Published in J. Environ. Qual. 32:1658-1668 (2003).
© 2003 ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Ecosystem Restoration
Evaluation of Water Treatment Sludge for Ameliorating Acid Mine Waste
L. Van Rensburg* and
T. L. Morgenthal
School of Environmental Sciences and Development, Potchefstroom University for Christian Higher Education, Potchefstroom 2520, South Africa
* Corresponding author (plblvr{at}puknet.puk.ac.za).
Received for publication July 30, 2002.
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ABSTRACT
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This study investigated the liming effect of water treatment sludge on acid mine spoils. The study was conducted with sludge from a water purification plant along the Vaal River catchments in South Africa. The optimum application rate for liming acid spoils and the speed and depth with which the sludge reacted with the mine waste were investigated. Chemical analysis indicated that the sludge is suitable as a liming agent because of its alkaline pH (8.08), high bicarbonate concentration (183.03 mg L-1), and low salinity (electrical conductivity = 76 mS m-1). The high cation exchange capacity of 15.47 cmolc kg-1 and elevated nitrate concentration (73.16 mg L-1) also increase its value as an ameliorative material. The soluble concentrations for manganese, aluminum, lead, and selenium were high at a pH of 5 although only selenium (0.83 mg L-1) warranted some concern. According to experimental results, the application of 10 Mg ha-1 of sludge to acid gold tailings increased the leach water pH from 4.5 to more than 7.5 and also increased the medium pH from 2.4 to 7.5. The addition of sludge further reduced the solubility of iron, manganese, copper, and zinc in the ameliorated gold tailings, but increased the electrical conductivity. The liming tempo was highest in the coal discard profile that had a coarse particle size distribution and took the longest to move through the gold tailings that had a fine particle size distribution. Results from this study indicate that the water treatment sludge investigated is suitable as a liming agent for rehabilitation of acid mine waste.
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INTRODUCTION
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A PRINCIPLE OF sustainable development is the effective recycling and reuse of waste streams, for example, through composting biodegradable waste (Miller, 1994). Since 1990 the use of wastewater sludge in agriculture has increased in popularity because of the increasing consciousness to be more sustainable in the use of natural resources (Frank, 1998). For this reason, wastewater treatment sludge has been publicized as a natural fertilizer because it can recycle organic matter, nitrogen, and phosphorus back into the natural system (Frank, 1998). Another potential municipal waste stream is water treatment sludge that is a by-product from the filtering and purification of drinking water. The disposal of water treatment sludge is, however, a problem for water purification authorities due to its continuous production, the limited area available for disposal, and the possible liabilities it may cause if disposed in sanitary landfill sites (Heil and Barbarick, 1989; Elliott et al., 1990; Viraraghavan and Ionescu, 2002). Rand Water, the water providing authority to the Gauteng Province (South Africa), produces 550 Mg of water treatment sludge daily. The reuse of sludge will therefore be environmentally and economically beneficial. Alternative uses of the sludge have been investigated (e.g., in brick manufacturing) but without success. Because water treatment sludge is rich in nutrients, it is often used as an ameliorative material or organic fertilizer (Heil and Barbarick, 1989; Frank, 1998; Viraraghavan and Ionescu, 2002). According to Ippolito et al. (1999), the co-disposal application of alum with biosolids on blue grama [Bouteloua gracilis (Kunth) Lag. ex Griffiths, nom. illeg.] and western wheat grass [Pascopyrum smithii (Rydb.) Á. Löve] increases the production of blue grama but results in a decrease in phosphate and aluminum shoot concentrations. A study by Heil and Barbarick (1989) also showed similar results. The properties of water treatment sludge are highly variable and depend on the characteristics of the water used (Heil and Barbarick, 1989) and the characteristics of the coagulation agent used during water purification, for example FeCl3, Al2(SO4)3·14H2O (alum), organic polymers, calcium oxides, and/or Ca(OH)2. General problems associated with the use of water treatment sludge are the possible occurrence of high concentrations of heavy metals such as cadmium, copper, chrome, nickel, lead, and zinc. According to Elliott et al. (1990), FeCl3 coagulant sludge is more likely to contain excessive concentrations of heavy metals, such as Ni, than other types of water treatment sludge. The fixation of plant-available phosphate may be a further potential problem at high application rates (>10 g kg-1 or 11–22 Mg ha-1) of alum and FeCl3 water treatment sludge (Heil and Barbarick, 1989). Due to the potential fixation of phosphate by alum, it has been used to combat eutrophication and algal blooms in lakes in Wisconsin (Ippolito et al., 1999). Dayton and Basta (2001) also reported on the possible occurrence of NO2–N toxicity in some water treatment sludge sources from Oklahoma (USA) that were evaluated as soil substitutes. Therefore, it is essential that the water treatment sludge be characterized and evaluated before it is used as organic fertilizer or as an ameliorative material.
