Method for reducing CH4 emission of constructed wetland, constructed wetland and construction method of constructed wetland

Abstract

The invention provides a method for reducing CH4 emission of a constructed wetland, the constructed wetland and a construction method of the constructed wetland, and particularly relates to the technical field of environmental protection. The method for reducing the CH4 emission of the constructed wetland comprises the following steps: doping biochar-loaded nano zero-valent iron into an upper-layer matrix of the constructed wetland; the doping amount of the charcoal-loaded nano zero-valent iron is 0.5-2% of the mass of the upper-layer substrate. According to the method, the concentration of soluble organic carbon (DOC) in the constructed wetland is improved by controlling the content of the biochar-loaded nano zero-valent iron, so that the diversity of CH4 oxidizing bacteria and the relative abundance of Guangmethylotrophicus are improved, the CH4 oxidation reaction is promoted, and finally the emission reduction of CH4 is realized. According to the data of the embodiment, the average discharge flux of CH4 of the constructed wetland prepared by the invention is 0.22-0.35 mg.C / m < 2 > / h, and is reduced by 35.6-59.8% compared with the contrast.

Description

Technical Field

[0001] The present invention belongs to the technical field of environmental protection, and particularly relates to a method for reducing CH4 emissions from constructed wetlands, a constructed wetland and a method for constructing the same. Background Art

[0002] As a wastewater treatment technology designed to imitate natural wetlands, constructed wetlands can effectively remove agricultural nitrogen pollutants mainly through the interaction among substrates, wetland plants and microorganisms. Due to its advantages such as low cost, low energy consumption, easy maintenance, high efficiency and environmental beautification, it has been widely used in the treatment of agricultural non-point source pollution. However, traditional constructed wetlands will release a certain amount of greenhouse gases, especially methane (CH4) during operation, resulting in the transfer of environmental problems from "water pollution" to "air pollution". CH4 emissions originate from a wide range of human activities and natural processes. Among them, wetland ecosystems constitute the most significant natural emission source, and constructed wetlands account for 82% of it. Given the continuous increase in the concentration of CH4 in the atmosphere, CH4 has gradually become the second most important anthropogenic greenhouse gas after carbon dioxide (CO2). How to effectively reduce CH4 emissions from constructed wetlands has become a research hotspot for scholars at home and abroad. Summary of the Invention

[0003] In view of this, the purpose of the present invention is to provide a method for reducing CH4 emissions from constructed wetlands, a constructed wetland and a method for constructing the same. The method provided by the present invention can effectively reduce CH4 emissions and increase the relative abundances of Methanobacterium and Methylotrophicus.

[0004] To solve the above technical problems, the present invention provides the following technical solutions:

[0005] The present invention provides a method for reducing CH4 emissions from constructed wetlands, wherein biochar-supported nano-zero-valent iron is incorporated into the upper substrate of the constructed wetland; the incorporation amount of the biochar-supported nano-zero-valent iron is 0.5-2% of the mass of the upper substrate.

[0006] The present invention also provides a constructed wetland, comprising a substrate, wherein the substrate includes an upper substrate, an intermediate substrate and a lower substrate; the upper substrate includes first gravel and biochar-supported nano-zero-valent iron;

[0007] The mass of the biochar-supported nano-zero-valent iron accounts for 0.5-2% of the mass of the upper substrate.

[0008] Preferably, the thickness of the upper substrate is 16-24 cm.

[0009] Preferably, the particle size of the first gravel is 0.5-1 cm, and the filling density of the first gravel is 1.80-1.90 t / m 3 .

[0010] Preferably, the intermediate layer matrix includes second gravel, and the thickness of the intermediate layer matrix is 13 - 17 cm.

[0011] Preferably, the particle size of the second gravel is 1 - 2 cm, and the filling density of the second gravel is 1.60 - 1.80 t / m 3 .

[0012] Preferably, the lower layer matrix includes quartz sand, and the thickness of the lower layer matrix is 3 - 7 cm.

[0013] Preferably, the particle size of the quartz sand is 0.5 - 1 cm, and the filling density of the quartz sand is 1.60 - 1.80 t / m 3 .

[0014] Preferably, the mass ratio of nano zero - valent iron to biochar in the biochar - loaded nano zero - valent iron is 0.5:1 - 2:1.

