Iron cycle-based method for reducing nitrogen and carbon emission in coastal wetland

Abstract

The present application relates to the technical field of wetland ecological restoration, and particularly relates to a nitrogen reduction, carbon fixation and emission reduction method for coastal wetlands based on iron cycle. The present application mixes biochar raw materials with water, and then performs hydrothermal reaction to obtain biochar precursors; the biochar precursors are immersed in a tannic acid modification solution to perform modification to obtain modified biochar precursors; and the modified biochar precursors are pyrolyzed under the condition of ferrocene vapor and inert atmosphere to obtain modified biochar. The present application uses green algae and cordgrass to prepare biochar, and realizes nitrogen reduction, carbon fixation and emission reduction of coastal wetlands based on iron cycle. After adding the modified biochar, the total nitrogen and greenhouse gas emission of the soil in the coastal wetland are reduced compared with those of the control group, and the total carbon content is increased.

Description

Technical Field

[0001] This invention relates to the field of wetland ecological restoration technology, and in particular to a method for reducing nitrogen and carbon emissions in coastal wetlands based on iron cycling. Background Technology

[0002] Coastal wetlands, as key ecosystems at the interface of land and sea, play a vital role as a buffer in the global carbon and nitrogen cycle. Their sedimentary environment effectively sequesters organic carbon through anaerobic processes and reduces nitrogen load through processes such as denitrification, which is of great significance in mitigating global climate change and eutrophication. Notably, coastal wetlands also possess strong iron reduction and enrichment capabilities due to abundant iron sources and ideal "oxidation barrier" retention conditions. Iron redox processes mediated by microorganisms and oxygen are closely coupled with the migration and transformation of carbon and nitrogen elements.

[0003] In the anaerobic environment during high tide, dissimilatory iron-reducing bacteria utilize ferric oxides as electron acceptors to convert unstable dissolved organic matter into their own stable organisms. This process drives organic matter mineralization and accumulates microbial biomass carbon. This process couples ferroautotrophic denitrification by releasing ferrous iron and inhibits nitrous oxide production during incomplete nitrate reduction. Simultaneously, electron competition at the bottom layer influences methanogenesis. Ferrous iron diffuses with the tides, either being utilized by iron-oxidizing bacteria in the reducing layer or receiving ample oxygen in the oxidizing layer, thus promoting iron mineral regeneration and the binding and protection of organic carbon.

[0004] However, coastal wetlands are facing the dual threats of climate change and human activities. Rising sea levels, eutrophication, and coastal engineering disturbances have severely weakened iron-mediated ecological functions: oxidizing conditions enhance the chemical stability of iron minerals, thereby disrupting their coupling with the nitrogen cycle by inhibiting denitrification efficiency; simultaneously, long-term exposure reduces the activity of iron minerals, weakening their complexing and protective effects on organic matter and their ability to suppress greenhouse gas emissions. Therefore, a new approach is needed to overcome the shortcomings of existing coastal wetland sediment improvement technologies, such as insufficient remediation technology reserves, large engineering scale, high cost, small remediation area, and single remediation target. Summary of the Invention

[0005] The purpose of this invention is to provide a method for reducing nitrogen and carbon emissions in coastal wetlands based on iron cycling, so as to solve the problems of excessive nitrogen accumulation and declining carbon sink function in coastal wetlands.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing modified biochar, comprising the following steps:

[0008] (1) After mixing biochar raw materials and water, a hydrothermal reaction is carried out to obtain biochar precursor;

[0009] (2) The biochar precursor was immersed in tannic acid solution for modification to obtain modified biochar precursor;

[0010] (3) The modified biomass precursor was pyrolyzed under ferrocene vapor and inert atmosphere to obtain modified biochar.

[0011] The biochar raw materials include one or more of the following: Ulva prolifera, Spartina alterniflora, and barnacles.

[0012] Optionally, when the biochar raw material is a mixture of Ulva prolifera and Spartina alterniflora, the mass ratio of Ulva prolifera to Spartina alterniflora is 1:1.5~2; when the biochar raw material is a mixture of Ulva prolifera, Spartina alterniflora and barnacles, the mass ratio of Ulva prolifera, Spartina alterniflora and barnacles is 1:1.5~2:1.

[0013] Optionally, the ratio of biochar raw material to water is 100~200g:1~2L; the temperature of the hydrothermal reaction is 150~180℃, and the time is 1~1.5h.

