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a Escuela Politécnica Superior, Departamento de Edafología y Química Agrícola, Universidad de Santiago de Compostela, Campus Universitario, 27002 Lugo, Spain
b Facultad de Farmacia, Departamento de Edafología y Química Agrícola, Universidad de Santiago de Compostela, Campus Sur, 15706 Santiago de Compostela, Spain
* Corresponding author (seoane{at}lugo.usc.es)
Received for publication May 22, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALSMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: FC, field capacity • SI, saturation index • WD, wetting–drying cycles
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALSMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Oxidation of sulfides produces sulfates and acidity (Smith and Shumate, 1970). Under certain conditions, the newly formed sulfates may react further with iron and/or aluminum to form hydroxysulfates and more acid (McSweeney and Madison, 1988).
Although some of the released acid is consumed in weathering reactions with spoil components such as carbonates, oxides, and silicates (Ulrich, 1991), this is often insufficient to neutralize all of the acidity produced. It is thus necessary to implement soil management practices, such as treatment of the affected sites with alkaline amendments or limestone (Caruccio et al., 1988; Katzur and Haubold-Rosar, 1996; Doolittle and Hossner, 1997).
Because of the very large amounts of acid to be neutralized, mine spoil recovery can be very costly. Therefore, the expensive, traditionally used pH-increasing agents such as calcium and magnesium oxides and carbonates are often replaced or supplemented with less efficient but cheaper agents, including various waste products (Simard et al., 1998) such as fly ash, cement kiln dust, and sugar beet waste. Use of these waste products is doubly attractive because it also provides a useful way to dispose materials that are themselves potential environmental hazards.
One such material is the fly ash that is generated in large quantities by coal-fired power stations, which are often located close to the mines from which the coal is obtained (Schuman and Sumner, 1999). The mineralogical, physical, and chemical characteristics of fly ash are highly variable (Carlson and Adriano, 1993; Schuman and Sumner, 1999), and depend on the characteristics of the coal and on the preparation and combustion methods. Generally, fly ash is a ferro–alumino–silicate mineral containing considerable quantities of Ca, K, and Na, with practically no C and N (Carlson and Adriano, 1993). In addition, it typically contains P and B, and trace elements including Cu, Zn, Mn, Mo, Ni, and Se (Furr et al., 1978; Jastrow et al., 1981). Electrical conductivity is generally high, because of the high salt content of the fly ash (Aitken et al., 1984). Fly ash has been used as a pH-increasing agent (Adams et al., 1972; Jastrow et al., 1981; Matsi and Keramidas, 1999) and even as a fertilizer (Schumann and Sumner, 1999; Matsi and Keramidas, 1999) because of its chemical properties.
In the present study, we carried out experiments to compare the acidification and neutralization processes occurring in lignite mine spoils amended with fly ash or agricultural limestone. High doses of fly ash were used, as is usual in studies of this type (Stewart et al., 1997; Hammermeister et al., 1998), with the aims of accelerating spoil recovery and disposing of the fly ash. We also treated spoils with a fly ash–limestone mixture. The three treatments (fly ash, limestone, and fly ash–limestone) were applied under two soil moisture regimes that were designed to simulate different field conditions: permanent field capacity or alternate wetting and drying.
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MATERIALS |
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TOPABSTRACTINTRODUCTIONMATERIALSMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The fly ash used was from a coal-fired thermal power station adjacent to, and served by, the As Pontes mine, which has a daily ash production of 9000 Mg. The main components of the fly ash were quartz, aluminosilicates (mullite), iron oxides (haematite), and aluminium oxides (Romero-Pena, 1993). Particle size analysis showed the following distribution: >0.2 mm, 22%; 0.2 to 0.05 mm, 58%; and <0.05 mm, 20%. The fly ash had low liming capacity (47.85 cmol CaCO3 kg-1).
The agricultural limestone had a very fine particles (with only 8% >0.2 mm and 63% <0.05 mm) and contained 93.8% (w/w) CaCO3 (liming capacity 938 cmol kg-1).
