| The hypothesis of this study was that a sulfate conjugated estrogen, i.e. 17beta-estradiol-17-sulfate (E2-17S), could be a precursor to free estrogens detected in the environment. The objectives of were to investigate the fate and transport processes of E2-17S in various soil-water systems. Radiolabeled E2-17S was synthesized using a series of chemical for the subsequent soil batch experiments. The batch experiment results showed that E2-17S dissipated more quickly from the aqueous phase of the topsoil compared to the subsoil, demonstrating that soil organic carbon played a significant role. The aqueous dissipation of E2-17S was attributed to sorption to the soil surface and transformation to form multiple metabolites. The non-linear sorption isotherms indicated limited sorption of E2-17S, and the concentration-dependent log KOC values were 2.20 and 2.45 for the sterile topsoil and subsoil, respectively. The total radioactive residue measured in the irreversible sites was greater than the reversible sites, demonstrating that irreversible sorption was the predominant sorption process. The observed multiple metabolites suggested that E2-17S underwent complex transformation pathways. For the aqueous phase speciation, mono- and di-hydroxy-E2-17S were consistently detected under all soil conditions, which indicated that hydroxylation was the major transformation process. Also, the hydroxyl metabolites were found at higher concentrations in the topsoil than the subsoil. In the reversibly sorbed phase, free estrogens (i.e. 17beta-estradiol and estrone) were detected at relatively low levels (≤ 2% of applied dose) for all soils, demonstrating that deconjugation/hydrolysis and subsequent oxidation did occur. Furthermore, both hydroxylation and hydrolysis of E2-17S took place under the non-sterile and sterile conditions. Although deconjugation was not a major pathway, E2-17S could be a precursor of free estrogens in the environment. A comprehensive one-site fully kinetic model was applied to simulate the overall governing processes in the soil-water systems and to describe the distribution of multiple metabolites in the aqueous, reversibly sorbed, and irreversibly sorbed phases.. The model gave rise to a satisfactory fit for all experimental data obtained from the batch studies, and the 36 estimated parameters were derived at relatively high confidence. |
