A method for treating wastewater using a molecular sieve

Abstract

This application discloses a method for treating wastewater using molecular sieves, belonging to the fields of wastewater utilization and solid waste resource utilization. This method uses residual sludge incineration ash as raw material, and through a two-stage activation process of acid washing and alkali dissolution, converts inert silica-alumina into hydrothermally active aluminosilicates. Low-temperature pyrolysis is used to pre-carbonize the residual sludge, converting organic matter into a stable biochar framework. The activated waste residue is mixed with residual sludge biochar in a specific ratio, and a carbon-zeolite composite material is prepared through hydrothermal crystallization. This carbon-zeolite composite material treats electroplating wastewater and dye wastewater, adsorbing heavy metal ions and organic pollutants. This method achieves resource utilization and recycling of residual sludge. The resulting composite material combines the ion exchange function of zeolite and the hydrophobic adsorption function of biochar, exhibiting excellent adsorption performance for both heavy metal ions and organic pollutants, and can be widely applied in the field of wastewater treatment.

Description

Technical Field

[0001] This application relates to the technical field of wastewater treatment, specifically to a method for treating wastewater using molecular sieves. More specifically, it involves preparing a carbon-zeolite composite molecular sieve using residual sludge, which combines the ion exchange / molecular sieving function of zeolite with the hydrophobic adsorption / π-π conjugation function of biochar. This method has a large adsorption capacity for heavy metal ions and organic dyes, and can effectively remove multiple pollutants from complex wastewater. Background Technology

[0002] With the acceleration of global urbanization, the annual production of municipal sludge worldwide is expected to reach 180 million tons by 2025, with China contributing 62 million tons, accounting for 34.4% of the global total. Sludge incineration has become a mainstream technology due to its significant volume reduction effect (over 90%), but the disposal of the resulting incineration ash (fly ash and bottom ash) is becoming increasingly prominent. Against this backdrop, treating sludge as an "urban mine" and recovering valuable resources from it has become an inevitable choice under the concepts of circular economy and sustainable development.

[0003] The ash obtained from sludge incineration, gasification, or thermal treatment is rich in silicon (SiO2), aluminum (Al2O3), iron (Fe2O3), phosphorus (P2O5), and some unburned carbon. The combined content of SiO2 and Al2O3 reaches 55%–77% (mass fraction), providing a potential raw material source for the preparation of various functional materials. Among these, utilizing the silicon and aluminum components to synthesize zeolite molecular sieves is a highly promising resource utilization pathway. Existing research shows that after acid washing and alkali melting activation, sludge incineration ash can be hydrothermally crystallized to prepare various zeolite molecular sieves such as NaSOD, Na-A, Na-X, and Na-P1. For example, Chinese patent CN202410037829.2 discloses a method for preparing zeolite adsorbents from urban sludge incineration ash, employing an acid leaching-microwave hydrothermal combined process to convert the incineration ash into zeolite materials. These technologies provide feasible pathways for the resource utilization of sludge incineration ash.

[0004] Synthesizing zeolite solely from sludge ash faces several challenges: First, the silica and aluminum in sludge ash mostly exist as inert crystalline phases such as quartz and mullite, resulting in low reactivity; second, the synthesized products are mostly single-phase microporous materials, limiting their application in treating large molecular pollutants or requiring rapid mass transfer. On the other hand, the remaining sludge itself is rich in organic matter (proteins, polysaccharides, lipids, etc.), which can be converted into biochar through low-temperature pyrolysis. Biochar possesses abundant oxygen-containing functional groups and aromatic structures, exhibiting good adsorption capacity for organic pollutants, but its selective adsorption capacity for heavy metal ions is limited, and its specific surface area is typically lower than that of traditional activated carbon.

