Environmental fate of endocrine disrupting growth promoters used in cattle production

Shen Qu

doi:10.17077/etd.b3z23r6j

Trenbolone acetate (TBA), melengestrol acetate (MGA) and zeranol are synthetic growth promoters extensively used in animal husbandry, yet despite occurrence in water and soil little is known about their environmental fate. The work provides some of the first details related to the persistence and mobility of these synthetic growth promoters and their metabolites (SGPM) in natural aquatic systems. Here, laboratory experimental studies explore their tendency to undergo phototransformation in surface waters and their interaction with soil via sorption or mineral-promoted reactions. In sunlit surface waters, results suggest that the families of TBA (including 17&beta-trenbolone, 17&alpha-trenbolone and trendione) and MGA (including melengestrol) will readily undergo direct photolysis. They exhibit half-lives between ~0.25-1 h in both natural and simulated sunlight that were largely insensitive to environmental variables (e.g., temperature, pH and cosolutes). In contrast, zeranol, &beta-zearalanol and zearalanone only degrade via indirect photolysis pathways mediated by dissolved organic matter. Their transformation is attributable primarily to hydroxyl radical and triplet DOM at neutral pH values, whereas the contribution of singlet oxygen becomes greater in more alkaline waters. An observed pH-dependence for rates of indirect photolysis suggests these photooxidants react primarily with the monodeprotonated form of zeranol, as well as its metabolites. Given their high rates of direct phototransformation, close attention was paid to elucidating the photoproducts for the TBA family. Generally, all metabolites of TBA photodegrade primarily to a species hydroxylated at the C12 position of the steroid ring. While this 12-hydroxy trenbolone species is photostable, it can hydrolyzes to di- and tri-hydroxy products. Most notably, however, it is also prone to dehydration, which results in the regeneration of the TBA metabolite from which it was derived. This dehydration pathway can be acid- and base-catalyzed, but it also occurs to an appreciable extent at neutral pH. It is also temperature dependent, with greatest rates of parent metabolite regeneration at higher temperature. Consequently, the rapid rate of photolysis for TBA metabolites observed in daylight is followed by photoproduct-to-parent reversion mechanism in the dark. This phenomenon has broad and important implications for the fate of TBA metabolites in surface waters. Generally, they will be far more persistent than currently realized, and their concentration will exhibit, pH, diurnal and seasonal dependencies. In soil systems, all SGPMs exhibit a high affinity for organic rich soil (e.g., peat), consistent with an important role for hydrophobic interactions in governing their uptake. However, the extent and reversibility of sorption in such systems is not predictable from common metrics for compound hydrophobicity (e.g., octanol-water partitioning coefficients), suggesting that specific chemical interactions also influence uptake. In model soils systems with lower organic carbon content (1-6% w/w), sorption occurs in parallel with transformation reactions that are mediated by the inorganic phases present. In suspensions of model metal oxides (e.g., SiO2, MnO2 and the iron oxide ferrihydrite), SGPM decay is observed, presumably from either hydrolysis or oxidation, although reaction products were not identifiable. Thus, in soil systems, especially those with moderate to low organic carbon content, most SGPMs will primarily undergo transformation rather than sorption. Results from this work can be used to more accurately predict the fate of synthetic steroid growth promoters in agriculturally impacted water and soil. This information will be critical for assessing the totality of the risks posed to ecosystem health by this emerging pollutant class.

Environmental fate of endocrine disrupting growth promoters used in cattle production

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