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Factors controlling acid mine drainage formation |
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The following equations show the generally accepted sequence of pyrite reactions:
2 FeS2 + 7 02 + 2 H2O -> 2 Fe2+ + 4 SO4 + 4 H+ (Equation 1)
4 Fe 2+ + O2 + 4 H+ -> 4 Fe3+ + 2 H2O (Equation 2)
4 Fe3+ + 12 H2O -> 4 Fe(OH)3 + 12 H+ (Equation 3)
FeS2 + 14 Fe3+ + 8 H2O -> 15 Fe2+ +2 SO42- + 16 H+(Equation 4)
In the initial step, pyrite reacts with oxygen and water to produce ferrous iron, sulfate and acidity. The second step involves the conversion of ferrous iron to ferric iron. This second reaction has been termed the "rate determining" step for the overall sequence.
The third step involves the hydrolysis of ferric iron with water to form the solid ferric hydroxide (ferrihydrite) and the release of additional acidity. This third reaction is Ph dependent. Under very acid conditions of less than about Ph 3.5, the solid mineral does not form and ferric iron remains in solution. At higher Ph values, a precipitate forms, commonly referred to as "yellowboy."
The fourth step involves the oxidation of additional pyrite by ferric iron. The ferric iron is generated by the initial oxidation reactions in steps one and two. This cyclic propagation of acid generation by iron takes place very rapidly and continues until the supply of ferric iron or pyrite is exhausted. Oxygen is not required for the fourth reaction to occur.
The overall pyrite reaction series is among the most acid-producing of all weathering processes in nature.
The pyrite weathering process is a series of chemical reactions, but also has an important microbiological component. The conversion of ferrous to ferric iron in the overall pyrite reaction sequence has been described as the "rate determining step" (Singer and Stumm, 1970). This conversion can be greatly accelerated by a species of bacteria, Thiobacillus ferroxidans. This bacteria and several other species thought to be involved in pyrite weathering, are widespread in the environment. T. ferroxidans has been shown to increase the iron conversion reaction rate by a factor of hundreds to as much as one million times (Singer and Stumm, 1970; Nordstrom, 1979).
The activity of these bacteria is Ph dependent with optimal conditions in the range of Ph 2 to 3. Thus, once pyrite oxidation and acid production has begun, conditions are favorable for bacteria to further accelerate the reaction rate. At Ph values of about 6 and above, bacterial activity is thought to be insignificant or comparable to abiotic reaction rates. The catalyzing effect of the bacteria effectively removes constraints on pyrite weathering and allows the reactions to proceed rapidly. The role of microbes in pyrite oxidation is described in more detail by Kleinmann and others (1981) and Nordstrom (1979).
Paleoenvironments under which coal bearing rocks formed can be characterized into three general categories: marine; freshwater; and brackish. Studies of Pennsylvanian age coal bearing rocks have shown that paleoenvironment can be used to broadly define acid drainage potential (Brady and others 1988; Hornberger and others 1981). Rocks formed in brackish water conditions are generally most prone to acid production; freshwater systems usually produce non-acid water, and marine systems produce variable drainage quality. In some coal measures, the paleoenvironment varies laterally and vertically within a single minesite and is a controlling factor in the inherent distribution of pyrite and carbonates.
Drainage and spoil quality is a product of two competing processes: acid formation from pyrite oxidation, and generation of alkalinity from dissolution of carbonates and other basic minerals.
The acid generation process consists of three phases: initiation; propagation; and termination. The initiation phase can begin as soon as pyritic materials are exposed to an oxidizing environment, however, the acid load generated is relatively small. In the propagation phase, and acid production increase rapidly. In the termination phase, acid production gradually declines. The actual times associated with these phases are, at present, ill-defined, but appear to be on the order of years to decades. Modeling predictions and comparison to a limited number of field sites indicate the peak acid load occurs 5 to 10 years after mining, followed by a gradual decline over 20 to 40 years (Ziemkiewicz and others, 1991, Hart and others, 1991). The same studies project very long decay curves for coal refuse (beyond 50 years) before acid leachate is depleted. Reliable acid generation/depletion predictions for underground mine discharges are not available.
The overall acid-producing process can proceed very rapidly with few chemical constraints. In contrast, dissolution or reaction rates of many common minerals is generally slow due to solubility limitations. Production of alkalinity tends to attain a constant value or level off with time so that the rate of acid production commonly may exceed the production of alkalinity.
The trends in reaction rates can be offset or enhanced by the mass balance between acid and alkaline producing minerals. A general relation between acid and basic minerals and resultant drainage quality is described as follows: o Low pyrite, low base content - Drainage may contain low levels of acidity, or maybe non-acid. Low concentrations of dissolved metals.
The conditions most conducive to acid formation are high pyrite contents with little base material present. Conversely, an excess of base relative to pyrite is most likely to preclude acid formation. Sites containing low quantities of pyrite and bases produce the most variable drainage quality and are the most difficult to assess with premining predictive techniques.
Lithology or rock type also influences spoil and drainage quality. Physical characteristics of the rock, such as porosity, and accessory minerals can exert various constraints or enhancements to the overall chemical weathering process. For example, pyritic sandstones tend to release their acid load rapidly (Ziemkiewicz, 1991). Argillaceous rocks tend to release their acid load over a longer period of time. Accessory minerals (clays and other silicates) may dissolve, form new minerals, or attenuate the acid and alkaline weathering products.
The mineral pyrite occurs in several different morphological forms and a range of grain sizes. The "framboidal" form is considered highly reactive and is characterized by a small grain size and large surface area (Caruccio and others, 1977). Pyrite can occur in grain sizes ranging from invisible to the eye up to several inches. Framboids and other fine grained pyrites with a large surface area are much more chemically reactive than the coarser forms. The reactivity of fine grained pyrites reflects the fact that acid generating reactions occur at the mineral surface.
Minesite hydrology plays a critical role in determining drainage quality, yet the flow mechanics of ground water in spoils are among the least understood aspects of AMD. The products of pyrite oxidation are free acid and soluble acid salts. If no percolating waters are present, the acid salts generated from the limited available moisture simply reside within the spoil. When excess moisture is present, the acid weathering products are dissolved and transported with the water moving through the material.
The chemistry of ground-water discharges can vary depending on the degree of flushing (Snyder and Caruccio, 1988) and time since the last precipitation event. Ground-water discharge can be "flashy" or rapid shallow interflow associated with high intensity short duration precipitation events or base flow. Underground mine discharges which drain from large volume pool storage typically exhibit a muted or seasonal response to precipitation patterns.
The position of a water table within the spoil also influences drainage quality. Water table elevations in spoils fluctuate in response to seasonal conditions forming a zone of cyclic wetting and drying. This provides optimal conditions for the oxidation and subsequent leaching of pyrite and associated weathering products. Ground-water flow paths and the location and elevation of saturated zones are often difficult to predict in mine spoils.
Numerous chemical, physical and biological factors interact to control the quality of mine drainage. Although the basic processes of acid mine drainage formation are universal, the importance of any single controlling factor is frequently specific to minesite conditions.