Acid Mine Drainage is acidic water (pH <5.0), laden with iron, sulfate and other metals, that forms under natural conditions when geologic strata containing pyrite are exposed to the atmosphere or oxidizing environments. AMD can form from coal mining, both in surface and in underground mines. Alkaline mine drainage is water that has a pH of 6.0 or above, contains alkalinity, but may still have dissolved metals that can create acid by oxidation and hydrolysis. The drainage quality (acid or alkaline) emanating from underground mines or backfills of surface mines is dependent on the acid (sulfide) and alkaline (carbonate material) minerals contained in the disturbed geologic material. In general, sulfide-rich and carbonate-poor materials are expected to produce acidic drainage. In contrast, alkaline-rich materials, even with significant sulfide concentrations, often produce net alkaline water.
Acid mine drainage (AMD) or acid rock drainage, collectively called acid drainage (AD), is formed when certain sulfide minerals in rocks are exposed to oxidizing conditions. Much of the acid drainage worldwide is commonly thought to be associated with coal mining, but acid drainage can occur under natural conditions or where sulfides in geologic materials are encountered in metal mining, highway construction, and other deep excavations. Iron sulfides common in coal regions are predominately pyrite and marcasite (FeS2), but other metals may be combined with sulfide in the form of chalcopyrite (CuFeS2), covellite (CuS), and arsenopyrite (FeAsS) (Table 1). Pyrite commonly occurs with other metal sulfides, potential causing acid drainage.
Table 1. Some important metal sulfides which occur in mining regions. The predominant acid producers are pyrite and marcasite.
|FeS2 – pyrite||MoS2 – molybdenite|
|FeS2 – marcasite||NiS – millerite|
|FexSx – pyrrhotite||PbS – galena|
|Cu2S – chalcocite||ZnS – sphalerite|
|CuS – covellite||FeAsS – arsenopyrite|
|CuFeS2 – chalcopyrite|
Upon exposure to oxidizing conditions and in the absence of alkaline materials, some sulfide minerals are oxidized in the presence of water and oxygen to form highly acidic, sulfate-rich drainage. Acidity levels, and metal composition and concentration depend on the type and amount of sulfide mineral and the presence or absence of alkaline materials. In the coal fields when sulfides are present, the oxidation of Fe disulfides and subsequent conversion to acid occur through several reactions. The following four chemical equations are accepted to explain the processes.
Equation 1 FeS2 + 7/2 O2 + H2O = Fe2+ + 2 SO42- + 2 H+
Equation 2 Fe2+ + 1/4 O2 + H+ = Fe3+ + ½ H2O
Equation 3 Fe3+ + 3 H2O = Fe(OH)3 + 3 H+
Equation 4 FeS2 + 14 Fe3+ + 8 H2O = 15 Fe2+ + 2 SO42- + 16 H+
In equation 1, Fe sulfide is oxidized, thereby releasing ferrous iron (Fe2+, the reduced form of iron), sulfate (SO42-), and acid. Ferrous iron in equation 2 can be oxidized to form ferric iron (Fe3+). Ferric iron can then either be hydrolyzed and form ferric hydroxide, Fe(OH)3, and H+ acidity (equation 1.3), or it can directly attack pyrite and act as a catalyst in generating much greater amounts of ferrous iron, sulfate, and acidity (equation 4). If any of the processes represented by the equations were slowed or altogether stopped, the generation of AMD would also slow or cease. Removal of air and/or water from the system, two of the three principal reactants, would stop pyrite from being oxidized. Almost complete absence of oxygen occurs in nature when pyrite is found beneath the water table where oxidizing conditions are limited. Under these conditions, the pyrite remains almost completely unreacted. When pyrite is enclosed within massive rock, only minimal amounts of pyrite are oxidized through natural weathering, thereby generating only small amounts of acid, and this acid is sometimes naturally diluted or neutralized by surrounding alkaline rocks. However, when large volumes of pyritic material are fractured and exposed to oxidizing conditions, which can occur in mining or other major land disturbances, the pyrite reacts, and water dissolves and moves the reaction products (Fe and other metals, sulfate, and acid) into ground and surface water sources.
Under many conditions, equation 2 is the rate-limiting step in pyrite oxidation because the conversion of ferrous iron to ferric iron is slow at pH values below 5 under abiotic conditions. However, Fe-oxidizing bacteria, principally Thiobacillus, greatly accelerate this reaction, so the activities of bacteria are crucial for generation of most AMD, and much work in bactericides has been conducted. In contrast, availability of oxygen may be the rate-limiting step in spoil of low porosity and permeability such as that composed of soft shale, so that oxidation is limited to the upper few meters of spoil. In porous and permeable spoil composed of coarse sandstone, air convection driven by the heat generated by pyrite oxidation may provide high amounts of oxygen deep into the spoil. The rate of pyrite oxidation depends on numerous variables such as reactive surface area of pyrite, form of pyritic sulfur, oxygen concentrations, solution pH, catalytic agents, flushing frequencies, the presence of Thiobacillus bacteria.
The natural base content of overburden materials (alkali and alkaline earth cations, commonly present as carbonates or exchangeable cations on clays) is important in evaluating the future neutralization potential (NP) of the materials. The amount of alkaline material in unweathered overburden may be sufficient to equal or overwhelm the acid-producing potential of the material. Of the many types of alkaline compounds present in rocks, carbonates (specifically calcite and dolomite) are the primary alkaline compounds which occur in sufficient quantities to be considered as effective deterrents to AMD generation. In overburden containing both alkaline and pyritic material, the alkaline material may be sufficient to reduce oxidation from occurring orto neutralize the acid formed from pyrite. Higher alkalinities also help control bacteria and restrict solubility of ferric iron, which are both known to accelerate acid generation. Although a number of factors must be considered, a balance of the acid-producing potential and neutralizing capacity of an overburden sample will indicate whether acidity or alkalinity is expected in the material upon complete weathering.