Ferrihydrite coating reduces microplastic induced soil water repellency

Title data

Cramer, Andreas ; Schmidtmann, Johanna ; Benard, Pascal ; Kaestner, Anders ; Engelhardt, Matthias ; Peiffer, Stefan ; Carminati, Andrea:
Ferrihydrite coating reduces microplastic induced soil water repellency.
In: Environmental Science : Processes & Impacts. Vol. 25 (10 May 2023) Issue 6 . - pp. 1094-1101.
ISSN 2050-7895
DOI: https://doi.org/10.1039/D3EM00077J

Project information

Abstract in another language

Addition of microplastics (MP) to soil has the potential to increase soil water repellency. However, coating of MP with soil abundant substances e.g., iron compounds, can reduce this effect. Here, we tested if pre-coating or in situ coating of MP with ferrihydrite (Fh) reduces soil water repellency. We applied hotspots of pristine and coated MP (20–75 μm, PS and PET) to sand and imaged capillary rise via neutron radiography. Capillary rise experiments in wetting–drying cycles were conducted using water and Fh suspension. Pristine MP hotspots were not wettable. Capillary rise of water into coated MP hotspots differed in wettability depending on polymer type. While coated PS was still non-wettable, water imbibed into the coated PET hotspot. Capillary rise of Fh suspensions in wetting and drying cycles also showed varying results depending on polymer type. MP hotspots were still non-wettable and local water content increased only marginally. Our results indicate that Fh coating of MP changes MP surface wettability depending on polymer type and therefore counteracts the hydrophobic properties of pristine MP. However, MP coating is likely to be slowed down by the initial hydrophobicity of pristine MP. Dynamics of MP coating and increasing wettability are key factors for biotic and abiotic degradation processes.

Environmental significance

The contamination of soil with microplastics (MP) is ubiquitous and occurs locally by deposition of larger plastic fragments with hotspots of high MP content leading to a loss of soil wettability. Consequently, locally reduced soil water content and air entrapment may occur, affecting soil functions such as capillary water flow as well as MP degradation. Here, we demonstrated that, depending on polymer type, coating of MP with iron (oxy)hydroxides reduces hydrophobicity of MP and allows for capillary infiltration of water. However, the initial low wettability of MP is likely to slow down the coating of MP. We conclude that the interaction of MP with surface-active environmental particles is important for the fate of MP and its impact on soil wettability.