Adsorption of organic pollutants by microplastics: Overview of a dissonant literature

Microplastic formation in aqueous systems is among the inevitable consequences of plastic pollution, which has cascading environmental and health implications. As the microplastic sizes get smaller over time, their surface areas increase, which creates an implicit, dynamic, and inflating new domain, called the microplastisphere. This study summarizes and critically reviews literature on the adsorption of organic compounds (OCs), which are already in our aquatic systems, onto microplastic surfaces. This investigation reviews a database of 91 articles, 68 of which are used in an adsorption analysis for 178 OCs via 770 isotherms. The four most prevalent polymer types throughout the literature are polyethylene (PE), polystyrene (PS), polypropylene (PP), and polyvinyl chloride (PVC). Our analysis revealed that the octanol-water partitioning coefficient, Kow, can be a good surrogate for adsorption capacity for some polymer and OC types; however, the hydrophobicity of the OC alone may not necessarily indicate adsorption affinity. The water chemistry also played notable roles on the adsorption process, evidenced by the dramatic enhancement of adsorption during per- and polyfluoroalkyl (PFAS) adsorption onto PS in saltwater conditions. However, our work concluded that the variability within the database indicates a strong need for defined microplastics characterization and testing procedures to better represent the interactions of microplastics with the environment.