A method for separating microplastics from wet sludge

Abstract

This invention discloses a method for separating microplastics from wet sludge, comprising the following steps: solid-liquid separation of wet sludge to obtain sludge and mud-water; electrostatic adsorption of the sludge to obtain a sludge mixture enriched with microplastics; digestion and flotation of the microplastic-enriched sludge mixture and mud-water separately to obtain the microplastics. This invention provides a method for separating microplastics from wet sludge that can quickly, simply, and effectively separate and extract microplastics from wet sludge, achieving a high microplastic recovery rate and good reproducibility.

Description

Technical Field

[0001] This invention belongs to the field of microplastic separation technology in wet sludge, and particularly relates to a method for separating microplastics in wet sludge. Background Technology

[0002] With the continuous increase in plastic usage and its inherent non-degradable nature, environmental pollution and risks caused by plastics are constantly increasing. Microplastics refer to plastic particles with a diameter of less than 5 mm that enter the ecosystem through various pathways. Based on their origin, they can be divided into primary microplastics and secondary microplastics. Primary microplastics mainly originate from microscale plastics produced in factories and are found in many cosmetics, facial scrubs, or aerosol cleansing media, as well as textile fibers generated during clothing washing. Secondary microplastics mainly originate from larger plastic fragments that are broken down into finer pieces under the influence of physicochemical factors such as photocatalysis, natural weathering, water flow, and chemical erosion. Existing research has shown that the presence of microplastics poses a significant threat to the safety of ecosystems. Microplastics can easily clog the feeding organs of aquatic animals, causing bodily harm. Furthermore, many microplastics contain toxic substances, which are released when ingested and enter the bodies of organisms, causing ecotoxicity.

[0003] Studies have shown that wastewater treatment plants are a significant source of microplastics in terrestrial ecosystems. Microplastics from personal care products, car tire wear, and household and laundry dust enter wastewater treatment plants through drainage pipes. While a small amount of microplastics is discharged into water bodies, over 90% is trapped in the sludge, resulting in sludge containing high levels of microplastics, far exceeding the abundance in water bodies and sediments. With the application of sludge in landscaping and landscaping, these microplastics directly enter the soil ecosystem, posing a potential threat. Therefore, the separation and recycling of microplastics from sludge requires serious attention.

[0004] Currently, the extraction of microplastics from surface water mainly employs density flotation, which separates the lighter microplastic components from the heavier impurities based on the density difference between the target component and the impurities. However, since wet sludge is a solid-liquid mixture containing a very high proportion of organic matter, clay, minerals, plankton, and other components, these components can become embedded in the microplastics, affecting the effectiveness of microplastic flotation and separation. Summary of the Invention

[0005] Based on the above-mentioned technical problems, the present invention proposes a method for separating microplastics from wet sludge. The method can quickly, simply and effectively separate and extract microplastics from wet sludge, with a high microplastic recovery rate and good reproducibility.

[0006] The present invention proposes a method for separating microplastics from wet sludge, comprising the following steps:

[0007] S1. After solid-liquid separation of wet sludge, sludge and muddy water are obtained;

[0008] S2. After electrostatic adsorption, the sludge is used to obtain a sludge mixture enriched with microplastics.

[0009] S3. The sludge mixture and mud-water enriched with microplastics are digested and then floated to obtain the microplastics.

[0010] In this invention, wet sludge is first subjected to solid-liquid separation, and then the resulting sludge and mud-water are treated separately. Considering that the sludge contains a large number of solid impurities, the principle of mechanical, physical and electromagnetic separation is used to effectively separate and enrich microplastics in advance, improve the pretreatment efficiency, reduce the pressure on subsequent digestion and flotation, and greatly reduce the separation time. After the sludge is digested and flotated, the impurities mixed in the microplastics are further eliminated, thereby efficiently and thoroughly separating the microplastics from the wet sludge.

[0011] Preferably, in step S1, the wet sludge is separated into solid and liquid phases after being filtered through multiple screens.

[0012] Preferably, the screen mesh size is 4-1000 mesh.

[0013] Preferably, in step S2, after the sludge is electrostatically adsorbed using an electrostatic generator, low-density microplastics and fine mud are adsorbed in the electrostatic generator. After the power is turned off, the sludge is recycled to obtain the sludge mixture enriched with microplastics.

[0014] Preferably, in step S2, before the sludge undergoes electrostatic adsorption, it is further dehydrated to be completely dried and then crushed into powder.

[0015] Preferably, the particle size of the powder is 1-5 mm.

