A method for sequential extraction and detection of fluorine in soil

Abstract

The application relates to a sequential extraction and detection method of fluorine in soil and belongs to the technical field of soil characteristic analysis. The method comprises the following steps: S1, mixing soil and water-soluble fluorine extraction liquid according to a certain solid-liquid ratio, ultrasonic treatment, centrifugation, and testing the content of water-soluble fluorine in supernatant; S2, when the soil is acidic or neutral, a weak alkali and then a weak acid extraction method is adopted; when the soil is alkaline, a weak acid and then a weak alkali extraction method is adopted, and the content of adsorbed fluorine is tested; S3, mixing the extraction residue after the step S2 extraction and oxidized combined fluorine extraction liquid according to a certain solid-liquid ratio, ultrasonic treatment, centrifugation, and testing the content of oxidized combined fluorine in supernatant; S4, drying and grinding the extraction residue after the step S3 extraction, and testing the content of fixed fluorine in the residue by an alkali fusion method. The method does not change the soil properties during the fluorine extraction process, the content of various forms is determined, and the ecological environment of fluorine in the soil can be most truly simulated.

Description

Technical Field

[0001] This invention relates to the field of soil characterization analysis technology, and in particular to a method for sequential extraction and detection of fluoride in soil. Background Technology

[0002] Fluorine is closely related to the ecological environment and human health. It is both an essential trace element for humans and animals and a toxic element with a low toxicity threshold. When the fluorine content in the soil is below or above a critical value, it can be transferred through water and food, leading to insufficient or excessive fluorine intake in humans, thus harming human health. It has been reported that the toxicity, migration and transformation behavior, and biological effects of fluorine do not simply depend on its total content, but rather on its specific chemical forms and binding states in the soil. The chemical characteristics of fluorine, the complexity of soil composition, and the diversity of various environmental influencing factors, such as climatic conditions and human activities, result in a wide variety of fluorine occurrence forms in the soil.

[0003] Currently, research on fluoride in soil mainly focuses on endemic fluorosis and environmental pollution caused by fluoride. Further research is needed on the occurrence forms and interconversions of fluoride in soil. To date, a unified technical standard system has not been established, and there is a lack of standard reference materials for soil fluoride speciation. How to determine the content of various speciations without altering soil properties during fluoride extraction, how to most realistically simulate the ecological environment of fluoride in soil, and how to establish methods for analyzing the chemical speciation of fluoride are among the current research hotspots and challenges. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a method for sequential extraction and detection of fluoride in soil, which solves the technical problem of difficulty in accurately analyzing the chemical forms of fluoride in soil.

[0006] (II) Technical Solution

[0007] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0008] In a first aspect, the present invention provides a method for sequential extraction of fluoride from soil, comprising the following steps:

[0009] S1. Extraction of water-soluble fluoride: Soil and water-soluble fluoride extract are mixed at a certain solid-liquid ratio, ultrasonicated, and centrifuged to obtain the first supernatant containing water-soluble fluoride.

[0010] S2. Extraction of adsorbed fluorine:

[0011] When the soil is acidic or neutral, the extraction residue after step S1 is mixed with the weak alkaline extract at a certain solid-liquid ratio, ultrasonicated, centrifuged, and the precipitate is collected. The weak acid extract is added to the collected precipitate at a certain solid-liquid ratio, ultrasonicated, and centrifuged to obtain a second supernatant containing adsorbed fluorine.

[0012] When the soil is alkaline, the extraction residue after step S1 is mixed with the weak acid extract at a certain solid-liquid ratio, ultrasonicated, centrifuged, and the precipitate is collected. The weak alkaline extract is added to the collected precipitate at a certain solid-liquid ratio, ultrasonicated, and centrifuged to obtain a second supernatant containing adsorbed fluorine.

[0013] S3. Extraction of oxidized fluorine: The extraction residue after step S2 is mixed with the oxidized fluorine extract at a certain solid-liquid ratio, sonicated, and centrifuged to obtain a third supernatant containing oxidized fluorine.

[0014] S4. Extraction of stationary fluorine: The extraction residue after step S3 is dried and ground, and a mixture containing stationary fluorine is obtained by alkaline melting method.

