Integrated separation unit for microplastics in the coastal sediments and collection method of microplastics

Abstract

The disclosure provides an integrated separation unit for microplastics in the coastal sediments and a collection method of microplastics, belonging to the technical field of water treatment. The unit includes: a holder, a separation cylinder, a collection bottle, a central baffle plate, a baffle plate control knob, a stirring propeller, a motor, a cylinder switch, a filtration screen, a welding nozzle, a filter membrane and a vacuum pump. Using this unit for microplastic collection has the advantages of easy operation, economical and environment-friendly, high separation efficiency, and high durability.

Claims

1. An integrated separation unit for microplastics in coastal sediments, comprising: a holder, a separation cylinder, a collection bottle, a central baffle plate, a baffle plate control knob, a stirring propeller, a motor, a cylinder switch, a filtration screen, a welding nozzle, a filter membrane and a vacuum pump; wherein the holder is threadedly connected to one end of the separation cylinder, and the other end of the separation cylinder is in a bayonet connection to the collection bottle; an inner bottom of the separation cylinder is provided with the stirring propeller, the stirring propeller is connected to the motor, the cylinder switch is arranged on an outer wall of the separation cylinder, and the motor is connected with the cylinder switch; the central baffle plate is arranged in an inner center of the separation cylinder, the central baffle plate is circular, a rotating shaft is arranged along a radial direction of the central baffle plate, one end of the rotating shaft passes through the separation cylinder and is connected with the baffle plate control knob; the filtration screen is arranged at a neck of the collection bottle, and the filtration screen is provided with the filter membrane, the welding nozzle is arranged at a lower side of the collection bottle, and the welding nozzle is connected with the vacuum pump.

2. The integrated separation unit for microplastics according to claim 1, wherein a diameter of the central baffle plate is equal to an inner diameter of the separation cylinder.

3. The integrated separation unit for microplastics according to claim 1, wherein the filter membrane is an aqueous filter membrane.

4. The integrated separation unit for microplastics according to claim 1, wherein apertures of the filter membrane are each of a size selected from 0.15 μm, 0.22 μm, 0.45 μm, 0.80 μm or 1.20 μm.

5. The integrated separation unit for microplastics according to claim 3, wherein the apertures of the filter membrane are each of a size selected from 0.15 μm, 0.22 μm, 0.45 μm, 0.80 μm or 1.20 μm.

6. A method of collecting microplastics using an integrated separation unit for microplastics, wherein the integrated separation unit for microplastics comprises: a holder, a separation cylinder, a collection bottle, a central baffle plate, a baffle plate control knob, a stirring propeller, a motor, a cylinder switch, a filtration screen, a welding nozzle, a filter membrane and a vacuum pump; wherein the holder is threadedly connected to one end of the separation cylinder, and the other end of the separation cylinder is in a bayonet connection to the collection bottle; an inner bottom of the separation cylinder is provided with the stirring propeller, the stirring propeller is connected to the motor, the cylinder switch is arranged on an outer wall of the separation cylinder, and the motor is connected with the cylinder switch; the central baffle plate is arranged in an inner center of the separation cylinder, the central baffle plate is circular, a rotating shaft is arranged along a radial direction of the central baffle plate, one end of the rotating shaft passes through the separation cylinder and is connected with the baffle plate control knob; the filtration screen is arranged at a neck of the collection bottle, and the filtration screen is provided with the filter membrane, the welding nozzle is arranged at a lower side of the collection bottle, and the welding nozzle is connected with the vacuum pump, the method comprising the following steps: (1) selecting water sampling sites to obtain sediment samples; (2) pre-treating debris by filtering debris from sediment samples via 1-5 mesh stainless steel screens, drying the sediment samples filtered at 100-110° C. for 1-2 h, removing the debris and cooling it to room temperature to obtain the sediment samples to be separated; (3) separating by adjusting the baffle plate control knob to cause the central baffle plate in the separation cylinder to be parallel to a barrel body of the separation cylinder; pouring the sediment samples to be separated obtained from step (2) and saturated NaCl solution into the separation cylinder; switching on the cylinder switch so that the stirring propeller stirs the sediment samples to be separated for 2-3 min; settling the sediment samples to be separated for solid-liquid stratification; adjusting the baffle plate control knob to cause the central baffle plate in the separation cylinder to be perpendicular to the barrel body of the separation cylinder; wherein a total volume of the sediment samples to be separated and the saturated NaCl solution do not exceed ⅔ of the volume of the separation cylinder; and, (4) collecting the sediment samples by laying the aqueous filter membrane over the filtration screen in the collection bottle; providing a bayonet connection between the separation cylinder and the collection bottle; connecting the welding nozzle with the vacuum pump; switching on the vacuum pump for suction filtration to enable collection of the microplastics as they are separated from the water in the sediment samples passing through the aqueous filter membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a structural representation showing the unit of the invention;

