Preparation method and application of gradient modified biomass doped sludge biochar

Abstract

The application discloses a preparation method and application of gradient modified biomass doped sludge biochar, and belongs to the technical field of biochar preparation.The step one is that sludge is taken, dewatered and air-dried, and bamboo biomass impregnated with KOH is doped into the sludge and crushed and sieved; the step two is that the sieved particles are again impregnated with KOH and pyrolyzed by using a tube furnace; and the step three is that the biochar obtained by pyrolysis is ultrasonically impregnated with hydrochloric acid, repeatedly washed with pure water, and dried to obtain a final sample. The sludge and biomass are used as pyrolysis raw materials for resource utilization, the enrichment of heavy metals on the biochar is reduced in the pyrolysis process, and the biochar is synthesized with lower energy consumption; after being reused for five times, the adsorption efficiency is still 72%, the adsorption capacity only decreases by about 10 mg / g, and the biochar has a good market application prospect.

Description

Technical Field

[0001] This invention relates to the preparation of biomass-doped sludge biochar, and particularly to a method for preparing and applying gradient-modified biomass-doped sludge biochar, belonging to the field of biochar preparation technology. Background Technology

[0002] Sludge is a semi-solid or solid substance produced during wastewater treatment. my country's annual production of surplus dry sludge is 13.69 million tons, and it is projected to exceed 18 million tons by 2050. Currently, conventional methods for disposing of urban surplus sludge in my country include sludge incineration for thermal energy utilization, sludge anaerobic digestion to produce biogas, sludge pyrolysis for resource utilization, and sludge composting for fertilizer. Among these methods, the pyrolysis gas has a low calorific value, and heavy metals tend to accumulate in biochar during the pyrolysis process.

[0003] my country produces 1 billion tons of agricultural waste annually. Co-pyrolyzing agricultural waste biomass with sludge can not only synthesize biochar more quickly with lower energy consumption, but also increase carbon reserves, specific surface area, enrich pore structure, and reduce the accumulation of heavy metals, thus mitigating the harm of heavy metals to some extent in practical applications.

[0004] Therefore, while utilizing sludge and biomass through co-pyrolysis, the properties of biochar itself can be improved. The biochar prepared by gradient modification of biomass and sludge has more than three times the adsorption capacity of tetracycline hydrochloride compared with the original biochar, and has good application prospects. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to realize a method for modifying sludge-based biochar by KOH gradient through co-pyrolysis of sludge and bamboo, which is low in cost, easy to operate and has excellent performance.

[0006] The objective of this invention can be achieved by adopting the following technical solution:

[0007] A method for preparing gradient-modified biomass-doped sludge biochar includes the following steps:

[0008] Step 1: Take sludge, dehydrate and air dry it, mix bamboo biomass impregnated with KOH into the sludge and crush and screen it. The sludge is the residual dewatered sludge from the secondary sedimentation tank of an urban sewage treatment plant with a moisture content of 80-85%. After crushing, the bamboo biomass is screened, impregnated with KOH activator, dried, and then mixed with the sludge and crushed into granules.

[0009] Step 2: The sieved particles are impregnated with KOH again and then pyrolyzed in a tube furnace;

[0010] Step 3: After ultrasonically impregnating the biochar obtained by pyrolysis with hydrochloric acid, rinse it repeatedly with pure water until neutral, and dry it to obtain the final sample.

[0011] Preferably, in step one, the sludge is taken from the residual dewatered sludge in the secondary sedimentation tank of the sewage treatment plant and air-dried to a moisture content of 60-70%.

[0012] Preferably, in step two, the particles are immersed in KOH again using a forced-air drying oven and dried at a temperature not exceeding 105°C for 6 hours to obtain sludge particles of fixed particle size. These particles are then placed in a high-temperature tubular furnace for oxygen-limited high-temperature pyrolysis, and subsequently removed and cooled to room temperature.

[0013] Preferably, in step three, the sludge-based biochar cooled to room temperature is ultrasonically impregnated at a solid-liquid ratio of 1:10 for 30 minutes.

