Adsorption of Antibiotics by Montmorillonite and Kaolinite

Agricultural antibiotics are commonly present in animal manures that are applied to soil. The objective of this study was to examine the influence of pH, ionic strength, background electrolyte cation type, and dissolved organic C (DOC) concentration on the adsorption of chlortetracycline (CTC), tylosin (TYL), and sulfamethazine (SMT) to montmorillonite and kaolinite. Surface complexation and isotherm models were used to analyze antibiotic adsorption. Adsorption of CTC and TYL decreased with increasing pH and ionic strength. Adsorption also decreased when Ca²⁺ was substituted for Na⁺ as the background electrolyte and when DOC was added. The CTC and TYL isotherms were Langmuirian. We concluded that ion exchange was the predominant process responsible for CTC and TYL retention. The adsorption of SMT generally decreased with increasing pH in response to the deprotonation of the phenolic amine group and the formation of the nonionic SMT molecule. The retention of SMT was generally unaffected by ionic strength and background electrolyte type, and the isotherms were either linear (montmorillonite) or S type; however, adsorption increased on the addition of DOC and the isotherm was Langmuirian. We postulated that ion exchange was the predominant retention mechanism for SMT in pH < 4 systems but that weak nonionic interactions predominated in the pH 5 to 6 range. Antibiotic adsorption as a function of pH was satisfactorily characterized by surface complexation models that considered outer sphere complexation (cation exchange), although these models were not unique (models that considered inner sphere complexation also satisfactorily described adsorption). Similarly, the adsorption isotherms were generally well characterized by the Dubinin–Radushkevich adsorption isotherm model.