Seite - 60 - in Freshwater Microplastics - Emerging Environmental Contaminants?

sedimentswere studied in theEuropean lakes [49]. Lower concentrations of plastic particles inAsian freshwaters were detected in the surface water samples of Lake Hovsgol (Mongolia). Microplastics were quantified in the range of 997–- 4435particles km 2, but it should be taken into account that the catchment area of LakeHovsgol is lesspopulatedcomparedtotheabovementionedlakes[50].Forcase study discussions onmicroplastic occurrence inAfrican andAsian freshwaters, see Kahnet al. [51] andChenxi et al. [52] of this volume. 3 EnvironmentalDegradationofSyntheticPolymers Oneof the reasons for thegreat versatility ofmany synthetic polymers is their high resistance against environmental influences. However, this fact leads to extremely lowdegradationand longresidence times for syntheticpolymersonce theyenter the environment.Degradationofsyntheticpolymerscangenerallybeclassifiedasbiotic or abiotic, following different mechanisms, depending on a variety of physical, chemical, or biological factors. During the degradation process, polymers are converted into smaller molecular units (e.g., oligomers, monomers, or chemically modifiedversions)andpossiblyarecompletelymineralized[53].Themost important processes for thedegradationof synthetic polymers canbedivided into (Fig. 2): • Physical degradation (abrasive forces, heating/cooling, freezing/thawing, wet- ting/drying) • Photodegradation (usually byUV light) • Chemical degradation (oxidationor hydrolysis) • Biodegradationbyorganisms (bacteria, fungi, algae) Mechanical degradation is an important factor with regard to plastics in the aquatic environment. Inmost cases, aging of the polymer by environmental influ- ences, suchasphotodegradationor chemical degradationof additives, changes the polymerpropertiesand leads toembrittlementof thepolymer [54].Therecalcitrant material is then shredded into smaller particles by friction forces occurringduring themovement throughdifferentenvironmentalhabitats (also seeKooietal. [55]of this volume for a discussion onmicroplastics fate and transportation). This degra- dationgenerally leads tosmallerplasticparticles,whichcanresult inparticleswith sizes between 1 and 5,000 μm. Such particles are classified as microplastics. However, themechanical degradation does not stop if the particles arewithin the size rangeofmicroplastics.Thus, the formationofevensmallerparticles, so-called nanoplastics, isvery likely [56].Thesenanoplasticscouldhavedifferentproperties compared to the original macroplastics or microplastics (for a discussion on nanomaterials, see Rist and Hartmann [57] of this volume). In both cases, the mechanical degradation leads to a decrease in particle size and consequently to an increase in the surface area of the polymer particles, which results in faster degradationdue to higher reactivity. 60 S.Klein et al.