Occurrences and characteristics of full-size microplastics derived from mulch films in major exorheic rivers of Hainan, China

doi:10.21203/rs.3.rs-991254/v1

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Occurrences and characteristics of full-size microplastics derived from mulch films in major exorheic rivers of Hainan, China

https://doi.org/10.21203/rs.3.rs-991254/v1

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Mulch film residues in farmland take risk of being transported to the wider environment, which may cause greater plastic pollution. Discharge of various sewage, as well as the releases from tourism, shipping, and aquaculture, was confirmed to important sources of microplastics (MPs) in rivers. However, the occurrences, characteristics and contribution of microplastics derived from mulch films (MMF) in rivers were still largely unknown. In this study, UV-Vis Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were sequentially used to identify the full-size MMF (1-5000 µm) in field samples. Studies confirmed that MMF, as a polyethylene-matrix composite doped with additives, is an important contributor of MPs in upstream farmland soil and riparian sediments, and even exists in a small amount in estuarine sediments. Specifically, the value of MMF in estuarine sediments ranged from 38 ± 8 to 82 ± 17 items/kg (including 54.9–74.2% small size (1-5 µm )), accounting for 9.0–13.7% of total MPs. Moreover, black and fragment were the dominant color and shape of these MMF, respectively. Our work first demonstrated that MMF was a neglected part of MPs in estuaries, especially in the drainage areas with high agricultural activity.

Environmental Policy

microplastics

MMF

full-size

exorheic rivers

farmland soils

Since it was invented, plastics have been widely used all over the world for their lightweight, inexpensive and excellent chemical stability. However, a large fraction of plastics that have not been recycled were decomposed under various actions and emitted MPs (< 5 mm) to the wider environment (Bergmann et al., 2017; Prata, 2018). Worse still, MPs were ingested by organisms, which can potentially cause blockage in the stomach and intestines of creatures and thus starvation or decreased energy supply (Cole et al., 2015; Wright et al., 2013). They can also enter the human body through the food chain and accumulate in large quantities, which will cause unpredictable harm to human health (Mercogliano et al., 2020; Tanaka et al., 2016).

Based on the existing studies, the ocean has been confirmed as the dominant destination for the migration of MPs (Unice, 2019; Wang and Zhao, 2020). Exorheic rivers, with their enormous carrying and transporting capacity, play an important role in the export of MPs from land to the ocean (Besseling et al., 2017; Xiong et al., 2019). Thus, the study of the source of MPs in exorheic rivers is not only helpful to reveal the potential source of MPs in the ocean, but also of great significance to deepening the understanding of the migration pathway and abundance of MPs. The reported sources of MPs in exorheic rivers include the discharge of domestic sewage and industrial wastewater, as well as the release from tourism, shipping, and aquaculture (Peng et al., 2017; Zhao et al., 2015). Notably, plastic debris or MPs in farmland has been neglected as a potential source of MPs in exorheic river. The abundance of MPs in farmland soil was generally high (He et al., 2018). A survey carried out in Wuhan City, central China, found that the abundance of MPs in the soil of vegetable plots reached a staggering 1.6 × 10⁵ items/kg (Zhou et al., 2019). Therefore, it is urgent to study the occurrences, characteristics and contribution of MPs in farmland soils and exorheic rivers (Rillig et al., 2020).

Among the plastic debris, mulch film may be the dominant fraction in farmland soil. Only in China, the consumption of mulch films in 2012 alone was estimated to reach 124.5 billion metric tons (National Bureau of Statistics of China, 2012). However, it is notable that an appreciable fraction of the annually produced mulch films are not yet recycled (Huang et al., 2020; Zhang et al., 2020). It has been reported that even as of 2018, the recycling rate of mulch films in China was still below 60% (Luo et al., 2018). Additionally, the mulch films used in China are more likely to break into MMF because their thickness is thinner than that in the developed countries of Europe and Japan (Luo et al., 2018). As time goes on, these residues will break into MMF and accumulate in farmland soil, posing a long-term threat to crop production (Qi et al., 2020). MMF also has the trend of becoming submicro-plastics and further transforming into nanoplastics. Once the particle size becomes smaller, they will be more easily ingested by organisms. Many reports indicate that MMF is a very important part of MPs in farmland soils (He et al., 2018; Xu et al., 2020). Moreover, under the effect of surface runoff, MPs in farmland soil have a high risk of migration, especially in irrigation canals, ditches, and cultivated soil with extensive surface runoff (Wang and Ge, 2020). However, little consideration has been given to whether the MPs (especially MMF) in farmland soils as a potential source of MPs in estuaries and their potential pollution to the ocean.

