Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved. Weathering of Pyrite and Sphalerite in Soils Contaminated with Pyritic Sludge Rau ´ l Hita and Jose ´ Torrent* ABSTRACT timeter scale, largely as a result of the residual sludge and industrial lime present (Burgos et al., 2003). The spillage of sphalerite-containing pyrite sludge in April 1998 A number of studies have dealt with the oxidation of contaminated 45 km 2 of Xerofluvents, Haploxerepts, and Calcixerepts this pyritic sludge and with its polluting effect on both in the Guadiamar valley, an area with a Mediterranean climate in the affected soils and the waters draining from them. southwestern Spain. The strong impact of sulfide oxidation on soil quality and phytotoxicity risks made it compulsory to investigate the Thus, Dome ` nech et al. (2002) studied the in vitro oxida- products and rate of weathering of pyrite and sphalerite remaining tive dissolution of the sludge, and Dorronsoro et al. in the soils after most of the sludge was mechanically removed and (2002) and Simo ´ n et al. (2002) characterized the reac- lime plowed in the autumn of 1998. To this end, 31 soil samples were tions of the acid liquid phase draining from a thin sludge collected in November 2000 and 32 in June 2001 (i.e., two and three cover with the underlying soil. To the authors’ knowl- rainy seasons, respectively, after the spillage). Based on concentrations edge, however, no studies on the long-term in situ of various extractable forms of S, Fe, and Zn, the soils contained up weathering of the sludge remaining in the remediated to 109 and 3.5 g kg - 1 of residual pyrite and sphalerite, respectively, soils have so far been conducted. Weathering in the immediately after remediation. About 51 and 69% of this pyrite had area occurs under a xeric hydric regime as it is under a weathered by November 2000 and June 2001, respectively, the higher warm Mediterranean climate; the mean annual temper- degree of weathering on the latter date being associated with an increased proportion of the resulting Fe oxides in poorly crystalline ature is approximately 18°C and the mean annual rain- forms. Sphalerite had weathered roughly to the same degree as pyrite fall approximately 600 mm (80% of which falls between and a significant proportion of Zn released was occluded in Fe oxides. October and April). The purpose of this study was to There was thus no evidence for preferential sphalerite weathering assess the degree of weathering of pyrite and sphalerite through galvanic effects as observed in other pyrite-sphalerite mix- (namely the two most abundant sulfides in the sludge) tures. An in vitro experiment with aerated soil-water suspensions in these contaminated/remediated soils after two and revealed limited oxidative weathering of the pyrite and sphalerite in three rainy seasons. The information thus obtained was the samples, probably because only the coarse less reactive particles expected to be useful with a view to predicting the remained after the sludge weathered in the field. direction and extent of future mineralogical and chemi- cal changes affecting the phytoavailability and potential toxicity of some metals present in the sulfides (particu- O n 25 Apr. 1998, the collapse of a dam wall enclosing larly Zn). In a companion study (Hita and Torrent, the tailings from a pyrite mine resulted in the re- 2005), this element was found to be present at poten- lease of about 4  10 6 m 3 of sulfide sludge that flooded tially phytotoxic levels in some sludge-affected soils. the valleys of the rivers Agrio and Guadiamar in south- western Spain (37°00-30' N, 6°10-20' W). The sludge MATERIALS AND METHODS solid phase consisted mainly of pyrite (75%), sphaler- ite (2%), galena (1%), chalcopyrite (1%), and ar- Soil Sampling senopyrite (1%), in addition to variable amounts of Soil samples were collected from two approximately 40-km quartz, silicate clays, and gypsum (Almodo ´ var et al., transects along the valley in November 2000 and June 2001. 1998; Dome ` nech et al., 2002). The 0.05- to 1-m thick The samples consisted of 1 to 2 kg of soil around the roots blanket of sulfide sludge, which covered approximately of plants of two wild species [namely, spreading pigweed (Am- 45 km 2 of arable soils, was mechanically removed in the aranthus blitoides S. Wats.), November 2000; common cockle- bur (Xanthium strumarium L.), June 2001] in areas affected following dry summer months. Beginning in the autumn by the spill that were remediated with industrial lime. We chose of 1998, the affected soils, which included Aquic, Typic, these species because, in a previous study on phytoavailability and Vertic Xerofluvents; Typic Haploxerepts; and Typic of Zn in the sludge-affected soils, we observed them to grow Calcixerepts (Cabrera et al., 1999; Clemente et al., 2000), on soils exhibiting widely different characteristics and contents were remediated mostly by ploughing in variable amounts in residual sludge and industrial lime. Small sludge lumps of organic matter and lime from sugar production factor- (5-10 cm) were seen at many sampling points. The samples, ies (85% CaCO 3 ). The remediated soils are at present which were moist at the time of collection, were transferred highly heterogeneous on the decimeter or even the cen- to the laboratory and processed within 24 h as described below. Soil Analyses Departamento de Ciencias y Recursos Agrı ´colas y Forestales, Uni- Soil samples were cleaned of roots, air-dried, and passed versidad de Co ´ rdoba, Edificio C4, Campus de Rabanales, 14071 Co ´ r- through a 2-mm sieve before analysis. Particle-size distribution doba, Spain. Received 20 Dec. 2004. *Corresponding author (torrent@ uco.es). Abbreviations: Ac, acetate; ACCE, active calcium carbonate equiva- lent; CCE, calcium carbonate equivalent; CEC, cation exchange ca- Published in Soil Sci. Soc. Am. J. 69:1314–1319 (2005). Soil Mineralogy pacity; EC, electrical conductivity. Subscripts for extractable Fe and Zn forms: t, total (aqua regia-extractable); c, citrate-extractable; cb, doi:10.2136/sssaj2004.0397  Soil Science Society of America citrate/bicarbonate-extractable; d, citrate/bicarbonate/dithionite (CBD)- extractable; o, acid oxalate-extractable. 677 S. Segoe Rd., Madison, WI 53711 USA 1314 Published online June 28, 2005