We all need metals to catalyse the enzyme and cellular reactions that form the basis of life. In mammals, copper for example is needed to set off the chain reactions that produce pigment. Pigment helps protect us from UV damage. Magnesium works synergistically with calcium to control the cellular actions that make our muscles expand and contract. Zinc is important for immune system health, for manufacturing DNA and for maintaining our senses of taste and smell. Iron is required by haemoglobin, the component in red blood cells that transports oxygen around the body; it also plays a role in producing cellular energy hence why an iron deficiency often leads to tiredness and lethargy. Manganese contributes to the metabolism of amino acids, carbohydrates, glucose and cholesterol; it's also important for blood clotting, bone growth, and fighting inflammation.
We mammals obtain the metals we need from dietary sources. Plants extract theirs from the soil they're growing in. Microorganisms make organic acids and metal binding compounds to leach and extract metal from minerals in their environment. It's this latter process that scientists have found ways to exploit in their quest to find safer, cleaner, greener ways of extracting valuable metals from a range of host mediums.
Biomining offers a range of advantages over traditional mining processes and bioleaching methods.
- It allows metals to be extracted from low-grade ores and mine tailings, which is highly significant given that high-grade ore reserves on earth are well on the way to depletion.
- It is far more environmentally friendly because the processing takes place in a controlled and contained environment, reducing or removing the risk of toxic leaks into surrounding ecosystems.
- It produces less pollutants, and what it does produce are microbial ie organic gases and acids.
- It means metals can be safely extracted from toxic ores and waste material.
- It considerably improves recovery rates because the microorganisms used in the process are highly efficient at what they do.
- It reduces operating costs notably because it uses far less energy.
Did you know that around 20% of the copper and 5% of the gold produced globally today isn't dug up out of the earth? Or that we're also producing a range of rare earth minerals as well as nickel, cobalt, and zinc without digging up any more fresh dirt! For the future, biomining's potential in the space of rare earth minerals is attracting the most attention as demand for these heats up.
The technological future of the planet, and thus many aspects of its overall health, relies heavily on rare earth elements. REEs though aren't particularly 'rare' and are commonly found alongside other 'more commercially significant' commodities (like uranium). There are already large mined quantities of some of them sitting in waste dumps around the globe, like the one at the Mary Kathleen uranium mine in Australia's Northern Territory. We 'just' have to reprocess the waste material to extract them!
Sounds simple in theory but the reality is that some of these waste dumps are highly toxic, not to mention radioactive. That makes reprocessing them via traditional methods highly dangerous. This is where biomining can help.
An Ancient 'Art'
Using 'nature' ie bacteria to extract precious metals isn't exactly modern technology. We've been doing it for centuries, or since at least 1000 BC, which is around about the beginning of the 'iron age' in some parts of the world. It's also (amongst many other significant events) the time of King David, ruler of the ancient United Kingdom of Israel, Israel's 'Golden Age', and the Chinese Zhou Dynasty. A notable example of our 'cleverness' back then is the way we were extracting copper from water that had passed through copper ore deposits. Whilst we don't know exactly how it was done, we do know it would have involved some type of bioleaching process.
Around 1510, Paracelsus the Great (real name Aureolus Phillipus Theophrastus Bombast) wrote the following in a chapter of "The Book Concerning the Tincture of the Philosophers":
"For truly, when the rustics in Hungary cast iron at the proper season into a certain fountain, commonly called Zifferbrunnen, it is consumed into rust and when this is liquefied with a blast-fire, it soon exists as pure Venus (copper), and nevermore returns to iron. Similarly in the mountain commonly called Kutenberg they obtain a lixivium out of marcasites, in which iron is forthwith turned into Venus (copper) of a high grade, and more malleable than the other produced by nature."
Modern science would understand the process described at Zifferbrunnen as being pretty standard operating procedure for processing low-grade copper ore dumps. Indeed, most copper mines around the world today use some type of liquid copper extraction technology that, more often than not, involves microorganisms.
