Method for hydrothermal conversion of municipal sludge into humic acid of mineral origin

Abstract

The present application relates to a kind of methods for hydrothermal conversion municipal sludge into mineral source humic acid, belong to sludge resource technology field.The method includes: sludge raw material is mixed with alkali agent uniformly, and mixed sludge precursor liquid is obtained;The alkali content of the mixed sludge precursor liquid is 0.1-2.0mol / L in terms of hydroxyl ion concentration;Mixed sludge precursor liquid is hydrothermally reacted at 120-210 ℃ for 0.5-6h, and mineral source humic acid with loose porous structure is obtained.This method simulates the genesis of natural humic acid underground, and the organic components in sludge are converted into high-quality mineral source humic acid by hydrothermal reaction, and the mineral source humic acid has more obvious quality advantage than biochemical humic acid produced by traditional compost in soil remediation, plant growth and nitrogen utilization.

Description

Technical Field

[0001] This invention relates to a method for hydrothermal conversion of municipal sewage sludge into mineral-based humic acid, belonging to the field of sludge resource utilization technology. Background Technology

[0002] Sludge is a major byproduct of domestic sewage treatment, produced in large quantities and continuously, making it a typical type of bulk solid waste. Sludge has a high water content and is rich in organic matter, with an organic matter content exceeding 50%, mainly including proteins, polysaccharides, and lipids. It is also rich in plant nutrients such as nitrogen, phosphorus, potassium, and carbon, making it a recyclable organic resource. Currently, the commonly used anaerobic digestion technology both domestically and internationally can degrade the organic matter in sludge into biomass energy such as biogas (methane) for reuse.

[0003] Land application of sludge is currently recognized as the most promising and feasible method for large-scale sludge treatment and disposal. In Europe and America, nearly 50% of sludge is used to improve soil by utilizing the residue from anaerobic treatment. However, sludge often contains heavy metals, pathogens, insect eggs, and various organic pollutants. Direct land application can easily lead to soil pollution, acidification, or eutrophication.

[0004] Humic acid is an aromatic carboxylic acid with a complex structure, widely found in natural materials such as soil, lignite, weathered coal, and peat. As is well known, humic matter is the most important component in soil organic matter for maintaining soil fertility. The degree of soil humification is usually evaluated by humic acid content; the higher the humic acid content, the more fertile the soil. Humic acid can be divided into mineral-derived humic acid and biochemical humic acid, etc. The two have different formation mechanisms. Mineral-derived humic acid is formed during geological sedimentation through underground mineralization and microbial decomposition of organic matter, and is commonly found in natural materials such as weathered coal, lignite, and peat. Biochemical humic acid is formed from organic matter through aerobic fermentation by microorganisms (composting), and is commonly found in biomass such as plant residues and animal manure after composting. However, composting requires a large area, easily produces odors, and pollutes the surrounding environment. Compared to biochemical humic acid, mineral humic acid typically contains more aromatic structures and active groups, which makes it more advantageous in soil remediation, saline soil improvement, plant growth promotion, pesticide use, and nitrogen utilization.

[0005] Chinese patent CN104478190B proposes a method for recovering humic acid from sludge, which involves adding chemical flocculants and undergoing multiple sedimentation processes to extract and recover the humic acid originally contained in the sludge. Chinese patent CN104231280B also proposes equipment and methods for extracting humic acid from domestic sewage sludge, using alkaline treatment and fermentation centrifugation. However, these methods only extract a small amount of humic acid naturally present in the sludge, without generating new, high-quality humic acid. Furthermore, the humic acid produced by these methods is similar to the low-quality biochemical humic acid produced by sludge composting or aerobic fermentation, failing to achieve efficient and high-quality reuse of sludge organic matter. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a method for hydrothermal conversion of municipal sewage sludge into mineral-like humic acid. This method simulates the formation of natural underground humic acid and converts the organic components in the sludge into high-quality mineral-like humic acid through a hydrothermal reaction. This mineral-like humic acid has significant quality advantages over biochemical humic acid produced by traditional composting in terms of soil remediation, plant growth, and nitrogen utilization.

[0007] In a first aspect, the present invention provides a method for hydrothermal conversion of municipal sewage sludge into mineral-based humic acid. The method includes: uniformly mixing sludge raw material with an alkali agent to obtain a mixed sludge precursor solution; the alkali content of the mixed sludge precursor solution, expressed as hydroxide ion concentration, is 0.1–2.0 mol / L; and subjecting the mixed sludge precursor solution to a hydrothermal reaction at 120–210°C for 0.5–6 h to obtain mineral-based humic acid with a loose, porous structure.

[0008] Preferably, the alkalinity of the mixed sludge precursor liquid is 0.1 to 1.0 mol / L, calculated as hydroxide ion concentration.

