The environmental damage caused by acid mine drainage (AMD) is a worldwide and growing problem in those countries that once, or are still, extracting coal and/or metals. What is AMD, what effect does it have on the environment, and what can be done about
The environmental damage caused by acid mine drainage (AMD) is a worldwide and growing problem in those countries that once, or are still, extracting coal and/or metals. What is AMD, what effect does it have on the environment, and what can be done about it?
Acid mine drainage (AMD) is the acidic water produced when rock containing sulphide minerals comes into contact with water and oxygen. Besides having a low pH, such water often contains high concentrations of dissolved solids, and often poses a serious threat to groundwater quality which, in turn, can affect drinking water;1 AMD destroys aquatic life and the problems can be long term - some Roman mine sites in Britain, for example, still generate acidic drainage.
Causes of AMD
Base metal ores, coal and mudstones associated with coal are high in iron pyrites or 'fool's gold' (FeS2). Normally this FeS2 is below the water table and so not liable to oxidation. However, in mining the water table is lowered by pumping, and now the pyrite can be oxidised:
FeS2(s) + 3½O2 + H2O → Fe2+ + 2SO42- + 2H+ (i)
While mining is going on, little leaching of the acid and Fe2+ takes place - the water is pumped out and so does not come into contact with the oxidised pyrite to any great extent. Additionally, the water is treated to neutralise it and the solids formed (usually metal hydroxides) are allowed to settle out.
When the mines are closed down the water being pumped out of the mine is stopped. The water level now rises to its original natural height above sea level and is in prolonged contact with the rock (and thus Fe2+), before it exits the mine via old adits (horizontal tunnels), springs and in the beds of streams and rivers which may have been dry throughout mining operations. Although the rise in the water level to its pre-mining height halts the oxidation of the pyrite, by restoring reducing conditions, the products of previous oxidation, ie sulphuric acid and iron sulphate now go into solution, resulting in a low pH (typically 2-3). The increased acidity results in the dissolution of more iron (often from siderite, FeCO3, which is common in coal sequences) plus other metals such as manganese and aluminium. Since Fe2+ is soluble, the water emerging from the mine is clear. At this point, however, or shortly before, this water will come into contact with oxygenated water or with the air. The Fe2+ is oxidised to insoluble Fe3+ and the reaction is accelerated (up to × 106) by the activities of bacteria - particularlyAcidithiobacillus ferrooxidans2 at pH<3:
2Fe2+ + ½O2 + 2H+ → 2Fe3+ + H2O (ii)
The newly formed Fe3+ further oxidises pyrite in a rapid reaction and the bacterium Acidithiobacillus thiooxidans catalyses the oxidation of S22- to SO42-:
FeS2(s) + 14Fe3+ + 8H2O → 15Fe2+ + 2SO42- + 16H+ (iii)
Reaction (iii) is not only much faster than (i) but also produces a higher ratio of acid to initial pyrite concentration.
As the acid solution leaves the mine, it becomes progressively diluted and the pH rises. As the pH values exceed 5.5 the Fe3+ is hydrolysed, resulting in the deposition of a red/orange/yellow precipitate composed mainly of iron(III) hydroxide (ochre or yellow boy). The hydrolysis generates protons:
Fe3+ + 3H2O → Fe(OH)3(s) + 3H+ (iv)
The 'overall' reaction is therefore:
4FeS2(s) + 15O2 + 14H2O → 4Fe(OH)3(s) + 8SO42- + 16H+ (v)
The same process occurs in mine spoil heaps and opencast sites and can seriously retard attempts to vegetate the tips.
Drainage from metal mines carries, in addition to iron, manganese and aluminium (see Table 1),3 metals such as zinc, copper, arsenic, cadmium and lead.