[0005] To overcome these limitations and further improve the overall performance of the material, researchers have proposed an innovative approach to construct "carbon-zeolite composite molecular sieves." This material aims to combine the regular micropores of zeolite with the well-developed mesoporous / macroporous network and rich surface chemistry of carbon materials (such as activated carbon and biochar) to achieve pore hierarchy and functional synergy. Zhu Jian et al. (WaterResearch 288 (2026) 124755) developed an integrated process that utilizes the solid-state melting effect of a high-temperature alkaline medium to convert inorganic materials in situ into highly reactive zeolite precursors and water-soluble phosphate components. Subsequently, these precursors are converted into SOD-type zeolite through a subboiling point crystallization process, ultimately obtaining a carbon-zeolite (CZ) composite material containing phosphorus and existing in the form of crystalline visverite.

[0006] However, current sludge resource utilization technologies have the following significant shortcomings: Limited product functionality; pure zeolite materials have weak adsorption capacity for organic pollutants, while pure biochar has poor selectivity for heavy metal ions. While simple physical mixing of the two can be complementary, it lacks structural synergy, resulting in limited improvement in adsorption performance. Fragmented processes; in existing technologies, the preparation of zeolite from sludge incineration ash and the preparation of biochar from sludge pyrolysis are usually two independent process routes, failing to achieve in-situ composite of the carbon and zeolite phases in the same system. Weak interfacial bonding; existing carbon-zeolite composite materials mostly employ physical mixing or a two-step method (first synthesizing zeolite then loading carbon), resulting in a lack of chemical bonding between the carbon and zeolite phases, weak interfacial bonding, and a tendency for carbon phase detachment during long-term use, affecting material stability.

[0007] Therefore, there is an urgent need to develop a method for the simultaneous high-value conversion of the inorganic-organic dual components of waste sludge. This method involves process coupling to allow zeolite crystals to grow in situ on a carbon matrix surface, forming a carbon-zeolite composite molecular sieve with CO-Si covalent bonds. The resulting composite material combines the ion exchange / molecular sieving functions of zeolite with the hydrophobic adsorption / π-π conjugation functions of biochar, exhibiting strong adhesion to heavy metal ions (Pb). 2+ Cd 2+ Organic dyes (such as methylene blue and crystal violet) have a large adsorption capacity and can effectively remove a variety of pollutants from complex wastewater.

[0008] Especially for complex wastewater containing heavy metals and organic matter, existing molecular sieves cannot adsorb heavy metals and organic matter at the same time, resulting in unsatisfactory treatment effects. Summary of the Invention

[0009] To achieve the above objectives, this invention provides a method for treating wastewater using molecular sieves. The method utilizes residual sludge to prepare a carbon-zeolite composite molecular sieve that combines the ion exchange / molecular sieving functions of zeolite with the hydrophobic adsorption / π-π conjugation functions of biochar, effectively treating heavy metal ions (Pb). 2+ Cd2+ Organic dyes (such as methylene blue and crystal violet) have large adsorption capacities and can effectively remove various pollutants from complex wastewater.

[0010] The present invention adopts the following technical solution: A method for treating wastewater using molecular sieves includes the following steps: (1) Acid washing activation: Grind the remaining sludge incineration ash through a 100-200 mesh sieve, add 0.5-2 mol / L hydrochloric acid or nitric acid solution at a solid-liquid ratio of 1:5-10, stir magnetically at room temperature for 4-8 hours, filter and wash with deionized water until neutral, dry to obtain acid-washed ash; (2) Alkali fusion activation: The pickled ash obtained in step (1) is mixed with NaOH powder at a mass ratio of 1:0.8-1.25, calcined at 500-700℃ for 1-2 hours, and then cooled to obtain alkali fused clinker; (3) Biochar preparation: The wastewater treatment plant residual sludge is pyrolyzed at 300-500℃ for 1-3 hours under an inert atmosphere at a heating rate of 5-10℃ / min. After cooling, it is washed with water until neutral, dried, and ground through a 100-200 mesh sieve to obtain sludge-based biochar. (4) Hydrothermal crystallization: The alkaline fused clinker obtained in step (2) is mixed with deionized water at a solid-liquid ratio of 1:5-10, and stirred and aged at 60-80℃ for 2-4 hours to obtain an active aluminosilicate solution. Add the biochar and aluminum source obtained in step (3) to the activated aluminosilicate solution with a SiO2 / Al2O3 molar ratio of 1.5-3.0, adjust the NaOH concentration to 2-5 mol / L, transfer to a polytetrafluoroethylene-lined hydrothermal reactor, and crystallize at 90-100℃ for 6-24h; filter the product, wash it with water until neutral, dry it at 60-80℃, grind and sieve it to obtain carbon-zeolite composite molecular sieve; wherein, the mass ratio of the biochar obtained in step (3) to the alkali-fused clinker obtained in step (2) is 1:1-3; (5) Wastewater pretreatment: Add coagulant to the wastewater and remove most of the hardness ions such as calcium and magnesium, colloidal suspended matter and large particulate suspended matter through coagulation and sedimentation. After sedimentation, take the supernatant of the wastewater. (6) Dynamic continuous column treatment: Carbon-zeolite composite molecular sieves are packed into adsorption columns, and the supernatant of wastewater is passed through the filter layer at a uniform speed from bottom to top. The ion exchange and adsorption of molecular sieves are used to continuously remove impurities from the water. (7) Molecular sieve regeneration stage: The carbon-zeolite composite molecular sieve after adsorption saturation is regenerated.