[0009] Preferably, the alkaline agent is one or a mixture of solid sodium hydroxide, potassium hydroxide, and aluminum hydroxide; or, the alkaline agent is a solution composed of one or a mixture of solid sodium hydroxide, potassium hydroxide, and aluminum hydroxide and water.

[0010] Preferably, the reaction temperature is 120–160°C; and the reaction time is 0.5–3 h.

[0011] Preferably, the carbon element in the mineral-derived humic acid is mainly aromatic carbon; more preferably, the carbon element in the mineral-derived humic acid includes 30-40% aromatic carbon and 25-35% aliphatic carbon.

[0012] Preferably, the mineral-derived humic acid has internal pores with a mesopore diameter mainly distributed in the range of 10–50 nm.

[0013] Preferably, the specific surface area of ​​the mineral-derived humic acid is larger than that of the sludge raw material; more preferably, the specific surface area of ​​the mineral-derived humic acid is 50-60 m². 2 / g.

[0014] Preferably, the method further includes the step of extracting mineral-like humic acid from the reaction solution after the hydrothermal reaction is completed; preferably, the pH of the hydrothermal reaction solution is adjusted to 12-13, mixed evenly and allowed to stand for a period of time, and the supernatant is taken; the pH of the supernatant is further adjusted to 1-2, and allowed to stand for a period of time to allow the solid product to precipitate; the solid product is washed and dried to obtain the mineral-like humic acid.

[0015] Secondly, the present invention provides the application of mineral-derived humic acid prepared by the method described in any of the above-mentioned methods in the adsorption of heavy metal pollutants.

[0016] Preferably, the application involves the adsorption of heavy metal copper ions (Cu) by mineral-derived humic acid in an environment with pH > 6. 2+ Application; or, the application is the adsorption of heavy metal chromium ions (Cr2O7) by mineral-derived humic acid in an environment with pH < 6. 2- Application of ). Attached Figure Description

[0017] Figure 1 It represents the yield of humic acid synthesized from sludge under different alkaline conditions via hydrothermal synthesis.

[0018] Figure 2 These are Fourier transform infrared absorption spectra of humic acid synthesized from sludge under different alkaline conditions via hydrothermal synthesis.

[0019] Figure 3 It represents the yield of humic acid synthesized from sludge hydrothermally at different reaction temperatures.

[0020] Figure 4 These are Fourier transform infrared absorption spectra of humic acid synthesized from sludge at different reaction temperatures using hydrothermal methods.

[0021] Figure 5 This represents the yield of humic acid synthesized from sludge by hydrothermal reaction at different reaction times.

[0022] Figure 6 These are Fourier transform infrared absorption spectra of humic acid synthesized from sludge at different reaction times using hydrothermal methods.

[0023] Figure 7 This is the microstructure of humic acid synthesized from sludge via hydrothermal synthesis under a scanning electron microscope.

[0024] Figure 8 It is the BJH pore size distribution and BET specific surface area of ​​humic acid synthesized from sludge hydrothermally.

[0025] Figure 9It is the X-ray photoelectron spectroscopy of humic acid synthesized from sludge hydrothermally.

[0026] Figure 10 The hydrothermal synthesis of humic acid from sludge affects Cu in different pH solution environments. 2+ Removal rate.

[0027] Figure 11 The hydrothermal synthesis of humic acid from sludge affects Cr2O7 under different pH solution conditions. 2- Removal rate. Detailed Implementation

[0028] The present invention is further illustrated by the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention. Unless otherwise specified, all percentages refer to mass percentages. The following exemplifies the method of hydrothermal conversion of municipal sewage sludge into mineral-based humic acid according to the present invention.

[0029] The sludge raw material is mixed evenly with the alkali agent to obtain a mixed sludge precursor liquid.

[0030] The sludge raw material of this invention is a fluid slurry with a high water content (above 50 wt%). That is, the sludge raw material is in the form of a slurry. The sludge raw material of this invention can be used directly without dewatering or drying treatment. In some technical solutions, the water content of the sludge raw material can be 50-90 wt%.

[0031] It should be understood that any type of sludge raw material can be used in this invention. The sludge raw material can be a combination of inorganic minerals and organic matter (organic matter). During experiments, it was found that the sludge raw material had high particle density, and plant fiber residues were present between the particles, indicating that the inorganic minerals and organic matter in the sludge raw material coexisted. In some technical solutions, the organic matter of the sludge raw material accounts for 50% or more of the total composition of the sludge raw material. The proportion of organic matter in the sludge does not affect the yield and quality of humic acid.

[0032] Preferred source material is municipal sludge. Compared to industrial sludge and other types of sludge, municipal sludge has a higher organic content (generally ≥50wt%), allowing for the full utilization of its organic components in hydrothermal reactions to convert and synthesize humic acid. Furthermore, due to its different source characteristics, municipal sludge contains lower levels of heavy metals and organic pollutants compared to industrial sludge. Therefore, municipal sludge ensures that the humic acid synthesized through hydrothermal reactions has stronger environmental safety and avoids the migration and diffusion of pollutants. Alternatively, municipal sludge from a slightly acidic to neutral environment can be used as the sludge source.

