Soil hexavalent chromium element determination sample pretreatment method

Abstract

The invention discloses a soil hexavalent chromium element determination sample pretreatment method, and belongs to the technical field of hexavalent chromium detection methods.The method specifically comprises the steps that a sample is collected and stored, a plastic or glass container is used, and a metal product is avoided; preparing a sample, adding an alkaline extracting solution and a buffer solution, heating, stirring, filtering and extracting hexavalent chromium; adjusting the solution, and adjusting the pH value with nitric acid to ensure that the detection environment is suitable. Compared with the prior art, the method has the advantages that the detection effect is obviously better than that of the prior art, the reutilization performance is high, and the method is suitable for efficient detection of soil hexavalent chromium and environmental monitoring.

Description

Technical Field

[0001] The present invention relates to the technical field of hexavalent chromium detection methods, and particularly to a sample pretreatment method for the determination of hexavalent chromium in soil. Background Art

[0002] Hexavalent chromium (Cr 6+ ) in soil is a common environmental pollutant with high toxicity, strong oxidizing property, and bioaccumulation, posing a serious threat to human health and the ecological environment. Therefore, accurately detecting the content of hexavalent chromium in soil is of great significance for environmental monitoring, pollution control, and ecological restoration. However, existing hexavalent chromium detection methods have some deficiencies during the sample pretreatment process, especially in terms of detection efficiency and accuracy.

[0003] When traditional methods extract hexavalent chromium, they usually require multiple steps such as ventilation, heating, acid-base extraction, and heating under reflux. These complex pretreatment steps are not only time-consuming and laborious but also prone to the loss or transformation of hexavalent chromium, thus affecting the accuracy and sensitivity of the detection results. Due to the cumbersome pretreatment steps, the entire detection process takes a long time and is difficult to meet the requirements of rapid detection. For example, some methods require long-term heating and stirring at high temperatures, which not only increases the operation difficulty but also may cause oxidation or reduction reactions of hexavalent chromium, affecting the detection results. During the extraction process, hexavalent chromium is also easily interfered by other ions, resulting in a decrease in the accuracy of the detection results. In addition, during the filtration process of traditional methods, the filter membrane is easily blocked by pollutants, leading to a decrease in flux and further affecting the detection efficiency. When separating hexavalent chromium, a large amount of chemical reagents are often required. These reagents not only have high costs but also may cause secondary pollution to the environment. At the same time, it is difficult to completely remove impurities during the separation process of these methods, resulting in insufficient accuracy of the detection results.

[0004] Chinese invention patent CN111077091A discloses a method for detecting the content of hexavalent chromium in soil. After pretreatment such as ventilation and heating, it can be used to detect hexavalent chromium in soil samples containing a large amount of reducing agents. This method uses iron oxide-based and sulfur-based reducing agents, does not oxidize trivalent chromium, and can maintain an oxidation environment under a certain air flow rate to inhibit the occurrence of reduction reactions; thereby reducing the interference of reducing agents on the alkali digestion results and protecting the digested hexavalent chromium to the greatest extent, so as to quickly and accurately measure the concentration of hexavalent chromium in the solution. The Cr 6+ detection method provided by this invention is simple and convenient to operate, fast in detection, low in cost, suitable for the acceptance detection of chromium-contaminated site restoration, and has wide application value. However, the detection effect of hexavalent chromium by this method is insufficient and the detection method is complex. Summary of the Invention

[0005] To address the deficiencies in the prior art, the present invention proposes a new method for preprocessing soil samples for the determination of hexavalent chromium. By optimizing the preprocessing process and introducing a modified filter membrane, the detection efficiency and accuracy of hexavalent chromium are significantly improved, and at the same time, the service life of the filter membrane is extended.

[0006] To achieve the above-mentioned invention objectives, the present invention adopts the following technical solutions:

[0007] A method for preprocessing soil samples for the determination of hexavalent chromium is as follows:

[0008] Step 1, Sample collection and preservation: Collect soil samples in accordance with the relevant requirements of the "Technical Specifications for Soil Environmental Monitoring" HJ / T166 - 2004. Use plastic or glass devices and containers, and do not use metal storage containers. Air-dry, crush, and sieve the collected samples through a nylon sieve and then store them.

[0009] Step 2, Sample preparation: Accurately weigh the soil sample passed through the nylon sieve into a polytetrafluoroethylene bottle, add an alkaline extraction solution, then add magnesium chloride and potassium hydrogen phosphate - potassium dihydrogen phosphate buffer solution. Tighten the bottle cap, place it on a horizontal shaker, shake at room temperature for 2 - 4 minutes, remove it, place it in an ultrasonic cleaner, set the ultrasonic power to 300 - 500 W, the temperature to 70 - 80 °C, ultrasonically extract for 20 - 40 minutes, remove the polytetrafluoroethylene bottle, cool to room temperature, filter with a filter membrane, and place the filtrate in a polytetrafluoroethylene bottle.

