Method for purifying nitrogen in nitrogen-containing water body by applying red mud iron separation residues to rice field wetland

Abstract

The invention discloses a method for applying red mud iron separation residues to purification of nitrogen in a nitrogen-containing water body in a rice field wetland, and belongs to the technical field of water pollution ecological restoration in environmental engineering. The red mud iron separation residues are washed with clear water to remove impurities attached to the surface until the washed water is clear and transparent; naturally airing the cleaned red mud iron separation residues, grinding the dried red mud iron separation residues by using a ball mill until the particle size is 0.1-1mm, irrigating the red mud iron separation residues by using river water with over-standard nitrogen in a manual irrigation manner until a flowerpot is full, standing for the hydraulic retention time of 1 day, 2 days, 4 days and 6 days set in the fourth step, and sampling and taking water, experimental variables are whether rice is used or not and whether red mud and brown iron residues are used or not, the other variables are consistent, and by applying the red mud iron separation residues in the wetland of the rice field, nitrogen-containing water can be effectively purified, nitrogen loss is reduced, and the pollution risk to surrounding water and soil is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of ecological restoration of water pollution in environmental engineering, and particularly relates to a method for applying red mud iron separation residue to purify nitrogen in nitrogen-containing water bodies in paddy field wetlands. Background Art

[0002] With the rapid development of the urbanization process, the environmental problems brought about by urbanization have gradually become prominent, including the pollution problems of small and medium-sized rivers. According to partial statistics of surface water [1] , the proportion of surface water with excellent (Class I-III) water quality sections reached 89.4%, an increase of 1.5 percentage points year-on-year, and the proportion of water quality sections of inferior Class V was 0.7%, remaining unchanged year-on-year. However, the total nitrogen concentration in some rivers flowing into the sea is still relatively high, and total nitrogen pollution has become an important factor affecting the water quality of the coastal waters. The water quality of the main stream remains stable at Class II throughout the year, and the water quality of the Haihe River Basin has improved from slightly polluted to good. However, in some areas with relatively serious agricultural non-point source pollution, the seasonal variation of the total nitrogen concentration is obvious. The pollution control in key river basins has achieved remarkable results, and the surface water environmental quality has continued to improve, but the nitrogen pollution problems of some rivers still need attention. During the urbanization process of surface water, the proportion of urban ground hardening increases, and the natural vegetation coverage rate decreases, resulting in an increase in rainwater runoff and an accelerated rate of pollutants flowing into rivers. Some researchers have studied the long-term accumulated nitrogen pollution in groundwater and rivers in some areas through isotopic methods, especially in agricultural intensive areas and rivers around cities, where the nitrogen levels are relatively high [2] . According to the research of Wang Mengru, she identified the sources of nitrates in groundwater and rivers in complex urban environments through isotopic and hydrochemical analyses, and pointed out that the nitrogen levels in urban rivers are significantly affected by the discharge of domestic sewage and industrial wastewater [3] . Human activities will affect the environment. Even though great efforts have been made in the treatment of environmental pollution and the improvement of environmental conditions, some pollution is still inevitable. During the period from 2021 to 2022, in-depth research and analysis were carried out on the surface water quality of a specific area. This area has a special geographical location, located at the intersection of farmland, industrial plants and coal mining subsidence areas, facing serious problems of exceeding the standard of total nitrogen TN content. According to relevant investigation reports, the TN content in this area not only exceeds the standard limit, but its existing forms are quite complex, covering various forms such as ammonia nitrogen, nitrite nitrogen and organic nitrogen. The total nitrogen exceeding the standard in the river poses a risk of water eutrophication.

[0003] For nitrogen in water bodies, the treatment methods are different in different types of wastewater. For wastewater with high ammonia nitrogen, alkaline substances can be added to change the pH value of the wastewater to remove ammonia nitrogen by converting it into ammonia gas through the stripping method [4] ; the membrane separation method that uses the high-efficiency selectivity of membrane technology to separate nitrogen [5] ; the breakpoint chlorination chemical method of adding chlorine to the water body to make ammonia nitrogen become nitrogen[6] ; The biological method that utilizes the substances in the water body through the action of microorganisms to carry out nitrification and denitrification to remove nitrogen [7] ; The ecological restoration method that constructs an artificial wetland and utilizes the synergistic effect of wetland plants and microorganisms to remove the substances in the water body [8] And so on. In the above technologies, if the stripping method is used, only part of the ammonia nitrogen may be removed, and a large amount of alkali is consumed, and the nitrogen-exceeding river water brings new problems; using the membrane separation method can achieve the separation of nitrogen, but the surface water discharge is large, and the membrane is prone to clogging and damage, which is tantamount to raising the operating cost; using the breakpoint chlorination method has a high cost and may produce harmful by-products; the biological method using the nitrification and denitrification of microorganisms has the problem of low treatment efficiency for low-pollution water due to insufficient carbon source; constructing an artificial wetland requires considering issues such as the floor area and whether the water body can meet the growth needs of wetland plants. For the river water with a large total amount of nitrogen exceeding the standard, directly using traditional methods cannot achieve an ideal treatment effect.

