A soil digestion method and a detection method for simultaneously determining multiple heavy metals in soil

Abstract

This invention provides a soil digestion method and a detection method for simultaneously determining multiple heavy metals in soil. The method involves mixing a soil sample with a mixed acid, performing thermal digestion according to a set temperature program, cooling the digestion product, removing the acid, adding hydrochloric acid solution to obtain a dissolved residue, and finally adjusting the volume to obtain the digestion solution. The mixed acid is nitric acid and hydrofluoric acid in a volume ratio of 3:1-1.2, and the ratio of soil sample to mixed acid is 0.2-0.25 g: 8-9 ml. This method, through the use of a mixed acid and a set temperature program, achieves rapid digestion with high precision and accuracy. It is applicable to soil sample digestion methods for seven heavy metal elements and three existing detection methods in laboratories, aiming to reduce the workload of soil digestion and improve the efficiency of heavy metal monitoring in soil.

Description

Technical Field

[0001] This invention belongs to the field of environmental protection technology, specifically relating to a soil digestion method and a detection method for simultaneously determining multiple heavy metals in soil. Background Technology

[0002] Heavy metals in soil are characterized by their difficulty in migration, long residual time, and high toxicity, making them a major soil pollutant for oil and gas field enterprises. Therefore, accurately measuring the content of heavy metals in soil is of great importance for establishing early warning systems for soil pollution in oil and gas fields and protecting the environmental quality of soil in oil and gas fields.

[0003] Soil sample analysis mainly involves sample collection, sample preparation, sample digestion, sample determination, and data processing. Due to the complex composition of soil, selecting a suitable digestion method and system to transform the soil into a detectable liquid state is a crucial step in determining the accuracy of heavy metal determination results. The soil sample digestion process is complex and time-consuming, taking anywhere from several hours to over ten hours to digest a single sample. Common digestion methods include hot plate digestion, microwave digestion, graphite furnace digestion, and dry ashing digestion. Because different metal elements have varying characteristics and exist in different forms in soil, pretreatment and detection methods also differ.

[0004] Pretreatment methods for heavy metal detection in soil, according to current national standards such as GB 17141-1997 Determination of Lead and Cadmium by Graphite Furnace Atomic Absorption Spectrophotometry, GB 17138-1997 Determination of Copper and Zinc by Flame Atomic Absorption Spectrophotometry in Soil, and GB 17140-1997 Determination of Lead and Cadmium by KI-MIBK Extraction Flame Atomic Absorption Spectrophotometry in Soil, generally employ a tetraacid digestion method (hydrochloric acid, nitric acid, hydrofluoric acid, and perchloric acid) in a polytetrafluoroethylene crucible. This method is cumbersome, highly dangerous, requires large amounts of reagents, demands high skill from the testers, and is unsuitable for large-scale soil sample testing. Furthermore, it is not applicable to the detection of all heavy metals.

[0005] 201510319094.3 A digestion method for detecting heavy metals in soil involves adding only small amounts of nitric acid and hydrofluoric acid to the sample, allowing it to stand overnight, and then digesting it at a certain temperature on a graphite digester to completely digest the soil sample. The sample can then be detected using graphite furnace atomic absorption spectrometry or inductively coupled plasma atomic emission spectrometry. However, this method requires overnight standing, resulting in a long digestion time and failing to improve the efficiency of heavy metal detection in soil. Summary of the Invention

[0006] The purpose of this invention is to provide a soil digestion method, which involves mixing a soil sample with a mixed acid, performing thermal digestion according to a heating program, cooling the thermal digestion product, removing the acid, adding hydrochloric acid solution to obtain a dissolved residue, and finally adjusting the volume to obtain a digestion solution. The mixed acid is nitric acid and hydrofluoric acid, with a volume ratio of 3:1-1.2. The ratio of soil sample to mixed acid is 0.2-0.25g:8-9ml.

[0007] The heating procedure is as follows: first, heat to 120℃ in 7-8 minutes and hold for 3-4 minutes; then, heat from 120℃ to 160℃ for 4-5 minutes and hold for 3-4 minutes; finally, heat from 160℃ to 200℃ for 5-6 minutes and hold for 25-30 minutes.

