Weathering and neoformation of minerals in carbonate soils affected by pyrite tailings
Dorronsoro C., Sim—n M., Mart’n F., Ort’z I., Aguilar J., Garcia I., Fern‡ndez J. & E. Fern‡ndez.
Dpto de Edafolog’a. Facultad de Ciencias. Universidad de Granada. Granada. Espa–a.
We have studied the oxidation of the tailings spilled when the retention walls of a pond containing the residues from a pyrite mine in Aznalc—llar (southern Spain) broke open, as well as the effects on the weathering and neoformation of minerals in carbonated soils (CaCO3 = 7.1%; pH = 7.9). The tailings were mainly constituted by S (397 g kg-1) and Fe (364 g kg-1), with intermediate values of Al (16.2 g kg-1), Pb (11.0 g kg-1), Zn (9.6 g kg-1), As (7.2 g kg-1), Mg (6.4 g kg-1), Ca (4.1 g kg-1), Cu (2.6 g kg-1) and Sb (1 mg kg-1), and minor quantities of Bi, Tl, Co y Cd (< 0.1 g kg-1).
In the first step, when drying took place (10 days after the spill), the soluble elements from the tailings rose by capillary action to the surface, precipitating and forming a white salty layer. This layer was mainly formed by bianchite [(Zn0.69Fe0.21Mg0.10)SO4. 6H2O], with minor quantities of beaverite [Pb(Cu,Fe,Al)3(SO4)2(OH)6], kieserite (MgSO4. H2O) and hexahydrite [(Mg0.72Zn0.15Fe0.13)SO4. 6H2O]. As a result of drying and consequently aeration of the tailings, sulphides oxidized to sulphates, the pH markedly fell, due to the formation of sulphuric acid, while the pollutants solubilized. The oxidation of the sulphides bonded to Zn, Cu and Cd (such as sphalerite or chalcopyrite) was very rapid and intense (25 days after the spill, 45% of the total Cu and 65% of the total Zn and Cd in the tailings were solubilized). Other sulphurs present in the tailings, such as arsenopyrite, galene or jaskolskiite oxidized more slowly and less intensely (40 days after spill, only around 2.5% of the total As, 2.0% of the total Bi, 0.4% of the total Sb and 0.1% of the total Pb were solubilized).
With the rainwater, the acidic pollutant solution and the salts that had previously formed on the surface of the tailings infiltrated the soil. As a result, in the first 12 mm of the soil, a reddish-yellow band formed, in which the weathering of the carbonates, the hydrolysis of the finest particles (silt and clay) and the precipitation of part of the pollutants took place. Two months after the spill, the pH of the band was 6.0 and the carbonate content diminished by 85% with respect to the underlying soil, while 15 months after the spill, the pH was 3.5 and the weathering of the carbonates was complete.
On this former date, the mean clay content in the reddish-yellow band was reduced by roughly 65%, fine silt by 60% and coarse silt by 30%, with respect to the underlying soil. The SO42- ions in the acid pollutant solution precipitated in this band, primarily as gypsum, as needle-like crystals of iron sulphate (melanterite), projecting from a mass of sulphate complexes with Zn, Fe and Mn, as well as raised scaly formations of aluminium sulphate (probably alunite) and aggregated microcrystals of iron and potassium sulphates (jarosite). Amorphous hydrous Fe-oxides also precipitated in this band, adsorbing As, Sb, Tl and Pb dissolved in the acid solution. The Al which had dissolved in the acid solution, although precipitated partially in the form of aluminium sulphate in the first 12 mm, almost precipitated as amorphous hydrous Al-oxides between 12 and 14 mm in depth (pH =6.0), adsorbing the Cu dissolved in the acid solution. The Zn and Cd also accumulated mainly at pH =6.0 (12-14 mm in depth).