[0015] The present invention also provides a construction method of the constructed wetland described in the above technical solution, including the following steps:

[0016] Mix first gravel and biochar - loaded nano zero - valent iron to obtain the upper layer matrix;

[0017] Load the upper layer matrix, the intermediate layer matrix and the lower layer matrix in sequence to obtain the constructed wetland.

[0018] The present invention provides a method for reducing CH4 emissions from a constructed wetland, by incorporating biochar - loaded nano zero - valent iron into the upper layer matrix of the constructed wetland; the incorporation amount of the biochar - loaded nano zero - valent iron is 0.5 - 2% of the mass of the upper layer matrix. The present invention controls the content of biochar - loaded nano zero - valent iron to increase the concentration of dissolved organic carbon (DOC) in the constructed wetland, thereby increasing the diversity of CH4 - oxidizing bacteria and the relative abundance of the genus Methylotrophs, promoting the CH4 oxidation reaction, and ultimately achieving the reduction of CH4 emissions. Through the data of the examples, it can be seen that for the constructed wetland provided by the present invention, the average CH4 emission flux is 0.22 - 0.35 mg·C / m 2 / h, which is reduced by 35.6 - 59.8% compared with the control. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0020] Figure 1Schematic diagram of the constructed wetlands prepared in Examples 1-2 and Comparative Example 1;

[0021] Figure 2 SEM image of the biochar-supported nano-zero valent iron prepared in the present invention;

[0022] Figure 3 Average effluent pH value a), redox potential b), and soluble organic carbon concentration c) in the constructed wetlands prepared in Examples 1-2 and Comparative Example 1;

[0023] Figure 4 CH4 dynamic emission flux a) and average CH4 emission flux b) in the constructed wetlands prepared in Examples 1-2 and Comparative Example 1;

[0024] Figure 5 Relative abundances of mcrA gene a) of CH4-producing archaea, pmoA gene b) of CH4-oxidizing bacteria, and ratio c) of the relative abundances of mcrA gene and pmoA gene in the constructed wetlands prepared in Examples 1-2 and Comparative Example 1;

[0025] Figure 6 Changes a) in the community structure of CH4-producing bacteria at the genus level and relative abundances b) of CH4-producing bacteria at the genus level in the constructed wetlands prepared in Examples 1-2 and Comparative Example 1;

[0026] Figure 7 Community structures a) of CH4-oxidizing bacteria at the genus level and community structures b) of CH4-oxidizing bacteria at the genus level in the constructed wetlands prepared in Examples 1-2 and Comparative Example 1;

[0027] Figure 8 Pearson correlation analysis of average CH4 emission flux, effluent physicochemical properties, α-diversity index of CH4-producing bacteria, and relative abundances of CH4-producing bacteria at the genus level in the constructed wetland prepared in Example 1;

[0028] Figure 9 Pearson correlation analysis of average CH4 emission flux, effluent physicochemical properties, α-diversity index of CH4-oxidizing bacteria, and relative abundances of CH4-oxidizing bacteria at the genus level in the constructed wetland prepared in Example 2. Detailed implementation manners

[0029] The present invention provides a method for reducing CH4 emissions from constructed wetlands by incorporating biochar-supported nano-zero valent iron into the upper substrate of the constructed wetland; the incorporation amount of the biochar-supported nano-zero valent iron is 0.5-2% of the mass of the upper substrate.

[0030] In the present invention, unless otherwise specified, the raw materials and equipment used are well-known commercially available products in the art.

[0031] In the present invention, the incorporation amount of the biochar-supported nano zero-valent iron is 0.5 to 2% of the mass of the upper substrate. In specific embodiments, the incorporation amount of the biochar-supported nano zero-valent iron is 0.5%, 1%, 1.5% or 2% of the mass of the upper substrate. In a specific embodiment of the present invention, when the incorporation amount of the biochar-supported nano zero-valent iron is 1% of the mass of the upper substrate, the effect of inhibiting CH4 emissions is the best.

[0032] In the present invention, the mass ratio of the nano zero-valent iron to the biochar in the biochar-supported nano zero-valent iron is preferably 0.5:1 to 2:1, and may be 1:1 in specific examples.

