Soil sample pretreatment method and soil total silicon determination method

Abstract

The invention belongs to the technical field of soil component analysis, particularly relates to determination of silicon content in soil, and particularly relates to a pretreatment method of a soil sample and a soil total silicon determination method. Wherein the pretreatment method comprises the following steps: weighing a soil sample, putting the soil sample into a digestion tank, adding a digestion solution, sealing, digesting according to a heating program set by parameters of a microwave digestion instrument, cooling to below 50 DEG C after digestion, opening a cover, quickly adding a boric acid solution, immediately sealing, uniformly shaking, putting on an acid-driving instrument, performing acid-driving treatment, cooling, opening the cover, and taking out the soil sample; transferring into a volumetric flask, and fixing the volume with pure water for measurement; the digestion solution comprises aqua regia, hydrofluoric acid and hydrogen peroxide; the molar ratio of the hydrofluoric acid to the boric acid is 1: 4. A microwave digestion-boric acid complexing pretreatment method is established, the method is used for measuring the content of all silicon in the soil, the detection method is rapid, and the measurement result is accurate.

Description

Technical Field

[0001] The present invention belongs to the technical field of soil component analysis, especially the determination of silicon content in soil, and specifically relates to a pretreatment method for soil samples and a method for determining total silicon in soil. Background Art

[0002] Silicon has a relatively high melting point and density, and its chemical properties are relatively stable. Except for being able to react with hydrofluoric acid and alkali solutions, silicon is insoluble in other inorganic acids. In nature, silicon mainly exists in the form of silicon dioxide or silicate. Silicon dioxide is an acidic oxide, insoluble in water, and can react with hydrofluoric acid to form gaseous SiF4. In the high-temperature molten state, silicon dioxide or silicate can also react with sodium peroxide, sodium hydroxide (potassium), lithium metaborate, anhydrous sodium carbonate, etc. to form soluble silicate. So far, the differential analysis method of alkali metal melting and silicon removal by hydrofluoric acid is still the most classic method for determining silicon dioxide.

[0003] Among them, the alkali metal melting method requires high-temperature melting treatment, which has problems such as complex operation, long time consumption, easy introduction of salts, and volatilization loss of silicon elements, resulting in unstable measurement results. Although the hydrofluoric acid digestion method can decompose silicate, the stability of the measurement results is poor, and the results often show a low value.

[0004] Therefore, there is an urgent need to develop a method for determining the total silicon content in soil with fast detection and accurate measurement results. Summary of the Invention

[0005] The purpose of the present invention is to provide a pretreatment method for soil samples and a method for determining total silicon in soil, so as to solve the problems of cumbersome detection methods and unstable measurement results for silicon dioxide in existing soil.

[0006] In the first aspect of the present invention, a pretreatment method for soil samples is provided, including the following steps:

[0007] Weigh a soil sample and place it in a digestion tank, add a digestion solution, seal it, and perform digestion according to the temperature increase program set by the microwave digester parameters. After digestion is completed, wait until it cools to below 50°C, quickly add boric acid solution with the lid open, immediately seal it, shake well, and then place it on an acid removal instrument for acid removal treatment. Wait until it cools, open the lid, transfer it to a volumetric flask, and make up the volume with pure water for measurement;

[0008] The digestion solution includes aqua regia, hydrofluoric acid, and hydrogen peroxide;

[0009] The molar ratio of the hydrofluoric acid to the boric acid is 1:4.

[0010] In the above technical solution, after digestion, boric acid solution is added, and the addition amount of boric acid has a great influence on the results. Considering the action mechanism of boric acid, boric acid reacts with free HF to form tetrafluoroborate (BF4 -) After adding boric acid to complex the remaining HF, the concentration of HF in the solution is extremely low, and there is almost no free HF in the system to maintain the equilibrium of the decomposition reaction. The forward progress of the decomposition reaction is restricted, and the reverse reaction (the re-dissolution of SiF4 to form H2SiF6) is also difficult to proceed due to the lack of sufficient HF in the system.

[0011] In addition, the closed system inhibits the volatilization of SiF4, both the forward and reverse directions of the decomposition reaction are inhibited, and the stability of H2SiF6 in the system dominates.

