Published online 28 June 2005
Published in Soil Sci Soc Am J 69:1314-1319 (2005)
DOI: 10.2136/sssaj2004.0397
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Soil Mineralogy
Weathering of Pyrite and Sphalerite in Soils Contaminated with Pyritic Sludge
Raúl Hita and
José Torrent*
Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Edificio C4, Campus de Rabanales, 14071 Córdoba, Spain
* Corresponding author (torrent{at}uco.es)
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ABSTRACT
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The spillage of sphalerite-containing pyrite sludge in April 1998 contaminated 45 km2 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.
Abbreviations: Ac, acetate • ACCE, active calcium carbonate equivalent • CCE, calcium carbonate equivalent • CEC, cation exchange capacity • EC, electrical conductivity. Subscripts for extractable Fe and Zn forms • t, total (aqua regia-extractable) • c, citrate-extractable • cb, citrate/bicarbonate-extractable • d, citrate/bicarbonate/dithionite (CBD)-extractable • o, acid oxalate-extractable
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INTRODUCTION
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ON 25 APR. 1998, the collapse of a dam wall enclosing the tailings from a pyrite mine resulted in the release of about 4 x 106 m3 of sulfide sludge that flooded the valleys of the rivers Agrio and Guadiamar in southwestern Spain (37°00–30' N, 6°10–20' W). The sludge solid phase consisted mainly of pyrite (
75%), sphalerite (<2%), galena (<1%), chalcopyrite (<1%), and arsenopyrite (<1%), in addition to variable amounts of quartz, silicate clays, and gypsum (Almodóvar et al., 1998; Domènech et al., 2002). The 0.05- to 1-m thick blanket of sulfide sludge, which covered approximately 45 km2 of arable soils, was mechanically removed in the following dry summer months. Beginning in the autumn of 1998, the affected soils, which included Aquic, Typic, and Vertic Xerofluvents; Typic Haploxerepts; and Typic Calcixerepts (Cabrera et al., 1999; Clemente et al., 2000), were remediated mostly by ploughing in variable amounts of organic matter and lime from sugar production factories (85% CaCO3). The remediated soils are at present highly heterogeneous on the decimeter or even the centimeter scale, largely as a result of the residual sludge and industrial lime present (Burgos et al., 2003).
A number of studies have dealt with the oxidation of this pyritic sludge and with its polluting effect on both the affected soils and the waters draining from them. Thus, Domènech et al. (2002) studied the in vitro oxidative dissolution of the sludge, and Dorronsoro et al. (2002) and Simón et al. (2002) characterized the reactions of the acid liquid phase draining from a thin sludge cover with the underlying soil. To the authors' knowledge, however, no studies on the long-term in situ weathering of the sludge remaining in the remediated soils have so far been conducted. Weathering in the area occurs under a xeric hydric regime as it is under a warm Mediterranean climate; the mean annual temperature is approximately 18°C and the mean annual rainfall approximately 600 mm (80% of which falls between October and April). The purpose of this study was to assess the degree of weathering of pyrite and sphalerite (namely the two most abundant sulfides in the sludge) in these contaminated/remediated soils after two and three rainy seasons. The information thus obtained was expected to be useful with a view to predicting the direction and extent of future mineralogical and chemical changes affecting the phytoavailability and potential toxicity of some metals present in the sulfides (particularly Zn). In a companion study (Hita and Torrent, 2005), this element was found to be present at potentially phytotoxic levels in some sludge-affected soils.
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MATERIALS AND METHODS
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Soil Sampling
Soil samples were collected from two approximately 40-km transects along the valley in November 2000 and June 2001. The samples consisted of 1 to 2 kg of soil around the roots of plants of two wild species [namely, spreading pigweed (Amaranthus blitoides S. Wats.), November 2000; common cocklebur (Xanthium strumarium L.), June 2001] in areas affected by the spill that were remediated with industrial lime. We chose these species because, in a previous study on phytoavailability of Zn in the sludge-affected soils, we observed them to grow on soils exhibiting widely different characteristics and contents in residual sludge and industrial lime. Small sludge lumps (<5–10 cm) were seen at many sampling points. The samples, which were moist at the time of collection, were transferred to the laboratory and processed within 24 h as described below.
