Soil micro-plastic rapid step-by-step flotation method capable of recycling reagent

Abstract

The invention discloses a rapid step-by-step flotation method for soil micro-plastics capable of recycling reagents. The method comprises a micro-plastic flotation separation step, a flotation liquid recovery step, an organic matter digestion removal step and a digestion liquid recovery step. According to the method, a sodium chloride solution with the density being 1.2 g / cm < 3 > and a sodium bromide solution with the density being 1.55 g / cm < 3 > are adopted as flotation solutions during flotation separation of the micro-plastics, and a digestion solution adopted during digestion is a Fenton reagent composed of H2O2 with the mass fraction being 30% and ferrous sulfate heptahydrate; the microplastics are separated and purified through membrane filtration, and reagents used in the process are recycled, so that the method is green and environment-friendly; and the equipment cost is low, and wide application is facilitated.

Description

Technical Field

[0001] The present invention belongs to the technical field of soil microplastic separation, and in particular, relates to a soil microplastic rapid step-by-step flotation method with recyclable reagents. Background Art

[0002] Microplastics generally refer to plastic particles smaller than 5 mm in size. They have high stability, small particle size and strong mobility in the environment. They can exist in the soil for a long time and be transmitted through the food chain, posing a potential threat to the environment and human health.

[0003] Existing microplastic separation technologies include density separation, pressurized fluid extraction, oil extraction, magnetic separation, digestion, etc. Among them, density separation uses the difference in density between microplastics and soil for separation. It is simple to operate but may not effectively remove organic matter and separate small plastic particles (less than 50μm); pressurized fluid extraction has a high degree of automation, low cost, and high efficiency, but it may damage the structure of microplastics; oil extraction and magnetic separation are relatively limited in application, and their applicability to soil samples needs to be studied; digestion methods of different intensities will cause different degrees of damage to the structure of microplastics, and enhancing the efficiency of organic matter digestion often comes at the expense of microplastic recovery. In addition, quantitative methods for identifying microplastics include visual identification with a microscope, spectral analysis methods (such as infrared spectroscopy and Raman spectroscopy), and thermal analysis methods. When these methods are applied to the identification of soil microplastics, they have problems such as long time consumption, limited microplastic size and sample size, or damage to the structure of microplastics.

[0004] Chinese patent application CN116060299A discloses a soil microplastic removal device based on airflow suspension separation-transportation technology and its use method, which includes a soil collecting device, a soil microplastic removal device and a microplastic collecting device; the soil collecting device includes a soil collecting device flip cover connecting shaft, a soil collecting device flip cover, and a microplastic blower; the soil microplastic removal device includes a soil microplastic removal device flip cover connecting shaft, a mixture discharge port, a soil microplastic removal device flip cover, a soil microplastic separation blower, a delivery pipeline, a visual display, etc.; the microplastic collection device includes a microplastic collection device flip cover connecting shaft, a microplastic collection device flip cover and a soil blower; the cost of its microplastic removal device is too high, and the initial investment is relatively large, which is not conducive to wide promotion.

[0005] Chinese patent application CN114273379A discloses a soil microplastic continuous flow separation flotation system, which realizes the continuous separation of soil microplastics 1-5mm through processes such as soil screening, liquid flotation overflow, microplastic screening and enrichment, and flotation liquid collection and circulation. Under the optimal parameters, the recovery rate of soil microplastics (1-5mm) is not less than 80%, and the recovery rate of microplastics is relatively low. Summary of the invention

[0006] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a soil microplastic rapid step-by-step flotation device and method with recyclable reagents, which can effectively improve the recovery of microplastics in soil, and achieve organic matter removal and flotation liquid recovery. The present invention separates and purifies microplastics through membrane filtration, and recycles and reuses the reagents used in the process, which is green and environmentally friendly; the equipment cost is also low, which is more conducive to wide application.

[0007] The technical solution of the present invention is specifically described as follows.

