Photo-controllable oxidation-reduction dual-mode biochar and application thereof in construction of self-purification biological buffer zone

Abstract

The application discloses a light-controlled oxidation-reduction dual-mode biochar and application of the biochar in construction of a self-purification biological buffer zone, and belongs to the field of environmental functional materials and agricultural non-point source pollution control technology. The application takes biomass resource utilization as a channel, and prepares a light-controlled oxidation-reduction dual-mode biochar by co-pyrolysis of a semiconductor material precursor and biomass rich in nitrogen / sulfur through a one-step pyrolysis method. The prepared biochar utilizes photo-generated holes to oxidize and degrade organic pollutants and nitrate ammonia nitrogen into nitrate nitrogen under daytime light, and stores photo-generated electrons in doped sites of the biochar skeleton at the same time. The stored electrons are released to reduce nitrate nitrogen into nitrogen in situ at night. The dual-mode biochar is used as a core functional filler to construct a self-purification biological buffer zone, and efficient and synergistic removal of nitrogen and phosphorus pollutants and pesticides is realized by utilizing natural day and night alternation, and the buffer zone is endowed with continuous self-purification capacity, so that the risk of saturation of traditional fillers and secondary pollution is solved.

Description

Technical Field

[0001] This invention relates to the fields of environmental functional materials and agricultural non-point source pollution control, and particularly to a photo-controlled oxidation-reduction dual-mode biochar and its application in constructing a self-purifying biological buffer zone. Background Technology

[0002] Agricultural non-point source pollution, particularly wastewater rich in nitrogen (N) and phosphorus (P) nutrients and pesticide residues discharged from crop farming and animal husbandry, is one of the main causes of eutrophication and ecological degradation in my country's water bodies. Biological buffer zones, as key ecological engineering projects for intercepting and purifying agricultural non-point source pollution, rely on the purification capacity of their internal packing materials for their core function. Biochar, due to its high specific surface area and porous structure, is often used as a reinforcing packing material in buffer zones to enhance the physical adsorption and retention of nitrogen, phosphorus, and organic pollutants.

[0003] However, traditional biochar faces two fundamental bottlenecks in the treatment of non-point source pollution: First, there are issues with adsorption saturation and secondary release. Biochar primarily removes pollutants through physical adsorption, but its adsorption sites are limited. Under continuous impact from non-point source pollution, it quickly becomes saturated and loses its purification function. More seriously, changes in environmental conditions (pH, rainfall runoff) can easily lead to the desorption of adsorbed pollutants such as nitrogen and phosphorus, resulting in more concentrated secondary pollution. Second, its capacity for removing total nitrogen (TN) is extremely limited. Water quality monitoring in the typical irrigation area of ​​Qingtongxia, my country, shows that during non-agricultural drainage periods, due to the influx of point source pollution, ammonia nitrogen (NH4) in the water... + The monthly average concentrations of total nitrogen (TN) and total phosphorus (TP) reached as high as 18.13 mg / L and 4.15 mg / L, respectively, far exceeding the surface water environmental quality standards. Existing ecological ditch technology, even with optimized construction (as shown in the literature "Treatment Capacity and Practical Application Effect of Ecological Ditches with Different Constructions in Farmland"), generally achieves removal rates of only 50%–80% for TN and TP, making it difficult to cope with high concentration shocks. Furthermore, it is ineffective against highly mobile nitrate nitrogen (NO3-). - The removal capacity for nitrogen (NH4+) is particularly insufficient, causing large amounts of nitrogen to penetrate the buffer zone and eventually enter the water body. Achieving efficient nitrogen removal requires aerobic nitrification (NH4+) processes. + -N→NO3 - -N) and "anoxic denitrification" (NO3) - The two steps (N-N→N2) make it difficult for traditional single fillers to create and maintain these two contradictory biochemical environments within the same spatiotemporal unit.

[0004] To address the aforementioned issues, existing technologies have explored various approaches. For instance, Chinese invention patent CN201310312559.3 discloses a method for zoned treatment of river pollution using rice straw and aquatic plants. This method involves setting up a rice straw "matrix absorption zone" upstream to degrade high-concentration COD, and planting *Myriophyllum spicatum* downstream to absorb nitrogen and phosphorus. While this method is adapted to local conditions, it is essentially still a physical interception and plant absorption process, which suffers from problems such as easy rotting and clogging of rice straw, seasonal influence on plant growth, and weak removal capacity for nitrate nitrogen. Other technologies primarily focus on modifying biochar to improve the adsorption selectivity for specific pollutants (such as phosphate or ammonia nitrogen), but fail to fundamentally change its passive adsorption nature, thus failing to achieve the conversion and removal of total nitrogen and the in-situ regeneration of the material itself.

[0005] Therefore, developing an intelligent functional material that can intelligently and sequentially complete the efficient transformation and removal of complex pollutants, especially the elimination of total nitrogen, within a system, and possess self-regeneration capabilities, is key to breaking through the current technological bottlenecks of biological buffer zones and achieving long-term control of agricultural non-point source pollution. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a novel light-controlled oxidation-reduction dual-mode biochar. This material can respond to natural day and night light changes and complete the entire process of oxidative degradation of organic matter such as pesticides, nitrification of ammonia nitrogen, adsorption of total phosphorus, and denitrification of nitrate nitrogen in a time-sequential and efficient manner within a reaction system, thereby realizing in-situ self-purification and continuous regeneration of the material's functions.