[0015] Optionally, in step S1, the water-soluble fluoride extract is water.

[0016] Water-soluble fluoride mainly refers to fluoride existing in soil as ions or complexes, including F. - HF 2- H2F 3- H3F 4- AlF6 3- This invention first extracts water-soluble fluoride from the soil, using water as the extraction solution. Under ultrasonic conditions, the water-soluble fluoride is fully dissolved in the water. After centrifugation, the content of water-soluble fluoride in the soil is determined by testing the content of water-soluble fluoride in the supernatant.

[0017] Optionally, in step S2, the weak acid extract is at least one of oxalic acid solution, acetic acid solution, dilute nitric acid solution, and citric acid solution, and the weak base extract is at least one of sodium bicarbonate solution, sodium carbonate solution, potassium hydroxide solution, and sodium bicarbonate solution.

[0018] Preferably, in step S2, the weak acid extract is an oxalic acid solution and the weak base extract is a sodium bicarbonate solution.

[0019] Optionally, in step S3, the oxidized fluoride extract is an acetic acid solution of 0.2-0.4 mol / L hydroxylamine hydrochloride.

[0020] Preferably, in step S3, the oxidized fluoride extract is a 0.25 mol / L acetic acid solution of hydroxylamine hydrochloride.

[0021] This invention uses an acetic acid solution of hydroxylamine hydrochloride as the extraction solvent for oxidized fluorine. Verification by this invention shows that the concentration of hydroxylamine hydrochloride in the extraction solvent has a significant impact on the extraction results. Using 0.25 mol / L NH3OHCl-C2H4O2 results in the lowest interference with the extraction of oxidized fluorine, the highest extraction rate, and the smallest error.

[0022] Optionally, in steps S1-S3, the ultrasound duration is at least 30 minutes.

[0023] Ultrasonic time has a significant impact on the extraction of different forms of fluoride from soil. When the ultrasonic time is less than 30 minutes, the extract cannot completely extract a certain form of fluoride from the soil, resulting in very low accuracy. When the ultrasonic time is 30 minutes or more, fluoride can be almost completely dissolved in the extract, and the error in the extraction results is essentially zero.

[0024] Optionally, in steps S1-S3, the temperature of the extract is 25±5℃ during the ultrasonic process.

[0025] Ultrasonic temperature affects extraction efficiency; higher temperatures increase solubility, but excessively high temperatures can damage soil structure, thus affecting accuracy. Therefore, this invention preferably uses an extraction solution temperature of 25±5℃ during ultrasonic treatment. At this temperature, both efficiency and extraction recovery rate can be balanced, reducing errors.

[0026] Optionally, in steps S1-S3, the solid-liquid ratio is 1:10-15 (m / V).

[0027] During the extraction process, both excessively high and excessively low solid-liquid ratios of the sample and extract will introduce significant errors into the test results. However, when the solid-liquid ratio is 1:10-15, the error in the measurement results is relatively small. If the solid-liquid ratio is too low, the large sample volume will result in insufficient solid-liquid contact in the extraction container, leading to lower measurement results. If the solid-liquid ratio is too high, the representativeness of the sample and the measurement will affect the low content values. Considering all factors, a solid-liquid ratio of 1:10-15, with a more preferred ratio of 1:10, is selected as the optimal extraction condition.

[0028] Optionally, in step S4, the reagent used in the alkali fusion method is one or more of sodium carbonate, sodium hydroxide, potassium hydroxide, sodium peroxide, lithium metaborate, and potassium pyrosulfate.

[0029] Optionally, in step S4, the mass ratio of the reagent used in the alkali fusion method to the dried and ground extraction residue is 2-8:1.

[0030] Preferably, in step S4, the reagent used in the alkali fusion method is sodium peroxide, and the mass ratio of sodium peroxide to the dried and ground extraction residue is 5:1.

[0031] Sodium peroxide is a strong oxidizing flux that ensures complete sample digestion and fully opens the mineral lattice, which is more conducive to the efficient extraction of stationary fluorine. Excess flux ensures complete digestion; too little flux results in insufficient dissolution and lower measurement results; too much flux leads to excessive salt content, affecting the measurement, and is also wasteful, increasing costs. The preferred flux-to-sample mass ratio in this invention is 2-8:1, more preferably 5:1.

