Factors Influencing Tetracycline Sorption by Smectite Clays from Water

Tetracyclines are a class of antimicrobials that have been extensively used to improve human and veterinary health. A large portion of tetracyclines used by humans and animals can be released into the environment, raising concerns regarding the potential risks to ecosystems and environmental quality. Sorption by soils is a determinant controlling transport, fate and hence exposure in the environment. Soil mineral fractions are the dominant sorptive domains for retaining these compounds. In this study, sorption of tetracycline by K+- and Ca2+-saturated smectite clays was measured using a batch equilibration method. Sorption manifested nonlinear isotherms, and was strongly suppressed by the presence of high levels of inorganic cations (e.g., K+ and Ca2+) in solution. Sorption of tetracycline by K-, and Ca-smectite was concomitant with the release of K+ and Ca2+ from the clays, suggesting that cation exchange process is responsible for the sorption of tetracycline. X-ray diffraction patterns showed that the interlayer distances of smectite clays increased as tetracycline sorption increased, and gradually approached a constant basal spacing of 17.5 to 18 Å. These results indicate that the clay-sorbed tetracyclines are intercalated in interlayer regions.