Segun A. Akinyemi

The leaching behaviour and geochemical fractionation of trace elements in hydraulically disposed weathered coal fly ash

A five-step sequential extraction (SE) procedure was used to investigate the leaching behaviour and geochemical partitioning of the trace elements As, Zn, Pb, Ni, Mo, Cr and Cu in a 20-year-old fly ash (FA) dump. The weathered FA, which was hydraulically co-disposed with salt laden brine in slurry form (FA: brine ratio of 1:5), was analyzed and compared with fresh FA. The weathered FA samples were collected from three cores, drilled at a coal-fired power station in the Republic of South Africa while the fresh FA sample was collected from the hoppers in the ash collection system at the power station. The FA samples were sequentially leached using: ultrapure water; ammonium acetate buffer solution (pH 7); ammonium acetate buffer solution (pH 5); hydroxylamine hydrochloride in nitric acid (pH 2) and finally the residues were digested using a combination of HClO4: HF: HNO3 acids. Digestion of as received (unleached) FA samples was also done using a combination of HClO4: HF: HNO3 acids in order to determine the total metal content. The trace element analysis was done using ICP-OES (Varian 710-ES). The SE procedure revealed that the trace elements present in the fresh FA and the weathered FA samples obtained from the three cores could leach upon exposure to different environmental conditions. The trace elements showed continuous partitioning between five geochemical phases i.e., water soluble fraction, exchangeable fraction, carbonate fraction, Fe and Mn fraction and residual fraction. Although the highest concentration of the trace elements (ranging 65.51%–86.34%) was contained in the residual fraction, a considerable amount of each trace element (ranging 4.42%–27.43%) was released from the labile phases (water soluble, exchangeable and carbonate fractions), indicating that the trace species readily leach from the dumped FA under environmental conditions thus pose a danger to the receiving environment and to groundwater.

Mineralogy and Mobility Patterns of Chemical Species in Weathered Coal Fly Ash

Abstract Chemical interactions of disposed coal fly ash with O2, CO2, and infiltrating rain-water lead to chemical alteration, flushing/leaching of soluble chemical species locked in different physico-chemical forms as the coal fly ash is aging. This study was carried out to gain insight into the chemical alterations and its effects on the mobility patterns of chemical species in weathered coal fly ash. Weathered coal fly ash samples of ages 1 year and 20 years were sampled at a coal burning power station in the Mpumalanga Province, South Africa. The chemical and mineralogical compositions of the weathered coal fly ash were investigated using X-ray fluorescence spectrometry, inductive coupled plasma-optical emission spectrometry, ion chromatography, X-ray diffraction, and Fourier transform infrared spectroscopy. X-ray fluorescence results showed the presence of major oxides, such as SiO2, Al2 O 3, Fe2 O 3, while CaO, K2O, TiO2, MnO, MgO, P2 O 5, and SO3 occur in minor concentrations. The major elements, such as Fe, Si, Mg, K, and Mn, showed increasing trends down the depth of the core, while Ti and Al show decreasing trends down the depth of the core (20-year-old coal fly ash). Trace metals, such as P and Zn, show increasing trends down the depth, while Ba, Ni, Pb, Sr, V, and Cr increase up the depth of the core (20-year-old ash). The trace metals distribution patterns in 1-year-old ash showed increasing trends down the depth of the core for Cr, Ni, Pb, Y, S, and P, while Ba and Sr decrease down the depth of the core. Major mineral phases as revealed by X-ray diffraction include quartz and mullite. Other minerals identified are hematite, calcite, lime, anorthite, mica, and enstatite. The pH of interstitial pore water for 1-year-old and 20-year-old coal fly ash ranged from 9.0–10.6 and 7.2–9.9, respectively, while in the 2-week-old coal fly ash ranged from 9.9–10.9. Analysis of extracted pore water shows that several toxic metals, such as B, Cr, As, Mo, and Se, are leaching from the weathered ash. The core ash samples in contact with the atmosphere and those at the saturation point record the highest availability of chemical species, such as Mg, Ca, Fe, K, Na, B, Cr, As, Mo, and Se in pore water. The 1-year-old and 20-year-old coal fly ash cores showed a lower pH and greater leaching/flushing of the soluble buffering constituents than the 2-week-old placed ash.