(2) FIG. 2 is the microscopic image of microplastics according to Embodiment 1;

(3) FIG. 3 is the microscopic image of microplastics according to Embodiment 2; and

(4) FIG. 4 is a schematic diagram showing a stirring blade, a cylinder switch and a motor.

(5) Wherein, A—holder, B—separation cylinder, C—collection bottle, 1—central baffle plate, 2—baffle plate control knob, 3—stirring blade, 4—filtration screen, 5—welding nozzle, 6—neck, 7—inner bottom of the separation cylinder, 8—filter membrane, 9—vacuum pump, 10—cylinder switch, 11—rotating shaft, 12—motor.

DESCRIPTION OF THE EMBODIMENTS

(6) The present invention will be further illustrated accompanying with the appended drawings and embodiments. The following embodiments are used to further explain the present invention, but not limit the scope of the invention.

(7) As shown in FIG. 1, it is a structural representation showing the integrated separation unit for microplastics of the invention. The unit includes: a holder A, a separation cylinder B, a collection bottle C, a central baffle plate 1, a baffle plate control knob 2, a stirring propeller 3, a motor 12, a cylinder switch 10, a filtration screen 4, a welding nozzle 5, a filter membrane 8 and a vacuum pump 9; the holder A is connected to one end of the separation cylinder B through screw threads, so that the separation cylinder B is stable and does not fall down when stirred; the other end of the separation cylinder B is connected to the collection bottle C through a bayonet. The collection bottle C used for vacuum suction filtration can be made of chemically inert glass material or PTFE material by technicians in this field. After the sediments are poured into the separation cylinder B, the central baffle plate 1 of the separation cylinder B is erected inside the separation cylinder B and is parallel to the separation cylinder B. The saturated NaCl solution is added into the separation cylinder B and mixed with the sediments. The motor is switched on to stir the sediments, so that microplastics can be centrifugally floated on the liquid surface; Solid-liquid stratification occurs after standing, microplastics and other low-density debris less than 5 mm can be separated from the sediments and suspended to the upper part of the separation cylinder B based on the density separation method; the central baffle plate 1 is rotated slowly until it is vertical to the separation cylinder B, so that the upper and lower chambers of the separation cylinder B are completely separated. Therefore, the integrated separation unit for microplastics is a device for separating microplastics based on centrifugal flotation and the density separation method. The inner bottom 7 of the separation cylinder B is provided with a stirring propeller 3. As shown in FIG. 4, the stirring propeller 3 is connected to a motor 12. The cylinder switch 10 is arranged on the outer wall of the separation cylinder B. The motor is connected with the cylinder switch. The motor is an n20 DC geared micro-motor (6V). The central baffle plate 1 is arranged in the inner center of the separation cylinder B, and the central baffle plate 1 is circular. A rotating shaft 11 is arranged along one diameter of the central baffle plate 1, one end of the rotating shaft passes through the separation cylinder B and connected with the baffle plate control knob 2. The baffle plate control knob 2 controls the rotation of the rotating shaft and drives the central baffle plate 1 to flip inside the separation cylinder B, so that the central baffle plate 1 can be flipped to parallel or vertical to the separation cylinder B. When the central baffle plate 1 is flipped to be vertical to the separation cylinder B, the separation cylinder B is equally divided into upper and lower chambers. A filtration screen 4 is arranged at the neck 6 of the collection bottle C. The filtration screen 4 is a glass sand core filtration screen. The filtration screen 4 is provided with a filter membrane. The filter membrane is an aqueous filter membrane. The aperture of the filter membrane is selected from 0.15 μm, 0.22 μm, 0.45 μm, 0.80 μm or 1.20 μm. A welding nozzle 5 is arranged at the lower side of the collection bottle C. The welding nozzle is made of the same material as the collection bottle C. The welding nozzle 5 is connected with the vacuum pump, by which microplastics are separated and adsorbed onto the filter membrane through suction filtration.

