A method for preparing high-quality sludge biochar and its application

Abstract

This invention pertains to the fields of wastewater treatment plant sludge treatment and livestock waste treatment. It discloses a method for preparing high-quality sludge biochar and its application. The method involves chemically activating sludge and cow manure, followed by sedimentation to obtain supernatant A and precipitate B. Precipitate B is then centrifuged at low speed to obtain supernatant C and precipitate D. Precipitate D is dried to obtain a pyrolysis precursor E. Precipitant E is pyrolyzed under a nitrogen atmosphere. The pyrolyzed biochar is washed with an acid solution, recovering some useful substances. After acid washing, the biochar and acid washing solution are separated. The biochar is then washed with clean water, dried, and sieved. The sludge biochar of this invention can generate superoxide anion free radicals through adsorption, transformation, and transfer, exhibiting good reusability and stability. This provides a new direction for improving the high-value utilization of sludge and cow manure, and also offers new ideas for designing efficient biochar catalysts for environmental remediation.

Description

Technical Field

[0001] This invention relates to the fields of wastewater treatment plant waste sludge treatment and livestock waste treatment, and to a method for preparing high-quality sludge biochar and its application, specifically to a method for the precise resource utilization of waste sludge and cow manure and its application. Background Technology

[0002] Sludge, as a byproduct of wastewater treatment, has a complex composition, mainly consisting of microorganisms, inert organic matter, and inorganic substances (primarily oxides of silicon, iron, aluminum, and calcium). It possesses dual attributes of "pollution" and "resource." The reduction, harmlessness, and resource utilization of wastewater sludge have always been key issues hindering the field of wastewater treatment and a hot topic of international research.

[0003] Due to the excellent properties of sludge-based biochar, different functions can be prepared by using different processes and modification methods for different types of sludge. These biochars vary greatly in the types of surface functional groups, defect structures, degree of graphitization, and porosity. Applying them to different catalytic systems can generate different active species, thus achieving good degradation effects. However, the application of sludge-based biochar materials is still in the research stage, and current sludge-based biochar catalytic technology still has many problems. For example, sludge biochar prepared by current methods often exhibits an aggregated state under mesophilic pyrolysis conditions, with relatively few pores and a smooth "protective film" on the surface. This protective film shows obvious traces of gas overflow and bubble rupture, which is caused by the high thermal stability of sludge under mesophilic pyrolysis conditions. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing high-quality sludge biochar and its application, which solves the problems of low specific surface area, insufficient porosity, and excessive heavy metal leaching that are common in sludge biochar materials.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A method for preparing high-quality sludge biochar includes the following steps:

[0007] Step 1. Activation: First, chemically activate the sludge and cow manure.

[0008] Furthermore, the chemical activation refers to mixing sludge and cow manure, adding an aqueous solution of zinc chloride (ZnCl2) and stirring, followed by natural settling.

[0009] Furthermore, the sludge is sludge from the thickening tank of a sewage treatment plant (treated with chemicals, with a moisture content of 97-98%), dewatered sludge from a sewage treatment plant (treated by a filter press, with a moisture content of 60-80%), and dried sludge. The cow manure is naturally air-dried.

[0010] Furthermore, the concentration of the ZnCl2 aqueous solution is 20–40% ω / ω.

[0011] Furthermore, the stirring time is 12–24 h, and the stirring speed is 50–200 rpm / min.

[0012] Furthermore, the natural settling time is 6–12 hours.

[0013] Step 2. Dehydration and Recovery: After sedimentation separation in Step 1, supernatant A and precipitate B are obtained. Precipitate B is then separated by low-speed centrifugation to obtain supernatant C and precipitate D. Precipitate D is then dried to obtain pyrolysis precursor E. Low-speed centrifugation not only effectively removes water from the mixture but also recovers some of the substances.

[0014] Furthermore, the rotation speed of the low-speed centrifugation is 4000–6000 rpm / min.

[0015] Furthermore, the drying temperature is 105–120°C, and the drying time is 4–6 hours, including the heating time.

[0016] Step 3. Pyrolysis: The above precursor is pyrolyzed under a nitrogen (N2) atmosphere. The gases produced during pyrolysis are recovered through a cooling and recovery unit to prevent environmental pollution. The biochar produced by pyrolysis is then pulverized and ground before proceeding to the next step. In this step, the biomass is heated to a certain temperature in an anaerobic or low-oxygen environment to induce thermal decomposition. This process causes the organic matter in the biomass to crack, forming new small organic molecules and gaseous products. These newly generated small organic molecules are rich in energy and can be further recycled. Simultaneously, the pyrolyzed biochar also possesses high reactivity and can be applied in various environments.

