Method for flotation separation of micro-plastics

Abstract

The invention relates to a method for flotation separation of micro-plastics, and belongs to the field of water body micro-plastic separation. The method comprises the steps that in the presence of a foaming agent, a hydrophobic collecting agent and a water body containing the micro-plastics are mixed, and then the micro-plastics are subjected to bubbling flotation; the hydrophobic collecting agent contains dodecahexaenoic acid, eicosapentaenoic acid, cis-oleic acid, cis-linoleic acid and palmitic acid. The method is simple in procedure, easy to operate, low in cost and high in practicability.

Description

Technical Field

[0001] The present invention relates to a method for flotation separation of microplastics, belonging to the field of separation of microplastics in water bodies, and particularly relates to the separation of microplastics in the ocean, rivers, lakes and aquaculture farms. Background Art

[0002] Plastic waste will form microplastics with a diameter less than 5 mm after aging and degradation in the environment. These microplastics are widely present in the ocean, rivers, lakes, fish farms and urban water treatment systems, and appear in various forms such as fragments, granules, microbeads and fibers.

[0003] Common types of microplastics include polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyethylene terephthalate, polyamide, polyurethane, etc.

[0004] Microplastic pollution can not only serve as a carrier for heavy metal ions and persistent organic pollutants, but also become a colonization platform for potential pathogens, posing a significant threat to the ecological environment and human health. Through the enrichment effect of the food chain, microplastics are ingested by zooplankton and fish and gradually accumulate to higher trophic level predators, ultimately affecting human health. Therefore, researchers are actively seeking efficient methods for treating microplastics in water bodies.

[0005] Currently, common microplastic treatment technologies include adsorption, electrocoagulation, membrane separation, advanced oxidation processes and biodegradation, etc. However, these technologies all have certain limitations. For example, biodegradation takes a long time and has low efficiency; although the removal efficiency of filters can usually reach more than 90%, most of them use polymer substrates, which may bring potential chemical risks, and the surface particles need to be cleaned frequently due to blockage; adsorption and magnetic separation technologies may cause ecological hazards due to the desorption behavior after the adsorbent is saturated or the release of nano-magnetic materials. The foam flotation process is applied to the separation of microplastics, in which hydrophobic microplastics can adhere to rising bubbles to form microbubble-bubble aggregates and be removed from the liquid. However, for naturally aged microplastics, due to the decrease in surface hydrophobicity, their binding ability with bubbles weakens, resulting in a decrease in flotation efficiency. Therefore, hydrophobic collectors are usually needed to improve the flotation efficiency.

[0006] Chinese Patent (Publication No. CN109909073A) discloses a method for separating microplastics based on the air flotation method. The specific steps include: (1) adding iodine-containing organic matter to the water body dispersed with microplastics and mixing well; (2) adding inorganic acid to the dispersion obtained in step (1) and mixing well, and then continuously introducing air or oxygen to form bubbles in the dispersion and rise. Among them, the iodine-containing organic matter is triheptyl dodecyl ammonium iodide. This method has the problems of large consumption of iodine-containing organic matter and potential harm to natural water bodies, which limits its practical application. Summary of the Invention

[0007] Aiming at the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide a method for flotation separation of microplastics. This method uses a hydrophobic collector containing docosahexaenoic acid, eicosapentaenoic acid, cis-oleic acid, cis-linoleic acid and palmitic acid to float and separate microplastics in water. This method has a simple procedure, is easy to operate, has low cost and strong practicability.

[0008] To achieve the above purpose, the first aspect of the present invention is to provide a method for flotation separation of microplastics. In the presence of a foaming agent, a hydrophobic collector is mixed with water containing microplastics, and then bubbling is carried out to float the microplastics; the hydrophobic collector contains docosahexaenoic acid, eicosapentaenoic acid, cis-oleic acid, cis-linoleic acid and palmitic acid.