[0032] Secondly, the present invention provides a method for detecting fluoride in soil, wherein the fluoride is extracted in the order described in the method, and the fluoride extracted in each step is quantitatively detected after each extraction.

[0033] (III) Beneficial Effects

[0034] The beneficial effects of this invention are as follows: This invention provides a sequential extraction method for different forms of fluoride in soil, namely a "four-step extraction method" involving water-soluble, adsorbed, oxidized, and fixed forms. This invention simulates changes in natural and anthropogenic environmental conditions, rationally utilizes a series of selective reagents, and follows a principle from weak to strong extraction to achieve a method for classifying and extracting analytes in different forms for speciation analysis. This invention selects the most suitable extraction solution for each form, employs ultrasonic extraction, and optimizes extraction conditions to ensure accurate and reliable data. It ensures that the soil properties are not altered during fluoride extraction to determine the content of various forms, and can most realistically simulate the ecological environment of fluoride in soil, establishing a chemical speciation analysis method for fluoride and forming a unified technical standard system.

[0035] In this invention, different extraction solutions are selected for soils with different pH levels when extracting adsorbed fluoride. Specifically, when the soil is acidic or neutral, a weakly alkaline extraction solution is used first, followed by a weakly acidic extraction solution. When the soil is alkaline, a weakly acidic extraction solution is used first, followed by a weakly alkaline extraction solution. This method achieves a higher extraction rate for adsorbed fluoride, resulting in more thorough extraction. Adsorbed fluoride is mainly adsorbed onto the surface of clay particles, organic matter particles, and hydrated oxides through electrostatic attraction. Due to the small ionic radius and high electronegativity of fluoride (F), various fluoride anions present in the soil solution are easily adsorbed through exchange with the hydroxyl groups (-OH) on layered silicates and amorphous iron and aluminum oxides. Among various environmental factors, activities such as plant root secretion, decomposition of plant and animal residues, microbial metabolism, fertilization, and atmospheric deposition promote the formation of organic acids in the soil. Low molecular weight organic acids (oxalic acid, H₂C₂O₄) were selected as the extraction agent for adsorbed fluorine to simulate the weakly acidic environment of plant roots and accurately describe the migration ability of the preforms under these conditions. Simultaneously, the stronger ion exchange capacity of oxalic acid ensured more complete extraction. As pH increased, OH⁻… - As concentration increases, F - With OH -Since the ions have similar radii and geochemical properties, they are prone to ion exchange, which gradually increases the activity of F. Therefore, sodium bicarbonate (NaHCO3) was selected as another extractant for adsorbed fluorine.

[0036] This invention optimizes extraction conditions. Experiments have shown that the solid-liquid ratio of the solid sample to the extractant, the temperature and time of ultrasonic extraction, the order of use of the extractant, and the concentration of the extractant all significantly affect the accuracy and efficiency of the extraction results. Using the method of this invention, the properties of the soil can be preserved, while accurately assessing the speciation and content of fluoride in the soil. Attached Figure Description

[0037] Figure 1 The effect of the solid-liquid ratio of solid samples and extracts on the determination of water-soluble fluoride;

[0038] Figure 2 The effect of ultrasonic time on the determination results of water-soluble fluoride;

[0039] Figure 3 The effect of ultrasonic temperature on the determination results of water-soluble fluorine;

[0040] Figure 4 The results show the determination of oxidized bound fluorine extracted from NH3OHCl-C2H4O2 at different concentrations. Detailed Implementation

[0041] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present invention can be understood more clearly and thoroughly, and that the scope of the present invention can be fully conveyed to those skilled in the art.

[0043] Example 1: Effect of solid-liquid ratio on water-soluble fluorine

[0044] For four national standard reference materials GBW07915, GBW07916, GBW07935, and GBW07938, the solid sample mass and extraction liquid (water) volume were varied. Taking the extraction of water-soluble fluoride as an example, the solid sample mass to water volume ratios were accurately weighed at 1:5, 1:10, 1:15, and 1:20, respectively. The solid sample was added to water, capped, and shaken well. The mixture was then sonicated at (22±5)℃ for 30 min, allowed to stand, and centrifuged at 4000 r / min for 20 min. The supernatant was accurately collected, and the water-soluble fluoride in the supernatant was extracted. The effect of different solid-liquid ratios on the extraction of water-soluble fluoride was studied.