[0033] In the present invention, the biochar-supported nano zero-valent iron is preferably prepared by conventional technical means in the art. Specifically, an Fe 2+ solution and biochar are mixed, and ultrasonic treatment is carried out under an inert atmosphere to obtain a mixture; the mixture is mixed with a NaBH4 solution, stirred under an inert atmosphere, and solid-liquid separation is carried out to obtain a solid, which is washed with water and dried to obtain the biochar-supported nano zero-valent iron. In the present invention, the inert atmosphere is preferably nitrogen.

[0034] In the present invention, the concentration of the NaBH4 solution is preferably 0.1 to 0.3 mol / L, and the mass ratio of the biochar to the volume of the NaBH4 solution is preferably 1 g:0.26 to 0.27 L.

[0035] The present invention has no special requirements for the method of solid-liquid separation, and conventional technical means in the art can be used.

[0036] In the present invention, the biochar is preferably prepared by conventional technical means in the art. Specifically, plants are crushed and carbonized under an inert atmosphere to obtain biochar. In the present invention, the temperature of the carbonization is preferably 600 °C, and the time of the carbonization is preferably 2 h.

[0037] In the present invention, the plants are preferably reeds, and preferably further include washing, cutting and drying before crushing.

[0038] The present invention also provides a constructed wetland, including a substrate, and the substrate includes an upper substrate, an intermediate substrate and a lower substrate; the upper substrate includes first gravel and biochar-supported nano zero-valent iron;

[0039] The mass of the biochar-supported nano zero-valent iron accounts for 0.5 to 2% of the mass of the upper substrate.

[0040] In the present invention, the thickness of the upper substrate is preferably 16 to 24 cm. In specific embodiments, the thickness of the upper substrate can be 16 cm, 18 cm, 20 cm, 22 cm or 24 cm.

[0041] In the present invention, the particle size of the first gravel is preferably 0.5 - 1 cm, and the packing density of the first gravel is preferably 1.80 - 1.90 t / m 3 , in a specific embodiment, the packing density of the first gravel can be 1.80 t / m 3 、1.82 t / m 3 、1.85 t / m 3 、1.88 t / m 3 or 1.90 t / m 3 .

[0042] In the present invention, the composition and preparation method of the biochar-supported nano-zero-valent iron have been discussed above and will not be elaborated here.

[0043] In a specific embodiment, the mass of the biochar-supported nano-zero-valent iron can account for 0.5%, 1%, 1.5% or 2% of the mass of the upper substrate. When the mass of the biochar-supported nano-zero-valent iron accounts for 1% of the mass of the upper substrate, the effect of inhibiting CH4 emissions is the best. In the present invention, the thickness of the intermediate layer substrate is preferably 13 - 17 cm, and in a specific embodiment, the thickness of the intermediate layer substrate can be 13 cm, 15 cm or 17 cm.

[0044] In the present invention, the intermediate layer substrate preferably includes second gravel, the particle size of the second gravel is preferably 1 - 2 cm, and the packing density of the second gravel is preferably 1.60 - 1.80 t / m 3 , in a specific embodiment, the packing density of the second gravel can be 1.60 t / m 3 、1.65 t / m 3 、1.70 t / m 3 、1.75 t / m 3 or 1.80 t / m 3 .

[0045] In the present invention, the thickness of the lower layer substrate is preferably 3 - 7 cm, and in a specific embodiment, the thickness of the lower layer substrate can be 3 cm, 5 cm or 7 cm.

[0046] In the present invention, the lower layer substrate preferably includes quartz sand, the particle size of the quartz sand is preferably 0.5 - 1 cm, and the packing density of the quartz sand is preferably 1.60 - 1.80 t / m 3 , in a specific embodiment, the packing density of the quartz sand can be 1.60 t / m 3 、1.65 t / m 3 、1.70 t / m 3 、1.75 t / m 3 or 1.80 t / m 3 .

[0047] In the present invention, the constructed wetland preferably has exhaust holes at a position 42 - 47 cm from top to bottom. In a specific embodiment, the exhaust holes can be at 42 cm, 43 cm, 44 cm, 45 cm, 46 cm or 47 cm from top to bottom of the constructed wetland. The provision of exhaust holes can ensure unobstructed water flow.

[0048] In the present invention, the bottom of the constructed wetland preferably further includes a catchment area, and the thickness of the catchment area is preferably 3 - 8 cm. In an embodiment of the present invention, the catchment area is at the lowermost end of the polyethylene plastic round barrel.

