Relevance of Iron Oxyhydroxide and Pore Water Chemistry on the Mobility of Nanoplastic Particles in Water-Saturated Porous Media Environments

The increasing use of plastic products and its inevitable decomposition after improper disposal has led to large numbers of nano- and microplastic in aqueous environments. There is currently a critical need to investigate the transport and retention mechanisms of nanoplastic particles in water-saturated porous media (e.g., aquifers or sediments) to better understand residence times and ecosystem exposure of these particles in aqueous environments. In this study, we performed a set of column experiments in order to investigate and understand the primary controls on the mobility of nanoplastics in a controlled laboratory environment. As part of the experiments, we used polystyrene nanoplastic particles (PS-NPs, 50 nm) in combination with iron oxyhydroxide–coated sand, which is known for its high surface reactivity and often can be found in natural systems in environmentally relevant amounts. We also adjusted pore water chemistry (pH, ionic strength, cation species) to represent non-uniform geochemical conditions in nature and to understand how these conditions quantitatively affect the transport of nanoplastics. Mobility and retention of PS-NPs were assessed by analyzing breakthrough curves. For negatively charged iron oxyhydroxide coatings (at pH > pHₚzc), only little retention of PS-NPs could be observed. In contrast, positively charged iron oxyhydroxide coatings (pH < pHₚzc) provided favorable deposition sites for the negatively charged PS-NPs. DLVO theory was used to show that high pH and low ionic strength increased the energy barriers between PS-NPs and the porous media. In contrast, low pH and high ionic strength decreased the barriers and thus increased retention in the columns. Finally, bridging agents, such as Ca²⁺ and Ba²⁺, resulted in the significant deposition of nanoplastics by forming bonds between O-containing functional groups on both the plastic and sediment surfaces. These findings indicate that the deposition and fate of nanoplastic particles are strongly affected by the water chemistry and soil components in subsurface environments.