A method for removing micro-nano plastics from water containing micro-nano plastics

Abstract

The application provides a method for removing micro-nano plastics from a water body containing the micro-nano plastics, which uses a divalent iron salt as a flocculant, controls an OH ‑ / Fe ratio of the water body and controls oxygen diffusion of the water body to generate magnetic Fe3O4 nanoparticles in situ on surfaces of the micro-nano plastics, promotes flocculation and sedimentation removal of the micro-nano plastics, or removes the micro-nano plastics through magnetic separation. The method for removing the micro-nano plastics from the water body containing the micro-nano plastics has the advantages of low flocculant cost, small dosage, good flocculation and sedimentation effect, and low residual flocculation ion concentration in the water body, and can be used for removing the micro-nano plastics in drinking water, sewage and natural water bodies.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water quality purification treatment, in particular to a method for removing micro-nano plastics from water bodies containing micro-nano plastics. BACKGROUND

[0002] Plastics have had a great impact on human production and life since their invention. In 2019, the global annual plastic production had reached 368 million tons. Discarded plastics will break into smaller fragments under the long-term action of sunlight, rainwater, wind, and biological factors.

[0003] Micro-nano plastics (MNPs) as a new type of water pollutant have received increasing attention. MNPs are a general term for plastic particles with a size of <5 mm. They are small in size and constantly migrate in the environment, and can enter organisms through water bodies and food chains, adversely affecting human and biological health. In order to reduce the exposure risk of humans and other organisms to microplastics, it is urgent to remove microplastics from water bodies through certain technical methods. SUMMARY

[0004] Therefore, the purpose of the present application is to provide a method for removing micro-nano plastics from water bodies containing micro-nano plastics. A divalent iron salt is used as a flocculant, and the OH - / Fe ratio of the water body is adjusted and the oxygen diffusion of the water body is controlled to generate magnetic Fe3O4 nanoparticles in situ on the surface of the micro-nano plastics, thereby promoting the flocculation and sedimentation removal of the micro-nano plastics or using magnetic separation for removal.

[0005] According to the method for removing micro-nano plastics from water bodies containing micro-nano plastics provided by the present application, the method comprises:

[0006] Under anoxic conditions, a divalent iron salt is used as a flocculant, and the divalent iron salt is added to the water body containing micro-nano plastics during stirring at a first preset stirring speed;

[0007] A sodium hydroxide solution is added during stirring at a second preset stirring speed to adjust the OH - / Fe ratio in the water body system;

[0008] The water body system is opened to allow oxygen to diffuse in, and the dissolved oxygen concentration is 0.1-1 mg / L. The water body is allowed to settle to separate the micro-nano plastics.

[0009] The mechanism of the divalent iron salt for removing micro-nano plastics from the water body is as follows: divalent iron ions dissolve in water, interact with micro-nano plastics in the water body, and adsorb and adhere to the surface of the micro-nano plastics; the OH -In the process of / Fe, the pH of the water body increases, and the divalent iron is hydrolyzed to produce Fe(OH)2; under the condition of limited oxygen, Fe(OH)2 is gradually oxidized, part of Fe(OH)2 is oxidized to Fe(OH)3, and adsorbs divalent iron ions, and gradually forms magnetite (Fe3O4) crystals in situ on the surface of micro-nano plastics. Experiments show that by controlling the OH - / Fe and oxygen diffusion can greatly promote the flocculation effect, when the dissolved oxygen concentration is 0.1-1mg / L, the water body can form a large amount of magnetite (Fe3O4) crystals; micro-nano plastics can be directly removed by sedimentation due to the increase in density caused by the in-situ growth of magnetite (Fe3O4) crystals on the surface, and can also be removed by magnetic separation.

[0010] Preferably, the dissolved oxygen concentration is 0.5mg / L.

[0011] Preferably, the concentration of the divalent iron salt is 0.01-10mmol / L.

[0012] Preferably, the concentration of the divalent iron salt is 0.1-0.5mmol / L.

[0013] Preferably, the first preset stirring speed is greater than 100r / min, and the second preset stirring speed is less than 50r / min.

[0014] Preferably, in the step of allowing oxygen to diffuse into the open water body system and allowing the water body to settle to separate micro-nano plastics in the water body:

[0015] The standing time is 2-24h.

[0016] Preferably, the OH - The ratio of / Fe is 1-3.

[0017] The method for removing micro-nano plastics in water bodies proposed by the application has the advantages of low cost of flocculant, small dosage, good flocculation and sedimentation effect, and low residual flocculation ion concentration in water bodies, and can be used for removing micro-nano plastics in drinking water, sewage and natural water bodies.