[0008] The present invention provides a method for rapid step-by-step sorting of soil microplastics with recyclable reagents, which adopts a soil microplastic rapid step-by-step flotation device for rapid step-by-step flotation; the device comprises an overflow column, a first peristaltic pump, a second peristaltic pump, a magnetic stirrer, a separator, an electric rotator, a scraper, a Buchner funnel filter, a vacuum pump and a glass sand core filter; a first filter membrane is laid in the Buchner funnel filter, and a second filter membrane is laid in the glass sand core filter; the method comprises the following steps: Step 1: Add a magnetic rotor to the overflow column, start the first peristaltic pump, add flotation liquid to the overflow column, and when the height of the flotation liquid is close to the water outlet of the overflow column, close the first peristaltic pump, take a quantitative dried soil sample and place it in the overflow column, start the magnetic stirrer, stir alternately at fast and slow speeds, then turn off the magnetic stirrer and let it stand; Step 2: Turn on the first peristaltic pump, slowly add the flotation liquid until it is flush with the liquid surface of the overflow column, then turn it off and let it stand, turn on the first peristaltic pump again, and pump the flotation liquid into the overflow column at a lower flow rate. At the same time, turn on the electric rotator, rotate the scraper to overflow the microplastics and flotation liquid floating on the upper layer to the separator, and then turn off the first peristaltic pump after the microplastics are completely transferred to the separator; Step 3: Start the second peristaltic pump to extract the microplastics in the separator into the Buchner funnel filter for filtration, use a cleaning solution to rinse the microplastics on the wall until all the microplastics are transferred, and recover the filtrate to make a usable solution, thereby completing the recycling of the flotation liquid; Step 4: Take out the first filter membrane, rinse the microplastic mixture on the filter membrane into an external beaker with pure water, and add digestion solution for digesting organic matter into the beaker for digestion. Step 5. After the digestion is completed, add the clarified solution in the beaker into the filter head of the glass sand core filter, turn on the vacuum pump, filter and rinse with cleaning solution to achieve the separation and collection of microplastics on the second filter membrane, make the filtrate into a usable solution, and complete the recycling of the digestion solution.

[0009] In the present invention, in step 1, the flotation solution has a density of 1.2 g / cm 3 The sodium chloride solution with a density of 1.55 g / cm 3 Sodium bromide solution.

[0010] In the present invention, in step 2, the first peristaltic pump first pumps the flotation liquid into the overflow column at a flow rate of 7-8 mL / s, and after the flotation liquid is flush with the liquid surface of the overflow column, the flotation liquid is pumped into the overflow column at a flow rate of 1-2 mL / s.

[0011] In the present invention, in step 3 and step 5, the cleaning solution is distilled water, and the rinsing operation is repeated at least 3 times.

[0012] In the present invention, in step three, sodium chloride or sodium bromide is added to the filtrate to prepare a usable solution.

[0013] In the present invention, in step 4, the digestion solution is Fenton's reagent, which is passed through a 30% mass fraction of H 2 O 2 When preparing Fenton's reagent, the pH value of the ferrous sulfate heptahydrate aqueous solution is first adjusted to between 2.5 and 3.5 with concentrated sulfuric acid, and then 30% H 2 O 2 and mixed with an aqueous solution of ferrous sulfate. In a specific embodiment, the concentration of the aqueous solution of ferrous sulfate heptahydrate is 20 g / L; the volume ratio of the aqueous solution of ferrous sulfate heptahydrate to 30% hydrogen peroxide is 1:1.

[0014] In the present invention, in step 5, H is added to the filtrate. 2 O 2 and Fe 2+ , prepared into a usable solution.

[0015] In the present invention, the first filter membrane is an organic filter membrane with a pore size of 1 μm.

[0016] In the present invention, the second filter membrane is an organic filter membrane with a pore size of 0.45 μm.

[0017] Membrane filtration is used in the present invention, mainly considering that the organic filter membrane is acid and alkali resistant and has more advantages in filtering organic matter and tiny particles. Before digestion, a large amount of organic matter is attached to the surface of the microplastics, and the particles are relatively large, so a 1μm organic filter membrane can be used. In order to fully separate the microplastics, a 0.45μm organic filter membrane is used as the second filter membrane.

