Adsorption Characteristics of Antibiotics on Microplastics: The Effect of Surface Contamination with an Anionic Surfactant

Microplastics and antibiotics are common typical pollutants, and where they coexist in the environment, they can cause compound pollution. Surfactants in the environment can change the interface characteristics of pollutants, and then cause the change of environmental behavior of pollutants. In this paper, we studied the physicochemical properties of complexes of polystyrene (PS) and polyethylene (PE) contaminated with sodium dodecyl benzene sulfonate (SDBS); the complexes are referred to as SPS and SPE, respectively. Taking oxytetracycline (OTC) and norfloxacin (NOR) as representatives broad-spectrum antibiotics, the effects of SDBS on the adsorption behavior of PS and PE were analyzed and possible mechanisms were proposed. The results showed that SDBS could effectively combine with PS and PE to enhance the surface electronegativity and reduce the Brunner−Emmet−Teller (BET) specific surface area and porosity. The crystal structure remained basically unchanged, and the surface functional groups changed slightly. SDBS greatly enhanced the saturated adsorption capacities of PS and PE for OTC and NOR, and made adsorption easier, which reduced the Gibbs free energy of the adsorption system. The adsorption behaviors of SPS and SPE for the two antibiotics were consistent with the Elovich kinetic model and Sips isothermal model. SDBS enhanced the hydrophilicity of the microplastics, which facilitated their adsorption of antibiotics dissolved in water. SDBS could directly combine with antibiotics to form a complex, which further increased the adsorption capacity of the microplastics for antibiotics. The -SO 3 H in SDBS could combine with oxygen-containing functional groups and -NH 2 in OTC and NOR. Non-ionic covalent bonds, electrostatic interactions, and hydrophobic attraction between the alkyl chain and benzene ring also played a role in adsorption.