The oxidation of pyrite tailings in relation to weathering and pollution of carbonate soils in a Mediterranean climate

M. SIMÓN, C. DORRONSORO, I. ORTÍZ, F. MARTÍN & J. AGUILAR
Departamento de Edafología y Quimica Agrícola. Facultad de Ciencias. Universidad de Granada. 18002 Granada. Spain.

Summary

The weathering and pollution caused by the infiltration of the acidic solution resulting of the oxidation of the tailings were studied in a carbonate soil affected by the toxic spill from a pyrite mine of Aznalcóllar (southern Spain). The acidic solution markedly affected only the first 12 mm of the soil, where the weathering of the carbonates, strong acidification and hydrolysis of the fine mineral particles took place. The SO42- ions present in the acidic solution precipitated almost entirely at this depth, forming gypsum, iron and aluminium sulphates and complex sulphates. The Fe3+ ions also precipitated at this depth, mainly in amorphous or poorly crystallized forms, adsorbing to As, Sb, Tl and Pb dissolved in the acidic solution. The Al3+ ions, though partially precipitating in the acidic layer, precipitated most when the soil pH exceeded 5.5 (12-14 cm in depth). These precipitated primarily as amorphous Al oxides adsorbed to Cu dissolved in the acidic solution. The Zn2+ and Cd2+ ions precipitated mainly at pH>7.0 (19-21 cm in depth), being adsorbed chiefly by clay mineral. The pollutants studied can be divided into 3 groups on the basis of their relative mobility in the soil: Zn and Cd, the most mobile; Bi and Cu, intermediate; Sb, As, Tl and Pb, the least mobile. The intermediate and least mobile elements did not penetrate beyond 100 mm in depth. These results imply that in cleaning up toxic spills of this type, the upper 10-cm layer of the soil must removed together with the toxic tailings.

Keywords: Pyrite mine, tailings oxidation, soil contamination, carbonate soils.