The interactions of oxytetracycline with model clay adsorbents were investigated as a function of suspension pH. The clay adsorbents used were native montmorillonite (SWy-2), Na-montmorillonite (Na-SWy-2), and hexadecyl trimethylammonium-montmorillonite (HDTMA-montmorillonite). 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 native and sodium forms of montmorillonite decreases with increasing pH in the order pH 1.5 > 5.0 > 8.7 > 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 over other mechanisms, as evident from the FT-IR spectrum of the HDTMA-montmorillonite and humic acid-montmorillonite adsorbed with oxytetracycline. The presence of a large amount of dissolved organic matter (DOM) was also found to decrease the sorption of oxytetracycline to 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 transform-infrared (FT-IR) analyses.

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The cation exchange capacity (CEC) of fine-grained materials, and especially clay minerals, is a fundamental property of these materials, and can be determined routinely. A search of the recent literature illustrates the great interest of this property to researchers. For example, a search of the GeoRef database for references to “cation exchange capacity” for the years 1980 to 1999 yields 2559 citations.

The worldwide emergence of infectious diseases, together with the increasing incidence of antibioticresistant bacteria, elevate the need to properly detect, prevent, and effectively treat these infections. The overuse and misuse of common antibiotics in recent decades stimulates the need to identify new inhibitory agents. Therefore, natural products like clays, that display antibacterial properties, are of particular interest.The absorptive properties of clay minerals are well documented for healing skin and gastrointestinal ailments. However, the antibacterial properties of clays have received less scientific attention. French green clays have recently been shown to heal Buruli ulcer, a necrotic or 'flesh-eating' infection caused by Mycobacterium ulcerans. Assessing the antibacterial properties of these clays could provide an inexpensive treatment for Buruli ulcer and other skin infections.Antimicrobial testing of the two clays on a broad-spectrum of bacterial pathogens showed that one clay promotes bacterial growth (possibly provoking a response from the natural immune system), while another kills bacteria or significantly inhibits bacterial growth. This paper compares the mineralogy and chemical composition of the two French green clays used in the treatment of Buruli ulcer.Mineralogically, the two clays are dominated by 1Md illite and Fe-smectite. Comparing the chemistry of the clay minerals and exchangeable ions, we conclude that the chemistry of the clay, and the surface properties that affect pH and oxidation state, control the chemistry of the water used to moisten the clay poultices and contribute the critical antibacterial agent(s) that ultimately debilitate the bacteria.