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METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALSMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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5 mm (without crushing), placed them on plastic trays (diameter 11.5 cm, depth 4.5 cm, filled to a depth of about 1.5 cm) and amended them with fly ash, agricultural limestone, or a mixture of both. The different treatments were mixed with the spoil. A control treatment did not receive any amendments. Taking into account that one of the aims was to dispose of the fly ash, high doses of 10 and 20 g per 100 g of spoil were used (Treatments A1 and A2). Treatments A1 and A2 increased the spoil pH to only 3.0 and 3.5, respectively, after 24 h of contact. To achieve the same pH increases with the limestone, it was necessary to add 1 and 2 g limestone per 100 g of spoil (Treatments L1 and L2). For studies of the fly ash–limestone mixture, a single dose comprising 10 g of fly ash and 1 g of limestone per 100 g of spoil (Dose AL1) was tested. The highest dose of fly ash (A2) should theoretically neutralize 53% of available acidity or 10% of the estimated potential acidity. The highest dose of limestone (L2) should theoretically neutralize all the available acidity or 21% of the estimated potential acidity. The mixture of fly ash and limestone (AL1) should theoretically neutralize 79% of the available acidity or 15% of the estimated potential acidity.
The experiment was carried out in the laboratory, at room temperature (15–20°C), under two moisture regimes: (i) field capacity (FC), that is, the water retained by the spoil at 1/3 bar pressure in a Richard apparatus (Guitián Ojea and Carballas, 1976) and (ii) alternate wetting–drying cycles (WD). These two regimes were designed to simulate the conditions to which spoils may be subjected: long periods at field capacity or alternate periods of waterlogging and drying out. The FC conditions were maintained by adding distilled water to the trays every two days. The WD conditions were achieved by covering the spoil samples with distilled water to a depth of 2 cm above the surface, then leaving them to dry to their initial mass before rewetting. The trays subject to WD conditions were flooded every 52 d, which allowed sufficient time for the samples to reach air-dryness moisture conditions (constant weight); seven such wetting–drying cycles were carried out.
A total of 48 separate trays, comprising four replicates of each of six treatments (control, A1, A2, L1, L2, AL1), under two moisture regimes (FC and WD), were prepared. Three of each group of four trays were used to study samples at different times (48, 210, and 360 d [t1, t2, t3]) in order to find how the treatments affected the spoils in the short, medium, and long term. The fourth tray was used to measure Eh and pH at shorter intervals.
At times t1, t2, and t3, the pH and Eh (±10 mV) were measured in a suspension of ratio 1:2.5, spoil to water. At the same times, spoil samples were extracted with distilled water (1:2 spoil to water ratio) for 24 h (Meiwes et al., 1986). Aluminum in the extract was determined by colorimetry (Frink and Peech, 1962), iron and calcium were determined by atomic absorption spectrophotometry, sulfates by turbidimetry (Bardseley and Landcaster, 1960), and soluble acid by titration to pH 7 with sodium hydroxide.
The pH and Eh were also measured at shorter intervals (in the fourth tray). In the WD experiments, pH and Eh were recorded at 1 and 10 d after flooding, and then no further measurements were made (because of lack of contact between the spoil and the electrodes). The pH and Eh of samples under FC conditions were measured on the same days as samples under WD conditions. All measurements were made directly in samples on the trays. Where the same value was obtained in two consecutive measurements, the value was represented by one point in the Eh–pH diagrams (Fig. 2–5). In addition, electrical conductivity was measured at the end of the experiment (t3), to allow estimation of ion activities and ionic strength. Ion activities were calculated from the corresponding activity coefficients using Davies' (1962) equation, and ionic strength was calculated from the electrical conductivity using Griffin and Jurinak's (1973) equation.
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Differences in treatments at the end of the experiment were compared with a one-way analysis of variance (ANOVA), using Tukey's least significant difference (LSD), with SPSS statistical software (SPSS, 2000).
