Migration of trace elements from pyrite tailings in carbonate soils.

In the carbonate soils contaminated by a toxic spill from a pyrite mine (Aznalcóllar, southern Spain), a study was made of a thin layer (thickness = 4 mm) 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 sulfates (from the oxidation of sulfides) 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, with concentration decreasing downward. The more mobile elements (Co, Cd, Zn, and Cu) tended to precipitate where the pH was basic, toward the bottom of the 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 the phyllosilicates as well as the carbonates. Also, within the reddish-yellow layer, gypsum formed autigenically, together with complex salts of sulfates of Fe, Al, Zn, Ca, and Mn, jarosite, and oxihydroxides of Fe.