Method for efficiently treating organic contaminated soil leaching organic wastewater

Abstract

The invention belongs to the technical field of environmental pollution treatment, and provides a method for efficiently treating organic contaminated soil leaching organic wastewater. The method comprises the following steps: crushing walnut shells, sieving, mixing with potassium hydroxide and urea, reacting for 2 hours at 500-800 DEG C under an argon condition, collecting the obtained product, and obtaining the biochar without washing. The organic polluted soil is eluted through SDS to obtain eluent containing organic pollutants, and the biochar and the PMS oxidizing agent are added into the eluted wastewater to remove the organic pollutants. According to the invention, agricultural wastes are utilized, and the prepared biochar material has a good removal effect on organic pollutants in soil leaching organic wastewater. Through the synergistic effect of biochar activation and alkali activation, the efficiency is improved by 30% compared with that of independent biochar activation, the addition amount of alkali liquor is reduced by 80%, and the biochar material has extremely high activity. The removal rate of organic pollutants in the leaching waste liquid reaches 90% or above.

Description

Technical Field

[0001] This invention belongs to the field of environmental pollution control technology, specifically relating to a method for efficiently treating organic wastewater from organic polluted soil leaching. Background Technology

[0002] With the acceleration of industrial modernization, leaks of petrochemical products, the extensive use of pesticides, urban sewage discharge, and atmospheric pollutant deposition have all caused serious organic pollution of water and soil. Typical pollutants include petroleum hydrocarbons (TPHs), organochlorine pesticides, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). These organic pollutants, characterized by high toxicity, high accumulation, reluctance to degrade, and long-distance migration, not only pose a serious threat to the ecological environment but also severely endanger human health. Therefore, the remediation of polluted water and soil is urgently needed to protect public health and the ecological environment.

[0003] Currently, my country's "three rural issues" (agriculture, rural areas, and farmers) are mainly manifested in three aspects: "agricultural ecological environment pollution," "weak rural economy," and "slow income growth for farmers," which have become important factors restricting the sustainable development of agriculture and rural areas in my country. The resource utilization of agricultural waste is an important means to drive rural economic development, transform the agricultural ecological environment, and increase farmers' income; it is also a concrete measure to coordinate the development of agriculture, rural areas, and farmers. The rational disposal and effective utilization of agricultural waste not only helps reduce rural land pollution and improve the environment but also promotes the sustainable use of agricultural resources. Transforming agricultural waste into value-added products for the treatment of polluted water bodies and soil is a clean process, cost-effective, and sustainable development, making it an inevitable choice for promoting the modernization of agricultural development.

[0004] In soil remediation technology, directly remediating organically contaminated soil by preparing persulfate (PMS) solutions is a common method, known as in-situ chemical oxidation (ISCO). While this technique successfully reduces the concentration of organic pollutants in the soil, the indiscriminate attack of free radicals generated after PMS activation leads to significant waste of PMS due to the consumption of free radicals by soil particles, inorganic salts, and humic acids. In contrast to direct oxidation, leaching, another treatment technology for organically contaminated soil, involves using specific leaching agents to wash the contaminated soil and remove pollutants. Therefore, leaching organically contaminated soil to remove pollutants, followed by oxidation of the leaching wastewater to meet discharge standards, is a feasible method. Furthermore, the unique functional group structure on the surface of biochar enables it to activate PMS and degrade organic pollutants. Therefore, using biochar as a purification material for leaching organic wastewater and organically contaminated soil can achieve comprehensive utilization of agricultural waste, treating waste with waste and improving resource recycling efficiency.

[0005] Biochar is a porous adsorbent material with a high surface area and good thermal stability. Using agricultural waste as a precursor for biochar preparation is a low-cost and technologically mature method. In research on using biochar to remediate polluted water and soil, the mainstream method for biochar preparation and utilization involves mixing agricultural waste with alkali to create pores and then activating it, followed by acid and water washing. However, this process requires the addition of large amounts of alkali (such as sodium hydroxide and potassium hydroxide) during pore creation and activation, and subsequent washing with large amounts of acid and pure water, generating substantial amounts of acid and alkali wastewater. On the other hand, research reports indicate that adding strong alkali can also activate some persulfates. Therefore, omitting the washing step during biochar preparation and directly using unwashed biochar to activate persulfates for purifying leaching organic wastewater can reduce waste acid and alkali emissions while improving removal efficiency, demonstrating foreseeable feasibility and advanced technology. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for efficiently treating organic wastewater from the leaching of organically contaminated soil.

