Microbiological Mining

 

Here we will discuss how is useful when we want to use it in the leeching of copper and many other metals. These bacteria are very useful in mining industry because of their role in leeching of different metals .In past many practices were performed for the mining of copper. The process is very difficult to perform but in the past years mining of copper was performed on large scale. In Japan during 18^th century mining of copper was very common. This process was performed by using traditional methods but 25 years ago this process started use of bacteria.

Bacteria actively take part in mining process as they convert copper into soluble form and this copper is than leached out. Now a days tons of copper is being recovered by using bacteria. This obtained copper is economically useful and it contributes about 10% to total US production .Along with the use of bacteria in leeching of copper it is also used in the recovery of many other metals like uranium.

Microorganisms have been modified genetically and are now being used in the recovery of many metals. Not only bacteria can be used in the recovery of different metals but also some microorganisms can also be used for the recovery purpose of different metals. Microorganisms can also be used in the treatment of contaminated water and bacteria are also used for bioremediation process specially during oil spills.

They start degrading oil that are normally spilled in rivers due to leakage. Microbiologists are working to develop new methods so that they can replace traditional methods with new methods for the recovery of different metals. There is continuous degradation of different metals so, during mining we have to recover metals from deep underground.

As we are facing environmental problems and also due to burning of fossil fuels that are rich in sulfur so ,because of these problems lot of energy is being consumed when we recover metals by using traditional methods so, there is requirement of new methods that will not consume much energy. The recent methods for the recovery of metals will take time to establish but these biotechnological methods are less energy consuming and don’t create pollution in the environment. That is the reason biotechnological methods are gaining importance.

Microbiology has greater applications in mineral industry and there are some questions that are linked. These questions are as follows:

·         What are the microorganisms that will play important role in the recovery of different metals?

·         Where these microorganisms will be present?

·         What type of functions will be performed by these microorganisms?

·         What type of nutrients are required for their function?

·         What will be their contribution towards environment?

·         What will be the effect of these tools on future biomining?

The most remarkable life forms of bacteria are being in use in the recovery of different metals. The word CHEMOLITHOTROPHIC can be used for different microorganisms which has meaning of ROCK-EATING. These organisms obtain energy when they oxidize different inorganic substances. Mostly of these organisms are autotrophic which means that they obtain their food themselves.

The bacteria that are involved in leeching process that are mostly present in those environments where other organisms don’t live because these are extreme conditions and they are not suitable for the survival of different organisms. Like bacteria also in those places where temperature is extremely high known as hot springs as they are heat loving species. They also live in those places where concentration of sulfur and other metals is very high and this place is inhabitable for other organisms.

The bacteria that was most commonly used in past for leeching of metals is Thiobacillus ferroxidans and this bacteria has rod shape. The early discovery place of these bacteria was acid water that was present in mines of coal. At that time it was observed that due to the presence of bacteria the metals were dissolved and this correlation indicated that bacteria is involved in the dissolution of metals.

When this information was obtained than this bacteria gained importance and was most commonly used in the recovery of different metals. As this bacteria was first obtained from acid water so nature of this bacteria is acidophilic. This specie is acid loving and it lives in different environments like volcanic fissures, hot springs or in those areas where concentration of sulfur is greater.

The temperature range for this bacteria is 20 to 35C.The energy source of this bacteria is oxidation of either sulfur or iron. The form in which iron is present must be Fe++ and than it is converted into ferric form by this bacteria. The form in which sulfur is present can be insoluble or elemental form. In these forms of sulfur and iron this bacteria attacks on them and obtains its energy requirement from these ions.

In these forms of sulfur and iron they contain less valence electrons .Carbon source of this bacteria can be fulfilled from atmospheric form of carbon dioxide. Although this bacteria is useful in the leeching process but its not only the bacteria that is useful in leeching process, many other organisms are present that are playing their role. One of these organisms is T-thiodoxins and this bacteria is also rod shaped but it is different from T-ferridoxins in such a way that it is not acid lover and is not present on that places where sulfur is present.

Research has shown that a mixture of bacteria is useful in recovery of different metals. For example if we use a mixture of T-ferridoxins and T-oxidants than this mixture will be much more useful than we use a single specie in recovery process. Another example is there if we use mixture of :

·         Leptospirllium ferrooxidans.