[0007] To achieve the above objectives, the technical solution provided by the present invention is as follows: In a first aspect, the present invention provides a photo-controlled oxidation-reduction dual-mode biochar, the core innovation of which lies in the construction of a microscopic, time-sequential nitrification-denitrification reactor, comprising: Porous biochar framework; A semiconductor photocatalytic material, wherein the semiconductor photocatalytic material is grown in situ and loaded onto the porous biochar framework, and is connected to the porous biochar framework by chemical bonds to form a heterojunction interface; The non-metallic dopant element includes one or two of nitrogen and sulfur; the non-metallic dopant element is embedded in the porous biochar framework carbon lattice to form an electron-rich micro-region with lone pair electrons in the porous biochar framework, which serves as a trap for capturing and storing photogenerated electrons. Under visible light irradiation, the photo-controlled oxidation-reduction dual-mode biochar utilizes the generated photogenerated holes to oxidize and degrade organic pollutants and convert ammonia nitrogen into nitrate nitrogen, while simultaneously capturing and storing photogenerated electrons; under dark conditions, it spontaneously releases the stored electrons to reduce the nitrate nitrogen into nitrogen gas.

[0008] Preferably, the specific surface area (Brunauer-Emmett-Teller, BET method) of the porous biochar framework is 300~800 m². 2 / g, total pore volume is 0.25~0.65 cm³ 3 / g, and the pore size distribution is mainly mesopores of 2~10 nm, with the mesopore volume accounting for ≥60% of the total pore volume.

[0009] Preferably, the semiconductor photocatalytic material includes at least one of graphitic carbon nitride (g-C3N4), titanium dioxide (TiO2), and cadmium sulfide (CdS).

[0010] The core optimization logic of semiconductor photocatalytic materials lies in the fact that semiconductor photocatalytic materials must meet the following common characteristics in order to form an effective dual-mode system with the doped biochar framework of this invention: First, a suitable band structure is required: the conduction band (CB) potential of the semiconductor must be higher than the defect levels formed by pyridine nitrogen and other components in the doped porous biochar framework, thus creating a driving force for electron transfer from the semiconductor to the biochar traps under illumination. Simultaneously, its valence band (VB) potential must be sufficiently positive to generate strongly oxidizing holes. g-C3N4, anatase TiO2, and CdS all meet this condition.

[0011] Secondly, the ability to form heterojunctions with biochar: the selected semiconductor precursor can react in situ with the biomass carbon source during pyrolysis, forming a tight chemically bonded heterojunction interface through CN bonds (such as graphitic carbon nitride) or CO-Ti bonds (such as titanium dioxide). This interface is the physical basis for achieving lossless and efficient transfer of photogenerated electrons to biochar.

[0012] Third, visible light responsiveness: In order to make full use of solar energy, the present invention preferably uses visible light responsive semiconductors such as graphitic carbon nitride and cadmium sulfide, or titanium dioxide modified by band engineering.

[0013] Guided by the above design logic, the selection of semiconductor photocatalytic materials used in this invention has general applicability, and those skilled in the art can choose appropriate types according to actual needs and conditions. As presented in one or more embodiments of this invention, both graphitic carbon nitride and oxide semiconductor titanium dioxide can construct high-performance dual-mode systems. The core inventive point of this invention lies in the synergistic design of the semiconductor-electron trap, rather than being limited to a specific semiconductor material. Therefore, based on the principles and embodiments disclosed above, those skilled in the art can anticipate that replacing the semiconductor with materials such as cadmium sulfide, which have similar band structure characteristics, can also achieve the technical effects claimed by this invention.

[0014] Preferably, the semiconductor photocatalytic material accounts for 5% to 30% of the total mass of the photocontrolled oxidation-reduction dual-mode biochar.

[0015] The loading of semiconductor photocatalytic material is one of the key factors for further optimizing the dual-mode function, and its mass percentage should be appropriately controlled within the aforementioned preferred range. If the percentage is less than 5%, the photogenerated charge density generated by the semiconductor component is insufficient, resulting in low daytime oxidation efficiency and too little total energy storage electrons available for reduction at night, making it impossible to effectively achieve the dual-mode function. If the percentage is greater than 30%, the excessive semiconductor material will agglomerate and excessively cover the pores and non-metallic doping sites on the surface of the biochar framework. On the one hand, this blocks the mass transfer channels of pollutants and reduces adsorption performance; on the other hand, it hinders the effective transfer and storage of photogenerated electrons to the doping sites, thus reducing electron storage efficiency.

[0016] Preferably, the non-metallic dopant element accounts for 1.5% to 6.5% of the total mass of the photocontrolled oxidation-reduction dual-mode biochar.

[0017] Preferably, when the non-metallic dopant element includes nitrogen, the nitrogen is in the form of pyridine nitrogen and graphitic nitrogen, and the atomic ratio of pyridine nitrogen to graphitic nitrogen is 1:1 to 3:1; CN covalent bonds are formed between the semiconductor photocatalytic material and the porous biochar framework.

[0018] In a second aspect of the invention, a method for preparing photo-controlled oxidation-reduction dual-mode biochar according to the first aspect of the invention is provided, employing a one-step pyrolysis process, comprising the following steps: Biomass containing non-metallic dopants is used as a carbon source and dopant source, mixed with a precursor of semiconductor photocatalytic material, and pyrolyzed in an inert atmosphere to obtain photo-controlled oxidation-reduction dual-mode biochar. During pyrolysis, biomass forms a porous biochar framework, and non-metallic dopants in the biomass are embedded in the carbon lattice of the porous biochar framework; the precursor forms a semiconductor photocatalytic material and is uniformly loaded on the surface of the porous biochar framework formed by biomass.

[0019] Preferably, the pyrolysis temperature is 475~700 ℃.

[0020] Nitrogen and sulfur, as non-metallic elements commonly found in biomass, can be released in situ during biomass pyrolysis and efficiently embedded into the formed biochar lattice, forming nitrogen / sulfur doping sites that serve as key electron traps.

[0021] Specifically, considering the application background of this invention in agricultural non-point source pollution control and wetland ecological restoration, wetland plant residues, which are widely distributed in southern regions and often cause ecological burdens, are prioritized as core biomass raw materials. For example, common aquatic plants such as *Myriophyllum spicatum*, *Vallisneria natans*, *Potamogeton crispus*, and *Hydrilla verticillata* (submerged plants), and emergent plants such as *Typha orientalis*, *Acorus calamus*, lotus, *Zizania latifolia*, and canna lily. These plants are rich in cellulose, lignin, and a certain amount of protein and alkaloids; their residues decaying in water bodies are a significant source of secondary pollution. By utilizing these discarded wetland plant resources and preparing the NS-BC precursor of this invention through pyrolysis, not only is the problem of wetland plant residue disposal solved, realizing "turning waste into treasure," but the prepared biochar material can also be combined with other materials, reducing pollution at the source while providing an efficient carrier for pollution control, forming a complete ecological closed loop.

