Monitoring of microplastics in road dust samples from Myanmar and Taiwan

ABSTRACT

Microplastics (MPs) are ubiquitous pollutants in various environmental matrices. In this study, 82 road dust samples collected from Myanmar and Taiwan between 2014 and 2018 were analyzed to investigate the status of MPs pollution and distribution. In Myanmar, the mean abundances of MPs in road dust were high in cities, such as Nay Pyi Taw (285 pieces/kg dry weight), Mandalay (79 pieces/kg), and Yangon (63 pieces/kg). Alternatively, no MPs were found in samples from the rural areas, Wundwin and Bagan. The mean MP abundance was 555 pieces/kg in road dust from Taiwan, which was two to nine times that of road dust from Myanmar. Polyethylene and polypropylene were the dominant polymers detected in road dust from Myanmar, whereas polyvinyl chloride was a major component in road dust from Taiwan. These results suggest that differences in economic and industrial activities and population densities are related to MP profiles in road dust. The plastic additives butylated hydroxytoluene, diethyl phthalate, and di-n-octyl phthalate were identified in a polyvinyl chloride MP in the road dust. These MPs originate from plastic products in the environment and lead to extensive pollution with hazardous chemicals. To our knowledge, this is the first study to report the occurrence of MPs in road dust from Myanmar.

INTRODUCTION

Plastics are used in various industrial applications, such as packaging, building, construction, automobiles, textiles and electrical materials, and agricultural, household, and health and safety products. However, 4.9 billion tons of plastic polymer resin and synthetic fiber, which corresponds to 60% of the total amount of plastic production, have been discarded into the environment since the 1950s (Geyer et al., 2017). Especially, low-income Asian countries, such as Myanmar and Bangladesh, are known as high emissions of mismanaged plastic litter into the ocean (Jambeck et al., 2015). Additionally, there have been increasing concerns in large number of the environmental loads of microplastic (MP) and plastic additives and their adverse effects on aquatic organisms. Considering that 80% of marine debits originate from land (Jambeck et al., 2015), it is important to identify their specific terrestrial sources to control/manage the discharge into the environment.

Recent studies have focused on road dust as an important source of MPs in the terrestrial environment because road signs and paints generally contain a various plastic polymers (Gaylarde et al., 2021). MPs have been detected in road dust in Australia (Su et al., 2020; O’Brien et al., 2021), India (Patchaiyappan et al., 2021), Iran (Abbasi et al., 2017), Japan (Kitahara and Nakata, 2020; Yukioka et al., 2020), Nepal, Vietnam (Yukioka et al., 2020), and Sweden (Järlskog et al., 2021). The mean abundances of MPs in road dust showed large variations, ranging from 68 pieces/kg (dry weight) in Okinawa, Japan (Kitahara and Nakata, 2020) to 165,800 pieces/kg in Bushehr, Iran (Abbasi et al., 2017). Additionally, organic plastic additives, including dioctyl adipate, 2-ethylhexyl diphenyl phosphate, bis(2-ethylhexyl) phthalate (DEHP), 2-ethylhexyl 2-cyano-3,3-diphenylacrylate, and 2-hydroxy-4-n-(octyloxo) benzophenone, were detected in MPs in road dust from Japan (Kitahara and Nakata, 2020). Yamahara et al. (2022) reported the occurrence of more than 100 additives in MPs collected from road dusts, and their sources are considered in road marking and paint, reflector, and braille block. These observations suggest that MPs and plastic additives in road dust could be a significant terrestrial source in various countries, but there are few studies on the occurrence and distribution of MPs in road dust in low-income Asian countries like Myanmar.

Myanmar is located at tropical monsoon climate area in Asia. The per capita gross domestic product (GDP) is low at US＄1,527 in 2020 (JETRO, 2020a), the total GDP had drastically increased at 6.8% in 2018 (Tang and Li, 2021). A previous study reported that Persistent Organic Pollutants (POP) concentrations in mussels and gross national product in Asian countries were well correlated, suggesting that environmental pollution is strongly linked to industrial and human activities (Monirith et al., 2003). Recently, the environmental monitoring of organic and inorganic pollutants has been reported in Myanmar. Mon et al. (2020) found that high polycyclic aromatic hydrocarbon (PAH) concentrations in road dust in urban areas and fossil fuel exhaust and biomass combustion are the major sources of PAHs. The artificial sweeteners acesulfame and sucralose were clearly detected in river water from Yangon and Pathein cities (Watanabe et al., 2016). The high concentrations of toxic elements have been detected in groundwater samples from Myingyan (Bacquart et al., 2015), and soils nearby gold mining in Sagaing Region (Tun et al., 2020). As for plastic study, the analysis of cosmetic products purchased in Myanmar showed polyethylene microbeads in facial cleansers (Mon and Nakata, 2020), but little information is available on the abundance and distribution of MPs in environmental matrices, including road dust in Myanmar.