·         T-organoparus.

This mixture will be more useful and it can degrade following elements:

·         Pyrite(FeS2).

·         Chalcopyrite(CuFeS2).

The above mentioned species cannot be applied alone, their mixture is useful in the degradation of above mentioned elements. When there is a need of recovery of metals from acidic environment than bacteria can isolate heterotrophic species like fungi.

Fungi is useful in such environments because they can scavenge organic matter in such environments and they also survive on this organic matter. Fungi use products this organic matter for their survival. We had discussed that there is greater role of autotrophic organisms in the leeching process .There is also role of heterotrophic organisms in leeching process but their greater role is still undetermined.

There is a specie Thiobacillus that attacks on:

·         They had attack on sulfide minerals.

·         They had attack on soluble sulfur that is present under neutral conditions.

·         They had attack on sulfide ores.

These Thiobacillus species increase level of acid and they promote growth of acidophilic bacteria. When temperature is in the range of 60 to 75C and when conditions are neutral than a bacterial specie Thermothrix thiopara is involved in the oxidation of :

·         Sulfite ions.

·         Thiosulfate ions.

·         Sulfhydral ions.

·         Elemental sulfur.

The role of this specie is to form sulfur ions. This specie is filamentous and is wide spread in volcanic fissures. Along with this specie many other filamentous species are present that are involved in the oxidation of sulfur.

In order to determine presence of species involved in leeching there is a need to test leeching sites. Just few leeching sites have been tested to determine the presence of these bacteria involved in leeching process. Most of these species are present in hot springs or volcanic fissures. They are present in the springs where sulfur is present and their presence indicates that they are involved in the colonizing of sulfide ores and they are also involved in the prepration of the environment that favors acidophilic species.

These microorganisms that are involved in leeching process are acidophilic as well as thermophilic. TH bacteria are another bacteria useful for this process and it has been shown that its strains grow on:

·         Ferrous iron.

·         Pyrite.

·         Chalcopyrite.

·         Covellite.

·         Pentlandite.

Organic supplement is required by these organisms for their survival .Heavy metals can be of different types. These are as follows:

·         CuS.

·         NiSO4.

·         ZnSO4.

All of these metals are extracted from water or different places and is then converted into soluble form and than our desired metals can be extracted by using microbes of different types.

This process can also be known as “biooxidation” as this process involves oxidation of different compounds. Normally the term biooxidation is used when we recover metal by microbial decomposition and the metal obtained from this process is insoluble. We can quote an example of when we recover gold from arsinopyrite and in this process gold is insoluble and process of oxidation is involved in the recovery of gold.

From ancient times the recovery of many metals has been started in which microbes are deposited and metals are obtained in insoluble form and mostly this process is linked with sulfur and iron cycles. The source for mineral deposits are sulfides and these sulfates are produced by bacteria that is sulfur reducing. Metal sulfates are formed when sulfides act as source of electrons for microbes that are sulfur oxidizing.

This process can be used to leech out metals from deposits because as these sulfates are present in soluble form. Many biomining industries are using this ability of microbes for the recovery of desired metals from deposits. These are of many advantages in different fields. The use of microbes for extraction processes does not harms the environment and this process is environment friendly.

The energy requirement for these microbes is low and they also don’t produce any chemicals that can harm our environment. The waste that is produced from this environment can be leeched out easily and they produce un wanted acid rain. The chemical activity of this process is least and in aspect of biological activity such type of chemicals are produced that are not harmful for the environment.

Such type of ores that are of low grade, from them metals can also be extracted by using this process. Example can be of recovery of copper from low grade ores. These metals cannot be recoverd for economical purpose by the use of non-biological processes so, biological processes must be used for the recovery of metals.

    In early biomining process there was no use of microbes and that was not economical process. Later on, the use of microbes was started in biomining process and that proved to be advantageous and were of economical importance. For the first time copper was extracted by using microbes. Copper and silver were recovered by Romans. Ferric iron imparts red colour and that is the reason the river in which ferric iron is present it is known as “Red River”.

