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Australian Journal of Soil Research, 2004, 42, 649–657
Acid neutralising capacity of two different bauxite residues (red mud) and their potential applications for treating acid sulfate water and soils Chuxia LinA,B,C , Greg MaddocksA , Jing LinA , Graham LancasterA , and Chengxing ChuB A School
of Environmental Science and Management, Southern Cross University, Lismore, NSW 2480, Australia. of Resources and Environment, South China Agricultural University, Guangzhou 510642, China. C Corresponding author; email:
[email protected]
B College
Abstract. The acid neutralising capacity (ANC) and potential beneficial uses of 2 different bauxite residues (red mud) were investigated. The results show that the ANC is much higher in the red mud disposed of using a dry stacking method at the Pingguo Alumina Refinery (China) than in the red mud disposed of by a wet method using seawater at the Queensland Alumina Ltd Refinery (Australia). The higher ANC in the Pingguo red mud is attributable to its high CaO and low SiO2 . An incubation experiment showed that leaching of alkaline materials from the lime-treated sample was much greater than that from the red mud-treated sample. This suggests that red mud may be superior to lime for treating potential acid sulfate soils, which contain sulfide minerals that could take a long time to oxidise and release soluble acid. The effects of 2 acid-filtering systems were tested, both of which used red mud as the main material for removal of acid from passing acidic water. The results showed that the red mud–CaCO3 filter performed better than the red mud–Mg(OH)2 filter. Results from pot trials in Australia further demonstrated that the application of combined red mud and sewage sludge significantly improved the soil conditions for the growth of 5 Australian native tree species, in addition to Eucalyptus paniculata, which successfully grew in the same mine soil amended with the red mud and sewage sludge in previous work of G. Maddocks et al. The results from the pot experiment in China showed that the application of combined neutralising agents (red mud/lime blends) and sewage sludge to the extremely acidic mine soil was insufficient for creating appropriate ecological conditions for the growth of vetiver grass. In this experiment, additional application of zeolitic rock powder significantly improved the growth performance of the plant.
Introduction There has been increasing interest in and research into the utilisation of industrial residues for remediation of degraded soils (e.g. Ho et al. 1989; Summers 1996; Vangronsveld et al. 1999; Lombi et al. 2001; Lin et al. 2002). The successful applications of such technologies create conduits for turning industrial wastes into products that can be used for ecological rehabilitation. Bauxite residue (red mud) from alumina production is an abundant industrial residue. Raw red mud frequently contains high levels of sodium hydroxide and is highly basic (pH >10). Costly engineering work is usually required for the storage of this potentially hazardous industrial waste. The high alkalinity of red mud makes it a potential acid neutralising agent for remediation of acidified environments (McConchie et al. 1996). Consequently, the acid neutralising capacity (ANC) is the most important chemical parameter of red mud when it is used for this purpose. Bauxite resides vary widely in mineralogical and chemical characteristics, depending on the source of bauxite ore, the refining process employed, and the disposal method for the red mud. In this © CSIRO 2004
paper, the ANC of red mud from 2 different sites, Queensland Alumina Ltd (QAL) (Australia) and Pingguo Aluminum Co. (China), was examined. The seawater-neutralised red mud (BauxsolTM ) in the QAL refinery has been intensively studied (McConchie and Clark 2000; Lin et al. 2002; Maddocks et al. 2004). Seawater neutralisation transforms strong bases to weak bases and thus significantly reduces the soluble basicity in the red mud. This allows the red mud to be used without the need for further chemical treatment. Previous research (Lin et al. 2002) has shown that BauxsolTM has a good capacity to immobilise soluble acid and environmentally significant metals in extremely acidified acid sulfate soils. It is likely that much of the acid-immobilising capacity of BauxsolTM is provided by deprotonated trivalent metal hydroxides, which are not soluble at slightly acidic and neutral pH. They only react with H+ when soils acidify. It has been suggested that BauxsolTM could be superior to lime for treating unoxidised sulfidic soils, because lime is slightly soluble and can be leached out before the acid that needs to be neutralised is generated (McConchie and Clark 2000; 10.1071/SR03080
