Environmental Impact of Abandoned Mine Waste: a review

2.2. Acid Mine Drainage (AMD)

As already mentioned, iron sulphide oxidation produces acidic drainage water (AMD). Prediction of AMD is the key factor in predicting the release of dissolved metals from active and past mining operations ( Salomons, 1995; Moncur et al., 2009). The prerequisite for AMD is the generation of protons at a faster rate than it can be neutralised by any alkaline materials in the waste (e.g. carbonate in the gangue), the access of oxygen and water, and a rate of precipitation higher than evaporation.

The most common mineral causing AMD is pyrite, but other metal sulphides may also contribute. The oxidation of pyrite, preceding iron hydrolysis, occurs in three steps. The first one occurs at pH above 4.3, with high sulphate and low iron concentrations, with little or no acidity, and slowly.
FeS₂+7/2O₂+H₂O=Fe²⁺+2SO₄^2-+2H⁺
The reaction may proceed both abiotically and by direct bacterial oxidation (Lindsay et al., 2009).

The second step occurs with a pH range between 2.5 and 4.15; there are high acidity and total iron increases. The Fe³⁺ /Fe²⁺ ratio is still low:

Fe²⁺ +1/4 O₂ +H⁺=Fe³⁺ + 1/2H₂O
This stage proceeds predominantly by direct bacterial oxidation determined by the activity of microorganisms of the genus Acidithiobacillus.

The third step occurs at pH values below 2.5, high sulphate and iron levels. The ratio of Fe³⁺/Fe²⁺ is high. The reaction is totally determined by bacterial oxidation, that enhances solubilisation of metal sulphides, catalyzed by chemolithoautotrophic acidophile microorganisms (e.g. Thiobacillus Ferrooxidans) (Trois et al, 2007):

FeS₂ + 14 Fe³⁺ + 8H₂O=15 Fe²⁺ + 2 SO₄^2- + 16H⁺
These three stages are the primary factors, directly involved in the acid production process (Ferguson and Erickson. 1988). The intensity of acid generation by these primary factors is determined (Salomons, 1995; Fanfani, 1997) by environmental (e.g. grain size, pH, temperature, oxygen concentration, metal activity) and biological parameters (population density of the bacteria, rate of bacterial growth, supply of nutrients).

Secondary factors control the consumption or alteration of the products from the acid generation reactions. Neutralisation of AMD can occur when an effective buffer system with relatively high pH is established, thus impeding Fe(III) mobilization (Fanfani, 1997). This occurs when carbonate minerals (calcite, dolomite or ankerite) are present. At pH <7.2, the carbonate-bicarbonate equilibrium turns towards bicarbonate:
CaCO₃ + H⁺  HCO₃^- + Ca²⁺_.
There occur four moles of carbonate to neutralize one mole of pyrite; pH is buffered at a range between 6.4 and 5.5 (Ritchie, 1994), a value at which iron in oxidizing environment precipitates as oxyhydroxide or as sulphate (jarosite):
4FeS +8CaCO₃ + 6O₂ + 4H₂O  4FeOOH + 4SO₄^2- + 8Ca²⁺ + 8CO₂
Neutralisation by carbonates is a relatively fast process and provides short-term buffering capacity.

Other buffering systems, as Fe and Al hydroxides, or silicate minerals, operate at much lower pH, and do not prove effective in controlling the metal release from sulphides, providing long-term buffering capacity. However, AMD neutralization could be not sufficient to eliminate contamination, since sulphides oxidation, although slowed down, is still active, and the release of pollutants simply occurs later than at lower pH. Submersion of waste in artificial impoundments with anoxic conditions could be an effective technique to prevent AMD production and pollutant release in the environment.

2.3. Flotation Tailingsx delete

As already mentioned, AMD contains elevated levels of metals. One way to attenuate, although not eliminate, environmental pollution by mine waste, is AMD neutralization. This may be attained through different methods and techniques. Yet, there are several physical, chemical and biological processes operating in the natural environment, that can contribute to contaminant attenuation. Physical processes include: physical mixing of waste particles with uncontaminated eroded soils and sediments particles; proportional dilution and dispersion of pollutants during high discharge and surface run-off, and metal confinement and sedimentation into confined basins (Figure 4). Chemical processes include solution (metal-soluble fraction), complexation (organic matter-bound fraction), precipitation (oxide-bound fraction), and adsorption by suspended particles (exchangeable fraction). These consist mainly of clay minerals, iron oxyhydroxides and organic matter (Salomons, 1995).

