Philosophiae Doctor - PhD / Coal combustion accounts for 95% of electricity generation in South Africa while globally coal combustion for energy generation stands at 42%. It has been predicted that coal utilization for energy generation will continue due to its low cost and availability in huge quantities in different parts of the world. Additionally brine and gaseous emissions are produced in the power generation and coal combustion processes. In fact, it has been established that CO2 emissions from power plants are the main cause of the green-house effect leading to global warming. Mitigation of the effects of disposal of fly ash, brine and CO2 emissions is critical for sustainable energy generation from coal and environmental protection. The study investigated whether South African coal fly ash could be used for brine remediation and CO2 capture using fly ash based hydrotalcites and zeolites. Four main objectives were investigated. These were; firstly, to compare the natural CO2 capture potential of a power station ash dam with an accelerated ex-situ mineral carbonation process. Secondly, to probe the effect of accelerated ex-situ mineral carbonation on brine quality with regards to major, minor and trace elements concentration. Furthermore, the study investigated the feasibility of synthesizing hydrotalcites from fly ash by optimizing the synthesis parameters such as acid concentration, aging time, aging temperature, pH during aging, crystallization time and crystallization temperature. Finally the study compared the CO2 adsorption capacities of the fly ash based hydrotalcites with fly ash based zeolites NaA, and NaX. The natural carbonation potential of the wet disposed ash dam at Secunda was investigated by coring a 20 year old dam. Three cores (SI, S2 and S3) were obtained by air flush coring the dam along a geophysical line and establishing the geophysical profile of the three cores. The surface of the three cores was of medium resistivity with values between 9.3 and 12.2 nm while the midsections were of low resistivity with values ranging between 4 and 7 nm. The base section of core SI had a resistivity of 28.3 nm, that of S2 was between 16.2 and 21.4 nm and that of S3 between 12.2 and 16.2 nm; implying that SI had the lowest salt load while S3 had the highest salt content. Moisture content was observed to be high deeper down the profiles of S2 and S3 with samples appearing water logged while SI had the highest moisture content at the surface showing the inhomogeneity of the ash dam. The morphology of fresh fly ash taken from the ash collection hoppers at Secunda was observed to be spherical. Weathered ash from the ash dam showed irregularly agglomerated particles while accelerated ex-situ mineral carbonation resulted in the formation of acicular particles of calcite. Fresh ash, weathered ash and the accelerated carbonated ash were all class F with a sum total of silica, alumina and iron oxide totaling more than 70%. A reduction in silica and alumina content with instability of fly ash. Dumping of spent iron catalyst (resulting from the petrochemical operations as Sasol) on the ash dam led to an increase in Fe203 content of the weathered ash. Enrichment of Nb, Sr, Y, Th, Na, Cl, S04, K and S with natural carbonation as well as during accelerated ex-situ mineral carbonation was observed and was due to the contact of ash with brine during these two processes. Reduction of Zr, Rb, Pb, Ni, Co and V content of ash was observed with weathering. Mineralogically, all the ash samples had main phases of mullite, quartz, magnetite and hematite, with weathered and accelerated carbonated ash having additional phases of calcite. The aluminosilicious nature of the three ashes was identified by structural evaluation using Fourier transform infrared analysis which revealed that, bands associated with C-O in-plane and out of plane bending of carbonates was only visible in weathered and carbonated ash.

The current study therefore aims to provide a comprehensive characterisation of weathered dry disposed ash cores, to reveal mobility patterns of chemical species as a function of depth and age of ash, with a view to assessing the potential environmental impacts. Fifty-nine samples were taken from 3 drilled cores obtained respectively from the 1 year, 8 year and 20-year-old sections of sequentially dumped, weathered, dry disposed ash in an ash dump site at Tutuka - a South African coal burning power station.

