A method for treating microplastics in water bodies

Abstract

This invention discloses a method for treating microplastics in water, belonging to the field of water environment management technology. This invention utilizes laser plasma to degrade microplastics. The laser plasma can not only directly act on the surface of microplastics to promote degradation, but also act on an iron substrate to generate iron / iron oxide nanoparticles adsorbed on the surface of the microplastics. These nanoparticles participate in the subsequent degradation reaction in the UV / H2O2 system. Compared with traditional Fenton reaction degradation, the pretreated microplastics show better degradation performance in the UV / H2O2 system, and can degrade PE microplastic particles into smaller particles more quickly, reducing the particle size to below nm.

Description

Technical Field

[0001] This invention belongs to the field of water environment management technology, and specifically relates to a method for treating microplastics in water. Background Technology

[0002] Plastics are macromolecules polymerized from organic monomers through addition or condensation polymerization. Most plastics are lightweight, chemically stable, impact-resistant, insulating, have low thermal conductivity, low processing costs, good transparency, and wear resistance. Due to these excellent properties, plastics are widely used in daily life. However, due to improper handling, large amounts of plastic waste enter the environment through various pathways, causing serious environmental pollution problems. Through physical, chemical, and biological processes, plastic waste entering the environment produces many small plastic particles; these particles, with a diameter of less than 5 mm, are called "microplastics," a new type of pollutant of widespread international concern. The toxicity of microplastics mainly stems from their chemical structure, physical properties, and the microorganisms they carry. First, because plasticizers, stabilizers, lubricants, colorants, and other additives are added during the manufacturing process, unreacted monomers, oligomers, and chemical additives will leak from the plastic due to prolonged mechanical friction, causing pollution. Second, because microplastics have a relatively large specific surface area, they will absorb existing hydrophobic pollutants in the water and carry them to other environments. Furthermore, microplastics can carry pathogens and other microorganisms into the environment, thus affecting health. Therefore, research on the degradation of microplastics is of great significance.

[0003] Currently known methods for microplastic degradation mainly include physical degradation, biodegradation, and chemical degradation. Physical degradation of microplastics primarily occurs through the abrasion of particles with other substances in the environment. The degree of mechanical abrasion varies depending on the polymer type. For example, polypropylene (PP) and PE particles are unlikely to degrade mechanically on their own, but expanded polystyrene (EPS) can break into many smaller fragments simply through friction. An experimental study by Enfrin et al. showed that microplastics can also undergo mechanical weathering in a water column when exposed to shear stress, but under low shear stress, microplastics will decompose into nanoplastics, thus introducing more plastic particles into the environment. Advanced oxidation and biodegradation methods have been extensively studied for degrading microplastics and have shown good performance. Biodegradation mainly follows two approaches: producing biodegradable plastics and using organisms capable of degrading plastics. The first strategy typically involves blending ordinary plastics with biopolymers to produce biodegradable plastics, while the second strategy involves researching organisms capable of converting complex polymers into simple molecules (such as carbon dioxide and water). These two bio-based degradation strategies can enable the natural degradation of microplastic particles remaining in the environment, but they require suitable conditions and microorganisms, and the efficiency of biodegradation needs further improvement. Advanced oxidation processes (AOPs) oxidize large, recalcitrant organic molecules into low-toxicity or non-toxic small molecules by generating highly oxidizing free radicals. Studies have shown that homogeneous and heterogeneous AOPs, such as UV photolysis, UV / H₂O₂, O₃, UV / Vis-induced photocatalysis, and thermally activated PS and PMS, can effectively degrade different types of microplastics (MPs). Photodegradation is considered a key process in polymer decomposition. When MPs are exposed to ultraviolet light for extended periods, due to the effects of free radicals and oxygen, they undergo chemical chain breakage or cross-linking, as well as some morphological changes. Ultraviolet light is considered the most important factor affecting this process. Therefore, effectively improving the utilization rate of ultraviolet light will enhance the degradation efficiency of MPs. Summary of the Invention

[0004] In view of the above-mentioned prior art, the present invention provides a method for treating microplastics in water, so as to solve the technical problem of difficult degradation of microplastics in the water environment in the prior art.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is to provide a method for treating microplastics in water, comprising the following steps:

[0006] S1: Place water containing microplastics into a reaction vessel, place an iron sheet into the reaction vessel, and then irradiate the iron sheet with a laser;

[0007] S2: After laser irradiation, the iron sheet is removed, filtered, microplastics are collected, dried, and laser plasma pretreated plastic particles are obtained.

