Migration of trace elements from pyrite tailings in carbonate soils

Dorronsoro, C.; Martin, F.; Ortiz, I.; García, I.; Simón, M.; Fernández, E.; Aguilar, J.; Fernández, J.
Dpto. Edafología. Facultad de Ciencias. Universidad de Granada. 18002 Granada. Spain.


ABSTRACT

In the carbonate soils contaminated by a toxic spill from a pyrite mine (Aznalcóllar, S Spain), a study was made of a thin layer (thickness=4mm) of polluted soil located between the pyrite tailings and the underlying soil. This layer, reddish-yellow in color due to a high Fe content, formed when sulphates (from the oxidation of sulphides) infiltrated the soil, causing acidification (to pH 5.6 as opposed to 8.0 of unaffected soil) and pollution (in Zn, Cu, As, Pb, Co, Cd, Sb, Bi, Tl and In). The less mobile elements (As, Bi, In, Pb, Sb and Tl) concentrated in the uppermost part of the reddish-yellow layer, this concentration decreasing downwards. The more mobile elements (Co, Cd, Zn and Cu) tended to precipitate where the pH was basic, towards the bottom of this layer or in the upper part of the underlying soil. The greatest accumulations occurred within the first 6 mm in overall soil depth, and were negligible below 15 mm. In addition, the acidity of the solution from the tailings degraded the minerals of the clay fraction of the soils, both of the phyllosilicates as well as the carbonates. Also, within the reddish-yellow layer, gypsum formed autigenically, together with complex salts of sulphates of Fe, Al, Zn Ca and Mn, jarosite and oxihydroxides of Fe.
Key words: heavy metals, soil pollution, tailings oxidation, pollutants solubilization.