<p>The management and disposal of huge volumes of coal combustion by products such as fly ash has constituted a major challenge to the environment. In most cases due to the inadequate alternative use of coal fly ash, the discarded waste is stored in holding ponds, slag heaps, or stock piled in ash dumps. This practice has raised concerns on the prospect of inorganic metals release to the surface and groundwater in the vicinity of the ash dump. Acceptable scientific studies are lacking to determine the best ash disposal practices. Moreover, knowledge about the mobility patterns of inorganic species as a function of mineralogical association or pH susceptibility of the dry disposed ash dump under natural weathering conditions are scarce in the literature. Fundamental understanding of chemical interactions of dry disposed ash with ingressed CO2 from atmosphere, percolating rain water and brine irrigation within ash disposal sites were seen as key areas requiring investigation. The mineralogical association of inorganic species in the dry disposed ash cores can be identified and quantified. This would provide a basis for understanding of chemical weathering, mineralogical transformations or mobility patterns of these inorganic species in the dry ash disposal scenario. The current study therefore aims to provide a comprehensive characterisation of weathered dry disposed ash cores, to reveal mobility patterns of chemical species as a function of depth and age of ash, with a view to assessing the potential environmental impacts. Fifty-nine samples were taken from 3 drilled cores obtained respectively from the 1 year, 8 year and 20-year-old sections of sequentially dumped,&nbsp / weathered, dry disposed ash in an ash dump site at Tutuka - a South African coal burning power station. The core samples were characterized using standard analytical procedures viz: X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transforms infrared (FTIR) techniques, Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) and Acid neutralisation capacity (ANC) test. A modified sequential extraction (SE) method was used in this study. The chemical partitioning, mobility and weathering patterns in 1 year, 8 year and 20-year-old sections of the ash dump were respectively investigated using this modified sequential extraction scheme. The sequence of the extractions was as follows: (1) water soluble, (2) exchangeable, (3) carbonate, (4) iron and manganese and (5) residual. The results obtained from the 5 steps sequential extraction scheme were validated with the total metal content of the original sample using mass balance method. The distribution of major and trace elements in the different liquid fractions obtained after each step of sequential extraction of the 59 drilled core samples was determined by inductively coupled plasma mass spectrometry (ICPMS). The data generated for various ash core samples were explored for the systematic analysis of mineralogical transformation and change in ash chemistry with ageing of the ash. Furthermore, the data was analyzed to reveal the impact of ingressed CO2 from atmosphere, infiltrating rain water and brine irrigation on the chemistry of ash core samples. Major mineral phases in original ash core samples prior to extraction are quartz (SiO2) and mullite (3Al2O3&middot / 2SiO2). Other minor mineral phases identified were hematite (Fe2O3), calcite (CaCO3), lime (CaO), anorthite (CaAl2Si2O8), mica (Ca (Mg, Al)3 (Al3Si) O10 (OH)2), and enstatite (Mg2Si2O6). X-ray diffraction results show significant loss of crystallinity in the older ash cores. The presence of minor phases of calcite and mica in dry disposed ash cores are attributed to reduction in the pore water pH due to hydration, carbonation and pozzolanic reactions. The X-ray diffraction technique was unable to detect Fe-oxyhydroxide phase and morealuminosilicate phases in ash core samples due to their low abundance and amorphous character. X-ray fluorescence results of the original ash core samples showed the presence of major oxides, such as SiO2, Al2O3, Fe2O3, while CaO, K2O, TiO2, Na2O, MnO, MgO, P2O5, and SO3 occur in minor concentrations. The ratio of SiO2/Al2O3 classified the original core samples prior to extraction as a silico-aluminate class F fly ash. The ternary plot of major elements in 1-year-old ash core samples was both sialic and ferrocalsialic but 8 year and 20-year-old ash core samples were sialic in chemical composition. It is