[0008] S3: Disperse the laser plasma pretreated plastic particles in a mixed solvent, which is obtained by mixing deionized water and hydrogen peroxide solution; then irradiate the dispersion system with ultraviolet light for 15-55 hours to complete the treatment of microplastics in the water.

[0009] Based on the above technical solution, the present invention can be further improved as follows.

[0010] Furthermore, the concentration of microplastics in water bodies containing microplastics is 0.8–1.2 mg / mL.

[0011] Furthermore, the laser used in S1 has a frequency of 5Hz.

[0012] Furthermore, the laser irradiation time is 1 to 3 hours.

[0013] Furthermore, the pore size of the filter membrane used for filtration in S2 is 0.1–0.22 μm.

[0014] Furthermore, the drying temperature in S2 is 40°C.

[0015] Furthermore, the concentration of the hydrogen peroxide solution is 30 wt%.

[0016] Furthermore, the volume ratio of deionized water to hydrogen peroxide solution in the mixed solvent is 1:1.

[0017] Furthermore, the ratio of the laser plasma pretreated plastic particles to the mixed solvent is 5 mg: 2 mL.

[0018] Furthermore, the ultraviolet irradiation time in S3 is 18 hours.

[0019] The beneficial effects of this invention are:

[0020] This invention utilizes laser plasma to degrade PE microplastics. Laser plasma has a degrading effect on PE microplastics and can accelerate their degradation within a UV / H2O2 system. The effects of laser plasma in the PE microplastic degradation process are mainly divided into two parts: First, the laser plasma directly acts on the surface of the PE microplastics, and the rapid temperature rise causes thermal ionization and thermal decomposition of the PE particles, resulting in degradation reactions such as dehydrogenation, carbon-carbon bond breaking, and recombination. Second, the laser plasma acts on an iron substrate, generating iron / iron oxide nanoparticles that adsorb onto the surface of the PE microplastics and participate in the subsequent degradation reaction in the UV / H2O2 system, accelerating the degradation process. In the first stage, the laser plasma directly acts on the surface of the PE microplastics, causing a rapid increase in surface temperature, resulting in ablation, melting, and vaporization. Due to the Gaussian distribution of laser energy, the temperature rise of microplastic particles at different locations varies, leading to different morphologies such as cracks, fragmentation, and layered structures. Simultaneously, the PE microplastic surface is broken into fragments under the influence of the laser plasma shock wave. These effects disrupt the PE microplastic structure, causing some chemical bonds to break and allowing additives to leach out, thus degrading the PE microplastics. In the second stage, iron / iron oxide nanoparticles are adsorbed onto the surface of the PE microplastics. When in the UV / H2O2 system, the iron nanoparticles directly participate in the Fenton-like reaction, while the iron oxide nanoparticles indirectly participate as catalysts. Compared with traditional Fenton reaction degradation, the pretreated PE microplastics show better degradation in the UV / H2O2 system, rapidly reducing the PE microplastic particles into smaller particles, with particle sizes decreasing to below nm. Attached Figure Description

[0021] Figure 1 A schematic diagram of the experimental setup used for a method to treat microplastics in water; wherein, 1. laser; 2. beam splitter; 3. laser energy detector; 4. condenser lens; 5. reaction vessel; 6. ultraviolet lamp irradiation chamber;

[0022] Figure 2 Here are SEM images of PE particles; where (a) and (b) are SEM images at different scales, respectively.

[0023] Figure 3 This is a particle size distribution diagram of PE particles;

[0024] Figure 4 SEM images of the sample obtained in Example 1 at different scales;

[0025] Figure 5 The infrared spectrum of the sample obtained in Example 1;

[0026] Figure 6 SEM images of the samples obtained in Examples 2 and 3;

[0027] Figure 7 Infrared spectra of the samples obtained in Example 3 and Example 2;

[0028] Figure 8 The nanoparticle size distribution diagrams are shown for the samples obtained in Examples 2 and 3.

[0029] Figure 9 The temperature (a) and thermal stress (b) distributions of PE particles under laser irradiation for different durations are shown in the diagram.

[0030] Figure 10 The curves show the temperature (a) and thermal stress (b) at different sites of PE particles as a function of time.

[0031] Figure 11 This diagram shows the changes that occur when laser plasma directly acts on PE microplastics.

