Natural weathering of pulverized fuel ash and porewater evolution

doi:10.1016/S0883-2927(97)00005-X

Applied Geochemistry

Volume 12, Issue 4, July 1997, Pages 367-376

https://doi.org/10.1016/S0883-2927(97)00005-X Get rights and content

Abstract

Borehole samples of pulverized fuel ash (PFA) were taken from the unsaturated zone in a disposal mound at a decommissioned power station in the UK. The aim was to investigate the long-term natural weathering reactions of PFA and the chemical evolution of the contained porewaters.

Concentrations of most species, including Al, Na, K, Ca, SO₄, B, Co, Cr, Li, Mo, Ni, Pb and Sr in the porewaters, increase with borehole depth, consistent with these elements being released from the PFA through continued weathering reactions with infiltrating porewaters. The concentration of Ba shows a near-constant value throughout the depth range investigated and this element is thought to have achieved equilibrium with respect to a sulphate phase. The Ca and S in the PFA show depletion near-surface, consistent with the higher porewater concentrations with depth. Using mass balance calculations for these two elements, approximate infiltration rates are obtained. Other elements which are depleted in near-surface samples are Cu, Mn, Ni, Pb and Zn. Higher concentrations, particularly of Na₂O and K₂O, in near-surface borehole samples demonstrate, however, that the ash was probably not homogeneous at the time of emplacement. Other elements in solution, such as Cl and NO₃, show peak concentrations in the depth profiles, which are thought to represent a time-dependent migration of an anthropogenic input, probably fertiliser. No significant changes were detected in the mineralogy using XRD and SEM.

Porewater analyses were processed using a geochemical modelling program, WATEQ4F, to investigate equilibrium relationships and to identify potential solubility controlling solid phases. Several solid phases were identified, including Al(OH)₃ for Al, Fe(OH)₃(am) for Fe and CaSO₄.2H₂O (gypsum) for Ca and SO₄.

