Pollution Characteristics of Microplastics in Soils in Southeastern Suburbs of Baoding City, China

Microplastics (MPs) are emerging pollutants that exist in different environmental media. Because of their wide range and large potential environmental hazards, they have attracted widespread attention in recent years. At present, the research on MP is mostly concentrated on the water ecosystems, and the impact on soil ecosystems is less studied. In this study, 12 typical soil samples from southeastern suburbs of Baoding city were investigated and characterized by time-of-flight secondary ion mass spectrometry (TOF-SIMS) combined with mass high resolution mode and positive and negative ion imaging mode. Four types of MPs, poly (propylene) (PP), poly (vinyl chloride) (PVC), poly (ethylene terephthalate) (PET), and poly (amide 6) (PA6), were quickly identified, of which PET and PA6 accounted for the largest proportion of both up to 30.2%; the particle size of the obtained MPs ranged from 0 to 35 μm, of which the proportion of <10 μm MPs was more than 26.3%, while that of 20–25 μm and 25–35 μm MPs was relatively small (17.83% and 9.3%, respectively). Risk assessment results of the MP in the soil showed that the risk level of MPs in the non-ferrous metal industrial parks and in concentrated with small workshops areas is relatively high, and attention should be paid to such areas. In addition, the study provides a reference method for the investigation and risk assessment of MPs in terrestrial soils, coastal beaches, and sediments.


Introduction
In recent years, microplastics (MPs), as a new type of environmental pollutant, have gradually attracted the attention of the public and of scholars [1][2][3][4]. MPs are usually defined as environmental plastic pollutants smaller than 5 mm, which exist widely in oceans, rivers, sediments, soils, and organisms [5][6][7][8]. Although MPs have attracted widespread attention in academic circles, most of studies have focused on marine MPs, and there are few studies on MPs in the terrestrial soil environment [9][10][11]. Until now, the research on soil environmental MPs has mainly focused on farmlands, such as research on farmlands in the suburbs of Shanghai and farmlands near Dianchi Lake [12,13]. Previous studies have examined the spatial distribution of MPs in riparian sediments, the soil microbial uptake, the sorption and bioaccumulation of persistent organic pollutants (POPs), and the transport of MPs in microbial communities and plants [14]. These effects may also be applicable to terrestrial soil systems, so more attention should be paid to the study of MPs in soils. A recent study conducted in the atmospheric environment confirmed that clothes drying in natural sunlight is a

Sample Collection
The study area was located near Baoding City in Hebei Province, northern China (38 • 70 -38 • 96 N, 115 • 67 -116 • 13 E). According to the different urban functional areas, 12 typical areas were selected (Table 1). Soil samples at a depth of about 10 cm from the surface were collected using a stainless steel sampling shovel. At least five soil samples (within 100 m 2 ) were collected at each sampling point; the samples were mixed and homogenized into a composite sample, wrapped with aluminum foil, put into a sampling bag, brought back to the laboratory, placed in a clean and light-proof place, dried, and preserved at low temperature.

Sample Pretreatment
At present, there are few detection methods for MPs with a particle size of 1-20 µm, and the maximum scanning area of TOF-SIMS is 500 µm 2 . The large particle size is not conducive to imaging characterization. It is considered that TOF-SIMS is suitable for detecting MPs smaller than 35 µm. After air drying, the soil samples were screened using a 35 µm sieve, and the preliminary separation samples (about 100 g) were obtained. About 1 g (dry weight) was weighed for MPs extraction. In total, 4 mL of 30% H 2 O 2 solution was added, then added to 10 mL ultrapure water, and the samples were sealed. The sample was then incubated at 60 • C constant temperature water bath for 6 h to remove natural organic matter, and the sample was left for 1 h to ensure aggregates dispersed in the sample. After standing for 24 h, a small amount of supernatant liquid was absorbed on the silicon chip by a liquid transfer gun, and then, the sample was put into a fume hood to dry naturally for 24 h. Before the experiment, the samples were stored in Petri dishes and kept away from light. According to this method, 12 soil samples were collected to be processed in turn.