[0045] The results are as follows Figure 1 As shown, it can be seen that during the extraction process, if the selected solid-liquid ratio is too large or too small, the relative error (RE / %) of the test results is relatively large, while the RE / % of the measured results is smaller when the solid-liquid ratio is 1:10 or 1:15. Taking all factors into consideration, a solid-liquid ratio of 1:10 is selected as the extraction condition.

[0046] Example 2: Effect of ultrasound time on water-soluble fluorine

[0047] Referring to the method in Example 1, with a solid-liquid ratio of 1:10, the effect of different ultrasonic times on water-soluble fluorine in the samples was studied. Specifically, water-soluble fluorine was measured for four national standard materials with ultrasonic times of 10, 20, 30, 40, and 50 min, respectively.

[0048] The results are as follows Figure 2 As shown, it can be seen that the extraction result of water-soluble fluoride shows an overall increasing trend as the ultrasonic time increases. When the time exceeds 30 min, the RE (%) of the extraction result is basically stable. Therefore, 30 min is selected as the optimal ultrasonic time parameter.

[0049] Example 3: Effect of ultrasonic temperature on water-soluble fluorine

[0050] Referring to the method of Example 1, the solid-liquid ratio was selected as 1:10, the ultrasonic time was 30 min, and the effect of different ultrasonic temperatures on water-soluble fluorine in the sample was studied. Specifically, 25±5℃ and 60±5℃ were selected as temperature variables, and water-soluble fluorine in four national standard materials was measured.

[0051] The results are as follows Figure 3 As shown, there is no significant difference between the two sets of data. Generally, temperature is closely related to the solubility of water-soluble fluoride; the higher the temperature, the greater the solubility. However, excessively high temperatures may damage the soil structure, thus affecting the accuracy of the experiment. Therefore, considering the simplicity of operation and the protection of the soil structure, room temperature extraction was chosen, specifically an ultrasonic temperature of 25±5℃.

[0052] Example 4: Effect of the order of extractants on adsorbed fluorine

[0053] This embodiment takes samples collected in southern Shaanxi Province as an example. Two extraction agents, weak acid (oxalic acid) and weak base (sodium bicarbonate), were used. The operation was carried out according to the steps, and the extraction order was adjusted to explore the effect of the extraction agents on the extraction of fluorine adsorbed in the soil.

[0054] Specifically, following the method of Example 1, the solid-liquid ratio was selected as 1:10, the ultrasonic time was 30 min, the ultrasonic temperature was 25±5℃, and the extraction solution was first extracted with a weak acid and then with a weak alkali; or first extracted with a weak alkali and then with a weak acid, to study the effect of the order of extraction on the adsorbed fluorine.

[0055] Table 1 shows the fluoride content measured in soils with different pH levels under different acid-base extraction sequences. The results indicate that soils with different pH levels will yield different amounts of adsorbed fluoride even without distinguishing between soil acidity and alkalinity. A higher fluoride extraction rate is achieved by using an extraction method that proceeds from alkaline to acidic soils and from acidic to alkaline soils. Therefore, the extraction sequence should be determined based on the soil's acidity or alkalinity to achieve optimal extraction and reduce subsequent detection errors.

[0056] Table 1. Effect of extraction sequence on fluoride extraction yield from soils at different pH levels.

[0057]

[0058] Example 5: Effect of different concentrations of hydroxylamine hydrochloride-acetic acid solution on oxidized fluorine.

[0059] Referring to the method in Example 1, the solid-liquid ratio was selected as 1:10, the ultrasonic time was 30 min, the ultrasonic temperature was 25±5℃, and two different concentrations of extraction solutions, 0.05 mol / L NH3OHCl / C2H4O2 or 0.25 mol / L NH3OHCl / C2H4O2, were selected as extraction agents to extract oxidized bound fluorine.