[0004] [1] Ministry of Ecology and Environment of the People's Republic of China. China Ecological Environment Bulletin 2023[M]. Beijing: Ministry of Ecology and Environment, 2024.

[0005] [2] CHEN W, ZHANG X, WU N, et al. Sources and transformations of riverine nitrogen across a coastal-plain river network of eastern China: New insights from multiple stable isotopes[J]. Science of the Total Environment, 2024, 924: 171671.

[0006] [3] Wang M, Bodirsky B L, Rijneveld R, et al. A triple increase in global river basins with water scarcity due to future pollution[J]. Nature Communications, 2024, 15(1): 880.

[0007] [4] Zhang Y Y, Xu X D, Wang K C, et al. Research progress on the resource utilization technology of nitrogen in sewage[J]. Water Purification Technology, 2024, 43(12): 18 - 26 + 159.

[0008] [5] Zhang Binyang, Zhu Xinhua, Zhou Jing, et al. Research progress on the application of membrane separation technology in industrial wastewater treatment [J]. Contemporary Chemical Industry Research, 2024, (15): 30 - 32.

[0009] [6] Yu Xinlong, Wan Qiang, Gao Zhen, et al. Application of breakpoint chlorination method in wastewater treatment plants [J]. Journal of Tianjin University of Science & Technology, 2021, 36(04): 47 - 50.

[0010] [7] Lin Hongjian, Liu Wei, Chen Yibin. Research progress on biological treatment technology for ammonia - nitrogen wastewater [J]. Guangzhou Chemical Industry, 2016, 44(11): 16 - 18.

[0011] [8] Ran Qiaoling, Yang Juan, Xu Haitao. Application and exploration of water ecological restoration technology in river pollution control [J]. Leather Manufacture and Environmental Science & Technology, 2024, 5(18): 131 - 133. Summary of the Invention

[0012] Technical problems to be solved:

[0013] Aiming at the deficiencies of the existing technology, this application solves the current problems that the stripping method only removes part of the ammonia - nitrogen and consumes a large amount of alkali, the membrane in the membrane separation method is prone to clogging and damage, which is tantamount to raising the operating cost, the breakpoint chlorination method has a high cost and may produce harmful by - products, the biological method has a low treatment efficiency for low - pollution water due to insufficient carbon source, and for the river water with a large total amount of nitrogen exceeding the standard, directly using traditional methods cannot achieve an ideal treatment effect. A method for applying red mud iron - selecting residue to purify nitrogen in nitrogen - containing water bodies in paddy field wetlands.

[0014] Technical solution:

[0015] To achieve the above - mentioned purpose, this application is realized through the following technical solutions:

[0016] A method for applying red mud iron - selecting residue to purify nitrogen in nitrogen - containing water bodies in paddy field wetlands, the steps are as follows:

[0017] The first step, pretreatment of red mud iron - selecting residue: Rinse the red mud iron - selecting residue with clean water to remove the impurities attached to the surface. After rinsing, continue to clean it by stirring and filtering until the washed water is clear and transparent; Place the washed red mud iron - selecting residue in a well - ventilated environment to dry naturally, and use a ball mill to grind the dried red mud iron - selecting residue to a particle size between 0.1 - 1 mm, and remove the particles that do not meet the particle size requirements through a screening device;

[0018] The second step, prepare the potted plants for the simulated paddy field wetland system: Prepare a group of potted plants with a radius of 20 cm, and the filling amount of natural soil is 14 kg; Uniformly lay the red mud iron - selecting residue at the bottom of the potted plants, and the laying thickness is 2 - 3 cm;

