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

Weinvestigated 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. Aconsiderable 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 Fe- and 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

A Geochemical Analytical Scheme for the Appraisal of Partitioning and Mobility of Major elements in Weathered Dry Disposed Coal Fly Ash

South Africa is endowed with significant deposits of coal which is utilized in electricity generation to meet the nation’s energy demand. A large volume of waste solid residue from the combustion of pulverized feed coal in power stations is dry disposed in stock piles or dumps. Chemical interactions of dry disposed fly ash with ingressed CO2 from the atmosphere and infiltrating rain water would cause dissolution of the soluble components in the fly ash matrix. Chemical partitioning and mobility of major elements in samples from cores drilled into serially stacked weathered dry disposed fly ash were investigated using a modified five steps sequential extraction scheme. A total acid digestion was carried out on the original ash core samples prior to extraction to validate the extraction procedure. The geochemical distribution of the investigated major elements in 59 drilled core samples was determined by x-ray fluorescence and inductively coupled plasma mass spectrometry. The relationship between SiO2 and chemical index of alteration (CIA) showed 8 year and 20-year-old core samples have a moderate to high degree of weathering. Conversely, 1-year-old cores samples showed characteristics between low and moderate-high degrees of weathering. A cluster and discriminant analysis of the major elements was also able to reveal the subtle chemical alteration differences of the core samples. Functional analysis revealed the disparities in the dissolution patterns of major soluble components in the matrix of the drilled core samples. Modified sequential extractions revealed high concentration of the major species in the leachates for every mineralogical fraction; although the bulk of the major elements are locked up in the insoluble phase of the core samples (i.e. residual fraction) which would not be released under normal environmental conditions. It is noteworthy that the concentration of major elements in the labile fractions (water soluble + exchangeable + carbonate) was high and this has implications for the long-term durability of residual mineral phases. Relative enrichment and depletion trends of major elements are promoted by heterogeneity in the ash dump (i.e. moisture content), gradual reduction of pore water pH and continuous brine and water irrigation.Key words: Coal fly ash; Weathering; Sequential extraction scheme; Cluster analysis; Factor Analysis

Laboratory study on the mobility of major species in fly ash–brine co-disposal systems: up-flow percolation test

Apart from the generation of fly ash, brine (hyper-saline wastewater) is also a waste material generated in South African power stations as a result of water re-use. These waste materials contain major species such as Al, Si, Na, K, Ca, Mg, Cl and SO4. The co-disposal of fly ash and brine has been practiced by some power stations in South Africa with the aim of utilizing the fly ash to capture the salts in brine. The effect of the chemical interaction of the species contained in both fly ash and brine, when co-disposed, on the mobility of species in the fly ash–brine systems is the focus of this study. The up-flow percolation test was employed to determine the mobility of some major species in the fly ash–brine systems. The results of the analysed eluates from the up-flow percolation tests revealed that some species such as Al, Ca and Na were leached from the fly ash into the brine solution while some species such as Mg, Cl and SO4 were removed to some extent from the brine solution during the interaction with fly ash. The pH of the up-flow percolation systems was observed to play a significant role on the mobility of major species from the fly ash–brine systems. The study showed that some major species such as Mg, Cl and SO4 could be removed from brine solution using fly ash when certain amount of brine percolates through the ash.Web of Scienc

Geochemical Partitioning of Major Elements in Brine Impacted Coal Fly Ash Residues

