Mining site

The Tao of Sulfate Reducers

Kendra Zamzow
Jul 28, 2013

This place stunk to high heaven, and I loved it.  The rotten egg smell of hydrogen sulfide meant the sulfate-reducing bacteria (SRB) in our remediation system were happy, happy, happy. 

SRB are part of the solution and part of the problem in mine waters.  They are a quintessential encapsulation of yin and yang: opposing, balanced forces. 

The old Leviathan copper and sulfur mine is located up in the pines of the high Sierras in California.  No electricity, no buildings. Some combination of truck, four-wheel ATV, and snow machine were needed just to get there every week.  What it did have was acid drainage: sulfuric acid with iron, aluminum, copper, nickel and zinc running out of the mine site and into two streams. 

The fish started dying in the 1950s when the mine transitioned from an underground mine to an open-pit mine.  The mine close in 1962, but the streams were still devoid of fish when the Washoe tribe asked for remediation in 1997.

SRB were a remediation option. If you give them sulfate,  which is part of sulfuric acid, they’ll reduce it to a respiratory waste product:  sulfide.  Sulfide is exactly what metals like iron, copper, and zinc love to bind to, forming fine particulate metal sulfides.  The particulates sink, leaving clean water for fish; the precipitate is dredged for disposal.

The University of Nevada Reno, where I was a graduate student, created a unique “semi-passive” bioreactor tailored specifically to SRB.  A series of ponds was carved into the mountainside: two rocky ponds for the bacteria, and two where precipitated metal sulfides accumulated.  A solar-powered system dripped an alkaline solution to raise stream pH from about 3.5 to 5.5, and dripped in a liquid carbon source.  SRB feed on “small” carbons like ethanol.

At the foot of the mountain was a farm, and on that farm they made their own biodiesel for their tractors.  The waste product from making biodiesel is mostly methanol with a hint of potassium hydroxide – the perfect SRB food.  The sludge of precipitated metals could be used by the farm as a soil amendment.  Although our “Grand Plan” of trading mine waste for biodiesel waste in a closed recycling loop never materialized, the bioreactor has been working well since 2003.  And fish are returning.

The heroes of the mining remediation story, SRB are the villains in mercury cycling.  SRB convert inorganic mercury into organic methyl-mercury (MeHg) which bioaccumulates in fish and poses neurological risks to fish-consumers. 

At the historic Malakoff Diggin’s hydraulic mining area, near Nevada City, California, mercury was used in gold extraction in the mid-1800s.  That mercury continues to move downstream, collecting in calm pools and potentially depositing in the Sacramento Delta.  Anaerobic sediments and wetlands are natural environments where SRB are found. No doubt SRB have something to do with the many fish advisories.

Ironically, SRB are cleaning the water for fish on one mountain in the Sierras, and increasing the toxicity of fish in a different part of the range.  The dual nature of sulfate reducers is just one of the complexities to consider in mining remediation. 

photo: K. Zamzow

Kendra Zamzow

Kendra Zamzow is an AAAS S&T Policy Fellow, class of 2012, at the EPA ORD Office of Science Policy. Prior to her fellowship, she was a public interest scientist in Alaska providing technical assistance on water quality at proposed, present, or abandoned mines.

Raised in California, she moved to Alaska in 1986 and became a commercial fisherman until 2001. She received her Ph.D. in 2007. Her interests are trace metal chemistry, and communication between local knowledge and the scientific community. When not in Washington DC, she lives in a yurt in the village of Nay’di’nae’ah’na (Chickaloon), Alaska.  Her website is http://www.earthchemistry.co.

Disclaimer

This blog does not necessarily reflect the views of AAAS, its Council, Board of Directors, officers, or members. AAAS is not responsible for the accuracy of this material. AAAS has made this material available as a public service, but this does not constitute endorsement by the association.

Comments (3)

Lynn Adams (not verified)
July 29, 2013 at 11:06 am
It is fascinating that you can use bacteria to clean up the water! About the methyl mercury problem, is the concentration of mercury in the Leviathan mine low enough that you don't have to worry about contaminating the fish that are returning to the site?
Kendra Zamzow (not verified)
July 29, 2013 at 12:03 pm
There is no mercury at the Leviathan site, so methylmercury (MeHg) in fish isn't an issue. But I am curious about the situation when SRB are producing sulfide and there is mercury present. Mercury binds VERY tightly to sulfide (makes HgS), so it would seem like there wouldn't be any ability to bind the methyl groups(s) (-CH3). I am guessing that where there are SRB, there is a mix of HgS and MeHg. I'd be interested in hearing from anyone that has thoughts on that.
Kendra Zamzow (not verified)
July 29, 2013 at 12:05 pm
By the way, these are natural soil bacteria, not "designer organisms". That's one of the cool things about this type of remediation -- you just need to tweak the environment to be a little more of what they like, and the populations will take off.

Leave a comment

CAPTCHA
This question is for testing whether or not you are a human visitor and to prevent automated spam submissions.

Subscribe to our blog