Qualification and Characterization of microplastics in selected water bodies in and around Dhaka city

Abstract

Today a world without plastics is unimaginable. But their non-biodegradability poses serious threat to the ecosystem, especially to the aquatic environment. Microplastics particles include either manufactured plastics of microscopic size (primary sources) or fragments or fibers of plastics resulting from the breakdown of larger plastic products (secondary sources). Degradation processes of plastics are extremely slow and thus microplastics tend to persist in the aquatic environment for very long periods of time. Exposure of freshwater organisms to microplastics has been found to cause mortality, neurotoxicity, oxidative stress and damage, decrease of individual and population fitness, and other adverse effects. Though freshwater environment is closely associated with the origin microplastics and their transfer to the seas and oceans, limited studies have been conducted on freshwater ecosystem compared to the marine environment. There is virtually no study/data on microplastics for any waterbody in Bangladesh. This study was aimed at identification, quantification and characterization of microplastics in selected water bodies in and around Dhaka City.

In this study, samples were collected from Dhanmondi and Ramna Lakes, Hatirjheel, Buriganga and Turag rivers in both Winter and Summer for identification, quantification and characterization of microplastics. Microplastics were identified by wet sieving where 4.75-mm and 0.3-mm sieves were used to isolate the solid material of appropriate size, then wet peroxide oxidation was carried out using hydrogen peroxide (H2O2) to digest labile organic matter while the plastic debris remained unchanged. Then density separation was done using sodium chloride (NaCl) solution to isolate the plastic debris through flotation. The floating solids were then separated from the denser undigested mineral components using a 0.3 mm filter, air dried and weighed. Finally, microplastics were detected using a magnifying glass. The quantity of microplastics was expressed as percentage mass of total solids, and other units (mass per unit area of water surface, and item/no. per unit area of water surface). The microplastics isolated from the samples were characterized through microscopic inspection, fractionation (through sieve analysis) and morphological analysis (size, shape, colour and texture).

The quantity of microplastics from the sampling locations in the five water bodies varied from 0.44% (in Dhanmondi Lake) to 9.34% (in Turag River) of total solids in Winter (dry) season; and 1.08% (in Hatirjheel) to 22.6% (in Turag River) in Summer (wet). In both Summer and Winter, the quantities of microplastics in the inland water bodies (Dhanmondi Lake and Ramna Lake) have been found to be relatively low, compared to those found in the two peripheral rivers – Buriganga and Turag. Size fractionation suggests that larger particles (in the size range of 1.18 mm to 4.75 mm) accounts for major fraction in term of weight of microplastics, while smaller size fractions account for majority of items of microplastics. Except for Hatirjheel, the quantity of microplastics (as % of total solids) at all the sampling locations increased in Summer compared to Winter. Morphological analysis suggests most of the microplastics identified in this study are irregular in shape with rough surface; this probably suggests that these have been introduced to the environment a long time ago. However, some microplastics with larger size and sharp/irregular edges were also identified, particularly in Dhanmondi Lake and Buriganga River, suggesting that these have probably been introduced to the respective water bodies relatively recently. This study gives an idea about the degree of microplastics pollution in different water bodies of Dhaka city in two different seasons.