Heavy metal form analysis method suitable for soil remediation through vulcanization method

Abstract

The invention discloses a heavy metal form analysis method suitable for soil remediation through a vulcanization method, and belongs to the technical field of soil analytical chemistry. According to the method, deionized water, acetic acid, hydroxylamine hydrochloride, a sodium persulfate-sodium bicarbonate composite extracting agent, hydrogen peroxide and a nitric acid-hydrofluoric acid-perchloric acid mixed acid are sequentially adopted for extracting six heavy metal forms in the soil, wherein the six heavy metal forms include the water soluble state, the weak acid extractable state, the iron and manganese oxide combined state, the organic combined state, the sulfide combined state and the residue state. By introducing the sodium persulfate-sodium bicarbonate composite extracting agent, the organic binding state heavy metal is independently separated in the continuous extraction process, the problem that the organic binding state of the soil heavy metal and the binding state of the sulfide cannot be distinguished in the prior art is solved, and accurate analysis of soil heavy metal form transformation and accurate evaluation of the remediation effect of the vulcanization method are achieved.

Description

Technical Field

[0001] This invention belongs to the field of soil analytical chemistry technology, specifically relating to a method for analyzing the speciation of heavy metals in soils remediated by sulfidation. Background Technology

[0002] The environmental behavior of heavy metals in soil (such as mobility, bioavailability, and ecotoxicity) is not only related to the total amount of heavy metals, but also closely related to their occurrence forms (such as exchangeable forms, iron-manganese oxide-bound forms, and residual forms). For example, exchangeable heavy metals are easily absorbed by plants and enter the food chain, while residual forms pose a lower environmental risk due to their high stability. Therefore, the analysis of heavy metal speciation in soil is a core basis for assessing pollution risk and remediation effectiveness.

[0003] Sulfidation is one of the most commonly used in-situ remediation technologies for heavy metals in soil. Sulfidation involves introducing sulfides (such as Na₂S and CaS) into contaminated soil. x (etc.), promotes the growth of heavy metal ions (such as Pb) 2+ Cd 2+ Zn 2+ The remediation process transforms heavy metals into insoluble heavy metal sulfides (such as PbS, CdS, ZnS, etc.), significantly reducing their mobility and bioavailability, thus achieving passivation / stabilization. This technology reconstructs the speciation of heavy metals in the soil, greatly increasing the proportion of sulfide-bound forms (i.e., the heavy metal sulfides generated during remediation), but traditional heavy metal speciation analysis methods cannot resolve this crucial speciation.

[0004] Current mainstream heavy metal speciation analysis methods (such as the Tessier and BCR sequential extraction methods) are primarily designed for naturally contaminated soils with low sulfide content, revealing significant limitations when analyzing sulfide-based soil remediation. The Tessier method includes exchangeable, carbonate-bound, iron-manganese oxide-bound, organically or sulfide-bound (including heavy metals bound to organic matter or sulfides), and residual speciation. The BCR method includes weakly acid-extractable, reducible, oxidizable (including heavy metals bound to organic matter or sulfides), and residual speciation. Existing methods use the strong oxidant H₂O₂ to simultaneously extract organically or sulfide-bound heavy metals, but cannot analyze sulfide-bound heavy metals separately. This major limitation severely restricts the accurate assessment of the effectiveness of sulfide-based soil remediation and hinders its engineering application and promotion.

[0005] In summary, there is an urgent need to develop a soil heavy metal speciation analysis method that can accurately distinguish between organically bound and sulfide-bound forms for soil remediation using sulfide methods. This would overcome the shortcomings of existing technologies and enable accurate assessment of the remediation effects of heavy metal pollution in sulfide-remediated soils. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and, in view of the surge in heavy metal sulfides in soil after sulfidation remediation, to provide a method for heavy metal speciation analysis suitable for sulfidation remediation of soil.

