Seite - 138 - in Freshwater Microplastics - Emerging Environmental Contaminants?

4.3 MultimediaModeling Multimediamodels for chemicals are built by setting up amass balance equation for each compartment that calculates the fluxes of transport via all exchange processes amongcompartments that are considered relevant.Thefluxes are calcu- lated based on first-order kinetic process rate parameters and concentration or fugacity gradients. Themodel equations are commonly solved by simple matrix algebra assuming steady state, but they also canbe temporally resolved.Common multimedia models for nanosized particles are MendNano [106] and SimpleBox4Nano(SB4N)[72,73].Thesemodelscalculate steady-stateconcentra- tions in the compartments atmosphere, surface water, soil, and sediment. In this reviewwediscussSB4Ninmoredetail,asafirstplastic implementationhasalready beenmadefor thismodel (Fig.3).SB4Nmodels thepartitioningbetweendissolved andparticulate formsof thechemicalasnonequilibriumcolloidalbehavior, instead of equilibriumspeciation.Within each compartment, particles can occur in differ- ent physical chemical forms (species): (a) freely dispersed, (b) heteroaggregated with natural colloidal particles, smaller than 450 nm, or (c) attached to natural particles larger than 450 nm.All these particle forms are subject to gravitational forces in aqueousmedia. Because SB4N is a spreadsheetmodel, it can easily be implemented for plastic debris of all sizes, as long as the parameter values are known.Oneof theadvantages is that themodelstemsfromSimpleBox,which isan established model already used in the risk assessment of chemicals [108]. A limitation is that themodel only calculates averagebackground concentrations. Fig. 3 Multimedia distribution of plastic debris of size 100 nm to 1mm, between atmosphere, soil,water, andsedimentona regional scale, for the riverRhinecatchment, simulatedwithSB4N [72,73].Concentrationsaregivenonthe logscale.Predictedenvironmentalconcentrations (PEC) assume a yearly emission in the catchment of 20 kt (based on data provided in [107]) in total, a (fouled) plastic density of 1,100 kg/m3, negligible degradation and fragmentation due to short particle residence time in the system, and an attachment efficiency for heteroaggregation of 0.01 [35, 36] 138 M.Kooi et al.