Editor's Note: The contents of this page are based on the text of a presentation given by the author to British Columbia high school science teachers at a seminar on Acid Rock Drainage at the Cordilleran Roundup, held at the Hotel Vancouver, Vancouver, B.C. in February 1995. The seminar was organized and presented by the Education Committee of the Mining Association of British Columbia.

Some of the text, therefore, refers specifically to British Columbia.

 

Introduction

The water flowing on the surface of the earth (creeks, rivers, lakes, etc.) and below the surface (groundwater) contains many chemical compounds in very small amounts. Many of these chemicals have natural sources, some are anthropogenic and others are derived from a combination of natural and anthropogenic sources.

Some examples of natural events that affect water chemistry are:

volcanic eruptions

hydrothermal activity (visible as hot springs)

formation of limestone caverns, stalactites and stalagmites

brine formation from soluble minerals (e.g. Great Salt Lake, Utah)

Acid Rock Drainage (ARD) generation

Examples of anthropogenic contributions to water chemistry are the industrial emissions that generate "acid rain" and the direct dumping of contaminants into water bodies.

While there are many examples of combined effects (e.g. the disposition of fertilizers in water bodies resulting from farming), the subject of this discussion is Acid Rock Drainage which results from natural events, and from the combination of human activity (such as highway construction, mining, quarrying, civil engineering works and logging) and natural events.

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 (FeS2) and pyrrhotite (FeS) in the presence of (catalyzed by) bacteria (Thiobacillus ferrooxidans), and any other products generated as a consequence of these oxidation reactions. To understand this more fully the basic chemistry must be examined.

Basic Chemistry of ARD Generation

The following chemical reactions describe the oxidation of pyrite (FeS2) to the products that constitute the contaminants generically termed Acid Rock Drainage (ARD), although similar equations may be written for the oxidation of pyrrhotite (FeS).

FeS2 + 7Fe2(SO4)3 + 8H2O = 15FeSO4 + 8H2SO4 (1)

FeS2 + Fe2(SO4)3 = 3FeSO4 + 2S (2)

4FeSO4 + O2 + 2H2SO4 bacteria = 2Fe2(SO4)3 + 2H2O (3)

2S + 3O2 + 2H2O bacteria = 2H2SO4 (4)

4FeS2 + 15O2 + 2H2O = 2Fe2(SO4)3 + 2H2SO4 (5)

S + 3Fe2(SO4)3 + 4H2O = 6FeSO4 + 4H2SO4 (6)

It may be seen that Equations 3 and 4 contain the term "bacteria" in addition to chemical formulae. The bacteria, which are usually site-specific strains of Thiobacillus ferrooxidans, utilize the sulphur present as their source of energy. They are autotrophic, obtaining their nutritional needs from the atmosphere (nitrogen, oxygen, carbon dioxide and water) and from minerals (sulphur and phosphorus). While these bacteria are not catalysts by true definition, they do act as accelerating agents if their habitat conditions are at or close to optimal and they are a most important factor in the generation of ARD. They are also capable of adaptation by mutation if their habitat is markedly changed.

It is thought that in the absence of bacteria (usually T. ferrooxidans) Equations 1,2 and 5 predominate, while in the presence of bacteria reactions are best described by Equation 5, which can be regarded as a combination of Equations 1 and 3, or 2,3 and 4, or 1,2,3 and 4. The formulae representations are:

FeS2 - pyrite; H2O - water; O2 - oxygen; S - sulphur; H2SO4 - sulphuric acid; FeSO4 - ferrous sulphate; Fe2(SO4)3 - ferric sulphate.

It may be seen that in addition to pyrite, the presence of both oxygen and water is required for process progression. This has important ramifications in that removal of the oxygen source (e.g. by total submersion under water) or the water source (e.g. conditions of aridity) will halt ARD production. ARD production would also be considerably slowed or halted by the termination of T. ferrooxidans reproduction by a bactericidal agent. The end products are sulphuric acid and ferric sulphate. Also, sulphuric acid is an important intermediate product. From the onset of pyrite oxidation, pH falls (acidity increases) quickly and then stabilizes, typically at values around pH 2.5 to 3.0. The pH of stabilization is normally determined by the optimal habitat requirement of the site-specific strain of bacteria.

If pyrite and/or pyrrhotite are the only sulphide minerals open to atmospheric oxidation then the products of the oxidation process are those described above. Depending upon the availability of water and oxygen, reactions may not always approach completion as indicated by equations 1 to 6, and in such cases intermediate phases of chemical compounds or minerals may remain at the oxidation site.