[0010] Step 3, Solution adjustment: Adjust the pH value of the solution to 7 - 8 with 1.2 - 1.5 g / mL nitric acid. If flocculent precipitates are generated during the adjustment process, filter with a filter membrane, transfer the solution to a 50 - 200 mL volumetric flask, make up to the mark with water, shake well, and wait for measurement.

[0011] The pore size of the filter membrane is 0.4 - 0.5 μm.

[0012] Preferably, the method for preprocessing soil samples for the determination of hexavalent chromium is as follows:

[0013] Step 1, Sample collection and preservation: Collect soil samples in accordance with the relevant requirements of the "Technical Specifications for Soil Environmental Monitoring" HJ / T166 - 2004. Use plastic or glass devices and containers, and do not use metal storage containers. Air-dry, crush, and sieve the collected samples through a nylon sieve and then store them.

[0014] Step 2, Sample Preparation: Accurately weigh the soil sample passed through the nylon sieve into a polytetrafluoroethylene bottle, add the alkaline extraction solution, then add magnesium chloride and potassium hydrogen phosphate - potassium dihydrogen phosphate buffer solution, tighten the bottle cap, place it on a horizontal shaker, shake for 2 - 4 min at room temperature, take it off, place it in an ultrasonic cleaner, set the ultrasonic power to 300 - 500 W, the temperature to 70 - 80 °C, perform ultrasonic extraction for 20 - 40 min, take off the polytetrafluoroethylene bottle, cool it to room temperature, filter it with a modified filter membrane, and place the filtrate in a polytetrafluoroethylene bottle;

[0015] Step 3, Solution Adjustment: Adjust the pH value of the solution to 7 - 8 with 1.2 - 1.5 g / mL nitric acid. If flocculent precipitates are produced during the adjustment process, filter it with a modified filter membrane, transfer the solution to a 50 - 200 mL volumetric flask, make up the volume to the mark with water, shake well, and wait for measurement.

[0016] The nylon sieve is 80 - 120 mesh.

[0017] The material - liquid ratio of the soil sample to the alkaline extraction solution is 1 g: 8 - 12 mL.

[0018] The weight ratio of the soil sample to magnesium chloride is 4 - 6: 0.3 - 0.5.

[0019] The volume ratio of the alkaline extraction solution to the potassium hydrogen phosphate - potassium dihydrogen phosphate buffer solution is 40 - 60: 0.4 - 0.6.

[0020] The alkaline extraction solution is prepared by dissolving 30 g of sodium carbonate and 20 g of sodium hydroxide in water, diluting and making up the volume to 1 L.

[0021] The potassium hydrogen phosphate - potassium dihydrogen phosphate buffer solution is prepared by dissolving 87.1 g of potassium hydrogen phosphate and 68.0 g of potassium dihydrogen phosphate in water, diluting and making up the volume to 1 L.

[0022] The preparation method of the modified filter membrane is as follows, by weight:

[0023] S1. Weigh 0.03 - 0.05 parts of zinc oxide, 3 - 5 parts of polyether ether ketone, and 0.3 - 0.5 parts of polyacrylamide. Add them to 30 - 50 parts of dimethyl sulfoxide, stir for 5 - 24 hours to obtain a casting solution. Filter it, then let the filtered casting solution stand for 10 - 30 hours. Subsequently, place it in a vacuum oven and conduct degassing treatment at 50 - 70 °C until complete degassing. After degassing is completed, pour the casting solution onto a glass plate covered with polypropylene non-woven fabric, and use a clean scraper to scrape it into a film. Under room temperature conditions, let the casting solution evaporate in the air for 30 - 50 seconds, then immerse it in water to form a film. Replace the water bath every 5 - 10 hours, and replace it 2 - 4 times in total. Store the obtained film in distilled water for later use to obtain a preformed film with a film thickness of 0.2 - 0.3 mm and a pore size of 0.4 - 0.5 μm.

[0024] S2. Add 8 - 12 parts of sodium alginate to 40 - 60 parts of water, place it under the condition of heating and stirring at 80 - 95 °C for 3 - 8 hours. Subsequently, add 5 - 10 parts of borax and 150 - 200 parts of water to form a mixed solution. Immerse the preformed film prepared in step S1 into this mixed solution. After maintaining it for 5 - 10 hours, take out the film and wash the film surface with water to obtain a modified filter membrane.

[0025] In the present invention, the functions of each substance are as follows:

[0026] The soil sample is used as the detection object, and the hexavalent chromium element in it is extracted through a pretreatment method for subsequent content determination.

[0027] The nylon sieve is used to screen the soil sample, remove larger particles, ensure the uniformity and fineness of the sample, and facilitate subsequent processing.

[0028] The alkaline extraction solution provides an alkaline environment, which helps in the extraction of hexavalent chromium, making hexavalent chromium exist in a dissolved state in the extraction solution, facilitating subsequent filtration and detection.

[0029] Magnesium chloride serves as an auxiliary extraction agent, which helps to improve the extraction efficiency of hexavalent chromium and enhance the adsorption capacity of the extraction solution for hexavalent chromium.