[0032] Figure 12 EDS images of the original PE particles (a) and the PE particles after laser plasma pretreatment (b);

[0033] Figure 13 The atomic percentages are those of the original PE particles and the PE particles after laser plasma pretreatment.

[0034] Figure 14 XRD (a) and Raman spectra (b) of iron oxides are shown.

[0035] Figure 15 The degradation mechanism of PE particles pretreated with laser plasma in a UV / H2O2 system. Detailed Implementation

[0036] The chemical reagents used in the experiments of this invention include 200-mesh polyethylene (PE), purchased from Hengfa Plastics Co., Ltd., with a density of 0.926 g / cm³. 3 It belongs to low-density polyethylene and has a melting temperature of 251.6K.

[0037] A schematic diagram of the apparatus used in the experiment of this invention is shown below. Figure 1As shown, the system includes a laser 1, a beam splitter 2, a laser energy detector 3, a condenser lens 4, a reaction vessel 5, and an ultraviolet lamp irradiation chamber 6. The laser 1 is a 1064nm Nd:YAG laser (Laserver, Wuhan, China), and the laser energy detector 3 is a laser energy meter (Coherent, USA). The laser emitted from the laser 1 passes through the beam splitter 2. A portion of the laser light travels through the optical path to the laser energy detector 3, thereby measuring the laser intensity in real time. The other portion of the laser light is focused by the condenser lens 4 (GCL-0108, Beijing, China) onto the iron metal plate of the reaction vessel 5 (preferably a test tube in this invention), which contains the sample, thus forming a laser plasma degradation reactor. The microplastics treated with laser plasma are then placed in the ultraviolet lamp irradiation chamber 6 for further degradation.

[0038] The specific embodiments of the present invention will be described in detail below with reference to examples.

[0039] Example 1: Laser plasma treatment of microplastics in water

[0040] Weigh 20 mg of PE particles into a test tube, add 2 mL of deionized water, and stir for 1 minute in a dark environment. Then add an iron sheet and continue stirring for 1 minute in a dark environment. Irradiate the solution containing PE microplastics using a 5 Hz laser, focusing the laser onto the iron substrate at a distance of approximately 1.5 cm from the liquid surface. The laser irradiation times are 1 h, 2 h, and 3 h, respectively. After laser irradiation, remove the iron sheet from the solution, and then filter the solution using a 0.1 μm or 0.22 μm pore size filter membrane as needed to obtain microplastic particles. Dry the particles in a 40 °C drying oven, collect the dried sample, and obtain the laser plasma-treated sample.

[0041] Example 2: Laser plasma + UV / H2O2 degradation treatment of microplastics in water

[0042] Weigh 20 mg of PE particles into a test tube, add 2 mL of deionized water, and stir for 1 minute in a dark environment. Then add an iron sheet and continue stirring for 1 minute in a dark environment. Next, irradiate the solution containing PE microplastics using a 5 Hz laser. The laser is focused on the iron substrate at a distance of approximately 1.5 cm from the liquid surface, and the laser treatment time is 1 hour. After irradiation, separate the metal plate from the solution, and then filter the microplastic particles using a 0.22 μm filter membrane. Dry the particles in a 40°C drying oven, and collect the dried sample to obtain the sample to be degraded using the UV / H2O2 system.

[0043] Weigh 10 mg of the sample to be degraded by the UV / H2O2 system, place it in a glass tube, add 1 mL of deionized water and 1 mL of 30% H2O2 solution, and stir for 1 minute in a dark environment; then place the glass tube containing the sample in a UV lamp irradiation chamber for degradation. After UV irradiation for 18 h, 36 h, and 54 h, filter the sample using a filter membrane with a pore size of 0.1 μm or 0.22 μm as needed to obtain microplastic particles. Dry the particles in a drying oven at 40 °C, collect the dried sample, and obtain the laser plasma + UV / H2O2 degradation treated sample.

[0044] Example 3: UV / H2O2 Degradation Treatment of Microplastics in Water

[0045] 10 mg of PE particles were weighed and placed in a glass tube. 1 mL of deionized water and 1 mL of 30% H₂O₂ solution were added, and the mixture was stirred for 1 minute in a dark environment. Then, an iron sheet was added as an iron source, and the mixture was stirred for another minute in a dark environment. The glass tube containing the sample was then placed in a UV lamp irradiation chamber for degradation. After UV irradiation for 18 h, 36 h, and 54 h, the microplastic particles were obtained by filtration using a 0.1 μm or 0.22 μm pore size filter membrane, as needed. The particles were then dried in a 40℃ drying oven, and the dried samples were collected to obtain the UV / H₂O₂ degradation-treated samples.