0004-9573/04/060649
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Lin et al. 2002). However, insufficient work has been done to test this hypothesis. This paper provides further evidence to support the above suggestion. BauxsolTM has also been considered as a potential material for construction of subsurface filtering walls to intercept soluble acid and environmentally significant metals contained in seepage water in acid sulfate soil areas (McConchie and Clark 2000). Davies-McConchie et al. (2002) showed positive effects when passing acidic mine water through a permeable reactive barrier consisting of a layer of BauxsolTM . However, given that the total alkalinity of red mud is much less than that of the commonly used liming materials, in the present study, we incorporated red mud with materials that have stronger ANC to maximise its capacity to remove soluble acid and potentially toxic metals from the passing water. In this paper, results of acid removal from acid sulfate waters by 2 designed acid-intercepting systems are also presented. Red mud contains very little organic matter and the concentrations of plant nutrients are generally very low. Therefore, red mud needs to be used with other soil conditioners in terms of soil improvement for plant growth. Sewage sludge is widely used to increase the fertility of soils (e.g. Polglase and Myers 1996; Bendfeldt et al. 1998; Brown et al. 1999). However, this material may contain excessive heavy metals that could contaminate the amended soils (e.g. Rule and Martin 1999; Whatmuff 1999). Our recent study (Maddocks et al. 2004) has shown that application of sewage sludge to acidic mine soils could present problems due to the remobilisation of the sludge-borne heavy metals under low pH conditions, and that the combined red mud and sewage sludge has more effects on improving conditions of acidified mined soils for the growth of Eucalyptus paniculata than either treatment alone. This paper further examines the effects of the combined red mud and sewage sludge on a wider range of Australian native tree species than was done by Maddocks et al. (2004). Unlike QAL, most of the world’s alumina refineries are located inland where seawater is not readily available for neutralising the raw red mud. Where wet disposal methods are used, the pH of the red mud could be >12. In such cases, moving red mud from the disposal sites is not permitted under the environmental protection regulations in many countries unless appropriate treatment processes are employed to reduce the soluble basicity of the red mud to a level that allows safe handling. This adds to the costs associated with the production of red mud-containing products and may become an economic constraint for the utilisation of the red mud. The Pingguo Alumina Refinery in China adopts a dry stacking method for the disposal of the red mud, coupled with a well-designed drainage system to remove caustic soda from the red mud during rainfall events for recycling. These techniques effectively reduce the concentration of residual NaOH in the red mud and allow its handling without the
C. Lin et al.
need for any chemical treatment. In this paper, a column leaching experiment was conducted to examine the effects of the Pingguo red mud on the neutralisation of soluble acid contained in a minesite acid sulfate soil in South China and consequently the minimisation of ‘acid mine drainage’. Further to our previous work (Maddocks et al. 2004), a new pot experiment on the treatment of this minesite’s acid sulfate soils for plant growth was also conducted in South China. In this experiment, soil conditioners additional to red mud and sewage sludge were used, and vetiver grass (Vetiveria zizanioides), which is widely used for remediation of degraded lands, was chosen as the test plant. Materials and methods The red mud samples Two red mud samples from different alumina refineries