[0019] The third step is transplanting rice: place the rice seeds in a dry and sunny environment to dry for 2 to 3 days, then immerse the seeds completely in clean water and stir to eliminate the empty grains, select the full-shaped seeds, continue to soak the seeds and change the clean water every 4-6 hours until the seeds germinate white roots and then transfer them to cultivated soil and natural soil for cultivation. The soil layer is 5 cm thick and fertilizers are added to meet the nutritional needs of rice before transplanting. After the seeds enter the soil cultivation stage, a thin layer of soil is sprinkled on the surface to cover the soil. During the planting period, the relative humidity of the soil was maintained at 70% to 80%, and the temperature was maintained at 25-35℃. Plastic film was covered at night to keep the temperature stable. When the rice developed to the three-leaf and one-heart stage, plants with the same growth condition and growth were selected for transplanting. The plants were transplanted into pots simulating the rice field wetland system. Three holes were planted in each pot, with one plant in each hole. After transplanting, the rice needed a 5-7 day greening period to adapt to the new environment. Subsequently, the water quality and rice plants in the rice field wetland system were monitored. The water quality monitoring items included ammonium nitrogen NH4 + -N, nitrate nitrogen NO3 - -N and total nitrogen TN;

[0020] Step 4: Phosphate fertilizer P2O5 dosage is 37.5kg / hm 2 , the dosage of potassium fertilizer K2O is 60kg / hm 2 , using TN as 9.40mg / L, NH4 + -N is 2.30mg / L, NO3 - -N was 3.54mg / L, and the rice in the pot was artificially flooded with river water with excessive nitrogen, and 10t / hm2 of zeolite was added 2 , equivalent to 20g / kg dry soil, set 4 different hydraulic retention times, namely 1d, 2d, 4d and 6d, let it stand for one day after each irrigation and then add new nitrogen-excessive river water, set two groups of potted plants without plants for control, and repeat irrigation three times for each treatment at the same retention time;

[0021] Step 5: Four different treatments were set up in the pot experiment, namely natural soil CK, natural soil treatment SA with rice, natural soil treatment SA-IRF with rice and red mud iron residue added, and natural soil treatment IRF without rice and red mud iron residue added;

[0022] Step 6: Simulate the rice field wetland experiment: artificial irrigation was adopted, using river water with excessive nitrogen content to irrigate until the flower pots were full, and then the water was sampled after the hydraulic retention time set in step 4, 1 day, 2 days, 4 days and 6 days. The experimental variables were whether rice was grown and whether red mud and black iron residue were used, and the other variables were kept consistent.

[0023] Step 7: During the vegetative growth stage, the concurrent stage, and the reproductive growth stage of rice, test the water quality at hydraulic retention times of 1 day, 2 days, 4 days, and 6 days respectively for each stage, and repeat three times.

[0024] Step 8: After the rice is mature and harvested, measure the related growth indexes of the plant, including root length, plant height, dry and fresh weights, number of effective panicles, rice panicle length, seed setting rate, tiller number determined by the artificial counting method, and 100-grain weight.

[0025] Furthermore, in Step 7, the vegetative growth stage of rice mainly focuses on the development of vegetative organs and has not entered the formation of reproductive organs. The vegetative organs of rice are one or several of roots, stems and leaves, and tillers. The reproductive organs of rice are young panicles.

[0026] Furthermore, in Step 7, the concurrent stage of rice is a transitional stage when both vegetative growth and reproductive growth occur simultaneously, with the stem growing rapidly and the young panicles beginning to differentiate.

[0027] Furthermore, in Step 7, the reproductive growth stage of rice mainly focuses on the development of reproductive organs, and the development of reproductive organs includes heading, flowering, filling, and maturity.

[0028] Furthermore, in the test of related growth indexes of root length, plant height, tiller number, and 100-grain weight of the plant in Step 8, the specific test methods are as follows:

[0029] Measurement of plant height: After the rice is completely mature, take out the plant and wash it thoroughly to remove the attached soil and moisture. Subsequently, use a tape measure to accurately measure the plant height in an indoor environment.

[0030] Measurement of root length: After the plant is washed, use a tape measure to measure the root length of the plant in an indoor environment.

[0031] Determination of dry and fresh weights: After measuring the plant height and root length, remove the soil from the roots of the plant, dry the surface of the plant with absorbent paper, and use an analytical balance with a precision of one ten-thousandth to measure the fresh weight of the plant. After the fresh weight measurement is completed, place the plant in an oven, perform a 0.5-hour blanching treatment at 105°C, then adjust the temperature to 80°C and dry it to a constant weight, and use the balance to measure the dry weight of the plant again.

