A method for estimating the risk threshold of ammonium nitrogen release from residual leaching agents in soil of rare earth in-situ leaching areas.

Abstract

This invention discloses a method for estimating the risk threshold of ammonium nitrogen release from residual leaching agents in soil of rare earth in-situ leaching areas. The method includes the following steps: developing a leaching experiment plan and designing and fabricating a leaching soil column device; selecting sampling points at different slopes of the rare earth mine and downstream riverbanks, and obtaining environmental and location information of the sampling points in the leaching area through field investigation; collecting fresh soil samples in layers; determining the ammonium nitrogen content and bulk density of the soil samples in laboratory experiments; screening leaching soil samples according to a certain concentration gradient based on the ammonium nitrogen concentration of the soil samples, and filling the soil column with fresh soil samples; preparing the leaching solution and adjusting its pH based on the main ion content and acidity of precipitation in the research site area; continuous leaching, collecting the leachate at regular intervals and monitoring the relevant properties of the leachate; and calculating the risk threshold of ammonium nitrogen release from the soil in the leaching area through data fitting. This invention avoids interference from uncontrollable factors in the field, ensuring the authenticity and accuracy of the ammonium nitrogen release risk threshold.

Description

Technical Field

[0001] This invention relates to the field of soil environmental monitoring technology, and in particular to a method for estimating the risk threshold of ammonium nitrogen release from residual leaching agents in soil of rare earth in-situ leaching areas. Background Technology

[0002] Ion-adsorption rare earth (IAE) deposits are the world's main source of medium and heavy rare earth elements (REEs), which are relatively scarce compared to light REEs. Therefore, the mining of IEE deposits has attracted significant attention. As a typical exogenous rare earth element, rare earth elements in IEE deposits can be extracted through ion exchange. After more than 50 years of improvement and development in rare earth mining technology, in-situ leaching has become the most favored technology for IEE mining.

[0003] When mining rare earth elements using the in-situ leaching process, at least 5 tons of ammonium sulfate are injected for every ton of rare earth extracted, resulting in a large amount of residual ammonium nitrogen in the soil of the rare earth leaching area. Even after top-water operation, the ammonium nitrogen content in the leaching area soil remains very high, several times, or even hundreds of times, higher than that of agricultural and natural soils. Although ammonium nitrogen is an essential nutrient element for plant growth, excessive ammonium can also harm plant growth. In addition, under certain conditions, ammonium nitrogen can be converted into easily leached nitrate nitrogen and nitrite nitrogen. The interconversion among these three forms makes it difficult to delineate the scope of nitrogen oxide pollution in soil and water bodies. At the same time, supersaturated ammonium nitrogen exceeds the soil's adsorption capacity and will be released into the surrounding soil, surface water, and groundwater over a long period of time, migrating with surface and groundwater runoff. Previous studies have found that the ammonia nitrogen content in the tributary water bodies of rare earth leaching areas is as high as 95 mg / L. -1 The concentration is 190 times the Class III ammonia nitrogen limit in the "Groundwater Quality Standard". Excessive nitrogen compounds cause a series of serious ecological and environmental problems in the mining area, including soil acidification, compaction, activation of heavy metals in the soil, poor plant root development, and eutrophication of water bodies. When humans ingest groundwater with excessive or excessive nitrogen compounds, it can cause vitamin A deficiency, decreased blood quality, dysentery, and other diseases. The release of excessive ammonium nitrogen from the soil will bring continuous nitrogen compound pollution to the leaching area and its surroundings, causing long-term harm to human health and production activities.

[0004] After environmental remediation of leaching areas, it is required that the ammonium nitrogen in the soil and water will not pose a threat to the surrounding environment. Therefore, establishing safety standards for the ammonium nitrogen content in soil and water is fundamental to environmental remediation. The National Standard for Pollutant Discharge from Rare Earth Industry (GB 26451-2011) stipulates that the limit for ammonia nitrogen content in water discharged from rare earth mining is 15 mg / L. -1The "Groundwater Quality Standard" (GBT14848-2017) also classifies the ammonium nitrogen and nitrate nitrogen content in water bodies with reference to the water quality requirements for drinking water, industrial and agricultural water. Ammonium nitrogen pollution in rare earth leaching areas originates from the soil, which is the source of pollution. However, currently, only the limits for ammonium nitrogen in water bodies are being considered, and there is a lack of safety standards for the ammonium nitrogen content in soil.

[0005] The safe limit for soil ammonium nitrogen is closely related to its adsorption and desorption processes in soil. The entire process of soil ammonium nitrogen release can be directly observed by analyzing the concentration changes of ammonium nitrogen in the leachate from simulated soil leaching experiments. However, various forms of nitrogen and their interaction with the surrounding medium are constantly undergoing transformation processes, including adsorption, nitrification, and denitrification. Furthermore, soil properties such as temperature, moisture, pH, and the number and activity of microorganisms all influence these transformation processes, thereby altering the ammonium nitrogen content and its adsorption capacity in the tested soil. Estimating the release risk of relatively reactive soil ammonium nitrogen remains a significant research challenge.

