Ferrihydrite coating reduces microplastic induced soil water repellency

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 mu m, 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.

Automated summary

Addition of microplastics (MP) to soil can increase soil water repellency, but coating MP with iron compounds such as ferrihydrite can reduce this effect. This study investigated whether pre-coating or in situ coating of MP with ferrihydrite reduces soil water repellency. The results showed that pristine MP hotspots were not wettable, but water could imbibed into the coated MP hotspots depending on the polymer type. The coating of MP with ferrihydrite can change MP surface wettability and counteract the hydrophobic properties of pristine MP.