[0032] Statistics of the number of effective panicles: Before harvesting the rice, record the number of panicles produced by each plant in each treatment group.

[0033] Measurement of rice panicle length: After the last irrigation and when the rice is mature, use scissors to cut off the tail of the rice panicle and measure the length of the rice panicle in an indoor environment.

[0034] Calculation of seed setting rate: For the rice in each treatment group, count the number of plump grains and shriveled grains respectively, and calculate using the following formula: Seed setting rate = Number of plump grains / (Number of plump grains + Number of shriveled grains) × 100%;

[0035] Determination of 100-grain weight: Use a balance with a precision of one ten-thousandth to accurately weigh the weight of 100 plump grains;

[0036] Determination of chlorophyll fluorescence parameters: Before the experimental system stops running, use a chlorophyll fluorometer to measure the fluorescence parameters of rice leaves. For each treatment group, select one leaf from each rice plant for measurement to ensure the representativeness and consistency of the data;

[0037] Number of tillers: Select representative sample plants such as single-plant transplanting areas, mark the main stem leaf age, and regularly record the tillering node positions, numbers, and growth and decline changes 56; Investigate once every 3 - 5 days during the tillering period until heading, and count the number of effective panicles at maturity.

[0038] Beneficial effects:

[0039] The present application provides a method for applying red mud iron ore dressing residue to purify nitrogen in nitrogen-containing water bodies in paddy field wetlands. Compared with the prior art, it has the following beneficial effects:

[0040] 1. The present application can not only resourcefully utilize nitrogen-exceeding water bodies (the nitrogen-exceeding river water has TN = 10.53 mg / L, and the nitrogen removal rate of the paddy field wetland with added red mud iron ore dressing residue is 78% when the hydraulic retention time is 6 days), but also have certain economic benefits (paddy field seed setting rate (91.14%) and 100-grain weight (2.89 g));

[0041] 2. The paddy field wetland of the present application, as a special ecosystem, plays an important role in the prevention and control of agricultural non-point source pollution. By applying red mud iron ore dressing residue in the paddy field wetland, nitrogen-containing water bodies can be effectively purified, nitrogen loss can be reduced, and the pollution risk to surrounding water bodies and soil can be lowered. Description of the drawings

[0042] Figure 1 It is the ammonia nitrogen removal rate diagram of different hydraulic retention times in the paddy field wetland of the present application;

[0043] Figure 2 It is the ammonia nitrogen removal rate diagram of different growth stages in the paddy field wetland of the present application;

[0044] Figure 3 It is the nitrate nitrogen removal rate diagram of different hydraulic retention times in the paddy field wetland of the present application;

[0045] Figure 4 It is the nitrate nitrogen removal rate diagram of different growth stages in the paddy field wetland of the present application;

[0046] Figure 5 This is the total nitrogen removal rate graph for different hydraulic retention times in the paddy wetland of this application;

[0047] Figure 6 This is the total nitrogen removal rate graph for different growth stages in the paddy wetland of this application. Detailed implementation manners

[0048] The following further illustrates the present invention in conjunction with embodiments. The following description is only for the preferred embodiments of the present invention and does not limit the present invention in other forms. Any person skilled in the relevant art may use the disclosed technical content to make equivalent embodiments with equivalent changes. Any simple modification or equivalent change made to the following embodiments based on the technical essence of the present invention without departing from the content of the present invention's solution falls within the protection scope of the present invention.

[0049] Embodiment 1:

[0050] A method for applying red mud iron ore dressing residue to purify nitrogen in nitrogen-containing water bodies in a paddy wetland, the steps are as follows:

[0051] First step, pretreatment of red mud iron ore dressing residue: Rinse the red mud iron ore dressing residue with clean water to remove impurities attached to the surface. After rinsing, continue to clean by stirring and filtering until the washed water is clear and transparent; Place the washed red mud iron ore dressing residue in a well-ventilated environment to dry naturally, and use a ball mill to grind the dried red mud iron ore dressing residue to a particle size between 0.1 - 1 mm, and remove particles that do not meet the particle size requirements through a screening device;

[0052] Second step, prepare potted plants for the simulated paddy wetland system: Prepare a group of potted plants with a radius of 20 cm, and the filling amount of natural soil is 14 kg; Uniformly lay red mud iron ore dressing residue at the bottom of the potted plants, and the laying thickness is 2 - 3 cm;

