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Plants absorb metals from tailings waste

Metals contaminating the soils surrounding mine sites can be slowly removed by the root systems of plants, a process known as phytoremediation.

West Kentucky University biology professor Shivendra Sahi spoke about the subject Oct. 24 at the Mining and the Environment International Conference held at Laurentian University in Sudbury.

He has discovered that a five-foot high bushy plant called sesbania drummondii (rattlebush) is especially good at extracting metals such as lead from mine tailings.

The metals bind themselves to the plant’s cell walls, and there’s also evidence they get into the plant’s vacuoles.
Sahi’s work was done in a laboratory using lead-saturated solutions, but he hopes to plant sesbania on mine sites some day.

“It is a genetic ability. This plant was collected from a contaminated site, so it has developed the genetic mechanism to grow (in contaminated areas),” said Sahi, who earned a Master’s degree in biology from Laurentian University in 1983.

“It has the ability to put the metals somewhere that does not interfere with the plant cells’ metabolism. Most plants will die if they are exposed to metals.”

Phytoremediation has really caught on in Europe, but still needs to gain popularity in North America, he said.
There are a few mining service companies offering phytoremediation technology in the United States, but Sahi is not aware of any in Canada.

Depending on the type of metals in the soil, it may be worthwhile to harvest the plants and extract the metals accumulated in their cells, he said.

“Let’s say you are working on gold tailings. There is low-quality gold and it’s not economically feasible for mining companies to get that amount of gold. This plant can do that work because it’s cheaper technology. You can burn the plant and get that gold.”

The sesbania plants used in the professor’s research were collected in the southern part of the United States, but they grow as far north as Colorado. Sahi is not sure if the plants would survive in Northern Ontario.

The mining industry should look more seriously at using phytoremediation on old mine sites, he said. It costs two cents to $1 to remediate a cubic metre of land using this process, as opposed to $10 to $1,000 per cubic metre using more traditional methods like excavation and soil flushing, Sahi said.

Phytoremediation is only good for extracting shallow contamination at the 30 to 60-centimetre level because plant roots can only reach so far.

It would take several crops of the plants and five to 10 years to remove all of the metals from the soil, he said.

“I think the mining industry should be using phytoremediation. Look at it this way. We took the time to vegetate the area (Sudbury), and now it looks a lot better. It is covered with vegetation, but the toxic stuff is still there.”

CVRD trial

CVRD Inco grounds manager Mike Peters, who attended Sahi’s presentation, said the company has experimented with phytoremediation in Sudbury in the recent past.

In 2004 and 2006, a Houston, Texas company called Veridian Resources got CVRD Inco to experiment with growing a flowering plant called alyssum in pots in a greenhouse. The pots contained tailings material.

Some of the plants were left outside during the winter to see if they would survive the cold and snow  with some success.

Peters said it has not been determined whether or not there will be upcoming field trials where alyssum is planted directly on CVRD Inco tailings.

“Basically what we’ve done so far is have two greenhouse trials with associated testing and results after there had been a certain growth period,” he said.

“There was some uptake of metals. The alyssum would grow in our tailings. If we were trying to grow it in some of the tailings, it would require an application of lime. A lot of the plants ended up needing some compost too.”

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