Considerations on the use of equilibrium models for the characterisation of HOC-microplastic interactions in vector studies

The association of hydrophobic organic contaminants (HOCs) to microplastics (MPs) in the aquatic environment and the possible perturbation of how biota and HOCs interact (i.e. 'MP vector effect') is a much researched topic in the emergent field of aquatic MP pollution. Consensus on whether the vector-effect is relevant can in part be ascertained using laboratory experimentation. Such studies, of which there are now many examples, have as a mandatory component a characterisation of the HOC-MP interaction. However, important considerations must be made when planning and executing such laboratory experiments, and subsequently when choosing equilibria models to fit sorption curves, as it is necessary to recognize that simplified conceptual models (i.e. Freundlich or Langmuir models) do not fit all HOC-MP interactions under all circumstances. The sorption equilibrium of HOCs to most plastic particles occurs as a combination of surface adsorption in the crystalline regions of the polymer (typically characterized by Langmuir models) and internal partition into amorphous regions (modelled with Freundlich relations), but this is rarely recognized. In this discussion we highlight some considerations needed when both characterizing the interactions between MPs and HOCs and improving the environmental realism of vector studies through the use of, for instance, weathered particles, adequate time for HOC-MP equilibria to be reached and working at lower concentrations. Increasing environmental realism of vector studies corresponds to a greater complexity in the equilibria model, but ultimately allows better understanding of any potential HOC-MP vector effect in nature.