Preparation of multi-layered time-released composite carbon source and its application in wastewater treatment

Abstract

This invention discloses the preparation of a multilayer time-release composite carbon source and its application in wastewater treatment, belonging to the field of wastewater treatment technology. The method includes the following steps: S1, preparing iron-nitrogen co-doped biochar using steel pickling sludge as raw material, melting and blending it with polycaprolactone and corn cob powder, then extruding and spheroidizing it to obtain core particles; S2, coating the surface of the core particles with a sodium alginate-chitosan composite adhesive containing maltose, and then subjecting it to Ca... 2+ Pre-crosslinking and Fe 3+ Secondary cross-linking forms a pH-responsive hydrogel intermediate layer; S3, glucose is covalently grafted onto the surface of the intermediate layer via Maillard reaction to form an outer layer, and dried to obtain the finished product; This invention achieves precise matching between carbon source release and microbial metabolic needs through a three-layer sequential release structure and a dual-signal triggering mechanism, and has the functions of promoting electron transfer, microbial attachment, and phosphorus removal after carbon source depletion, and has broad application prospects in the field of deep denitrification of wastewater.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology and relates to the preparation of multilayer time-release composite carbon sources and their application in wastewater treatment. Background Technology

[0002] With the increasing severity of eutrophication in water bodies, the requirements for total nitrogen in wastewater discharge standards are constantly being raised. Currently, the most widely used biological nitrogen removal technology is denitrification, which works by denitrifying bacteria using organic carbon sources as electron donors to reduce nitrate and nitrite nitrogen into nitrogen gas under anaerobic conditions. However, urban wastewater and some industrial wastewater in my country generally suffer from low carbon-to-nitrogen ratios, and insufficient organic carbon sources severely limit denitrification efficiency. Therefore, adding external carbon sources to the anoxic stage has become an important technical measure to ensure that the total nitrogen in effluent consistently meets standards.

[0003] While traditional liquid carbon sources offer rapid carbon release and quick denitrification start-up, they also suffer from drawbacks such as difficulty in precisely controlling dosage, susceptibility to COD rebound in effluent, high transportation and storage costs, and significant safety risks. In recent years, solid slow-release carbon sources have become a research hotspot due to their advantages, including continuous in-situ carbon release, no need for online dosing equipment, and high carbon source utilization.

[0004] Solid slow-release carbon sources are typically composed of biodegradable polymers and natural biomass materials. They leverage the complementary properties of the stable degradation characteristics of synthetic polymers and the enzymatic hydrolysis characteristics of natural biomass to construct a multi-stage carbon source release system. Chinese patent CN121554109A discloses a composite slow-release carbon source and its preparation method, comprising a three-layer structure design: a rapid-start layer, a stable maintenance layer, and a long-term protection layer. The outer layer is a fast-acting carbon source, the middle layer is agricultural waste powder and biodegradable polymer, and the core is a mixture of lignin and the fast-acting carbon source, achieving sequential carbon source release. However, the coating of the middle layer in this scheme uses a physical method, alternating spraying of binder and coating powder. The uniformity and thickness of the coating layer are difficult to control precisely, leading to a mismatch risk between carbon source release and the actual metabolic needs of microorganisms. In practical applications, the fast-acting layer may be depleted before the slow-release layer is fully activated, resulting in denitrification stagnation and nitrite accumulation.

[0005] To address the mismatch between carbon source release and microbial demand, intelligent carbon sources with environmental responsiveness are gaining increasing attention. Chinese patent CN121470706A discloses an intelligent responsive filter media and its preparation method. Its coating layer uses a pH-responsive polymer, automatically adjusting the carbon source release rate based on pH changes caused by denitrification, achieving intelligent on-demand carbon source supply. This solution determines whether to release the carbon source solely based on system pH changes. However, in actual wastewater treatment plant operation, influent pH fluctuates frequently and significantly due to various factors such as industrial emissions and rainfall runoff. A single pH response mechanism is highly susceptible to false triggering due to influent pH fluctuations, prematurely releasing carbon sources when microorganisms do not require them, resulting in carbon source waste and excessive COD in the effluent.

[0006] Therefore, how to achieve precise matching between carbon source release and microbial metabolic needs in a multi-layered time-series release structure, and improve the accuracy of environmental response of carbon source release, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to provide a multilayer time-release composite carbon source for preparation and its application in wastewater treatment. By constructing a three-layer time-release structure consisting of a steel pickling sludge-based iron-nitrogen co-doped biochar composite core, a sodium alginate-chitosan pH-responsive hydrogel intermediate layer, and a glucose covalently grafted outer layer, the outer layer glucose rapidly initiates denitrification, the intermediate layer accurately responds and releases maltose under dual signal conditions of glucose depletion and pH increase, and the core continuously provides a long-lasting carbon source and performs electron transfer and end-of-pipe adsorption functions. This solves the technical problems in the prior art, such as the misalignment between carbon source release and microbial metabolic needs, the susceptibility to false triggering of the single pH response mechanism, and the limited function of solid carbon sources. This improves the denitrification efficiency, carbon source utilization rate, and effluent quality stability of the denitrification system.

[0008] The technical solution adopted in this invention is a method for preparing a multilayer time-release composite carbon source, which mainly includes the following steps: S1. Iron-nitrogen co-doped biochar, polycaprolactone and corn cob powder are melted, blended and then extruded and rolled into spherical shapes to obtain core particles; the above-mentioned iron-nitrogen co-doped biochar is a porous magnetic carbon material obtained by high-temperature pyrolysis after acid washing to remove impurities, ammonium chloride nitrogen doping and potassium hydroxide activation. S2. A sodium alginate-chitosan composite solution and a maltose solution are mixed to obtain a maltose-containing composite solution. This maltose-containing composite solution is then coated onto the surface of the core particles to obtain core particles coated with the composite solution. The core particles are then subjected to Ca... 2+ Pre-crosslinking and Fe 3+ Secondary cross-linking forms a composite hydrogel intermediate layer encapsulating maltose, resulting in intermediate layer coated particles; S3. Glucose is covalently grafted onto the surface of the intermediate layer coated particles via Maillard reaction to form a glucose outer layer, which is then dried to obtain carbon source particles.

[0009] Furthermore, the preparation method of the iron-nitrogen co-doped biochar in step S1 is as follows: Using steel pickling sludge as an iron and carbon source, the steel pickling sludge is washed with water, pickled, dried, and pulverized to obtain dry sludge powder. The above-mentioned dry sludge powder, ammonium chloride and potassium hydroxide are mixed at a mass ratio of 40:(1~3):(40~120), water is added to make a paste, and after drying, the mixture is pyrolyzed at 650℃~850℃ for 1h~3h under nitrogen protection by heating at 8℃ / min~12℃ / min. After acid washing, water washing and drying, iron-nitrogen co-doped biochar is obtained.

[0010] Specifically, during the process of adding water to form a paste, the amount of water used is 20% to 40% of the total mass of dry sludge powder, ammonium chloride, and potassium hydroxide.

[0011] Further, in step S1, the specific operation of the above melt blending and molding is as follows: polycaprolactone and corn cob powder are melt blended at a mass ratio of 1:(0.6-1) at 120℃-150℃ and 80r / min-120r / min for 15min-30min, and then the above iron-nitrogen co-doped biochar is added so that the mass ratio of iron-nitrogen co-doped biochar to polycaprolactone is (4-8):1, and the blending is continued for 10min-20min to obtain a composite melt material; the composite melt material is extruded and spheroidized, and dried to obtain core particles.

[0012] Furthermore, in step S2, the sodium alginate-chitosan composite solution is obtained by mixing and stirring a sodium alginate base solution and a chitosan acetic acid solution at a mass ratio of 1:(0.2-0.6); the sodium alginate base solution has a sodium alginate mass concentration of 15 g / L to 25 g / L; the chitosan acetic acid solution is obtained by dissolving chitosan in an acetic acid aqueous solution with a volume concentration of 1% to 2% to obtain a chitosan acetic acid solution with a chitosan mass concentration of 5 g / L to 15 g / L, and then adjusting the pH to 5.0 to 5.5.

[0013] Furthermore, in step S2, the maltose-containing composite adhesive is obtained by mixing a maltose solution with a sodium alginate-chitosan composite adhesive at a mass ratio of (30-60):100, and the mass concentration of the maltose solution is 100g / L to 200g / L.

[0014] Furthermore, in step S2, Ca 2+ Pre-crosslinking and Fe 3+The specific operation of secondary crosslinking is as follows: the core particles coated with the composite adhesive are placed in a calcium chloride solution with a mass concentration of 15 g / L to 25 g / L for crosslinking for 30 min to 60 min, washed, and then immersed in a ferric chloride solution with a mass concentration of 10 g / L to 20 g / L for crosslinking for 2 h to 6 h. After washing, the intermediate layer coated particles are obtained; the above Fe 3+ Secondary cross-linking forms a COO--Fe3+ and NH2-Fe3+ coordination network within the coating layer, which makes the intermediate layer of the above-mentioned composite hydrogel stable when pH≤7.0, and disintegrates and releases maltose when pH rises to 7.5 and above.

[0015] It needs to be explained that the specific operation of the Maillard reaction covalent grafting is as follows: the intermediate layer coated particles are mixed with glucose solution at a mass ratio of 1:(1~2), and reacted at 90℃~120℃ for 2h~6h. After the reaction is completed, the mixture is washed with deionized water and dried in a ventilated environment at 35℃~40℃ for 2h~4h to obtain carbon source particles.

[0016] Specifically, the glucose concentration in the above glucose solution is 100g / L to 200g / L, and the pH is 4.0 to 5.0.

[0017] The key to the application of the multilayer time-release composite carbon source prepared by the above method in wastewater treatment lies in the fact that when the glucose concentration in the outer layer decreases to 50 mg / L or below and the pH of the system increases to 7.5 or above, the intermediate layer of the composite hydrogel is triggered to disintegrate and release maltose to continue the denitrification process; the polycaprolactone and corn cob powder in the core particles degrade, providing a long-lasting carbon source for denitrification; after the carbon source is released, the remaining iron and nitrogen co-doped biochar framework adsorbs phosphate and COD in the water.