[0006] Traditional simulated leaching methods use dry soil samples sieved through a 2.00mm sieve. Exogenous pollutants are added at specific concentration gradients according to research needs to simulate soil pollution in mining areas. However, the air-drying process alters some soil properties, affecting the accuracy and reliability of the simulation results. Furthermore, residual leaching agents in rare earth mining areas are unsuitable for artificial addition. This is because the residual leaching agent in rare earth mining areas is ammonium sulfate, which readily transforms during soil wetting and drying. The artificially added substances after soil air-drying differ significantly from the actual residual leaching agents in the field, reducing the practical application value of the experimental results. In-situ field leaching experiments involve designating a mined area as the test site in the rare earth mining area, setting up leaching fluid transport pipelines, injection holes, collection pipes, and other facilities, and periodically collecting and measuring the composition of the leaching fluid. This method yields results that best reflect reality and can directly guide pollution control. However, the experiments are large-scale and long-term, requiring several years of leaching, resulting in high costs. Moreover, there are many uncontrollable factors, making it difficult to control variables to explore the release mechanism of ammonium nitrogen in the soil. How to investigate the release process of ammonium nitrogen in soil in leaching areas in a real, accurate, efficient, and low-cost manner is the second major challenge in current research. Summary of the Invention

[0007] Purpose of the invention: The purpose of this invention is to provide a method for estimating the risk threshold of ammonium nitrogen release from residual leaching agents in soil in rare earth leaching areas, so as to explore the release process of ammonium nitrogen in soil and estimate the content of ammonium nitrogen in soil with environmental risks; and to use this risk threshold as a standard for assessing the environmental risks of rare earth leaching areas, or as a method for calculating the amount of leaching agent to be added when optimizing the treatment of residual ammonium nitrogen in soil.

[0008] Technical solution: The present invention provides a method for estimating the risk threshold of ammonium nitrogen release from residual leaching agents in soil of rare earth in-situ leaching areas, comprising the following steps:

[0009] S1, determine the number of leaching samples, leaching volume, leaching cycle, and composition of the leaching solution, and design and fabricate a leaching soil column device, including a soil column frame and an acrylic soil column tube;

[0010] S2. Select sampling points at different slopes of the rare earth mine and its downstream riverbank to collect fresh soil samples and soil samples of known volume.

[0011] S3. Use a constant temperature drying oven to dry a soil sample of known volume. Calculate the soil moisture content based on the soil sample mass before and after drying. At the same time, calculate the soil bulk density based on the total volume of the soil sample before drying and the dry soil weight.

[0012] S4, weigh 10.0g of fresh soil sample and place it in a 200mL Erlenmeyer flask, then add 50.0mL of a 2mol / L solution. -1 The potassium chloride solution was sealed, shaken for 30 minutes, filtered, and the ammonium nitrogen concentration was detected using a flow analyzer to calculate the soil ammonium nitrogen content.

[0013] S5. Based on the soil ammonium nitrogen concentration determined in step S4, leached soil samples are screened according to a certain concentration gradient. Fresh soil samples are used to fill the soil column. Based on the soil bulk density determined in step S3, the soil is made close to the natural compaction level.

[0014] S6. Prepare the leaching solution and adjust the pH of the leaching solution according to the main ions and pH range of precipitation in the area where the research site is located;

[0015] S7, using a peristaltic pump for continuous leaching, collecting the leachate periodically and monitoring its relevant properties;

[0016] S8. Using soil ammonium nitrogen content as the independent variable and leachate ammonium nitrogen content as the dependent variable, data fitting was performed; the independent variable value corresponding to the inflection point on the data fitting curve was used as the risk threshold for soil ammonium nitrogen release in the leaching area.

[0017] Furthermore, in step S1, by collecting environmental information of the leaching area and the soil ammonium nitrogen pollution status, the leaching amount, leaching cycle, and leaching solution composition of the soil column experiment are determined based on the local rainfall conditions. At the same time, the amount of leaching sample is determined based on the range of ammonium nitrogen content in the soil of the leaching area.

[0018] Furthermore, in step S2, sampling points need to be set up at least three on different slopes of the mine, based on the range of leaching agent injection during rare earth mining and the location of mining facilities; at the same time, sampling points are set up along the direction of water flow on the banks of rivers near the mine; soil samples are collected layer by layer from the surface to the bottom of the ore body using a Luoyang shovel, with each layer divided into 50cm sections; if the soil texture, color and structural morphology change significantly within a 50cm thickness at a certain depth, the samples are divided into two sections.

[0019] Furthermore, in step S5, the total amount of fresh soil samples required for each soil column is calculated based on the average bulk density of the soil in the leaching area, the corresponding soil sample moisture content, the diameter of the leached soil column, and the filling height:

[0020]

[0021] In the formula, m 柱 Total wet soil weight (g) for each soil column of fresh soil sample; ρ b Soil bulk density (g / cm³) -3 ); w represents soil moisture content (%); V 柱 h is the volume of the soil column filling; r is the height of the soil column filling; h is the radius of the soil column.

[0022] When filling the soil column, ensure that the soil is compacted evenly to reduce the creation of large artificial holes, while avoiding excessive compaction that would cause the soil to lose its original size of pores, so that the soil is close to its natural compaction.

[0023] Furthermore, in step S8, based on the limited ability of soil to fix and adsorb ammonium nitrogen, and the principle that ammonium nitrogen is easily released into the environment when it exceeds the soil's retention capacity range, it is concluded that when the soil ammonium nitrogen content exceeds the risk threshold, the release of ammonium nitrogen enriched in the leaching area soil will increase sharply.

[0024] Compared with the prior art, the significant advantages of this invention are as follows:

[0025] 1. This invention uses a small indoor soil column to simulate the ammonium nitrogen leaching process in rare earth leaching areas under natural conditions. Fresh soil samples are used to maintain the original state of the soil as much as possible, while avoiding the influence of uncontrollable factors in the field. This allows for the realistic observation of the ammonium nitrogen release process in the soil, improving the accuracy of the ammonium nitrogen release risk threshold estimation. This risk threshold can be used as a standard for assessing the environmental risk of rare earth leaching areas, and can also be used to calculate the amount of leaching agent to be added when treating residual ammonium nitrogen in the soil.