[0012] Therefore, while fixing HF, boric acid does not promote the decomposition of H2SiF6, but instead inhibits the formation of SiF4 through the dual effects of thermodynamics and kinetics.

[0013] According to the reaction mechanism, the F- that needs to be complexed by boric acid comes from the excess hydrofluoric acid that did not participate in the main reaction in the solution and the re-free F- released by side reactions. Not all fluoride ions need to be fixed by boric acid. And the introduction of boric acid may produce matrix effects, causing problems such as nebulizer blockage. Therefore, the addition amount of boric acid needs to be comprehensively considered in view of the above situations.

[0014] Within the addition amount range of the present invention, boric acid can fully complex the excess hydrofluoric acid, and the addition amount is relatively small, reducing the matrix effect and the possibility of nebulizer blockage.

[0015] In the above scheme, the addition timing of boric acid also has a great impact on the results. When added simultaneously with reagents such as hydrofluoric acid and aqua regia at the beginning, the test results are significantly lower; when added boric acid when the reaction temperature reaches 120 °C and hydrofluoric acid begins to accelerate the decomposition of silicate, the measured results are still significantly lower; when the silicate is completely decomposed by excess hydrofluoric acid and boric acid is added before starting the acid expulsion procedure, the measured values meet the requirements of the uncertainty range. Before the silicate in the sample is completely decomposed by hydrofluoric acid, if boric acid is added in advance, due to the stronger affinity of boric acid for hydrofluoric acid, it will react with hydrofluoric acid preferentially, reducing the effective concentration of hydrofluoric acid. When the boric acid is consumed, the remaining hydrofluoric acid will be used to decompose the silicate, resulting in insufficient decomposition of the silicate and thus causing the measured value to be low. Therefore, it is more reasonable to choose the way of adding reagents step by step, that is, first add hydrofluoric acid, aqua regia, and hydrogen peroxide. After digestion is completed, hydrofluoric acid has fully reacted with silicate to form H2SiF6 and SiF4 (volatile). At this time, adding boric acid can neutralize the excess hydrofluoric acid, fix fluoride ions, and inhibit the formation and volatilization of SiF4, and relatively accurate measurement results can be obtained.

[0016] In the above solution, hydrogen peroxide is also added to the digestion solution. When hydrogen peroxide is not added, the test results are significantly lower. As a strong oxidant, hydrogen peroxide can effectively decompose organic matter in the soil (such as humic acid, lipids, proteins, etc.), preventing organic residues from interfering with subsequent analysis. At the same time, it can also act synergistically with hydrofluoric acid (HF) to enhance the decomposition efficiency of silicate minerals. Under acidic conditions, H2O2 decomposes to produce reactive oxygen species such as hydroxyl radicals, accelerating the oxidation reaction.

[0017] Preferably, the concentration of the boric acid solution is 5%.

[0018] Preferably, the digestion solution is: aqua regia, hydrofluoric acid, and hydrogen peroxide in a volume ratio of 2:1:1.

[0019] In the second aspect of the present invention, a method for determining total silicon in soil is provided, including the following steps:

[0020] S1: Weigh a soil sample and place it in a digestion tank. Add the digestion solution, seal it, and perform digestion according to the temperature rise program set by the microwave digestion instrument parameters. After digestion is completed, wait until it cools to below 50 °C, quickly add the boric acid solution with the lid open, immediately seal it, shake well, and then place it on an acid removal instrument for acid removal treatment. Wait until it cools, open the lid, transfer it to a volumetric flask, and make up the volume with pure water for measurement.

[0021] S2: Prepare a series of standard series working solutions with different silicon concentrations respectively, and draw a standard curve.

[0022] S3: Use ICP-OES to measure the response value of silicon in the test sample prepared in step S1, and calculate the content of silicon in the test sample according to the standard curve drawn in step S2.

[0023] Preferably, during the detection process, an internal standard element is also added.

[0024] Preferably, the internal standard element is Rh.

[0025] Preferably, the digestion parameters are: the first stage: 80 °C, temperature rise for 5 min, hold for 10 min; the second stage: 120 °C, temperature rise for 5 min, hold for 20 min; the second stage: 170 °C, temperature rise for 5 min, hold for 30 min.