(8) The operation principle of the integrated separation unit for microplastics of the invention is as below: microplastics usually refer to plastic products or plastic scraps less than 5 mm; based on the principle of centrifugal flotation and density separation, using the unit of the invention, the sediments to test are firstly settled naturally, allowing particles or fibers less than 5 mm to float up to the upper part of the separation cylinder B, thus achieving preliminary separation by means of the central baffle plate 1; after then, suction filtration is performed through the microporous filter membrane with the purpose of removing water, therefore allowing microplastic particles or fibers to be separated and adsorbed onto the microporous filter membrane for collection in order to be further determined by microscopy and Fourier transform infrared spectroscopy.

Embodiment 1

(9) A microplastic collection experiment conducted on the coastal sediments from waters of Zhoushan, Zhejiang, East longitude 122°09.344, North latitude 29°51.531, was taken as an example:

(10) (1) Sampling: A stretch of coast was randomly selected for sampling to obtain sediment samples.

(11) (2) Pre-treatment: The debris in the sediment samples, such as large plastics, dead branches, rocks, glass or others, was filtered out over 5-mesh stainless steel screens. The sediments were identified as quartz sand. 100.00 g of the sediments were weighed, dried at 110° C. for 2 h, taken out and cooled to obtain the sediments to be separated, which was weighed again as 96.41 g, so the water content was calculated as 3.59%.

(12) (3) Separation: The baffle plate control knob 2 was adjusted to make the central baffle plate 1 in the separation cylinder B parallel to the barrel body of the separation cylinder B; the above obtained sediments to be separated were poured into the separation cylinder B, into which was also added the saturated NaCl solution; the cylinder switch was switched on so that the stirring propeller 3 stirred the sediments for 2 min; the sediments to be separated were then settled for solid-liquid stratification; the baffle plate control knob 2 was adjusted to make the central baffle plate 1 in the separation cylinder B vertical to the barrel body of the separation cylinder B; the total volume of the sediments to be separated and the saturated NaCl solution was ⅔ of the volume of the separation cylinder B.

(13) (4) Collection: the aqueous filter membrane of 0.15 μm was laid over the center of the filtration screen 4 in the collection bottle C, which was wetted with distilled water so that it fitted perfectly with the screen; the separation cylinder B was inverted and connected with the collection bottle C through a bayonet; the welding nozzle 5 of the collection bottle C is connected to the rubber hose of the vacuum pump; the vacuum pump was switched on for suction filtration for 5 min. The rubber hose of the vacuum pump was removed and the vacuum pump was shut off. The collection bottle C was taken out. The aqueous microporous filter membrane on the collection bottle C was washed with distilled water for three times. The suction filtration was conducted by the vacuum pump again for 5 min and then the vacuum pump was shut off. Microplastics were collected on the 0.15 μm aqueous filter membrane.

(14) The microplastics collected by the above method were observed under a microscope, with the results shown in FIG. 2. They were determined as microfibers, which were linear in shape, without bending or winding, and the thickness of each fiber was uniform.

Embodiment 2

(15) A microplastic collection experiment conducted on the coastal sediments from waters of Nanji Island, Zhejiang, East longitude 122°06.805, North latitude 27°46.045, was taken as an example:

(16) (1) Sampling: A stretch of coast was randomly selected for sediment sampling to obtain sediment samples.

(17) (2) Pre-treatment: The debris in the sediment samples, such as large plastics, dead branches, rocks, glass or others, was filtered out over 1-mesh stainless steel screens. The sediments were identified as mixtures of quartz sand and gravel. 300.00 g of the sediments were weighed, dried at 100° C. for 1 h, taken out and cooled to obtain the sediments to be separated, which was weighed again as 290.02 g, so the water content was calculated as 3.32%.

(18) (3) Separation: The baffle plate control knob 2 was adjusted to make the central baffle plate 1 in the separation cylinder B parallel to the barrel body of the separation cylinder B; the above obtained sediments to be separated were poured into the separation cylinder B, into which was also added the saturated NaCl solution; the cylinder switch was switched on so that the stirring propeller 3 stirred the sediments for 3 min; the sediments to be separated were then settled for solid-liquid stratification; the baffle plate control knob 2 was adjusted to make the central baffle plate 1 in the separation cylinder B vertical to the barrel body of the separation cylinder B; the total volume of the sediments to be separated and the saturated NaCl solution was ⅔ of the volume of the separation cylinder B.

