Seasonal dynamics of micro- and nanoplastics and associated pollutants in Aarin River: Insights into abundance and digestion impact

Despite decades of rapid plastic production in Nigeria, lack of sufficient data limits knowledge of pollution contribution from micro- and nanoplastics (MNPs). This study investigates the abundance, shapes and colour of MNPs alongside associated pollutants such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals (HMs) in Aarin River, Oyo town, Southwestern Nigeria, over two seasons (rainy and dry). The MNPs were of four size categories - 125, 75, 53 and 0.45 μm. The MNP level was notably higher in the rainy season (130.5 ± 3.5 particles/L (pts/L) in water and 30,384.5 ± 55.0 particles/kg (dry weight) in sediment) compared to the dry season (113.0 ± 5.0 pts/L in water and 2763.5 ± 28.5 pts/kg in sediment), with 53 and 75 μm-sized MNPs predominant in both seasons. Most of the MNPs were fragment-shaped and transparent white in colour. X-ray diffraction, scanning electron microscopy, and Fourier Transform Infrared spectroscopy revealed polyethylene, polypropylene, polyethylene terephthalate, and polysulfone to be prominent MNP constituents. The associated PAHs ranged from 43.40 to 615.77 μg/L and 21.90–599.84 μg/kg (dw) in the predigested water and sediment samples, respectively, with naphthalene having the highest concentration. Iron (Fe), lead (Pb), and chromium (Cr) account for most HMs in both water and sediment. Following organic and inorganic matter digestion, HMs and PAHs were reduced by 99 % and 83 %, respectively. The multivariate analysis of variance (MANOVA) and principal component analysis (PCA) indicated significant effects of seasonal variations and sample type on MNPs and pollutant concentrations. The results emphasize the critical role MNPs play in transporting pollutants. • Seasonal variations and sample type affected the abundance, sizes, shapes, and colors of micro- and nanoplastics (MNPs). • Fragment-shaped and transparent white-coloured MNPs were the most prevalent in water and sediment. • Nanoplastics were dominant in the dry season, while larger MNPs were more prevalent in the rainy season. • Adsorption of associated pollutants (PAHs and HMs) by MNPs contributed to Aarin River pollution. • MANOVA and PCA revealed seasonal and sample type influences on MNP levels and associated pollutants.