Studies with freshwater coagulant sludge have shown that the sludge may be an effective liming agent (Heil and Barbarick, 1989; Dayton and Basta, 2001). This study was initiated to investigate the feasibility of utilizing the alkaline properties of water treatment sludge from the Rand Water purification plant as a liming agent on waste produced from nearby gold and coal mines. Acid formation and the occurrence of acid mine drainage due to the oxidation of pyrite are serious problems in gold (Groves, 1974; Hilson and Murck, 2001) and coal mining (Pietz et al., 1998a; Bell et al., 2001). Bell et al. (2001) reported pH values as low as 1.8 to 3.0 for leachate from a coalmine in the Witbank coal field, South Africa. The role of waste material as an ameliorative material in mine rehabilitation has been illustrated by Pietz et al. (1998b). Sewage sludge applied at 542 Mg ha-1 was effective in ameliorating acidic coal discard and reducing leaching of heavy metals for a period of 5 yr (Pietz et al., 1998b).
Although investigations to revegetate gold mine tailings in South Africa were initiated as early as the 1930s (Phillips, 1936; Groves, 1974), no long-term solution has been found for the rehabilitation of gold tailings. Evaluating alternative methods is therefore a high priority for rehabilitation of gold tailings. Currently, all coal waste dumps are rehabilitated by covering the waste with a 30- to 50-cm topsoil layer. This practice is, however, expensive and the availability of suitable topsoil is limited. Existing resources, like Rand Water sludge, can minimize the costs involved in the rehabilitation of mine heaps and put to use the large quantities of "useless" sludge that Rand Water produces every year.
The objective of this study was to evaluate the water treatment sludge as a liming agent for the neutralization of acidic mine waste by evaluating potential problems that may arise from the use of the sludge and its effectiveness as a liming material.
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MATERIALS AND METHODS
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Two greenhouse experiments were conducted to evaluate the effectiveness of the water treatment sludge from Rand Water as a liming material during rehabilitation of acid mine waste. The first experiment tested the alkalizing capacity of the sludge at different sludge application ratios to determine the optimum sludge application volume, and obtain some idea of the time limit to alkalize the sludge. The second experiment determined the speed with which the alkalizing properties move through the different media and identified the potential limitations both in terms of penetration depth and duration of the alkalizing effect.
The experiments were conducted in a greenhouse at the School for Environmental Sciences and Development, Potchefstroom University for Christian Higher Education, Potchefstroom, South Africa. The water treatment sludge originated from the Rand Water purification plant near Sasolburg. This purification plant draws its water from the Vaal River catchment, which is a major tributary in the Orange River catchment system. Rand Water supplies most of its water to the highly urbanized and industrial areas of Gauteng and North West Provinces. The dolomitic geology, intensive urbanization, industrialization, and agriculture have a considerable impact on the water resource (Vaal River catchment), as well as the coagulation methods used, and therefore regulate the chemical properties of the sludge. The coagulation agents mostly used by Rand Water include activated silica, lime, and FeCl3. In general the sludge is therefore an alkaline, eutrophic sludge with a high silt and organic content (due to sporadic algal blooms). The chemical properties are described in more detail in the Results and Discussion section.
Liming Effect of the Sludge
A factorial layout was followed with mine waste (four levels: sand medium, clay medium, coal discard, and gold tailings) and sludge rate (four levels: 5, 10, 20, and 40 Mg sludge ha-1) as factors. Early trials with different applications indicated that 5 Mg ha-1 sludge is the minimum application for effective results and the higher applications were chosen as increments of that value. The experiment, therefore, consisted of four treatments with four sludge applications that were replicated three times. The gold tailing material was from a mine in the Klerksdorp area, North West Province and the coal discard was from a colliery in the Secunda area, Mpumalanga Province. The clay soil originated from a Vertisol with a silt clay texture and the sand was from an Oxisol with a sandy loam texture.