[0014] Optionally, the concentration of the tannic acid solution is 5 g / L; the modification method is oscillation followed by ultrasound; the oscillation rate is 120-160 rpm, and the time is 2-4 h; the ultrasound frequency is 40-50 kHz, the power is 50-60 W, the temperature is 25-30 °C, and the time is 1-2 h.

[0015] Optionally, the method for preparing ferrocene vapor includes: heating ferrocene under a nitrogen atmosphere and continuously introducing nitrogen to sublimate the ferrocene vapor;

[0016] The mass ratio of ferrocene to modified biochar precursor is 1:20~30, the heating rate is 5~10℃ / min, the temperature is 230~250℃, the time is 4~6h, and the nitrogen gas introduction rate is 0.6~0.8L / min.

[0017] Optionally, the pyrolysis heating rate is 3~5℃ / min, the temperature is 350~375℃, and the time is 3~4h.

[0018] Optionally, the biochar precursor prepared in step (1) can be directly pyrolyzed under an inert atmosphere to obtain biochar;

[0019] The pyrolysis heating rate is 3~10℃ / min, the temperature is 350~850℃, and the time is 3~4h.

[0020] The present invention also provides modified biochar prepared by the above preparation method.

[0021] The present invention also provides biochar prepared by the above-described preparation method.

[0022] The present invention also provides a method for reducing nitrogen, sequestering carbon and reducing emissions in coastal wetlands based on iron cycle, wherein the modified biochar or the above-mentioned biochar is added to the soil of coastal wetlands affected by tides to achieve nitrogen reduction, carbon sequestration and emission reduction.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] This invention utilizes *Ulva prolifera* and *Spartina alterniflora* to prepare biochar, achieving nitrogen reduction, carbon sequestration, and emission reduction in coastal wetlands based on iron cycling. The coastal wetland soil after the addition of modified biochar showed reduced total nitrogen and greenhouse gas emissions compared to the control group, while increasing total carbon content. This invention achieves comprehensive utilization of the invasive species *Ulva prolifera* and *Spartina alterniflora*. The prepared biochar is green, environmentally friendly, and not easily lost. This method of nitrogen reduction, carbon sequestration, and emission reduction is green and economical, filling the gap in the preparation and application technology of effective functional materials in the geological restoration of coastal wetlands. Attached Figure Description

[0025] Figure 1 Here is a SEM-EDS image of the modified biochar from Example 2;

[0026] Figure 2 This is a SEM-EDS image of the original biochar from Example 1. Detailed Implementation

[0027] 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.

[0028] 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. Every smaller range between any stated value or intermediate value within a stated range, and 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.

[0029] 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.

[0030] 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 obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0031] 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.

[0032] All raw materials used in this invention can be obtained commercially or prepared using existing technologies.

[0033] This invention provides a method for preparing modified biochar, comprising the following steps:

[0034] (1) After mixing biochar raw materials and water, a hydrothermal reaction is carried out to obtain biochar precursor;

[0035] (2) The biochar precursor was immersed in tannic acid solution for modification to obtain modified biochar precursor;

[0036] (3) The modified biomass precursor was pyrolyzed under ferrocene vapor and inert atmosphere to obtain modified biochar.

[0037] The biochar raw materials include one or more of the following: Ulva prolifera, Spartina alterniflora, and barnacles.

[0038] This invention first mixes nearshore invasive species biochar raw materials and performs simple dehydration treatment, then washes them with deionized water to remove excess impurities and fragments. The biochar raw materials are then dried, pulverized, and passed through a 100-mesh sieve. The pulverized biochar raw materials are then placed in water and subjected to a hydrothermal reaction in a reactor to reduce unstable nitrogen components in the mixed raw materials and stabilize the biomass carbon structure, thereby obtaining a biochar precursor.

[0039] In an embodiment of the present invention, the drying temperature is 55°C and the drying time is 48 hours.

[0040] In this invention, when the biochar raw material is a mixture of Ulva prolifera and Spartina alterniflora, the mass ratio of Ulva prolifera to Spartina alterniflora is 1:1.5 to 2, for example, it can be 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9 or 1:2, etc.

