Characteristics of microplastics in sediment of the Vaal River, South Africa: implications on bioavailability and toxicity

The aim of this study was to examine the physical characteristics and chemical composition of microplastics in sediments of the Vaal River, South Africa. Microplastics were detected in all samples, with abundance ranging from 29.12 to 1095.89 particles/kg dw. The physical identification of microplastics revealed dominance of small-sized particles of less than 0.5 mm, which accounted for 31.75% of the total microplastics detected in all samples. Fragments and fibres were significantly abundant compared to pellets, representing 63% and 35%, respectively. Microplastics were observed in different colours, among which blue, white and green were the most dominant. Raman analyses of microplastics showed the presence of high density polyethylene, low density polyethylene, polyurethane foam, polypropylene, polyethylene co-vinyl acetate, and poly(ethylene-co-1-hexene). Additionally, two pigments (vine black and smalt), one dye (saffron), three minerals (orthoclase, carbon, and microcline), and one additive (cis-13-docosanol) were also identified. The dominance of fragments and fibres, with the clear signs of fragmentation implied that microplastics in the Vaal River are mostly from secondary sources. The study reported the first data on microplastic pollution and characteristics in sediments of the Vaal River, thus, providing a benchmark and reference platform for relevant formulation and decision-making regarding this essential water source.