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  Section: General Biotechnology / Biotechnology & Environment
 
 
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Environmental Biotechnology

 
     
 

Microbial Leaching (Bioleaching, Biomining)
Microbial leaching is the process by which metals are dissolved from ore bearing rocks using microorganisms. For the last 10 centuries, microorganisms have assisted in the recovery of copper dissolved in drainage from water. Thus biomining has emerge as an important branch of biotechnology in recent years. Microbial technology renders helps in case of recovery of ores which cannot be economically processed with chemical methods, because they contain low grade metals. Therefore, large quantity of low grade ores are produced during the separation of high grade ores. The low grade ores are discarded in waste heaps which enter in the environment. The low grade ores contain significant amount of nickel, lead, and zinc ores which could be processed by microbial leaching. Bioleaching of uranium and copper has been widely commercialized. But large scale leaching process may cause environmental problems when dump is not managed properly. This results in seepage of leach fluids containing large quantity of metals and low pH into nearby natural water supplies and ground water.

Thus, biomining is, economically sound hydrometallurgical process with lesser environmental problem than conventional commercial application. However, it is an inter-disciplinary field involving metallurgy, chemical engineering, microbiology and molecular biology. It has tremendous practical application. In a country like India biomining has great national significance where there is vast unexploited mineral potential (Mogal and Desai, 1992).

Miroorganisms used for Leaching
The most commonly used microorganisms for bioleaching are Thiobacillus thiooxidans and T.ferrooxidans. The other microorganisms may also be used in bioleaching viz., Bacillus licheniformis, B. luteus, B. megaterium, B. polymyxa, Leptospirillum ferrooxidans, Pseudomonas fluorescens, Sulfolobus acidocaldarius, Thermothrix thioparus, Thiobacillus thermophilica, etc.

Chemistry of Microbial Leaching
T. thiooxidans and T. ferrooxidans have always been found to be present in mixture on leaching dumps. Thiobacillus is the most extensively studied Gram-negative bacillus bacterium which derives energy from oxidation of Fe2+ or insoluble sulphur. In bioleaching there are two following reaction mechanisms:

Direct Bacterial Leaching
In direct bacterial leaching a physical contact exists between bacteria and ores and oxidation of minerals takes place through several enzymatically catalyzed steps. For example, pyrite is oxidized to ferric sulphate as below:

 
T. ferrooxidans
 
2FeS2 + 7O2 + 2H2O 2FeSO4 + 2H2SO4

 
Indirect Bacterial Leaching
In indirect bacterial leaching microbes are not in direct contact with minerals but leaching agents are produced by microorganisms which oxidize them.

FeS2 + Fe2(SO4 3FeSO4 + 2S°

2S° + 3O2 + 2H2O 2H2SO4

Oxidation of ferrus (Fe2+) to ferric (Fe3+) by T. ferrooxidans at low pH is given below:

 
T. ferrooxidans
 
4FeSO4 + 2H2SO4 + O2 2Fe2(SO4)3 + 2H2O


Leaching Process
There are three commercial methods used in leaching:
(i) Slope Leaching. About 10,000 tonnes of ores are ground first to get fine pieces. It is dumped in large piles down a mountain side leaching dump. Water containing inoculum of Thiobacillus is continuously sprinkled over the pile. Water is collected at bottom. It is used to extract metals and generate bacteria in an oxidation pond.
(ii) Heap Leaching. The ore is dumped in large heaps called leach dump. Further steps of treatment are as described for slope leaching.
(iii) In situ Leaching. In this process ores remain in its original position in earth. Surface blasting of rock is done just to increase permeability of water. Thereafter, water containing Thiobacillus is pumped through drilled passage to the ores. Acidic water seeps through the rock and collects at bottom. Again from bottom water is pumped, mineral is extracted and water is reused after generation of bacteria.

 

 Content

Bioremediation

 

In situ bioremediation

 

 

Intrinsic bioremediation

 

 

Engineered in situ bioremediation

 

Ex situ bioremediation

 

 

Solid phase system (composting, composting process)

 

 

Slurry phase system (aerated laggons, low shear airlift reactor)

 

 

Factors affecting slurry phase bioremediation

 

Bioremediation of hydrocarbon

 

 

Use of mixture of bacteria

 

 

Use of genetically engineered bacterial strains 

 

Bioremediation of Industrial wastes

 

 

Bioremediation of dyes

 

 

Bioremediation of heavy metals

 

 

Bioremediation of coal waste through VAM fungi

 

Bioremediation of xenobiotics

 

 

Microbial degradation of xenobiotics

 

 

Gene manipulation of pesticide-degrading microorganisms

Utilization of sewage, and agro-wastes

 

Production of single cell protein

 

Biogas from sewage

 

Mushroom production on agro-wastes

 

Vermicomposting

Microbial leaching (bioleaching)   