[0053] Step 3: Transplanting of rice: Place the rice seeds in a dry and sunny environment for 2 to 3 days of sun drying. Then, completely immerse the seeds in clean water and stir to eliminate underfilled and shrivelled grains, and screen out seeds with plump shapes. The seeds continue to be soaked and the water is changed every 4 - 6 hours until white roots germinate from the seeds, and then they are transferred to cultivated soil and natural soil for cultivation. The soil layer thickness is 5 cm, and fertilizers are incorporated to meet the nutritional requirements of the rice before transplantation. After the seeds enter the soil cultivation stage, a thin layer of soil is spread on the surface as covering soil. During the soil cultivation stage, maintain the relative humidity of the soil at 70% - 80%, and keep the temperature at 25 - 35 °C. Cover with a plastic film at night to keep the temperature stable. When the rice develops to the three - leaf and one - heart stage, select plants with consistent growth status and growth vigor for transplantation. Transplant the plants into pots in the simulated paddy wetland system, plant 3 holes in each pot, with 1 plant in each hole. After transplantation, the rice needs a 5 - 7 - day green - turning period to adapt to the new environment, and then start monitoring the water quality in the paddy wetland system and the rice plants. The water quality monitoring items include ammonium nitrogen NH4 + -N, nitrate nitrogen NO3 - -N and total nitrogen TN;

[0054] Step 4: The application rate of phosphorus fertilizer P2O5 is 37.5 kg / hm 2 , and the application rate of potassium fertilizer K2O is 60 kg / hm 2 , and use river water with excessive nitrogen content (

[0055] TN is 9.40 mg / L, NH4 + -N is 2.30 mg / L, NO3 - -N is 3.54 mg / L) for artificial flooding irrigation of the rice in the pots. Add 10 t / hm 2 of zeolite, equivalent to 20 g / kg of dry soil. Set 4 different hydraulic retention times, which are 1 d, 2 d

[0056] , 4 d and 6 d. After each irrigation, let it stand for one day and then add new river water with excessive nitrogen content. Set two groups of plant - free pots as controls

[0057] , and repeat the irrigation three times for each treatment at the same retention time;

[0058] Step 5: The pot experiment sets 4 different treatments, namely natural soil CK, natural soil treatment SA with rice planted

[0059] , natural soil treatment SA - IRF with rice planted and red mud iron - selected residue added, and natural soil treatment IRF without rice planted but with red mud iron - selected residue added;

[0060] Step 6: Simulate paddy wetland experiment: By means of artificial irrigation, irrigate with river water exceeding the nitrogen standard until the flower pots are full. Then let it stand for the hydraulic retention time set in Step 6, namely 1 day, 2 days, 4 days, and 6 days, and then sample and take water. The experimental variables are whether there is rice and whether red mud and praseodymium iron residue are used, and other variables are ensured to be the same;

[0061] Step 7: During the vegetative growth stage, progressive stage, and reproductive growth stage of rice, test the water quality at hydraulic retention times of 1 day, 2 days, 4 days, and 6 days respectively for each stage, and repeat three times; The vegetative growth stage of rice is mainly characterized by the development of vegetative organs and has not entered the formation of reproductive organs; The vegetative organs of rice are one or several of roots, stems and leaves, and tillers; The reproductive organs of rice are young panicles; The progressive stage of rice is the transitional stage of simultaneous vegetative growth and reproductive growth, with the rapid elongation of the stem and the beginning of differentiation of young panicles; The reproductive growth stage of rice is mainly characterized by the development of reproductive organs, and the development of the reproductive organs includes heading, flowering, filling, and maturity; Testing the water quality specifically means testing the ammonium nitrogen NH4 + -N, nitrate nitrogen NO3 - -N and total nitrogen TN;

[0062] Step 8: After the rice is mature and harvested, measure the related growth indexes of root length, plant height, dry and fresh weight, effective panicle number, rice panicle length, seed setting rate, tiller number measured by artificial counting method, and 100-grain weight of the plants.