To understand the present status of MPs pollution in Myanmar, this study focused on Taiwan, which is also located in Asia but has a different economic background from Myanmar, as a reference site. Taiwan is highly industrial and urbanized in the Asian region, and the per capita GDP is US＄28,371 in 2020 (JETRO, 2020b), approximately 20-fold greater than Myanmar. Electronic and chemical industries are developed, which is different from Myanmar which textile and agriculture are major industry (JETRO, 2020a, 2020b). A comparison of the MP pollution between Myanmar and Taiwan suggests the effects of industrial structure for their specific pollution. In Taiwan, the occurrence and distribution of MPs have been reported in water (Tien et al., 2020; Wong et al., 2020; Shiu et al., 2021), sediments (Kurz et al., 2016; Bancin et al., 2019; Chen et al., 2021), and aquatic organisms (Liao et al., 2021). The mean MP abundance in harbor sediments was 79.3 items/kg (dry weight), which was three to five times that in coastal regions (Chen et al., 2021). The MP abundance in freshwater sediments was 508–3,987 pieces/kg in Fengshan River, Kaohsiung, southern Taiwan (Tien et al., 2020), but little data are available on MP abundances in road dust, although it may be a potential source of MPs.

Prompted by this background, we aimed to monitor the abundance and distribution of MPs in road dust samples collected in Myanmar and Taiwan. We analyzed the MP polymer types and plastic additives to determine their potential sources and evaluate their toxicological implications in the environment.

MATERIALS AND METHODS

SAMPLE COLLECTION

Seventy-two road dust samples were collected in Yangon (n=23), Mandalay (n=13), Nay Pyi Taw (n=11), Pathein (n=9), Bagan (n=3), Wundwin (n=3), Chaungthar (n=2), Myingyan (n=2), and other rural sites (n=6) in Myanmar between 2014 and 2018 (Fig. S1). Nay Pyi Taw is the capital of Myanmar, and Yangon and Mandalay are the first and second largest cities by population density, respectively. Road dust samples were also collected from Tainan in southern Taiwan in 2018 (n=10). The details of the sampling sites are given in Table S1. All samples were collected using a clean nonplastic brush and dust pan and stored at −20°C until required for analysis.

ABUNDANCE AND POLYMER IDENTIFICATION

MP abundances and polymer types in the road dust samples were determined using a method (Tun et al., 2022) with slight modifications. Briefly, 20–50 g of road dust was sieved using a stainless steel sieve (1-mm mesh). The sieved sample was poured in a beaker containing 30% hydrogen peroxide for 3 days at room temperature to remove the organic matrix. The solid sample was recovered using a nylon filter (100-μm mesh) and treated with a 60% sodium iodide solution (1.8 g/cm³) for density separation. The solids floating on the solution were removed by filtration, and the MP candidates were collected and analyzed individually using Fourier transform infrared spectroscopy (IR Affinity 1S, Shimadzu, Japan). Each spectrum was compiled from 20 scans recorded over 500–4,000 cm⁻¹ at a resolution of 4 cm⁻¹. The spectra were compared with a library database to identify the polymers. The threshold for MP polymer identification was a 75% or greater match with the reference spectrum. Smaller MPs (ø=100–300 μm) and rubber MPs (e.g., styrene-butadiene rubber) were not analyzed because of sample handling limitations in this study. The sample masses and total abundances of the MPs identified are shown in Tables S2 and S3.

PLASTIC ADDITIVES ANALYSIS

Plastic additives were analyzed in polyvinyl chloride (PVC) MP and polypropylene (PP) fragments collected from Taiwan road dust. To avoid the elution of additives during the digestion process, the sample was not treated with H₂O₂ before extraction. Also, samples were slightly wiped by isopropyl alcohol (IPA) to remove chemicals adsorbed in the surface layer. The samples was placed in a glass tube containing 1-mL dichloromethane and ultrasonicated for 10 min. The sample extract was concentrated under a stream of nitrogen gas and then injected into a gas chromatograph mass spectrometer (Agilent 7890A-5975C, Agilent Technologies, Santa Clara, CA) in the scan mode. The initial gas chromatography oven temperature was 80°C. This was then increased to 160°C at 20°C/min, held at 160°C for 10 min, increased to 300°C at 3°C/min, and held at 300°C for 10 min. The separation was achieved using an HP-5MS column (30 m×0.25 mm i.d., 0.25 μm film thickness; Agilent Technologies). Helium was used as the carrier gas at a flow rate of 1 mL/min. The NIST 2014 library was used to identify the plastic additives.

