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Micro- and nanoplastics

Definition and properties

Although there is no single definition, conventionally, plastic waste has been divided into four classes based on its size (macroplastics, mesoplastics, microplastics and nanoplastics) to which additional criteria such as shape and structure, colour and origin are added (see figure below, Hartmann et al., 2019). In addition to size, these particles are subdivided on a compositional basis (chemical composition of the polymer). The most common plastic polymers are polyethylene terephthalate (PET), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene (PE). These materials are characterised by high heterogeneity of shape and colour and long persistence in the environment. The classification by origin comprises two classes: primary and secondary particles. Primary MPs and NPs are industrially manufactured, such as abrasives (acrylic acid or polyester beads), microspheres used in cosmetic products (e.g. scrubs, toothpaste, sunscreens, etc.) and decorative materials, including sequins and glitter. Secondary ones originate from the degradation of larger plastic objects when exposed to various physical, chemical and biological processes in the natural environment, which fragment into smaller, irregular pieces such as synthetic textile fibres and tyre wear particles. In fresh waters, typically MP particles originate from the latter category (Parker et al. 2022).

Examples of categorisation of plastic particles reported in scientific literature and institutional reports. Figures reprocessed from Hartmann et al. 2017 (DOI: 10.1021/acs.est.8b05297)

The characteristics of various plastic particles, such as size, density or shape, have a direct effect on the processes of particle sedimentation and resuspension and, therefore, influence the abundance of microplastics in the water column and sediments. Furthermore, the hydrophobic surface of plastics can absorb related contaminants from the surrounding environment such as persistent organic pollutants (POPs), as well as traces of metals, or leach certain chemicals used in their production to give them special properties (durability, strength, flexibility, low weight, ignition inhibition). This makes MPs a source and vector of other toxic contaminants (Bakir et al. 2014) or various organisms such as pathogens and alien species that colonise the surface of MPs, facilitating their entry into the aquatic ecosystem or trophic networks. Other risks associated with MPs are bioavailability to aquatic organisms, bioaccumulation and trophic transfer through food chains, with consequences for ecological and human health.

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