Behavior and distribution of polystyrene foams on the shore of Tuul River in Mongolia

https://doi.org/10.1016/j.envpol.2020.113979Get rights and content

Highlights

  • Polystyrene foams (PSFs) are dominant plastic debris in the studied river shore.

  • Wide range of surface oxidation of PSFs can be related to their surface status.

  • Abundance of adhered microplastics on PSFs was different between size fractions.

  • PSFs have a great potential to transport various types of microplastics.

Abstract

Foamed plastic debris in aquatic systems has become one of the emerging global contaminants. In this study, the behavior of polystyrene foam (PSF) and microplastics (MPs) adhered on the PSFs were investigated on the Tuul River shore in Ulaanbaatar, the capital city of Mongolia. The micro-sized (<5 mm) PSF, which was the dominant PSF over 600 pieces in 100 m2, have accumulated along the shoreline of Tuul River. Carbonyl index (CI) was calculated to evaluate the surface oxidation of macro-sized (20–100 mm), meso-sized (5–20 mm), and micro-sized PSFs and confirm the relative aging depending on photodegradation. CI ranged from 0.00 to 1.09 in the sampled PSFs, whereby the degraded PSFs with high CI were distributed on the shore of downstream of sewer drainage. Micro-sized PSFs showed a wide range of CI and a relatively high average value of CI as compared to those of meso- and macro-sized PSFs. Most of PSFs aggregated with MPs and the adhered MPs have been ubiquitously detected from the surface of PSFs. Adhered micro-sized plastics explored from the surface of PSFs with various sizes, except for mega-sized (>100 mm) PSF, ranged from 5 to 141 items per piece of PSF fragment. The aggregates of PSFs and MPs were common status of PSFs during their transportation. The present findings, which indicated a high concentration of adhered MPs, raise an environmental concern about the widespread aquatic plastic pollution.

Introduction

Environmental plastics have been widely recognized as emerging pollutants. The annual world plastic production has reached more than 300 million tons since 2014 (PlasticsEurope, 2018), mainly consisting of packaging material, which is the leading industrial sector occupying 44.8% of the plastic market (Geyer et al., 2017). With a growing population, concerns about consumption of single-use plastic materials have emerged in many countries, and the nations have faced a large amount of generated plastic waste without proper recycling managements (Reisser et al., 2013; UNEP, 2018). Improper management of excessive plastic wastes produced plastic debris in the environment, which end up in the ocean through river transportation (Lebreton et al., 2018, 2017; Miller et al., 2017). Recently, scientists particularly addressed the accumulation of plastic debris, changes in their morphology, distribution, and degradation in the aquatic environments in order to figure out the human plastic footprint in the global environment (Brandon et al., 2016; Eriksen et al., 2014; Huerta Lwanga et al., 2017; Koelmans et al., 2017).

Despite their durability, environmental plastics break down into smaller size fractions of micro-size (<5 mm) and nanometric size (<100 nm) depending on the environmental conditions (Besseling et al., 2017; Wagner et al., 2014). In the early stage of fragmentation, under the prolonged exposure to ultraviolet (UV) light and other physical conditions (freezing-thawing, dry–wet cycles) and mechanical forces lead to plastic degradation (Cooper and Corcoran, 2010; Kaiser et al., 2017). Plastics have various durability with different degradation rates based on the source of plastics, types of polymerization, chemical additives, and environmental conditions (Gewert et al., 2015). Common signs of degradation include yellowing, cracking, and embrittlement of plastic surface character. Changes in the surface chemical structure accompanying the formation of oxygen-containing functional groups and a decrease in molecular mass are other signs of plastic degradation (Bandow et al., 2017). Concurrently, changes in surface properties of plastics are important indicators of aging of environmental plastic polymers which are characterized by different degrees of plastic resistance to various physical (environmental) conditions. For example, acrylonitrile butadiene styrene and polystyrene (PS) plastics have poor resistance against UV light, and these plastics can be significantly damaged by environmental factors compared with other polymer materials (Castillo et al., 2016; Weinstein et al., 2016). Scanning electron microscope images revealed that the surface of aged PS was filled with microcracks, differing from that of pristine PS (Liu et al., 2019b; Zhang et al., 2018). Additionally, change in surface color has been widely detected in environmental plastics (Brandon et al., 2016; Endo et al., 2005; Lv et al., 2017). Yellowing discoloration is mainly attributed to formation of the chromophore as a result of formation by oxidation of polymer itself or byproducts with quinoidal structures which are formed by oxidation of phenolic antioxidants (Acosta-Coley and Olivero-Verbel, 2015; Andrady, 2017). Furthermore, aging and weathering of plastics increase their sorption potential of pollutant chemicals through microcracks and oxygen containing functional groups, resulting in the ingestion of toxic chemicals by aquatic organisms (Gao et al., 2019; Vroom et al., 2017).