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Effect of weathering transformations of coal combustion residuals on trace element mobility in view of the environmental safety and sustainability of their disposal and use. I. Hydrogeochemical processes controlling pH and phase stability
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This was confirmed further by the research of Donahoe (2004) on actual CCRs disposal environments, who identified a number of secondary phases, among them metastable aluminosilicate and Fe-oxyhydroxide phases being precursors of clay minerals and Fe-oxide minerals. Formation of these phases was documented also in earlier and later studies (Lee and Spears, 1995, 1997; Warren and Dudas, 1985; Adamczyk and Nowak, 2012). The geochemistry of formation/dissolution of mineral phases in the course of CCR weathering is associated with internal processes regulating the pH of the pore waters.
Coal combustion residuals (CCRs) are one of the most abundant high-volume waste materials disposed in impoundments worldwide. Some methods of CCR recycling, e.g. their use as structural fill for low lying areas or as soil amendment, also expose this material to atmospheric conditions. Combustion processes result in concentration of trace elements in CCRs at about an order of magnitude compared to coal. In order to assess an effect of long-term weathering transformations of CCRs on trace element binding/release, a study has been carried out. It is based on the chemical composition of real pore solutions extracted from the most abundant primary alkaline Class F bituminous CCRs, 0 to >40 years old, sampled from the surface layer and vertical profiles at four different impoundments. In this part of the study, results of a hydrogeochemical simulation of the saturation state of real pore solutions with respect to mineral phases of CCRs with use of the PHREEQC program, related to actual pH values reflecting the full cycle of weathering transformations, have been discussed. This study is the first geochemical proof of the general trend towards a progressive acidification up to pH < 4 of primary alkaline CCRs due to release of protons during internal processes of formation of gibbsite and aluminosilicate minerals, buffered by carbonates at the alkaline - near-neutral stages, and followed by parallel dissolution and buffering by aluminosilicates at pH < 7 after carbonate depletion, to the level up to pH∼3.5–4.0. The intrinsic geochemical changes have resulted in the different susceptibility of trace elements to release and associated changes in risk to the environment at consecutive stages of weathering.
Chemical, mineralogical and morphological changes in weathered coal fly ash: A case study of a brine impacted wet ash dump
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This shows that the salt holding capacity of the ash dump is very low which has a major implication for using it as a salt sink. According to Lee and Spears (1997), the interaction of dumped FA with infiltrating pore water leads to the depletion of CaO as a result of leaching and weathering. This explains why Ca was depleted at most of the depths along the drilled cores S1, S2 and S3 compared to the fresh FA.
The mobility of species in coal fly ash (FA), co-disposed with brine using a wet ash handling system, from a coal fired power generating utility has been investigated. The study was conducted in order to establish if the wet ash dump could act as a salt sink. The ash was dumped as a slurry with 5:1 brine/ash ratio and the dam was in operation for 20 years. Weathered FA samples were collected along three cores at a South African power station's wet ash dump by drilling and sampling the ash at 1.5 m depth intervals. A fresh FA sample was collected from the hoppers in the ash collection system at the power station. Characterization of both fresh FA and weathered FA obtained from the drilled cores S1, S2 and S3 was done using X-ray diffraction (XRD) for mineralogy, X-ray fluorescence (XRF) for chemical composition and scanning electron microscopy (SEM) for morphology. Analysis of extracted pore water and moisture content determination of the fresh FA and the weathered FA obtained from the drilled cores S1, S2 and S3 was done in order to evaluate the physico-chemical properties of the FA. The XRD analysis revealed changes in mineralogy along cores S1, S2 and S3 in comparison with the fresh FA. The SEM analysis revealed spherical particles with smooth outer surfaces for the fresh FA while the weathered ash samples obtained from cores S1, S2 and S3 consisted of agglomerated, irregular particles appearing to be encrusted, etched and corroded showing that weathering and leaching had occurred in the ash dump. The moisture content (MC) analysis carried out on the fresh FA (1.8%) and the weathered FA obtained from the drilled cores S1 (41.4–73.2%), S2 (30.3–94%) and S3 (21.7–76.2%)indicated that the ash dump was water logged hence creating favourable conditions for leaching of species. The fresh fly ash (n = 3) had a pH of 12.38 ± 0.15, EC value of 4.98 ± 0.03 mS/cm and TDS value of 2.68 ± 0.03 g/L, the pH of the drilled core S1 (n = 45) was 10.04 ± 0.50, the EC value was 1.08 ± 0.14 mS/cm and the TDS value was 0.64 ± 0.08 g/L. Core S2 (n = 105) had pH of 10.04 ± 0.23; EC was 1.08 ± 0.06 mS/cm and TDS was 0.64 ± 0.04 g/L, while core S3 (n = 66) had pH of 11.04 ± 0.09; EC was 0.99 ± 0.03 mS/cm and TDS was 0.57 ± 0.01 g/L. The changes in pH values can be attributed to the dissolution and flushing out of alkaline oxides like CaO and MgO from the dumped ash. The variations in pH values shows that the fly ash is acidifying over time and metal mobility can be expected under these conditions. The large decrease of EC in the drilled ash cores S1, S2 and S3 compared to the fresh ash indicated a major loss of ionic species over time in the ash dump. The XRF analysis showed the progressive dissolution of the major aluminosilicate ash matrix which influenced the release of minor and trace elements into the pore water enhancing their mobility as the ash dam acidified over time. Brine co-disposal on the ash may have been responsible for the slight enrichment of some species such as Na (0.27–0.56%), SO₄²⁻ (0.06–0.08%), Mg (0.57–0.96 %) and K (0.02–0.34%) in the disposed weathered FA. However, there was no significant accumulation of these species in the disposed FA despite continuous addition of large volumes of highly saline brine over the 20 year period that the dump existed, indicating that the ash dam was incapable of holding salts and continually released elements to the environment over the lifetime of the dam.
Studies on rheological and leaching characteristics of heavy metals through selective additive in high concentration ash slurry
2012, Journal of Hazardous Materials
Citation Excerpt :
Further the disposal of dilute slurry with excess water causes leaching of heavy metals from the ash matrix in the ash ponds/dykes [15]. Terrestrial disposal of the fly ash has been regarded as a potential source of contamination due to the enrichment and surface association of trace elements in the ash particles [16–23]. Much concern has been paid to the leaching behaviour and possible contamination, especially for the aquatic environment, when ash is in contact with water.