[0022] In addition, those skilled in the art may select other nitrogen- and sulfur-rich waste biomass, including but not limited to: Protein-rich agricultural processing wastes: such as soybean residue, peanut meal, rapeseed cake, corn gluten meal, and distiller's grains; Waste from mushroom production: such as waste shiitake mushroom substrate and oyster mushroom substrate; Animal-derived waste: such as discarded shrimp shells, crab shells, feathers, and silkworm excrement; Plant waste rich in sulfur compounds: such as garlic peels, onion peels, and mustard stalks.

[0023] This invention is not limited to the specific biomass listed above. Any biomass that can form a porous carbon structure with a certain amount of nitrogen and sulfur doping after pyrolysis is within the scope of protection of this invention.

[0024] In the preparation of photocontrolled oxidation-reduction dual-mode biochar, the selection of appropriate biomass and precursor materials can achieve the desired results. For example, in practice, protein-rich agricultural processing waste is used as biomass to provide both carbon and dopant sources, and is mixed with melamine as a precursor for graphitic carbon nitride. Subsequently, during high-temperature pyrolysis under an inert atmosphere, based on a one-step thermal polymerization method, melamine undergoes in-situ polymerization and decomposition to form graphitic carbon nitride nanosheets, which are uniformly loaded onto the surface of the biochar. Graphitic carbon nitride is a visible-light-responsive semiconductor, responsible for the "photocontrolled switch" function. Nitrogen and sulfur elements in the biomass are embedded into the carbon lattice of the biochar at high temperatures, forming structures such as pyridine nitrogen and graphitic nitrogen. These structures possess lone pairs of electrons, serving as excellent "electron storage repositories."

[0025] By adjusting the pyrolysis conditions, the ratio of the two key nitrogen configurations, pyridine nitrogen and graphitic nitrogen, can be precisely controlled, with the preferred atomic ratio being 1:1 to 3:1. These two nitrogen configurations play different but indispensable roles in the "electron storage-release" mechanism of this invention: Firstly, pyridine nitrogen is located at the edges or defects of the carbon lattice, and its lone pair electrons do not participate in the π-conjugated system, forming a highly electron-rich microregion with strong electronegativity. It can instantaneously and efficiently capture high-energy photogenerated electrons transferred from semiconductors, making it the first key node in the electron storage process.

[0026] Secondly, graphitic nitrogen replaces carbon atoms within the carbon lattice, providing an electron to the π-conjugated system and modulating the electron cloud density of surrounding carbon atoms. It can stabilize and delocalize electrons trapped by pyridine nitrogen and provide a transport path for the orderly release of electrons at night.

[0027] Based on the above functional division, the ratio between the two is crucial. If the ratio of pyridine nitrogen is too low (<1:1), the capture efficiency of photogenerated electrons is low, and most electrons recombine with holes before they can be stored. If the ratio of pyridine nitrogen is too high (>3:1), that is, if there is too little graphite nitrogen, after a large number of electrons are captured, there is a lack of effective stabilization and transport channels, which makes them prone to reverse recombination, resulting in the stored electrons not being able to be released smoothly and effectively at night.

[0028] In a third aspect of the invention, the application of the photo-controlled oxidation-reduction dual-mode biochar of the first aspect of the invention is provided, including: as an environmental functional material for removing total nitrogen, total phosphorus and / or pesticide-like organic pollutants from water bodies.

[0029] In a fourth aspect of the present invention, based on an application of the third aspect of the present invention, a self-purifying biological buffer zone is provided, comprising: An interception ditch and a composite filler system filling the interception ditch; The composite packing system comprises, from bottom to top: A graded gravel support layer is used for water permeability and physical interception; the thickness ratio of the graded gravel support layer, the dual-mode biochar functional layer and the soil-aquatic plant surface layer is 12:35:1~2. The dual-mode biochar functional layer is made by mixing and compacting photocontrolled oxidation-reduction dual-mode biochar with sand at a mass ratio of 1:5 to 1:20. Soil - the surface layer of aquatic plants, with a soil cover layer planted with emergent plants.

[0030] In a fifth aspect of the invention, a method for using a self-purifying biological buffer zone, as described in the fourth aspect of the invention, to purify farmland wastewater containing compound pollution is provided. This method utilizes the natural alternation of day and night to achieve sequential treatment of different pollutants, and includes the following steps: (1) During the daytime when there is sunlight, the oxidation mode of photo-controlled oxidation-reduction dual-mode biochar is used to degrade organic pollutants in the water, while simultaneously reducing ammonia nitrogen (NH4) + -N) is oxidized to nitrate nitrogen (NO3). - -N), and adsorb phosphates; (2) At night when there is no light, the reduction mode of the photo-controlled oxidation-reduction dual-mode biochar is used to release the electrons stored during the day and reduce the nitrate nitrogen in the water to nitrogen gas (N2) in situ, thereby achieving deep removal of total nitrogen.

[0031] Preferably, the hydraulic retention time (HRT) of farmland drainage flowing through the self-purifying biological buffer zone is controlled to be 12-24 h to ensure that the polluted water entering the buffer zone can undergo at least one complete light-dark cycle.

[0032] Preferably, the organic pollutants include at least one of herbicides, pesticides, and fungicides; the nitrogen and phosphorus pollutants include total nitrogen (TN) and ammonia nitrogen (NH4). + -N), nitrate nitrogen (NO3) - At least one of (-N) and total phosphorus (TP).