As one of the most important agricultural provinces in China, Hainan Province supplies a tremendous number of anti-season vegetables to the northern region every year. Also, in Hainan, mulch films were widely used in agricultural production. In 2011 alone, the plastic film mulched land area and the amount of mulch films used in Hainan reached 32,233 hectares and 10,342 metric tons respectively (Chen et al., 2013). In this study, the four major exorheic rivers, Nandu River, Wenlan River, Zhubi River, and Changhua River in Hainan Province of China were selected as hot spots to evaluate the occurrences, characteristics and contribution of MMF. The objectives of this comprehensive study were (i) to provide field evidence and use UV-Vis Raman spectroscopy combined with XPS to explore whether MMF is a neglected part of MPs in exorheic rivers, (ii) to deepen our understanding of the occurrences, characteristics and contribution of full-size MMF (1-5000µm) in estuaries, upstream riversides and farmlands.

2.1 Sample collection

Samples were collected from four major exorheic rivers in Hainan Province, China, including sediments at the estuaries, soil samples from the upstream riparian farmland, and some riverside sediments close to the above farmland. The sampling sites are shown in Fig. 1 (19°18’N-20°05’N, 108°40’E-110°25’E). The N1, W1, Z1, and C1 were the estuary sites of Nandu River, Wenlan River, Zhubi River, and Changhua River respectively; and the N3, N5, N7, W2, Z2, and C2 were the upstream riparian farmland sites. The N2, N4, and N6 were selected as adjacent riverside sites. Some other detailed information about the sampling procedures and devices were presented in the Supporting Information.

2.2 Density separation and microplastic extraction

Using a method similar to Zhang et al. (2017), the MPs in the samples were extracted using three experimental procedures: (1) oxidative digestion, (2) density separation/filtration, and (3) microscopic inspection/identification. Firstly, the samples collected from the sampling sites were dried at 333.15 K and sieving with 5 mm meshes. Then, the relatively small particles that passed through the sieve were placed in a 2-liter conical flask and treated three times with 20 mL of 30% hydrogen peroxide (H₂O₂) for about 48 hours.

Secondly, 1.5 L potassium formate aqueous solution with the concentration of 1.5 g/cm³ was poured into the conical flask, stirred with a glass rod, and precipitated for 24 h by three times. Then, the supernatant was sequentially filtered by the 5.0 µm and 0.45 µm GF/C glass microfiber filter (Whatman, UK). The residues on the 0.45 µm filter were air-dried for 6 h and then collected by a 50 mL liquid loading port with a sealing cover. After that, 10 mL methanol was injected into the port and shaken at the ultrasonic oscillator for 1 h.

At last, the residues on the 5.0 µm filter membrane were also air-dried for 6 hours. Then, the particles on the filter membrane were observed and counted under the stereomicroscope (Olympus SZ53, Japan) at 3-4 magnification. After that, the suspected and typical microplastic particles were captured by the charge coupled device (Andor Zyla 4.2 s CMOS) and carefully extracted from the filter membrane.

2.3 Identification method of MPs

2.3.1 Detection of the small-size MPs (1-5 µm) by UV Raman spectroscopy

The transparent quartz slide without any Raman background signal under different laser excitation wavelength, instead of commonly used glass slide, were carefully cleaned with distilled water, Milli-Q water and chromatogram class ethanol for three times. After air drying, special zones with the dimensions of 1 cm×2 cm (width×length) were selected. Then, different content of small-size MPs in methanol were introduced onto these special zones using 20 µL micropipettes and slowly coating to ensure homogeneous distribution of the MPs onto the silicon slide.

The Raman spectra and images were conducted using a Renishaw InVia laser confocal micro-Raman microscopy equipped with the 15X/0.40 and 50X/0.75 objective. The parameters of this instrument were as follows: excitation laser wavelength and energy, 325 nm, 532 nm and 785 nm; emission wave number, 130-3500 cm⁻¹; exposure time, 5.0 s; six different micro-zones on the particles were selected and scanned to finally identify the particle type.