The generation and disposal level of thermal power plant ash in India is a challenging task. The conventional mode of dilute phase ash slurry (10–20% solids by weight) transport through pipelines being practiced in majority of these plants not only consumes huge amount of precious water and pumping energy but also causes serious environmental problem at the disposal site. The present study investigates the rheological and leaching characteristics of an Indian ash samples at high solids concentrations (>50% by weight) using sodium silicate as an additive. The flow behaviour of ash slurry in the concentration range of 50–60% by weight is described by a Bingham-plastic model. It was indicated that the addition of sodium silicate (0.2–0.6% of the total solids) could able to reduce both the slurry viscosity and the yield stress. The analysis of the ash samples for the presence of heavy metals such as Fe, Cd, Ni, Pb, Zn, Cu, Co, As and Hg were carried out following Hansen and Fisher procedure. The addition of sodium silicate affected the leaching characteristics of the ash samples over a period of 300 days resulting in the reduction of leaching of heavy metals.
Geochemistry and partitioning of trace metals in paddy soils affected by metal mine tailings in Korea
2006, Geoderma
Soil solution samples were extracted from paddy soil near mine tailing dumps in an abandoned mine in Korea. Trace metals in the soil solution and soil solid (As, Cd, Cr, Cu, Ni, Pb and Zn) were analysed, along with the mineralogical composition of the soil, using XRD. Sequential extraction was also undertaken for speciation of different fractions bound to the soil solid. Kaolinite was the main clay minerals and the CEC value was relatively low, recording between 11.0 and 27.5 meq/100 g. Comparison of the paddy soil with the control soil samples collected from an uncontaminated area revealed that nearly all the paddy soil samples were enriched for trace metal contents. Cadmium, Pb and Zn concentrations exceeded the intervention values of the Dutch standards, which requires remediation action. Arsenic, Ni and Cr were below the values, indicating little or no contamination for these metals. The trace metal contents in the paddy varied with distance from the mine tailing site. In addition to the changes with distance, higher proportions of relatively easily releasable fractions, such as exchangeable forms, were found to be proportional to the total trace metal contents in the soil sample. Although most paddy soil samples were enriched with respect to some trace elements, concentrations of Cd, Cu, Ni and Pb in the solution were very low, recording maximum levels of up to only several milligrams per liter. Geochemical equilibrium modelling using PHREEQC indicates the presence of solubility controlling solid phases for Cd and Pb, whereas Zn and Cu seem to be controlled by adsorption/desorption processes. We presumed that either iron sulphides or oxides removed dissolved trace elements. Under reducing conditions, iron may be dissolved as Fe²⁺ in the soil solution and then the reduced iron possibly would form iron sulphide precipitates with sulphur from the mine tailings. On the contrary, in an oxidizing condition, iron exists as the oxidised form, Fe³⁺, and again forms iron oxides, which is a good adsorbent for trace elements in soil environments. A Cycle of dry and flooded conditions is characteristic for paddy fields. Therefore, either iron sulphide or iron oxide is believed to provide a good adsorbing site for trace metals and so explains the subsequent low concentrations in the soil solution. Despite the low trace metal concentrations in the soil solution, higher portions of the trace metals still exist as relatively easily available form in the soil solid. It should be stressed that controlling of the redox condition and pH would be important in managing the transport of trace metals in the paddy soil that was affected by the mine tailings.
A thermodynamic equilibrium comparison of the mobilities of trace elements when washed and unwashed coals are burnt under pf firing conditions
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Thermodynamic equilibrium predictions of trace element partitioning between the gaseous and condensed products of combustion have been made for four coals as supplied and after washing to remove further mineral. The calculations modelled the products of combustion of the whole coal with excess oxygen (1% excess for each coal with additional calculations at 3% for Harworth coal and 5% for Gascoigne Wood); the trace elements examined were As, Cr, Cu, Mn, Ni, Pb and Zn. The calculations indicated that almost all thermodynamic equilibrium distributions included a greater percentage of the trace element in the gas phase at 1300–2100 K for feedstock with extra washing as compared with the standard feedstock. When the amounts volatilised were converted to moles per 100 g of coal burnt it was found that in some cases the relative amount of some elements volatilised from washed compared with unwashed coal was lower, in other cases the relative amount volatilised increased.
Step cooling calculations were carried out using as initial concentrations the amounts of each element in the gas phase at 1300 K for washed and unwashed Gascoigne Wood coal. The results indicated deposition of sulphates in the lower part of the temperature range 1300–600 K, with a greater tendency to form alkali sulphate based melts for the washed coal.
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2002, Fuel
Currently only 20% of the fly ash produced in Korea is utilised for industry, and the remainder is disposed as waste in landfill sites. Both anthracite and sub-bituminous coals are burnt in Korea. Fly ash and coal samples were collected from five different coal-fired power stations in Korea and analysed for their chemistry and mineralogy. Batch leaching tests were also carried out to investigate the leaching behaviour of selected fly ashes. The fly ash leachate chemistry was compared with the groundwater taken directly from the monitoring well installed in one of the power stations. The anthracite coals contain illite, pyrophyllite and kaolinite whereas kaolinite is the representative clay mineral for the sub-bituminous coals. Anthracite coals were higher in Si, Al and K than the sub-bituminous coals, reflecting higher mineral matter contents in the anthracite coals. Mullite and quartz are the main mineral phases for two different types of the fly ashes, with some iron oxides. The chemical compositions of the anthracite and sub-bituminous fly ashes are comparable with each other, except for extraordinary high concentrations of Cr for one anthracite fly ash. Most of the trace elements in the ash were enriched in the finer fraction, indicating surface associations. Although, some elements including Na, K, Ca and Cu were released rapidly in the initial stage of leaching, measurable amounts of metals were still detectable in the fly ash leachate treated several times with distilled water. Such leaching behaviour indicates slow and long-term leaching of elements associated with the glass fractions of the ash particle. This was confirmed by leaching of weathered fly ash, which had been disposed of for several years. Comparison of the ash leachate, treated with 0.1N-HCl, fly ash slurry in the ash pond and the groundwater indicate the influence of the ash leachate from the ash disposal mound on the groundwater composition.

