INVESTIGATING THE MOLECULAR INTERACTIONS OF OXYTETRACYCLINE IN CLAY AND ORGANIC MATTER: INSIGHTS ON FACTORS AFFECTING ITS MOBILITY IN SOIL

Residual concentrations of tetracycline antibiotics in the environment have been found to contaminate surface, ground and drinking water, aquatic sediments and soils. This has also led to increased emergence of resistant strains of pathogenic bacteria that could potentially impact human health. The interactions of oxytetracycline with model clay adsorbents and organic matter were investigated as a function of suspension pH. The clay adsorbents used were the montmorillonite clay (SWy-2), Na-montmorillonite clay (Na-SWy-2), and hexadecyl trimethyl ammonium-montmorillonite clay (HDTMA-clay). The adsorption of oxytetracycline to the clay could be described by Freundlich type adsorption isotherms. It was observed that the adsorption of oxytetracycline in the natural and sodium forms of montmorillonite clay decreases with increasing pH in the order pH 1.5 > pH 5.0 > pH 8.7 > pH 11.0. This trend is consistent with cationic exchange interactions that are dominant at lower pH values when oxytetracycline has a net positive charge. On the other hand, hydrophobic interactions when oxytetracycline is zwitterionic (at pH 5.0) are predominant, as evident from the FT-IR spectrum of the HDTMA-clay adsorbed with oxytetracycline. The presence of large amount of dissolved organic matter (DOM) was also found to decrease the sorption of oxytetracycline on clay, suggesting that DOM may increase its mobility in the natural environment. Several mechanisms of interaction of oxytetracycline in clay are proposed based on the adsorption isotherms and the results from X-ray diffraction (XRD) and Fourier transformed infrared (FT-IR) analyses.