[0008] The heating process is repeated twice.

[0009] A soil decomposition method, wherein the acid removal process is as follows: Step 1) After cooling, transfer the digestion solution to a polytetrafluoroethylene crucible and add perchloric acid, wherein the volume ratio of perchloric acid to mixed acid is 1:4; Step 2) Cover and heat with an electric heating plate for 1-1.5 hours until the black particles disappear and a large amount of white smoke is produced; Step 3) Open the lid, expel the white smoke, and steam until the contents become viscous.

[0010] The hydrochloric acid solution contains 50% water by volume, and the volume ratio of the hydrochloric acid solution to the mixed acid is 5-4:8.

[0011] A detection method for simultaneously determining multiple heavy metals in soil, using atomic absorption spectrometry to detect multiple heavy metals in the digestion solution obtained by the digestion method.

[0012] The atomic absorption spectrometry methods include flame atomic absorption, graphite furnace atomic absorption, and atomic fluorescence.

[0013] Various heavy metals, including copper, zinc, lead, nickel, chromium, cadmium, and mercury.

[0014] The beneficial effects of this invention are: The soil digestion method provided by this invention, through mixing acid and a set heating program, not only achieves rapid digestion but also boasts high precision and accuracy. It is applicable to soil sample digestion methods for seven existing heavy metal elements and three detection methods in laboratories, thereby reducing the workload of soil digestion and improving the efficiency of heavy metal monitoring in soil. Attached Figure Description

[0015] Figure 1 The examples illustrate the effects of different digestion methods on copper in samples 202 and 204. Figure 2 The examples illustrate the effects of different digestion methods on lead in samples 202 and 204. Figure 3 The examples illustrate the effects of different digestion methods on chromium in samples 202 and 204. Figure 4 The examples illustrate the effects of different digestion methods on cadmium in samples 202 and 204. Figure 5 The examples illustrate the effects of different digestion methods on mercury concentration in samples 202 and 204. Figure 6 These are digestion solutions from different digestion methods in the examples. Detailed Implementation

[0016] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0017] Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the drawings is not intended to limit the invention.

[0018] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning. Example 1

[0019] This invention provides a soil digestion method, which involves mixing a soil sample with a mixed acid, performing thermal digestion according to a heating program, cooling the thermal digestion product, removing the acid, adding hydrochloric acid solution to obtain a dissolved residue, and finally adjusting the volume to obtain a digestion solution. The mixed acid is nitric acid and hydrofluoric acid, with a volume ratio of 3:1-1.2. The ratio of soil sample to mixed acid is 0.2-0.25g:8-9ml. Example 2

[0020] Based on Example 1, this example provides a soil digestion method, the specific steps of which are as follows: Step 1) Weigh 0.25g of soil sample (accurate to 0.0001g), measure 6ml of nitric acid and 2ml of hydrofluoric acid, mix them to obtain a mixed acid; Step 2) Add the soil sample to the mixed acid for thermal digestion, and repeat the following heating program twice: 0-120℃, heating time 7min, holding time 3min; 120-160℃, heating time 5min, holding time 3min; 160-200℃, heating time 5min, holding time 25min. Step 3) Acid removal: After cooling, transfer the solution to a 50mL polytetrafluoroethylene crucible and add 2mL of perchloric acid; Cover and heat the electric heating plate at a medium-low setting (around 140V) for 1-1.5 hours, until the black particles disappear and a large amount of white smoke is produced; Open the lid, expel the white smoke, and steam until the contents become viscous. Step 4) Dissolve the residue: Remove the crucible and let it cool slightly, then add 5 mL of hydrochloric acid solution (water and hydrochloric acid volume ratio of 1:1); Step 5) Volume adjustment: Transfer the entire volume to a 50mL graduated centrifuge tube, cool, and then dilute to the mark with pure water. Shake well.