[0018] Compared with the prior art, the present invention has the following advantages: First, this invention addresses the core technical problems of existing solid slow-release carbon sources, such as mismatch between carbon source release and microbial metabolic needs, simplistic and easily mistriggered environmental response mechanisms, limited functionality, and questionable safety. It innovatively constructs a composite core of iron and nitrogen co-doped biochar based on steel pickling sludge, containing Fe... 3+ A three-layer time-release structure consisting of a cross-linked sodium alginate-chitosan pH-responsive hydrogel intermediate layer and a glucose Maillard-grafted outer layer. Unlike existing technologies that use physically coated or single pH-responsive layers for sustained-release carbon sources, this invention is the first to introduce a dual-signal AND gate logic based on low glucose concentration and increasing pH into the carbon source release regulation system, through Fe... 3+ Secondary cross-linking forms COO within the intermediate layer. - -Fe 3+ and NH2- Fe 3+The coordination network ensures that the intermediate layer only disintegrates and releases maltose when the glucose concentration drops below 50 mg / L and the system pH rises above 7.5, fundamentally avoiding the false release problem caused by fluctuations in influent water quality in a single pH response mechanism. The dual-signal response verification experiment of this invention shows that sample 1 achieves a maltose release rate of over 65% under conditions where both signals are satisfied, while the release rate in the control group meeting only one condition is less than 15%, exhibiting typical on / off jump characteristics, proving the accuracy and reliability of this triggering mechanism.

[0019] Secondly, this invention uses iron and nitrogen co-doped biochar as raw material from steel pickling sludge, achieving high-value resource utilization of hazardous waste. Steel pickling sludge contains Cr... 6+ and F - Untreated, it is considered hazardous waste. This invention utilizes a high-temperature pyrolysis process, through the synergistic effect of carbothermic reduction and KOH activation, to transform Cr... 6+ Reduced to low-toxicity Cr 3+ The fluoride forms stable mineral crystals with iron, and undergoes significant volatilization and stabilization after high-temperature treatment. The leaching toxicity of the resulting iron-nitrogen co-doped biochar is far below the limit of GB 5085.3-2007 standard. Compared with existing technologies that use pure chemical reagents or municipal sludge to prepare iron-carbon materials, this invention not only eliminates the cost of adding an external iron source but also opens up a new resource utilization path for steel pickling sludge. Safety evaluation results show that even with relatively high leaching toxicity of the iron-nitrogen co-doped biochar intermediate, the leaching toxicity of the finished particles is further reduced after melt blending and embedding with polycaprolactone and corn cob powder, and after multi-layer physical isolation. Under standard operating conditions, the effluent quality meets the GB8978-1996 Class I standard, achieving environmental safety and controllability throughout the entire life cycle from raw materials to application.

[0020] Third, this invention endows the composite carbon source particles with multiple synergistic functions by introducing iron-nitrogen co-doped biochar into the core. The high specific surface area and porous structure of the iron-nitrogen co-doped biochar provide an ideal carbon source reservoir for polycaprolactone and corn cob powder, and regulate the carbon source release rate through physical adsorption and spatial confinement effects, making the carbon source supply curve smoother; Fe 3+ / Fe 2+ The redox cycle acts as an electron shuttle, accelerating nitrate reduction and nitrite conversion. In the denitrification efficiency test, the nitrate removal rate of samples 1–5 was over 98% after 72 hours, and the peak nitrite concentration was below 3.0 mg / L, with all nitrites eliminated after 48 hours. The porous framework provides abundant attachment sites for microorganisms, and Fe... 3+ / Fe 2+The circulation promotes the secretion of extracellular polymers, which is beneficial to the rapid formation and stability of biofilms. After the carbon source is released, the remaining iron-carbon skeleton continues to remove residual phosphate and COD from the water through ligand exchange and electrostatic adsorption, achieving a phosphate removal rate of 71.7%. This realizes a functional transformation from carbon source supply to end-of-pipe adsorption. Compared with the shortcomings of existing solid carbon sources with single functions, this invention achieves the integration and progressive utilization of four functions: carbon source storage, electron shuttle, microbial carrier, and end-of-pipe adsorption.

[0021] Fourth, this invention uses the Maillard reaction to covalently graft glucose onto the surface of the intermediate layer to form an outer layer. Compared with physical coating methods, this avoids the initial explosive release problem caused by the rapid dissolution of glucose upon contact with water. The glucose outer layer is released steadily within 0-4 hours after addition, providing an immediate carbon source for denitrification. Its rapid consumption also provides a signal condition for glucose depletion to trigger the subsequent pH response. The introduction of chitosan not only participates in Fe... 3+ The formation of the cross-linked coordination network also provides free amino groups for glucose Maillard grafting, ensuring the chemical bonding between the outer and middle layers. Control experiments show that the absence of chitosan reduces the density of the middle layer structure, increases passive maltose leakage, and significantly reduces the dual-signal triggering efficiency, demonstrating that chitosan plays a crucial role in maintaining the structural integrity of the middle layer and the grafting efficiency of the outer layer.

[0022] Fifth, in the core of this invention, polycaprolactone is compounded with corn cob powder, leveraging the stable degradation characteristics of the synthetic polymer and the enzymatic hydrolysis characteristics of natural biomass to form a complementary relationship. Polycaprolactone continuously releases small-molecule organic acids through ester bond hydrolysis, while corn cob powder is gradually hydrolyzed into soluble sugars under the action of cellulase. Together, they provide a long-lasting carbon source for denitrification for more than 72 hours, avoiding the problem of insufficient supply in the later stages of a single carbon source. After the carbon source is released, the remaining iron-carbon skeleton can still continue to remove phosphates through ligand exchange and electrostatic adsorption, achieving functional continuity from carbon source supply to end-stage adsorption and phosphorus removal.

[0023] In summary, this invention, through the synergistic design of a three-layer time-release structure, a dual-signal intelligent response intermediate layer, iron-nitrogen co-doped biochar based on steel pickling sludge, and a glucose Maillard-grafted outer layer, systematically solves a series of technical problems in existing solid slow-release carbon sources, such as the mismatch between carbon source release and microbial metabolic needs, the simplistic and easily mistriggered response mechanism, limited functionality, and questionable safety. The prepared multi-layer time-release composite carbon source particles possess comprehensive advantages, including precise and controllable carbon source release, high denitrification efficiency, low risk of nitrite accumulation, excellent microbial adhesion performance, and the ability to continue phosphorus removal and magnetic separation recovery after carbon source depletion. It has broad application prospects in areas such as deep denitrification in urban wastewater treatment plants, denitrification treatment of industrial wastewater, and in-situ remediation of polluted water bodies. Attached Figure Description

[0024] Figure 1This is a curve showing the change in nitrate removal rate over time.

[0025] Figure 2 This is a curve showing the change in nitrite concentration over time. Detailed Implementation

[0026] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0027] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0028] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0029] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0030] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0031] Unless otherwise specified in the examples, the standard conditions shall apply. Unless otherwise specified, the reagents or instruments used are all commercially available products.

[0032] Steel pickling sludge is a solid waste generated during the pickling process of steel surfaces. The quality standards for the steel pickling sludge used in this invention include: total iron content (calculated as Fe, determined by referring to HG / T 5811-2020 "Method for Determination of Iron Content in Iron-Containing Chemical Sludge") of 40%–55%, moisture content of 45%–60%, and impurity content ≤10%. Impurities include, but are not limited to, valuable metal elements such as Ni and Cr, as well as impurities such as CaF2, CaO, SiO2, and CaSO4. It should be noted that steel pickling sludge contains Cr... 6+ and F - Its leaching toxicity (refer to GB 5085.3-2007 "Identification Standard for Hazardous Waste: Leaching Toxicity Identification") usually exceeds the standard for hazardous waste and is generally classified as hazardous waste.

[0033] Polycaprolactone is a commercially available product with an average weight-average molecular weight of 50 kDa to 80 kDa.

[0034] Sodium alginate is a commercially available product with a viscosity range of 200 mPa·s to 500 mPa·s.

[0035] Chitosan is a commercially available product with a degree of deacetylation ≥85%, a viscosity of 100 mPa·s to 500 mPa·s in 1% acetic acid solution, and an average weight-average molecular weight of 100 kDa to 300 kDa.

[0036] Corn cob powder is a natural agricultural byproduct. Before use, it is screened to remove stones and moldy particles, washed with water to remove surface dust and impurities, dried to a moisture content of ≤12%, and then mechanically crushed and passed through a 100-mesh sieve. After the average particle size is measured to be ≤150μm, it meets the usage standards.

[0037] Example 1 The quality standards of the steel pickling sludge used in this embodiment are as follows: total iron content 52.7%, water content 45.2%, impurity content 2.1%; polycaprolactone with a weight-average molecular weight of 70 kDa; sodium alginate viscosity 350 mPa·s; corn cob powder with a water content of 9.6% and an average particle size of 128.7 μm; chitosan with a degree of deacetylation of 95%, a viscosity of 250 mPa·s in 1% acetic acid solution, and an average weight-average molecular weight of 150 kDa.

