Between 2023 and 2025, more than 30 million hectares burned in Canada due to wildfires. The threat from increasingly frequent and intense wildfires goes beyond fire and smoke - the heat can also transform naturally occurring metals in soil into more toxic forms that could pose a threat to human health.
Researcher Derek Peak and colleagues used the powerful X-ray beamlines at the Canadian Light Source (CLS) at the University of Saskatchewan (USask) to study how chromium can change into a toxic form, and how it interacts with iron in soils during fires. The interaction with iron is important because the two elements “tend to travel in the same circles” in nature, says Peak, a soil chemist in USask’s Soil Science department.
“This project was looking at fundamental mechanisms under controlled conditions so that we can start to predict or model the process better,” he said. The CLS’s SXRMB beamline was vital to this work, Peak says, because it is difficult to study the oxidation states of iron and chromium without ultrabright synchrotron light.
The researchers heated both lab minerals and soils naturally rich in chromium and iron to see how they were transformed by the heat. In both cases benign chromium-3 turned into toxic chromium-6 at temperatures up to around 600oC -- but then switched back into the safe form as the temperature rose to 800oC. Wildfires typically burn at temperatures ranging from 800°C and 1200°C. The team’s findings, which show iron in soil controls when and how chromium becomes toxic during fires, are published in the journal Environmental Science and Technology.
“Fires, depending on their temperature and severity, can oxidize and create chromium-6, but [the chromium] can also undergo a variety of reactions to re-reduce and return back into soils,” said Peak. “Where it ends up and how toxic it is is going to be site- or fire-specific, and this research shows it couples with iron oxidation.”
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