QUALITY ASSURANCE AND CONTROL

For quality assurance and control, a recovery test using MPs and standard sand was performed. Ten particles each of polyethylene (PE), polystyrene (PS), and polymethyl methacrylate (PMMA) with particle sizes between 250 and 1,000 μm were spiked into 5 g of standard sand and analyzed according to the procedure described earlier. The recoveries of PE, PP, and PMMA were 97%±15%, 80%±17%, and 77%±21%, respectively.

RESULTS AND DISCUSSION

ABUNDANCE AND DISTRIBUTION OF MPs

MPs were detected in 42 of the 72 road dust samples collected from Myanmar (Figs. 1 and S2 and Tables 1 and S2). The detection frequencies of MPs were high in cities, such as Nay Pyi Taw (73%), Yangon (70%), Pathein (56%), and Mandalay (54%), whereas no MPs were detected in samples from the rural areas, Wundwin and Bagan (Tables 1 and S2). The highest abundance of MPs was found at site MMR-63 (2,119 pieces/kg) in Nay Pyi Taw, which was located between a highway bus stop and market in a downtown area (Table S2). In Yangon, the MP abundance was highest at site MMR-37 (365 pieces/kg), followed by site MMR-20 (198 pieces/kg) and site MMR-21 (197 pieces/kg) (Fig. 2). Sites MMR-37 and -20 were located in the city center of Yangon and near a domestic ferry port crossing the Yangon River, respectively.

Fig. 1

FT-IR spectrum of microplastics in road dust samples collected from Myanmar

Table 1 Microplastic abundance (piece/kg dry wt.) in road dusts collected from Myanmar and Taiwan between 2014 and 2018

Country	N	Detection frequency (%)	N of MP identified	Median (piece/kg dry wt.)	Mean (piece/kg dry wt.)	Min-max (piece/kg dry wt.)	Major polymers (%)
Myanmar (n=72)
Pathein	9	56	6	20	17	0–40	PE (50)>PS (17)=PET (17)=PVC (17)
Chaungthar	2	50	1	10	10	10	PVC (100)
Wundwin	3	0	0	0	0	0	NA
Bagan	3	0	0	0	0	0	NA
Myingyan	2	50	1	14	14	14	PES (100)
Yangon	23	70	36	40	63	0–365	PE (50)>PS (17)=PET (17)=PVC (17)
Yangon-Pathein	5	60	5	20	84	20–200	PE (40)=PC (40)
Mandalay	13	54	41	20	79	0–414	PE (37)>PP (32)
Mandalay-Nay Pyi Taw	1	100	2	192	192	192	PP (100)
Nay Pyi Taw	11	73	62	141	285	0–2,119	PP (45)>PE (34)
Taiwan (n=10)
Tainan	10	90	151	363	555	0–2,065	PVC (32)>PE (9.3)

NA=Not available

Fig. 2

Distribution of microplastics in road dust samples from Yangon, Myanmar

The high abundances of MPs in road dust were also found at sites MMR-48 (414 pieces/kg) and MMR-51 (238 pieces/kg), which were located near Mandalay Technological University and the Mandalay downtown area, respectively (Table S2). Numerous MPs in road dust from sites MMR-2 and -4 were also relatively high (40 pieces/kg) in Pathein. The MP abundances were generally low in rural towns, such as Chaungthar, Bagan, Wundwin, and Myingyan, and ranged from 0 to 14 pieces/kg (Tables 1 and S2). These results suggest there are major MPs sources in urban areas with high traffic and population densities. The population and the density were 7,360,703 people and 716 people/km² in Yangon city in 2014, which are greater than those in Mandalay region and Ayeyawady State in Myanmar (Ministry of Immigration and Population, 2015). The percentage of urban population is 70% in Yangon, which is also more than 2-fold higher than that in rural states and regions. Our results are similar to those reported in India (Patchaiyappan et al., 2021), Iran (Abbasi et al., 2017), Japan (Kitahara and Nakata, 2020), and Nepal (Yukioka et al., 2020). MPs were also abundant in highway road dust collected between Yangon and Pathein near the bridge crossing the Ayeyarwady River (site MMR-45, Table S2).