[0038] In this embodiment, a multilayer time-release composite carbon source is prepared. The specific preparation process is as follows: S1. Preparation of iron-nitrogen co-doped biochar core: S11, Preparation of iron-nitrogen co-doped biochar: Mix steel pickling sludge with an equal mass of deionized water, wash at 100 r / min for 8 min, let stand, pour off the supernatant, and repeat 3 times. The sludge was soaked in 1.5 mol / L dilute hydrochloric acid at 25℃, and then acid-washed at 150 r / min. The mass ratio of dilute hydrochloric acid to acid-washed sludge was 1.5:1. The sludge was then filtered and washed with deionized water until the pH reached 7.0. The acid-washed sludge was dried in an oven at 105℃ for 18 hours, then pulverized and passed through a 100-mesh sieve to obtain dry sludge powder. Mix dry sludge powder, ammonium chloride, and potassium hydroxide in a mass ratio of 40:2:80, and add deionized water to adjust the material to a paste-like consistency. The amount of deionized water used is 30% of the total mass of the dry material. Mix at 100 r / min for 45 min, and dry in an oven at 95 ℃ for 18 h; The material is then transferred to a tube furnace and pyrolyzed at 750°C for 2 hours under nitrogen protection at a rate of 10°C / min. After pyrolysis, the sample was allowed to cool naturally to room temperature, washed four times with 0.15 mol / L dilute hydrochloric acid, then washed with deionized water until neutral, and dried in an oven at 105℃ for 18 h to obtain iron-nitrogen co-doped biochar, which was designated as biochar sample 1.

[0039] Take biochar sample 1 and prepare leachate according to HJ / T 299-2007 "Leaching Toxicity of Solid Waste - Leaching Method - Sulfuric Acid and Nitric Acid Method". Detect the concentration of hazardous components in the leachate according to GB 5085.3-2007 "Identification Standard for Hazardous Waste - Leaching Toxicity Identification". The leachate should contain a total chromium concentration of <15 mg / L, a hexavalent chromium concentration of <5 mg / L, and an inorganic fluoride concentration of <100 mg / L.

[0040] In this embodiment, the leachate of biochar sample 1 contained a total chromium concentration of 3.2 mg / L, a hexavalent chromium concentration of 0.8 mg / L, and an inorganic fluoride concentration of 28 mg / L. This is because the chromium content in the steel pickling sludge was relatively high. 6+ Under high-temperature pyrolysis and carbothermic reduction, it has been fully reduced to low-toxicity Cr. 3+ It forms stable mineral crystals with iron, and the fluoride volatilizes and stabilizes significantly after high-temperature treatment. The resulting iron-nitrogen co-doped biochar is no longer considered hazardous waste, meets safety and environmental protection requirements, and can be safely used as a carbon source carrier for denitrification in wastewater treatment.

[0041] S12, Melt blending and molding of composite core particles Polycaprolactone and corn cob powder were mixed at a mass ratio of 1:0.8 and melt-blended at 135℃ and 100 r / min for 22 min to obtain a melt mixture. Biochar sample 1 and polycaprolactone were added to the above molten mixture at a mass ratio of 6:1. The mixture was then continuously melted and mixed for 15 minutes while maintaining the rotation speed to ensure that biochar sample 1 was uniformly dispersed, thus obtaining iron-nitrogen co-doped biochar composite molten material. The iron-nitrogen co-doped biochar composite molten material was transferred to an extrusion spheroidizer. It was first extruded into strips through a 1.0 mm sieve at a speed of 400 r / min, and then spheroidized at a speed of 1000 r / min for 4 min to obtain spherical core particles. After being dried under vacuum at 38℃ and -0.085 MPa for 36 h, the iron-nitrogen co-doped biochar core particles were obtained.

[0042] S2. Preparation of the intermediate layer of sodium alginate-chitosan composite hydrogel and maltose encapsulation S21. Preparation of maltose-containing composite adhesive solution Sodium alginate was dissolved in deionized water to prepare a sodium alginate basic solution with a mass concentration of 20 g / L. Chitosan was dissolved in a 1.5% (v / v) aqueous acetic acid solution to prepare a chitosan acetic acid solution with a chitosan mass concentration of 10 g / L. After stirring until completely dissolved, the pH was adjusted to 5.2 with a 10% (v / v) sodium hydroxide solution. Maltose was dissolved in deionized water to prepare a maltose solution with a maltose concentration of 150 g / L. The sodium alginate base solution and chitosan acetate solution were mixed at a mass ratio of 1:0.4 and stirred at 35°C and 100 r / min for 45 min. The carboxyl groups of sodium alginate and the protonated amino groups of chitosan formed a polyelectrolyte complex through electrostatic interaction, thus obtaining a composite adhesive solution. Mix the maltose solution and the composite adhesive solution at a mass ratio of 45:100, and continue stirring for 22 minutes to ensure uniform dispersion of the maltose, thus obtaining a composite adhesive solution containing maltose.

[0043] S22, Ca 2+ Pre-crosslinking and core embedding Seventy-five portions of iron-nitrogen co-doped biochar core particles prepared by S12 were placed in an extrusion spheronizer; The above-mentioned maltose-containing composite adhesive solution is dripped onto the surface of the core particles using a peristaltic pump at a flow rate of 8 mL / min, and then extruded and mixed at a speed of 400 r / min to uniformly coat the surface of the core particles with the composite adhesive solution, forming a coating layer. The coated particles were transferred to a 20 g / L calcium chloride solution and immersed at 28°C for 45 min for crosslinking. 2+ It forms a coordination bond with the carboxyl group of sodium alginate, while the amino group of chitosan also participates in the Ca2+ complexation. 2+ Coordination enables the composite adhesive to cross-link and solidify, forming core-shell structured composite hydrogel microspheres. The composite hydrogel microspheres were obtained by filtration and washed three times with deionized water to remove uncrosslinked Ca on the surface. 2+ And free maltose.

[0044] S23, Fe 3+ Secondary cross-linking: The core-shell structured composite hydrogel microspheres and a ferric chloride solution with a mass concentration of 15 g / L were mixed at a mass ratio of 1:8 and soaked at 28 °C for secondary cross-linking for 4 h. After filtration, the microspheres were washed four times with deionized water to obtain intermediate layer coated particles.

[0045] S3, Glucose outer coating: Take 100 portions of the intermediate layer coated particles and place them in a reaction vessel; Adjust the pH of a glucose solution with a glucose concentration of 150 g / L to 4.5; The pH-adjusted glucose solution was added to the reaction vessel, with a mass ratio of glucose solution to intermediate coated particles of 1.5:1. The reaction was heated at 105℃ and 100r / min for 4 hours. After the reaction was completed, the composite carbon source particles with glucose chemical grafting were obtained by filtration, washed three times with deionized water, and dried in a ventilated environment at 38°C for 3 hours to obtain the finished particles of multilayer time-release composite carbon source, which were designated as sample 1.

[0046] Example 2 The quality standards of the steel pickling sludge, sodium alginate, corn cob powder, and chitosan used in this embodiment are the same as those in Embodiment 1.

[0047] In this embodiment, a multilayer time-release composite carbon source is prepared. The specific preparation process is as follows: S1. Preparation of iron-nitrogen co-doped biochar core: S11, Preparation of iron-nitrogen co-doped biochar: Mix steel pickling sludge with an equal mass of deionized water, wash at 120 r / min for 5 min, let stand, pour off the supernatant, and repeat twice. The sludge was soaked in 2 mol / L dilute hydrochloric acid at 15℃ and acid washed at 100 r / min. The mass ratio of dilute hydrochloric acid to acid-washed sludge was 1:1. The sludge was filtered and washed with deionized water until the concentration reached 6.8. The acid-washed sludge was dried in an oven at 100℃ for 12 hours, then pulverized and passed through a 100-mesh sieve to obtain dry sludge powder. Mix dry sludge powder, ammonium chloride, and potassium hydroxide in a mass ratio of 40:3:120, and add deionized water to adjust the material to a paste-like consistency. The amount of deionized water used is 40% of the total mass of the dry material. Mix at 120 r / min for 30 min, then dry in an oven at 105 ℃ for 12 h. The material is then transferred to a tube furnace and pyrolyzed at 850°C for 1 hour under nitrogen protection at a rate of 12°C / min. After pyrolysis, the sample was allowed to cool naturally to room temperature, washed three times with 0.1 mol / L dilute hydrochloric acid, then washed with deionized water until neutral, and dried in an oven at 100℃ for 12 h to obtain iron-nitrogen co-doped biochar, which was designated as biochar sample 2.

[0048] Take biochar sample 2, and test it using the same methods and applicable standards as in Example 1. The total chromium concentration in the leachate of biochar sample 2 in this example is 1.8 mg / L, the hexavalent chromium concentration is 0.3 mg / L, and the inorganic fluoride concentration is 12 mg / L, which meets the safety and environmental protection requirements and can be safely used as a carbon source carrier for denitrification in wastewater treatment.

[0049] S12, Melt blending and molding of composite core particles Polycaprolactone and corn cob powder were mixed at a mass ratio of 1:1 and melt-blended at 120 r / min for 15 min at 150℃ to obtain a melt mixture. Biochar sample 2 and polycaprolactone were added to the above molten mixture at a mass ratio of 8:1. The mixture was then continuously melted and blended for 10 minutes while maintaining the rotation speed to ensure that biochar sample 2 was uniformly dispersed, thus obtaining iron-nitrogen co-doped biochar composite molten material. The iron-nitrogen co-doped biochar composite molten material was transferred to an extrusion spheroidizer. It was first extruded into strips through a 0.5 mm sieve at a speed of 500 r / min, and then spheroidized at a speed of 1200 r / min for 3 min to obtain spherical core particles. After being dried under vacuum at 35℃ and -0.09 MPa for 24 h, the iron-nitrogen co-doped biochar core particles were obtained.

[0050] S2. Preparation of the intermediate layer of sodium alginate-chitosan composite hydrogel and maltose encapsulation S21. Preparation of maltose-containing composite adhesive solution Sodium alginate was dissolved in deionized water to prepare a sodium alginate basic solution with a mass concentration of 25 g / L. Chitosan was dissolved in a 2% (v / v) aqueous acetic acid solution to prepare a chitosan-acetic acid solution with a chitosan concentration of 15 g / L. After stirring until completely dissolved, the pH was adjusted to 5.5 with a 10% (v / v) sodium hydroxide solution. Maltose was dissolved in deionized water to prepare a maltose solution with a maltose concentration of 200 g / L. The sodium alginate base solution and chitosan acetate solution were mixed at a mass ratio of 1:0.6 and stirred at 40℃ and 120r / min for 30min. The carboxyl groups of sodium alginate and the protonated amino groups of chitosan formed a polyelectrolyte complex through electrostatic interaction, thus obtaining a composite adhesive solution. Mix the maltose solution and the composite adhesive solution at a mass ratio of 30:100, and continue stirring for 15 minutes to ensure uniform dispersion of the maltose, thus obtaining a composite adhesive solution containing maltose.