[0060] The results are as follows Figure 4 As shown, 0.25 mol / L NH3OHCl-C2H4O2 has a better extraction effect. To reduce the influence of metal ions on the determination of oxidized bound fluorine, 0.25 mol / L NH3OHCl-C2H4O2 was used as the extraction solvent.

[0061] Example 6

[0062] This embodiment selects soil from tea gardens in southern Shaanxi as the research object, and extracts water-soluble fluorine, adsorbed fluorine, oxidized fluorine and fixed fluorine from the soil in sequence to study the migration and transformation of various forms of fluorine in the soil.

[0063] (1) Water-soluble fluoride: Weigh 2.50 g of soil sample into an extraction bottle, add 25.0 mL of boiled and cooled ultrapure water (18.25 MΩ·cm), cover and shake well, and extract ultrasonically at (25 ± 5) ℃ for 30 min. After standing, centrifuge at 4000 r / min for 20 min, accurately transfer 20.0 mL of supernatant, and retain the extraction residue. Add 5.0 mL of 1.0 mol / L trisodium citrate buffer solution to the supernatant, and measure the potential response value (mV) of the sample using an ion meter under stirring conditions.

[0064] (2) Adsorbed fluorine: Weigh 2.50 g of the extraction residue obtained in step (1), add 25.0 mL of 0.11 mol / L oxalic acid to the extraction residue, and extract ultrasonically at (25 ± 5) °C for 30 min. After standing, centrifuge at 4000 r / min for 20 min, accurately transfer 20.0 mL of supernatant, add 5.0 mL of 1.0 mol / L trisodium citrate buffer solution, and measure the potential response value (mV) of the sample using an ion meter under stirring conditions. Wash the remaining solid twice with pure water, centrifuge, weigh 2.50 g, add 25.0 mL of 0.5 mol / L sodium bicarbonate solution, and extract by ultrasonication at (25 ± 5) ℃ water bath for 30 min. After standing, centrifuge at 4000 r / min for 20 min, accurately transfer 20.0 mL of supernatant, add 5.0 mL of 1.0 mol / L trisodium citrate buffer solution, and measure the potential response value (mV) of the sample using an ion meter under stirring conditions.

[0065] (3) Oxidized fluorine: Weigh 2.50 g of the extraction residue obtained by centrifugation after extraction with sodium bicarbonate solution in step (2). Add 25.0 mL of 0.25 mol / L NH3OHCl / C2H4O2 solution to the extraction residue and extract ultrasonically at (25 ± 5) ℃ for 30 min. After standing, centrifuge at 4000 r / min for 20 min. Accurately transfer 20.0 mL of supernatant and retain the extraction residue. Add 5.0 mL of 1.0 mol / L trisodium citrate buffer solution to the supernatant. Under stirring conditions, measure the potential response value (mV) of the sample using an ion meter.

[0066] (4) Fixed fluorine: After washing the extraction residue obtained in step (3) with pure water, the remaining solid was evaporated to dryness in a water bath. The residue was transferred to a container, ground, and then placed in a desiccator. 0.5g of the residue sample was weighed and 2.5g of sodium peroxide was added for alkaline fusion to completely open all the forms of fluorine in the mineral lattice. The test was performed according to the procedure for determining perfluorine in soil (reference standards: GB / T22104-2008, DZ / T 0279.21-2016, HJ 873-2017).

[0067] Precision and accuracy experiments were conducted on the national standard reference material GBW07935 and the collected actual sample AK04, and the results are shown in Tables 2 and 3. The results show that the precision (RSD / %) of the standard reference material GBW07935 for each form and total amount ranged from 2.02% to 22.8%, the accuracy (RD%) ranged from 0.42% to 6.92%, and the recovery rate ranged from 87.1% to 99.2%. For the actual sample AK04, the precision (RSD / %) for each form ranged from 2.41% to 22.6%, the accuracy (RD%) ranged from 0.79% to 10.5%, and the recovery rate ranged from 81.0% to 98.4%. This demonstrates that the method of this invention can accurately extract and test the content of various forms of fluorine in soil, with high accuracy and high recovery rate.

