After a big rainstorm has sent water rushing down the street, you may have noticed the oily sheen left behind. Unfortunately, this everyday scene—caused by contaminants such as metals and fossil fuels that collect on road surfaces—is a major source of water pollution in Minnesota.
But new research from the University of Minnesota Duluth’s Natural Resources Research Institute (NRRI) is exploring how a common waste product could act as a “super-sponge” to absorb pollutants from stormwater, helping to protect the state’s lakes and rivers.
The current solution to this problem is roadway bioswales and basins, which use vegetation to slow down the water flow and filter out pollutants, says Brian Barry, NRRI’s chemistry and materials science program leader and the project’s principal investigator. “While these current designs are certainly helpful, we hope to make them even better, more effective, and longer lasting with the help of a new ingredient.”
This new ingredient is biochar—a charcoal-like substance made by heating biomass, such as wood waste, in a low-oxygen environment. Because it’s extremely porous, biochar acts as a super-absorbent sponge to contain pollutants. In addition, it improves water infiltration into the engineered soils. For this project, researchers are investigating how adding biochar to bioswales could improve their performance.
This first phase of the research aimed to determine what makes the best spongy biochar for this specific job. The team discovered that the production process is crucial; the temperature used to make the biochar should be at least 675 degrees Celsius (about 1,247 Fahrenheit) to form the best-performing structure for trapping contaminants, especially hydrocarbons such as motor oil. The researchers also found that the most effective biochar must have a “clean” surface, devoid of oily residue and containing plenty of vascular pores to trap contaminants.
To help assess biochar’s quality, the team recommended several testing methods, making sure to include some simple and affordable options. “For example, we found that smelling biochar to ensure there was no ‘campfire’ scent correlated to a clean surface, as determined in a more onerous test,” Barry says. “And viewing the biochar’s surface through a powerful scanning electron microscope allowed us to see if the right sizes of vascular pores for water uptake were present.”
A second key finding of the project was that the best material for this type of biochar could be sourced locally from woody biomass—such as sawmill waste or wood from pest-infested trees. Researchers found that woody material naturally creates more of the right kind of vascular pores needed to capture pollutants compared to other feedstocks such as grasses or agricultural waste. This finding is important for both environmental and economic reasons: it eliminates the cost of shipping in biochar source material from afar while putting a waste product to good use.
“From this project phase alone, we likely have enough information to develop standard specifications for using biochar for stormwater management. Field tests will illustrate its multiple benefits even more,” notes Dwayne Stenlund, erosion control specialist with MnDOT’s Office of Environmental Stewardship.
The work in the project’s next phase will move from the lab to the field, as researchers test biochar-enhanced soils in real-world conditions. “Our hope is that this research will turn a local, abundant waste resource into a powerful tool for environmental protection,” Barry says. “This could lead to a more effective, durable, and cost-efficient way to keep Minnesota’s lakes and rivers clean all while sequestering carbon through biochar production.”
—Megan Tsai, contributing writer