Buckets connected to a bottle via a 12-mm-diameter tube were used in the experiment. Each bucket was lined with a biddum filter to keep silt from leaching into the bottle. The contents of the buckets were saturated with ±4.2 L distilled water per watering occasion (watering was done when the content in the buckets appeared dry). The pH and electrical conductivity (EC) of the leachate from each bucket were measured daily for a 30-d period using HI 991301 and HI 9033 meters (Hanna Instruments, Woonsocket, RI), respectively.
A 1:2 water extraction analysis was done on a composite sample of the content of each of the treatment and application volumes. Because the data did not conform to normality a Kruskal–Wallis rank test was performed using STATISTICA (StatSoft, 2001) to test the effect of different treatments on leachate pH and electrical conductivity
Alkalizing Tempo of the Water Treatment Sludge
The experimental layout to test the alkalizing tempo consisted of one factor (type of media: sand, clay, coal discard, and gold tailings) with three replicates. Two treatments served as controls and the two remaining treatments were gold tailings and coal discard, similar to the first method. Leach tubes with 10 extraction outlets, spaced 10 cm, were used for this experiment. Rand Water–generated sludge (60 Mg ha-1) was mixed into the top layer of each medium. The leaching tempo was measured by adding 7.5 L distilled water and timing the water drainage through the leach tubes at each outlet. The leachate at each outlet was collected and the pH measured.
Soil Chemical Analysis
Samples (approximately 500 g) collected of the sludge, media, and media–sludge mixtures at the end of the experiment were analyzed using 1:2 water extraction procedures. An ammonium acetate procedure was used to determine the cation exchange capacity and exchangeable bases (Ca2+, Mg2+, K+, and Na+) of the different treated samples. As the sludge presumably contains high concentrations of micro and trace elements that may become available with a decrease in sludge pH, a 1:5 water extraction analysis (acidified at pH 5) and an ethylenediaminetetraacetic acid (EDTA) analysis were performed on the sludge.
The 1:2 (v/v) extraction procedure as described by Peech (1965) was used to determine the water-soluble basic cation (Ca2+, Mg2+, K+, and Na+), trace element (Fe, Mn, Cu, and Zn), and heavy metal (As, Se, Al, Cr, Co, Ni, Pb, and Cd) fractions. Quantification was done by means of atomic absorption spectrophotometry with a Spectr AA-250 (Varian Australia Pty. Ltd., Mulgrave, VIC, Australia) (Ramiriz-Munoz, 1968).
The anions phosphate (PO-34), nitrate (NO-3), sulfate (SO2-4), and chloride (Cl-) were determined using an ion chromatograph (Model 761; Metrohm, Herisau, Switzerland). Ammonium (NH+4) concentrations were determined using an ammonium-selective electrode (Banwart et al., 1972). The bicarbonate (HCO-3) content of the media extract was determined by the potentiometric titration method with a pH end point of 4.5 using a standard 0.005 M HCl solution (Skougstad et al., 1979). Boron concentrations were determined by the azomethine-H method described by Barrett (1978)(p. 435–436).
Exchangeable bases were determined using a Spectr AA-250, following extraction with an ammonium acetate solution (Thomas, 1982).
The cation exchange capacity (CEC) was determined by stepwise replacement of the cations from exchange sites by adding sodium acetate followed by ammonium acetate. The suspension was placed in leach tubes and leached with ammonium acetate to replace the Na2+ on the exchange complex with ammonium. The Na concentration of the leachate, determined with a Spectr AA-250, was then used to calculate the cation exchange capacity.
The pH (water) and pH (KCl) were determined by adding 50 mL deionized water or KCl to 20 g media. The suspension was stirred for 2 h and the pH was measured with a calibrated pH/conductivity meter (pHM 80; Radiometer A/S, Brønshøj, Denmark). The supernatant for determining the electrical conductivity was prepared by adding 50 mL of deionized water to 50 g of medium. The suspension was shaken and left to equilibrate for 12 h after which it was centrifuged for 10 min at 2000 rpm and filtrated through Whatman (Maidstone, UK) no. 540 filter paper. The electrical conductivity of the supernatant was measured with a WTW (Weilheim, Germany) LF92 conductivity meter. Available phosphate was determined by adding 75 mL Bray P solution to 10 g medium. The phosphate concentration was analyzed with a continuous flow analysis system (Skalar, Breda, the Netherlands) at 340 to 650 nm in a 15-mm tubular flow cell.