[0033] Based on the above technical solutions and applications, the design concept and principle of this invention are as follows: Existing research on biochar modification or semiconductor composites (such as constructing Z-scheme structures) aims to accelerate the transfer and recombination of photogenerated electrons and holes, minimizing the residence time of electrons within the material. A bias in existing technologies is that lattice defects are typically considered recombination centers for photogenerated carriers and should be eliminated. This invention takes the opposite approach, artificially creating numerous controllable defects (pyridine nitrogen) in the porous biochar framework using nitrogen and / or sulfur doping. Instead of eliminating these "electron traps," these defects are used as microscopic "electron reservoirs," achieving absolute separation of photogenerated charges over time, thus obtaining an unexpected dual-mode self-purification effect.

[0034] 1. A novel synergistic purification mechanism targeting nitrogen, phosphorus, and pesticides: Daytime: Oxidation / nitrification and adsorption mode (photocatalysis dominant) Under daylight, semiconductor components generate high-energy electron-hole pairs. Among them, strongly oxidizing holes (h... + It undertakes a dual task: first, it efficiently degrades recalcitrant organic matter such as pesticides in water through direct oxidation or the generation of reactive oxygen species (such as ·OH); second, with the participation of oxygen enriched on the surface of biochar, it decomposes adsorbed ammonia nitrogen (NH4+). + -N) is oxidized to nitrate nitrogen (NO3). --N), to complete the nitrification process in the traditional denitrification process.

[0035] Meanwhile, photogenerated electronics (e - The phosphorus is efficiently captured and stored by the nitrogen / sulfur doping sites in the biochar framework, acting as a "recharge" for the nighttime reaction. The well-developed porous structure of the biochar itself continuously adsorbs phosphates (total phosphorus) from the water.

[0036] 2. Nighttime: Reduction / denitrification mode (energy storage and release dominance) As night falls and sunlight disappears, the photocatalytic process ceases. At this time, the large number of electrons stored in the nitrogen / sulfur doping sites during the day begin to be released slowly and continuously. These electrons act as efficient electron donors, converting the nitrate nitrogen (NO3) generated during the day into nitrogen and sulfur doping sites. - -N acts as an electron acceptor, reducing it in situ to harmless nitrogen gas (N2), thus completing the denitrification process. This process requires no additional carbon source and is a highly efficient form of autotrophic denitrification. The adsorbed phosphate is then stably immobilized in the biochar carrier.

[0037] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1. Achieved complete nitrogen removal within a single material. This invention cleverly utilizes the presence or absence of light to realize the sequential switching between "aerobic (aerobic oxidation-like environment) - anoxic (reducing environment)" at the microscopic scale, completing the entire conversion path from ammonia nitrogen to nitrogen gas on a single material, thus solving the problem of low total nitrogen removal rate in traditional buffer zones.

[0038] 2. Synergistic purification of complex pollution: This invention couples multiple processes such as organic matter degradation, nitrification, denitrification and phosphorus removal into one system, achieving "one-stop" efficient treatment of agricultural non-point source complex pollutants, far exceeding the effect of simply mixing multifunctional materials.

[0039] 3. True self-purification and sustainable operation. Through nighttime denitrification, nitrogen on the material surface is converted and removed, effectively clearing away temporary residues and allowing it to continue efficiently processing newly introduced pollutants the following day. This "daytime operation - nighttime cleanup" cycle gives the buffer zone continuous and renewable purification capabilities, completely eliminating the limitations of adsorption saturation.

[0040] 4. Green and energy-saving. The energy for the entire advanced oxidation and reduction process comes entirely from solar energy, requiring no external power source or chemical reagents, resulting in extremely low operating costs and perfectly aligning with the green and sustainable concept of ecological restoration projects. Attached Figure Description

[0041] Figure 1This is a schematic diagram illustrating the diurnal working mechanism of the photocontrolled oxidation-reduction dual-mode biochar described in this invention against agricultural non-point source pollution. The diagram shows the functional switching of the material in (a) daytime mode and (b) nighttime mode: Daytime mode (a): Under light, semiconductor (9) generates electrons (10) and holes (8). Holes (8) oxidize and degrade organic pollutants (7) such as pesticides into non-toxic products (5), and oxidize ammonia nitrogen (not separately labeled in the figure, but can be considered as part of the pollutants) into nitrate nitrogen (NO3). - Meanwhile, electrons (10) are stored in nitrogen / sulfur doping sites (11), and phosphate (TP) is adsorbed by the biochar framework.

[0042] Night mode (b): In the dark, the stored electrons are released, reducing the nitrate nitrogen (3) in the water to harmless nitrogen gas (13), which escapes from the water body (4), thus completing denitrification.

[0043] Figure 2 The N2 adsorption-desorption isotherm and BJH (Barrett-Joyner-Halenda) pore size distribution diagram of the photocontrolled oxidation-reduction dual-mode biochar in Example 2 are shown. Figure 3 The N 1s high-resolution X-ray photoelectron spectroscopy (XPS) spectrum of the photocontrolled oxidation-reduction dual-mode biochar in Example 2 is shown. Figure 4 This is a time-series test diagram of electron paramagnetic resonance (EPR) of the photo-controlled oxidation-reduction dual-mode biochar in Example 2; Figure 5 The time-current (it) curve of the dark discharge stage obtained by electrochemical testing in Example 4; In the above diagram, 1: Daytime mode; 2: Photo-controlled oxidation-reduction dual-mode biochar; 3: NO3 - ; 4: Water body; 5: Non-toxic products (CO2, H2O); 6: Organic pollutants with preliminary decomposition and chain breaks; 7: Organic pollutants; 8: Holes (h + ); 9: g-C3N4; 10: electron (e-); 11: nitrogen / sulfur doping site; 12: NH4 + 13: N2; 14: Night mode. Detailed Implementation

[0044] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0045] The present invention is further illustrated below through examples. To more systematically demonstrate the technical advantages of the present invention, the following examples compare the photocontrolled oxidation-reduction dual-mode biochar (g-CN / NS-BC) prepared by the present invention with several key comparative materials.