[0017] Furthermore, the pyrolysis step is carried out in a box-type atmosphere furnace, rather than a tubular atmosphere furnace.

[0018] Furthermore, after the pyrolysis precursor is placed in the box-type atmosphere furnace, it is necessary to evacuate the vacuum, then introduce nitrogen gas at a flow rate of 200 mL / min to depressurize, and then evacuate the vacuum again.

[0019] Furthermore, the pyrolysis temperature of the box-type atmosphere furnace is 400–700℃, the heating rate is 5–20℃ / min, and the cooling rate does not exceed 20℃ / min.

[0020] Furthermore, the N2 flow rate is 50–200 mL / min during the heating phase, 200–2000 mL / min during the isothermal phase, and 50–200 mL / min during the cooling phase.

[0021] This study found through experiments that controlling the gas flow rate can promote the development of pore size inside sludge biochar. The gas flow rate during the heating stage has little impact on pore size development and should be controlled at 50-200 mL / min to save resources. The gas flow rate during the isothermal stage has a greater impact on pore size development, and the experimental study found that maintaining it at 200-2000 mL / min is the most suitable.

[0022] Furthermore, the biochar is pulverized and ground using a planetary ball mill, and the grinding jar and grinding balls are made of agate.

[0023] Step 4. Acid washing: The pyrolyzed biochar is washed with an acid solution to remove impurities and harmful substances from its surface and to recover some useful substances. After acid washing, the biochar and acid washing solution are separated. The biochar is rinsed with clean water until the pH of the final rinse water is neutral. The acid washing solution is recovered for subsequent use.

[0024] The strong oxidizing properties of acid can react with organic pollutants on the surface of biochar, decomposing them into soluble substances that can then be removed by the washing solution. After acid washing, we obtain pure biochar with a high specific surface area and porosity, which is beneficial to improving its application performance.

[0025] Furthermore, the acid solution is a hydrochloric acid (HCl) solution with a concentration of 0.05–0.2 mmol / L.

[0026] Furthermore, the pickling time is 30–60 min.

[0027] Step 5. Drying and Sieving: The acid-washed biochar is placed in a drying device for drying. The dried biochar can be stored for a long time and is not easily deteriorated. Then, we use a specific sieve to sieve it to obtain biochar with different particle size distributions to meet the needs of different application scenarios.

[0028] In this invention, cow dung mixed with sludge is used as a precursor. The cow, acting as a natural "grinding machine," loosens the structure of the cellulose, and the cow dung exhibits a multi-folded state under a scanning electron microscope. These unique folds provide a larger effective contact area, significantly improving the dispersibility of ZnCl2 nanoparticles and sludge on the cellulose surface.

[0029] Secondly, the addition of cow dung reduces the overall thermochemical stability of the precursor, allowing it to decompose and transform under mesophilic pyrolysis conditions. This results in biochar surfaces typically exhibiting a cracked appearance rather than a smooth one. This characteristic further increases the complexity of the biochar pore size, as well-developed mesopores and micropores evolve within the macropores, forming a multi-level, interconnected, and coarse-structured pore network.

[0030] Then, using an aqueous ZnCl2 solution as an activator, after thorough stirring and natural sedimentation, ZnCl2 nanoparticles can be fully dispersed on the surface of cow dung and sludge. The pore structure of the biochar surface is further increased due to the activation effect of Zn particles during the pyrolysis reaction, resulting in a morphology of dimple cracks on its surface, accompanied by many fine bumps, wrinkles, and defects.

[0031] Finally, acid washing removed ash impurities, tar, polycyclic aromatic hydrocarbons, and heavy metals from the surface of the biochar, and further increased the specific surface area of ​​the biochar.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. Preparing sludge-based biochar using waste sludge as a precursor is an important method for sludge resource utilization. This invention employs a one-step pyrolysis technology to synthesize a novel composite sludge-cow dung biochar. Compared with existing sludge biochar, this composite sludge biochar has a higher specific surface area and a higher content of oxygen-containing functional groups on its surface; it also has a more adaptable proportion of graphitization defects; and it exhibits lower heavy metal leaching rates, lower polycyclic aromatic hydrocarbon content, and lower biotoxicity. Furthermore, it can be used as a non-metallic catalyst in catalytic ozone oxidation (COP), catalytic persulfation, multiphase Fenton oxidation, and photocatalysis, and can also be used as an adsorbent to adsorb and remove heavy metals and antibiotics from wastewater.

