Molecular-level insight into ciprofloxacin adsorption on goethite: I. Approach and non-specific binding

Abstract

Ciprofloxacin (CIP) is an antibiotic extensively used in both human medicine and the livestock industry, leading to significant environmental contamination. This study investigated the adsorption mechanisms of CIP on four important crystallographic faces of goethite (GT; α-FeOOH), a key nanomineral in determining the fate and transport of antibiotics in natural waters. We employed classical molecular dynamics simulations to explore the initial stages of CIP adsorption on GT. These stages include (i) the approach of the three main aqueous of CIP (CIP⁺, CIP^-/+ and CIP⁻) species, and (ii) the establishment of non-specific interactions in the form of hydrogen bonds between CIP (carboxyl, ketone) and GT (hydroxyl) functional groups. Simulations revealed that the medium-range (< 1.5 nm) approach was not significantly influenced by crystallographic orientation, but primarily by local positive charges induced by the orientation of OH groups at the GT/water interface. The anionic (CIP⁻) CIP species achieved the highest densities and residence times near GT surfaces, followed by the zwitterionic (CIP^−/+), then the cationic (CIP⁺) species. Additionally, protonated, the CIP⁺ species formed more stable hydrogen bonds with the (100) and (110) faces, and facilitated access to the (010) face by integration into a tightly-bound water layer. These insights provide a foundational understanding of CIP-GT interactions, essential for further studies on specific adsorption mechanisms between CIP functional groups and surface Fe sites. This molecular-scale understanding also contributes to the broader field of contaminant fate and transport in natural systems.