(19) (4) Collection: the aqueous filter membrane of 1.20 μm was laid over the center of the filtration screen 4 in the collection bottle C, which was wetted with distilled water so that it fitted perfectly with the screen; the separation cylinder B was inverted and connected with the collection bottle C through a bayonet; the welding nozzle 5 of the collection bottle C is connected to the rubber hose of the vacuum pump; the vacuum pump was switched on for suction filtration for 5 min. The rubber hose of the vacuum pump was removed and the vacuum pump was shut off. The collection bottle C was taken out. The aqueous microporous filter membrane on the collection bottle C was washed with distilled water for three times. The suction filtration was conducted by the vacuum pump again for 5 min and then the vacuum pump was shut off. Microplastics were collected on the 1.20 μm aqueous filter membrane.

(20) The above collected microplastics were observed under a microscope, with the results shown in FIG. 3. They were determined as microfibers, which were linear in shape, without bending or winding, and the thickness of each fiber was uniform.

Embodiment 3

(21) The separation performance of the integrated separation unit of the invention is compared with those of pipette filtration, density method and direct dumping method.

(22) (1) Sample Collection

(23) The sampling sites and grouping of sediments are shown in Table 1:

(24) TABLE-US-00001 TABLE 1 Sampling sites and grouping of submarine sediments Sampling Longitude Total Amount sites and Latitude <0.1 mm 0.1~0.315 mm 0.315~0.5 mm 0.5~1 mm Separation Method 1 East longitude \ 100 g 500 g 76 g Integrated 122°09.344 separation unit North latitude 7 g 200 g 500 g 76 g Pipette filtration 29°51.531 7 g 100 g 500 g \ Direct dumping \ \ \ 76 g Density method 2 East longitude \ 70 g 500 g 170 g Integrated 122°09.761 separation unit North latitude 3 g 70 g 500 g 170 g Pipette filtration 29°50.501 3 g 70 g 500 g \ Direct dumping \ \ \ 170 g Density method 3 East longitude \ 200 g 500 g 170 g Integrated 122°10.251 separation unit North latitude 11 g  200 g 500 g 170 g Pipette filtration 29°51.552 11 g  200 g 500 g \ Direct dumping \ \ \ 170 g Density method

(25) (2) Experimental Methods

(26) All the samples were divided into large particle size (>0.5 mm) and small particle size (<0.5 mm) through the screen, from which microplastics were extracted by employing four methods including separation with the integrated separation unit, pipette filtration, density method, and direct dumping method.

(27) Method {circle around (1)}: Separation with the integrated separation unit, employing the method of Embodiment 1

(28) Method {circle around (2)}: Pipette filtration

(29) The debris in the tested sediments, such as large plastics, dead branches, rocks, glass or others, was filtered out over 5-mesh stainless steel screens. The sediments were identified and weighed, dried at 110° C. for 2 h, taken out and cooled, and then weighed again to calculate the water content. Then, 15 mL soil samples were taken each time and placed in a test tube, into which was added an appropriate amount of saturated NaCl solution (1000 mL distilled water, 360 g sodium chloride crystal) and stirred uniformly. After adjusting the quality of each test tube (ensuring that the quality of each test tube is equal as far as possible), they were placed into a centrifuge for centrifugation (4000 rpm, 5 min). The suction filter device was provided with a filter membrane. An appropriate amount of supernatant was taken using a pipette and passed through the filter membrane, then suction filtered with the vacuum pump and further suction filtered by adding distilled water to wash NaCl particles repeatedly for several times, to obtain the filter membrane samples, which were packaged in a filter membrane storage box and observed under a microscope.

