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Environmental Pollution 159 (2011) 3324e3333

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Environmental Pollution

journal homepage: www.elsevier .com/locate/envpol

The internal microstructure and fibrous mineralogy of fly ash from coal-burningpower stations

Patrick Browna,b,*, Tim Jonesa,1, Kelly BéruBéb,1

a School of Earth and Ocean Sciences, Cardiff University, Park Place, CF10 3YE Cardiff, UKb School of Biosciences, Cardiff University, Museum Avenue, CF10 3US Cardiff, UK

a r t i c l e i n f o

Article history:Received 10 June 2011Received in revised form5 August 2011Accepted 21 August 2011

Keywords:Coal fly ashMineralogyGeochemistryMicrostructureFibrous mullite

* Corresponding author.E-mail addresses: brownpd@cf.ac.uk (P. Brown),

berube@cf.ac.uk (K. BéruBé).1 These authors contributed equally to this work.

0269-7491/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.envpol.2011.08.041

a b s t r a c t

Coal fly ash (CFA) is a significant environmental pollutant that presents a respiratory hazard whenairborne. Although previous studies have identified the mineral components of CFA, there is a paucity ofinformation on the structural habits of these minerals. Samples from UK, Polish and Chinese powerstations were studied to further our understanding of the factors that affect CFA geochemistry andmineralogy. ICP-MS, FE-SEM/EDX, XRD, and laser diffraction were used to study physicochemical char-acteristics. Analysis revealed important differences in the elemental compositions and particle sizedistributions of samples between sites. Microscopy of HF acid-etched CFA revealed the mullite presentpossesses a fibrous habit; fibres ranged in length between 1 and 10 mm. Respirable particles (<10 mm)were frequently observed to contain fibrous mullite. We propose that the biopersistence of theserefractory fibres in the lung environment could be contributing towards chronic lung diseases seen incommunities and individuals continually exposed to high levels of CFA.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Over 60% of global electricity is generated by coal-fired powerstations, resulting in vast quantities of coal fly ash (CFA) beingproduced annually Worldwide; approximately 600 million tons(Johnson, 2009). During coal combustion in a modern powerstation furnace, minerals contained within the raw pulverised feedcoal are melted to ‘glass’; the amount of incombustible mattercontained within raw coal differs largely depending on source andquality, ranging from between 2 and 50 wt%. Subsequently, fineliquid droplets are released and lifted by the flue gases, where theyundergo rapid solidification forming small glassy spheres, knownas CFA. In addition, a small amount of particles also form throughnucleation, a sublimation reaction of gaseous phases present in theflue gases (Seames, 2003). The five main elemental constituents ofCFA are Si, Al, Fe, Ca and O. The Si, Al and O combine together toform an amorphous aluminosilicate glass matrix (SixAlyOz); Fe canbe substituted into this matrix in place of Al. The SiO2:Al2O3 ratio iscommonly close to 2:1, and is governed by the mineral content ofthe feed coal; which is a function of the local geology, extraction

jonestp@cf.ac.uk (T. Jones),

All rights reserved.

and cleaning methods. The concentration of metals in CFA is highlydependent on the coal; with an enrichment factor usually between10 and 15 times the levels found in the coal (Vassilev and Vassileva,2007). Trace elements commonly found in CFA include Ni, V, As, Be,Cd, Cu, Zn, Pb, Hg, Se, Rn and Mo (Donaldson et al., 2005; Jones andBéruBé, 2006). CFA often possesses a surface layer, nmemm inthickness, formed from the readily-leachable material depositedwhilst cooling (Ugurlu, 2004). CFA also contains recrystallizedminerals, such as quartz (SiO2) and mullite (Al4þ2xSi2�2xO10�x),which are derived from the elements in the minerals originallypresent in the feed coal. Crystalline silicates, most notably the SiO2polymorph minerals quartz and cristobalite, are of great concern torespiratory health (Fubini and Fenoglio, 2007). However, little isknown about the respiratory hazards of mullite, although one studycited chronic bronchitis, silicotuberculosis and pneumoconiosis(mullitosis) in workers exposed to highly aluminiferous clay(Artamonova et al., 2000).

Despite CFA not being classified as a toxic waste hazard, thereare concerns with the current methods of storage, given that CFA isknown to contain a range of metals and other elements of envi-ronmental concern, such as Cd, Hg, Pb, As, and Se. Commonly, CFA isstock-piled in ash mounds for storage, where it can be re-suspended by winds and contribute to ambient particulate airpollution. Numerous epidemiological studies have associatedexposure to small particles, such as combustion-generated fineparticles, with lung cancer, heart disease, asthma and/or increased

P. Brown et al. / Environmental Pollution 159 (2011) 3324e3333 3325

mortality (Zanobetti and Schwartz, 2007; Knox, 2008). It is notedthat strictly speaking CFA is not a combustion particle, but moreprecisely a ’melt’ particle. This study examines CFA samples fromthe UK, Poland and China. The main objective was to further detailour current understanding of the geochemistry and structure ofCFA, with special attention paid to the crystalline mineral compo-nent. This paper considers the various factors that can influenceCFA composition, structure and morphology by comparing CFAsourced from power stations that differ in their operationalprocedures, including the quality and source of the feed coal.

