Microplastic abundance and characteristics in seawater and inter-tidal sediments of the Tokin Bay Coast (North Vietnam)

<title>Abstract</title> Microplastics (MPs, plastic items from 1 µm to 5 mm in size) are present in all the environmental compartments. The evaluation of their concentration, fate, and spatial distribution is still a challenge for the scientific community. This concern is just debuting in developing countries, <italic>i.e.</italic> Asia, South America and Africa. This study deals with the MP contamination in the abiotic marine compartments of the Northern of Vietnam: seawater and intertidal sediments. Four sites located in the intertidal zone or near the coastline in the Tonkin bay, Vietnam were studied. A total of 16 samples (8 for each compartment) were collected in July 2020 (rainy season) and January 2021 (dry season). Plastic-like items were found at levels ranging from 3 to 303 items/m³ in seawater and from 63 to 955 items/kg dry weight in sediments. Most of these were fibers less than 300 µm long. Higher levels in seawater were found in the rainy season compared to the drier one. As the river flow was estimated six times higher during the rainy season than during the dry season, these results suggest the river discharge as a potential source of contamination for coastal zone. The temporal variability was lower for the sediments than for the seawater, demonstrating the long-term integration of the plastic-like items in this compartment. While the nature of fibers wasn’t investigated, the plastic-like fragments were analyzed on µFTIR imaging spectrometer. The determination of polymer composition indicated that most of the fragments were made of polypropylene (PP, 82%), followed by polyethylene (PE, 9%) and polystyrene (PS, 9%). The fragment size was similar in two studied compartments but it was depending on polymer types, since PS fragments (140 ± 17 µm) were smaller than those made of PE (622 ± 123 µm) and PP (869 ± 905 µm).