[0068] Table 2 Precision and accuracy data of standard reference GBW07935

[0069]

[0070]

[0071] In Table 2, RD% = C 总 -C 全 / C 全 ×100%, C 全 For total fluorine, C 总 This represents the total amount of fluorine in its current form. Recovery rate % = C 总 / C 全 ×100%, C 全 For total fluorine, C 总 This represents the total amount of fluorine in its various forms.

[0072] Table 3 Precision and accuracy data of actual sample AK04

[0073]

[0074]

[0075] In Table 3, RD% = C 总 -C 全 / C 全 ×100%, C 全 For total fluorine, C 总 This refers to the total amount of fluorine in its various forms.

[0076] Recovery rate % = C 总 / C 全 ×100%, C 全 For total fluorine, C 总 This refers to the total amount of fluorine in its various forms.

[0077] RE(%=(Ci -C j ) / (C i +C j )×100%, C i and C j These represent the amount of fluorine in a specific form.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for sequential extraction of fluoride from soil, characterized in that, Includes the following steps: S1. Extraction of water-soluble fluoride: Soil and water-soluble fluoride extract are mixed at a certain solid-liquid ratio, ultrasonicated, and centrifuged to obtain the first supernatant containing water-soluble fluoride. S2. Extraction of adsorbed fluorine: When the soil is acidic or neutral, the extraction residue after step S1 is mixed with the weak alkaline extract at a certain solid-liquid ratio, ultrasonicated, centrifuged, and the precipitate is collected. The weak acid extract is added to the collected precipitate at a certain solid-liquid ratio, ultrasonicated, and centrifuged to obtain a second supernatant containing adsorbed fluorine. When the soil is alkaline, the extraction residue after step S1 is mixed with the weak acid extract at a certain solid-liquid ratio, ultrasonicated, centrifuged, and the precipitate is collected. The weak alkaline extract is added to the collected precipitate at a certain solid-liquid ratio, ultrasonicated, and centrifuged to obtain a second supernatant containing adsorbed fluorine. S3. Extraction of oxidized fluorine: The extraction residue after step S2 is mixed with the oxidized fluorine extract at a certain solid-liquid ratio, sonicated, and centrifuged to obtain a third supernatant containing oxidized fluorine. S4. Extraction of stationary fluorine: The extraction residue after step S3 is dried and ground, and a mixture containing stationary fluorine is obtained by alkaline melting method.

2. The method for sequential extraction of fluoride from soil according to claim 1, characterized in that, In step S1, the water-soluble fluoride extract is water.

3. The method for sequential extraction of fluoride from soil according to claim 1, characterized in that, In step S2, the weak acid extract is at least one of oxalic acid solution, acetic acid solution, dilute nitric acid solution, and citric acid solution, and the weak base extract is at least one of sodium bicarbonate solution, sodium carbonate solution, potassium hydroxide solution, and sodium bicarbonate solution.

4. The method for sequential extraction of fluoride from soil according to claim 1, characterized in that, In step S3, the oxidized fluoride extract is an acetic acid solution of 0.2-0.4 mol / L hydroxylamine hydrochloride.

5. The method for sequential extraction of fluoride from soil according to claim 1, characterized in that, In steps S1-S3, the ultrasound duration is at least 30 minutes.

6. The method for sequential extraction of fluoride from soil according to claim 1, characterized in that, In steps S1-S3, the temperature of the extract is 25±5℃ during the ultrasound process.

7. The method for sequential extraction of fluoride from soil according to claim 1, characterized in that, In steps S1-S3, the solid-liquid ratio is 1:10-15 (m / V).

8. The method for sequential extraction of fluoride from soil according to claim 1, characterized in that, In step S4, the reagents used in the alkali fusion method are one or more of sodium carbonate, sodium hydroxide, potassium hydroxide, sodium peroxide, lithium metaborate, and potassium pyrosulfate.

9. The method for sequential extraction of fluoride from soil according to claim 1, characterized in that, In step S4, the mass ratio of the reagent used in the alkali fusion method to the dried and ground extraction residue is 2-8:

1.

10. A method for detecting fluoride in soil, characterized in that, The fluorine is extracted sequentially according to any one of claims 1-9, and the fluorine extracted in each step is quantitatively detected after each extraction.

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