Mineralogical transformations in carbonate soils affected by a spill of pyrite tailings

García I.; Martín F.; Dorronsoro C.; Simón M.; Diez M.; Bouza P.; Aguilar J.

Introduction

When the tailings from a pyrite mine are exposed to oxygen and water, sulphides oxidise to sulphates, the pH falls markedly due to the formation of sulphuric acid, and the pollutants solubilize (Förstner & Wittmann, 1983). In the case of pyrite, the most abundant sulphide in these tailings, the oxidation can be represented by the following reactions:

            2FeS2(s) + 7O2(g) + 2H2O     <====>       2Fe2+(aq) + 4SO42-(aq) + 4H+(aq) (1)

            4Fe2+(aq) + O2(g) + 4H+(aq)    <====>   Fe3+(aq) + 2H2O (2)

The Fe3+ released in Reaction (2) may hydrolyze to form ferric hydroxide:

Fe3+(aq) + 3H2O    <====>    Fe(OH)3 + 3H+(aq) (3)

or may oxidize additional pyrite by the reaction:

FeS2(s) + 14Fe3+(aq) + 8H2O    <====>   15Fe2+(aq) + 2SO42-(aq) + 16H+(aq) (4)

Reaction (2) is very slow at pH<4.0 and has been described as the rate-determining step in pyrite oxidation; nevertheless, Fe-oxidizing bacteria (e.g. Thiobacillus ferrooxidans, Thiobacillus thiooxidans) increased the oxidation rate of Fe2+ by 105 (Singer & Stumn, 1970) and thus, oxidation rates for pyrite are 10- to 20-fold higher than those resulting from purely chemical oxidation (Battaglia et al., 1998; Boon & Heijnen, 1998).

When CaCO3 is present in the soils, the acidity is neutralised, the oxidation of Fe2+ to Fe3+ proceeds rapidly (Singer & Stumm, 1968), iron precipitates and the calcium and sulphate ions form gypsum (Ritsema & Groenenberg, 1993; Kashir & Yanful, 2000):

Fe3+(aq) + 2SO42-(aq) + H+(aq)+ 2CaCO3(s) + 5H2O  <====>   Fe(OH)3(s) + 2CaSO4.2H2O(s) + 2CO(g) (5)

On 25 April 1998 the retention walls in a pond containing the residues from the pyrite mine of Aznalcóllar (southern Spain) broke open, and approximately 4x106 m3 of polluted water (solution phase) and 2x106 m3 of toxic tailings (solid phase) were spilled into the Agrio and Guadiamar River Basin, affecting some 55 km2 (Grimalt et al., 1999). The solid phase consisted of different sulphides such as pyrite (75-80%), sphalerite and galene (5%), chalcopyrite (1.5%), arsenopyrite (1%), as well as minor amounts of bournonite, boulangerite, nuffieldite jaskolkiite and numerous trace metals (Almodóvar et al., 1998; López-Pamo et al., 1999). The principal pollutants were: Zn, Pb, Cu, As, Sb, Bi, Cd and Tl (Cabrera et al.., 1999; Vidal et al., 1999; Simón et al., 1999).

From the spill, the soils were covered by a layer of tailings of variable  thickness averaging 7 cm (López-Pamo et al., 1999). When the tailings from a pyrite mine are exposed to oxygen and water, sulphides oxidise to sulphates, the pH falls markedly due to the formation of sulphuric acid, and the pollutants solubilize (Nordstrom, 1982; Förstner & Wittmann, 1983, Nordstrom et Alpers, 1999; Alastuey et al., 1999).

The aim of this work is to analyze the effect of the pyrite tailing oxidation, over time, on soil mineralogy.

Material and Methods

In some places in these soils, two months after the spill, a thin layer of reddish-yellow soil (7.5YR 6/8) a 4 mm thick developed immediately underneath the tailings. This layer appeared a few weeks after the spill, the colour being owed to abundance of Fe in the tailings (Simón et al., 1999). At 15 months the layer became 15 mm thick and at 4 years, 60 mm, with the uppermost 5 mm clearly discoloured (2.5Y 7/4). In each date, the layers were sampled each millimetre and, below the reddish-layer, each 5 cm up to reach the unaffected soil. All samples were sieved to 2 mm. 

