Sorption of Tetracycline, Oxytetracycline, and Chlortetracycline in Illite and Kaolinite Suspensions

The sorption interaction of three widely used tetracycline antibiotics, namely, tetracycline, chlortetracycline, and oxytetracycline, under various conditions of time, pH, temperature, ionic strength, and exchangeable cations on illites and kaolinites was examined. First-order reaction indicated diffusion-controlled adsorption, maximum adsorption occurring at pH values of 3.5, 3, and 4 for tetracycline, oxytetracycline, and chlortetracycline, respectively. Adsorption isotherms of “L” type showed that the adsorption of antibiotics was in the order of chlortetracycline > oxytetracycline > tetracycline. Adsorption was maximum for Al-saturated complexes and followed the order of Al- > Na- > K- Ca. The adsorption varied inversely with changes in temperature and ionic strength up to 0.6 after which it became constant. Free energy changes (ΔG) were negative signifying a spontaneous reaction; the values of ΔG suggest a partial physical adsorption. Enthalpy changes showed that the process is exothermic. The positive values of entropy change suggest that adsorption of tetracyclines molecules is in disordered arrangement on clay surfaces. The data of these parameters with IR and X-ray studies revealed the existence of protonation and/or coordination between exchangeable cation and oxygen of >C=O group of antibiotics. The amount of cations desorbed indicated that cation exchange phenomena played an important role during adsorption.