Interaction dynamics of agricultural fungicides at water-nanoplastics interfaces and the effects of dissolved natural organic matter

Polystyrene microplastics and nanoplastics, which are commonly detected in agricultural runoff, often occur in the presence of agricultural pesticides. However, there is limited mechanistic understanding of the fate of these pesticides in relation to the presence of these plastic surfaces. Here, we used molecular dynamics simulations to investigate the adsorption mechanisms of four common triazole fungicides-flusilazole, hexaconazole, myclobutanil, and triadimenol-on polystyrene nanoplastics, with and without dissolved natural organic matter. In the absence of organic matter coating, simulated adsorption of the fungicide compounds on the polystyrene surface was driven primarily by van der Waals interactions, which were correlated with the hydrophobicity of the compounds and the polarity of their associated functional groups. Accordingly, flusilazole and hexaconazole exhibited both the highest hydrophobicity, as characterized by octanol-water coefficients and the most favorable interaction energies on the polystyrene nanoplastics in the molecular simulations. Consistent with these theoretical results, subsequent adsorption experiments revealed two-fold higher adsorbed amount of flusilazole on polystyrene plastics, compared to myclobutanil and triadimenol. When the model polystyrene nanoplastics were coated with representative plant-derived organic matter compounds in the molecular simulations, the interaction energy of the fungicides was decreased by 150% due to the hydrophilic nature of the organic matter-plastic interface that was unfavorable to the binding of the fungicides. However, this theoretical prediction was not corroborated by adsorption experiments with a river-isolated dissolved natural organic matter, likely due to insufficient coating or relatively weak interactions of the organic matter components on the PS surface. We highlight the importance of considering the role of natural organic matter of different chemistries in relation to the environmental fate of fungicides with nanoplastics.