[0033] Alkali agents can be solid, including but not limited to solid sodium hydroxide, potassium hydroxide, and aluminum hydroxide. They can also be alkaline solutions, including but not limited to solutions composed of one or more of solid sodium hydroxide, potassium hydroxide, and aluminum hydroxide mixed with water. The function of alkali agents is to accelerate the decomposition of organic matter in sludge, allowing its organic components to participate more fully in the hydrothermal reaction, thereby synthesizing humic acid.

[0034] The alkalinity of the mixed sludge precursor liquor, expressed as hydroxide ion concentration, ranges from 0.1 to 2.0 mol / L. An appropriate alkalinity concentration accelerates the decomposition of sludge organic matter, enhances the reaction process, and allows the organic components to participate more fully in the hydrothermal reaction, ultimately synthesizing humic acid. More specifically, an appropriate alkalinity concentration effectively breaks down the macroscopic particles and flocculent structure of the sludge, releasing internal macromolecules such as sugars, lipids, and proteins, and further decomposing large organic molecules into smaller ones, thus providing sufficient precursors for the hydrothermal synthesis of new large organic molecules (humic acid). If the alkalinity is too low, it may not be sufficient to release all the organic matter from the sludge, or even to decompose the large organic molecules into smaller organic groups (essential for humic acid synthesis), requiring the addition of organic conditioners to provide more organic groups.

[0035] When the alkali agent is in the form of an alkaline solution, an alkaline solution with a hydroxide ion concentration of 0–2.0 mol / L can be used. The solid-liquid ratio of the sludge raw material to the alkaline solution can be 1 g: 1–9 mL. By controlling the solid-liquid ratio of the sludge raw material to the alkaline solution, it is possible to ensure that the mixed sludge precursor liquid is in a fluid state (solid-liquid mixed state) during the hydrothermal reaction, achieving uniform and sufficient contact and reaction in a homogeneous reactor. As an example, the solid-liquid ratio of the sludge raw material to the alkaline solution is 1 g: 9 mL. The addition of the alkaline solution has virtually no effect on the change in the moisture content of the sludge raw material. As an example, the moisture content of the mixed sludge precursor liquid is 50–90%.

[0036] As an optional technical solution, sodium hydroxide is used as an alkaline agent to control the alkalinity of the mixed sludge precursor liquid. For example, the alkaline agent is an aqueous solution of sodium hydroxide with a concentration of 0.1–2.0 mol / L. More preferably, the alkaline agent is an aqueous solution of sodium hydroxide with a concentration of 0.5 mol / L.

[0037] The pH value of the mixed sludge precursor liquor is 11-13. The higher the alkalinity of the mixed sludge precursor liquor, the easier it is for the macromolecular organic matter such as sugars, lipids, and proteins in the sludge raw material to be decomposed by hydrothermal processes, and the higher the degree of aromatization of humic acid products. However, if the alkalinity of the mixed sludge precursor liquor is excessive, the aromatization of humic acid products will be weakened.

[0038] From the perspective of the reaction system, the hydrothermal system of this invention does not require the addition of additional conditioning agents; sludge is the sole primary reaction material. The sludge material of this invention is in a fluid slurry state, meaning it can be used for reaction without deep dewatering or drying, offering significant advantages: a. It avoids preliminary processes such as sludge dewatering, drying, grinding, and screening, simplifying the sludge pretreatment process and reducing overall energy consumption and treatment costs. b. It maintains the initial solid-liquid mixture state of the sludge slurry to the greatest extent possible, ensuring a homogeneous reaction state in the material system. For example, a reaction material system prepared by completely drying the sludge material and then adding water for stirring disrupts the initial homogeneous solid-liquid state of the sludge. Although water is subsequently added for stirring, the solid-liquid contact is inevitably less than in the initial state, further leading to insufficient / incomplete reaction. This invention directly uses the initial sludge slurry, maintaining its original homogeneous solid-liquid state, ensuring sufficient contact and reaction between the solid and liquid phases in the hydrothermal reaction. c. During the hydrothermal reaction, the existing moisture in the fluid sludge can serve as the medium for the hydrothermal reaction, eliminating the need for additional water. Therefore, the material handling process of the reaction system of this invention is simpler, has lower overall costs, and is more environmentally friendly.

[0039] The mixed sludge precursor liquid is subjected to hydrothermal reaction at 120–210℃ for 0.5–6 hours. During this hydrothermal reaction, the organic matter of the sludge raw material is fully decomposed into small molecules, and the small molecules are directionally recombined under hydrothermal conditions.