[0041] When the biochar raw material is a mixture of Ulva prolifera, Spartina alterniflora and barnacles, the mass ratio of Ulva prolifera, Spartina alterniflora and barnacles is 1:1.5 to 2:1, for example, it can be 1:1.5:1, 1:1.6:1, 1:1.7:1, 1:1.8:1, 1:1.9:1 or 1:2:1, etc.;

[0042] The ratio of biochar raw material to water is 100~200g:1~2L, for example, it can be 100g:1L, 150g:1L, 200g:1L, 100g:2L, 150g:2L or 200g:2L, etc.

[0043] The hydrothermal reaction temperature is 150~180℃, for example, 150℃, 160℃, 170℃ or 180℃, and the time is 1~1.5h, for example, 1h, 1.2h or 1.5h.

[0044] The present invention involves immersing a dried biochar precursor in a tannic acid solution, vibrating and sonicating to fully mix the two, vacuum filtering, washing and drying to obtain a modified biochar precursor.

[0045] Understandably, the present invention modifies the biochar precursor with tannic acid solution, which can add phenolic hydroxyl functional groups to the surface of biochar, thereby improving the redox activity and electron transfer capacity of biochar.

[0046] In this invention, the tannic acid solution is prepared by dissolving tannic acid in water, and the concentration of the tannic acid solution is 5 g / L;

[0047] The modification method is oscillation followed by ultrasound; the oscillation rate is 120~160 rpm, for example, 120 rpm, 140 rpm, 150 rpm or 160 rpm, etc., and the time is 2~4 h, for example, 2 h, 3 h or 4 h, etc.; the ultrasound frequency is 40~50 kHz, for example, 40 kHz, 45 kHz or 50 kHz, etc., the power is 50~60 W, for example, 50 W, 55 W or 60 W, etc., the temperature is 25~30℃, for example, 25℃, 28℃ or 30℃, etc., and the time is 1~2 h, preferably 2 h.

[0048] The present invention involves pyrolyzing a modified biomass precursor under ferrocene vapor and an inert atmosphere, and then cooling it to room temperature to obtain modified biochar.

[0049] Understandably, during pyrolysis, ferrocene steam is continuously introduced to form zero-valent iron nanoparticles on the surface of the graphite carbon shell. This allows the iron in the modified biochar to participate in the biogeochemical cycle of iron together with the primary iron minerals in the coastal environment, thereby coupling iron cycle-driven carbon sequestration and nitrogen removal. Carbon sequestration is achieved through the biological assimilation of iron-related microorganisms and the preservation of mineral-related organic carbon, while nitrogen removal is related to iron autotrophic denitrification.

[0050] In this invention, the method for preparing ferrocene vapor includes: placing ferrocene powder in the low-temperature zone of a dual-temperature zone tube furnace, continuously introducing nitrogen gas throughout the heating and holding process, and the continuous introduction of nitrogen gas causing the ferrocene vapor to sublimate and diffuse into the high-temperature zone of pyrolysis.

[0051] In this invention, the mass ratio of ferrocene to modified biochar precursor is 1:20~30, for example, it can be 1:20, 1:25 or 1:30, etc.

[0052] The heating rate is 5~10℃ / min, for example, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min, etc.; the temperature is 230~250℃, for example, 230℃, 240℃ or 250℃, etc.; the time is 4~6h, for example, 4h, 5h or 6h, etc.; and the nitrogen gas introduction rate is 0.6~0.8L / min, for example, 0.6L / min, 0.7L / min or 0.8L / min, etc.

[0053] In this invention, the heating rate of the pyrolysis is 3~5℃ / min, for example, 3℃ / min, 4℃ / min or 5℃ / min, the temperature is 350~375℃, for example, 350℃, 360℃ or 375℃, and the time is 3~4h, preferably 3h; the inert atmosphere includes nitrogen.

[0054] In this invention, the biochar precursor prepared in step (1) is directly pyrolyzed under an inert atmosphere to obtain biochar;

[0055] The heating rate of the pyrolysis is 3~10℃ / min, for example, 3, 5, 8 or 10, the temperature is 350~850℃, for example, 350℃, 500℃, 600℃, 800℃ or 850℃, and the time is 3~4h, preferably 3h.

[0056] The present invention also provides modified biochar prepared by the above preparation method.

[0057] The present invention also provides biochar prepared by the above-described preparation method.

