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THE SULPHUR CYCLE
Figure 1: A Simplified Sulphur Cycle for Mineral Deposits
The environmental sulphur cycle involves many physical, chemical and biological agents. A simplified schematic diagram of the cycle is shown above (Figure 1), which has been prepared to show the major phases of sulphur cycling with relation to mineral deposits. As such, the figure indicates the relationships between sulphur, S, hydrogen sulphide, H2S, sulphur dioxide, SO2, and the sulphate ion, SO4--. In mineral form sulphur may be present as sulphides (e.g. pyrite, FeS2, chalcopyrite, FeS.CuS, pyrrhotite, FeS) and/or sulphates (e.g. gypsum, CaSO4.2H2O, barite, BaSO4). Sulphur in minerals may move through the cycle as a result of the oxidation of sulphides to sulphate and/or the dissolution of sulphates. For example, oxidation of pyrite to sulphuric acid may be immediately followed, in situ, by acid neutralization by calcium carbonate (calcite) to form calcium sulphate (gypsum). The reaction of hydrogen sulphide with dissolved metal ions may precipitate metallic sulphides which are chemically indistinguishable from naturally occurring sulphide minerals.
At some mines, sulphur is added to the cycle as sulphur dioxide in processes such as the Inco/SO2 process for cyanide destruction in the treatment of tailings. This added sulphur is oxidized to sulphate ion (Ingles & Scott, 1987), most of which remains free, but some of which combines with lime, CaO, in the tailings to form gypsum.
For information on the sulphur cycle with respect to water quality monitoring see Canadian Council of Environment Ministers (1987).
The Role of Micro-organisms in the Sulphur Cycle
Micro-organisms (most frequently bacteria) are often integrally involved in the chemical alteration of minerals. Minerals, or intermediate products of their decomposition, may be directly or indirectly necessary to their metabolism. The dissolution of sulphide minerals under acidic conditions (ARD), the precipitation of minerals under anaerobic conditions, the adsorption of metals by bacteria or algae, and the formation and destruction of organometallic complexes are all examples of indirect micro-organism participation. Where minerals are available as soluble trace elements, serve as specific oxidizing substrates, or are electron donors/acceptors in oxidation-reduction reactions, they may be directly involved in cell metabolic activity.
There are three categories of oxidation-reduction reactions for minerals with micro-organisms:
Oxidation by autotrophic (cell carbon from carbon dioxide) or mixotrophic (cell carbon from carbon dioxide or organic matter) organisms. Energy derived from the oxidation reaction is utilized in cell synthesis.
Electron acceptance by minerals (reduction) for heterotrophic (cell carbon from organic matter) and mixotrophic bacteria. Chemical energy is used to create new cell material from an organic substrate.
Electron donation by minerals (oxidation) for bacterial or algal photosynthesis (reaction is fuelled by photon energy).
Natural Oxidation in the Sulphur Cycle
Oxidation of sulphur or sulphides for energy production is restricted to the bacterial genus Thiobacillus, the genus Thiomicrospira, and the genus Sulfolobus. These bacteria all produce sulphuric acid (i.e. hydrogen ions, H+, and sulphate ions, SO4-- ) as a metabolic product. Extensive reviews of these bacteria and their behaviour have been written by Brierley (1978) and Trudinger (1971).
It is these bacteria that are known to accelerate the generation of Acid Rock Drainage (ARD) from pyritic and pyrrhotitic rocks under suitable conditions. Evangelou & Zhang (1995) report that sulphide oxidation catalysed by bacteria may have reaction rates six orders of magnitude (i.e. 1,000,000 times) greater than the same reactions in the absence of bacteria. Photomicrographs 1, 2 and 3, from LeRoux, North & Wilson (1973), illustrate the shape and appearance of T. ferrooxidans: The bacteria develop flagella only if they are required for mobility in accessing energy sources.
Photomicrographs 1 (left), 2 (centre) and 3 (right): Thiobacilli from bacterial generator (no flagella) - left & centre - and grown on ferrous iron (flagella) - right. Magnification about x5000 (left), x20,000 (centre), x15000 (right) (from LeRoux, North & Wilson, 1973)
ARD is the product formed by the atmospheric (i.e. by water, oxygen and carbon dioxide) oxidation of the relatively common iron-sulphur minerals pyrite and pyrrhotite in the presence of (catalysed by) bacteria (Thiobacillus ferrooxidans), and any other products generated as a consequence of these oxidation reactions.
An important reaction involving T. ferrooxidans is the oxidation of ferrous to ferric iron (Fe++ to Fe+++)
4Fe++ + O2 + 4H+ = Fe+++ + 2H2O
Ferric iron is a powerful oxidizing agent. Even at a Fe+++/Fe++ ratio of 1:1,000,000, a Redox potential of greater than +0.4 V is generated which is sufficient for the attack of most base metal sulphides (Dutrizac & MacDonald, 1974). The general equation for the ferric ion reaction with base metal sulphides is:
MS + nFe+++ = Mn+ + S + nFe++
Consequently T. ferrooxidans, in generating Fe+++, is indirectly responsible for the dissolution of base metal sulphide minerals and the mobilization of metallic cations such as Cu++, Zn++, Pb++ and Cd++. Base metal sulphides react only very slowly with sulphuric acid in the absence of ferric iron (Roman & Benner, 1973).
