Diodegradation

1. Biodegradation
MBH-203: Environmental Microbiology

2. Bioleaching
 The extraction of metals from their ores by microorganisms.
 Conversion of insoluble metal sulfides into water-soluble
metal sulfates.
 Microbial leaching is increasingly applied for metal recovery
from low grade ores that cannot be processed economically
by conventional methods

3. Factors affecting Bioleaching

4. Bioleaching Mechanisms
 There are two major mechanisms of bacterial leaching:
 physical contact of the organism with the insoluble sulphide (direct or
contact mechanism)
 the ferric-ferrous cycle (indirect or non-contact mechanism)
 Direct mechanism:
 MS+2O2 → MSO4
 Where M is a bivalent heavy metal.
 The following equations describe the “direct” mechanism for the
oxidation of pyrite:
 2FeS2+7O2+2H2O → 2FeSO4+2H2SO4

5. Bioleaching Mechanisms
 Indirect mechanism:
 the reaction mechanism for the bio-oxidation of sulphide minerals-
indirect mechanism:
 4FeSO4+O2+2H2SO4 → 2Fe2(SO4)3+2H2O
 FeS2+Fe2(SO4)3 → 3FeSO4+2S
 2S+3O2+2H2O → 2H2SO4
 The indirect mechanisms can be demonstrated, i.e., for uranium
leaching as follows:
 UO2+Fe2(SO4)3 → UO2SO4+2Fe2SO4

6. Types of Bioleaching
 In situ: leached directly from the ore without excavating the
ore prior to leaching
 Dump: ore is taken directly from the mine and stacked on the
leach pad without crushing where, and is irrigated with a
dilute cyanide solution that percolates through the ore to
dissolve the metal.
 Heap: ore is usually crushed into small chunks and heaped
on an impermeable plastic and/or clay lined leach pad where
it can be irrigated with a leach solution to dissolve the
valuable metals

7. Types of Bioleaching
 Vat: involves placing ore, usually after size reduction and
classification, into large tanks or vats at ambient operating
conditions containing a leaching solution and allowing the
valuable material to leach from the ore into solution.
 Reactor: Metal sulfide concentrates are generally bioleached
in stirred tank reactors (agitation leaching)

8. Benefits of Bioleaching
 Simple and inexpensive process
 No sulfur dioxide emissions as in smelters
 No need for high pressure or temperature
 Leaching residues less active than in physico-chemical
processes
 Ideal for low grade sulfide ores – lower cut-off rate possible

9. Bioleaching of Iron
 Microorganisms involved: Thiobacillus ferrooxidans (now called
Acidithiobacillus ferrooxidans), and Thiobacillus thiooxidans (now called
Acidithiobacillus thiooxidans)
 The most important player in the bioleaching process is
Acidithiobacillus ferrooxidans. It is a chemoautotrophic.
 Its unique ability to oxidise ferrous to ferric, and sulphur and reduced
sulphur compounds to sulphuric acid, leads to leaching of metals from
their oxide and sulphide ores.
 Penicillium verruculosum can also bioleach iron
 Mineral ore: Pyrite (FeS2)

10. Pyrite leaching
 Pyrite is spontaneously oxidized to thiosulfate by Fe3+
 The ferrous ion is then oxidized by bacteria (iron oxidizers)
using oxygen:
 Thiosulfate is also oxidized by bacteria (sulfur oxidizers) to
give sulfate:
 Net reaction:

11. Bioleaching of Copper
 Microorganisms involved: Thiobacillus ferrooxidans (now called
Acidithiobacillus ferrooxidans), and Thiobacillus thiooxidans (now called
Acidithiobacillus thiooxidans)
 Many different microorganisms have been isolated from
copper dumps. (These include a variety of mesophilic, aerobic iron and
sulphur oxidising microorganisms; thermophilic iron and sulphur oxidising
microorganisms; and anaerobic sulphate reducing bacteria. Some are
heterotrophic bacteria, which indirectly affect metal solubilisation by affecting the
growth and activity of metal solubilising bacteria. Others are protozoa, which
interact with and prey on different types of bacteria.)
 Mineral ore: Chalcopyrite (CuFeS2)

12. Chalcopyrite leaching
 Chalcopyrite is spontaneously oxidized
 The ferrous ion is then oxidized by bacteria (iron oxidizers)
using oxygen:
 Sulfur is also oxidized by bacteria (sulfur oxidizers) to give
sulfate:
 Net reaction:

13. Cu2+ processing
 The dissolved copper (Cu2+) ions are removed from the
solution by ligand exchange solvent extraction, which leaves
other ions in the solution.
 The copper can also be concentrated and separated by
displacing the copper with Fe from scrap iron.

14. Bioleaching of Uranium
 Mineral ore: Uraninite (UO2)
 Uraninite is a radioactive, uranium-rich mineral and ore with a
chemical composition that is largely UO2, but due to oxidation the
mineral typically contains variable proportions of U3O8.
 Uranium leaching proceeds by the indirect mechanism as
Acidithiobacillus ferrooxidans does not directly interact with
uranium minerals.
 Indirect mechanism:
 UO2+Fe2(SO4)3 → UO2SO4+2Fe2SO4

15. Bioleaching of Gold
 Also called Bioliberation of Gold.
 Iron- and sulphur-oxidising acidophilic bacteria are able to
oxidise certain sulphidic ores containing encapsulated
particles of elemental gold, resulting in improved accessibility
of gold to complexation by leaching agents such as cyanide.
 Bio-oxidation involves treatment with Acidithiobacillus
ferrooxidans to oxidise the sulphide matrix prior to cyanide
extraction.

16. Bioleaching of Gold
 Commercial exploitation has made use of heap leaching
technology for refractory gold ores.
 Refractory sulphidic gold ores contain mainly two types of
sulphides: pyrite and arsenopyrite.
 Since gold is usually finely disseminated in the sulphide
matrix, the objective of biooxidation of refractory gold ores is
to break the sulphide matrix by dissolution of pyrite and
arsenopyrite.