[0046] Experimental Example

[0047] I. The effect of laser plasma treatment on the degradation of microplastics

[0048] Figure 2 The SEM images of PE particles at different scales show that their surfaces are rough, formed by an alternating combination of granular and filamentous structures. The original PE particle sizes range from 100 to 300 μm, and their particle size distribution is as follows. Figure 3 As shown, the particle sizes corresponding to the cumulative particle size distribution numbers reaching 10%, 50%, and 90% are d[0.1] = 104.068 μm, d[0.5] = 144.871 μm, and d[0.9] = 200.994 μm, respectively.

[0049] Figure 4 The images show SEM images of the samples obtained in Example 1 at different scales. (a), (b), and (c) are samples after 1 hour of laser irradiation; (d), (e), and (f) are samples after 2 hours of laser irradiation; and (g), (h), and (i) are samples after 3 hours of laser irradiation. It can be observed that the original granular material on the PE surface gradually melts and adheres together, the adhered parts become smoother, and the filamentous structure is gradually torn apart, forming cracks on the PE surface, which is conducive to further photodegradation. At the same time, various plastic fragments were found, such as... Figure 4(c)(f)(i) This is due to the breakage of some particles. Particles closer to the laser's action area are subjected to a stronger effect, resulting in a faster degradation process. PE particles are melted and broken under high temperature and pressure, forming many fragments of varying sizes that float in the liquid environment. The fragments have inconsistent morphologies, mostly sheet-like, with sizes ranging from micrometers to nanometers, and micrometer-sized fragments being the most numerous. Additionally, the laser plasma induced by the iron sheet brings some additional effects. The ablation of the iron substrate by the laser forms some iron oxide nanoparticles. These particles adsorb onto the surface of the PE microplastics and even enter surface crevices, such as... Figure 3 As shown in (b)(e)(h), these iron oxide nanoparticles will play a role in the subsequent degradation of PE microplastics.

[0050] The functional group changes in the sample obtained in Example 1 were detected by Fourier transform infrared spectroscopy, with a measurement range of 4000–400 cm⁻¹. -1 .like Figure 5 As shown, the original PE microplastic particles were observed at 1460 cm⁻¹. -1 The region shows a characteristic peak (indicating bending deformation) at 2919 cm. -1 2857cm -1 (Can be assigned to CH2 as asymmetric stretching) and 719cm -1 (Indicating swaying deformation), these peaks can be considered characteristic peaks of PE particles. It can be observed that after the action of laser plasma, the characteristic peaks of PE become less pronounced, which may be due to the surface melting caused by laser ablation. (1620 cm⁻¹) -1 At point C = C and 1000cm -1 ~1100cm -1 A slight increase in the CO peak is observed, which may be due to the breakage and oxidation of carbon chains caused by the high-energy impact of the laser plasma. The formation of C=C double bonds reflects the breaking of chemical bonds on the surface of PE particles, while the formation of CO bonds indicates surface oxidation. (3400 cm⁻¹) -1 The hydroxyl group (OH) is located at 3100–3600 cm⁻¹ -1 The stretching region is likely due to free radical substitution formed under laser plasma. Overall, after laser plasma treatment, the disappearance of PE's characteristic peaks and the formation of hydroxyl (OH) groups are more pronounced, which are the main changes throughout the process. The thermal effect of the laser on the PE surface is the primary reason.

[0051] The main effects of laser plasma on PE microplastics are ablation, fracture, and fragmentation of the particle surface. These effects promote the breaking of carbon chains and the release of additives within the microplastics, thereby accelerating their degradation. Comparison of laser plasma degradation results after 1 hour, 2 hours, and 3 hours revealed that the degradation effect did not change significantly over time. Subsequent studies will use PE microplastics filtered after 1 hour of laser treatment as the experimental subject.

