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    The Chemistry of Acid Mine Drainage

    Richard T. Lowson
    Australian Nuclear Science and Technology Organization
    Brian J. Reedy and James K. Beattie
    University of Sydney

    Acid Mine Drainage (AMD) is a major environmental hazard for the mining industry. The magnitude of the hazard grows as the mining industry grows to meet modern material demand. The hazard arises due to the exposure of reduced sulfidic ores and gangue material to the natural elements of air and water. The sulfides have a thermodynamic capacity to react with the air and water to form sulfuric acid. In uncontrolled situations the product is discharged as an acid liquor, hence the descriptive name. The hazard may be exacerbated by the acid leaching out base metals from the associated rock The concentration of elements such as copper can exceed 100 mg/L. The toxic limit of copper for some fresh water biota can be as low as 0.05 mg/L.

    The dominant sulfide mineral contributing to AMD is pyrite and use of the phenomenon as a source of vitriol and sulfuric acid may be traced back to Roman and Greek writings(1). The process has been used down the ages to modern times to recover base metals, and it forms the basis for a number of modern heap-reach and in situ operations for economic recovery of base metals from low grade ores. Recognition of the process as an environmental hazard developed in the late 1950s and early 1960s as the diverse occurrence of the problem became documented. Not limited to base metal mines, AMD is an environmental hazard for a range of industries including high sulfur coal mines and agriculture. In the Australian context, it is a severe problem for the base metal mines in the temperate and tropical areas, and at the abandoned sites down the west coast of Tasmania, in southern South Australia, eastern New South Wales and the tropics of the Northern Territory. Progress on unravelling the controlling parameters in the environmental context has been slow. To a certain extent this was because many of the investigations were single discipline based, with reports detailing the results on highly focused topics such as reaction kinetics, morphology, or bacteriology. While both relevant and competent, in the context of AMD the results provided only a small fragment of the total information required for understanding the overall process.

    A major development in describing the process was made by Dr Ian Ritchie's group at ANSTO(2). Using temperature as a measure of the product heat, they were able to determine the overall reaction rate of an AMD leaching waste heap and identify the controlling parameters for the hazard. They established that in a heap or in situ situation, the overall rate of reaction is controlled by the bulk transport of reactant (oxygen) into the heap or in situ site. This is the 'slow' and therefore rate determining step. Application of the faster chemical or bacteriological mediated reaction rate determined in the laboratory was inappropriate to the environmental situation. The main contribution for the chemist is the definition of the overall stoichiometry.

    Stoichiometry of acid mine drainage
    Unfortunately defining the overall stoichiometry is not a simple matter. This is because the oxidation of the disulfide ion S22- to sulfate is a seven electron process, and prone to a number of side reactions. Consequently authors have proposed a range of either partial or complete oxidation pathways. For example: Allan and Johnston(3) in 1910 proposed equation 1 for oxidation of only the sulfur species (S22-) by molecular oxygen. If the oxidation of Fe2+ is included, then the overall stoichiometry is equation 2.

    2FeS2 + 7O2 + 2H2O → 2Fe2+ + 4SO42- + 4H+ (1)
    4FeS2 + 15O2 + 2H2O → 4Fe3+ + 8SO42- + 4H+ (2)
    FeS2 + 14Fe3+ + 8H2O → 15Fe2+ + 2SO42- + 16H+ (3)
    4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H2O (4)

    In 1960 Garrels and Thompson(4) proposed oxidation of the sulfur species (S22-) by Fe3+ (equation 3). The ferric ion is supplied through the oxidation of ferrous ion by molecular oxygen (equation 4). None of the above equations took into account side reactions. The dominant identified side reaction is the formation of stable sulfur, S0, which can occur at up to 40% yield. This reaction is best represented as a half-cell reaction (equation 5, 5a). Formation of S0 was first reported by Gmelin(5) in 1851 and widely confirmed by other workers(1).