[0040] In some technical solutions, the reaction temperature is 130–170°C. Within this temperature range, the aromatization of the humic acid product is more complete. When the temperature exceeds this range, the aromatic ring structure of the humic acid product may decompose at high temperatures, thus affecting the aromatization degree of the humic acid product. Preferably, the reaction temperature is 150°C.

[0041] In some technical solutions, the reaction time is 1 to 6 hours. When the reaction time exceeds this range, the excessively long reaction time will cause the humic acid reaction products to denature and decompose, especially the aromatic ring skeleton of the humic acid products will be decomposed and cannot exist stably, ultimately leading to a weakening of the aromatization degree of the humic acid products. Preferably, the reaction time is 1 hour.

[0042] The method further includes the step of extracting mineral-like humic acid from the reaction solution after hydrothermal reaction. Preferably, the pH of the hydrothermal reaction solution is adjusted to 12-13 (optimal 13.0), mixed thoroughly, and allowed to stand for a period of time before the supernatant is collected. The purpose of the first pH adjustment is to provide a higher alkaline environment to ensure that the humic acid is fully dissolved in the liquid phase. This step can be repeated multiple times. That is, alkali solution can be added to the solid to adjust the pH to 12-13 (optimal 13.0), mixed thoroughly, and allowed to stand for a period of time before the supernatant is collected; the supernatants obtained multiple times are mixed for later use. Then the pH of the supernatant is adjusted to 1-2 (optimal 1.0), and allowed to stand for a period of time to allow the solid product to precipitate. The purpose of the second pH adjustment is to provide a higher acidic environment to ensure that the humic acid is fully precipitated from the liquid phase and precipitates as a flocculent solid phase. The solid product is washed and dried to obtain the mineral-like humic acid.

[0043] This invention constructs suitable hydrothermal reaction conditions, which enable the organic matter of sludge raw materials to be rapidly dissolved and reconstituted into mineral-derived humic acid, and endow the humic acid with a large specific surface area and abundant mesoporous structure, thereby providing sufficient adsorption sites for adsorbing pollutants such as heavy metals.

[0044] In some technical solutions, the specific surface area of ​​the mineral-derived humic acid is 50-60 (e.g., 54.955) m². 2 / g. The specific surface area of ​​the mineral-derived humic acid is greater than that of the sludge raw material.

[0045] Microscopic morphology images reveal that the mineral-derived humic acid obtained by the method of this invention possesses a loose, porous structure. Its surface is covered with interconnected micron-sized channels, and its interior contains fine pores. These channels and / or pores provide ample adsorption sites. In some technical solutions, the mineral-derived humic acid has internal pores with mesopore diameters primarily distributed between 10 and 60 nm.

[0046] Further characterization and analysis revealed that the humic acid synthesized in this invention exhibits a higher degree of aromatization, acidification, and condensation, meaning it displays a more significant degree of humification overall. Its properties are highly similar to those of natural mineral-derived humic acid, resulting in higher humic acid quality. For example, this mineral-derived humic acid possesses a significant aromatic ring structure and active oxygen-containing groups such as carboxyl, carbonyl, and phenolic hydroxyl groups.

[0047] In some technical solutions, the carbon element of the mineral-derived humic acid exists mainly in the form of aromatic carbon; preferably, the carbon element of the mineral-derived humic acid exists in the form of 30-40% aromatic carbon and 25-35% aliphatic carbon.

[0048] It should also be noted that this invention not only reduces the hydrothermal reaction temperature and shortens the hydrothermal reaction time (hydrothermal reaction temperature 120–210℃, preferably 120–180℃, hydrothermal reaction time 0.5–6 h, preferably 0.5–5 h), but also achieves a humic acid yield of 30%–50% after the reaction, significantly improving the humic acid yield. In other words, this invention achieves a higher humic acid yield through a hydrothermal reaction at a shorter reaction time and lower hydrothermal temperature. The formula for calculating the humic acid yield is as follows:

[0049]

[0050] In the following embodiments, the specific steps of the method for synthesizing mineral-derived humic acid from municipal sludge using hydrothermal conversion according to the present invention are as follows:

[0051] 1. Hydrothermal synthesis of mineral-derived humic acid: Weigh the sludge raw material, add sodium hydroxide, and stir to mix evenly to prepare a mixed sludge precursor solution (the alkali content of the mixed sludge precursor solution is 0.1-2.0 mol / L based on the sodium hydroxide concentration). Then, place the mixed sludge precursor solution into a 50 mL polytetrafluoroethylene liner, place the liner into a high-pressure reactor, and then place it in a homogeneous reactor. Control the rotation speed at 20 r / min and perform a hydrothermal reaction at 120-210℃ for 0.5-6 h.

