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Mining with Microbes

Sampling from acidic saline drains in Western Australia.

Sampling from acidic saline drains in Western Australia.

By Carla Zammit

High salt concentrations in Western Australian groundwater have restricted the mining industry’s use of microorganisms to extract metals from their ores. Until now.

Microorganisms first appeared on Earth 3–4 billion years ago. Since then, microbes have evolved to inhabit close to every corner of the world.

Microorganisms are capable of living in environments that are far too harsh for any other Earth-based life form. Some are capable of growth at a pH as low as 0 (the pH of gastric acid is 1), can withstand levels of heavy metals that are lethal to most other life forms, or live in temperatures above the boiling point of water.

Many microorganisms also make significant contributions to environmental cycles, such as the fixation of nitrogen, the cycling of carbon and the transformation of a range of metals.

Throughout history, microorganisms have been used to assist us in industrial systems. With our broadening knowledge about the capability of microorganisms we are finding new applications for our micro-coinhabitants.

Ever since ancient times, microorganisms have been utilised by humans for the manufacturing of alcohol and bread. However, around 3000 years BC, a less well-known use for microorganisms was first reported. This process employed a variety of quite remarkable microorganisms to assist in the breakdown of sulphide-containing ores.

The Rio Tinto River in Spain was the seed of this technology. Here, miners would source ore and place it in beds along with water from the local river. Unbeknown to workers at the time the microorganisms from this river would assist in the deposition of copper, which would eventually be collected and used.

It was not until the mid 1940s that it was realised that micro­organisms played a role in the dissolution of metals from their ores. This process was termed bioleaching.

In 1959 the first commercial bioleaching operation began at the Bingham Canyon Mine in Utah, USA. Since that time it has been an expanding industry, driven heavily by the benefits of using the technology over traditional mineral processing methods.

Scientists have since been able to use their knowledge to expand the application of these microorganisms to the extraction of many types of metals. Currently it is estimated that ore processed using microorganisms accounts for 20% of the world’s copper supply. A number of other metals have now been extracted using bioleaching, including gold, silver, uranium, nickel, zinc, lead and cobalt.

The microorganisms most commonly associated with bioleaching are chemolithoautotrophs. This means that their energy is obtained from inorganic iron and/or sulfur (chemo) by oxidising these compounds and using the freed electrons in energy production (litho), and using carbon dioxide from the atmosphere as their source of carbon (auto).

Generally, the pH of bioleaching operations is within the range of 1.2–2.0, hence these microorganisms are also acidophilic. As a result of the oxidation of iron and/or sulfur at low pH by the bioleaching microorganisms, minerals trapped in the ore become soluble. These solubilised metals can then be processed using traditional methods, such as solvent extraction and electro­winning.

Western Australia’s economy is heavily driven by the mining industry, yet there has been a relatively low take-up of bioleaching technologies. In part, this has been due to the limitations of the bioleaching microorganisms that are currently in use commercially. Western Australian soils and waters have an extremely high salt content, with brackish to saline water (>0.5% NaCl) making up 36% of the state’s groundwater supply. However, the microorganisms that are commercially used in bioleaching operations tend to have no tolerance for NaCl past 0.1%, which has limited the use of bioleaching in places that have limited access to fresh water.

This problem formed the basis of research that was undertaken at Curtin University, CSIRO Minerals and the Parker Cooperative Research Centre for Integrated Hydrometallurgy Solutions.

To solve the dilemma of limited NaCl tolerance in bioleaching microorganisms, we turned to the environment to look for systems that supported similar conditions to those found in a salty bioleaching system. However, environments that are low in pH, high in heavy metals and high in NaCl are a rarity on Earth. Island volcanoes and marine harbour sediments have been previously investigated as potential sources of NaCl-tolerant bioleaching microorganisms but yielded little of commercial interest.

Fortunately, Western Australia is spotted with environments that could serve as a source of potentially NaCl-tolerant bioleaching microorganisms. Acidic saline drains found throughout Western Australia provide the perfect conditions for the growth of possibly novel NaCl-tolerant bioleaching micro­organisms.

Saline waters are rising in many agricultural areas in Western Australia due to the clearing of deep-rooted vegetation. When these waters approach the surface, a condition called “dryland salinity” occurs, thus causing the salinisation of the soil and above groundwater supplies. A currently practised solution to dryland salinity is to gully the rising groundwaters. Drains are drilled to bring the water to the surface in a controlled manner, and the water is drained into rivers or lakes or evaporated off in man-made channels called saline drains.

In the south of Western Australia, the saline groundwater typically has a pH of 3–4.5, making the drains acidic and increasing the levels of heavy metals, thus serving as an environmental analogue to bioleaching systems.

We selected 16 different acidic saline drain sites for investigation based on their geochemical characteristics: low pH, high levels of metals (in particular iron and sulfur) and high levels of NaCl were all desired. These environments were sampled for the presence of bioleaching microorganisms.

Samples were enriched in the laboratory to encourage the growth of micro­organisms that could be used in bioleaching. From the original 16 samples, three contained microorganisms that could oxidise iron – a key trait when looking for bioleaching microorganisms. One sample was eventually chosen for further research based on its superior ability to oxidise iron and sulfur under high levels of NaCl, as well as some other traits it shared with other commercially available bioleaching microorganisms.

The selected sample was tested extensively for its ability to bioleach at different temperatures and conditions. The sample contained four different microorganisms, two of which were isolated for further investigation. We found that one of these microorganisms was able to tolerate very high levels of NaCl, certainly the highest levels of NaCl that have ever been seen in an acidophile. Additionally, this micro­organism was also able to grow in the absence of NaCl, but preferentially grew at 1.3% NaCl. This was another first, as no other acidophile has been found with these properties.

Using molecular techniques we were able to decipher more detailed information about the four microbes isolated from the acidic saline drain. We found that when these four microorganisms were put into a mixture with different levels of NaCl, particular microorganisms dominated at various levels of NaCl. This means that the mixture of microorganisms is pliable and will shift to adjust to the different environmental factors seen in bioleaching operations.

Additional testing showed that these four microorganisms fell into two groups of more closely related species. One of the microorganisms identified was similar to a microorganism that has only ever been isolated from the base of an island volcano in Italy. The base of this volcano is exposed to geothermally heated seawater, has a pH of 6.5 and is rich in minerals. This micro­organism is able to oxidise iron in the presence of up to 3.5% NaCl, but preferentially grows in the absence of NaCl, making the micro­organism isolated from Western Australia superior in terms of its bioleaching capabilities.

Research in the future will determine how these microorganisms are able to tolerate such high levels of NaCl. Additionally, we hope to study the differences between the Italian and Western Australian microorganisms.

The ultimate goal of this research is to use these microorganisms in actual bioleaching operations within Western Australia. Hopefully this will come to fruition in the near future.

Carla Zammit completed this study during her PhD at Curtin University and is currently a postdoctoral researcher at The University of Adelaide.