[0026] 2. The risk threshold for the release of ammonium nitrogen in soil in rare earth leaching areas proposed in this invention can provide an evaluation standard for the ecological and environmental health risk assessment of rare earth mines and the acceptance of mine pollution control projects.

[0027] 3. This invention provides a theoretical basis for exploring the release mechanism of ammonium nitrogen in soil. Based on the exploration of the release process of ammonium nitrogen in soil, combined with the morphological characteristics and physicochemical properties of ammonium nitrogen in soil, a model can be constructed and extended to other areas except for ion-type rare earth leaching areas. The basic properties of soil can be used to predict the release of ammonium nitrogen in different types of soil.

[0028] 4. The experimental apparatus, theory, and method of this invention can be used for risk threshold assessment of residual leaching agents or other soil pollutants in other types of mining areas;

[0029] 5. This invention can be used in basic theoretical research and applied research in environmental science, soil science, land reclamation, etc., and provides important methods and necessary data support for environmental assessment of contaminated sites and soil remediation, and has broad application prospects. Attached Figure Description

[0030] Figure 1 This is the overall flowchart of the present invention;

[0031] Figure 2 This is a schematic diagram of an acrylic soil column used in a leaching experiment.

[0032] Figure 3 This is a schematic diagram of the soil column frame device for leaching experiments;

[0033] Figure 4 This is a diagram of the sampling tool;

[0034] Figure 5 This is a fitted diagram of the soil ammonium nitrogen release process;

[0035] Figure 6 A graph showing the estimated risk threshold for soil ammonium nitrogen release;

[0036] In the diagram: 1. Dust cover, 2. Sealing film, 3. Acrylic glass column, 4. Porous acrylic glass partition, 5. Connection port, 6. Drain pipe (silicone rubber conduit), 7. Quartz sand, 8. Filter cotton, 9. Non-woven filter screen, 10. Peristaltic pump hose, 11. Soil column frame, 12. Peristaltic pump, 13. Leachate collection bottle. Detailed Implementation

[0037] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0038] like Figure 1 The diagram shown is the overall flowchart of this invention, which mainly includes: collecting fresh soil samples in layers in the rare earth leaching area, determining the ammonium nitrogen content and bulk density of the soil samples; screening and packing leaching samples according to a certain ammonium nitrogen concentration gradient; preparing leaching solution according to the main components of local precipitation; continuous leaching, while monitoring the change process of ammonium nitrogen in the leaching solution, and data fitting to estimate the release risk threshold of ammonium nitrogen in the soil of the rare earth leaching area. Detailed implementation steps are as follows:

[0039] Step 1: Based on the relevant information collected in the rare earth leaching area in the early stage, formulate a leaching plan, determine the number of leaching samples, leaching volume, leaching cycle, and composition of the leaching solution, and design and manufacture the leaching soil column frame and plexiglass soil column tube.

[0040] (1.a): Collect data on the ammonium nitrogen content in the soil of the rare earth leaching area, including relevant papers and environmental monitoring information during rare earth mining, to understand the range of ammonium nitrogen content in the soil of the study area, which is 0–50 mg / kg. -1 Select at least 5 samples within the range of 50–300 mg / kg. -1 Select at least 10 samples within the range, >300mg / kg -1 Appropriately increase the screening gradient and set the number of leaching samples accordingly to avoid missing the change point of soil ammonium nitrogen release due to excessive density between ammonium nitrogen concentration gradients.

[0041] (1.b): A soil column tube containing leached soil samples is made of transparent plexiglass. Figure 2 The soil column is 10cm in diameter and 30cm in height. A silicone rubber conduit is connected to a hole at the bottom of the soil column. A circular porous acrylic partition is placed at the bottom, covered with a non-woven fabric filter to prevent leakage of filter material and soil sample. A removable dust cover is made at the top of the soil column, with a hole cut to the size of the peristaltic pump hose. Correspondingly, soil column racks with dimensions of 60cm × 30cm × 35cm (length × width × height) are constructed, each rack holding 8 soil columns, spaced 5cm apart. Figure 3 ).

[0042] (1.c): Consult local meteorological bureaus or academic papers focusing on this region to determine meteorological information such as the average annual precipitation and precipitation intensity of the rare earth leaching area. First, determine the total amount of leaching solution required for the leaching experiment based on the average annual precipitation, multiplying the amount by the top surface area of ​​the soil column (5 mm) based on the intensity of moderate to heavy rain (25 mm of rainfall in 24 hours). 2 ×πcm 2 The calculated daily leaching volume for the leaching experiment was approximately 196 mL. The total leaching volume divided by the daily leaching volume yielded the leaching experiment period.

[0043] (1.d): Identify the main cations (Ca) in precipitation in the study area through data or preliminary investigations. 2+ K + Na + Mg 2+ The content of ) and its pH range are used to determine the composition and acidity / alkalinity of the leachate.

[0044] Step two: Field investigation. Select sampling points at different slopes of the mine and the downstream riverbank to collect fresh soil samples and soil samples of known volume.

[0045] (2.a): Taking into account the topography, vegetation, and artificial facilities of the study area, six sampling points were selected along the mountain slopes at the top, middle, and bottom, and along the river at the upstream, midstream, and downstream sections. Sampling points on the rare earth mining area were placed as close to the slope as possible, with even spacing between points, avoiding large trees and mining facilities. Sampling points along the riverbank were located close to the water flow, with the upstream sampling point at the foot of the leaching mine. Two additional sampling points were set along the river's flow direction.