[0058] The present invention also provides a method for reducing nitrogen, sequestering carbon and reducing emissions in coastal wetlands based on iron cycle, wherein the modified biochar or the above-mentioned biochar is added to the soil of coastal wetlands affected by tides to achieve nitrogen reduction, carbon sequestration and emission reduction.

[0059] The modified biochar of this invention has more phenolic hydroxyl functional groups and zero-valent iron intercalation, which can couple and drive microbial iron cycling, producing microbial biomass carbon through assimilation. Iron cycling makes the crystalline state of iron more unstable, thus generating more iron-bound organic carbon in the oxidized state, reducing carbon dioxide production. The iron reduction process promotes the iron autotrophic denitrification process, accelerating the removal of nitrate nitrogen, and the iron cycling inhibits methanogenesis and nitrous oxide processes.

[0060] In this invention, the dosage of the modified biochar is 2.5 kg / m³. 2 .

[0061] In this invention, the improved coastal wetland is affected by tides, and the experimental area experiences two semi-diurnal tide cycles every day, with a tidal range of about 1 to 2 m.

[0062] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0063] Example 1

[0064] (1) Invasive species *Ulva prolifera* and *Spartina alterniflora* were collected from degraded coastal wetlands in Wenzhou Bay as raw materials for biochar. After simple dehydration, *Ulva prolifera* and *Spartina alterniflora* were mixed at a mass ratio of 1:1.5 and washed with deionized water to remove excess impurities and debris. The mixed raw materials were dried at 55°C for 48 hours, pulverized, and passed through a 100-mesh sieve.

[0065] (2) Place 100 g of the mixed raw material of Ulva prolifera and Spartina alterniflora treated in step (1) into 1 L of water and put it into a reaction vessel. Set the heating temperature to 180°C and the pressure to the pressure of the reaction vessel itself. Continue the reaction for 1.5 h to obtain biochar precursor.

[0066] (3) Place a quartz boat containing 200 g of biochar precursor powder in a tube furnace, and keep the nitrogen flow rate at 0.6 L / min. Set the pyrolysis temperature to 350℃ and keep it constant for 3 h. Control the heating rate at 3℃ / min. After cooling to room temperature, the original biochar is obtained.

[0067] Example 2

[0068] (1) Invasive species *Ulva prolifera* and *Spartina alterniflora* were collected from degraded coastal wetlands in Wenzhou Bay as raw materials for biochar. After simple dehydration, *Ulva prolifera* and *Spartina alterniflora* were mixed at a mass ratio of 1:1.5 and washed with deionized water to remove excess impurities and debris. The mixed raw materials were dried at 55°C for 48 hours, pulverized, and passed through a 100-mesh sieve.

[0069] (2) Place 100 g of the mixed raw material of Ulva prolifera and Spartina alterniflora treated in step (1) into 1 L of water and put it into a reaction vessel. Set the heating temperature to 180°C and the pressure to the autogenous pressure in the reaction vessel. Continue the reaction for 1.5 h to obtain biochar precursor.

[0070] (3) Prepare a 5 g / L tannic acid solution and add 300 g of dried biochar precursor. Mix the solution by shaking at 120 rpm for 4 h. The subsequent ultrasonic treatment is set at 25 °C, 40-50 kHz, and 50-60 W for 2 h. The tannic acid-modified biochar precursor is then vacuum filtered and washed with ultrapure water. The washed composite material is then dried in an oven at 55 °C for 48 h to obtain the modified biochar precursor.

[0071] (4) A quartz boat containing 10 g of ferrocene powder was placed in the low-temperature zone of a dual-temperature zone tubular furnace. Nitrogen gas was introduced into the furnace at a fixed flow rate of 0.6 L / min. The continuous introduction of nitrogen gas caused the ferrocene vapor to sublimate and diffuse into the high-temperature zone. The parameters for the low-temperature zone were set as follows: heating rate of 10 °C / min, heating to 250 °C and holding for 4 h. A quartz boat containing 200 g of modified biochar precursor powder was placed in the high-temperature zone of the dual-temperature zone tubular furnace. Nitrogen gas was introduced into the furnace at a fixed flow rate of 0.6 L / min to allow it to undergo pyrolysis and carbonization under a nitrogen atmosphere. The heating rate for the high-temperature zone was 3 °C / min. After heating to 350 °C, the temperature was kept constant for 3 h. After cooling to room temperature, modified biochar powder was obtained.