[0007] This invention is achieved by the following technical solution: a method for efficiently treating organic wastewater from leaching organic polluted soil, comprising the following steps: (1) Preparation of biochar: Walnut shells were crushed, ultrasonically cleaned with deionized water, and then dried in an oven at 80°C for 24 hours. Biomass was obtained by passing through a 100-200 mesh sieve. 1g of the obtained biomass was dispersed in 20mL of deionized water, and then mixed with potassium hydroxide and urea at a mass ratio of biomass:potassium hydroxide:urea of ​​1:1.5:3. The mixture was stirred evenly to obtain a suspension. The suspension was dried at 80°C for 12h and then at 120°C for 2h. Finally, the suspension was heated to 500-800°C for 2h under argon conditions at a heating rate of 3°C / min. After natural cooling to room temperature, the product obtained was biochar. (2) Obtaining leachate containing organic pollutants: Organically polluted soil is leached with sodium dodecyl sulfate (SDS) to obtain leachate containing organic pollutants. The concentration of SDS is 0-7.5 g / L; the concentration of organic matter in the leachate is 0-10 mg / L. (3) Removal of organic pollutants: Biochar and persulfate oxidant PMS are added to the rinsing solution containing organic pollutants to remove organic pollutants. The concentration of biochar is 0.1-1 g / L, the concentration of PMS is 1-10 mM, the pH of the system is controlled at 3-11, the reaction temperature is 5-35℃, and the treatment time of the rinsing solution containing organic pollutants is 1-100 min.

[0008] In step (1), the walnut shells are dried and passed through a 100-mesh sieve; the mixture is heated to 600℃ and reacted for 2 hours. In step (2), the concentration of SDS is 5 g / L; the organic compound is naphthalene with a concentration of 10 mg / L. In step (3), the concentration of biochar is 0.2-0.3 g / L; the concentration of PMS is 5 mM; and the reaction temperature is 5-20℃.

[0009] The biochar activated persulfate oxidant prepared by this invention has a good catalytic degradation effect on aqueous solutions of various organic pollutants and leaching wastewater, which improves the value-added utilization of agricultural waste. The preparation method is simple, easy to industrialize, and has industrial application prospects.

[0010] In this invention, agricultural waste is prepared into biochar, which can turn agricultural by-products into valuable resources. After the biochar is prepared, there is no need to acid wash and water wash the product, which reduces the discharge of acid and alkaline wastewater and lowers the preparation cost. It has certain innovation and application potential. Using biochar to activate typical persulfate to purify leaching organic wastewater has the dual advantages of environmental protection and resource utilization.

[0011] This invention reduces steps in biochar preparation, thereby lowering costs and increasing efficiency. It combines chemical rinsing with chemical oxidation, allowing for the efficient removal of organic pollutants and the reuse of the rinsing solution, thus avoiding waste and having a wide range of applications. Attached Figure Description

[0012] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 The N2 adsorption-desorption curve of the biochar prepared in this invention is shown. Figure 2 This is a pore size distribution diagram of the biochar prepared in this invention; Figure 3 The graph shows the catalytic removal effect of different dosages of biochar on naphthalene in Test Example 1 of washing organic wastewater. Figure 4 The catalytic removal effect of biochar prepared by different doping processes on naphthalene is shown in the figure. Figure 5 The graph shows the effect of annealing temperature on the catalytic removal of naphthalene during the preparation of Example 1. Figure 6 The graph shows the effect of PMS concentration on the catalytic removal of naphthalene in Example 1. Figure 7 The graph shows the effect of solution pH on the catalytic removal of naphthalene in Example 1. Figure 8 The graph shows the effect of reaction temperature on the catalytic removal of naphthalene in Example 1. Figure 9The graph shows the effect of SDS concentration on the catalytic removal of naphthalene in Example 1. Figure 10 This image shows the catalytic removal effect of naphthalene in the leachate from naphthalene-containing soil using SDS leaching. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0015] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and all materials publicly cited herein and cited by them are incorporated herein by reference.

