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

The fate of the antibiotic ciprofloxacin (CIP) in natural waters can be strongly affected by interactions with nanominerals, such as goethite (GT; α-FeOOH). Using classical molecular dynamics, this study resolved the early stages of CIP adsorption on the four main crystallographic faces of GT nanoparticles, which is otherwise difficult to study experimentally. These early stages are driven by (i) electrostatic attraction, and (ii) the establishment of non-specific bonds between CIP (carboxyl, keto, amine) and GT (surface OH) functional groups. Simulations revealed that the medium-range (<1.5 nm) approach was not influenced by crystallographic orientation, but primarily by local positive charges generated by the interfacial orientation of GT surface OH groups. As a result, the deprotonated CIP^- species reached the highest densities and residence times near GT surfaces, followed by the zwitterionic CIP^-/+ species, and finally the protonated CIP⁺ species. Hydrogen bond numbers follow the same trend, and result from interactions between CIP carboxyl and GT surface O(H) groups. However, the protonated CIP⁺ species formed more hydrogen bonds and the most stable hydrogen bonds with the reactive OH groups of the (100) and (110) faces of GT. Additionally, protonation facilitated access to the (010) face by allowing the carboxyl group to fit into a tightly-bound water layer. By resolving the very first stages of CIP-GT interactions, this study established a foundational understanding of the precursor species which ultimately lead to redox-active Fe-bonded CIP species that can alter antimicrobial resistance in nature. These findings should thus contribute to a deeper understanding of mineral-organic interactions and to antibiotic transport in nature.