Caracterização dos microplásticos ambientais presentes nos sedimentos inconsolidados das águas costeiras de 9 praias do litoral do Rio de Janeiro, Brasil

Abstract

Plastic is a synthetic organic polymer that emerged in the mid-19th century, becoming essential due to its versatility, lightweight, durability, and resistance. Global plastic production has significantly increased over the last decade. Due to mechanical, physical, chemical, and biological processes, plastics can be fragmented into increasingly smaller sizes, classified as: macroplastics (>25 mm), mesoplastics (5-25 mm), microplastics (<5 mm), and nanoplastics (<1 mm). This study aimed to evaluate and characterize environmental microplastics present in the sands of nine beaches and the waters along the coast of the State of Rio de Janeiro (RJ), providing information on quantity, size, color, morphotype, type of polymer, and adsorbed metals. The methodology consisted of conducting microplastic characterization tests in the sands of six beaches along the RJ coast and three in the insular region of Ilha Grande. The tests involved quantitative and qualitative characterization by size, shape, color, and composition of the polymer and adsorbed metals in the microplastics extracted from the collected samples. In the 2022 campaigns, a visual screening methodology for microplastics was used, resulting in the extraction of 1,517 items, including 732 microplastics or 8.4 microplastics/kg of dry sand in the summer and 785 microplastics or 8.9 microplastics/kg of dry sand in the winter. For the 2023 campaigns, the screening methodology involved flotation followed by filtration, yielding 5,961 items, including 3,187 microplastics or 39.0 microplastics/kg of dry sand during the rainy period and 2,774 microplastics or 32.2 microplastics/kg of dry sand during the dry period, proving more suitable for the characterization study by recovering a larger number of items. During the rainy period, the beaches presented the following number of items: Botafogo (973), Flamengo (370), Dois Rios (35), Urca (308), Ipanema (287), Santo Antônio (283), Pontal (269), Lopes Mendes (249), and Praia Vermelha (113). In the dry period, the highest incidence remained at Botafogo beach (552 items), followed by Urca (402), Lopes Mendes (350), Praia Vermelha (296), Dois Rios (257), Pontal (243), Ipanema (240), Flamengo (238), and Caxadaço (196). Based on the results, an evaluation of the presence of microplastics (size class, morphotype, and color) was made by periods (dry and rainy) for each beach. The most abundant morphotype was fiber (56.28%), followed by fragments (28.2%), foams (12%), pellets (2.7%), and films (0.87%). The most abundant size class was <1 mm, comprising 66.7% of the microplastics. The most prominent color was blue (32.3%), followed by black (25.5%). The nine beaches were grouped by geographical similarity (insular, sheltered, and exposed) to assess the presence of microplastics along the RJ coast, with the largest quantities found in the group of sheltered beaches. The evaluation of the composition of environmental microplastics using Fourier-transform infrared (FTIR) spectroscopy showed that the three most commonly found polymers were: PE, PP, and PS. The investigation of adsorbed metals in microplastics using X-ray fluorescence analytical techniques identified the following elements: S, Cl, K, Ca, Ti, Cr, Fe, Cu, Zn, and Pb, with polyethylene (PE) showing the greatest variety of adsorbed metals. The analysis of the interaction between polymers and metals and metalloids and their interface with the environment requires an understanding of various synergistic processes that will be studied in the future.