[0016] Equivalent technologies of the specific embodiments described herein that are readily apparent to those skilled in the art through routine experimentation are included in this application.

[0017] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the instruments and equipment used in the following examples are all standard laboratory instruments and equipment; unless otherwise specified, the experimental materials used in the following examples were all purchased from regular biochemical reagent stores.

[0018] The specific experimental conditions and experiment numbers for walnut shell treatment are shown in Table 1.

[0019] Table 1: Comparison of Treatment Effects of Leaching Organic Wastewater Treatment Schemes I. Preparation of walnut shell biochar material: Walnut shells were crushed, ultrasonically cleaned with deionized water, and then dried in an 80℃ forced-air oven for 24 hours. Biomass was obtained by passing the biomass through a 100-mesh sieve. 1g of the obtained biomass was dispersed in 20mL of deionized water, then mixed with 1.5g of potassium hydroxide and 3.0g of urea, and stirred for 30min to obtain a uniform suspension. The suspension was dried at 80℃ for 12h, and then dried at 120℃ for 2h. Finally, the mixture was heated to 600℃ under argon atmosphere at a heating rate of 3℃ / min for 2h. After naturally cooling to room temperature, the product obtained was biochar. II. Pore Structure of Walnut Shell Biochar Material: A suitable pore structure can provide appropriate specific surface area and reaction sites, which is an important parameter for evaluating biochar.

[0020] The N2 adsorption-desorption curve of biochar is shown in the figure below. Figure 1As shown, the results indicate that the prepared walnut shell biochar exhibits rapid nitrogen adsorption at low relative pressures, suggesting the presence of a large number of microporous structures. The isotherms of the biochar at relative pressures... P / P After a gradual rise between 0.8 and 1.0, the value continues to increase and forms a hysteresis loop of shape H1, indicating the presence of a small number of mesoporous and macroporous structures in the biochar.

[0021] Aperture distribution curve as shown Figure 2 The diagram shows the pore structure of walnut shell biochar. Further calculations based on adsorption isotherms yielded a specific surface area of ​​1767 m² for the graded biochar. 2 / g, total pore volume is 0.81cm³ 3 / g, with a micropore volume of 0.60cm³. 3 / g, accounting for 74% of the total pore volume, indicates that the main pore distribution of this biochar is micropores, with mesopores as a secondary component.

[0022] III. Mechanism of Walnut Shell Biochar Treatment of Leaching Organic Wastewater: Persulfate (PMS) is a moderately strong oxidant. Before activation, its oxidizing capacity for organic matter is limited. After activation, it generates various highly reactive species (such as free radicals like hydroxyl radicals and sulfate radicals, as well as non-free radicals like singlet oxygen). These highly reactive species can rapidly degrade organic pollutants, thus having wide applications in the field of organic wastewater purification. Activation of biochar materials is a common method, characterized by low energy consumption and no metal leaching. During the preparation of biochar, alkali modification and activation can produce biochar with richer pore sizes and a larger specific surface area. Simultaneously, the spp at defect sites on the biochar... 2 The covalent carbon structure and oxygen-containing functional groups facilitate the transfer of electrons to the oxidant at the solid-liquid interface, thereby generating an activation effect. Essentially, the activation of biochar materials utilizes the electron-rich structure, diverse functional groups, and defect sites generated after the carbonization and activation of biomass.

[0023] IV. Testing of Walnut Shell Biochar Treatment of Leaching Wastewater: Leaching is one of the effective methods for treating organically contaminated soil. It involves adding a leaching agent to the soil to remove organic pollutants. Leaching agents are mostly surfactants, commonly including cationic surfactants, anionic surfactants, nonionic surfactants, and biosurfactants. Sodium dodecyl sulfate (SDS) is one of the most common anionic surfactants. Because the high concentration of leaching agent in the wastewater coexists with the eluted organic pollutants, the treatment of leaching wastewater is challenging, making its research particularly meaningful.

