Bioleaching: Microorganisms, Types, Factors, Applications microbiologystudy

Bioleaching, also called biomining or microbial bioleaching is a biotechnological process that employs microorganisms to extract metal from their ores and the solid metallic waste.

The traditionally used method not being economically feasible in recent days, the bioleaching method has gained worldwide acceptance. Bioleaching for the extraction of metals, uses a specific group of microorganisms, bacteria or archaebacteria, which interact with the ore-containing rocks, transforming the solid metal compounds into the extractable forms.

It is a part of hydrometallurgy, which studies the interactions between organisms and ores to obtain valuable metals from low-grade ores, as a more efficient and sustainable alternative to conventional extraction methods.

The metals mostly extracted with the application of bioleaching are: 

• Gold
• Copper
• Silver
• Cobalt
• Uranium
• Zinc
• Nickel

Microorganisms Involved and Role in Metal Recovery

Bioleaching utilizes certain microorganisms for the extraction of metals from the ores. Some of them are discussed below:

1. Thiobacillus Species

• Thiobacillus ferrooxidans and Thiobacillus thiooxidans are the most active bacteria in bioleaching.
• These Gram-negative, non-spore-forming rods thrive in highly acidic, aerobic environments (pH 1.5 to 3).
• They are chemolithoautotrophic, using atmospheric carbon dioxide as a carbon source and deriving energy from the oxidation of sulfur compounds, including sulfides and elemental sulfur, which are converted into soluble sulfates.

2. Leptospirillum ferrooxidans

• This acidophilic bacterium oxidizes ferrous iron and is highly tolerant of extreme conditions, including low pH and high concentrations of metals like uranium, molybdenum, and silver.
• Although Leptospirillum ferrooxidans cannot oxidize sulfur compounds alone, it works synergistically with Thiobacillus species to enhance the bioleaching process.

3. Thiobacillus-like and Thermophilic Bacteria

• Thiobacillus-like bacteria, known as Th-bacteria, are moderately thermophilic and grow on minerals such as pyrite and chalcopyrite at temperatures around 50°C.
• Acidianus brierleyi, an extremely thermophilic archaeon, can thrive at temperatures above 60°C, growing on ferrous iron and elemental sulfur. It can also reduce sulfur to hydrogen sulfide under anaerobic conditions.

These microorganisms convert insoluble metal sulfides into water-soluble metal sulfates through biochemical oxidation, which is crucial for bioleaching.

Mechanism of Bioleaching

Bioleaching involves two primary mechanisms:

• Direct Bacterial Leaching 
• Indirect Bacterial Leaching

1. Direct Bacterial Leaching (Contact leaching)

• In direct bacterial leaching, microorganisms directly interact with the mineral surface.
• Here, the physical contact between bacterial cells and the mineral sulfide surface occurs.
• Organisms such as Thiobacillus ferrooxidans, catalyze the oxidation of minerals through a series of enzymatic reactions.
• The bacteria tend to attach to specific sites on the mineral surface, often where there are imperfections in the crystal structure.

For example, pyrite (FeS₂) is oxidized to ferric (III) sulfate via the following reaction: 

2FeS₂ + 7O₂ + 7H ₂O → 2FeSO4 + 2H₂SO₄ 

Direct oxidation can also be applicable for other non-iron metal sulfides such as covellite (CuS), chalcocite (Cu₂S), and sphalerite (ZnS).

2. Indirect Bacterial Leaching (Non- contact leaching)

• In Indirect bacterial leaching, no direct contact between the microorganisms and the mineral occurs.
• Here, the strong oxidizing agents produced by bacteria, such as ferric ions and sulfuric acids
• Instead, bacteria produce strong oxidizing agents, such as ferric ions and sulfuric acid, which chemically oxidize the metal sulfides in the ore.

In the bioleaching of uranium, the following reaction occurs: 

U₂O + Fe₂ (SO₄ )₃ → UO₂SO₄ + 2FeSO₄

In this mechanism, ferric iron is generated in an acidic environment (pH < 5.0) and acts as a lixiviant, oxidizing the sulfide minerals. The ferrous iron produced in this reaction can be reoxidized to ferric iron by T. ferrooxidans or L. ferrooxidans, thus perpetuating the leaching process. The presence of Thiobacillus thiooxidans further accelerates the oxidation of sulfur to sulfuric acid, creating a favorable acidic environment for continued metal solubilization.

In practice, both direct and indirect leaching mechanisms often occur simultaneously, particularly in natural environments and industrial applications.