[0051] S22, Ca 2+ Pre-crosslinking and core embedding Fifty portions of iron-nitrogen co-doped biochar core particles prepared by S12 were placed in an extrusion spheronizer; The above-mentioned maltose-containing composite adhesive solution is dripped onto the surface of the core particles at a flow rate of 5 mL / min using a peristaltic pump, and then extruded and mixed at a speed of 500 r / min to uniformly coat the surface of the core particles with the composite adhesive solution, forming a coating layer. The coated particles were transferred to a calcium chloride solution with a mass concentration of 25 g / L and immersed at 30 °C for 60 min for crosslinking. 2+ It forms a coordination bond with the carboxyl group of sodium alginate, while the amino group of chitosan also participates in the Ca2+ complexation. 2+ Coordination enables the composite adhesive to cross-link and solidify, forming core-shell structured composite hydrogel microspheres. The composite hydrogel microspheres were obtained by filtration and washed twice with deionized water to remove uncrosslinked Ca²⁺ and free maltose on the surface.

[0052] S23, Fe 3+ Secondary cross-linking: The core-shell structured composite hydrogel microspheres and a ferric chloride solution with a mass concentration of 10 g / L were mixed at a mass ratio of 1:5 and immersed at 25 °C for secondary cross-linking for 2 h. After filtration, the microspheres were washed three times with deionized water to obtain intermediate layer coated particles.

[0053] S3, Glucose outer coating: Take 100 portions of the intermediate layer coated particles and place them in a reaction vessel; Adjust the pH of a glucose solution with a glucose concentration of 200 g / L to 5.0; The pH-adjusted glucose solution was added to the reaction vessel, with a mass ratio of glucose solution to intermediate layer coated particles of 2:1. The reaction was heated at 120℃ and 120r / min for 2 hours. After the reaction was completed, the composite carbon source particles with glucose chemical grafting were obtained by filtration, washed twice with deionized water, and dried in a ventilated environment at 35°C for 2 hours to obtain the finished particles of multilayer time-release composite carbon source, which were designated as sample 2.

[0054] Example 3 The quality standards of the steel pickling sludge, sodium alginate, corn cob powder, and chitosan used in this embodiment are the same as those in Embodiment 1.

[0055] In this embodiment, a multilayer time-release composite carbon source is prepared. The specific preparation process is as follows: S1. Preparation of iron-nitrogen co-doped biochar core: S11, Preparation of iron-nitrogen co-doped biochar: Mix steel pickling sludge with an equal mass of deionized water, wash at 80 r / min for 10 min, let stand, pour off the supernatant, and repeat 4 times. The sludge was soaked in 1 mol / L dilute hydrochloric acid at 30℃ and acid washed at 200 r / min. The mass ratio of dilute hydrochloric acid to acid-washed sludge was 2:1. The sludge was filtered and washed with deionized water to a concentration of 7.2. The acid-washed sludge was dried in an oven at 110℃ for 24 hours, then pulverized and passed through a 100-mesh sieve to obtain dry sludge powder. Mix dry sludge powder, ammonium chloride, and potassium hydroxide in a mass ratio of 40:1:40, add deionized water to adjust the material to a paste-like consistency, and use deionized water at a rate of 20% of the total mass of the dry material. Mix at 80 r / min for 60 min, and dry in an oven at 80 ℃ for 24 h. The material is then transferred to a tube furnace and pyrolyzed at 650°C for 3 hours under nitrogen protection at a rate of 8°C / min. After pyrolysis, the sample was allowed to cool naturally to room temperature, washed five times with 0.2 mol / L dilute hydrochloric acid, then washed with deionized water until neutral, and dried in an oven at 110℃ for 24 h to obtain iron-nitrogen co-doped biochar, which was designated as biochar sample 3.

[0056] Take biochar sample 3, and test it using the same methods and applicable standards as in Example 1. The total chromium concentration, hexavalent chromium concentration, and inorganic fluoride concentration in the leachate of biochar sample 3 in this example are 8.6 mg / L, 2.1 mg / L, and 56 mg / L, which meet the safety and environmental protection requirements and can be safely used as a carbon source carrier for denitrification in wastewater treatment.

[0057] S12, Melt blending and molding of composite core particles Polycaprolactone and corn cob powder were mixed at a mass ratio of 1:0.6 and melt-blended at 120℃ and 80 r / min for 30 min to obtain a melt mixture. Biochar sample 3 and polycaprolactone were added to the above molten mixture at a mass ratio of 4:1. The mixture was then continuously melted and blended for 20 minutes while maintaining the rotation speed to ensure that biochar sample 3 was uniformly dispersed, thus obtaining iron-nitrogen co-doped biochar composite molten material. The iron-nitrogen co-doped biochar composite molten material was transferred to an extrusion spheroidizer. It was first extruded into strips through a 1.5mm sieve at a speed of 300r / min, and then spheroidized at a speed of 800r / min for 5min to obtain spherical core particles. After being dried under vacuum at 40℃ and -0.08MPa for 48h, the iron-nitrogen co-doped biochar core particles were obtained.

[0058] S2. Preparation of the intermediate layer of sodium alginate-chitosan composite hydrogel and maltose encapsulation S21. Preparation of maltose-containing composite adhesive solution Sodium alginate was dissolved in deionized water to prepare a sodium alginate basic solution with a mass concentration of 15 g / L. Chitosan was dissolved in a 1% (v / v) aqueous acetic acid solution to prepare a chitosan-acetic acid solution with a chitosan concentration of 5 g / L. After stirring until completely dissolved, the pH was adjusted to 5.0 with a 10% (v / v) sodium hydroxide solution. Maltose was dissolved in deionized water to prepare a maltose solution with a maltose concentration of 100 g / L. The sodium alginate base solution and chitosan acetate solution were mixed at a mass ratio of 1:0.2 and stirred at 30°C and 80 r / min for 60 min. The carboxyl groups of sodium alginate and the protonated amino groups of chitosan formed a polyelectrolyte complex through electrostatic interaction, thus obtaining a composite adhesive solution. Mix the maltose solution and the composite adhesive solution at a mass ratio of 60:100, and continue stirring for 30 minutes to ensure uniform dispersion of the maltose, thus obtaining a composite adhesive solution containing maltose.

[0059] S22, Ca 2+ Pre-crosslinking and core embedding One hundred portions of iron-nitrogen co-doped biochar core particles prepared by S12 were placed in an extrusion spheronizer; The above-mentioned maltose-containing composite adhesive solution is dripped onto the surface of the core particles at a flow rate of 10 mL / min using a peristaltic pump, and then extruded and mixed at a speed of 300 r / min to uniformly coat the surface of the core particles with the composite adhesive solution, forming a coating layer. The coated particles were transferred to a calcium chloride solution with a mass concentration of 15 g / L and immersed at 25 °C for 30 min for crosslinking. 2+ It forms a coordination bond with the carboxyl group of sodium alginate, while the amino group of chitosan also participates in the Ca2+ complexation. 2+ Coordination enables the composite adhesive to cross-link and solidify, forming core-shell structured composite hydrogel microspheres. The composite hydrogel microspheres were obtained by filtration and washed three times with deionized water to remove uncrosslinked Ca on the surface. 2+ And free maltose.

[0060] S23, Fe 3+ Secondary cross-linking: The core-shell structured composite hydrogel microspheres and a ferric chloride solution with a mass concentration of 20 g / L were mixed at a mass ratio of 1:10 and immersed at 30 °C for secondary cross-linking for 6 h; after filtration, the microspheres were washed 5 times with deionized water to obtain intermediate layer coated particles.

[0061] S3, Glucose outer coating: Take 100 portions of the intermediate layer coated particles and place them in a reaction vessel; Adjust the pH of a glucose solution with a glucose concentration of 100 g / L to 4.0; The pH-adjusted glucose solution was added to the reaction vessel, with a mass ratio of glucose solution to intermediate layer coated particles of 1:1. The reaction was heated at 90℃ and 80r / min for 6 hours. After the reaction was completed, the composite carbon source particles with glucose chemical grafting were obtained by filtration, washed three times with deionized water, and dried in a ventilated environment at 40°C for 4 hours to obtain the finished particles of multilayer time-release composite carbon source, which were designated as sample 3.

[0062] Example 4 The quality standards of the steel pickling sludge used in this embodiment are as follows: total iron content 42.8%, water content 52.9%, impurity content 4.3%; polycaprolactone with a weight-average molecular weight of 80 kDa; sodium alginate viscosity 200 mPa·s; corn cob powder with a water content of 10.3% and an average particle size of 145.8 μm; chitosan with a degree of deacetylation of 90%, a viscosity of 500 mPa·s in 1% acetic acid solution, and an average weight-average molecular weight of 300 kDa.

[0063] In this embodiment, a multilayer time-release composite carbon source was prepared. The specific preparation process was the same as in Example 1, except that the iron-nitrogen co-doped biochar prepared in step S11 was designated as biochar sample 4. Biochar sample 4 was tested using the same methods and applicable standards as in Example 1. The leachate of biochar sample 4 in this embodiment had a total chromium concentration of 5.5 mg / L, a hexavalent chromium concentration of 1.3 mg / L, and an inorganic fluoride concentration of 35 mg / L, which met the safety and environmental protection requirements and could be safely used as a carbon source carrier for denitrification in wastewater treatment. The finished particles of the obtained multilayer time-release composite carbon source were designated as sample 4.

