The Adsorption of Tylosin by Montmorillonite and Vermiculite: Exchange Selectivity and Intercalation

Tylosin (Tyl) is a broad-spectrum veterinary antibiotic that may enter the environment through animal manure applications. The compound bears a methylamine functional group that is protonated in acidic to circumneutral pH, and cation exchange of Tyl through this functional group has been identified as an important soil retention mechanism. The objectives of this study are to investigate the exchange interactions of Tyl with the expansive phyllosilicates, montmorillonite and vermiculite. Tylosin is retained on the phyllosilicates by exchange and nonexchange mechanisms. Exchange isotherms show that Tyl is preferred relative to Na throughout a broad range of exchanger composition on STx-1. Tylosin is also preferred relative to Ca on STx-1, but only when Ca is the dominant exchangeable cation. Tylosin does not intercalate Libby vermiculite, total adsorption is minor, and exchangeable Tyl is at or below detectable levels. X-ray diffraction (XRD) indicates that Tyl intercalates STx-1, but not vermiculite. A comparison of experimental to simulated XRD profiles of Tyl intercalated smectite suggests that demixing of exchangeable cations occurs with the random interstratification of Tyl layers. In situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analysis confirmed the importance of the exchange mechanism for Tyl retention. In addition, other polar and covalent bonding surface interactions were inferred, particularly in the Ca-smectite, where the nonexchangeable forms of adsorbed Tyl predominate. The findings directly establish the intercalation of Tyl into smectite, the distribution of Tyl between exchangeable and nonexchangeable forms, and the selectivity of Tyl relative to common soil cations for the smectite surface.