[0049] In the present invention, the constructed wetland preferably further includes emergent plants, and the emergent plants are preferably iris seedlings, and the planting density is preferably 30 plants / m 2 . In the present invention, the roots of the emergent plants are preferably planted in the middle - layer substrate and / or the upper - layer substrate.

[0050] The constructed wetland obtained in the present invention controls the content of biochar - loaded nano - zero - valent iron to increase the concentration of DOC in the constructed wetland, thereby increasing the diversity of CH4 - oxidizing bacteria and the relative abundance of the genus Methylotrophs, thus promoting the CH4 oxidation reaction and ultimately achieving CH4 emission reduction.

[0051] The present invention also provides a method for constructing the constructed wetland according to the above - mentioned technical solution, including the following steps:

[0052] Mix the first gravel and biochar - loaded nano - zero - valent iron to obtain the upper - layer substrate;

[0053] Load the upper - layer substrate, the middle - layer substrate and the lower - layer substrate in sequence to obtain the constructed wetland.

[0054] The present invention has no special requirements for the mixing method, and common technical means in the art can be adopted.

[0055] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. The described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Any modification, equivalent replacement, improvement, etc. made to the embodiments of the present invention without creative work based on the technical essence and general principles of the present invention shall fall within the protection scope of the present invention.

[0056] Preparation of Biochar and Biochar - Loaded Nano - Zero - Valent Iron

[0057] Collect the wetland plant Phragmites australis from the riverbanks in Shanghai. Wash, cut, dry, and pulverize the collected Phragmites australis, place it in a tubular carbonization furnace, introduce nitrogen (N₂) for 30 min, heat from room temperature to 600 °C at a rate of 10 °C / min, and keep it at a constant temperature for 2 h to fully pyrolyze the Phragmites australis. The entire pyrolysis process is carried out under anaerobic conditions. After the pyrolysis is completed, wait for the carbonization furnace to cool to room temperature to obtain biochar.

[0058] Weigh 2.78 g of FeSO₄·7H₂O and dissolve it in 100 mL of deionized water to prepare a 0.1 mol / L Fe 2+ solution, then mix it with 0.56 g of biochar and ultrasonicate for 1 h under a N₂ atmosphere to obtain a mixture. Place the mixture under a N₂ atmosphere, and gradually add 150 mL of 0.2 mol / L NaBH₄ solution dropwise to the mixture with a syringe and stir for 30 min. Centrifuge to obtain solid particles, wash them 3 times each with deionized water and absolute ethanol, and dry the solid particles by N₂ purging to prepare biochar-supported nano zero-valent iron, denoted as nZVI-BC, where the mass ratio of nano zero-valent iron to biochar is 1:1.

[0059] Perform electron microscopy scanning on the prepared nZVI-BC to obtain Figure 2 , from Figure 2 it can be seen that nano zero-valent iron adheres inside the biochar.

[0060] Construction of constructed wetland

[0061] Example 1

[0062] Construct a small-scale constructed wetland system in a polyethylene plastic round barrel with a diameter of 35.5 cm, a height of 50 cm, and a substrate layer thickness of 40 cm. There is a 5-cm-high catchment area under the constructed wetland. To ensure smooth water flow, there is an exhaust hole at 45 cm from the top to the bottom of the round barrel, as Figure 1 shown.

[0063] The substrate of the constructed wetland bed is divided into three layers: the upper substrate is 20 cm, the middle substrate is 15 cm, and the lower substrate is 5 cm. Mix gravel (particle size 0.5 - 1 cm; density 1.84 t / m 3 ) and nZVI-BC evenly as the upper substrate, where the mass of nZVI-BC accounts for 1% of the mass of the upper substrate; gravel (particle size 1 - 2 cm; density 1.70 t / m 3 ) is used as the middle substrate; quartz sand (particle size 0.5 - 1 mm, density 1.70 t / m 3 ) is used as the lower substrate. The obtained constructed wetland is as Figure 1 shown in b) of

[0064] Example 2

[0065] The mass of nZVI-BC accounted for 2% of the mass of the upper substrate, and other conditions were the same as those in Example 1. The obtained constructed wetland was as shown in Figure 1 c) below.

[0066] Comparative Example 1

[0067] Only gravel (particle size 0.5 - 1 cm; density 1.84 t / m 3 ) was used in the upper substrate, and other conditions were the same as those in Example 1. The obtained constructed wetland was as shown in Figure 1 a) below.