[0009] The innovation of the present invention lies in that the hydrophobic collector containing docosahexaenoic acid, eicosapentaenoic acid, cis-oleic acid, cis-linoleic acid and palmitic acid can adsorb on the surface of microplastics, enhance the ability of bubbles to capture microplastics, and the hydrophobic collector can adsorb on the surface of microplastics and enhance the surface hydrophobicity of microplastics, thereby improving the flotation separation efficiency of microplastics in water. The method of the present invention has excellent flotation effects on both hydrophilic and hydrophobic microplastics in water.

[0010] In a preferred embodiment, in the hydrophobic collector, the content of docosahexaenoic acid is 0.3 - 1 wt%, the content of eicosapentaenoic acid is 0.2 - 1 wt%, the content of cis-oleic acid is 1 - 3 wt%, the content of cis-linoleic acid is 1 - 3 wt%, and the content of palmitic acid is 1 - 4 wt%. When the contents of docosahexaenoic acid, eicosapentaenoic acid, cis-oleic acid, cis-linoleic acid and palmitic acid are within this range, it can enhance the surface adsorption with microplastics and enhance the surface hydrophobicity of microplastics, thereby strengthening the ability of bubbles to capture microplastics and resulting in an increase in the flotation separation efficiency of microplastics in water.

[0011] In a preferred embodiment, the hydrophobic collector is fish feed and / or fish feed residues. Fish feed and fish feed residues can be used as a natural and harmless hydrophobic collector, which can enhance the flotation efficiency of aged microplastics. This method has a simple procedure, is easy to operate, has low cost and strong practicability. At the same time, this method can not only efficiently separate microplastics in water, but also avoid the risk of secondary pollution of natural water by traditional collectors, has no biological toxicity, and the presence of an appropriate amount of fish feed and / or fish feed residues in open waters will not have a negative impact on the environment. This strategy combines environmental protection and efficient treatment, providing a new idea for the improvement of microplastic separation technology.

[0012] In a preferred embodiment, the concentration of the hydrophobic collector in water is 40 - 150 g / L.

[0013] In a preferred embodiment, the concentration of the foaming agent in the water body is 0.01 - 0.05 mg / L.

[0014] In a preferred embodiment, the foaming agent is sodium dodecyl sulfate.

[0015] In a preferred embodiment, the hydrophobic collector is fish feed residue. Fish feed residue refers to the part of fish feed that remains unconsumed after fish preferentially ingest the edible part when the fish feed is put into the water body. Using the fish feed residue existing in the water body can not only achieve waste utilization but also reduce the flotation cost.

[0016] In a preferred embodiment, the flotation operation includes: using a flotation column for flotation, introducing air from the bottom of the flotation column, and the air flow rate is 40 - 60 L / h. If the flow rate is too low, the bubble density will be low, and the probability of bubble collision and capture with microplastics will be low. If it is too high, the stability of the bubbles in the flotation column will become poor, reducing the flotation efficiency.

[0017] In a preferred embodiment, the flotation operation further includes: starting to scrape the foam 10 - 20 s after bubbling, and the total scraping time is 300 s - 500 s.

[0018] Compared with the prior art, the present invention has at least the following advantages: (1) The method provided by the present invention has a simple procedure, is easy to operate, and has a low cost, and can effectively remove microplastics in water bodies (oceans, rivers, lakes, fish farms).

[0019] (2) The present invention can make full use of fish feed and / or fish feed residues in the aquaculture water body, has no biological toxicity, has strong practicability, and can be directly applied to natural waters and aquaculture waters.

[0020] (3) The method of the present invention can not only float hydrophobic microplastics but also float hydrophilic microplastics with high flotation efficiency under the condition of low foaming agent dosage.

[0021] (4) This method can not only efficiently separate microplastics in the water body but also avoid the risk of secondary pollution of natural water bodies by traditional collectors, has no biological toxicity, and the presence of an appropriate amount of fish feed and / or fish feed residues in open waters will not have a negative impact on the environment. Description of the Drawings

[0022] Figure 1 is the flotation result diagram of microplastics of each particle size level in Example 1; Figure 2 is the flotation result diagram of microplastics of each particle size level in Example 2; Figure 3 is the flotation result diagram of microplastics of each particle size level in Example 3; Figure 4 It is the flotation result diagram of microplastics at each particle size level in Comparative Example 1. Specific embodiments

[0023] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0024] The present invention will be further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited to the following specific embodiments. Obviously, the embodiments described below are only a part of the embodiments, and all other embodiments obtained by those of ordinary skill in the art without creative efforts still fall within the protection scope of the present invention.

