A soil hexavalent chromium extraction method based on composite lye dispersion and fractional centrifugation coupling
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- ZHENGZHOU TONGBIAO ENVIRONMENTAL TESTING CO LTD
- Filing Date
- 2026-05-25
- Publication Date
- 2026-08-07
AI Technical Summary
然而，这些改进多为单因素的局部优化，尚未形成针对高粘性土壤样品类型的系统性解决方案，特别是在如何在提取过程中兼顾分散效能与抗干扰能力的协同匹配等方面，仍缺乏深入研究和可靠的技术手段
提高了高粘性土壤中六价铬的提取效率：本发明通过引入六偏磷酸钠作为分散剂，并采用分步提取策略，解决了高粘性土壤中六价铬解吸不完全的难题；
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Abstract
Description
Technical Field
[0001] This invention relates to the field of hexavalent chromium extraction and detection technology, specifically to a method for extracting hexavalent chromium from soil based on the coupling of composite alkaline solution dispersion and graded centrifugation. Background Technology
[0002] Hexavalent chromium is a highly oxidizing and toxic form of heavy metal, classified as a Group 1 human carcinogen. Compared to trivalent chromium, hexavalent chromium has weaker adsorption capacity and stronger mobility in soil, readily entering groundwater systems through leaching. It can also be rapidly absorbed and accumulated in organisms, posing a serious threat to human health and the ecological environment. Therefore, accurate and rapid determination of hexavalent chromium content in soil is of fundamental importance for contaminated site identification, risk assessment, and subsequent remediation.
[0003] Currently, the standard method for determining hexavalent chromium in soil is the "Determination of Hexavalent Chromium in Soil and Sediments by Alkaline Solution Extraction-Flame Atomic Absorption Spectrophotometry" (HJ 1082-2019) issued by the Ministry of Ecology and Environment of my country. This method uses a sodium carbonate-sodium hydroxide mixed solution as the alkaline extractant, and extracts by heating and stirring at 90℃~95℃ for 60 min. The alkaline environment stabilizes the hexavalent chromium form and precipitates trivalent chromium. Simultaneously, magnesium chloride and phosphate buffer are used to form a co-precipitation system to suppress interference from trivalent chromium. The extracted mixture is filtered through a 0.45 μm filter membrane, and the filtrate is adjusted to pH with nitric acid before being analyzed by flame atomic absorption spectrophotometry.
[0004] In the practical application of standard methods, the preprocessing stage has revealed several common technical challenges: Firstly, the magnetic heating and stirring device has a limited number of samples that can be processed at one time, which is difficult to meet the testing needs of large batches of samples. Moreover, the beaker is not easy to fix during the heating process, and the stir bar jumps and samples splash out from time to time, resulting in insufficient operational stability. Secondly, the filtration process is time-consuming, especially for soil samples with high clay content, where the filter membrane is easily clogged by fine particles, becoming the efficiency bottleneck of the entire analysis process. Third, for highly viscous soils, due to the large specific surface area and strong adsorption capacity of clay minerals, chromate ions are not only adsorbed on the mineral surface, but may also be embedded between the mineral crystal layers. The standard extraction solution has limited desorption capacity for this type of hexavalent chromium, which can easily lead to incomplete extraction and lower test results.
[0005] To address the aforementioned issues, some improvement studies have been conducted within the industry. Regarding extraction methods, some studies have replaced magnetic stirring with constant-temperature water bath oscillation, increasing the sample throughput per batch. Regarding separation methods, some studies have introduced a centrifugation pretreatment step to reduce the burden on the filter membrane. However, these improvements are mostly local optimizations of single factors and have not yet formed a systematic solution for highly viscous soil sample types. In particular, there is a lack of in-depth research and reliable technical means regarding how to synergistically match dispersion efficiency and anti-interference ability during the extraction process.
