Predicting Cyanide’s Role in Making Mercury More Dangerous

Shannon Plunkett, a PhD student in civil and environmental engineering at Duke, won the “Best Student Presentation” award at the International Conference for Mercury as a Global Pollutant conference held this past summer in Cape Town, South Africa.

Working in the laboratory of Helen Hsu-Kim, professor of civil and environmental engineering at Duke, and Emily Bernhardt, the James B. Duke Distinguished Professor in the Nicholas School of the Environment, Plunkett has spent time studying the use of mercury in small-scale, artisanal gold mining operations at multiple sites throughout South America. Despite how common this practice is and the knowledge that it can create environmental hazards for wildlife and humans alike, not enough is known about what happens to the mercury as it travels downstream from the site of its initial discharge.

Depending on how the mercury interacts with the environment around it, it can be more or less toxic to humans. A particularly toxic form of mercury is called methylmercury, which is produced by microbes that take in inorganic mercury and transform its overall structure.

It is illegal to use mercury and cyanide together in these mining operations because the combination produces strongly bonded, soluble mercury-cyanide complexes that can extend the reach of the mercury contamination downstream. However, Plunkett had a hunch that the most dangerous areas for methylmercury might not be at the initial dumping site where these two elements first come together.

Her research presented in “Fate Transport and Bioavailability of Mercury Downstream of a Gold Mining-Affected River” gives credence to this theory. After testing the concentrations of these chemicals over 175 miles of river past mining operations, she found the greatest concentrations of methylmercury to be 35-90 miles downstream of the mercury and cyanide inputs, further downstream than ever explored in this type of mining.

When cyanide and mercury are first released together, the amount of free cyanide is extremely toxic to local bacteria. As it travels downstream, the free cyanide dissipates, leaving strongly bonded cyanides complexes, like those formed with mercury, in solution and available for microbial uptake.

“A lot is known about how chlorides, sulfates and organic matter affect the bioavailability of mercury, but cyanide hasn’t been explored before,” Plunkett said. “And it seems to be a major controller of the production of methylmercury in these areas, which is a useful thing to know so we can predict which areas will be more at risk.”