Microplastic in soil - waterdynamics, hydrophobicity, and interactions

Abstract

Soils are considered as large sinks of microplastic (MP, diameter < 5 mm). Sources of MP contamination are identified as mismanagement of plastic containing waste, the use of plastics in agriculture as well as the neglected life cycle of plastic containing products. The long-range transport of airborne MP, its subsequent deposition and the degradation of larger plastic fragments are a potential source and pathway of MP input into soils. Mechanisms of MP incorporation into the bulk soil include among others the plowing of agricultural fields and bioturbation. Since MP exhibits a low wettability, it has the potential of increasing the water repellency of soils. Various effects on plant-soil systems, water dynamics, soil respiration and soil aggregation have been reported depending on plastic type, size, shape, and state of degradation. Furthermore, MP seems to have the potential of being retained in soil, which might be related to its low wettability. The low wettability of MP acts to reduce the contact between MP and water. Water would bypass soil pores containing MP. Additionally, MP water repellency would drive MP to the air-water interface and could play a role in the displacement of MP. However, over time MP becomes part of the soil matrix and degradation processes are expected to increase MP wettability. Nevertheless, when pristine MP particles enter the soil, their low wettability might prevent extensive contact with soil water, and thus, degradation processes may be prevented and delayed. These wettability and transport dynamics need to be understood to predict the fate of MP in soils. We hypothesize that MP increases soil water repellency. MP renders soil pores hydrophobic by creating a lower ratio of wettable to non-wettable surfaces. Thus, infiltrating and imbibing water bypasses those regions (s. CHAPTER 1 and 3). Bypassing low wettability areas prevents extensive contact between water and MP and diminishes MP transport with water flow (s. CHAPTER 3). Additionally, the wettability of MP in soils changes over time. Adsorption of soil water constituents like ferrihydrite increases the wettability of MP (s. CHAPTER 2). Contact angle (CA) measurements, either derived from capillary rise or the sessile drop method, were employed to quantify the wettability of MP in porous media. Additionally, neutron radiography and neutron combined X-ray tomography were applied to image and quantify water dynamics during repeated wetting and drying cycles in sand mixed with MP in increasing contents. Neutron imaging is a non-destructive method highly sensitive to hydrous materials and, thus, optimal to image water and MP distribution in porous media. Generally, we observed that hydrophobic MP particles induce soil water repellency and that such an effect is MP content dependent. The interplay between wettable and non-wettable surfaces in soils determines the availability of flow paths for water. Imbibition and infiltration of water are impeded, resulting in decreased water contents and decelerated water flow. In imbibition experiments (s. CHAPTER 1) we observed a gradual reduction of water saturation and extended time for water to flow to the top of samples with increasing in MP content. Water needs to bypass low wettability regions and entraps air. In infiltration experiments (s. CHAPTER 3), we observed that increasing MP contents gradually restrict water infiltration. Infiltrating water bypasses MP contaminated regions, leading to preferential and delayed but eventually rapid water infiltration and to lower water contents. Furthermore, MP content did not move significantly in the vertical direction of water flow. Water bypassing low wettability soil regions is considered a factor limiting MP transport in soils. Additionally, we observed that naturally occurring coating agents like ferrihydrite can change the wettability of MP in porous media (s. CHAPTER 2). Depending on polymer type, capillary driven imbibition of water into MP hotspots was facilitated by pre-coating the MP with ferrihydrite. We infer, an effective coating of particles via adsorption depends on the presence of functional groups on the MP surface. In contrast, in situ coating of MP hotspots with ferrihydrite suspensions during capillary rise did not change the wettability regardless of ferrihydrite concentrations, MP type or wetting and drying cycle. We argue that MP, due to its hydrophobic properties, had limited contact to ferrihydrite suspensions and adsorption of ferrihydrite only occurred on MP particles at the boundary of the MP hotspot. Translated into a natural process, our experiments with wetting and drying cycles mimic fluctuating moisture conditions in the vadose zone. In nature, moisture conditions change numerous times and therefore might allow MP in soil to get in extensive contact with soil water. However, the initial low wettability of pristine MP would delay this process. The temporal dynamics of overcoming MP low wettability in natural soil environments is unclear. It most likely relates to its wettability since e.g., hydrolytic degradation processes in soils require water and furthermore, the presence of water is necessary for microorganisms and their enzymes to migrate towards MP. As the presence of water accelerates biotic and abiotic degradation processes, future research should focus on surface combined wetting kinetics of MP as a key factor to predict MP degradation under natural conditions and to understand transport mechanisms of environmentally altered MP.