[0016] In this invention, the sludge is dried and pulverized to facilitate better electrostatic adsorption.

[0017] Preferably, the sludge mixture enriched with microplastics is directly added to the digestion solution for digestion, and after filtration, microplastics that do not agglomerate with sludge are obtained; the mud-water is filtered through a filter membrane, and the resulting filtrate is then added to the digestion solution for digestion, and after filtration, microplastics that do not agglomerate with sludge are also obtained.

[0018] In this invention, by digesting the sludge mixture and the sludge water separately, organic matter and plankton can be eliminated, which helps to remove microplastics and sludge particles, allowing subsequent flotation to proceed fully.

[0019] Preferably, the digestion solution includes Fenton's reagent, the pH of which is 3-5, and the molar ratio of hydrogen peroxide to ferrous ions is 1:2-5;

[0020] In this invention, the digestion solution is selected from Fenton's reagent, which has a high digestion rate for organic impurities in microplastics and improves the recovery rate of microplastics in wet sludge.

[0021] In this invention, in addition to Fenton digestion, the digestion method can also be acid digestion (HCl, HNO3, HSO4), alkaline digestion (KOH, NaOH), enzymatic digestion (protease), hydrogen peroxide (30%) digestion, etc. The appropriate method is selected based on the physicochemical properties of the impurities to ensure maximum separation of microplastics from impurities.

[0022] Preferably, the digestion solution further includes an N-acetyl activator and an anionic surfactant, wherein the molar ratio of the N-acetyl activator, the anionic surfactant, and the hydrogen peroxide in Fenton's reagent is 0.05-0.2:0.01-0.05:1;

[0023] Preferably, the N-acetyl activator is tetraacetylethylenediamine, and the anionic surfactant is sodium dodecylbenzenesulfonate or sodium dodecyl sulfate.

[0024] In this invention, the inventors discovered that wet sludge usually contains a certain amount of plankton. Traditional strong oxidants such as Fenton's reagent are ineffective against these plankton. However, by adding a certain amount of N-acetyl activator and anionic surfactant to the digestion solution, the plankton can be effectively killed and decomposed without damaging the microplastics. Therefore, it can ensure the full digestion of impurities and help improve the recycling efficiency of microplastics.

[0025] Preferably, the ratio of the microplastic-enriched sludge mixture to the digestion solution is 0.1-5 g / 100 mL; the ratio of the filtrate obtained after sludge-water filtration to the digestion solution is also 0.1-5 g / 100 mL.

[0026] Preferably, the digestion temperature is controlled between 60-70℃, and the digestion time is 1-10h.

[0027] Preferably, in step S3, the microplastics that do not agglomerate with sludge obtained after digestion are added to the flotation solution for flotation, stirred and mixed, and then allowed to stand. The resulting supernatant is then filtered to obtain the microplastics.

[0028] In this invention, when a mixture of materials of different densities (sludge and microplastics) is placed in a medium-density flotation solution (saturated salt solution), the material with a density lower than that of the flotation solution (microplastics) will float above the flotation solution, while the material with a density higher than that of the flotation solution (sludge) will sink. Therefore, in this invention, the microplastics are separated by adding the obtained digested filter material to the flotation solution for density separation.

[0029] For those skilled in the art, the separation operations that are inseparable from the digestion process mainly include density separation and oil extraction. The main principle is to use density difference and the oleophilic properties of microplastics for separation. In this invention, density separation achieved by flotation solution is the most suitable.

[0030] Preferably, the flotation solution is a saturated sodium chloride or saturated zinc chloride solution;

[0031] Preferably, the saturated sodium chloride solution or saturated zinc chloride solution is further filtered before use.

[0032] In this invention, by filtering the saturated sodium chloride solution or saturated zinc chloride solution before use, the separation efficiency is reduced due to the inclusion of crystal particles in the microplastics when they are used as flotation solutions, and the interference of crystals in the detection of microplastics is also reduced.

[0033] Preferably, the filter membrane used for filtration has a pore size of 0.22-0.45 μm.

[0034] Compared with the prior art, the present invention has the following technical effects:

[0035] (1) The method described in this invention can effectively separate and extract microplastics from wet sludge. It is not only simple and fast to operate, but also has a high microplastic particle recovery rate and strong practicality.

