A composite metal oxide adsorbent, its preparation method and application

Abstract

This invention relates to the field of sewage sludge treatment technology, and discloses a composite metal oxide adsorbent, its preparation method, and its application. The preparation method includes the following steps: S1. Pre-treating the sewage sludge raw material; S2. Leaching the pre-treated sewage sludge with an acid solution, obtaining a leachate after solid-liquid separation; the molar concentration of the acid solution is 0.5-3 mol / L; S3. Adding an alkaline solution to the leachate to adjust the pH to 9-11, and carrying out a co-precipitation reaction to obtain a hydrotalcite precursor; S4. Calcining the hydrotalcite precursor to obtain the composite metal oxide adsorbent. The composite metal oxide adsorbent provided by this invention has a large specific surface area and abundant active sites, exhibiting excellent adsorption efficiency for anionic pollutants in water. After adsorption saturation, some adsorption products can be directly used as fertilizer for soil.

Description

Technical Field

[0001] This invention relates to the field of sewage sludge treatment technology, specifically to a composite metal oxide adsorbent, its preparation method, and its application. Background Technology

[0002] With the continuous improvement of drainage pipe network systems, the cleaning and maintenance of urban drainage pipes generates a large amount of waste known as "ditch sludge." Ditch sludge has a complex composition, being a mixture of particulate matter, organic matter, heavy metals, pathogens, and gravel from sewage. Its treatment and disposal is a challenge in urban environmental governance. Currently, the main methods for treating ditch sludge are "washing and screening-incineration" or "dewatering-sanitary landfill," which not only consume large amounts of land resources but may also cause secondary pollution. Meanwhile, the eutrophication problem caused by excessive anions (such as phosphates and nitrates) in water bodies is becoming increasingly serious. Developing efficient and low-cost adsorbents for deep water purification has become a research hotspot in the field of environmental engineering. Therefore, seeking a technological path to treat waste with waste and turn waste into treasure, transforming low-value waste such as ditch sludge into high-value-added adsorbent materials, has extremely important practical significance.

[0003] Existing technologies using sludge from drainage ditches to prepare adsorbents generally suffer from a limited adsorption range and restricted practical applications. Furthermore, the final product of the adsorbent is only used as an adsorbent, and the saturated adsorbent after use still needs to be disposed of as hazardous waste, failing to achieve a complete resource loop. As a result, the degree of resource utilization is low and the added value of the product is low. Summary of the Invention

[0004] This invention provides a composite metal oxide adsorbent, its preparation method, and its application, which solves the problems of limited adsorption range, low resource utilization, and low added value of products of adsorbents prepared from sewage sludge in the prior art.

[0005] In a first aspect, the present invention provides a method for preparing a composite metal oxide adsorbent, comprising the following steps: S1. Pre-treatment of sewage sludge raw materials from drainage ditches; S2. The pretreated ditch sludge is leached with an acid solution, and the leachate is obtained after solid-liquid separation. The molar concentration of the acid solution is 0.5-3 mol / L; for example, the concentration of the acid solution in S2 is any value among 0.5 mol / L, 1 mol / L, 2 mol / L, and 3 mol / L, or any value within a range formed by any two of these values. The metal cations in the leachate include Mg 2+ Al 3+ and Fe 3+ The Mg2+ Al 3+ and Fe 3+ The molar ratio is (1.5-4):(1-1.5):1; S3. Add an alkaline solution to the leachate to adjust the pH to 9-11, and perform a co-precipitation reaction to obtain the hydrotalcite precursor; exemplarily, the pH adjustment can be any value among 9, 10, and 11, or any value within a range formed by any two of these values. Within this pH range, sufficient OH- can be provided. - To meet the acid-base neutralization requirements for the precipitation of divalent and trivalent metal hydroxides, a hydrotalcite precursor (LDH) framework is formed. This pH range allows for precise control of the reaction process, preventing incomplete metal ion precipitation due to insufficient alkali or the induction of amphoteric metal (such as Al) precipitation due to excessive alkali. 3+ The redissolved impurity phase ensures the crystallinity and chemical homogeneity of the LDH precursor, and enhances the reliability and repeatability of the process, resulting in a uniform distribution of interlayer anions.

[0006] S4. The hydrotalcite precursor is calcined to obtain the composite metal oxide adsorbent.

[0007] In one alternative implementation, the pretreatment includes dehydration, drying, and pulverization.

[0008] In one optional embodiment, the drying temperature in S1 is 70-110℃, and the drying time is 8-24h; for example, the drying temperature is any value among 70℃, 80℃, 90℃, 100℃, and 110℃, or any value within the range formed by any two of these values; for example, the drying time is any value among 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h, and 24h, or any value within the range formed by any two of these values.

[0009] In one alternative implementation, the equipment used for pulverizing in S1 includes a ball mill or a planetary ball mill.