2. Material and methods

2.1. Fly ash sources

The two UK-sourced CFA samples were obtained (anonymous donation) froma coastally-located coal-burning power station; UK1 was taken from the top ofa compacted fly-ash pile (w50m high) formed from CFA that had been ‘conditioned’(sprayed) with local seawater, whilst UK2 was taken directly from the electrostaticprecipitators (ESP). The power station burned a mixture of UK and overseas-sourcedbituminous coal, which was supplemented with biomass from Scandinavia(5e10 wt%, wood chippings/sawdust). The two Polish CFA samples were collectedfrom power stations located in the Ma1opolska district. The first sample, PL1, wastaken from themain ESP unit of an old 590MW coal-fired power station. The secondsample, PL2, was collected from the final filter of a three-stage ESP unit that serveda combined electricity (460 MW) and heat (1236 MW) power station. The stationsboth co-fired biomass (10e15 wt%) in a 60:40 split of sawdust and crops (corn/sunflower seeds) with bituminous coal from numerous sources, themajority comingfrom the nearby open-cast mines of Katowice. The Chinese CFA samples wereobtained from power stations that used locally-sourced coal that is found inter-bedded in late Permian limestones (CaCO3). Both were collected from the powerstation’s ESP units. The first sample, CH1, was collected from a very small, relativelynew power station (50 MW) located in the Henan province of China. The secondChinese sample, CH2, was obtained from a significantly older and larger (1260 MW)coal-fired power station located in the Shandong province. The stations burn locally-sourced sub-bituminous coal known for having a high sulphur content, approxi-mately 1.8e4.5%, and high overall ash content, 35e39% (Shao et al., 2003).

2.2. Fly ash preparation

All samples were dried in a Heraeus oven at 50 �C for 24 h. For each sample, the‘respirable’ PM10 fraction was dry separated from the bulk via a rotating drum witha HEPA-filtered air flow of 30 L/min. The PM10 fraction was then collected ona 0.67 mm polycarbonate filter in a NEGRETTI collection head (BéruBé et al., 2006).

2.3. Fly ash heating experiments

Heat-treated samples were prepared for the UK and Polish CFA in order toidentify the relative thermostability of each sample’s mineral content via X-raydiffraction (XRD). The rationale behind the heating experiments was to mimickconditions possibly experienced by particles collected on ESP plates, but still beingexposed to hot flue gases. Samples were weighed into ceramic crucibles and placedinto a muffle furnace at 400 �C for 7 days, on removal samples were left to coolnaturally at room temperature and pressure.

2.4. Laser diffraction particle sizing

Laser diffractionwas used to establish a particle size distribution profile for bothbulk and PM10 fractions of each sample. An approximate dry mass of 200 mg of eachsample was analysed using a Scirocco 2000 Mastersizer (Malvern Instruments); allmeasurements were made on the same day under the same conditions. A refractiveindex (RI) of 1.60 with an adsorption of 0.001 was used; as recommended by themanufacturer for use with CFA samples. Median mass diameter (MMD) and specificsurface area (SSA) values for each samplewere calculated from the distribution data;a reference particle density value of 2.46 g/cm3 was used when calculating MMDvalues (Gurupira et al., 2001).

2.5. Scanning Electron Microscopy

Particle morphology and microstructure was studied via Field Emission-Scanning Electron Microscopy (FE-SEM). Samples were mounted on standardaluminium SEM stubs using sticky carbon tabs before being gold/palladium coatedto a thickness of 20 nm using a BIO-RADMicroscience Division SC500 sputter coater.The samples were analysed using a Phillips XL30 FE-SEM, equipped with an OxfordInstruments INCA ENERGY Energy Dispersive X-ray analyser (EDX). The workingdistance was 10 mm, beam voltage 20.0 kV, aperture 4 and spot size 5. The imageswere captured via secondary electron detection. All images were saved as *.TIF files.

INCA ENERGY analysis package was used to determine the composition of specificsites of interest; spot analysis was used for verifying the composition of individualitems of interest.

