Microplastic Interaction with Soil Water - Visualization and Quantification with Neutron and X-ray Imaging

Soil is considered the largest sink of microplastics (MP) in terrestrial ecosystems. Among the expected effects of MP as hydrophobic surface addition is the likelihood that MP enhances soil water repellency. So, crucial for MP fate in soils is the interaction between MP and water. If MP is translocated by water flow and, vice versa, MP impacts water flow, to what extent? Water flow on the pore scale will be impacted with feedbacks on transport and retention of MP. However, we don’t know the extent of and conditions under which MP are transported through porous media and, if deposited, how they interplay with soil water dynamics. We hypothesize that: (i) isolated MP are displaced and translocated by air-water interfaces and (ii) local accumulation of MP is facilitated by bypassing water flow. To approach this question, neutron and x-ray imaging of MP and water in soils was utilized.

Dual neutron and x-ray imaging at the beamlines ICON (Paul-Scherrer-Institute) during repeated wetting-drying cycles was applied to trace MP-water interactions in aluminum cylinders filled with sand (0.7-1.2 mm) and MP (PET, 20-75 µm) in gravimetric contents of 0.35, 1.05 and 2.10%. The contents refer to static contact angle estimations of the mixtures resembling < 90°, 90° and > 90°. First, simultaneous neutron and x-ray tomography captured the initial dry MP configuration in samples. Subsequently, neutron radiographies of deuterated water flow through the sample of 1 ml min^-1 were recorded for 200s. After drying, repeated tomography gave insights into MP translocation.

Neutron and x-ray imaging results showed that regions of major MP content are water repellent. Water flow bypasses and MP is mainly retained. Resultant air entrapments lead to reduced water contents. In regions of minor MP content water can infiltrate. Here, the air-water interface collects isolated MP and shifts their distribution towards an enhanced accumulation.

Extrapolation of these results to natural soil systems suggests that vertical transport of MP can be limited especially at hotspots of high MP contents. Water bypasses here. This might limit the water dependent degradation processes of MP due to reductions in hydrolysis, coating and colonization by microorganisms even elongating the process of natural attenuation.