[0010] In one optional implementation, the sludge in S1 includes one or more of the following: rainwater pipe network sludge, sewage pipe network sludge, and combined sewer system sludge.

[0011] In one optional embodiment, the particle size of the pretreated sludge in S1 is 50-150 μm; exemplaryly, the particle size of the pretreated sludge is any value from 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm or any value within a range formed by any two values.

[0012] In one optional embodiment, based on the total mass of the ditch sludge raw material, the Mg content of the ditch sludge raw material is... 2+ The mass percentage content is 1%-5%, Al 3+ The mass percentage content is 0.5%-3%, Fe 3+ The mass percentage content is 0.5%-4%.

[0013] In one optional embodiment, the dewatering in S1 includes mechanically dewatering the ditch sludge to convert it into sludge cake with a moisture content of 60%-75%. Optionally, the mechanical dewatering instrument includes one of a plate and frame filter press, a belt filter press, a centrifugal dewatering machine, and a screw press dewatering machine.

[0014] In one alternative implementation, S2 satisfies at least one of the following conditions: (1) The acidic substance in the acid solution includes one of hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), citric acid (C6H8O7), and acetic acid (CH3COOH); (2) The leaching temperature is 50-90℃ and the leaching time is 1-4h; for example, the leaching temperature in S2 is any value of 50℃, 60℃, 70℃, 80℃, 90℃ or any value within the range formed by any two values; for example, the leaching time is any value of 1h, 2h, 3h, 4h or any value within the range formed by any two values.

[0015] (3) The ratio of the mass of the pretreated ditch sludge to the volume of the acid solution is 1:(2-8); for example, the ratio of the mass of the pretreated ditch sludge to the volume of the acid solution in S2 is any value among 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8 or any value within the range formed by any two values.

[0016] In one optional embodiment, the alkaline solution in S3 is prepared from alkaline materials and deionized water; Optionally, the alkaline material includes one or more of sodium hydroxide (NaOH), sodium carbonate (Na2CO3), potassium hydroxide (KOH), potassium carbonate (K2CO3), NH3·H2O, and urea (CO(NH2)2).

[0017] In an optional embodiment, the OH- in the alkaline solution of S3 - The molar concentration is 0.5-3 mol / L. For example, the OH- in the alkaline solution in S3... -The molar concentration is any value among 0.5 mol / L, 1 mol / L, 2 mol / L, and 3 mol / L, or any value within a range formed by any two of these values.

[0018] OH- of different alkaline materials - The method for calculating the amount of substance is as follows: When the alkaline material is at least one of sodium hydroxide (NaOH) and potassium hydroxide (KOH), the amount of alkaline material = OH - The amount of substance; When the alkaline material is NH3·H2O, for the sake of simplifying calculations, this application uses the amount of alkaline material = OH... - The amount of substance; When the alkaline material is at least one of sodium carbonate (Na₂CO₃) and potassium carbonate (K₂CO₃), the amount of alkaline material is equal to twice the amount of OH⁻. - The amount of substance; When the alkaline material is urea (CO(NH2)2), it does not directly provide OH-. - However, under heating conditions, it can hydrolyze to produce NH3 and CO2. NH3 further reacts with water to produce NH3·H2O and provides OH-. - To simplify calculations, this application assumes that 1 mol of urea can provide 2 mol of OH. - Estimate (based on CO(NH2)2+3H2O→2NH3·H2O+CO2, 2NH3·H2O→2NH4) + +2OH - ).

[0019] In one optional embodiment, the alkaline solution contains OH... - The molar concentration is 0.5-3 mol / L.

[0020] In one optional embodiment, the coprecipitation reaction time in S3 is 0.5-4h; exemplaryly, the coprecipitation reaction time in S3 is any value among 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, and 4h, or any value within a range formed by any two values.

[0021] In one optional embodiment, the calcination atmosphere in S4 is a protective atmosphere; Optionally, the protective atmosphere includes one of nitrogen, argon, and helium.

[0022] In one optional embodiment, the calcination temperature in step S4 is 500-800℃, the calcination time is 2-4h, and the heating rate is 5-10℃ / min. For example, the calcination temperature in step S4 can be any value from 500℃, 600℃, 700℃, and 800℃, or any value within the range formed by any two of these values; the calcination time can be any value from 2h, 3h, and 4h, or any value within the range formed by any two of these values; and the heating rate can be any value from 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, and 10℃ / min, or any value within the range formed by any two of these values.

[0023] Secondly, the present invention provides a composite metal oxide adsorbent, which is prepared by the aforementioned preparation method.

[0024] In one optional embodiment, the total mass of oxides of magnesium, aluminum and iron metals in the adsorbent accounts for 60%-90% of the dry basis mass of the adsorbent; the dry basis mass refers to the mass of the prepared adsorbent after removing moisture, calculated purely on a solid basis.