[0011] Furthermore, in step (7), a sodium chloride or calcium chloride solution with a mass fraction of 5%-10% is used to remove heavy metals, and the organic residues in the pores are decomposed by calcination at 200-300℃.

[0012] Furthermore, the residual sludge is municipal sludge from an urban wastewater treatment plant, and the wastewater is heavy metal wastewater, dye wastewater, or complex wastewater containing heavy metals and organic matter.

[0013] Furthermore, the heavy metal wastewater is electroplating wastewater containing one or more of Pb(II), Cd(II) and Cu(II); the dye wastewater is organic dye wastewater containing methylene blue, crystal violet or rhodamine-B.

[0014] Furthermore, in step (2), the calcination temperature is 600-700℃ and the alkali-to-fuss mass ratio is 1:1.

[0015] Furthermore, in step (3), the pyrolysis temperature is 350-400℃, the moisture content of the remaining sludge is 80%, and the inert atmosphere is a nitrogen atmosphere.

[0016] Further, in step (4), the mass ratio of biochar to alkali-fused clinker is 1:2, the molar ratio of SiO2 / Al2O3 is 2.0, the NaOH concentration is 3 mol / L, the crystallization temperature is 90℃, and the crystallization time is 12h; The aluminum source mentioned in step (4) is one or more of NaAlO2, Al(OH)3 and sodium aluminate solution.

[0017] Furthermore, the carbon-zeolite composite molecular sieve comprises a composite structure of zeolite crystal phase and carbon phase, wherein the zeolite crystal phase is one or more of Na-A type, Na-X type, Na-P type and SOD type, and the zeolite crystal phase is embedded in the carbon matrix and bonded by CO-Si covalent bonds.

[0018] Furthermore, the BET specific surface area of ​​the carbon-zeolite composite molecular sieve is 200-905 m². 2 / g.

[0019] Furthermore, the adsorption capacities of the carbon-zeolite composite molecular sieve for Pb(II) and Cd(II) are 400-450 mg / g and 300-333 mg / g, respectively; and the adsorption capacity for methylene blue is 100-230 mg / g.

[0020] Compared with existing technologies, this invention has the following advantages: Using incineration ash from waste sludge as a silicon-aluminum source and low-temperature pyrolysis biochar from waste sludge as a carbon source, the inorganic and organic components in the sludge are simultaneously converted into high-value-added carbon-zeolite composite molecular sieves, truly realizing the resource utilization of sludge; through a hydrothermal crystallization process, zeolite crystal phases grow in situ on the surface of biochar, forming a composite structure with CO-Si covalent bonds, resulting in strong interfacial bonding and avoiding the disadvantages of easy carbon phase detachment in physical mixing or two-step methods. The resulting composite material combines the ion exchange / molecular sieving function of zeolite with the hydrophobic adsorption / π-π conjugation function of biochar, effectively targeting heavy metal ions (Pb). 2+ Cd 2+ Both the adsorption capacity of the acid washing-alkali fusion activation section and the low temperature pyrolysis section can be processed in parallel. The hydrothermal crystallization section can be completed under normal pressure at 90-100℃ without high pressure or high temperature sintering. The overall process has low energy consumption and is easy to scale up industrially. Attached Figure Description

