Selenium isotopes are a good indicator of cleaning efforts

A new testing technique developed using synchrotron light could significantly improve how we monitor the effectiveness of remediation practices aimed at removing selenium contamination from mining operations.

Selenium is a natural nutrient that humans and animals need, in small amounts, to stay healthy. However, exposure to higher concentrations can cause neurological problems in humans as well as death and sterility in wildlife and livestock.

Mining can cause selenium and other substances to runoff into nearby soils and water bodies, and accumulate over time, even if mitigation strategies such as constructed wetlands or selenium-eliminating bacteria are in place.

“We need mining to extract certain resources from the ground,” says Heather Shrimpton, a postdoctoral fellow at the University of Waterloo (Department of Earth and Environmental Sciences). “We can’t yet rely 100% on recycled materials, so it’s important that we have techniques that can reduce the impacts of mining on people and the environment, and my technique can help with that.”

Until now, there has been no way to determine whether selenium is likely to dissipate permanently as a result of remediation efforts or whether it is absorbed into nearby streams or river banks.

Shrimpton and his colleagues discovered that isotopes of selenium (which are the same element as selenium but have different atomic masses) can be used to determine what removes this contaminant from water. Changes in isotopes indicate whether selenium is eliminated and whether the elimination is permanent. Shrimpton’s study is published in the journal Environmental science and technology.

“We need a technique like mine to check whether cleaning systems are working. It’s about testing whether or not we need to do better,” Shrimpton said.

In the lab, Shrimpton and his team replicated a well-known remediation strategy called reduction, which uses sulfur-reducing bacteria to trap selenium in solid form. In nature, reduction causes selenium to adhere to gravel and sand found in bodies of water.

Using the Canadian Light Source (CLS) at the University of Saskatchewan (USask), Shrimpton analyzed the isotopes of these solid selenium samples. She found that adding sulfur in specific amounts to selenium prevents the contaminant from mixing with liquids again, meaning the water removal can be permanent. The magnitude of the change in isotopes, she said, confirmed that it was the reduction process alone that was responsible for the change.

“The Canadian Light Source allowed me to gather additional information at the molecular level. So I knew what was happening and I could say, ‘That’s it, that’s what did it’,” Shrimpton said. “It’s a piece to solve the puzzle.”

Now that the technique has proven effective in the laboratory, Shrimpton and his team plan to test it at mining sites and expand their study to include other mining environmental pollutants such as mercury.