[0007] The specific technical solution adopted in this invention is as follows: This invention provides a method for heavy metal speciation analysis suitable for soil remediation via sulfide treatment, the specific steps of which are as follows: S1: Obtain soil samples containing insoluble heavy metal sulfides after sulfidation remediation, extract water-soluble heavy metals from the soil samples using deionized water, and obtain the first extract and the first residue after centrifugation. S2: Add acetic acid solution to the first residue obtained in step S1 to extract the weakly acid-extractable heavy metals. After centrifugation, the second extract and the second residue are obtained. S3: Add hydroxylamine hydrochloride solution to the second residue obtained in step S2 to extract the iron-manganese oxide-bound heavy metals, and centrifuge to obtain the third extract and the third residue. S4: Add sodium persulfate-sodium bicarbonate composite extractant to the third residue obtained in step S3 to selectively oxidize and degrade the organic matter in the third residue, extract the organically bound heavy metals, and centrifuge to obtain the fourth extract and the fourth residue; the molar ratio of sodium persulfate to sodium bicarbonate in the sodium persulfate-sodium bicarbonate composite extractant is 1:(2~4), the concentration of sodium persulfate is 0.10~0.30 mol / L, and the concentration of sodium bicarbonate is 0.40~0.80 mol / L; S5: Add acidified hydrogen peroxide solution to the fourth residue obtained in step S4, and ensure that the pH of the reaction system is 2±0.1 to extract sulfide-bound heavy metals. After centrifugation, the fifth extract and the fifth residue are obtained. S6: The fifth residue obtained in step S5 is digested with a mixed acid of nitric acid, hydrofluoric acid and perchloric acid. The residual heavy metals are then extracted to obtain the sixth extract. S7: The concentration of heavy metals in the first, second, third, fourth, fifth, and sixth extracts was measured using an inductively coupled plasma atomic emission spectrometer, and the content of different forms of the heavy metals was calculated.

[0008] Preferably, the mass ratio of the soil sample to deionized water in step S1 is 1:(20~25); after shaking the mixture of deionized water and soil sample at 25±2℃ for 1~2 hours, it is centrifuged at 8000 rpm for 5~10 minutes to obtain the first residue and the first extract containing water-soluble heavy metals.

[0009] Preferably, the concentration of the acetic acid solution in step S2 is 0.1~0.3 mol / L.

[0010] Furthermore, the extraction of weakly acid-extractable heavy metals in step S2 is specifically as follows: Add excess acetic acid solution to the first residue, shake at 25±2℃ for 3-4 hours, centrifuge at 8000 rpm for 5-10 minutes and collect the supernatant; then add deionized water to the centrifuged residue, shake for 30 minutes to wash, centrifuge and obtain the supernatant; combine all supernatants to obtain the second extract containing weakly acid-extractable heavy metals.

[0011] Preferably, the concentration of the hydroxylamine hydrochloride solution in step S3 is 0.5 mol / L.

[0012] Furthermore, the extraction of iron-manganese oxide-bound heavy metals in step S3 is specifically as follows: Add excess hydroxylamine hydrochloride solution to the second residue, shake at 25±2℃ for 16-18 hours, centrifuge at 8000 rpm for 5-10 minutes and collect the supernatant; then add deionized water to the centrifuged residue, shake for 30 minutes to wash, centrifuge and obtain the supernatant; combine all supernatants to obtain the third extract containing iron-manganese oxide-bound heavy metals.

[0013] Preferably, the initial pH of the sodium persulfate-sodium bicarbonate composite extractant in step S4 is controlled at 8.5~9.5, and the time interval between its preparation and use is less than or equal to 4.0 hours.

[0014] As a preferred embodiment, the extraction of organically bound heavy metals in step S4 is specifically as follows: Add excess sodium persulfate-sodium bicarbonate composite extractant to the third residue, and react with shaking in a water bath at 75℃~85℃ for 2~3 hours. After cooling, add sulfuric acid solution with a concentration of 0.5~0.6 mol / L to adjust the pH to 3~4. Then, centrifuge at 8000 rpm for 5~10 minutes and collect the supernatant. Then, add deionized water to the centrifuged residue, shake for 30 minutes to wash, and centrifuge to obtain the supernatant. Combine all supernatants to obtain the fourth extract containing organically bound heavy metals.

[0015] As a preferred embodiment, the extraction of sulfide-bound heavy metals in step S5 is specifically as follows: An acidified hydrogen peroxide solution was added to the fourth residue in batches for oxidation. The pH was adjusted to 2.0±0.1 using a 0.1 mol / L nitric acid solution. After each addition of acidified hydrogen peroxide solution, the reaction was shaken in a water bath at 85±2℃ for 2-3 hours to ensure complete oxidation. After the oxidation reaction was completed, the reaction system was cooled, and then an ammonium acetate-nitric acid composite solution and deionized water were added sequentially. The system was shaken at 25±2℃ for 30-45 minutes for two washes. The ammonium acetate-nitric acid composite solution was a 20% nitric acid solution containing 3-4 mol / L ammonium acetate. After each wash, the supernatant was collected by centrifugation at 8000 rpm for 5-10 minutes. All supernatants were combined to obtain the fifth extract containing the sulfide-bound heavy metals.

