Interactions of Tylosin with Smectite Clay.

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 as a feed additive in swine and poultry production and can be found in manure and litter leachates, with concentrations as high as 4.0 mg L⁻¹. 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, as retention is influenced by background electrolyte and ionic strength. The objective of this study was to examine the exchange selectivity of Tyl in competition with Na⁺ and Ca²⁺ in Libby vermiculite and STx smectite (montmorillonite), to determine the location of the adsorbed Tyl in the mineral structures, and to use ATR-FTIR to examine the surface bonding mechanism. Homoionic (NaX−TylX) and heteroionic (CaX₂−TylX) binary exchange studies were performed and exchange isotherms were developed to establish cation preference and to determine the Vanselow selectivity coefficient (K_V) as a function of exchange phase composition. X-ray diffraction was used to determine the d-values of the 00l spacing of the reference clays saturated with varied ratios of NaX−TylX and CaX₂−TylX. Both freeze-dried and aqueous presentations of TylX samples were examined using ATR-FTIR. In addition, in situ ATR-FTIR experiments were carried out to understand the bonding mechanisms. In both the NaX−TylX and CaX₂−TylX systems, Tyl⁺ is preferred by the STx exchange phase. The K_V values are positive and vary with exchange phase composition in the CaX₂−TylX system, but are relatively invariant and positive in the NaX−TylX system. Tylosin adsorption is greater in the Na systems relative to the Ca systems. The bulk of the adsorbed Tyl in the Na system is exchangeable, while only a small portion of the adsorbed Tyl in the Ca system is exchangeable. Total Tyl adsorption by the vermiculite was very minor, approximately 10% of that observed for smectite adsorption. Nearly none of the adsorbed Tyl was exchangeable; exchange sites in the vermiculite were not accessible to Tyl due to the limited expandability of the 2:1 structure. 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. The impact of Tyl⁺ on the d-value of Ca²⁺-saturated clay 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. In both cation systems, random and segregated interstratification occurs in STx. FTIR analysis corroborates the exchangeable nature of the adsorbed tylosin. For the in situ ATR-FTIR experiments with 10 and 100 μM Tyl and Na-saturated STx, the IR bands corresponding to ν(Si-O) vibrations (~1075, 1049, 1026, and 996 cm^-1) increased with time until equilibrium. This spectral region (1075-996 cm^-1) reflected a broad band, which most likely is suggesting weak electrostatic interactions of Tyl in the smectite interlayer. The findings indicate that Tyl is intercalated into smectite interlayers, but not into vermiculite, potentially affecting the environmental fate, bioaccessibility, and protection of the antibiotic in smectite-containing soils.