Can sulfide mining be environmentally friendly?

April 26, 2018

 

David Glenn
Zenith News

Earlier this year, the press was abuzz with the “discovery” of a bacterium that eats heavy metals and craps gold nuggets. This too-good-to-be true description belies the true nature of the multifaceted wonder-bug Cupriavidus metallidurans.


Identified decades ago, researchers at Martin Luther University in Germany recently discovered the exact mechanism by which it can precipitate gold through its cell wall. Using specialized enzymes, C. metallidurans can render inert, toxic copper and gold compounds that have accumulated in its cellular interior.


A robust little bugger, it can operate in zero-oxygen, using hydrogen, carbon dioxide fixation and reduction, or inorganic sulfur compounds (copper sulfide) as energy sources. It is one of a handful of extremophiles that can thrive in otherwise toxic environments like heavy metal contaminated soils and mining pits. Its potential use as a replacement for the mercury used in small, artisanal gold mining operations should be of interest to all concerned with maintaining a toxin-free environment.


According to the Environmental Protection Agency, gold mining is the leading source of mercury emissions, followed by coal/fossil fuel combustion, with all other forms of metal and mineral extraction in third place, and then cement production. Elemental mercury is used to form an amalgam with the gold found in various mineral ores. This amalgam is then heated which turns the mercury into a gas, leaving behind pure gold.


Atmospheric mercury becomes a problem when it settles into aquatic environments and is converted by other organisms into methylmercury. This form of mercury (CH₃Hg+) is a bioaccumulative toxicant in that it is slow to be eliminated from the body and tends to concentrate in apex predators, such as people who eat a lot of fish.


In the human body, methylmercury combines with the amino acid cysteine, which is then falsely recognized as the essential amino acid methionine. This camouflage allows it to pass into critical regions of the body such as the brain and the placenta. Methylmercury causes developmental issues in children exposed to it in utero, and a variety of health problems in adults as well.


Thankfully, coal-fired power plants are slowly being replaced by zero-emissions technologies such as wind and solar. Eliminating the mercury in gold production through biomining techniques would go a long way towards reducing another major contributor. Biomining is one facet of the developing field of biohydrometallurgy. Stripped of the fancy name, it is a method of employing microbes and fungi to extract specific elements from mineral resources. Biomining techniques have been used to leach copper, gold, silver, zinc, cobalt, and even uranium from raw ore.


The Escondida mine in Chile employs biomining to recover copper from its lower grade sulfide ores. The ore is first crushed, then heaped into acidic water-filled tanks containing two strains of the metal-reducing microorganism Acidithiobacillus. Iron is added, and the interchange of electrons provides the bacteria with the energy to sustain life and reproduce. During this process, catalytic metabolites oxidize sulfur, separating it from the copper and producing sulfuric acid and copper ions as byproducts. An electric charge is then passed through the solution, concentrating the ions onto the cathode.


This emerging technology is not a panacea. It cannot compete with cheaper and faster ore separation processes such as froth flotation. It does not lend itself well to in situ extraction, as most metal reducing microbes contribute to environmentally unfriendly acid mine drainage.


Biomining does offer substantial energy savings compared to smelting, which currently consumes three to five percent of the world’s energy. Biomining is also more environmentally friendly than agglomeration techniques involving arsenic and other solvents.


A number of these metal-loving proteobacteria are also electrogenic, meaning they produce and respirate electrons.  One such species, Geobacter sulfurreducens, can be found in the sediments of Minnesota wetlands and waterways. Its cousin, G. metallireducens, has been used in the development of electricity-generating biofuel cells.


Researchers at the University of California, Santa Barbara have created a chemical additive called DFSO+ that not only boosts cellular metabolism but also mimics the membrane proteins responsible for electron transfer. This was demonstrated in both the naturally occurring form as well as two deliberately disabled non-electrogenic strains. Facilitating and/or amplifying electron respiration is a next-level advance as it opens up a broad range of microorganisms for use in bioelectrical applications.


This breakthrough is good news for bioremediation projects such as wastewater treatment. First, this technique requires no Frankenstein-like genetic modifications. Changes are not permanent as DFSO+’s effect fades as the individual cells divide and the size of the bacterial colony increases. Second, there is the potential of reclaiming a portion of the energy expended transporting and processing contaminated water through the use of fuel cell technology. Third, it increases the bacteria’s metabolic rate, promoting growth, thus speeding up detoxification.  


The discovery of DFSO+ should make biomining more attractive and profitable for similar reasons. The precious metals in tailings can now produce income.


Outside mining and remediation, DFSO+’s ability to enable electron transport is important to the study of biology in general. Enabling electrical contact will allow researchers to measure and probe the inner workings of a variety of life forms at a sub-cellular level.
 

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