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Published online 31 May 2006
Published in J Environ Qual 35:1032-1039 (2006)
DOI: 10.2134/jeq2005.0371
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Heavy Metals in the Environment

Experimental Oxidative Dissolution of Sphalerite in the Aznalcóllar Sludge and Other Pyritic Matrices

Raúl Hitaa, José Torrenta,* and Jerry M. Bighamb

a Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Edificio C4, Campus de Rabanales, 14071 Córdoba, Spain
b School of Environment and Natural Resources, The Ohio State University, 2021 Coffey Road, 210 Kottman Hall, Columbus, OH 43210-1085

* Corresponding author (torrent{at}uco.es)

Received for publication September 26, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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.

Abbreviations: DTPA, diethylenetriaminepentaacetic acid • SSA, specific surface area


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE COLLAPSE of the tailings pond dike of a zinc mine near Aznalcóllar, in southwestern Spain, on 25 Apr. 1998 resulted in the release of several million tons of sulfide sludge that flooded 45 km2 of the Agrio and Guadiamar river valleys (37°00' to 37°30' N, 6°10' to 6°20' W). Pyrite (FeS2) made up ~75% of the sludge solid phase, which also contained minor amounts of sphalerite (<2% ZnS), galena (<1% PbS), chalcopyrite (<1% CuFeS2), and arsenopyrite (<1% FeAsS), plus variable amounts of quartz, silicate clays and gypsum (Almodóvar et al., 1998; Domènech et al., 2002). Because of the threat posed by release of toxic metals (mainly Zn, Pb, Cu, and As) to the soil and ground water as the result of sulfide oxidation, the sludge blanket was mechanically removed during the following summer months. Still later, the soils were remediated by plowing in organic amendments (sugarbeet [Beta vulgaris L.] wastes that were ~85% CaCO3) and iron oxide–rich sediments. These amendments proved to be effective in reducing the solubility of the main pollutant elements (Aguilar et al., 2004).

After the spill, a number of studies were undertaken to examine the mobility and phytoavailability of toxic elements in the sludge-affected areas (Alastuey et al., 1999; Cabrera et al., 1999; Díaz-Barrientos et al., 1999; Domènech et al., 2002; Madejón et al., 2002; Simón et al., 2002; Hita and Torrent, 2005a, b). The biogeochemistry of Zn attracted particular attention because the sludge contained sphalerite, whose oxidative dissolution resulted in locally phytotoxic levels of Zn in wild plants (Madejón et al., 2002; Hita and Torrent, 2005a).

In vitro oxidation experiments with Aznalcóllar sludge samples in a flow-through reactor showed that sphalerite dissolved faster than pyrite in the 2.5 to 4.7 pH range (Domènech et al., 2002). This result can be partly attributed to a galvanic interaction between pyrite and sphalerite that is usually observed in sulfide mixtures (Mehta and Murr, 1983, 2004; Baláz et al., 1994; Cruz et al., 2001, 2005). Hita and Torrent (2005b) examined samples of the contaminated soils and concluded that: (i) weathering via oxidative dissolution of pyrite was fast, so 51 and 69% of this mineral had weathered by November 2000 and June 2001(that is, two and three rainy seasons after the spillage), respectively; (ii) sphalerite had weathered to approximately the same extent as pyrite; (iii) further weathering was unlikely to be significant because only the coarse, less reactive sludge particles remained in the soil; and (iv) a significant portion of the Zn released appeared to be occluded in Fe oxides resulting from the oxidation of pyrite.