    FeS2 → Fe2+ + 2S0 + 2e- (5)
    Eh = 0.5266 + 0.0296log(aFe2+) (5a)


    Although referred to as a minor product, it should be noted that the formation of sulfur is a major stumbling block in the industrial removal of sulfur from pyritic coal. The only mechanism to account for the variable production of sulfur is that of Bailey and Peters(6) who interpreted the overall process as a set of half-cell anodic reactions for the production of sulfur (equation 5, 5a), sulfate (equation 6, 6a) and ferric ion (equation 7, 7a). The anodic reactions are balanced by the cathodic oxygen reduction reaction (equation 8, 8a).

    FeS2 + 8H2O → Fe2+ + 2SO42- + 16H+ + 14e- (6)
    Eh = 0.362 - 0.05917pH + 0.0296log (aFe2+ a2SO42-) (6a)
    Fe2+ + e- (7)
    Eh = 0.770 + 0.05917log(aFe2+/aFe2+) (7a)
    O2 + 4H+ + 4e- → 2H2O (8)
    Eh = 1.23 - 0.05917pH (8a)

    Chemical parameters contributing to AMD
    The aqueous oxidation of pyrite is inherently a complex process because:
    • The first step is a heterogeneous surface reaction,
    • The overall reaction involves the transfer of 7 electrons per mole of sulfur,
    • The natural material has three well defined morphologies: cubic pyrite. orthorhombic marcasite and framboidal amorphous material,
    • The sudace has a passivating surface film,
    • Pyrite has a variable elemental composition from FeS2.00 to FeS1.94
    • Pyrite has trace metal impurities, and
    • Pyrite occurs as both n- and p-type semiconductor material.
    Laboratory studies have shown that the oxidation rate has the following attributes.
    • It is between fractional and first order with respect to oxygen and is independent of solution composition. This includes the concentration of H+, Fe2+, and Fe3+; Cu2+ as a catalyst and SO42- except for gross variations and then only marginally.
    • It is first order or greater with respect to water in under-saturated systems.
    • The activation energy indicates a chemical rather than physical rate determining step.
    • The activation energy decreases with increasing pH.
    • The production of elemental sulfur reaches a maximum at about 100-150°C; overall production is reduced above 150°C and below 100°C.
    • The formation of sulfate is favoured with increasing overpotential.
    • The formation of thiosulfate intermediates is possible.

    18O labelling studies
    While the ultimate oxidation source is molecular oxygen, it is still uncertain whether or not all of the molecular oxygen is directly associated with the formation of sulfate. For example, if reaction 3 is solely responsible for the formation of sulfate, then all the sulfate oxygen atoms are derived from water. A solution is to isotopically mark the oxygen containing reactants (O2 or H2O) with 18O and observe the isotopic composition of the product sulfate.

    Experimental method
    The experimental method was deceptively simple. Pyrite was reacted with water and gaseous/dissolved oxygen under controlled conditions of pH and temperature. The pH was controlled with hydrochloric acid. Either the water or the gaseous oxygen was marked with the stable isotope 18O. At the end of the experiment the sulfate was precipitated as barium sulfate and analysed by Fourier Transform Infra-Red spectrometry(FTIR). The introduction of 18O into the SO4 causes a spectral shift in the IR spectra.

    Crystalline BaS16O4 has four formula units per unit cell of space group D2h16 (Vh16). The site symmetry of each of the tetrahedral sulfate ions is CS, and crystal field effects lead to a splitting of the v1 vibration in the infra-red into two components, B1u and B3u. This splitting cannot be observed in the standard infra-red spectra measurements and the weakly active v1 vibrational mode is observed as a single band at 981 cm-1 that shifts to 927 cm-1 for BaS18O4.

    There is a change in crystal symmetry on moving from a pure isotope system to a system containing a heterogeneous mixture of the isotopomers S16O4-n18On (where n = 1 to 4). This lowered symmetry leads to further band splitting. Figure 1 is a calibrating FTIR spectrum in the v1 region, for powdered BaSO4 with a random distribution of 50% 18O across the complete set of 5 isotopomers. The bands for S16O218O22-(954 cm-1) and S16O18O32- (944 cm-1) exhibit some additional fine structure. It was found that for these mixed systems the IR intensity of each isotopomer no longer varies linearly with concentration; that is to say, the individual absorbances do not follow Beer's law. This is because there is a continuous change in symmetry with changing isotope concentration. Thus while infra-red spectroscopy is a convenient qualitative technique for observing mixed sulfate isotopomer distributions, it has little quantitative use(7), and Raman spectroscopy on larger samples is required.

