Environmental Impact of Abandoned Mine Waste: a review

1.3. Chance

As already mentioned, active and abandoned mining activities are widely diffused at worldwide level. Many countries have abandoned mine workings since the ‘70s of last century, and from that time are developing actions to minimize the environmental impact during and after exploitation, and projects aimed at restoration of contaminated/degraded areas. Examples of these research projects are reported in current literature (see f.i. Berger et al., 2000; Mendez and Maier, 2008, and references therein).

Once mines have been closed, and waste abandoned on the land or discharged into surface waters, (since mining industry was completely unregulated until half 19^th century, as reported by Davies, 1987), decision makers, after claiming the environmental damage had been done, have discovered that these sites could constitute a challenge, or rather a chance, to rehabilitate the contaminated land. Archaeologists and geologists joined their effort to discover ancient settlements nearby former mining sites, and to understand the organization of ancient societies, their evolution with time, and the metallurgical works that characterized the economy of the interested areas (Francovich, 1985; Costagliola et al., 2008; Dill, 2009). To date, under the stimulus of modern historigraphy that pays particular attention to mining and metallurgical concerns, many mine-archaeological parks have been established, particularly in Europe (France, England, Austria, Germany, Poland). In Italy, where many mining and metallurgical monuments of pre-industrial times are located, studies about this subject flourished since the end of 19^th century (see Cipriani and Tanelli, 1983, and references therein). Since that time, many initiatives succeeded, giving a profound insight into historical, archaeological, socio-economical and industrial (metallurgical) aspects of former mining sites (e.g. D’Achiardi, 1927; Francovich, 1985; Tanelli, 1989; Costagliola et al, 2008, and references therein), aimed at the valorisation of the land with the opening of several mine-archaeological parks, recreational itineraries and museums in Tuscany, Sardinia, Veneto (Figure 3).

Figure 3. Archaeological Mine Park at Rio Elba (Elba Island). Tourists looking for minerals. (Photo Bini).

Archaeological investigations carried out in these areas have lead to discoveries of human activities during three millennia (Casini, 1993). The most significant discoveries are related to the extraction, processing, and commerce of metals. For example, excavation of the ruins of the village of Rocca S. Silvestro (a Middle Age village in Central Italy) suggests that it was inhabited by at least 300 persons, devoted to the processing of lead and copper (Francovich,1985).

Archaeological studies also indicate four major periods of settlement and human activity in the territory (Heimann et al., 1998). First, mining debris and stony artifacts (scrapes, tips) of pre-historical and proto-historical periods (Middle Palaeolithic-Neolithic) have been found close to shelters. Second, excavation of settlements of the Etruscan and Roman period revealed intensive metal mining and limestone quarrying activity. Third, in the Medieval period lead and copper processing proved an important activity at different sites in various countries (Costagliola et al., 2008; Dill, 2009; Forel et al., 2010). Finally, in more recent times (16^th-19^th century), ore exploitation has been carried out by both local population and foreign people, such as the Germans, as demonstrated by local nomenclature (Francovich, 1985).

Another chance offered by formerly mining sites is the fact that such sites host wild vegetation genetically tolerant to high metal concentrations. According to Baker (1981), plants may be classified into three groups on the basis of their ability to accumulate metals in their aerial parts. Excluders are those plants whose metal concentrations remain unaffected by metal concentration in soils up to a critical level, when toxic symptoms appear.

Indicator plants are those whose metal concentrations reflect those of the related soil.

Accumulator plants have the ability to take up and concentrate metals from soils containing both low and high levels of metals. Among the species that may tolerate high metal concentrations in their tissues, plants presenting exceptional accumulating ability are referred to as hyperaccumulators. More than 400 wild plants have been reported as metal hyperaccumulators (Bini et al., 2000). A well known hyperaccumulator species for Ni, for example, is Alyssum bertoloni (Baker and Brooks, 1989), for Zn Viola calaminaria and several Thlaspi species (Baker and Brooks, 1989), for Pb Brassica napus (Mc Grath, 1995); Calendula officinalis has been discovered to accumulate chromium (Bini et al, 2000), and Pteris vittata arsenic (Bettiol et al., 2010).

