The bioleaching of sulphide minerals with emphasis on copper sulphides — A review

1. The electron and the proton content (measured as electrode potential [Eh] and pH) of an environment characterize this environment in many ways. In this paper the electrode potential and the pH are used as empirical parameters rather than as electrochemical data capable of thermodynamic interpretation. From published and unpublished work by the authors and from the literature, more than 6,200 pairs of characteristics were gathered, covering most types of the aqueous environment as well as the potential milieu of the chief actors in these environments: algae and bacteria. 2. It appears that the Eh-pH limits of biological systems and of the naturally occurring aqueous environment almost coincide. This would indicate that there are few, if any, sterile terrestrial environments caused by limiting Eh-pH characteristics. 3. As it seems unlikely that environments will be found outside the limits outlined in this paper, physico-chemical speculations on the sedimentary environment should be limited by this outli...

Limits of the Natural Environment in Terms of pH and Oxidation-Reduction Potentials

The 'iron bacteria' are a collection of morphologically and phylogenetically heterogeneous prokaryotes. They include some of the first micro-organisms to be observed and described, and continue to be the subject of a considerable body of fundamental and applied microbiological research. While species of iron-oxidizing bacteria can be found in many different phyla, most are affiliated with the Proteobacteria. The latter can be subdivided into four main physiological groups: (i) acidophilic, aerobic iron oxidizers; (ii) neutrophilic, aerobic iron oxidizers; (iii) neutrophilic, anaerobic (nitrate-dependent) iron oxidizers; and (iv) anaerobic photosynthetic iron oxidizers. Some species (mostly acidophiles) can reduce ferric iron as well as oxidize ferrous iron, depending on prevailing environmental conditions. This review describes what is currently known about the phylogenetic and physiological diversity of the iron-oxidizing proteobacteria, their significance in the environment (on the global and micro scales), and their increasing importance in biotechnology.

The iron-oxidizing proteobacteria

Leaching bacteria such as Thiobacillus ferrooxidans attach to pyrite or sulfur by means of extracellular polymeric substances (EPS) (lipopolysaccharides). The primary attachment to pyrite at pH 2 is mediated by exopolymer-complexed iron(III) ions in an electrochemical interaction with the negatively charged pyrite surface. EPS from sulfur cells possess increased hydrophobic properties and do not attach to pyrite, indicating adaptability to the substrate or substratum.

Importance of Extracellular Polymeric Substances from Thiobacillus ferrooxidans for Bioleaching

The Tinto River (Huelva, southwestern Spain) is an extreme environment with a rather constant acidic pH along the entire river and a high concentration of heavy metals. The extreme conditions of the Tinto ecosystem are generated by the metabolic activity of chemolithotrophic microorganisms thriving in the rich complex sulfides of the Iberian Pyrite Belt. Molecular ecology techniques were used to analyze the diversity of this microbial community. The community’s composition was studied by denaturing gradient gel electrophoresis (DGGE) using 16S rRNA and by 16S rRNA gene amplification. A good correlation between the two approaches was found. Comparative sequence analysis of DGGE bands showed the presence of organisms related to Leptospirillum spp., Acidithiobacillus ferrooxidans, Acidiphilium spp., “Ferrimicrobium acidiphilum,” Ferroplasma acidiphilum, and Thermoplasma acidophilum. The different phylogenetic groups were quantified by fluorescent in situ hybridization with a set of rRNA-targeted oligonucleotide probes. More than 80% of the cells were affiliated with the domain Bacteria, with only a minor fraction corresponding to Archaea. Members of Leptospirillum ferrooxidans, Acidithiobacillus ferrooxidans, and Acidiphilium spp., all related to the iron cycle, accounted for most of the prokaryotic microorganisms detected. Different isolates of these microorganisms were obtained from the Tinto ecosystem, and their physiological properties were determined. Given the physicochemical characteristics of the habitat and the physiological properties and relative concentrations of the different prokaryotes found in the river, a model for the Tinto ecosystem based on the iron cycle is suggested.

Microbial Ecology of an Extreme Acidic Environment, the Tinto River

As late as 1945 Starkey called attention to the incompleteness of the information concerning the true autotrophic nature of iron bacteria, and to the confusion existing as to whether any bacteria in iron-bearing waters are concerned directly or indirectly with the precipitation of iron from the waters. The recent critical summary of researches on the iron bacteria by Pringsheim (1949) indicates a continuing interest in this member group of the Chlamydobacteriales, and, as Pringsheim states, "The general interest in iron bacteria is apparently greater than that bestowed upon other biological groups of comparable standing. This may partly be due to the fact that deposits of iron bacteria are readily perceived by the naked eye." Carpenter and Herndon (1933) investigated the drainage produced by bituminous coal mines on different coal seams. They reported some waters were alkaline or near neutrality, whereas others were very acid, and suggested that the reaction responsible for the production of the acid waters was:

Melvin E. Hinkle

Papers

An iron-oxidizing bacterium from the acid drainage of some bituminous coal mines