Forensic determination of adhesive vinyl microplastics in urban soils

Plastic production and consumption hubs are mainly concentrated in urban centers, causing the soil in these places to become sinks of plastic fragments. Adhesive vinyl polymers are widely used in various commercial sectors and, to the best of our knowledge, this is the first study to investigate the potential for this type of material to form microplastics in urban soils. This proof-of-concept work started by studying the soil around a sign made of adhesive vinyl that had been exposed to the weather for eight years and showed evident signs of degradation, like cracking and color fading. We separated the microplastics with a two-step density separation protocol and selected only the microplastics targeted by this research, finding up to 5,570 fragments produced from 1 cm² of adhesive vinyl film. In the soil below the sign, we registered 5.6 × 10⁴ fragments kg^-1 of dry soil on its topsoil layer (0-10 cm), 1.2 × 10⁴ fragments kg^-1 in the 10-20 cm layer, and 1 × 10⁴ fragments kg^-1 in the 20-30 layer. At a distance of 1 and 2 m from the sign, the highest concentration of fragments was also in the topsoil, respectively 9.3 × 10³ and 5.3 × 10² fragments kg^-1. We also observed that vertical and horizontal transport was not favored, causing the formation of hotspots near the source and that the area of the fragments did not influence vertical transport. Another important finding regarding the characterization technique is that degraded polyvinyl chloride is unlikely to be identified through FTIR without comparison to the source. Here, we presented a low-cost forensic assessment of the association between the presence of MPs and its source that can be used both for the development of public policies and for setting up quality controls for polluting companies.The results here presented reveal the need to rethink the use and types of materials used for visual identities and signage in urban environments.