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, Domè nech et al. (2002) studied the in vitro oxidaproducts 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 Simó n et al. (2002) characterized the reaclime 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' knowlrainy 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 Ϫ 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 temperdegree of weathering on the latter date being associated with an increased proportion of the resulting Fe oxides in poorly crystalline ature is approximately 18ЊC and the mean annual rainforms. 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 pyriteϪsphalerite mix-(namely the two most abundant sulfides in the sludge) tures. An in vitro experiment with aerated soilϪwater 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. Soil Analyses Departamento de Ciencias y Recursos Agrícolas y Forestales, Uni-Soil samples were cleaned of roots, air-dried, and passed versidad de Có rdoba, Edificio C4, Campus de Rabanales, 14071 Có ...
After the collapse on 25 Apr. 1998 of the Aznalcóllar mine tailings dike in southwestern Spain, 45 km2 of the Guadiamar valley were covered by a pyritic sludge containing up to 2% sphalerite (ZnS). Later, the sludge was mechanically removed and calcium carbonate was plowed into the soil to immobilize heavy metals. By June 2001 more than 60% of the sulfides in the residual sludge had oxidized and soil Zn contents reached locally phytotoxic levels. Therefore, the oxidative dissolution of sphalerite in the sludge and other pyritic samples was examined. Flow-through oxidation experiments showed that: (i) about 5 and 17% of the sludge Fe and Zn were in soluble form, respectively, because the sludge sample had been partly oxidized in the field; (ii) the oxidation rates of the residual pyrite and sphalerite were similar; (iii) the overall sulfide oxidation rate was relatively unaffected by the addition of calcite; and (iv) poorly crystalline Fe (hydr)oxides containing Zn in occluded form and Zn (hydroxi)carbonates were formed in the presence of calcite. The rate of oxidation of reference sphalerite greatly increased when it was incorporated in the sludge or in a reference pyrite matrix. This enhancement was due to galvanic interaction because pyrite oxidation was depressed in the presence of sphalerite. Oxidation by Fe3+ ions was less important because the oxidation rates of native sphalerite were not greater at low than at high pH. The fast oxidation rate of sphalerite in the Aznalcóllar sludge indicates a need for quick adoption of remediation measures in similar accidents elsewhere. The use of calcite amendments has little influence on the oxidation rate but does result in the accumulation of Zn in relatively insoluble forms.
Zinc can be toxic to plants growing on soils in areas of the Guadiamar River valley (southwestern Spain) affected by the spillage of pyritic sludge in April 1998. The shoots and the soil around the roots of two wild plants (viz. Amaranthus blitoides S. Wats., November 2000; and Xanthium strumarium L., June 2001) growing in the sludge-affected areas were sampled with the purpose of relating Zn phytoavailability to soil properties. The soils were calcareous and non-calcareous Entisols and Inceptisols which, after remediation, contained ploughed-in residual sludge and unevenly distributed industrial lime. Chemical extracts from the soils suggested that much of the sphalerite (ZnS) originally present in the sludge had weathered and Zn was partly bound to carbonates and Fe oxides, the total Zn concentration ranging from 37 to 2407 mg kg )1 . To identify the soil properties that influenced Zn phytoavailability under controlled conditions, the soil samples (n ¼ 63) were homogenized and oilseed rape (Brassica napus var. Karola) was pot-grown on them in a growth chamber. The concentrations of Zn in oilseed rape shoots and roots were below phytotoxic levels, with mean ± standard deviation values of 142 ± 128 and 244 ± 328 mg kg )1 dry matter, respectively. Citrate/bicarbonate-extractable Zn in soil (Zn cb ) was found to be the best predictor for the Zn concentration in both shoots and roots. Also, the Zn cb /Olsen P ratio exhibited a high predictive power for Zn in shoots as the likely result of the Zn-P interaction in soil. The shoot Zn concentration in the wild plants, generally lay below phytotoxic levels (the mean ± standard deviation values were 261 ± 255 and 200 ± 228 mg kg )1 dry matter for Amaranthus blitoides and Xanthium strumarium, respectively) and was not correlated with soil properties -by exception, there was slight correlation between the Zn concentration in Amaranthus blitoides and Zn cb /Olsen P. Such a lack of correlation can be ascribed to the local small-scale soil heterogeneity caused by remediation practices. The Zn concentration in wild plants growing on CaCO 3 -poor soils was weakly correlated with Zn cb /Olsen P; no similar correlation was found in CaCO 3 -rich soils, however. The wild plants growing on CaCO 3 -poor and CaCO 3 -rich soils differed little in Zn concentration; this suggests that further addition of lime to reduce Zn phytoavailability may be unjustified.Abbreviations: ACCE -active calcium carbonate equivalent; CCE -calcium carbonate equivalent; CEC -cation exchange capacity; EC -electrical conductivity. Subscripts for extractable Zn forms: t -total
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