[0052] II. The Effects of Laser Plasma + UV / H2O2 Degradation Treatment on Microplastic Degradation

[0053] Figure 6 The images show SEM images of the samples obtained in Examples 2 and 3, where (a), (b), and (c) are SEM images of the samples obtained in Example 3 after 18h, 36h, and 54h of UV irradiation, respectively; and (d), (e), and (f) are SEM images of the samples obtained in Example 2 after 18h, 36h, and 54h of UV irradiation, respectively. As can be seen from the images, the samples obtained in Example 3 underwent a process of fragmentation-fusion-re-fragmentation during degradation. After 18h of degradation, the surface of the PE particles showed fragmentation and larger voids, with more filamentous material remaining and a tendency to unfold. After 36h of degradation, the PE particles unfolded into a wrinkled shape, and the original granular and filamentous structures changed, with more wrinkled structures appearing. These structures fused and accumulated together. After 54h of degradation, the fused structure began to break down again, and the particle size after the second fragmentation decreased to about 1 / 5 of the original size. After degradation, the particle size of the samples obtained in Example 2 changed significantly, from micrometers to nanometers. Figure 5 As shown in (df), after 18 hours of treatment, the PE particles had been degraded into nanoparticles, which accumulated together. This degradation process was faster than that of the control group. However, after this point, the morphology and size of the PE particles did not change significantly, indicating that the degradation process slowed down after 18 hours. This is because dissolved organic matter was released as the degradation reaction proceeded, and these dissolved substances were oxidized faster than the solid particles. In addition, no additional hydrogen peroxide was added during the experiment, which could not provide enough hydroxyl radicals to oxidize the PE microplastics. This may be the reason for the slow degradation in the subsequent process.

[0054] Figure 7 (a) and (b) are the infrared spectra of the samples obtained in Example 3 and Example 2, respectively. As can be seen from the figures, with the progress of photodegradation, the FTIR results of the samples obtained in Example 3 and Example 2 show the same trend, with the characteristic peak values ​​gradually decreasing, down to 1620 cm⁻¹. -1 3400cm -1 Peak growth at 1620cm -1 The formation of the C=C peak at 3400 cm⁻¹ is likely due to dehydrogenation during photolysis. -1The generation of the OH peak is likely due to the formation of hydroperoxides and alcohols during photo-oxidation. (1000 cm⁻¹) -1 ~1100cm -1 The CO peak is also a result of carbon chain oxidation. If these increased peak values ​​are considered as characteristic peaks of degradation, it can be found that the degradation characteristic peaks of PE microplastics pretreated with laser plasma are more obvious, especially the OH peak and C=C peak, indicating that the oxidation process of PE after pretreatment proceeds faster. It is worth noting that the characteristic peaks of PE particles degraded after laser plasma treatment do not decrease significantly in the initial degradation stage. This may be because the surface ablated by laser plasma is degraded and sloughed off first, exposing new unablated surfaces, making the characteristic peaks more obvious, before gradually being oxidized.

[0055] Figure 8 (a) and (b) are the nanoparticle size distribution diagrams of the samples obtained in Examples 2 and 3, respectively. In Example 2, the PE particles after laser plasma pretreatment had a particle size distribution between 400-900 nm. After 18 hours of degradation in the UV / H2O2 system, the pretreated PE particles exhibited two particle size distributions: 60-130 nm and 290-720 nm. In Example 3, the untreated particles had a particle size distribution between 190-220 nm after 18 hours of degradation in the UV / H2O2 system. Furthermore, the number of samples observed after pretreatment was significantly greater than that of the control group, indicating that the pretreated PE particles degraded in greater numbers, with a greater reduction in particle size and a better degradation effect. After 54 hours of degradation in the UV / H2O2 system, the pretreated PE particles had a particle size distribution between 390-620 nm, while the control group had a particle size distribution between 450-620 nm. The number of samples in both groups was similar, indicating that the degradation degree of the two groups was similar at this point. Overall, the pretreated PE particles degraded faster and in greater numbers in the early stages of the UV / H2O2 system, then gradually approached the degradation rate of the control group. More organic matter was generated in the water in the early stages, and then gradually decreased. Therefore, it can be concluded that the effect of the laser plasma pretreatment process on the degradation of PE particles in the UV / H2O2 system is mainly concentrated in the early stage of degradation (before 18 hours).

[0056] The degradation process of microplastics treated with laser plasma in the UV / H2O2 system differs from that of microplastics in the UV / H2O2 system alone. Morphological, spectral, and compositional analyses reveal that the degradation of microplastics treated with laser plasma in the UV / H2O2 system proceeds faster in the initial stage, with more organic matter leached out, and then gradually slows down in the later stages.