[0030] The potassium hydrogen phosphate - potassium dihydrogen phosphate buffer solution maintains the stability of the pH value of the extraction solution, ensures that hexavalent chromium maintains a stable form of existence during the extraction process, and prevents it from undergoing chemical changes during the extraction process.

[0031] The polyethylene film is used for sealing to prevent the loss of volatile substances during the extraction process and ensure the sealing of the reaction system.

[0032] The present invention utilizes the ultrasonic function of an ultrasonic cleaner and the certain pressure generated during the heating process to ensure complete extraction of hexavalent chromium in soil, shorten the extraction time of the original method, is applicable to the detection of batch samples, and effectively improves the detection efficiency.

[0033] The filter membrane is used to filter the extract, remove insoluble impurities, ensure the purity of the filtrate, and facilitate subsequent detection.

[0034] Nitric acid is used to adjust the pH value of the solution to 7 - 8 to ensure that the solution is in a neutral or weakly acidic environment, which is suitable for the detection of hexavalent chromium.

[0035] The modified filter membrane provides high - efficient filtration performance, enhances the adsorption and separation ability of hexavalent chromium, and at the same time has self - cleaning and self - repair functions, extending the service life of the filter membrane.

[0036] Zinc oxide improves the hydrophilicity and self - cleaning ability of the filter membrane, can degrade the pollutants attached to the filter membrane, and reduce membrane fouling.

[0037] Polyetheretherketone, as the matrix material, provides good mechanical properties and chemical stability, ensuring the structural integrity and durability of the filter membrane.

[0038] Polyacrylamide, as an additive, improves the rheological properties of the casting solution and enhances the film - forming performance and uniformity of the filter membrane.

[0039] Dimethyl sulfoxide, as a solvent, is used to dissolve polyetheretherketone and polyacrylamide to prepare a uniform casting solution.

[0040] Polypropylene non - woven fabric, as a support material, is used for casting the filter membrane, provides mechanical support, and ensures the structural stability of the filter membrane.

[0041] Sodium alginate and borax form a hydrogel, which fills the pores of the filter membrane, endows the filter membrane with self - repair performance, and extends the service life of the filter membrane.

[0042] Borax and sodium alginate form a cross - linked network, enhancing the stability and self - repair ability of the hydrogel.

[0043] Water, as a solvent, is used to prepare various solutions and clean the filter membrane to ensure the smooth progress of the experiment.

[0044] Through the synergistic effect of these substances, the present invention realizes efficient and accurate detection of hexavalent chromium elements, and significantly improves the service life and reusability of the filter membrane.

[0045] By introducing zinc oxide particles with photocatalytic activity, the present invention significantly improves the hydrophilicity of the modified filter membrane, enabling it to efficiently degrade the attached pollutants, thereby greatly enhancing the flux and separation performance of the filter membrane, ensuring the full separation of hexavalent chromium and its entry into the filtrate for subsequent detection. Although the degradation process may cause slight aging of the filter membrane matrix material, the present invention ingeniously uses the hydrogel formed by sodium alginate and borax to fill the pores of the filter membrane. With its water absorption and swelling properties, self-repair of the damaged part of the filter membrane is achieved, restoring the flux and rejection rate to the initial level and endowing the filter membrane with self-repair function. This modified filter membrane not only has excellent separation performance but also outstanding self-cleaning, anti-pollution, anti-aging, and self-repair capabilities, significantly extending its service life and having broad application prospects in the detection field.

[0046] Compared with the prior art, it has the following beneficial effects:

[0047] 1) By optimizing the sample pretreatment method, especially by introducing inorganic zinc oxide particles, the present invention not only enhances the filtration effect of hexavalent chromium, enabling it to enter the filtrate more effectively, but also improves the detection sensitivity, so that the concentration of hexavalent chromium can be more accurately detected in the final content determination.

[0048] 2) The activity of zinc oxide in the present invention enables it to degrade the pollutants attached to the filter membrane, reduce membrane pollution, and enhance the separation performance and self-cleaning ability of the filter membrane. This self-cleaning function effectively extends the service life of the filter membrane, reduces the detection error caused by membrane pollution, and improves the stability and reliability of detection.

[0049] 3) The present invention introduces the hydrogel formed by sodium alginate and borax, which can fully fill the pores of the modified filter membrane. Utilizing its water absorption and swelling properties, the hydrogel can self-repair the damaged part after the membrane is damaged, restoring the water flux and rejection rate of the membrane, thereby endowing the filter membrane with self-repair performance. This innovative design greatly extends the service life of the filter membrane, reduces the detection cost, and improves the detection efficiency. Specific embodiments

[0050] Main sources of substances:

[0051] Polyetheretherketone, grade: 450FC30, material: PEEK, brand: Victrex, UK.

[0052] Zinc oxide, particle size: 50 nm, purchased from Shanghai Nano Technology Co., Ltd.

[0053] Polyacrylamide, anionic, molecular weight: 6 million, mesh number: 60 - 80, purchased from Henan Chuangyi Environmental Protection Technology Co., Ltd.