The red river don’t contains any fish and water of this river cannot be used for drinking purpose as it contains high concentration of ferric iron. When we want to recover dissolved form of copper from aqueous solutions than a process known as “Cementation” is used.

Characteristics of biomining microbes:

The features of biomining microbes can be as follows:

Ø  The source of electron donors for these microbes is reduced sulfur or ferrous iron.

Ø  They all are “Chemolithotrophic”.

Ø  They all are acidophilic.

Ø  Biomining bacteria use source of electron donor other than oxygen.

Ø  They fix Co2 and their efficiency in fixing Co2 is different.

Ø  Those organisms that have low efficiency of fixing Co2 they use yeast for their growth.

Ø  Their resistance for metals is different .

These are properties for which microbes are used in process of biomining. Air is their source of Co2 and electron source.

Water is their medium for growth. These microbes also use air to obtain nitrogen and fix it but they are unable to do so in highly aerated environment. Water and minerals are source of trace elements. In order to avoid nutrient limitation ammonium sulfate and potassium phosphate are added.

Biodiversity of microbes involved in mining

 The microbes that are involved in mining process grow rapidly. Many techniques are there to use microbes in this process and there are also many environments which contain mineral deposits. In order to test these acidophilic species there are many problems associated with this testing.

There is difficulty in cultivating these species and those mediums that contain sugar and polysaccharides these species are very sensitive to these environments. Sugar inhibits the growth of cells and the process in which agarose is used it has proven successful because the amount of sugar that is released it is of low quantity. A variety of gelling agents had also been used but they had not met with much success and it is difficult to work with these gelling agents so, agarose is much useful to use it in this process.

The most useful technique for preparing lab medium is the use of double layer plate technique. Sulfur is used for the growth of heterotrophic bacteria that are also acidophilic. In double layer plate technique when first layer is prepared than microbes are poured on this layer. The microbes that are starving they absorb sugar and other waste products for the sake of growth.

If we use isolation media of best quality even then all the problems associated with medium cannot be solved out properly. Some bacteria also grow in mineral environments and they adhere to biofilms for the sake of their growth as they absorb food from these biofilms. There are also some bacteria that cannot be cultivated on the lab media.

PCR technique can be used for biomining process and it’s a great breakthrough. Dna samples are extracted and they are amplified by using PCR technique. This method also has drawbacks that are as follows:

·         All microbes cannot be lysed equally.

·         If microbes are attached to solid medium than its difficult to separate them.

·         PCR technique is also not a quantitative so, we can’t estimate the total number of organisms present in our sample.

There is advantage of PCR method that is it can also can also separate bacteria from heaps and  also from different compositions and also there is no need to grow this bacteria in labs.

One of the disadvantage of biodiversity is that we can use just selective methods and most of these methods are unreliable. These methods require cultivation of microbes on lab media and most of the microbes cannot be cultivated and there are about 1% total species that have been cultivated so, these methods cannot be commonly used.

Another drawback associated with these methods is that sometimes mixtures of different cultures is applied and minerals are provided there are some species that may compete for these nutrients and as a result of this competition the dominant species consume these nutrients and result in the death of those species that were unable to consume nutrients.

This decomposition of minerals takes place in a range of different temperatures. Microbes can be either:

·         Mesophiles.

·         Thermopiles’.

·         Moderate thermophiles.

There are no minimum temperatures in case of heaps and deposits. Exception is that biooxidation process takes place in the temperature range of 20_35C. When we perform biooxidation processes in biooxidation tanks than temperature is controlled in these tanks.

In biooxidation tanks the temperature is mostly maintained in the range of 40C. The species that has temperature 70 or 78C for these species different processes are developed. Different species have different ranges of temperatures but when we have mixture of:

·         Sulfur oxidizing species.

·         Iron oxidizing species.

These species are helpful as they have same temperatures. Microorganisms are also present in acid environment and we can use them to extract metals from these environment.

We cannot make a list of all those microbes that are involved biomining as few of the species has been discovered that are involved in biomining and some species are under process they will serve us in future. This method has following drawbacks:

·         The species that operate at temperature of 40C they have been just reported.

·         Those species that operate at temperature of 50C there is just a single publication of these species.

·         Those species that have temperature of 70C or above they are under development.