09-11-2012, 04:04 PM
GENERAL INTRODUCTION and LITERATURE REVIEW
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INTRODUCTION
The term bioleaching refers to the conversion of an insoluble metal (usually a metal sulfide, e.g., CuS, NiS, and ZnS) into a soluble form (usually the metal sulfate, e.g., CuSO4, NiSO4, ZnSO4). When this happens, the metal is extracted into water; this process is called bioleaching (Kelly, et al. 1979, Torma. 1977). Because these processes are oxidations, this process may also be termed biooxidation. However, the term biooxidation is usually used to refer to processes in which the recovery of a metal is enhanced by microbial decomposition of the mineral, but the metal being recovered is not solubilized. An example is the recovery of gold from arsenopyrite ores where the gold remains in the mineral after biooxidation and is extracted by cyanide in a subsequent step. The term bioleaching is clearly inappropriate when referring to gold recovery (although arsenic, iron, and sulfur are bioleached from the mineral). Biomining is a general term that may be used to refer to both processes.
BIOLEACHING MICROORGANISMS
The most important mineral-decomposing microbes are the iron- and sulfur-oxidizing chemolithrophs, which grow autotrophically by fixing CO2 from the atmosphere. Not all of the mineral-oxidizing organisms are equally efficient at CO2 fixation, and some grow better when provided with air that has been enriched with 0.5–5.0% (v/v) carbon dioxide (Clark at al., 1996). Unlike most autotrophic organisms that use radiant energy from sunlight, chemolithotrophs obtain their energy by using either ferrous iron or reduced inorganic sulfur compounds (some use both) as an electron donor, and oxygen as the electron acceptor. As sulfuric acid is produced during the oxidation of inorganic sulfur, these organisms grow in low-pH environments. Most mineral bio-oxidation processes operate at a pH between 1.4 and 1.6. At low pH, ferric iron is soluble and many of the sulfuroxidizing organisms are able to use ferric iron in place of oxygen as an electron acceptor. This ability is relevant in non aerated heap reactors in which oxygen might not penetrate to the bottom of the heap.
MESOPHILES
ACIDITHIOBACILLUS
Biomining bacteria belonging to this genus were previously included in the genus Thiobacillus. As a result of 16S rRNA sequence analysis, it became clear that the genus Thiobacillus (as described prior to 2000) included sulfur-oxidizing bacteria that belonged to α-, β-, and γ -divisions of the Proteobacteria. To solve this anomaly, the genus Thiobacillus was subdivided (Kelly et al., 2000) and a new genus, Acidithiobacillus, was created to accommodate the highly acidophilic members of the former genus. These members include Acidithiobacillus ferrooxidans (previously Thiobacillus ferrooxidans), At. thiooxidans (previously T. thiooxidans), and At. caldus (previously T. caldus). Phylogenetically, the genus Acidithiobacillus is situated very close to the branch point between the β-, and γ -subdivisions of the Proteobacteria (Lane et al., 1992; Rawlings. 2001).
LEPTOSPIRILLUM
Bacteria of this genus are similar to acidithiobacilli in that they are also highly acid-tolerant (optimum pH 1.5–1.8), gram-negative, chemolithoautrotrophic bacteria. However, in most other respects they are very different (Johnson., 2001). Based on 16S rRNA sequence data, they are not members of the Proteobacteria but belong to the division Nitrospira. An unusual and unifying characteristic of the leptospirilli is that they are capable of using only ferrous iron as an electron donor.