[0046] Comparative Example 1: Ordinary biochar without any modification (made from rice husks, pyrolyzed at 800℃).

[0047] Comparative Example 2: Pure semiconductor photocatalyst g-C3N4.

[0048] Comparative Example 3: Ordinary biochar and g-C3N4 were simply physicomechanically mixed (mass ratio 7:3).

[0049] Example 1 This embodiment provides a photocontrolled oxidation-reduction dual-mode biochar (g-CN / NS-BC) co-doped with g-C3N4 and nitrogen and sulfur, and its preparation method. The purification efficiency of the dual-mode biochar for typical farmland runoff compound pollution was also tested.

[0050] The preparation method of photocontrolled oxidation-reduction dual-mode biochar in this embodiment is as follows: 10 g of dried soybean residue powder was uniformly ground and mixed with 10 g of melamine. The mixture was placed in a tube furnace and heated to 550 °C at a rate of 5 °C / min under nitrogen protection, and held at that temperature for 2 h. After naturally cooling to room temperature, the resulting black powder was removed, ground, and passed through a 100-mesh sieve to obtain g-C3N4 / nitrogen-sulfur co-doped photocontrolled oxidation-reduction bimodal biochar, denoted as g-CN / NS-BC.

[0051] This embodiment simulates a real farmland runoff environment to evaluate the material's synergistic purification capacity for combined nitrogen, phosphorus, and pesticide pollution.

[0052] Purification experiment: 0.1 g of material and 100 mL of simulated agricultural runoff wastewater were added to a batch reactor. Wastewater parameters were configured as follows: Total nitrogen (TN) 15 mg / L (including NH4+). + -N accounts for 12 mg / L, NO3 - The concentrations of nitrogen (N) were 3 mg / L, total phosphorus (TP) was 1.0 mg / L, and the representative herbicide atrazine (ATZ) was 200 μg / L. A 24-hour cycle test (12h light / 12h darkness) was conducted using a 300 W xenon lamp to simulate sunlight. The results are shown in Table 1.

[0053] Table 1. 24-hour removal rate (%) of each material for simulated farmland runoff combined pollution

[0054] The material of this invention (Example 1) exhibits extremely high synergistic removal efficiency for all three pollutants. Its TN removal rate (88.6%) is far higher than that of all control groups, proving the successful realization of the dual-mode nitrogen removal pathway of "daytime nitrification + nighttime denitrification". In contrast, the control groups can only degrade organic matter (Comparative Example 2) or can only perform limited physical adsorption (Comparative Examples 1 and 3), and cannot achieve efficient removal of total nitrogen.

[0055] Example 2 This embodiment characterizes and verifies the microstructure and photocontrolled electron storage capacity of photo-controlled oxidation-reduction dual-mode biochar (g-CN / NS-BC) to confirm the existence and working mechanism of the "electron trap" in this invention from a microscopic mechanism perspective.

[0056] The microstructure and mechanism are characterized as follows: I. Pore Structure Analysis: The photo-controlled oxidation-reduction dual-mode biochar was tested according to the method and procedures provided in GB / T 19587-2017 "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method". The N2 adsorption-desorption isotherms and BJH pore size distribution of the photo-controlled oxidation-reduction dual-mode biochar are shown in the figure below. Figure 2 As shown.

[0057] Figure 2 The results show that the adsorption-desorption isotherm is a typical type IV curve, accompanied by an H3 type hysteresis loop, indicating that the material contains a large number of cracks and pores formed by the accumulation of lamellar particles. N2 adsorption-desorption tests show that the specific surface area of ​​g-CN / NS-BC is as high as 452 m². 2 / g, total pore volume is 0.41 cm³ 3 / g. Its BJH pore size distribution curve shows that the material is dominated by mesopores of 3~8 nm (accounting for 68% of the total pore volume). This abundant mesoporous structure not only provides an anchor point for the in-situ growth of g-C3N4, but also facilitates the growth of phosphate and NH4. + -N provides ample adsorption sites. These results demonstrate that the material prepared in this invention possesses a high specific surface area and abundant mesoporous structure, providing an excellent physical basis for efficient mass transfer and adsorption of pollutants, as well as the full exposure of active sites.

[0058] II. X-ray photoelectron spectroscopy interface and defect analysis: X-ray photoelectron spectroscopy was used to characterize the photocontrolled oxidation-reduction dual-mode biochar. Figure 3 The image shows the XPS N 1s high-resolution spectrum of this material.

[0059] Figure 3The information shows that the N 1s spectrum can be deconvoluted into two main peaks, corresponding to pyridine nitrogen with a binding energy of 398.5 eV and graphitic nitrogen with a binding energy of 401.2 eV, respectively. Calculations using peak area integration show that the atomic ratio of pyridine nitrogen to graphitic nitrogen is approximately 1.6:1, with pyridine nitrogen accounting for 62%. This high content of pyridine nitrogen forms strong electronegative microregions (lone pairs of electrons) at the carbon lattice edges, becoming ideal traps for capturing photogenerated electrons. This spectrum directly confirms that the present invention successfully constructed a high-proportion pyridine nitrogen structure as an "electron trap" within the biochar framework, providing crucial structural evidence for the efficient capture and storage of photogenerated electrons. It is also a key structural basis for constructing an "electron trap" to store electrons required for denitrification.

[0060] Furthermore, the XPS C 1s spectrum showed a distinct NC=N covalent bond characteristic peak at 288.2 eV (not shown in the figure), confirming the tight chemical bond between the semiconductor and the biochar framework.

[0061] III. Electron Paramagnetic Resonance Test: In-situ photo-controlled oxidation-reduction dual-mode biochar was characterized using in-situ photoelectron paramagnetic resonance (EPR) technology. Figure 4 This is the EPR timing test diagram for this material.

[0062] like Figure 4 As shown, EPR tests directly observed the phenomenon of a large number of electrons being captured under illumination and slowly released in the dark. This provides the most direct evidence for "carbon-free" autotrophic denitrification at night, namely that the electron donor comes from photogenerated electrons stored during the day.