The purpose of this study was to investigate the in vitro oxidation of pyrite and sphalerite in the Aznalcóllar sludge, either pure or mixed with calcium carbonate. The results were compared with those from similar experiments using reference pyrite and sphalerite and their mixtures.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sample Collection and Preparation
About 1 kg of sludge was collected in October 2000 in an area close to the Guadiamar River where the original sludge blanket was thick (>30 cm), had not been disturbed, and showed no visual signs of oxidation in the form of yellow or brown colors typical of Fe(III) compounds. The sample was taken from the middle part of the blanket, thus discarding the upper few cm and that part adjacent to the underlying soil, placed in a polyethylene bag, transferred immediately to the laboratory, freeze-dried, and stored at 4°C before analysis. This dry product (henceforth called sludge) was analyzed according to the methods described by Hita and Torrent (2005b), and was found to contain 400 g kg–1 Fe, 480 g kg–1 S, 6.9 g kg–1 Pb, 6.5 g kg–1 Al, 3.8 g kg–1 As, 2.6 g kg–1 Ca, 1.32 g kg–1 Zn, 630 mg kg–1 Cu, 245 mg kg–1 Mn, and other elements in lesser concentrations. Its BET specific surface area (SSA) was ~1500 m2 kg–1, which is consistent with its particle size distribution (>45% and about 15% of the sludge consisted of particles >30 µm and <2 µm, respectively) and is similar to that reported previously for sludge samples collected immediately after the spill (Domènech et al., 2002).

Reference pyrite and sphalerite samples were obtained from Ward's Natural Science Establishment (Rochester, NY) and checked for the presence of impurities by X-ray diffraction. They were ground before the experiment with a disk mill to <1 mm; in the resulting product, ~58% of the particles were >0.25 mm and ~2% were <0.05 mm. Accordingly, the SSA values of the reference sulfides were <1500 m2 kg–1 and thus smaller than those of sulfides in the sludge.

Flow-Through Experiment
This experiment was performed to compare the oxidation rates of pyrite and sphalerite in the presence and absence of calcite and with alternate wetting and drying. The work was conducted using a commercial mechanical extractor (obtained from Concept Engineering Co., Lincoln, NE, and schematized in Fig. 1) with 10 different types of samples (Table 1) and five replicates per sample type. Each sample (2 or 3.2 g) was placed over a paper pulp filter in the sample tube and sprayed with 10 mL of deionized water to allow homogeneous moistening. The reservoir tube was then put in place, filled with 50 mL of deionized water, and the extractor was activated to collect leachate in the extraction syringe over a period of 16 h. This process was repeated after 10 d and then at 7-d intervals six more times, allowing the sample to dry at room temperature (~22°C) after each leaching to mimic drying–wetting cycles in the field. The leachates were filtered through a 0.2-µm Millipore filter and the filtrate split into two portions, the first being used for immediate pH measurement (glass electrode), and the second being acidified with HCl and kept at 4°C until analysis. For each sample type, the samples that had undergone four and eight leachings were air dried. Then the X-ray powder diffraction patterns of these samples and that of the unleached material were acquired using a Philips PW 1316/90 diffractometer equipped with CuK{alpha} radiation, a {theta}-compensating slit, a 0.2-mm receiving slit, and a diffracted-beam monochromator. Data were collected at 0.05°2{theta} steps in the 3 to 80°2{theta} range. The color of the ground products was measured by diffuse reflectance with a Minolta CR-300 Chroma Meter (Konica Minolta Sensing, Osaka, Japan) and expressed in Munsell notation. Diethylenetriaminepentaacetic acid (DTPA)-extractable Fe and Zn were determined according to Lindsay and Norvell (1978), and pH 6 citrate- and citrate–ascorbate-extractable Fe and Zn according to Reyes and Torrent (1997). Acetate-extractable Fe and Zn were determined after shaking 0.1-g samples in 4 mL of 1 M, pH 5 Na acetate–acetic acid at 25°C for 16 h in a reciprocating shaker set at 2 Hz. Iron and Zn in solution were determined by inductively coupled plasma–atomic emission spectrometry (ICP–AES) using a Perkin-Elmer Optima 300 spectrometer (PerkinElmer, Wellesley, MA).


Figure 1
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  		Fig. 1. Cross-section of the mechanical extractor used in the flow-through experiments (not to scale: syringe and sample tube diameter is ~4 cm).

 

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  		Table 1. Samples used in the flow-through experiment.