After adjusted the instrument to the optimal conditions, the quartz slide was put onto the sample holder of the microscopy. Firstly, the images of single small-size MPs were scanned under the bright field light path of microscopy (50X/0.80) to acquire their characteristics (size, color and shape). Secondly, the instrument was adjusted to the light path of micro-Raman spectroscopy and acquired the corresponding images and spectra of the single small-size MPs using the 50X UV objective (Žukovskaja et al., 2018). At last, the excitation emission matrixes of the Raman spectra were further processed by the previously established chemometrics algorithms method (min-max signal adaptive zooming approach) to smooth and eliminate the interference of non-target (Ralbovsky et al., 2019).

2.3.2 Detection of the larger-size MPs (5-5000 µm) by Vis Raman spectroscopy

The selected particles (5-5000 µm) in section 2.2 were placed on quartz glass slides and identified by the micro-Raman spectrometer (Renishaw inVia, UK). The parameters of the micro-Raman spectrometer were adjusted to reduce the influence of background fluorescence signal: Laser energy, 15 mW (5%); Laser wavelength, 532 nm; Exposure time, 10.0 s; and Emission wave number, 300-3200 cm⁻¹. Three scans were accumulated for each spectrum and each particle needs to scan its six different micro-zones.

2.3.3 X-ray Photoelectron Spectroscopy

To further identify whether the suspected MMF and mulch film were the same substance, the chemical composition of them were scanned by X-ray Photoelectron Spectroscopy (Kratos, UK). Spectra were recorded with 16 mA, 12.5 kV, and binding energies were normalized at C_1s 284.8 eV. The scanned area was set to 1 × 1 mm. Finally, an average spectrum of six particles was recorded.

2.4 Quality assurances (QA) and quality controls (QC)

The following QA and QC measures described in previous studies were employed by this study (Lozoya et al., 2016).

1. Soil and sediment samples were collected and stored in metal samplers and containers to avoid possible contamination.

2. All flasks and beakers were rinsed three times with distilled water before use.

3. Before identification and quantification of the microplastics, the sample holders of the stereomicroscope and micro-Raman spectroscope were carefully cleaned and inspected.

4. Blank tests were performed to adjust the results obtained during sample collection and laboratory analysis; microplastics were not found in the blank samples.

2.5 Statistical analysis

One-way analysis of variance tests (ANOVA) were used to compare the mean values and relative standard deviation (RSD) of the replicates. As a bivariate correlation, all statistical analyses were conducted using SPSS Statistics version 22.0, and the Pearson correlation analysis method was selected. (p > 0.05 was considered statistically insignificant; p < 0.05 was considered as low correlation; p < 0.01 was considered as moderate correlation; p < 0.001 was considered as significantly relevant.)

3.1 Identification of full-size MPs (1-5000 µm)

3.1.1 Larger-size MPs (5-5000 µm)

Clear spectra with well-defined peaks and little spectral noise were obtained for all MPs that were greater than 5 µm. The spectrum of each particle was compared to a reference library (Renishaw, UK) for identification. Similar to the previous reports (Di et al., 2018; Elert et al., 2017), such chemometrics approaches as the classical least squares (CLS), support vector machine (SVM) and asymmetric least-squares (ALS) regression approaches have been applied to eliminate the interference of the background fluorescence signal, which made the matching degree of Raman spectrum and standard spectrum reached more than 90 % (Fig. S1). Fig. S2 shows the exemplary MPs detected in this study.

3.1.2 Small-size MPs (1-5 µm)

Limited by the sensitivity and selectivity of methods, it is difficult to identify small-size MPs at present. Small-size MPs and complex matrix in field samples need us to improve the S/N values as high as possible. Increased the probability of internal conversion may be a better choice for improving the lower S/N, and the UV Raman spectroscopy can realize this goal to a certain degree. After all of the other instrument conditions were adjusted to their optimal values, the maximum S/N values of small-size PC particles significantly increased to 42.5 when detected by the 325 nm excitation wavelength, exceeding the second one (532 nm) by more than an order of magnitude (S/N=3.9). The S/N values increased to 112.6, to 82.8 and to 73.3 for the PP, PS and PE, respectively (Table S1). What needs to be explained was that although the normal Raman scattering intensity is proportional to the fourth power of the scattered light frequency in theory, both the Raman signal of small-size MPs and background fluorescence intensity decreased for the high depletion of UV laser in the light path of micro-Raman spectroscopy, and the drop of formed one was more obviously than the latter one, which increased the S/N values. Compared with the traditional used excitation wavelength (532 nm and 785 nm), the UV laser excitation wavelength was more reliable to investigate the MPs with strong background fluorescence signal in environmental samples.

