Microplastic induces soil water repellency and limits capillary flow

Abstract

Soils are considered the largest sink of microplastic (MP) in terrestrial ecosystems. However, little is known about the implications of MP on soil physical properties.We hypothesize that low wettability of MP induces soil water repellency, depending on MP content and size of MP and soil particles. We quantified wettability of mixtures of MP and sand. The sessile drop method (SDM) was applied to measure static contact angle (CA) of MP and glass beads at contents ranging from 0 to 100% (w/w). The results are extrapolated to varying combinations of MP and soil particle sizes based on specific surface area. Capillary rise was imaged with neutron radiography quantifying the effect of MP on dynamic CA, water imbibition, and water saturation distribution in sand. At 5% (w/w) MP content, static CA exhibited a steep increase to 80.2˚for MP 20-75 μm and 59.7˚for MP 75-125 μm. Dynamic CAs were approximately 40% lower than static CAs. Capillary rise experiments showed that MP 20-75 μm reduced water imbibition into sand columns (700-1,200 μm), with average dynamic CA of 40.3˚at 0.35% (w/w) MP content and 51.8˚at 1.05%. Decreased water saturation and increased tortuosity of flow paths were observed during imbibition peaking at 3.5% (w/w) local MP content. We conclude, in regions with high MP content. water infiltration and thus MP transport are hindered. Local low wettability induced by MP is expected to limit soil wettability and impede capillary rise.

Minimal Impacts of Microplastics on Soil Physical Properties under Environmentally Relevant Concentrations

“…Similar to that found in our study, Botyanszká et al reported a generally insignificant increase in soil water repellency in terms of water drop penetration time, when 5% w/w of polyethylene (<400 μm), polyvinyl chloride (<400 μm), and polystyrene (<400 μm) granules were added into a silt loam soil. However, a significant increase in water repellency has been reported by Cramer et al, who found that the contact angle of quartz sand increased from 30° to 40° and 50° by the amendment of polyethylene terephthalate granules (25–75 μm) at concentrations of 0.35 and 1.05% w/w, respectively. Qi et al reported that the water drop penetration time of a sandy soil significantly increased due to the incorporation of macro-sized (5 mm) polyethylene film debris at both 1 and 2% w/w.…”

Ferrihydrite coating reduces microplastic induced soil water repellency

“…In samples mixed with MP, dynamic CA are typically lower compared to static CA. 12 Instead of water not being able to spread beyond low wettability of MP during static CA measurements on a quasi-two-dimensional packing, water, during imbibition into a three-dimensional volume, has the possibility of bypassing into more favorable flow paths for imbibition and enclosing non-wettable locations. 12 Furthermore, forces like inertia of the imbibing liquid can also lead to overcome locations of low wettability.…”

“…Furthermore, hydrogen atoms are abundant in polymers allowing neutron imaging to distinguish PET and PS from dry soil material. 12,33 Differing material neutron attenuation coefficients can be used to quantify sample constituents in space and time. 32…”

Microplastics and nanoplastics in agriculture—A potential source of soil and groundwater contamination?

“…For example, Wang et al (2020) showed that soil bulk density, water holding capacity and soil structure were affected by MnP concentration. Cramer et al (2022) showed that for areas with high MP content, water infiltration and thus transport were significantly reduced and decreasing water saturation paired with entrapped air could lead to more tortuous flow paths. MnP-induced changes in available pore space (e.g., reduction in pore size due to filling of pore spaces with MnPs) have been shown to lead to both enhanced or reduced MnP transport (Scheurer and Bigalke 2018;Zhang et al 2019).…”