 
Sludge Suspension Experiment
This experiment was performed to compare the oxidation rates of pyrite and sphalerite in the presence and absence of calcite without the effect of drying. Suspensions were prepared with 240 mL of deionized water and (i) 6 g of sludge, (ii) 5.7 g of sludge + 0.3 g of reference sphalerite, (iii) 6 g of sludge + 3.6 g of reagent-grade CaCO3, and (iv) 5.7 g of sludge + 0.3 g of sphalerite + 3.6 g of reagent-grade CaCO3. The suspensions, placed in beakers, were magnetically stirred and oxidized with a 94 mL min–1 flow of O2 previously passed through a column of Ascarite II (a NaOH-coated inert material) to remove CO2, and a column of deionized water equipped with a porous diffuser to saturate it with water. At eight preset times during a period of 25 d, one 15-mL and eight 0.5-mL portions of the suspension were taken and centrifuged at an acceleration of 7000 to 9000 m s–2. The supernatant of the 15-mL portion was filtered through a 0.2-µm Millipore filter and the filtrate split into two portions, one being used to measure pH and the other to determine the Fe, Zn, and S in solution by ICP–AES. The sediment was resuspended in 1 mL of deionized water, deposited on a quartz plate and allowed to air dry before acquiring its X-ray diffraction pattern and measuring color as described before. The sediments corresponding to the eight 0.5-mL portions were used to determine (in duplicate) the DTPA-, acetate-, citrate-, and citrate–ascorbate-extractable Fe and Zn as described before.

Experiments with Suspensions of Reference Pyrite and Sphalerite
Two experiments were designed to examine the influence of particle contact between pyrite and sphalerite on the oxidative dissolution of these minerals. The first was based on four suspensions: (i) 1.25 g of pyrite and 100 mL of deionized water in a 150-mL Erlenmeyer flask; (ii) as (i) but with 1.25 g of sphalerite; (iii) 2.5 g of pyrite, 2.5 g of sphalerite and 200 mL of deionized water in a 250-mL Erlenmeyer flask; (iv) two suspensions, one with 2.5 of pyrite and the other with 2.5 g of sphalerite in 100 mL of deionized water, placed in a 200-mL Erlenmeyer-shaped glass vessels that were connected with a tube equipped with two Pyrex porous (10–16 µm) plates that allowed electrolyte but not particle diffusion between the two suspensions. The suspensions were magnetically stirred to prevent particle sedimentation and to favor oxygen diffusion from the air, and deionized water was periodically added to make up for evaporation losses. At preset times between 0 and 111 d the suspensions were sampled and the solution analyzed for Fe and Zn by atomic absorption spectrometry.

The second experiment consisted of the oxidation of suspensions containing pyrite and sphalerite in different proportions prepared using 1.8 g of solids and 35 mL of deionized water. The suspensions were stirred (36 d), sampled, and the solutions analyzed for Fe and Zn as before.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Flow-Through Oxidation Experiment
The pH of the first leachate was 2.9 for the sludge and 5.6 for the sludge–calcite mixture. Subsequent respective values of ~3.0 and ~7.2 were recorded in the leachates over an 8-wk period (Fig. 2). The initial pH values indicate that the sludge had been partially oxidized, consistent with the fact that the undisturbed sludge blanket from which the sample was collected had been exposed to changes in temperature and several drying–wetting cycles for about 30 mo. They also indicate that the proportion of calcite in the sludge–calcite mixtures was sufficient to buffer the pH at a circumneutral value.


Figure 2
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  		Fig. 2. The pH of leachates of sludge and sludge–calcite samples as a function of the number of leaching steps in the flow-through experiment. Each value is the mean of two replicates. The standard error bar (two replicates) is not shown when its length is similar to or smaller than the size of the corresponding marker.

