Thermodynamics of Equilibrium Adsorption of Antibiotics by Clay Minerals and Humic Acid–Clay Complexes

Abstract

The sorption interaction of three widely used tetracycline antibiotics, tetracycline (TC), chlortetracycline (CTC) and oxytetracycline (OTC), has been determined at three different temperatures from dilute solutions of montmorillonite, humic acid and humic acid-montmorillonite complex saturated with Na+, Ca2+ and Al3+ cations using batch equilibrium technique. The presence of humic acid enhanced the adsorption of antibiotics while rise in temperature decreases the adsorption. Adsorption isotherms were of ‘L’ type. The adsorption of antibiotics was in the order OTC > CTC > TC. The adsorption was maximum for Al-saturated complexes and followed the order Al- > Na- > Ca. The standard free energy changes (ΔG0) for the adsorption reactions were negative, signifying a spontaneous reaction. The enthalpy changes (ΔH0) calculated from the temperature coefficient of the equilibrium constant showed that process is exothermic. The adsorption of tetracyclines on adsorbents give rise to an entropy loss as antibiotics molecules adsorbed on surface had fewer translational and rotational degrees of freedom than antibiotics molecules in solution. Various studies e.g. IR and X-ray, as well as thermodynamic parameters and Freundlich adsorption isotherms revealed the existence of protonation or coordination (or both) between exchangeable cation and oxygen of >C=O group of antibiotics. The values of thermodynamic parameters indicated towards a partial physical adsorption. Antibiotics present in soil/water in small amount encouraging the development of antibiotics resistant bacteria pose a threat to human health. The results of present study showed that antibiotics are more retained in soils which contain higher amount of humic acid. The sorption on motmorillonites was spontaneous, exothermic and depends on nature of tetracycline and cation. The values of thermodynamic parameters indicated a partial physical adsorption and partial chemisorption.