[0064] Example 5 The quality standards of the steel pickling sludge used in this embodiment are as follows: total iron content 48.3%, water content 45.2%, impurity content 6.5%; polycaprolactone with a weight-average molecular weight of 50 kDa; sodium alginate viscosity of 500 mPa·s; corn cob powder with a water content of 11.6% and an average particle size of 113.5 μm; chitosan with a degree of deacetylation of 85%, a viscosity of 100 mPa·s in 1% acetic acid solution, and an average weight-average molecular weight of 100 kDa.

[0065] In this embodiment, a multilayer time-release composite carbon source was prepared. The specific preparation process was the same as in Example 1, except that the iron-nitrogen co-doped biochar prepared in step S11 was designated as biochar sample 5. Biochar sample 5 was tested using the same methods and applicable standards as in Example 1. The leachate of biochar sample 5 in this embodiment had a total chromium concentration of 4.3 mg / L, a hexavalent chromium concentration of 1.1 mg / L, and an inorganic fluoride concentration of 26 mg / L, which met the safety and environmental protection requirements and could be safely used as a carbon source carrier for denitrification in wastewater treatment. The finished particles of the obtained multilayer time-release composite carbon source were designated as sample 5.

[0066] Comparative Example 1 The comparative example preparation of the composite carbon source is carried out in the same manner as in Example 1, except that the preparation of iron-nitrogen co-doped biochar in step S11 is omitted. That is, only polycaprolactone and corn cob powder are used in the core, and no iron-nitrogen co-doped biochar is added. The specific operation of melt blending and molding of the composite core particles in this comparative example is as follows: polycaprolactone and corn cob powder are mixed at a mass ratio of 1:0.8 and melt blended at 135°C and 100 r / min for 37 min to obtain a melt mixture, which is then directly extruded and spheroidized. The subsequent steps S2 and S3 are the same as in Example 1. The finished particles of the multilayer time-release composite carbon source are designated as control 1.

[0067] Comparative Example 2 The comparative preparation of the composite carbon source is carried out in the same manner as in Example 1, except that corn cob powder is omitted in the core composition of step S1, that is, the core is composed only of iron-nitrogen co-doped biochar and polycaprolactone; the specific operation of melt blending and molding of composite core particles in S12 is as follows: polycaprolactone is melted at 100 r / min at 135℃, and then biochar sample 1 and polycaprolactone are added at a mass ratio of 6:1, and melt blended for 37 min to obtain iron-nitrogen co-doped biochar composite melt material; the subsequent extrusion, spheroidization and drying steps are the same as in Example 1; steps S2 and S3 are the same as in Example 1, and the finished particles of the obtained multilayer time-release composite carbon source are designated as control 2.

[0068] Comparative Example 3 The comparative example preparation of the composite carbon source is carried out in the same manner as in Example 1, except that the Fe step in step S23 is omitted in step S2. 3+Secondary crosslinking process, i.e., without Fe 3+ Secondary crosslinking is directly performed in subsequent operations; subsequent step S3 is the same as in Example 1, and the resulting multilayer time-release composite carbon source particles are designated as control 3. It should be noted that in this comparative example, Ca... 2+ The crosslinking process conditions (such as calcium chloride concentration, crosslinking temperature, and crosslinking time) remained the same as in Example 1, except that the step names were changed from "Ca" to "C"". 2+ "Pre-crosslinked" changed to "Ca" 2+ "Cross-linking" is used, and after washing, it is directly used as an intermediate layer to coat the particles, without proceeding to step S23 for Fe. 3+ Secondary cross-linking.

[0069] Comparative Example 4 The comparative example uses a composite carbon source prepared in the same manner as in Example 1, except that in step S3, the glucose Maillard reaction covalent grafting process is replaced by a physical coating method.

[0070] The specific steps are as follows: Take 100 portions of the intermediate layer coated particles and place them in a coating pan; A glucose solution with a glucose concentration of 150 g / L does not require pH adjustment; Glucose solution was uniformly sprayed onto the surface of the intermediate layer coated particles using a spraying method, with the amount of glucose solution used being the same as in Example 1. The glucose solution was evenly sprayed onto the surface of the intermediate layer of coated particles using a multi-coating method. After each spraying, the particles were ventilated at 38°C for 15 minutes before the next spraying was performed. This process was repeated until all the glucose solution was sprayed. After all the spraying was completed, it was continued to be dried in a ventilated environment at 38℃ for 2 hours to obtain composite carbon source particles physically coated with glucose, which was designated as control 4.

[0071] Comparative Example 5 The comparative preparation of the composite carbon source is carried out in the same manner as in Example 1, except that in step S2, only sodium alginate is used as the gelling material and chitosan is not added.

[0072] The specific steps are as follows: S21. Preparation of sodium alginate solution containing maltose: Sodium alginate was dissolved in deionized water to prepare a sodium alginate basic solution with a mass concentration of 20 g / L. Maltose was dissolved in deionized water to prepare a maltose solution with a maltose concentration of 150 g / L. The maltose solution and the sodium alginate base solution were mixed at a mass ratio of 45:100 and stirred at 35℃ and 100r / min for 45min to obtain a sodium alginate gel reference solution containing maltose.

[0073] The sodium alginate solution containing maltose was used as a reference standard in the subsequent process. Steps S22, S23, and subsequent step S3 were the same as in Example 1. The resulting multilayer time-release composite carbon source particles were designated as reference standard 5.

[0074] Analysis and Testing To verify the technical effect of the present invention and to compare it with the comparative scheme, the obtained composite carbon source samples 1-5 and reference samples 1-5 were systematically analyzed and tested.

[0075] I. Test of the denitrification efficiency of composite carbon source granules To evaluate the application effect of various composite carbon source particles in actual denitrification, a static denitrification experiment was designed.

[0076] Artificially prepared simulated wastewater (NO3) - With an initial N concentration of 35 mg / L, C / N = 3.5, and pH = 7.0 ± 0.2, equal amounts (1.0 g per liter of simulated wastewater) of samples 1-5 and controls 1-5 were added to the reaction flasks containing simulated wastewater. Acclimated denitrifying sludge (MLSS = 3000 mg / L) was inoculated, and denitrification experiments were conducted at 25℃ under constant temperature, in the dark, and with stirring at 100 r / min. Samples were taken at 0 h, 2 h, 4 h, 8 h, 12 h, 24 h, 48 h, and 72 h, and NO3 was determined spectrophotometrically. - -N, NO2 - The nitrate removal rate (NRE) was calculated based on the -N concentration according to Equation 1. The results are shown in Tables 1 and 2. A curve showing the nitrate removal rate versus time was plotted based on the data in Table 1, and a curve showing the nitrite concentration versus time was plotted based on the data in Table 2. The results are shown in Tables 1 and 2, respectively. Figure 1 and Figure 2 .

[0077] The method for calculating NRE at each time point is: NRE(%) = (C0 - C t Formula 1: ) / C0×100% In Equation 1, C0 is the initial NO3. - -N concentration (mg / L), C t NO3 at time t - -N concentration (mg / L).

[0078] Table 1: Nitrate removal rates (%) of composite carbon source samples and control samples at different treatment times Table 2: Nitrite concentration (mg / L) of composite carbon source samples and controls at different treatment times Regarding the nitrate removal effect, in Table 1, samples 1-5 of this invention all achieved a nitrate removal rate of over 98% within 72 h, and NO3 at 72 h... - The -N concentrations were all below 0.65 mg / L, demonstrating excellent denitrification nitrogen removal effect.

[0079] Figure 1 In the study, the removal rates of all samples rapidly increased within 0–12 hours. Sample 1 achieved a removal rate of 42.3% at 4 hours and 82.4% at 12 hours, with a 40.1% increase between 4 and 12 hours. This indicates that the continuous release of glucose from the outer layer and maltose from the middle layer provided a sustained and sufficient carbon source for denitrification. After 12 hours, the rate of increase slowed, indicating the entry into a deep denitrification phase, where the long-term carbon release from the core polycaprolactone and corn cob powder maintained denitrification until completion. Sample 2, due to its highest pyrolysis temperature, the largest specific surface area of ​​the resulting iron-nitrogen co-doped biochar, and the highest electron transfer efficiency, achieved a removal rate of 99.6% at 72 hours, with residual NO3. - -N concentration was only 0.14 mg / L; Sample 3, due to its lower pyrolysis temperature, had a slightly lower removal rate, but residual NO3 remained. - The -N concentration was the highest among all samples, but it was still significantly better than that of the control samples.

[0080] Reference standard 1 showed significantly lower nitrate removal rates than the sample of this invention at all time points, with a removal rate of only 90.8% after 72 hours, and residual NO3. - The -N concentration was as high as 3.22 mg / L. This is because the reference standard lacked iron-nitrogen co-doped biochar during preparation, resulting in a lack of porous framework regulation for carbon source release and a deficiency of Fe. 3+ / Fe 2+ Electron shuttle effect reduces electron transfer efficiency.

[0081] Reference standard 2 showed a removal rate of only 95.2% after 72 hours, with residual NO3. - The -N concentration was 1.68 mg / L. This is because after the corn cob powder was missing, the kernel relied solely on the hydrolysis of polycaprolactone ester bonds to release carbon. In the later stages (48-72 h), the total carbon source supply was insufficient to support the completion of denitrification to a deep level. This proves that the enzymatic hydrolysis of corn cob powder plays an important role in maintaining the terminal denitrification effect.

[0082] The nitrate removal rate of reference standard 3 increased at a lower rate than that of sample 1 from 4 to 12 hours, with a removal rate of 96.5% at 72 hours and residual NO3. -The -N concentration was 1.23 mg / L, approximately 4.9 times the residual amount in sample 1. This is because in sample 1, the depletion of the outer glucose layer led to an increase in system pH, triggering the disintegration of the intermediate layer and the release of maltose, ensuring a smooth carbon source supply. In contrast, reference standard 3 lacked Fe... 3+ Cross-linking, the lack of pH responsiveness in the intermediate layer, and the hindered release of maltose lead to insufficient carbon source supply and decreased denitrification efficiency over 4–12 hours. These results demonstrate that Fe... 3+ The pH-responsive mechanism constructed by secondary crosslinking plays a crucial role in ensuring the sequential connection of carbon sources.