[0034] 2. The cow dung doping method used in this invention has a significant positive impact on the material properties and catalytic activity of sludge biochar. The method of this invention reduces the thermochemical stability of the sludge precursor, enabling the precursor to decompose and transform under medium-temperature pyrolysis conditions, effectively generating more active sites (graphite C and graphite N), and promoting the balance between defects and graphitization in carbon materials.

[0035] The sludge biochar obtained by this invention exhibits excellent ozone activation performance, capable of generating superoxide anion free radicals through adsorption, conversion, and transfer. · O2 - Furthermore, it exhibits good reusability and stability. This invention provides a new direction for improving the high-value utilization of sludge and cow manure, and also offers new ideas for designing highly efficient biochar catalysts for environmental remediation. Attached Figure Description

[0036] The accompanying drawings described in the following embodiments are only a part of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0037] Figure 1 A flowchart illustrating a method for preparing high-quality sludge biochar.

[0038] Figure 2 A detailed flowchart of the method for preparing a high-quality sludge biochar;

[0039] Figure 3 a and 3b are scanning electron microscope images of Zn@SBC-CD at different magnifications; Figure 3 c and 3d are scanning electron microscope images of Zn@SBC at different magnifications; Figure 3 e, 3f are scanning electron microscope images of SBC at different magnifications; Figure 3 g, 3h are scanning electron microscope images of BC-CD at different magnifications;

[0040] Figure 4 a is the nitrogen desorption-adsorption isotherm of four types of biochar; Figure 4 b is the BJH mesopore distribution diagram of the four biochar species; Figure 4 c is the DFT micropore distribution diagram of the four types of biochar;

[0041] Figure 5 a shows the X-ray diffraction patterns of four types of biochar; Figure 5 b shows the Fourier transform infrared (FTIR) spectra of four types of biochar. Figure 5 c shows the Raman spectra of the four types of biochar; Figure 5 d shows the full X-ray photoelectron spectra of the four biochar species;

[0042] Figure 6 a is the refined C1s spectrum of X-ray photoelectron spectroscopy for four types of biochar; Figure 6 b is the refined X-ray photoelectron spectroscopy (XPS) spectrum of four types of biochar, N1s spectrum;

[0043] Figure 7 The adsorption and removal of methylene blue by four types of biochar;

[0044] Figure 8 The efficacy of four types of biochar in catalytic ozone oxidation for the degradation of methylene blue was investigated. Detailed Implementation

[0045] The design scheme of the present invention will be further described in detail below with reference to specific embodiments. It should be noted that the following detailed description is exemplary and intended to provide further explanation of the present invention, and should not be construed as limiting the scope of protection of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0046] Example 1

[0047] This embodiment provides a method for preparing high-quality sludge biochar.

[0048] Please refer to the appendix for a method for preparing high-quality sludge biochar. Figure 1 Follow these steps:

[0049] Step 1. Activation

[0050] First, we selected dewatered sludge from a wastewater treatment plant with a moisture content of approximately 80.56%. This sludge was then dried in a well-ventilated, sunny environment to remove excess moisture. Similarly, we dewatered cow manure from a beef cattle farm under the same conditions. After drying, both the sludge and manure were crushed and sieved, with the fraction smaller than 100 mesh selected for the next step.

[0051] Furthermore, the sludge and cow dung were mixed at a mass ratio of 1:4, that is, 2.00 g of cow dung and 8.00 g of sludge were added to 100 mL of ZnCl2 solution (20%, ω / ω), and activated at 50 rpm / min for 12 h at room temperature to allow Zn to react. 2+ The activated mixture was fully complexed on the surface of cow dung and sludge, and then allowed to settle for 24 hours.

[0052] Step 2. Dehydration and Recycling

[0053] After sedimentation and separation, the precipitate was placed in a benchtop centrifuge and centrifuged at 5000 rpm for 5 min to remove water. The supernatant was poured into a recovery tank and the precipitate was placed in an oven and dried at 105±5℃ for 4 h until constant weight. The dried substance is called the pyrolysis precursor.