[0026] Preferably, the ICP-OES parameter settings are: wavelength 251.6 nm, power 1500 W.

[0027] By implementing the above technical solution, compared with the prior art, the present invention has established a pretreatment method of microwave digestion - boric acid complexation for determining the total silicon content in soil. Through screening and verification in aspects such as whether to select the addition of boric acid complexing agent, the addition amount and addition timing of boric acid, the addition amount of hydrogen peroxide, and whether to use internal standard correction, four national certified reference materials, namely GBW07407a, GBW07401a, GBW07402a, and GBW07382, were determined, and the precision and accuracy were verified. The RSD was between 0.14% and 0.60%, and the relative error was between -0.2% and 0.4%. The detection method of the present invention is fast and convenient, and is more suitable for the detection of a large number of samples than the alkali fusion method. It not only solves the problem of easy volatilization of silicon elements during the pretreatment process, but also introduces fewer salt ions, making the subsequent ICP - OES analysis process smoother. At the same time, it also greatly reduces the possibility of clogging of the nebulizer and torch tube. Detailed implementation manners

[0028] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0029] In the following description, if not otherwise specified, the reagents used are conventional commercially available products, and the methods used are well - known means in the art.

[0030] The main instruments and reagents are shown in Table 1.

[0031] Table 1 Main instruments and reagents used in the embodiments of the present invention

[0032]

[0033] The instrument conditions are shown in Table 2 and Table 3.

[0034] Table 2 ICP - OES parameter settings

[0035]

[0036] Table 3 Microwave digestion parameter settings

[0037]

[0038]

[0039] Sample solution preparation:

[0040] Weigh 0.10 g (accurate to 0.0001 g) of soil sample and put it into the digestion tank. Add 4 mL of aqua regia, 2 mL of hydrofluoric acid, and 2 mL of hydrogen peroxide. After sealing, carry out digestion according to the temperature rising program set by the microwave digestion instrument parameters. After digestion is completed, wait for it to cool to below 50 °C. Open the lid and quickly add 5 mL of 5% boric acid solution. Immediately seal it, shake well, and place it on the acid expelling instrument at 150 °C for 20 min. Wait for it to cool, open the lid, transfer it to a 250 mL plastic volumetric flask, and make up the volume with pure water for measurement.

[0041] Effect of Boric Acid on Sample Digestion

[0042] After determining the microwave digestion conditions, in order to determine the effect of boric acid on sample digestion, two groups of standard samples with different silicon contents, GBW07402a and GBW07407a, were selected as examples. On the premise that other conditions are the same, comparative tests were carried out on the digestion effects of samples with different boric acid addition amounts. The test results are shown in Table 4 (GBW07402a) and Table 5 (GBW07407a): Calculate the theoretical amount of boric acid added: 2 mL of hydrofluoric acid is added for sample digestion, about 0.046 moL, and the boric acid demand (by a molar ratio of 1:4) is about 0.0115 moL, about 0.71 g, and the volume of 5% boric acid solution is about 14.2 mL.

[0043] Table 4 Effect of Boric Acid with Different Addition Amounts on Sample Digestion (Taking GBW07402a as an Example)

[0044]

[0045] Table 5 Effect of Boric Acid with Different Addition Amounts on Sample Digestion (Taking GBW07407a as an Example)

[0046]

[0047]

[0048] Under the same microwave conditions, when no boric acid is added, the test results are significantly lower. After adding 1 mL of boric acid, the results increase to a certain extent but are still lower than the theoretical value; after adding 3 mL of boric acid, the measured value of the standard sample GBW07402a with relatively high silicon content is within the theoretical value range, while the standard sample GBW07407a with relatively low silicon content is still lower than the theoretical value; when 5 mL, 10 mL, and 15 mL of boric acid are added, the test results are all consistent with the theoretical value.

[0049] Considering the action mechanism of boric acid, boric acid reacts with free HF to form tetrafluoroborate (BF4 - ):

[0050] H3BO3 + 4HF → BF4 - + H3O ++H2O

[0051] After adding boric acid to complex the remaining HF, the concentration of HF in the solution is extremely low, and there is almost no free HF in the system to maintain the equilibrium of the decomposition reaction. The forward progress of the decomposition reaction is restricted, and the reverse reaction (SiF4 redissolves to form H2SiF6) is also difficult to proceed due to the lack of sufficient HF in the system.