In Taiwan, the detection frequency and mean abundance of MPs in road dust was 90% and 555 pieces/kg, respectively (Tables 1 and S3). The results were higher than those found in Myanmar, probably because of the different degrees of urbanization and lifestyle. The highest MP abundance occurred at site TWN-10 (2,065 pieces/kg), which was located on a narrow street in the Tainan downtown area (Fig. 3 and Table S3). High MP abundances were also found at sites TWN-1 (468 pieces/kg) and TWN-7 (1,364 pieces/kg), where were located in front of Tainan Station and on the main street in Tainan, respectively. Chen et al. (2021) analyzed MPs in coastal sediments and suggested that hydrodynamic forces influence the distribution of MPs in Taiwan.

Fig. 3

Distribution of microplastics in road dust samples from Tainan, Taiwan

Previous studies have reported on the abundances of MPs in road dust from various countries. These values are of the same order of magnitude as those observed in Myanmar and Taiwan. Alternatively, extremely high MP abundances were detected in road dust from Bushehr (4,400–165,800 pieces/kg; Abbasi et al., 2017) and Tehran (2,900–20,166 pieces/kg; Dehghani et al., 2017) in Iran. These differences could partly be attributed to the analytical instruments used and the target size of MPs, which will affect the comparison of MP abundance in road dust.

POLYMER TYPES

PE and PP were the dominant polymers in MPs in road dust samples from cities in Myanmar (Fig. 4). In Nay Pyi Taw, PP contributed to 45% of the total MPs (n=62), followed by PE (34%), polydiallyl phthalate (PDAP; 6.5%) and polyethylene terephthalate (PET; 4.8%) (Fig. 4). PP and PET were abundant in the MMR-63 sample collected near a highway bus terminal and market (Table S2). As described earlier, the highest MP abundance occurred at this site (Table S2). These results imply that vehicles and human activities are the major sources of MPs. PP was dominant in the road dust sample from site MMR-66, which was located near the entrance of Uppatasanti Pagoda in Nay Pyi Taw (Table S2). Similar profiles were also found in road dust samples from Mandalay, where PE (37%) and PP (32%) were dominant, followed by PET (4.9%) (Fig. 4). The high contributions of PE and PP were detected in the samples from site MMR-48 near Mandalay Technological University and site MMR-51 on a major street in the downtown area of Mandalay. Except for the Nay Pyi Taw and Mandalay samples, PVC was a major MP polymer in the Yangon road dust samples. The contributions of PE, PVC, and PP to the total MPs were 28%, 22%, and 19%, respectively (Fig. 4).

Fig. 4

Polymer compositions of microplastics in road dust samples from Nay Pyi Taw, Yangon, Mandalay in Myanmar and Tainan in Taiwan

PE: Polyethylene, PP: Polypropylene, PS: Polystyrene, PET: Polyethylene terephthalate, PVC: Polyvinyl chloride, PMMA: Polymethyl methacrylate, PDAP: Polydiallyl phthalate, PES: Polyester, PMM: Polymethyl acrylate, PBM: Polybutyl methacrylate

In Taiwan, various polymers were identified in the MPs from road dust. PVC was the dominant polymer, accounting for 32% of the total MPs, followed by PE (9.8%), PP (8.6%), and PS (7.9%) (Fig. 4). PVC was detected in 16 of 31 MPs found in the sample from TWN-10, which had the highest abundance of MPs in Tainan (Table S3). Three acrylates, PMMA, polymethyl acrylate (PMA), and polybutyl methacrylate (PBM), were dominant (>12% of the total MPs). As described earlier, Taiwan is a highly economic and industrially developed country, while Myanmar is low-income and agricultural country. A PVC, which is frequently used for industrial purposes, accounted for a high percentage of the polymer composition in Taiwan, whereas major polymers identified in Myanmar are PE and PP, which are frequently used in household products. The differences in the MP abundances and polymer types between Taiwan and Myanmar could be explained by the different backgrounds and sources of MPs between the countries. However, further study using various statistical data is needed to evaluate the relationship between plastic pollution and economic growth in the Asian countries.