Soil Analyses
Soil samples were cleaned of roots, air-dried, and passed through a 2-mm sieve before analysis. Particle-size distribution was determined by the pipette method. Organic C was determined with an EuroVector EA3000 CHN analyzer (EuroVector S.p.A., Milan, Italy), with correction for C in CaCO3 when necessary. Cation exchange capacity (CEC) was determined by using 1M ammonium acetate (NH4OAc) buffered at pH 7. Soil pH was measured in a 1:2 soil/water mixture and the electrical conductivity (EC) of the soil solution in a 1:5 soil/water extract. The total CaCO3 equivalent (CCE) was determined from the weight loss in 2 g of sample treated with 6 M HCl. Finally, the active calcium carbonate equivalent (ACCE) or "active lime" was determined with NH4–oxalate as described by Drouineau (1942).
The total soil Fe and Zn contents (Fet, Znt) were taken to be the amounts of Fe and Zn dissolved by aqua regia; for Zn, the results were checked against BCR Certified Reference Material 601 (Rauret et al., 2000). Total S was determined by combustion using a LECO Model 521 induction furnace (LECO Corporation, St Joseph, MI) after mixing the sample with a small amount of tin metal and iron chip accelerator to ensure complete conversion of S to SO2, which was titrated iodometrically on a LECO Model 517 titrator. The quantification of SO4 present as gypsum and soluble sulfates was based on the gypsum determination method of Berigari and Al-Any (1994). Briefly, 2-g soil samples were suspended in 6 mL of 0.5 M Na2CO3 in a 10-mL polyethylene tube and the suspension sonicated for 5 min to convert all gypsum Ca into CaCO3 and release gypsum SO4. The suspension was centrifuged and the supernatant passed through a 0.2-µm Millipore filter. Then, 4 mL of the filtrate was slowly acidified with 1 mL of 6 M HCl to remove CO2 and centrifuged again before treating 4 mL of the supernantant with 1 mL of 200 g L–1 BaCl2 to precipitate SO4 as BaSO4. The precipitate was finally washed with water, dried, and weighed to quantify SO4. The detection limit of this method was 0.05 mg SO4–S g–1 soil and the recovery of gypsum added to different samples 98 to 99%. Citrate/bicarbonate/dithionite (CBD)-extractable Fe and Zn (Fed, Znd) were determined according to Mehra and Jackson (1960) except that extraction was performed at 25°C for 16 h. Citrate/bicarbonate-extractable Zn (Zncb) was also determined following the former procedure except that no dithionite was added to the citrate/bicarbonate solution. Acid NH4 oxalate-extractable Fe and Zn (Feo, Zno) were determined according to Schwertmann (1964), and pH 6 citrate-extractable Zn (Znc) according to Reyes and Torrent (1997). Iron in solution was determined by the o-phenanthroline method (Olson and Ellis, 1982) using a wavelength of 508 nm. Full color development in the oxalate extracts was accomplished by using only 0.8 cm3 of extract in a final volume of 25 cm3, acetate buffer at pH 5.25 and three times as much o-phenanthroline as in the usual procedure. Zinc in solution was quantified by atomic absorption spectrophotometry. All analyses were performed at least in duplicate, and reagent grade chemicals used throughout.
Sulfide Oxidation Experiment
An in vitro soil sulfide oxidation experiment was performed by aerating a suspension of 1.5 g of soil in 15 mL of water placed in a 25-mL polypropylene tube for 60 d. The suspension was stirred by injecting air through a polyethylene tube of 1 mm i.d. at a flow of 2 mL s–1. At the end of the experiment, the total amount of SO4 in the suspension was determined as described before.
Statistical Analyses
Statistical analyses were performed by using Costat (Cohort Software, 1995) or Statistix 8 (Analytical Software, 2003). Unless otherwise stated, the term "significant" is used here to denote significance at the 5% level.
