Bacteria, glass, and cement could be long-term Giant Mine solutions

Researchers have identified naturally occurring bacteria at Giant Mine that could prove useful in permanently dealing with the site’s toxic legacy.

Giant Mine operated on the outskirts of Yellowknife from 1948 to 2004. The former gold mine now sits on 237,000 tonnes of arsenic trioxide – a highly toxic dust stored in underground chambers.

Arsenic trioxide dissolves easily in water, but it’s possible to convert the substance into a mineral known as arsenic sulfide that is up to 10,000 times less soluble, potentially providing a safer means of long-term storage.

A key ingredient in the formation of the arsenic-containing mineral is sulfide. Last summer, a team of scientists found microbes that can produce sulfide in samples of bacteria living in the mine’s polluted environment.

“These bacteria naturally produce chemicals that combine with dissolved arsenic to create arsenic-trapping minerals,” said Carol Ptacek, a professor at the University of Waterloo, who is involved in a project exploring whether naturally occurring bacteria at the mine might play a role in cleaning it up.

The project was one of seven discussed at a meeting hosted last week by the Giant Mine Oversight Board (GMOB), an independent body that serves as a watchdog for the mine’s ongoing multi-billion-dollar remediation.

Since 2014, GMOB has also been tasked with supporting research to find a long-term strategy for dealing with the arsenic dust stored underground.

For now, scientists have concluded the best approach is to freeze the arsenic in place. Part of the remediation plan involves installing hundreds of thermosyphons that cool the rock around arsenic-containing chambers, turning water to ice before it can reach the arsenic inside.

But the “freeze plan” was never considered a lasting solution.

Nine years into GMOB’s mission to come up with a more permanent plan, the board has shared preliminary results of the ongoing research.

“We’re very excited to have the first really public session where we share some information,” Marc Lange, a member of the oversight board, said at the event, which fell on the same night as the territorial election.

“The ideas that we’re testing here, and that we’ll be presenting today, they’re still very early-stage,” he said. “But at the same time, the results are actually very promising.”

Where the bacteria come in

Two different approaches are being researched to stabilize the arsenic at Giant.

The first approach tries to stop the arsenic trioxide reaching the outside environment. The second tries to transform it into something safer.

In the second category are two projects that look at turning arsenic trioxide into arsenic sulfide – the more stable, mineral form.

The arsenic at Giant actually originated in a sulfide form before it was mined and transformed by the ore roasting process, said Tom Al, a professor at the University of Ottawa. 

“If you put it back underground in the sulfide form, then it should be stable for a very long time,” he said at the event.

To undertake this chemical transformation, the arsenic dust would have to be extracted from underground chambers and processed in a facility at the surface. The resulting mineral could then be stored deep underground.

“We’re a long way from that,” Al said.

As a start, he and his colleagues have been trying to figure out how to efficiently perform the chemical conversion in the lab. The process involves dissolving dust samples from Giant in 200C water, then treating the solution with hydrogen sulfide.

So far, Al and his colleagues have found that most of the arsenic trioxide can be dissolved in hot water. After the treatment, a residue is left behind that largely contains a different form of arsenic, which Al said his team is further investigating.

A major drawback is that the approach requires hydrogen sulfide, an industrial gas that smells like rotten eggs and is toxic at high concentrations.

One option would be to truck in the chemical. Another option would be to produce it using bacteria found at Giant.

That’s where Ptacek’s project comes in.

After finding bacteria that can turn sulfate in contaminated mine water into sulfide, Ptacek and her colleagues started growing the bacteria in a lab. They are trying to find the bacteria that are most effective at producing hydrogen sulfide in high arsenic conditions.

They’ve also been feeding the bacteria with locally sourced food waste, including a beer-brewing byproduct from Yellowknife’s NWT Brewing Co, and municipal compost. The goal is to produce sulfide with locally available materials, Ptacek said, adding that producing the chemical must be done safely.

Ptacek and her colleagues suspect a second group of bacteria may also be present at Giant Mine that could help with the formation of arsenic-trapping minerals.

“These bacteria are able to eat and breathe the arsenic and use it for energy,” Ptacek said, converting arsenic into arsenic sulfide in the process.

She and her colleagues intend to search their samples for this type of bacteria as their research continues.

Also options: cement and glass

Another possibility involves mixing arsenic dust with cement to create a paste that can be stored underground.

A similar approach is often used in mine remediation, according to Nicholas Beier, associate professor at the University of Alberta. Typically, cement is mixed with tailings to form a paste that can be used to backfill voids, providing both structural support and long-term contaminant storage.

While the technique is widely used, arsenic is not usually added to the paste.  

“We’re not sure how it’s going to behave,” Beier said at the meeting.

To find the best formulation, Beier and his colleagues have been testing hundreds of different arsenic-containing mixtures and testing their strengths.  

“We found what we think is an ideal recipe,” he said.

He and his colleagues are now examining how the paste stands up to the extreme temperatures the material would encounter at Giant Mine, which could weaken the paste and its ability to hold on to arsenic.

Yet another approach for physically trapping the arsenic is turning it into glass.

Researchers from the University of Waterloo have been working with Dundee Sustainable Technologies (DST), a Montreal-based company that developed a method for locking arsenic waste into glass.

The process, known as vitrification, involves heating materials that act like glue alongside industrial waste in a high-temperature furnace.

The technique has been used at a demonstration plant in Namibia. The company is working on a full-scale plant in Ghana.

For this research, DST made three types of arsenic-containing glass out of samples taken from Giant Mine, as the CBC previously reported.

To assess long-term safety, researchers first examined the samples under a microscope.

The vitrification process appeared to shift the arsenic from a more toxic form to a less toxic form, said Alana Ou Wang, a postdoctoral fellow at the University of Waterloo, who is involved in the research.

Next, Wang and her colleagues placed the glass in eight different conditions and examined the amount of arsenic released. Conditions ranged from mild ­(such as water or a slightly saline solution) to extreme (placing the glass in a strong acid, pressurizing the vessel and then microwaving it).

Arsenic concentrations released under the milder conditions fall below the current limit used at Giant Mine’s wastewater treatment plant, preliminary results show. Even under the most extreme conditions, the concentration of arsenic released was 75 percent lower than the maximum concentration observed in underground mine water, Wang said.  

Although the results have yet to be published in a peer-reviewed journal, Wang said they suggest vitrification can produce similar outcomes to the water treatment system already in place at Giant.

Wang said a final storage location for the glass would have to be carefully determined. With the current formulation, the glass is roughly four times larger than the arsenic itself, according to David Blowes, a professor at the University of Waterloo who is also involved in the work.

The volume of arsenic stored at the site is already massive. According to a 2017 report, it would fill downtown Yellowknife’s 11-storey Precambrian building seven times.

GMOB’s Lange said questions about what implementing any of these possible solutions might entail – from storage to costing to transportation or manufacturing – have yet to be answered.

It’s also too early to say which method is the most promising, according to Lange.

“The amount of change since the labs reopened post-Covid, from one year to another, is mind-boggling,” he said of the research projects. Even a year ago, he said, each project was at a very different stage.

“Waiting for this to be completed will really change the picture.”

Lange added it’s possible solutions could be combined, with one approach cleaning the site to a certain level, then another approach cleaning it to the next level.

Nonetheless, he said the research is “moving forward in a very promising way,” adding it will likely only be a matter of years before the board is able to provide recommendations on a permanent solution.