[0052]

[0053] In addition, the closed system inhibits the volatilization of SiF4, both the forward and reverse directions of the decomposition reaction are inhibited, and the stability of H2SiF6 in the system dominates.

[0054] Therefore, while fixing HF, boric acid does not promote the decomposition of H2SiF6, but instead inhibits the formation of SiF4 through the dual effects of thermodynamics and kinetics.

[0055] According to the reaction mechanism, the F- that needs to be complexed by boric acid comes from the excess hydrofluoric acid that did not participate in the main reaction in the solution and the re-free F- released by the side reaction. Not all fluoride ions need to be fixed by boric acid. Moreover, the introduction of boric acid may produce matrix effects, causing problems such as nebulizer blockage. Therefore, when determining the addition amount of boric acid, the above situations need to be considered comprehensively.

[0056] Therefore, in the case of a 5 mL addition amount, the excess hydrofluoric acid can be fully complexed, and the addition amount is relatively small. Setting the volume to 250 mL can make the boric acid concentration in the solution ≤ 0.2%, reducing the matrix effect and the possibility of nebulizer blockage.

[0057] Effect of the addition timing of boric acid on the sample digestion effect

[0058] After the microwave digestion conditions are determined, to determine the effect of the addition timing of boric acid on the sample digestion effect, taking the GBW07402a standard sample as an example, under the same reagent concentration, addition amount, closed conditions and reaction time, the test results of adding boric acid at different times are shown in Table 6.

[0059] Table 6 Comparison of the effect of the addition timing of boric acid on the sample digestion effect

[0060]

[0061] When added simultaneously with reagents such as hydrofluoric acid and aqua regia at the beginning, the test results are significantly lower; when boric acid is added when the reaction temperature reaches 120 °C and hydrofluoric acid begins to accelerate the decomposition of silicate, the measured results are still significantly lower; when the silicate is completely decomposed by excessive hydrofluoric acid and added before starting the acid expulsion procedure, the measured values meet the requirements of the uncertainty range. Before the silicate in the sample is completely decomposed by hydrofluoric acid, if boric acid is added in advance, due to the stronger affinity of boric acid for hydrofluoric acid, it will react with hydrofluoric acid preferentially, reducing the effective concentration of hydrofluoric acid. When the boric acid is consumed, the remaining hydrofluoric acid will be used to decompose the silicate, resulting in incomplete decomposition of the silicate and thus lower measured values. Therefore, it is more reasonable to choose the method of adding reagents step by step, that is, first add hydrofluoric acid, aqua regia, and hydrogen peroxide. After digestion is completed, hydrofluoric acid has fully reacted with silicate to form H2SiF6 and SiF4 (volatile). At this time, adding boric acid can neutralize the excess hydrofluoric acid, fix fluoride ions, and inhibit the formation and volatilization of SiF4, and more accurate measurement results can be obtained.

[0062] The reactions that occur are:

[0063] SiO2 + 6HF → H2SiF6 + 2H2O

[0064]

[0065] H3BO3 + 4HF → BF4 - + H3O + + H2O

[0066] The influence of hydrogen peroxide on the digestion effect

[0067] After determining the microwave digestion conditions, to determine the influence of hydrogen peroxide on the sample digestion effect, taking the GBW07402a standard sample as an example, under the same other conditions, different hydrogen peroxide addition amounts were selected to conduct comparative tests on the sample digestion effect. The test results are shown in Table 7.

[0068] Table 7 Influence of hydrogen peroxide with different addition amounts on the sample digestion effect (taking GBW07402a as an example)

[0069]

[0070] Under the same microwave conditions, the test results are significantly lower without adding hydrogen peroxide; when 1 mL of hydrogen peroxide is added, the test results improve, but are still lower than the standard value; after adding 2 mL and 3 mL of hydrogen peroxide, the measured results are within the standard value range.

[0071] As a strong oxidant, hydrogen peroxide can effectively decompose organic matter in the soil (such as humic acid, lipids, proteins, etc.) to prevent organic residues from interfering with subsequent analysis. CxHyOz + H2O2 → CO2↑ + H2O

[0072] It can also act synergistically with hydrofluoric acid (HF) to enhance the decomposition efficiency of silicate minerals.