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RESULTS AND DISCUSSION
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Soil Properties
The soils collected in both November 2000 and June 2000 (i.e., two and three rainy seasons after the spill, respectively) ranged widely in particle-size distribution, organic matter, CCE and ACCE contents, and CEC (Table 1). The wide range spanned by the high values of EC of the 1:5 extract in some soils suggests the presence of gypsum (resulting from sulfide oxidation and precipitation of released SO4 with soil Ca). No significant differences as per Student's t test were found in any property between the two soils populations and the same applies to the soil total contents in S, Fe, and Zn (Table 1). The concentrations of these elements in the soils are supposed to be closely related to the amount of residual sludge because the native (unaffected) soils exhibit Fe, S, and Zn concentrations much lower than those found in the average sludge (Cabrera et al., 1999).
The mean contents in the different forms of extractable Fe and Zn, as well as in SO4–S, were higher in the November 2000 soils than in the June 2001 soils (Table 2). However, the only significant difference was that in acid oxalate-extractable Fe. The meaning of these differences is discussed later on.
Estimation of the Sludge Sulfide Content in the Remediated Soil
The Fet contents in the sludge samples analyzed by different authors differ little, with an average of 360 g kg– 1 (Domènech et al., 2002; Ginhas, 2003; Hita and Torrent, unpublished data, 2003). This is consistent with a proportion of stoichiometric pyrite (FeS2; 46.6 wt.% Fe) of approximately 75% in the sludge, in addition to 0.5 to 1% chalcopyrite (FeCuS2; 34.9 wt.% Fe) and minor amounts of arsenopyrite and Fe-bearing clay minerals, as reported by Domènech et al. (2002). By contrast, the proportion of sphalerite in the sludge ranged more widely (0.7–1.6 wt.%; Cabrera et al., 1999).
One can assume the acid solutions resulting from sulfide oxidation in the remediated soils to have been neutralized by the surrounding soil, Fe3+, Zn2+, and other metal ions being adsorbed/precipitated by the soil matrix within a short distance (few centimeters) of the sludge mass, as found by Dorronsoro et al. (2002) and Simón et al. (2002) in soils covered with a sludge blanket. In fact, the acid neutralizing capacity of the soils studied was high enough for this phenomenon to occur (none of the soils exhibited a pH value below 5.5). On these grounds, one can obtain a reasonably accurate estimate of the initial (sludge) pyrite content in the polluted/remediated soil from the current Fet content—no Fe was lost from the soil—after subtracting the native soil Fet content and using the stoichiometric ratio between pyrite and Fe. We adopted a value of 10 g kg– 1 for native Fet; this was the lowest concentration found in the soil samples low in Znt, which were therefore unlikely to have been significantly contaminated by sulfides. This difference, henceforth referred to as 
Fet, can slightly overestimate the initial pyrite content, but has the advantage that it can never be negative. Similarly, the initial content of sphalerite in the soil was estimated by computing the difference between the actual Znt content of the soil and a background (native) Znt content of 37 mg kg– 1, which was the lowest level found in the samples, and then using the stoichiometric ratio between sphalerite and Zn. Slight overestimation is also possible here because the average Znt content of non-polluted soils in the area is 109 mg kg– 1 (Cabrera et al., 1999); again, however, no negative values for sphalerite were obtained. Table 3 gives the pyrite and sphalerite contents estimated under the previous assumptions. The weak correlation between the pyrite and sphalerite contents in the polluted/remediated soils (Fig. 1)
reflects the highly variable content of sphalerite in the contaminating sludge.
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Table 3. Estimated initial contents of pyrite and sphalerite in soils sampled in remediated sludge-affected areas.
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Fig. 1. Relationship between the estimated initial contents of pyrite and sphalerite in all soil samples.
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The soil St content can provide one other estimate of the initial content of pyrite in the soil. This was estimated on the assumptions that (i) all S could be assigned to pyrite (i.e., the much smaller amounts of S in other sulfides were ignored), and (ii) the native soil St content was 0.45 g kg– 1 (namely, the lowest value found in the two soil populations). The initial pyrite values thus estimated (Table 3) were substantially lower than those estimated from the soil Fet contents. This obviously reflects the fact that a significant portion of the SO4 ion resulting from sulfide oxidation had been leached from the soil before sampling.
