Seite - 133 - in Freshwater Microplastics - Emerging Environmental Contaminants?

mechanistically explainedbybiofoulingcausinganoverall increase indensity and attachmentefficiencywithotherparticles.Heteroaggregationwithnatural colloids, clays, and other high-density suspendedparticleswill lead to faster sedimentation of the plastic particles that are captured in the aggregate [35, 36] (Fig. 2). Another unique feature is the high persistence of plastic. Other particles with similar density and size, e.g., wood, algae, detritus, or other natural organicmatter solids, disappear through degradation and mineralization within rather short time scales.Plasticdebrishowever,onceburied in the sediment,willonlybemineralized onvery longtimescales, rendering themhighlyaccumulative,bioavailable, andalso subject to further transport. As long as plastic particles are close to the sediment surface, they can be resuspended if the flowvelocity is high enough to exceed the critical shearstress [86].However,afterprolongedsedimentation, theparticlescould become“buried.”Buriedplastic debriswouldnot resuspendanymore, unless turbu- lencewould increase sharplydue to stormeventsorflashfloods, for example. In the laboratory under accelerated weathering conditions, plastics have been shown to becomebrittle and fragment [28], and it is likely that this also occurs in freshwater systems.Thisprocesshowever isvery slow innature [87]andprobably much slower than the typical residence times of plastic in rivers. In lakes with a large retention time,weathering is potentially important though. Fragmentation is caused by photodegradation, thermo-oxidation, hydrolysis, physical abrasion, and/orbiodegradation [1,88].Mostof theseprocesses require either light, friction, or oxygen to act on the surfaces of theparticles,which implies that onceburied in anoxic sediment layers, plasticwill be preserved for at least decades [89]. Several model categories exist that canuse theabovemechanistic evidence to simulate the fate of plastic debris in rivers, some ofwhich already have been published in the literature [36, 37, 72]. 4 Models forFate andTransport ofMicroplastics inFreshwaterSystems In this section, four categories ofmodelswill be discussed: emission-basedmass balance modeling, global modeling, multimedia modeling, and spatiotemporally explicitmodeling.Themodelsdiffer in their aim,design, scale, levelofdetail, and state of validation (Table 1). We classified the models based on their major characteristics,butsomeoverlap in theseclassificationscanbefound.Forexample, a globalmodel can also be referred to as spatiotemporally explicit yet on amuch larger scale, and a small-scale spatiotemporally explicit model can cover plastic transport inwater and sediment, rendering it “multimedia.” Modeling theFate andTransport of PlasticDebris inFreshwaters:Reviewand. . . 133