[0054] Polypropylene non-woven fabric, gram weight: 200 g / m 2 , Wujiang Chenghu Non-woven Fabric Co., Ltd.

[0055] Borax, item number: 01, Jinan Yongtai Chemical Co., Ltd.

[0056] Yeast β-glucan, model: SG90, Angel Yeast Co., Ltd.

[0057] Polyvinyl alcohol, model: 117, brand: Kuraray, origin: Japan.

[0058] The remaining raw materials in the examples and comparative examples of the present invention are all commercially available products.

[0059] The design idea of the present invention is to improve the detection efficiency and accuracy of hexavalent chromium by optimizing the pretreatment method of soil hexavalent chromium element determination samples, and at the same time extend the service life of the filter membrane. Specifically, the present invention introduces inorganic zinc oxide particles with photocatalytic activity. These particles not only improve the hydrophilicity of the modified filter membrane, enhance the filtration ability of hexavalent chromium ions, but also can degrade the pollutants attached to the filter membrane, reduce membrane fouling, and improve the separation performance and self-cleaning ability of the filter membrane. In addition, the present invention also introduces a hydrogel formed by sodium alginate and borax. This hydrogel can fully fill the pores of the modified filter membrane. Using its water absorption and swelling properties, it can self-repair the damaged parts after the membrane is damaged, restore the water flux and rejection rate of the membrane, thereby endowing the filter membrane with self-repair performance and greatly extending the service life of the filter membrane. Through these innovative designs, the present invention not only improves the detection effect of hexavalent chromium, but also significantly enhances the reuse performance of the filter membrane, providing strong technical support for the efficient detection of soil hexavalent chromium and environmental monitoring.

[0060] Example 1

[0061] A pretreatment method for soil hexavalent chromium element determination samples is as follows:

[0062] Step 1, sample collection and preservation: Collect soil samples according to the relevant requirements of the "Technical Specification for Soil Environmental Monitoring" (HJ / T166). Use plastic devices and containers, and do not use metal storage containers. Air-dry, crush, and pass the collected samples through a nylon sieve (100 mesh) before preservation;

[0063] Step 2, Sample Preparation: Accurately weigh 5.0 g of the soil sample passed through a nylon sieve into a 100 mL polytetrafluoroethylene bottle, add 50.0 mL of the alkaline extraction solution. The alkaline extraction solution is prepared by dissolving 30 g of sodium carbonate and 20 g of sodium hydroxide in water and diluting and making up the volume to 1 L; add 400 mg of magnesium chloride and 0.5 mL of potassium hydrogen phosphate - potassium dihydrogen phosphate buffer solution. The potassium hydrogen phosphate - potassium dihydrogen phosphate buffer solution is prepared by dissolving 87.1 g of potassium hydrogen phosphate and 68.0 g of potassium dihydrogen phosphate in water and diluting and making up the volume to 1 L. Tighten the bottle cap, place it on a horizontal shaker, shake at room temperature for 3 min, take it down, place it in an ultrasonic cleaner, set the ultrasonic power to 400 W and the temperature to 75 °C, perform ultrasonic extraction for 30 min, take down the polytetrafluoroethylene bottle, cool it to room temperature, filter it by suction with a 0.45 μm filter membrane, and place the filtrate in a 250 mL polytetrafluoroethylene bottle;

[0064] Step 3, Solution Adjustment: Adjust the pH value of the solution to 7.5 with 1.42 g / mL nitric acid. If flocculent precipitates are produced during the adjustment process, filter it with a 0.45 μm filter membrane, transfer the solution to a 100 mL volumetric flask, make up the volume to the mark with water, shake well, and wait for measurement.

[0065] Example 2

[0066] A method for pre - treating samples for the determination of hexavalent chromium in soil is as follows:

[0067] Step 1, Sample Collection and Preservation: Collect soil samples in accordance with the relevant requirements of the "Technical Specifications for Soil Environmental Monitoring" (HJ / T166). Use plastic devices and containers, and do not use metal storage containers. Air - dry, crush, and pass the collected samples through a nylon sieve (100 mesh) and then store them;

[0068] Step 2, Sample Preparation: Accurately weigh 5.0 g of the soil sample passed through a nylon sieve into a 100 mL polytetrafluoroethylene bottle, add 50.0 mL of the alkaline extraction solution. The alkaline extraction solution is prepared by dissolving 30 g of sodium carbonate and 20 g of sodium hydroxide in water and diluting and making up the volume to 1 L; add 400 mg of magnesium chloride and 0.5 mL of potassium hydrogen phosphate - potassium dihydrogen phosphate buffer solution. The potassium hydrogen phosphate - potassium dihydrogen phosphate buffer solution is prepared by dissolving 87.1 g of potassium hydrogen phosphate and 68.0 g of potassium dihydrogen phosphate in water and diluting and making up the volume to 1 L. Tighten the bottle cap, place it on a horizontal shaker, shake at room temperature for 3 min, take it down, place it in an ultrasonic cleaner, set the ultrasonic power to 400 W and the temperature to 75 °C, perform ultrasonic extraction for 30 min, take down the polytetrafluoroethylene bottle, cool it to room temperature, filter it by suction with a modified filter membrane, and place the filtrate in a 250 mL polytetrafluoroethylene bottle;

[0069] Step 3, Solution Adjustment: Adjust the pH value of the solution to 7.5 with 1.42 g / mL nitric acid. If flocculent precipitates form during the adjustment process, filter with a modified filter membrane, transfer the solution to a 100 mL volumetric flask, make up the volume to the mark with water, shake well, and wait for measurement.