noteworthy that the mass % of SiO2 varies through the depth of the core with an increase of nearly 3 %, to 58 mass % of SiO2 at a depth of 6 m in the 1-year-old core whereas in the case of the 8-year-old core a 2 % increase of SiO2 to a level of 57.5 mass % can be observed at levels between 4-8 m, showing dissolution of major components in the matrix of older ash cores.. The Na2O content of the Tutuka ash cores was low and varied between 0.6-1.1 mass % for 1-year-old ash cores to around 0.6-0.8 mass % for 8-year-old ash cores. Sodium levels were higher in 1-year-old ash cores compared to 8 year and 20-year-old ashcores. Observed trends indicate that quick weathering of the ash (within a year) leached out Na+ from the ash dump. No evidence of Na+ encapsulation even though the ash dump was brine irrigated. Thus the dry disposal ash placement method does not result in a sustainable salt sink for Na-containing species over time. The total content of each of the elements in 1 year and 20-year-old ash cores was normalised with their total content in fresh ash from same power station to show enrichment and depletion factor. Major elements such as K+, Mn showed enrichment in 1-year-old ash cores whereas Al, Si, Na+, Ti, Ca, Mg, S and Fe showed depletion due to over time erosion. Trace elements such as Cr, Sr, P, Ba, Pb, V and Zn showed enrichment but Ni, Y, Zr showed depletion attributed to over time erosion. In 20-year-old ash cores, major elements such as Al, Na+ and Mn showed enrichment while Si, K+, Fe, Mg and Ca showed depletion highlighting their mobility. Trends indicated intensive flushing of major soluble components such as buffering constituents (CaO) by percolating rain water. The 1-year-old and 20-year-old coal ash cores showed a lower pH and greater loss/depletion of the soluble buffering constituents than the 2-week-old placed ash, indicating significant chemical weathering within a year. Based&nbsp / on ANC results the leaching behaviours of Ca, Mg, Na+, K+, Se, Cr, and Sr were found to be controlled by the pH of the leachant indicating high mobility of major soluble species in the ash cores when in contact with slightly acid rain water. Other investigated toxic metals such as As, Mo and Pb showed amphoteric behaviour with respect to the pH of the leachant. Chemical alterations and formation of transient minor secondary mineral phases was found to have a significant effect on the acid susceptibility and depletion pattern of chemical species in the core ash samples when compared to fresh ash. These ANC results correlated well with the data generated from the sequential extraction scheme. Based on sequential extraction results elements, showed noticeable mobility in the water soluble, exchangeable and carbonate fractions due to adsorption and desorption caused by variations in the pore water pH. In contrast, slight mobility of elements in the Fe and Mn, and residual fractions of dry disposed fly ashes are attributed to the co-precipitation and dissolution of minor amount of less soluble secondary phase overtime. The 1-year-old dry disposed ash cores were the least weathered among the 3 drilled ash cores. Therefore low concentration of toxic metals in older ash cores were ascribed to extensive weathering with slower release from residual mineral phases over time. Elements were found to associate with different mineral phases depending on the age or depth of the core samples showing greater heterogeneity in dispersion. For instance the average amount of total calcium in different mineral associations of 1-year-old ash cores is as follows / water soluble (10.2 %), exchangeable (37.04 %), carbonate (37.9 %), Fe and Mn (7.1 %) and residual (2.97 %). The amount of total Na+ in different mineral phases of 1-year-old ash cores followed this trend: water soluble (21 %), exchangeable (11.26 %), carbonate (2.6 %), Fe and Mn (4.7 %) and residual (53.9 %). The non-leachable portion of the total Na+ content (namely that contained in the residual fraction) in the 1-year-old ash core samples under conditions found in nature ranged between 5-91 %. This non-leachable portion of the Na+ showed the metastability of the mineral phases with which residual Na+ associates. Results showed older ash cores are enriched in toxic elements. Toxic elements such as As, B, Cr, Mo and Pb are enriched in the residual fraction of older ash cores. For instance As concentration in the residual fraction varied between 0.0003- 0.00043 mg kg-1 