Intercalation of Phyllosilicate Minerals By Tylosin: Influence of Solution and Exchange Phase Composition.

Tylosin (Tyl) is a broad-spectrum antibiotic used to treat livestock. Only a small amount of Tyl is actually absorbed by animals, while the remainder is excreted in urine and feces. Tylosin is a large molecule (~1,000 g mol⁻¹) that bears a protonated methylamine group when solution pH < 7.5. Adsorption studies indicate that an important soil retention mechanism for Tyl is cation exchange. Further, exchange isotherms show the Tyl⁺ is preferred by soil and smectite clays, relative to Na⁺ and Ca²⁺. These studies also show that adsorbed Tyl is distributed between non-exchangeable and exchangeable forms, and that adsorbed Tyl reduces the CEC of clay minerals. The objective of this study was to determine the location of adsorbed Tyl in Na- and Ca-saturated source clays (STx and SWy) and soil smectite. Solutions having differing Tyl-Na or Tyl-Ca ratios (with a total normality of 4 mM) were equilibrated with Na- or Ca-saturated solids. X-ray diffraction was then preformed to determine d-values of the 00l spacing. The d-value of STx increased from 1.26 nm for Na⁺-saturated to 3.13 nm when Tyl⁺ was 75% of the exchange complex. Similarly, the d-value of SWy increased from 1.24 nm to 2.69 nm. The impact of Tyl⁺ on the d-value of Ca²⁺-saturated clays was minor due to the competitive nature of Ca²⁺ for exchange sites. The d-value of STx increased from 1.54 nm for Ca²⁺-saturated to 1.72 nm when Tyl⁺ was approximately 10% of the exchange complex. Similarly, the d-value of SWy increased from 1.51 nm to 1.69 nm. In the Na-saturated soil clays, the d-value shifted from 1.30 nm to approximately 2.7 nm when Tyl⁺ occupied 40% of the exchange complex. The d-value shifted from 1.45 nm to 1.6 nm when Tyl⁺ occupied 10% of the exchange complex in Ca-saturated soil clays. The findings indicate that Tyl is intercalated into smectite interlayers, affecting the environmental fate and bioavailability of the antibiotic.