[0073] Under acidic conditions, H2O2 decomposes to generate reactive oxygen species such as hydroxyl radicals, accelerating the oxidation reaction.

[0074] The decomposition of hydrogen peroxide may release gas, resulting in an increase in the pressure inside the digestion tank. It is necessary to control the addition amount to prevent the pressure in the digestion tank from being too high. Therefore, it is more appropriate to add 2 mL of hydrogen peroxide.

[0075] The influence of internal standard on test results

[0076] After determining the pretreatment conditions, to determine the influence of using an internal standard on the sample measurement results, three national standard reference materials with low, medium, and high silicon contents, namely GBW07407a, GBW07402a, and GBW07382, were selected for determination. Under the same other conditions, a comparative test was conducted on the influence of whether to add the internal standard Rh on the measurement results. The test results are shown in Table 8.

[0077] Table 8 Influence of adding internal standard on sample test effect

[0078]

[0079] When the three national standard reference materials did not add Rh as the internal standard element, the measurement results were on the low side and the stability was poor; after adding the internal standard Rh to the three standard reference materials, the measured values were within the theoretical value range and the measurement results had good stability.

[0080] Due to the introduction of boric acid, it is easy to cause a decrease in the atomization effect of the nebulizer. To suppress the influence of the changes in the sample introduction process on the analysis, adding Rh as an internal standard can effectively correct the instrument fluctuations caused by differences in sample introduction volume, signal drift, etc., and the interference of matrix effects on silicon detection, and at the same time track and compensate the recovery rate after silicon loss caused by the digestion step.

[0081] The internal standard can be added online, or the same concentration of internal standard can be added to the standard solution, the test solution, and the blank solution.

[0082] Therefore, it is more appropriate to use the internal standard method to add the internal standard Rh for calibration of the measurement results.

[0083] Calibration curve and method detection limit

[0084] Accurately pipette 0.00, 1.00, 2.00, 5.00, 8.00, 10.00 mL of the silicon standard solution into 6 100-mL volumetric flasks, add 4 mL of aqua regia, and make up the volume with pure water. The silicon concentrations are 0.00, 10.0, 20.0, 50.0, 80.0, 100 mg / L respectively.

[0085] According to the instrument reference conditions, sequentially measure the intensities of the calibration series solutions from low concentration to high concentration. Use the intensities of the calibration series solutions after blank subtraction as the ordinate and the corresponding element content (mg / L) as the abscissa to plot the calibration curve. The measurement data of the calibration curve are shown in Table 9.

[0086] Table 9 Test Data of Calibration Curve

[0087]

[0088] According to the full steps of sample analysis, under the optimal conditions of instrument measurement, select the intensity at 251.6 nm and perform 7 parallel determinations of the whole-process blank samples to calculate the method detection limit. See Table 10 for details.

[0089] Table 10 Test Data of Method Detection Limit and Determination Lower Limit

[0090]

[0091] Precision and Accuracy Verification

[0092] Select four national certified reference materials with different silicon contents, namely GBW07407a, GBW07401a, GBW07402a, and GBW07382, and perform 6 consecutive determinations for each of them. Calculate their standard deviation and relative standard deviation to illustrate the method precision; calculate the relative error between the calculation result and the standard value to measure the method accuracy. The specific values are shown in Table 11.

[0093] Table 11 Precision and Accuracy Verification

[0094]

[0095] As can be seen from Table 11, the RSD is between 0.14% and 0.60%, indicating that the method has good precision; the relative error is between -0.2% and 0.4%, indicating that the method has good accuracy.

[0096] Results Comparison of Different Methods

[0097] To verify the feasibility of this method, select 3 actual soil samples with different silicon contents, and perform 5 parallel determinations for each sample. Simultaneously determine them using this method and the alkali fusion-inductively coupled plasma emission spectrometry method for the determination of 11 elements in soil and sediment (HJ 974-2018). The results are shown in Table 12.

[0098] Table 12 Test Results of Method Comparison for Actual Samples

[0099]

[0100] According to the data in Table X, when the actual samples were determined by the method described in this paper and the alkali fusion method of HJ 974, the relative error between the two methods was in the range of -2.4% to 0.52%, indicating that the results of the two methods had a small deviation and were both applicable to the detection of total silicon in soil.

