Seite - 61 - in Freshwater Microplastics - Emerging Environmental Contaminants?

Under normal environmental conditions in aquatic systems, the temperature is not high enough to start chemical changes of synthetic polymers; thus, thermal degradation is not significant for freshwaters [58, 59]. Thedegradationofsyntheticpolymers intheenvironmentonamolecularbasis is usually initiated by photooxidation (with UV radiation) or by hydrolysis and is eventually followed by chemical oxidation [60]. The predominant mechanisms stronglydependonthetypeofpolymer,astherearenumerousdifferentcompositions of synthetic polymers produced (i.e., polyolefins, polyesters, polyamides).After the initial reactions, themolecularweight of the polymer is decreased, and the reacted groupsbecomeavailable formicrobial degradation. Photooxidation is usually a fast process,but thedegradationratealsodependsontheextentofadditivesinaparticular polymer that could prevent oxidation processes (i.e., antioxidants). Moreover, the photodegradationofplasticsfloating in theaquatic environment is slowercompared to degradation in terrestrial exposure [61]. Experiments on the disintegration of PE and PS showed faster degradation on the water surface compared to plastics that partially or completely submerged, likely related to the decreasing intensity of light and thus to the lower rateofphotooxidation [62].For this reason,manyplastics can stay in theaquatic environment for decadesorhundredsofyears. Biodegradation of synthetic polymers can occur in two different environments (aerobicandanaerobic).TheextentofthedegradationofpolymersintoCO2,H2O,N2, H2,CH4,salts,minerals,andbiomass(mineralization)canbefullorpartial [63].Par- tialorprimarydegradationof thepolymerchain leads to stableor temporarilystable transformationproducts.Biodegradation iscoupled to threeessentialcriteria: 1. Microorganismsmustbepresent that candepolymerize the target substanceand mineralize the monomeric compounds with enzymes of an appropriate meta- bolic pathway. 2. The environmental parameters, such as temperature, pH,moisture, and salinity must provide conditions that are necessary for biodegradation. Fig. 2 Degradationpathwaysof synthetic polymers in theaquatic environmentwithdegradation processes involved and intermediate steps until completemineralization Analysis,Occurrence, andDegradation ofMicroplastics in theAqueous. . . 61