[0025] In one optional embodiment, the molar ratio of magnesium, aluminum, and iron in the adsorbent is (1.5-4):(1-1.5):1; exemplaryly, the molar ratio of magnesium, aluminum, and iron in the adsorbent is any value from 1.5:1:1, 1.5:1.1:1, 1.5:1.2:1, 1.5:1.3:1, 1.5:1.4:1, 1.5:1.5:1, 3:1:1, 3:1.1:1, 3:1.2:1, 3:1.3:1, 3:1.4:1, 3:1.5:1, 4:1:1, 4:1.1:1, 4:1.2:1, 4:1.3:1, 4:1.4:1, 4:1.5:1 or any value within a range formed by any two values.

[0026] In one optional embodiment, the specific surface area of ​​the adsorbent is 50-200 m². 2 / g; for example, the specific surface area of ​​the adsorbent is 50m². 2 / g、60m 2 / g、70m 2 / g、80m 2 / g、90m 2 / g, 100m 2 / g、110m 2 / g、120m 2 / g、130m 2 / g, 140m 2 / g, 150m 2 / g、160m 2 / g、170m 2 / g、180m 2 / g、190m 2 / g、200m 2 Any value in / g or any value within a range formed by any two values.

[0027] Thirdly, the present invention provides an application of the adsorbent described above in the removal of anionic pollutants in water; Optionally, the anionic pollutant includes one or more of phosphate ions, arsenate ions, fluoride ions, and chromate ions. Optionally, the solid product obtained after the adsorbent adsorbs phosphate ions and / or fluoride ions can be used as a fertilizer; the fertilizer can be applied directly to the soil or mixed with other fertilizers.

[0028] In one optional embodiment, this application can obtain products with different adsorption characteristics by adjusting the preparation process parameters for sewage sludge raw materials with different compositions: high magnesium content adsorbent (in the adsorbent, Mg... 2+ The amount of substance in Mg 2 + Al 3+ Fe 3+ (The total amount of iron in the adsorbent exceeds 50%) exhibits excellent selectivity and high adsorption capacity for phosphate, and its saturated product is rich in phosphorus and magnesium nutrients, making it particularly suitable for the improvement and fertilization of phosphorus-deficient soils; high-iron content adsorbents (in which Fe...) have excellent selectivity and high adsorption capacity for phosphate, and their saturated products are rich in phosphorus and magnesium nutrients, making them particularly suitable for the improvement and fertilization of phosphorus-deficient soils; 3+ The amount of substance in Mg 2+ Al 3+ Fe 3+ (It accounts for more than 30% of the total amount of matter) and is specifically used to adsorb toxic anions such as arsenate / chromate. Its saturated product needs to be stabilized before it can be used as a soil conditioner for forestry land or mining area reclamation, based on the heavy metal leaching toxicity assessment results.

[0029] In one alternative implementation, when treating polluted water bodies of different concentrations, a direct addition-collection mode can be used for low-concentration pollution scenarios (anionic pollutant concentration ≤10 mg / L), and the adsorbed saturated products can be returned to the field on-site; while for high-concentration industrial wastewater treatment scenarios (anionic pollutant concentration >50 mg / L), a fixed-bed column adsorption-regeneration cycle mode can be used, and the saturated adsorbent can be processed into granular slow-release fertilizer products in batches.

[0030] In one alternative implementation, the final product can be applied not only directly, but also by mixing it with organic fertilizer to form compound fertilizer, or by granulation and encapsulation for use in professional fields such as golf courses and nurseries where slow-release effects are required.

[0031] The technical solution of this invention has the following advantages: 1. The preparation method of the composite metal oxide adsorbent provided by the present invention includes the following steps: S1. Pretreatment of sewage sludge raw material; S2. Leaching treatment of the pretreated sewage sludge with an acid solution, and obtaining leachate after solid-liquid separation; the molar concentration of the acid solution is 0.5-3 mol / L; the metal cations in the leachate include Mg 2+ Al 3+ and Fe 3+ The Mg 2+ Al 3+ and Fe 3+ The molar ratio is (1.5-4):(1-1.5):1; S3. Add an alkaline solution to the leachate to adjust the pH to 9-11 and carry out a co-precipitation reaction to obtain the hydrotalcite precursor; S4. Calcinate the hydrotalcite precursor to obtain the composite metal oxide adsorbent.

[0032] This invention employs an acid leaching-coprecipitation-calcination method. First, acid leaching selectively extracts valuable metal elements (Mg, Al, Fe) from the complex sludge matrix and transfers them to the liquid phase, achieving enrichment and purification of the target components. This effectively removes interference from organic matter and inert impurities in the sludge, transforming heterogeneous solid waste into a homogeneous metal leachate with controllable components. Subsequently, coprecipitation is performed at pH 9-11, allowing these metal ions to reassemble at the molecular level, generating a layered hydrotalcite precursor (LDH) with a unique layered structure. This structure determines its large specific surface area and abundant surface hydroxyl sites. Finally, calcination transforms the LDH precursor into a complex metal oxide (LDO). This process removes interlayer water molecules and anions, causing the layered structure to collapse, resulting in an even higher specific surface area and more porous structure, especially exposing a large number of unsaturated coordinated metal sites. These sites have a strong affinity for anionic pollutants in water and can achieve efficient adsorption through mechanisms such as surface coordination and ion exchange.