[0052] 2. Extraction of mineral-like humic acid: After the hydrothermal reaction is completed, wait for the reactor to cool to room temperature, transfer the reaction product to a plastic beaker, add 2 mol / L sodium hydroxide aqueous solution to adjust the pH to 13, seal and shake on a vibrating plate for 4 hours, then let stand for 8 hours, centrifuge, and retain the supernatant A at 4℃. Add 30 mL of 0.1 mol / L sodium hydroxide aqueous solution to the solid, seal and shake on a vibrating plate for 4 hours, then let stand for 20 hours and centrifuge to collect the supernatant B. Mix supernatant A and supernatant B and pour into a beaker, add 6 mol / L HCl solution to adjust the pH to 1, seal and let stand for 12 hours to allow the precipitate to completely precipitate, then centrifuge in a centrifuge tube, pour off the supernatant, add deionized water to the precipitate and wash repeatedly by centrifugation, and finally freeze-dry the obtained solid product for 24 hours to obtain mineral-like humic acid.

[0053] This invention explores multiple factors to ensure that humic acid products have both high yield and good quality. These factors include: (1) a homogeneous sludge reaction system: directly reacting the homogeneous sludge system to ensure sufficient contact between the solid and liquid phases and a thorough reaction. (2) providing a sufficient and suitable alkaline reaction environment, ensuring that the original organic macromolecules of the sludge can be completely decomposed during the hydrothermal reaction, and ensuring that the alkali agent is not excessive, thus having an adverse effect on the reaction process and the characteristics of the humic acid products. (3) providing a suitable reaction temperature and reaction time. Too low a reaction temperature or too short a reaction time may lead to incomplete / incomplete reaction; too high a reaction temperature or too long a reaction time may lead to unstable (decomposition) humic acid products, manifested as reduced yield and poor quality (reduction of characteristic functional groups). Conversely, a suitable reaction temperature and reaction time promote high humic acid yield and good quality.

[0054] In summary, this invention provides a method for synthesizing mineral-derived humic acid from sewage sludge through hydrothermal conversion by simulating the formation of underground mineral-derived humic acid. This method uses municipal sewage sludge as the main raw material, rapidly dissolving and reconstructing the organic components of the sludge into mineral-derived humic acid under low-temperature hydrothermal conditions. The humic acid yield can reach 30%–50% (e.g., 46.7%), fully and efficiently converting and reusing the organic matter of the sludge, and greatly improving the degree of humification and quality of the sludge. This humic acid can be applied as a soil conditioner or fertilizer, and its loose, porous structure can effectively adsorb pollutants such as heavy metals, simultaneously purifying the natural environment and realizing the high-value utilization of sewage sludge organic matter.

[0055] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the appropriate range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below.

[0056] Example 1: Characterization of sludge raw material properties

[0057] The sludge came from a large municipal wastewater treatment plant in Shanghai and was the residual sludge after activated sludge treatment, ensuring the representativeness and universality of the sludge raw material. The sludge raw material underwent pretreatment processes including natural air drying, vacuum freeze drying, crushing, grinding, and sieving. Then, the sludge raw material was tested and characterized to analyze its chemical composition and basic physicochemical properties.

[0058] The initial moisture content of the sludge raw material was 79.1%, which was reduced to an absolutely dry state (moisture content <0.5%) after drying. The sludge loss on ignition was 54.1%, and thermogravimetric analysis (TG-DTA) showed that the sludge reached equilibrium after thermal weight loss at 550℃, with a weight loss rate of approximately 50%, indicating that organic matter accounted for approximately 50% of the total composition of the sludge. The initial pH of the sludge was 6.37, indicating that the sludge raw material was in a slightly acidic-neutral environment. Nitrogen adsorption results showed that the BET specific surface area of ​​the sludge raw material was 0.297 m². 2 The sludge contains a large number of particles with a size of 100 micrometers (<100 μm) and intact surface morphology with high density. Rod-shaped plant fiber residues are present between the particles, confirming the coexistence of inorganic minerals and organic matter. ICP-OES results show that the main heavy metals (mg / kg) in the sludge are As (~13.88), Cr (~19.16), Cd (~0.037), Cu (~73.28), Pd (~17.76), and Zn (~275.42). The content of all heavy metals meets the Class A standard for agricultural use of sludge, thus ensuring the environmental safety of sludge resource utilization.

[0059] Example 2: Hydrothermal synthesis of humic acid from sludge under different alkaline environments

[0060] This embodiment investigated the effect of different alkali concentrations (the alkali content of the mixed sludge precursor liquid was 0.1–2.0 mol / L, calculated as sodium hydroxide concentration) on the hydrothermal synthesis of humic acid from sludge, while controlling other conditions: the reaction temperature was 150°C and the reaction time was 1 hour.

