SORPTION AND TRANSPORT OF SILDENAFIL IN NATURAL SOILS

Trace levels of pharmaceuticals enter ecosystems mainly through the discharge of treated wastewater and may have direct effects on the ecological health and water quality of that area. Sildenafil citrate (Viagra) is one such pharmaceutical which has been found in natural streams. Although one study has shown that sildenafil is not harmful in bacterial and fungal environments, its fate elsewhere in ecosystems remains unknown. Sildenafil is a complex, highly water-soluble organic molecule with two amino groups and pKa’s of 7.27 and 5.97. As sildenafils use and its concurrent disposal is on the rise, it is essential that its behavior in the natural environment be better understood. The main objective of this study was to quantify the sorption and transport of sildenafil in natural soils. Pristine A- (organic matter-rich) and B- (clay mineral-rich) horizon soil samples with variable organic matter content (0.6-7.6%), clay content (6-20%), and soil pH (4-5) were used in this study. Batch kinetic experiments showed a fast reaction rate in both clay-rich and organic-rich soils and an equilibration time of less than 24 h. Batch reactor sorption isotherms were nonlinear and were fit using Freundlich model. Sildenafil sorbed more strongly to clay-rich soils than to organic-rich soils. The negative charge on clay mineral surfaces likely form ionic bonds with positively charged amines in sildenafil in acidic pHs (soil solutions were naturally acidic). Transport experiments were conducted using glass chromatography columns, homogenously packed with the soils. Sildenafil was significantly retarded in clay-rich soil columns confirming trends observed in sorption experiments. Overall data indicate stronger sorption of sildenafil to clay-rich soils. These results suggest that natural soil acts as a natural filter for sildenafil, due to strong sorption to specific components present in soil, and may prevent contamination of groundwater. Given the high metabolic rate and long shelf life of sildenafil, further research is needed to better understand how sildenafil’s metabolites behave in ecosystems.