) of lanthanum nitrate hexahydrate and 15.00 g (0.04 mol) of aluminum nitrate nonahydrate were weighed according to a La:Al molar ratio of 1:4 and dissolved in deionized water. The solution was then diluted to 1 L with water, and the pH of the solution was adjusted to 6.5 with NaOH to obtain the defluorinating agent La-Al.

[0046] Take 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4 with a concentration of 50 mg / L), and adjust the pH of the wastewater to 7.0. Add the defluorinating agent La-Al to the wastewater (the dosage of defluorinating agent La-Al is based on the total molar amount of metal ions (La, Al) in the wastewater). The dosage of defluorinating agent La-Al in the wastewater is 1.2 mmol / L. After adding the defluorinating agent, stir rapidly at 200 rpm for 5 minutes, then stir slowly at 40 rpm for 5 minutes. After settling for 40 minutes, take the supernatant and filter it through a 0.45 μm filter membrane. Measure the residual fluoride ion concentration (using the fluoride ion selective electrode method). The residual fluoride ion concentration is 4.35 mg / L, and the removal rate is 78.3%. The formed flocs are small and loose, and it is necessary to settle for more than 40 minutes to achieve effective solid-liquid separation.

[0047] Comparative Example 3

[0048] The La-Ce binary defluorinating agent was prepared as follows: 4.33 g (0.01 mol) of lanthanum nitrate hexahydrate and 5.34 g (0.01 mol) of cerium nitrate hexahydrate were weighed according to a La:Ce molar ratio of 1:1 and dissolved in deionized water. The solution was then diluted to 1 L with water, and the pH of the solution was adjusted to 6.5 with NaOH to obtain the defluorinating agent La-Ce.

[0049] Take 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4 with a concentration of 50 mg / L), and adjust the pH of the wastewater to 7.0. Add the defluorinating agent La-Ce to the wastewater (the dosage of defluorinating agent La-Ce is based on the total molar amount of metal ions (La, Ce) in the wastewater). The dosage of defluorinating agent La-Ce in the wastewater is 1.2 mmol / L. After adding the defluorinating agent, stir rapidly at 200 rpm for 5 minutes, then stir slowly at 40 rpm for 5 minutes. After settling for 35 minutes, take the supernatant and filter it through a 0.45 μm filter membrane. Measure the residual fluoride ion concentration (using the fluoride ion selective electrode method). The residual fluoride ion concentration is 5.42 mg / L, and the removal rate is 72.9%. The formed pale yellow flocs have poor settling performance and require about 35 minutes to be effectively separated.

[0050] Comparative Example 4

[0051] Single lanthanum salt defluorinating agent: Weigh 4.33 g (0.01 mol) of lanthanum nitrate hexahydrate and dissolve it in deionized water. Add water to make up the volume of the solution to 1 L. Adjust the pH of the solution to 6.5 with NaOH to obtain the defluorinating agent La.

[0052] Take 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4 with a concentration of 50 mg / L), and adjust the pH of the wastewater to 7.0. Add 1.2 mmol / L of defluorinating agent La to the wastewater (the amount of defluorinating agent La added is based on the molar amount of La ions in the wastewater). After adding the defluorinating agent, stir rapidly at 200 rpm for 5 minutes, then stir slowly at 40 rpm for 5 minutes. After settling for 40 hours, take the supernatant and filter it through a 0.45 μm filter membrane. Measure the residual fluoride ion concentration (using the fluoride ion selective electrode method). The residual fluoride ion concentration is 7.10 mg / L, and the removal rate is 64.5%. The solution remains turbid, and the turbidity of the supernatant is still greater than 30 NTU after 40 hours of settling, making it difficult to achieve solid-liquid separation by natural sedimentation.

[0053] Comparative Example 5

[0054] The only difference between Comparative Example 5 and Example 1 is that the pH of the defluorinating agent was not adjusted to 6.5. Specifically, 17.32g of La(NO3)3·6H2O (0.04mol), 11.25g of Al(NO3)3·9H2O (0.03mol), and 2.96g of Zn(NO3)2·6H2O (0.01mol) were weighed and dissolved in deionized water according to the metal ion molar ratio La:Al:Zn=4:3:1. The solution was then diluted to 1L with water to obtain the defluorinating agent (pH approximately 3.5).