[0033] In the acid leaching process of this invention, metals such as magnesium, aluminum, and iron in the sludge from the drainage ditch usually exist in the form of oxides, hydroxides, or combined with organic matter. The acid solution passes through protons (H... + ) attacks and breaks the chemical bonds of these metal compounds, causing them to exist in ionic form (Mg 2+ Al 3+ Fe 3+ It dissolves, effectively strips away the encapsulation of organic matter, and dissolves some carbonate impurities.

[0034] In the coprecipitation reaction process of this invention, by precisely controlling the pH value (pH 9-11), the Mg in the acid leaching solution is reduced. 2+ Al 3+ Fe 3+ Metal ions and hydroxyl groups (OH) - In the liquid phase, a layered hydrotalcite precursor with a regular layered structure spontaneously assembles. This process first achieves molecular-level structural customization; by controlling the type of alkali, the proportion of metal elements in the layers and the type of anions between layers can be precisely controlled, thus directionally synthesizing a hydrotalcite precursor (LDH) with specific adsorption properties. Secondly, it possesses excellent compositional inclusiveness; the reaction system allows for the spontaneous assembly of Fe... 3+ Partial replacement of Al 3+ Entering the LDH layer not only enhances the material's adsorption capacity for anions but also avoids the formation of impurity phases, ensuring the integrity of the crystal structure. Furthermore, it completes the standardized preparation of the product. Regardless of the fluctuations in the composition of the raw sludge, this step can transform leachates with varying compositions into LDH precursor materials with highly uniform structure, morphology, and chemical properties.

[0035] This invention removes interlayer water molecules from LDH through calcination, causing dehydration of the hydroxyl groups in the layers. This leads to the collapse and recombination of the layered structure, forming an LDO with abundant mesopores and numerous unsaturated coordination metal sites. This ensures complete decomposition of LDH while preventing the sintering and agglomeration of metal oxides, thus preserving a high specific surface area and active sites. A suitable heating rate promotes an ordered structural transformation, preventing abrupt pore collapse. An inert atmosphere prevents the oxidation of active components such as ferrous iron. Calcination under specific conditions precisely activates adsorption sites. The Mg-O, Al-O, and Fe-O unsaturated bonds generated after calcination act as strong Lewis acid sites, exhibiting extremely strong coordination adsorption capacity for anions in water, further enhancing the ion exchange capacity.

[0036] The present invention provides a simple, low-cost, low-energy-consumption treatment and preparation process for sewage sludge from drainage ditches, and does not generate secondary pollution.

[0037] This invention prepares a highly efficient anion adsorbent mainly composed of magnesium, aluminum, and iron oxides from complex sewage sludge. The adsorbent obtained by this invention has a large specific surface area and abundant active sites, exhibiting excellent adsorption efficiency for anion pollutants such as phosphate and arsenate in water.

[0038] In the preparation method provided by this invention, the Mg in the leachate 2+ Al 3+ and Fe 3+ The molar ratio falling within the range of (1.5-4):(1-1.5):1 ensures the successful synthesis of a layered hydrotalcite precursor (LDH) with a stable layered structure and high adsorption activity, which is the thermodynamically stable basis for constructing the LDH layered structure. An appropriate amount of Fe... 3+ Partial replacement of Al 3+Entering the laminations can introduce additional Lewis acid sites and potentially create crystal defects, thereby enhancing the adsorption capacity for specific anions while avoiding Fe... 3+ Excessive content leads to the formation of independent iron oxide phases, which disrupts the uniform structure of the hydrotalcite precursor and its unique "memory effect". Ultimately, this optimized ratio enables the composite metal oxide adsorbent obtained after calcination to have both high specific surface area, rich pore structure and multi-metal synergistic active sites, laying the compositional foundation for achieving efficient adsorption and subsequent slow-release fertilizer function.

[0039] This invention provides the application of the adsorbent prepared by the method in removing anionic pollutants from water bodies; the anionic pollutants include one or more of phosphate ions, arsenate ions, fluoride ions, and chromate ions; optionally, the solid product obtained after using the adsorbent to adsorb phosphate ions and / or fluoride ions can be used as fertilizer; the fertilizer can be applied directly to the soil or mixed with other fertilizers for use.

[0040] This invention opens up a new avenue for the high-value utilization of sewage sludge from drainage ditches, transforming it from an environmental burden into a material with adsorption capabilities. It is used to treat anionic pollutants in water bodies, purifying water quality as an adsorbent. Some of the adsorbed products can also be used as slow-release fertilizer to return the adsorbed nutrients to the soil, thus constructing a complete closed loop of "pollution control-resource recycling" that can generate both environmental and economic benefits.