[0021] This invention is applicable to soil sample digestion methods for three existing laboratory detection methods, aiming to reduce the workload of soil digestion and improve the efficiency of soil heavy metal monitoring. Example 3

[0022] To further illustrate the effectiveness of the digestion method of this invention, 14 0.25g (accurate to 0.1mg) soil standard samples ERM-S-510202 and ERM-S-510204 (hereinafter referred to as 202 and 204, soil standard sample information is shown in Table 1) were weighed and digested using digestion methods labeled A, B, C, D (method of this invention), E, ​​F, and G, respectively. One set of parallel samples was prepared for each digestion method. No matrix modifier was added to the digested solution. After making up to volume and standing for at least 2 hours, the supernatant was collected, and the heavy metal elements in the soil digestion solution were determined according to national standards. If the concentration of the analyte exceeded the upper limit of the standard curve, the digestion solution was diluted with 1% nitric acid before measurement.

[0023] Table 1 Information on Soil Standard Samples

[0024] Among them, digestion method A takes 360 minutes, and the specific process is as follows: ① Accurately weigh 0.1-0.3g of soil sample into a polytetrafluoroethylene container, moisten with water, add 5mL of hydrochloric acid, and heat on a hot plate at low temperature to allow the sample to decompose initially; ② When the volume evaporates to about 2-3 mL, remove it and let it cool slightly. Then add 5 mL of nitric acid, 4 mL of hydrofluoric acid, and 5 mL of perchloric acid. Cover the container and heat it on a hot plate at medium temperature for about 1 hour. ③Then open the lid and continue heating to remove silicon; ④ When heating to the point of emitting thick, highly acidic white fumes, cover the container to decompose the black organic carbon compounds; ⑤ After the black organic matter on the crucible disappears, open the lid to drive away the white smoke and steam until the contents become viscous; ⑥ Remove and let cool slightly, rinse the pliers cover and inner wall with water, add 1mL of nitric acid solution and warm to dissolve the residue; ⑦ Then transfer the solution to a 25 mL volumetric flask, cool, and dilute to volume.

[0025] Digestion method B takes 300 minutes and the specific process is as follows: ① Weigh 0.2-0.3 g of soil sample into a polytetrafluoroethylene crucible, moisten with water, add 10 mL of hydrochloric acid, and heat at 100 °C on a hot plate in a fume hood to preliminarily decompose the sample; ② When the volume of the evaporator reaches about 3 mL, add 9 mL of nitric acid, cover and heat until no obvious particles remain; ③ Add 5-8 mL of hydrofluoric acid, open the lid, and heat at 120℃ for 30 min; ④ After cooling slightly, add 1 mL of perchloric acid and heat at 150-170℃ until white fumes are emitted; ⑤ Add 3 mL of nitric acid solution and warm to dissolve the soluble residue; ⑥ Transfer the entire volume to a 25 mL volumetric flask, dilute to the mark with nitric acid solution, shake well, store in a polyethylene bottle, let stand, and take the supernatant for testing; ⑦ The analysis shall be completed within 30 days.

[0026] The C digestion method takes 300 minutes and the specific process is as follows: ① Accurately weigh 0.2-0.5g of soil sample into a polytetrafluoroethylene crucible, moisten it with water, add 10 mL of hydrochloric acid, and heat it at a low temperature on a hot plate in a fume hood to allow the sample to decompose initially. ②When the volume has evaporated to about 3 mL, let it cool slightly, then add 5 mL of nitric acid, 5 mL of hydrofluoric acid, and 3 mL of perchloric acid. Cover the container and heat it on a hot plate at medium temperature for about 1 hour. ③ Open the lid, keep the temperature of the heating plate at 150℃, and continue heating to remove silicon; ④ When heating until thick white perchloric acid fumes are emitted, cover the container to decompose the black organic carbon compounds; ⑤ After the black organic matter on the crucible wall disappears, open the lid, drive away the white smoke, and steam until the contents become viscous; ⑥ Remove the crucible and let it cool slightly. Add 3 mL of hydrochloric acid solution and warm it to dissolve the soluble residue. ⑦ Transfer the entire volume to a 50 mL volumetric flask, add 5 mL of ammonium chloride aqueous solution, cool, and then dilute to the mark with water. Shake well.