 
Figure 3 shows the cumulative amounts of Fe, Zn, and S leached from the solids as percentages of their total content; X-ray diffraction data (not shown) indicated that pyrite and sphalerite were the primary sources of Fe and Zn in the sludge. The proportion of Zn released to the first leachate (~17%) was much higher than those of Fe or S (3.5–5.5%) for both the sludge and sludge–calcite indicating that sphalerite had weathered (via oxidative dissolution) in the field to a greater extent than pyrite. In contrast, the rates at which Fe and S (as SO4) were later released from the sludge (~0.23 and ~0.21% leaching–1, respectively) were slightly higher than that for Zn (0.19% leaching–1). Little (<0.1%) Fe and only ~0.4% Zn was released after the first leaching from the sludge–calcite samples because precipitation of Fe (hydr)oxides (Cornell and Schwertmann, 2003) and Zn hydroxicarbonates and carbonates (Uygur and Rimmer, 2000) occurs readily in calcareous environments. The amount of S in the first leachate was higher for sludge–calcite than for the sludge (4.6% vs. 3.5%), which may be due to decreased SO4 adsorption with increased pH. The S leaching rates tended to the same value after several leaching steps (~0.21% leaching–1), indicating the overall rate of sulfide oxidation to sulfate to be relatively unaffected by pH.


Figure 3
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  		Fig. 3. Cumulative amounts of (a) Fe, (b) Zn, and (c) S leached from sludge and sludge–calcite samples in the flow-through experiment. Each value is the mean of two replicates. The standard error bar (two replicates) is not shown when its length is similar to or smaller than the size of the corresponding marker. Despite the large standard errors for the sludge samples (solid symbols), the rates of leaching of either Fe, Zn, or S (as measured by the slope of the corresponding y–x regression line) did not differed significantly (P < 0.05) between replicates.

 
Figure 4 shows the amounts of Fe and Zn extracted by different reagents in the sludge and in the sludge–calcite after zero, four, and eight leaching steps. It must be recalled that (i) DTPA and citrate are complexing agents and the acetate extractant is mildy acidic, so they can dissolve small amounts of poorly crystalline Fe oxides; (ii) both the citrate and the acetate extractants are acidic and can thus dissolve carbonates; and (iii) the citrate–ascorbate extractant has both complexing and reducing properties allowing it to dissolve poorly crystalline Fe (hydr)oxides (Reyes and Torrent, 1997). None of the extractants can dissolve sulfides to a significant extent; therefore, little Fe and Zn were removed by these reagents from the sludge samples subjected to four or eight leachings, because the Fe and Zn ions released by sulfide oxidation were soluble at low pH and were leached away (Fig. 3a and 3c). In contrast, the sludge–calcite solids contained substantial amounts of extractable Fe and Zn after leaching (Fig. 4b and 4d). In this instance, citrate–ascorbate was the most effective extractant for Fe, suggesting that much of the Fe released in the oxidation of pyrite had precipitated in the form of poorly crystalline Fe (hydr)oxides. Citrate–ascorbate extracted significantly more Zn than the acidic and/or complexing reagents, which suggests that some Zn was occluded in the Fe (hydr)oxides (either structural or adsorbed on micropore surfaces). The significant amounts of citrate- or acetate-extractable Zn support the idea that some Zn also precipitated as hydroxi(carbonates) that are readily soluble in acidic/complexing reagents.


Figure 4
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  		Fig. 4. Diethylenetriaminepentaacetic acid (DTPA)-, acetate-, citrate-, and citrate–ascorbate-extractable Fe and Zn before leaching and after four and eight leaching steps (flow-through experiment): (a) Fe in sludge, (b) Fe in sludge–calcite; (c) Zn in sludge, and (d) Zn in sludge–calcite. Error bars are standard errors (two replicates).

 
The X-ray diffraction patterns of the leached solids did not differ significantly from those of the unleached ones as for as the number, position, and relative intensity of the different pyrite and sphalerite peaks, and they did not exhibit any reflections attributable to crystalline Fe (hydr)oxides. The color of the sludge samples (9.4YR 4.9/0.6, i.e., gray) did not change on leaching. That of the sludge–calcite samples changed from 7.5Y 5.8/0.4 to 5.2Y 4.9/1.5, that is, from gray to grayish brown, consistent with the presence of significant amounts of Fe (hydr)oxides. Of these, ferrihydrite and poorly crystalline lepidocrocite are the best candidates because they are soluble in citrate–ascorbate (Reyes and Torrent, 1997).

