Episode 6: Can Soil Bacteria Clean Up Our Toxic Messes?

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Thousands of sites across the US are contaminated with chemical solvents that have been used for decades in industrial processes. These solvents can leach into groundwater and create plumes up to several miles long. 1,4-dioxane, a probable human carcinogen, is often present in groundwater contaminant plumes because of its historical use in degreasing heavy machinery, but it’s also present in trace amounts in products as varied as laundry detergents, deicing agents, cosmetics, and even in food.

There’s good news and bad news here: The Resource Conservation and Recovery Act, enacted in 1980, established laws for the management and disposal of hazardous wastes, meaning new releases to the environment have diminished considerably. Decontamination of chlorinated solvents often involves pumping groundwater to the surface and removing the contamination through volatilization or adsorption. However, this process is expensive, time- and energy-consuming, and ineffective at removing some chemicals, like water-soluble 1,4-dioxane.

Some jobs require the help of friends. In this case, for Hannah Rolston, a fifth-year PhD student in the Department of Environmental Engineering working with Dr. Lewis Semprini, these friends are soil bacteria that are able to naturally degrade this carcinogen. Bioremediation, or the practice of putting these bacteria to work to degrade contaminants, offers some hope in cases like these. Sometimes they can degrade certain pollutants all by themselves (called natural attenuation), but when you’re dealing with carcinogens in areas with people nearby, you want to use an engineered approach to make sure this process goes as quickly and efficiently as possible.

Hannah explained to us that not all compounds are easily degraded by bacteria, and even though some will consume 1,4-dioxane as food, environmental concentrations are not enough to sustain their growth (though remain harmful to humans). To work around this, she has been using a strategy called cometabolism. This involves adding a different carbon source into the groundwater plume for the microbes to eat–ideally, one that will cause the bacteria to produce enzymes that not only degrade the food source, but the 1,4-dioxane as well. This can be tricky, and not only in an engineering sense: you need to know enough microbial metabolism to be sure they’re not converting the hazardous compound into something even worse.

Using soil samples from two contaminated sites in Colorado and California, Hannah and the Semprini group are using isobutane (yes, the same gas you use for your camp stove) to nourish the native microbial communities so that they produce a type of enzyme called a monooxygenase. She has observed the 1,4-dioxane levels decrease in these enrichments. Preliminary work shows the bacteria convert 1,4-dioxane all the way to carbon dioxide–completely benign compared to what we started with.

Hannah began her undergraduate at Seattle University as an international studies major interested in a career in diplomacy. Feeling her first year of humanities classes provided her a wide breadth of knowledge but didn’t give her applicable skills, she transferred to environmental engineering, where she became interested in groundwater and hazardous waste remediation. After graduation, she worked for the US Army Environmental Command, working with army installations across the country to comply with environmental regulations.. When the spreadsheets and desk work didn’t quite live up to its expectations, she knew it was time to seek out graduate programs where she could put her engineering background and interest in hazardous waste remediation to work.

When she’s not tricking microbes into consuming carcinogenic contaminants, Hannah can be found road biking and doing ceramics at the OSU craft center. She is also involved in the OSU Chemical, Biological, and Environmental Engineering Graduate Student Association and the OMSI Science Communication Fellowship program.

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