Seite - 256 - in Freshwater Microplastics - Emerging Environmental Contaminants?

Usually, regulation of pollutants in freshwater systems refers to dissolved chemicals, which are different to particulate matter with regard to their environ- mental fate (e.g., homogeneousversus inhomogeneousdistribution).Therefore,we have to critically evaluate the transferability of regulatory options for dissolved chemicals to the issue ofMPs. This represents a similar challenge aswe know it from engineered nanomaterials. The development of regulation strategies forMP should considermoreoptions than the simple adaptationof the existing regulation strategies for dissolved chemicals or suspended matter. Possibly, entirely new regulation strategies for MP in freshwater need to be developed. To start with, this requires a commonly accepteddefinitionof “microplastics.” 3.2 APrecondition forRegulation:TheDefinition ofMicroplastics The term“microplastics” turned into akindofbuzzword inpublic communication andmedia, and it is understood as one specific type of pollutant. Hence, expecta- tions rose to find solutions and regulations, which could consider all materials summarized by this single term. In contrast, the term “microplastics” refers to a largegroupofpolymerswithvariouschemical andphysicalproperties, originating from different sources and entering the environment via different pathways (see [3, 59]). Accordingly, these differences amongMP particles apply to their envi- ronmental fate and persistence and, consequently, also to their bioavailability and potential impacts to organisms. Verschoor [61] identifiedfivecommonly applied criteria todefineMP: (1) syn- theticmaterialswithhighpolymercontent, (2)solidparticles, (3)<5mm,(4) insol- uble in water, and (5) not degradable. However, several points are still under discussion;e.g., someexpertsarestilldebatingif tireabrasionshouldbeconsidered as “microplastics” as the monitoring guidance documents for marine litter [62] categorize rubberoriginating fromtires separately fromplastics (discussed inmore detail in [61]).Thisdecisionwould significantly influence themeasurement results of total environmentalMPconcentration. Thesameapplies to thedefinitionofa lower limit forparticle size,which is still underdiscussion.While it is commonlyaccepted todefineall plastic items<5mm asMPs [e.g., 63], some authors categorize MPs into size-based subgroups. The MSFD Technical Subgroup on Marine Litter [62], for instance, differentiates between largerMPs (1–5mm)and smallerMPs (20μmto1mm).Various studies set particular methodical limits as a lower size limit – e.g., mesh size of the sampling net or analytical detection limits. As “nano” refers to particles of 1–100nm[64], the size limit forMPs should consequently startwith a lower size limitof100nm.Miklosetal. [65]base their sizedefinitiononthis ideaandsuggest a size rangeon“microscale” from100nmto100μm.Dependingon the thresholds defined for these criteria, completely different field concentrations would be 256 N.Brennholt et al.