Impacts of acid mine drainage
Acid mine drainage into streams and rivers has several physical and biological implications. Inevitably, the appearance of a watercourse is altered by AMD, making the immediate area look unpleasant. High concentrations of metal ions and acidity render the water unsuitable for drinking, irrigation and industrial applications (it would damage pipes and machinery). Prior to any mining operations, the water may have been used to maintain the flow in rivers during dry spells. Contaminated water would no longer be suitable for this purpose. From a biological viewpoint, AMD may cause a river to be essentially lifeless for long distances below the discharge point of the drainage. More specifically:
- the numbers and diversity of the benthonic (bottom dwelling) species in a stream are depleted because photosynthetic activities are affected - suspended solids, such as precipitated Fe(OH)3, cut down the amount of light available. Also, an acidic environment will neutralise bicarbonate ions in the water and plants will be deprived of a carbon source, which is essential for growth;
- the oxidation of Fe2+ markedly reduces the amount of oxygen available in the water, which affects organisms sensitive to low oxygen levels;
- the increased acidity of the stream into which the drainage water is flowing may cause gill damage to fish and affect the ionic and osmotic composition of their body fluids. Additionally, any benthonic life with carbonate shells, for example snails, suffer since the acidity eventually erodes their shells;
- heavy metals other than iron which may be present in the water, such as copper and lead, can be lethal to water life, especially fish;4
- any deposits can block the gaps within the gravel used by fish (especially trout and salmon) for spawning. This would reduce the water circulation around the eggs and the eggs would fail to develop.
Remediation of acid mine drainage
Treatments for AMD are categorised as active or passive, ie they do or do not require added chemicals and maintenance respectively. The aim of any remediation work is to remove the metals and to adjust the pH of the water. This can be done in one of three ways.
Engineered cascades or waterfalls are used to impart turbulence to the water draining from the mine or tip. The turbulence leads to assimilation of O2 and results in rapid oxidation of Fe2+ and a consequent precipitation of sludge rich in iron(III) hydroxide. Initially the capital costs for such treatments may be high, but much of this cost can be recovered from disposal of the sludge as an iron ore (though the cost of dewatering the sludge is an important consideration).
The acidic discharge from mines can be buffered by adding alkalis,5 for example limestone (CaCO3), quicklime (CaO), hydrated lime (Ca(OH)2), caustic soda (NaOH) and soda ash (Na2 CO3). The sodium salts are the most effective (because they are more soluble) but are also the most expensive.5 If calcium salts are used and sulphate concentrations are high, insoluble CaSO4 precipitates out and may clog pipes used to move the water after treatment. Metal hydroxides/oxides precipitate out as the pH rises, from Fe(OH)3 at pH 5.5 to MnO2 at pH 10. The metals present, and therefore what has to be removed in the AMD, to some extent dictate the choice of alkaline reagent to be used. For example, hydroxide rather than carbonate would be used to remove manganese. The resulting metal salts are allowed to settle out in a lagoon, the lagoon is drained periodically and the alkaline sludge removed for disposal. A Tennessee surface coal mine is an example of the effectiveness of alkaline treatment - treatment with limestone resulted in a 10-fold decease in iron concentration and a two-fold decrease in manganese concentration.6
Phosphates in the form of the mineral apatite, which dissolves in acidic conditions, may be added to coal spoil,7 though no large-scale trials of this control mechanism have been done to date. The presence of phosphate inhibits oxidation of the pyrite because oxidising Fe3+ ions are precipitated as insoluble iron phosphates. This process would have the advantage of stopping the production of acid rather than neutralising it after formation.
Organic matter, such as sewage sludge, sawdust and newspaper, raises the pH of water as it decomposes.8
CH2O + 2MnO2 + 4H+ → 2Mn2+ + 3H2O + CO2 (vi)
CH2O + 4FeO(OH) + 8H+→ CO2 + 4Fe2+ + 7H2O (vii)
2CH2O + SO42- + 2H+ → 2CO2 + H2S + 2H2O (viii)
Addition of municipal organic matter has been used to revegetate thousands of acres of mine land, and improve groundwater quality, in Pennsylvania for example.8
Biological methods for treating AMD include:
- using selected microorganisms, ie Acidithiobacillus thiooxidans, to oxidise the iron into its insoluble oxides: for example, over 90 per cent of iron(II) in the drainage from the Matsuo mine in Japan was oxidised to iron(III) following controlled use of bacteria;9
- processes for inhibiting the activities of Acidithiobacillus (the bacterium that speeds up the oxidation of the pyrite)10 - for example, using relatively non-toxic bacterial surfactants such as those used in the manufacture of detergents; these attack the lipid component of the Acidithiobacillus cell wall which protects the bacteria against the acidic water. Once this protective component is removed, the water enters the bacterial cell and destroys it. Surfactant treatment of a site in Pennsylvania resulted in an approximately 80 per cent decrease in both acidity and iron concentration,11 saving hundreds of thousands of dollars a year.