[0018] Compared with the prior art, the present invention has the following beneficial effects: The present invention adopts a density of 1.2g / cm 3 The sodium chloride solution with a density of 1.55 g / cm 3 Sodium bromide solution was used as the flotation solution to gradually separate microplastics, which not only improved the efficiency of microplastic separation, but also solved the preliminary screening of different types of microplastics. 2 O 2The Fenton reagent composed of ferrous sulfate heptahydrate is digested at room temperature, and concentrated sulfuric acid is used to adjust the pH to 3.0, which can further remove organic impurities contained in the floated microplastics; digestion and separation are carried out in two steps, and new samples can be separated during digestion to achieve the separation of a large number of samples; the waste liquid generated during the transition experiment is recycled through the waste liquid recovery container, while the flotation liquid is recycled and reused without affecting the experimental process; The present invention adopts two-stage separation, first flotation and then digestion separation, to obtain pure microplastics; at the same time, the filtrate of the present invention can be recycled and continued to be used for microplastic flotation; The flotation operation of the present invention is simple, the flotation performance of the flotation liquid is good, and the reasonable control of the addition speed of the flotation liquid can promote rapid flotation, and the magnetic stirring and filtration process can also accelerate the separation of microplastics; the present invention can achieve efficient and rapid separation and purification of microplastics, and the recovery efficiency is above 90%; The invention recycles the reagents used in the process, is environmentally friendly, has low cost and is easy to promote and apply. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic structural diagram of a soil microplastic rapid step-by-step flotation device with recyclable reagents according to the present invention.

[0020] Figure 2 Flow chart of a rapid step-by-step flotation method for soil microplastics with recyclable reagents.

[0021] Numbers in the figure: 1-flotation liquid storage container, 2-first peristaltic pump, 3-magnetic stirrer, 4-stirring bar, 5-electric rotator, 6-scraper, 7-overflow column, 8-overflow port, 9-separator, 10-second peristaltic pump, 11-Buchner funnel suction filter, 12-peristaltic pump, 13-vacuum pump, 14-glass sand core suction filter. DETAILED DESCRIPTION

[0022] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0023] like Figure 1As shown, the present invention provides a soil microplastic rapid step-by-step flotation device with recyclable reagents, which includes a flotation liquid storage container 1, a first peristaltic pump 2, a second peristaltic pump 10, an overflow column 7, a magnetic stirrer 3, a Buchner funnel filter 11, a vacuum pump 13, and a glass sand core filter 14. The overflow column 7 is used to load the microplastic contaminated soil to be separated by flotation, and the wall of the overflow column 7 is provided with a flotation liquid inlet, and the flotation liquid storage container 1 is connected to the flotation liquid inlet through a pipeline; the upper end of the overflow column 7 is provided with a microplastic overflow port; the electric rotator 5 is fixed above the iron frame; the scraper 6 is below the electric rotator 5, and is used to scrape the floated microplastics to the overflow port 8 in time. The waste liquid recovery container is connected to the Buchner funnel filter 11 through a pipeline, and the flotation liquid can be quickly recovered by filtration to achieve the initial separation of microplastics. The microplastics after the initial separation are digested with Fenton reagent, and the glass sand core filter 14 is connected to the vacuum pump 13 to achieve the separation of microplastics.

[0024] In the embodiment, the preparation method of Fenton's reagent is as follows: First, ferrous sulfate solution was prepared by mixing 20 g of ferrous sulfate heptahydrate with 1 L of pure water, and the pH of the solution was adjusted to 3.0 using concentrated sulfuric acid. Then, the acidic ferrous sulfate solution and 30% H2O were mixed in a volume ratio of 1:1. 2 O 2 The solutions were mixed at room temperature to prepare Fenton's reagent.

[0025] The following are specific embodiments.

[0026] Example 1

[0027] In this example, polystyrene (hereinafter referred to as PS) was used as microplastics. 60-mesh polystyrene (particle size of about 0.25 mm) was fully mixed with 20 g of sand (floatation was repeated 3 times to remove microplastics) to simulate soil samples. Three replicates were set. The masses of PS before mixing were ① 0.2009 g, ② 0.2013 g, and ③ 0.2002 g, respectively. Six blank controls were set to eliminate interference. Three of them directly filtered the flotation liquid, and the other three were respectively subjected to the following steps using 20 g of sand without PS added.

[0028] Add a stirrer 4 to the overflow column 7, start the first peristaltic pump 2, and successively pump the liquid with a density of 1.2 g / cm 3 The sodium chloride solution with a density of 1.55 g / cm 3 Sodium bromide solution is added to the overflow column 7 at a flow rate of 8 mL / s. When the height of the flotation liquid is 3 cm from the water outlet, the first peristaltic pump 2 is turned off. 20 g of the dried soil sample is placed in the overflow column 7. The magnetic stirrer 3 is turned on and the speed is slowly increased. After stirring alternately at fast and slow speeds, the magnetic stirrer 3 is turned off and allowed to stand. The first peristaltic pump 2 is turned on, and the flotation liquid is slowly added to the level with the liquid surface of the overflow column 7, and then the pump is turned off and left to stand. The first peristaltic pump 2 is turned on again to pump the flotation liquid into the overflow column 7 at a rate of 2 mL / s. At the same time, the electric rotator 5 is turned on, and the scraper 6 rotates to overflow the microplastics and the flotation liquid floating on the upper layer to the separator 9, and the first peristaltic pump 2 is turned off after the microplastics are completely transferred to the separator 9.

