According to research by Staci Simonich, Ph.D., some of the products formed during the breakdown of pollutants called polycyclic aromatic hydrocarbons (PAHs) may be more toxic than the original compounds.
In an NIEHS Keystone Science Lecture Sept. 20, Simonich described her work to predict the formation of these PAH breakdown products, measure their presence, and determine their toxicity.
Engineers use a variety of approaches to break down PAHs in soil at Superfund and other hazardous waste sites. “From a chemistry point of view, PAHs are interesting because they can be metabolized and transformed into new products,” said Simonich, from Oregon State University (OSU). “We are starting to think much more broadly about the structure of PAHs and how that impacts their fate and toxicity.”
Simonich described her research at the OSU Superfund Research Program (SRP) Center(https://tools.niehs.nih.gov/srp/programs/Program_detail.cfm?Project_ID=P42ES0164650106) where she studies the fate of PAHs in soil following common treatment methods like bioremediation, which refers to the use of microbes to break down contaminants.
“Using various chemistry techniques and computational models, we are identifying compounds formed during remediation of soil, including what increased in concentration as well as what was present following remediation that wasn’t there before,” she said. “We work with colleagues to test the toxicity of these compounds and determine whether they may have made the soil more toxic.”
Identifying toxic breakdown products
Simonich and her team identified and measured breakdown products formed when a microorganism metabolized phenanthrene, a common and relatively nontoxic PAH. She collaborated with OSU SRP Center Director Robert Tanguay, Ph.D., to measure the developmental toxicity of these metabolites in zebrafish.
The researchers reported that the metabolite mixtures were more toxic than phenanthrene, and they identified specific metabolites that produced the toxic response. Further studies shed light on the developmental toxicity of PAH breakdown products.
In a soil sample with more complex contamination, they identified PAH metabolites and toxic high-molecular-weight PAHs that remained after treatment. The team also measured samples before and after treatment, and reported that the soil toxicity increased following bioremediation.
“Sometimes we work at the end of a simple system, looking at one compound and one microorganism, to understand the fundamentals. Then we also work at the opposite end, looking at a complex mixture of PAHs in Superfund soil,” Simonich added. “Working at these two ends, we can start to connect the dots.”
Implications for remediation
According to Simonich, if engineers focus on reducing specific PAH compounds during remediation, they may not see the whole picture.
Her findings shed light on the fate of PAHs in realistic conditions and may inform risk assessment, which typically does not consider all PAH components in treated soils. Understanding the breakdown products and which of them are driving toxicity may be the first step toward reducing these harmful compounds.
“The idea is not that bioremediation is bad. But as we learn more about the process, we may be able to improve it,” she explained. “Moving forward, we want to look at different approaches and combinations to see if we can minimize the formation of these toxic byproducts and reduce toxicity.”
Schrlau JE, Kramer AL, Chlebowski A, Truong L, Tanguay RL, Simonich SLM, Semprini L. 2017. Formation of developmentally toxic phenanthrene metabolite mixtures by Mycobacterium sp. ELW1. Environ Sci Technol 51(15):8569–8578.
Chlebowski AC, Garcia GR, La Du JK, Bisson WH, Truong L, Massey Simonich SL, Tanguay RL. 2017. Mechanistic investigations into the developmental toxicity of nitrated and heterocyclic PAHs. Toxicol Sci 157(1):246–259.
Chibwe L, Davie-Martin CL, Aitken MD, Hoh E, Massey Simonich SL. 2017. Identification of polar transformation products and high molecular weight polycyclic aromatic hydrocarbons (PAHs) in contaminated soil following bioremediation. Sci Total Environ 599-600:1099–1107.
(Sara Amolegbe is a research and communication specialist for MDB Inc., a contractor for the NIEHS Division of Extramural Research and Training.)