Page - 55 - in Freshwater Microplastics - Emerging Environmental Contaminants?

1.2.1 SeparationofMicroplastics fromSedimentSamples In contrast to microplastics in water, which are easily filtered from the sample during the samplingprocess,microplastics in sediment samplesmust be separated in the first step of the sample preparation. A commonly used technique for the separationofplasticparticles fromsedimentparticles is thedensity separation. Ina solution of high density, the microplastic particles float, while the very dense sedimentparticles settle.Numerousdifferent techniquesaredescribed in literature, many of thembased on the separation introduced byThompson et al. [15].Alter- ations to thismethodincludetheuseofdifferentsalts tocreate thedenseliquidused for separation and the development of different instrumental setups and different pretreatment andposttreatment steps of the samples (compareFig. 1). In addition to sodium chloride, which was used by Thompson et al. [15] and others, theapplicationofsodiumiodideandzincchloridehasalsobeenreported[16– 18]. Sodium iodide, sodium tungstate, and zinc chloride offer the possibility to produce solutions with higher densities than sodium chloride. As the density of a saturatedsodiumchloridesolution(ρ 1.2gcm 3)isratherlimitedanddoesnotoffer consistentseparationofhigherdensitypolymerssuchaspolyoxymethylene,polyvinyl chloride (PVC), and polyethylene terephthalate (PET), sodium iodide, sodium polytungstate (ρ 1.6 g cm 3), and zinc chloride are viable choices. Density separations in themicroplastic research rarely use sodiumpolytungstate despite the possibilityofsolutionswithhighdensity(ρupto1.6gcm 3),asit istooexpensivefor theapplication in largevolumesamples [11].Sodiumiodide(ρ 1.6–1.8gcm 3) is usuallycombinedwithapre-separation,basedonelutriationthatseparates lessdense particles fromheavier particles in an upward directed streamof gas orwater. This procedure isnecessary tominimize thevolumeneededfor thedensityseparationdue to thehighcosts of sodiumiodide [14, 16].Theapplicationofzincchlorideenables solutionswith densities ofρ> 1.6–1.7 g cm 3 and is suitable for the separationof mostpolymertypes.Duetothelowercostscomparedtosodiumtungstateandsodium iodide, zinc chloride is frequently reported in recent studies [8].However, the eco- logical hazards of zinc chloride complicate the disposal of used solutions and con- taminated sediments. Thus, the recycling of solutions containing zinc chloride, sodium iodide, or sodiumpolytungstate offers a possibility to overcome thewaste management problemand reduce thematerial costs.To improve the effectivity, the repeatability, and the ease of handling for the density separationmethod, different setupsweredeveloped.TheinitialuseofbeakersorErlenmeyerflaskwassubstituted bytheuseofseparationfunnels,vacuum-enhancedseparationof theplasticparticles, or stainless steel separatorswithhighsamplevolumecapacity [9,10,13]. Recent developments focus on alternatives to density separation techniques. Elutriation seems to be a suitable and cost-effective alternative even without followingdensity separation, yielding ingood recoveries for polymerswith densi- ties of up to ρ¼1.4, and the versatility of thismethodmight be improvedwith a pre-size fractionationof the sample [19]. A different approach includes accelerated solvent extraction (ASE) for the sep- aration of plastics from soils. The extraction by ASE is carried out under higher Analysis,Occurrence, andDegradation ofMicroplastics in theAqueous. . . 55