[0063] Furthermore, in the measurement of related growth indexes of root length, plant height, tiller number, and 100-grain weight of the plants in Step 8, the specific measurement methods are as follows:

[0064] Measurement of plant height: After the rice is completely mature, take out the plants and thoroughly clean them to remove the attached soil and moisture. Subsequently, accurately measure the plant height of the plants using a tape measure in an indoor environment;

[0065] Measurement of root length: After the plants are washed, measure the root length of the plants using a tape measure in an indoor environment;

[0066] Determination of dry and fresh weight: After the measurement of plant height and root length is completed, remove the soil from the roots of the plants, dry the surface of the plants with blotting paper, and measure the fresh weight of the plants using an analytical balance with a precision of one ten-thousandth. After the fresh weight measurement is completed, place the plants in an oven, conduct a blanching treatment at 105°C for 0.5 hours, then adjust the temperature to 80°C and dry to a constant weight, and measure the dry weight of the plants using the balance again;

[0067] Statistics of effective panicle number: Before the rice is harvested, record the number of panicles produced by each plant in each treatment group;

[0068] Measurement of rice panicle length: After the last irrigation and when the rice is mature, use scissors to cut off the tail of the rice panicle and measure the length of the rice panicle in an indoor environment;

[0069] Calculation of seed setting rate: For the rice in each treatment group, count the number of plump grains and shriveled grains respectively, and calculate using the following formula: Seed setting rate = Number of plump grains / (Number of plump grains + Number of shriveled grains) × 100%;

[0070] Measurement of 100-grain weight: Use a balance with a precision of one ten-thousandth to accurately weigh the weight of 100 plump grains; Measurement of chlorophyll fluorescence parameters: Before the experimental system stops running, use a chlorophyll fluorometer to measure the fluorescence parameters of rice leaves. For each treatment group, select one leaf from each rice plant for measurement to ensure the representativeness and consistency of the data;

[0071] Number of tillers: Select representative sample plants such as the single-plant transplanting area, mark the main stem leaf age, and regularly record the tillering node position, quantity, and growth and decline changes 56. Investigate once every 3 - 5 days during the tillering stage until heading, and count the number of effective panicles at maturity.

[0072] The nitrogen removal effect of the paddy wetland system on surface water, such as Figure 1 and Figure 2 The ammonia nitrogen removal efficiency diagrams of the paddy wetland at different hydraulic retention times and different growth stages as shown. The ammonia nitrogen removal efficiency increases with the increase of hydraulic retention time

[0073] , and the removal rate in the vegetative growth stage is higher than other stages. When the hydraulic retention time is 6 days, the ammonia nitrogen removal rate of CK reaches 71.2%, that of SA is 83.2%, that of SA-IRF is 88.7%, and that of IRF is 77.2%.

[0074] As Figure 3 and Figure 4 The nitrate nitrogen removal efficiency diagrams of the paddy wetland at different hydraulic retention times and different growth stages as shown. When the hydraulic retention time is 6 days, the ammonia nitrogen removal rate of CK reaches 29.4%, that of SA is 54.2%, that of SA-IRF is 70.4%, and that of IRF is 63.2%.

[0075] As Figure 5 and Figure 6As shown in the figure, the graph of the removal efficiency of total nitrogen by paddy field wetlands at different hydraulic retention times and different growth stages shows an upward trend in the total nitrogen removal rate with the increase of hydraulic retention time. There is no significant difference between HRT = 1d and HRT = 2d, but it shows an upward trend. There is a significant difference between HRT = 4d and other hydraulic retention time groups. HRT = 6d has the highest removal efficiency and there is a significant difference. The total nitrogen removal efficiency of the paddy field wetland experimental group at the same growth stage at different growth stages

[0076] , showing that the total nitrogen removal rate in the vegetative growth stage of rice is greater than that in other stages, with significant differences. When the hydraulic retention time is 6 days, the ammonia nitrogen removal rate of CK reaches 55.5%, the ammonia nitrogen removal rate of SA is 68.8%, the removal rate of SA-IRF is 82.1%, and the removal rate of IRF is 71.0%

[0077] Table 1 Relevant indicators for rice harvest in this application

[0078] Group Plant height / cm Root length / cm Tiller number / plant Fresh weight / g Dry weight / g SA 80.00±1.93b 23.30±0.92b 6.53±0.34b 210.70±21.64b 120.01±22.03b SA-IRF 95.00±1.91a 30.87±0.99a 7.67±0.19a 299.27±20.29a 155.00±36.29a

[0079] These physiological indicators can reflect the growth situation of rice plants. In the SA-IRF experimental group, the plant height, tiller number, fresh weight, dry weight, and root length in the physiological indicators of rice plants are significantly higher than those of the ordinary planted rice treatment.

