Weathering of Pyrite and Sphalerite in Soils Contaminated with Pyritic Sludge

The spillage of sphalerite‐containing pyrite sludge in April 1998 contaminated 45 km 2 of Xerofluvents, Haploxerepts, and Calcixerepts in the Guadiamar valley, an area with a Mediterranean climate in southwestern Spain. The strong impact of sulfide oxidation on soil quality and phytotoxicity risks made it compulsory to investigate the products and rate of weathering of pyrite and sphalerite remaining in the soils after most of the sludge was mechanically removed and lime plowed in the autumn of 1998. To this end, 31 soil samples were collected in November 2000 and 32 in June 2001 (i.e., two and three rainy seasons, respectively, after the spillage). Based on concentrations of various extractable forms of S, Fe, and Zn, the soils contained up to 109 and 3.5 g kg − 1 of residual pyrite and sphalerite, respectively, immediately after remediation. About 51 and 69% of this pyrite had weathered by November 2000 and June 2001, respectively, the higher degree of weathering on the latter date being associated with an increased proportion of the resulting Fe oxides in poorly crystalline forms. Sphalerite had weathered roughly to the same degree as pyrite and a significant proportion of Zn released was occluded in Fe oxides. There was thus no evidence for preferential sphalerite weathering through galvanic effects as observed in other pyrite−sphalerite mixtures. An in vitro experiment with aerated soil−water suspensions revealed limited oxidative weathering of the pyrite and sphalerite in the samples, probably because only the coarse less reactive particles remained after the sludge weathered in the field.