[0041] 2. The preparation method of the composite metal oxide adsorbent provided by the present invention, wherein the pretreatment includes dehydration, drying and pulverization; and / or, the particle size of the sludge in the drainage ditch after pretreatment in S1 is 50-150 μm.

[0042] The drying and pulverizing step of the sludge in this invention is to expose the active sites. The adsorption capacity of the adsorbent is mainly contained in the intricate pore structure inside the particles. If the particles are too large, pollutant molecules will have difficulty reaching them. Pulverizing to the micron level will greatly shorten the internal diffusion path of pollutant molecules, so that the active surface inside the pores is fully exposed. Attached Figure Description

[0043] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0044] Figure 1 This is a SEM image of the composite metal oxide adsorbent of Example 1 of the present invention. Detailed Implementation

[0045] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0046] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0047] The sources of raw materials in the embodiments and comparative examples provided by this invention are as follows: The sewage sludge was taken from a sewage sludge treatment plant in Songjiang District, Shanghai. Based on the total mass of the sewage sludge raw material, the Mg content of the sewage sludge raw material was... 2+ The mass percentage content is 3.2%, Al 3+ The mass percentage content is 1.8%, Fe 3+ The mass percentage content is 2.1%.

[0048] Example 1 This embodiment provides a method for preparing a composite metal oxide adsorbent, comprising the following steps: S1. The sludge from the drainage ditch is mechanically dewatered using a centrifugal dewatering machine until it reaches a sludge cake with a moisture content of 65%. It is then dried at 80℃ for 10 hours and finally pulverized using a ball mill to a particle size of 75μm.

[0049] S2. The pretreated ditch sludge was leached with a 3 mol / L sulfuric acid solution (the mass ratio of sludge to sulfuric acid solution was 1:4). The leaching temperature was 60℃ and the leaching time was 2 hours. After solid-liquid separation by centrifugation, metal cations (Mg) were obtained. 2+ Al 3+ and Fe 3+ The leachate of the sample was tested, and the Mg content in the leachate was found to be... 2+ The concentration was 2.68 g / L, Al 3+ Concentration of 0.78 g / L, Fe 3+ The concentration is 1.6 g / L, which is Mg 2+ Al 3+ and Fe 3+ The molar ratio is 3.85:1.01:1; S3. Using the leachate as a metal source, the pH of the leachate was adjusted to 10 with 1 mol / L sodium hydroxide (NaOH) solution, and a co-precipitation reaction was carried out for 1 h to obtain the hydrotalcite precursor LDH. S4. The hydrotalcite precursor was calcined at 600°C for 3 hours in an argon atmosphere at a heating rate of 6°C / min to obtain the composite metal oxide adsorbent LDO.

[0050] The total mass of magnesium, aluminum, and iron oxides in the prepared adsorbent material accounts for 89.6% of the dry weight of the adsorbent (calculated from ICP-OES elemental content), with the remainder being other metal oxide impurities besides magnesium, aluminum, and iron. The molar ratio of magnesium, aluminum, and iron in the adsorbent is 3.95:1.20:1.00, and the specific surface area is 198 m². 2 / g (Detection method: BET nitrogen adsorption method).

[0051] The above adsorbent was used in 1L of arsenic-containing wastewater at a dosage of 2g / L, with a reaction pH of 7, a reaction temperature of 24℃, and a reaction time of 1h. The resulting arsenate ion removal efficiency was 96%.

[0052] Figure 1 This is a SEM image of the composite metal oxide adsorbent of Example 1 of the present invention. Figure 1 The morphological characteristics of the composite metal oxide adsorbent are as follows: the adsorbent particles exhibit a typical plate-like or layered stacked structure, which is a characteristic morphology of the hydrotalcite precursor after calcination into composite metal oxides. The edges of the plates are clear, and the particle distribution is relatively uniform. Figure 1 This directly demonstrates that the composite metal oxide adsorbent prepared by this invention has a high specific surface area and a sheet-like or layered morphology, which provides a structural basis for its excellent anion adsorption performance.

[0053] Example 2 This embodiment provides a method for preparing a composite metal oxide adsorbent. The difference from Embodiment 1 is that in step S2, a 0.5 mol / L sulfuric acid solution is used to leach the pretreated drainage sludge (the mass ratio of drainage sludge to the volume of sulfuric acid solution is 1:2), and the leaching time is 1 hour. Measurements showed that the Mg content in the leachate of this embodiment was... 2+ The concentration was 2.25 g / L, Al 3+ Concentration of 0.72 g / L, Fe 3+ The concentration is 1.45 g / L, which is Mg 2+ Al 3+ and Fe 3+ The molar ratio was 3.57:1.03:1. In the final adsorbent material, the total mass of magnesium, aluminum, and iron oxides accounted for 70% of the dry weight of the adsorbent, with a molar ratio of 3.30:1.10:1. The specific surface area was 151 m². 2 / g.