[0072] Figure 1 This is a SEM-EDS image of the modified biochar from Example 2; Figure 2 The image shown is a SEM-EDS image of the original biochar from Example 1. Figure 1 and Figure 2 The comparison shows that the modified biochar has a higher iron content on its surface.

[0073] Test Example 1

[0074] The experimental area is a coastal wetland located in Dongtou District, Wenzhou City, Zhejiang Province. Situated at the confluence of the Oujiang River estuary and the East China Sea, this wetland is influenced by typical semi-diurnal tides, meaning it experiences two high tides and two low tides daily, with a tidal cycle of approximately 12 hours and 25 minutes and an estimated tidal range of 1-2 meters. Before applying biochar, 15 experimental partitions (20*20 cm each) were installed. The experimental area was protected by planning regulations, free from human interference and vegetation disturbance; plant residues, large debris, and stones on the wetland surface were all removed.

[0075] Test method:

[0076] The experiment was conducted during low tide. 100 g of raw biochar was evenly spread within a 20*20 cm partitioned experimental area. Immediately after spreading, the top 10 cm of wetland soil was tilled for preliminary mixing and compacted. This process was repeated twice to ensure thorough mixing of the biochar and soil. A systematic grid method was used, with parallel sections spaced 10 m apart, for a total of five parallel partitioned experiments to represent the study area equally and independently.

[0077] Soil samples were collected from the partition at 0, 60, 120 and 180 days after the application of biochar, and the soil at different heights in the sampler was homogenized.

[0078] Weigh 10 g of freeze-dried soil powder (passed through a 100-mesh sieve) into a reagent bottle, add 100 ml of 1 M potassium chloride solution, and thoroughly mix the soil and potassium chloride solution using a vortex mixer. Then, place the mixture in a constant-temperature shaker and shake at 250 rpm for 1 h. Transfer the soil-potassium chloride mixture to a 50 ml centrifuge tube and centrifuge at 5000 rpm for 10 min. Collect the supernatant and determine its nitrate, nitrite, and ammonia nitrogen content using a spectrophotometer.

[0079] The total nitrogen and total carbon contents of freeze-dried soil powder passed through a 100-mesh sieve were determined directly using an elemental analyzer. 1 g of soil was weighed and subjected to three acid treatments with 7 mL of 1 M HCl, followed by centrifugation at 6000 rpm for 20 min. After washing and drying, the organic carbon content was determined using an elemental analyzer. 2 g of soil was weighed and extracted with 20 mL of ultrapure water through shaking at 24 h and 240 rpm, filtered through a 0.45 μm membrane, and the dissolved organic carbon content was determined using a total organic carbon analyzer. Two 0.5 g soil samples were accurately weighed. One sample was mixed with a buffer solution containing 0.11 M NaHCO3 and 0.27 M C6H5Na3O7 at pH 7.3, preheated in an 80°C water bath for 15 min, and 0.5 g of Na2S2O4 powder was accurately added. The mixture was then kept at 80°C with continuous shaking for 15 min. The other sample was mixed with a pH 7.3 solution of 1.6 M NaCl and 0.11 M NaHCO3, and NaCl was added. All mixtures were centrifuged six times at 4000 rpm for 20 minutes each time, washed with ultrapure water, and then freeze-dried. The carbon content in the residues was determined using an elemental analyzer. The percentage of iron-bound organic carbon in the soil was calculated using formula (1):

[0080] .

[0081] Weigh 5 g of fresh soil and place it in a 25 ml glass beaker. Fumigate with chloroform for 24 h in a sealed container under light-protected conditions. Then extract the fumigated soil with 30 ml of 0.05 M K₂SO₄ solution at 160 rpm for 4 h. Filter the extract through a 0.45 μm filter membrane and determine the dissolved organic carbon content using a total organic carbon analyzer. The microbial biomass carbon content in the soil is determined by formula (2), where the effective extraction ratio is 0.38.

[0082] .

[0083] Portable greenhouse gas analyzers were used to monitor the fluxes of carbon dioxide, methane, and nitrous oxide within the partition of the experimental area at 0, 60, 120, and 180 days after biochar application. Gas collection was conducted in a dark environment at 25°C. The partition was sealed with a closed polyvinyl chloride film and then fixed in a rectangular static chamber, while the air inside the chamber was uniformly mixed during the greenhouse gas flux measurement. The flux data for 20 minutes were compiled, a stable slope was selected, and the fluxes of the three gases were calculated using formula (3):

[0084] .