Pyrite Weathering
Weathering of pyrite increases the CBD-extractable Fe content (Fed) of the soil because (i) Fe3+ ions produced in the oxidation of this mineral are likely to precipitate as various Fe oxides—which are CBD-extractable—at pH values above 2 to 3 such as those found in the sampled soils; and (ii) CBD dissolves pyrite to a negligible extent. Accordingly, the ratio between Fed (obtained by subtracting the native Fed from the actual soil Fed) and the estimated initial content in pyrite-Fe (Fet) provided a measure of the degree of weathering of pyrite. We adopted a value of 3.4 g kg– 1 for the native Fed, which was the lowest level for the whole soil population; this generally resulted in overestimation of Fed; however, as with Fet, no negative values were obtained. Figure 2
illustrates the relationship between Fed (y axis) and Fet (x axis). The regression line for each soil population was calculated without a constant term to account for the fact that Fed tended to zero as Fet approached zero (i.e., when the soil was not polluted). The slope of the regression line was significantly (P < 0.001) higher for the soils collected in June 2001 than for those collected in November 2000 (Fig. 2), consistent with the fact that the average degree of pyrite weathering increased with time; on average, 51 and 69% of pyrite had weathered by November 2000 and June 2001, respectively. The degree of pyrite weathering as estimated from the Fed/Fet ratio was negatively, but weakly, correlated with the soil contents in CCE (r = –0.26; P < 0.05) and ACCE (r = –0.26; P < 0.05). Thus, the presence of carbonate and its associated high pH seems to somewhat hinder pyrite oxidation. An inverse relationship between the rate of in vitro sludge weathering and pH over a narrow acid pH range was previously reported by Domènech et al. (2002).
Insight into the nature of the neoformed Fe oxides can be gleaned from the increase in acid oxalate-extractable Fe (Feo) as calculated by subtracting the native Feo content from the current Feo content. This difference provides an estimate of the amount of poorly crystalline Fe oxides produced in the course of pyrite weathering. Computations were based on a value of 0.32 g kg– 1 for native Feo, which, similarly to Fet and Fed, was the lowest level in the whole soil population. Figure 3
illustrates the relationship between Feo and Fet for the two soil populations. As with Fed, the slope of the regression line was significantly (P < 0.001) higher for the June 2001 than for the November 2000 soils (0.263 vs. 0.127). The average Feo/Fed ratio was 0.25 for November 2000 and 0.40 for June 2001.
In summary, as pyrite weathering proceeded, the proportion of newly formed Fe oxides that were poorly crystalline increased. The degree of crystallinity as measured by the Feo/Fed ratio was weakly correlated with ACCE (r = 0.313; P < 0.05), consistent with the predominance of ferrihydrite in the products of in vitro pyrite oxidation in a carbonate medium (Caldeira et al., 2003).
We also assessed the degree of pyrite weathering by comparing the residual S content with the initial pyrite-S content in the soil. The residual pyrite content was estimated by subtracting the content in native S (0.45 g kg– 1 as discussed above) and the amount of SO4–S from St, thus excluding the negligible amounts of S in other sulfides. The initial pyrite-S content was estimated by multiplying the pyrite-Fe (Fet) content by the pyrite stoichiometric S/Fe ratio (1.15). The degree of weathering calculated from the (initial pyrite-S – residual pyrite-S)/(initial pyrite-S) ratio ranged widely (mean = 0.85); unlike the degree of weathering based on the Fed/Fet ratio, it differed little between the two soil populations and was not correlated with the soil carbonate content. One possible source of these discrepancies is the error involved in the estimation of the residual pyrite-S content.
Sphalerite Weathering
Acid oxalate-extractable Zn (Zno) provides an estimate of the amount of Zn released from sphalerite because oxalate dissolves poorly crystalline Fe oxides and carbonates, the main sinks for Zn2+ (Montilla et al., 2003) but not sphalerite—at least not to a significant extent. Therefore, the degree of sphalerite weathering was assessed by comparing Zno with Znt; this rested on the assumption that Zn was not lost from the soil during weathering and that the relatively low content of native soil Zn was negligible. The slope of the regression line of Zno on Znt (Fig. 4)
was 0.42 for the November 2000 soils and 0.55 for the June 2001 soils. Therefore, as with pyrite, the degree of sphalerite weathering increased with time.