The importance of Redox potential in determining metal solubility and transport can be clearly seen for copper in the Eh-pH diagram for the Cu-H2O-O2-S-CO2 system (Figure 2, Garrels & Christ, 1965). The effects of bacteria upon the rate of dissolution of copper from chalcopyrite are highly pronounced, as demonstrated by Malouf & Prater (1961), Figure 3.
Figure 2: Eh-ph Diagram for Cu-H2O-O2-S-CO2 System (from Garrels and Christ, 1965)
Figure 3: Effects of Bacteria upon the Rate of Disolution of Copper from Chalcopyrite (from Malouf and Prater, 1961)
In general, for substantial metal mobilization from base metal sulphides the following conditions must be met:
Ferric iron for sulphide oxidation
T. ferrooxidans and oxygen for ferrous to ferric oxidation
pH compatible with T. ferrooxidans habitat requirements, typically pH 1.5-3.5 (Roman & Benner, 1973)
The typical habitat pH of T. ferrooxidans of 1.5 to 3.5 is not one that develops spontaneously. It is currently believed (Béchard, 1996) that these conditions are produced by a consortium of bacteria acting in succession. Such a succession may include T. thioparus at neutral pH, giving way to dominance by metallogenium bacteria under mildly acid conditions (pH 3.5 to 4.5) (Walsh & Mitchell, 1972), and finally T. ferrooxidans dominance at low pH.
The metabolic activity of T. ferrooxidans is temperature dependent, peaking at about 30-35 degrees Celsius, and falling with both increasing and decreasing temperature (Roman & Benner, 1973).
Leduc & Ferroni (1994) have demonstrated that T. ferrooxidans strains are site-specific.
From the above discussion it is clear that consideration of bacterial behaviour is most important in understanding the process of ARD generation. This is particularly so when "kinetic" tests are used to predict the rate of generation of ARD in the field. Only if the bacterial conditions of testwork are identical to those in the field, can rates of ARD generation and/or metal solubilization be taken from laboratory kinetic testwork and used to predict field behaviour with any degree of confidence.
Natural Reduction in the Sulphur Cycle
The direct reduction of sulphate ions to hydrogen sulphide is effected in nature by specialized, strictly anaerobic bacteria of the genera Desulfovibrio and Desulfotomaculum.
These sulphate reducing bacteria (SRB) are heterotrophic (cell carbon from organic compounds) organisms that utilize sulphate, thiosulphate, S2O3--, sulphite, SO3--, or other reducible sulphur-containing ions as terminal electron acceptors in their respiratory metabolism. In the process these sulphur-containing ions are reduced to hydrogen sulphide.
The bacteria require an organic substrate which is usually a short chain acid such as lactic or pyruvic acid. In nature such substrates are generated by the fermentation activities of other anaerobic bacteria on more complex organic substrates. Thus in natural systems, the specific requirement for a short chain acid by the SRB is met by the availability of a complex organic source and a mixed bacterial system. Lactate is used by the SRB during anaerobic respiration to produce acetate according to the reaction (Cork and Cusanovich, 1979):
2CH3CHOHCOO- + SO4-- = 2CH3COO- + 2HCO3- + H2S
This is the major natural process for the conversion (destruction) of sulphate ion. However, the process may be adapted to a controlled engineering process by the use of anaerobic reaction vessels and carbon monoxide, CO, and hydrogen, H2, or partially oxidized propane or natural gas, as the energy source for the bacteria (Warkentin and Rowley, 1994). This process of sulphate ion to hydrogen sulphide conversion is the first stage of a pilot plant that operated in 1996 at the former Britannia mine site in British Columbia, where the second stage utilized the hydrogen sulphide generated to precipitate copper (Cu++), Zinc (Zn++), and cadmium (Cd++) ions from ARD as metallic sulphides (Warkentin and Rowley, 1994). Further information, including details of the performance of this pilot plant may be found on the NTBC Research Corporation web site.
Steffen, Robertson and Kirsten (B.C.) Inc. (1991) have proposed the use of the underground mine at Faro, YT, as a giant underground SRB reactor to convert ARD sulphate ion to hydrogen sulphide, and precipitate zinc contained in the ARD as zinc sulphide. The proposed system would use the mixed bacterial system present in liquid cow manure as an SRB source, and sugar as a bacterial carbohydrate (energy) source.
SRB activity in natural wetlands is capable of metal sulphide precipitation from ARD as a result of sulphate to hydrogen sulphide reduction, and this concept may be extended to constructed wetlands. Dr. André Sobolewski treats this subject in considerable detail on his web site Wetlands for Treatment of Mine Drainage. Reviews of this subject have also been published by the Mine Environment Neutral Drainage (MEND) program (MEND, 1990, 1993).
Other Micro-organism Reactions in the Sulphur Cycle
Sulphate ion is taken up from soil by plants, which incorporate it into protein, and plant protein is consumed by animals that convert plant protein to animal protein. Death of plants and animals allows bacterial decomposition of protein in remains to produce hydrogen sulphide and other products , in processes involving many fungi, actinomycetes and bacteria such as the heterotroph Proteus vulgaris.
Some bacteria can function in the transition zone between aerobic and anaerobic environments,. Hydrogen sulphide may be oxidized to sulphur by such bacteria which deposit elemental sulphur in their cells while using oxygen as the terminal electron acceptor.
Hydrogen sulphide may also be oxidized to sulphate photosynthetically by the bacteria, Chromtiacceae and Chlorobiaceae.
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