[0024] Polycyclic aromatic hydrocarbons (PAHs) are a significant class of organic pollutants, drawing considerable attention due to their widespread presence, carcinogenicity, and well-documented adverse health and ecological impacts. The U.S. Environmental Protection Agency (EPA) alone lists 16 parent PAHs (PAH16) on its Priority Pollutant List, while hundreds of PAH homologues exist in the environment. PAHs exist in the environment as mixtures of numerous compounds, each with varying degrees of toxicity. Naphthalene is a typical PAH and also one of the simplest in molecular structure, making its removal research crucial.

[0025] Example 1 of organic wastewater rinsing test: 100 mL of 10 mg / L naphthalene solution (containing 5 g / L sodium dodecyl sulfate), 0.02 g of biochar, and 5 mM PMS were placed in a 150 mL Erlenmeyer flask. The reaction was carried out in a constant-temperature shaker at 25 °C and a shaking rate of 160 r / min. Samples were taken at regular intervals, and the supernatant was filtered through a 0.22 μm filter membrane. The residual naphthalene concentration was determined using high-performance liquid chromatography (HPLC).

[0026] The results are as follows Figure 3 As shown in Table 1, the results indicate that in the presence of the biochar prepared in Example 1, the catalytic removal of naphthalene reached 91% within 60 minutes, at a biochar dosage of 0.3 g / L. Reducing the biochar dosage to 0.2 g / L resulted in a catalytic efficiency decrease to 86%. Considering the 33% reduction in catalyst dosage while the catalytic efficiency only decreased by 5%, this demonstrates that excessive catalyst dosage is not economically viable. Furthermore, the biochar was not subjected to acid washing and water washing after activation, whereas Comparative Example 1 underwent repeated acid washing (0.01 M HCl) and water washing. Figure 3 The results show that Comparative Example 1 achieved a catalytic removal rate of 55% for naphthalene, which is 31% lower than that of the unwashed biochar. This indicates that the washing process removes residual alkali from the biochar preparation process, reducing the potential activation effect of alkali on PMS. Therefore, even with Comparative Example 1 (0.2 g / L) and sodium hydroxide (0.2 g / L) as co-catalysts, the removal rate of naphthalene can only reach 73%. Clearly, eliminating the need for washing is equivalent to reducing or eliminating the need for additional sodium hydroxide for alkali activation in the catalytic reaction.

[0027] Example 2 of organic wastewater leaching test: In the preparation of biochar, nitrogen doping is generally achieved using urea and melamine. Based on the preparation process, in Example 1 of Table 1, walnut shells, potassium hydroxide, and urea are mixed together to simultaneously complete the carbonization and activation processes. Comparative Example 2, however, replaces urea with another common nitrogen-doped precursor, melamine. From... Figure 4It can be seen that although melamine has more nitrogen atoms in its molecule, when the overall nitrogen atom content (amount of substance) of the precursor is the same (at this time, the mass ratio of walnut shell to urea is 1:3, and the mass ratio of walnut shell to melamine is 1:2.1), the catalytic removal of naphthalene in Comparative Example 2 is only 62%, indicating that the biochar prepared from urea has a better catalytic effect.

[0028] Comparative Example 3 was obtained by carbonizing and activating biochar prepared by mixing walnut shells with potassium hydroxide, then mixing it with urea and annealing. The difference from Example 1 is that in Example 1, urea was directly mixed with walnut shell biomass, while in Comparative Example 3, it was mixed with walnut shell biochar. The catalytic removal of naphthalene in Comparative Example 3 only reached 57%, indicating that the effect of urea doping on biochar is lower than that of directly adding urea at the biomass stage. This may be because biomass itself contains a large amount of organic matter and moisture, making it easy for urea to penetrate and form a unified mixture during the mixing reaction. However, after high-temperature annealing, biochar loses a large amount of organic matter and moisture, making nitrogen doping via urea more difficult.

