TREATING ACIDITY IN COAL PIT LAKES USING SEWAGE AND ...

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Treating acidity in pit lakes using sewage and greenwaste In natural systems and in old mine lakes which have accumulated organic materials in the sediments, sulfate is removed by SRB reduction in carbon-rich and anaerobic conditions producing hydrogen sulfide (Frömmichen et al., 2003). Hydrogen sulfide is chemically highly reactive and binds strongly to heavy metals such as iron, copper, cadmium and zinc forming insoluble precipitates (Kleeberg, 2000; Brown et al., 2003). Furthermore, elevated metal ions may also be removed by direct sorption and complexation to organic matter (Peiffer et al., 1999; Fyson, 2000). Controlled organic enrichment (saprobisation) of lake sediments in laboratory microcosm experiments has been found to increase pH and decrease acidity as a result of SRB processes (Castro et al., 1999; Frömmichen et al., 2003). Sulfate and iron reducing bacteria are a diverse and ubiquitous group of micro-organisms (Johnson & Hallberg, 2003). However, their activity is directly limited by the low carbon availability typical of waters in acid pit lakes (Kafper, 1998). In addition, to sufficient available carbon SRB require an anoxic (no oxygen) environment to reduce sulfate. In many pit lakes, which stratify over summer, the hypolimnion (bottom waters) can take a considerable time to become anoxic due to low biological oxygen demand (BOD) in the sediments (Boland & Padovan, 2002). Addition of organic matter can provide available carbon and high BOD providing suitable conditions for SRB activity (Klapper et al., 1998). Most of the research into organic matter additions has come from the northern hemisphere and has focused on expensive and largely unavailable sources of carbon for an Australian mine site; such as glucose, ethanol, molasses, whey, potato processing waste and cattle effluent (Castro et al., 1999; Frömmichen et al., 2003). Nonetheless, other than establishing that there is a requirement for an easily accessible supply of carbon, little is understood about differences in the quality of organic substrate for bacterially-mediated sulfate reduction (Blodau et al., 2000). In Collie (Western Australia) pit lakes, experiments have trialled a number of organic matter types in experiments at scales ranging from laboratory, to ponds to mesocosms in pit lakes (Lund et al., 2000; Phillips et al., 2000; Lund, 2001). Readily available organic materials such as mulch, hay, manure (cow) and sawdust have been tested (Thompson, 2000). Although there have been noticeable improvements in biodiversity within the experiments with organic matter additions, there has been little long term change in pH. This has been 12
13 McCullough, Lund and May (2008) attributed to low sulfate concentrations (typically
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8.2.1 Risk Assessment 65 McCullough
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8.2.5 Pit lakes 69 McCullough, Lund
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(a). (b). (c). (d). Depth (m) Depth
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(a). (b). (c). (d). Depth (m) Depth
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8.3.4 Metal/metalloid concentration
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Concentration (mg L -1) 93 McCullou
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8.4 Discussion 95 McCullough, Lund
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9 Conclusions 99 McCullough, Lund a
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11 References 103 McCullough, Lund
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