Distinct chemical adsorption behaviors of sulfanilamide as a model antibiotic onto weathered microplastics in complex systems

Having proved to be harmful to plants, fishes and other species, microplastics as an emerging class of pollutants are raising public concern worldwide. Microplastics that harbor invasive substances in aquatic environment have further attracted scientific attention, due to potential synergistic toxicity greater than that produced individually. This study investigates the fundamental aspect of the adsorption behavior of sulfanilamide, one of the mostly adopted and extensively abused antibiotics, to weathered microplastics via hydrophobic interaction in different contexts. By exerting repeated UV-irradiation/temperature-variation, polyamide (PA), polyvinyl chloride (PVC) and polyethylene terephthalate (PET) microplastics are surface-roughed to display deep furrows depending on composition, while preserving the original chemical groups. Kinetics evaluation reveals an adsorption equilibrium achieved in 2 days. Satisfying fitting with pseudo-second-order model indicates chemical adsorption behavior for sulfanilamide onto all the aged microplastics. Probably due to the distinct surface roughness, a characteristic of Langmuir adsorption for the aged PA and PVC was observed with larger adsorption capacities; While the aged PET microplastics with relatively flat surfaces follow Freundlich adsorption model. We clarified the competition of the antibiotic with other species (e.g., salt, surfactant) and enhanced adsorption by interacting with protein in complex systems. These findings suggest limited adsorption of sulfanilamide antibiotics on microplastics in environment, but incremental toxicity of microplastics to organisms and even human beings that intake antibiotics and microplastics from environment and through food chain.