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RESULTS |
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The pH of spoil treated with the fly ash–limestone mixture was always higher than that of spoil treated with the corresponding dose of ash or limestone alone.
The effect of moisture conditions on spoil pH was the same for the control and limed samples: spoil subjected to wetting–drying cycles (WD) had a higher pH than spoil maintained at field capacity (FC), except with the L1 treatment, in which there was no significant difference. In contrast, spoil treated with ash or with the fly ash–limestone mixture generally had lower pH under WD conditions than under FC conditions.
Eh
Measurement of redox potentials showed all of the amended spoils except L1 to be more reducing than the control spoil under WD conditions (although it should be noted that these measurements were only made when the moisture content was above field capacity). Spoils amended with limestone were slightly more oxidizing than the control spoil under FC conditions (Table 1). In all cases, spoils amended with fly ash were more reducing than spoils amended with limestone, for the corresponding doses and moisture conditions.
Soluble Elemental Forms
Aluminum
Spoil treated with fly ash or with the fly ash–limestone mixture only released aluminum under WD conditions, whereas under FC conditions, the soluble aluminum content in the water extracts decreased (Table 1). The control spoil and the limed spoil released aluminum under both sets of moisture conditions. The limestone-amended spoil released the most aluminum at the lower dose of limestone (L1).
Iron
Most iron was released under WD conditions, for all treatments and the controls. The iron content of the spoil-water extract for spoil treated with fly ash was low throughout the experiment (Table 1) and decreased even more at the end of the experiment. There was also a small amount of iron extracted from spoil treated with the higher quantity of fly ash (A2). The fly ash–amended spoils had less iron extracted than the limestone-amended spoils or the controls.
In contrast, the iron content of the water extracted from spoil treated with limestone increased with time. Levels in L1-treated spoils were similar to the controls under FC moisture conditions but greater than the controls under the WD conditions. The amount of iron released after treatment with the higher doses of limestone was much less than with the lower dose of limestone.
In the spoil treated with the fly ash–limestone mixture, iron contents in the extracts were very low or undetectable throughout the experiment.
Calcium
Amendment of the spoil with fly ash induced an increase in soluble calcium content over time (Table 1). The increase was gradual under FC conditions, but under WD conditions there was a sharp increase between t1 and t2, followed by a decrease from t2 to t3. However, for a given fly ash treatment (A1 or A2), final calcium contents in the extracts were similar, regardless of moisture conditions.
As expected, liming the spoil immediately increased its soluble calcium content compared with the control sample. This increase was greatest and most sustained at the higher dose (L2).
Spoil treated with the fly ash–limestone mixture released calcium throughout the experiment, and, by t3, it had a calcium content similar to that of spoil that received the higher dose of limestone (L2) under WD moisture conditions or rather more under the corresponding FC conditions.
Sulfates
The control spoil contained more sulfates than the treated spoils, and slightly higher levels of sulfates were found under WD conditions than under FC conditions (Table 1).
Treating the spoil with limestone or coal fly ash reduced the amount of sulfates present, particularly at the higher doses and under FC conditions. The amount of sulfates present were similar to those in the control sample only in spoil treated with the lower dose of limestone and subjected to WD conditions. Following treatment of the spoil with the fly ash–limestone mixture, the amount of sulfates found was low under both moisture regimes. The amount of sulfate in AL1 was also similar to that in the spoil that had received treatment A2 of fly ash and had been subjected to FC conditions.
Neutralization of Acidity
The proportion of soluble acid that was extracted from the control samples and neutralized by treatment with each amendment is shown in Fig. 1. In the spoils treated with fly ash, the proportion of acidity neutralized generally increased with time and, by t3, the A1 and A2 treatments had neutralized roughly the same proportion of acidity.
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The results for the fly ash–limestone mixture show the positive effects of combining the two amendments. After 48 d (t1), the proportion of extractable acid neutralized was similar to that neutralized by L2, and by the end of the experiment this proportion was roughly equal to that neutralized by fly ash (A1 or A2) and limestone treatment dose L2.