[0016] As a preferred embodiment, the extraction of residual heavy metals in step S6 is specifically as follows: The fifth residue was transferred to a polytetrafluoroethylene digestion vessel, and concentrated hydrochloric acid was added. The mixture was evaporated at 120℃~130℃ until the volume of the concentrated hydrochloric acid decreased by more than 60%. Subsequently, concentrated nitric acid with a mass concentration of 65~68%, hydrofluoric acid with a mass concentration of more than 40%, and perchloric acid with a mass concentration of 70%~72% were added in sequence. After adding the digestion cap, the mixture was digested at 190℃~200℃ until all particles in the fifth residue were dissolved. Finally, the digestion cap was removed, and the digestion was accelerated at 240℃~250℃. The digestion solution in the digestion vessel and the rinsing solution used to rinse the digestion vessel were combined to obtain the sixth extract containing residual heavy metals.

[0017] Compared with the prior art, the present invention has the following advantages: (1) This invention uses sodium persulfate-sodium bicarbonate composite extractant to extract organically bound heavy metals separately, and then uses hydrogen peroxide to extract sulfide-bound heavy metals, thus achieving accurate differentiation between the two and solving the problem that sulfide-bound heavy metals in soil cannot be analyzed separately. It also overcomes the problem of morphological confusion caused by existing methods that combine organically bound and sulfide-bound heavy metals into "organic / sulfide-bound" or "oxidizable".

[0018] (2) This invention also optimizes the problem of easy interference between adjacent forms in continuous extraction of heavy metals from soil. By optimizing parameters such as reagent sequence, drug concentration, reaction temperature, solution pH, and shaking time in continuous extraction, and standardizing the residue washing process at each stage, the extraction efficiency is significantly improved and the risk of interference between adjacent forms is reduced. Detailed Implementation

[0019] The present invention will be further described and illustrated below with reference to specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly, provided there are no mutual conflicts. This invention addresses the characteristic of a large amount of heavy metal sulfides generated in soil after sulfidation remediation. It utilizes a sodium persulfate-sodium bicarbonate composite extractant to separately extract organically bound heavy metals, and then uses hydrogen peroxide to extract sulfide-bound heavy metals, achieving precise differentiation between the two.

[0020] Example 1 This embodiment uses a small-scale soil test to remediate three types of copper-contaminated soil using the sulfidation method, and obtains soil samples after remediation, as follows: (1) The heavy metal pollution status of the soil to be remediated is as follows: Three types of copper-contaminated soil (labeled as contaminated soil A, contaminated soil B, and contaminated soil C, respectively) were collected at 1.0 kg each, air-dried, and then passed through a 100-mesh sieve to remove particulate matter such as plastic, gravel, and plant roots. The total copper content of contaminated soil A, contaminated soil B, and contaminated soil C were 143.8±14.3 mg / kg, 72.0±7.5 mg / kg, and 94.2±9.7 mg / kg, respectively.

[0021] (2) The steps for remediating copper-contaminated soil using calcium polysulfide are as follows: Each soil sample was divided into two groups: an experimental group with added calcium polysulfide and a control group without added calcium polysulfide. In the experimental group, 400 mg / kg calcium polysulfide and 150 mL / kg deionized water were added, stirred thoroughly, and incubated at room temperature (25±2℃) for 5 days, yielding soil samples A, B, and C. In the control group, only 150 mL / kg deionized water was added, stirred thoroughly, and incubated at room temperature (25±2℃) for 5 days, yielding soil samples D, E, and F. After incubation, the soils were air-dried again for subsequent heavy metal speciation analysis.

[0022] Example 2 This embodiment uses the heavy metal speciation analysis method of soil samples A, B, and C after sulfide remediation in Example 1. The specific steps are as follows: (1) Extraction of water-soluble heavy metals: Three samples, each weighing 1.00 g, were randomly selected from each group of soil samples. 20 mL of deionized water was added to each soil sample, and the mixture was shaken for 1.0 hour at room temperature (25±2℃). The mixture was then centrifuged at 8000 rpm for 5 minutes to obtain the first supernatant. 20.0 mL of deionized water was added to the residue after centrifugation, and the mixture was shaken for 30 minutes and then centrifuged at 8000 rpm for 5 minutes to obtain the second supernatant. All supernatants were combined and diluted to 40.0 mL with deionized water to obtain the first extract containing water-soluble heavy metals and the first residue.

