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TECHNICAL REPORTS

Organic Compounds in the Environment

Adsorption and Clay-Catalyzed Degradation of Erythromycin A on Homoionic Clays

Div. of Chemistry, National Center for Toxicological Research, U.S. Food and Drug Admin., Jefferson, AR 72079

^* Corresponding author (ccerniglia{at}nctr.fda.gov).

Received for publication April 8, 2003. Erythromycin has been widely used in food-producing animals and in humans, and is frequently detected as an organic pollutant in U.S. streams. In batch experiments with homoionic clays, the Freundlich isotherms were determined at 10 and 25°C. The adsorption of erythromycin A was strongly influenced by clay type, exchanged cations, the pH of the bulk solutions, and the acidity of clay surfaces. The formation of clay–erythromycin A complexes was thermodynamically favorable except for K⁺– and Fe³⁺–exchanged montmorillonites, since the reactions were exothermic (H° > 0) and the systems became stable (S° > 0). Clays catalyzed the erythromycin A degradation by the hydrolysis of the neutral sugar and the multiple dehydrations. The surface acidity of clay surface enhanced the rate of clay-catalyzed degradation of erythromycin A. In addition, the Fe³⁺–exchanged clay minerals seemed to have an electrostatic interaction with the erythromycin A molecule, by which the hydrolysis of the neutral sugar was influenced.

Abbreviations: HPLC, high-performance liquid chromatography