[0055] Take 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4 with a concentration of 50 mg / L). Add a defluorinating agent to the wastewater (the dosage of the defluorinating agent is based on the total molar amount of metal ions (La, Al, Zn) in the wastewater). The dosage of the defluorinating agent is 1.2 mmol / L. After adding the defluorinating agent, stir rapidly at 200 rpm for 5 minutes, then stir slowly at 40 rpm for 5 minutes. After settling for 30 minutes, take the supernatant and filter it through a 0.45 μm filter membrane. Measure the residual fluoride ion concentration (using the fluoride ion selective electrode method). The residual fluoride ion concentration is 2.18 mg / L, and the removal rate is 89.1%. Meanwhile, the resulting flocs have poor structure and slow settling, requiring at least 40 minutes of settling time. This demonstrates that adjusting the defluorinating agent to a weakly acidic state (pH 6-6.5) is a necessary means to stabilize the hydroxyl polymerization form of metal ions and fully expose the active sites, thereby achieving a good defluorination rate and sedimentation properties.

[0056] Comparative Example 6

[0057] The only difference between Comparative Example 6 and Example 1 is that the molar ratio of metal ions La:Al:Zn is 1:1:1. The specific preparation method is as follows: 10.825g of La(NO3)3·6H2O (0.025mol), 9.375g of Al(NO3)3·9H2O (0.025mol) and 7.4g of Zn(NO3)2·6H2O (0.025mol) are weighed and dissolved in deionized water. Water is added to make up the volume of the solution to 1L. The pH of the solution is adjusted to 6.5 with NaOH to obtain the defluorinating agent.

[0058] Take 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4 with a concentration of 50 mg / L), and adjust the pH of the wastewater to 7.0. Add a defluorinating agent to the above wastewater (the dosage of the defluorinating agent is based on the total molar amount of metal ions (La, Al, Zn) in the wastewater). The dosage of the defluorinating agent in the wastewater is 1.2 mmol / L. After adding the defluorinating agent, stir rapidly at 200 rpm for 5 minutes, then stir slowly at 40 rpm for 5 minutes. After settling for 20 minutes, take the supernatant and filter it through a 0.45 μm filter membrane. Measure the residual fluoride ion concentration (using the fluoride ion selective electrode method). The residual fluoride ion concentration is 1.95 mg / L, and the removal rate is 90.3%. The flocs formed have poor settling performance and require more than 20 minutes to achieve effective solid-liquid separation.

[0059] Comparative Example 7

[0060] The only difference between Comparative Example 7 and Example 3 is that the ZnFe2O4 nanoparticles used in step (2) are commercially available (purity > 99%, average particle size > 100 nm and exhibiting hard agglomeration), specifically:

[0061] (1) Weigh 4.33g La(NO3)3·6H2O (0.01mol), 15g Al(NO3)3·9H2O (0.04mol) and 7.4g Zn(NO3)2·6H2O (0.025mol) and dissolve them in deionized water to obtain a salt solution;

[0062] (2) Weigh 1.205g of ZnFe2O4 nanopowder (0.005mol, purity >99%, average particle size >100nm and hard agglomerates) and disperse it in 100mL of deionized water. Sonicate the dispersion for 30 minutes to obtain a dispersion.

[0063] (3) Add the salt solution from step (1) to the dispersion from step (2), bring the volume to 1L, adjust the pH to 6.5, and then stir magnetically for 1 hour to obtain the magnetic defluorinating agent.

[0064] Take 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4 with a concentration of 50 mg / L), and adjust the pH of the wastewater to 7.0. Add a defluorinating agent to the above wastewater (the amount of defluorinating agent added is based on the total molar amount of metal elements (La, Al, Zn) in the wastewater). The amount of defluorinating agent added to the wastewater is 1.2 mmol / L. After adding the defluorinating agent, stir rapidly at 200 rpm for 5 minutes, then stir slowly at 40 rpm for 5 minutes. After settling for 25 minutes, take the supernatant and filter it through a 0.45 μm filter membrane. Measure the residual fluoride ion concentration (using the fluoride ion selective electrode method). The residual fluoride ion concentration is 1.85 mg / L, and the removal rate is 90.8%. Due to the poor dispersibility of commercial particles, the supernatant is still turbid after 25 minutes of settling, and incomplete solid-liquid separation can only be achieved after more than 10 minutes under an external magnetic field.

[0065] Comparative Example 8

[0066] A method for preparing a composite defluorinating agent, specifically comprising:

[0067] (1) Weigh 4.33g La(NO3)3·6H2O (0.01mol), 15g Al(NO3)3·9H2O (0.04mol) and 4.44g Zn(NO3)2·6H2O (0.015mol) and dissolve them in deionized water to obtain a salt solution;

[0068] (2) Weigh 3.615g of ZnFe2O4 nanopowder (0.015mol, particle size ~50nm) and disperse it in 100mL of deionized water. Sonicate the dispersion for 30 minutes to obtain a dispersion.