[0046] (2.b): Estimate the total amount of soil sample required for the leaching experiment. The inner diameter of the plexiglass soil column is 10 cm, and the planned filling height is 20 cm. The total filling volume of the soil column is approximately 1570.80 cm³. 3 The maximum bulk density of soil in China's terrestrial ecosystems does not exceed 2 g / cm³. 3 It is estimated that a maximum of 3.14 kg of dry soil is needed for the leaching experiment. Therefore, at least 5 kg of fresh soil samples need to be collected in the field.

[0047] (2.c): When collecting soil from the mine, remove surface dead branches and leaves, and use a Luoyang shovel with a diameter of 10cm. Figure 4 Soil samples are collected vertically downwards from the surface. Layers are defined with a 50cm boundary; if there are significant changes in soil texture, compaction, color, or weathering degree within 50cm, the soil should be divided into two layers. Soil samples are collected down to the bedrock surface. When approaching the bedrock, significant resistance will be encountered during downward hammering with the Luoyang shovel, and large stones with clearly visible rock structures will appear in the collected soil. When collecting riverbank soil, use a shovel to collect the top and subsurface soil layers, stopping when significant seepage occurs.

[0048] (2.d): Each layer of soil collected was first placed on a 1m x 1m white tarpaulin. Figure 4 Wearing rubber gloves, thoroughly mix the soil samples from each layer, seal them in a plastic resealable bag, and then place them in a sampling cloth bag for storage (collect at least 5 kg of soil sample from each layer). Mark the outside of the sample bag with a black marker, including: sampling point, sample depth, sampling time, sampling location, and the person who took the sample. Transport the collected soil samples to the laboratory in a low-temperature refrigerator and keep them in a refrigerator at a low temperature (below 4°C) until the experiment is conducted.

[0049] (2.e): Collect fresh soil samples. Simultaneously, collect soil samples of known volume using a ring cutter or Luoyang shovel to determine the soil bulk density and moisture content of the study area. Record the sampling depth, number of samples, sampling point, sampling time, and sampling location. Place the soil samples directly into self-sealing bags (pre-weighed), label them, and store them at low temperature.

[0050] Step 3: Dry a soil sample of known volume using a constant temperature drying oven at 105℃ to test its moisture content and bulk density. Calculate the soil moisture content based on the soil sample mass before and after drying; calculate the soil bulk density based on the total volume before drying and the weight of the dried soil.

[0051] (3.a): Weigh the total volume (m) of the known wet soil collected in the field using a balance. Then, take a portion of it and place it in a pre-numbered and weighed aluminum drying box, weighing it to an accuracy of 0.01g. Place the aluminum box containing the soil sample (without a lid) in a drying oven heated to 105±2℃ and dry it to constant weight, which takes about 6-8 hours. Wearing heat-resistant gauze gloves, remove the sample and transfer it to a desiccator to cool to room temperature (about 30 minutes). Weigh it immediately to an accuracy of 0.01g. Substitute the data into formula (1) to calculate the soil moisture content:

[0052]

[0053] In the formula, w is the soil moisture content (%); m0 is the mass of the dried empty aluminum box (g); m1 is the mass of the aluminum box and wet soil sample before drying (g); and m2 is the mass of the aluminum box and dry soil sample after drying (g).

[0054] (3.b): Substitute the above parameters into formula (2) to calculate the soil bulk density of the corresponding sample:

[0055]

[0056] In the formula, ρ b Soil bulk density (g / cm³) -3 m is the total weight of wet soil (g); w is the soil moisture content (%); V is the total volume of the soil sample (cm³). 3 ).

[0057] Step 4: Weigh 10.0g (accurate to 0.1g) of fresh soil sample and place it in a 200mL Erlenmeyer flask. Add 50.0mL of 2mol / L solution. -1 A potassium chloride (KCl) solution was sealed, shaken for 30 minutes, filtered, and the concentration of ammonium nitrogen (calculated as nitrogen) in the leachate was detected using a flow analyzer. The ammonium nitrogen content in the soil was then calculated.

[0058] (4.a): Prepare 2 mol L -1 To prepare the KCl solution, first weigh 149.1g of KCl (analytical grade) solid into a beaker, then dissolve it in distilled water, pour it into a 1L volumetric flask, dilute it to approximately 1L, and then bring the volume to 1L with distilled water.

[0059] (4.b): Weigh 10.0 g of fresh soil sample, accurate to 0.01 g. Place the sample into a 200 mL Erlenmeyer flask, avoiding contact with the flask walls. Measure 50 mL of 2 mol / L solution using a 50 mL graduated cylinder. -1 Slowly add the KCl solution along the wall of the flask into the Erlenmeyer flask. Tightly stopper the flask with the rubber stopper and shake well. Prepare parallel and blank samples simultaneously.

[0060] (4.c): Set the vibration amplitude of the vibrator to 180±20 r / min. -1 The temperature was 20℃. The Erlenmeyer flask was placed in a centrifuge and shaken for 30 minutes. After thorough mixing, it was filtered through medium-speed qualitative filter paper into a 50mL Erlenmeyer flask.

[0061] (4.d): The content of ammonium nitrogen in the extract was determined using a flow analyzer.

[0062] (4.e): Based on the ammonium nitrogen concentration of the extract and the measured soil moisture content, substitute into formula (3) to calculate the ammonium nitrogen content of the soil sample:

[0063]

[0064] In the formula, ρ is the mass concentration of ammonium nitrogen in the extract (mg / L). -1 );V 液 The volume of the extract (mL); m 干 The dry soil mass (g) of a 10.0g fresh soil sample; 10 3 Convert mL to L; 1000 is the conversion of ammonium nitrogen content per kg of soil.