[0070] The preparation method of the modified filter membrane is as follows:

[0071] S1. Weigh 0.04 g of zinc oxide, 4 g of polyether ether ketone, and 0.4 g of polyacrylamide, add them to 40 g of dimethyl sulfoxide, stir for 12 hours to obtain a casting solution, filter, then let the filtered casting solution stand for 24 hours, and then place it in a vacuum oven for degassing at 60 °C until complete degassing. After degassing, pour the casting solution onto a glass plate covered with polypropylene non-woven fabric, use a clean scraper to scrape into a film, and keep the casting solution evaporating in the air for 40 seconds at room temperature, then immerse it in water to form a film. Replace the water bath every 8 hours, for a total of 3 replacements. Keep the prepared film in distilled water for later use to obtain a preformed film with a film thickness of 0.25 mm and a pore size of 0.45 μm;

[0072] S2. Add 10 g of sodium alginate to 50 g of water, heat and stir at 90 °C for 5 hours, then add 7 g of borax and 180 g of water to form a mixed solution. Immerse the preformed film prepared in step S1 in this mixed solution, keep it for 8 hours, then take out the film and wash the film surface with water to obtain a modified filter membrane.

[0073] Example 3

[0074] A method for pretreating samples for the determination of hexavalent chromium in soil is basically the same as that in Example 1, except that the preparation method of the modified filter membrane is different.

[0075] The preparation method of the modified filter membrane is as follows:

[0076] S1. Weigh 0.04 g of zirconium oxide, 4 g of polyether ether ketone, and 0.4 g of polyacrylamide, add them to 40 g of dimethyl sulfoxide, stir for 12 hours to obtain a casting solution, filter, then let the filtered casting solution stand for 24 hours, and then place it in a vacuum oven for degassing at 60 °C until complete degassing. After degassing, pour the casting solution onto a glass plate covered with polypropylene non-woven fabric, use a clean scraper to scrape into a film, and keep the casting solution evaporating in the air for 40 seconds at room temperature, then immerse it in water to form a film. Replace the water bath every 8 hours, for a total of 3 replacements. Keep the prepared film in distilled water for later use to obtain a preformed film with a film thickness of 0.25 mm and a pore size of 0.45 μm;

[0077] S2. Add 10 g of sodium alginate to 50 g of water, heat and stir at 90 °C for 5 hours, then add 7 g of borax and 180 g of water to form a mixed solution. Immerse the preformed membrane prepared in step S1 into this mixed solution. After maintaining for 8 hours, take out the membrane and wash the surface of the membrane with water to obtain a modified filter membrane.

[0078] Example 4

[0079] A method for pretreating a sample for the determination of hexavalent chromium in soil is basically the same as that in Example 1, and the only difference is the preparation method of the modified filter membrane.

[0080] The preparation method of the modified filter membrane is as follows:

[0081] S1. Weigh 0.04 g of alumina, 4 g of polyetheretherketone, and 0.4 g of polyacrylamide, add them to 40 g of dimethyl sulfoxide, stir for 12 hours to obtain a casting solution, filter it, and then let the filtered casting solution stand for 24 hours. Subsequently, place it in a vacuum oven and perform degassing treatment at 60 °C until complete degassing. After degassing is completed, pour the casting solution onto a glass plate covered with polypropylene non-woven fabric, use a clean scraper to scrape it into a film, and keep the casting solution evaporating in the air for 40 seconds at room temperature. Then immerse it in water to form a film. Replace the water bath every 8 hours, and replace it 3 times in total. Keep the prepared film in distilled water for later use to obtain a preformed membrane with a film thickness of 0.25 mm and a pore size of 0.45 μm.

[0082] S2. Add 10 g of sodium alginate to 50 g of water, heat and stir at 90 °C for 5 hours, then add 7 g of borax and 180 g of water to form a mixed solution. Immerse the preformed membrane prepared in step S1 into this mixed solution. After maintaining for 8 hours, take out the membrane and wash the surface of the membrane with water to obtain a modified filter membrane.

[0083] Example 5

[0084] A method for pretreating a sample for the determination of hexavalent chromium in soil is basically the same as that in Example 1, and the only difference is the preparation method of the modified filter membrane.