Fly ash-brine co-disposal technique has been considered as a way of disposing fly ash and brine (hyper-saline water) by some power stations in South Africa. This practice was aimed at using the fly ash to capture most of the elements in brine. However, the geochemical partitioning of the major elements in the waste materials after the fly ash-brine interaction has not been fully understood. This study focuses on understanding the geochemical partitioning of the major elements captured in the fly ash solid residues after the fly ash-brine interaction experiment. XRF and sequential extraction procedure were respectively applied to determine the chemical composition and partitioning of the major elements in fresh fly ash and the solid residues recovered after fly ash-brine interaction. The comparison of the results of the XRF analysis carried out on the fresh fly ash and the solid residues showed that the major elements such as Si, Ca, Mg and Na increased in the solid residues after the fly ash-brine interaction. This indicates that Ca, Mg and Na in the brine solution were captured by the fly ash during the interaction. However, the sequential extraction results showed that significant concentrations of Ca, Na and Mg were released into the water soluble, exchangeable and carbonate fractions. The results show that significant amounts of the elements captured in the fly ash solid residues during fly ash-brine interaction exist in the form which can be easily leached out when in contact with aqueous solution

An Investigative Study on the Chemical, Morphological and Mineralogical Alterations of Dry Disposed Fly Ash During Sequential Chemical Extraction

The hazardous elements associated with various physicochemical forms in coal fly ash are of environmental concern due to their leaching potential and subsequent contamination of surface and groundwater in the vicinity of the ash dump. Selective sequential extraction was performed on dry disposed fly ash samples from a coal-fired power station in Mpumalanga province, South Africa. The alteration of the chemical, morphological and mineralogical species of weathered fly ash during the selective sequential extraction was investigated using X-ray fluorescence (XRF), Nano-scan electron microscopy (NANOSEM) and X-ray powder diffraction (XRD). Insoluble residue from the water-soluble fraction is composed of amorphous alumino-silicate. The residues from exchangeable carbonate and Fe and Mn fractions consisted mostly of amorphous alumino-silicate spheres with a lesser quantity of iron-rich spheres. The iron-rich spheres are surrounded by amorphous alumino-silicate spheres.  The leaching behavior of trace metals (such as Ce, Y, Nb, Rb, U, and Tl) in weathered dry disposed fly ash was considered to have a dependency relationship with the components of SiO2, CaO, MgO, P2O5, and amount of unburned carbon. The decrease in the quantities of calcite with successive extraction could be considered as a marker of progress of sequential extraction technique. At the same time, the increase in the quantities of quartz could be also considered as an indicator of progress of the sequential extraction scheme. Trace elements bound to exchangeable or carbonate fraction during sequential chemical extraction were found associated with calcite. The elemental concentrations, as determined by electron dispersive x-ray spectroscopy (EDS), were consistent with XRF and XRD data. Therefore, the chemical extractant used in this study proved efficient for extraction of inorganic metals associated with various physicochemical forms in weathered fly ash.Key words: Coal fly ash; Selective sequential extraction; Major components; Trace elements; Morphology; Mineralogical compositio

Spectroelectrochemical reactivities of novel polyaniline nanotube pesticide biosensors

Summary: The preparation and characterisation of electrosynthetic polyaniline nanomaterials doped with phenanthrene sulphonic acid (PSA) is being presented. The polymeric nanomaterials prepared include processable poly(o-methoxy aniline) (POMA) and poly(2,5-dimethoxy aniline) (PDMA). Spectroelectrochemical reactivities of the electroactive polymeric nanotube systems as well as the nanobiosensor systems were studied by SEM, FTIR, UV-Vis and Subtractively Normalised Fourier Transformed Infrared Spectroscopy (SNIFTIRS) techniques. Furthermore, cyclic and differential pulse voltammetric studies of the nanomaterials were also performed using platinum or thiol-modulated gold electrodes. The SEM studies confirmed the nanorod morphology of the polyanilines. The heterogeneous rate constant, k o , for the nanopolymeric material and the diffusion coefficient of electrons, D e , was calculated and found to be in agreement with values expected for electron hopping along conducting polymer chains. Organophosphate pesticide nanobiosensor devices were prepared by encapsulating acetylcholinesterase (AChE) in the nanopolymeric composite. The biosensor amperometric response to the organophosphate pesticide called diazinon and the carbamate pesticide called carbofuran were studied. The sensor responses to pesticides followed typical electrochemical Michaelis-Menten kinetics