The EDTA buffer extractions were prepared by adding 50 mL 0.02 mol dm-3 diammonium EDTA solution to 5 g of medium. The microelement concentrations of the supernatant were analyzed with a Spectr AA-250.
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RESULTS AND DISCUSSION
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Chemical Characterization of Water Purification Sludge
With the exception of the Mg2+, NO-3, HCO-3, and B, the concentrations of the macro- and microelements were low in the samples (Table 1). The slightly low electrical conductivity of the sample is consistent with the generally low nutritional status of the sample, specifically in terms of the essential macroelements.
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Table 1. Results of the 1:2 water extraction and ammonium acetate analyses on a water treatment sludge sample from the Rand Water Purification Plant, South Africa.
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As indicated, the sludge sample contained high Mg2+ and slightly elevated NO3- concentrations (Table 1). The sludge had an alkaline pH that is a consequence of the high HCO-3 content of the sample. This property, together with the low SO2-4 concentration of the sample, makes the sludge a suitable liming agent. The alkaline nature of the sample is also corroborated by the ammonium acetate–extracted pH values. The high cation exchange capacity of the sample is a further positive attribute for using the sludge as a soil ameliorative material. According to the Department of Water Affairs and Forestry (1996), the Mg2+ concentration constitutes an environmental health risk (water concentrations greater than 400 mg L-1 will result in severe scaling problems and diarrhea in all new users) if it leaches into the ground water system and therefore the sludge must be carefully applied.
Because the sludge will be used as a liming material for acid mine waste, the contribution of the sludge to heavy metal concentration in the acid mine waste has to be considered. According to the Department of Water Affairs and Forestry (1996), Se (the target water quality for Se is 0.02 mg L-1) and Pb (the target water quality for Pb is 0.01 mg L-1) concentrations warrant some concern (Table 2). Because of the potential dilution effect and the fact that such acidified water is unlikely to be directly used for domestic purposes, metal toxicity from the sludge becomes negligible. The Mn and Se concentrations in the EDTA extract were found to be dramatically lower and Al and Pb concentrations similar to those in the acidified extract.
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Table 2. Average micro and heavy metal concentrations from the 1:5 acid extraction procedure and EDTA analysis on two water treatment sludge samples from the Rand Water Purification Plant, South Africa.
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Alkalizing Effect of the Sludge
The increasing sludge application had, according to Kruskal–Wallis analysis of variance (ANOVA) tests (H), a significant effect on the pH of the sand (H = 61.54; N = 341; p < 0.000), clay (H = 60.09; N = 348; p < 0.000), coal discard (H = 19.872; N = 348; p < 0.000), and gold tailings medium (H = 193.218; N = 348; p < 0.000). The effect of different applications of water treatment sludge on the pH of the sand, clay, coal discard, and gold tailings media are presented in Fig. 1 through 4
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Fig. 1. Average pH values measured in the leachate from the sand medium at different water treatment sludge application rates, over time. Vertical bars present standard error of the mean.
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Fig. 4. Average pH values measured in the leachate from the gold tailings medium at different water treatment sludge application rates, over time. Vertical bars present standard error of the mean.
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Fig. 2. Average pH values measured in the leachate from the clay medium at different water treatment sludge application rates, over time. Vertical bars present standard error of the mean.
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Fig. 3. Average pH values measured in the leachate from the coal discard medium at different water treatment sludge application rates, over time. Vertical bars present standard error of the mean.