Differences between direct bioleaching and indirect bioleaching

Character	Direct Bioleaching	Indirect bioleaching
Mechanism	Microorganisms directly interact with the mineral substrate where their enzymatic activity catalyzes the oxidation of metal	Microorganisms generate highly effective oxidizing agents that oxidize metal sulfides.
Microbial action	Organism directly attaches to metal sulfide ores	For example, an organism oxidizes ferrous to ferric ions, which reacts with the metal sulfides
Reaction (Example)	2FeS2​+7O2​+2H2​O→2FeSO4​ + 2H2​SO4​	FeS2+14Fe3++8H2O→15Fe2++2SO42−​+16H+
Application	Used in direct extraction of metals from sulfide ores.	Used in large scale, such as heap leaching.

Besides bacteria, fungi, and cyanogen are also responsible for bioleaching activities.

Fungal Bioleaching

• Fungi used: Aspergillus niger and Penicillium simplicissimum 
• Fungal activity:  Produce organic acids like gluconic, citric, and oxalic acids from glucose or sucrose. These acids dissolve metals from ores or waste.
• Process: The fungi convert glucose into organic acids through enzymatic reactions in the cytosol and mitochondria. The acids then dissolve metals via pathways like acidolysis (proton release), complexolysis (metal-ion stabilization with organic acids), redoxolysis (minor role for fungi, major for chemolithoautotrophic microbes), and bioaccumulation (active transport of metals into cells).

Cyanogenic Bioleaching

• Cyanogenic Microorganisms: Proteobacteria like Chromobacterium violaceum, Pseudomonas putida, and Pseudomonas fluorescens, etc.
• Fungal activity: Produce cyanide from glycine, which is an effective leaching agent for precious metals.
• Process: Cyanide is produced via HCN synthase, encoded by the hcnABC operon. The cyanide forms complexes with metals like copper, silver, and gold, facilitating their dissolution and recovery from ores. This bioleaching is typically an indirect process where biologically generated HCN creates soluble metallic complexes.

Types of Bioleaching

Three major processes are prevalent in bioleaching

1. Slope Leaching

• In slope leaching, fine ore is ground and placed in a large, slope-shaped dump.
• The process involves continuously sprinkling water containing Thiobacillus bacteria over the ore. 
• As the solution trickles down through the ore, it gathers at the bottom, rich with dissolved metals.
•  This metal-rich solution is then collected and processed further to recover the metals and regenerate the bacterial culture in an oxidation pond.

2. Heap Leaching

• Heap leaching is similar to slope leaching but involves the ore being arranged in large heaps, known as leach heaps. 
• An aqueous solution containing Thiobacillus is sprayed over these heaps. The solution percolates through the heap, dissolving metals as it goes. 
• The metal-laden solution is collected at the heap’s base and then processed to extract the metals and sustain the bacterial culture in an oxidation pond.

3. In-situ Leaching

• In-situ leaching does not require the ore to be removed from its natural location. Instead, after surface blasting to increase permeability, water containing Thiobacillus is pumped through drilled passageways within the ore body. 
• The solution slowly seeps through the rock, dissolving the metals. The metal-rich solution is then collected from the base of the ore body, and the water is reused after the bacterial culture has been regenerated.

Metal-Specific Bioleaching Processes

Copper Leaching

• Copper is extracted from ores such as chalcocite (Cu₂S), chalcopyrite (CuFeS₂), and covellite (CuS) using both heap leaching and in-situ leaching processes. 
• Dilute sulfuric acid is percolated through the ore piles, dissolving the copper into the solution. The liquid collected from the base of the pile is rich in copper and is transported to a precipitation plant, where the metal is purified and precipitated.

Uranium Leaching

• Uranium extraction is primarily an indirect bioleaching process. 
• Insoluble tetravalent uranium is oxidized with a hot H₂SO₄/FeSO₄ solution, converting it into a soluble hexavalent uranium sulfate. 
• The bacteria, particularly T. ferrooxidans, do not directly attack the uranium ore but instead assist in the production of ferric iron (Fe³⁺), which acts as an oxidant. 
• The process is optimized at a pH of 1.5-3.5 and a temperature of around 35°C.

Gold and Silver Leaching

• The bioleaching of gold and silver involves the microbial breakdown of refractory ores, such as arsenopyrite and pyrite, to enhance metal recovery.
•  Gold is typically extracted from arsenopyrite or pyrite, while silver is more readily solubilized during the microbial leaching of iron sulfides. 
• This process is particularly promising for recovering precious metals from low-grade ores.

Factors Affecting Bioleaching

1. Choice of Bacteria

• The bacteria must survive and thrive under extreme conditions such as high temperatures, acidic environments, and elevated concentrations of heavy metals. Their efficiency directly impacts the bioleaching process. Most used microorganisms are chemolithotrophic acidophiles. It also employs fungi along and some cyanogenic bacteria. E.g. Acidithiobacillus ferrooxidans, and Acidithiobacillus thiooxidans.
• Fungal bioleaching is done using Aspergillus niger and Penicillium simplicissium

2. pH and Temperature

• Optimal pH (around 2.0–2.5) and temperature (28–30°C) are crucial for maximizing microbial activity and metal solubilization. Since acidophilic chemolithotrophs are used for the bioleaching process, an acidic environment (low pH) is required. Deviations from these conditions can inhibit bacterial growth and decrease metal extraction rates.