[0065] Step 5: Based on the soil ammonium nitrogen content determined in Step 4, leached soil samples are screened according to a certain gradient. The total mass of soil samples required is calculated based on the average soil bulk density, water content and soil column specifications of the leaching area. Fresh soil samples are used to fill the soil column in layers to make the soil as close as possible to its natural compaction.

[0066] (5.a): Substitute the average bulk density of the soil in the leaching area, the corresponding soil sample moisture content, the diameter of the leached soil column, and the filling height into formula (4) to calculate the total amount of fresh soil sample required for each soil column:

[0067]

[0068] In the formula, m 柱 Total wet soil weight (g) for each soil column of fresh soil sample; ρ b Soil bulk density (g / cm³) -3 ); w represents soil moisture content (%); V 柱 ρ is the volume of the soil column; h is the height of the soil column, h = 20cm; r is the radius of the soil column, r = 5cm.

[0069] (5.b): Before filling the soil column, first lay a layer of non-woven fabric at the bottom of the soil column, then lay a 2cm thick layer of glass wool to prevent the loss of fine particles, and then fill a 2cm thick layer of quartz sand to prevent the loss of coarse particles.

[0070] (5.c): Weigh the appropriate weight of soil sample and fill it with tools to ensure that the soil is compacted evenly, reduce the generation of artificial large holes, and avoid excessive compaction that would cause the soil to lose its original size of pores. Try to make the height of each soil column close to 20cm, that is, the soil close to the natural compaction of the study area.

[0071] Step Six: Based on the Ca of precipitation in the area where the research site is located... 2+ K + Na + Mg 2+ Prepare the solution by concentration and adjust the pH.

[0072] (6.a): Simulated rainwater composition in the study area (Ca 2+ K + Na + Mg 2+ To avoid introducing sulfate and other interfering ions from the leaching agent, the leaching solution was prepared using CaCl2, KCl, NaCl, and MgCl2 (analytical grade).

[0073] (6.b): Calculate the mass of reagents required in 1L of leaching solution based on the content of the main cations, weigh the quantitative amount of reagents using a balance with an accuracy of 1 / 100,000, dissolve them in ultrapure water and make up to volume.

[0074] (6.c): To appropriately accelerate the leaching of ammonium nitrogen, acid rain was simulated for leaching. Therefore, hydrochloric acid was used to adjust the pH of the leaching solution to be close to the pH of the local acid rain. A multi-day leaching solution was prepared at once and stored in a sealed container for later use.

[0075] Step 7: Prepare a peristaltic pump and its auxiliary devices with a matching flow rate, use the peristaltic pump for continuous leaching, collect the leaching liquid at regular intervals, and monitor its relevant properties.

[0076] (7.a): Taking into account factors such as the settable flow rate of the peristaltic pump and water infiltration, the final set flow rate was 16.3 mL / h. -1 The daily leaching time is 12 hours, ensuring a total daily leaching volume of 196 mL.

[0077] (7.b): Collect all leachate at regular intervals each day and weigh it. Use a flow analyzer to determine the ammonium nitrogen content. The main observations are the differences in ammonium nitrogen content in leachate from different soil columns and the dynamic changes in ammonium nitrogen concentration in the same soil column.

[0078] Step 8: Based on the ammonium nitrogen content in the soil and leachate, perform data fitting to estimate the risk threshold of ammonium nitrogen release from the soil in the leaching area.

[0079] Note: If soil ammonium nitrogen content or precipitation composition information is unavailable in Step 1, pre-sampling is required. Soil sampling methods remain consistent with Step 2, and the collected soil samples are analyzed for ammonium nitrogen content according to Step 4. Precipitation samples must be collected and transported according to specifications. After filtration through a 0.25 μm filter membrane, the main cations (Ca) in rainwater are determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). 2+ K + Na + Mg 2+ The content of ).

[0080] The following example is a rare earth in-situ leaching area in southern my country.

[0081] The study area is located at the border of Jiangxi and Guangdong provinces and has a subtropical monsoon climate. It experiences four distinct seasons and abundant rainfall, with the rainy season concentrated from April to June. The region possesses rich ion-adsorption rare earth mineral resources and has a long history of rare earth mining. The ion-adsorption rare earth ore selected for this study was mined using in-situ ammonium sulfate leaching technology. The mine was completed in 2015, and sampling for this study was conducted in 2019.

[0082] (I) Preliminary Data Collection and Plan Formulation

[0083] Literature review revealed that the ammonium nitrogen content in the soil of this leaching area ranged from 2.32 to 1056.44 mg / kg. –1 The plan is to set up 21 concentration gradients, and based on these, 21 acrylic soil columns and soil column frames will be customized.

[0084] The survey found that the average annual precipitation in the study area is about 1500 mm. Multiplying this by the surface area of ​​the top of the soil column, the total leaching volume in the leaching experiment was calculated to be 11781 mL, with a daily leaching volume of 196 mL. Therefore, the leaching experiment period was 60 days. The main cation in the precipitation in the study area is Ca. 2+ K + Na + Mg 2+ The content was 0.00976 mmol / L. -1 0.00465 mmol / L -1 0.00442 mmol / L -1 0.0015 mmol / L -1 The leaching solution was prepared using CaCl2, KCl, NaCl, and MgCl2 (analytical grade). The mass content (mg / L) of each reagent in the leaching solution was calculated based on their molar mass. -1(Table 1). According to the information published in the local environmental quality annual report, the pH value of precipitation in the study area in 2019 ranged from 4.88 to 7.46. In order to accelerate the leaching of ammonium nitrogen in the soil and facilitate the control of the pH of the leaching solution, the pH of the leaching solution was adjusted to 5.0 ± 0.1 with HCl.