for 1-year-old ash cores to around 0.0003-0.0015 mg kg-1 for 20-year-old ash cores. This suggests that the older ash is enriched in toxic elements hence dust from the ash dump would be toxic to human health. The knowledge of mobility and ecotoxicological significance of coal fly ash is needed when considering its disposal or reuse in the environment. The mobility and ecotoxicology of inorganic metals in coal fly ash are determined by (i) mineralogical associations of inorganic species (ii) in-homogeneity in the ash dumps (iii) long and short term exposure to ingress CO2 and percolating rain water. Management issues such as inconsistent placement of ash in the dumps, poor choice of ash dump site, in-homogeneity in brine irrigation, no record of salt load put on the ash dumps and lack of proper monitoring requires improvement. The thesis provides justification for the use of the modified sequential extraction scheme as a predictive tool and could be employed in a similar research work. This thesis also proved that the dry ash disposal method was not environmental friendly in terms of overall leaching potential after significant chemical weathering. Moreover the study proved that the practice of brine co-disposal or irrigation on ash dumps is not sustainable as the ash dump did not act as a salt sink.</p>

<p>The management and disposal of huge volumes of coal combustion by products such as fly ash has constituted a major challenge to the environment. In most cases due to the inadequate alternative use of coal fly ash, the discarded waste is stored in holding ponds, slag heaps, or stock piled in ash dumps. This practice has raised concerns on the prospect of inorganic metals release to the surface and groundwater in the vicinity of the ash dump. Acceptable scientific studies are lacking to determine the best ash disposal practices. Moreover, knowledge about the mobility patterns of inorganic species as a function of mineralogical association or pH susceptibility of the dry disposed ash dump under natural weathering conditions are scarce in the literature. Fundamental understanding of chemical interactions of dry disposed ash with ingressed CO2 from atmosphere, percolating rain water and brine irrigation within ash disposal sites were seen as key areas requiring investigation. The mineralogical association of inorganic species in the dry disposed ash cores can be identified and quantified. This would provide a basis for understanding of chemical weathering, mineralogical transformations or mobility patterns of these inorganic species in the dry ash disposal scenario. The current study therefore aims to provide a comprehensive characterisation of weathered dry disposed ash cores, to reveal mobility patterns of chemical species as a function of depth and age of ash, with a view to assessing the potential environmental impacts. Fifty-nine samples were taken from 3 drilled cores obtained respectively from the 1 year, 8 year and 20-year-old sections of sequentially dumped,&nbsp / weathered, dry disposed ash in an ash dump site at Tutuka - a South African coal burning power station. The core samples were characterized using standard analytical procedures viz: X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transforms infrared (FTIR) techniques, Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) and Acid neutralisation capacity (ANC) test. A modified sequential extraction (SE) method was used in this study. The chemical partitioning, mobility and weathering patterns in 1 year, 8 year and 20-year-old sections of the ash dump were respectively investigated using this modified sequential extraction scheme. The sequence of the extractions was as follows: (1) water soluble, (2) exchangeable, (3) carbonate, (4) iron and manganese and (5) residual. The results obtained from the 5 steps sequential extraction scheme were validated with the total metal content of the original sample using mass balance method. The distribution of major and trace elements in the different liquid fractions obtained after each step of sequential extraction of the 59 drilled core samples was determined by inductively coupled plasma mass spectrometry (ICPMS). The data generated for various ash core samples were explored for the systematic analysis of mineralogical transformation and change in ash chemistry with ageing of the ash. Furthermore, the data was analyzed to reveal the impact of ingressed CO2 from atmosphere, infiltrating rain water and