Risk Assessment
In this study, the chemical composition and relative abundance of MPs were considered to assess the potential risk of MPs in surface soil of the study area. Following Lithner et al. the chemical toxicities of MP polymers were used as an important indicator for assessing their ecological hazards. Therefore, the hazard score of Lithner's plastic polymers and the type of polymers used to evaluate the risk of MPs were used in this study (Table 3) [24]. The formula is as follows: where H is the calculated polymer risk index of MPs, Pn is the percentage of different types of MPs collected at each sampling point, and Sn is Lithner's score for a polymer compound consisting of MP particles. PET lacks ecological toxicity data, therefore its hazard score cannot be determined. The value for the score of each polymer is taken from Lithner [24].
This study provides a classification standard of risk level of MPs (Table 4), which can initially be used to understand the risk level of MP pollution in the study area and also to provide support for risk management of MP pollution in soil [30,31].

Data Analysis
Using the respective peak lists as described above the raw data sets were further processed using ION-TOF Surface Lab 6.6. Particle counting and size measurement was executed with Nano Measurer 1.2.

TOF-SIMS Analysis
Taking XA1 sample as an example, the typical fragments of four MPs were analyzed by ion mass spectrometry, and the representative data of PP, PVC, PET, and PA6 were collected with imaging information. Accordingly, the characteristic ions of MPs in were obtained ( Figure 1). In addition, the characteristic ions of clay minerals such as Na + , Al + , Si + and Ca + were also detected on the surface of MPs (Figure 2o The color shading presented by the ion imaging mass spectrometry indicates a change in the magnitude of the ion count values at different sites and is a reflection of the change in the concentration distribution of specific ion composition in the 500 μm × 500 μm microzone (Figure 2). The characteristic ions corresponding to PP, PVC, PET, and PA6 were all present as single particles with certain morphologies in the target region, and the relative signal intensity was high. The trend of changes differed among the characteristic ions of all MPs, thus denoting that the characteristic ions of these four MPs can be distinguished from each other. To a large extent, this information also shows    The color shading presented by the ion imaging mass spectrometry indicates a change in the magnitude of the ion count values at different sites and is a reflection of the change in the concentration distribution of specific ion composition in the 500 µm × 500 µm microzone ( Figure 2). The characteristic ions corresponding to PP, PVC, PET, and PA6 were all present as single particles with certain morphologies in the target region, and the relative signal intensity was high. The trend of changes differed among the characteristic ions of all MPs, thus denoting that the characteristic ions of these four MPs can be distinguished from each other. To a large extent, this information also shows that most of the MPs in farmland soil in this area are single polymers. C 4 H 9 + , C 5 H 11 + , C 6 H 9 + , C 6 H 11 + , and

Type, Particle Size, Abundance and Distribution of MPs
The particle size and quantity information of the four types of MPs in soil can be obtained by using Nano Measurer 1.2 software to deal with the superposition map. In this study, a stainless-steel screen with a pore diameter of 35 μm was used for the preliminary separation, so the particle size of