[0080] Table 2 Yield and its related indicators after rice harvest

[0081] Group Panicle length / cm 100-grain weight / g Seed setting rate / % Panicle number Estimated yield / (kg / a) SA 23.13±0.45a 2.19±0.14b 73.65±1.23b 33 342.15 SA-IRF 24.00±2.02a 2.81±0.10a 83.64±1.11a 30 462.46

[0082] To sum up, the nitrogen removal effect of paddy field wetlands is the best when HRT = 6d. SA-IRF can achieve efficient nitrogen removal, and its effluent removal rates of NH4 + -N, NO3 - -N and TN reach 77.3%, 63.3% and 71.0% respectively. The seed setting rate of 83.64% and 100-grain weight of 2.81g indicate that adding red mud iron ore dressing residue can produce a synergistic effect with paddy field wetlands and has a positive impact on the yield of rice. The red mud iron ore dressing residue reduces the concentration of nitrate nitrogen in the water body through adsorption, providing a more suitable growth environment for rice; while rice further removes nitrogen in the water body through biological absorption, and its root exudates may promote the adsorption performance of the red mud iron ore dressing residue.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not restrictive. Other modifications or equivalent replacements made by those of ordinary skill in the art to the technical solutions of the present invention should be covered within the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for applying red mud iron - selecting residue to purify nitrogen in nitrogen - containing water bodies in paddy field wetlands, characterized in that, The steps are as follows: The first step is the pretreatment of the iron - selected residue of red mud: Rinse the iron - selected residue of red mud with clear water to remove the impurities attached to the surface. After rinsing, continue to clean it by stirring and filtering until the washed water is clear and transparent. Place the washed iron - selected residue of red mud in a well - ventilated environment to dry naturally. Use a ball mill to grind the dried iron - selected residue of red mud to a particle size between 0.1 - 1 mm, and remove the particles that do not meet the particle size requirements through a screening device; The second step is to prepare the potted plants for the simulated paddy wetland system: Prepare a group of potted plants with a radius of 20 cm, and the filling amount of natural soil is 14 kg. Evenly lay the iron - selected residue of red mud at the bottom of the potted plants, and the laying thickness is 2 - 3 cm; Step 3, transplanting of rice: Place the rice seeds in a dry and sunny environment and sun-dry them for 2 to 3 days. Then, completely immerse the seeds in clean water and stir to eliminate underfilled and shriveled grains, and screen out seeds with plump morphology. The seeds continue to be soaked and the water is changed every 4 - 6 hours until white roots germinate from the seeds, and then they are transferred to cultivated soil and natural soil for cultivation. The soil layer thickness is 5 cm, and fertilizers are incorporated to meet the nutritional requirements of rice before transplantation. After the seeds enter the soil cultivation stage, a thin layer of soil is spread on the surface as covering soil. During the soil cultivation stage, maintain the relative humidity of the soil at 70% - 80%, and keep the temperature at 25 - 35°C. Cover with a plastic film at night to keep the temperature stable. When the rice develops to the three-leaf and one-heart stage, select plants with consistent growth conditions and growth vigor for transplantation. Transplant the plants into pots in the simulated paddy wetland system, plant 3 holes in each pot, and 1 plant in each hole. After transplantation, the rice needs a 5 - 7-day green-recovery period to adapt to the new environment, and then start monitoring the water quality in the paddy wetland system and the rice plants. The water quality monitoring items include ammonium nitrogen NH4 + -N, nitrate nitrogen NO3 - -N and total nitrogen TN; Step 4: The application rate of phosphate fertilizer P2O5 is 37.5 kg / hm 2 , and the application rate of potassium fertilizer K2O is 60 kg / hm 2 . Use the river water with excessive nitrogen content with TN of 9.40 mg / L, NH4 + -N of 2.30 mg / L, and NO3 - -N of 3.54 mg / L to conduct artificial flooding irrigation on the rice in the potted plants. Add 10 t / hm of zeolite 2 , equivalent to 20 g / kg of dry soil. Set 4 different hydraulic retention times of 1 d, 2 d, 4 d, and 6 d respectively. After each irrigation, let it stand for one day and then add new river water with excessive nitrogen content. Set two groups of potted plants without plants for comparison. Each treatment is repeatedly irrigated three times at the same retention time; The fifth step: The potted plant experiment sets 4 different treatments, namely natural soil CK, natural soil treatment SA with rice planted, natural soil treatment SA - IRF with rice planted and iron - selected residue of red mud added, and natural soil treatment IRF with iron - selected residue of red mud added without rice planted; The sixth step is the simulated paddy wetland experiment: Adopt the method of artificial irrigation, use the river water with excessive nitrogen for irrigation until the flower pots are full, and then stand for the hydraulic retention times of 1 day, 2 days, 4 days, and 6 days set in the fourth step before sampling and water intake. The experimental variables are whether there is rice and whether to use the iron - selected residue of red mud, and the other variables are kept consistent; The seventh step: During the vegetative growth stage, the progressive stage, and the reproductive growth stage of rice, test the water quality according to the hydraulic retention times of 1 day, 2 days, 4 days, and 6 days in each stage, and repeat three times; The eighth step: After the rice is mature and harvested, measure the relevant growth indexes of the plant, such as root length, plant height, dry and fresh weight, effective panicle number, rice panicle length, seed - setting rate, measure the tiller number by the artificial counting method, and 100 - grain weight; 2. The method for applying red mud iron-selection residue to purify nitrogen in nitrogen-containing water bodies in paddy field wetlands according to claim 1, characterized in that: In the seventh step, the vegetative growth stage of rice is mainly the development of vegetative organs and has not entered the formation of reproductive organs. The vegetative organs of rice are one or several of roots, stems and leaves, and tillers. The reproductive organs of rice are young panicles.