Sphalerite had weathered to a greater extent than pyrite in the sludge left in the field but this result does not necessarily prove that the pristine dissolution rate of the former was higher than that of the latter on a surface area basis—although previous studies indicated that the two sulfides have similar specific surface areas in the Aznalcóllar sludge (Domènech et al., 2002). To support the idea that sphalerite was oxidized faster than pyrite in the sludge, and that galvanic effects are largely responsible for this result, we examined data from the parallel flow-through experiments with (i) reference pyrite and sphalerite having similar particle size (and thus similar SSA) and (ii) mixtures of sludge and sphalerite.

Figure 5 shows the evolution of pH in the leachates of pyrite, sphalerite, 95% pyrite–5% sphalerite and their corresponding mixtures with calcite, and Fig. 6 the cumulative amounts of Fe, Zn, and S leached from them. Significant sulfide oxidation, as indicated by the amounts of S leached (Fig. 6c), took place in the pyrite and pyrite–sphalerite samples, particularly in the presence of calcite. More than 10% of the total Zn but <4% of the total Fe was released from the pyrite–sphalerite mixtures (Fig. 6a and 6b). The hypothesis that this difference was mainly due to galvanic interaction was supported, on one hand, by the depressing effect of sphalerite on the dissolution of pyrite—less Fe was leached from pyrite–sphalerite than from pyrite (Fig. 6a). On the other hand, the fast dissolution of sphalerite in the pyrite–sphalerite mixture cannot be attributed strictly to the low pH of the latter relative to that of the sphalerite-only sample (~3 vs. ~6, Fig. 5). This contention was supported by the observation that the amounts of extractable Zn forms were: (i) similar for the sphalerite and the sphalerite–calcite solids (Fig. 7a and 7b) despite the contrasting pH values of their leachates (~6 vs. ~8.5, Fig. 5a and 5b); and (ii) one order of magnitude greater in the pyrite–sphalerite–calcite than in the sphalerite–calcite mixture (Fig. 7b and 7c), even though the corresponding leachate pH values differed by less than 1.5 units. The fact that citrate–ascorbate was more efficient than other extractants for the pyrite–sphalerite–calcite mixture suggests that part of the Zn released in the oxidation of sphalerite was occluded in neoformed Fe (hydr)oxides.


Figure 5
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  		Fig. 5. The pH of leachates of pyrite, sphalerite, and 95% pyrite–5% sphalerite mixtures either with or without calcite as a function of the number of leaching steps in the flow-through experiment. Each value is the mean of two replicates. The standard error bar (two replicates) is not shown when its length is similar to or smaller than the size of the corresponding marker.

 

Figure 6
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  		Fig. 6. Cumulative amounts of (a) Fe, (b) Zn, and (c) S leached from pyrite, sphalerite, and 95% pyrite–5% sphalerite mixtures either with or without calcite as a function of number of leaching steps in the flow-through experiment. Each value is the mean of two replicates. The standard error bar (two replicates) is not shown when its length is similar to or smaller than the size of the corresponding marker.

 

Figure 7
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  		Fig. 7. DTPA-, acetate-, citrate-, and citrate–ascorbate-extractable Zn before leaching and after four and eight leaching steps (flow-through experiment) in (a) sphalerite, (b) sphalerite–calcite, and (c) pyrite–sphalerite–calcite mixtures. Error bars are standard errors (two replicates).

 
Experiments with 95% sludge–5% sphalerite mixtures also demonstrated an enhancement of the sludge (pyritic) matrix on sphalerite oxidation: after eight leaching steps, ~15% of the total Zn had been released from the sludge–sphalerite (Fig. 8a) and about 4% of the total Zn was citrate–ascorbate-extractable in the sludge–sphalerite–calcite mixture (Fig. 8b). As with the native sludge sphalerite, the different forms of extractable Zn suggested that Zn in the solids was in the form of (hydroxi)carbonates and occluded in Fe (hydr)oxides.


