Although most plastic pollution originates on land, current research largely remains focused on aquatic ecosystems. Studies pioneering terrestrial microplastic research have adapted analytical methods from aquatic research without acknowledging the complex nature of soil. Meanwhile, novel methods have been developed and further refined. However, methodical inconsistencies still challenge a comprehensive understanding of microplastic occurrence and fate in and on soil. This review aims to disentangle the variety of state-of-the-art sample preparation techniques for heterogeneous solid matrices to identify and discuss best-practice methods for soil-focused microplastic analyses. We show that soil sampling, homogenization, and aggregate dispersion are often neglected or incompletely documented. Microplastic preconcentration is typically performed by separating inorganic soil constituents with high-density salt solutions. Not yet standardized but currently most used separation setups involve overflowing beakers to retrieve supernatant plastics, although closed-design separation funnels probably reduce the risk of contamination. Fenton reagent may be particularly useful to digest soil organic matter if suspected to interfere with subsequent microplastic quantification. A promising new approach is extraction of target polymers with organic solvents. However, insufficiently characterized soils still impede an informed decision on optimal sample preparation. Further research and method development thus requires thorough validation and quality control with well-characterized matrices to enable robust routine analyses for terrestrial microplastics.
Plastic pollution
is increasingly perceived as an emerging threat
to terrestrial environments, but the spatial and temporal dimension
of plastic exposure in soils is poorly understood. Bioturbation displaces
microplastics (>1 μm) in soils and likely also nanoplastics
(<1 μm), but empirical evidence is lacking. We used a combination
of methods that allowed us to not only quantify but to also understand
the mechanisms of biologically driven transport of nanoplastics in
microcosms with the deep-burrowing earthworm
Lumbricus terrestris
. We hypothesized that ingestion and subsurface excretion drives
deep vertical transport of nanoplastics that subsequently accumulate
in the drilosphere, i.e., burrow walls. Significant vertical transport
of palladium-doped polystyrene nanoplastics (diameter 256 nm), traceable
using elemental analysis, was observed and increased over 4 weeks.
Nanoplastics were detected in depurated earthworms confirming their
uptake without any detectable negative impact. Nanoplastics were indeed
enriched in the drilosphere where cast material was visibly incorporated,
and the reuse of initial burrows could be monitored via X-ray computed
tomography. Moreover, the speed of nanoplastics transport to the deeper
soil profile could not be explained with a local mixing model. Earthworms
thus repeatedly ingested and excreted nanoplastics in the drilosphere
calling for a more explicit inclusion of bioturbation in nanoplastic
fate modeling under consideration of the dominant mechanism. Further
investigation is required to quantify nanoplastic re-entrainment,
such as during events of preferential flow in burrows.
The studies aimed at examining the influences of the particle size distribution and surface charge on the behaviour of sludge in dewatering. It was possible to show that defined size dispersions of sludge particles as well as surface charge are necessary to reach better dewatering results and that the found correlations were independent of the type of sludge and the sewage treatment plant (Friedrich et al., 1990 and 1991). Based on laser diffraction measurements to determine panicle size distribution it was found that it was necessary to set a specific proportion between fine and large sludge particles in order to produce the best possible dewatering results and to get the required shear resistant flocs. To characterize the surface charge of the sludge particles various measuring methods were used. Determining the zeta-potential is a suitable means to describe the kinetics of degradation process in the sludge.
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