Generally, moisture conditions had only minor effects on the proportion of extractable acid neutralized by each amendment. By the end of the experiment, however, slightly more extractable acid had been neutralized under FC than WD conditions (especially for spoils that had received the L1 treatment, in which the difference between FC and WD was greater).
Analysis of Effloresced Salts
During the experiments, salts effloresced on all the tray walls, most notably in samples amended with fly ash. In samples maintained under FC conditions, the salts were redissolved when water was added to restore moisture content to field capacity. On the other hand, a significant proportion of the effloresced salts migrated out of the samples in trays maintained under WD conditions. The X-ray fluorescence analysis of effloresced salts indicated the presence of sulfur, iron, and small amounts of calcium, but did not allow the precise stoichiometry of these salts to be established. The X-ray diffraction analysis of the salts suggested that they were hydroxysulfates.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALSMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Acidification Processes
Acidification processes cannot be quantified simply on the basis of net pH changes, because protons are consumed during neutralization processes. A more effective measurement can be achieved by the quantification of processes that generate protons (i.e., the oxidation of sulfides and the formation of hydroxysulfates). These proton-generating processes are the most likely to occur, given the nature of the spoil and the alkaline amendments used.
An illustration of the importance of proton release from sulfide oxidation can be obtained using sulfur-form stability field diagrams (Fig. 2 and 3). These figures show the positions of each sample in an Eh–pH relationship. In the case of WD samples, measurements were not taken during the dry phases (more than half of the total time), when Eh would have been higher. The plots indicate that sulfate was the most stable sulfur form in both the control and treated spoils, regardless of moisture conditions. In addition, the control and spoils amended with L1, L2, or A1 entered the HSO-4 stability domain at some stages during the experimental periods; therefore, the rate of sulfide–oxidation proton release in these spoils during these periods would have been lower than in the other treatments.
In spite of the above, the only certainty is that there was oxidation of sulfides (an increase in sulfates at the end of the experiment) in the control spoil. This process possibly also occured in the treated spoils but the participation of sulfates in other processes makes it impossible to show that oxidation took place.
Sulfides can be oxidized to sulfates by O2 or by Fe3+. In the case of Fe3+ the release of large amounts of acid occurs, according to the following equation (Caruccio et al., 1988):
 | [1] |
To illustrate the rate of proton release due to oxidation of sulfides by Fe3+, stability field diagrams can again be used (Fig. 4 and 5). The data for the WD samples are for the wet phases only. These plots indicate that the stability domain for ionic forms of Fe(III) was entered at some stages during the experimental period by control and by limestone- and fly ash–amended samples maintained under FC conditions. In these samples, proton release due to oxidation of sulfides by Fe3+ may have occurred. In contrast, this domain was not entered by samples maintained under WD conditions or by samples amended with the higher fly ash treatment (A2) or with the fly ash–limestone mixture (AL1). In control spoils, limed spoils, and spoils amended with the lower fly ash treatment, conditions were also favorable for oxidation of sulfides by ionic Fe(III), which generates more acidity than oxidation of sulfides by oxygen.
Hydroxysulfates may be formed from sulfates and any iron or aluminum released under certain conditions causing release of more protons (Reactions 2–5):
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 | [3] |
 | [4] |
 | [5] |
We established which of these salts were formed in the treated spoils by calculating saturation indices (SIs) for some hydroxysulfate minerals commonly found in mine environments (David and Driscoll, 1984; Mulder et al., 1987) and for which thermodynamic data were available (Fillipek et al., 1987; Sullivan et al., 1988). The large negative SIs obtained for jarosite [KFe3(OH)6(SO4)2] indicate that jarosite concentrations were well below saturation in all the treated spoils. Similarly, alunite [KAl3(OH)6(SO4)2] content was below saturation or close to equilibrium in most of the spoils, except those amended with L1, in which precipitation may have occurred. The SIs obtained for jurbanite [Al(OH)SO4] were close to zero but positive in all of the treated spoils (Table 2), and indicate a smaller contribution to acidification by the formation of this mineral. In contrast, the SIs for FeOHSO4 were positive in all the treated spoils, indicating that formation of this mineral was favored, thus contributing to acidification.
