Pharmaceuticals on the Soil, Oh My!
I spend my time in lab mixing up different pharmaceuticals in water, shaking up that pharmaceutical/water solution with soil and then using a complicated instrument called the HPLC (high performance liquid chromatography instrument) to see how much of each pharmaceutical stays in the water and how much latches onto the soil (through a process called sorption). Our lab focuses on the movement of pharmaceuticals in the environment because, frankly, there are lots of pharmaceuticals floating around out there. Studies show that a large number of pharmaceuticals have been detected at measurable (μg/L and ng/L) concentrations in ground water and drinking water worldwide.1-9 Pharmaceuticals typically enter the environment through animal manure containing pharmaceuticals or excretions from the human body after ingestion of medication. Some examples of these pharmaceuticals, specifically the negatively charged ones which I focus on in my research, can be found in Figure 1. We’re interested in studying the movement of pharmaceuticals once they enter the environment because understanding this movement is key to understanding the risks that these compounds pose to human and environmental health.
Figure 1. Examples of negatively charged pharmaceuticals that have been detected in natural waters.
These compounds are present in our natural waters like streams and lakes and they tend to stick onto soil particles in the environment. Soil particles are made of three primary components (organic matter, aluminosilicate clays and surficial iron and aluminum) relevant to sorption (the movement of the pharmaceutical from water to soil). These three soil components provide five receptor sites relevant to sorption and of those, two are relevant to the sorption of the negatively charged pharmaceuticals I focus on: negatively charged sites with positively charged molecules adjacent to them and iron or aluminum on the soil’s surface.10
Onto these receptor sites, pharmaceuticals can attach or sorb via different mechanisms based upon the chemical structure of the pharmaceutical. Negatively charged pharmaceuticals can sorb via two mechanisms: cation bridging and surface complexation. In cation bridging, the pharmaceuticals interact with positively charged sites present on the soil, whereas in surface complexation, the pharmaceuticals interact with iron and aluminum on the soil’s surface. Which mechanism predominates depends upon the soil’s makeup and the pharmaceutical’s structure. Ideally, we would be able to predict sorption via these mechanisms, because we would like to know how these pharmaceuticals are moving in the environment. However, at this point in time, there is not an established predictive model for the sorption of negatively charged pharmaceuticals. Our lab is working to find one!
It was previously hypothesized that to predict the sorption of negatively charged compounds, sorption via cation bridging and surface complexation could be quantified by probe compounds. Probe compounds give us a measure of how many soil receptor sites are available to bind negatively charged pharmaceuticals for either cation bridging or surface complexation.10 Scaling factors could then be used to move from small and simple probe compounds to larger and more complex pharmaceuticals. With the addition of other predictive factors, the sorption of negatively charged compounds can be predicted.
In my specific research, I’m working to find and validate the use of small and simple probe compounds to capture baseline driving forces and receptor site densities for sorption by cation bridging and surface complexation. Building on the results found by a previous member of my lab, I have confirmed the use of salicylic acid as a potential probe for compounds that sorb primarily via surface complexation and hydratropic acid as a potential probe for compounds that sorb primarily via cation bridging (Figure 2). From here, I plan to test the predictive capacity of salicylic acid and hydratropic acid for larger and more complex pharmaceuticals. This will ultimately contribute to the predictive model for the sorption of negatively charged pharmaceuticals and will help us better predict human and environmental exposure to those pharmaceuticals.
Figure 2. Proposed probes for compounds that sorb primarily via surface complexation (salicylic acid) and cation bridging (hydratropic acid).11
Author Bio: Leah is a senior at Bowdoin College who enjoys walking dogs, splashing in puddles and jumping on trampolines. You will likely find Leah in the Craft Center, exploring the Bowdoin Commons or babysitting her professors’ children. A Chemistry and Sociology major, Leah hopes to someday attend medical school and eventually become a pediatrician. She would also like to have a dog and an unlimited supply of toffee. The project described in this piece was completed as part of Honors research with Prof. Dharni Vasudevan at Bowdoin College and is funded by the Hughes Family Fellowship, the James Stacy Coles Fellowship, and the National Science Foundation.
- Heberer, T., Occurrence, fate and removal of pharmaceutical residues in the aquatic environment: a review of recent research data. Toxicology Letter 2002, 131 (1-2), 5-17.
- Lishman, L., et al., Occurence and Reductions of Pharmaceuticals and Personal Care Products and Estrogens by Municipal Wastewater Treatment Plants in Ontario, Canada. Science of the Total Environment 2006, 367 (2), 554-558.
- Ternes, T. A., Occurrence of Drugs in German Sewage Treatment Plants and Rivers. Water Research 1998, 32 (11), 3245-3260.
- Blair, B. D., et al., Pharmaceuticals and personal care products found in the Great Lakes above concentrations of environmental concern. Chemosphere 2013, 93 (9), 2116-2123.
- Figueroa-Diva, R. A., et al., Trends in Soil Sorption Coefficients Within Common Antimicrobial Families. Chemosphere 2010, 79 (8), 786-793.
- Kodešová, R., et al., Pharmaceuticals’ sorptions relative to properties of thirteen different soils. Science of the Total Environment 2015, 511, 435-443.
- Li, W. C., et al., Occurrence, sources and fate of pharmaceuticals in aquatic environment and soil. Environmental Pollution 2014, 187, 193-201.
- Taylor, D.; Senac, T., Human pharmaceutical products – The “problem” in perspective. Chemosphere 2014, 115, 95-99.
- Tolls, J., Sorption of Veterinary Pharmaceuticals in Soil: A Review. Environmental Science & Technology 2001, 35 (17), 3397-3406.
- MacKay, A. A.; Vasudevan, D., Polyfunctional Ionogenic Compound Sorption: Challenges and New Approaches to Advance Predictive Models. Environmental Science & Technology 2012, 46 (17), 9209-9223.
- Lopez, A., Sorption of Polyfunctional Ionogenic Organic Compounds to Soils via Surface Complexation and Cation Bridging: Evaluation of Probe Compounds. Honors Thesis for the Bowdoin Department of Chemistry 2016.