[0006] Therefore, there is an urgent need to develop a hexavalent chromium pretreatment method that can adapt to high-viscosity soil types and has both high extraction efficiency and high throughput processing capacity to meet the growing demand for soil environmental monitoring. Summary of the Invention
[0007] The purpose of this invention is to provide a method for extracting hexavalent chromium from soil based on the coupling of compound alkaline solution dispersion and graded centrifugation, so as to solve the problems mentioned in the background art.
[0008] To achieve the above objectives, the present invention provides the following technical solution: A method for extracting hexavalent chromium from soil based on the coupling of compound alkaline solution dispersion and fractional centrifugation includes the following steps: S1. After air-drying and grinding the soil sample, pass it through a 100-mesh sieve. Weigh 5.0g of the treated soil sample and place it in a 100mL capped plastic centrifuge tube. Add 50mL of compound alkali solution and shake well. S2. Place the centrifuge tube in an ultrasonic instrument preheated to 60°C and sonicate for 10 minutes. Then add 400 mg MgCl2 and 0.5 mL potassium dihydrogen phosphate-dipotassium hydrogen phosphate buffer solution to the centrifuge tube and shake well. S3. Transfer the centrifuge tubes to a preheated water bath shaker at 93±2℃ and shake at 200 rpm for 50 minutes. S4. After shaking, remove the centrifuge tube and cool it to room temperature. Place it on a centrifuge for centrifugation. The centrifugation conditions are as follows: first centrifuge at 1000 r / min for 5 min, transfer all the supernatant, discard the precipitate, and then centrifuge at 4000 r / min for 5 min. S5. After centrifugation, take all the supernatant and adjust the pH to 7.5±0.5 with nitric acid. Transfer the solution to a 100mL volumetric flask and make up to volume. This is the solution to be tested.
[0009] Preferably, in step S1, the composite alkaline solution comprises the following components by weight: 25-35 parts KOH, 28-38 parts Na2CO3, 3-8 parts sodium hexametaphosphate, and 800-1000 parts deionized water, and the pH value of the composite alkaline solution is ≥12.5.
[0010] Preferably, in step S1, particles with a diameter less than 0.075 mm account for more than 50% of the total volume of the soil sample.
[0011] Preferably, in step S2, the preparation method of the potassium dihydrogen phosphate-dipotassium hydrogen phosphate buffer solution is as follows: weigh 87.1 g of dipotassium hydrogen phosphate and 68.0 g of potassium dihydrogen phosphate, dissolve them in an appropriate amount of deionized water, make up to 1 L, shake well, and the pH is 7.0~7.5.
[0012] The beneficial effects of the above-mentioned technical solution of the present invention are as follows: Improved extraction efficiency of hexavalent chromium in highly viscous soil: This invention solves the problem of incomplete desorption of hexavalent chromium in highly viscous soil by introducing sodium hexametaphosphate as a dispersant and adopting a stepwise extraction strategy. In the first extraction stage, sodium hexametaphosphate anions fully exert their electrostatic dispersion and steric hindrance effects on clay particles, breaking down the colloidal structure of the clay particles and allowing the extract to penetrate into the mineral micro-regions. Simultaneously, the K+ in KOH... + With less than Na + The hydration radius can further promote the release of hexavalent chromium into the solution; In the second extraction stage, magnesium chloride and phosphate buffer were added after ultrasonic extraction to initiate a co-precipitation mechanism, removing interfering substances such as trivalent chromium without affecting the already dissolved hexavalent chromium. The two stages worked synergistically to improve the extraction efficiency of hexavalent chromium from highly viscous soil.
[0013] Improved pretreatment efficiency and sample throughput: This scheme replaces magnetic stirring with ultrasonic and constant temperature water bath oscillation, which can process more samples per batch than magnetic stirrers, providing a feasible path for batch detection; the pre-ultrasonic pretreatment combined with the dispersing effect of sodium hexametaphosphate accelerates the desorption and mass transfer process of hexavalent chromium, and the extraction time is controllable overall; differential centrifugation replaces traditional direct filtration, eliminating the problem of filter membrane clogging, significantly shortening the separation time of a single sample, and improving the efficiency of batch sample detection. Detailed Implementation
[0014] The preferred embodiments of the present invention are described below. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.
