Microscopic adsorption mechanism of montmorillonite for common ciprofloxacin emerging contaminant: Molecular dynamics simulation and Multiwfn wave function analysis

Many studies have proved that montmorillonite (Mt) has good adsorption properties for some antibiotic pharmaceuticals, but the detailed research on the microscopic sorption mechanism is very limited from the atomic level. In the present work, Mt was chosen as an adsorbent for common emerging contaminant-ciprofloxacin (CIP) to investigate the essence of the interaction between Mt and different CIP species at different pH. X-ray diffraction (XRD) and Fourier Transform Infrared spectroscopy (FTIR) characterization after adsorption under different pH were utilized to explore the difference of adsorption mechanism with the changed pH. Especially outstanding, for the first time, Molecular dynamics simulation (MD) was carried out to investigate the arrangement and dynamic behavior of cationic CIP in the interlayer of Mt, Multiwfn wave function analysis program was performed to quantitatively analyze the molecular surface electrostatic potential (ESP) distribution of different CIP species to evaluate the electrostatic interaction. Furthermore, for the first time, Hirshfeld surface analysis and independent gradient model (IGM) analysis were applied to probe and visualize the interaction sits between Mt and CIP zwitterion(CIP+/-) or CIP anion (CIP-), which can reveal the adsorption mechanism of Mt for CIP+/- and CIP- from the atomic level microscopic perspective.