[0023] (2) Extraction of heavy metals in weakly acidic extractable states: Add 10.0 mL of 0.11 mol / L acetic acid solution to the first residue obtained in step (1), and shake at room temperature (25±2℃) for 3.0 hours. Centrifuge at 8000 rpm for 5 minutes to obtain the first supernatant. Add 15.0 mL of deionized water to the centrifuged residue for washing, shake for 30 minutes, and centrifuge at 8000 rpm for 5 minutes to obtain the second supernatant. Repeat the washing once more to obtain the third supernatant. Combine all supernatants and dilute to 40.0 mL with deionized water to obtain the second extract containing weakly acid-extractable heavy metals and the second residue.

[0024] (3) Extraction of heavy metals bound to iron and manganese oxides: Add 10.0 mL of 0.5 mol / L hydroxylamine hydrochloride solution to the second residue obtained in step (2), and shake at room temperature (25±2℃) for 16.0 hours. Centrifuge at 8000 rpm for 5 minutes to obtain the first supernatant. Add 15.0 mL of deionized water to the residue for washing, shake for 30 minutes, and centrifuge at 8000 rpm for 5 minutes to obtain the second supernatant. Repeat the washing once more to obtain the third supernatant. Combine all supernatants and dilute to 40.0 mL with deionized water to obtain the third extract containing iron-manganese oxide-bound heavy metals and the third residue.

[0025] (4) Extraction of organically bound heavy metals: 1) Dissolve sodium persulfate and sodium bicarbonate in water, with the sodium persulfate concentration being 0.2 mol / L and the sodium bicarbonate concentration being 0.6 mol / L, to prepare a sodium persulfate-sodium bicarbonate composite extractant with a pH of 8.90. It should be noted that the time interval between preparing the sodium persulfate-sodium bicarbonate composite extractant and using it should be less than or equal to 4 hours.

[0026] 2) Add 10.0 mL of sodium persulfate-sodium bicarbonate composite extractant to the third residue obtained in step (3). React by shaking in an 80℃ water bath for 2.0 hours, then add 0.5 mol / L sulfuric acid solution to adjust the pH to 4.0. Centrifuge at 8000 rpm for 5 minutes to obtain the first supernatant. Add 15.0 mL of deionized water to the residue and wash, shake for 30 minutes, then centrifuge at 8000 rpm for 5 minutes to obtain the second supernatant. Repeat the washing process once more to obtain the third supernatant. Combine all supernatants and dilute to 40.0 mL with deionized water to obtain the fourth extract containing organically bound heavy metals and the fourth residue.

[0027] (5) Sulfide-bound heavy metals: Add 3.0 mL of 0.01 mol / L nitric acid solution and 5.0 mL of 30% hydrogen peroxide solution to the fourth residue obtained in step (4). Adjust the pH to 2.0 with 0.1 mol / L nitric acid solution and react in an 85℃ water bath with shaking for 2.0 hours. Add another 5.0 mL of hydrogen peroxide solution and continue reacting in an 85℃ water bath with shaking for 2.0 hours. After cooling to room temperature (25±2℃), add 5.0 mL of ammonium acetate-nitric acid composite solution (ammonium acetate molar concentration of 3.2 mol / L and nitric acid mass concentration of 20%) and continue shaking for 30 minutes. Centrifuge at 8000 rpm for 5 minutes to obtain the first supernatant. Add 20.0 mL of deionized water to the residue, shake for 30 minutes, and then centrifuge at 8000 rpm for 5 minutes to obtain the second supernatant. Combine all supernatants and bring the volume to 40.0 mL with deionized water to obtain the fifth extract containing sulfide-bound heavy metals and the fifth residue.