were used for this study. The first sample was a seawater-neutralised red mud collected from the red mud dam of the QAL refinery, Gladstone, Queensland, Australia. Previous research results show that the red mud (BauxsolTM ) consisted of a mixture of minerals dominated by hematite, with abundant quartz, boehmite, gibbsite, brucite, calcite, sodalite, para-aluminohydrocalcite, hydrotalcite, and whewellite (McConchie and Clark 2000). The major chemical properties are presented in Table 1. The second red mud sample was collected from the red mud impoundment area of the Pingguo Alumina Refinery, China. The red mud contained fine materials dominated by imogolite, hematite, calcite, ilmenite, and rutile. The major chemical components in the bauxite residue (expressed as weight percentage of the oxides) are in the following order: Fe2 O3 > Al2 O3 > CaO > SiO2 > TiO2 > MgO > ZrO2 > Cr2 O3 > MnO. The red mud had a pH of 10.5. The unoxidised coastal acid sulfate soil sample A potential coastal acid sulfate soil used for this study was collected from the Emerald Lakes Development Area, Gold Coast, Queensland. The soil contained about 1.5% reduced inorganic S (chromium-reducible S; Sullivan et al. 2000) and had a pH and EC of 6.9 and 2.55 dS/m, respectively. The sample contained significant amounts of shell, which contribute to its high self-ANC (256 mmol/kg). The sample had a sulfate concentration of 2055 mg/kg (Table 1). The minesite acid sulfate soil sample from the Mt Carrington Mine The soil sample was collected from a waste rock/soil stockpile near the Tailings Dam in the Mt Carrington mine site, northern New South Wales, Australia. It is characterised by materials ranging in size from large boulders of ≥100 mm to fine clay soil/BauxsolTM > control. If we assume that the difference in alkalinity between each leachate of the soil/lime sample and the control is the added lime-derived alkalinity leached out with each leachate, then the accumulative alkalinity of the added lime source removed from the soil/lime sample with leachate during the period March 2001–September 2002 was about 126 mmol, accounting for >6% of the total alkalinity added to the soil as lime. In contrast, the accumulative alkalinity of the added BauxsolTM source removed from the soil/BauxsolTM sample with leachate during the same period was only 46 mmol, accounting for 5.5 for the first 8.25 L (the sum of 33 additions of 250 mL) of 0.005 M H2 SO4 solution. The sum of the alkalinity released from the BauxsolTM –CaCO3 filter during this period was about 212 mmol. However, only 79 mmol of the released alkalinity was needed to neutralise the added acidity carried by the 0.005 M H2 SO4 solution, and 133 mmol of the released alkalinity was leached out with the filtrate. The estimated ANC of the BauxsolTM – CaCO3 filter at pH 5.5 was about 270 mmol. Therefore, the results indicate that at pH 5.5, only about 29% of the BauxsolTM –CaCO3 filter’s ANC was used for the removal of added soluble acid; about 49% of the ANC was lost by leaching; and about 22% of the ANC was still retained in the filter. Probably, the remaining ANC was too weak to completely neutralise the further added sulfuric acid under the flow conditions in this experiment. This resulted in a drop in pH of the filtrate to 61% of the ANC was retained in the filter. Further release of alkalinity (61 mmol) after the pH dropped to 7 and added organic matter and nutrients to the soil, the growth performance was poor. This is in contrast with Treatment B where zeolitic rock powder was added in addition to red mud, lime, and sewage sludge (Table 4). Probably, the added zeolitic rock powder in Treatment B played a key role in adsorbing Na+ introduced by application of red mud and releasing additional K+ , which benefited the growth of the plant. The concentrations of heavy metals in the plant materials were generally higher in Treatment A than Treatment B (Table 4). This appears to suggest that the zeolitic rock powder also had some effects on reducing the
