Spectroscopic investigation of ciprofloxacin speciation at the goethite-water interface

We investigated ciprofloxacin (a fluoroquinolone antibiotic) speciation as a function of pH in aqueous solution and in the presence of dissolved ferric ions and goethite using ATR-FTIR and UV-vis spectroscopy. The presence of dissolved and surface bound ferric species induced the deprotonation of the ciprofloxacin carboxylic acid group at pH < pKa1. The resultant ciprofloxacin zwitterions appeared to interact via both carboxylate oxygens to form bidentate chelate and bridging bidentate complexes within colloidal iron oxide-ciprofloxacin precipitates and bidentate chelates on the goethite surface. However, the structure of the aqueous ferric-ciprofloxacin complexes remains unclear. Our evidence for bidentate chelates (involving only the carboxylate oxygens) on the goethite surface was distinct from previous IR studies of fluoroquinolone sorption to metal oxides that have proposed surface complexes involving both the keto and the carboxylate groups. We find that the distinct ciprofloxacin surface complex proposed at the goethite-water interface may be a result of differences in metal oxide mineralogy or assignment of the carboxylate antisymmetric stretch in the metal oxide-fluoroquinolone spectra.