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4 Particles as aVector forCo-pollutants One of the possible environmental processes, often discussed for both engineered nanomaterials andmicroplastics, is their ability to act as vectors for other pollut- ants. Through their use in, for example, consumer products and medical and industrial applications, engineered nanomaterials and primary microplastics will comeintocontactwithotherchemicalsubstances,suchaspreservatives,surfactants and active ingredients in pharmaceutical drugs. Finally, throughdifferent disposal routes, the particles will come into contact with environmental contaminants present in, for example, waste water streams and landfill leachate. As a conse- quence, intentional and unintentionalmixing of the particleswith other chemical compounds takesplacebefore, duringandafter their intendeduse.By thisprocess, an otherwise inert and non-toxic particle potentially becomes a carrier of toxic compounds. At the same time hydrophobic pollutantswith a lowwater solubility becomemore mobile when sorbed to plastic particles, whichmay increase their transport andconsequently impact their distributionandbioavailability . It has beenshownthatengineerednanomaterialscansorbandtransportorganicpollutants in the aquatic environment [35–37]. Similarly, nano- andmicroplastics have the potential toact asvectors forhydrophobicorganicchemicals, as recently reviewed byRochman . With an increased surface area-to-volume ratio, smaller particleswill generally havea largercapacity for adsorptionofchemical substances (onan ‘adsorptionper particle mass’ basis). At the same time, their small sizemay facilitate uptake by organisms and even potential translocation into different parts and organs. This vector function is governed by the properties of the pollutant and the particle . Important particle properties include chemical composition, porosity, size and surface properties (coating, charge).Weathering processes can both increase and decrease sorption . The formation of cracks and increased surface rough- ness leads to an increased surface area and, therefore, a potentially increased sorption capacity. Counteracting this, weathering may also change crystallinity, increasedensity andhardness and change surface charge. For instance, changes in surface charge as a result of weathering can increase the sorption of some sub- stances anddecrease the affinity for others . Plastic towaterpartitioningcoefficients (logKpw) forvariousorganicchemicals (logKow from0.90 to8.76)havebeencollected forpolydimethylsiloxane (PDMS), lowdensityPE (LDPE), highdensityPE (HDPE), ultra-highmolecularweight PE (UHMWPE), PP, PS and PVC . Regression analysis showed generally good correlationsbetween logKowand logKpwand linear proportionality forLDPEand HDPE. This analysis suggests that the partitioning of chemicals into plastics is driven by hydrophobic interactions – similar to the partitioning of chemicals into animal lipids .At thesametime,pollutantsmayadhere to theparticle surfaces. For example, it has been found that nanoplastics have a capability to adsorb hydrophobicpollutants,aprocesswhichcanpotentiallybeexploited in theremoval of chemicals from contaminated soil and water . Hence, for nano- and 32 S.Rist andN.B.Hartmann
Freshwater Microplastics Emerging Environmental Contaminants?