Seite - 136 - in Freshwater Microplastics - Emerging Environmental Contaminants?

4.1 Emission-BasedMassFlowModeling Emission-basedmass flowormass balancemodels have been used for chemicals [97] and have recently been implemented for engineered nanoparticles as well [45, 98, 99]. The latter category ofmodels is of particular interest for this review, becausemass flowmodels for plastic particles can relatively easily be developed along the same lines.Basedonestimates of nanoparticle emissions fromproducts, environmentalfluxesarecalculated to themajorcompartments likeair, soil,water, sediment,andseveral technicalcompartments[45].Thecompartments typicallyare considered homogeneous and well mixed [45, 98]. Deposition and removal of particles within compartments are modeled as constant annual flows into a sub-compartment of each box considered. Similar mass flowmodel applications that calculate environmental concentration for plastic debris in allmedia (air, soil, water, and sediment) have not been published yet. However, the essence of the approachhasbeenused toestimateconcentrationsofmicroplastics fromcosmetics inWWTP effluents in the Netherlands [90] andmass emissions ofmicroplastics fromcosmetics fromEurope to theNorthSea [100].Thefirst study is discussed in detail below. MassFlowModelingofMicroplasticConcentrations inWWTPEffluents With the useofamassflowmodelingapproach,VanWezeletal. [90]estimatedtheemission ofmicroplastics fromconsumerproducts to the surfacewater viaWWTPeffluents (Table1).Basedon theknownuseofmicroplastics in cosmetics andpersonal care products, cleaning agents, and paints and coatings, emissionswere estimated. Per product category, data on the use of the product, the market penetration, and concentration ofmicroplastics in the productwere collected. Itwas estimated that during thewastewater treatment, between 40 and 96%of themicroplasticswould be retained by theWWTP. Themodel calculated the predicted concentration of microplastics in a WWTP effluent as the product of the concentration of microplastics in a product, the daily usage of that product, the fraction of microplastics removedduringthewastewater treatment,andthemarketpenetration of the products, divided by the volume of wastewater produced. The estimated effluent concentrationofmicroplastic ranged from0.2μgL 1 for the conservative estimate to 66μgL 1 for themaximumscenario. Measured concentrations ofmicroplastics inWWTPeffluents range from20 to 150 particles L 1, as reported after a Dutch monitoring campaign [90]. These particle numbers were converted to mass, based on the size range, the volume assuming cubic shapes, and an average density. To validate themodel, themodel outcomes were compared with the observations of the monitoring [90]. Three different particle number-to-mass conversion categories were used, classified at “little and light,” “intermediate,” and“bigandheavy”particles, thenames relating to the assumed particle size, volume, and density. Themodel coincided bestwith observationswhen“bigandheavy”particleswereassumed tobemeasured, that is, particleswith a relativelyhighdensity, large size, and largevolume.However, the 136 M.Kooi et al.