[0014] The beneficial technical effects of this invention are as follows: The method for preparing and applying gradient-modified biomass-doped sludge biochar according to this invention utilizes simple tools and reagents, and incorporates agricultural waste into the sludge as part of the modified biochar. This not only achieves resource utilization but also reduces the accumulation of heavy metals on the biochar and synthesizes biochar with lower energy consumption. It achieves the goal of treating urban waste sludge and expanding its resource utilization through an ecological method, while also providing superior performance. The modification method uses gradient modification. First, the biomass is modified with KOH and then doped into the sludge. The pulverized biomass-sludge mixture particles are then modified again with KOH. Finally, the carbonized product is ultrasonically impregnated with HCl. Through this gradient modification method, the prepared biochar exhibits a high adsorption effect on the target pollutant tetracycline hydrochloride, forming a dual gain. Simultaneously, it provides new ideas for the preparation, modification, and application of sludge-based biochar, and has broad market prospects. Attached Figure Description

[0015] Figure 1 This is a flowchart illustrating the preparation process of the gradient-modified biomass-doped sludge biochar of the present invention.

[0016] Figure 2 Bar chart showing the adsorption efficiency of different modification methods of the present invention;

[0017] Figure 3 Bar and line graphs showing the condition optimization experiments of five single factors for the modified biochar of the present invention, as well as the effect of dosage on adsorption efficiency and adsorption capacity.

[0018] Figure 4 The bar chart shows the adsorption efficiency and adsorption capacity of the modified biochar of the present invention under different pH conditions, and the line graph shows the change of Zeta potential on the surface of the biochar.

[0019] Figure 5 This is a bar chart showing the effect of different concentrations of coexisting cations on the adsorption efficiency of tetracycline hydrochloride according to the present invention.

[0020] Figure 6This is a bar chart showing the effect of different concentrations of coexisting anions on the adsorption efficiency of tetracycline hydrochloride according to the present invention.

[0021] Figure 7 The bar chart shows the effect of different temperatures and different initial concentrations on the adsorption efficiency of tetracycline hydrochloride according to the present invention.

[0022] Figure 8 The images are SEM topographic images of BC, BBC, K-HBBC and K-HBBC (TCH) of the present invention. Detailed Implementation

[0023] To enable those skilled in the art to understand the technical solution of the present invention more clearly, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0024] like Figure 1 As shown, the method for preparing gradient-modified biomass-doped sludge biochar provided in this embodiment includes dewatering sludge, particle sieving, using a modifying agent, drying, and pyrolysis to obtain sludge-bamboo co-pyrolysis KOH gradient modification; wherein the raw materials include urban surplus dewatered sludge with a moisture content of 80-85% and biomass bamboo.

[0025] The specific steps for preparing sludge granules are as follows:

[0026] (1) Spread the remaining dewatered sludge from the city on a stainless steel drying tray. After removing stones, plastics and other impurities, put the sludge into a mixer and stir slowly and air dry naturally to ensure that the moisture content of each part is uniform until the average moisture content of each part of the sludge is 60-70%.

[0027] (2) Bamboo biomass was pulverized using a pulverizer and passed through a 100-mesh sieve. A mixture of biomass and potassium hydroxide at a mass ratio of 1:0.5 (1 mol / L) and biomass and pure water at a solid-liquid ratio of 1:10 was placed in a shaking incubator and soaked at 100 r / min for 24 h. Afterward, it was placed in a 105℃ oven for 6 h. Then, it was mixed into sludge at a 10% addition ratio. The mixing method involved repeatedly kneading a portion of the biomass into the sludge by hand until its morphology was no longer visible. This process was repeated until all the biomass was incorporated, ensuring a relatively uniform distribution within the sludge. This method differs from the previous co-pyrolysis method of separately pulverizing and mixing the sludge and biomass. The mixture was then placed in a 105℃ oven for 6 h, pulverized again, and sieved through a 35-50 mesh sieve to maintain a uniform particle size.

[0028] (3) The sieved particles were placed in a shaking incubator at a mass ratio of 1:0.5 and a solid-liquid ratio of 1:10 for 24 hours at 100 r / min. Then, they were repeatedly rinsed with pure water until the particles settled naturally and remained neutral. After that, they were dried in an oven at 105℃ for 2 hours. The dried sample was placed in a covered boat and placed in a tube muffle furnace. Nitrogen gas was introduced for protection. The other end of the furnace was connected to a rubber hose and a 1 mol / L NaOH solution was introduced to absorb the tail gas. The temperature was increased from 20℃ to 700℃ at a rate of 10℃ / min. The sample was then pyrolyzed at this temperature for 60 minutes and then cooled. After the pyrolysis was completed, the sample was taken out and placed in a glass desiccator to cool to room temperature.

