Sorption of Tetracycline and Chlortetracycline on Na- and Ca-saturated Montmorillonites, Humic Substances and Clay-humic Complexes

To elucidate the environmental fate of two antibiotics tetracycline (TC) and chlortetracycline (CTC) that are extensively used for growth promotion and therapeutic purposes in livestock, sorption of these antibiotics was studied on Na- and Ca-montmorillonite, humic substance (HS) and clay-HS complexes. The sorption of CTC was more than TC and sorption was stronger on clays, followed by HS and least on clay-HS. The sorption isotherms were of ‘L’ type and denoted greater sorption in Na-system than in Ca-system. Desorption followed the order: Clay-HS > HS > clay and desorption of TC was more than CTC. The adsorption isotherms and desorption data provided evidence for a partly chemisorption and partly physical adsorption of antibiotics on clay surfaces. The X-ray diffraction analyses showed that antibiotics were sorbed in the interlayer of montmorillonite and then humic substances well in the interlayer of clay-HS. The X-ray, IR and Freundlich isotherms studies revealed the existence of protonation and/or coordination (or both) between metallic cations of clay and oxygen of amide group of antibiotics.