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In terms of its liming effect it is evident from Fig. 1 and 2 that the leachate from the sand and clay media have high initial pH values. The leachate pH from the 40 Mg ha-1 sludge application was more stable in the sand medium (pH = 8.5) than for the lower applications of the sludge to the sand medium (Fig. 1). Although the addition of the sludge (irrespective of application volume) only elevated the pH of the coal discard leach water by one unit (Fig. 3), it is interesting to note that this effect only became apparent after the second water application (5 d) but lasted for a minimum of 26 d. In all probability very much the same trend will be followed for an extended period of time since the average pH value of the leachate from the coal discard medium was still greater than 7.5 after 65 d. Although the liming effects of the different sludge applications to the coal discard were statistically significant, the 5 Mg ha-1 applications to the coal discard followed the same trend as the 40 Mg ha-1 application (Fig. 3). The most dramatic effect of the sludge on the pH of the leachate was visible in the gold tailings (Fig. 4). After Day 6 (second water addition) the 5 Mg ha-1 sludge application volume treatment started to differ significantly from the rest of the treatments, and was no longer able to alkalize the growing medium. The 10 to 40 Mg ha-1 application volumes did not differ significantly among themselves in terms of their ability to alkalize the acidic growing medium. The sludge application volumes to the gold tailings medium were, however, capable of elevating the pH of the leach water from approximately 4.0 to an average of 7.5, and maintained it at that level for at least 28 d. The different media were watered during Days 1, 5, and 18, which had significant effects on the pH and electrical conductivity of the leachate (Fig. 1–4). The effect of watering was visible first by an increase in the pH after Day 5 and thereafter by a sudden drop in pH and electrical conductivity after Day 18. Watering especially influenced the leachate from the lower application rates (5–20 Mg ha-1), indicating that high applications are needed to sustain the liming effect. The sudden drop in the leachate at Day 18 from the lower application ratios in the sand and clay media can be attributed to the dilution effect after watering. The gold and coal media behaved totally different from the sand and clay media and did not have a sudden drop in pH and electrical conductivity after watering. This was unexpected considering that the gold had an acidic pH and presumably did not have the buffering capacity to sustain a high pH.
According to the Kruskal–Wallis ANOVA the different sludge applications had an overall statistically significant effect on the electrical conductivity of the sand (H = 115.622; N = 228; p < 0.000), clay medium (H = 180.743; N = 228; p < 0.000), coal discard (H = 85.73; N = 224; p < 0.000), and gold tailings (H = 143.07; N = 228; p < 0.000). Larger applications of water treatment sludge increased the electrical conductivity of leachate from the four media (Fig. 5–8)
by adding macroelements, particularly magnesium (Table 1) and other solubles to the growth media.
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Fig. 5. Average electrical conductivity values measured in the leachate from the sand medium at different water treatment sludge application rates, over time. Vertical bars present standard error of the mean.
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Fig. 8. Average electrical conductivity values measured in the leachate from the gold tailings medium at different water treatment sludge application rates, over time. Vertical bars present standard error of the mean.
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Fig. 6. Average electrical conductivity values measured in the leachate from the clay medium at different water treatment sludge application rates, over time. Vertical bars present standard error of the mean.
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Fig. 7. Average electrical conductivity values measured in the leachate from the coal discard medium at different water treatment sludge application rates, over time. Vertical bars present standard error of the mean.
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Sludge application contributed to the Mg2+ concentration, led to a decrease in the SO2-3 concentration (not much in the sand as there was not a lot to start with nor at the 5 Mg ha-1 application rate in the gold tailings), and elevated the HCO-3 concentration (not the 5 Mg ha-1 application rate in the gold tailings) in the leach water of all four the potential growing media (Table 3). A slight decrease in the soluble Fe concentration became apparent (although at a lesser extent than in the clay and coal discard growing media) as the pH values of the leach water increased or was kept above 5.5. With the exception of the electrical conductivity value in the gold tailings treatment, all the electrical conductivity values of the different growing media can be considered low. The addition of the sludge did result in an increase in salinity. Too much sludge may therefore give rise to saline conditions and to Ca to Mg imbalances.
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Table 3. Chemical properties of the different substrates before and after application of different rates of water treatment sludge.
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Alkalizing Tempo of the Water Treatment Sludge
Water moved fastest through the coal discard profile (as can be seen from the steeper slope of the curve, y = 0.029x3 - 0.48x2 + 5.2x - 4.63) and took the longest to move through the gold tailings profile (y = 0.001x3 + 0.17x2 - 2.33x + 10.62) (Fig. 9– 12
and Table 4). The rate of water movement was to a large extent a function of the particle size and therefore the porosity of the potential growing medium. The curve fitting for time indicated a negligible deviation from the experimental curve in all four growing media. Except for the time against depth equation, all other polynomial equations were related to the distribution curves and had R2 values greater than 0.8 (Table 4). At this stage it is important to note that no rewatering occurred in this phase to determine the depth at which alkalinization will occur after a single water application. Although a 60 Mg kg-1 water treatment sludge application had little effect on the sand, clay, and coal discard media, as these were originally alkaline, the effect of the single water treatment sludge was clearly visible at different depths (Table 4) in the tubes filled with gold tailings. The results presented in Fig. 9 through 12 and Tables 4 and 5 indicate the predictive value of the study. For example, in the case of acid-generating gold mine tailings typified by a particular particle size distribution and chemical properties, the alkalizing effects of the sludge would be found to a depth of 0.3 m in the profile so that the pH of the growing medium per se would not be a growth-limiting factor.