[0015] For experiments not specified in this protocol, the procedures and conditions described in the literature in this field should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products. Example 1
[0016] A method for extracting hexavalent chromium from soil based on the coupling of compound alkaline solution dispersion and fractional centrifugation includes the following steps: S1. After air-drying and grinding the soil sample, pass it through a 100-mesh sieve. Weigh 5.0g of the treated soil sample and place it in a 100mL capped plastic centrifuge tube. Add 50mL of compound alkali solution and shake well. S2. Place the centrifuge tube in an ultrasonic instrument (power 300W, frequency 40Hz) preheated to 60℃, sonicate for 10 minutes, then add 400mg MgCl2 and 0.5 mL potassium dihydrogen phosphate-dipotassium hydrogen phosphate buffer solution to the centrifuge tube and shake well. S3. Transfer the centrifuge tubes to a preheated water bath shaker at 93±2℃ and shake at 200 rpm for 50 minutes. S4. After shaking, remove the centrifuge tube and cool it to room temperature. Place it on a centrifuge for centrifugation. The centrifugation conditions are as follows: first centrifuge at 1000 r / min for 5 min, transfer all the supernatant, discard the precipitate, and then centrifuge at 4000 r / min for 5 min. S5. After centrifugation, take all the supernatant and adjust the pH to 7.5±0.5 with 1.42 g / mL nitric acid. Transfer the solution to a 100mL volumetric flask and make up to volume. This is the test solution.
[0017] In step S1, the composite alkaline solution comprises the following components by weight: 28 parts KOH, 31 parts Na2CO3, 5 parts sodium hexametaphosphate, and 930 parts deionized water, with a pH value ≥ 12.5.
[0018] In step S2, the preparation method of potassium dihydrogen phosphate-dipotassium hydrogen phosphate buffer solution is as follows: weigh 87.1 g of dipotassium hydrogen phosphate and 68.0 g of potassium dihydrogen phosphate, dissolve them in an appropriate amount of deionized water, make up to 1 L, shake well, and the pH is 7.0.
[0019] Hexavalent chromium standard working solution: standard value is 100 mg / L, purchased from Tanmo, product number BW30030-100-20. Example 2
[0020] The difference between this and Example 1 is that the composite alkaline solution includes the following components by weight: 25 parts KOH, 28 parts Na2CO3, 3 parts sodium hexametaphosphate, and 800 parts deionized water. Example 3
[0021] The difference between this and Example 1 is that the composite alkaline solution includes the following components by weight: 35 parts KOH, 38 parts Na2CO3, 8 parts sodium hexametaphosphate, and 1000 parts deionized water. Comparative Example 1
[0022] The difference between it and Example 1 is that: The compound alkaline solution comprises the following components by weight: 20 parts NaOH, 31 parts Na2CO3, 5 parts sodium hexametaphosphate, and 930 parts deionized water. The pH value of the compound alkaline solution is ≥12.5. Comparative Example 2
[0023] The difference between it and Example 1 is that: The compound alkaline solution comprises the following components by weight: 28 parts KOH, 31 parts Na2CO3, 1 part sodium hexametaphosphate, and 930 parts deionized water. Comparative Example 3
[0024] The difference between it and Example 1 is that: The compound alkaline solution comprises the following components by weight: 28 parts KOH, 31 parts Na2CO3, 15 parts sodium hexametaphosphate, and 930 parts deionized water. Comparative Example 4
[0025] The difference between it and Example 1 is that: Remove the sonication process in step S2 and replace it with: add 400 mg MgCl2 and 0.5 mL potassium dihydrogen phosphate-dipotassium hydrogen phosphate buffer solution to the centrifuge tube and shake well; Replace centrifugation in step S4 with vacuum filtration, using a filter membrane with a pore size of 0.45 μm. Comparative Example 5
[0026] The test solution was prepared using the method described in HJ1082.
