Tiny plastics, big impact? Towards an improved understanding of the interaction between microplastics and sedimentary rocks.

In recent years, microplastics have become a pollutant of global concern: they have been identified in virtually every environment, including the hydrosphere, atmosphere and pedosphere. A growing amount of research now focuses on their impact on these environments. In this context, however, rocks have largely been overlooked.

In particular, sedimentary rocks constitute the majority of Earth’s exposed surface and are widely used as building materials, notably in cultural heritage. Serving as an important interface between the atmosphere, the terrestrial environment and human influences, they are likely susceptible to microplastic pollution and could potentially act as (temporary) storage media for microplastics. This could have implications for the long-term durability and weathering behaviour of the rocks.

The goal of this research, funded by the Research Foundation - Flanders (FWO), is therefore to gain more insight into the interaction between sedimentary rocks and microplastics. Specifically, we aim to investigate the factors that control microplastic adhesion to rock surfaces and examine how these pollutants might modify the physical and water transport properties of the rock. Given the inherent heterogeneity of sedimentary rocks and the many anticipated factors involved in this interaction (e.g. rock specific properties, microplastic specific properties, environmental conditions), this is a challenging task that requires a systematic approach.

Our experimental setup involves four types of sedimentary rock with variable physical properties that commonly occur in Belgium: Lede sandy limestone, Bentheimer sandstone, Maastricht limestone and Belgian Blue limestone. The rocks were treated with a selection of the most prevalent microplastic types (polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polyethylene terephthalate (PET)) and studied using advanced visualization and characterization methods, including light microscopy, micro-computed tomography and 3D profilometry. Contact angle measurements were also performed to evaluate changes in rock-water interaction after microplastic exposure.

The preliminary findings of this study indicate that microplastics can alter the roughness of the rock surface, though this effect depends on the type of rock. We also observed that microplastics tend to reduce the wettability of rock surfaces. This effect is most likely due to the hydrophobic nature of the microplastics. 

Furthermore, the preliminary findings suggest that rock surface roughness and (surface) porosity can facilitate microplastic adherence to the rock surface. Moreover, certain microplastic types seem to have a greater affinity to attach to rocks than others. We anticipate that additional factors, such as environmental conditions, microplastic characteristics and rock characteristics, play a role in this interaction as well. Further study is required to determine the extent of their influence.