Polystyrene polymer production has reached 7.6% of the global plastic demand, which is mostly used for packaging (2.3%) and building and construction materials (2.2%) in the industrial sectors (Geyer et al., 2017). An extensive use of those PS materials promotes scattering of these plastic wastes as marine (Didier et al., 2017; Fok et al., 2017) and freshwater (Battulga et al., 2019; Moore et al., 2011) debris. The released PS wastes are ingested by organisms and have harmful effects on the food web through the biological system in the environment (Gambardella et al., 2017; Hu et al., 2016; Hüffer et al., 2018; Schlummer et al., 2015).

In recent years, MPs in the aquatic environment have received attention from interdisciplinary scientific fields due to severe problems, such as the MP distribution in freshwater ecosystems and their transportation to marine ecosystems (Li et al., 2016; Peng et al., 2018), the bioaccumulation through the ingestion of MPs by organisms (Pazos et al., 2017; Windsor et al., 2019), and the interactions between organic pollutants and MP particles (Bakir et al., 2016; Wang and Wang, 2018). Although the number of studies on the aquatic environment, particularly on river systems, has increased to evaluate the MP behavior affecting the marine environment, the dynamics of environmental plastics in any freshwater compartments, as well as their degradation processes, are still unknown (Horton et al., 2017). Despite various MP studies, a comprehensive understanding of environmental degradation, plastic aging, and interactions between aged plastics and other MPs is still rather limited (Hüffer et al., 2018; Lee et al., 2014). Although we found interaction of two major microplastics (PS and PE) with PS-foamed (PSF) particles in the Selenga River system of Mongolia (Battulga et al., 2019), the potential of PSF as a carrier of invisible MPs has never been evaluated to explain the behavior of the aggregates of MPs and PSFs. To understand the characterization of the adhered MPs, we focused on the distribution of PSFs and their interactions with adhered MPs. The photooxidation status is also evaluated for PSF plastics. The properties of aggregates can be useful in understanding the behavior of degraded plastics in freshwater ecosystems. The aims of this current study are as follows: 1) to understand the behavior of plastics, particularly polystyrene foam (PSF), in the urban river in Ulaanbaatar (UB) city, 2) to identify the photooxidation status of PSF using carbonyl index (CI), and 3) to determine the amount of MPs that formed aggregates with PSFs and characterize them.

Section snippets

Study area and field research

The study area was set along the Tuul River running through the city center of UB, the capital city of Mongolia (Fig. 1). Although the upstream of the river supplies the primary drinking water for UB citizens, sewage has been drained into the Tuul River around S3, just west of the city center. Excessive wastewater released from the wastewater treatment plant (WWTP) located at S5 (Fig. 1) has been directly released into the Tuul River for approximately last 50 years. During the past 50 years,

Surface oxidation of PSFs

The most common plastic debris in the Tuul River shore was PSF because of the consumption as a heat insulator in this colder region. The PSFs were commonly found as foams with brownish color (Fig. 2), mainly covered with organic substances, oxides, and/or fine minerals during their transport by river. On the other hand, PSFs were linearly distributed along the river, probably due to the effect of seasonal or annual water level changes (Fig. S1, Supplementary materials). The highest distribution

Conclusion

In this study, the distribution and characterization of PSFs on the river shore were evaluated, and the MPs adhered onto the surface of PSFs were also examined. The behavior of PSFs and the distribution along the river shore should be affected by both environmental and anthropogenic factors. Seasonal changes in water level could leave floating plastic debris on the shore, which could again float on the water surface followed by their downward transportation. The potential of transporting the

CRediT authorship contribution statement

Batdulam Battulga: Investigation, Formal analysis, Validation, Visualization, Resources, Data curation, Writing - review & editing. Masayuki Kawahigashi: Supervision, Project administration, Funding acquisition, Conceptualization, Writing - review & editing. Bolormaa Oyuntsetseg: Data curation, Resources.

Acknowledgement

This study was supported by the Sumitomo Foundation no. 173495, Japan and partly supported by National University of Mongolia. The authors would like to thank many students from National University of Mongolia for their assistance in the sampling of plastics. We acknowledge the Mongolian National Waste Recycling Association for their supply for information about the Mongolian waste management.

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    This paper has been recommended for acceptance by Eddy Y. Zeng.

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