Every living thing needs energy. We get our energy from eating food and breathing oxygen, but some bacteria eat sulfur for energy. The way they process this sulfur is by oxidizing it. In oxidizing that sulfur, these microbes produce sulfuric acid.
In mining, sulfuric acid helps break down rocks that contain metals used in phones, computers and cars. It can be dangerous to transport sulfuric acid across long distances. If it spills, it can ruin water and crops, and cause financial losses to both the company and the community. This means that companies have a vested interest in finding safer, cheaper sources of sulfuric acid. These sulfuric acid-generating bacteria may be a potential resource that could efficiently produce acid at a mine without dealing with the dangers associated with transporting sulfuric acid in trucks.
With an eye toward biomining applications, researchers in Wales experimented with sulfuric acid production by sulfur-eating bacteria that live in acidic conditions (pH 1). They wanted to figure out if there is a way to make them produce more acid at a faster rate. Yellow elemental sulfur is an extremely cheap and plentiful commodity, so its addition to a mining operation would be inexpensive. If the bacteria were able to produce a large quantity of sulfuric acid, the cost of sulfur would be much cheaper than the costs to buy and transport sulfuric acid. To do this, these researchers built two big bioreactors to measure sulfur oxidation. They set one of the reactors to 30°C (86°F) and the other to 50°C (122°F) and added the sulfur-oxidizing bacteria. They used a mixture of several types of well-studied microorganisms available from culture collections: the bacteria Acidithiobacillus albertensis, Acidithiobacillus caldus, Sulfobacillus thermosulfooxidans, and the archaeon Sulfolobus metallicus. These organisms are all known sulfur-oxidizers and many have been found in extremely acidic conditions including acid mine drainage sites and other naturally acidic environments.
They found that the bioreactor at 50°C (122°F) did not produce as much acid as the reactor at 30°C (86°F). They also found that by adding another chemical called sulfate, they more than doubled the acid production in the bioreactors. They also characterized the microbial community that developed during the experiment. While all of the bacteria that they initially added were still there at the end of the experiment, Acidithiobacillus albertensis made up the majority of the community in the bioreactors at both temperatures. They also found a little surprise – a bacterium that they did not add but that grew from the sulfur! It is a new species they called Acidibacillus ambivalens. It is also a sulfur oxidizer that contributed to acid production in the bioreactors.
This study shows that some microbes can produce a lot of acid and can produce it quickly. With a little bit of engineering, they could provide an alternative to transporting concentrated sulfuric acid over long distances in trucks. This could also be a cheaper option for mining companies, since they could just add in the bacteria and they produce all of the acid for free. Sulfur is a cheap and plentiful commodity, so it would be easy and inexpensive to add to a biomining operation to jumpstart acid production.