[0054] The above adsorbent was used in 1L of arsenic-containing wastewater at a dosage of 2g / L, with a reaction pH of 7, a reaction temperature of 24℃, and a reaction time of 1h. The resulting arsenate ion removal efficiency was 92%.

[0055] Example 3 This embodiment provides a method for preparing a composite metal oxide adsorbent. The difference from Embodiment 1 is that in step S2, a 1.5 mol / L sulfuric acid solution is used to leach the pretreated drainage sludge (the mass ratio of drainage sludge to the volume of sulfuric acid solution is 1:8), and the leaching time is 4 hours. Measurements showed that the Mg content in the leachate of this embodiment was... 2+ The concentration was 2.35 g / L, Al 3+ Concentration of 1.05 g / L, Fe 3+ The concentration is 1.50 g / L, which is Mg 2+ Al 3+ and Fe 3+ The molar ratio of magnesium, aluminum, and iron was 3.60:1.45:1. In the final adsorbent material, the total mass of magnesium, aluminum, and iron oxides accounted for 80% of the dry weight of the adsorbent, with a molar ratio of 3.52:1.41:1. The specific surface area was 159 m². 2 / g.

[0056] The above adsorbent was used in 1L of arsenic-containing wastewater at a dosage of 2g / L, with a reaction pH of 7, a reaction temperature of 24℃, and a reaction time of 1h. The resulting arsenate ion removal efficiency was 94%.

[0057] Example 4 This embodiment provides a method for preparing a composite metal oxide adsorbent. The difference from Embodiment 1 is that the prepared adsorbent is used in fluoride-containing wastewater, and the fluoride ion removal efficiency is 85%.

[0058] Example 5 This embodiment provides a method for preparing a composite metal oxide adsorbent. The difference from Embodiment 1 is that the prepared adsorbent is used in phosphorus-containing wastewater, and the phosphate ion removal efficiency is 95%.

[0059] Comparative Example 1 This comparative example provides a method for preparing a composite metal oxide adsorbent. The difference from Example 1 is that in step S2, a 0.1 mol / L sulfuric acid solution is used to leach the pretreated ditch sludge. The final adsorbent material contains magnesium, aluminum, and iron oxides, accounting for 40% of the dry weight of the adsorbent. The molar ratio of magnesium, aluminum, and iron in the adsorbent is 0.83:2.24:1, and the specific surface area is 18 m². 2 / g.

[0060] The above adsorbent was used in 1L of arsenic-containing wastewater at a dosage of 2g / L, with a reaction pH of 7, a reaction temperature of 24℃, and a reaction time of 1h. The resulting arsenate ion removal efficiency was 66%.

[0061] Experimental Example 1 The composite metal oxide adsorbents prepared in Examples 1-5 and Comparative Example 1 were tested for elemental content, anionic pollutant adsorption efficiency, and anionic pollutant concentration. The test methods are as follows: 1. Element content test: Using X-ray fluorescence spectroscopy (XRF), the dried and pulverized composite metal oxide adsorbent is pressed into uniform, flat discs under high pressure, which can then be directly tested on the instrument.

[0062] 2. Adsorption efficiency of anionic pollutants: η= ×100%, where C0 refers to the initial concentration of anionic pollutants in the wastewater (mg / L), and C1 refers to the concentration of anionic pollutants after treatment with the addition of composite metal oxide adsorbent (mg / L). The concentration of anionic pollutants was tested using ion chromatography (IC).

[0063] The test results are shown in Table 1.

[0064] Table 1

[0065] Comparing Examples 1-3, it is evident that by using the method of this application to leach pretreated drainage sludge with sulfuric acid of different concentrations, a more effective increase in the sulfuric acid concentration can dissolve magnesium, aluminum, iron, and other metal ions from the drainage sludge, resulting in a leachate with a higher concentration and better proportion of metal ions, closer to the ideal molar ratio of 4:1:1. This leads to a higher quality hydrotalcite precursor (LDH) formed through subsequent co-precipitation, and the resulting composite metal oxide (LDO) after calcination exhibits a higher metal oxide content (89.6%), a more ideal crystal structure, and a larger specific surface area. These superior physicochemical properties collectively determine its higher number of active sites and stronger adsorption capacity during arsenic adsorption and removal.