[0029] (4) The sample was ultrasonically impregnated with 3 mol / L hydrochloric acid solution at a solid-liquid ratio of 1:10 for 30 min, rinsed repeatedly with pure water until neutral, and finally dried in an oven at 105℃ for 2 h to obtain the final sample. It was then stored in a black-capped glass bottle for later use.

[0030] The drying and pyrolysis times are not fixed and depend on the drying temperature and the desired performance of the pyrolysis preparation. Generally, a higher drying temperature and lower residual moisture content result in a shorter drying time, and vice versa. However, once the mass of the dried sludge particles is observed to be constant, the moisture is considered completely dried, and drying can be stopped for storage or pyrolysis can proceed. Those skilled in the art can set certain time intervals to weigh the sludge particles during drying to confirm whether the mass is constant. In this embodiment of the invention, for the sake of consistency, the drying time is set to more than 24 hours, as 24 hours is generally sufficient to completely obtain dried sludge particles.

[0031] In this embodiment of the invention, after obtaining the dried sludge, it needs to be subjected to oxygen-limited high-temperature pyrolysis to obtain sludge-based biochar. There are no specific requirements for the pyrolysis conditions, so either a muffle furnace or a tubular furnace can be used.

[0032] Optionally, the pyrolysis temperature for preparing gradient-modified biomass-doped sludge biochar is 300-700℃.

[0033] In this embodiment of the invention, when preparing gradient-modified biomass-doped sludge biochar, the pyrolysis temperature is in the range of 300-700℃. The pyrolysis temperature can be any temperature within the range of 300-700℃, such as 300℃, 400℃, 500℃, 600℃, or 700℃. Generally speaking, the carbon content of the biochar after carbonization is related to the pyrolysis temperature. Those skilled in the art can select a suitable pyrolysis temperature according to the required carbon content and performance of the material. This invention does not impose specific limitations on this.

[0034] Optionally, the pyrolysis time for preparing gradient-modified biomass-doped sludge biochar is 60-180 min.

[0035] In this embodiment of the invention, when preparing gradient modified biomass-doped sludge biochar, the pyrolysis time is in the range of 60-180 min. The pyrolysis time can be 60 min, 90 min, 120 min, 150 min, 180 min, etc., any time within the range of 60-180 min. Generally speaking, the structure and properties of biochar after carbonization are related to the pyrolysis time. Those skilled in the art can select a suitable pyrolysis temperature as needed, and this invention does not impose specific limitations on this.

[0036] Optionally, the pyrolysis heating rate for preparing gradient-modified biomass-doped sludge biochar is 5-10℃ / min.

[0037] In this embodiment of the invention, when preparing gradient-modified biomass-doped sludge biochar, the pyrolysis heating rate is within the range of 5-10℃ / min. The pyrolysis heating rate can be any heating rate within the range of 5-10℃ / min, such as 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, etc. Generally speaking, the structure of the biochar after carbonization is related to the pyrolysis heating rate. Those skilled in the art can select a suitable heating rate as needed, and this invention does not impose specific limitations on this.

[0038] Optionally, when preparing gradient modified biomass-doped sludge biochar, under the premise that the solid-liquid ratio of sludge to pure water is 1:10, the mass ratio of sludge to potassium hydroxide is 1:0.5-1:2.5 (1-5 mol / L), and the hydrochloric acid concentration is 1-5 mol / L.

[0039] In this embodiment of the invention, when preparing gradient-modified biomass-doped sludge biochar, the mass ratio of sludge to potassium hydroxide is within the range of 1:0.5-1:2.5. The mass ratio can be 1:0.5 (1 mol / L), 1:1 (2 mol / L), 1:1.5 (3 mol / L), 1:2 (4 mol / L), or 1:2.5 (5 mol / L), and the hydrochloric acid concentration can be 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, or 5 mol / L, etc. Generally, the surface functional group content of the prepared gradient-modified biomass-doped sludge biochar is related to the KOH and hydrochloric acid modification concentrations. Those skilled in the art can select appropriate KOH and hydrochloric acid modification solution concentrations as needed, and this invention does not impose specific limitations on this.

[0040] Optionally, when preparing gradient-modified biomass-doped sludge biochar, the impregnation time is 12-24 hours.