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Fig. 4. Relationship between acid oxalate-extractable Zn (Zno) and total Zn (Znt) in the soil samples collected in November 2000 and June 2001.
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A comparison of Fig. 2 and 4 reveals that sphalerite weathered a lesser extent than did pyrite. This contradicts the results of in vitro oxidation experiments with sludge (Domènech et al., 2002) and other sulfide mine tailings (Gleisner and Herbert, 2002). One reason for this discrepancy is that many in vitro tests were done at an acid pH, whereas most of the soil samples studied here were alkaline or only slightly acidic. One other reason may be that part of the Zn2+ ions released in the course of sphalerite weathering were occluded in the newly formed crystalline Fe oxides, which acid oxalate cannot dissolve significantly. Accordingly, we estimated the total amount of Zn released from sphalerite as combination of (i) (Znd – Zncb), which represents the amount of Zn occluded in Fe oxides, both crystalline and poorly crystalline; and (i) Znc, as the acid citrate extractant dissolves the Zn in carbonates (the other major Zn sink in most soils). The slope of the regression line of (Znd – Zncb + Znc) on Znt (Fig. 5)
was significantly (P < 0.001) higher for the June 2001 soil than for the November 2000 soil (0.672 vs. 0.506); this supports the previous finding that the degree of sphalerite weathering increases with time. The absolute values of these slopes are (i) higher than those of Fig. 4, which suggests that the newly formed crystalline Fe oxides incorporated Zn either in structural form or adsorbed on the surfaces of internal inter- or intraparticle pores; and (ii) similar to those for pyrite (Fig. 2).
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Fig. 5. Relationship between the amount of Zn released from sphalerite [calculated as (Znd – Zncb + Znc)] and total Zn (Znt) in the soils collected in November 2000 and June 2001.
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Our results show that in situ weathering of pyrite and sphalerite occurred at comparable mean rates during the first 3 yr of soil-sludge contact, so 65 to 70% of the sulfides on average had weathered by June 2001. The increase in SO4–S resulting from the in vitro oxidation of soil suspensions (SO4–S, Table 2) was relatively modest (mean for all soils = 0.37 g kg– 1), with no significant differences between the two soil groups. On average, only 28 and 19% of the sulfide-S present in the soil was converted into SO4–S in the November 2000 and June 2001 samples, respectively. One reason for such low values is that the sulfides remaining in the soil after field exposure to two to three rainy seasons were the least reactive, coarsest particles, which were likely to weather at a slow rate. This is supported by the fact that the sludge was relatively heterometric and contained a significant proportion of coarse fractions. Thus, more than 45% and about 15% of the sludge mass consisted of particles >30 and <2 µm, respectively; also, the specific surface area was 1 to 1.5 m2 g–1 in one sample analyzed by Domènech et al. (2002) and two samples analyzed by the authors (detailed results not shown). However, in some areas where the sludge was not removed, the sediment left on the soil surface was stratified and exhibited a more variety in particle-size distribution than that of the former analyses.
The similarity in the 3-yr average weathering rates of pyrite and sphalerite does not necessarily mean that the initial weathering rate was also similar for the two minerals. It may simply reflect the fact that, after a relatively long time, all the easily weatherable (fine sized) particles, whether pyrite or sphalerite, had weathered. Thus, preferential sphalerite weathering, as generally observed in pyrite–sphalerite mixtures because of galvanic effects (Balá
et al., 1994), cannot be ascertained.
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ACKNOWLEDGMENTS
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This study was partly funded by Spain's Ministerio de Ciencia y Tecnología/FEDER Project 1FD97–2101. The senior author thanks Spain's Ministerio de Educación y Cultura for a study grant from April 2001 to March 2003, Prof. J.M. Bigham of Ohio State University for his advice on the analyses for total S in soil, and Dr. A.M. Ginhas for supplying some sludge samples.
Received for publication December 20, 2004.
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R. Hita, J. Torrent, and J. M. Bigham
Experimental Oxidative Dissolution of Sphalerite in the Aznalcollar Sludge and Other Pyritic Matrices
J. Environ. Qual.,
May 31, 2006;
35(4):
1032 - 1039.
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ABSTRACT
INTRODUCTION