Molecularly imprinted polypyrrole sensors for the detection of pyrene in aqueous solutions

Recently, electrochemical sensors have emerged as tools for polyaromatic hydrocarbons (PAH) detection that are cost-effective, easy to produce and use, highly selective and sensitive, and with good reproducibility. Polypyrrole may be easily produced from polymerization of pyrrole, by chemical as well as electrochemical methods, to produce dimensionally stable semi-conductive polymer materials, under mild synthesis conditions. In this study, polypyrrole was used as the stable molecular framework within which to create an imprint of the desired polyaromatic hydrocarbon, in situ, at glassy carbon electrodes. The molecularly imprinted polymer (MIP) sensors were washed to remove the imprint and subsequently characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), and cyclic voltammetry (CV). The MIP sensors were then applied to the detection of pyrene and non-imprinted polymers (NIP) sensors were also evaluated for comparison with the MIP sensors. Calibration curves obtained for the detection of the pyrene at the MIP sensors in aqueous media reported limits of detection (LOD) of 2.28 × 10−7 M for pyrene and limit of quantification (LOQ) of 6.92 × 10−7 M (n = 3). The sensitivity of the MIP sensors (32.53 A/M) determined from the slopes of the calibration curves reported twice the value measured for NIP sensors (14.48 A/M)

Long-term brine impacted fly ash, Part II: Mobility of major species in the ash residues

The leaching of major species from fly ash is a function of the pH of the solution in contact with the fly ash. The aim of this study was to determine the effect of the pH of the leachant on the leaching of species from the ash residues recovered after the long-term fly ash-brine interactions. Acid neutralization capacity (ANC) tests using solutions of different pH values ranging from the initial pH of the ash residues (11-12) to pH 4 were employed in the leaching experiments. The ANC tests revealed that the release of major species from the ash residues depended on the pH of the leachants except for Na and Cl, where the significant concentrations leached were independent of the solution pH. The concentrations of Al and Si in the ANC leachates were very high at pH below 6 while Ca, K, Sr, Mg and B were immediately mobilized from the brine impacted fly ash when in contact with de-ionized water, and leaching increased as the pH decreased. The concentration of SO4 leached from the brine impacted ash residues at high pH was high, and the leaching increased with decrease in the pH of the leachant. This study shows that most of the major elements captured in the ash residues could be mobilized when in contact with aqueous solutions of various pH. This reveals that the co-disposal of fly ash and brine may not be the best practice as the major elements captured in the ash residues could leach over time. © 2013 Islamic Azad University (IAU)

Long-term Brine Impacted Fly Ash. Part 1: Chemical and Mineralogical Composition of the Ash Residues

The co-disposal of brine and fly ash has become a common practice in South African power utilities. This study focuses on the effects of the long-term fly ash–brine interaction on the chemical and mineralogical composition of fly ash and the quality of the brine solution after the interaction test. Long-term fly ash–brine interaction test was carried out by contacting fly ash and brine for a period of time varying from 1 week to 12 months under static and closed conditions. The results of the chemical composition of the brine decanted after the interaction test revealed that species such as B, Co, Cu, Pb, Mg, Mn, Zn, Cl and SO4 were removed to a certain extent from brine during the fly ash–brine interaction test while Al, Si, Ca, K, Ba, Sr, Fe, As, Cr and Mo were significantly leached out of the fly ash into the brine. The X-ray fluorescence results showed that the concentrations of Na, Mg, Cl and SO4 (as S) in the ash residues were somewhat higher than their concentrations in the fresh fly ash. Secondary mineral phases such as wairakite, charlesite, spinel and celestine which were missing in the X-ray diffraction analysis of the fresh fly ash were identified in the ash residues. This study shows that some species contained in the brine solution could be captured by the fly ash through secondary mineralization during co-disposal in a closed static environment, while many other elements could be significantly leached into the brine