Soil heavy metal digestion method comparison (soil digestion instrument)

Abstract: Digestion is a key step affecting the accuracy of soil heavy metal determination. Comparing the operation procedures of three kinds of digestion methods: electrothermal plate digestion, microwave digestion and fully automatic graphite digestion, and comparing the contents of Cu, Zn, Pb, Cd, Cr and Ni in different types of soil samples, the results show that the electric heating The plate digestion equipment is simple, but the steps are complicated, and the operation is easy to cause component loss; the microwave digestion speed is fast, the precision and accuracy of the measurement result are good, but the manual acid pick-up procedure is required and the tank position is small, which is not suitable for large batch samples. Analysis; Fully automatic graphite digestion not only has good precision and accuracy, but also has high degree of automation, can achieve unattended operation, and greatly saves manpower, especially suitable for analysis of large quantities of samples. Key words: electrothermal plate digestion; microwave digestion; automatic graphite digestion; soil; heavy metal CLC number: X833 Document code: A Article ID: 1002-6002( 2013) 01-0123-04
With the continuous development of industry and agriculture, a large amount of waste residue and waste water are produced. Due to the random dumping and stacking of solid waste, the arbitrary discharge of harmful waste water and the large-scale use of pesticides, the types and concentrations of pollutants in the soil are on the rise. Among the soil environmental problems, heavy metal pollution has multi-source, concealment and long-term effects, and the pollution consequences are serious. It is an important pollution investigation and evaluation object in environmental pollution investigation and evaluation research, and is listed as a priority control pollutant list by countries [1- 2]. In recent years, the Ministry of Environmental Protection has organized national soil pollution surveys and rural soil environmental quality supervision and monitoring, showing the importance the country attaches to soil environmental issues. Therefore, in the face of increasing soil heavy metal monitoring tasks, how to quickly and accurately analyze its content, in order to determine the degree of soil pollution is particularly important [3-5]. Digestion is the key in the determination of soil heavy metals. Different digestion methods have different operational difficulties, and the digestion effects are sometimes different [6-8]. In this paper, three kinds of digestion methods, namely, hot plate digestion, microwave digestion and automatic graphite digestion, were used to digest the soil samples. The ease of operation of various digestion methods was compared, and the three digestion methods were used to analyze Cu, Zn, Pb, Cd, The influence of the determination results of 6 heavy metal elements of Cr and Ni provides a reference for the selection of digestion methods in the determination of heavy metals in soil samples. 1 Experimental part 1. 1 Main instruments and reagents ETHOS 1 Microwave digestion instrument, Italy; EH45B high temperature graphite electric heating plate, Beijing; DEENAII automatic graphite digestion instrument, USA; AA240Duo Atomic Absorption Apparatus, USA. Cu, Zn, Pb, Cd, Cr, Ni standard solution 1 000 mg / L, National Institute of Environmental Protection Standard Sample Research Institute; ESS-3 environmental soil standard sample, China Environmental Monitoring Station; all experimental water is ultrapure water, All experimental acids were excellent grades. 124 China Environmental Monitoring Volume 29 Number 1 February 2013 1. 2 Preparation of Soil Samples The collected soil samples (not less than 500 g) were mixed and divided by a quarter method to approximately 100 g. After the shrinking soil sample is air-dried (naturally dried or freeze-dried), remove foreign matter such as stones and animal and plant residues in the soil sample, and grind with a wooden rod (or agate rod) through a 2 mm nylon sieve (remove 2 mm) Mix the above gravel). The soil sample that had passed through a 2 mm nylon mesh was ground to a nylon mesh with a pore size of 0.150 mm using an agate mortar, and mixed for use. 1. 3 digestion of soil samples 1. 3. 1 electric heating plate digestion method accurately weighed 0. 5 g (accurate to 0. 000 1 g) sample in 50 mL of Teflon, wet with water and then add 10 mL hydrochloric acid, heated at low temperature on the hot plate, the sample is initially decomposed, and evaporated to about 3 mL. After a little cold, add 5 mL of nitric acid, 5 mL of hydrofluoric acid, 3 mL of perchloric acid, cover, and heat on a hot plate. Heat for about 1 h, then open the lid to continue heating to remove silicon. In order to achieve a good flying silicon effect, it should be vibrated frequently. When heated to thick white perchloric acid, the black organic carbide is decomposed. After the black organic matter on the wall disappears, open the lid, drive away the white smoke and steam until it is viscous (depending on the digestion, add 3 mL of nitric acid, 3 mL of hydrofluoric acid, 1 mL of perchloric acid, repeat the above digestion process) . Remove the sputum and add a little cold. Add 1 mL of nitric acid solution and a small amount of distilled water to dissolve the soluble residue warmly. After cooling, transfer the whole amount to a 50 mL volumetric flask, and dilute to the mark with water and shake well. 1. 