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^†: Present address: Korea Environmental Technology Research Institute, 9-2 Samsung-Dong, Kangnam-Ku, Seoul, Korea, 135-090.

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Applied Geochemistry

Abstract

References (28)

The solubility of (Ba, Sr) SO₄ precipitates: Thermodynamic equilibrium and reaction path analysis

Geochimica et Cosmochimica Acta

Leaching characteristics of a high calcium fly ash as a function of pH: A potential source of selenium toxicity

Applied Geochemistry

Chemical characterization of fossil fuel wastes

WATEQ4F — A personal computer FORTRAN translation of the geochemical model WATEQ2 with revised database

U.S. Geological Survey. Open-file report 87-50

Environmental impacts of coal combustion residues

Journal of Environmental Quality

Annual abstract of statistics

Chemical weathering of basalts and andesites: Evidence from weathering rinds

U.S. Geological Survey Professional Paper 1246

Comparison of levels of trace elements extracted from fly ash and levels found in effluent waters from a coal fired power plant

Environmental Science and Technology

Long-term leachability of selected extracts from fly ash

Environmental Science and Technology

Geochemical factors controlling the mobilization of inorganic constituents from the fossil fuel combustion residue: II — Review of the minor elements

Journal of Environmental Quality

Centrifuge extraction and chemical analysis of interstitial waters

Environmental Science and Technology

Identification of solubility-controlling solid phases in a large fly ash field lysimeter

Environmental Science and Technology

Leachate chemistry at the Montour fly ash test cell

Solutions, Minerals, and Equilibria

Cited by (12)

Effect of weathering transformations of coal combustion residuals on trace element mobility in view of the environmental safety and sustainability of their disposal and use. I. Hydrogeochemical processes controlling pH and phase stability

Chemical, mineralogical and morphological changes in weathered coal fly ash: A case study of a brine impacted wet ash dump

Studies on rheological and leaching characteristics of heavy metals through selective additive in high concentration ash slurry

Geochemistry and partitioning of trace metals in paddy soils affected by metal mine tailings in Korea

A thermodynamic equilibrium comparison of the mobilities of trace elements when washed and unwashed coals are burnt under pf firing conditions

Leaching characteristics of selected Korean fly ashes and its implications for the groundwater composition near the ash disposal mound