If metallic minerals (such as galena [lead sulphide, PbS], chalcopyrite [iron-copper sulphide, FeS.CuS], sphalerite [zinc sulphide, ZnS]) in addition to pyrite and pyrrhotite are present (as is usually the case in the natural oxidation of a mineral deposit and the oxidation of products from the mining of a mineral deposit) then there may be a secondary effect of the oxidation of the iron-sulphur minerals to sulphuric acid and ferric iron.

The stable pH developed (2.5 to 3.0) and the products of sulphuric acid and ferric sulphate create conditions where the ferric iron ion itself can act as an oxidant (above about pH 3 the ferric ion is itself hydrolyzed to ferric hydroxide, which precipitates as the familiar rust-coloured stain associated with ARD). In the absence of ferric iron at pH 2.5-3.0, sulphuric acid will dissolve some heavy metal carbonate and oxide minerals, but has little reactive effect on heavy metal sulphides. However, ferric iron ion is capable of dissolving many heavy metal sulphide minerals, including those of lead, copper, zinc, and cadmium , by the general reaction:-

MS + nFe+++ = Mn+ + S + nFe++ (7)

Where: MS = solid heavy metal sulphide; Fe+++ = aqueous ferric iron ion; Mn+ = aqueous heavy metal ion; S = sulphur; Fe++ = aqueous ferrous iron ion.

It is by this process that significant amounts of heavy metals may be solubilised by ARD. It has been customary to call Acid Rock Drainage with the dissolution of metals by Equation 7 Acid Mine Drainage even though this term implies that the process occurs only in association with mining activity.

In addition, many metallic elements are often present at trace levels within the minerals pyrite and pyrrhotite. Oxidation of these minerals can therefore release and mobilize these trace elements

Since these processes occur naturally, geologists who measure the levels of metals in surface or ground waters (Geochemists) can often infer the presence of a mineral deposit and predict its location. This procedure is frequently a powerful exploration tool. The chemical processes of ARD are directly related to metal transport and the formation of metallic mineral deposits via the natural generation and neutralization of ARD - see gossans in the following section.

Natural processes are equally important when a new mine is planned. For example, in British Columbia the regulatory process for new mine development under the British Columbia Environmental Assessment process requires a "Baseline" study of water chemistry (in addition to many other requirements) before mine development begins, to determine the "natural" levels of metals and other constituents in surface and ground waters potentially affected by development.

Untreated (not neutralized) ARD creates two quite distinct environmental problems - the acidity from sulphuric acid (which is invariably a product by definition) and the heavy metal solubilization caused by ferric iron (which may occur under the conditions described above). It is important that these two effects be recognized as separate, since their consequences to ecosystems are distinct, and because ARD generation and heavy metal transport are separate processes..

Basic Chemistry of ARD Neutralization

If the pH of ARD is increased, as would happen with contact with basic minerals such as calcite (CaCO3) or dolomite (Ca,MgCO3) or entry into a water system of higher pH (e.g. a saltwater fjord such as Howe Sound), then metallic ions such as Fe+++ and Cu++, Zn++, Pb++ and As+++ will react to eventually form hydroxides as precipitates by the general reaction:-

Mn+ + nOH- = M(OH)n (8)

where: OH- = hydroxyl ion; M(OH)n = metal hydroxide.

This over-simplification represents chemical neutralization as it occurs by human intervention, rather than an accurate portrayal of natural occurrence, where the precipitation products are usually carbonates and sulphates and their hydrated and/or hydroxy-complex forms. In nature, acid generating minerals such as pyrite often occur in close association with acid neutralizing minerals such as calcite (CaCO3) and dolomite (Ca,MgCO3), and acid produced from pyrite is neutralized, in situ, by these minerals. The sulphate most commonly formed is gypsum (CaSO4.2H2O), which is sparingly soluble in water, and which therefore contributes to elevated sulphate levels in ground and surface waters.

Other minerals, including some silicates such as plagioclase feldspar, may also neutralize acid produced by sulphide mineral oxidation. However, this process is relatively slow compared with neutralization by carbonates.

Geologists determine both the acid producing and acid neutralizing mineral contents of many samples from a proposed mine site and analyze the results by a method called Acid-Base Accounting (ABA). This method of evaluating ARD generation potential (in addition to other methods) is a requirement of regulatory guidelines in British Columbia. Proposed new mines are required to evaluate potential ARD generation in considerable detail, and to demonstrate comprehensive planning to prevent or suppress ARD generation at all phases of mine operation, from development to closure and post-closure. Such evaluation must include pit walls, overburden, waste rock, tailings, and any other material produced by the mining process.