In each sample, the particle-size distribution was determined by the pipette method after elimination of organic matter with H2O2 and dispersion with sodium hexametaphosphate (Loveland and Whalley, 1991). The pH was measured potentiometrically in a 1:2.5 soil-water suspension. The CaCO3 equivalent was determined by a manometric method (Williams, 1948). The cation-exchange capacity (CEC) was determined with 1N Na-acetate at pH 8.2. Pills of soil and lithium tetraborate (0.6:5.5) were prepared and the total content in Fe was measured by X-ray fluorescence using a Philips PW-1404 instrument. A Zeiss-950 scanning electron microscope with a Tracor Northern 523 X-ray energy-scattering microanalyser (SEM-EDS) was used to examine the morphology and analyse the composition of certain minerals present in the first 6 mm of the soil. For X-ray diffraction, a Philips PW-1710 instrument with CuKa radiation was used. The climate of the study area is typically Mediterranean (hot, dry summers; cold, wet winters; temperate autumns; and springs with variable rainfall). The average annual rainfall is 630 mm, the average temperature 17.9 °C and the potential evapotranspiration 975 mm.

Results and discussion

            The soils in which the reddish-yellow layers (RYL) develop are carbonated (8%) and sandy (46%) soils, with low organic matter content (1.7%), alkaline pH (8.0) and slightly developed profile (Xerorthent and Xerofluvent (Soil Survey Staff, 1999)).

Over time, the pH decrease with the progress of the contamination (table 1), reaching the value of 2.5 at five years from the staying of the tailings in soil, causing the decrease and even the dissolution of the carbonates; in this sense, two month after the spill carbonate content is reduced up to 58%, the samples from 15 months and 5 years are completely decarbonated, having a strong fall in the cation exchange capacity values. The iron within this layer strongly increased, indicating a high degree of pollution. Also, from this Fe distribution, we deduce that this metal was responsible for the reddish-yellow coloration.

Mineralogy of uncontaminated soil was dominated by quartz, feldspars, calcite and dolomite (figure 1), and the infiltration of the acidic solution produced and intense weathering of the calcite, totally disappearing in soil at 15 months from the pollution. The other constituents have less substantial variations. In this sense, feldspars and phyllosilicates are reduced approximately to half at five years from the spill. On the other hand, the peaks in the X-ray diffractograms of the resistant-to-hydrolysis minerals, like quartz, increase its intensity.

Very important are the neoformations produced by the high quantity of iron and sulphates dissolved in the contaminant solution (table 2). In this way, the calcium released in the weathering of the calcite reacts with the sulphates to form gypsum (Ritsema and Groenenberg, 1993) which increases gradually until reach the 15% of the total soil; meanwhile the sulphate, iron and potassium form jarosite which, as the contamination progressing, trend to change to plumbojarosite. So the plumbojarosite is relatively abundant in the decoloured zone of the layer formed at 5 years. The electron microscopy (SEM_EDS) reveals, both crystals of gypsum and jarosite, as other neoformations like the needled crystals of S and Fe (melanterite ¿?) or the plated-habit crystals of S and Al (alunite ¿?) figure 2.

According to the clay mineralogy, the oriented aggregates of the clay fraction in the samples from the soil underlying the reddish-yellow layer gave diffractograms showing illite, kaolinite, smectite, interstratified complexes of chlorite/smectite, calcite and feldspars. In the samples of the reddish-yellow layer, the peaks become broader and lower, indicating a generalized breakdown of minerals, both phyllosilicates as well as feldspars. The presence and alteration of the components in the soil (mainly, feldspars, micas and clays) boosted the capacity of acid neutralization and retention of released metals. Identification of such mineral alteration is not easy, the phyllosilicates and particularly smectites apparently being the most active in neutralizing the acidity (Pons et al, 1982; van Breemen, 1980).

Conclusions

            The infiltration in soil of the solution coming from the pyrite tailing oxidation hydrolyze the silicates, weather the carbonates, acidify the soil and neoform minerals, mainly gypsum and other minerals of the jarosite group, melanterite and alunite.

Acknowledgements

            This work has been realized by the financial support of the project REN2003-03615 Ministerio Educacion y Ciencia.

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