[0068] Application Example

[0069] The experiment was carried out in a plastic greenhouse at the Zhuanghang Comprehensive Experimental Base of the Shanghai Academy of Agricultural Sciences (30°53′24″N, 121°23′15″E). The four sides of the greenhouse were open, which not only avoided the influence of precipitation on the experiment but also kept the temperature inside the greenhouse consistent with the outside ambient temperature. This area belongs to the subtropical monsoon climate, with an average annual precipitation of 1191.5 mm and an average annual temperature of 16.1°C. Examples 1 - 2 and Comparative Example 1 were repeated 3 times each, and there were a total of 9 groups of constructed wetlands. On July 2, 2023, Iris seedlings with consistent growth in the nursery of the experimental station were transplanted into the constructed wetlands, and the initial planting density was 3 plants / barrel (30 plants / m 2 ). Since nZVI-BC is very unstable, after using the river water in the experimental area to stabilize the constructed wetland system for 7 days, the experiment officially began. Referring to the local farmland runoff discharge situation, the experimental water was configured using the river water in the experimental area, with an influent TN concentration of 8.51 mg / L and a TOC of 10.88 mg / L.

[0070] Operation of the Constructed Wetland, Collection and Analysis of Methane Gas

[0071] The hydraulic retention time of the constructed wetland was 7 days, and the average hydraulic load was 150 mm / d. Every week, a peristaltic pump was used to pump the simulated runoff into each constructed wetland from above once, and the influent flow rate was 0.43 m 3 / h. Water samples and CH4 gas were collected on the 1st, 2nd, 4th, and 6th days of each hydraulic retention period. A portable multi-parameter water quality detector (HI9829, HAN-NA, Italy) was used to measure the physicochemical properties of the effluent from the constructed wetland on-site, including the oxidation-reduction potential (ORP) and pH. The DOC in the effluent was measured using a total organic carbon analyzer (Vario EL III, Elementar, Germany).

[0072] The collection of CH4 gas was carried out using a statically sealed plexiglass box and a self-developed gas automatic sampling device. The height of the plexiglass box was 50 cm and the diameter was 36.5 cm. Each time of sampling, four gas samples were collected from each plexiglass box at 6-min intervals using the automatic sampling device, and each gas sample was stored in a 1-L aluminum foil gas bag (Dalian Delin Gas Packaging Co., Ltd., Dalian, China). The gas automatic sampling device consisted of the following parts: a 12-V rechargeable battery (NP7-12); a gas pump (FAY4002, 2 L / min, Chengdu Qihai Electromechanical Manufacturing Co., Ltd., Chengdu, China); a circuit board box (Nanjing Weina Electronics Co., Ltd., Nanjing, China); a three-way direct-acting solenoid valve (VDW23-6G-1, SMC Pneumatics Co., Ltd., Tokyo, Japan). The collected samples were sent to the laboratory for the determination of CH4 concentration by a gas chromatograph (7820A, Agilent, USA). The CH4 standard gas was purchased from the National Standard Substances Center.

[0073] The calculation formula for CH4 emission flux is:

[0074] F = ρ×V / S×dC / dt×273 / (273 + T) Equation 1;

[0075] In Equation 1, F is the CH4 (mg CH4 ·m -2 ·h -1 ) emission flux, ρ is the density of CH4 under standard conditions (kg·m- 3 ), V is the effective volume of the closed box (m 3 ), S is the base area (m 2 ), dC / dt represents the change in CH4 concentration in the closed box per unit time, and T is the average temperature in the closed box (°C). A linear correlation coefficient R2 > 0.90 between the concentrations of the four gas samples collected within 18 min and time was regarded as valid.

[0076] The average CH4 emission flux was calculated using the following formula:

[0077]

[0078] In Equation 2, E CH4 is the average CH4 emission flux (mg CH4 ·m -2 ·h -1 ); F i and F i+1 are the emission fluxes of CH4 at the i-th and (i + 1)-th samplings, respectively, with the unit of mg CH4 ·m -2 ·h -1 ; t i+1 and t iThey are the (i + 1)-th and i-th sampling dates respectively, with the unit of d, and n is the total number of determinations during the cumulative emission observation period.