Natural weathering in dry disposed ash dump: Insight from chemical, mineralogical and geochemical analysis of fresh and unsaturated drilled cores

Some existing alternative applications of coal fly ash such as cement manufacturing; road construction; landfill; and concrete and waste stabilisation use fresh ash directly collected from coal-fired power generating stations. Thus, if the rate of usage continues, the demand for fresh ash for various applications will exceed supply and use of weathered dry disposed ash will become necessary alternative. As a result its imperative to understand the chemistry and pH behaviour of some metals inherent in dry disposed fly ash. The bulk chemical composition as determined by XRF analysis showed that SiO2, Al2O3 and Fe2O3 were the major oxides in fresh ash and unsaturated weathered ashes. The unsaturated weathered ashes are relatively depleted in CaO, Fe2O3, TiO2, SiO2, Na2O and P2O5 due to dissolution and hydrolysis caused by chemical interaction with ingressing CO2 from the atmosphere and infiltrating rain water. Observed accumulations of Fe2O3, TiO2, CaO, K2O, Na2O and SO3 and Zn, Zr, Sr, Pb, Ni, Cr and Co in the lower layers indicate progressive downward movement through the ash dump though at a slow rate. The bulk mineralogy of unsaturated weathered dry disposed ash, as determined by XRD analysis, revealed quartz and mullite as the major crystalline phases; while anorthite, hematite, enstatite, lime, calcite, and mica were present as minor mineral phases. Pore water chemistry revealed a low concentration of readily soluble metals in unsaturated weathered ashes in comparison with fresh ash, which shows high leachability. This suggests that over time the precipitation of transient minor secondary mineral phases; such as calcite and mica might retard residual metal release from unsaturated weathered ash. Chloride and sulphate species of the water soluble extracts of weathered ash are at equilibrium with Na+ and K+; these demonstrate progressive leaching over time and become supersaturated at the base of unsaturated weathered ash. This suggests that the ash dump does not encapsulate the salt or act as a sustainable salt sink due to over time reduction in pore water pH. The leaching behaviours of Ca, Mg, Na+, K+, Se, Cr and Sr are controlled by the pH of the leachant in both fresh and unsaturated weathered ash. Other trace metals like As, Mo and Pb showed amphoteric behaviour with respect to the pH of the leachant. The precipitation of minor quantities of secondary mineral phases in the unsaturated weathered ash has significant effects on the acid susceptibility and leaching patterns of chemical species in comparison with fresh ash. The unsaturated weathered ash had lower buffering capacity at neutral pH (7.94-8.00) compared to fresh (unweathered) ash. This may be due to the initial high leaching/flushing of soluble basic buffering constituents from fly ash after disposal. The overall results of the acid susceptibility tests suggest that both fresh ash and unsaturated weathered ash would release a large percentage of their chemical species when in contact with slightly acidified rain. Proper management of ash dumps is therefore essential to safeguard the environmental risks of water percolation in different fly ashes behaviour.

The Leachability of Major Elements at Different Stages of Weathering in Dry Disposed Coal Fly Ash