[0061] Figure 1 The yield of humic acid synthesized from sludge under different alkaline reaction environments via hydrothermal synthesis is given. The formula for calculating the humic acid yield (%) is as follows:

[0062]

[0063] The results showed that as the alkalinity of the reaction system increased, the yield of humic acid synthesized from sludge by hydrothermal reaction increased significantly (14.8%→46.7%), and then gradually decreased (46.7%→18.9%), with the highest yield of 46.7% obtained under alkaline conditions of 0.5 mol / L NaOH.

[0064] Figure 2 FTIR (Fourier Transform Infrared) spectra of humic acid products synthesized from sludge under different alkaline reaction environments were obtained, characterizing the changes in various characteristic functional groups in the humic acid. The humic acid products were observed at 2921 cm⁻¹. -1 and 2852cm -1The absorption peaks at 1513 cm⁻¹ represent the stretching vibrations of the methyl (-CH₃) and methylene (-CH₂) groups in the aliphatic chain structure, respectively. As the alkalinity of the reaction environment increases (0.1→2.0 mol / L NaOH), the intensity of the corresponding absorption peaks significantly weakens, indicating that more saturated aliphatic chain structures in the product are hydrothermally decomposed, meaning the degree of aliphaticization is decreasing. Meanwhile, the absorption peak intensity at 1513 cm⁻¹... -1 The absorption peak at 1644 cm⁻¹ also weakens, indicating that more amide-NH is hydrothermally decomposed. Furthermore, as the alkalinity of the reaction environment increases, the absorption peak at 1644 cm⁻¹ also decreases. -1 The C=C skeletal vibration of the aromatic ring first increases and then decreases, indicating that the degree of aromatization (i.e., unsaturation) of the humic acid product first increases and then decreases, with the highest degree of aromatization of humic acid under 0.5 mol / L NaOH conditions. Furthermore, the humic acid exhibits obvious characteristic absorption peaks corresponding to carboxyl, phenolic hydroxyl, and alcoholic hydroxyl groups, further indicating that the humic acid product synthesized under suitable alkaline conditions (0.5 mol / L NaOH) has a higher degree of humification.

[0065] The above results indicate that an alkaline environment of appropriate concentration is conducive to the full decomposition of sludge organic matter (macromolecules such as proteins, cellulose, polysaccharides, and fats) into smaller molecules, and further promotes the directional recombination of these smaller molecules into macromolecules of humic acid under hydrothermal conditions. Therefore, considering both the yield and the degree of humification of humic acid synthesized hydrothermally, 0.5 mol / L NaOH was selected as the optimal alkaline environment for the hydrothermal reaction.

[0066] Example 3: Hydrothermal synthesis of humic acid from sludge at different reaction temperatures

[0067] This embodiment investigated the effect of different reaction temperatures (120℃~210℃) on the hydrothermal synthesis of humic acid from sludge, while controlling other conditions: the alkaline environment of the reaction environment was 0.5mol / L NaOH, and the reaction time was 1 hour.

[0068] Figure 3 The yield of humic acid synthesized from sludge by hydrothermal reaction at different reaction temperatures is shown. As the hydrothermal reaction temperature increases, the yield of humic acid synthesized from sludge by hydrothermal reaction increases significantly (39.5%→46.7%), and then gradually decreases (46.7%→31.8%), with the highest yield of 46.7% obtained at 150℃.

[0069] Figure 4 The FTIR (Fourier Transform Infrared) spectra of the humic acid products synthesized from sludge at different reaction temperatures were used to characterize the changes in various characteristic functional groups in the humic acid. As the reaction temperature increased (130℃→210℃), the aliphatic chain structure (2921 cm⁻¹) changed. -1 and 2852cm -1 ) and amide (1513cm) -1The intensity of the corresponding absorption peak did not change significantly, indicating that the saturated aliphatic chain structure had been fully hydrothermally decomposed under these conditions. When the hydrothermal reaction temperature was in the range of 130℃ to 170℃, the 1644 cm⁻¹ value corresponding to the C=C skeleton of the aromatic ring was [missing information]. -1 The high and relatively constant absorption peak intensity indicates a relatively high degree of aromatization; however, when the temperature continues to rise from 170℃ to 210℃, the peak intensity reaches 1644 cm⁻¹. -1 The absorption peak at this point showed a significant and continuous decrease, indicating that the aromatic ring C=C skeleton was decomposed at high temperature, and the degree of aromatization was reduced. Therefore, considering both the yield of hydrothermal synthesis of humic acid and the degree of humification, 150℃ was selected as the optimal hydrothermal reaction temperature.

[0070] Example 4: Hydrothermal synthesis of humic acid from sludge at different reaction times

[0071] This embodiment investigated the effect of different reaction times (0h to 6h) on the hydrothermal synthesis of humic acid from sludge, while controlling other conditions: the alkaline environment of the reaction environment was 0.5 mol / L NaOH, and the reaction temperature was 150℃.