[0027] The establishment of the working curve is based on HJ1082: Weigh 0 mL, 0.10 mL, 0.20 mL, 0.50 mL, 1.00 mL, and 2.00 mL of hexavalent chromium standard working solution into 100 mL capped centrifuge tubes, respectively. Add 50 mL of compound alkali solution according to step S1 above, and then prepare the working curve solution according to steps S2 to S5. The reference concentrations are 0 mg / L, 0.10 mg / L, 0.20 mg / L, 0.50 mg / L, 1.00 mg / L, and 2.00 mg / L. Use the blank sample to zero the instrument, and measure the absorbance in order of increasing concentration. Plot the hexavalent chromium concentration on the x-axis and the absorbance on the y-axis to establish the working curve, as shown in Table 1. The blank sample is the sample prepared according to steps S1 to S5 without adding any sample.
[0028] Table 1. Working curves of hexavalent chromium
[0029] Preparation of model soil samples containing hexavalent chromium: Prepare model soils according to the following three formulations: Formula A (Kaolin-based high-cohesive soil): 70% kaolin, 30% quartz sand; Formula B (Illite-based high-cohesive soil): 65% illite, 35% quartz sand; Formula C (Montmorillonite-based high-viscosity soil): 60% montmorillonite, 40% quartz sand; Weigh 5.0 g (accurate to 0.01 g) of each of the three soil formulations mentioned above, and prepare three spiking concentration levels for each formulation: 2.0 mg / kg, 20.0 mg / kg, and 40.0 mg / kg, with 6 parallel samples prepared for each concentration level. Spiking method: Add 2 mL of hexavalent chromium standard solution (5 mg / L) diluted 20 times with deionized water, 2 mL of hexavalent chromium standard solution (50 mg / L) diluted 2 times with deionized water, and 2 mL of hexavalent chromium standard solution (100 mg / L) to the weighed soil to achieve the target concentration of hexavalent chromium in the final soil. After spiking, mix thoroughly and age in the dark at room temperature (25±2℃) for 48 h to allow the hexavalent chromium to fully react with the soil particles. At the same time, prepare unspiked blank soil as a matrix control.
[0030] Before use, the soil sample was dried at 105℃ for 2 hours, ground and passed through a 100-mesh sieve. The particle size was measured by a laser particle size analyzer (Mastersizer 2000). The proportion of particles with a diameter <0.075mm in each formulation was greater than 50%.
[0031] The absorbance values of the test solutions prepared in Examples 1-3 and Comparative Examples 1-5 were determined by flame atomic absorption spectrophotometry. The concentration of the sample solution was found from the standard working curve, and the content of hexavalent chromium in the soil sample was further calculated. The spiked recovery rate of hexavalent chromium in the three groups of soil samples was calculated, and the results are summarized in Table 2.
[0032] Table 2. Spike recoveries (%) of hexavalent chromium in three soil samples
[0033] As shown in Table 2, the extraction methods provided in Examples 1-3 of this invention exhibited significantly better recoveries of hexavalent chromium than the comparative examples in three typical high-cohesive soils (kaolin-based, illite-based, and montmorillonite-based). Compared to Comparative Example 1, Example 1 showed higher recoveries in all three soils, demonstrating that K... + With its smaller hydration radius, it can effectively displace chromate, resulting in a higher extraction efficiency than Na+. + better.