(30) Method {circle around (3)}: Direct dumping method

(31) The debris in the tested sediments, such as large plastics, dead branches, rocks, glass or others, was filtered out over 5-mesh stainless steel screens. The sediments were identified and weighed, dried at 110° C. for 2 h, taken out and cooled, and then weighed again to calculate the water content. Then, 15 mL soil samples were taken each time and placed in a test tube, into which was added an appropriate amount of saturated NaCl solution (1000 mL distilled water, 360 g sodium chloride crystal) and stirred uniformly. After adjusting the quality of each test tube (ensuring that the quality of each test tube is equal as far as possible), they were placed into a centrifuge for centrifugation (4000 rpm, 5 min). The suction filter device was provided with a filter membrane. The supernatant of the centrifuge tube was collected and suction filtered with the vacuum pump, and further suction filtered by adding distilled water to wash NaCl particles repeatedly for several times, to obtain the filter membrane samples, which were packaged in a filter membrane storage box and observed under a microscope.

(32) Method {circle around (4)}: Density method

(33) The debris in the tested sediments, such as large plastics, dead branches, rocks, glass or others, was filtered out over 5-mesh stainless steel screens. The sediments were identified and weighed, dried at 110° C. for 2 h, taken out and cooled, and then weighed again to calculate the water content. Then, soil samples were poured into a volumetric flask, into which was also added an appropriate amount of saturated NaCl solution (1000 mL distilled water, 360 g sodium chloride crystal) and stirred uniformly. The volumetric flask was inverted slowly and placed on the iron stand to set. When the solution was obviously stratified, the lower sediments were removed, the upper solution was taken and filtered with filter papers (distilled water was added to remove NaCl particles). The filtered samples were transferred into alcohol. The lower solution and sediments were taken and filtered with filter papers. Finally, the filter papers were dried in a culture dish to obtain the suspected microplastic samples, which were observed under a microscope.

(34) (3) Experimental results

(35) The abundances (n/kg) of each group of microplastics obtained through experiments were shown in Tables 2-5:

(36) TABLE-US-00002 TABLE 2 Abundance (n/kg) of microplastics obtained by the integrated separation unit Integrated separation unit <0.1 mm 0.1~0.315 mm 0.315~0.5 mm 0.5~1 mm 1 180 267 351 2 678 210 209 3 381 182 214

(37) TABLE-US-00003 TABLE 3 Abundance (n/kg) of microplastics obtained by the pipette filtration method Pipette filtration <0.1 mm 0.1~0.315 mm 0.315~0.5 mm 0.5~1 mm 1 2142 271 170 131 2 1865 514 156 176 3 1090 437 100 82

(38) TABLE-US-00004 TABLE 4 Abundance (n/kg) of microplastics obtained by the direct dumping method Direct dumping method <0.1 mm 0.1~0.315 mm 0.315~0.5 mm 0.5~1 mm 1 1285 267 160 2 2053 328 207 3 727 233 90

(39) TABLE-US-00005 TABLE 5 Abundance (n/kg) of microplastics obtained by the density method Density method <0.1 mm 0.1~0.315 mm 0.315~0.5 mm 0.5~1 mm 1 289 2 215 3 127

(40) Microplastics were classified according to their morphological and structural characteristics by visual inspection (i.e. direct visual observation or with the assistance of microscope). Generally, three experimenters were randomly selected for observation: linear microplastics with uniform thickness and greater length than width were identified as fibrous microplastics; microplastics with clear boundary, uniform color and similar length and width were identified as granular microplastics; flaky or laminar microplastics were identified as membranous microplastics. It was shown from the experimental results that in all the samples, fibrous microplastics accounted for the largest proportion (46.91%), followed by membranous microplastics (40.74%) and granular microplastics (12.35%), wherein fibrous microplastics had the highest abundance value; with the increase of the particle size of microplastics, the number of microplastics decreased. Different separation methods were compared, with the conclusions shown below:

(41) Methods separation with the integrated separation unit, {circle around (2)} pipette filtration, {circle around (3)} direct dumping method and {circle around (4)} density method all can achieve effective separation of microplastics in the submarine sediments. It can be seen from the analysis of Table 6 that, for the main research group of microplastics at present (particle sizes of 0.5˜1 mm), the overall recovery efficiency and operating efficiency of the method {circle around (1)} separation with the integrated separation unit were superior to those of other three methods.

(42) TABLE-US-00006 TABLE 6 Comparison of abundances (n/kg) of the main research group of microplastics (particle sizes of 0.5~1 mm) collected by different methods Pipette Integrated Density 0.5~1 mm filtration separation unit method 1 131 351 289 2 176 209 215 3 82 214 127