Growth promoters (GPs) are antibiotic or hormone compounds given to livestock at sub-therapeutic levels in order to promote growth rates and feed efficiency. Many GPs have limited bioavailability, meaning that a fraction of the compound will pass through the target animal without being metabolized. The application of manure containing the unmetabolized fraction as fertilizer can result in the introduction of GPs to the environment. The subsequent transport and environmental fate of GPs is largely controlled by the underlying soil matrix. Tylosin (a macrolide antibiotic) and progesterone (a naturally produced hormone) are both GPs with limited bioavailability that have been detected in surface waters in the United States. Despite their widespread use, the interactions of both of these compounds with soils are poorly understood. The purpose of this work was to help elucidate the fate of these two compounds in soil systems.
Batch sorption experiments were conducted with five different sterilized soils and both tylosin and progesterone in order to determine their interactions with soils. Kinetics experiments were completed in order to determine the amount of time required to reach quasi-equilibrium between the sorbed and aqueous phase. Sorption isotherms were also completed, with initial aqueous concentrations ranging from 0.05-5.0 µM and 0.5-5.0 µM for progesterone and tylosin, respectively. Analysis of the aqueous concentration after quasi-equilibrium was reached was conducted via reverse phase high performance liquid chromatography (RP-HPLC). Both tylosin and progesterone were found to undergo strong non-linear sorption based upon Freundlich transformations of the isotherm data. Concentration dependent partition coefficients were calculated for both compounds using the Freundich isotherm fits and an initial aqueous concentration of Caq=0.5 µM. This resulted in average log Koc=2.94 ¿¿¿¿ 0.10 for progesterone and log Koc=2.95 ¿¿¿¿0.18 for tylosin. While no work has been published regarding the sorption of progesterone to soils, these results are similar to values reported for tylosin.
Mass balances yielded acceptable recoveries for progesterone; however, almost 40% of the added tylosin was lost by the time quasi-equilibrium were reached (7 days). Based upon the hypothesis that this loss could be due to abiotic metal-oxide mediated degradation, the fate of tylosin in the presence of goethite (α-FeOOH) and birnessite (δ-MnO2) (two minerals ubiquitous to soils) was investigated. Tylosin was added to a suspension of the metal oxide at pH=6.5 and its aqueous concentration measured over time by reverse phase high pressure liquid chromatography (RP-HPLC) in order to determine the rate of tylosin transformation. Tylosin appeared to undergo pseudo-first order transformation kinetics, with half lives of 27 h and 139 h with goethite and birnessite, respectively. Possible transformation pathways and products were hypothesized based upon UV-Vis absorbance and liquid chromatography-mass spectrometry results. The transformation of tylosin in the presence of metal oxides has not previously been reported, and suggests an additional degradation pathway for tylosin in soils. These results, in addition to a better understanding of the sorption of tylosin and progesterone, help us to better comprehend the ultimate environmental fate of both compounds in soils.