Surface Complexation of the Zwitterionic Fluoroquinolone Antibiotic Ofloxacin to Nano-Anatase TiO2 Photocatalyst Surfaces

The surface complexation behavior of ofloxacin (OFX), a zwitterionic fluoroquinolone antibiotic, to nano-anatase titanium dioxide (TiO2) was characterized. OFX adsorption in aqueous TiO2 suspensions was measured as a function of pH, OFX concentration, and electrolyte type and concentration, and structural information was derived from in situ spectroscopic observations. An ultraviolet-visible spectral red shift upon OFX adsorption indicated formation of inner-sphere coordination complexes. Fourier transform infrared spectra of TiO2-adsorbed OFX were invariable over a wide concentration and pH range and were similar to measured spectra of dissolved species wherein the carboxylate group is deprotonated. A charge distribution surface complexation model constrained by spectroscopic observations was developed to describe macroscopic adsorption trends. A tridentate mode of adsorption involving bridging bidentate inner-sphere coordination of the deprotonated carboxylate group and hydrogen bonding through the adjacent carbonyl group on the quinoline ring resulted in successful predictions of observed adsorption trends. In NaClO4 electrolyte, spectroscopic data and model fitting suggested that OFX ion pairing with ClO4- enhanced adsorption under acidic conditions. Moreover, comparison of OFX adsorption data with the pH trend in the kinetics of OFX degradation by visible light (lambda > 400 nm) photocatalysis suggested that adsorbed OFX-CO4- ion pairs inhibit photodegradation.