Adsorption and intercalation of ciprofloxacin on montmorillonite

Fluoroquinolones are a group of antibiotics commonly used in human and veterinary medicine. Highly enriched in sewage sludge as well as animal manure they can enter the environment by surface application of the sludge and manure. In this study, the interaction and adsorptive removal of ciprofloxacin (CIP) with a sodium montmorillonite (MMT) were studied in batch tests and supplemented by XRD and FTIR analyses. The adsorption of CIP on MMT was instantaneous with a large rate constant and a high initial rate. Quantitative desorption of exchangeable cations confirmed cation exchange as the most important mechanism of CIP adsorption on MMT. Higher CIP adsorption was achieved when solution pH was less than the pKa2 value of CIP, above which adsorption coefficient decreased significantly. Fitting of experimental data to adsorption of different CIP species indicated that both cationic and zwitterion forms of CIP could adsorb on MMT. An increase in basal spacing after CIP uptake indicated interlayer adsorption, i.e. intercalation of CIP. The substantial adsorption of CIP when pH was greater than the pKa2 together with the disappearance of C = O stretching vibration of the –COOH group in FTIR analyses suggested that complexation may be the dominant mechanism at high pH.

Research Highlights
► Montmorillonite is adsorbent for ciprofloxacin (CIP) removal from water.
► Solution pH had strong influence on the amount and speciation of CIP adsorbed.
► Cation exchange is the dominant mechanism for CIP removal when pH < 8.
► The adsorbed CIP was in the interlayer position, resulting in an intercalation.