[0085] Test Example 2

[0086] Test method:

[0087] The experiment was conducted during low tide. 100 g of modified biochar was evenly spread within a 20*20 cm partitioned test area. Immediately after spreading, the top 10 cm of wetland soil was tilled for preliminary mixing and compacted. This process was repeated twice to ensure thorough mixing of the biochar and soil. A systematic grid method was used, with parallel sections spaced 10 m apart, for a total of five parallel partitioned tests to represent the study area equally and independently.

[0088] Soil samples were collected from the partition at 0, 60, 120 and 180 days after the application of biochar, and the soil at different heights in the sampler was homogenized.

[0089] Weigh 10 g of freeze-dried soil powder (passed through a 100-mesh sieve) into a reagent bottle, add 100 ml of 1 M potassium chloride solution, and thoroughly mix the soil and potassium chloride solution using a vortex mixer. Then, place the mixture in a constant-temperature shaker and shake at 250 rpm for 1 h. Transfer the soil-potassium chloride mixture to a 50 ml centrifuge tube and centrifuge at 5000 rpm for 10 min. Collect the supernatant and determine its nitrate, nitrite, and ammonia nitrogen content using a spectrophotometer.

[0090] The total nitrogen and total carbon contents of freeze-dried soil powder passed through a 100-mesh sieve were determined directly using an elemental analyzer. 1 g of soil was weighed and subjected to three acid treatments with 7 mL of 1 M HCl, followed by centrifugation at 6000 rpm for 20 min. After washing and drying, the organic carbon content was determined using an elemental analyzer. 2 g of soil was weighed and extracted with 20 mL of ultrapure water through shaking at 24 h and 240 rpm, filtered through a 0.45 μm membrane, and the dissolved organic carbon content was determined using a total organic carbon analyzer. Two 0.5 g soil samples were accurately weighed. One sample was mixed with a buffer solution containing 0.11 M NaHCO3 and 0.27 M C6H5Na3O7 at pH 7.3, preheated in an 80°C water bath for 15 min, and 0.5 g of Na2S2O4 powder was accurately added. The mixture was then kept at 80°C with continuous shaking for 15 min. The other sample was mixed with a pH 7.3 solution of 1.6 M NaCl and 0.11 M NaHCO3, and NaCl was added. All mixtures were centrifuged six times at 4000 rpm for 20 minutes each time, washed with ultrapure water, and then freeze-dried. The carbon content in the residues was determined using an elemental analyzer. The percentage of iron-bound organic carbon in the soil was calculated using formula (1):

[0091] .

[0092] Weigh 5 g of fresh soil and place it in a 25 ml glass beaker. Fumigate with chloroform for 24 h in a sealed container under light-protected conditions. Then extract the fumigated soil with 30 ml of 0.05 M K₂SO₄ solution at 160 rpm for 4 h. Filter the extract through a 0.45 μm filter membrane and determine the dissolved organic carbon content using a total organic carbon analyzer. The microbial biomass carbon content in the soil is determined by formula (2), where the effective extraction ratio is 0.38.

[0093] .

[0094] Portable greenhouse gas analyzers were used to monitor the fluxes of carbon dioxide, methane, and nitrous oxide within the partition of the experimental area at 0, 60, 120, and 180 days after biochar application. Gas collection was conducted in a dark environment at 25°C. The partition was sealed with a closed polyvinyl chloride film and then fixed in a rectangular static chamber, while the air inside the chamber was uniformly mixed during the greenhouse gas flux measurement. The flux data for 20 minutes were compiled, a stable slope was selected, and the fluxes of the three gases were calculated using formula (3):

[0095] .

[0096] result:

[0097] Table 1 shows the changes in various nitrogen components (nitrate nitrogen, nitrite nitrogen, ammonia nitrogen, and total nitrogen) in coastal wetlands after the application of raw and modified biochar at 0d, 60d, 120d, and 180d.

[0098] Table 1. Changes in the content of nitrate nitrogen, nitrite nitrogen, ammonia nitrogen, and total nitrogen in coastal wetland soils.