Figure 8
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  		Fig. 8. (a) Cumulative amounts of Zn leached from 95%–5% sludge–sphalerite mixtures either with or without calcite, and (b) extractable forms of Zn in 95% sludge–5% sphalerite mixtures mixed with calcite, as a function of number of leaching steps in the flow-through experiment. The standard error bar (two replicates) is not shown when its length is similar to or smaller than the size of the corresponding marker.

 
If we assume that the Zn released from sphalerite is mostly leached (for calcite-free samples) or is citrate–ascorbate-extractable (for samples with calcite), the data in Fig. 3, Fig. 4, and Fig. 8 yield the following rates of Zn release between the fourth and eighth leaching steps: (i) 0.2 and 1.5% per leaching step, respectively, for native and reference sphalerite in calcite-free samples, and (ii) 1 and 0.2% leaching step, respectively, for native and reference sphalerite in the presence of calcite. Because the specific surface area of reference sphalerite was lower than that of sphalerite in the sludge, the data for calcite-free samples support the previously formulated hypothesis that a portion of the finer, more reactive fraction of the native sphalerite had weathered in the field before sample collection. The reason why Zn release rate in the presence of calcite was substantially higher for native than for reference sphalerite is not straightforward. One possible explanation is that the addition of calcite reduced the number of contact points, and so galvanic interaction, between the sludge and the reference sphalerite particles in the sludge–sphalerite–calcite but not in the sludge–calcite mixtures given that native sphalerite particles were finer and homogeneously distributed in the sludge sample.

The reason why the apparent rate of Zn release from native sphalerite increases by a factor of 5 when calcite is added to the sludge (cf. Fig. 3b [sludge] with Fig. 4d [citrate–ascorbate-extractable Zn]) is unclear. It can be speculated that the difference in electrode potentials for the ZnS–FeS2 pair increases with increasing pH and so does galvanic interaction. This increase also suggests that oxidation by Fe3+ released from pyrite (McKibben and Barnes, 1986) is secondary in importance to galvanic interaction since Fe precipitates as (hydr)oxides in calcite-containing mixtures.

Oxidation of Suspensions of Sludge–Sphalerite
The amounts of both Fe and Zn released to solution from the sludge and sludge–sphalerite suspensions after 25 d of oxidation (Fig. 9) were greater than the corresponding ones in the flow-through experiment, probably because oxygen diffusion to the surface of the sulfide particles was faster in stirred suspensions than it was under the flow-through conditions. We observed, in accordance with the flow-through experiments, that addition of sphalerite to pyrite depressed dissolution of the latter slightly (but not significantly), as the likely consequence of galvanic interaction (which can occur even in the suspension because of the existence of aggregates of small sulfide particles and occasional particle contact during stirring). We also observed that (i) sphalerite had partly dissolved before sample collection, as discussed before, and (ii) reference sphalerite was dissolved to a substantially greater extent than was the native one, because either they differed in crystallochemical properties (e.g., isomorphic Fe–Zn substitution), or the finer, more reactive sphalerite particles in the fresh sludge had been dissolved before the sample was collected.


Figure 9
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  		Fig. 9. Amounts of Fe and Zn in the solution of suspensions of sludge and 95% sludge–5% sphalerite mixtures as a function of oxidation time. A logarithmic scale is used for the y axis to accommodate data better. According to the standard error values (not shown), differences between sludge and sludge + sphalerite are significant for Zn, but not Fe in solution.

 
The concentrations of Fe and Zn in the solutions from the oxidized sludge–calcite and sludge–sphalerite–calcite suspensions were negligible (data not shown). However, substantial oxidation of sphalerite occurred as deduced from the high percentage of total Zn that was extracted with the various reagents (Fig. 10). A higher proportion of total Zn was extracted from sludge–calcite than from sludge–sphalerite–calcite suspension solids, consistent with the results of the flow-through experiment. A significant fraction of the Zn released from the native sphalerite was occluded in Fe oxides (i.e., the difference between citrate–ascorbate- and citrate-extractable Zn was positive and significant) in the solids of the sludge–calcite suspensions. This was not the case for the sludge–sphalerite–calcite samples, which were rich in sphalerite (~5% of the sulfides) and released substantial amounts of Zn. The presence of calcite and excess sphalerite apparently favored rapid precipitation of Zn hydroxi(carbonates) rather than the incorporation of Zn ions into the newly formed Fe hydr(oxides).


