Heteroaggregation of PS microplastic with ferrihydrite leads to rapid removal of microplastic particles from the water column

The pollution of the environment by plastics has become one of the most emerging environmental issues over the past years. Especially micro- and nanosized colloidal particles are of environmental concern since they can be easily taken up by organisms and accumulate in the food chain. Hitherto, only little attention has been paid to the transformation and elimination processes of colloidal microplastic (MP) in the environment. In aquatic environments, colloidal MP will interact with natural constituents, such as metal (oxyhydr)oxides or organic matter. The reaction of those particles is strongly controlled by the surface properties of both, MP particles and the environmental particles. In this study, we investigated the interactions of polystyrene (PS) particles (diameter 1 µm) and ferrihydrite, a common ferric (oxyhydr)oxide. PS particles were allowed to react with ferrihydrite for one week at different pH values (3-11) and constant ionic strength (10 mM). The surface properties of PS were examined before and after reaction with ferrihydrite using dynamic light scattering techniques. Furthermore, we determined the sedimentation rate of PS in presence and absence of ferrihydrite. The results demonstrate that the presence of ferrihydrite increases the sedimentation of PS at all pH values. At neutral pH, we observe not only the strongest sedimentation but also maximum heteroaggregation between PS and ferrihydrite. Overall, our research suggests that ferric (oxyhydr)oxides are highly important reactants to control the environmental behaviour of MP particles. Heteroaggregation with ferric (oxyhydr)oxides and subsequent sedimentation can remove microplastic particles from the water column.