[0018] Additional aspects and advantages of the application will be in part apparent and in part pointed out hereinafter in the description. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 a is the turbidity removal efficiency of water bodies containing 10mg / L of 500nm plastic microspheres under the condition of not regulating the water body environment by adding 0.1, 0.5, 1, 5 and 10mmol / L of FeSO4 flocculant respectively; Figure 1b is the turbidity removal efficiency graph of water containing 10 mg / L 100 nm plastic microspheres under the condition of not regulating the water body environment, respectively adding 0.1, 0.5, 1, 5 and 10 mmol / L FeSO4 flocculants;

[0020] Figure 2 a is the turbidity removal rate contrast graph of water containing 10 mg / L 500 nm plastic particles under the condition of not regulating the water body environment and regulating the water body environment (controlling oxygen diffusion, adding alkali to regulate OH - / Fe ratio is 2), respectively adding 0.1 and 0.5 mmol / L FeSO4 flocculants; Figure 2 b is the turbidity removal rate contrast graph of water containing 100 nm plastic particles under the condition of not regulating the water body environment and regulating the water body environment (controlling oxygen diffusion, adding alkali to regulate OH - / Fe ratio is 2), respectively adding 0.1 and 0.5 mmol / L FeSO4 flocculants;

[0021] Figure 3 a is a schematic diagram of magnetic flocs formed by using FeSO4 as a flocculant and regulating the water body dissolved oxygen and Fe / OH ratio; Figure 3 b is an X-ray diffraction pattern of the magnetic flocs;

[0022] Figure 4 a is an electron microscope morphology diagram of the flocs formed in Example 1, Figure 4 c and Figure 4 d is a transmission electron microscope morphology diagram of the flocs formed in Example 1.

[0023] Figure 5 is the turbidity removal rate curve of water containing 10 mg / L 500 nm and 100 nm under the condition of natural sedimentation, respectively adding 0.1 mM FeSO4 flocculant and regulating the water body environment (controlling oxygen diffusion, adding alkali to regulate OH - / Fe=2).

[0024] Figure 6 is the turbidity removal rate of water containing 10 mg / L 100 nm under the condition of different sedimentation time under the action of magnetic field, respectively adding 0.1 mM FeSO4 flocculant and regulating the water body environment (controlling oxygen diffusion, adding alkali to regulate OH - / Fe=2).

[0025] The following specific embodiments will further illustrate the present application in conjunction with the above figures. DETAILED DESCRIPTION

[0026] For the purposes of promoting an understanding of the principles of the application, reference will now be made to the embodiments illustrated in the drawings. There is shown in the drawings several embodiments of the application. However, it should be understood that the application can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete. It will fully convey the scope of the application to those skilled in the art.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0028] In some preferred embodiments of the application, the method for removing micro-nano plastics in a water body comprises the following steps:

[0029] Step S01: under anoxic conditions, a divalent iron salt is added to the water body containing micro-nano plastics as a flocculant under stirring at a first preset stirring speed;

[0030] In this step, the water body needs to be kept in anoxic or even anaerobic environment, and then a divalent iron salt is used as a flocculant. The divalent iron salt can be ferrous chloride, ferrous sulfate, or a mixture of ferrous chloride and ferrous sulfate. The concentration of the divalent iron salt is 0.01-10 mmol / L, preferably 0.1-0.5 mmol / L. For example, in some preferred embodiments of the application, the concentration of the divalent iron salt can be 0.1 mmol / L, 0.3 mmol / L, 0.5 mmol / L, etc.

[0031] It should be noted that the OH - The ratio of OH - The ratio of OH

[0032] Step S02: a sodium hydroxide solution is added to adjust the ratio of OH - The ratio of OH

[0033] It should be noted that the first preset stirring speed is generally greater than the second preset stirring speed, that is, the step S01 needs to maintain a high stirring speed, and the step S02 needs to maintain a low stirring speed, the first preset stirring speed is greater than 100 r / min, and the second preset stirring speed is less than 50 r / min, for example, in some preferred embodiments of the application, the first stirring speed can be 180 r / min, 200 r / min, 220 r / min, etc., and the second preset stirring speed can be 30 r / min, 40 r / min, 50 r / min, etc.

[0034] Step S03: Open the water body system, let oxygen diffuse into it, and let it stand and settle to separate micro-nano plastics in the water body.

[0035] The standing time is 2-24 h, for example, the standing time can be 10 h, 12 h, 14 h, etc.

[0036] Example 1

[0037] FeSO4 as flocculation, control oxygen, add alkali to control OH - / Fe ratio

[0038] a) Micro-nano plastic dispersion: uniformly disperse PS microspheres with particle sizes of 500 and 100 nm in water, control the concentration to be 10 mg / L, and remove dissolved oxygen in water by N2 exposure to simulate anoxic / anaerobic environment in water;

[0039] b) Add flocculant: add 0.1 and 0.5 mmol / L FeSO4 flocculant under magnetic stirring at 200 r / min;

[0040] c) Control the water environment: slowly add 0.1 mol / L NaOH solution under slow stirring to adjust the OH - / Fe ratio to 1-3, and the above steps are carried out in an anoxic environment; then open the system to let oxygen diffuse freely into it, and control the dissolved oxygen concentration to be 0.5 mg / L.

[0041] d) Turbidity and floc characterization: after standing for 12 h, take water samples to test turbidity and calculate turbidity removal rate; take the flocculation body, dry it, and perform electron microscope morphology and X-ray diffraction (XRD) characterization.