[0063] Figure 4 The data shows that under dark conditions, the electron paramagnetic resonance signal (corresponding to free electrons) of the material is weak; however, after 30 minutes of simulated sunlight irradiation, the signal intensity rapidly increases and reaches a plateau, exhibiting an extremely strong symmetrical singlet carbon radical signal at g = 2.0034, indicating that a large number of single electrons are trapped in carbon lattice defects. Subsequently, after the light was turned off, the signal intensity did not disappear immediately, but rather exhibited a slow decay process lasting several hours. These results demonstrate that the material of this invention can capture and stabilize a large number of electrons under light and release them slowly in the dark. This is the most direct and crucial experimental evidence for the core "daytime energy storage-nighttime discharge" working mechanism of this invention.

[0064] Example 3 This embodiment, as a degradation example of herbicides and toxic elements, provides a photocontrolled oxidation-reduction bimodal biochar (TiO2 / N-BC) and its preparation method, and provides the purification of the herbicide atrazine (ATZ) and pentavalent arsenic (As(V)) by the material. This test can be regarded as an application example of photocontrolled oxidation-reduction bimodal biochar.

[0065] The preparation method of photocontrolled oxidation-reduction dual-mode biochar in this embodiment is as follows: 10 g of corn stalk powder was impregnated in 50 mL of 28% ammonia water for 12 h and then dried. It was then mixed thoroughly with 5 g of tetrabutyl titanate and pyrolyzed at 500 °C for 2 h under a nitrogen atmosphere. The resulting material is anatase-type titanium dioxide in-situ grown and nitrogen-doped photocontrolled oxidation-reduction bimodal biochar, denoted as TiO2 / N-BC.

[0066] The control group materials in this embodiment include: Pure biochar (BC): Corn stalk powder treated only with ammonia water is pyrolyzed under the same conditions.

[0067] The purification application experiment steps in this embodiment are as follows: A miniature ecological ditch model was constructed, filled with TiO2 / N-BC as the matrix. Simulated farmland drainage containing 100 μg / L atrazine and 50 μg / L was introduced. Long-term observation was conducted for 30 days using natural light. The reduction product of As(V) was trivalent arsenic (As(III)), which is less toxic and more easily adsorbed and fixed. The test results are shown in Table 2.

[0068] Table 2. Experimental test data for TiO2 / N-BC purification applications

[0069] Thirty days later, samples were taken from the matrix of the model of this invention, and the valence state of arsenic was analyzed by X-ray photoelectron spectroscopy. It was found that 85% of the arsenic existed in the form of As(III) and was firmly locked on the surface of biochar. In contrast, the arsenic in the pure BC matrix of the control group was still mainly As(V), and the release risk was found to be 3 times higher than that of the group of this invention through leaching experiments.

[0070] The above tests demonstrate that the proposed solution is universally applicable, and the same excellent day-night dual-mode purification effect can be achieved by replacing the semiconductor (TiO2) and the pollutants (atrazine, As(V)). Long-term operational data proves that this technology can effectively reduce the risk of toxic pollutant accumulation in the buffer zone and achieve long-term stable purification.

[0071] Example 4 This embodiment quantifies the dual core functions of photo-controlled oxidation-reduction dual-mode biochar, namely (1) photo-controlled electron storage and release capacity, and (2) efficient adsorption capacity for phosphate, providing quantitative scientific basis for the "daytime energy storage-nighttime nitrogen removal" and "efficient phosphorus removal" mechanisms of this invention.

[0072] (1) Quantification of photoelectric storage and release capabilities The testing steps are as follows: The g-CN / NS-BC prepared in Example 1, along with Comparative Example 1 (ordinary biochar BC) and Comparative Example 3 (physical mixture Mix), were used as working electrodes. Chronoamperometry (it) tests were performed in a three-electrode system. First, the electrodes were "charged" under simulated light at an open-circuit potential for 30 min to store electrons. Then, the system was switched to a dark environment and "discharged" at +0.25 V vs. RHE (this potential ensures that only stored electrons are released without inducing water oxidation), and the discharge current was recorded. The total released charge was obtained by integrating the discharge curve. The test results and theoretical conversion capacity calculations are shown in Table 3.

[0073] Table 3. Photocontrolled electron storage and release capacity and theoretical nitrogen removal efficiency of various materials

[0074] Electronic storage capacity analysis: Figure 5 This figure shows the chronocurrent (it) curve of the dark discharge stage obtained by electrochemical testing in this embodiment. The information in the figure shows that the g-CN / NS-BC electrode of this invention can release a strong and sustained anodic current in the dark, with an integrated charge (35.8 C / g) far exceeding that of the control group. Combined with the test results in Table 3, the amount of charge released by the material of this invention in the dark is nearly 30 times that of the physical mixing group and more than 70 times that of pure biochar. This demonstrates that a highly efficient heterojunction is formed between the nitrogen-sulfur doped biochar framework and g-C3N4, enabling photogenerated electrons to be efficiently captured, stored, and released in the dark.

[0075] Theoretical nitrogen removal efficiency analysis: Based on the core chemical equation of the denitrification reaction 2NO3 - +12H + +10e - →N2+6H2O, it can be seen that the reduction of 1 mole of nitrate nitrogen (NO3) - -N), requires the consumption of 5 moles of electrons. Based on the Faraday constant (F≈96485 C / mol), we can perform the following calculations: The molar amount of electrons that g-CN / NS-BC can provide = 35.8 C / g / 96485 C / mol = 3.71 * 10 -4 mol / g = 371 μmol / g.

[0076] Theoretically reducible NO3 - -N molar amount = (371 μmol / g) / 5 = 74.2 μmol / g.

[0077] Theoretically reducible NO3 - -N mass = 74.2 μmol / g * 14 g / mol (molar mass of N) = 1039 μg / g = 1.04 mg / g.