There are also accounts of copper extraction via bioleaching in Hungary, Germany and Spain dating back to the 16th century, and in the USSR and USA from the late 19th century / early 20th century respectively.
The Study Of Microorganisms In Metal Recovery Gets Scientific
In 1922 Rudolfs and Helbronner, and Waksman and Joffe, studied bacteria that could create sulphuric acid by oxidising sulphur compounds. Carpenter and Herndon did further work on the topic in 1933. In 1947 Colmer and Hinkle 'found' Acidithiobacillus (originally Thiobacillus) ferrooxidans in coalmine water, the first bioleaching microorganism to be officially identified. The body of work produced by these researchers went on to lay the foundation for ongoing studies into the use of microorganisms in metal solubilisation processes.
One significant early study that built on this research was the work done by Bryner et al in the early 1950's. In 1954, they reported on acidophilic Thiobacilli-induced oxidisation of copper sulphides and iron pyrites from the Bingham Canyon open pit mine-drainage water. A patent for 'the use of iron oxidising acidophilic bacteria in a cyclic leaching process' was issued to the mine's owner Kennecott Copper Corporation in 1958, and was to become the first of many such patents.
Acidithiobacillus ferrooxidans And Other Helpful Microorganisms
Acidithiobacillus ferrooxidans is a bacterial chemoautotrophic acidophile. In plain English this means a bacteria (bacillus) that uses a chemical reaction (chemo) to generate its own (auto) food ie energy (troph) and loves extremely acidic environments (acidophile).
Chemoautotrophs are further categorised as organic (chemoorganotrophs) or inorganic (chemolithotrophs) depending on whether they get their energy from organic or inorganic sources respectively. Carbonates, oxides, phosphates, and silicates are classed as organic sources whilst inorganic sources include ammonia, ferrous iron, elemental sulphur, hydrogen sulphide, molecular hydrogen etc.
As a chemolithotroph, Acidithiobacillus ferrooxidans gets its energy (food) by oxidising iron (ferro) and sulphur. Today it is one of a group of commercially important microorganisms with bioleaching properties that includes A. thiooxidans, Leptospirillum ferrooxidans and several species of Sulpholobus (notably S. thermosulphidoxidans and S. brierleyi). Sulpholobus species have both acidophile and thermophile properties (a thermophile is a microorganism that likes high temperatures).
Despite the discovery of other bioleaching microorganisms, Acidithiobacillus ferrooxidans remains one of the most significant. This is due to its unique ability to oxidise both ferrous iron and sulphur/reduced sulphur compounds, making it ideal for leaching metals like gold and copper from oxide and sulphide ores.
Bioreactors, Bacteria And Biomining
The biomining process starts with bioreactors, a fancy name for big closed tanks with a stirring mechanism inside. The tanks contain water and microorganisms like fungi, archaea or bacteria, plus an energy source for the microorganisms. The metal-containing material (usually waste ore but it can also be old computer hard drives and mobile phone components) is added to this mix. As the microorganisms do their work, the metals separate out and are retrieved.
One of the critical components in this process is supplying the right type of energy/food source. As mentioned above, chemolithotrophic microorganisms like Acidithiobacillus ferrooxidans get their energy from inorganic sources (by oxidising iron and sulphur) whilst chemoorganotrophs require organic sources. For the latter, this is usually supplied by adding sugars to the mix.
Fun question – is there a difference between a microorganism and a microbe?
You've probably noticed most people use the two terms interchangeably and to a degree, that's correct. They both refer to microscopic organisms and according to some authorities (like dictionaries), microorganism is simply the plural of microbe. Technically though 'microbe' covers the entire spectrum of these microscopic organisms whilst 'microorganism' is reserved for those that are living. This includes bacteria, fungi, archaea, and protists. The 2 other identified types of microbes – viruses and prions, are non-living and therefore not classed as microorganisms. Thus, in relation to biomining, we're referring specifically to microorganisms.