Microplastic water repellency reduced by ferrihydrite coating

Pathways of Microplastic (MP) into ecosystems are manifold and range from agricultural mulching practices to atmospheric deposition with soil being considered the largest sink of MP in terrestrial ecosystems. Once deposited there, MP is posing a hydrophobic surface addition. Former experiments showed that pristine MP can cause lower water saturation of pore spaces and so change the liquid configuration within a porous network. If water cannot reach MP, biotic degradation might be hindered. However, in natural soil systems MP can be coated over time by soil abundant substances e.g., iron compounds with the potential effect of decreasing their hydrophobicity. We hypothesize that: 1) ferrihydrite pre-coated MP shows reduced hydrophobicity; 2) in-situ wetting and drying cycles with ferrihydrite leads to partial coating of MP.

We tested these hypotheses by applying hotspots of MP, pre-coated and pristine, to sand in rectangular columns and performed neutron imaging during capillary rise. Neutron imaging allowed for visualizing and quantifying liquid dynamics and configuration. Water was used for the pre-coated MP (n=6) variants and ferrihydrite suspension (100 mg L^-1) in three wetting and drying cycles for the pristine MP (n=6) variants. The utilized MP are polystyrene (PS, 20-75 µm) and polyethylene terephthalate (PET, 20-75 µm). The grain size of sand was 0.7-1.2 mm. Pre-coating was achieved by shaking the raw material for 3 h in a 100 mg L^-1 ferrihydrite suspension and subsequent drying in a sieve supported by a vacuum pump.

Capillary rise of water into pristine MP variants exhibited zero water saturation at the hotspot and water movement around the MP aggregation was observed. Capillary rise of water into pre-coated MP variants differ in result by polymer type. While pre-coated PS is still hydrophobic, the pore space of pre-coated PET was completely water saturated. The rising water accelerated towards the hotspot due to its lower matric potential compared to sand.

Capillary rise of ferrihydrite suspension in wetting and drying cycles also showed varying results according to polymer type. While there is no effect on water saturation on PS in the hotspot after three wetting cycles, PET exhibits a slightly higher water saturation during the second wetting but stagnating in the third.

Our results suggest that ferrihydrite coating, being only one of numerous potential coating agents, can bond to MP and change its surface polarity. Differences in completeness of coating can be explained by inherent chemical and physical properties of different polymer types. But once hydrophilic, completely, or only part of the surface, water flow induced colonization and migration of microorganisms and their enzymes can proceed and biotic degradation can take place. The open question lies within the time frame necessary to overcome MP’s inherent hydrophobicity.