3. 2 Microwave digestion method accurately weighs 0. 5 g (accurate to 0. 000 1 g) The sample is in a microwave digestion tank, wetted with a small amount of water, then added 5 mL of nitric acid, 3 mL of hydrofluoric acid, 1 The mL hydrogen peroxide was digested according to a certain heating procedure (see Table 1). After cooling, the solution was transferred to 50 mL of Teflon and heated at 150 °C to drive off the white smoke and steam until it became viscous. Remove the sputum and let it cool. Add 1 mL of nitric acid solution, dissolve the soluble residue warmly, transfer it to a 50 mL volumetric flask. After cooling, dilute to the mark and shake. Table 1 Microwave digestion procedure step temperature / ° C time / min power / kW1 180 10 1. 52 210 20 1. 51. 3. 3 automatic graphite digestion method accurately weighed 0. 5 g (accurate to 0. 000 1 g) The sample was dissolved in a fully automatic graphite digestion Teflon digestion tube. After wetting with a small amount of water, add 10 mL of nitric acid and hydrochloric acid, shake for 10 s, and heat at 150 °C for 90 min; after cooling for 30 min, add 5 mL of hydrofluoric acid and 3 mL of perchloric acid, shake 10 s, heat at 150 °C for 40 min; add 1 mL of nitric acid and 5 mL of distilled water, continue heating at 150 °C for 20 min; cool for 30 min, dilute to 50 mL with distilled water, shake for 30 s. 1. 4 Determination of sample The prepared digestion solution is determined by atomic absorption method according to the standard method [9-12], and Cu, Zn, Pb, Cd, Cr, Ni are determined by atomic absorption method, and the sample is calculated according to the standard curve. content. 2 Results and discussion 2. Comparison of the operation flow of 3 kinds of digestion methods By comparison (see Table 2), the advantages of the electrothermal plate digestion method are that the instrument and equipment are simple, and more samples can be analyzed at one time, and the application is extensive; Obviously, it takes a long time, and the whole process needs manual operation. The open digestion, high temperature and large amount of acid mist generated are harmful to the operator and the environment, and the digestion process is susceptible to external interference and operator level. The advantages of microwave digestion are short time-consuming, low acid consumption, and less harm to people and the environment. The disadvantage is that it is not fully automated, and the sample must be acid-adjusted after microwave digestion, and the sample amount is one-time digestion. Not suitable for analysis of large samples. The advantage of automatic graphite digestion is that it takes less time and the whole digestion process is fully automatic, no need to be on duty, which saves manpower, avoids the harm of acid mist to the experimenter, and is suitable for the analysis of large batch samples; The amount is large and the instrument is expensive. Table 2 Comparison of the operation procedures of the three digestion methods. Comparison of the contents of the electrothermal plate digestion method, microwave digestion, automatic graphite digestion, digestion of acid species, HCl-HNO3 -HF-HClO4, HNO3 -HF-H2, O2, HCl-HNO3 -HF-HClO4 Less than one time, the number of samples to be digested, the number of samples, the number of times, the number of samples, the number of time-consuming, the amount of time, the amount of time, the amount of time, the amount of time, the amount of time. No automation, the whole process needs to be semi-automatically completed, the microwave program and the cooling process are completed automatically. The whole process is fully automated when the acid and the constant volume are digested with the electric heating plate. The unattended impact on the staff is great, and the acid mist harms the human respiratory tract. High temperature operation is easy to scald. Acid acid process produces a small amount of acid mist. It has little effect on people and acid mist, but it has no effect on operators. Longjiahong, etc.: Comparison of different methods for determination of soil heavy metal content 1252. 2 3 kinds of digestion methods According to the three methods of electric heating plate digestion method, microwave digestion method and automatic graphite digestion method, the accuracy and precision of the standard soil samples were respectively weighed 5 parts of GSBZ 50013-1988 (ESS-3). Digestion experiments were carried out as described in Section 1.3. The results are shown in Table 3. It can be seen from Table 3 that 13 methods have good digestion effects for Cu, Zn, Pb and Ni. Due to the low content of 2Cd, the precision of the three methods is relatively poor, and the accuracy is not in the standard range. 