[0054] Step 3. Pyrolysis

[0055] The pyrolysis reaction is carried out in a box-type atmosphere furnace. First, the above-mentioned precursor is placed in a corundum crucible and then placed in the box-type atmosphere furnace. After the furnace door is closed, a vacuum is drawn, and then nitrogen gas is introduced at a gas flow rate of 200 mL / min to release the pressure. After that, a vacuum is drawn again.

[0056] Furthermore, the pyrolysis reaction was carried out under a nitrogen (N2) atmosphere at a temperature of 600℃, a heating rate of 15℃ / min, and a cooling rate of 15℃ / min. The N2 flow rate was 50 mL / min during the heating phase, 1000 mL / min during the isothermal phase, and 50 mL / min during the cooling phase. The gases generated during pyrolysis were recovered through a cooling and recovery unit to prevent environmental pollution. After pyrolysis, the resulting biochar powder and agate grinding balls were placed in an agate grinding jar and ground using a planetary ball mill.

[0057] Step 4. Pickling

[0058] The ground biochar was washed with 0.1 mmol / L hydrochloric acid solution for 30 min by stirring. The solid biochar was separated by filtration and repeatedly washed with deionized water until the pH of the filtrate was neutral. The acid washing solution was recovered for subsequent use.

[0059] Step 5. Drying and Sieving

[0060] The acid-washed biochar is then placed in a drying device for drying.

[0061] Example 2

[0062] This embodiment introduces the application of a high-quality sludge biochar. Experiments verified that the adsorption and catalytic activation performance of the sludge biochar prepared by the method in Example 1 was improved. Therefore, sludge biochar can be used as an adsorption and activation material in wastewater treatment, thereby achieving precise resource utilization of sludge.

[0063] To more clearly describe the technical solutions in the embodiments of the present invention, the sludge biochar prepared by the present invention is named Zn@SBC-CD. For comparison, the activated sludge biochar without cow dung is named Zn@SBC. For comparison, the unactivated sludge biochar without cow dung is named SBC. For comparison, the unactivated sludge biochar with cow dung but no added is named SBC-CD. Unless otherwise specified, the four types of biochar referred to in the accompanying drawings are the four types of biochar described above.

[0064] By comparing and characterizing the various properties of the above-mentioned sludge biochar, it is concluded that the sludge biochar prepared in this invention has improved performance in adsorption and advanced oxidation, as detailed below.

[0065] 1. Morphological analysis of biochar

[0066] The morphology and microstructure of Zn@SBC-CD, Zn@SBC, SBC, and BC-CD catalysts were observed and analyzed using scanning electron microscopy (SEM). The experimental results are attached. Figure 3 As shown.

[0067] BC-CD exhibits a typical cellulose morphology with a smooth surface and abundant macroporous structure. SBC is generally aggregated with relatively fewer pores, but its surface is covered with a smooth "protective film" showing signs of bubble rupture. Zn@SBC surface exhibits dimple crack morphology, accompanied by fine bumps, wrinkles, and defects, indicating that Zn nanoparticles are uniformly distributed on the SBC surface. The porous structure of BC can increase the specific surface area and provide more active sites. Zn@SBC-CD further increases the pore size complexity, with well-developed mesopores and micropores, forming a coarse pore network that provides even more active sites. In summary, cow dung doping improves the elemental composition of Zn@SBC, forming a complex carbon framework and pore structure, resulting in high porosity.

[0068] 2. Specific surface area and pore size distribution

[0069] The specific surface area, average pore size, pore density, and pore size distribution of four types of biochar were analyzed using a specific surface area and pore size analyzer. The experimental results are attached. Figure 4 As shown in Table 1.

[0070] Table 1 Specific surface area and pore size distribution of biochar

[0071]