[0079] Determination of methanogenic archaea and methane-oxidizing bacteria

[0080] After the experiment, five positions close to the plant roots were selected, and root matrix samples were collected using UV-sterilized forceps and 10 mL centrifuge tubes. After mixing the matrix samples at these five points evenly, they were stored in a -80 °C refrigerator, and Guangdong Magigene Technology Co., Ltd. was entrusted to determine the functional genes and community compositions of CH4-producing archaea and CH4-oxidizing bacteria. Before the experiment, the gravel samples were placed in a beaker and shaken, and the precipitate was taken out after high-speed centrifugation, followed by DNA extraction.

[0081] Using the extracted DNA of CH4-producing archaea as a template, the forward primer MLf (5′-GGTGGTGTMGGATTCACACARTAYGCWACAGC-3′, denoted as SEQ ID NO.1) and the reverse primer MLr (5′-TTCATTGCRTAGTTWGGRTAGTT-3′, denoted as SEQ ID NO.2) were used to amplify the mcrA gene sequence of CH4-producing archaea. Using the extracted DNA of CH4-oxidizing bacteria as a template, the forward primer pmof1 (5′-GGGGGAACTTCTGGGGITGGAC-3′, denoted as SEQ ID NO.3) and the reverse primer pmor (5′-GGGGGRCIACGTCITTACCGAA-3′, denoted as SEQ ID NO.4) were used to amplify the pmoA gene sequence of CH4-oxidizing bacteria. The qPCR program was: 95 °C for 5 min, 40 cycles × (94 °C for 15 s, 60 °C for 15 s), 72 °C for 30 s. The products obtained by PCR amplification were purified, and the MiSeq library was constructed according to the standard procedure of NE Ultra TM (New England Biolabs, USA), and the constructed amplicon library was sequenced by PE250 on the Illumina Nova 6000 platform. The obtained sequences were clustered with 97% similarity through OTUs (operational taxonomic units).

[0082] The data was analyzed using SPSS 22.0 software. The effects of the physicochemical indices and CH4 cumulative emissions of the effluents from the constructed wetlands prepared in Examples 1-2 and Comparative Example 1 were tested using one-way ANOVA and Tukey's HSD (p < 0.05). Origin 2018 software was used for mapping. The Mantel test and plotting were performed using the ggplot2, vegan, dplyr, linkET, scales, and RColorBrewer packages in R 4.3.3.

[0083] The data of the average pH value, redox potential, and soluble organic carbon (DOC) concentration of the effluents from the constructed wetlands prepared in Examples 1-2 and Comparative Example 1 were recorded eight times, and Figure 3 . From Figure 3 a) in it, it can be seen that the addition of nZVI-BC has a significant effect on the pH value of the effluent from the constructed wetland, and generally shows that 0% > 1% > 2% (P < 0.05). The addition of nZVI-BC has no significant effect on the redox potential of the effluent from the constructed wetland ( Figure 3 b) in it). Compared with Comparative Example 1, the DOC concentration in the constructed wetland prepared in Example 1 increased significantly (P < 0.05), while Example 2 had no significant effect on the DOC concentration ( Figure 3 c) in it), among which, the average DOC concentration in Example 1 was 83.4 mg / L.

[0084] The CH4 dynamic emission flux and CH4 average emission flux of the constructed wetlands prepared in Examples 1-2 and Comparative Example 1 were tested, and Figure 4 was obtained. The peak of CH4 emission in Comparative Example 1 was on July 10 (1.33 mg·C / m 2 / h), July 16 (0.63 mg·C / m 2 / h), and July 19 (0.70 mg·C / m 2 / h). The peaks of CH4 emission in Example 1 were on July 14 (0.20 mg·C / m 2 / h) and July 19 (0.45 mg·C / m 2 / h); the peaks of CH4 emission in Example 2 were on July 10 (0.50 mg·C / m 2 / h), July 16 (0.33 mg·C / m 2 / h), and July 21 (0.60 mg·C / m 2 / h) ( Figure 4 a) in it). Compared with Comparative Example 1, Examples 1-2 reduced the CH4 emission peak on July 10 and July 16, and the reduction in Example 1 was more significant (P < 0.05) ( Figure 4b)). During the entire experimental run, the average CH4 emission flux of Comparative Example 1 was 0.55 mg·C / m 2 / h, that of Example 1 was 0.22 mg·C / m 2 / h, and that of Example 2 was 0.35 mg·C / m 2 / h. Therefore, compared with Comparative Example 1, both Examples 1 to 2 significantly reduced the average CH4 emission flux, and the reduction rate of Example 1 (59.8%) was significantly higher than that of Example 2 (35.6%) ( Figure 4 b)).