[0041] In this embodiment of the invention, when preparing gradient-modified biomass-doped sludge biochar, the impregnation time is within the range of 12-24 hours. The time can be any impregnation time within the range of 12-24 hours, such as 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, or 24h. Generally speaking, the pore structure of the prepared gradient-modified biomass-doped sludge biochar is related to the impregnation time. Those skilled in the art can select an appropriate impregnation time as needed, and this invention does not impose specific limitations on this.

[0042] Optionally, in the preparation of gradient modified biomass-doped sludge biochar, the proportion of biomass added is 2.5%-20%.

[0043] In this embodiment of the invention, when preparing gradient-modified biomass-doped sludge biochar, the proportion of biomass added is in the range of 2.5%-20%, which can be 2.5%, 5%, 10%, 15%, 20%, etc. Generally speaking, the addition of biomass will affect the carbon storage and specific surface area of ​​the biochar. Those skilled in the art can select an appropriate impregnation time as needed, and this invention does not impose specific limitations on this.

[0044] Furthermore, to demonstrate the feasibility of gradient modification in this invention, three modifying reagents of the same concentration—NaOH, KOH, and NaHCO3—were used in the embodiments of this invention. Two modification methods were employed for each corresponding reagent. The first method involved directly modifying the unmodified biomass with KOH by mixing it into the sludge. The second method involved first modifying the biomass with KOH, then mixing the modified biomass into the sludge, pulverizing and sieving it, and finally modifying it again with KOH (gradient modification). The three reagents and modification methods were designated as Na-HBBC-T (gradient modification), Na-HBBC-O (direct modification), K-HBBC-T (gradient modification), K-HBBC-O (direct modification), C-HBBC-T (gradient modification), and C-HBBC-O (direct modification), respectively. The optimal modified sludge-based biochar was screened out by pyrolysis at 700°C. The results are as follows:

[0045] Table 1 Effect of different modification methods on the adsorption efficiency of tetracycline hydrochloride

[0046]

[0047] K-HBBC-T was chosen as the final modification reagent method.

[0048] The optimal modification method was determined, and subsequent experiments used a particle size of 0.355 mm-0.5 mm and a tetracycline hydrochloride concentration of 50 mg / L.

[0049] The preparation was optimized using a design of five groups of single-factor variables, as detailed below:

[0050] The study investigated the effect of biochar prepared using five biomass additives (2.5%-20%) at a 1:1 (sludge: potassium hydroxide) impregnation mass ratio, continuous pyrolysis at 500℃ for 60 min, and ultrasonic impregnation with 1 mol / L HCl on the adsorption efficiency of TCH.

[0051] The study investigated the effect of biochar prepared by ultrasonic impregnation with 1 mol / L HCl on TCH adsorption efficiency under five pyrolysis conditions (300℃-700℃) with a biomass addition ratio of 10%, a 1:1 impregnation mass ratio, continuous pyrolysis for 60 min at five different pyrolysis temperatures (300℃-700℃).

[0052] This study investigated the effect of biochar prepared by ultrasonic impregnation with 10% biomass, a 1:1 impregnation mass ratio, five different pyrolysis times (60 min-180 min) at 700℃, and 1 mol / L HCl on TCH adsorption efficiency.

[0053] The study investigated the effect of biochar prepared by adding 10% biomass, using five impregnation mass ratios (1:0.5, 1:1, 1:1.5, 1:2, 1:2.5), continuous pyrolysis at 700℃ for 60 min, and ultrasonic impregnation with 1 mol / L HCl on the adsorption efficiency of TCH.

[0054] This study investigated the effect of biochar prepared by ultrasonic impregnation with five concentrations (1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, and 5 mol / L) of HCl at a biomass addition ratio of 10%, an impregnation mass ratio of 1:1, continuous pyrolysis at 700℃ for 60 min, and ultrasonic impregnation with HCl.

[0055] like Figure 3 The final optimized preparation conditions were determined to be: 10% biomass addition, continuous pyrolysis at 700℃ for 60 min, impregnation mass ratio of 1:0.5 (sludge: potassium hydroxide), and ultrasonic impregnation with 3 mol / L HCl.

