BINDING OF NAPROXEN AND IBUPROFEN IN ORGANIC- AND CLAY-RICH SOILS

Pharmaceutical compounds, several of which are known endocrine disruptors in mammals and aquatic life, are some of the most ubiquitous chemicals found in natural surface waters across the world. However, it is not very clear as to how these chemicals behave in soil materials and how they may migrate into groundwater systems. The nature of the physical and chemical interactions between soils and these pharmaceutical compounds, therefore, needs to be understood. In this project, we studied sorption of two pharmaceutical chemicals, ibuprofen and naproxen, in several soils that have different physical and chemical properties (e.g., soil pH, organic content, particle size, etc.). Ibuprofen and naproxen are both aromatic organic molecules that have a carboxylic acid functional group which ionizes as a function of solution pH. Clean soil samples were taken from two different locations within the Francis Marion National Forest near Charleston, SC, a site dominated by lowland watersheds and wetlands. These soil samples were systematically spiked with varying concentrations (10^-3-10 mg/L) of ibuprofen and naproxen in aqueous suspensions. Using batch sorption studies, sorption isotherms were created for these chemicals in the soil samples. A sorption isotherm quantitatively describes sorption of chemicals onto solid surfaces resulting in compound-specific sorption parameters such as distribution coefficients. Our experiments concluded that both chemicals more readily sorbed to organic-rich soil and less to the lower organic, relatively clay-rich soil. It is hypothesized that the ionized forms of the chemical have the ability to interact with the positively charged sites on soil organic matter (SOM) causing sorption. Naproxen sorbs more strongly than ibuprofen to all soil samples due to a lower octanol water partition coefficient, which describes their polarity. Naproxen has a lower partition coefficient of the two and hence is more polar than ibuprofen. The higher polarity of naproxen also likely causes it to create more hydrogen bonds with the SOM than the ibuprofen. This indicates that in areas of organic-rich soil one can expect naproxen and ibuprofen to accumulate, whereas in soils with lower organic content, the chemicals are not expected to be retained in soil environments.