[0029] Turn on the second peristaltic pump 10, extract the microplastics in the separator 9 into the Buchner funnel filter 11 for filtration, use a cleaning solution to rinse the microplastics stuck to the wall until all the microplastics are transferred, recover the filtrate to the waste liquid recovery container, make the recovered waste liquid into a usable solution, add it to the flotation liquid storage container, and complete the flotation liquid recycling.

[0030] Take out the first filter membrane, rinse the microplastic mixture on the filter membrane into an external beaker with pure water, and add digestion solution for digesting organic matter into the beaker.

[0031] After the impurities are digested, a clear solution is formed. The clear solution in the beaker is added to the filter head of the glass sand core filter 14. The vacuum pump 13 is turned on to filter the microplastics onto the filter membrane. The beaker and the filter head are rinsed with pure water, the cleaning liquid is filtered, and the second filter membrane is placed in a glass dish. After marking, it is dried and tested to complete the separation of microplastics.

[0032] When the liquid level of the filtrate in the suction flask is 2 cm away from the filter tip, stop the suction filtration, make the filtrate into a usable solution, add it into the digestion solution storage container, and complete the digestion solution recycling.

[0033] After recovery, no microplastics were detected in the blank controls. The masses of PS were ①0.1936g, ②0.1953g, and ③0.1957g, respectively. The recovery rates were 96.37%, 97.02%, and 97.75%, respectively, and the average recovery rate was 97.05%.

[0034] Example 2 (PP) The only difference between this embodiment and Example 1 is that the density of polypropylene (hereinafter referred to as PP) is smaller than that of PS. PP of different particle sizes are fully and evenly mixed with 20 g of sand (floatation has been repeated 3 times to remove microplastics) to simulate soil samples. Three different particle sizes are set: less than 1 mm, 1-2 mm, and 2-5 mm. The masses of PP before mixing are ①0.2001 g, ②0.2012 g, and ③0.2006 g, respectively. Six blank controls are set to eliminate interference, three of which directly filter the flotation liquid, and the other three are respectively subjected to the same steps as in Example 1 with 20 g of sand without PP added.

[0035] After recovery, no microplastics were detected in the blank control. The masses of PP were ①0.1935g, ②0.1969g, and ③0.1944g, respectively. The recovery rates were 96.70%, 97.86%, and 96.90%, respectively, and the average recovery rate was 97.15%. The flotation efficiency of microplastic particles with a particle size between 1-2 mm was higher, followed by microplastic particles with a particle size of 2-5 mm. Among them, the flotation efficiency of microplastic particles with a particle size of less than 1 mm was the lowest, which may be because microplastics with a particle size too small are easily wrapped by organic matter in the soil, making it difficult to float out.

[0036] Example 3 (PVC) The only difference between this embodiment and Example 1 is that the density of polyvinyl chloride (hereinafter referred to as PVC) is greater than that of PS. PVC of different particle sizes are fully mixed with 20g of sand (floatation has been repeated 3 times to remove microplastics) to simulate soil samples. Three different particle sizes are set: less than 1mm, 1-2mm, and 2-5mm. The masses of PVC before mixing are ①0.2002g, ②0.2003g, and ③0.2001g, respectively. Six blank controls are set to eliminate interference, three of which directly filter the flotation liquid, and the other three are respectively subjected to the same steps as in Example 1 with 20g of sand without PVC added.

[0037] After recovery, no microplastics were detected in the blank control, the masses of PVC were ①0.1941g, ②0.1956g, ③0.1943g, and the recovery rates were 96.95%, 97.65%, 97.10%, and the average recovery rate was 97.23%. The results were similar to those in Example 2. The flotation efficiency of microplastic particles with a particle size between 1-2 mm was higher, and the flotation efficiency of microplastic particles with a particle size of 2-5 mm was second, while the flotation efficiency of microplastic particles with a particle size of less than 1 mm was the lowest, which may be because microplastics with a particle size too small are easily wrapped by organic matter in the soil, making it difficult to float out.