3. The method for applying red mud iron-selection residue to purify nitrogen in nitrogen-containing water bodies in paddy field wetlands according to claim 1, wherein: In the seventh step, the progressive stage of rice is the transitional stage of simultaneous vegetative growth and reproductive growth, with the rapid elongation of the stem and the beginning of young panicle differentiation.

4. The method for applying red mud iron-selection residue to purify nitrogen in nitrogen-containing water bodies in paddy field wetlands according to claim 1, characterized in that: In the seventh step, the reproductive growth stage of rice is mainly the development of reproductive organs, and the development of reproductive organs includes heading, flowering, filling, and maturity.

5. The method for applying red mud iron-selection residue to purify nitrogen in nitrogen-containing water bodies in paddy field wetlands according to claim 1, characterized in that: In the seventh step, the water quality test specifically refers to the test of ammonium nitrogen NH4 + -N, nitrate nitrogen NO3 - -N and total nitrogen TN.

6. The method for applying red mud iron - selecting residue to purify nitrogen in nitrogen - containing water bodies in paddy - field wetlands according to claim 1, wherein: In the eighth step, when testing the relevant growth indexes of root length, plant height, tiller number, and 100 - grain weight of the plant, the specific test methods are as follows: Measurement of plant height: After the rice is completely mature, take out the plant and thoroughly clean it to remove the attached soil and moisture. Subsequently, accurately measure the plant height of the plant using a tape measure in an indoor environment; Measurement of root length: After the plant is washed, measure the root length of the plant using a tape measure in an indoor environment; Determination of dry and fresh weight: After measuring the plant height and root length, remove the soil from the roots of the plant, use blotting paper to dry the surface of the plant, measure the fresh weight of the plant using an analytical balance with a precision of one ten - thousandth. After measuring the fresh weight, place the plant in an oven, conduct a 0.5 - hour blanching treatment at 105 °C, then adjust the temperature to 80 °C and dry it to a constant weight, and measure the dry weight of the plant using the balance again; Effective panicle number statistics: Before rice harvest, record the number of panicles produced by each rice plant in each treatment group; Rice panicle length measurement: After the last irrigation and when the rice is mature, use scissors to cut off the tail of the panicle and measure the length of the panicle in an indoor environment; Calculation of seed setting rate: For the rice in each treatment group, count the number of plump grains and shriveled grains separately, and calculate using the following formula: Seed setting rate = Number of plump grains / (Number of plump grains + Number of shriveled grains) × 100%; Determination of 100-grain weight: Use a balance with a precision of one ten-thousandth to accurately weigh the weight of 100 plump grains; Determination of chlorophyll fluorescence parameters: Before the experimental system stops running, use a chlorophyll fluorometer to measure the fluorescence parameters of rice leaves. For each treatment group, select one leaf from each rice plant for measurement to ensure the representativeness and consistency of the data; Tiller number: Select representative sample plants such as single-plant transplanting areas, mark the main stem leaf age, and regularly record the tillering node positions, numbers, and growth and decline changes 56; Conduct investigations every 3 - 5 days during the tillering stage until heading, and count the effective panicle number at maturity.

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