3. Population Density of Microorganisms

• The higher the bacterial population densities, the higher the leaching rate due to an increased number of microorganisms to accelerate the oxidation reactions.

4. Metal Tolerance of Microorganisms

• The leaching process often leads to high metal concentrations in the leachate. 
• Organisms with high tolerance to increased metal concentration are effective, as elevated metal concentration can be toxic to most bacteria. 
• The use of tolerant/resistant organisms avoids inhibition and ensures effective metal extraction. Bacteria, particularly, thiobacilli tolerate high levels of heavy metals and can be used as best option.

5. Oxidation Reactions and Compositions

• Effective bioleaching requires maintaining appropriate levels of oxidation reactions to keep ferric ions and metals in solution. Low oxidation levels are necessary for achieving the fastest leaching rates.

6. Surface Area of the Ore/ Particle size

• The higher the surface area (lesser ore particle size) of the ore, higher the bacterial oxidation due to increased area for bacterial action
• An optimal particle size of about 42 µm is often used to maximize leaching efficiency.

7. Nutrients

• Chemolithoautotrophic bacteria require only inorganic compounds for growth. They require inorganic compounds such as iron and sulfur to promote their growth. Essential nutrients such as iron, sulfur, ammonium, phosphate, and magnesium salts must be provided to support bacterial activity and metal extraction.

8. Oxygen and Carbon Dioxide

• Oxygen: An adequate oxygen supply (1.5-4mg/l dissolved oxygen)  is crucial for high bacterial activity. 
• Carbon dioxide: CO2 is the sole carbon source required for bioleaching.

9. Mineral Substrate

• Leaching efficacy is affected by the mineral constituents.
•  High carbonate content can increase pH and inhibit bacterial activity, requiring acid addition to maintain low pH. 

10. Surfactants and Organic Extractants

• Surfactants and organic compounds used in solvent extraction can inhibit bacterial activity by decreasing surface tension and reducing oxygen transfer. 
• Solvent extraction must be managed to avoid adverse effects on the bioleaching process.

11. Catalyst

• The addition of catalysts like silver ions (Ag+), graphene, and biochar can significantly enhance bioleaching performance.

Applications of Bioleaching

• Environmental Stabilization: Bioleaching stabilizes sulfate toxins, minimizing environmental impact.
• Emission Reduction: It eliminates harmful sulfur dioxide emissions associated with traditional smelting, improving health and environmental outcomes.
• Cost-Effective Extraction: Bioleaching is a more cost-efficient method compared to smelting, particularly for low-grade ores.
• Optimized Recovery Processes: It allows for two-phase optimization, involving initial characterization and microorganism acclimatization, followed by scaling up to intermediate bioleaching techniques using bioreactors or columns.

Advantages of Bioleaching

• Simplicity: The process is simple and does not require complex technology.
• Cost Efficiency: It is economical, and particularly suited for low-grade sulfide ores.
• Environmental Friendliness: Bioleaching avoids harmful emissions and operates under milder conditions, making it eco-friendly.
• Low Energy Requirement: The process functions in ambient conditions, reducing energy consumption compared to smelting.

Limitations of Bioleaching

• Time-Consuming: Bioleaching can be slow, taking between 6 to 24 months or longer for metal recovery.
• Low Mineral Yield: The process often yields a lower amount of extracted metals compared to other methods.
• Space Requirement: It requires a large area, particularly for heap or in-situ leaching setups.
• Process Control Challenges: There is a risk of contamination and inconsistent yields due to variable bacterial growth and activity.

References

1. Alam, A. Biomining and Bioleaching. Retrieved from https://www.ramauniversity.ac.in/online-study-material/fet/biotechnology/m.sc/ii-semester/environmentalbiotechnology/lecture-6.pdf
2. Bioleaching Definition and Process. AngloAmerican. Retrieved from https://www.angloamerican.com/futuresmart/stories/our-industry/mining-explained/mining-terms-explained-a-to-z/bioleaching-definition-and-process
3. Bosecker, K. (1997). Bioleaching: metal solubilization by microorganisms. FEMS Microbiology reviews, 20(3-4), 591-604. https://doi.org/10.1111/j.1574-6976.1997.tb00340.x
4. Tezyapar Kara, I., Kremser, K., Wagland, S. T., & Coulon, F. (2023). Bioleaching metal-bearing wastes and by-products for resource recovery: a review. Environmental Chemistry Letters, 21(6), 3329-3350.
5. Tributsch, H. (2001). Direct versus indirect bioleaching. Hydrometallurgy, 59(2-3), 177-185.