3Cr is greatly affected by acid and temperature, and the electrothermal plate digests the volatile loss. The result is lower than the standard lower limit of 27.6% to 16.5%, and the precision is poor. The accuracy and precision of the microwave digestion method are the best; Automatic graphite digestion is slightly worse than microwave digestion, but can also be controlled within standard limits. Table 3 Accuracy and precision of three kinds of digestion methods Comparison element standard value / ( μg · g - 1 ) Electrothermal plate digestion method Microwave digestion method Fully automatic graphite digestion method measurement value / ( μg · g - 1 ) RSD /% determination Value / ( μg·g - 1 ) RSD /% Measured value / ( μg·g - 1 ) RSD /%Cu 29. 4 ± 1. 6 28. 5 ～ 30. 5 2. 5 28. 8 ～ 30. 7 2. 4 28. 6 ～ 29. 8 1. 8Zn 89. 3 ± 4. 0 88. 5 ～ 92. 5 4. 7 87. 5 ～ 91. 7 3. 2 86. 8 ～ 92. 8 4. 1Pb 33. 3 ± 1. 3 32. 3 ～ 34. 0 3. 6 32. 5 ～ 33. 8 1. 7 32. 6 ～ 34. 1 3. 3Cd 0. 044 ± 0. 014 0. 041 ～ 0. 063 6. 8 0. 042 ～ 0. 060 5. 4 0. 039 ～ 0. 059 5. 9Cr 98. 0 ± 7. 1 65. 8 ～ 75. 9 7. 8 93. 2 ～ 97. 8 3. 6 89. 2 ～ 94. 2 5. 2Ni 33. 7 ± 1. 3 33. 5 ～ 34. 6 3. 9 32. 8 ～ 34. 6 3. 6 32. 9 ～ 34. 3 2. 82. 3 Determination of actual soil samples and calculation of spiked recovery rate 3 Digestion method was used to analyze different types of soils. Three samples were tested in parallel for each sample. The results are shown in Table 4. It can be seen from Table 4 that the measured values ​​of Cu, Zn, Pb, and Ni are parallel; the difference between the measured values ​​of Cd is slightly larger; for the measurement results of Cr, the measured values ​​of microwave digestion and fully automatic graphite digestion are close. However, the measured value of the hot plate digestion is lower, and the result is much lower. At the same time, the farmland soil is spiked and the standard is added in parallel. The standard addition rate is measured (see Table 5) and the deviation between the actual sample and the standard sample. More consistent. Table 4 Actual soil sample measurement results μg / g sample digestion method Cu Zn Pb Cd Cr Ni electric heating plate digestion method 32. 6 101 37. 2 0. 346 71. 8 25. 2 farmland soil microwave digestion method 31. 5 104 38. 5 0. 366 98. 2 26. 5 automatic graphite digestion method 32. 9 99. 8 37. 4 0. 318 96. 5 26. 4 hot plate digestion method 39. 4 124 33. 1 0. 203 72. 8 27. 3 vegetable soil microwave digestion method 38. 2 128 34. 5 0. 238 97. 1 28. 1 automatic graphite digestion method 39. 7 131 32. 8 0. 253 91. 8 27. 0 hot plate digestion 29. 7 73. 7 19. 8 0. 054 58. 6 21. 5 Forest soil microwave digestion method 29. 8 75. 2 20. 5 0. 041 82. 4 22. 8 automatic graphite digestion method 28. 8 72 9 20. 6 0. 060 79. 8 20. 9 Table 5 Farmland soil samples spiked recovery Experimental element digestion method Pre-standardization measurement value / μg Sizing amount / μg After spiked value / μg Recovery amount / μg Recovery rate /% 电热板解法16. 3 30. 0 45. 1 28. 8 96. 0Cu microwave digestion method 15. 7 30. 0 45. 2 29. 5 98. 3 automatic graphite digestion method 16. 4 30. 0 47. 3 30. 9 103. 0 electric heating plate digestion method 50. 5 30. 0 82. 5 32. 0 106. 7Zn microwave digestion method 52. 0 30. 0 82. 9 30. 9 103. 0 automatic graphite digestion 49. 9 30. 0 81. 6 31. 7 105. 7 Electrothermal plate digestion method 18. 6 30. 0 46. 8 28. 2 94. 0Pb Microwave digestion method 19. 2 30. 0 49. 6 30. 4 101 3 automatic graphite digestion method 18. 7 30. 0 47. 7 29. 0 96. 7 electric heating plate digestion method 0. 173 0. 100 0. 264 0. 091 91. 0Cd microwave digestion method 0. 183 0. 100 0. 281 0. 098 98. 0 automatic graphite digestion method 0. 159 0. 100 0. 252 0. 093 93. 0 heating plate digestion method 35. 9 30. 0 58. 4 22. 5 75. 0Cr microwave digestion Method 49. 1 30. 0 78. 1 29. 0 96. 7 Fully automatic graphite digestion method 48. 2 30. 0 75. 7 27. 5 91. 7 Electrothermal plate digestion method 12. 6 30. 0 40. 7 28 1 93. 7Ni Microwave Digestion Method 13. 2 30. 0 42. 1 28. 9 96. 3 Fully Automatic Graphite Digestion Method 13. 2 30. 0 44. 3 31. 1 103. 7126 China Environmental Monitoring Volume 29 Phase 1 February 2013 3 Conclusions Through the comparison of the operation process and a series of comparative experiments, it can be seen that the heating and dissolving method of the electric heating plate is simple and low in cost, but it takes a long time and is cumbersome to operate, and a large amount of acid mist is generated to harm the operator. If the manual operation is improper, it is easy to cause component loss, especially the Cr element is difficult to control. The microwave digestion method has high accuracy, good precision, small acid consumption, and convenient operation, and can greatly shorten the digestion time. The disadvantage is that it is still placed on the hot plate to heat up the acid, and the digestion tank is small, no Great for large sample analysis. Fully automatic graphite digestion can realize acid addition, heating temperature control, cooling and constant volume, etc. The steps are all automatically completed, unattended, greatly saving manpower, and the generated acid mist will not pose a hazard to the operator, especially suitable for large batch samples. Analysis. Fully automatic graphite digestion is the process control of all steps, avoiding the impact of improper manual operation, so the precision and accuracy are also good.