[0101] Conclusion

[0102] A pretreatment method of microwave digestion - boric acid complexation was established for the determination of total silicon content in soil. Through screening and verification in aspects such as whether to select the addition of boric acid complexing agent, the addition amount and addition timing of boric acid, the addition amount of hydrogen peroxide, and whether to use internal standard correction, four national certified reference materials, namely GBW07407a, GBW07401a, GBW07402a, and GBW07382, were determined to verify the precision and accuracy. The RSD was in the range of 0.14% to 0.60%, and the relative error was in the range of -0.2% to 0.4%. Through the detection of actual samples, the method described in this paper was compared with the alkali fusion method of HJ 974, and the relative error was in the range of -2.4% to 0.52%. In summary, the method described in this paper is rapid and convenient, and is more applicable to the detection of a large number of samples than the alkali fusion method. It not only solves the problem of easy volatilization of silicon elements during the pretreatment process, but also introduces fewer salt ions, making the subsequent ICP - OES analysis process smoother and greatly reducing the possibility of clogging of the nebulizer and torch tube.

Claims

1. A pretreatment method for soil samples, characterized in that, It includes the following steps: Weigh a soil sample and place it in a digestion tank. Add the digestion solution, seal it, and perform digestion according to the temperature-rising program set by the microwave digestion instrument parameters. After digestion is completed, wait until it cools to below 50 °C, quickly open the lid and add boric acid solution, immediately seal it, shake well, and place it under the condition of 150 °C ± 5 °C for reaction. After the reaction ends, wait until it cools, open the lid, transfer it to a volumetric flask, and make up the volume with pure water for measurement; The digestion solution includes aqua regia, hydrofluoric acid, and hydrogen peroxide; The molar ratio of the hydrofluoric acid to the boric acid is 1:

4.

2. The pretreatment method of a soil sample according to claim 1, characterized in that, The concentration of the boric acid solution is 5%.

3. The pretreatment method of a soil sample according to claim 1, characterized in that, The digestion solution is: the volume ratio of aqua regia, hydrofluoric acid, and hydrogen peroxide is 2:1:

1.

4. A method for determining total silicon in soil, characterized in that, It includes the following steps: S1: Weigh a soil sample and place it in a digestion tank. Add the digestion solution, seal it, and perform digestion according to the temperature-rising program set by the microwave digestion instrument parameters. After digestion is completed, wait until it cools to below 50 °C, quickly open the lid and add boric acid solution, immediately seal it, shake well, and place it on a fume eliminator for fume elimination treatment. After it cools, open the lid, transfer it to a volumetric flask, and make up the volume with pure water for measurement; S2: Prepare a series of standard series working solutions with different silicon concentrations respectively, and draw a standard curve; S3: Use ICP-OES to measure the response value of silicon in the sample to be measured prepared in step S1, and calculate the content of silicon in the sample to be measured according to the standard curve drawn in step S2; In step S1, the digestion solution includes aqua regia, hydrofluoric acid, and hydrogen peroxide; the molar ratio of the hydrofluoric acid to the boric acid is 1:

4.

5. A method for determining total silicon in soil according to claim 4, characterized in that, The concentration of the boric acid solution is 5%.

6. The method for determining total silicon in soil according to claim 4, wherein The digestion solution is: the volume ratio of aqua regia, hydrofluoric acid, and hydrogen peroxide is 2:1:

1.

7. According to the method for determining total silicon in soil as described in claim 4, characterized in that During the detection process, an internal standard element is also added.

8. A method for determining total silicon in soil according to claim 7, characterized in that, The internal standard element is Rh.

9. A method for determining total silicon in soil according to claim 4, characterized in that, The digestion parameters are: the first stage: 80 °C, temperature rise for 5 min, hold for 10 min; the second stage: 120 °C, temperature rise for 5 min, hold for 20 min; the second stage: 170 °C, temperature rise for 5 min, hold for 30 min.

10. A method for determining total silicon in soil according to claim 4, characterized in that, The ICP-OES parameter settings are: wavelength 251.6 nm, power 1500 W.

Images