[0066] Comparing Examples 1, 4, and 5, it can be seen that the same composite metal oxide adsorbent prepared using the preparation method of Example 1 of this application adsorbs different ions. As shown in the above experiments, in Example 5, this adsorbent effectively adsorbs phosphate (PO4) ions. 3- The removal rate of arsenate (AsO4) is as high as 95%, which is comparable to the removal rate of arsenate (AsO4) in Example 1. 3-The effect of fluoride ions (F) was comparable (96%), mainly because both are trivalent anions with a strong affinity for the LDO layer metal and readily form stable inner-layer complexes through ligand exchange, even generating insoluble phosphate / arsenate precipitates. In contrast, in Example 4, the effect of fluoride ions (F) was significantly reduced. - The removal rate of F was slightly lower (85%), mainly because F - It is a monovalent ion with a low charge density, resulting in a weak electrostatic attraction with the adsorbent; meanwhile, F - Small ionic radius and high hydration energy make them more effective in interacting with other anions (such as Cl-, which is commonly found in water). - SO4 2- It may be at a disadvantage when competing for adsorption sites. Therefore, this adsorbent exhibits a highly efficient adsorption capacity specifically for multivalent oxygen-containing anions, making it an excellent material with broad application potential.

[0067] Comparing Example 1 and Comparative Example 1, it can be seen from the above experiments that, using the method of this application, high-concentration acid (3 mol / L) can effectively dissolve target metals such as magnesium, aluminum, and iron in the sludge from the drainage ditch, obtaining a leachate with high ion concentration and ideal ratio (molar ratio Mg:Al:Fe ≈ 4:1:1), laying the foundation for the subsequent synthesis of a well-structured and high-performance hydrotalcite precursor (LDH). After calcination, the precursor was successfully transformed into a product with a high specific surface area (198 m² / L). 2 The composite metal oxide (LDO) with high metal oxide content (89.6%) and abundant adsorption sites and "memory effect" exhibited a removal rate of up to 96% for arsenate. Conversely, the extremely low concentration of acid (0.1 mol / L) in Comparative Example 1 could not effectively leach the metals from the sludge, resulting in insufficient total ion content and an imbalanced ratio in the leachate (Mg:Al:Fe = 0.83:2.24:1, severely deficient in magnesium). This prevented the formation of a good hydrotalcite structure, resulting in a final product with low metal content (40%) and small specific surface area (18 m²). 2 The structure is disordered, and its adsorption efficiency for arsenate is only 66%.

[0068] Experimental Example 2 The composite metal oxide adsorbents prepared in Examples 4 and 5 exhibited excellent adsorption effects on fluoride ions. After adsorption saturation, they can be used as slow-release fertilizer for land resource utilization. However, their environmental safety and agricultural value need to be assessed before practical application. Toxicity leaching tests (using the acetic acid buffer solution method according to HJ / T 300-2007) were conducted on Examples 4 and 5 to evaluate their fluoride release characteristics or available phosphorus content after adsorption saturation. Pot experiments were also performed.

[0069] (1) Test process and results of Example 4 After the composite metal oxide adsorbent prepared in Example 4 was saturated with fluoride ions, a toxicity leaching test was conducted (using the acetic acid buffer solution method of HJ / T 300-2007). The test results showed that the concentrations of various heavy metals in the leachate were all lower than the limits in GB 5085.3-2007 "Identification Standard for Hazardous Waste - Leaching Toxicity Identification", and the fluoride ion concentration in the leachate was 0.5 mg / L, which, according to GB 5085.3-2007 "Groundwater Quality Standard" Class III water limit, did not exceed 1 mg / L.

[0070] The fluoride release characteristics were evaluated using a soil column leaching experiment. Saturated adsorbent was packed into a glass column with an inner diameter of 5 cm and a height of 30 cm, and leached with deionized water at a flow rate of 0.5 mL / min. The fluoride ion concentration in the leachate was collected periodically. The results showed that on days 1, 3, 7, 14, 21, and 28, the fluoride ion concentrations in the leachate were 1.8 mg / L, 2.1 mg / L, 2.0 mg / L, 1.9 mg / L, 1.7 mg / L, and 1.5 mg / L, respectively. The adsorbent could maintain the fluoride concentration in the leachate between 1.5 and 2.1 mg / L for a relatively long period (28 days), far below the standard of 10 mg / L.

[0071] Finally, a pot experiment was conducted using Chinese cabbage as the experimental subject. The results showed that the biomass of the Chinese cabbage with added saturated adsorbent was significantly increased compared to the control group (without added saturated adsorbent), and the content of metals such as cadmium and lead in the Chinese cabbage was below the limit. In conclusion, saturated adsorbents that adsorb fluoride ions can be effectively used as slow-release fertilizers for land resource utilization.

[0072] (2) Test process and results of Example 5 The composite metal oxide adsorbent prepared in Example 5 was saturated with phosphate ions and subjected to a toxicity leaching test (using the acetic acid buffer solution method according to HJ / T 300-2007). The test results showed that the concentrations of various heavy metals in the leachate were all lower than the limits in GB 8978-2002 "Integrated Wastewater Discharge Standard". Since the phosphating wastewater used contained a small amount of arsenic (As), the arsenic content in the leachate was also determined, and it was found to be 0.04 mg / L, which is less than the standard of 0.1 mg / L.