Type, Particle Size, Abundance and Distribution of MPs
The particle size and quantity information of the four types of MPs in soil can be obtained by using Nano Measurer 1.2 software to deal with the superposition map. In this study, a stainless-steel screen with a pore diameter of 35 µm was used for the preliminary separation, so the particle size of the MPs obtained ranged from 0 to 35 µm. The overall particle size distribution is shown in Figure 4b. More than 49% of the total MP particles were smaller than 15 µm; 26.4% and 23.3% were in the range of 0-10 µm and 10-15 µm, respectively. Very few MPs larger than 25 µm were observed, accounting for only 9.3% of the total MP concentrations. The proportion of MPs with different particle size ranges in the study area is 0-10 µm > 10-15 µm ≈ 15-20 µm > 20-25 µm > 25-35 µm. This is consistent with the results of most studies which state that the relative abundance of MPs decreases with increasing particle size. Because of the ubiquity of MPs smaller than 25 µm and their potential environmental significance over large size MPs, many studies have focused on MPs with particle sizes below 25 µm, such as in the mangrove wetlands of Singapore, where the particle size range of MPs is mainly below 20 µm [32]. There were significant differences in particle size distribution of MPs among the sampling sites. The MPs at XA1, XA5, XA7, XA8, XA9, and XA10 were mainly lower than 15 µm probably because the land use types of these sampling areas are mostly farmlands. In the past few decades, sewage sludge is usually recycled to use as fertilizer in farmland soil. The plastic concentrations in sludge were assayed in the range of 1500 to 24,000 items kg −1 [33][34][35][36]. Sewage sludge treated by a sewage treatment plant usually has a high abundance of MPs and a small particle size (µm level). At the same time, there are a large number of plastic microbeads in pesticides and fertilizers, namely, the original MPs. Through decades of agricultural activities, a large number of MPs with smaller particle sizes have accumulated in the region. Plastic products in soil are easier to decompose into MPs with smaller particle size under the effects of agricultural activities, ultraviolet radiation, and microbial decomposition [37][38][39]. However, the MPs at XA2, XA3, XA4, and XA11 were mainly larger than 20 µm. This may be because the MPs in these areas are less affected by external disturbances, and the MPs discharged in the environment are accumulated continuously, making further degradation more difficult. the MPs obtained ranged from 0 to 35 μm. The overall particle size distribution is shown in Figure  4b. More than 49% of the total MP particles were smaller than 15 μm; 26.4% and 23.3% were in the range of 0-10 μm and 10-15 μm, respectively. Very few MPs larger than 25 μm were observed, accounting for only 9.3% of the total MP concentrations. The proportion of MPs with different particle size ranges in the study area is 0-10 μm > 10-15 μm ≈ 15-20 μm > 20-25 μm > 25-35 μm. This is consistent with the results of most studies which state that the relative abundance of MPs decreases with increasing particle size. Because of the ubiquity of MPs smaller than 25 μm and their potential environmental significance over large size MPs, many studies have focused on MPs with particle sizes below 25 μm, such as in the mangrove wetlands of Singapore, where the particle size range of MPs is mainly below 20 μm [32]. There were significant differences in particle size distribution of MPs among the sampling sites. The MPs at XA1, XA5, XA7, XA8, XA9, and XA10 were mainly lower than 15 μm probably because the land use types of these sampling areas are mostly farmlands. In the past few decades, sewage sludge is usually recycled to use as fertilizer in farmland soil. The plastic concentrations in sludge were assayed in the range of 1500 to 24,000 items kg −1 [33][34][35][36]. Sewage sludge treated by a sewage treatment plant usually has a high abundance of MPs and a small particle size (μm level). At the same time, there are a large number of plastic microbeads in pesticides and fertilizers, namely, the original MPs. Through decades of agricultural activities, a large number of MPs with smaller particle sizes have accumulated in the region. Plastic products in soil are easier to decompose into MPs with smaller particle size under the effects of agricultural activities, ultraviolet radiation, and microbial decomposition [37][38][39]. However, the MPs at XA2, XA3, XA4, and XA11 were mainly larger than 20 μm. This may be because the MPs in these areas are less affected by external disturbances, and the MPs discharged in the environment are accumulated continuously, making further degradation more difficult.  The abundance of different types of MPs is different (Figure 4b). In general, the main types of MPs in this area are PA6 and PET, which account for more than 30% of the detected MPs, followed by PP, which accounts for 23.3%, and PVC, which accounts for 16.3%. Thus, the main MPs in the soil in this area are PA6 and PET. There are many small workshops in this area, most of which develop the down industry of shoes and clothing, and PA6, as one of the main raw materials of shoes and clothing production, produces a large amount of plastic waste in the processing and production processes. The non-ferrous metal industries are a local pillar industry, and related supporting industries such as pipe processing, automotive parts, hardware products, and other enterprises. Related industries in the non-ferrous metals industry, such as auto parts factories and electronic device factories, require a large amount of PET powder in the production process, and some PET waste will be directly discharged into the nearby soil, even with the rainwater runoff and soil sedimentation MPs will be discharged to Farther areas [40]. Therefore, PA6 and PET are the most common in the soil of this area.
Comprehensive analysis of the type and particle size of MPs shows that the particle size composition of different types of MPs is quite different (Figure 4c). PET and PVC have a higher abundance in particle sizes less than 15 µm, PA6 has a higher proportion in the 15-20 µm MPs, and PP has a uniform distribution in each particle size distribution range. Due to the poor toughness of PET, it exhibits certain brittleness and poor alkali resistance, so its properties in the soil are unstable and it is more likely to be broken and decomposed. Smaller MPs (µm grade) are easy to adsorb POPs (such as organochlorine pesticides, polycyclic aromatic hydrocarbons, and their derivatives) in soil and are more likely to enter biological tissues and even cells [6,41,42]. Therefore, in the future, more attention should be paid to these MPs with higher abundance and finer particles.

