Spatial and seasonal abundance and characteristics of microplastics along the Red River to the Gulf of Tonkin, Vietnam.

This study aimed to examine the occurrence of microplastics in surface water and sediment samples collected from Hanoi to the Ba Lat estuary along the Red River, the second-largest river in Vietnam (surface area: 156,451 km). 21 stations were sampled during the dry (March 2023) and rainy (September 2023) seasons. The analytical procedure involved: digestion with hydrogen peroxide, flotation with potassium carbonate, and overflow filtration. The filters were analyzed by microscopy (Nikon SMZ645) to describe shapes and colors and by μ-FTIR (PerkinElmer Spotlight 400) to determine polymer types and abundances. Results showed that microplastic quantities throughout the river ranged from 10 to 203 items.m in surface water and from 653 to 8069 and 990 to 21,610 items.kg dried weight (d.w.) in sediment during the rainy and dry seasons, respectively. MPs were classified into two main shape groups: fiber and fragment, with fibers being predominant, representing 82.0 % and 75.5 % of microplastics in water and sediment, respectively. The primary colors identified were white/transparent, black, and blue. Particles between 13 and 200 μm were the predominant size class, accounting for 64.1 % and 72.4 % of the microplastics in water and sediment, respectively. Polyethylene, polypropylene, and polyethylene terephthalate were the main polymers, accounting for 71.5 % and 72.2 % of the microplastics in water and sediment, as revealed by μ-FTIR analyses. Overall, in the Red River, the MP pollution load is moderate, but the type of particles detected represents a high to dangerous polymer risk, resulting in a very high potential ecological risk on the river.