brine irrigation on the chemistry of ash core samples. Major mineral phases in original ash core samples prior to extraction are quartz (SiO2) and mullite (3Al2O3&middot / 2SiO2). Other minor mineral phases identified were hematite (Fe2O3), calcite (CaCO3), lime (CaO), anorthite (CaAl2Si2O8), mica (Ca (Mg, Al)3 (Al3Si) O10 (OH)2), and enstatite (Mg2Si2O6). X-ray diffraction results show significant loss of crystallinity in the older ash cores. The presence of minor phases of calcite and mica in dry disposed ash cores are attributed to reduction in the pore water pH due to hydration, carbonation and pozzolanic reactions. The X-ray diffraction technique was unable to detect Fe-oxyhydroxide phase and morealuminosilicate phases in ash core samples due to their low abundance and amorphous character. X-ray fluorescence results of the original ash core samples showed the presence of major oxides, such as SiO2, Al2O3, Fe2O3, while CaO, K2O, TiO2, Na2O, MnO, MgO, P2O5, and SO3 occur in minor concentrations. The ratio of SiO2/Al2O3 classified the original core samples prior to extraction as a silico-aluminate class F fly ash. The ternary plot of major elements in 1-year-old ash core samples was both sialic and ferrocalsialic but 8 year and 20-year-old ash core samples were sialic in chemical composition. It is noteworthy that the mass % of SiO2 varies through the depth of the core with an increase of nearly 3 %, to 58 mass % of SiO2 at a depth of 6 m in the 1-year-old core whereas in the case of the 8-year-old core a 2 % increase of SiO2 to a level of 57.5 mass % can be observed at levels between 4-8 m, showing dissolution of major components in the matrix of older ash cores.. The Na2O content of the Tutuka ash cores was low and varied between 0.6-1.1 mass % for 1-year-old ash cores to around 0.6-0.8 mass % for 8-year-old ash cores. Sodium levels were higher in 1-year-old ash cores compared to 8 year and 20-year-old ashcores. Observed trends indicate that quick weathering of the ash (within a year) leached out Na+ from the ash dump. No evidence of Na+ encapsulation even though the ash dump was brine irrigated. Thus the dry disposal ash placement method does not result in a sustainable salt sink for Na-containing species over time. The total content of each of the elements in 1 year and 20-year-old ash cores was normalised with their total content in fresh ash from same power station to show enrichment and depletion factor. Major elements such as K+, Mn showed enrichment in 1-year-old ash cores whereas Al, Si, Na+, Ti, Ca, Mg, S and Fe showed depletion due to over time erosion. Trace elements such as Cr, Sr, P, Ba, Pb, V and Zn showed enrichment but Ni, Y, Zr showed depletion attributed to over time erosion. In 20-year-old ash cores, major elements such as Al, Na+ and Mn showed enrichment while Si, K+, Fe, Mg and Ca showed depletion highlighting their mobility. Trends indicated intensive flushing of major soluble components such as buffering constituents (CaO) by percolating rain water. The 1-year-old and 20-year-old coal ash cores showed a lower pH and greater loss/depletion of the soluble buffering constituents than the 2-week-old placed ash, indicating significant chemical weathering within a year. Based&nbsp / on ANC results the leaching behaviours of Ca, Mg, Na+, K+, Se, Cr, and Sr were found to be controlled by the pH of the leachant indicating high mobility of major soluble species in the ash cores when in contact with slightly acid rain water. Other investigated toxic metals such as As, Mo and Pb showed amphoteric behaviour with respect to the pH of the leachant. Chemical alterations and formation of transient minor secondary mineral phases was found to have a significant effect on the acid susceptibility and depletion pattern of chemical species in the core ash samples when compared to fresh ash. These ANC results correlated well with the data generated from the sequential extraction scheme. Based on sequential extraction results elements, showed noticeable mobility in the water soluble, exchangeable and carbonate fractions due to adsorption and desorption caused by variations in the pore water pH. In contrast, slight mobility of elements in the Fe and Mn, and residual fractions of dry disposed fly ashes are attributed to the co-precipitation and dissolution of minor amount of less soluble secondary phase overtime. The 1-year-old dry disposed ash cores were the least weathered among the 3 drilled ash cores. Therefore