[0057] III. Mechanism Analysis

[0058] 1. Mechanism of laser-plasma action

[0059] In this invention, a laser is used to irradiate water containing microplastics. When the laser is directed at the liquid containing microplastics, it first interacts with the microplastic particles floating on the surface of the liquid. The most significant effect is thermal effect. The high-energy laser directly acts on the PE particles, causing them to heat up rapidly and conduct downwards. The rapid heating causes the PE particles to undergo thermal ionization and thermal decomposition, while simultaneously producing degradation reactions such as dehydrogenation, carbon-carbon bond breaking, and recombination.

[0060] According to Beer-Lambert's law, the intensity of the incident laser beam at position z below the surface of the PE particle is:

[0061] I(r,z,t)=(1-R)I(r,t)exp(-αz)

[0062] Where R is the reflectivity of the PE surface to the laser, α is the absorption coefficient of the PE to the laser, and I(r,t) is the incident laser intensity.

[0063]

[0064] Where r0 is the laser spot radius and τ is the laser pulse width. Therefore, the heat conduction of the laser energy absorbed by the PE particles can be expressed as:

[0065]

[0066] Where k is the thermal conductivity. This solution yields the temperature distribution of the PE particles under laser irradiation. With temperature changes, thermal expansion occurs within the PE particles, generating stress. The thermal stress per unit area is:

[0067]

[0068] Where Y is Young's modulus and ζ is strain. The thermal expansion displacement length is:

[0069] ΔL=LγΔT(r,z,t)

[0070] Where γ is the coefficient of thermal expansion of PE particles, therefore:

[0071] σ=YγΔT(r,z,t)

[0072] The temperature and thermal stress distribution of PE particles under different laser irradiation times were obtained through COMSOL simulation. The results are as follows: Figure 9 As shown in the figure, the temperature gradually rises from the surface and conducts inward as the laser is applied to the PE particles. Figure 9The arrow in (a) indicates the direction of temperature conduction. After 12 ns, the surface temperature no longer increases, but temperature conduction continues. This shows that extremely high temperatures can be conducted to the bottom of the particle, meaning the thermal effect can affect the entire particle. The melting point of PE particles is 395.15 K, and their thermal decomposition temperature ranges from 573.15 K to 723.15 K. Therefore, under the thermal effect of the laser, the PE particles may have already melted, vaporized, or even thermally decomposed. Figure 9 As shown in (b), the direction of the arrow in the figure indicates the direction of thermal stress. The direction of thermal stress caused by temperature difference gradually changes from inside to outside to tangent to the surface. The thermal force from inside to outside will cause the PE particle surface to loosen and crack, while the stress tangent to the surface can cause the PE surface to fall off and form layered fragments. Figure 10 The curves showing the temperature and thermal stress changes over time at different sites on PE particles reveal a strong correlation between stress and temperature changes. During the first 12 ns of continuous laser pulse application, both temperature and stress rise rapidly, with the surface temperature change being the most significant. This indicates that heat is gradually conducted from the particle surface to the interior, and the rapid temperature rise on the surface causes a sharp increase in thermal stress. Therefore, the surface forces are stronger, and the morphological changes are more pronounced. After 12 ns, the laser pulse no longer acts on the PE particles, and the temperature begins to diffuse outwards. The upper surface temperature drops rapidly. Because the bottom of the particle is in contact with the liquid, the temperature diffuses towards the bottom of the particle, eventually cooling down. As the temperature difference decreases, the stress also decreases rapidly, and the outward expansion of the particles weakens, eventually reaching equilibrium.

[0073] Figure 11 This describes the changes that occur when laser plasma directly acts on PE microplastics. When the laser acts on the surface of PE particles, the surface temperature rises rapidly, theoretically reaching up to 2000K. Under high temperature and pressure, the PE particles gradually undergo ablation, melting, and vaporization. Because the laser spot size is larger than the size of the PE microplastic particles, the Gaussian distribution of the laser has different effects on PE particles at different locations. The surface temperature of particles near the center of the spot is much higher, far exceeding the vaporization temperature of the PE particles. Plasma is easily generated on the particle surface, and the plasma shock wave will tear or break the PE particle surface, forming some surface cracks or directly breaking into smaller particles. When the surface temperature of particles far from the center of the spot is higher than the melting point, the PE particles will melt. The molten PE particles will recrystallize in water, forming some irregular layered structures. The particle size of the particles after surface melting is smaller, and the surface functional group characteristics gradually weaken. At the same time, some chemical bonds will break during the melting and fragmentation process, and some additives in the PE particles will leach out and dissolve in the liquid environment. Finally, the PE microplastics will be gradually degraded.