uptake of heavy metals by the plant and this further benefited the growth of the plant. Conclusion and recommendations (1) The ANC of red mud varies significantly, depending on the source of red mud. Assessment of the ANC of red mud by slow titration is time-consuming. Reasonably accurate methods for rapid determination of this chemical parameter is needed for practical applications of red mud in environmental remediation. (2) The new experimental results reported in this paper support the previous assumption that red mud could be superior to lime in terms of treatment of unoxidised sulfidic soils (McConchie and Clark 2000; Lin et al. 2002). (3) Comparison between 2 designed acid-filtering systems based on previous finding that red mud had positive effects on removing soluble acid and environmentally important metals from the passing acidic water (Davies-McConchie et al. 2002) shows that the red
Table 4. Comparison of soil pH, various growth criteria, and concentrations of some heavy metals in dry plant materials among the control, Treatment A (red mud, lime and sewage sludge), and Treatment B (red mud, lime, sewage sludge, and zeolitic rock powder) in pot experiment in China Parameter Soil pH Survival rate No. of new shoots Dry biomass Plant height Zn in root Zn in shoot Cu in root Cu in shoot Pb in root Pb in shoot
Unit (%) g cm mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg
Control
Treatment A
Treatment B
2.63 ± 0.15 0 0 0 0 – – – – – –
7.71 ± 0.35 25 1 2.6 47 303 89.3 43.6 5.1 9.3 0
7.25 ± 0.23 100 5 ± 2.58 8.25 ± 2.96 119 ± 20.9 97.6 ± 35.5 17.6 ± 7.9 26.6 ± 9.33 6.92 ± 2.36 6.63 ± 3.35 0
Acid neutralising capacity of bauxite residues
Australian Journal of Soil Research
mud–CaCO3 filter had better performance than the red mud–Mg(OH)2 filter for treating acid sulfate water. (4) Mixing red mud with acidified sulfidic mine soils could be a simple but effective method for reducing acid mine drainage. However, for highly acidic mine soils, incorporation of stronger acid neutralising agents (e.g. quick lime) with red mud may be more effective than sole application of red mud. (5) On the basis of our previous finding that application of combined red mud and sewage sludge improved the mine soil for the growth of a tree species more than either treatment alone (Maddocks et al. 2004), a further pot experiment demonstrated that the application of combined red mud and sewage sludge was also effective for a range of native Australian tree species. However, a new pot experiment in China shows that application of combined red mud, lime, and sewage sludge to highly acidic mine soil was insufficient for the creation of appropriate ecological conditions for growth of vetiver grass, and another soil conditioner was needed for effective soil improvement. Acknowledgments Work in this paper was partly supported by grants from Southern Cross University, Virotec International Ltd., and South China Agricultural University. References Bendfeldt ES, Burger JA, Daniels WL, Feldhake CM (1998) Dynamics and characterisation of soil organic matter in mine soils sixteen years after amendment with native soil, sawdust and sludge. In ‘Proceedings of the 16th Annual National Meeting of the American Society for Surface Mining and Reclamation’. (Eds SA Bengson, DM Bland) pp. 225–235. (ASSMR: Scottsdale, AZ) Brown S, Chaney RL, Ryan JA (1999) The phytoavailability of Zn and Cd in long term biosolids—amended soils. In ‘Proceedings of the 5th International Conference on the Biogeochemistry of Trace Elements’. (Eds WW Wenzel, DC Adriano, B Alloway, HE Doner, C Keller, NW Lepp, M Mench, R Naidu, GM Pierzynski) pp. 684–685. (International Society for Trace Element Research: Vienna) Davies-McConchie F, McConchie D, Clark MW, Lin C, Pope S, Ryffel T (2002) A new approach to the treatment and management of sulphidic mine tailings, waste rock and acid mine drainage. New Zealand Mining 31, 7–15. Ho GE, Mathew K, Newman PWG (1989) Leachate quality from gypsum neutralised red mud applied to sandy soils. Water, Air, and Soil Pollution 47, 1–18. Lombi E, Zhao FJ, Zhang G, Suna B, Fitza W, Zhang H, McGrath S (2001) In situ fixation of metals in soils using bauxite residue: chemical assessment. Environmental Pollution (Barking, Essex: 1987) 118, 435–443. Lin C, Clark MW, McConchie D, Lancaster G, Ward N (2002) Effects of BauxsolTM in the immobilisation of soluble acid and environmentally significant metals in acid sulfate soils. Australian Journal of Soil Research 40, 805–815. doi: 10.1071/SR01060
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Manuscript received 16 May 2003, accepted 10 March 2004
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