[0034] Sodium hexametaphosphate, as an anionic dispersant, allows its long-chain polyphosphate ions to adsorb onto the surface of clay particles. Through electrostatic repulsion and steric hindrance, it prevents particle aggregation, allowing the extract to fully penetrate between the fine particles. When the dosage is insufficient (Comparative Example 2), the clay particles remain in a flocculated state, and the desorption sites of hexavalent chromium are physically encapsulated, resulting in a lower recovery rate than in the example. When the dosage is excessive (Comparative Example 3), the ionic strength is too high, compressing the electric double layer and weakening the electrostatic repulsion effect, leading to a decrease in recovery rate.
[0035] Comparative Example 4 shows that fractional centrifugation not only improves the separation throughput, but also avoids the secondary adsorption of hexavalent chromium by fine particles and the filter membrane during the filtration process.
[0036] In terms of pretreatment efficiency and sample throughput, Example 1 only requires 10 minutes for graded centrifugation of a single sample, while Comparative Example 4 requires 20-25 minutes for filtration. Furthermore, the filtration process requires attention to whether the filter membrane is ruptured, resulting in lower batch processing efficiency. Meanwhile, the single-batch processing capacity of ultrasonic and constant temperature water bath oscillation is significantly higher than that of the magnetic heating stirring device in the HJ1082 standard, greatly improving the detection throughput of large batches of soil samples.
[0037] The above data fully verifies the overall technical effect of the "composite alkali dispersion-ultrasound-gradient centrifugation coupling" method of the present invention: sodium hexametaphosphate and K + Synergistically disrupting the colloidal structure and crystalline layer blockage of highly cohesive soils, ultrasound in the absence of Mg 2+ The process involves thorough dispersion under interference, followed by high-throughput, clogging-free separation via staged centrifugation. All three steps are indispensable and together solve the two core problems raised in the background technology: "incomplete desorption of hexavalent chromium in highly viscous soil" and "clogging and low throughput of filtration." This provides a new technical solution for the rapid and accurate detection of large batches of samples from contaminated sites.
[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.
Claims
1. A method for extracting hexavalent chromium from soil based on a combination of compound alkaline solution dispersion and fractional centrifugation, characterized in that, Includes the following steps: S1. After air-drying and grinding the soil sample, pass it through a 100-mesh sieve. Weigh 5.0g of the treated soil sample and place it in a 100mL capped plastic centrifuge tube. Add 50mL of compound alkali solution and shake well. S2. Place the centrifuge tube in an ultrasonic instrument preheated to 60°C and sonicate for 10 minutes. Then add 400 mg MgCl2 and 0.5 mL potassium dihydrogen phosphate-dipotassium hydrogen phosphate buffer solution to the centrifuge tube and shake well. S3. Transfer the centrifuge tubes to a preheated water bath shaker at 93±2℃ and shake at 200 rpm for 50 minutes. S4. After shaking, remove the centrifuge tube and cool it to room temperature. Place it on a centrifuge for centrifugation. The centrifugation conditions are as follows: first centrifuge at 1000 r / min for 5 min, transfer all the supernatant, discard the precipitate, and then centrifuge at 4000 r / min for 5 min. S5. After centrifugation, take all the supernatant and adjust the pH to 7.5±0.5 with nitric acid. Transfer the solution to a 100mL volumetric flask and make up to volume. This is the test solution.
2. The extraction method according to claim 1, characterized in that, In step S1, the composite alkaline solution comprises the following components by weight: 25-35 parts KOH, 28-38 parts Na2CO3, 3-8 parts sodium hexametaphosphate, and 800-1000 parts deionized water, with a pH value ≥12.
5.
3. The extraction method according to claim 1, characterized in that, In step S1, particles with a diameter of less than 0.075 mm account for more than 50% of the total volume of the soil sample.
4. The extraction method according to claim 1, characterized in that, In step S2, the preparation method of potassium dihydrogen phosphate-dipotassium hydrogen phosphate buffer solution is as follows: weigh 87.1 g of dipotassium hydrogen phosphate and 68.0 g of potassium dihydrogen phosphate, dissolve them in an appropriate amount of deionized water, make up to 1 L, shake well, and the pH is 7.0.