Solving the Chemical-Dirt Dilemma
This past summer, I worked in Bowdoin College Professor Dharni Vasudevan’s lab (funded by an NSF grant), researching soils interactions with chemicals. One of my favorite things about the research is its applicability and importance. While our research isn’t the easiest to explain to a non-scientist audience, I find the practicality of the research to be very easy to explain, which I believe is a testament to its importance.
Modern society uses a lot of chemicals in our day to day life, and this has resulted in a lot of these chemicals escaping into the environment, where more and more are being detected . While it’s easy to think about these chemicals simply as the pesticides sprayed on crops, chemicals like the hormones given to livestock or everyday drugs like ibuprofen are largely not metabolized or filtered out in waste treatment plants, making them potentially just as problematic in the environment and thus relevant to our research. With these pollutants growing increasingly prevalent in the environment, understanding the risk of exposure to these chemicals is critical to maintaining human and ecological health. Unfortunately, we don’t know a whole lot about what happens to these chemicals or where they end up once they’re released into the environment. With over 83,000 registered chemicals in the United States, predictive models are needed to expedite the process of determining the environmental fate of a chemical . In the Vasudevan lab, we’re attempting to build a model to predict the interaction between a chemical in water and soil it comes into contact with, based on the structure of the chemical. This interaction is best described as the “sticking” and “un-sticking” of the chemical to the soil, and is called sorption. Sorption can filter contaminants out of water, but a contaminant in soil may degrade slower in soil than in water, and thus remain in the environment longer. Regardless, understanding the sorption of a contaminant is an important component of understanding the human and ecological effects of the contaminant.
Thus far, the my research has focused on the interactions between soil and cationic pyridines. While “cationic pyridines” may sound intimidating, they’re relatively simple compounds compared to most of the chemicals whose sorption we’re trying to predict. Cationic pyridines are comprised of carbon, hydrogen, and nitrogen, and contain a permanent positive charge in typical environmental conditions. The EPA already has a model to predict the sorption of neutral (no charge) compounds, but no such model exists for charged organic (carbon-based) compounds. We’re aiming to eventually be able to predict how complex compounds that contain a pyridine substructure sorb, using cationic pyridines as a building block for our model.
My personal research has been very focused on dirt! While dirt may look simple, soils are actually very complex. Soil is composed largely of sand, metal oxides, organic matter, and aluminosilicate clays. While it would be easy to assume our model is universal across soils, soils can vary significantly, and thus our model needs to account for these differences should they prove relevant to sorption. For example, the red soils found in the American Southeast look nothing like the sandy soil around Bowdoin College in Midcoast Maine. These soils have different compositions, and may interact differently with, say, loratidine (otherwise known as Claritin) in water.
Because my research has focused entirely on positively charged compounds thus far, I isolate the aluminosilicate clay components of soil. Aluminosilicate clays possess a mineral structure that allows the clay to carry a negative charge. Our compounds of interest, which possess a positive charge, can bind to these negatively charged clay sites, and thus bind to the soil. These charge sites are usually satisfied by naturally occurring cations-which are often things you’ve probably heard of like sodium and potassium. When a positively charged contaminant comes into contact with the receptor sites on a clay molecule, it swaps out with the naturally occurring cation (be it sodium, potassium, etc.) through a process called cation exchange. My portion of this research is focused entirely on predicting sorption due to cation exchange, as most positively charged compounds, including our cationic pyridines, will attach to soils primarily through cation exchange . Cation exchange can vary with the composition and structure of the clay, and thus my goal for this research is to determine how our predictive model can best account for these differences.
My research uses a high-performance liquid chromatography (HPLC) instrument to test the sorption of these compounds using specially modified HPLC columns packed with clay and non-sorbing material. Sorption between the chemical and the clay occurs while the chemical flows through the column, and the HPLC reading, which gives the time it takes the chemical to flow through, can be used to indicate interactions between the chemical and the clay that we use to calculate sorption. Thus far, I’ve been finding that different concentrations of pyridines often result in different amounts of sorption. These results are unexpected and may suggest that secondary interactions beyond cation exchange occur due to the dislocation of charge in the structure of pyridines.
Author Bio: Sam Shaheen is a Junior at Bowdoin College. Sam is an Earth & Oceanographic Science major and a Chemistry minor. Beyond science, Sam’s interests include cross country skiing, record collecting, and exploring of any kind.
 Kolpin, D. W.; Furlong, E. T.; Meyer, M. T.; Thurman, E. M.; Zaugg, S. D.; Barber, L. B.; Buxton, H. T., Pharmaceuticals, hormones, and other organic wastewater contaminants in US streams, 1999-2000: A national reconnaissance. Environmental science & technology 2002, 36 (6), 1202-1211.
 Toxic Substances Control Act, non-confidential TSCA inventory, 2015.
 Schwarzenbach, Rene P., Philip M. Gschwend, and Dieter M. Imboden. Environmental Organic Chemistry. 2nd ed. Hoboken: John Wiley & Sons, 2003. Print.