Previous studies reported PE, PP and PET as the major polymers in water and sediment samples in Kaohsiung, southern Taiwan, and PVC is minor (Tien et al., 2020; Chen et al., 2021). The polymer profiles of MPs may be linked to the environmental matrices analyzed. PE and PP were the dominant components of MPs in road dust (Kitahara and Nakata, 2020; Yukioka et al., 2020) and lake sediment (Era and Nakata, 2020) collected from an urban area in Japan. This suggests that consumer products is a major source of MPs in the Asian countries.

PLATICS ADDITIVES ANALYSIS

A piece of PVC MP and PP fragment in Taiwan road dust was extracted using dichloromethane and analyzed for plastic additives. The chromatogram and mass spectra suggested the PVC MP contained BHT, diethyl phthalate (DEP), and di-n-octyl phthalate (DnOP) (Fig. 5). BHT and tris (2,4-di-tert-butylphenyl) phosphite were clearly identified in a PP fragment analyzed (Fig. S3). BHT is a representative antioxidant generally used in foods, cosmetics, and various plastic products. It is frequently detected in PE plastic waste at a maximum concentration of 33,000 ng/g (Nurlatifah et al., 2021). However, the zebrafish embryo toxicity tests of BHT have shown that the acute toxicity after 96 h of exposure during the embryogenic stage is 4.4 mg/L and the effective concentration is 1.4 mg/L (Sarmah et al., 2020). BHT can also affect the expression of the dopamine system, which is hypothesized to be related to the abnormal anxiety-associated behavior of larval zebrafish (Liang et al., 2020). BHT is generally considered safe for humans, but its ubiquitous distribution in plastic waste implies frequent exposure in ecosystems, which could result in adverse effects in the developmental stages of wildlife.

Fig. 5

GC-MS chromatogram and mass spectrum of a PVC microplastic from Taiwan

DEP and DnOP are both phthalate plasticizers present in several plastic products and are suspected to have endocrine-disrupting effects in human (Bang et al., 2012). The European Union banned the use of six phthalates, including DnOP in toys and children’s products that might be placed in the mouth (EPC, 2005). DEP has been detected in plastic waste at concentrations up to several thousand nanograms per gram (Nurlatifah et al., 2021). It has also been detected in MPs found in coastal sand (Tanaka et al., 2019; Fred-Ahmadu et al., 2020). As for tris(2,4-di-tert-butylphenyl) phosphite, it has been used as antioxidant and the production volume is estimated to be 200 tons in Japan (Japan Chemical Daily, 2022). Considering that MPs containing hazardous plastic additives can be washed into aquatic ecosystems, the occurrence and distribution of MPs in road dust should be monitored as a potential source of additives in the terrestrial environment.

CONCLUSION

Eighty-two road dust samples collected from Myanmar and Taiwan were analyzed for MPs. The highest MP abundances was detected in urban road dust samples from Nay Pyi Taw (2,119 pieces/kg), Mandalay (414 pieces/kg), Yangon (365 pieces/kg) in Myanmar. The mean abundance of MPs in road dust was relatively high in Taiwan at 555 pieces/kg. PE, PP, and PVC were major polymers in the MPs in road dust, which implies that consumer products and road paints may be the potential sources of MPs in the road dust. Additionally, the difference in economic and industrial activities between Myanmar and Taiwan may be related to the abundance and distribution of MPs in the road dust. The GC-MS analysis of an MP particle found in the road dust showed that it contained BHT and phthalates. These results imply that MPs in road dust transport hazardous chemicals into the environment, resulting in their bioaccumulation and adverse effects in ecosystems. This is the first study to determine MPs in road dust from Myanmar.

ACKNOWLEDGMENTS

The authors thank the academic staff and students at Pathein University and Mandalay Technological University for assisting with sample collection in Myanmar. This study was partly supported by the Bilateral Open Partnership Joint Research Project (Grant # JPJSBP120209934) and KAKENHI (Grant # 19K12372 and 22H03762), both funded by the Japan Society for the Promotion of Science and SATREPS by Japan Science and Technology Agency.

SUPPLEMENTARY MATERIAL

Fig. S1, Sampling sites of road dust samples in Myanmar and Taiwan; Fig. S2, Pictures showing microplastics detected in road dust samples from Myanmar; Fig. S3, GC-MS chromatograms and mass spectrum of a PP fragment from Taiwan road dust; Table S1, Sampling sites of road dust samples collected from Myanmar and Taiwan during 2014 and 2018; Table S2, Abundances and polymer type of microplastics in road dust samples collected from Myanmar; Table S3, Abundances and polymer type of microplastics in road dust samples collected from Taiwan.

This material is available on the Website at https://doi.org/10.5985/emcr.20220008.

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