The cation exchange behavior of tylosin in loess-derived soil

Tylosin (Tyl) is a veterinary antibiotic commonly used in swine and poultry production. Due to metabolic inefficiencies, it enters the environment through manure applications. Ion exchange is an important retention mechanism for Tyl, particularly for smectite clay. The objectives of this study are to characterize the exchange interactions of Tyl with common soil cations in subsoil horizons that contain smectite and to investigate the interactions using in situ Fourier transform infrared (FTIR) spectroscopy. Adsorbed Tyl in pH neutral, smectitic subsoil horizons is divided into exchangeable and nonexchangeable forms. The percentage of adsorbed Tyl that is exchangeable varies from 36% to 43% when Na⁺ is the competing cation, and from 57% to 66% when Ca²⁺ competes. In NaX-TylX binary exchange systems, neither Na⁺ nor Tyl⁺ is preferred by the clay exchange phase, and the Vanselow selectivity coefficients (K_V) for the NaX→TylX exchange reaction range between 0.79 and 1.41. In the CaX₂-TylX systems, Tyl⁺ is preferred by the clay exchange phase when the equivalent fraction of TylX (E_TylX) is less than 0.4. The K_V values for the CaX₂→TylX exchange reaction are at a maximum at the lowest E_TylX values, with 17.6 V < 58.1, then decrease with increasing E_TylX to 1.34 V < 6.28. Adsorbed Tyl masks the CEC of the soil clays; the effect is greatest in systems that are initially Tyl-saturated, and is attributed to the steric effects of the large Tyl molecule. In situ FTIR indicates that Tyl interacts with soil iron oxides through the dimethylamine moiety.