To further improve the S/N values, the spectra data should be treated with chemometrics approach. Then, the CLS, SVM and ALS regression approaches were sequentially used with the S/N values of small size PC improved to 118.8, to 164.5 and to 232.8, respectively. The characteristics peak of micro-Raman spectra become shape and narrow with the half width of 1-5 cm^-1 (Fig. 2). Similar results were also obtained for the other three types MPs (Fig. S3 and S4). It can be concluded that the combinations of UV micro-Raman spectra and ALS were the better choices to determine the small-size MPs (1-5 μm). The minimum MPs particle size that can be identified by the laser confocal Raman spectroscopy were evaluated under different laser excitation wavelength with the criterion of S/N>6 and the results were listed in Table S2. The smallest MPs particles size detected by 325 nm were much low than the 532 nm and 785 nm in the ranges of 1.2-2.2 μm.

3.2 Identification of MMF in full-size MPs (1-5000 µm)

As shown in Fig. 3, there was a broad and strong peak band at 1296 cm^-1 and in the range of 2800-2900cm^-1 in Raman spectra of mulch film standard sample and polyethylene. This shows that mulch film is a kind of polyethylene-matrix substance.However, the positions of many other peaks are obviously shifted. This phenomenon may be mainly due to the addition of other substances such as dyestuff in addition to polyethylene. These results indicate that mulch film is a polyethylene-matrix composite doped with additives. After comparing the Raman spectra of particles selected from section 2.2 with the spectrum of mulch film standard sample, we found that some particles have a broad and strong band in the range of 2800-2900 cm^-1 as in the standard sample, and also have the same peaks at 1128, 1296, 2851 and 2884 cm^-1. The Raman spectra of other typical suspected MMF were shown in Fig. S5. Although it was time-consuming and depended on equipment availability, UV-Vis Raman spectroscopy produced the most reliable results from the tested techniques as it was the only to provide definitive proof of the particle’s nature (Prata et al., 2020).

To further identified that these suspected MMF were derived from mulch film,we analyzed the correlation between their surface elemental composition and the surface elemental composition of mulch film standard sample. Detailed surface elemental composition data were shown in Table 1. Specifically, as shown in Table S3, there is a significant correlation between the surface elemental composition of the suspected MMF identified by UV-Vis Raman from each site and that of mulch film standard sample (p-values are all less than 0.001;R²-values are all greater than 0.980).The identification method of XPS combined with UV-Vis Raman has successfully confirmed the presence of MMF in the MPs collected from section 2.2.

Table 1

Surface elemental composition data of mulch film standard samples and suspected MMF samples from four different estuaries. ^a standard deviations derived from duplicate measurements.
Surface elemental composition (%)
				C (%)
	Total	C-C	C-O		C=O	COOH	N	O	O/C	(O+N)/C
N1	88.9	60.2±0.47^a	15.7±0.08^a		10.1±0.33^a	2.9±0.01^a	3.5±0.18^a	7.6±0.12^a	0.09	0.12
W1	82	55.5±0.09	16.9±0.22		8.6±0.07	1±0.27	2.7±0.00	15.3±0.25	0.19	0.22
Z1	75.6	49.8±0.65	17.8±0.05		7.5±0.05	0.5±0.00	1.1±0.15	23.3±0.33	0.31	0.32
C1	81.1	53.9±1.06	19.5±0.28		7.4±0.12	0.3±0.01	5.3±0.07	13.6±0.09	0.17	0.23
Standard sample	89	59.6±0.18	18.4±0.76		9.5±0.21	1.5±0.05	2.2±0.09	8.8±0.05	0.10	0.12

3.3 Abundance

Our study found that the abundance of MPs ranged from 2750 ± 18 items/kg to 5030 ± 46 items/kg at six farmland sites (Fig. 4). This level was significantly higher than that of the sediment samples collected from estuaries and adjacent riversides, and similar to that found in the fiveother provinces, China,at 2784-6366 items/kg (Wang et al., 2021). The content of MMF ranged from 1240 ± 125 items/kg to 2860 ± 77 items/kg, accounting for more than 44.1% of the total amount of MPs, with a maximum of 59.5%.Therefore, the MMF pollution of farmland soil in Hainan should not be ignored, especially in the recovery of mulch films and the treatment of residues.