[0072] Appendix Figure 4 The N2 adsorption-desorption isotherms for Zn@SBC-CD, Zn@SBC, SBC, and BC-CD are shown. All materials exhibit typical type IV isotherms with a distinct type H1 hysteresis loop, confirming the presence of mesoporous structures. The BJH mesopore and DFT micropore distributions of the materials are shown in the attached figure. Figure 4 As shown in b and c, it can be observed that BC-CD and SBC have relatively low specific surface areas, mainly composed of microporous structures. This may be because the pyrolysis reaction is slow at this temperature, preventing the material from converting into gas and escaping to form pores. The activation effect of ZnCl2 can increase the specific surface area and pore volume of the material, while also significantly increasing the mesoporous structure. Table 1 shows that compared to SBC, the BET specific surface area of ​​Zn@SBC is 66.11 cm². 2 / g increased to 669.42cm 2 / g, pore volume from 0.035cm 3 / g increased to 0.200cm 3 / g. This is because the organic matter in the raw materials decomposes at higher temperatures, exposing it to larger pores and channels, thus increasing the specific surface area. Surprisingly, the doping of cow dung further increases the specific surface area of ​​SBC, with Zn@SBC-CD exhibiting an even higher BET specific surface area of ​​835.69 cm². 2 / g, with a larger pore volume of 0.419cm³ 3 / g. It has been reported that more microporous and mesoporous structures can promote the adsorption of contaminants and O3 molecules in solution onto the active sites of the catalyst, thereby accelerating electron transfer and improving catalytic activity.

[0073] 3. Crystal structure and surface chemical composition analysis

[0074] The crystal structure and surface chemical composition of four types of biochar were analyzed by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectric spectroscopy. The results are attached. Figure 5 Appendix Figure 6 As shown in Tables 2 and 3.

[0075] Table 2. Determination of organic element (EA) content in four types of biochar

[0076]

[0077] Table 3. Elemental analysis of XPS surface of four types of biochar.

[0078]

[0079] XRD determination (see attached) Figure 5 a) The crystal phase composition of the material was determined. The four types of BC exhibited similar peak shapes under different doping compositions, with the strongest peak observed at 2θ = 26.58°, which is a characteristic peak of amorphous graphitic carbon. The graphitic C diffraction peaks showed differences with the change of doping composition, with Zn and cow dung doping increasing the degree of graphitization. A peak coinciding with the (110) plane of Zn crystal was found at 46.3°. The intensity of the Zn diffraction peak in Zn@SBC-CD was not obvious, possibly because some Zn nanoparticles were etched and encapsulated in the graphite layer of BC, demonstrating the better biocompatibility of Zn@SBC-CD. Raman spectroscopy (see attached) Figure 5 b) The graphitization degree of BC materials was measured, and it was found that BC-CD had a better graphitization degree, possibly because the high carbon content in cow dung further enhanced the graphitization degree. Zn doping is beneficial for improving the defect level of carbon materials, while cow dung doping contributes to the enhancement of graphitization degree and promotes ozone activation. FT-IR spectroscopy (see attached) was used to further analyze the graphitization. Figure 5 c) The composition of functional groups on the material surface was studied. Zn@SBC-CD showed the highest number of oxygen-containing and unsaturated functional groups, providing more active sites. XPS analysis (see attached) Figure 5 d) shows that the C and O contents of Zn@SBC-CD are 67.35% and 16.73%, respectively, which are 1.02 and 1.08 times higher than those of Zn@SBC. Elemental analysis further confirmed that C, N, O, and Zn elements are aggregated and distributed in the catalyst (Table 2).

[0080] Figure 5 Fine C1s and N1s spectra of Zn-doped cow dung carbon sphere-shell composites (Zn@SBC-CD), Zn-doped cow dung carbon spheres (Zn@SBC), pure cow dung carbon spheres (SBC), and cow dung carbon-shell composites (BC-CD) are presented. Figure 5 In graphite a, the C1s peaks correspond to CC, COC, C=O, and π-π*, respectively, located at 284.72, 285.92, 288.42, and 291.33 eV. Notably, Zn doping promotes the increase of graphite C content and charge transfer, accelerating the decomposition and transformation of O3. Cow dung doping reduces the destruction of oxygen-containing groups and is beneficial to increasing their content. Figure 5 In step b, the graphite N content increased from 37.9% (SBC) to 58.26% (Zn@SBC-CD). Pyridine N and pyrrole N were more readily converted to Zn-N and graphite N, and graphite N was considered a more efficient electron-transfer N species, which could also effectively enhance the catalyst's activation of O3. Based on the above experimental results, it was found that the doping of cow dung into the Zn@SBC-CD biochar obtained in this embodiment effectively improved the catalytic performance of the biochar. This is attributed to the more developed mesoporous structure of Zn@SBC-CD, a more balanced ratio of carbon material defects to graphitized C structures, and more surface functional groups. The amorphous graphite structure, material defects, C=C, and C=O on the Zn@SBC-CD surface are the main catalytic active sites.

[0081] Example 3

[0082] This embodiment verifies a method for preparing high-quality sludge biochar, which produces sludge biochar with good performance in adsorption and catalytic applications.

