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

Polystyrene (PS) microplastics and nanoplastics, which are common in agricultural runoff, often co-occur with agricultural pesticides. However, the mechanisms governing pesticide association with plastic surfaces are poorly understood. Here, we used molecular dynamics simulations to investigate the adsorption of four triazole fungicides-flusilazole, hexaconazole, myclobutanil, and triadimenol-on PS nanoplastics, with and without dissolved natural organic matter. In the absence of organic matter coating, simulated adsorption of the fungicides on PS was driven primarily by van der Waals interactions, in accordance with the hydrophobicity of the fungicide compounds and the polarity of their functional groups. Therefore, flusilazole and hexaconazole, with the highest octanol-water coefficients, exhibited the most favorable interaction energies in the simulations. Consistent with these theoretical predictions, subsequent adsorption experiments revealed a twofold higher adsorbed amount of flusilazole on PS relative to myclobutanil and triadimenol. In simulations with plant-derived organic matter coating, the interaction energies were decreased because the hydrophilic nature of the organic matter-plastic interface was less favorable for binding of the fungicides. However, this latter 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. The findings of this study highlight the importance of considering the role of natural organic matter of different chemistries in relation to the environmental fate of agricultural pesticides interacting with nanoplastics.