[0028] (6) Extraction of heavy metals from residues: The fifth residue obtained in step (5) was transferred to a polytetrafluoroethylene digestion vessel, and 10.0 mL of concentrated hydrochloric acid was added. The mixture was evaporated at 120°C to a volume of approximately 3.0 mL. Then, 5.0 mL of 68% concentrated nitric acid, 5.0 mL of 42% hydrofluoric acid, and 3.0 mL of 70% perchloric acid were added sequentially. The digestion vessel was covered and digested at 200°C for 60 minutes until no solid particles remained in the residue. After removing the cover, the mixture was further evaporated at 250°C to remove white fumes and until the contents became viscous, yielding the digestion solution. The digestion solution was allowed to cool naturally to room temperature (25±2°C), and the inner wall and cover of the polytetrafluoroethylene digestion vessel were rinsed with deionized water. The digestion solution and rinsing solution were combined and diluted to 40.0 mL with deionized water to obtain the sixth extract containing residual heavy metals.

[0029] (7) The concentration of heavy metal copper in the first, second, third, fourth, fifth and sixth extracts was measured by inductively coupled plasma atomic emission spectrometry, and the content of copper in different forms was calculated.

[0030] For soil samples D, E, and F, which were not treated with calcium polysulfide remediation, the same extraction method was used to analyze the speciation of heavy metals. The results are shown in Table 1.

[0031] As shown in Table 1, the heavy metal speciation analysis method provided by this invention reveals that after soil remediation via sulfidation, the sulfide-bound state of copper increases significantly from 8.7-9.0% to 20.9-31.5%, the organic-bound state increases slightly from 1.3-1.6% to 1.6-2.2%, the iron-manganese oxide-bound state decreases sharply from 13.3-28.8% to 3.2-8.8%, and the weakly acid-extractable state decreases from 5.8-11.2% to 3.1-5.1%. The water-soluble and residual states show no significant changes. Compared to soil not remediated via sulfidation, it is evident that a large amount of iron-manganese oxide-bound and weakly acid-extractable states are converted into sulfide-bound states during soil remediation. This invention's method can accurately analyze the aforementioned conversion process involving heavy metal sulfide-bound states, which current methods such as Tessier and BCR cannot achieve.

[0032] Table 1. Comparison of copper speciation after soil remediation by sulfidation method The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained through equivalent substitution or transformation fall within the protection scope of the present invention.

Claims

1. A method for analyzing the speciation of heavy metals in soil remediated by sulfidation, characterized in that, The specific steps are as follows: S1: Obtain soil samples containing insoluble heavy metal sulfides after sulfidation remediation, extract water-soluble heavy metals from the soil samples using deionized water, and obtain the first extract and the first residue after centrifugation. S2: Add acetic acid solution to the first residue obtained in step S1 to extract the weakly acid-extractable heavy metals. After centrifugation, the second extract and the second residue are obtained. S3: Add hydroxylamine hydrochloride solution to the second residue obtained in step S2 to extract the iron-manganese oxide-bound heavy metals, and centrifuge to obtain the third extract and the third residue. S4: Add sodium persulfate-sodium bicarbonate composite extractant to the third residue obtained in step S3 to selectively oxidize and degrade the organic matter in the third residue, extract the organically bound heavy metals, and centrifuge to obtain the fourth extract and the fourth residue; the molar ratio of sodium persulfate to sodium bicarbonate in the sodium persulfate-sodium bicarbonate composite extractant is 1:(2~4), the concentration of sodium persulfate is 0.10~0.30 mol / L, and the concentration of sodium bicarbonate is 0.40~0.80 mol / L; S5: Add acidified hydrogen peroxide solution to the fourth residue obtained in step S4, and ensure that the pH of the reaction system is 2±0.1 to extract sulfide-bound heavy metals. After centrifugation, the fifth extract and the fifth residue are obtained. S6: The fifth residue obtained in step S5 is digested with a mixed acid of nitric acid, hydrofluoric acid and perchloric acid. The residual heavy metals are then extracted to obtain the sixth extract. S7: The concentration of heavy metals in the first, second, third, fourth, fifth, and sixth extracts was measured using an inductively coupled plasma atomic emission spectrometer, and the content of different forms of the heavy metals was calculated.

2. The method for heavy metal speciation analysis of soil remediated by sulfidation method according to claim 1, characterized in that, The mass ratio of soil sample to deionized water in step S1 is 1:(20~25); after shaking the mixture of deionized water and soil sample at 25±2℃ for 1~2 hours, centrifuge at 8000 rpm for 5~10 minutes to obtain the first residue and the first extract containing water-soluble heavy metals.

3. The method for heavy metal speciation analysis of soil remediated by sulfidation method according to claim 1, characterized in that, The concentration of the acetic acid solution in step S2 is 0.1~0.3 mol / L.

