Adsorption of zwitterionic fluoroquinolone antibacterials to goethite: A charge distribution-multisite complexation model

Fluoroquinolone (FQ) antibacterials are aquatic contaminants of emerging concern (CEC), and adsorption to mineral surfaces is expected to play an important role in the fate, transport, and treatment of FQs. This study characterizes and models the adsorption of a zwitterionic FQ, ofloxacin (OFX), to goethite (α-FeOOH) over a wide range of pH (3-11), OFX concentration (20-500μM), and electrolyte compositions (0.001-0.1M NaCl and NaClO₄). Comparing OFX adsorption to structural analogues demonstrates that the carboxylate group is essential for binding to goethite. ATR-FTIR measurements indicate that FQs complex to goethite surfaces through carboxylate and carbonyl oxygen atoms, and that ClO₄^- co-adsorbs with OFX. Adsorption of the zwitterionic OFX increases with increasing ionic strength and is enhanced in NaClO₄ relative to NaCl electrolyte, whereas adsorption of a non-zwitterionic analogue is insensitive to ionic strength. A CD-MUSIC (charge distribution-multisite complexation) model, incorporating multiple modes of surface complexation constrained by spectroscopic measurements and the crystallographic distribution of goethite surface sites, yields accurate predictions over wide-ranging solution conditions. According to the model, OFX adsorbs predominantly by inner-sphere complexation on terminal surfaces of the rod-shaped goethite crystals in NaCl electrolyte, and OFX-ClO₄^- ion pairing in NaClO₄ induces formation of additional inner- and outer-sphere surface complexes on multiple crystal faces of goethite.