[0074] 2. Degradation mechanism of UV / H2O2 system

[0075] Based on the above results, after 1 hour of laser plasma treatment, the size of some PE particles decreased, but the surface structure of some unbroken PE particles did not change significantly, and nanoparticles were adsorbed on the surface. Electron spectroscopy scanning was performed on the original PE particles and the pretreated PE microspheres, as shown... Figure 12 As shown, it was confirmed that the adsorbed particles on the surface of PE particles are iron oxide nanoparticles, and the adsorbed iron oxide nanoparticles can provide target sites for photo-Fenton reaction in the UV / H2O2 degradation system.

[0076] Elemental analysis was performed on the samples before and after laser plasma treatment, focusing primarily on the changes in the atomic percentages of C, O, and Fe. The results are as follows: Figure 13 As shown, carbon (C) is the dominant element in PE particles before and after treatment. This is determined by the PE process. After treatment, the proportion of C in the PE particles decreases, while the proportions of O and Fe increase. Most notably, the Fe content increases from 0% to 1.58%. This is likely because the plasma generated by the laser focused on the iron substrate carries out the iron-based nanoparticles, which are then adsorbed onto the surface of the PE particles. These iron-based nanoparticles will play a role in the subsequent degradation process.

[0077] The composition of iron-based nanoparticles on the surface of PE was studied. Figure 14 (a) The XRD results of an iron sheet used as a laser focusing substrate, iron-based nanoparticles generated after 1 hour of laser irradiation, and a silicon wafer without diffraction peaks loaded with the sample are shown. The XRD diffraction peaks of the iron sheet are located at 44.89°, 65.25°, and 82.52°, consistent with the characteristics of iron metal. The diffraction peaks of the iron nanoparticles are almost identical to those of the silicon wafer loaded with the sample, with a faint peak between 20° and 40°, indicating that the iron-based nanoparticles generated by laser plasma are amorphous. The crystalline state of the iron-based nanoparticles can be determined from the XRD results, but the specific composition cannot be determined. Therefore, Raman spectroscopy was added to determine its composition. Figure 14 (b) The Raman spectra of the iron sheet used as the laser focusing substrate and the iron-based nanoparticle sample generated after 1 hour of laser treatment are shown respectively. It can be found that the iron nanoparticles at 800 cm⁻¹... -1 The overlapping of the left and right peaks indicates the presence of Fe in the sample. (680 cm⁻¹) -1 and 960cm -1 The peak at that location belongs to the Fe-O bond, indicating that the iron-based nanoparticles generated by laser plasma are a mixture of iron and iron oxides. These iron-based nanoparticles will have a certain impact on the subsequent degradation process.

[0078] The above text confirms that the iron-based nanoparticles generated during the laser plasma treatment stage are a mixture of iron and iron oxides. These iron-based nanoparticles adsorbed on the microplastics play a catalytic degradation role in the UV / H2O2 degradation system. In the UV / H2O2 system, the iron nanoparticles act as an iron source, primarily by releasing Fe... 2+ and Fe 3+ It participates in a Fenton-like reaction to generate active oxides, thereby achieving the purpose of degrading microplastic particles. The reaction process is shown below.

[0079] Fe + 2H+ → Fe 2+ +H 2 ↑

[0080] 2Fe + O₂ + 4H + →2Fe 2+ +2H2O

[0081] 4Fe 2+ +O2+4H + →4Fe 3+ +2H2O

[0082] Fe + H₂O₂ → Fe 2+ +·OH+OH -

[0083] Fe 2+ +H₂O₂→Fe 3+ +·OH+OH -

[0084] Fe 3+ +H₂O₂→Fe 2+ +HO2·+H +

[0085] Fe + 2Fe 3+ →3F e2+

[0086] Iron oxide nanoparticles, acting as catalysts in the UV / H2O2 system, primarily degrade target compounds by reacting surface active iron species with hydrogen peroxide to generate active oxides. The process by which iron oxides participate in Fenton-like reactions is more complex, mainly involving three steps: first, the adsorption of H2O2 molecules on the iron-based catalyst surface, followed by the reaction with surface iron species (≡Fe). II or ≡Fe III Precursor surface complexes ≡Fe are formed between them. II -H2O2 or ≡Fe III -H2O2; then, the resulting surface complex undergoes a simple intramolecular electron transfer to produce ≡Fe III And the surface-bound ·OH radicals, at the same time, ≡Fe II-H2O2 complex is converted to ≡Fe II The formation of ·OOH may further lead to ≡Fe I The regeneration process continues. Finally, the generated ·OH radicals oxidize the surface of the microplastic particles adsorbed with active iron species. The specific reaction process is shown below:

[0087] ≡Fe Ⅱ +H₂O₂→≡Fe Ⅱ -H2O2

[0088] ≡Fe Ⅲ +H₂O₂→≡Fe Ⅲ -H2O2

[0089] ≡Fe Ⅱ -H₂O₂→≡Fe Ⅲ +·OH+OH -

[0090] ≡Fe Ⅲ -H₂O₂→≡Fe Ⅱ +·OOH+H +

[0091] ≡Fe Ⅲ +·OOH→≡Fe Ⅱ +O2+H +

[0092] ≡Fe Ⅱ +·OH→≡Fe Ⅲ +OH -

[0093] ≡Fe Ⅱ +·OOH+H + →≡Fe Ⅲ +H2O2

[0094] H₂O₂ + ·OH → ·OOH + H₂O

[0095] ·OH + ·OOH → H₂O + O₂

[0096] Figure 15This study describes the degradation mechanism of PE microplastics pretreated with laser plasma in a UV / H2O2 system. The pretreated PE microplastics adsorb iron / iron oxide nanoparticles onto their surface. In the UV / H2O2 system, the iron nanoparticles directly participate in a Fenton-like reaction, while the iron oxide nanoparticles act as catalysts. Because PE particles are hydrophobic, the PE microplastics float on the liquid surface during degradation in the UV / H2O2 system, limiting their surface area and thus the effectiveness of traditional Fenton degradation. However, the pretreated PE microplastics self-adsorb both the iron source and the iron oxide catalyst. These nanoparticles are hydrophilic and actively adsorb the liquid, thus fully utilizing the hydrogen peroxide in contact with the PE plastic to generate a large number of hydroxyl radicals. These hydroxyl radicals concentrate near the nanoparticles and oxidize and degrade substances at these sites. Therefore, the sites on the surface of PE microplastics where nanoparticles are adsorbed will be degraded more quickly. These sites are interconnected and eventually form small particles that fall off. If there is sufficient oxidant, even smaller PE particles will enter a new round of degradation. The degradation rate of smaller particles will be faster than that of larger particles, and the whole process is faster than the degradation of traditional UV / H2O2 systems.

[0097] Although specific embodiments of the present invention have been described in detail with reference to the accompanying drawings, this should not be construed as limiting the scope of protection of this patent. Various modifications and variations that can be made by those skilled in the art without inventive effort within the scope described in the claims are still within the scope of protection of this patent.

Claims

1. A method for treating microplastics in water, characterized in that, Includes the following steps: S1: Place water containing microplastics into a reaction vessel, place an iron sheet into the reaction vessel, and then irradiate the iron sheet with a laser; S2: After laser irradiation, the iron sheet is removed, filtered, microplastics are collected, dried, and laser plasma pretreated plastic particles are obtained. S3: Disperse the laser plasma pretreated plastic particles in a mixed solvent, which is obtained by mixing deionized water and hydrogen peroxide solution; then irradiate the dispersion system with ultraviolet light for 15-55 hours to complete the treatment of microplastics in the water.

2. The method for treating microplastics in water according to claim 1, characterized in that: The concentration of microplastics in the water containing microplastics is 0.8–1.2 mg / mL.

3. The method for treating microplastics in water according to claim 1, characterized in that: The laser used in S1 has a frequency of 5 Hz.

4. The method for treating microplastics in water according to claim 3, characterized in that: The laser irradiation time is 1 to 3 hours.

5. The method for treating microplastics in water according to claim 1, characterized in that: The pore size of the filter membrane used in S2 is 0.1–0.22 μm.

6. The method for treating microplastics in water according to claim 1, characterized in that: The drying temperature in S2 is 40℃.

7. The method for treating microplastics in water according to claim 1, characterized in that: The concentration of the hydrogen peroxide solution is 30 wt%.

8. The method for treating microplastics in water according to claim 7, characterized in that: The volume ratio of deionized water to hydrogen peroxide solution in the mixed solvent is 1:

1.

9. The method for treating microplastics in water according to claim 8, characterized in that: The ratio of the laser plasma pretreated plastic particles to the mixed solvent is 5 mg: 2 mL.

10. The method for treating microplastics in water according to claim 1, characterized in that: The UV irradiation time in S3 is 18 hours.

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