Page - 38 - in Freshwater Microplastics - Emerging Environmental Contaminants?

standard ecotoxicity tests with particles, rather than soluble chemicals, are chal- lengedbythedynamicnatureofparticlessuspendedinaqueousmedia[90].Particle properties and behaviour may change as a function of time or as a result of interactions with test organisms and emitted biomolecules (e.g. exudates) [7]. It has therefore been recognised as essential in the work with engineered nano- materials to conduct a particle and exposure characterisation before and during a laboratorytest [26,92].This includesananalysisof thesize,shape,surfaceareaand surface chemistry of the tested particle, as well as aggregation/agglomeration, sedimentation and dissolution behaviour in the test system, thereby providing information on exposure in both qualitative and quantitative terms. Furthermore, appropriatewaysofdispersing theparticles inaquaticmediahavebeenhighlighted as an important area of future testmethoddevelopment [93].The rationale behind thorough characterisation and carefully considered sample preparation methods relates to data interpretation and avoidance of the introduction of test artefacts. Such activities are currently rarely undertaken in theworkwith nano- andmicro- plastics, but should be included in order to gain an insight into the behaviour of theparticles inexposuremediaand the resulting influenceon their interactionwith test organisms. Another aspect that needs tobe taken into account is the leachingofmolecules fromparticles.Forengineerednanomaterials,work isongoingwithin theOECDto develop test guidelines for investigating the dissolution ofmetal ions frommetal- containing nanomaterials [94]. In the case of plastic particles, the leaching of chemicals from the polymermatrix (e.g. additives ormonomers) and the release ofadheredco-pollutantscan influence the test results.Appropriate testmethodsare thereforeneededtoinvestigate theactual releaseofplasticadditivesfromnano-and microplastics under relevant conditions (media, temperature, pH, etc.), and a control for the effects of chemicals and released additives or adhered pollutants needs to be included as a reference. Transformation processes, such as oxidation/reduction, interactionwithmacro- molecules, lightexposureandbiological transformation,cansignificantly influence the integrity, behaviour and persistence of nanomaterials in aquatic media [95– 97]. Depending on the specific conditions, dissolution and degradation can be enhanced or reduced. Enhanced dissolution may result in increased toxicity of, forexample,metal andmetaloxidenanomaterials.At the sametime itmaycausea gradual decrease in particle size [97]. For nano- and microplastics, aging/ weathering processes should also be accounted for as they may change particle properties(e.g. surfacechemistry,polarityanddensity)andenhancefragmentation. It should be emphasised that a complete degradation of plastic particles under realistic environmental conditions has not yet been demonstrated [6, 98, 99]. While aging is potentially important for nanomaterials, and for nano- and microplastics, in the environment, the relevant aging processes and kinetics may differ. Based on current knowledge, nano- andmicroplastics may have a higher core persistence and lower release of soluble compounds than certain engineered nanomaterials (especiallymetal andmetal oxide nanomaterials such as ZnO and Ag).However, this isclearlyanareaoffutureresearch–forbothnanomaterialsand 38 S.Rist andN.B.Hartmann