Surface erosion by water or wind, or direct discharge of waste materials in rivers, may result in the introduction of metals in particulate form into aquatic ecosystems, and the heavy metals can be transported considerable distances downstream, causing extended contamination. The leaching time of sulphides from oxygenated spoils is estimated to be about 11 years on average (Helios-Rybicka, 1996). The water discharge supplies 100 tonnes of total dissolved soils per day, with base metals (mostly Pb and Zn) up to 2gm^{-3 n}(Helios-Rybicka, 1996). However, some spoil dumps can be persistent sources of contamination with products of sulphide oxidation, which may affect the environment for decades.
Figure 4. A typical flotation basin in the Sulcis mine district, Sardinia, Italy. (Photo Bini).

Initially, in former mine works, AMD was convoyed to nearby streams, increasing metal concentrations in water and overbank sediments for more kilometres downstream. Successively, although too late to avoid environmental damages (in particular to the aquatic ecosystem) (Davies, 1987), owing to the major sensitivity of population, apposite flotation basins were built, with the goal to limit water and surface contamination. Flotation impoundments have been long utilized to reduce the environmental impact of fine particles produced by metal processing, in such a way that pollutant attenuation may occur.

When the AMDs reach the impoundment, a wider dispersion of the metals both in solution and (after adsorption) in particulate form is possible. Benvenuti et al. (1999) found that there is a gradation in grain size from sand and silt close to the mouth of the drains, to mud and clay at the opposite site; during wet season, the whole impoundment may be flooded, and tailings alteration is less pronounced, presumably because oxidation of the waste is impeded by water saturation in reducing conditions (Neel et al., 2003), when Eh drops to <250mV.

A similar gradient in metal concentration was observed, due to dilution connected to adsorption/precipitation processes occurring in the flotation basins (Benvenuti et al., 1999). Near the dumps, the water is acid (average pH 3.0) and tailings have an high metal content (average Pb 369 ppm; average Zn 176 ppm); the pH increases dramatically (up to 8.0) with increasing distance from the dumps, and the metal content decreases so far. Berger et al. (2000) point out that natural attenuation in drainage from a historic mining district may be related to two distinct pathways: metals (Al, Cu, Fe, Pb) precipitate directly from carbonate-rich solution, whereas Zn, Mg, Mn and SO₄ concentrations decrease primarily through mixing (i.e. dilution) with tributary streams.

Sulphide weathering (oxidation, adsorption or coprecipitation by iron hydroxides) was identified in tailing ponds in the unsaturated proximal areas beside the earthen dams (Heikkinen and Raisanen, 2009) at an active mine site in Finland, where the raised water table contributed to desorption and remobilization of metals, probably through dissolution of iron precipitates.

Sequential extractions applied to mine tailings (Fanfani et al., 1997; Conesa et al, 2008; Perez-Lopez et al., 2009) showed that a relevant part of the total amount of metals convoyed in flotation basins (around 90-100% of total S, Zn, Co and Ni, 60-70% of Mn and Cd, 30-40% of Fe and Cu, and 5% of As and Pb) was estimated to be in the bio-available fraction, i.e. potentially harmful to the aquatic ecosystems.

Leaching experiments carried out by Da Pelo et al. (2009) in Sardinia mining sites, and by Palumbo-Roe et al. (2009) on mine tailings in Wales, show that where surface waters interact with mineral assemblages of the alteration zone, this corresponds to a marked increase in pH concomitant with a decrease in dissolved metals. A comparative slow reaction rate results in the release of a harmful amount of contaminants (Musu et al., 2007). Solute transport in the tailings is governed by unsaturated flow and is controlled by the seasonal precipitation–evapotranspiration cycle. It is envisaged that the seasonal movement of the saturated/unsaturated surface in the tailings in response to seasonal capillary pressure changes is responsible for causing the solute transport. The results of the percolation tests are consistent with control of metal concentrations by mechanisms of dissolution/precipitation/sorption, whereas there is no evidence of sulphide oxidation during the leaching.

The percolation test best describes the seasonal flushing of the secondary minerals, products of metal sulphide oxidation, from the surface layers of the tailings, whereas it does not address the sensitivity to redox changes of the waste. This aspect becomes significant during periods of exposure of the tailings to alternating wet and dry periods.