[0027] The E digestion method takes 180 minutes and the specific process is as follows: ① Weigh 0.25-0.5g of the air-dried and sieved sample and place it in a digestion vessel, then moisten it with a small amount of experimental water; ② Add 6 mL of nitric acid, 3 mL of hydrochloric acid, and 2 mL of hydrofluoric acid in sequence to thoroughly mix the sample and digestion solution; ③ Perform microwave digestion according to the heating program. After digestion, transfer the solution from the digestion vessel to a polytetrafluoroethylene crucible and gently boil it on a temperature-controlled heating device to remove the acid. Heating program: 0-120℃, heating time 7min, holding time 3min; 120-160℃, heating time 5min, holding time 3min; 160-190℃, heating time 5min, holding time 25min. Gently boil to remove the acid; warm to dissolve the residue.

[0028] ④ When the liquid becomes viscous, remove it and let it cool slightly. Then rinse the inside of the crucible with a small amount of nitric acid. ⑤ Transfer to a 25mL volumetric flask, dilute to the mark, let stand for 60 minutes, and collect the supernatant for testing.

[0029] The F-digestion method takes 60 minutes and the specific process is as follows: ① Weigh 0.25-0.5g of the air-dried and sieved sample and place it in a digestion vessel, then moisten it with a small amount of experimental water; ② In an acid-proof fume hood, add 2 ml of nitric acid and 6 ml of hydrochloric acid in sequence, ensuring the sample and digestion solution are thoroughly mixed. If a violent chemical reaction occurs, wait until the reaction is complete before sealing and tightening the lid. ③ After placing the digestion vessel into the digestion vessel support, put it into the furnace cavity of the microwave digestion device and confirm that the temperature sensor and pressure sensor are working properly; ④ Perform microwave digestion according to the heating program, and cool after the program is completed. Heating program: 0-120℃ Heating time: 7 min, holding time: 3 min; Heating time: 10 min, holding time: 15 min. After the temperature inside the vessel drops to room temperature, remove the digestion vessel from the acid-proof fume hood, slowly depressurize and release the gas, and open the digestion vessel lid. ⑤ Transfer the solution in the digestion vessel to a 25ml volumetric flask. Rinse the digestion vessel and lid with a small amount of laboratory water and pour them into the volumetric flask. Then, dilute to the mark with laboratory water, mix well, and let stand for 60 minutes. Take the supernatant for testing.

[0030] The G digestion method takes 60 minutes and the specific process is as follows: ① Weigh 0.1-0.5g of the air-dried and sieved sample and place it in a sample dissolving cup, then moisten it with a small amount of experimental water; ② Add 6 ml of hydrochloric acid, then slowly add 2 ml of nitric acid, mix well to ensure the sample is fully in contact with the digestion solution; ③ After placing the digestion vessel into the digestion vessel support, put it into the furnace cavity of the microwave digester and connect the temperature sensor and pressure sensor; ④ Perform microwave digestion and cool after the program ends; Temperature program: 0-100℃, heating time 5min, holding time 2min, 100-150℃, heating time 5min, holding time 3min, 150-180℃, heating time 5min, holding time 25min. ⑤ Insert the small glass funnel into the mouth of the 50mL volumetric flask, filter the digested solution using slow quantitative filter paper, transfer it into the volumetric flask, wash the sample cup and precipitate with experimental water, add all the washing liquid into the volumetric flask, and finally dilute to the mark with experimental water and mix well.

[0031] I. Seven digestion methods for determining certified standard samples

[0032] Soil standard samples 202 and 204 were digested using seven different digestion methods. Based on the characteristics of different elements and the detection methods, five heavy metal elements—copper, chromium, lead, cadmium, and mercury—were selected from the seven for determination. The results were compared with the certificate reference values ​​to analyze the impact of each digestion method on the accuracy of the determination of different heavy metal elements. The accuracy of the test results is demonstrated if the soil standard sample test results are within the upper and lower limits of the certificate reference values, or if the relative error between the measured value and the reference value meets the requirements of the detection method.

