Microplastics in dumping site soils from six Asian countries as a source of plastic additives

Highlights

• Microplastics (MP) was analyzed in 54 dumping site soils from 6 Asian countries.

• The highest abundance of MP was identified in Cambodian samples at 218,182 pieces/kg.

• PE and PP were dominant polymers, suggesting daily-used plastics are main sources of MP.

• Soil samples were divided into floating and sedimentation fractions for additive analysis.

• The half amount of plastic additives in dumping site soils are derived from MP.

Abstract

The ubiquitous distribution of microplastics (MP) is a serious environmental issue in Asian countries. In this study, 54 open-dumping site soils collected from Cambodia, India, Indonesia, Laos, the Philippines, and Vietnam were analyzed for MP. Soil samples were also divided into light (floating) and heavy (sedimentation) fractions by density separation and analyzed for plastic additives. The highest abundance of MP was found in a soil from Cambodia at 218,182 pieces/kg. The median of MP in soils ranged from 1411 pieces/kg in India to 24,000 pieces/kg in the Philippines, suggesting that dumping sites are a major source of MP into the environment. Polyethylene, polypropylene, and polyethylene terephthalate were dominant polymers in soil samples analyzed. This indicates that daily-used plastic products are main sources of MP in dumping site soils in Asian countries. The high concentrations and burdens of phthalates and an antioxidant were detected in floating fraction accounting for 40 to 60% of the total additives in soils. Previous studies on soil pollution have assumed that the organic hydrophobic chemicals analyzed are adsorbed on the surface of soil particles. However, this result indicates that approximately half of the additives in dumping site soils were derived from MP, not soil particle. Monitoring study on soil pollution should be considered the occurrence of MP in the matrices.

Introduction

During the past 10 years, the wide spread distribution of microplastics (MP) and its adverse effects in the ecosystem have been investigated in the world. These studies reported the present status of MP pollution in water, sediment and biological samples, and recently soil pollution of MP has also been focused on a global scale from a viewpoint of agricultural plastic film mulching (Qi et al., 2020). MP including low density polyethylene (PE) is detected in farmland soils from China, at 107 items/kg (Zhang et al., 2020). Recently the MP abundance in vegetable-grown soils ranged from 320 to 12,560 pieces/kg (Chen et al., 2020), suggesting that agriculture multi film contributes a large number of MP production in farmland. The abundance of MP in the floodplain soils were found ranging from 0 to 593 particles/kg in Switzerland (Scheurer and Bigalke, 2018). The high abundance of MP was detected in road dusts collected from Iran (Abbasi et al., 2019), Nepal (Yukioka et al., 2020) and Japan (Kitahara and Nakata, 2020). The abundance of MP was significantly correlated with traffic density, and PE and polypropylene (PP) were dominant polymers of MP. This suggests that vehicles and commonly used consumer products are major source of MP in urban road dusts (Kitahara and Nakata, 2020).

Recent studies on soil pollution have been conducted in dumping sites and their surrounding areas. Chai et al. (2020) analyzed 33 soils from an e-waste dismantling area in Guangdong Province, China. MP were detected in 30 soils, and the highest abundance was 34,100 pieces/kg, implying that e-waste dumping site has become the MP hotspot. The abundance of MP ranged from 600 to 14,200 pieces/kg in soils from an e-waste recycling site (Zhang et al., 2021). The halogenated flame retardants (HFR) were also detected, and BDE209 showed the highest concentrations at 718 μg/g in soils and 30,446 μg/g in plastic debris. These findings suggest that plastic particles have greatly contributed to total HFR burden in soils. However, to date, limited information is available on the occurrence and abundance of MP in dumping site soils. Furthermore, few studies have been reported the relationship between additives concentrations and MP abundance in dumping site soils from the perspective of source identification.

The amount of mismanaged plastic waste was the highest in China at 8.82 million tones in 2010, and 7 of the top 10 waste dumping counties were located in Asia (Jambeck et al., 2015). The economy has rapidly expanded in these countries, but waste management infrastructure is seriously lacking. Therefore, large amounts of municipal solid wastes are dumped into open dumping sites without adequate management. For example, the local government has a responsibility for municipal solid waste management in Indonesia, but they give low priority to the services because of the low allocation in their budget and insufficient maintenance and investment (Lyons et al., 2020). Almost half of the counties in Indonesia continues to operate open dumping sites (The Jakarta Post, 2019). In Thailand, 20% of total amount of urban waste were mismanaged, resulting in waste disposition in open dumping sites and open burning in uncertified facilities (Toyama Kankyo Seibi Co. Ltd, 2017). It has been reported that MP abundance in marine sediments in Brazil (Neto et al., 2019) and wild plants in Italy (Loppi et al., 2021) was high close to dumping sites compared with those at more distant sites. This suggests that dumping site may be a potential source of MP in the environment.

Plastic additives are of great concerns to evaluate the adverse effects of plastic debris and MP in the environment. Phthalate plasticizers such as dibutyl phthalate (DBP), benzyl butyl phthalate (BBP), diisobutyl phthalate (DiBP), bis (2-ethylhexyl) phthalate (DEHP) are included in candidate list for substance of very high concern (SVHC) which require authorization before using (European Chemical Agency, n.d.). Traditional phthalates plasticizers, such as dimethyl phthalate (DMP), dibutyl phthalate (DBP), and di(2-ethylhexyl) phthalate (DEHP) have been found at high concentrations in soils in India (Chakraborty et al., 2019) and in indoor dusts in China (Deng et al., 2021) collected from both e-waste recycling facilities. This implies that MP containing in soil and indoor dust contribute to their total burdens of plastic additives, but it is unclear because previous studies analyzed plastic additives in a whole soil. To evaluate the contribution of MP, it is necessary to divide a soil sample into floating fraction and sedimentation fraction including MP and other soil residues, respectively, and analyzed both. If additive concentrations in floating fraction is higher than that of sedimentation fraction, MP could be a source of plastic additives in dumping site soils.

In this study, we analyzed MP and plastic additives in dumping site soils collected from six Asian countries. Before the additive analysis, soils were divided into two fractions; ‘floating including MP’ and ‘Sedimentation including soil residues’ by density separation and analyzed. The aims of this study are as follows: (1) to understand the status of MP and plastic additives pollution, and (2) to evaluate a contribution of MP to the total burden of additives in soils of Asian dumping sites. Information on the concentrations and burdens of plastic additives in two fractions could be suggested a potential source of additives in dumping site soils.