[0042] The turbidity removal results are shown in Figure 2 , the electron microscope morphology of the floc is shown in Figure 4 , and the XRD results are shown in Figure 3 b.

[0043] Comparative Example 1

[0044] FeSO4 flocculation, without controlling oxygen diffusion;

[0045] d) Micro-nano plastic dispersion: PS microspheres with particle sizes of 500 and 100 nm were uniformly dispersed in the water body, and the concentration was controlled to be 10 mg / L;

[0046] e) Add flocculants: Under the magnetic stirring of 200 r / min, 0.1, 0.5, 1, 5, and 10 mmol / L FeSO4 flocculants were added, respectively;

[0047] f) Turbidity and floc characterization: After standing for 12 h, the water sample was taken for turbidity test.

[0048] The turbidity removal results are shown in Figure 1 .

[0049] Figure 1 a shows that when the dosage of FeSO4 flocculant is 0.1-1 mmol / L, without adjusting the water body environment, it has a certain removal efficiency on the PS microspheres with a particle size of 500 nm, and when the concentration is increased to 5 mmol / L, the turbidity removal rate can exceed 90%. However, Figure 1 b shows that only when the dosage of FeSO4 is more than 1 mmol / L, it has a certain removal efficiency on the PS microspheres with a particle size of 100 nm in the water body, and when the concentration is 10 mmol / L, the removal efficiency exceeds 40%. Figure 2 a shows that when the dosage of FeSO4 flocculant is 0.1 and 0.5 mmol / L, and the water body environment is adjusted (controlling oxygen diffusion, adding alkali to control OH - / Fe ratio is 2-3), the removal efficiency of PS microspheres with a particle size of 500 nm is greatly improved, even reaching 100%. Moreover, Figure 2 b shows that the removal efficiency of PS microspheres with a particle size of 100 nm also changes from negative to positive, even exceeding 80%. The above results show that the method of the present application, using FeSO4 as a flocculant, by controlling oxygen diffusion and adjusting OH - / Fe ratio, can greatly improve the flocculation and sedimentation removal efficiency of micro-nano plastics, and can reduce the dosage of flocculants while maintaining high removal efficiency, thereby greatly reducing the flocculation cost of micro-nano plastics.

[0050] Figure 3 a shows that the flocs formed by using FeSO4 as a flocculant and adjusting the dissolved oxygen and OH - / Fe ratio of the water body can be attracted by a magnet, indicating that the flocs have magnetism. Figure 3 The XRD characterization results of b show that the flocs contain magnetic Fe3O4 components. Figure 4 The electron microscope morphology graph shows that Fe3O4 nanoparticles are generated in situ on the surface of the PS microspheres.

[0051] Figure 5 It is shown that, by adding 0.1 mM FeSO4, controlling oxygen diffusion, adjusting OH - / Fe=2. For 500 nm PS microspheres, the turbidity removal rate can reach nearly 60% after 2 h of settling, and reach 80% after 8 h of settling; while for 100 nm PS microspheres, the required settling time is longer, and only 40% is reached after 24 h. Figure 6 It is shown that, by adding 0.1 mM FeSO4, under the action of magnetic force, the removal rate of 100 nm PS microspheres is significantly accelerated, and the turbidity removal rate can reach 47% after 2 h of standing, and the turbidity removal rate is more than 87% after 6 h. It is shown that the magnetic method can be combined to strengthen the removal of micro-nano plastics in water bodies.

[0052] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0053] The above-described embodiments only express several implementation manners of the present application, which are described in a more specific and detailed manner, but cannot be understood as a limitation on the scope of the patent of the present application. It should be noted that, for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, which are all within the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for removing micro-nano plastics from water containing micro-nano plastics, characterized in that: The method comprises: Under anoxic conditions, using a divalent iron salt as a flocculant, the divalent iron salt is added to a water body containing micro-nanoplastics during stirring at a first preset stirring speed; and sodium hydroxide solution is added during stirring at a second preset stirring speed to adjust the OH- / Fe ratio in the water system; The water system was opened to allow oxygen to diffuse in, with a dissolved oxygen concentration of 0.5 mg / L, and allowed to settle to separate micro-nanoplastics in the water. The divalent iron salt is at least one of ferrous chloride and ferrous sulfate, and the concentration of the divalent iron salt is 0.01-10 mmol / L; The OH- / Fe ratio is 1-3; The particle size of the micro-nano plastic is 100-500nm.

2. A method for removing micro-nano plastics from water containing micro-nano plastics according to claim 1, characterized in that: The concentration of the divalent iron salt is 0.1-0.5 mmol / L.

3. The method for removing micro-nano plastics from water containing micro-nano plastics according to claim 2, characterized in that: The first preset stirring speed is greater than 100 r / min, and the second preset stirring speed is less than 50 r / min.

4. The method for removing micro-nano plastics from water containing micro-nano plastics according to claim 1, characterized in that: In the step of allowing oxygen to diffuse into the open water system and allowing the system to settle to separate micro-nano plastics from the water: The standing time is 2-24h.

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