Retention of Tylosin and Chlortetracycline By Soil Colloids: Exchange Selectivity.

Agricultural antibiotics are used extensively in livestock production. Because antibiotics are slowly absorbed by animals, a large amount (> 80%) is excreted in urine and feces, potentially impacting soil and other aquatic systems. One such antibiotic is tylosin (Tyl). Tylosin is used heavily as a feed additive in swine production and can be found in swine manure leachates, with concentrations as high as 4.0 mg L⁻¹. Tylosin is predominately a cationic species, due to the protonation of a dimethylamine moiety in pH < 7.5 solutions. The soil adsorption of Tyl is influenced by pH, background electrolyte, and ionic strength, suggesting that ion exchange is an important retention mechanism. The objective of this study was to examine the exchange selectivity of Tyl in competition with sodium and calcium in surface and subsurface horizons of a vermiculitic soil. Binary exchange studies were performed and exchange isotherms were developed to establish preference and to determine the Vanselow selectivity coefficient (K_V) as a function of exchange phase composition. The CEC of the soils was a function of the initial cation saturation; 15.6 cmol_c kg⁻¹ for Na⁺-saturated soils, and 4.2 cmol_c kg⁻¹ for Tyl⁺-saturated, indicating that a portion of the adsorbed Tyl occupies exchange locations, but is non-exchangeable. For NaX-TylX exchange, the exchange isotherms indicate that Tyl⁺ is preferred by the soil exchange phase. The K_V values are generally invariant with exchange phase composition, with an average K_V = 1.32 for the Tyl⁺ + NaX = TylX + Na⁺ exchange reaction.