[0085] The preparation method of the modified filter membrane is as follows:

[0086] S1. Weigh 0.04 g of zinc oxide, 4 g of polyether ether ketone, and 0.4 g of polyacrylamide, add them to 40 g of dimethyl sulfoxide, stir for 12 hours to obtain a casting solution, filter it, then let the filtered casting solution stand for 24 hours, and then place it in a vacuum oven for degassing at 60 °C until complete degassing. After degassing is completed, pour the casting solution onto a glass plate covered with polypropylene non-woven fabric, and use a clean scraper to make a film. At room temperature, let the casting solution evaporate in the air for 40 seconds, and then immerse it in water to form a film. Replace the water bath every 8 hours, for a total of 3 replacements. Store the obtained film in distilled water for later use to obtain a preformed film with a film thickness of 0.25 mm and a pore size of 0.45 μm.

[0087] S2. Add 10 g of yeast β-glucan to 50 g of water, heat and stir at 90 °C for 5 hours, then add 7 g of borax and 180 g of water to form a mixed solution. Immerse the preformed film prepared in step S1 in this mixed solution. After 8 hours, take out the film and wash the film surface with water to obtain a modified filter membrane.

[0088] Example 6

[0089] A method for pretreating samples for the determination of hexavalent chromium in soil is basically the same as that in Example 1, and the only difference lies in the different preparation methods of the modified filter membrane.

[0090] The preparation method of the modified filter membrane is as follows:

[0091] S1. Weigh 0.04 g of zinc oxide, 4 g of polyether ether ketone, and 0.4 g of polyacrylamide, add them to 40 g of dimethyl sulfoxide, stir for 12 hours to obtain a casting solution, filter it, then let the filtered casting solution stand for 24 hours, and then place it in a vacuum oven for degassing at 60 °C until complete degassing. After degassing is completed, pour the casting solution onto a glass plate covered with polypropylene non-woven fabric, and use a clean scraper to make a film. At room temperature, let the casting solution evaporate in the air for 40 seconds, and then immerse it in water to form a film. Replace the water bath every 8 hours, for a total of 3 replacements. Store the obtained film in distilled water for later use to obtain a preformed film;

[0092] S2. Add 10 g of hydroxyethyl cellulose to 50 g of water, heat and stir at 90 °C for 5 hours, then add 7 g of borax and 180 g of water to form a mixed solution. Immerse the preformed film prepared in step S1 in this mixed solution. After 8 hours, take out the film and wash the film surface with water to obtain a modified filter membrane.

[0093] Example 7

[0094] A method for pre - treating samples for the determination of hexavalent chromium in soil is basically the same as Example 1, and the only difference lies in the preparation method of the modified filter membrane.

[0095] The preparation method of the modified filter membrane is as follows:

[0096] S1. Weigh 0.04 g of zinc oxide, 4 g of polyetheretherketone, and 0.4 g of polyacrylamide, add them to 40 g of dimethyl sulfoxide, stir for 12 hours to obtain a casting solution, filter it, then let the filtered casting solution stand for 24 hours. Subsequently, place it in a vacuum oven and perform degassing treatment at 60 °C until complete degassing. After degassing is completed, pour the casting solution onto a glass plate covered with polypropylene non - woven fabric, use a clean scraper to scrape it into a film. At room temperature, let the casting solution evaporate in the air for 40 seconds, then immerse it in water to form a film. Replace the water bath every 8 hours, for a total of 3 replacements. Store the obtained film in distilled water for later use to obtain a pre - formed film;

[0097] S2. Add 10 g of polyvinyl alcohol to 50 g of water, heat and stir at 90 °C for 5 hours. Subsequently, add 7 g of borax and 180 g of water to form a mixed solution. Immerse the pre - formed film prepared in step S1 into this mixed solution. After 8 hours, take out the film and wash the film surface with water to obtain the modified filter membrane.

[0098] Comparative Example 1

[0099] Preparation of blank sample

[0100] Blank sample: Without adding soil samples, prepare blank samples according to the same steps as in Example 1 for blank tests in subsequent analyses.

[0101] Test Example 1

[0102] Determination of the content of hexavalent chromium in soil samples

[0103] Determine the dry matter content of soil samples according to HJ613, and determine the moisture content of sediment samples according to GB17378.5. The soil samples are collected from Gansu Geological Mine.

[0104] Respectively pipette 0 ml, 0.10 ml, 0.20 ml, 0.50 ml, 1.00 ml, and 2.00 ml of the hexavalent chromium standard stock solution into 250-ml polytetrafluoroethylene bottles, and prepare working curve solutions according to the steps of sample preparation. 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 adjust the instrument zero point, and measure the absorbance in ascending order of concentration. Establish a working curve with the hexavalent chromium concentration as the abscissa and the absorbance as the ordinate. Perform the determination of the sample under the same analytical conditions as those for the establishment of the working curve.

[0105] The content w (mg / kg) of hexavalent chromium in the soil sample is calculated according to formula (1):

[0106] w = (ρ × V × D) / (m × W dm ) (1)

[0107] In the formula:

[0108] w - the content of hexavalent chromium in the soil sample, mg / kg;

[0109] ρ - the concentration of hexavalent chromium in the sample, mg / L;

[0110] V - the volume of sample constant volume, mL;

[0111] D - the dilution factor of the sample;

[0112] m - the weight of the soil sample taken, g;

[0113] W dm - the dry matter content of the soil sample, %.