The metal-enriched areas, therefore, represent an ideal natural laboratory where to study the processes in order to provide descriptive models of the interactions between the toxic elements, the pedosphere, the biosphere and the hydrosphere (Ritchie, 1994). The assessment of soil contamination has been extensively carried out through plant analysis (Ernst, 1996). Wild and cultivated plant species (catchfly, dandelion, plantain, marigold, willow, common reed, fescue, maize) have been used as (passive accumulative) bioindicators for large scale and local soil contamination (Bini, 2009). Based on current knowledge, in the last decades, attention has been deserved to plants as tools to clean up metal-contaminated soils, and restoration plans have been addressed to these sites, with application of low cost and environmental friendly phytoremediation technologies (Bini, 2009; 2010).

Chapter 2

Processes Occurring
at the Mine Sites

2.1. Weathering of Mine Spoils

Weathering of metal sulphides in exogene environment, and the consequent release of pollutants, is the geochemical process responsible for the contamination of former mining areas. The process occurs even in absence of exploitation, when ore deposits are exposed to atmospheric agents, but is particularly environmentally relevant with extensive exploitation, or when mining operations ceased, and uncontrolled mine waste is abandoned on the land.

Although sulphide alteration constitutes a common process in contamination of wide mine areas, various sources may contribute to environmental pollution: acid mine drainage (AMD), flotation tailings, mine dumps, crushing, grinding and milling plants. Wind, gravity, runoff, surface and ground-waters are the agents that contribute PHEs to the environment as soluble, suspended or transported material. The spectrum of mobilized PHEs varies depending on the ore deposits composition: Cd, Pb, Zn are the most common, while As, Cu, Co, Hg, Ni, Sb, Se, Te are less frequent.

Among the mixed sulphide deposits, the most hazardous to the environment are those bearing Fe-, Zn-, Pb-minerals in both the exploited and the raw material (gangue). The alteration of these minerals proceeds via an oxidation reaction that involves the sulphide or disulphide (transformed in sulphate), together with oxidation of iron to Fe³⁺, and subsequent hydrolysis to Fe(OH)₃. The role of iron in oxidation and hydrolysis is particularly important, given the abundance of Fe-minerals in ore deposits.

Iron sulphide oxidation reactions (mainly pyrite and pyrrhotite), tend to create an acidic environment, releasing protons, as observed in the oxidation reaction of pyrite:
FeS₂ + 3,75 O₂ +3,5 H₂O  Fe(OH)₃(s) + 2 SO₄^2- + 4H⁺
Pyrrhotite is a not stoichiometric iron sulphide with different polytypes (Fe(1-x)S, where x = 0 - 0,125). The reaction kinetics depends on pH, temperature and surface area, besides the polytype, being the monocline pyrrhotite more reactive than the hexagonal one (Salomons, 1995).

The oxidation reaction of pyrrhotite can be syntetized as follows:

Fe(1-x)S_(S) + (2-x/2) O₂ + x H₂O  (1-x) Fe²⁺ + SO₄^2- + 2x H⁺
As it is evident, the higher the S/Fe ratio, the higher is the proton release (i.e. acidification).
Iron sulphides oxidation, as observed by Fanfani (1997), creates an acidic environment that enhances metal mobilisation from mixed sulphides, according to the following reaction:
MeS + Fe₂(SO4)₃ + 1,5 O₂ +H₂O  MeSO₄ +2FeSO₄ + H₂SO₄ ( with Me = Cu, Pb, Zn…)
To the acidification process contribute all the mixed sulphides (Fe, Cu, Zn, etc.): for example, in the case of chalcopyrite the reaction is:
4CuFeS₂ + 17O₂ + 6H₂O  4FeOOH + 8SO₄^2-+ 4Cu²⁺ + 8H⁺

Oxidation of not-iron bearing sulphides, as sphalerite or galena, runs in such a way that base metal sulphates form, but no acidification occurs:

MeS + 2O₂ Me²⁺+ SO₄ ^2-  MeSO₄ (with Me: Cu,Pb,Zn…)
Instead, if iron sulphides are present in the environment, as it happens frequently, hydrogen ions are produced, according to the reaction:
MeS+8Fe³⁺+4H₂OMe²⁺+8Fe²⁺+SO₄^2-+8H^+.