First experimental evidence of fast leaching of small (1.7 µm) microplastics added to soil in field conditions

Research on microplastics (MP) in soils is much complicated due to the lack of dedicated (extraction) methodologies and strong matrix interferences for MP detection, and there is almost no research on the dynamics of the smallest MP in soil. In our research we first compared the possible detection of the smallest MP fraction (1-2 µm) by µ-Raman spectroscopy and fluorescence microscopy in matrices of highly varying complexity. Subsequently, we have demonstrated that it is possible to use fluorescent MP to monitor and measure the rate of the leaching process of small MP in soils under field conditions.

In a first experiment, samples of pure quartz sand, soils with variable texture (sandy loam, silt loam, clay loam) and removal of native soil organic matter (SOM), and a sandy loam soil with native SOM still present were amended with fluorescent polystyrene (PS) microparticles (diameter 1.7 µm) in different concentrations ranging from 0.1 to 0.001%. After mixing and compaction both the Raman spectra and fluorescence microscopy images were obtained. Characteristic PS Raman fingerprint peaks (main peak at 1001 cm^-1) were visible in quartz sand (all concentrations) as well as in sandy and silt loam soils without SOM (some concentrations), but not in the other situations, whereas fluorescence microscopy clearly visualized the MPs at all concentrations in all matrices.

In a second experiment, fluorescent PS microparticles were amended under field conditions to a sandy loam soil on a small surface area (circles of ± 0.2 m diameter), to a depth of 3 cm and at a rate of 70 mg kg^-1 soil. At regular time intervals, samples were taken up to a depth of maximally 100 cm. The results of the experiment demonstrate the fast process of downward transport of the MP, reaching a depth of 30 cm after only 40 days and subsequently moving further through the vadose zone to get into range of the fluctuating groundwater table. Unambiguous fluorescent MP detection in real soil thus opens up new avenues for monitoring the vertical redistribution of the smallest MP fractions in the soil profile.