To assess strategies aimed at minimizing the release of trace elements and the impact of disposal of coal waste materials on the environment, two long-term leaching experiments of up to five months duration were performed using waste materials from two plants cleaning high and low sulfur bituminous coal. The tests evaluated the mobility of major trace elements under different disposal scenarios: (i) a static leaching test designed to simulate the quiescent conditions encountered by coal waste material stored under water in a stable impoundment, and (ii) a dynamic test to simulate waste materials exposed to the atmosphere, either in variable wet/dry storage conditions, or in unusual circumstances like those resulting from breaching of an impoundment containment wall. The results indicate that different refuse streams have different leaching characteristics due to difference in their mineralogy and the mobility of most elements is enhanced under highly alkaline or acidic conditions with a few being mobilized under both conditions, suggesting that the minimization of element mobility requires the pH value of the medium to be maintained around neutral. In addition, most of heavy metals were associated with the illite and pyrite minerals. Two strategies of treating coal refuse were evaluated: fly ash mixed with coarse refuse and co-disposal of coarse and fine refuse. Both methods were found to neutralize the pH conditions and thus reduce mobility of the trace elements in static leaching tests whereas the opposite was found from dynamic experiments. The results indicate that such controlled storage under water could retard acid generation and the mobility of trace elements.

>Magister Scientiae - MSc / The waste products acid mine drainage (AMD), formed during coal mining and fly ash (FA) from coal burning power generation, pose substantial environmental and economic problems for South Africa. Eskom has developed a remediation system employing alkaline FA to neutralize and precipitate heavy metals from toxic acidic AMD streams. The aim of this study was to assess the microbial diversity in and microbial impact on this remediation system. The total microbial diversity was assessed by well-established molecular phylogenetic analyses using 16S rDNA gene sequences. The results obtained from the AMD confirmed the presence of acidophilic organisms, such as Acidithiobacillus ferrooxidans (At. ferrooxidans). After co-disposal of FA and AMD, microbial cell growth was not detected and microbial genomic DNA could not be extracted. The absence of microbial communities in the co-disposal phase is beneficial to the continuation of the development of such a co-disposal process. Results of this project will assist in the effective implementation of FA-AMD co-disposal systems, which may improve water quality in effected regions of South-Africa.

Landfill leachate is generated as a result of reactions between water percolating through the landfill and wastes. Under normal conditions leachate is found at the bottom of landfills and from there, its movement can be controlled with collection systems to be treated, discharged, or recirculated. Landfill leachate collection systems are positioned above the liner and are designed to collect liquid under gravitational flow for the entire active, closure, and post-closure periods. Clogging of any portion of the system can lead to higher hydraulic heads and increase the potential for leakage through the liner. To reduce the quantity of municipal solid wastes (MSW) requiring landfilling, many municipalities have adopted waste-to-energy (WTE) facilities that yield energy in the form of combustible gases and noncombustible residues. Disposal practices for WTE residuals include landfilling in monofills or co-disposal with MSW and other materials such as residues from water and wastewater treatment facilities. There has been concern about co-disposal practices, because the impacts on leachate quality and waste interactions are not well known yet. This research was conducted to evaluate clogging of leachate collection systems due to co-disposal of MSW and combustion residues from WTE facilities. The use of laboratory lysimeters in conjunction with batch tests to predict short-term and long-term leaching characteristics of noncombustible residues from WTE facilities was also evaluated. Laboratory lysimeters were used to simulate monofills (WTE residues and MSW) and co-disposal practices. Relationships between waste composition and leachate quality were evaluated over a seven month period.

Waste rocks (WRs) from coal mining and fly ash (FA) from coal combustion were studied to determine the potential of the WRs to generate AMD, FAs to neutralise it and estimate their impacts on environment. The ultimate goal was to develop a methodology based on co-disposal to mitigate the environmental problems associated to both wastes. Two methods for co-disposal were tested: i) Mixing WRs and FAs and ii) covering WRs with FAs. WRs were sampled from the Lakhra coal mines in Pakistan (PK), which has an estimated coal reserves of 1.3 Btonne, varying from lignite to sub-bituminous quality. The FA was sampled from a power plant utilising coal from Lakhra coal mines and is situated in close vicinity (15km) of the mine site. For comparative purposes a bituminous FA from Finland (FI) and biomass FA from Sweden (SE) were also characterised. The WRs and FA samples were characterised by mineralogical and geochemical methods. Besides organic material (coal traces), quartz, pyrite, kaolinite, hematite, gypsum and traces of calcite, lime, malladerite, spangolite, franklinite and birnessite were identified in WRs by XRD. The major elements Si, Al, Ca and Fe were in the range (wt. %) of 8 – 12, 6 – 9, 0.3 – 3 and 1 – 10, respectively, with high S concentrations (1.94 – 11.33 wt. %) in WRs. The AMD potential of WRs ranged from -70 to -492 kg CaCO3 tonne-1. All FAs contained quartz, with iron oxide, anhydrite and magnesioferrite in PK, mullite and lime in FI and calcite and anorthite in SE. The Ca content in SE was 6 and 8 times higher compared to PK and FI, respectively. FAs were enriched in As, Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn compared to continental crust. The acid neutralising potential of PK was equivalent to 20 kg CaCO3 tonne-1 compared to 275 kg CaCO3 tonne-1 (SE) and 25 kg CaCO3 tonne-1 (FI). During the period of 192 days in weathering cell experiments (WCE), the pH of leachates from most acidic WRs was maintained from 1 to 2.5, whereas, the less acidic WRs produced leachates of mildly acidic (2.7) to neutral (7.3) pH. The leachates from very acidic WRs ranged in the concentrations of Fe, SO24− and Al from mg L-1 to g L-1. The samples were subjected to column leaching experiments (CLE) in which mixture (FA:WR; 1:3) and cover (FA:WR; 1:5) cases were mimicked (with 10mm particle size) and effects of particle size (2, 5 and 10mm) on element leaching were studied. Despite having the lowest acid-neutralisation potential compared to FI and SE, co-disposal of PKFA as mixture readily provides acid buffering minerals, resulting in better start-up pH conditions and leachate quality. However, acidity produced by secondary mineralisation contributes towards the acidification of the system, causing stabilisation of pH at around 4.5−5. Secondary mineralisation (especially Fe- and Al-mineral precipitation) also removes toxic elements such as As, Pb, Cu, Zn, Cd, Co, Ni and Mn, and these secondary minerals can also buffer acidity when the pH tends to be acidic. In contrast, the pH of the leachates from the PKFA cover scenario gradually increased from strongly acidic to mildly acidic and circumneutral along with decrease in EC and elemental leaching in different WRs. Gradually increasing pH can be attributed to the cover effect, which reduces oxygen ingress, thus sulphide oxidation, causing pH to elevate. Due to the fact that pH~4–5 is sufficient for secondary Fe- and Al-mineral precipitation which also removes toxic elements (such as Cd, Co, Cu, Zn and Ni) by adsorption and/or co-precipitation, the FA cover performs well enough to achieve that pH until the conclusion of the CLE. However, due to the slower reactivity of the buffering system (additional to the initial flush-out), leaching in the beginning could not be restricted. The co-disposal of FA as cover and/or mixture possesses potential for neutralisation of AMD and improving leachate quality significantly. Particle size of the WRs affected the leaching of the sulphide related elements (such as Fe, S, Zn, Co, Cr, Cu, Mn and Ni) in CLE and WCE. Experiments with ≤1mm particle size constantly produced acidic and metal laden leachates. Co-disposal of FA and WRs as cover and mixture need to be investigated on pilot-scales before full-scale application.