[0099]

[0100] As shown in Table 1, compared with the blank control group, the application of raw and modified biochar reduced nitrate nitrogen, ammonia nitrogen, and total nitrogen in coastal wetland sediments. The reduction in each nitrogen component was greater in the modified biochar group than in the raw biochar group.

[0101] Table 2 shows the changes in the content of various carbon components (total carbon, organic carbon, microbial biomass carbon, and iron-bound organic carbon) in the soil at 0d, 60d, 120d, and 180d after the application of raw biochar and modified biochar to coastal wetlands.

[0102] Table 2. Changes in the content of total carbon, organic carbon, microbial biomass carbon, and iron-bound organic carbon in coastal wetland soils.

[0103]

[0104] As shown in Table 2, compared with the blank control group, the total carbon, organic carbon, microbial biomass carbon, and iron-bound organic carbon in both the original biochar group and the modified biochar group were increased. Compared with the original biochar group, the modified biochar group showed a greater increase in each carbon component, especially microbial biomass carbon and iron-bound organic carbon.

[0105] Table 3 shows the trends of greenhouse gas (carbon dioxide, methane, nitrous oxide) fluxes in coastal wetland soils at 0d, 60d, 120d, and 180d after the application of raw and modified biochar.

[0106] Table 3. Flux changes of carbon dioxide, methane, and nitrous oxide in coastal wetland soils

[0107]

[0108] As shown in Table 3, compared with the blank control group, both the original biochar group and the modified biochar group showed a decrease in nitrous oxide. However, unlike the original biochar, the modified biochar simultaneously reduced the emission fluxes of carbon dioxide and methane over 180 days.

[0109] Example 3

[0110] Different biochar raw materials.

[0111] (1) Invasive species *Ulva prolifera*, *Spartina alterniflora*, and barnacles were collected from degraded coastal wetlands in Wenzhou Bay as raw materials for biochar. After simple dehydration, the *Ulva prolifera*, *Spartina alterniflora*, and barnacles were mixed in a mass ratio of 1:1.5:1 and washed with deionized water to remove excess impurities and debris. The mixed raw materials were dried at 55°C for 48 hours, pulverized, and passed through a 100-mesh sieve.

[0112] (2) Place 100 g of the mixed raw material of Ulva prolifera, Spartina alterniflora and barnacles treated in step (1) into 1 L of water and put it into a reaction vessel. Set the heating temperature to 180℃ and the pressure to the pressure of the reaction vessel itself. Continue the reaction for 1.5 h to obtain biochar precursor.

[0113] (3) Place a quartz boat containing 200 g of biochar precursor powder in a tube furnace, and keep the nitrogen flow rate at 0.6 L / min. Set the pyrolysis temperature to 350℃ and keep it constant for 3 h. Control the heating rate at 3℃ / min. After cooling to room temperature, new mixed biochar is obtained.

[0114] Example 4

[0115] Different biochar preparation conditions.

[0116] (1) Invasive species *Ulva prolifera* and *Spartina alterniflora* were collected from degraded coastal wetlands in Wenzhou Bay as raw materials for biochar. After simple dehydration, *Ulva prolifera* and *Spartina alterniflora* were mixed at a mass ratio of 1:1.5 and washed with deionized water to remove excess impurities and debris. The mixed raw materials were dried at 55°C for 48 hours, pulverized, and passed through a 100-mesh sieve.

[0117] (2) Place 100 g of the mixed raw material of Ulva prolifera and Spartina alterniflora treated in step (1) into 1 L of water and put it into a reaction vessel. Set the heating temperature to 180°C and the pressure to the pressure of the reaction vessel itself. Continue the reaction for 1.5 h to obtain biochar precursor.

[0118] (3) Place a quartz boat containing 200 g of biochar precursor powder in a tube furnace, and keep the nitrogen flow rate at 0.6 L / min. Set the pyrolysis temperature to 850℃ and keep it constant for 3 h. Control the heating rate at 10℃ / min. After cooling to room temperature, high-temperature biochar is obtained.

[0119] The tests were conducted using the methods described in Test Case 1 and Test Case 2, and the results are shown below:

[0120] Table 4 shows the changes in each nitrogen component (nitrate nitrogen, nitrite nitrogen, ammonia nitrogen, and total nitrogen) at 0d, 60d, 120d, and 180d after the application of mixed and high-temperature biochar in coastal wetlands.