The use of reed beds (a passive AMD treatment) in an integrated chemical and biological system is relatively inexpensive and is becoming popular in the UK and in the US. The precise configuration of a reed bed facility is largely determined by the specific properties of the water from the mine/spoil tip being treated, but the scheme outlined here is one of the more common examples of this technology.
Initially a storage lagoon is constructed so that the drainage can be stored to avoid any overloading of the treatment facility. The water is then usually passed through an anoxic limestone drain (ALD), prior to treatment, to increase its alkalinity (Fig 1). Dissolution of calcium carbonate in the limestone neutralises the acidity and raises the pH of the water.12 The drain is anoxic, or capped, to prevent contact with oxygen, which prevents the precipitation of iron(III) hydroxide, which in turn would coat the limestone and halt the reaction. The water is then run through an aerobic reed bed (or wetland) where the iron(II) sulphate is oxidised to iron(III), which is precipitated as the hydroxide (Fig 2). Manganese precipitation is slow and is inhibited by the presence of Fe2+; consequently it occurs after the bulk of the iron has been precipitated out. The precise mechanism whereby such reed beds can assist in removing the iron as iron(III) hydroxide is unclear. Processes such as: the formation of organic complexes; ion exchange; neutralisation by carbonate; adsorption onto algae; and microbial attack are known to take place, though the relative importance of each is obscure. The possibility that the reeds act as a pathway for oxygen to travel from the atmosphere to the bottom of the wetland has also been suggested. We know that in the waterlogged soils in which reeds flourish, metals such as iron and manganese tend to be in the reduced state and are thus soluble. As such, they may be absorbed by plants in toxic quantities. The transport of oxygen to the root area, which leads to the precipitation of such metals, thus rendering them less harmful, may be a defence mechanism of the plant.
Heavy metals (particularly common in the drainage from base metal mines) are removed by passing the water through an anaerobic wetland. This consists of a one-metre layer of organic material ('CH2O') - typically a mixture of spent mushroom compost, sawdust and manure which must contain the sulphate-reducing bacteria Desulfovibrio. As the water passes through the organic layer it becomes anaerobic, owing to the high biological oxygen demand (BOD). The sulphate in the mine water is reduced to hydrogen sulphide. The hydrogen sulphide then reacts with the heavy metals in the water and sulphides are precipitated.
SO42- + 2CH2O → H2S + 2HCO3-
Zn2+(aq) + H2S(aq) → ZnS(s) + 2H+(aq)
The reduction of the sulphate produces bicarbonate, which neutralises the hydrogen ions produced and buffers the system to over pH 5.
The treatment of acid mine drainage with reed beds has the following advantages over most other methods:
- little or no day-to-day control is needed;
- capital and operational costs are low;
- the recovery of metals is a possible option for the future;
- the lagoons and pools can provide good habitats for wildlife.
Disadvantages of reed beds relative to chemical treatment are:
- longer retention times are required;
- more space is needed;
- treatment is normally restricted to AMD where the iron content of the water is <10 ppm. For higher levels of iron, chemical treatment is normally used though wetland technology may follow as a 'polish'.
As an example of the success of passive treatment, seven kilometres of the River Pelenna in South Wales had been polluted for a significant length of time by acid mine drainage from abandoned mine workings. This resulted in the polluted reaches of the river being essentially lifeless. Passive wetland treatment of the discharges resulted in 82-95 per cent iron removal and increases in pH of between 1.05 and 1.57.13 After treatment, a marked increase in the fauna of the river was noticeable.
Although AMD is an immediate and growing problem in the world, especially in developed countries with their legacies of an industrial past, the technology exists to remediate the problem. However, it will require determination, resources and political will to make this a success, as well as the ability to develop further innovative and novel methods of dealing with the situation.
Dr Stuart Baskerville is a geologist and Dr Wynne Evans is a chemist in the department of physical sciences in the school of applied sciences at the University of Glamorgan, Pontypridd, Wales CF37 1DL.
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