[0056] The effect of dosage on adsorption efficiency and adsorption capacity was investigated using dosages ranging from 0.1 to 1.25 g / L, specifically 0.1 g / L, 0.25 g / L, 0.5 g / L, 0.75 g / L, 1.0 g / L, and 1.25 g / L. The results are as follows:

[0057] Table 2. Effects of different dosages on adsorption efficiency and adsorption capacity of tetracycline hydrochloride

[0058]

[0059] From Table 2 and Figure 3As shown in f, the dosage significantly affects the adsorption efficiency and capacity of TCH. When the adsorbent dosage gradually increases from 0.1 g / L to 1.25 g / L, the TCH removal rate increases from 15.37% to 92.87%, while the adsorption capacity drops sharply from 86.65 mg / g to 42.38 mg / g. With increasing dosage, the adsorption efficiency gradually increases, while the adsorption capacity shows a negative correlation. The higher removal efficiency may be due to the increase in functional groups and effective contact area on the biochar surface, leading to an increase in active sites. The decrease in TCH adsorption capacity with increasing biochar dosage is due to the overlap of active sites reducing the effective contact area and increasing the TCH diffusion path. When the dosage increases from 0.75 g / L to 1.0 g / L, the rate of increase in TCH adsorption efficiency decreases. Therefore, considering the economic cost of biochar in practical applications, the subsequent biochar dosage is 0.75 g / L.

[0060] The effects of different pH values ​​on the adsorption efficiency and adsorption capacity of the prepared gradient-modified biomass-doped sludge biochar were investigated. The results at pH values ​​of 3, 4, 5, 6, 7, 8, 9, 10, and 11 are as follows:

[0061] Table 3. Effects of different pH values ​​on the adsorption efficiency and adsorption capacity of tetracycline hydrochloride.

[0062]

[0063] Tetracycline antibiotics have three ionization equilibrium constants under different pH conditions, exhibiting four ionization forms: positively charged at pH < 3.3, simultaneously positively and negatively charged at pH 3.3 < pH < 7.7, and negatively charged at pH > 7.7. At pH 3, both the biochar surface and tetracycline are positively charged, resulting in repulsion and low adsorption efficiency. Within the pH range of 3 to 7, both biochar and tetracycline exist in simultaneously positively and negatively charged forms, with little impact on adsorption efficiency. At pH > 8, both the biochar surface and tetracycline exist in negatively charged forms, resulting in repulsion and low adsorption efficiency. The prepared gradient-modified biomass-doped sludge biochar showed no significant change in adsorption efficiency under different pH conditions, demonstrating good adaptability to actual wastewater treatment.

[0064] In practical applications, interference from coexisting ions exists. Therefore, the effects of different concentrations of typical cations and anions on the adsorption efficiency of tetracycline hydrochloride were investigated. The reagents selected for cations were sodium chloride (NaCl), potassium chloride (KCl), ammonium chloride (NH4Cl), and calcium chloride (CaCl2), while the reagents selected for anions were sodium chloride (NaCl), sodium carbonate (Na2CO3), sodium dihydrogen phosphate (NaH2PO4), and sodium sulfate (Na2SO4).

[0065] Table 4. Effect of different concentrations of coexisting cations on the adsorption efficiency (%) of tetracycline hydrochloride

[0066]

[0067] Table 5. Effect of different concentrations of coexisting anions on the adsorption efficiency (%) of tetracycline hydrochloride

[0068]

[0069]

[0070] The presence of varying concentrations of saline ions and metal ions in actual wastewater affects the adsorption capacity of the prepared K-HBBC biochar. Therefore, the adsorption capacity of typical cations (K+) at different concentrations (0-1 mol / L) was investigated. + Na + NH4 + Ca 2+ ) and anions (Cl) - CO3 2- SO4 2- H2PO4 - The effect of K-HBBC on TCH adsorption. It can be seen that K-HBBC has a specific inhibitory effect on TCH adsorption. For example... Figure 5 cation K + Na + and NH4 + The added Ca had little effect on the adsorption efficiency of TCH. 2+ The adsorption efficiency of TCH decreased significantly with increasing concentration, which may be due to Ca 2+ It reacts with the functional groups on the surface of biochar, occupying active adsorption sites and hindering the adsorption of TCH. For example... Figure 6 anion Cl - and SO4 2- It had no significant effect on the adsorption efficiency of TCH, while H2PO4... - and CO3 2- The presence of certain substances causes a shift in the solution towards acidity or alkalinity, and this shift becomes more pronounced with increasing concentration, thus affecting the adsorption efficiency of TCH. This confirms the influence of different pH values ​​on TCH adsorption efficiency. In summary, some coexisting anions and cations can affect TCH adsorption, but gradient-modified biomass-doped sludge biochar exhibits good resistance to interference.