[0114] For each example, 20 samples are tested respectively, and the average value is taken. The specific test data are shown in Table 1.

[0115] Table 1

[0116] Experimental Scheme Content of hexavalent chromium w (mg / kg) Example 1 31.2 Example 2 32.7 Example 3 32.1 Example 4 32.3 Example 5 32.3 Example 6 32.0 Example 7 31.9

[0117] Test Example 2

[0118] Homogeneity test

[0119] Repeatedly use the method of Test Example 1 to perform sample pretreatment for the determination of soil hexavalent chromium elements on the filter membrane or modified filter membrane in the example. The number of repetitions is 30 times, and the variance of the data is calculated to evaluate the reusability of the filter membrane in the sample pretreatment for determination. The smaller the variance, the stronger the reusability of the filter membrane in the sample pretreatment for determination.

[0120] The variance is calculated according to formula (2):

[0121] var = Σ(x - μ) 2 / N (2)

[0122] Among them, var represents variance, Σ represents summation, x represents each data point, μ represents the average value of all data points, and N represents the number of data points in the dataset. The test results are shown in Table 2.

[0123] Table 2

[0124] Experimental Scheme Variance Example 1 0.972 Example 2 0.417 Example 3 0.561 Example 4 0.513 Example 5 0.506 Example 6 0.486 Example 7 0.483

[0125] From the data of Test Examples 1 - 2, it can be seen that the sample pretreatment method for the determination of hexavalent chromium in soil in Example 2 has the best detection effect on hexavalent chromium and the best performance of repeated utilization of the modified filter membrane obtained.

[0126] In the present invention, the modified filter membrane prepared with zinc oxide in Example 2 shows the best detection effect and repeated utilization performance in the sample pretreatment for the determination of hexavalent chromium in soil. Zinc oxide has a high specific surface area and good hydrophilicity, which can significantly improve the hydrophilicity of the modified filter membrane, thereby enhancing the filtering ability for hexavalent chromium ions, enabling hexavalent chromium to enter the filtrate and be detected in the final content determination, improving the detection sensitivity. In addition, zinc oxide can degrade the pollutants attached to the filter membrane, reduce membrane fouling, and further enhance the separation performance and self-cleaning ability of the filter membrane. In contrast, although zirconia and alumina also have certain adsorption capacity and activity, their specific surface areas are relatively small, and the surface charge distribution under alkaline conditions is not as uniform as that of zinc oxide, resulting in a weaker filtering ability for hexavalent chromium. Therefore, the modified filter membrane prepared with zinc oxide is more excellent in terms of detection effect and repeated utilization performance, can separate hexavalent chromium to the maximum extent, enabling it to be accurately detected in the final content determination, and at the same time, by introducing the hydrogel formed by sodium alginate and borax, endowing the filter membrane with self-repairing performance, greatly extending the service life of the filter membrane.

[0127] In the present invention, the modified filter membrane prepared with sodium alginate in Example 2 exhibits the best detection effect and reusability performance in the pretreatment of soil hexavalent chromium element determination samples. The hydrogel formed by sodium alginate and borax has good water absorption and swelling properties and can fully fill the pores of the modified filter membrane. This hydrogel can automatically fill the damaged parts of the membrane after the membrane is damaged, restoring the water flux and rejection rate of the membrane to the original level, thereby endowing the filter membrane with self-healing performance. In contrast, although yeast β-glucan, hydroxyethyl cellulose, and polyvinyl alcohol also have certain film-forming and cross-linking properties, the water absorption and swelling properties of their hydrogels are not as good as those of sodium alginate. Therefore, they perform slightly weaker in terms of self-healing performance and reusability performance. In addition, sodium alginate has good chemical stability and biocompatibility, can maintain stable performance during multiple uses, and further improves the service life and detection effect of the modified filter membrane.

Claims

1. A soil hexavalent chromium element determination sample pretreatment method, characterized in that: Here’s how: Step 1. Sample collection and storage: Soil samples shall be collected in accordance with the relevant requirements of the Technical Specifications for Soil Environmental Monitoring HJ / T166-2004. Plastic or glass devices and containers shall be used, and metal storage containers shall not be used. The collected samples shall be air-dried, crushed, and stored after being sieved through a nylon sieve in the laboratory. Step 2, sample preparation: accurately weigh the soil sample after passing through a nylon sieve into a polytetrafluoroethylene bottle, add an alkaline extract, then add magnesium chloride and dipotassium hydrogen phosphate-potassium dihydrogen phosphate buffer solution, tighten the bottle cap, place it on a horizontal oscillator, oscillate at room temperature for 2 to 4 minutes, remove it, place it in an ultrasonic cleaning machine, set the ultrasonic power to 300 to 500 W, the temperature to 70 to 80°C, ultrasonically extract for 20 to 40 minutes, remove the polytetrafluoroethylene bottle, cool it to room temperature, filter it with a filter membrane, and place the filtrate in a polytetrafluoroethylene bottle; Step 3, solution adjustment: Use 1.2-1.5 g / mL nitric acid to adjust the pH value of the solution to 7-8. If flocculent precipitation is produced during the adjustment process, it is necessary to filter it with a filter membrane. Transfer the solution to a 50-200 mL volumetric flask, dilute to the mark with water, shake well, and test.

