Which parameters influence the infiltration behaviour of microplastic particles into fluvial sediment?

, Waldschläger Kryss.

To answer this question, an experimental test series was conducted and the results were compared to principles of classical sediment transport. An infiltration column was filled with glass spheres with different diameters (1.5 - 11 mm), in either uniform or bimodal packings. Subsequently, 21 microplastic particles with different properties (density, shape, diameter) were applied to the top of the glass spheres and a constant water flow was simulated for one hour. Finally, maximum and average infiltration depths of the individual microplastic particles were recorded. The infiltration depth of the particles increased with decreasing particle diameter (dMP) and with increasing diameter of the glass spheres (dGS). An examination of the differences in infiltration depth due to particle shape (pellets, fragments, fibres) has shown that spherical particles infiltrate deepest and that for fibres, a thinner diameter leads to a deeper infiltration. Density does not seem to influence the infiltration depth of microplastics. Additionally, it was assessed to what extent the ratio of the diameters of microplastics and glass spheres influences the infiltration depth. It was shown that the boundary values from fine sediment infiltration (dMP/dGS ¿ 0.32 = fine surface sealing; 0.11 ¿ dMP/dGS ¿ 0.32 = finite depth infiltration; dMP/dGS ¡ 0.11 = unimpeded static percolation) can also be applied to microplastics. Formulas were developed which can be used for defining the necessary depth of a sediment sampling, so that a comprehensive representation of microplastic contamination can be generated. The study therefore offers a first insight into microplastic infiltration in sediment. In the future, more studies should be carried out to determine depth-variable microplastic concentrations in sediment. In addition, studies should pay attention to the grain size of the sampled sediment, which has a strong influence on the infiltration depths of microplastics and thus strongly influences the microplastic concentrations in sediment.

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