GEOCHEMICAL FATE AND TRANSPORT OF FLUOXETINE IN NATURAL SOIL

Pharmaceutical chemicals have become a worldwide concern due to increasing presence on our water resources. They are increasingly being discharged into natural streams from wastewater treatment plant and as run-off from sludge-applied fields and unintended leaks from farm waste holding ponds. Discharges can also result in soil, sediment, and groundwater contamination. Fluoxetine, a commonly prescribed antidepressant, has been found in trace amounts in municipal drinking water and in natural streams. It is a relatively non-polar organic molecule with a reactive amine functional group and has aqueous solubility of 14 g/L and pKa of 10.1 resulting in a positively charged molecule at low pH.

The main objective of this study was to determine sorption and transport behavior of fluoxetine in natural soils. Laboratory studies were conducted using two types of uncontaminated, natural soils – organic-rich A-horizon soils (organic matter 6-8%) and clay-mineral-rich B-horizon soils (clay mineral content ~20%). These soils were used to conduct batch sorption isotherm and column transport experiments. Fluoxetine was measured using HPLC and LC-MS techniques.

Batch kinetic experiments showed that fluoxetine sorption was a 2^nd order reaction in both soils, but B-horizon soil had a slightly higher reaction rate than the A-horizon soil. In both soils fluoxetine sorption reached equilibrium in 72 hours. Sorption isotherms measured in both soils were nonlinear and were fit using Freundlich and Langmuir sorption models. The data showed fluoxetine sorbed more strongly to the clay-rich soils than the organic-rich soils. This implied that positively-charged fluoxetine molecule preferentially sorbed with negatively-charged clay minerals that are abundant in B-horizon soils. Transport behavior of fluoxetine was measured in glass chromatography columns packed homogenously with the soil. Breakthrough curves indicate that fluoxetine was strongly retarded in clay-rich soils as was predicted from the sorption isotherms. Overall, the experiments confirm that fluoxetine reacts more strongly with clay minerals in soils. These results have strong implications for environmental management of pharmaceutical chemicals effluents and discharges.