[0038] The density of the flotation liquid used is higher than that of microplastics. In theory, all microplastics can be floated out. However, organic matter in the soil may wrap around the surface of microplastics, increasing their density and making them unable to be effectively floated out. The flotation efficiency of microplastics of different materials is slightly different. The flotation efficiency of PS, PP and PVC is almost the same, and the average recovery rate is over 90%.

Claims

1. A rapid step-by-step flotation method for soil microplastics with recyclable reagents, characterized in that: The method adopts a soil microplastic rapid step-by-step flotation device for rapid step-by-step flotation; the device comprises an overflow column, a first peristaltic pump, a second peristaltic pump, a magnetic stirrer, a separator, an electric rotator, a scraper, a Buchner funnel filter, a vacuum pump and a glass sand core filter; a first filter membrane is laid in the Buchner funnel filter, and a second filter membrane is laid in the glass sand core filter; the method comprises the following steps: Step 1: Add a magnetic rotor to the overflow column, start the first peristaltic pump, add flotation liquid to the overflow column, and when the height of the flotation liquid is close to the water outlet of the overflow column, close the first peristaltic pump, take a quantitative dried soil sample and place it in the overflow column, start the magnetic stirrer, stir alternately at fast and slow speeds, then turn off the magnetic stirrer and let it stand; Step 2: Turn on the first peristaltic pump, slowly add the flotation liquid until it is flush with the liquid surface of the overflow column, then turn it off and let it stand, turn on the first peristaltic pump again, and pump the flotation liquid into the overflow column at a lower flow rate. At the same time, turn on the electric rotator, rotate the scraper to overflow the microplastics and flotation liquid floating on the upper layer to the separator, and then turn off the first peristaltic pump after the microplastics are completely transferred to the separator; Step 3: Start the second peristaltic pump to extract the microplastics in the separator into the Buchner funnel filter for filtration, use a cleaning solution to rinse the microplastics on the wall until all the microplastics are transferred, recover the filtrate, add sodium chloride and sodium bromide, adjust the concentration, make a usable solution, and complete the recycling of the flotation solution; Step 4: Take out the first filter membrane, rinse the microplastic mixture on the filter membrane into an external beaker with pure water, and add digestion solution into the beaker to digest the organic matter; Step 5. After the digestion is completed, add the clarified solution in the beaker into the filter head of the glass sand core filter, turn on the vacuum pump, filter and rinse with cleaning solution to achieve the separation and collection of microplastics on the second filter membrane, add the corresponding reagents to the filtrate to make a usable solution, and complete the recycling of the digestion solution.

2. The method for rapid step-by-step flotation of soil microplastics with recyclable reagents according to claim 1, characterized in that: In step 1, the flotation solution has a density of 1.2 g / cm 3 The sodium chloride solution with a density of 1.55 g / cm 3 Sodium bromide solution.

3. The rapid step-by-step flotation method for soil microplastics with recyclable reagents according to claim 1 is characterized in that: In step 2, the first peristaltic pump first pumps the flotation liquid into the overflow column at a flow rate of 7-8 mL / s. After the flotation liquid is flush with the liquid surface of the overflow column, the flotation liquid is pumped into the overflow column at a flow rate of 1-2 mL / s.

4. The rapid step-by-step flotation method for soil microplastics with recyclable reagents according to claim 1 is characterized in that: In step 3 and step 5, the cleaning solution is distilled water, and the rinsing operation is repeated at least 3 times.

5. The method for rapid step-by-step flotation of soil microplastics with recyclable reagents according to claim 1, characterized in that: In step 4, the digestion solution is Fenton's reagent, which is prepared by 30% by mass of H2O2 and acidic ferrous sulfate heptahydrate solution.

6. The method for rapid step-by-step flotation of soil microplastics with recyclable reagents according to claim 5, characterized in that: In step 4, when preparing Fenton's reagent, the pH value of the ferrous sulfate heptahydrate solution is first adjusted to between 2.5 and 3.5 with concentrated sulfuric acid, and then 30% H2O2 and the ferrous sulfate heptahydrate solution are mixed.

7. The rapid step-by-step flotation method for soil microplastics with recyclable reagents according to claim 1, characterized in that: The first filter membrane has an organic filter membrane with a pore size of 1 μm.

8. The method for rapid step-by-step flotation of soil microplastics with recyclable reagents according to claim 1, characterized in that: The second filter membrane has an organic filter membrane with a pore size of 0.45 μm.

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