Page - 9 - in Freshwater Microplastics - Emerging Environmental Contaminants?

structure and the use of structural features that are not easy to biodegrade. Here, crystallinity is an important polymer property because the crystalline region con- sists of more ordered and tightly structured polymer chains. Crystallinity affects physical properties such as density and permeability. This in turn affects their hydration and swelling behaviour, which affects accessibility of sorption sites for microorganisms. Stabilisers such as antioxidants and antimicrobial agents act to prolong the life of plastics, whereas biological ingredients act to decompose the plastic in shorter time frames. Overall, environmental degradation processes will involveMP fragmentation into increasingly smaller particles includingnanoplastics, chemical transformation of the plastic fragments, degradation of the plastic fragments into non-polymer organicmolecules, and the transformation/degradation of these non-polymermol- ecules intoother compounds [65].Theenvironmental degradationof plasticmate- rials is also further discussed inKlein et al. [20]. 5.3 InteractionswithOtherCompounds Thesorptionofhydrophobicpollutants toMPs isconsideredan important environ- mental process, because this will affect the mobility and bioavailability of these pollutants. It iswellknownthatMPsinmarineenvironmentsconcentratepersistent organic pollutants (POPs) such asDDT, PCBs, and dioxins [78–80]. In addition, Ashton et al. [81] also found concentrations ofmetals in composite plastic pellet samples retrieved from the high tide line along a stretch of coastline inSouthwest England.To investigatewhether themetalswere in factassociatedwithnonremov- able fine organic matter associated with the pellet samples, new polypropylene pellets were suspended in a harbour for 8 weeks and were found to accumulate metals from the surrounding seawater, from lowof0.25μgg 1 forZn toahighof 17.98μgg 1 forFe [81].So far, littledata is availableonfreshwater and terrestrial ecosystems, which will have a pollutant makeup very different to that found in marine environments. In the freshwater environmentMPs are likely to co-occur withotheremergingcontaminantssuchaspharmaceuticals,personalcareproducts, flame retardants, and other industrial chemicals, which enter the environment as parts of complex solid and liquidwaste streams. Sorptionprocesseswill occur throughphysical andchemical adsorptionaswell as pore-filling processes. Physical adsorption is the reversible sorption to surfaces of the polymer matrix and does not involve the formation of covalent bonds. Chemical adsorption involves chemical reactions between the polymer surface and thesorbate.This typeof reactiongeneratesnewchemicalbondsat thepolymer surface andmaydependonhowaged the polymer surface is. These processes can be influenced by changes in pH, temperature, and ionic strength of the localised environment [82]. Pore-filling occurs when hydrophobic pollutants enter the MicroplasticsAreContaminants ofEmergingConcern inFreshwater. . . 9