Figure 10
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  		Fig. 10. Extractable Zn forms in the solids of suspensions of sludge and 95% sludge–5% sphalerite after 25 d of oxidation in the presence of calcite. Error bars are standard errors (two replicates).

 
Oxidation of Suspensions of Pyrite–Sphalerite
The fact that galvanic interactions are also strong in suspensions of sulfide particles was demonstrated by the experiments in which suspensions of reference pyrite and sphalerite, either unconnected, connected through a porous bridge permeable to ions, or mixed were oxidized. After 111 d of oxidation, we observed that the degree of sphalerite oxidation was >95% in the mixed suspension, ~3% when ion (Fe3+, Zn2+, SO42–) diffusion between individual suspensions was permitted, and <0.5% for the sphalerite-only suspension. These results support the idea that oxidation via galvanic effect dominates over direct oxidation by Fe3+ ions produced in the course of pyrite oxidation (Konishi et al., 1992; Kai et al., 2000). Moreover, oxidation experiments with suspensions containing pyrite and sphalerite in different proportions (Fig. 11) showed that: (i) the fraction of sphalerite Zn that went into solution increased with decreasing sphalerite/pyrite ratio, that is, with increasing number of contact points (galvanic bridges) between sphalerite and the neighboring pyrite particles; and (ii) increasing sphalerite/pyrite ratios depressed pyrite dissolution, as dictated by galvanic interaction.


Figure 11
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  		Fig. 11. Amounts of Fe and Zn in the solution of pyrite–sphalerite suspensions (1.8 g of solids in 35 mL of water) after 36 d of oxidation as a function of the percentage of sphalerite in the mixture.

 
The sludge sample used in all our experiments was not sterilized; therefore, it might be argued that oxidation was catalyzed by acidophilic bacteria, which are common in sulfide-rich environments (e.g., Acidithiobacillus ferrooxidans, A. thiooxidans). We can dismiss this hypothesis because SO4 production was not depressed in alkaline media, that is, in mixtures containing calcite (Fig. 3c and Fig. 6c), where these organisms are unlikely to be active. This result is in accordance with other oxidation experiments with pyritic tailings in which abiotic sulfide oxidation was found to be relatively independent of pH (Gleisner and Herbert, 2002). Moreover, the amount of SO4 released from the pyrite–sphalerite mixtures was of the same order of magnitude as that released from the sludge subjected to the same oxidation treatment. Because the reference pyrite and sphalerite are unlikely to contain significant concentrations of sulfide-oxidizing bacteria and/or nutrients for microbial growth one can safely assume this is also the case with the sludge we used.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The results of our experiments indicate, in summary, that oxidative dissolution of sphalerite was relatively fast in the Aznalcóllar sludge mainly because of the pyrite–sphalerite galvanic interaction. For this reason, water soluble Zn constituted a significant proportion of total Zn in the sample collected from a sludge blanket that had been lying in the field for 30 mo. Fast oxidation of sphalerite in, and Zn release from, pyrite matrices points thus to the need for a quick implementation of remediation practices in soils contaminated by sulfide mixtures. In the presence of calcium carbonate, the Zn released by oxidation of sphalerite was not susceptible to leaching, as it was precipitated as Zn (hydroxi)carbonate and occluded in the Fe (hydr)oxides resulting from the oxidation of pyrite. The addition of calcite to the sludge resulted in faster oxidation of native but not of the added reference sphalerite. This result, as well as those obtained from our experiments with suspensions of reference pyrite and sphalerite, indicates that the factors affecting particle contact—and, therefore, galvanic interaction—strongly influence the rate of sphalerite oxidation in pyritic matrices.


   ACKNOWLEDGMENTS
 
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, and the School of Environment and Natural Resources, The Ohio State University, in whose laboratories most of this work was carried out.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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