PhD Thesis

Interactions of microplastic particles with iron oxyhydroxides in aqueous phase: The role of surface properties on aggregation and sedimentation

Johanna Schmidtmann (01/2020-05/2024)

Support: Stefan Peiffer, Ben Gilfedder, Michael Sander

Summary

The pollution of the environment with microplastics (MP) is ubiquitous. Therefore, it is essential to understand how MP behaves in the environment, how it interacts with widely distributed environmental colloids, and how these interactions affect the fate and transport of MP. This dissertation investigated how interactions of MP with naturally occurring iron oxyhydroxides influence the surface properties of MP particles, the heteroaggregation and the sedimentation in water.

In Study 1, the heteroaggregation of 1 μm polystyrene (PS) and ferrihydrite, a naturally occurring iron oxyhydroxide, was investigated in dependency of the pH. Interactions between the particles strongly depended on electrostatic interactions. At acidic pH values, the negatively charged PS particles were coated with positively charged ferrihydrite particles, leading to charge reversal. At alkaline pH, no aggregation occurred since both particle types were negatively charged and repelled each other. At neutral pH values, PS was also coated with ferrihydrite particles. However, the low charge of ferrihydrite in this pH range, contrary to the acidic range, caused charge neutralization of coated PS particles leading to strong heteroaggregation, resulting in rapid sedimentation. After just one day, PS particles had almost completely sedimented. In the acidic and alkaline pH range, only slight sedimentation occurred because repulsion forces prevented the formation of larger aggregates. Concluding, interactions with natural colloids influence not only the surface properties of MP in aquatic environments but also drive sedimentation and therefore influence MP transport in water.

In Study 2, aggregation and sedimentation were examined for UV-weathered PS particles, in addition to pristine particles. UV-irradiation led to a decrease in particle size, roughening and "shrinking" of the surfaces, reduction of negative surface charge and the formation of dissolved and particulate weathering products. These altered properties of the PS particles also affected the heteroaggregation with ferrihydrite. With increasing weathering, the isoelectric point shifted from neutral to acidic. Remarkably, maximum heteroaggregation and sedimentation for weathered PS with ferrihydrite were observed not only at the isoelectric point but over a wider pH range, presumably due to the increased surface reactivity of the irradiated particles. The formation of functional groups on the surface due to weathering might have allowed not only electrostatic interactions between the particles to take place but also additional interactions such as hydrogen bonding. It is furthermore likely that PS weathering products, such as oligomers, additionally influenced the aggregation behavior through interactions with PS and ferrihydrite. In summary, UV-induced changes of the PS surface increased the interactions with ferrihydrite and the subsequent sedimentation of PS. Additionally, with increasing UV-weathering the formation of dissolved weathering products was observed and eventually 90% of the PS escaped from the suspension, most likely as gas.

Since strong interactions between PS and ferrihydrite were observed in the first two studies, Study 3 examined how MP coating with ferrihydrite affects the initially hydrophobic properties of MP in soil. For this purpose, MP hotspots (PS and polyethylene terephthalate (PET), 20-75 μm) were introduced into sand, and the capillary rise of water was imaged using time-series neutron radiography. Uncoated MP hotspots were non-wettable. For coated MP hotspots, differences were observed depending on polymer type: while coated PS remained non-wettable, water was attracted into the coated PET hotspots. Our results suggest that the ferrihydrite coating of MP alters surface wetting properties depending on the polymer type and thus counteracts the hydrophobic properties of pristine MP. The dynamics of MP coating and wettability are key factors for biotic and abiotic degradation processes.

Study 4 developed an analytical method for quantifying PS particles with diameters in the lower micrometer range from aqueous solution. For laboratory experiments with MP (e.g. sedimentation experiments from Study 1 and 2) a fast and simple quantification method is essential and often associated with lower requirements than for environmental samples. Here it was shown that a Total Organic Carbon (TOC) analyzer is suitable for quantifying PS in the lower micrometer range. For successful oxidation of PS to CO2 in the instrument, the addition of iron or aluminum hydroxides as an additional catalyst was necessary. This increased the recovery from 52.9% to 89.7%. Thus, the presented TOC method offers a simple and fast alternative for quantifying MP samples in the lower micrometer range when no other organic substances are present in the sample. In summary, this dissertation shows that the surface properties of MP particles in the environment and their sedimentation strongly depend on interactions with natural particles and colloids. In the aqueous phase, strong heteroaggregation with iron oxyhydroxides caused MP sedimentation. In soil, MP coating with iron oxyhydroxides rendered the initial hydrophobic surface of MP more wettable. Therefore, the consideration of interactions with environmental colloids is essential to draw predictions about MP behavior and its fate in the environment.