[0083] Reference standard 4 showed a nitrate removal rate of 22.5% from 0 to 2 hours, higher than sample 1. Its nitrate removal rate after 72 hours was 99.1%, with residual NO3. - The -N concentration was 0.32 mg / L, which is close to that of sample 1. It is evident that the physically coated glucose layer dissolves rapidly in the initial stage. Although this does not affect the final denitrification effect, the excessively high initial carbon source release rate can easily lead to excessive COD in the effluent and the risk of sludge bulking. This invention employs a Maillard reaction covalent grafting method to effectively control the glucose layer release rate and avoid the aforementioned problems.

[0084] The nitrate removal rate of reference standard 5 after 72 hours was 97.2%, and the residual NO3 was... - The -N concentration was 0.98 mg / L, approximately 3.9 times the residual amount in sample 1. This indicates that the absence of chitosan leads to a decrease in the stability of the intermediate layer structure, some maltose is lost during the cross-linking process, and the glucose Maillard grafting reaction is less efficient due to the lack of amino groups provided by chitosan. This demonstrates that chitosan plays an important role in maintaining the integrity of the intermediate layer structure and the efficiency of the outer layer grafting.

[0085] Analysis of nitrite accumulation characteristics showed that the peak nitrite concentrations of samples 1-5 were all below 3.0 mg / L, with the peak occurring at 4 h. After 48 h, nitrite was not detected. Sample 2, due to its highest pyrolysis temperature and highest potassium hydroxide ratio, exhibited the largest degree of graphitization and specific surface area in its iron-nitrogen co-doped biochar. 3+ / Fe 2+ The redox cycle has the highest electron shuttle efficiency, and the nitrite produced by nitrate reduction can be rapidly reduced to nitrogen gas, resulting in the best match between the rate of nitrite generation and consumption.

[0086] The nitrite concentration of reference standard 1 peaked at 8.2 mg / L at 8 h, and remained at 1.5 mg / L until 72 h, approximately 4.6 times that of sample 1, with a slow decline. This is because the reference standard lacked iron-nitrogen co-doped biochar during preparation, resulting in Fe... 3+ / Fe 2+ The absence of electron shuttle activity leads to a lag in nitrite reduction compared to nitrate reduction, resulting in the continuous accumulation of intermediate products.

[0087] The peak nitrite concentration of reference standard 2 was 4.8 mg / L, which was about 2.7 times that of sample 1. The residual concentration after 72 hours was 0.2 mg / L, indicating that insufficient carbon source supply in the middle stage led to the prolonged accumulation time of nitrite.

[0088] Reference standard 3 showed the most significant nitrite accumulation, reaching a concentration of 7.8–8.5 mg / L within 8–12 hours, with the peak occurring at 12 hours. This peak concentration was approximately 4.7 times that of sample 1, and 0.8 mg / L remained after 72 hours. This result corresponds to the characteristic of a decreasing rate of nitrate removal within 4–12 hours, indicating that glucose was depleted after 4 hours, but the intermediate layer lacked Fe. 3+ Cross-linking prevents the release of maltose in response to changes in system pH, interrupts carbon source supply, lacks electron donors, and causes nitrite to accumulate in large quantities as it cannot be reduced in time.

[0089] The peak nitrite concentration of reference standard 4 was 3.5 mg / L, which was about 1.9 times that of sample 1. This is because the initial burst of glucose release caused a mismatch between the release and consumption rate of carbon source, resulting in an instantaneous increase in nitrite accumulation, which then rapidly decreased due to the rapid consumption of carbon source.

[0090] The peak nitrite concentration of reference standard 5 was 3.2 mg / L, which was about 1.8 times that of sample 1. The decreased stability of the intermediate layer structure led to a reduction in the effective carbon source loading and affected the carbon source connection efficiency.

[0091] In summary, the composite carbon source of this invention utilizes an iron-nitrogen co-doped biochar composite core and Fe... 3+ The three-layer structure design of the secondary cross-linked sodium alginate-chitosan hydrogel intermediate layer and glucose Maillard grafted outer layer achieved a coordinated match between the sequential release of carbon source and the metabolic needs of microorganisms within a 72-h denitrification cycle. The nitrate removal rate was significantly better than that of the control products, and the risk of nitrite accumulation was effectively controlled.

[0092] II. Verification Experiment of Dual-Signal Response Mechanism To verify the dual-signal triggering mechanism of low glucose concentration and increased pH required for the release of maltose from the intermediate layer in this invention, the following crossover experiment was designed.

[0093] Take 2.0 g each of sample 1 and reference standards 1-5, and place them separately in buffer solutions with the following conditions, and shake for 12 h at 25℃ and 100 r / min: Condition A (neither signal is met): glucose concentration 200 mg / L, pH = 7.0; Condition B (only pH increase is satisfied): glucose concentration 200 mg / L, pH = 8.0; Condition C (only satisfied at low glucose concentration): glucose concentration 20 mg / L, pH = 7.0; Condition D (neither signal is met, close to the threshold): glucose concentration 60 mg / L, pH=7.3; Condition E (both signals are satisfied, lower threshold): glucose concentration 50 mg / L, pH = 7.5; Condition F (both signals are satisfied): glucose concentration 30 mg / L, pH = 7.8; Condition G (both signals are satisfied): glucose concentration 20 mg / L, pH = 8.0.

[0094] The maltose content in the solution was determined by the dinitrosalicylic acid colorimetric method, and the cumulative release rate of maltose over 12 h was calculated according to Equation 2. The results are shown in Table 3.

[0095] Maltose release rate (%) = C t ×V / M0×100% Equation 2 In Equation 2, C t Let t be the maltose concentration in the solution at time t (mg / L), V be the solution volume (L), and M0 be the theoretical total maltose encapsulation in the particles (mg). M0 is determined as follows: Take a sample or reference standard prepared in the same batch, place it in a sodium hydroxide solution at pH 9.0, and shake it at 25℃ and 100 r / min for 24 h to completely disintegrate the hydrogel layer. The total maltose content in the supernatant is then determined using the dinitrosalicylic acid colorimetric method, which is the theoretical total maltose encapsulation M0 of that batch of particles.

[0096] Table 3: Cumulative maltose release rate (%) of Sample 1 and Control 3 under different conditions over 12 hours As shown in Table 3, under condition A (high glucose concentration, neutral pH), the maltose release rate of sample 1 was only 6.5% after 12 hours, indicating that under conditions of sufficient carbon source and no increase in pH, the intermediate Fe... 3+ The cross-linked coordination network remained stable, and maltose was effectively encapsulated. Under condition B (pH increased only), the release rate was 12.8%, only slightly higher than under condition A, indicating that even at higher glucose concentrations, when the pH increased to 8.0, Fe... 3+ The coordination network remained relatively stable, and maltose was not significantly released. Under condition C (low glucose concentration only), the release rate was 10.2%, slightly higher than under condition A but still at a low level, indicating that simply reducing the glucose concentration to 20 mg / L was insufficient to trigger the disintegration of the intermediate layer. Under condition D (glucose concentration 60 mg / L, pH=7.3), although both parameters approached the threshold, neither crossed the critical value (glucose concentration ≥50 mg / L, pH≤7.5), resulting in a release rate of only 14.3%, which was still at a low level. This demonstrates that both signals must simultaneously meet the threshold requirements to trigger a response.

[0097] When both conditions of low glucose concentration and elevated pH are met and exceed the threshold, the maltose release rate exhibits a significant jump. Under condition E (glucose concentration 50 mg / L, pH=7.5), the release rate reaches 65.8%, approximately 4.6 times that under condition D, showing a clear jump characteristic; under conditions F and G, the release rates further increase to 72.4% and 78.6%, respectively. These results indicate that when the glucose concentration decreases to 50 mg / L or below and the system pH increases to 7.5 or above, Fe... 3+ Hydrolysis occurs to form Fe(OH)3 precipitate, COO - -Fe 3+ and NH2-Fe 3+ Coordination bonds break, the hydrogel network disintegrates, and maltose is rapidly released. The release rate increases with further decrease in glucose concentration and further increase in pH, demonstrating a dose-response effect synergistically regulated by two signals.

[0098] The maltose release rates of reference standards 1, 2, and 4 under various conditions were basically consistent with those of sample 1, and they also exhibited the characteristic of significant release only under conditions E, F, and G. This is because the intermediate layer preparation process of reference standards 1, 2, and 4 was exactly the same as that of sample 1, all involving Ca... 2+ Pre-crosslinking and Fe 3+ Secondary cross-linking forms a complete COO - -Fe 3+ and NH2-Fe 3+ The coordination network, acting as a dense chemical cross-linking barrier, exhibits swelling-disintegration behavior controlled solely by the pH and ionic strength of the external liquid environment, and is not directly related to the composition of the core particles (whether or not they contain biochar or corn cob powder). It also isolates the intermediate layer's internal structure from interference by the outer glucose coating method (physical spraying or chemical grafting). This result further verifies that in the three-layer structure design of this invention, the intermediate layer, as a functional unit with independent response logic, possesses good chemical stability and anti-interference capabilities due to its dual-signal triggering characteristics, and each layer performs its respective temporal function without interfering with the others. It should be noted that the difference in the outer glucose coating method is already reflected in the initial carbon source release behavior: the physical coating of control 4 leads to an initial explosive release and causes a transient increase in nitrite accumulation. However, this is due to the difference in the outer layer's own release kinetics and does not change the coordination network structure and dual-signal response threshold already formed in the intermediate layer.