[0033] The experimental results show that, Figure 1 As shown, methods A, D, and G can accurately determine different concentrations of copper in two soil standard samples; Figure 2 As shown, methods B and D can accurately determine lead at different concentrations; Figure 3 As shown, methods B, C, and D can accurately determine chromium at different concentrations; for example... Figure 4 As shown, methods D and G can accurately determine cadmium at different concentrations, such as... Figure 5 As shown, Method D can accurately determine different concentrations of mercury in two soil standard samples.

[0034] II. Accuracy and precision of the digestion method

[0035] 1. Accuracy Evaluation The digestion methods were ranked from lowest to highest based on the relative errors of the determination results for different elements (Table 2), and the accuracy of different digestion methods was compared. The results show that method D has good digestion effects for all five elements at different concentrations, and the errors and relative errors meet the requirements of the standard sample uncertainty and the analytical method.

[0036] Table 2. Ranking and comparison of accuracy of different digestion methods

[0037] 2. Precision evaluation The relative deviations between two parallel samples using the same digestion method were calculated (Table 3) to evaluate the precision of different digestion methods. According to relevant standards, the upper limits for relative deviation between parallel samples are 20% (copper, lead, chromium, cadmium) and 25% (mercury). As shown in the table, except for some elements measured by digestion method E whose relative deviations exceeded the upper limits, the relative deviations of most test results met the relevant requirements. Method D showed relatively small inter-group deviations, with the largest being only 2.86% among all different concentrations of the analyte, indicating good method precision, small inter-group deviations, and good repeatability. Methods B and G were next best, followed by methods A and F.

[0038] Table 3. Inter-group bias of different digestion methods

[0039] 3. Evaluation of the experimental process In terms of acid usage, hot plate digestion (methods A, B, and C) uses a large amount of acid, mostly over 20 mL per sample; while microwave digestion (methods D, E, F, and G) uses less acid, usually around 10 mL per sample. Regarding digestion time, hot plate digestion is time-consuming, typically requiring about 6 hours to digest a batch of samples, and requires constant monitoring by the experimenter; microwave digestion increases the digestion rate and shortens the digestion time, requiring 1-3 hours to digest a batch of samples. From the experimental process, methods A, E, and G result in more thorough digestion, removing most of the solids, and producing a clear, transparent digest with only trace amounts of incompletely decomposed residue. Methods F and G have simpler acid systems and shorter digestion times, but the digest is cloudy, leaving a large amount of solids, such as… Figure 6 As shown.

[0040] Commonly used acids for soil digestion include hydrochloric acid, nitric acid, hydrofluoric acid, and perchloric acid, each with a different boiling point and role in digestion. Nitric acid, with a boiling point of approximately 118°C, is mainly used to decompose organic matter in samples; its ratio with hydrochloric acid (boiling point approximately 108°C) can improve the decomposition effect. Hydrofluoric acid (boiling point approximately 112°C) is often used with other acids to decompose samples containing silicon and silicates, easily causing metal elements to dissolve from the mineral lattice. Perchloric acid has a high boiling point (approximately 203°C) and is an extremely strong oxidizing agent, capable of completely decomposing organic matter and the carbon produced during organic matter decomposition. It can also be used to remove excess hydrofluoric acid.

[0041] In summary, for five analytes of different concentrations and types, the digestion method (method D) of this invention can meet the relevant requirements in terms of precision and accuracy. Moreover, it uses less acid and has a shorter digestion time, thus meeting the requirement that one digestion method can be applied to different elements and detection methods.

[0042] Using the digestion method of this invention, zinc and nickel in soil standard samples were determined. The results were all within the uncertainty range of the certificate reference values, with a maximum inter-group deviation of 6.1% (Table 4), meeting the relevant requirements and demonstrating good accuracy and precision. Therefore, it can be used for the determination of zinc and nickel.

