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weather conditions were not accounted for given the short simulation times
required to reach steady state in thewater column.
The scenario studies showed that the attachment efficiency to suspended solids
or other plastic particles, biofilm formation on the plastic particles, and polymer
typeof theplasticparticleshadonlyasmalleffectonthemodeledfateandretention
[36]. Particle size, however, had amuch stronger effect. Both the occurrence of
accumulation“hot spots” in river sediment andoverall retention in the40kmriver
stretch were found to be highly affected by particle size. The larger micro- and
millimeter-sizedplasticparticles showedhighup tocomplete retention in the river
due to direct Stokes settling [36]. Nanoplastic appeared to be retained and
transported to an equal extent, due to the predicted fast heteroaggregation with
natural solids. These solids have a higher density than the plastic and the water,
causing subsequent sedimentation of these aggregates that captured the plastic
particles.Retention, however,was lowest for the intermediate size class of plastic
particles around 5 (1–10) μm due to the trade-off between these “direct” and
“indirect heteroaggregate” Stokes settling mechanisms. The authors emphasized
the importance of this size selection mechanism in rivers. The model was also
applied to particles with a density equal to water, which resulted in no particle
settling.This scenariowas taken as a proxy for buoyant particles.
Freshwater organisms might be exposed relatively more to such intermediate
size classes, whereas they would be depleted in the mixture of particles that is
exported to sea under discontinuous input regimes [36]. The fact that the model
accounted for all known processes that are relevant and that it was in agreement
with data for another particle type (nano-CeO2 submicron particles [96]) contri-
butes to the credibility of the results. Themodel however was set up for (near-)
spherical particles. This means that it is already applicable for spherules, micro-
beads, or secondary plastics (e.g., car tire dust) that can be assumed to approach
sphericity, but itmaynotyet simulateparticleswithdivergingshapes likefibersor
thin films with the same level of accuracy. Furthermore, parameters for hetero-
aggregation are still poorly known,which also calls for further refinement.
Modeling the Transport of Microplastic Debris in the Thames River Catchment
(UK) Nizzetto et al. presented a spatiotemporally explicitmodel thatwas applied
to theThamesRiver catchment [37]. The study is framed as purely theoretical as
empirical data on microplastic emissions and concentrations were not available.
Themodel isbasedonanexistinghydrobiogeochemicalmultimediamodel, INCA-
contaminants [93],witha sediment transportmodule [95], a rainfall-runoffmodule
[94], and thepossibility toadddirect effluent inputs from, for instance,WWTPs. It
isa lumpedmodelas itassumeshomogeneousrainfalland temperaturedistributions.
Themodelaccountedforsurface runoffandeffluent inputsandreentrance to the
system by resuspension. Whether particles are transported by surface runoff
dependedonthemicroplasticpoolavailableformobilization, the transportcapacity
of the overland flow both formicroplastics and sediment, and the detachment of
plastics through splash erosion and flow erosion. In the stream, the particles are
140 M.Kooi et al.
Freshwater Microplastics
Emerging Environmental Contaminants?
- Title
- Freshwater Microplastics
- Subtitle
- Emerging Environmental Contaminants?
- Authors
- Martin Wagner
- Scott Lambert
- Publisher
- Springer Open
- Date
- 2018
- Language
- English
- License
- CC BY 4.0
- ISBN
- 978-3-319-61615-5
- Size
- 15.5 x 24.1 cm
- Pages
- 316
- Categories
- Naturwissenschaften Chemie