Seasonal assessment and characterization of microplastics in two urban (Balu) and peri-urban (Shitalakshya) rivers of Bangladesh

Abstract

This study presents a comprehensive assessment of microplastic (MP) contamination in the Shitalakshya and Balu Rivers, analyzing abundance, characteristics, and seasonal variations in both water and sediment compartments. Sediment samples showed significantly higher concentrations than water samples, with the Balu River exhibiting greater pollution levels (dry season: 5673.33 ± 762.17 MPs per kg; wet season: 3438.33 ± 328.83 MPs per kg) compared to the Shitalakshya River (dry: 3894.1 ± 388.17 MPs per kg; wet: 2671.6 ± 287.33 MPs per kg). Water samples exhibited seasonal fluctuations, with the Shitalakshya showing increased MP abundance during wet seasons (4.65 ± 0.56 MPs per L) while the Balu demonstrated opposite trends. Size distribution analysis revealed dominance of <0.1 mm particles, while morphological characterization identified fragments (53.78–95.36% abundance) and transparent particles as most prevalent. Fourier-transform infrared spectroscopy identified polyethylene as the dominant polymer (56–82%), followed by polypropylene and polyvinyl chloride. Scanning electron microscopy revealed advanced weathering in sediment-derived MPs, featuring cracks and surface erosion indicative of prolonged environmental exposure. Pollution Load Index values (1.34–3.19) classified both rivers as low risk (Category I), though notably lower than regional urban waterways. These findings establish critical baseline data on MP pollution in these economically and ecologically significant rivers, highlighting the urgent need for improved waste management strategies and regulatory frameworks to address plastic pollution in the freshwater ecosystems of Bangladesh.