[0043] Table 4. Effect of Selected Digestion Methods on the Determination of Zinc and Nickel

[0044] Actual soil sample testing

[0045] Three types of soil samples were randomly selected from the laboratory. Two portions of each sample were weighed simultaneously, moistened with a small amount of water, and one sample was digested and analyzed simultaneously with the other unspecified sample after adding an appropriate amount of standard solution. The results are shown in Table 5. The spiked amounts of copper, chromium, lead, zinc, and nickel were 15 μg, cadmium was 0.15 μg, and mercury was 0.0057 μg. The results show that the digestion method of this invention achieves recoveries of all seven elements in the three soil samples that meet the requirements of the detection method or relevant quality standards.

[0046] Table 5 Summary of Spike Recovery Results

[0047] The digestion method of this invention is applicable to three detection methods: flame atomic absorption, graphite furnace atomic absorption, and atomic fluorescence. It can be used to detect seven heavy metal elements, including copper, zinc, lead, nickel, chromium, cadmium, and mercury, in soil. This breaks the limitation of detection standards and literature that only one digestion method is applicable to one or a few heavy metal elements.

[0048] This method can detect both certified standard soil samples and actual soil samples, and its accuracy and precision meet the requirements of the detection method or relevant technical specifications. It uses less acid and has a shorter digestion time, reducing the digestion time for a batch of soil samples from 28 working days to 10 working days, thus reducing the workload. The amount of acid used in the digestion process has been reduced from 3000 mL to 1200 mL, reducing the generation of acidic waste liquid containing heavy metals and lowering the environmental and health risks that may arise from soil heavy metal monitoring, resulting in significant environmental and social benefits.

[0049] The above examples are merely illustrative of the present invention and do not constitute a limitation on the scope of protection of the present invention. All designs that are the same as or similar to the present invention are within the scope of protection of the present invention.

Claims

1. A method for soil digestion, characterized in that: After mixing the soil sample with the mixed acid, thermal digestion was carried out according to the heating program. Then, the thermal digestion product was cooled and the acid was removed. Hydrochloric acid solution was added to obtain the dissolved residue. Finally, the volume was adjusted to obtain the digestion solution. The mixed acid is nitric acid and hydrofluoric acid, and the volume ratio of the two is 3:1-1.

2. The ratio of soil sample to mixed acid is 0.2-0.25g:8-9ml.

2. The soil digestion method according to claim 1, characterized in that: The heating procedure is as follows: first, heat to 120℃ in 7-8 minutes and hold for 3-4 minutes; then, heat from 120℃ to 160℃ for 4-5 minutes and hold for 3-4 minutes; finally, heat from 160℃ to 200℃ for 5-6 minutes and hold for 25-30 minutes.

3. The soil digestion method according to claim 1, characterized in that: The heating process is repeated twice.

4. The soil digestion method according to claim 1, characterized in that: The acid removal process is as follows: Step 1) After cooling, transfer the digestion solution to a polytetrafluoroethylene crucible and add perchloric acid, wherein the volume ratio of perchloric acid to mixed acid is 1:4; Step 2) Cover and heat with an electric heating plate for 1-1.5 hours until the black particles disappear and a large amount of white smoke is produced; Step 3) Open the lid, expel the white smoke, and steam until the contents become viscous.

5. The soil digestion method according to claim 1, characterized in that: The hydrochloric acid solution contains 50% water by volume, and the volume ratio of the hydrochloric acid solution to the mixed acid is 5-4:

8.

6. A method for simultaneously determining multiple heavy metals in soil, characterized in that: Atomic absorption spectrometry was used to detect multiple heavy metals in the digestion solution obtained by the digestion method according to any one of claims 1-5.

7. The method for simultaneously determining multiple heavy metals in soil according to claim 6, characterized in that: The atomic absorption spectrometry methods include flame atomic absorption, graphite furnace atomic absorption, and atomic fluorescence.

8. The method for simultaneously determining multiple heavy metals in soil according to claim 6, characterized in that: Various heavy metals, including copper, zinc, lead, nickel, chromium, cadmium, and mercury.

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