The science and understanding of ARD generation is still developing and there are some mining operations in British Columbia that were developed and operated without the benefit of current knowledge. As a consequence, these mines do generate ARD which has to be treated by neutralization. In commercial practice ARD is neutralized with lime (CaO) to reduce acidity and to precipitate metals - an expensive and long-term necessity.

In addition, there are some former mines in British Columbia that are abandoned , and that are ARD generating (e.g. Britannia). The solution to this problem is currently being studied by the government.

In nature, the neutralization of ARD, either in-situ or at some distance from the source of generation, results in a difference in rock appearance (oxidized, and different in texture and colour from the host rock) and/or additions to bottom sediments or sediment particle surfaces (including rocks and boulders in watercourse beds). Here again, geologists use natural effects as an aid in mineral exploration since the oxidized rock (called gossans) and sediment chemistry are indicative of potential mineral deposits. Gossans, in particular, have been a target of mineral prospectors (as they generally have a reddish-brown to yellow-ochre colour) throughout human history and there are many examples of gossan discovery leading to important economic mining operations. These red-brown to yellow-ochre soils, when mixed with water, have traditionally been used as paint by aboriginal peoples, and by some modern-day artists.

Economic mineral deposits such as bauxite (ore of aluminum), laterite (ore of nickel) and supergene oxide zones (that may contain economic deposits of copper, gold and silver) are products of natural acid rock generation developed over geologic time (several thousands of years). They are derived from the breakdown of minerals (which may occur in several rock types) at surface. Minerals are also formed as a result of ARD processes, some of which are rare and highly prized by mineral collectors (e.g. boleite), some of which are used for jewelry (e.g. malachite), and some of which form mineral deposits (e.g. chalcocite).

It must be understood that the above is a highly simplified description of the basic chemistry of ARD. For example, the precipitation of hydroxides or related chemicals is dependent upon the aqueous concentration of a metal, the pH and the reduction/oxidation (Redox) potential of the solution as well as the concentrations of other ions. The chemistry of real ARD systems is extremely complex.

Basic ARD Kinetics

While chemical equations and their related thermodynamic data are a useful concept in the understanding of ARD generation, they refer to equilibrium conditions at infinite time and give no indication of the kinetics, or speed, with which a reaction will take place. The kinetics of chemical reactions are considered to be either chemically or transport controlled. A chemically controlled reaction is one controlled only by the concentrations of the chemicals reacting with one another, while a transport controlled reaction is dependent upon the rate of transfer of reagents or products to or from the reaction site. It is also reasonable to consider reaction rates controlled by biological activity to be a separate case.

ARD generation is a complex group of reactions which involve all three of these possibilities vying for dominance. Thus in conditions of atmospheric oxidation it is probable that ARD generation is predominantly transport controlled during periods of high rainfall and runoff, biologically controlled during low rainfall and optimal temperature for bacterial growth, and chemically controlled at other times. At any given time control may involve all three mechanisms.

In addition temperature affects these three rate processes differently, and temperature effects are controlled not just by atmospheric variations but also by the heat generated by the (exothermic) chemical reactions and the rate of dissipation of this heat. Thus seasonal variations of precipitation may affect the transport characteristics of the process by water flow, and temperature control through heat transfer.

It follows that prediction of ARD generation rates is a complex, difficult process involving the interaction of a large number of system variables. The prediction of metal hydroxide precipitation from ARD is equally difficult because of the complexity of the reaction kinetics.

Conclusion

This brief discussion has addressed Acid Rock Drainage generation and its associated technical issues. The understanding of ARD, its prediction and treatment are the subject of a substantial research effort by government, the mining industry, universities and research establishments with input from the general public and environmental groups. Much of this work is conducted under the auspices of MEND (Mine Environment Neutral Drainage) which is a cooperative program of the mining industry, the governments of eight provinces (including B.C.) and the government of Canada. In British Columbia, the Ministry of Employment and Investment, Energy and Minerals Division has taken an active role in the formulation of ARD guidelines as part of its responsibility for mining and the environment.

Research in British Columbia involves members of a variety of professions and disciplines, such as geologists, geochemists, biologists, mining engineers, analytical chemists, mineralogists, chemical engineers and many more, and there is certainly much to be learned. It is hoped that the preceding short summary will assist the reader in their understanding of Acid Rock Drainage.

 

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