Enrofloxacin sorption on smectite clays: effects of pH, cations, and humic acid.

Enrofloxacin (ENR) occurs widely in natural waters because of its extensive use as a veterinary chemotherapeutic agent. To improve our understanding of the interaction of this emerging contaminant with soils and sediments, sorption of ENR on homoionic smectites and kaolinite was studied as a function of pH, ionic strength, exchangeable cations, and humic acid concentration. Batch experiments and in situ ATR-FTIR analysis suggested multiple sorption mechanisms. Cation exchange was a major contributor to the sorption of cationic ENR species on smectite. The decreased ENR sorption with increasing ionic strength indicated the formation of outer-sphere complexes. Exchangeable cations significantly influenced the sorption capacity, and the observed order was Cs<Ca<K. The peak shifts of the COO(-) stretch, coupled CO stretch and OH deformation, and the result of irreversible desorption FTIR spectra indicated that OH of the carboxyl group was involved in the formation of inner-sphere surface complexes. The XRD analysis confirmed that interlayer intercalation is an important contributor to ENR sorption, while humic acid had a negligible contribution to the interlayer intercalation. The results of this study provide new insight into the molecular mechanisms of ENR sorption on clay minerals.