[0029] Annealing Temperature Test: Annealing temperature is one of the key factors determining biochar preparation, influencing the specific surface area and pore volume of the biochar, as well as the active sites on it. This is because higher annealing temperatures result in more organic matter (including urea) and moisture loss from the biomass, forming porous structures, leading to a larger specific surface area and pore volume, and thus better adsorption of organic pollutants. On the other hand, higher annealing temperatures also destroy the organic functional groups on the biomass, and the precursor urea decomposes to form ammonia gas, which is lost instead of forming nitrogen-doped functional groups. This is detrimental to PMS activation and the removal of organic pollutants; therefore, maintaining a suitable annealing temperature is crucial. Figure 5 It can be seen that, within the temperature range of 500-800℃, the samples prepared at each temperature all exhibited a certain degree of catalytic removal of naphthalene within 60 minutes, with the biochar prepared at 600℃ showing the best effect.

[0030] PMS Dosage Test: Oxidants commonly used in the remediation of organically contaminated soils include persulfate (PMS), perdisulfate (PDS), and hydrogen peroxide (H2O2). The activation reaction of PDS is generally slower than that of PMS, mainly due to the symmetrical structure of PDS, with an -SO3 group on each of its two OO bonds. Furthermore, the inherent nonpolarity and negatively charged OO bonds in PDS hinder electron transfer activation. In contrast, asymmetric PMS can be adsorbed onto the surface of a heterogeneous electron donor via OOH terminals. The O atoms on the peroxy bonds are located within the first coordination sphere of the transition metal, which facilitates charge transfer from the metal to the inner sphere of the OO bond. On the other hand, PMS is also more advantageous than the traditional Fenton-like oxidant H2O2 because PMS activation generates sulfate radicals (SO42-). •− PMS possesses advantages such as a longer lifespan (30-40 μs), higher redox potential (2.5-3.1 V), wider applicable pH range (2-9), and selective reactivity. Furthermore, PMS carries lower transportation and storage risks compared to H2O2, making it more practically applicable; therefore, PMS was used as the oxidant in this study. Considering that the consumption of PMS as an oxidant during the reaction is directly proportional to the amount of organic pollutants, and that even the leachate itself, some coexisting ions in the water and soil, and humic acids can consume PMS, the dosage of PMS must be investigated and optimized. Figure 6 It can be seen that the removal rate of naphthalene increases with the increase of PMS dosage, but it reaches the optimal level at 5 mM. Increasing the dosage of PMS to 6 mM does not enhance the removal effect of naphthalene.

[0031] pH testing: pH is a crucial factor affecting advanced oxidation reactions in aqueous phases and the removal of organic pollutants. It determines the adsorption effect of biochar on pollutants in leaching organic wastewater, the activation effect of PMS, and ultimately the pollutant removal rate. Traditional Fenton-like reactions are suitable for acidic conditions, while reactions using PMS as an oxidant have a wider pH range. Figure 7 It can be seen that the reaction system has a high removal rate of naphthalene within the pH range of 3-11, and the removal effect is less affected by pH, which means that the reaction system has a wide range of applications.

[0032] Reaction temperature testing for the removal of organic pollutants: Reaction temperature has a significant impact on the practical application of the reaction system. This is because laboratory reactions are generally carried out at room temperature (25℃), while the temperature of outdoor water bodies is often lower than this, and even much lower in northern winters. Therefore, investigating the reaction temperature is essential. Figure 8It can be seen that the reaction system has a considerable naphthalene removal rate in the range of 5-20℃. However, excessively high temperatures (>30℃) are not conducive to the removal of naphthalene. This may be because the volatility of organic pollutants in water and rinsing solution increases at high temperatures.