To better reveal the occurrences, characteristics and contribution of MMF in rivers, we also selected the estuarine sediments of four rivers as the research objects. Our study found that the abundance of MMF ranging from 38 ± 11 items/kg to 82 ± 15 items/kg, accounting for 9.0–13.7% of the total MPs. Puzzling is that the abundance of MPs and MMF at site N1, the largest urban estuary in Hainan, was 620 ± 21 items/kg and 76 ± 13 items/kg respectively, both of which were lower than those at the upstream sites. This result may be mainly affected by the marine dynamics of Qiongzhou Strait, such as frequent typhoons, storm surges, and turbulent coastal currents, which interfered with the settlement of MPs/MMF at the estuary (Onink et al., 2019; Lo et al., 2020; Osinski et al., 2020).

Even though the watershed area of Wenlan river is far less than that of Nandu River and Changhua River, and the scale of cities flowing through is far less than that of Haikou City, the abundances of MPs/MMF at the W1 site were the highest among all estuary sites, reaching 910 ± 24 items/kg and 82 ± 15 items/kg respectively. We speculate that this is because the M1 site was located in a semi-closed harbor (Bopu Port), and the relatively closed environment was conducive to the subsidence of MPs/MMF (Zheng et al., 2019). Moreover, the MPs pollution caused by busy port operations was also an important reason that can not be ignored. Whether it is a substantial global Port such as the port of Durban and the Port of Melbourne, or a small port such as the Port of stenonson in Sweden and the Port of tanjungprio in Indonesia, relevant reports demonstrated that port environments have high MPs pollution levels (Firdaus et al., 2020; Preston-Whyte et al., 2021; Su et al., 2020).

The results of correlation analysis suggested that the abundance of MPs/MMF in the farmland soil was not correlated with that in the sediments from estuaries (p = 0.864, R² = 0.335; p = 0.895, R² = 0.295) and adjacent riverside (p = 0.280, R² = 0.819; p = 0.944, R² = 0.215). This phenomenon mainly resulted in two reasons. Firstly, the amount of surface runoff at each site was different, leading to a difference in the ability of transporting MPs/MMF from farmland soil. Secondly, not all MPs/ MMF in riverside sediments and estuarine sediments came from nearby or upstream farmland soils. The settlement of upstream MPs/MMF carried by rivers and human activities nearby were important influencing factors.

3.4 Characteristic analysis

Similar compositions of MPs were observed in the topsoil samples collected at the three farmland sites (N3, N5, and N7) of Nandu River, with PE, PP, PS, and others accounted for in the total, which ranged from 54.3% to 68.7%, from 18.0% to 26.3%, from 1.9% to 14.4%, and from 3.1% to 10.9%, respectively (Fig. S6). Nevertheless, the PE shares significantly decreased to 32.3%, 35.8%, 34.3%, and 47.8% at the N1, N2, N4, and N6 sites of Nandu River, respectively. The difference in the source of MPs was the primary reason for this phenomenon. Specifically, the major contributor to MPs in farmland soils was the PE-based MMF, while the sources of MPs in riverside sediments and estuarine sediments were relatively more diversified.

Additionally, many studies have reported that small-size fibers and/or particles account for a large proportion of MPs ingested by marine organisms (Steer et al., 2017; Su et al., 2019). Small-size MPs are not only relatively easier to be retained and accumulated in organisms, but also have been proved to have stronger adsorption capacity for POPs, which may pose a greater potential threat to organisms (Rodrigues et al., 2019). Therefore, it was essential to further confirm and analyze the information of small-size MMF. Our results show that there were significant differences in the proportion of small-size MMF between farmland soils and estuarine sediments. The proportion of MMF with a diameter of less than 5 µm in farmland soils ranged from 31.4–47.6% (Fig. 5a). There are many factors known to affect the formation of small-size MMF in the farmland soils that has been mulched with plastic films, such as crop type, mulching time, climate difference, soil texture, film thickness, and so on (Zhang et al., 2020). However, the specific reasons for the low proportion of small-size MPs in farmland soils in Hainan need to be further studied. In contrast, MMF with a diameter of less than 5 µm accounted for more than 54.9% of total MMF in the studied estuarine sediments. We speculate that marine dynamic factors accelerate the further fragmentation of MMF in the estuarine sediments of Hainan, which is an important reason for the high proportion of small-size MMF.