    Results of isotope labelling experiment
    Figure 2 shows the FTIR spectrum of the v1 region of BaSO4 from pyrite oxidized at pH 1, 70°C for 60 hours under 18O2 in natural abundance water, acidified with HCl. Figure 3 shows the results of the reverse experiment using ordinary air (essentially 16O2) and H18O2. The spectra is the mirror image of Figure 2. The band assignments were based on the earlier calibrations.

    The major conclusion to be drawn, is that at the end of the experiment the majority of the oxygen in the sulfate is derived from the water. The production of three different sulfate isotopomers, indicates the existence of at least three different pathways for pyrite oxidation in acid solution in achieving the end result. A range of sulfur species of intermediate oxidation state (between S22- and SO42-) have been proposed by a number of workers. Some conclusions as to the nature of these species may be drawn from these experiments.(8)

    Sulfite, SO32- is known to exchange oxygen atoms very rapidly with water and reflects the isotopic composition of the solvent, water. Thiosulfate, S2O32-, also exchanges oxygen with water, but not as quickly. Thus, while these species may be precursors to S16O42- and S16O318O2- in the oxidation with 18O2, there must be other species present to account for the presence of S16O218O22-. Isotropic tracer studies on the oxidation of intermediate species such as sulfite, thiosulfite and tetrathionite may resolve this problem.

     Summary

    Acid Mine Drainage (AMD) is a major environmental hazard of 20th century mining practice. The controlling parameters are defined by the physics of the site. The chemistry is complex and not completely identified. Some Australian work using 18O in near isotopically pure O2 and H2O has identified the general direction of the reaction path. The results indicate that while oxygen is the primary oxidant, water plays a pivotal role as the disulfide species is oxidized to sulfate. Important areas in understanding the role of intermediates remain remains undefined and is an area which will have to be developed before an overall understanding of the environmental hazard is reached.

    References

    1. Lowson, R.T., Chem. Rev. 1982, 82, 461.
    2. Pantelis, G. and Ritchie, A.I.M., Appl. Math. Modelling, 1992, 16, 553.
    3. Allen, E.T. and Johnston, J.J., J. Ind. Eng. Chem.,1910, 2, 196.
    4. Garrels, R.M. and Thompson, M.E., Amer. J. Sci.,1960, 258-A, 57.
    5. Gmelin, Handbuch Chemie, 1851, 5, 234.
    6. Bailey, L.K. and Peters, E., Canad. Metall. Quart., 1976, 15, 333.
    7. Reedy, B.J., Beattie, J.K. and Lowson, R.T., Spectrochimica Acta, 1990, 10A, 1513.
    8. Reedy, B.J., Beattie, J.K. and Lowson, R.T., Geochim. Cosmochim. Acta, 1991, 55, 1609.

    About the authors
    James Beattie (FRACI) is Associate Professor of Chemistry at The University of Sydney where he has been on the staff since 1972. A migrant from the USA, he was educated at Princeton, Cambridge and Northwestern where his Ph.D. research with Fred Basolo developed a continuing interest in inorganic reaction mechanisms.

    Brian Reedy obtained First Class Honours at The University of Sydney and a Ph.D. degree there in 1990. Presently he is pursuing his interest in bioinorganic chemistry as a postdoctoral fellow at the Oregon Graduate Institute in the laboratories of Professor Ninian Blackburn.

    Richard Lowson is a Senior Principal Research Scientist and Project Manager of the Environmental Chemistry group at ANSTO. His group is focused on environmental remediation which includes AMD, and geochemistry.

    The Leighton Address 1996 Cerebral Chemistry
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