[0021] Figure 1 This is a scanning electron microscope (SEM) image of the carbon-zeolite composite molecular sieve obtained in Example 1 of the present invention; Figure 2 This is a process flow diagram for preparing molecular sieves according to the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. However, this invention is not limited to the following embodiments, and any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

[0023] Example 1 S1: Acid pickling and activation Ash residue from the incineration of residual sludge from a municipal wastewater treatment plant was ground and passed through a 150-mesh sieve. 50g of the ash residue was weighed and added to 400mL of 1mol / L hydrochloric acid solution at a solid-liquid ratio of 1:8. The mixture was magnetically stirred at room temperature for 6 hours. After vacuum filtration, the residue was repeatedly washed with deionized water until the filtrate was neutral. It was then dried in an oven at 105℃ to constant weight to obtain acid-washed ash residue. Analysis showed that the SiO2+Al2O3 content increased from 55.48% in the raw material to 77.21% after acid washing.

[0024] S2: Alkali fusion activation Take 20g of the pickling ash obtained from S1, mix it evenly with 20g of NaOH powder (mass ratio 1:1), place it in a ceramic crucible, put it in a muffle furnace, heat it to 700℃ at 5℃ / min, keep it at the temperature for 1.5h, and cool it naturally to room temperature to obtain alkali-fused clinker.

[0025] S3: Biochar Preparation Excess sludge (approximately 80% moisture content) from the same wastewater treatment plant was chopped into small pieces of approximately 1cm × 1cm and placed in a tubular furnace. N2 was introduced (flow rate 100 mL / min), and the temperature was increased to 350℃ at a rate of 5℃ / min, and pyrolyzed for 2 hours. After cooling to room temperature, the sludge was removed, washed with ultrapure water until neutral, filtered, dried at 80℃, and ground through a 150-mesh sieve to obtain sludge-based biochar. The biochar yield was determined to be approximately 35%, and the BET specific surface area was approximately 180 m². 2 / g.

[0026] S4: Hydrothermal crystallization Take 15g of the alkali-fused clinker obtained from S2, add 120mL of deionized water (solid-liquid ratio 1:8), and age it magnetically for 3h in a 70℃ water bath to obtain an activated aluminosilicate solution. Add 7.5g of biochar obtained from S3 (biochar to alkali-fused clinker mass ratio 1:2) to this solution, and add an appropriate amount of NaAlO2 to adjust the SiO2 / Al2O3 molar ratio to 2.0. Then adjust the NaOH concentration of the system to 3 mol / L with NaOH particles. Transfer the mixed slurry to a 200mL polytetrafluoroethylene-lined hydrothermal reactor, seal it, and crystallize it in a 90℃ oven for 12h. After the reaction, allow it to cool naturally to room temperature, vacuum filter the product, wash it with deionized water until the filtrate is neutral, dry it at 80℃, and grind it through a 20-mesh sieve to obtain a carbon-zeolite composite molecular sieve, denoted as TF-1. The results are shown in the scanning electron microscope. Figure 1 The preparation process of molecular sieves is shown in [link to process flow chart]. Figure 2 The molecular sieve has a carbon content of 18.2% and a BET specific surface area of ​​486 m². 2 / g, total pore volume 0.32 cm³ 3 / g.

[0027] Adsorption experiment of Pb(II) on carbon-zeolite composite molecular sieve TF-1: initial concentration 200 mg / L, pH=5.5, adsorbent dosage 0.5 g / L, shaking at 30℃ for 24 h, the equilibrium adsorption capacity was measured to be 412.3 mg / g.

[0028] Adsorption experiment of methylene blue (MB) on carbon-zeolite composite molecular sieve TF-1: initial concentration 200 mg / L, pH=7.0, adsorbent dosage 0.5 g / L, shaking at 30℃ for 24 h, the equilibrium adsorption capacity was measured to be 186.5 mg / g.