low concentration of toxic metals in older ash cores were ascribed to extensive weathering with slower release from residual mineral phases over time. Elements were found to associate with different mineral phases depending on the age or depth of the core samples showing greater heterogeneity in dispersion. For instance the average amount of total calcium in different mineral associations of 1-year-old ash cores is as follows / water soluble (10.2 %), exchangeable (37.04 %), carbonate (37.9 %), Fe and Mn (7.1 %) and residual (2.97 %). The amount of total Na+ in different mineral phases of 1-year-old ash cores followed this trend: water soluble (21 %), exchangeable (11.26 %), carbonate (2.6 %), Fe and Mn (4.7 %) and residual (53.9 %). The non-leachable portion of the total Na+ content (namely that contained in the residual fraction) in the 1-year-old ash core samples under conditions found in nature ranged between 5-91 %. This non-leachable portion of the Na+ showed the metastability of the mineral phases with which residual Na+ associates. Results showed older ash cores are enriched in toxic elements. Toxic elements such as As, B, Cr, Mo and Pb are enriched in the residual fraction of older ash cores. For instance As concentration in the residual fraction varied between 0.0003- 0.00043 mg kg-1 for 1-year-old ash cores to around 0.0003-0.0015 mg kg-1 for 20-year-old ash cores. This suggests that the older ash is enriched in toxic elements hence dust from the ash dump would be toxic to human health. The knowledge of mobility and ecotoxicological significance of coal fly ash is needed when considering its disposal or reuse in the environment. The mobility and ecotoxicology of inorganic metals in coal fly ash are determined by (i) mineralogical associations of inorganic species (ii) in-homogeneity in the ash dumps (iii) long and short term exposure to ingress CO2 and percolating rain water. Management issues such as inconsistent placement of ash in the dumps, poor choice of ash dump site, in-homogeneity in brine irrigation, no record of salt load put on the ash dumps and lack of proper monitoring requires improvement. The thesis provides justification for the use of the modified sequential extraction scheme as a predictive tool and could be employed in a similar research work. This thesis also proved that the dry ash disposal method was not environmental friendly in terms of overall leaching potential after significant chemical weathering. Moreover the study proved that the practice of brine co-disposal or irrigation on ash dumps is not sustainable as the ash dump did not act as a salt sink.</p>

The current study therefore aims to provide a comprehensive characterisation of weathered dry disposed ash cores, to reveal mobility patterns of chemical species as a function of depth and age of ash, with a view to assessing the potential environmental impacts. Fifty-nine samples were taken from 3 drilled cores obtained respectively from the 1 year, 8 year and 20-year-old sections of sequentially dumped, weathered, dry disposed ash in an ash dump site at Tutuka - a South African coal burning power station.

Philosophiae Doctor - PhD / The management and disposal of huge volumes of coal combustion by products such as fly ash has constituted a major challenge to the environment. In most cases due to the inadequate alternative use of coal fly ash, the discarded waste is stored in holding ponds, slag heaps, or stock piled in ash dumps. This practice has raised concerns on the prospect of inorganic metals release to the surface and groundwater in the vicinity of the ash dump. Acceptable scientific studies are lacking to determine the best ash disposal practices. Moreover, knowledge about the mobility patterns of inorganic species as a function of mineralogical association or pH susceptibility of the dry disposed ash dump under natural weathering conditions are scarce in the literature. Fundamental understanding of chemical interactions of dry disposed ash with ingressed CO2 from atmosphere, percolating rain water and brine irrigation within ash disposal sites were seen as key areas requiring investigation. The mineralogical association of inorganic species in the dry disposed ash cores can be identified and quantified. This would provide a basis for understanding of chemical weathering, mineralogical transformations or mobility patterns of these inorganic species in the dry ash disposal scenario. The current study