[0085] The relative abundances of the mcrA gene of CH4-producing archaea and the pmoA gene of CH4-oxidizing bacteria in the artificial wetlands prepared in Examples 1 to 2 and Comparative Example 1 were tested, and the results were obtained Figure 5 . From Figure 5 a) and b), it can be seen that as the addition amount of nZVI-BC increases, the relative abundances of both the mcrA gene and the pmoA gene show a downward trend. By calculating the relative abundance ratio of the mcrA gene and the pmoA gene ( Figure 5 c) in the text), Example 1 reduced the ratio of the mcrA gene to the pmoA gene, while Example 2 increased this ratio.

[0086] The Alpha (α) diversity indices of CH4-producing archaea and CH4-oxidizing bacteria in the artificial wetlands prepared in Examples 1 to 2 and Comparative Example 1 were tested, and Table 1 was obtained.

[0087] Table 1 Alpha diversity indices of CH4-producing archaea and CH4-oxidizing bacteria in the artificial wetlands prepared in Examples 1 to 2 and Comparative Example 1

[0088]

[0089] Note: Different lowercase letters indicate significant differences between different treatments (P<0.05).

[0090] Compared with Comparative Example 1, Examples 1-2 had no significant effect on the α-diversity index of methane-producing archaea. Different from methane-producing archaea, Example 1 significantly increased the α-diversity index of methane-oxidizing bacteria, especially the Shannon index and Simpson index. Specifically, Example 1 increased the Shannon index of methane-oxidizing bacteria in the constructed wetland by 52.3%, while decreasing the Simpson index by 74.4%, indicating a significant increase in the diversity of methane-oxidizing bacteria in Example 1 (a large Shannon index indicates high diversity, and a small Simpson index indicates high diversity). Compared with Comparative Example 1, Example 1 increased the Chao index of methane-oxidizing bacteria in the constructed wetland by 15.6%, while Example 2 decreased it by 34.3%, indicating that Example 1 enhanced the richness of methane-oxidizing bacteria, while Example 2 decreased its richness.

[0091] The changes in the community structure of methane-producing archaea at the genus level in the constructed wetlands prepared in Examples 1-2 and Comparative Example 1 were explored by NMDS analysis ( Figure 6 as shown in a)). Different constructed wetlands were separated along Axis 2 according to the addition amount. The distance between Comparative Example 1 and Examples 1-2 in the NMDS space was relatively far, indicating that the genus-level community composition of Comparative Example 1 was different from that of Examples 1-2. The distances between Examples 1-2 in the NMDS space were relatively close, indicating that their genus-level community compositions were relatively similar.

[0092] The community structure of methane-producing archaea at the genus level in Examples 1-2 was as Figure 6 shown in b). In Examples 1-2 and Comparative Example 1, the genus with the highest relative abundance was Methanobacterium, accounting for 65.9% - 89.0%. The relative abundance of Methanobacterium in Example 1 increased by 1.76%, while that in Example 2 decreased by 24.7%.

[0093] The NMDS analysis of the community structure of methane-oxidizing bacteria at the genus level in the constructed wetlands prepared in Examples 1-2 and Comparative Example 1 was as Figure 7 shown in a). Different constructed wetlands were separated along Axis 2 according to the addition amount. The distance between Comparative Example 1 and Example 2 in the NMDS space was relatively close, which reflected that their genus-level community compositions had a certain similarity. While the distance between Example 1 and Comparative Example 1 and Example 2 in the NMDS space was significantly farther, indicating that its genus-level community composition was significantly different from the other two.

[0094] The community structure of methane-oxidizing bacteria at the genus level in the constructed wetlands prepared in Examples 1-2 and Comparative Example 1 was as Figure 7As shown in b) of [the Chinese text]. In Comparative Example 1 and Example 2, the genus with the highest relative abundance was Methylomonas, accounting for 89.4% and 99.7% respectively. While in Example 1, the genus with the highest relative abundance was Methylomagnum, accounting for 55.1%. Therefore, compared with Comparative Example 1, the relative abundance of Methylomonas in Example 1 increased, and the relative abundance of Methylomagnum increased.