[0085] Table 1. Content of each component in the leachate

[0086]

[0087]

[0088] (II) Sample screening, soil column filling and leaching

[0089] Soil samples were collected in stratified layers in the field, and their ammonium nitrogen content and moisture content were determined promptly after low-temperature transportation. The ammonium nitrogen content of soil samples collected in the study area ranged from 9.35 to 904.93 mg / kg. -1 The soil in rare earth leaching areas contains a large amount of ammonium nitrogen, and the content of ammonium nitrogen in the soil varies greatly between different layers.

[0090] Two ring cutter samples were collected from the surface layer of the mountain, and two samples of known volume from the mineral-bearing soil layer were collected from the deeper layers using a Luoyang shovel. The average soil bulk density of the study area was approximately 1.20 g / cm³. -3 The bulk density of the soil column was used as the filling weight. 21 soil samples were selected according to the principle of "small gradient for low concentration and large gradient for high concentration". The corresponding soil column numbers and their ammonium nitrogen concentration and water content are shown in Table 2. Since the water content of the selected soil samples varied, the data were substituted into formula (4) to calculate the wet soil weight of each soil column (Table 2).

[0091] After layered filling, a peristaltic pump was used to simulate acid rain leaching. Water was continuously injected for 12 hours daily, followed by a 12-hour wait for the leachate to infiltrate and leach out. The leachate from the previous day was collected and tested periodically before turning on the peristaltic pump the following day. The changes in ammonium nitrogen concentration in the leachate were observed. It was found that the release of ammonium nitrogen from the soil in the early stage of leaching was significantly higher than in the later stage, and the release rate gradually decreased with increasing leaching time and volume. Figure 5 The initial content of ammonium nitrogen in soil varies among different soil columns, but the release process of all columns follows a power-law decreasing trend.

[0092] Table 2. Ammonium nitrogen concentration, moisture content, and filling weight of the experimental soil samples.

[0093]

[0094]

[0095] (III) Estimation of the ammonium nitrogen release process and release risk threshold in the soil of the leaching area

[0096] Using soil ammonium nitrogen content as the independent variable and ammonium nitrogen release as the dependent variable for fitting, it was found that under acid rain leaching in the simulated study area, the release of ammonium nitrogen from the soil in the leaching area had two inflection points. Figure 6 When the ammonium nitrogen content is greater than 260 mg / kg -1 At that time, the increase in ammonium nitrogen released into the environment gradually slowed down. Figure 6 This is because the amount of leaching solution input in the simulation experiment is limited, and there is also an upper limit to the amount of ammonium nitrogen that can be removed in a single leaching process. However, each leaching process will cause a large amount of ammonium nitrogen to migrate out of the soil. If there is no human intervention, each natural precipitation will cause serious ammonia nitrogen pollution to the water bodies around the leaching area.

[0097] When the soil ammonium nitrogen content exceeds 40 mg / kg -1 At that time, the release rate of ammonium nitrogen increased significantly. Figure 6 This is because the soil's capacity to fix and adsorb ammonium nitrogen is limited; once the soil's retention capacity is exceeded, ammonium nitrogen is easily released into the environment. Therefore, in this example, 40 mg / kg is considered appropriate. -1 This represents the risk threshold for ammonium nitrogen in the soil of the leaching area. When the soil ammonium nitrogen content exceeds 40 mg / kg... -1 At this time, the risk of release of ammonium nitrogen enriched in the soil of the leaching area increases sharply, which will cause greater harm to the surrounding environment. Therefore, the environmental risk threshold for ammonium nitrogen in the soil is 40 mg / kg. -1 .

[0098] Application Example 1: Recommendations for the Formulation of Environmental Protection Standards for Soil in Rare Earth Leaching Areas

[0099] While standards such as the "Groundwater Quality Standard" and the "Rare Earth Industry Pollutant Discharge Standard" have been established, a nationally applicable mandatory environmental protection standard for ion-adsorption rare earth in-situ leaching areas is currently lacking, particularly a standard for soil environmental risk assessment. Studies have found that ammonium nitrogen pollution is the primary pollutant in this area, leading to a series of subsequent environmental problems such as soil and water nitrate pollution, acidification, and heavy metal activation. Accurately assessing the ammonium nitrogen pollution status and environmental risks in leaching areas is fundamental to efficient remediation and rapid restoration of the fragile ecological environment. The soil ammonium nitrogen release risk threshold of this invention can be used for ecological risk assessment of soil in rare earth leaching areas. This value can serve as one of the bases for determining whether the residual ammonium nitrogen content in the leaching area soil poses an environmental hazard. Therefore, based on the release process and risk threshold of soil ammonium nitrogen, combined with relevant environmental protection standards, a basis can be provided for formulating suitable soil environmental protection standards for ion-adsorption rare earth in-situ leaching areas.

[0100] If the environmental risk assessment of soil ammonium nitrogen is based on the background value of soil ammonium nitrogen in the leaching area, the ammonium nitrogen content in undisturbed mine soils outside the leaching area ranges from 0 to 3.08 mg / kg. -1 The highest value was 3.08 mg / kg.-1 This is the most stringent ecological screening value. When the ammonium nitrogen content in the soil of the leaching area does not exceed this value, the ammonium nitrogen in the soil will not have any adverse effects on the surrounding environment. However, this standard is too harsh, as the total amount of ammonium nitrogen that the soil can retain is higher than this value. In addition, this standard is not realistic, because the local natural soil is nitrogen-deficient. Appropriately increasing the nitrogen content in the soil can actually provide nitrogen nutrition for plants and promote plant growth.