[0033] Surfactant SDS Dosage Test: Surfactants increase the transfer of contaminants to the aqueous phase by reducing the interfacial tension between water and hydrophobic contaminants and by accumulating hydrophobic compounds. Based on the dissociation of hydrophilic groups in the aqueous phase, surfactants can be classified as anionic, cationic, nonionic, or amphoteric. Sodium dodecyl sulfate (SDS, CH3(CH2)) 11 SO4Na is an anionic surfactant with a critical micelle concentration (CMC) of 8 mM. It is food-grade and readily biodegradable by soil and / or aquatic microorganisms. Figure 9 It can be seen that naphthalene in the naphthalene-containing eluent can be effectively removed within the SDS concentration range of 0-7.5 g / L. However, excessively high SDS concentrations will clog the active sites on the biochar, hindering the contact between PMS and naphthalene molecules and the active sites, resulting in a decrease in catalytic activity.

[0034] V. Performance Testing of Organic Wastewater from Soil Leaching: A 1 kg sample of uncontaminated soil was collected from outdoors and air-dried at room temperature for 20 days. The soil sample was then ground and sieved (0.15 mm). A certain amount of soil was then added to an acetone solution containing naphthalene (100 mg / kg) and dried in a fume hood for 7 days. 10 g of the naphthalene-containing soil sample was leached with 100 mL of 5 g / L SDS. The leaching solution was collected, filtered, and the solid matter was added to a 150 mL Erlenmeyer flask. 0.02 g of the biochar obtained in Example 1 and 5 mM PMS were added to the Erlenmeyer flask, and the sample was placed in a constant-temperature shaker and shaken at 200 r / min. The concentration of residual naphthalene in the aqueous phase was determined by high-performance liquid chromatography (HPLC) at set time intervals.

[0035] from Figure 10 It can be seen that the reaction system has a 75% removal rate of naphthalene pollutants in soil leached by naphthalene-containing SDS. This removal rate is slightly lower than that of naphthalene in SDS-containing naphthalene solutions. This is because there are a large number of humic acids, anions and cations, and reducing inorganic substances in actual soil samples. These components can consume PMS or react with free radicals generated after PMS activation, thus resulting in a slight decrease in removal efficiency.

[0036] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for efficiently treating organic wastewater from leaching organically contaminated soil, characterized in that: The steps include the following: (1) Preparation of biochar: Walnut shells were crushed, ultrasonically cleaned with deionized water, and then dried in an oven at 80°C for 24 hours. Biomass was obtained by passing through a 100-200 mesh sieve. 1g of the obtained biomass was dispersed in 20mL of deionized water, and then mixed with potassium hydroxide and urea at a mass ratio of biomass:potassium hydroxide:urea of ​​1:1.5:

3. The mixture was stirred evenly to obtain a suspension. The suspension was dried at 80°C for 12h and then at 120°C for 2h. Finally, the suspension was heated to 500-800°C for 2h under argon conditions at a heating rate of 3°C / min. After natural cooling to room temperature, the product obtained was biochar. (2) Obtaining leachate containing organic pollutants: Organically polluted soil is leached with sodium dodecyl sulfate (SDS) to obtain leachate containing organic pollutants. The concentration of SDS is 0-7.5 g / L; the concentration of organic matter in the leachate is 0-10 mg / L. (3) Removal of organic pollutants: Biochar and persulfate oxidant PMS were added to the eluent containing organic pollutants to remove organic pollutants. The concentration of biochar was 0.1-1 g / L, the concentration of PMS was 1-10 mM, the pH of the system was controlled at 3-11, the reaction temperature was 5-35℃, and the treatment time of the eluent containing organic pollutants was 1-100 min.

2. The method for efficiently treating organic wastewater from leaching organically contaminated soil according to claim 1, characterized in that: In step (1), the walnut shells are dried and passed through a 100-mesh sieve; then heated to 600℃ and reacted for 2 hours.

3. The method for efficiently treating organic wastewater from leaching organically contaminated soil according to claim 1, characterized in that: In step (2), the SDS concentration is 5 g / L; the organic compound is naphthalene with a concentration of 10 mg / L.

4. The method for efficiently treating organic wastewater from leaching organically contaminated soil according to claim 1, characterized in that: In step (3), the concentration of biochar is 0.2-0.3 g / L; the concentration of PMS is 5 mM; and the reaction temperature is 5-20℃.

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