 

Microorganisms used in leaching

 

Chemistry of leaching

 

 

Direct leaching

 

 

Indirect leaching

 

Leaching process (slope leaching heap leaching in situ leaching)

 

Examples of bioleaching

 

 

Copper leaching

 

 

Uranium leaching

 

 

Gold and silver leaching

 

 

Silica leaching

Hazards of environmental engineering

 

Survival of released GMMs in the environment

 

 

Adaptive mutagenesis in GMMs

 

 

Gene transfer from GMMs into other microorganisms

 

 

Gene transfer via conjugative transposons

 

 

Effect of environmental factors on gene transfer

 

Ecological impact of GMMs released into the environment

 

 

Growth inhibition of natural strains

 

 

Growth stimulation of indigenous strains

 

 

Replacement of natural strains

 

Monitoring of GEMs in the environment

 

 

Risk assessment of the GEMs released into the environment


Examples of Bioleaching
Bioleaching has been discussed with copper, uranium, gold, silver and silica.

Copper Leaching
Throughout the world copper leaching plants have been widely used for many years. It is operated as simple heap leaching process or combination of both heap leaching and in situ leaching process. Dilute sulphuric acid (pH 2) is percolated down through the pile. The liquid coming out of the bottom of pile reach in mineral. It is collected and transported to precipitation plant, metal is reprecipitated and purified. Liquid is pumped back to top of pile and cycle is repeated. For removal of copper the ores commonly used are chalcocite (Cu2S), chalcopyrite (CuFeS2) or covellite (CuS). Several other metals are also associated with these ores. Chalcocite is oxidized to soluble form of copper (Cu2+) and covellite by T. ferrooxidans.

Cu2S + O2CuS + Cu2+ + H2O

Covellite is oxidized to copper sulphate chemically or by bacteria.
  Microbial leaching of copper.
 

Fig. 21.9. Microbial leaching of copper.

2CuFeS2 + 8½ O2 + H2SO4a2CuSO4 + Fe2(SO4)3 + H20


Thereafter, strictly chemical reaction occurs which is the most important reaction in copper leaching.
CuS + 8Fe3+ + 4H2OaCu2+ + SFe2+ + SO42- + 8H+



Copper is removed as below:

Fe2+ + Cu2+Cu° + Fe2+

Fe2+ is transferred in oxidation pond
 
T. ferrooxidans
 
Fe2+ + ¼O2 + H+ Fe3+ + ½O2


The Fe3+ ions produced is an oxidation of ores; therefore, it is pumped back to pile. Sulphuric acid is added to maintain pH. An outline of microbial leaching of copper is shown in Fig. 21.9. Microbial leaching of copper has been widely used in the USA. Australia, Canada, Mexico, South Africa and Japan. In the USA 200 tonnes of copper is recovered per day.

Uranium Leaching
Uranium leaching is more important than copper, although less amount of uranium is obtained than copper. For getting one tonne of uranium, a thousand tonne of uranium ore must be handled. In situ uranium leaching is gaining vast acceptance. However, uranium leaching from ore on a large scale is widely practiced in the USA, South Africa, Canada and India.

Insoluble tetravalent uranium is oxidized with a hot H2SO4/Fe3+ solution to make soluble hexavalent uranium sulfate at pH 1.5-3.5 and temperature 35°C (Crueger and Crueger, 1984).

UO2 + Fe2(SO4)3 UO2SO4 + 2FeSO4

Uranium leaching is indirect process. T. ferrooxidans does not directly attack on uranium ore, but on the iron oxidant. The pyrite reaction is used for the initial production of Fe3+ leach solution.  

  T. ferrooxidans  
2FeS + H2O +7½ O2 Fe2(SO4)3 + H2SO4

Gold and Silver Leaching
Today's microbial leaching of refractory precious metal ores to enhancegold and silver recovery is one of the most promising applications. Gold is obtained through bioleaching of arsenopyrite/pyrite ore and its cyanidation process. Silver is more readily solubilized than gold during microbial leaching of iron sulfide.

Silica Leaching
Magnesite, bauxite, dolomite and basalt are the ores of silica. Mohanty et al (1990) isolated Bacillus Ucheniformis from magnesite ore deposits. Later it was shown to be associated with bioleaching, concomitant mineralysis and silican uptake by the bacterium. It was concluded that silican uptake was restricted adsorption of bacterial cell surface rather than internal uptake through the membrane. The bioleaching technology of silica magnesite by using B. licheniformis developed at Bose Institute, Calcutta is being used for the first time for commissioning a 5 billion tonnes capacity of pilot plant at Salem Works of Burn, Standard Co. Ltd, Tamil Nadu, in collaboration with the Department of Biotechnology, Govt of India (Haider et al., 1994).

 
     
 
 
     



     
 
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