According to Wright and Thompson (2013), aquatic organisms tend to ingest MPs with high color similarity to prey items. Moreover, the toxic effects and growth effects of MPs with different colors on aquatic organisms were also different (Chen et al., 2020). Therefore, it is worthwhile to analyze the color difference of MPs. In this study, transparent, black, and green MMF were detected in estuarine sediments. More significantly, the percentage of colored MMF reached the highest value of 81.6% at Z1 (Fig. 5b), which was much higher than that of transparent MMF. Similar characteristics were observed at other estuarine sites W1, Z1, and C1. It may be due to the extensive use of colored plastic mulching films in farmland, the proportion of colored MMF in the upstream farmland increased by 21.5-34.3% compared with the estuary site.

Finally, this study found that the shape of MPs in estuarine sediments was predominantly fiber, while the vast majority of the MMF found was in the shape of the fragment. In sharp contrast to the situation in estuaries, the content distribution of MPs with different shapes in farmland soil following the sequences of fragment > film > fiber > foam (Fig. S6), and the shapes of MMF, which accounted for the vast proportion of MPs in farmland soil, were similarly dominated by fragment and film (Fig. 5c). The reason for this phenomenon may be that the process of transporting MMF by surface runoff was selective for MMF with different shapes, or it may be that external forces change the shape of MMF in estuarine sediments and break the shape of MMF from the film into fragments.

This study confirmed that MMF is a neglected part of MPs in estuarine sediments of the four main exorheic rivers in Hainan, China. The key to this conclusion is that there is a certain amount of MMF (1-5000µm) found in the estuarine sediments, with abundance ranging from 38 ± 11 items/kg to 82 ± 15 items/kg, accounting for 9.0–13.7% of the total MPs. In addition, the study also found that the abundance and characteristics of MMF in farmland soils are significantly different from those in estuarine sediments. Based on some existing reports, we speculate that the difference in sources and the effects of marine dynamics might be two important factors. Finally, the findings of this study provide new insights into the source of MPs in the ocean, as well as the migration mode and fate of MPs in farmland soils.

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Availability of data and materials

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Competing interests

The authors declare that they have no competing interests

Funding

Hainan Provincial Key Research and Development Projects (ZDYF2021SHFZ066), Natural Science Foundation of China (No. 21806026), Science and Technology Base, Talent Special Project of Guangxi Province (No. AD19245023). Central Public-interest Scientific Institution Basal Research Fund for Chinese Academy of Tropical Agricultural Sciences (No. 163004201802, No. 16300420190041).

Authors' contributions

MJ: Investigation, Data analysis, Writing- Original draft preparation.

BL: Conceptualization, Funding acquisition, Writing-Reviewing and Editing.

LW: Writing-Reviewing and Editing.

LR: Writing - review & editing.

LW: Methodology, Data curation.

WM: Investigation, Data curation, Editing.

RL: Conceptualization, Writing-review & editing. Validation, Funding acquisition.

All authors read and approved the final manuscript.

Acknowledgements

The authors are grateful for financial support from the Hainan Provincial Key Research and Development Projects (ZDYF2021SHFZ066), Natural Science Foundation of China (No. 21806026), Science and Technology Base, Talent Special Project of Guangxi Province (No. AD19245023). Central Public-interest Scientific Institution Basal Research Fund for Chinese Academy of Tropical Agricultural Sciences (No. 163004201802, No. 16300420190041).

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Occurrences and characteristics of full-size microplastics derived from mulch films in major exorheic rivers of Hainan, China

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Abstract

Figures

1. Introduction

2. Materials And Methods

2.1 Sample collection

2.2 Density separation and microplastic extraction

2.3 Identification method of MPs

2.3.1 Detection of the small-size MPs (1-5 µm) by UV Raman spectroscopy

2.3.2 Detection of the larger-size MPs (5-5000 µm) by Vis Raman spectroscopy

2.3.3 X-ray Photoelectron Spectroscopy

2.4 Quality assurances (QA) and quality controls (QC)

2.5 Statistical analysis

3. Results And Discussion

3.1 Identification of full-size MPs (1-5000 µm)

3.1.1 Larger-size MPs (5-5000 µm)

3.1.2 Small-size MPs (1-5 µm)

3.2 Identification of MMF in full-size MPs (1-5000 µm)

3.3 Abundance

3.4 Characteristic analysis

4. Conclusion

Declarations

References

Supplementary Files

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