Large quantities of solid residue are generated by coal-fired power stations in many parts of the World. The disposal and management of the unused fly ash remains a major problem to the environment. The weathered dry disposed ash cores comprise of major element constituents such as Al, Si, Ca, Mg, Fe, Mn, Na and K. The mobility patterns and mineralogical associations of major elements in weathered dry disposed ash dumps aged 1-year-old, 8-year-old and 20-year-old from a coalfired power station in South Africa were investigated using a modified sequential extraction scheme. The extraction sequence was as follows: (1) water soluble, (2) exchangeable, (3) carbonate, (4) Fe and Mn and (5) residual. A total acid digestion was carried out on the original sample prior to extraction to validate the extraction procedure. The distribution of Si, Fe, Mn, Ca, Mg, Na, and K in 59 drilled ash core samples was determined by inductively coupled plasma mass spectrometry. The leachability of the seven elements from different fractions proved to be different; so various distribution patterns have been achieved. The highest concentration of analytes is recorded in the water soluble, exchangeable, and carbonates of 1-year-old ash cores hence it is the least leached. The concentration of each element in each fraction was calculated as a percentage of the total metal content for the 1-year-old ash cores. The average amount of the major elements in the easily soluble fractions of 1-year-old ash core samples are: water soluble: Na (21%) . Ca (10.2%) . Mn (8.4%) . Si (4.0%) . K (2.58%) . Mg (0.05%) . Al (0.003%) . Fe (0.001%), exchangeable: Ca (37.04%) . Mg (12.6%) . Na (11.26%) . Mn (10.3%) . K (3.17%) . Si (1.6%) . Al (0.27%) . Fe (0.33%), carbonate: Mn (41.21%) . Ca (37.9%) . Mg (32.9%) . Al (29.25%) . Si (25.39%) . Fe (21.39%) . Na (2.6%) . K (2.23%). The mobility of major elements in the weathered ash dumps are influenced by heterogeneity in the ash dump, inhomogeneous continuous brine irrigation and chemical interaction of ash cores with ingressed CO2 from atmosphere and percolating rain water. f 2011 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved. A R T I C L E I N F O Article history: Received 15 March 2011; Received in revised form 30 May 2011; Accepted 2 June 2011

Source rock potential of selected Cretaceous shales, Orange Basin, South Africa

Twenty Cretaceous shale samples from two wells in the Orange Basin of South Africa were evaluated for their source rock potential. They were sampled from within a 1400 m-thick sequence in boreholes drilled through Lower to Upper Cretaceous sediments. The samples exhibit total organic carbon (TOC) content of 1.06–2.17%; Rock-Eval S2 values of 0.08–2.27 mg HC/g; and petroleum source potential (SP), which is the sum of S1 and S2, of 0.10–2.61 mg HC/g, all indicating the presence of poor to fair hydrocarbon generative potential. Hydrogen index (HI) values vary from 7 to 128 mg HC/g organic carbon and oxygen index (OI) ranges from 37 to 195 mg CO2/g organic carbon, indicating predominantly Type III kerogen with perhaps minor amounts of Type IV kerogen. The maturity of the samples, as indicated by T max values of 428–446°C, ranges from immature to thermally mature with respect to oil generation. Measured vitrinite reflectance values (%Ro) of representative samples indicate that these samples vary from immature to mature, consistent with the thermal alteration index (TAI) (spore colour) and fluorescence data for these samples. Organic petrographic analysis also shows that amorphous organic matter is dominant in these samples. Framboidal pyrite is abundant and may be indicative of a marine influence during deposition. Although our Rock-Eval pyrolysis data indicate that gas-prone source rocks are prevalent in this part of the Orange Basin, the geochemical characteristics of samples from an Aptian unit at 3318 m in one of the wells suggest that better quality source rocks may exist deeper, in more distal depositional parts of the basin.

Human bioaccessibility of Fe, Mn, Zn and Cu from consumed earth materials in Vhembe District, South Africa

ABSTRACT Earth materials, such as clays, anthills and dried-up pond sediments, are consumed by humans as a means of supplementing essential elements and for protection against toxins. A two-stage physiologically based extraction test (PBET), simulating the human stomach and intestine, was used to estimate the human bioaccessibility of Fe, Mn, Zn and Cu. Extracts from these earth materials were analysed by inductively coupled plasma mass spectroscopy (ICP-MS). Principal component analysis (PCA) was used to evaluate the relationships amongst bioaccessible element concentrations. The mean concentrations of elements for the stomach and intestinal phases were as follows; Fe, 0.94 and 0.20 ppm; Zn, 81.44 and 22.42 ppm; Mn, 10.10 and 1900 ppm; Cu, 24.93 and 230.04 ppm. Iron and Zn showed higher bioaccessibility under gastric conditions whilst Cu was bioaccessible only in the intestinal phase extract. Concentrations of Mn were the same in gastric and intestinal phases. There were strong associations between Zn, Fe and Mn. Some of the consumed earth materials could contribute to the daily recommended nutrient intake of Fe and provide a significant source of Mn and Cu for consumers of earth materials.

Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa

© 2012 Akinyemi et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Mineralogy and Geochemistry of Sub-Bituminous Coal and Its Combustion Products from Mpumalanga Province, South Africa

Mineralogy and Mobility Pattern of Trace Metals in Brine Irrigated Dry Disposed Weathered Coal Fly Ash

This study determines mineralogical association of trace elements in the ash core samples and provides insight into various factors that promote their mobility and eventual release into surface and groundwater systems. Fly ash from a power station ash dump in Mpumalanga Province (South Africa) was characterized using X-ray diffraction and X-ray fluorescence. A five-step sequential extraction procedure was used to determined mineralogical association of elements as a function of depth in the ash dump. The dry disposed ash cores are sialic in chemical composition. The mineral peak height as revealed by X-ray diffraction analysis showed variations due to heterogeneity in the ash dump and in homogenous continuous brine irrigation. The sequential extraction results showed that some trace elements are moving in a leaching pathway through the fly ash in the water soluble fraction. Toxic analytes are present in water soluble, exchangeable, and carbonate fractions. Observed Pb concentration maxima in the carbonate fraction of ash cores suggests mobility due to reduction in the pore water pH. A relatively high concentration of As and Se in the exchangeable fraction indicates their possible release when fly ash comes in contact with slightly acidic rain water. This is of major environmental concern for possible contamination of surface and groundwater systems. The relative enrichment of As and Se in the residual fraction suggests that the dust from 8-year-old brine irrigated ash dump would be toxic to human health. This investigative study demonstrates that reduction in pore water pH, inhomogeneity in the ash dump, continuous brine irrigation, leachate from topsoil, moisture content level, ingressed CO2 from atmosphere, and percolating rain water are factors that promote trace metal mobility and leaching in the brine irrigated dry disposed ash dump.

Chemical Partitioning and Mobility of Trace Elements in Dry Disposed Weathered Ash Conditioned with high-Saline Effluents

We investigated the mobility of inorganic elements in the ash dump due to chemical interaction of weathered dry disposed ash conditioned with high-saline effluents and ingressed CO2 from atmosphere and percolating rainwater. Drilled ash core samples collected from dry disposed ash dump at a South African coal-burning power station were characterized using X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses. A modified sequential extraction procedure was selected to determine the mineralogical association of the investigated elements in the 1-year-old drilled ash cores. Major mineral phases are quartz and mullite and other minor phases included hematite, lime, calcite, anorthite, mica, and enstatite. Mica and calcite in the ash cores is attributed to carbonation process, which led to long-term reduction in pore water pH. The 2-week-old ash cores are sialic but the 1-year-old ash cores were both sialic and ferrocalsialic in chemical composition. The ferrocalsialic nature of 1-year-old ash cores could be attributed to drastic changes in feed coal in the power station. The chemical index of alteration (CIA) and chemical index of weathering (CIW) values suggest a relatively high degree of weathering. The changes in CIA and CIW values in the ash cores depend on the pore water pH, leaching rate, carbonation process, and possibly the conversion of the alkali and alkali earth metals into carbonates. Multivariate analysis results suggest that the major oxides and carbon percent show differences and have a greater contribution to the differentiation in the 1-year-old ash cores. Trace elements such as Mo, Cr, and Se show high mobility but As, B, and Pb exhibit fairly low mobility in the water-soluble fraction. A considerable mobility of trace elements in the exchangeable and carbonate fractions is attributed to the alkaline nature and pore water pH. A decreasing response of As, Mo, Cr, and Pb in the reducible fraction with depth suggests immobility attributed to coprecipitation with the Feand Mn-oxide phases. On the contrary, the increasing response of Pb, Mo, and B in the carbonate fraction with depth implies mobility due to dissolution and flushing of soluble major chemical phases, as evident in the pore water pH. A considerable amount of B and Pb are concentrated in the residual fraction of 1-year-old ash cores. This suggests that, under natural settings, these elements will not be released in solution over a long time; therefore, they are not considered to be an environmental risk. f 2013 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved. A R T I C L E I N F O Article history: Received 16 March 2013; Received in revised form 16 September 2013; Accepted 17 September 2013