[0078] The calculation results clearly show that, theoretically, each gram of the material of this invention stores enough electrons after one light cycle to completely reduce 1.04 mg of nitrate nitrogen to nitrogen gas. This powerful intrinsic reducing ability is the core of this invention's efficient, carbon-source-free denitrification, and its fundamental advantage over all existing adsorption or microbial technologies.

[0079] (2) Quantification of phosphate adsorption capacity To characterize the material's response to phosphorus pollutants (in the form of PO4) 3- To assess the removal capacity of phosphate (based on P), this section presents phosphate adsorption isotherm experiments.

[0080] 0.1 g of each material was added to 100 mL of phosphate solutions with different initial concentrations (5-100 mg / L), and the solutions were incubated at 25 °C with shaking for 24 h until adsorption equilibrium was reached. The remaining phosphate concentration in the solution after equilibrium was measured, the adsorption capacity was calculated, and the Langmuir adsorption isotherm model (Q) was used. e =(Q_max*K_L*C e ) / (1+K_L*C e The data were fitted to obtain the key parameter—the theoretical maximum adsorption capacity (Q_max). The results are shown in Table 4.

[0081] Table 4. Maximum Langmuir adsorption capacity (Q_max) of phosphate for each material

[0082] Adsorption capacity comparison: As shown in Table 4, the maximum adsorption capacity (Q_max) of the material g-CN / NS-BC of the present invention for phosphate is as high as 8.55 mg / g, which is 2.74 times and 2.20 times that of ordinary biochar and physical mixture, respectively.

[0083] Mechanism Correlation: This significant advantage is attributed to the synergistic design of this invention. The one-step pyrolysis method not only constructs the optoelectronic functionality but also optimizes the physical structure of the material. As shown in Example 2, g-CN / NS-BC exhibits a high 452 m... 2The high specific surface area ( / g) and abundant mesoporous structure provide a large number of active adsorption sites for phosphates. Furthermore, nitrogen and sulfur doping may introduce Lewis basic sites, further enhancing the ability to capture phosphate anions through electrostatic attraction or coordination.

[0084] This embodiment comprehensively demonstrates the dual-functional advantages of the material of this invention through quantitative characterization from both electrochemical and adsorption kinetic dimensions: on the one hand, its unique electronic structure makes it a highly efficient "photocontrolled electron pool" with a theoretical nitrogen removal potential of 1.04 mg / g; on the other hand, its optimized physicochemical surface makes it an excellent phosphorus adsorbent with a phosphorus removal capacity of 8.55 mg / g. The organic combination of these two functions enables this invention to exhibit a synergistic purification effect and practical value far exceeding existing technologies in the treatment of nitrogen and phosphorus combined pollution.

[0085] Example 5 This embodiment provides a mechanism verification experiment for the nitrogen conversion pathway, aiming to accurately verify the two-step denitrification mechanism of the present invention, namely, "daytime nitrification-nighttime denitrification", by controlling the initial nitrogen source form.

[0086] Experimental setup: Two parallel experiments were set up, both using g-CN / NS-BC prepared in Example 1.

[0087] Group A (pure ammonia nitrogen source): Wastewater contains 15 mg / L TN (all as NH4). + -N).

[0088] Group B (pure nitrate nitrogen source): Wastewater contains 15 mg / L TN (all NO3). - -N).

[0089] Test procedure: Both groups underwent a 24-hour cycle (12 hours of light + 12 hours of darkness). Samples were taken at the 12th hour (end of light) and the 24th hour (end of darkness) to analyze the NH4 in the water. + -N, NO3 - The concentrations of -N and TN were determined. The results are shown in Table 5.

[0090] Table 5. Nitrogen form transformation and total nitrogen removal rate under different initial nitrogen sources

[0091] In group A, after 12 hours of light exposure, NH4 + -N is efficiently oxidized to NO3. - -N, but the total nitrogen removal rate was extremely low at this time, proving that the main function of the light stage was nitrification. However, during the subsequent 12 hours of darkness, NO3... - A significant amount of -N was removed, and the total nitrogen removal rate soared to 88.0%, demonstrating that efficient denitrification occurred during the dark phase.

[0092] In group B, the initial pollutant is NO3. - During the light exposure phase, there is almost no TN removal. However, during the dark exposure phase, the total nitrogen removal rate is still as high as 90.0%.

[0093] This comparative experiment perfectly confirms the relay-style nitrogen removal mechanism of "daytime nitrification and nighttime denitrification" of the present invention, demonstrating its universality and high efficiency for different forms of nitrogen sources.

[0094] Example 6 This embodiment provides a simulation of high-concentration aquaculture wastewater purification. By simulating the treatment of high-concentration wastewater from intensive aquaculture areas, the performance of the present invention under high load is verified.

[0095] Wastewater preparation: Referring to the high-load scenario described in patent CN201310312559.3, wastewater was prepared with a COD of 300 mg / L and NH4+. + -N is 50 mg / L, and TP is 5 mg / L.

[0096] Experimental procedure: The material dosage was increased to 0.5 g / L, and a 24-hour cycle treatment (12 h light + 12 h darkness) was performed. The results are shown in Table 6.

[0097] Table 6. Purification effect of dual-mode biochar on high-concentration aquaculture wastewater

[0098] Faced with high concentrations of complex pollution far exceeding that of ordinary farmland runoff, the material of this invention can still achieve an excellent removal effect of over 90% by increasing the dosage, demonstrating strong resistance to shock loads and wide applicability.

[0099] Example 7 This embodiment demonstrates the engineering application effect of dual-mode biochar in a self-purifying biological buffer zone, completing the engineering application of the self-purifying biological buffer zone and optimizing the hydraulic retention time. A buffer zone system identical to the original scheme (10 m long, 2 m wide, 1 m deep) was constructed, with the functional layer using g-CN / NS-BC.

[0100] Operating parameters: Simulated farmland runoff wastewater (TN 15 mg / L, TP 1.0 mg / L, ATZ 200 μg / L) from Example 1 was introduced. Different hydraulic retention times (HRTs) were set by adjusting the inlet valve.