[0099] The maltose release rate of reference standard 3 remained between 38% and 46% under all seven conditions, with no significant differences between conditions, and no jump between conditions D and E. This result indicates that reference standard 3 was not subjected to Fe... 3+ Secondary crosslinking, in which the intermediate layer consists only of Ca 2+The structure consists of a cross-linked calcium alginate single network. Due to the relatively large pore size of the calcium alginate gel network and the fact that maltose is a small-molecule disaccharide, single Ca... 2+ The cross-linked network failed to form a sufficiently dense physical barrier layer, leading to significant initial leakage of the embedded maltose during the 12-hour soaking cycle via concentration gradient-driven passive diffusion. In sample 1, Fe... 3+ The secondary cross-linking not only introduces COO - -Fe 3+ and NH2-Fe 3+ The coordination bonds, and more importantly, the coordination of high-valence metal ions, further shrink and densify the pore size of the gel network, significantly reducing the passive diffusion rate of maltose molecules. Therefore, the performance of control 3 not only demonstrates its lack of environmentally responsive disintegration ability, but also, conversely, verifies the role of Fe. 3+ Secondary cross-linking plays an indispensable role in improving the static physical embedding density of small molecule carbon sources in the hydrogel interlayer.

[0100] The maltose release rate of reference standard 5 under various conditions was between that of sample 1 and reference standard 3. Under conditions A, B, C, and D, its basal release rate was higher than that of sample 1, indicating that the density of the intermediate network decreased after the absence of chitosan, weakening its passive encapsulation ability for maltose. Under conditions E, F, and G, its triggered release rate was significantly lower than that of sample 1, and the jump from condition D to condition E was much smaller than that of sample 1, indicating that the absence of the amino group provided by chitosan reduced the NH2-Fe... 3+ Coordination networks cannot be formed, relying solely on COO - -Fe 3+ The coordination network was insufficient to achieve adequate pH-responsive disintegration, resulting in a significant reduction in the efficiency of the dual-signal triggering mechanism. This demonstrates that chitosan not only participates in constructing a complete dual-signal-responsive coordination network but also plays a crucial role in maintaining the structural compactness of the hydrogel and reducing the passive leakage of maltose.

[0101] The above results systematically verify the Fe of the present invention. 3+ COO constructed by secondary crosslinking - -Fe 3+ and NH2-Fe 3+ The coordination network endows the intermediate layer with a strict dual-signal response characteristic: the hydrogel network only disintegrates and releases maltose when the glucose concentration drops below 50 mg / L and the system pH rises above 7.5; both conditions are indispensable. A comparison of conditions D and E clearly demonstrates the on / off jump before and after crossing the threshold, proving that the response has a distinct threshold-triggered characteristic.

[0102] III. Evaluation of Biofilm Formation and Microbial Attachment Performance This experiment was designed to evaluate the promoting effect of various composite carbon source particles on microbial attachment and biofilm formation, and to verify the function of the porous framework of iron-nitrogen co-doped biochar as a microbial carrier.

[0103] Equal amounts (5.0 g) of the composite carbon source samples 1–5 and controls 1–5 were placed in reaction flasks containing 1 L of simulated wastewater (formulation as in “I. Test of Denitrification Efficiency of Composite Carbon Source Particles”) and inoculated denitrifying sludge (MLSS = 3000 mg / L) after inoculation and acclimatization. The flasks were cultured at 25 °C and 100 r / min for 7 days. After culture, the particles were removed and gently rinsed with deionized water to remove any unattached sludge. The volatile suspended solids (VSS) content of the biofilm attached to the surface was determined by gravimetric method. The polysaccharide content in the extracellular polymeric material (EPS) was determined by the heat extraction-anthrone sulfate method. The protein content in the EPS was determined by the Coomassie Brilliant Blue G-250 method. The results are shown in Table 4.

[0104] Among them, MLSS is the mixed liquor suspended solids concentration, which refers to the mass of suspended solids in a unit volume of mixed liquor. It is the core indicator for measuring the total amount of microorganisms in the activated sludge system, and the unit is mg / L. Acclimation refers to the pre-cultivation of inoculated sludge under specific carbon source and nitrate conditions for a certain period of time, so that the denitrifying bacteria can fully adapt to the experimental carbon source substrate and restore their metabolic activity.

[0105] Table 4: Test results of surface biofilm characteristics of composite carbon source samples and reference standards Note: In Table 4, “Attached VSS content” refers to the mass of volatile suspended solids attached to the surface of each gram of composite carbon source particles; “EPS protein content” and “EPS polysaccharide content” refer to the mass of protein and polysaccharide contained in each gram of VSS, respectively; “Total EPS” is the sum of protein and polysaccharide content.

[0106] As shown in Table 4, the attached VSS content of samples 1–5 of this invention ranged from 78.2 to 92.3 mg / g, and the total EPS content ranged from 172.9 to 214.4 mg / g VSS. Among them, sample 2, due to its highest pyrolysis temperature and highest potassium hydroxide ratio, produced the iron-nitrogen co-doped biochar with the largest specific surface area and the most developed pore structure, providing the richest attachment sites for microorganisms. Simultaneously, Fe... 3+ / Fe 2+The redox cycle had the strongest promoting effect on EPS secretion, resulting in the highest amounts of attached VSS and total EPS. Sample 3, due to its lowest pyrolysis temperature and potassium hydroxide ratio, had a relatively small specific surface area of ​​iron-nitrogen co-doped biochar. Although its attached VSS and total EPS were the lowest, they were still significantly higher than control 1. Samples 4 and 5 used a medium-limit process, but with slightly lower total iron content in the raw materials. Their attached VSS and total EPS were between those of samples 1 and 3, consistent with the variation of specific surface area of ​​iron-nitrogen co-doped biochar with pyrolysis temperature and iron content.

[0107] The amount of attached VSS in reference standard 1 was only 42.5 mg / g, approximately 49% of that in sample 1; the total EPS was 110.8 mg / g VSS, approximately 57% of that in sample 1. This is because the core of reference standard 1 consists only of polycaprolactone and corn cob powder, lacking the high specific surface area porous framework of iron-nitrogen co-doped biochar, resulting in a significant reduction in microbial attachment sites; at the same time, it lacks Fe 3+ / Fe 2+ The redox cycle promotes EPS secretion, leading to a significant reduction in total EPS. This result directly demonstrates the crucial role of iron-nitrogen co-doped biochar as a microbial carrier.

[0108] The amount of attached VSS in reference standard 2 was 72.8 mg / g, approximately 84% of that in sample 1; the total amount of EPS was 167.1 mg / g VSS, approximately 86% of that in sample 1. The fibrous structure of corn cob powder has a certain surface roughness, which helps microbial attachment. The attachment performance decreased slightly after its absence, but because the kernel still contains iron-nitrogen co-doped biochar as the main carrier, the amount of attached VSS and the total amount of EPS remained at a high level.

[0109] The attached VSS levels of reference standards 3, 4, and 5 were 82.5 mg / g, 84.2 mg / g, and 80.5 mg / g, respectively, and the total EPS levels were 181.7 mg / g VSS, 186.1 mg / g VSS, and 177.7 mg / g VSS, respectively. The differences compared to sample 1 were all within ±7%. This is because the cores of reference standards 3, 4, and 5 all contained iron-nitrogen co-doped biochar prepared using the same process as sample 1. Changes in the intermediate and outer layers did not affect the porous framework structure of the core and the Fe... 3+ / Fe 2+ The circulation promotes EPS secretion, therefore the microbial adhesion performance is not significantly affected. Control 5, due to the absence of chitosan, exhibits decreased interlayer density and slightly altered particle surface roughness. The amount of attached VSS and total EPS are slightly lower than in sample 1, but the decrease is limited.

[0110] The above results indicate that the microbial attachment and biofilm formation capabilities of the composite carbon source particles are mainly determined by the porous framework structure of the iron-nitrogen co-doped biochar core. The high specific surface area of ​​the iron-nitrogen co-doped biochar provides abundant microbial attachment sites, and the Fe³⁺ / Fe²⁺ redox cycle promotes EPS secretion. The synergistic effect of these two factors facilitates the rapid formation and stabilization of the biofilm, providing a favorable carrier environment for denitrifying bacteria.

[0111] IV. Test of residual phosphate adsorption capacity after carbon source depletion This experiment verifies the adsorption function of the composite carbon source of the present invention on phosphate by the remaining skeleton after the carbon source is released, thus demonstrating that the present invention achieves a functional transformation from carbon source supply to end-of-pipe adsorption.

[0112] The residual particles of Sample 1 and Reference 1 after 72h denitrification efficiency testing were recovered and repeatedly washed with deionized water until the COD of the washing solution was <5mg / L. They were then dried to constant weight in an oven at 105℃. 2.0g of each of the dried residual skeletal particles were weighed and placed in 100mL of KH₂PO₄ solution with an initial phosphorus concentration of 10mg / L (particle dosage 20g / L). Adsorption was carried out at 25℃ and 100r / min for 4h under constant temperature shaking. After adsorption, the solution was filtered through a 0.45μm filter membrane. The phosphorus concentration in the filtrate was determined according to GB / T 11893-1989, and the phosphate removal rate was calculated according to Equation 3. The results are shown in Table 5.

[0113] Phosphate removal rate (%) = (P0 - P) e Equation 3: ) / P0×100% In Equation 3, P0 is the initial phosphorus concentration (mg / L), P e The phosphorus concentration (mg / L) is after adsorption equilibrium.

[0114] Table 5: Adsorption test results of phosphate by the residual framework after carbon source depletion As shown in Table 5, the phosphate removal rate of the residual skeleton in Sample 1 reached 71.7%, while that in Control 1 was only 14.8%. The iron-nitrogen co-doped biochar in the core of Sample 1 contained Fe3O4 and Fe... 0 The active sites can effectively remove phosphate ions through ligand exchange and electrostatic adsorption, and the porous carbon framework with a high specific surface area also provides abundant adsorption sites for phosphates. The core of control 1 consists only of polycaprolactone and corn cob degradation residues, lacking iron oxide active sites. Relying solely on the weak physical adsorption of residual organic matter, it has almost no phosphorus removal capacity. These results fully demonstrate that introducing iron-nitrogen co-doped biochar into the core can endow the carbon source material with the ability to continue its phosphorus removal function after the carbon source is depleted, realizing a functional transformation from single carbon source supply to end-of-pipe adsorption and phosphorus removal.