[0072] Figure 5 The yield of humic acid synthesized from sludge by hydrothermal reaction at different reaction times is shown. A reaction time of 0 h represents the initial humic acid content of the sludge, which is only 14.8%. As the hydrothermal reaction time increases, the yield of humic acid synthesized from sludge by hydrothermal reaction increases significantly (14.8% → 46.7%), with the highest yield of 46.7% obtained at a reaction time of 1 h. As the reaction time is further extended to 6 h, the yield gradually decreases (46.7% → 34.9%), indicating that excessively long reaction times lead to denaturation and decomposition of humic acid products.

[0073] Figure 6 The FTIR (Fourier Transform Infrared) spectra of the humic acid products synthesized from sludge at different reaction times characterize the changes in various characteristic functional groups in humic acid. With increasing reaction time (1 h → 6 h), the 1644 cm⁻¹ value corresponding to the C=C skeleton of the aromatic ring increases. -1 The continuously decreasing absorption peak intensity indicates that the aromatic C=C skeleton has gradually decomposed and cannot remain stable after a relatively long reaction time, resulting in a weakened aromatization degree of the humic acid product. Therefore, considering both the yield and the degree of humification in the hydrothermal synthesis of humic acid, 1 hour was selected as the optimal hydrothermal reaction time.

[0074] Example 5: Microstructural characteristics of humic acid synthesized from sludge via hydrothermal synthesis

[0075] This embodiment selects humic acid synthesized under the optimal reaction conditions in Examples 2, 3, and 4. That is, humic acid synthesized from sludge as raw material is synthesized by hydrothermal reaction at 150°C for 1 hour in an alkaline environment of 0.5 mol / L NaOH. The pore structure characteristics of humic acid synthesized from sludge by hydrothermal reaction were investigated to support its adsorption performance for pollutants such as heavy metals.

[0076] Figure 7 The microstructure of humic acid synthesized from sludge via hydrothermal microscopy was characterized using scanning electron microscopy (SEM). Within a high-resolution field of view (50 μm scale), the humic acid product exhibited a distinctly loose and porous structure, with numerous micron-sized channels arranged on its surface. These channels were interconnected, and the interior contained a large number of tiny pores, providing ample adsorption sites. Figure 8 The results of nitrogen adsorption tests show the BJH pore size distribution of humic acid synthesized from sludge via hydrothermal synthesis. The results indicate that the internal pore diameter of the humic acid is mainly distributed in the mesoporous range of 10–50 nm, with an average pore size of 24 nm. This confirms that the internal pores of the humic acid are primarily mesoporous, providing excellent adsorption performance. Simultaneously, the BET specific surface area of ​​the humic acid product was calculated to be 54.955 m². 2 / g, its specific surface area is similar to that of sludge raw material (0.297m²). 2 The size of the sample is significantly increased compared to the previous size, which can provide sufficient adsorption sites for pollutants (such as heavy metal ions).

[0077] Example 6

[0078] This embodiment selects the humic acid synthesized under the optimal reaction conditions in Examples 2, 3, and 4, that is, humic acid synthesized by hydrothermal reaction at 150°C for 1 hour in an alkaline environment of 0.5 mol / L NaOH using sludge as raw material. Figure 9 X-ray photoelectron spectroscopy (XPS) was used to characterize the products of hydrothermal synthesis of humic acid from sludge. Carbon elements were selected for fine spectral analysis, thereby enabling semi-quantitative analysis of carbon structure under different chemical environments and clarifying the different chemical states of carbon elements in mineral-like humic acid.

[0079] XPS deconvolution results showed that carbon in humic acid exists mainly in six chemical states, including aromatic carbon, aliphatic carbon, alcohol-ether carbon (carbon bonded to alcohols or ethers), amino carbon (carbon bonded to amino groups), carbonyl carbon, and carboxyl carbon. Aromatic carbon accounted for the highest proportion (35.9%) among all carbon structures, indicating that the humic acid products synthesized from sludge via hydrothermal synthesis contain a large number of aromatic ring structures, exhibiting a significant degree of aromatization. Simultaneously, many carbon structures in the humic acid products exist in the form of active oxygen-containing groups, including acidic groups such as carboxyl groups, indicating that humic acid has significant activity and acidification. Furthermore, the presence of a certain amount of amino carbon in humic acid indicates that nitrogen in sludge, after hydrothermal humification, forms pyrroles and amides, which, as nitrogen-containing structures in humic acid, can provide a certain nitrogen source for humic acid land application.

[0080] Example 7: Adsorption behavior of heavy metals by humic acid synthesized from sludge via hydrothermal synthesis

[0081] Examples 2, 3, and 4 demonstrate that humic acid products possess abundant negatively charged functional groups such as carboxyl and hydroxyl groups, exhibiting excellent chemisorption performance, particularly for positively charged heavy metal ions. Example 5 shows that humic acid has a large specific surface area and abundant mesoporous structures, providing ample adsorption sites. Therefore, humic acid can serve as an excellent regenerable adsorbent for adsorbing pollutants such as heavy metals, thereby achieving efficient utilization.