[0101] Observation results: When HRT < 8 h, the water body does not experience a complete dark phase before flowing out of the buffer zone, resulting in insufficient denitrification at night and a still high total nitrogen in the effluent.

[0102] When the HRT was set to 12–24 h, all water flows underwent at least one complete “day (oxidation / nitrification) – night (denitrification)” cycle. Detailed data are shown in Table 7.

[0103] Table 7. Effects of different HRTs on the purification efficiency of self-purifying biological buffer zones

[0104] Table 7 clearly shows that a 12-24 hour HRT (Heat Retention Time) is a key engineering parameter for activating and fully utilizing the dual-mode function of this invention. Only by ensuring that the polluted water undergoes a complete diurnal cycle within the buffer zone can efficient and deep removal of total nitrogen be achieved. This provides a direct scientific basis for practical engineering design and operation management.

[0105] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A photo-controlled oxidation-reduction dual-mode biochar, characterized in that, include: Porous biochar framework; A semiconductor photocatalytic material, wherein the semiconductor photocatalytic material is grown in situ and loaded onto the porous biochar framework, and is connected to the porous biochar framework by chemical bonds to form a heterojunction interface; The non-metallic dopant element includes one or two of nitrogen and sulfur; the non-metallic dopant element is embedded in the porous biochar framework carbon lattice to form an electron-rich micro-region with lone pair electrons in the porous biochar framework, which serves as a trap for capturing and storing photogenerated electrons. Under visible light irradiation, the photo-controlled oxidation-reduction dual-mode biochar utilizes the generated photogenerated holes to oxidize and degrade organic pollutants and convert ammonia nitrogen into nitrate nitrogen, while simultaneously capturing and storing photogenerated electrons; under dark conditions, it spontaneously releases the stored electrons to reduce the nitrate nitrogen into nitrogen gas.

2. The photo-controlled oxidation-reduction dual-mode biochar according to claim 1, characterized in that: The specific surface area of ​​the porous biochar framework is 300~800 m². 2 / g, total pore volume is 0.25~0.65 cm³ 3 / g, and the pore size distribution is mainly mesopores of 2~10 nm, with the mesopore volume accounting for ≥60% of the total pore volume.

3. The photo-controlled oxidation-reduction dual-mode biochar according to claim 1, characterized in that: The semiconductor photocatalytic material includes at least one of graphitic carbon nitride, titanium dioxide, and cadmium sulfide.

4. The photo-controlled oxidation-reduction dual-mode biochar according to claim 1, characterized in that: The semiconductor photocatalytic material accounts for 5% to 30% of the total mass of the photocontrolled oxidation-reduction dual-mode biochar; the non-metallic dopant element accounts for 1.5% to 6.5% of the total mass of the photocontrolled oxidation-reduction dual-mode biochar.

5. The photo-controlled oxidation-reduction dual-mode biochar according to claim 1, characterized in that: When the non-metallic dopant element contains nitrogen, the nitrogen is in the form of pyridine nitrogen and graphitic nitrogen, and the atomic ratio of pyridine nitrogen to graphitic nitrogen is 1:1 to 3:1; CN covalent bonds are formed between the semiconductor photocatalytic material and the porous biochar framework.

6. A method for preparing photocontrolled oxidation-reduction dual-mode biochar as described in any one of claims 1 to 5, characterized in that, The process employs a one-step pyrolysis technique, including the following steps: Biomass containing non-metallic dopants is used as a carbon source and dopant source, mixed with a precursor of semiconductor photocatalytic material, and pyrolyzed in an inert atmosphere to obtain photo-controlled oxidation-reduction dual-mode biochar. The pyrolysis temperature is 475~700 ℃; During pyrolysis, biomass forms a porous biochar framework, and non-metallic dopants in the biomass are embedded in the carbon lattice of the porous biochar framework. The precursor forms a semiconductor photocatalytic material and is uniformly loaded onto the surface of a porous biochar framework formed from biomass.

7. An application of photo-controlled oxidation-reduction dual-mode biochar as described in any one of claims 1 to 5, characterized in that, include: It is used as an environmental functional material to remove total nitrogen, total phosphorus and / or pesticide-like organic pollutants from water bodies.

8. A self-purifying biological buffer zone, characterized in that, It includes an interception ditch and a composite filler system filling the interception ditch, the composite filler system comprising, from bottom to top: A graded gravel support layer is used for water permeability and physical interception; the thickness ratio of the graded gravel support layer, the dual-mode biochar functional layer and the soil-aquatic plant surface layer is 12:35:1~2. The dual-mode biochar functional layer is formed by mixing and compacting photocontrolled oxidation-reduction dual-mode biochar as described in any one of claims 1 to 5 with sand at a mass ratio of 1:5 to 1:

20. Soil - the surface layer of aquatic plants, with a soil cover layer planted with emergent plants.

9. A method for using a self-purifying biological buffer zone as described in claim 8 to purify farmland mixed pollution drainage, characterized in that, The time-series treatment of different pollutants is achieved by utilizing the natural day-night cycle, including the following steps: (1) During the day when there is light, the oxidation mode of photo-controlled oxidation-reduction dual-mode biochar is used to degrade organic pollutants in water, while oxidizing ammonia nitrogen to nitrate nitrogen and adsorbing phosphate. (2) At night when there is no light, the reduction mode of the photo-controlled oxidation-reduction dual-mode biochar is used to release the electrons stored during the day and reduce the nitrate nitrogen in the water to nitrogen gas in situ, thereby achieving deep removal of total nitrogen.

10. The method for using a self-purifying biological buffer zone according to claim 9 to purify farmland mixed pollution drainage, characterized in that: The hydraulic retention time of farmland drainage flowing through the self-purifying biological buffer zone is controlled to be 12-24 h; the organic pollutants include at least one of herbicides, insecticides, and fungicides; the nitrogen and phosphorus pollutants include at least one of total nitrogen, ammonia nitrogen, nitrate nitrogen, and total phosphorus.

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