Abundance, Characteristics and Ecological Risks of Microplastics in River Sediments around Dhaka City

Abstract

Microplastics (MPs), the small particles of plastics with a size less than 5 mm have been identified as an emerging pollutant in recent decades. Microplastics pose a higher risk in the aquatic environment and also a potential threat to human health. The aquatic species ranging from invertebrates to fishes can easily ingest microplastics along with other contaminants considering MPs as food sources due to their diverse characteristics (size, shape, and color), which accumulate in digestive tracts of aquatic species. Finally, MPs enter into the human body through gastrointestinal ingestion of aquatic species as well as from water consumption and thus create human health risks depending on their toxicity level. Microplastics (MPs) pollution has become an escalating problem in Bangladesh also due to its rapid urbanization, economic growth, and excessive uses of plastics, however data of MPs pollution of fresh water resources is very limited in Bangladesh. This study investigated microplastics pollution in the riverbed sediments in the peripheral rivers of Dhaka city. In total, 28 sediment samples were collected from the selected stations of Buriganga, Turag, and Balu River. A total of about 1 kg of riverbed sediment, 5-10 m away from the shoreline was sampled using an Ekman grab sampler (15×15×15 cm) from top 10 cm of the riverbed at each sampling station. Density separation and wet-peroxidation methods were employed to extract microplastic particles. Attenuated total reflectance-Fourier transform infrared spectroscopy was used to identify the polymers. Scanning electron microscopy (SEM) analysis was performed to examine the surface characteristics of weathered MPs. MPs in the river sediment were found to vary with sampling locations and the abundance of MPs varies from 46 to 534 items per kilogram (kg) of dry sediment. The mass concentration of MPs varies from 13.56 mg/kg to 430.65 mg/kg with an overall average value of 106.52 ± 73.17 mg/kg. The results indicated a medium-level abundance of microplastics in the riverbed sediment in comparison to other studies in the freshwater sediments around the world. The observed MPs particles were shorted into three shapes: films, fragments, and fibers. Films (53.89%) were the most abundant shapes followed by fragments (37.57%), and fiber (8.54%).. The white (18.77%) MPs were major abundance followed by transparent (14.90%), yellow (14.37%), blue (14.37%), red (12.03%), green (11.27%), black (8.40%) and grey (5.87%). Larger quantities of the plastics are generally used in Bangladesh for shopping bags, package products and textile materials, which are white or transparent in color. MPs are categorized into small microplastics (<1 mm) and large microplastics (1-5 mm). The results of this x investigation found that on an average, the riverbed sediments contain large sized MPs (67%) much higher than small sized MPs (33%). The most abundant polymers were polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET). The pollution load index (PLI) values more than 1 were observed indicating that all sampling sites were polluted with microplastics. An assessment of ecological risks, using the abundance, polymer types, and toxicity of MPs in the sediment samples suggested a medium to very highlevel ecological risks of microplastics pollution of the rivers. The average ecological risk index (ERI) value suggested that both BR and TR have high ecological risk and BaR has medium ecological risk. In some sampling locations of both BR and TR, ERI value more than 1200 was observed, indicating very high ecological risk to those sampling locations. Higher abundance of MPs and presence of highly hazardous polymers such as polyurethane, acrylonitrile butadiene styrene, polyvinyl chloride, epoxy resin, and polyphenylene sulfide were caused the higher ecological risks. SEM images revealed that the PE, PP, and PET polymers with the carbonyl group had linear fractures, cracks, pits, grooves, granules, and flakes and along with some crystalline formation. However, the same types of particles without carbonyl group had experienced relatively stable surfaces but still contained rough and irregular textures. This textural analysis suggested that MPs particles in riverine sediment were weathering by various processes, producing smaller MPs, which are caused more potential ecological hazards in these river ecosystems. This study indicated that the river ecosystem of the peripheral rivers of Dhaka city is polluted by MPs from the anthropogenic sources both point and non-point in nature. MPs pollution of freshwater bodies is a new dimension of the widespread pollution because of increased use of plastic products, reckless and uncontrolled disposal of municipal solid wastes including plastic wastes, disposal of untreated industrial wastewater including plastic industries and excessive urbanization. Finally, this investigation provided a baseline information on microplastics pollution in the riverine freshwater ecosystem for more in-depth study on risk assessment and developing strategies for controlling microplastics pollution in the country.