[0069] (3) Add the salt solution from step (1) to the dispersion from step (2), bring the volume to 1L, adjust the pH to 6.5, and then stir magnetically for 1 hour to obtain the magnetic defluorinating agent.

[0070] The preparation method of ZnFe2O4 nanoparticles in this comparative example is the same as in Example 3.

[0071] Take 500 mL of simulated wastewater with an initial fluoride ion concentration of 20.0 mg / L (the simulated wastewater was prepared with NaF, and the background ion in the simulated wastewater was Na2SO4 with a concentration of 50 mg / L), and adjust the pH of the wastewater to 7.0. Add a defluorinating agent to the above wastewater (the amount of defluorinating agent added is based on the total molar amount of metal elements (La, Al, Zn) in the wastewater). The amount of defluorinating agent added to the wastewater is 1.2 mmol / L. After adding the defluorinating agent, stir rapidly at 200 rpm for 5 minutes, and then stir slowly at 40 rpm for 5 minutes. Place a magnet at the bottom of the beaker, and after magnetic adsorption and settling for 5 minutes, take the supernatant and filter it through a 0.45 μm filter membrane. Measure the residual fluoride ion concentration (using the fluoride ion selective electrode method). The residual fluoride concentration is 1.69 mg / L, and the removal rate reaches 91.55%.

[0072] Table 1 shows the performance data for Examples 1-3 and Comparative Examples 1-8.

[0073] .

Claims

1. A composite defluoridating agent for deep defluorination of water, characterized in that: The defluorinating agent is an aqueous solution of a metal salt or an aqueous solution of a metal salt containing dispersed nanoparticles. The active ingredient of the defluorinating agent contains at least three metal ions: lanthanum, aluminum, and zinc, and the molar ratio of La, Al, and Zn is 1~4:3~5:1~3. The pH of the aqueous solution is 6.0~6.

5.

2. The composite defluorinating agent according to claim 1, characterized in that: When the defluorinating agent is an aqueous solution of a metal salt with dispersed nanoparticles, the nanoparticles are ZnFe2O4 nanoparticles with a particle size of ~50nm.

3. The composite defluorinating agent according to claim 2, characterized in that: The amount of ZnFe2O4 nanoparticles added is 1 / 6 to 1 / 5 of the total molar amount of Zn in the system, based on the mass of Zn.

4. The composite defluorinating agent according to claim 2, characterized in that: In the defluorinating agent, the molar ratio of the metal elements is La:Al:Zn = 1:4:

3.

5. The composite defluorinating agent according to claim 1, characterized in that: When the defluorinating agent is an aqueous solution of a metal salt, the molar ratio of the metal ions in the aqueous solution is La:Al:Zn = 4:3:1; or the molar ratio of the metal ions is La:Al:Zn = 2:5:

1.

6. The composite defluorinating agent according to claim 1, characterized in that: When the defluorinating agent is an aqueous solution of a metal salt, the concentration of the active ingredient is 0.075~0.08 mol / L; when the defluorinating agent is an aqueous solution of a metal salt with dispersed nanoparticles, the concentration of the active ingredient is 0.075~0.08 mol / L.

7. The method for preparing the composite defluorinating agent according to claim 2, characterized in that, Includes the following steps: (1) Add lanthanum salt, aluminum salt and zinc salt to deionized water, dissolve them and obtain a salt solution; (2) Disperse ZnFe2O4 nanoparticles in deionized water to obtain a nanoparticle dispersion; (3) Add the salt solution to the dispersion, add alkali solution to adjust the pH of the solution to 6~6.5, and stir magnetically to obtain the composite defluorinating agent.

8. The preparation method according to claim 7, characterized in that: In step (3), the magnetic stirring time is 1 to 2 hours.

9. The method for preparing the composite defluorinating agent according to claim 5, characterized in that, Specifically, lanthanum salt, aluminum salt, and zinc salt are added to deionized water, dissolved, and then an alkaline solution is added to adjust the pH of the solution to 6-6.5 to obtain a composite defluorinating agent.

10. The preparation method according to claim 7 or 9, characterized in that: The lanthanum salt, aluminum salt, and zinc salt are one of the corresponding nitrate, chloride, or sulfate.

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