[0036] (2) This invention clarifies the feasibility of physical electromagnetic separation and chemical digestion flotation separation of microplastics in wet sludge, and verifies by simulation experiments that it can reduce the time of traditional separation methods and improve the pretreatment performance; at the same time, the integrated concept of the method described in this invention provides a reference for the whole system treatment of microplastic recycling. Attached Figure Description

[0037] Figure 1 This is a microscopic morphology image of the microplastics on the filter membrane obtained in Example 1. Detailed Implementation

[0038] The present invention will now be described in detail through specific embodiments. However, these embodiments are clearly provided for illustrative purposes and are not intended to limit the scope of the present invention.

[0039] Example 1

[0040] A method for separating microplastics from wet sludge, the specific steps of which are as follows:

[0041] (1) The wet sludge (water content 98%) is filtered layer by layer through stainless steel screens of 4 mesh (5mm), 18 mesh (1mm), 600 mesh (25μm) and 1000 mesh (15μm) to achieve solid-liquid separation of sludge and water, and obtain semi-dry sludge (water content 64%-76%) and sludge water.

[0042] (2) The semi-dry sludge (moisture content 64%-76%) is placed in an oven to dehydrate and dry completely, and then crushed into powder to obtain powder sludge (particle size ≤5mm); the powder sludge is then electrostatically adsorbed using an electrostatic generator rod, and the low-density microplastics and fine mud are repeatedly collected and adsorbed on the electrostatic generator rod. After the power is turned off, the sludge is recycled to obtain a sludge mixture enriched with microplastics.

[0043] (3) The mud-water mixture is first filtered using a 0.45 μm polytetrafluoroethylene (PTFE) membrane to obtain a filter membrane that intercepts the filtrate; then, the microplastic-enriched sludge mixture and the filter membrane that intercepts the filtrate are added to beakers respectively, and a digestion solution is slowly added at a material-to-liquid ratio of 1 g / 100 mL. The digestion solution is Fenton's reagent (Fenton's reagent is prepared by mixing FeSO4·7H2O aqueous solution and H2O2 water to obtain a solution with a concentration of 6 g / L, wherein H2O2... The mass fraction of O2 water was 30%, and the molar ratio of Fe(II) to H2O2 was 1:3. During the digestion process, the beaker was sealed with tin foil, and the temperature was controlled between 60-70℃. The reaction was carried out with magnetic stirring for 6 hours until no obvious bubbles appeared in the beaker. After cooling to room temperature, the digested solution was filtered through a 0.45μm polytetrafluoroethylene (PTFE) membrane. The inner wall of the beaker was then wetted with a small amount of digestion solution. After standing for 6 hours, a clean PTFE membrane (Disk diam 50mm) was used to scrape the wetted inner wall of the beaker to obtain a scraped filter membrane. The residual liquid in the beaker was repeatedly rinsed with MilliQ water, and the resulting rinsing solution was also filtered through a 0.45μm PTFE membrane.

[0044] (4) The filter membranes obtained from the two filtrations and the scraped filter membrane were added to a beaker containing flotation solution. After shaking and stirring for 24 hours, the supernatant was collected. The residue in the beaker was added back to the flotation solution, which was a saturated sodium chloride solution (the saturated sodium chloride solution needed to be filtered with a 0.45 μm polytetrafluoroethylene (PTFE) membrane before use). After shaking for 24 hours, the supernatant was collected. The above operation was repeated once more. The supernatants were combined and filtered using a 0.45 μm polytetrafluoroethylene (PTFE) membrane. The sample on the obtained filter membrane was the microplastic. The obtained filter membrane was observed, identified, and photographed using a microscope. The results were referred to... Figure 1 As shown, by Figure 1 The size, shape, and color distribution characteristics of the recycled microplastics can be observed.

[0045] Recovery rate calculation: 50 polymer plastic particles were mixed with 50g of relatively clean sludge (98% water content) as a wet sludge control sample. Using the method described in Example 1, three parallel experiments were set up to separate filter membranes containing microplastics. After air drying, the microplastics on the filter mesh were counted and statistically analyzed using microscopy and infrared spectroscopy. The microplastic recovery rate in the method described in Example 1 was calculated to be 84.2%.

[0046] Example 2

[0047] A method for separating microplastics from wet sludge, the specific steps of which are as follows:

[0048] (1) The wet sludge (water content 98%) is filtered layer by layer through stainless steel screens of 4 mesh (5mm), 18 mesh (1mm), 600 mesh (25μm) and 1000 mesh (15μm) to achieve solid-liquid separation of sludge and water, and obtain semi-dry sludge (water content 64%-76%) and sludge water.