[0095] For the factors affecting CH4 emissions in the constructed wetlands prepared in Examples 1 - 2 and Comparative Example 1, Mantel test was used for analysis, and the result was Figures 8 - 9 . From Figure 8 it can be seen that there was no significant correlation between the average CH4 emission flux and the α - diversity index of CH4 - producing bacteria, indicating that the average CH4 emission flux was not affected by the diversity and richness of CH4 - producing bacteria.

[0096] The average CH4 emission flux was significantly negatively correlated with the Shannon index of CH4 - oxidizing bacteria, and significantly positively correlated with the Simpson index ( Figure 9 ), indicating that the higher the diversity of CH4 - oxidizing bacteria, the lower the CH4 emission. The effluent DOC concentration was significantly positively correlated with the Shannon index of CH4 - oxidizing bacteria, and significantly negatively correlated with the Simpson index ( Figure 9 ), indicating that the diversity of CH4 - oxidizing bacteria was more easily affected by DOC. The average CH4 emission flux was significantly negatively correlated with the relative abundance of the genus Methylomagnum ( Figure 9 ), indicating that the genus Methylomagnum was beneficial to CH4 emission reduction in the constructed wetland. In addition, the effluent DOC concentration was significantly positively correlated with the relative abundance of the genus Methylomagnum ( Figure 9 ), indicating that DOC was the key environmental driving factor for the genus Methylomagnum. Therefore, in Example 1, the DOC concentration reached the highest. This environmental change not only increased the diversity of CH4 - oxidizing bacteria, but also significantly increased the relative abundance of the genus Methylomagnum in CH4 - oxidizing bacteria, thus accelerating the CH4 oxidation process and ultimately effectively reducing CH4 emissions.

[0097] The CH4 emissions in the constructed wetland provided by the present invention were significantly reduced. Examples 1 - 2 reduced by 59.8% and 35.6% respectively. Among them, the constructed wetland prepared in Example 1 mainly promoted the occurrence of CH4 oxidation reaction by increasing the content of dissolved organic carbon (DOC) in the constructed wetland, increasing the diversity of CH4 - oxidizing bacteria, and increasing the relative abundance of the genus Methylomagnum (CH4 - oxidizing bacteria), and ultimately effectively reduced CH4 emissions.

[0098] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for reducing CH4 emissions from artificial wetlands, characterized in that: Biochar-loaded nanometer zero-valent iron is added to the upper matrix of the artificial wetland; the amount of the biochar-loaded nanometer zero-valent iron added is 0.5-2% of the mass of the upper matrix.

2. An artificial wetland, characterized in that: The method comprises a matrix, wherein the matrix comprises an upper matrix, a middle matrix and a lower matrix; the upper matrix comprises first gravel and biochar-loaded nano zero-valent iron; The mass of the biochar loaded nano zero-valent iron accounts for 0.5-2% of the mass of the upper matrix.

3. The artificial wetland according to claim 2, characterized in that: The thickness of the upper matrix is ​​16 to 24 cm.

4. The artificial wetland according to claim 2, characterized in that: The particle size of the first gravel is 0.5-1 cm, and the packing density of the first gravel is 1.80-1.90 t / m 3 .

5. The artificial wetland according to claim 2, characterized in that: The intermediate layer matrix includes second gravel, and the thickness of the intermediate layer matrix is ​​13-17 cm.

6. The artificial wetland according to claim 5, characterized in that: The particle size of the second gravel is 1-2 cm, and the packing density of the second gravel is 1.60-1.80 t / m 3 .

7. The artificial wetland according to claim 2, characterized in that: The lower layer matrix includes quartz sand, and the thickness of the lower layer matrix is ​​3 to 7 cm.

8. The artificial wetland according to claim 7, characterized in that: The particle size of the quartz sand is 0.5-1 cm, and the packing density of the quartz sand is 1.60-1.80 t / m 3 .

9. The artificial wetland according to claim 2, characterized in that: The mass ratio of nano zero-valent iron to biochar in the biochar-loaded nano zero-valent iron is 0.5:1 to 2:

1.

10. The method for constructing an artificial wetland according to any one of claims 2 to 9, characterized in that: The following steps are involved: The first gravel and the biochar loaded with nano-zero-valent iron are mixed to obtain an upper matrix; The upper layer matrix, the middle layer matrix and the lower layer matrix are filled in sequence to obtain the artificial wetland.

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