Three-dimensional excitation-emission matrix (EEM) fluorescence approach to probing the binding interactions of polystyrene microplastics to bisphenol A

Microplastics (MPs) are readily to bind with contaminants, hence becoming the mobile composite pollutions with the migration of MPs in the environment. However, the binding mechanisms between MPs and various contaminants are still unclear due to the scarcity of investigation methods. In this work, three-dimensional excitation-emission matrix fluorescence spectroscopy approach is developed to probe the binding interactions between polystyrene MPs and a typical hydrophobic comtaminant bisphenol A (BPA). The binding affinity and thermodynamic parameters of the binding interactions are accurately and rapidly determined. Roles of environmental conditions in the binding interactions of MPs and BPA are discriminated. Results show that the binding strength of MPs to BPA is relatively higher under neutral (logK=4.96, pH 7) and weak acidic (logK=4.63, pH 6) conditions. The binding strength significantly grows with the increasing temperature from 4 to 35 °C. The binding process is barely influenced by ionic strength, but being promoted in the presence of divalent ions (Ca²⁺ and Mg²⁺). The binding process of MPs to BPA is solely driven by entropy. Under neutral condition, hydrophobic forces dominate the binding interactions between MPs and BPA, with the assistance of weak hydrogen bonds. The predominant hydrophobic interactions are weakened under acidic conditions, resulting in lower binding strength. Under alkaline conditions, both hydrophobic forces and hydrogen bonds are weakened, while electrostatic repulsions are magnified, leading to the obviously decreased binding strength. The results benefit to a better understanding of the composition pollutions of MPs and hydrophobic pollutants in aquatic environments.