2. The soil hexavalent chromium element determination sample pretreatment method according to claim 1, characterized in that: The pore size of the filter membrane is 0.4-0.5 μm.

3. The soil hexavalent chromium element determination sample pretreatment method according to claim 1, characterized in that: The soil hexavalent chromium element determination sample pretreatment method is as follows: Step 1. Sample collection and storage: Soil samples shall be collected in accordance with the relevant requirements of the Technical Specifications for Soil Environmental Monitoring HJ / T166-2004. Plastic or glass devices and containers shall be used, and metal storage containers shall not be used. The collected samples shall be air-dried, crushed, and stored after being sieved through a nylon sieve in the laboratory. Step 2, sample preparation: accurately weigh the soil sample after passing through a nylon sieve into a polytetrafluoroethylene bottle, add an alkaline extract, then add magnesium chloride and dipotassium hydrogen phosphate-potassium dihydrogen phosphate buffer solution, tighten the bottle cap, place it on a horizontal oscillator, oscillate at room temperature for 2 to 4 minutes, remove it, place it in an ultrasonic cleaning machine, set the ultrasonic power to 300 to 500 W, the temperature to 70 to 80°C, perform ultrasonic extraction for 20 to 40 minutes, remove the polytetrafluoroethylene bottle, cool it to room temperature, filter it with a modified filter membrane, and place the filtrate in a polytetrafluoroethylene bottle; Step 3, solution adjustment: Use 1.2-1.5 g / mL nitric acid to adjust the pH value of the solution to 7-8. If flocculent precipitation is produced during the adjustment process, it is necessary to filter with a modified filter membrane. Transfer the solution to a 50-200 mL volumetric flask, dilute to the mark with water, shake well, and test.

4. The soil hexavalent chromium element determination sample pretreatment method according to claim 1 or 3, characterized in that: The nylon sieve has a mesh size of 80 to 120.

5. The method for pretreating soil hexavalent chromium samples according to claim 1 or 3, characterized in that: The solid-liquid ratio of the soil sample to the alkaline extract is 1 g: 8-12 mL.

6. The soil hexavalent chromium element determination sample pretreatment method according to claim 1 or 3, characterized in that: The weight ratio of the soil sample to magnesium chloride is 4-6:0.3-0.

5.

7. The method for pretreating soil hexavalent chromium samples according to claim 1 or 3, characterized in that: The volume ratio of the alkaline extract to the dipotassium hydrogen phosphate-potassium dihydrogen phosphate buffer solution is 40-60:0.4-0.

6.

8. The soil hexavalent chromium element determination sample pretreatment method according to claim 1 or 3, characterized in that: The alkaline extract is prepared by dissolving 30 g of sodium carbonate and 20 g of sodium hydroxide in water and diluting the solution to 1 L.

9. The soil hexavalent chromium element determination sample pretreatment method according to claim 1 or 3, characterized in that: The dipotassium hydrogen phosphate-potassium dihydrogen phosphate buffer solution is prepared by dissolving 87.1 g of dipotassium hydrogen phosphate and 68.0 g of potassium dihydrogen phosphate in water and diluting the solution to a constant volume of 1 L.

10. The soil hexavalent chromium element determination sample pretreatment method according to claim 3, characterized in that: The preparation method of the modified filter membrane is as follows, in parts by weight: S1, weigh 0.03-0.05 parts of zinc oxide, 3-5 parts of polyetheretherketone and 0.3-0.5 parts of polyacrylamide, add them to 30-50 parts of dimethyl sulfoxide, stir for 5-24 hours to obtain a casting solution, filter, and then let the filtered casting solution stand for 10-30 hours, then place it in a vacuum oven, degas it at 50-70°C until it is completely degassed, after degassing, cast the casting solution on a glass plate covered with a polypropylene non-woven fabric, use a clean scraper to scrape it into a film, at room temperature, let the casting solution evaporate in the air for 30-50 seconds, then immerse it in water to form a film, replace the water bath at intervals of 5-10 hours, and replace it 2-4 times in total, store the obtained film in distilled water for standby use, and obtain a preformed film with a film thickness of 0.2-0.3 mm and a pore size of 0.4-0.5 μm; S2. Add 8 to 12 parts of sodium alginate to 40 to 60 parts of water, heat and stir at 80 to 95° C. for 3 to 8 hours, then add 5 to 10 parts of borax and 150 to 200 parts of water to form a mixed solution, immerse the preformed membrane prepared in step S1 in the mixed solution, keep it for 5 to 10 hours, take out the membrane, wash the membrane surface with water, and obtain a modified filter membrane.