[0121] Table 4. Changes in the content of nitrate nitrogen, nitrite nitrogen, ammonia nitrogen, and total nitrogen in coastal wetland soils.

[0122]

[0123] As shown in Table 4, compared with the blank control group, the application of mixed and high-temperature biochar reduced nitrate nitrogen, ammonia nitrogen, and total nitrogen in coastal wetland sediments. Furthermore, the reduction rates of each nitrogen component in the mixed and high-temperature biochar groups were not significantly different.

[0124] Table 5 shows the changes in the content of various carbon components (total carbon, organic carbon, microbial biomass carbon, and iron-bound organic carbon) in the soil at 0d, 60d, 120d, and 180d after the application of mixed biochar and high-temperature biochar to coastal wetlands.

[0125] Table 5. Changes in the content of total carbon, organic carbon, microbial biomass carbon, and iron-bound organic carbon in coastal wetland soils.

[0126]

[0127] As shown in Table 5, compared with the blank control group, both the mixed biochar group and the high-temperature biochar group showed increases in total carbon, organic carbon, microbial biomass carbon, and iron-bound organic carbon. However, compared with the mixed biochar group, the high-temperature biochar group only showed a greater increase in total carbon in the sediment. For derived persistent organic carbon, especially microbial biomass carbon and iron-bound organic carbon, the mixed biochar group showed a better effect in increasing these parameters.

[0128] Table 6 shows the trends of greenhouse gas (carbon dioxide, methane, nitrous oxide) fluxes in coastal wetland soils at 0d, 60d, 120d, and 180d after the application of mixed biochar and high-temperature biochar.

[0129] Table 6. Flux changes of carbon dioxide, methane, and nitrous oxide in coastal wetland soils

[0130]

[0131] As shown in Table 6, compared with the blank control group, both the mixed biochar group and the high-temperature biochar group showed reduced carbon dioxide and methane emissions. However, both groups increased nitrous oxide emission fluxes over 180 days.

[0132] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing modified biochar, characterized in that, Includes the following steps: (1) After mixing biochar raw materials and water, a hydrothermal reaction is carried out to obtain biochar precursor; (2) The biochar precursor was immersed in tannic acid solution for modification to obtain modified biochar precursor; (3) The modified biochar precursor was pyrolyzed under ferrocene vapor and inert atmosphere to obtain modified biochar. The biochar raw materials include one or more of the following: Ulva prolifera, Spartina alterniflora, and barnacles.

2. The preparation method according to claim 1, characterized in that, When the biochar raw material is a mixture of Ulva prolifera and Spartina alterniflora, the mass ratio of Ulva prolifera to Spartina alterniflora is 1:1.5~2; when the biochar raw material is a mixture of Ulva prolifera, Spartina alterniflora and barnacles, the mass ratio of Ulva prolifera, Spartina alterniflora and barnacles is 1:1.5~2:

1.

3. The preparation method according to claim 1, characterized in that, The ratio of biochar raw material to water is 100~200g:1~2L; the hydrothermal reaction temperature is 150~180℃ and the time is 1~1.5h.

4. The preparation method according to claim 1, characterized in that, The concentration of the tannic acid solution is 5 g / L; the modification method is oscillation followed by ultrasound; the oscillation rate is 120~160 rpm, and the time is 2~4 h; the ultrasound frequency is 40~50 kHz, the power is 50~60 W, the temperature is 25~30 °C, and the time is 1~2 h.

5. The preparation method according to claim 1, characterized in that, The method for preparing ferrocene vapor includes: heating ferrocene under a nitrogen atmosphere and continuously introducing nitrogen to sublimate the ferrocene vapor; The mass ratio of ferrocene to modified biochar precursor is 1:20~30, the heating rate is 5~10℃ / min, the temperature is 230~250℃, the time is 4~6h, and the nitrogen gas introduction rate is 0.6~0.8L / min.

6. The preparation method according to claim 1, characterized in that, The pyrolysis heating rate is 3~5℃ / min, the temperature is 350~375℃, and the time is 3~4h.

7. A modified biochar prepared by the preparation method according to any one of claims 1 to 6.

8. A method for nitrogen and carbon sequestration and emission reduction in coastal wetlands based on iron cycling, characterized in that, The modified biochar described in claim 7 is applied to coastal wetland soil affected by tides to achieve nitrogen reduction, carbon sequestration, and emission reduction.

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