Bacteria: Converting toxic metals into useful derivatives

Dr. Neil Canter, Contributing Editor | TLT Tech Beat May 2014

A bacterial strain transforms a suspected pollutant, antimony (V), through anaerobic respiration into antimony (III) oxide which is used in commercial applications. 

KEY CONCEPTS
• Antimony and many of its oxide derivatives are classified as pollutants of priority interest by the EPA and the EU.
• A new bacterium strain designated as MLFW-2 has been identified as converting antimonite to a more usable form known as antimony (III) oxide that is used in commercial applications.
• The antimony (III) oxide produced by the bacterium is superior in quality to material currently produced commercially.

THE MANY SPECIES OF BACTERIA present in our environment have both negative and positive impacts on lubrication systems. Negatively, bacteria can grow in metalworking fluid systems using components of the lubricant as food. The net result is premature fluid degradation and eventual failure and potential health and safety concerns unless bacterial growth is controlled.

On the positive side, bacteria species have been highlighted for unique contributions in specific areas. For example, a previous TLT article discussed research about a bacterium, Mariprofundus ferrooxydans strain PV-1 that is known to oxidize ferrous [Fe (II)] ions (1). In a process known as electrochemical cultivation, this bacterium was able to grow using electrons as the sole energy source. Potentially, the energy used by this organism can be used to reduce carbon dioxide and become a source for manufacturing organic compounds including fuels.

Biologically mediated redox chemistry also can be applied by bacteria in converting poisonous metals to a desired state where a compound is produced that is less hazardous and might even be useful in a commercial application. James Hollibaugh, Franklin College of Arts and Sciences Distinguished Research Professor of Marine Sciences at The University of Georgia in Athens, Ga., says, “Our research in this area began when I was working with scientists at the U.S. Geological Survey in California to determine why birds living near agriculture drainage impoundments in the Central Valley were being poisoned by selenium. We determined that the water soluble and relatively low toxicity oxidized form of the metalloid, selenate, was being reduced by microbial anaerobic respiration to more toxic and more bio-available forms including selenite and elemental selenium.”

Hollibaugh then decided to focus on better understanding other elements with similar redox chemistry from the same region of the Periodic Table, including arsenic. Another of the elements under study is antimony.

Antimony is a metalloid that can be used in a variety of applications, including in the electronics industry. This element and many of its oxide derivatives found naturally as minerals are classified as pollutants of priority interest by the EPA and the EU.

Hollibaugh says, “Antimony derivatives are found in acid mine drainage such that from an open pit mine in Stibnite, Idaho. No simple treatment method has been found to convert antimony species to safer derivatives or to remove them from the wastewater. Treatment primarily involves minimizing, isolating and containing the drainage stream.”

Work in Hollibaugh’s lab by graduate student Christopher Abin has led to a new process that has the potential to use a bacterium to convert antimonite, which is an antimony (V) oxyanion into the antimony (III) oxide that is currently used commercially.

MLFW-2
Abin has identified a new bacterial species that utilizes antimony (V) as an electron acceptor for anaerobic respiration and converts it to antimony (III) oxide, also known as antimony trioxide. This compound is used commercially as a flame retardant, pigment and ceramic opacifier.

The project was initiated with the collection of samples at a site known to be rich in metals and metalloids. Hollibaugh says, “We went to an area adjacent to Mono Lake, Calif., where there is a good deal of geothermal activity, many hot springs and high concentrations of the ores of precious metals such as gold and silver. Other minerals present are sulfidic ores that can contain such metals and metalloids as lead and metalloids such as lead, arsenic and antimony.”

Antimony was found to be present at a concentration approximately two to three times normal background levels found elsewhere on Earth. The researchers figured that there would be microorganisms present in this environment that would use antimony for respiration.

Since the likely bacterium is an anaerobe, sediments were collected by extracting a 35-centimeter core and storing slices of the core in sealed mason jars to keep them anoxic, and they were kept at a temperature of 4 C to minimize microbial growth.

Back in the lab, the researchers initiated work to isolate a bacterium through a series of growth experiments. Hollibaugh says, “We placed samples of the sediment in bottles in an anaerobic chamber and started to cultivate organisms by adding water, lactate and antimony (V).”

Lactate is the sole carbon and energy source for the bacteria and is oxidized to acetate as part of the process. The samples were incubated in the dark at 30 C and those organisms growing and turning the culture turbid were separated out by growing them on a petri dish. Eventually, three to four organisms were cultivated and the individual species isolated by separating individual colonies.

Over a six-month period, further work eventually led to the isolation of a pure bacterial strain that was designated as MLFW-2 and identified as reducing antimony (V) and oxidizing lactate. Further testing was done using controls that lacked lactate or in which the bacteria were heat-killed and did not lead to the formation of antimony (III) oxide, indicating that the reaction was driven by this bacterium.

After sufficient antimony (III) oxide is formed, it will precipitate as two crystal phases. Figure 2 shows a scanning electron micrograph of the crystals with the more common prismatic phase represented by “bowtie” shaped crystals and the less common phase displayed as cubic crystals.


Figure 2. A scanning micrograph shows the two crystal phases of antimony (III) oxide formed by the new bacterium strain, MLFW-2. (Courtesy of The University of Georgia)

Hollibaugh says, “We have isolated a bacterium that is an obligate anaerobe that can convert toxic antimony (V) into antimony (III) oxide that is superior in quality to material currently produced commercially. We believe that the process is more sustainable, uses less energy and generates less toxic waste than the commercially used technique.”

The researchers are determining if MLFW-2 can be used to treat other dissolved metals in wastewater under anaerobic condition. Hollibaugh adds, “We believe that MLFW-2 may be effective on other metals/metalloids such as arsenic, selenium and tellurium. A question we are trying to answer is does the bacterium prefer some metals over others and, if so, which metal/metalloid will it use preferentially?”

Wastewater treatment could prove to be an important application of this discovery because these metals/metalloids can build up in areas near mines and refineries and the process provides an opportunity to recover valuable metals that would otherwise be lost.

Hollibaugh envisions that the bacterium can be genetically engineered so that it can be tailored to specific applications. The researchers are seeking to license this technology and looking to find other applications with industrial partners. Further information can be found in a recent article (2) or by contacting Hollibaugh at aquadoc@uga.edu.

2. Abin, C. and Hollibaugh, J. (2014), “Dissimilatory Antimonate Reduction and Production of Antimony Trioxide Microcrystals by a Novel Microorganism,” Environmental Science & Technology, 48 (1), pp. 681-688.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.