[0083] The heterogeneous catalytic ozone oxidation experimental setup employed a self-made 2L plexiglass column reactor, with the reaction conducted at room temperature. O3 was generated from an ozone generator. In a typical experiment, the adsorption performance of the catalyst was first investigated by placing the catalyst in an MB solution with an initial concentration of 200 mg / L and an initial pH of 5.8, and slowly stirring for 2 hours to allow adsorption. The catalytic performance of the catalyst was then then evaluated.

[0084] Furthermore, the adsorption experiment was switched to a catalytic ozone oxidation process, with O3 continuously injected into the titanium alloy microporous diffuser at the bottom of the reactor at a gas flow rate of 200 mL / min. The residual O3 in the reaction was recovered using a 2% (w / w) KI solution.

[0085] Further, 1 mL of sample was periodically collected and injected with 50 μL of Na₂S₂O₃ solution to remove free radicals. The reaction solution was then filtered through a 0.22 μm polytetrafluoroethylene filter and used to detect pollutant concentrations and chemical oxygen demand (COD). See the appendix for experimental results. Figure 7 and attached Figure 8 .

[0086] This experiment investigated the effects of adsorbents BC-CD, SBC, Zn@SBC, and Zn@SBC-CD on MB removal. The four BC adsorbents reached adsorption saturation within one hour, with static adsorption capacities of 21.3%, 38.4%, 63.7%, and 82.7%, respectively, indicating limitations in their MB removal capabilities. The adsorption experiment was further converted to a catalytic ozone oxidation process. After 10 minutes of reaction, the MB removal rates of the Zn@SBC-CD / O3, Zn@SBC / O3, SBC / O3, and BC-CD / O3 systems reached 100%, 90.45%, 74.81%, and 68.32%, respectively, while the COD removal rates reached 70.24%, 49.15%, 32.28%, and 29.37%, respectively. Conversely, when only O3 was present in the reaction system, the MB removal rate was only 32.41%, and the COD removal rate was only 22.25%, indicating that O3 could not be catalytically activated, leading to the low MB removal rate.

[0087] Furthermore, in the BC-CD / O3 and SBC / O3 systems, O3 exhibits a certain activation capacity, possibly due to the presence of functional groups such as semiquinones, which can participate in the reaction as redox ligands. The Zn@SBC / O3 and Zn@SBC-CD / O3 systems demonstrate better MB degradation performance because the composite materials possess higher crystallinity, larger specific surface area, and a well-defined graphitized structure. The addition of Zn promotes the formation of a porous carbon network, increasing the contact area between pollutant molecules and the catalyst.

[0088] Meanwhile, the addition of cow manure enriches the elemental composition of the sludge precursor, avoids the reduction in reactive sites caused by Zn particle aggregation, and increases metal active sites. These characteristics enhance the ability to activate ozone and directly accelerate the generation of free radicals. Therefore, the Langmuir-Hinshelwood kinetic model was used for fitting, and it was found that the degradation of pollutants by the Zn@SBC / O3 system conforms to the adsorption oxidation model, possibly through adsorption, conversion, and mass transfer to generate ROS and degrade pollutants.

[0089] Example 4

[0090] This embodiment experimentally verifies a method for preparing high-quality sludge biochar and its application. The biochar prepared meets safety standards in terms of biotoxicity and medium metal leaching characteristics.

[0091] Polycyclic aromatic hydrocarbons (PHAs) are a class of toxic, mutagenic, and carcinogenic organic pollutants, primarily derived from the incomplete combustion or pyrolysis of fossil fuels and organic matter. PHA detection is crucial for the biotoxicity assessment of biochar.

[0092] Table 4 shows the content of 16 PHAs in Zn@SBC-CD. The Σ16PAHs in Zn@SBC-CD are far below the safety standard for high-quality biochar (≤4 mg / kg) stipulated by the Swiss Chemical Risk Reduction Management Regulations.

[0093] Table 4. Content of 16 Polycyclic Aromatic Hydrocarbons (PHAs)

[0094]

[0095] The ∑16PAHs content in Zn@SBC-CD is 0 (μg / kg).

[0096] HDXRF was used to detect the changes in heavy metal leaching concentrations after the catalytic degradation reaction of the Zn@SBC-CD / O3 system with different initial pH values, and to simulate the leaching of harmful components from the catalyst using the sulfuric acid-nitric acid method and the horizontal shaking method. The results are shown in Table 5.

