Non-invasive 3D analysis of microplastic particles in sandy soil — Exploring feasible options and capabilities

Increasingly, environmental research efforts seek to understand how the continuous input of microplastics into terrestrial environments alters soil physicochemical properties and affects plants and other soil biota. However, fundamental understanding is hampered by the destructive nature of current analytical techniques, which typically require the disruption of soil samples and often the removal of soil organic matter. This results in the irretrievable loss of essential information about soil microstructure and the spatial distribution of microplastic particles. We showed that the non-invasive approach of dual neutron and X-ray tomography is capable of detecting and localizing microplastics embedded in soil environments with organic components, here tested with peat, charcoal, and bark mulch additions. We explored how the number of microplastic particles can be determined on intact samples, even accompanied by add-on information on the size, shape and distribution of microplastic particles. For some combinations of plastic types and organic material amendments, the basic approach was not successful, but could be enhanced by soaking the sample in hydrogen peroxide solution while largely preserving the integrity of the microstructure, or by including shape parameters into the image analysis. By segmenting images using region growing, we were able to identify all microplastic particles without false positives, even in the presence of organic material. We also succeeded in analyzing small-sized microplastic particles, such as film or fibers, embedded in natural sandy soil. 3D visualization of plastic film fragments together with the soil matrix made it obvious that larger fragments can have a significant impact on soil hydraulic properties. It has also been shown that a group of microplastic fibers can induce a planar crack in the soil matrix. Finally, roots and microplastics could be differentiated and visualized in a soil sample, demonstrating the leeway for the non-invasive study of potential interactions between roots and microplastics.