[0029] The carbon-zeolite composite molecular sieve described above is used to treat landfill leachate wastewater containing various heavy metal ions and organic matter. The specific process steps are as follows: The preprocessing stage includes the following steps: Large particle impurity interception: Wastewater is first treated by passing it through a screen to remove floating objects and large suspended particles, thus preventing subsequent clogging of the molecular sieve filter layer.

[0030] Homogenization, temperature and pH adjustment: The water enters the equalization tank to stabilize the water quality and quantity, strictly control the pH in the range of 6-8, and adjust the water temperature to room temperature (20-35℃) to avoid high temperature affecting the stability of the molecular sieve structure.

[0031] Softening and deep impurity removal: Adding coagulants such as polyaluminum chloride (PAC) removes most of the hardness ions such as calcium and magnesium and colloidal suspended matter through coagulation and sedimentation, preventing these impurities from occupying the exchange sites of the molecular sieve and improving the processing capacity of the molecular sieve.

[0032] Dynamic continuous column processing includes the following steps: Carbon-zeolite composite molecular sieves are packed into adsorption columns, and wastewater passes through the filter layer at a uniform speed from bottom to top. By utilizing the ion exchange and adsorption properties of the molecular sieves, metal ions and organic matter in the water are continuously removed, with a removal rate of ≥80% for multiple pollutants.

[0033] The molecular sieve regeneration stage includes the following steps: Once the molecular sieve is saturated with adsorption, it must be regenerated to restore its processing capacity in order to avoid packing failure and soaring operating costs.

[0034] Using 8% sodium chloride as a regenerator, the heavy metals adsorbed in the molecular sieve pores are replaced by a low-speed countercurrent flow through the adsorption column, and the regeneration time is controlled within 2-4 hours.

[0035] The recalcitrant organic matter on the surface of the molecular sieve is completely decomposed by low-temperature (200-300℃) calcination, thus fully restoring the adsorption performance of the molecular sieve.

[0036] Example 2 The difference from Example 1 is that the pyrolysis temperature in step S3 was changed to 400℃, while the other conditions remained the same. The resulting sample was designated TF-1, with a carbon content of 15.6%. A higher pyrolysis temperature is beneficial for zeolite crystallization, but at the same time, the carbon phase content decreases slightly. The carbon-zeolite composite molecular sieve TF-2 has a BET specific surface area of ​​512 m². 2 / g, the adsorption capacity for Pb(II) is 398.7 mg / g, and the adsorption capacity for MB is 172.4 mg / g.

[0037] Example 3 The difference from Example 1 is that the calcination temperature in step S2 was changed to 600℃, the mass ratio of biochar to alkali-fused clinker in step S4 was changed to 1:1, the SiO2 / Al2O3 molar ratio was changed to 1.5, the crystallization temperature was changed to 100℃, and the crystallization time was changed to 8 hours. All other conditions remained the same. The resulting sample was designated as carbon-zeolite composite molecular sieve TF-3. The BET specific surface area of ​​carbon-zeolite composite molecular sieve TF-3 was 387 m². 2 / g. The adsorption capacity for Pb(II) was 356.8 mg / g, and the adsorption capacity for MB was 218.3 mg / g.

[0038] The results showed that the carbon-zeolite composite molecular sieve exhibited excellent removal capabilities for both organic pollutants and heavy metals in wastewater, fully demonstrating the advantages of the synergistic effect of carbon-zeolite.