[0071] Table 6. Effects of different temperatures and initial concentrations on the adsorption efficiency (%) of tetracycline hydrochloride

[0072]

[0073] As shown in Table 6, the adsorption efficiency of the prepared gradient modified biomass-doped sludge biochar significantly increased with increasing temperature when adsorbing tetracycline hydrochloride, and it had a better adsorption effect on low concentrations of tetracycline hydrochloride.

[0074] The specific surface area and pore size of the prepared gradient modified biomass-doped sludge biochar were analyzed.

[0075] Table 7. Specific surface area, average pore size, and pore volume of different biochars

[0076]

[0077] The specific surface area of ​​BBC biochar with added biomass was slightly increased compared to the original BC biochar, indicating that the addition of biomass can increase the carbon storage and improve the pore structure of biochar. The specific surface area of ​​K-HBBC modified biochar was more than three times that of the original BC biochar, demonstrating the feasibility of this modification method. Figure 8 Scanning electron microscopy (SEM) analysis was performed on BC, BBC, K-HBBC, and K-HBBC(TCH) biochar prepared by pyrolysis at 700℃. Compared with BC, the surface roughness of BBC was slightly increased, but it was difficult to observe a significant porous structure. In contrast, the surface of K-HBBC showed significant corrosion, with numerous pits found on the rough pore walls, indicating that the porous structure was formed after KOH activation. Many fine particles were observed on the surface and inside of K-HBBC(TCH), indicating that TCH was successfully adsorbed by the gradient-modified biomass-doped sludge biochar.

[0078] The changes in adsorption effect of the prepared gradient-modified biomass-doped sludge biochar after several recycling cycles were investigated.

[0079] Table 8. Changes in adsorption efficiency and adsorption capacity after five cycles of biochar recycling.

[0080]

[0081] The stability and reusability of adsorbents are important factors in evaluating their performance and are significant in practical applications due to their impact on economic costs. The prepared gradient-modified biomass-doped sludge biochar showed a decrease in TCH (50 mg / L) adsorption efficiency from 86.79% to 71.56% after five reuses, but still maintained a large adsorption capacity (48.39 mg / g), thus demonstrating its good practical application potential.

[0082] The above description is merely a further embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope disclosed in the present invention, based on the technical solution and concept of the present invention, shall fall within the scope of protection of the present invention.

Claims

1. A method for preparing gradient-modified biomass-doped sludge biochar, characterized in that, Includes the following steps: Step 1: Take sludge, dehydrate and air dry it, then mix the bamboo biomass soaked in KOH into the sludge and crush and sieve it. Step 2: The sieved particles are impregnated with KOH again and then pyrolyzed in a tube furnace; Step 3: After ultrasonically impregnating the biochar obtained by pyrolysis with hydrochloric acid, rinse it repeatedly with pure water and dry it to obtain the final sample; In step one, when preparing gradient modified biomass-doped sludge biochar, the proportion of bamboo biomass added is 2.5% to 20%. In steps one and two, when preparing gradient-modified biomass-doped sludge biochar, the mass ratio of sludge to potassium hydroxide is 1:0.5 to 1:2.5; in steps one and two, when preparing gradient-modified biomass-doped sludge biochar, the impregnation time is 12 to 24 hours; in step three, when preparing gradient-modified biomass-doped sludge biochar, the hydrochloric acid concentration is 1 to 5 mol / L.

2. The method for preparing gradient-modified biomass-doped sludge biochar according to claim 1, characterized in that, In step one, the sludge is taken from the residual dewatered sludge in the secondary sedimentation tank of the sewage treatment plant and air-dried to a moisture content of 60 to 70%.

3. The method for preparing gradient-modified biomass-doped sludge biochar according to claim 1, characterized in that, In step two, the particles are immersed in KOH again using a forced-air drying oven and dried at a temperature not exceeding 105°C for 6 hours to obtain sludge particles with a fixed particle size. These particles are then placed in a high-temperature tubular furnace for oxygen-limited high-temperature pyrolysis, and subsequently removed and cooled to room temperature.

4. The method for preparing gradient-modified biomass-doped sludge biochar according to claim 1, characterized in that, In step three, the sludge-based biochar cooled to room temperature is ultrasonically impregnated at a solid-liquid ratio of 1:10.

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