GEOCHEMICAL SORPTION AND TRANSPORT OF SILDENAFIL AND VARDENAFIL IN NATURAL SOILS

The presence of pharmaceutical drugs in our water resources is becoming of increasing concern to our ecosystems. In this study, we focus on the geochemical behavior of sildenafil (SDF) and vardenafil (VDF), drugs prescribed to treat erectile dysfunction. SDF and VDF are complex polar organic molecules with good water solubility (3.5 to 110 mg/L) and multiple amine functional groups, which give them both acid-base functionality. The pKa’s of these molecules range from 4.7 to 6.2. Our main goal is to examine the sorption and transport behavior of SDF and VDF in natural organic matter- (OM) and clay-rich soils, both individually and together. Soils used for this study were collected from undisturbed forested areas in Francis Marion National Forest, Charleston, SC and characterized for physical and chemical properties. A series of fixed- and variable-pH batch sorption isotherm and column transport experiments were conducted in these soils. SDF and VDF were analyzed using a high-performance liquid chromatography (HPLC).

Preliminary batch sorption experiments with SDF and VDF produced nonlinear trends for both OM- and clay-rich soil types. Though, both compounds sorbed more strongly to the clay-rich soils than to the OM-rich soils. This suggests that SDF and VDF were positively-charged and preferentially sorbed with the negatively-charged clay minerals (e.g. kaolinites and montmorillonites). The transport behavior of SDF and VDF as determined by experiments with soil-packed glass chromatography columns confirmed this behavior. The resulting breakthrough curves show that SDF and VDF are strongly retarded in clay-rich soils. We are investigating if SDF and VDF compete to sorb with same sorption sites and, if that is the case, whether the sorption of each compound would be reduced compared to sorption occurring when only one compound is present in the soil. Further, empirical modeling based on multi-sorption site models will help determine relative distribution of OM- and clay mineral-sorption sites that control the geochemical fate of these compounds. The results from this study have strong implications for the environmental management of pharmaceutical chemical effluents and their disposal.