therefore aims to provide a comprehensive characterisation of weathered dry disposed ash cores, to reveal mobility patterns of chemical species as a function of depth and age of ash, with a view to assessing the potential environmental impacts. Fifty-nine samples were taken from 3 drilled cores obtained respectively from the 1 year, 8 year and 20-year-old sections of sequentially dumped, weathered, dry disposed ash in an ash dump site at Tutuka - a South African coal burning power station. The core samples were characterized using standard analytical procedures viz: X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transforms infrared (FTIR) techniques, Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) and Acid neutralisation capacity (ANC) test. A modified sequential extraction (SE) method was used in this study. The chemical partitioning, mobility and weathering patterns in 1 year, 8 year and 20-year-old sections of the ash dump were respectively investigated using this modified sequential extraction scheme. The sequence of the extractions was as follows: (1) water soluble, (2) exchangeable, (3) carbonate, (4) iron and manganese and (5) residual. The results obtained from the 5 steps sequential extraction scheme were validated with the total metal content of the original sample using mass balance method. The distribution of major and trace elements in the different liquid fractions obtained after each step of sequential extraction of the 59 drilled core samples was determined by inductively coupled plasma mass spectrometry (ICPMS). The data generated for various ash core samples were explored for the systematic analysis of mineralogical transformation and change in ash chemistry with ageing of the ash. Furthermore, the data was analyzed to reveal the impact of ingressed CO2 from atmosphere, infiltrating rain water and brine irrigation on the chemistry of ash core samples. Major mineral phases in original ash core samples prior to extraction are quartz (SiO2) and mullite (Al2O3·2SiO2). Other minor mineral phases identified were hematite (Fe2O3), calcite (CaCO3), lime (CaO), anorthite (CaAl2Si2O8), mica (Ca (Mg, Al)3 (Al3Si) O10 (OH)2), and enstatite (Mg2Si2O6). X-ray diffraction results show significant loss of crystallinity in the older ash cores. The presence of minor phases of calcite and mica in dry disposed ash cores are attributed to reduction in the pore water pH due to hydration, carbonation and pozzolanic reactions. The X-ray diffraction technique was unable to detect Fe-oxyhydroxide phase and morealuminosilicate phases in ash core samples due to their low abundance and amorphous character. X-ray fluorescence results of the original ash core samples showed the presence of major oxides, such as SiO2, Al2O3, Fe2O3, while CaO, K2O, TiO2, Na2O, MnO, MgO, P2O5, and SO3 occur in minor concentrations. The ratio of SiO2/Al2O3 classified the original core samples prior to extraction as a silico-aluminate class F fly ash. The ternary plot of major elements in 1-year-old ash core samples was both sialic and ferrocalsialic but 8 year and 20-year-old ash core samples were sialic in chemical composition. It is noteworthy that the mass % of SiO2 varies through the depth of the core with an increase of nearly 3 %, to 58 mass % of SiO2 at a depth of 6 m in the 1-year-old core whereas in the case of the 8-year-old core a 2 % increase of SiO2 to a level of 57.5 mass % can be observed at levels between 4-8 m, showing dissolution of major components in the matrix of older ash cores.. The Na2O content of the Tutuka ash cores was low and varied between 0.6-1.1 mass % for 1-year-old ash cores to around 0.6-0.8 mass % for 8-year-old ash cores. Sodium levels were higher in 1-year-old ash cores compared to 8 year and 20-year-old ashcores. Observed trends indicate that quick weathering of the ash (within a year) leached out Na+ from the ash dump. No evidence of Na+ encapsulation even though the ash dump was brine irrigated. Thus the dry disposal ash placement method does not result in a sustainable salt sink for Na-containing species over time. The total content of each of the elements in 1 year and 20-year-old ash cores was normalised with their total content in fresh ash from same power station to show enrichment and depletion factor. Major elements such as K+, Mn showed enrichment in 1-year-old ash cores whereas Al, Si, Na+, Ti, Ca, Mg, S and Fe showed depletion due to over time erosion. Trace elements such as