[0049] (2) The semi-dry sludge (moisture content 64%-76%) is placed in an oven to dehydrate and dry completely, and then crushed into powder to obtain powder sludge (particle size ≤5mm); the powder sludge is then electrostatically adsorbed using an electrostatic generator rod, and the low-density microplastics and fine mud are repeatedly collected and adsorbed on the electrostatic generator rod. After the power is turned off, the sludge is recycled to obtain a sludge mixture enriched with microplastics.

[0050] (3) The mud-water mixture is first filtered using a 0.45 μm polytetrafluoroethylene (PTFE) membrane to obtain a filter membrane that intercepts the filtrate; then, the microplastic-enriched sludge mixture and the filter membrane that intercepts the filtrate are added to beakers respectively, and a digestion solution is slowly added at a ratio of 1 g / 100 mL. The digestion solution includes Fenton's reagent, tetraacetylethylenediamine, and sodium dodecylbenzenesulfonate (Fenton's reagent is prepared by mixing FeSO4·7H2O aqueous solution and H2O2 water to obtain a solution with a concentration of 6 g / L, wherein the mass fraction of H2O2 water is 30%). The molar ratio of Fe(II) to H2O2 was 1:3, and the molar ratio of H2O2 to tetraacetylethylenediamine and sodium dodecylbenzenesulfonate was 1:0.1:0.03. During the digestion process, the beaker was sealed with tin foil, and the temperature was controlled between 60-70℃. The reaction was magnetically stirred for 6 hours until no obvious bubbles appeared in the beaker. After cooling to room temperature, the digested solution was filtered through a 0.45μm polytetrafluoroethylene (PTFE) membrane. The inner wall of the beaker was then wetted with a small amount of digestion solution. After standing for 6 hours, a clean PTFE membrane (Disk diameter 50mm) was used to scrape the wetted inner wall of the beaker to obtain a scraped filter membrane. The residual liquid in the beaker was repeatedly rinsed with MilliQ water, and the resulting rinsing solution was also filtered through a 0.45μm PTFE membrane.

[0051] (4) Add the filter membranes obtained from the two filtrations and the scraped filter membrane into a beaker containing flotation solution. After shaking and stirring for 24 hours, take the supernatant. Add the residue in the beaker back into the flotation solution. The flotation solution is a saturated sodium chloride solution (the saturated sodium chloride solution needs to be filtered with a 0.45μm polytetrafluoroethylene (PTFE) membrane before use). After shaking for 24 hours, stir and take the supernatant. Repeat the above operation once more. After combining the supernatants, filter them with a 0.45μm polytetrafluoroethylene (PTFE) membrane. The sample on the obtained filter membrane is the microplastic.

[0052] Recovery rate calculation: 50 polymer plastic particles were mixed with 50g of relatively clean sludge (98% water content) as a wet sludge control sample. Using the method described in Example 2, three parallel experiments were set up to separate filter membranes containing microplastics. After air drying, the microplastics on the filter mesh were counted and statistically analyzed using microscopy and spectroscopy. The microplastic recovery rate in the method described in Example 2 was calculated to be 92.7%.

[0053] Comparative Example 1

[0054] A method for separating microplastics from wet sludge, the specific steps of which are as follows:

[0055] (1) The wet sludge (98% water content) was placed in an oven to dehydrate and dry completely, and then crushed into powder to obtain a sludge mixture.

[0056] (2) The sludge mixture was added to a beaker, and the digestion solution was slowly added at a rate of 1 g / 100 mL. The digestion solution was Fenton's reagent (Fenton's reagent was prepared by mixing FeSO4·7H2O aqueous solution and H2O2 water to obtain a solution with a concentration of 6 g / L, wherein the mass fraction of H2O2 water was 30%, and the molar ratio of Fe(II) to H2O2 was 1:3). During the digestion process, the beaker was sealed with tin foil, and the temperature was controlled between 60-70℃. The reaction was magnetically stirred for 6 hours until no obvious bubbles appeared in the beaker. After cooling to room temperature, the digested solution was filtered through a 0.45 μm polytetrafluoroethylene (PTFE) membrane. Then, a small amount of digestion solution was used to wet the inner wall of the beaker, and after standing for 6 hours, a clean polytetrafluoroethylene (PTFE) membrane (Disk diamter) was clamped. A 50mm membrane was used to scrape the wetted inner wall of the beaker to obtain a scraped filter membrane; the residual liquid in the beaker was repeatedly rinsed with MilliQ water, and the resulting rinsing liquid was also filtered using a 0.45μm polytetrafluoroethylene (PTFE) membrane.

