Occurrence of microplastics derived from tyres in bottom sediments of Guanabara Bay, Brazil: a form of pollution that is neglected or difficult to detect?

Aim: Tyre wear particles (TWPs) have been indicated as one of the main sources of microplastics (MPs) in aquatic environments, yet they receive little attention in the published literature. They have never been reported in Guanabara Bay in spite of several published analyses of the abundance, distribution, type, chemical composition and color of MPs in this area. We aimed to develop a method to allow ready detection of these particles in sediments. Methods: Nine sediment samples were taken from locations over a wide geographic cover of the bay. They were collected from different depths using a Van Veen grab sampler. MPs were separated using peroxide degradation of organic matter followed by floatation and filtration. TWPs were identified using a binocular stereo microscope,Scanning Electron Microscopy (SEM) and Fourier transformed infrared spectroscopy. Results: The combined use of these MP detection methods allowed the visualization and identification of high numbers of TWPs in the bay. Exact quantification was impossible because of the large size range of the particles and the use of a sedimentation system already shown to be sub-optimal. Conclusion: High levels of TWPs were detected in Guanabara Bay sediments. Future studies to quantify these particles in coastal environments will necessitate the development of more robust methods, possibly using specific TWP markers. Various options are discussed.