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Even though FeOHSO4 formation was possible in practically all of the treatments, the salts formed under FC conditions were redissolved every time that water was added to maintain field capacity; therefore, protons were alternately released and consumed. In contrast, under WD conditions, only those salts that remained on the tray walls at the end of each cycle were redissolved. Therefore, proton release due to this process was greater under WD conditions than under FC conditions.
Neutralization Processes
Even though acidification and neutralization processes occur simultaneously, neutralization processes can be measured by evaluating the weathering of components from alkaline amendments added (Doolittle and Hossner, 1997).
Regarding the weathering processes of the fly ash, the minerals most susceptible to weathering were probably aluminosilicates. Dissolution of these minerals consumes hydrogen ions, as indicated in Eq. [6] for sillimanite (Lindsay, 1979):
 | [6] |
To estimate the extent to which dissolution of aluminosilicates may have contributed to neutralization of the spoil acid, the saturation index was calculated for sillimanite (Al2SiO5). Sillimanite is a mineral similar to mullite (the main aluminosilicate in fly ash) and for which thermodynamic constants are available (Lindsay, 1979). The SI was calculated using two values of pH4SiO04 (5.4 and 7.0), corresponding to the limits of the range of values in surrounding native soils. The SIs obtained (Table 3) were negative, indicating that sillimanite was unstable in the spoil and would thus have dissolved, consuming protons in the process. Note that neutralization by dissolution of sillimanite was more strongly favored under WD conditions and when the lower dose of fly ash (A1) was applied. This neutralization effect was slower but more sustained than that due to limestone (Fig. 1).
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 | [7] |
Among the factors influencing dissolution of carbonates are particle size (Caruccio et al., 1988), the amount of CO2 dissolved in the medium, and pH (Garrels and Christ, 1965; Lindsay, 1979). Particle size should not have limited dissolution, as the agricultural limestone used was fine enough to not have played a role (Caruccio et al., 1988). Similarly, increases in carbonate dissolution due to increased CO2 concentration (Garrels and Christ, 1965) were likely to have been minor, because if an increase in CO2 occurred, it took place in samples maintained under the WD regime during the wet phase. Accumulation of dissolved CO2 during the dry phase of the WD regime or under the FC regime would have been prevented by atmospheric exchange.
The very low initial pH (1.98) of the spoil would have favored dissolution of carbonates (Eq. [7]). Subsequently, spoils treated with limestone showed a rapid increase in soluble calcium content (Table 1) and rapid neutralization of spoil acid (Fig. 1), although pH remained very low (below 3.5). These effects were more marked and sustained in spoils amended with the higher quantities of limestone.
The limestone rapidly neutralized the most labile forms of acids in the spoil, but this effect was not sustained, as reported previously by Tedesco et al. (1999). In contrast, the fly ash had a more sustained neutralizing effect, as reported by Stewart et al. (1997). This difference may be due to differences in the rates of weathering of the two amendments.
To examine this hypothesis, the data for soluble calcium released during the experiments were fitted to the following kinetic equation (Sposito, 1994):
 | [8] |
In most cases there was a good fit of data to the equation, though with varied statistical significance (Table 4). The values obtained for Ca0 and k were consistent with the observed differences between limestone- and fly ash–amended samples. Therefore, k and Ca0 were higher for the limestone-amended samples than for the fly ash–amended samples; the value of k increased with increasing quanties of alkaline amendment, while Ca0 was similar for the two limestone treatments, indicating rapid solubilization at the beginning of the experiment. For fly ash, both k and Ca0 increased with the quantity of fly ash applied, though the latter was generally small. Finally, k for spoil treated with the fly ash–limestone mixture was higher than that for spoils treated with the corresponding doses of fly ash or limestone alone (A1, L1).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALSMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALSMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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