[0115] V. Leaching Toxicity and Safety Evaluation of Composite Carbon Source Particles To verify the environmental safety of the composite carbon source particles of the present invention during use, the composite carbon source particles prepared from sample 3 (total chromium 8.6 mg / L, hexavalent chromium 2.1 mg / L, inorganic fluoride 56.3 mg / L), which has the highest leaching toxicity of iron and nitrogen co-doped biochar, were selected for testing.

[0116] The leachate from Sample 3 was prepared according to HJ / T 299-2007 "Leaching Toxicity of Solid Waste - Leaching Method: Sulfuric Acid and Nitric Acid Method". The concentration of hazardous components in the leachate was tested according to GB 5085.3-2007 "Identification Standard for Hazardous Waste - Leaching Toxicity Identification". Simultaneously, a water sample from Sample 3 after the 72-hour denitrification experiment was taken from "I. Test of Denitrification Efficiency of Composite Carbon Source Particles". The concentrations of various pollutants were tested according to the methods specified in GB 8978-1996 "Integrated Wastewater Discharge Standard". The results are shown in Table 6.

[0117] Table 6: Leaching toxicity of sample 3 finished product particles and water quality test results after 72 hours of denitrification. Note: In Table 6, "Not detected" means that the test result is below the detection limit of the corresponding method; "—" means that there is no limit to the value requirement for this item in the corresponding standard or that this test is not required.

[0118] As shown in Table 6, the leaching toxicity study of the composite carbon source finished particles of this invention showed that the total chromium concentration in the leachate of Sample 3 was 5.2 mg / L, the hexavalent chromium concentration was 1.3 mg / L, and the inorganic fluoride concentration was 38.6 mg / L, all far below the standard limits. Compared with the leaching toxicity test results of biochar Sample 3 in Example 3, the various leaching indicators of the finished particles were further reduced, namely, total chromium decreased by about 40%, hexavalent chromium decreased by about 38%, and fluoride decreased by about 31%. This is because in step S12, the iron-nitrogen co-doped biochar was melt-blended and embedded by polycaprolactone and corn cob powder, forming a dense physical isolation layer; the multi-layer coating of the sodium alginate-chitosan composite hydrogel intermediate layer in step S2 and the glucose outer layer in step S3 further played a physical barrier role, effectively reducing the leaching risk of harmful substances. It is evident that even with the relatively high leaching toxicity of iron-nitrogen co-doped biochar intermediates, the leaching toxicity of the composite carbon source particles obtained after multi-layer coating is still significantly lower than the standard limit, demonstrating that the solid material itself possesses environmental safety.

[0119] The results of the study on the effluent quality of denitrification application showed that after the 72-hour denitrification experiment, the concentrations of total chromium, hexavalent chromium and inorganic fluoride in the effluent sample of sample 3 were all below the method detection limit, and the COD concentration was 42.5 mg / L. All indicators were far below the first-class standard limit of GB 8978-1996.

[0120] It is important to note that there is no contradiction between the leaching concentration of the finished product particles and the "not detected" result in the effluent quality. This is because the leaching toxicity test uses a nitric acid / sulfuric acid mixed solution as the extractant. The acidic conditions are designed to simulate the maximum possible leaching amount of waste under extreme scenarios such as landfill and stockpiling, when affected by acidic precipitation. This falls under the conservative safety assessment required by regulations. In contrast, the actual water pH in the denitrification experiment is consistently maintained in the neutral-to-alkaline range of 7.0–8.0. Under these conditions, the dissolution rate and total amount of pollutants are far lower than under acidic leaching conditions. Furthermore, the solid-liquid ratio in the leaching toxicity test is 1:10, while the particle dosage in the denitrification experiment is 1.0 g / L. The solid-liquid ratio difference between the two is 100 times; the dilution effect alone is enough to reduce the theoretical concentration to near the detection limit. More importantly, the iron-nitrogen co-doped biochar in the composite carbon source particles of this invention has the function of actively adsorbing, reducing and complexing heavy metals and fluorides. Therefore, even if a trace amount of pollutants dissolves from the core during the 72-hour use, they will be immediately captured and fixed by the iron-nitrogen co-doped biochar and biofilm. The residual concentration in the final effluent is lower than the method detection limit, and it will not cause secondary pollution to the receiving water body.

[0121] The above results demonstrate that, under regulated use conditions, the trace amounts of leached substances from the composite carbon source particles of this invention are far below the environmental safety threshold. Therefore, the composite carbon source particles prepared from steel pickling sludge, when used under regulated conditions, can meet safety and environmental protection requirements throughout their entire life cycle, thus achieving a closed-loop resource recovery system that treats waste with waste.

[0122] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing a multilayer time-release composite carbon source, characterized in that, Includes the following steps: S1. Iron-nitrogen co-doped biochar, polycaprolactone and corn cob powder are melt-blended and then extruded and spherically shaped to obtain core particles; the iron-nitrogen co-doped biochar is a porous magnetic carbon material obtained by high-temperature pyrolysis after acid washing to remove impurities, ammonium chloride nitrogen doping and potassium hydroxide activation, using steel pickling sludge as raw material. S2. A sodium alginate-chitosan composite adhesive solution and a maltose solution are mixed to obtain a maltose-containing composite adhesive solution. The maltose-containing composite adhesive solution is then coated onto the surface of the core particles to obtain core particles coated with the composite adhesive solution. This is followed by Ca... 2+ Pre-crosslinking and Fe 3+ Secondary cross-linking forms a composite hydrogel intermediate layer encapsulating maltose, resulting in intermediate layer coated particles; S3. Glucose is covalently grafted onto the surface of the intermediate layer coated particles via Maillard reaction to form a glucose outer layer, which is then dried to obtain carbon source particles. The method for preparing iron-nitrogen co-doped biochar in step S1 is as follows: Using steel pickling sludge as an iron and carbon source, the steel pickling sludge is washed with water, pickled, dried, and pulverized to obtain dry sludge powder. The dry sludge powder, ammonium chloride and potassium hydroxide are mixed at a mass ratio of 40:(1~3):(40~120), water is added to make a paste, and after drying, the mixture is pyrolyzed at 650℃~850℃ for 1h~3h under nitrogen protection by heating at 8℃ / min~12℃ / min. Then, after acid washing, water washing and drying, iron-nitrogen co-doped biochar is obtained. In step S2, the sodium alginate-chitosan composite adhesive is obtained by mixing and stirring a sodium alginate base solution and a chitosan acetic acid solution at a mass ratio of 1:(0.2-0.6); the sodium alginate base solution has a sodium alginate mass concentration of 15 g / L to 25 g / L; the chitosan acetic acid solution is obtained by dissolving chitosan in an acetic acid aqueous solution with a volume concentration of 1% to 2% to obtain a chitosan acetic acid solution with a chitosan mass concentration of 5 g / L to 15 g / L, and then adjusting the pH to 5.0 to 5.

5.

2. The production method according to claim 1, characterized by, During the process of adding water to form a paste, the amount of water used is 20% to 40% of the total mass of dry sludge powder, ammonium chloride, and potassium hydroxide.

3. The preparation method according to claim 1, characterized in that, In step S1, the specific operation of melt blending and molding is as follows: Polycaprolactone and corn cob powder are melt blended at a mass ratio of 1:(0.6-1) at 120℃-150℃ and 80r / min-120r / min for 15min-30min, and then the iron-nitrogen co-doped biochar is added so that the mass ratio of iron-nitrogen co-doped biochar to polycaprolactone is (4-8):

1. The blending is continued for 10min-20min to obtain a composite melt material; the composite melt material is extruded and spheroidized, and dried to obtain core particles.

4. The method of claim 1, wherein, In step S2, the maltose-containing composite adhesive is obtained by mixing maltose solution and sodium alginate-chitosan composite adhesive at a mass ratio of (30-60):100, and the mass concentration of the maltose solution is 100g / L-200g / L.

5. The preparation method according to claim 1, characterized in that, In step S2, Ca 2+ Pre-crosslinking and Fe 3+ The specific operation of secondary crosslinking is as follows: the core particles coated with the composite adhesive are placed in a calcium chloride solution with a mass concentration of 15 g / L to 25 g / L for crosslinking for 30 min to 60 min, washed, and then immersed in a ferric chloride solution with a mass concentration of 10 g / L to 20 g / L for crosslinking for 2 h to 6 h. After washing, the intermediate layer coated particles are obtained; the Fe 3+ Secondary cross-linking forms COO within the coating layer. - - Fe 3+ and NH2-Fe 3+ The coordination network ensures that the intermediate layer of the composite hydrogel remains stable at pH ≤ 7.0, but disintegrates and releases maltose when the pH rises to 7.5 or above.

6. The method of claim 1, wherein, The specific operation of the Maillard reaction covalent grafting is as follows: the intermediate layer coated particles are mixed with glucose solution at a mass ratio of 1:(1~2), and reacted at 90℃~120℃ for 2h~6h. After the reaction is completed, the particles are washed with deionized water and dried in a ventilated environment at 35℃~40℃ for 2h~4h to obtain carbon source particles.

7. The production method according to claim 6, wherein The glucose solution has a glucose concentration of 100 g / L to 200 g / L and a pH of 4.0 to 5.

0.

8. The use of the multilayered time-released composite carbon source prepared according to the preparation method of any one of claims 1-7 in sewage treatment, characterized in that, When the glucose concentration in the outer layer decreases to 50 mg / L or below and the pH of the system increases to 7.5 or above, the intermediate layer of the composite hydrogel is triggered to disintegrate and release maltose to continue the denitrification process; the polycaprolactone and corn cob powder in the core particles degrade, providing a long-term carbon source for denitrification; after the carbon source is released, the remaining iron and nitrogen co-doped biochar framework adsorbs phosphate and COD in the water.

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