Cr, Sr, P, Ba, Pb, V and Zn showed enrichment but Ni, Y, Zr showed depletion attributed to over time erosion. In 20-year-old ash cores, major elements such as Al, Na+ and Mn showed enrichment while Si, K+, Fe, Mg and Ca showed depletion highlighting their mobility. Trends indicated intensive flushing of major soluble components such as buffering constituents (CaO) by percolating rain water. The 1-year-old and 20-year-old coal ash cores showed a lower pH and greater loss/depletion of the soluble buffering constituents than the 2-week-old placed ash, indicating significant chemical weathering within a year. Based on ANC results the leaching behaviours of Ca, Mg, Na+, K+, Se, Cr, and Sr were found to be controlled by the pH of the leachant indicating high mobility of major soluble species in the ash cores when in contact with slightly acid rain water. Other investigated toxic metals such as As, Mo and Pb showed amphoteric behaviour with respect to the pH of the leachant. Chemical alterations and formation of transient minor secondary mineral phases was found to have a significant effect on the acid susceptibility and depletion pattern of chemical species in the core ash samples when compared to fresh ash. These ANC results correlated well with the data generated from the sequential extraction scheme. Based on sequential extraction results elements, showed noticeable mobility in the water soluble, exchangeable and carbonate fractions due to adsorption and desorption caused by variations in the pore water pH. In contrast, slight mobility of elements in the Fe and Mn, and residual fractions of dry disposed fly ashes are attributed to the co-precipitation and dissolution of minor amount of less soluble secondary phase overtime. The 1-year-old dry disposed ash cores were the least weathered among the 3 drilled ash cores. Therefore low concentration of toxic metals in older ash cores were ascribed to extensive weathering with slower release from residual mineral phases over time. Elements were found to associate with different mineral phases depending on the age or depth of the core samples showing greater heterogeneity in dispersion. For instance the average amount of total calcium in different mineral associations of 1-year-old ash cores is as follows; water soluble (10.2 %), exchangeable (37.04 %), carbonate (37.9 %), Fe and Mn (7.1 %) and residual (2.97 %). The amount of total Na+ in different mineral phases of 1-year-old ash cores followed this trend: water soluble (21 %), exchangeable (11.26 %), carbonate (2.6 %), Fe and Mn (4.7 %) and residual (53.9 %). The non-leachable portion of the total Na+ content (namely that contained in the residual fraction) in the 1-year-old ash core samples under conditions found in nature ranged between 5-91 %. This non-leachable portion of the Na+ showed the metastability of the mineral phases with which residual Na+ associates. Results showed older ash cores are enriched in toxic elements. Toxic elements such as As, B, Cr, Mo and Pb are enriched in the residual fraction of older ash cores. For instance As concentration in the residual fraction varied between 0.0003- 0.00043 mg kg-1 for 1-year-old ash cores to around 0.0003-0.0015 mg kg-1 for 20-year-old ash cores. This suggests that the older ash is enriched in toxic elements hence dust from the ash dump would be toxic to human health. The knowledge of mobility and ecotoxicological significance of coal fly ash is needed when considering its disposal or reuse in the environment. The mobility and ecotoxicology of inorganic metals in coal fly ash are determined by (i) mineralogical associations of inorganic species (ii) in-homogeneity in the ash dumps (iii) long and short term exposure to ingress CO2 and percolating rain water. Management issues such as inconsistent placement of ash in the dumps, poor choice of ash dump site, in-homogeneity in brine irrigation, no record of salt load put on the ash dumps and lack of proper monitoring requires improvement. The thesis provides justification for the use of the modified sequential extraction scheme as a predictive tool and could be employed in a similar research work. This thesis also proved that the dry ash disposal method was not environmental friendly in terms of overall leaching potential after significant chemical weathering. Moreover the study proved that the practice of brine co-disposal or irrigation on ash dumps is not sustainable as the ash dump did not act as a salt sink. / South Africa