[0057] (3) Add the filter membranes obtained from the two filtrations and the scraped filter membrane into a beaker containing flotation solution. After shaking and stirring for 24 hours, take the supernatant. Add the residue in the beaker back into the flotation solution. The flotation solution is a saturated sodium chloride solution (the saturated sodium chloride solution needs to be filtered with a 0.45μm polytetrafluoroethylene (PTFE) membrane before use). After shaking for 24 hours, stir and take the supernatant. Repeat the above operation once more. After combining the supernatants, filter them with a 0.45μm polytetrafluoroethylene (PTFE) membrane. The sample on the obtained filter membrane is the microplastic.

[0058] Recovery rate calculation: 50 polymer plastic particles were mixed with 50g of relatively clean sludge (98% water content) as a wet sludge control sample. Using the method described in Comparative Example 1, three parallel experiments were set up to separate filter membranes containing microplastics. After air drying, the microplastics on the filter mesh were counted and statistically analyzed using microscopy and infrared spectroscopy. The microplastic recovery rate in the method described in Comparative Example 1 was calculated to be 57.8%.

[0059] As can be seen from the above, the method described in this invention can effectively extract microplastic particles from wet sludge systems. It is not only simple and fast to operate, but also has a high microplastic particle recovery rate and strong practicality.

[0060] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for separating microplastics from wet sludge, characterized in that, Includes the following steps: S1. After solid-liquid separation of wet sludge, sludge and muddy water are obtained; S2. After electrostatic adsorption, the sludge is obtained as a sludge mixture enriched with microplastics. S3. The sludge mixture and mud-water enriched with microplastics are digested and floated separately to obtain the microplastics. The digestion solution includes Fenton's reagent, which has a pH of 3-5 and a molar ratio of hydrogen peroxide to ferrous ions of 1:2-5. The digestion solution also includes an N-acetyl activator and an anionic surfactant, and the molar ratio of the N-acetyl activator, the anionic surfactant and the hydrogen peroxide in Fenton's reagent is 0.05-0.2:0.01-0.05:1; The N-acetyl activator is tetraacetylethylenediamine, and the anionic surfactant is sodium dodecylbenzenesulfonate or sodium dodecyl sulfate.

2. The method for separating microplastics from wet sludge according to claim 1, characterized in that, In step S1, the wet sludge is separated into solid and liquid phases after being filtered through multiple screens.

3. The method for separating microplastics from wet sludge according to claim 2, characterized in that, The sieve has an aperture of 4-1000 mesh.

4. The method for separating microplastics from wet sludge according to any one of claims 1-3, characterized in that, In step S2, the sludge is electrostatically adsorbed using an electrostatic generator. Low-density microplastics and fine mud are adsorbed in the electrostatic generator. After the power is turned off, the sludge is recycled to obtain a sludge mixture enriched with microplastics.

5. The method for separating microplastics from wet sludge according to claim 4, characterized in that, In step S2, before the sludge undergoes electrostatic adsorption, it is further dehydrated to be completely dried and then crushed into powder; the particle size of the powder is 1-5 mm.

6. The method for separating microplastics from wet sludge according to any one of claims 1-3, characterized in that, In step S3, the sludge mixture enriched with microplastics is directly added to the digestion solution for digestion. After filtration, microplastics that do not agglomerate with sludge are obtained. The mud-water is then filtered through a filter membrane, and the resulting filtrate is added to the digestion solution for digestion. After filtration, microplastics that do not agglomerate with sludge are also obtained.

7. The method for separating microplastics from wet sludge according to claim 1, characterized in that, The ratio of the microplastic-enriched sludge mixture to the digestion solution is 0.1-5 g / 100 mL; the ratio of the filtrate obtained after sludge-water filtration to the digestion solution is also 0.1-5 g / 100 mL.

8. The method for separating microplastics from wet sludge according to claim 1 or 7, characterized in that, The digestion temperature is controlled between 60-70℃, and the digestion time is 1-10h.

9. The method for separating microplastics from wet sludge according to claim 1, characterized in that, In step S3, the microplastics that do not agglomerate with sludge obtained after digestion are added to the flotation solution for flotation. After stirring and mixing, the mixture is allowed to stand, and the resulting supernatant is filtered to obtain the microplastics.

10. The method for separating microplastics from wet sludge according to claim 9, characterized in that, The flotation solution is a saturated sodium chloride or saturated zinc chloride solution.

11. The method for separating microplastics from wet sludge according to claim 10, characterized in that, The saturated sodium chloride or saturated zinc chloride solution also includes filtration before use.

12. The method for separating microplastics from wet sludge according to any one of claims 9-11, characterized in that, The filtration process uses a filter membrane with a pore size of 0.22-0.45 μm.

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