[0082] This embodiment selects the humic acid synthesized under the optimal reaction conditions in Examples 2, 3, and 4, i.e., the humic acid synthesized from sludge as raw material under an alkaline environment of 0.5 mol / L NaOH and a hydrothermal reaction at 150°C for 1 h, as the adsorbent; Cu was selected respectively. 2+ Solution (Cu(NO3)2) and Cr2O7 2- A (K₂Cr₂O₇) solution was used as the adsorption solution, with an initial concentration of 100 ppm. The adsorption temperature was controlled at 298.15 K and the adsorption time at 24 h to investigate the effect of humic acid on Cu₂O₇ heavy metal ions in the solution environment under different pH conditions. 2+ and Cr2O7 2- Removal rate.

[0083] Figure 10 , Figure 11 This indicates that the initial pH value of the solution environment has a significant impact on the adsorption effect of heavy metal ions, and that humic acid exhibits different removal effects on the two heavy metal ions. Humic acid on Cu 2+ The removal rate first increased with increasing pH value and then gradually remained constant, while for Cr2O7... 2- The removal rate gradually decreases with increasing pH. This is because at very low pH, the solution contains a large amount of H₂. + These H + Will with Cu2+ Competition for the adsorption sites of humic acid leads to the synthesis of humic acid for Cu. 2+ The removal rate is low; conversely, at lower pH, the surface of synthetic humic acid adsorbs H+. + This results in its surface carrying a positive charge, making it more attractive to anions Cr2O7. 2- This improved the resistance to Cr2O7. 2- The adsorption effect is significant. As the pH gradually increases to a certain range, the carboxyl and hydroxyl groups in the synthesized humic acid undergo deprotonation: X≡OH + OH⁻ → X≡O - +H2O results in a negative charge on its surface, while competing ions H in the environment... + It is also less, resulting in humic acid affecting Cu 2+ The removal rate of Cr2O7 was further improved; conversely, humic acid had a lower removal rate than Cr2O7. 2- The removal rate is significantly reduced. In summary, at pH > 6, the humic acid synthesized from sludge hydrothermally significantly affects Cu. 2+ The removal rates were high, all exceeding 74%, with a maximum of 84.7%; at pH < 6, the humic acid synthesized from the hydrothermal sludge had a high removal rate for Cr2O7. 2- The removal rates are high, all greater than 50%, with the highest reaching 80%.

[0084] This embodiment confirms that humic acid synthesized from sludge via hydrothermal synthesis has a good adsorption effect on different types of heavy metal ions and can be used as an adsorbent to effectively remove heavy metal ions from the environment.

Claims

1. A method for hydrothermal conversion of municipal sewage sludge into highly aromatic, porous, mineral-derived humic acid, characterized in that, The method includes: mixing sludge raw material with an alkaline agent to obtain a mixed sludge precursor liquid; the alkalinity of the mixed sludge precursor liquid is 0.5 mol / L, calculated as hydroxide ion concentration; hydrothermally reacting the mixed sludge precursor liquid at 150℃ for 1 hour; extracting mineral-like humic acid from the reaction solution after the hydrothermal reaction; the extraction of mineral-like humic acid from the reaction solution after the hydrothermal reaction includes: adjusting the pH of the reaction solution after the hydrothermal reaction to 12-13, mixing evenly and allowing it to stand, and taking the supernatant; then... The pH of the supernatant was adjusted to 1-2, and the mixture was allowed to stand to allow the solid product to precipitate. The solid product was washed and dried to obtain a highly aromatic, porous, mineral-derived humic acid. The surface of the mineral-derived humic acid is covered with cross-linked micron-sized channels, and the interior contains mesopores with a diameter mainly between 10 and 60 nm. The carbon in the mineral-derived humic acid consists of 30-40% aromatic carbon and 25-35% aliphatic carbon. The specific surface area of ​​the mineral-derived humic acid is 50-60 m². 2 / g.

2. The method according to claim 1, characterized in that, The alkaline agent is one or a mixture of solid sodium hydroxide, potassium hydroxide, and aluminum hydroxide; or, the alkaline agent is a solution composed of one or a mixture of solid sodium hydroxide, potassium hydroxide, and aluminum hydroxide and water.

3. The method according to claim 1, characterized in that, The specific surface area of ​​the mineral-derived humic acid is greater than that of the sludge raw material.

4. The application of the mineral-derived humic acid prepared by any one of claims 1 to 3 in the adsorption of heavy metal pollutants.

5. The application according to claim 4, characterized in that, The application is the adsorption of heavy metal copper ions by mineral-derived humic acid in an environment with pH > 6; or, the application is the adsorption of heavy metal chromate ions by mineral-derived humic acid in an environment with pH < 6.

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