[0097] Table 5. Heavy metal leaching of Zn@SBC-CD (mg / L)

[0098]

[0099]

[0100] The levels of all seven metal ions (Cu, As, Cd, Zn, Ni, Cr, and Pb) were below the Class A standard of China's "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB 18918-2002).

[0101] On the one hand, the content of these metal elements in the original sludge may be relatively low, and the catalyst after calcination is acid-washed, which greatly reduces the leaching of elements such as Zn and Cu from the catalyst. On the other hand, high-temperature pyrolysis may have played a role in removing or solidifying heavy metals in the sludge. For example, As, Cd, and Pb are highly volatile, and when the temperature rises to a certain level, they can volatilize directly or form metal oxides with low melting and boiling points, such as As2O3(g), PbCl2(g), PbO(g), CrO2(OH)2(g), NiO(g), etc.

[0102] The above results indicate that Zn@SBC-CD, as a catalyst, has good stability, reusability, and safety, while also possessing good economic benefits and strong potential for wastewater treatment.

[0103] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the definitions herein are not intended to limit them to the embodiments shown herein, but rather to be accorded the widest scope consistent with the principles and novel features of the claims herein.

Claims

1. A method for preparing high-quality sludge biochar, characterized in that, Includes the following steps: Step 1. Activation: Chemically activate the sludge and cow manure, mix them together, add a ZnCl2 aqueous solution with a concentration of 20~40% w / w and stir, then allow it to settle naturally. Step 2. Dehydration and recovery: After the sedimentation separation in Step 1 above, supernatant A and precipitate B are obtained. The precipitate B is separated by low-speed centrifugation to obtain supernatant C and precipitate D. Then, precipitate D is dried to obtain pyrolysis precursor E. Step 3. Pyrolysis: The above precursor is pyrolyzed under an N2 atmosphere in a box furnace. The pyrolysis step is carried out under vacuum, and then nitrogen is introduced at a gas flow rate of 200 mL / min to depressurize. After vacuuming, the flow rate of N2 is 50~200 mL / min during the heating stage, 200~2000 mL / min during the isothermal stage, and 50~200 mL / min during the cooling stage. The pyrolysis temperature in the box furnace is 400~700℃, the heating rate is 5~20℃ / min, and the cooling rate does not exceed 20℃ / min. The gas generated by pyrolysis is recovered through a cooling and recovery unit. The biochar generated by pyrolysis is pulverized and ground before entering the next step. Step 4. Acid washing: The pyrolyzed biochar is washed with a hydrochloric acid solution with a concentration of 0.05~0.2mmol / L to remove impurities and harmful substances from its surface and recover some useful substances. The acid washing time is 30~60min. After acid washing, the biochar and acid washing solution are separated. The biochar is washed with water until the pH value of the final washing water is neutral. The acid washing solution is recovered for subsequent use. Step 5. Drying and sieving: The acid-washed biochar is placed in a drying device for drying, and then sieved with a specific sieve to obtain biochar with different particle size distributions to meet the needs of different application scenarios.

2. The method for preparing high-quality sludge biochar according to claim 1, characterized in that, The sludge in Step 1 is dewatered sludge from a wastewater treatment plant, which has been processed by a filter press and has a moisture content of 60-80%.

3. The method for preparing high-quality sludge biochar according to claim 1, characterized in that, The stirring time in Step 1 is 12~24h, and the stirring speed is 50~200r / min.

4. The method for preparing high-quality sludge biochar according to claim 1, characterized in that, The natural settling time in Step 1 is 6~12 hours.

5. The method for preparing high-quality sludge biochar according to claim 1, characterized in that, In Step 2, the speed of low-speed centrifugation is 4000~6000 r / min.

6. The method for preparing high-quality sludge biochar according to claim 1, characterized in that, The drying temperature in Step 2 is 105~120℃, and the drying time is 4~6 hours, including the heating time.

7. The method for preparing high-quality sludge biochar according to claim 1, characterized in that, In Step 3, the biochar is pulverized and ground using a planetary ball mill. The grinding jar and grinding balls are made of agate.

8. The biochar prepared by the method for preparing high-quality sludge biochar according to claim 1.

9. The application of the biochar prepared by the method for preparing high-quality sludge biochar according to claim 1, characterized in that, It is used in the field of wastewater treatment.

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