Chemical Risk Assessment of MP
According to the risk index (PI) of chemical characteristics of MP, the study area is facing serious MP pollution. The classification of MP pollution risk index is shown in Table 4. MP risks that cannot be ignored have been found in different urban functional areas of the study area ( Figure 5). The high chemical risk index in some areas is due to the presence of MPs (such as PVC) with high hazard scores [30,31]. It can be seen from the figure that the risk level of MP pollution in the northern part of the study area is low, while the risk level of MP in the southern part of the study is relatively high, which is related to the distribution of urban functional areas in the study area. The northern functional area is the central urban area and commercial area and is away from the garbage treatment plant. Plastic waste that is closer can be processed centrally, so this area is less polluted by MPs. The southern and eastern regions are non-ferrous metal industrial parks and small workshops. During production and processing, a large amount of plastic waste is generated and most of it is discharged directly into the environment without treatment. Therefore, the risk level from MPs in the southern and eastern regions is relatively high. Different from other refractory organic pollutants, the distribution of MPs in soil is uneven. Particle size and chemical composition of MPs are important factors affecting their distribution. More than 80% of the MPs studies here have similar shape characteristics, so they can be used to compare the risk of MPs in different regions. Some studies have also used similar risk assessment methods in the MP risk assessment of Shanghai river sediments and Yangtze River Estuary waters, which Different from other refractory organic pollutants, the distribution of MPs in soil is uneven. Particle size and chemical composition of MPs are important factors affecting their distribution. More than 80% of the MPs studies here have similar shape characteristics, so they can be used to compare the risk of MPs in different regions. Some studies have also used similar risk assessment methods in the MP risk assessment of Shanghai river sediments and Yangtze River Estuary waters, which showed that high chemical toxicity can result in a high environmental risk of MPs [30].
In view of the chemical properties of MPs, it is believed that polymers are inert and often do not pose environmental risks to the soil environment [43]. However, plastic is not a single polymer, and unreacted monomers and other additives exist in plastics that are released into the environment as plastics decompose, posing a threat to ecosystems and human health. Polymers in MPs may also pose a threat to the environment. PVC is the most hazardous polymer and one of the most widely used plastic raw materials, with a global production of about 38.5 million tons [24]. Once PVC enters the soil, it releases carcinogenic monomers and other additives such as plasticizers, stabilizers, pigments, thus seriously affecting soil microorganisms and plants. At the same time, MPs also can adsorb POPs and transfer them to aquatic and terrestrial organisms, resulting in the absorption and storage of POPs in organisms to produce a complex ecological effect [44,45].
To date, MP contamination of terrestrial soils has not been part of environmental risk assessment. With the increasingly serious problem of MP pollution, the analysis of its distribution characteristics will help to better assess the environmental risk of MP, while gradually improved risk assessment methods and indicators will help to fully understand its impact on the ecological environment and on human health [30,46].

Conclusions
In this study, four kinds of MPs (PP, PVC, PET, PA6) in the surface soil of Baoding Suburbs were characterized by TOF-SIMS, and their pollution risks were preliminarily assessed. Twelve surface soil samples from various functional areas contained different degrees of MPs. The particle size of MPs in this area is mainly between 0 and 10 µm, and PA6 and PET account for the highest proportion. PVC is an important source of high pollution risk for MPs. According to the chemical risk assessment index for MP, non-ferrous metal industrial parks and small workshops were identified as the most polluted areas, reaching the highest risk level of V. Meanwhile, this study shows that the risk level of MP pollution is closely related to human production activities, and the region should be closely monitored. At the same time, MP evaluation indicators still need to be increased, which is the basis of future risk assessment. It is suggested that the academic circles should carry out the work on environmental risks of MPs as soon as possible to help control MPs pollution.