[0101] Soil ammonium nitrogen environmental risk assessment, if based on the Class IV water limit (1.5 mg / L) in the "Groundwater Quality Standard" (GB / T 14848-2017) and the "Surface Water Environmental Quality Standard" (GB 3838-2002), -1 The soil ammonium nitrogen release threshold limit was calculated to be approximately 23 mg / kg using a soil ammonium nitrogen release threshold fitting model. -1 This value applies to general industrial water use areas and can only be used as drinking water after proper treatment. However, this is only a standard for the ammonium nitrogen content in water bodies and cannot accurately determine the environmental limit for ammonium nitrogen content in soil.

[0102] If the environmental risk assessment of soil ammonium nitrogen is based on the water pollution discharge limits specified in the "Rare Earth Industry Pollutant Discharge Standard" (GB 26451-2011) to estimate the corresponding ammonium nitrogen content limit in the soil, the standard stipulates that the ammonium nitrogen content in the water discharged by rare earth enterprises during mining shall not exceed 15 mg / L. -1 The calculated soil ammonium nitrogen content is approximately 50 mg / kg. -1 This value represents the minimum requirement for the treatment of residual ammonium nitrogen in tailings soil. When the soil ammonium nitrogen content exceeds this value, the ammonium nitrogen content in the solution discharged from the soil after rainfall infiltration in the leaching area will exceed the discharge standards, posing a significant ecological hazard and requiring further treatment. This standard does not apply to wastewater discharged from ion-adsorption rare earth mining using in-situ leaching or heap leaching methods. Although it does not explicitly prohibit the use of this standard to assess the environmental risk of ammonium nitrogen pollution in mining area waters, there is an urgent need to determine a more suitable standard for assessing whether all soil ammonium nitrogen in ion-adsorption rare earth in-situ leaching areas poses an environmental risk.

[0103] The above three values ​​serve as a reference for the environmental risk assessment of ammonium nitrogen in soil from rare earth in-situ leaching areas, but each has its own limitations. Based on the release characteristics of ammonium nitrogen from the ore-bearing soil in this embodiment, and considering the soil ammonium nitrogen release risk threshold (40 mg / kg) estimated by referring to simulated leaching experiments... -1 When the soil ammonium nitrogen content exceeds 40 mg / kg -1 At this time, ammonium nitrogen in the soil is easily released, and the content of ammonium nitrogen migrating from the soil to the water increases sharply during leaching. When the ammonium nitrogen content in the soil is 40 mg / kg... -1At that time, under moderate rainfall conditions, the ammonium nitrogen content in the water body was less than 3.39 mg / L. -1 Moreover, at 40mg / kg -1 When this value is the target for the treatment of ammonium nitrogen in the soil of the leaching area, it is lower than the ammonium nitrogen discharge standard (15 mg / L) for rare earth enterprises during production. -1 The estimated soil ammonium nitrogen content is 50 mg / kg. -1 This is safer for the environment. Therefore, using the risk threshold of ammonium nitrogen release in soil as the soil safety limit for rare earth leaching areas can be used as a basis for environmental risk assessment.

[0104] Application Example 2: Remediation of Ammonia Nitrogen Pollution in Rare Earth Leaching Areas

[0105] Currently, the treatment of residual ammonium in in-situ leaching areas commonly employs the in-situ leaching method. This method utilizes mining facilities left over from the completion of mining operations to inject a certain amount of leaching agent solution into the tailings to wash away residual ammonium in the soil. To control treatment costs and achieve efficient and green leaching, it is necessary to determine the total amount of leaching agent input, which depends on the total amount of ammonium nitrogen to be removed from the soil in the in-situ leaching area.

[0106] To obtain this data, the leaching mine can be divided into three parts: the top, middle, and bottom of the slope, and soil samples can be collected and their ammonium nitrogen content determined at each location. The sampling and sample testing requirements are consistent with those of this invention. The total enrichment of ammonium nitrogen in the soil of the leaching area can be obtained by summing the products of the volumes of different soil layers in each topographic location and the ammonium nitrogen content (Formula (5)). The total volume of the mine soil in this embodiment is calculated to be 1.44 × 10⁻⁶. 5 m 3 The total amount of ammonium nitrogen in the soil is 38.0 tons. If the goal of remediation is to completely remove water-soluble and exchangeable ammonium nitrogen from the soil, the required amount of leaching agent can be calculated based on the total enrichment amount. However, this is time-consuming, labor-intensive, and material-intensive. For ammonia nitrogen remediation in mining areas, it is sufficient to ensure that residual ammonium in the soil no longer poses an environmental risk. Based on the calculation results of this example, it is considered that when the soil ammonium nitrogen content decreases to 40 mg / kg... -1 Leaching can be stopped at this time. Therefore, if the ammonium nitrogen content in a certain soil layer is greater than 40 mg / kg... -1 Then only the excess portion needs to be removed; if it is less than 40 mg / kg... -1 If the soil in the leaching zone is not in the leaching zone, then no treatment is required. Based on this rule and combined with formulas (5) and (6), the total amount of ammonium nitrogen to be removed from the soil in the leaching zone can be calculated, that is, the total potential release of ammonium nitrogen from the soil in the leaching zone (formulas (7) and (8)). The total amount of ammonium nitrogen in the soil in the leaching zone that poses a significant environmental risk is 32.8 tons, which needs to be treated in a timely manner.