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Fig. 9. The relationship between time (minutes) after application and penetration depth (cm) as well as the relationship between pH and penetration depth (cm) to evaluate the effectiveness of 60 Mg kg-1 sludge in a sand medium.
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Fig. 12. The relationship between time (minutes) after application and penetration depth (cm) as well as the relationship between pH and penetration depth (cm) to evaluate the effectiveness of 60 Mg kg-1 sludge in a gold tailings medium.
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Fig. 10. The relationship between time (minutes) after application and penetration depth (cm) as well as the relationship between pH and penetration depth (cm) to evaluate the effectiveness of 60 Mg kg-1 sludge in a clay medium.
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Fig. 11. The relationship between time (minutes) after application and penetration depth (cm) as well as the relationship between pH and penetration depth (cm) to evaluate the effectiveness of 60 Mg kg-1 sludge in a coal tailings medium.
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Table 4. The polynomial relationship between penetration depth and time as well as penetration depth and pH after an application of 60 Mg kg-1 water treatment sludge (see Fig. 9–12).
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Table 5. Results of the time required for water to move through the leach tubes for different substrates and corresponding pH values of the leachate collected at the different depth intervals to determine the effective alkalizing depth when 60 Mg kg-1 water treatment sludge is applied.
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Spontaneous germination of weeds, for example, lamb's-quarters (Chenopodium album L.) and Mexican marigold (Tagetes minuta L.), also occurred in both the gold and coal media, during the first phase of the experiment. Spontaneous germination was also visually correlated with increasing applications of the water treatment sludge. During an initial germination trial with a mixture of indigenous grass seeds the ameliorated coal discard had similar germination success as the ameliorated sand medium (Fig. 13)
. Germination in the ameliorated gold tailings was considerably lower in comparison with the other media.
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Fig. 13. Number of seedlings 30 and 60 d after seeding in the sand, clay, coal discard, and gold tailings media treated with 60 Mg kg-1 water treatment sludge.
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CONCLUSIONS
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The Rand Water sludge could be used as an ameliorative material in acid-generating mine tailings, but refinement of the application volume might be needed for different substrates (i.e., differing in terms of their particle size distribution) and under different ecological conditions. The chemical characteristics of the sludge material produced by Rand Water give this material the combined characteristics of dolomitic lime (with a low Ca fraction) and inorganic MgNO3. This study confirms earlier findings on the usefulness of water treatment sludge as an ameliorative material or topsoil substitute from Heil and Barbarick (1989), Elliott et al. (1990), and Dayton and Basta (2001). None of the problems indicated by these authors were found in this study, probably because lime, activated silica, and FeCl3 were used in combination as coagulates, whereas alum was mostly used in their studies. Possible chemical growth-limiting factors associated with the water treatment sludge used in this study are elevated Mg concentrations, the occurrence of Ca to Mg imbalances, and an increase in the salinity of the limed medium. This can, however, partially be rectified by combining the sludge application with calcitic lime. Although an increase in salinity was experienced, this was, except for gold tailings, not serious. Most grass species can tolerate saline conditions of 200 mS m-1 (Havlin et al., 1999).
The next logical step would be to conduct an experiment on one or more tailings dams to further identify any possible environmental and site-specific variables and constraints that might influence the large-scale implementation of this concept. Specifically, the practical and logistical aspects of the sludge application, such as the incorporation of the material on a slope, the effectiveness of the treatment in terms of water quality when only applied to the soil surface layer, and the best times of the year to perform each action need to be tested further. Pot experiments are also too short to indicate the viability of the liming material over an extended period.
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ACKNOWLEDGMENTS
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The School of Environmental Sciences and Development gratefully acknowledge the technical and financial support received from Mr. J. Geldenhuys and Rand Water that made this study possible.
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ABSTRACT
INTRODUCTION