Claims

1. A method for treating wastewater using molecular sieves, characterized in that, Includes the following steps: (1) Acid washing activation: Grind the remaining sludge incineration ash through a 100-200 mesh sieve, add 0.5-2 mol / L hydrochloric acid or nitric acid solution at a solid-liquid ratio of 1:5-10, stir magnetically at room temperature for 4-8 hours, filter and wash with deionized water until neutral, dry to obtain acid-washed ash; (2) Alkali fusion activation: The pickled ash obtained in step (1) is mixed with NaOH powder at a mass ratio of 1:0.8-1.25, calcined at 500-700℃ for 1-2 hours, and then cooled to obtain alkali fused clinker; (3) Biochar preparation: The wastewater treatment plant residual sludge is pyrolyzed at 300-500℃ for 1-3 hours under an inert atmosphere at a heating rate of 5-10℃ / min. After cooling, it is washed with water until neutral, dried, and ground through a 100-200 mesh sieve to obtain sludge-based biochar. (4) Hydrothermal crystallization: The alkaline fused clinker obtained in step (2) is mixed with deionized water at a solid-liquid ratio of 1:5-10, and stirred and aged at 60-80℃ for 2-4 hours to obtain an active aluminosilicate solution. Add the biochar and aluminum source obtained in step (3) to the active aluminosilicate solution with a SiO2 / Al2O3 molar ratio of 1.5-3.0, adjust the NaOH concentration to 2-5 mol / L, transfer it into a polytetrafluoroethylene-lined hydrothermal reactor, and crystallize it at 90-100℃ for 6-24h. The product is filtered, washed with water until neutral, dried at 60-80℃, ground and sieved to obtain carbon-zeolite composite molecular sieve; wherein, the mass ratio of biochar obtained in step (3) to alkali-fused clinker obtained in step (2) is 1:1-3; (5) Wastewater pretreatment: Add coagulant to the wastewater and remove most of the calcium and magnesium hardness ions, colloidal suspended matter and large particulate suspended matter through coagulation and sedimentation. After sedimentation, take the supernatant of the wastewater. (6) Dynamic continuous column treatment: Carbon-zeolite composite molecular sieves are packed into adsorption columns, and the supernatant of wastewater is passed through the filter layer at a uniform speed from bottom to top. The ion exchange and adsorption of molecular sieves are used to continuously remove impurities from the water. (7) Molecular sieve regeneration stage: The carbon-zeolite composite molecular sieve after adsorption saturation is regenerated.

2. The method for treating wastewater according to claim 1, characterized in that: In step (7), a sodium chloride or calcium chloride solution with a mass fraction of 5%-10% is used to remove heavy metals, and the organic residues in the pores are decomposed by calcination at 200-300℃.

3. The method for treating wastewater according to claim 1, characterized in that: The residual sludge is municipal sludge from urban sewage treatment plants, and the wastewater is heavy metal wastewater, dye wastewater, or complex wastewater containing heavy metals and organic matter.

4. The method for treating wastewater according to claim 3, characterized in that: The heavy metal wastewater is electroplating wastewater containing one or more of Pb(II), Cd(II) and Cu(II); the dye wastewater is organic dye wastewater containing methylene blue, crystal violet or rhodamine-B.

5. The method for treating wastewater according to claim 1, characterized in that: In step (2), the calcination temperature is 600-700℃ and the alkali-to-fuss mass ratio is 1:

1.

6. The method for treating wastewater according to claim 1, characterized in that: In step (3), the pyrolysis temperature is 350-400℃, the moisture content of the remaining sludge is 80%, and the inert atmosphere is a nitrogen atmosphere.

7. The method for treating wastewater according to claim 1, characterized in that: In step (4), the mass ratio of biochar to alkali-fused clinker is 1:2, the molar ratio of SiO2 / Al2O3 is 2.0, the NaOH concentration is 3 mol / L, the crystallization temperature is 90℃, and the crystallization time is 12h. The aluminum source mentioned in step (4) is one or more of NaAlO2, Al(OH)3 and sodium aluminate solution.

8. The method for treating wastewater according to claim 1, characterized in that: The carbon-zeolite composite molecular sieve comprises a composite structure of zeolite crystal phase and carbon phase, wherein the zeolite crystal phase is one or more of Na-A type, Na-X type, Na-P type and SOD type, and the zeolite crystal phase is embedded in the carbon matrix and bonded by CO-Si covalent bonds.

9. The method for treating wastewater according to claim 1, characterized in that: The BET specific surface area of ​​the carbon-zeolite composite molecular sieve is 200-905 m². 2 / g.

10. The method for treating wastewater according to claim 1, characterized in that: The carbon-zeolite composite molecular sieve has an adsorption capacity of 400-450 mg / g for Pb(II) and 300-333 mg / g for Cd(II), and an adsorption capacity of 100-230 mg / g for methylene blue.

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