[0107]

[0108] m 浸矿区(kg)=m 坡顶 +m 坡中 +m 坡底 (6)

[0109] In the formula, m a The total amount of residual ammonium nitrogen at slope position a (kg); S 1a The area of ​​the top surface at slope position a (m²) 2 );S 2a The area of ​​the base at slope position a (m²) 2 ). ; Δh i ρb represents the thickness (m) of the i-th soil layer at slope position a, i.e., the interval sampling depth; i The bulk density of the i-th soil layer at slope position a (g / cm³) -3 );c i The amount of ammonium nitrogen (mg / kg) to be removed from the i-th soil layer at slope position a -3 ). 1 to n represent the first layer (the soil layer closest to the surface) to the nth layer (the soil layer closest to the bedrock), respectively; m 浸矿区 Total residual ammonium nitrogen in the soil of rare earth leaching area (kg); m 坡顶 m 坡中 m 坡底 The figures represent the total residual ammonium nitrogen (kg) in the soil at the top, middle, and bottom of the closed mine slope.

[0110]

[0111] m 潜在总量 (kg)=m 坡顶潜在 +m 坡中潜在 +m 坡底潜在 (8)

[0112] In the formula, m a潜在 The total potential release of residual ammonium nitrogen at topographic location a; c i The residual ammonium nitrogen content (mg / kg) in the i-th soil layer of topographic location a -1 When c i ≥40mg kg -1 When participating in the accumulation, c i <40mg kg -1 Time is not included in the calculation; m 潜在总量 The total potential release of residual ammonium nitrogen from the soil in the rare earth leaching area (kg); m 坡顶潜在 m 坡中潜在 m 坡底潜在 These represent the total potential release (kg) of residual ammonium nitrogen in the soil at the top, middle, and bottom of the leaching area.

Claims

1. A method for estimating the risk threshold of ammonium nitrogen release from residual leaching agents in soil of rare earth in-situ leaching areas, characterized in that, Includes the following steps: S1, determine the number of leaching samples, leaching volume, leaching cycle, and composition of the leaching solution, and design and fabricate a leaching soil column device, including a soil column frame and an acrylic soil column tube; S2. Select sampling points at different slopes of the rare earth mine and its downstream riverbank to collect fresh soil samples and soil samples of known volume. S3. Use a constant temperature drying oven to dry a soil sample of known volume. Calculate the soil moisture content based on the soil sample mass before and after drying. At the same time, calculate the soil bulk density based on the total volume of the soil sample before drying and the dry soil weight. S4, weigh 10.0g of fresh soil sample and place it in a 200mL Erlenmeyer flask, then add 50.0mL of a 2mol / L solution. -1 The potassium chloride solution was sealed, shaken for 30 minutes, filtered, and the ammonium nitrogen concentration was detected using a flow analyzer to calculate the soil ammonium nitrogen content. S5. Based on the soil ammonium nitrogen concentration determined in step S4, leached soil samples are screened according to a certain concentration gradient; the total amount of fresh soil sample required for each soil column is calculated based on the average bulk density of the soil in the leaching area, the corresponding soil sample moisture content, the diameter of the leached soil column, and the filling height. In the formula, m 柱 Total wet soil weight (g) of fresh soil sample filled into each soil column; ρ b Soil bulk density, g / cm³ -3 w represents soil moisture content, %; V 柱 h is the volume of the soil column filling; r is the height of the soil column filling; h is the radius of the soil column. Use fresh soil samples to fill the soil column, and make the soil close to its natural compaction level according to the soil bulk density measured in step S3; when filling the soil column, ensure that the soil is compacted evenly, reduce the generation of artificial large holes, and avoid excessive compaction that would cause the soil to lose its original size of pores. S6. Prepare the leaching solution and adjust the pH of the leaching solution according to the main ions and pH range of precipitation in the area where the research site is located; S7, using a peristaltic pump for continuous leaching, collecting the leachate periodically and monitoring its relevant properties; S8. Using soil ammonium nitrogen content as the independent variable and leachate ammonium nitrogen content as the dependent variable, data fitting was performed; the independent variable value corresponding to the inflection point on the data fitting curve was used as the risk threshold for soil ammonium nitrogen release in the leaching area.

2. The method for estimating the risk threshold of ammonium nitrogen release from soil in rare earth in-situ leaching areas according to claim 1, characterized in that, In step S1, by collecting environmental information of the leaching area and soil ammonium nitrogen pollution, the leaching amount, leaching cycle and leaching solution composition of the soil column experiment are determined according to the local rainfall. At the same time, the amount of leaching sample is determined according to the range of ammonium nitrogen content in the soil of the leaching area.

3. The method for estimating the risk threshold of ammonium nitrogen release from soil in rare earth in-situ leaching areas according to claim 1, characterized in that, In step S2, sampling points need to be set up at least three on different slopes of the mine, based on the range of leaching agent injection during rare earth mining and the location of mining facilities. At the same time, sampling points should be set up along the direction of water flow on the banks of rivers near the mine. Fresh soil samples should be collected layer by layer from the surface to the bottom of the ore body using a Luoyang shovel, with each layer divided into 50cm sections. If the soil texture, color, and structural morphology change significantly within a 50cm thickness at a certain depth, the samples should be divided into two sections.

4. The method for estimating the risk threshold of ammonium nitrogen release from soil in rare earth in-situ leaching areas according to claim 1, characterized in that, In step S8, based on the limited ability of soil to fix and adsorb ammonium nitrogen, and the principle that ammonium nitrogen is easily released into the environment when it exceeds the soil's retention capacity, it is concluded that when the soil ammonium nitrogen content exceeds the risk threshold, the release of ammonium nitrogen enriched in the leaching area soil will increase sharply.

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