Page - 130 - in Freshwater Microplastics - Emerging Environmental Contaminants?

advective, dispersive, anddiffusivemass transfer.Advection refers to the longitu- dinal transport based on the averageflowvelocity.Dispersivemass transfer is the turbulent spreading of mass from highly concentrated areas to less concentrated areas [47].This results fromnonidealflowpatterns (i.e.,deviations fromplugflow) and is amacroscopic phenomenon.Dispersion ismultidirectional, as it covers the distribution of all flowdirections and velocities [48].Diffusion is the transport of particles from a high to a low concentration caused by random molecular (Brownian)motion,which is amicroscopic phenomenon.The combined transport of substances in rivers due to advection and dispersion is commonly described by the one-dimensional advection-dispersion equation [49, 50]. Besides being transportedinthewater, suspendedsolidscanberemovedfromthefluidbysettling, which canbemodeledwithStokes lawor amodificationof that law [51, 52]. Par- ticles and substances can reenter thewater columnby erosion/resuspension of the riverbed. Advection, dispersion, diffusion, settling, and resuspension depend on particleproperties suchassize,density, shape, fractaldimension,andporosity [53– 55]. During transport, the aforementioned particle properties can change due to aggregationorbiofouling,whichwill further influence their fate [56–58].Aggrega- tionisusuallymodeledusingavonSmoluchowskiparticle interactionmodelwhere the formation of aggregates is described kinetically as a function of the colliding particle concentrations, their sizes and densities, their collision frequencies, and attachment efficiencies [56, 59, 60]. Many studies assume spherical particles or aggregates, although it has been suggested that fractal dimensions should be includedinthesemodels[52,61].Foraggregates, thiswouldresult inmorerealistic collision radii and hydrodynamics, therewith providing better collision frequency andsedimentationestimates [61].Therelative importanceof thedifferent transport processes isdependenton theparticleproperties.Forexample,Brownianmotion is important fornanoparticle aggregation [36,56],whereas itwill benegligiblewhen studying the transport of larger particles [47]. Likewise, biofouling has a faster effect on the buoyancy of microscopic particles, which have a large surface-to- volume ratio, compared tomacroscopic particles [62]. Type of Aquatic System Several system properties affect the occurrence of the abovementioned processes. Transport of particles in rivers and lakes differs in manyaspects.Rivershaveadownstreamdischargedrivenbyanelevationgradient. Although natural rivers are turbulent, the time-averagedmotion of thewater is in the longitudinaldirection. Inrivers, theadvectionflowcomponent isusuallyhigher than thedispersioncomponent [47].Due to sedimentation andburial of suspended solidsandassociatedcontaminants, riversoftenactasasinkfor thesecontaminants. Compared to rivers, lakes have a very low, if any, flow velocity, enhancing sedimentation processes. Water residence times can be days to>103 years [63] compared todaysorweeks in rivers.Therefore, biological andchemical processes are usuallymore important for the fate of particles in lakes.Mixing processes in lakes, and therefore the importance of sedimentation versus resuspension, can be complex tomodelbecauseofvertical stratification, theeffectofwind, and the lake geometry [47]. Dams in rivers or lakes can increase thewater retention time and 130 M.Kooi et al.