Seite - 132 - in Freshwater Microplastics - Emerging Environmental Contaminants?

Theuniquenatureofplasticdebriscanbe illustratedbycomparisonwithproperties ofother typesofparticlespresent inwater systems.Plastic canbeconsidered tobe uniquewith respect to fate processes because: • Other particles can be similar sized but then have higher density (metal-based nanoparticles and colloids, suspended sediments, clays,minerals). • Other particles canhave similar density but are far less persistent (wood, algae, detritus, exopolymers, organicmatter flocs, or organic colloids). • Other particlesdonot exist in anmto>cmsize rangewith all otherproperties being similar to those of plastics. We argue that the combination of low density (often near that of water), persistence, wide size range, and variable shape is what makes plastic particles andthusfatemodelsimulationresultsdifferentfromthoseforotherparticles.At the same time, low-density nanomaterials (fullerenes, carbon nanotubes) or natural organic particles like cellulose can have a hydrodynamic behavior similar to that of some specificplastic particles. Processes Specifically Relevant for the Modeling of Plastic Debris Once in the aquatic environment, plastics will be transported downstream. Floating macroplastic can be assumed to be transported with the flow (Fig. 1), i.e., to estuaries, to sea, or to lake reservoirs, where reduced flow conditions, fouling, embrittlement,andfragmentationmaytriggersedimentationandfurtherdispersion. Larger itemswill also accumulate on riverbanks due to wind or reduced flow or dispersive flow patterns in river bends. Vegetation or trees near the shores may serve as a temporary sink for large plastic debris [82], which later on may be released again to the main stream. Non-buoyant plastic debris is subject to the advective, dispersive, and sedimentation processes as described in the previous section. A unique feature here is that a high proportion of the plasticwill have a density not that different from that of water, in contrast to natural suspended (mineral) solidparticlesof the samesize.Thevarietyofplastic sizesanddensities, however, still varies enormously, leading toawidevarietyof transport patterns for individual particles in themixture. Biofoulingofplasticshasbeenreportedfor freshwatersamples [83,84]andalso isawell-researchedphenomenoninmarinewaters [57,58,62,84].Plasticdebrisof all sizes anddensitieswill be fouled andcolonizedbymicrobes, formingbiofilms, whichcan lead to significant changes inparticlebuoyancy.For instance, increased settling as a result of biofouling has recently been shown for marine particles [57, 58, 62], and it is plausible that the same holds for plastics in the freshwater environment (Fig. 2). The recent detection of microplastics in rivers and lake sediment [15, 20, 85] confirms that particles with a density higher as well as lower thanwater can settle and be buried in the sediment. Recentmodel analysis showed that this also can be explained on a theoretical basis [35, 36]. Buoyant plastics will only settle when they are incorporated in aggregates with a density larger than the water density. This is an important phenomenon, which is 132 M.Kooi et al.