Seite - 10 - in Freshwater Microplastics - Emerging Environmental Contaminants?

polymermatrixandwill bedependenton theporediameterofaparticularpolymer structure and themolecular size of the chemical.Here, pollutantswith lowermol- ecularweightswillmore easilymove through apolymermatrixwith larger pores. Adsorption kineticswill depend on polymer type, polymer characteristics such as density and crystallinity, the surrounding environment, and the pollutants pre- sent.For instance, thesorptionanddiffusionofhydrophobiccontaminantsaremost likely to takeplace in theamorphousareaofaplasticmaterial,because thecrystal- line region consists ofmore ordered and tightly structured polymer chains. Poly- mers that have structures with short and repeating units, a high symmetry, and strong interchain hydrogen bondingwill have a lower sorption potential. A good example is low-density polyethylene (LDPE) and high-density polyethylene (HDPE; Table 3). LDPE contains substantial concentrations of branches that prevent the polymer chains frombeen easily stacked side by side. This results in a lowcrystallinityandadensityof0.90–0.94gcm 3[83].Whereas,HDPEconsists primarilyof linear unbranchedmolecules and is chemically theclosest in structure to pure polyethylene. The linearity HDPE has a high degree of crystallinity and higher densityof 0.94–0.97gcm 3 [83].LDPE isoftenused for passive sampling devices to determine dissolved polycyclic aromatic hydrocarbons (PAH), poly- chlorinatedbiphenyls (PCB),andotherhydrophobicorganiccompounds inaquatic environments [84–88]. Batch sorption experiments were also used to determine PAH sorption to LDPE and HDPE pellets, and LDPE was identified to exhibit higher diffusion coefficients than HDPE meaning shorter equilibrium times for low-density polymers [89]. This indicates that PE is of interest from an envi- ronmental viewpoint because of its high sorption capacity. In addition, particle size will influence the sorption parameters because the higher surface to volume ratioof smaller particleswill shortendiffusion times. Isolatingandquantifying the sorption mechanisms for all polymer types in use today will be challenging, because sorption behaviour may differ within polymer classification depending on the type and quantity of additive compounds the polymer is compoundedwith andtheeffects that thismayhaveonpolymercrystallinityanddensity.These issues are discussed in further detail inScherer et al. [26] andRist andHartmann [58] in relation toMPandnanoplastics, respectively. An interesting question is towhat extent does irreversible sorption play a role? Some evidence in the pesticides literature suggests that a proportion of pesticides bind irreversibly soils [90, 91]. The study of sorption equilibrium isotherms is an important step in investigating the sorption processes that exist between different polymer typesandco-occurringhydrophobiccontaminants.Thiswillmakeitpossi- ble to identify the sorption and diffusion relationships between case study co-occurringcontaminants andMPs.Another interestingquestion is towhat extent sorbed chemicals become bioavailable in the water column due to the continued breakdownanddegradationof theMPs,ordue tochanges inenvironmental condi- tions, such as changes in pH, temperature, or systemchemistry. 10 S.Lambert andM.Wagner