[0025] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present invention can be obtained through market purchases or can be prepared by existing methods.

[0026] The unsaturated fatty acids in fish feed residues were determined using the national standard "GB5009.168 - 2016", and the results are as follows: Fish feed residues: The content of docosahexaenoic acid is 0.59 wt%, the content of eicosapentaenoic acid is 0.47 wt%, the content of cis - oleic acid is 1.83 wt%, the content of cis - linoleic acid is 1.95 wt%, and the content of palmitic acid is 2.03 wt%.

[0027] In the following examples, the microplastics were hydrophilically treated. Since the stronger the hydrophobicity of the microplastics, the better the flotation effect, hydrophilically treated microplastics with greater flotation difficulty were used for flotation experiments in the following examples.

[0028] Example 1 (1) Select waste polystyrene plastic as the raw material, and crush the polystyrene plastic to 0.125 - 0.45 mm, 48 - 72 μm, 35 - 48 μm to obtain microplastics at different particle size levels; (2) Add 0.5 g of microplastics at different particle size levels respectively, and add 100 mL of tannic acid with a concentration of 1000 mg / L for hydrophilization treatment, adjust the pH to 7, and the treatment time is 1 h; (3) Filter and separate the above liquid to obtain hydrophilic microplastics at different particle size levels; (4) Flotation of microplastics with different particle size levels: Add 1000 mL of deionized water into the flotation column, add 50 g of fish feed and stir evenly, then add hydrophilic microplastics into the flotation column, and add 0.02 mg of sodium dodecyl sulfate as a foaming agent; (5) Inject gas from the bottom of the flotation column using an air pump, set the air pump flow rate at 50 L / h, and set the flotation time at 480 s. The volume of the flotation column is 1 L, the height is 30 cm, and it is equipped with a porous medium with a pore size of 10 - 30 μm. Start skimming the foam 15 s after bubbling, and collect the flotation microplastics at 30, 60, 120, 180, 240, and 480 s respectively. Filter, dry, and weigh the flotation-separated microplastics respectively.

[0029] The flotation results of this example are shown in Figure 1 , at 300 s, the flotation separation rate of microplastics in the particle size range of 0.125 - 0.45 mm is 85 wt%, the flotation separation rate of microplastics in the particle size range of 48 - 72 μm is 80 wt%, and the flotation separation rate of microplastics in the particle size range of 35 - 48 μm is 75 wt%.

[0030] Example 2 (1) Select waste polystyrene plastic as the raw material, crush the polystyrene plastic to 0.125 - 0.45 mm, 48 - 72 μm, and 35 - 48 μm to obtain microplastics with different particle size levels; (2) Take 0.5 g of microplastics with different particle size levels respectively, add 100 mL of tannic acid with a concentration of 1000 mg / L for hydrophilization treatment, adjust the pH to 7, and the treatment time is 1 h; (3) Filter and separate the above liquid to obtain hydrophilic microplastics with different particle size levels; (4) Flotation of microplastics with different particle size levels: Add 1000 mL of deionized water into the flotation column, add 100 g of fish feed and stir evenly, then add hydrophilic microplastics into the flotation column, and add 0.02 mg of sodium dodecyl sulfate as a foaming agent; (5) Inject gas from the bottom of the flotation column using an air pump, set the air pump flow rate at 50 L / h, and set the flotation time at 480 s. The volume of the flotation column is 1 L, the height is 30 cm, and it is equipped with a porous medium with a pore size of 10 - 30 μm. Start skimming the foam 15 s after bubbling, and collect the flotation microplastics at 30, 60, 120, 180, 240, and 480 s respectively. Filter, dry, and weigh the flotation-separated microplastics respectively.