4. The method for heavy metal speciation analysis of soil remediated by sulfidation method according to claim 3, characterized in that, The extraction of weakly acid-extractable heavy metals in step S2 is as follows: Add excess acetic acid solution to the first residue, shake at 25±2℃ for 3-4 hours, centrifuge at 8000 rpm for 5-10 minutes and collect the supernatant; then add deionized water to the centrifuged residue, shake for 30 minutes to wash, centrifuge and obtain the supernatant; combine all supernatants to obtain the second extract containing weakly acid-extractable heavy metals.

5. The method for heavy metal speciation analysis of soil remediated by sulfidation method according to claim 1, characterized in that, The concentration of the hydroxylamine hydrochloride solution in step S3 is 0.5 mol / L.

6. The method for heavy metal speciation analysis of soil remediated by sulfidation according to claim 5, characterized in that, The extraction of iron-manganese oxide-bound heavy metals in step S3 is as follows: Add excess hydroxylamine hydrochloride solution to the second residue, shake at 25±2℃ for 16-18 hours, centrifuge at 8000rpm for 5-10 minutes and collect the supernatant; then add deionized water to the centrifuged residue, shake for 30 minutes to wash, centrifuge and obtain the supernatant; combine all supernatants to obtain the third extract containing iron-manganese oxide-bound heavy metals.

7. The method for heavy metal speciation analysis of soil remediated by sulfidation method according to claim 1, characterized in that, The initial pH of the sodium persulfate-sodium bicarbonate composite extractant in step S4 is controlled at 8.5~9.5, and the time interval between its preparation and use is less than or equal to 4.0 hours.

8. The method for heavy metal speciation analysis of soil remediated by sulfidation method according to claim 1, characterized in that, The extraction of organically bound heavy metals in step S4 is as follows: Add excess sodium persulfate-sodium bicarbonate composite extractant to the third residue, and react with shaking in a water bath at 75℃~85℃ for 2~3 hours. After cooling, add sulfuric acid solution with a concentration of 0.5~0.6 mol / L to adjust the pH to 3~4. Then, centrifuge at 8000rpm for 5~10 minutes and collect the supernatant. Then, add deionized water to the centrifuged residue, shake for 30 minutes to wash, and centrifuge to obtain the supernatant. Combine all supernatants to obtain the fourth extract containing organically bound heavy metals.

9. The method for heavy metal speciation analysis of soil remediated by sulfidation according to claim 1, characterized in that, The extraction of sulfide-bound heavy metals in step S5 is as follows: An acidified hydrogen peroxide solution was added to the fourth residue in batches for oxidation. The pH was adjusted to 2.0±0.1 using a 0.1 mol / L nitric acid solution. After each addition of acidified hydrogen peroxide solution, the reaction was shaken in a water bath at 85±2℃ for 2-3 hours to ensure complete oxidation. After the oxidation reaction was completed, the reaction system was cooled, and then an ammonium acetate-nitric acid composite solution and deionized water were added sequentially. The system was then shaken at 25±2℃ for 30-45 minutes for two washes. The ammonium acetate-nitric acid composite solution was a 20% nitric acid solution containing 3-4 mol / L ammonium acetate. After each washing, the supernatant was collected by centrifugation at 8000 rpm for 5-10 minutes; all supernatants were combined to obtain the fifth extract containing the sulfide-bound heavy metals.

10. The method for heavy metal speciation analysis of soil remediated by sulfidation according to claim 1, characterized in that, The specific steps for extracting residual heavy metals in step S6 are as follows: The fifth residue was transferred to a polytetrafluoroethylene digestion vessel, and concentrated hydrochloric acid was added. The mixture was evaporated at 120℃~130℃ until the volume of the concentrated hydrochloric acid decreased by more than 60%. Subsequently, concentrated nitric acid with a mass concentration of 65~68%, hydrofluoric acid with a mass concentration of more than 40%, and perchloric acid with a mass concentration of 70%~72% were added in sequence. After adding the digestion cap, the mixture was digested at 190℃~200℃ until all particles in the fifth residue were dissolved. Finally, the digestion cap was removed, and the digestion was accelerated at 240℃~250℃. The digestion solution in the digestion vessel and the rinsing solution used to rinse the digestion vessel were combined to obtain the sixth extract containing residual heavy metals.

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