Soil alteration by continued oxidation of pyrite tailings

Abstract This work examines the alteration processes triggered after the oxidation of pyrite tailings deposited for 3 years over a carbonate soil. The infiltration of the acidic solution into the soil is causing important morphological, compositional and mineralogical changes in the profile. After 3 years of continued action of such alteration, a considerable degradation of the main soil properties was evident, the most notable being the decline in the cation-exchange capacity (caused by the decreases in clay and organic matter content), texture variation, greater electrical conductivity (10-fold greater than in unaffected soil), and the appearance of horizons with colorations strongly differing from those of the original soil (a discoloured layer with greyish tonalities in the first 5 mm, followed by a reddish-brown layer to a depth of 65–70 mm). At the same time, the carbonates have weathered, disappearing completely from the upper 35 mm and partially to 80 mm in depth. There has also been an intense acidification of the soil (with pH values close to 2.0 within the greyish layer) as well as a partial hydrolysis of the primary silicates (mainly feldspars and phyllosilicates), causing extreme infertility of the soil. The resulting products in this process give rise to intense neoformation of gypsum and hydroxysulphates of Fe and Al, which, together with the acidic conditions of the medium, determine the distribution of the main elements of the soil, both in their total and soluble forms.