[0073] The available phosphorus content was determined using the sodium bicarbonate method. Based on relevant standards such as "Determination of Phosphorus Content in Fertilizers," the available phosphorus content in the saturated adsorbent reached 20%, meeting the basic requirements for use as a phosphate fertilizer. The slow-release performance of the saturated adsorbent was verified using the leaching rate determination method specified in "Slow-Release Fertilizers" (National Standard, GB / T 23348-2021). The initial nutrient release rate after 24 hours was only 8%, lower than the standard of 15%, and the cumulative nutrient release rate after 28 days was 70%, lower than the standard of 80%.

[0074] Finally, a pot experiment was conducted using ryegrass as the experimental subject. The results showed that the biomass of the ryegrass with added saturated adsorbent increased by 50% compared to the control group, and the arsenic content in the ryegrass was 0.001 mg / kg, far below 0.5 mg / kg. In conclusion, saturated adsorbents that adsorb phosphate ions can be effectively utilized as slow-release fertilizers for land resource recovery.

[0075] 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. A method for preparing a composite metal oxide adsorbent, characterized in that, Includes the following steps: S1. Pre-treatment of sewage sludge raw materials from drainage ditches; S2. The pretreated ditch sludge is leached with an acid solution, and the leachate is obtained after solid-liquid separation. The molar concentration of the acid solution is 0.5-3 mol / L; The metal cations in the leachate include Mg. 2+ Al 3+ and Fe 3+ The Mg 2+ Al 3+ and Fe 3+ The molar ratio is (1.5-4):(1-1.5):1; S3. Add an alkaline solution to the leachate to adjust the pH to 9-11, and carry out a co-precipitation reaction to obtain the hydrotalcite precursor; S4. The hydrotalcite precursor is calcined to obtain the composite metal oxide adsorbent.

2. The preparation method of the composite metal oxide adsorbent according to claim 1, characterized in that, The pretreatment includes dehydration, drying, and pulverization; And / or, the sludge in the drainage ditch in S1 includes one or more of the following: rainwater pipe network sludge, sewage pipe network sludge, and combined sewer system sludge; And / or, the particle size of the pretreated sludge in S1 is 50-150 μm; And / or, based on the total mass of the sludge raw material, the Mg content in the sludge raw material is... 2+ The mass percentage content is 1%-5%, Al 3+ The mass percentage content is 0.5%-3%, Fe 3+ The mass percentage content is 0.5%-4%.

3. The method for preparing the composite metal oxide adsorbent according to claim 1, characterized in that, S2 satisfies at least one of the following conditions: (1) The acidic substance in the acid solution includes one of hydrochloric acid, sulfuric acid, nitric acid, citric acid, and acetic acid; (2) The leaching temperature is 50-90℃ and the leaching time is 1-4h; (3) The ratio of the mass of the pretreated ditch sludge to the volume of the acid solution is 1:(2-8).

4. The method for preparing the composite metal oxide adsorbent according to claim 1, characterized in that, The alkaline solution in S3 is prepared from alkaline materials and deionized water; Optionally, the alkaline material includes one or more of sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, NH3·H2O, and urea; And / or, the alkaline solution contains OH- - The molar concentration is 0.5-3 mol / L.

5. The method for preparing the composite metal oxide adsorbent according to any one of claims 1-4, characterized in that, The coprecipitation reaction in S3 takes 0.5-4 hours.

6. The method for preparing the composite metal oxide adsorbent according to any one of claims 1 or 4, characterized in that, The calcination atmosphere in S4 is a protective atmosphere; Optionally, the protective atmosphere includes one of nitrogen, argon, and helium.

7. The method for preparing the composite metal oxide adsorbent according to claim 1, characterized in that, The calcination temperature in S4 is 500-800℃, the calcination time is 2-4h, and the heating rate is 5-10℃ / min.

8. A composite metal oxide adsorbent, characterized in that, It is prepared by any one of the preparation methods described in claims 1-7.

9. The composite metal oxide adsorbent according to claim 8, characterized in that, At least one of the following conditions must be met: (1) The total mass of oxides of magnesium, aluminum and iron in the adsorbent accounts for 60%-90% of the dry mass of the adsorbent; (2) The molar ratio of magnesium, aluminum and iron in the adsorbent is (1.5-4):(1-1.5):1; (3) The specific surface area of ​​the adsorbent is 50-200 m². 2 / g.

10. The application of the adsorbent according to claim 8 or 9 in the removal of anionic pollutants from water; Optionally, the anionic pollutant includes one or more of phosphate ions, arsenate ions, fluoride ions, and chromate ions. Optionally, the solid product obtained after the adsorbent adsorbs phosphate ions and / or fluoride ions can be used as a fertilizer; the fertilizer can be applied directly to the soil or mixed with other fertilizers.

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