Sometimes it’s best to embrace the complexities of environmental science.
That’s one big takeaway from research that sheds light on how manganese oxides break down contaminants in water.
Fourth-year civil and environmental engineering PhD student Jenna Swenson conducted the research under Civil and Environmental Engineering Professor Christy Remucal and Associate Professor Matthew Ginder-Vogel. Swenson is in the Environmental Chemistry and Technology program.
Manganese oxides are naturally occurring minerals that react with organic and inorganic materials, including chemicals that are contaminants in the natural environment. Swenson’s research focused on six contaminants’ reactions with manganese oxides, and how adjusting different variables in the environment—to emulate, for example, a natural water environment or a wastewater system—impacts those interactions.
Through the research, Swenson found that the water’s chemistry—especially the concentration of ions—and a contaminant’s chemical structure play big roles in determining how quickly manganese oxides break down contaminants.
The journal Environmental Science & Technology published Swenson’s research findings in January 2024.
Swenson tested bisphenol A (BPA), triclosan, phenol, 4-chlorophenol, 4-bromophenol, and 4-tert-octylphenol across experiments that emulated an array of different water environments. The research team collected water samples from sources across Minnesota and Wisconsin, including Trout Lake in northern Wisconsin and Lake Wingra in Madison, as well as wastewater effluent from the Madison Metropolitan Sewerage District wastewater treatment plant.
“We scooped up our water, put in our manganese oxide with a buffer to make sure the pH held constant, added the contaminant, and we let it run,” she says. “The cool thing about our paper is that we’re using real-life environmental samples, whereas a lot of research tends to use synthetic organic matter, model compounds, or things that have been tested but aren’t realistic in terms of the environment.”
Chemicals with one aromatic ring—a structure of atoms joined by their covalent bonds—behaved similarly across all the tests in all environments. Those chemicals—phenol, 4-chlorophenol, 4-bromophenol and 4-tert-octylephenol—took longer to degrade in the presence of manganese oxide.
BPA and triclosan, the two remaining compounds, have structures with two aromatic rings. They degraded faster in some tests and more slowly in others.
Sample bottles with brownish manganese oxide. They’re being used in research to study how manganese oxide affects the breakdown of various contaminants in water samples collected from different sources. Credit: Joel Hallberg.
Across all tests, Swenson’s experiments showed that wastewater dramatically reduced the rate at which manganese oxide broke down the contaminants; the researchers found those reactions slowed down due to the high presence of ions in wastewater.
However, the researchers have yet to discover what drives manganese oxide to break down contaminants more quickly.
“Ideally, we’d like to see these reactions happen faster,” Swenson says. “If we could find a way to recreate those conditions in the experiments where they did speed up, then we could make our contaminant removal processes really efficient.”
Swenson says it was surprising to see the chemicals’ behavior in reactions split into groups by their number of aromatic rings. The divide’s significance remains to be seen—but it has prompted another, undergraduate-led project to test dozens more chemicals in similar conditions to gather more data.
Importantly, the testing also demonstrates the value of using real-world materials to model real-world reactions. Manganese oxides are already used for water treatment, where manganese oxide-coated sand can be used as a filter. Understanding how a real water environment affects the reactions that happen in those systems means we may one day be able to make them more effective and efficient.
Featured image caption: Civil and environmental engineering PhD student Jenna Swenson checks bottles with samples of manganese oxide in them. Photos by Joel Hallberg.