[0031] The flotation results of this example are shown in Figure 2, at 300 s, the flotation separation rate of microplastics in the particle size range of 0.125 - 0.45 mm is 65 wt%, the flotation separation rate of microplastics in the particle size range of 48 - 72 μm is 90 wt%, and the flotation separation rate of microplastics in the particle size range of 35 - 48 μm is 90 wt%.

[0032] Example 3 This example was carried out by referring to a similar method as in Example 1, except that the fish feed was replaced with an equal mass of fish feed residues.

[0033] The flotation results of this example are shown in Figure 3 , at 300 s, the flotation separation rate of microplastics in the particle size range of 0.125 - 0.45 mm is 50 wt%, the flotation separation rate of microplastics in the particle size range of 48 - 72 μm is 79 wt%, and the flotation separation rate of microplastics in the particle size range of 35 - 48 μm is 77 wt%.

[0034] Comparative Example 1 This comparative example was carried out by referring to a similar method as in Example 1, except that no fish feed was added in step (4).

[0035] The flotation results of this comparative example are shown in Figure 4 , at 300 s, the flotation separation rate of microplastics in the particle size range of 0.125 - 0.45 mm is 10 wt%, the flotation separation rate of microplastics in the particle size range of 48 - 72 μm is 5 wt%, and the flotation separation rate of microplastics in the particle size range of 35 - 48 μm is 30 wt%.

[0036] Test Example The fish feed was crushed to 200 - 300 mesh, separated by sieving, 50 g of the undersize powder was placed in water, and rotated and stirred to obtain a fish feed suspension. The suspension was separated using a centrifuge, and the supernatant was taken to obtain a fish feed residue liquid.

[0037] Deionized water and the fish feed residue liquid were respectively dropped onto the surface of hydrophilic microplastics, and dried with an ear syringe. The contact angle was measured. The contact angle of the hydrophilic microplastics was 25.4°, and the contact angle of the fish feed residue liquid was 37.5°.

[0038] From the above results, it can be seen that after adding the fish feed residue liquid to the surface of hydrophilic microplastics, the surface contact angle increases, and the hydrophobicity is enhanced. The unsaturated fatty acids in the fish feed are adsorbed on the surface of hydrophilic microplastics and enhance the hydrophobicity, improving the flotation separation efficiency of microplastics.

[0039] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including the combination of each technical feature in any other suitable manner. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for flotation separation of microplastics, characterized in that: In the presence of a frother, a hydrophobic collector is mixed with water containing microplastics and then the microplastics are floated by bubbling; the hydrophobic collector contains docosahexaenoic acid, eicosapentaenoic acid, oleic acid, linoleic acid and palmitic acid.

2. A method for flotation separation of microplastics according to claim 1, characterized in that: In the hydrophobic collector, the content of docosahexaenoic acid is 0.3-1wt%, the content of eicosapentaenoic acid is 0.2-1wt%, the content of oleic acid is 1-3wt%, the content of linoleic acid is 1-3wt%, and the content of palmitic acid is 1-4wt%.

3. A method for flotation separation of microplastics according to claim 1 or 2, characterized in that: The hydrophobic collector is fish feed and / or fish feed residue.

4. A method for flotation separation of microplastics according to claim 1 or 2, characterized in that: The concentration of the hydrophobic collector in the water body is 40-150 g / L.

5. A method for flotation separation of microplastics according to claim 1 or 2, characterized in that: The concentration of the foaming agent in the water body is 0.01-0.05 mg / L.

6. A method for flotation separation of microplastics according to claim 1 or 2, characterized in that: The foaming agent is sodium lauryl sulfate.

7. A method for flotation separation of microplastics according to claim 1 or 2, characterized in that: The hydrophobic collector is fish feed residue.

8. A method for flotation separation of microplastics according to claim 1 or 2, characterized in that: The flotation operation includes: flotation using a flotation column, blowing air from the bottom of the flotation column, and the flow rate of the air is 40-60 L / h.

9. A method for flotation separation of microplastics according to claim 8, characterized in that: The flotation operation also includes: scraping bubbles starting from 10 to 20 seconds after bubbling, and the total scraping time is 300 to 500 seconds.

Images