Natural Organic Matter Decreases the Sorption Capacityof Fipronil and Its Degradation Products onto Polyethylene Microplastics:Combined Experimental and Theoretical Insights

Microplastics (MPs) can sorb hydrophobic contaminants, such as pesticides, in water. Herein, we aimed to assess the interactions of fipronil, a toxic insecticide, and its degradation products fipronil sulfone and fipronil sulfide with polyethylene (PE) MPs in ultrapure and river water. Kinetics experiments showed that, in ultrapure water, fipronil and its degradation products were greatly sorbed by PE MPs. At equilibrium, we observed adsorption of 86, 86, and 76% of fipronil, fipronil sulfone, and fipronil sulfide, respectively. In river water, the adsorption capacity of PE MPs was substantially reduced. In fact, a negative correlation was observed between the dissolved organic carbon concentration in river water and the PE MP adsorption capacity. Similarly, adding humic acids (HAs) to ultrapure water led to a lower PE adsorption capacity for the three contaminants. Density functional theory revealed that the low binding energy between HA and fipronil and its degradation products inhibits their adsorption onto MP surfaces coated with HAs, decreasing the adsorption capacity of PE MPs. Our results show that PE MPs may not play a very significant role in the removal and transport of fipronil and its degradation products in waters with high dissolved organic matter concentrations, affecting the bioavailability of these compounds to aquatic organisms.