2.6. Resin-embedded polished blocks

The UK and Polish CFA was embedded and polished in resin blocks usingstandard coal petrology techniques (Jones and Rowe, 1999). A portion of dry sample(w5 g) was mixed with low viscosity epoxy resin in a 5 cm diameter circular mould.The blocks were then set in a vacuumed oven tominimise trapped air bubbles in theresin. The blocks were polished following the British Standard guidelines for prep-aration for coal petrology, finishing with a diamond paste abrasive to achieve a highpolish (Jones and Rowe, 1999). This revealed the cross-sections of individual CFAspheres and agglomerates. Acetone was used to clean the surface of the blocks,which were carbon-coated to a thickness of 15 nm using an EMITECH K450 sputtercoater prior to analysis. The resin blocks were examined under SEM using backscatter detection, with site-specific composition determined by EDX.

2.7. Hydrofluoric acid etching experiments

In order to study the structural habits of CFA mineral components in detail, theamorphous glass matrix must first be removed by an etching process. Bulk CFAsamples were placed on 0.45 mm Whatman cellulose nitrate membrane filters(47 mm diameter) inside a separable Millipore filter unit attached to a syringe. Thesamples were then suspended in 40% hydrofluoric acid (HF) for 30 s, enough time toallow the acid to etch away a layer of the amorphous aluminosilicate glass matrix.Following completion of the etching time, each sample was flushed with deionisedwater to ensure removal of surface HF acid. The resin-embedded polished blockswere also subjected to the same etching process for cross-sectional comparison ofthe spheres; the resin is unaffected by the acid.

2.8. X-ray diffraction

Sample mineralogy was determined by XRD analysis carried out using a PhilipsPW1710 diffractometer with Co-Ka radiation (from a Cobalt tube) generated at 35 kVand 40 mA. All samples were packed into a cell using double-sided tape andmeasurements were taken between 2 and 70 2q at a scan speed of 0.040�2q/sec witha count time of 0.5 s/step with a total of 3400 steps.

2.9. Inductively Coupled Plasma-Mass Spectrometry

The major and selective trace elemental compositions of bulk and PM10 CFAsamples were determined using Inductively Coupled Plasma-Mass Spectrometry(ICP-MS). The Claisse Fluxy fusion system was used to prepare samples for analysis.1 g (þ/� 1%) of each sample was weighed and placed into a platinum crucible. Ineach crucible, 0.6 g of 50% lithium metaborate and 50% lithium tetraborate flux wasadded; 6e9 drops of lithium iodide were also added to each crucible as a wettingagent. The crucibles were then heated to between 800 �Ce1000 �C. The resultingfluxwas then tipped into a beaker containing diluted nitric acid and 1ml of 100 ppmrhodium solution (internal standard). This was automatically mixed until all the fluxhad dissolved. The resulting solution was made up to 100 ml with deionised(>18 UM) H2O and analysed using a Thermo Elemental X-series ICP-MS equippedwith a Cetac AS-500 autosampler. Raw data was corrected for blanks, controls anddilutions. The international standards, JB 1a, Japanese basalt, and MRG-1, a rockstandard, were run in duplicate as external controls.

3. Results

3.1. Particle size distribution

The values obtained represent equivalent spherical diameter(ESD); although the majority of CFA particles are spheres, irregularshaped particles are also common. The median mass diameter(MMD, mm) and specific surface area (SSA, m2 g�1) values weredetermined for each sample (Table 1). The bulk sample representsthe full size-distribution of the sample, including spheres,composites/aggregates and large agglomerates, or grains, whereasthe PM10 fraction is composed almost exclusively of individualspheres. The separation of the PM10 fraction from the bulk wassuccessful, with the average sphere size of the respirable fractionsranging from between 1.98 and 5.64 mm. The Chinese PM10 samplesdisplayed both smaller particle size and greater SSA values than theEuropean PM10 samples, at 1.98 mm/3.01m2 g�1 (CH1) and 2.42 mm/2.96 m2 g�1 (CH2), compared to the European samples particle size

Table 1Laser diffraction analysis for UK, Polish and Chinese bulk and PM10 coal fly ashsamples. The median mass diameter (MMD, mm) and specific surface area (SSA,m2 g�1) values are given for all samples.

Sample MMD (mm) SSA (m2 g�1)

UK1 21.57 0.73UK1a 3.50 2.20UK2 13.09 0.61UK2a 5.64 1.72PL1 15.94 1.00PL1a 5.16 1.92PL2 6.80 2.20PL2a 4.36 2.64CH1 28.16 0.61CH1a 1.98 3.01CH2 97.58 0.27CH2a 2.42 2.96

MMD ¼ Median Mass Diameter, SSA ¼ Specific Surface Area.a PM10 fraction.

P. Brown et al. / Environmental Pollution 159 (2011) 3324e33333326

range of 3.50 mme5.64 mm and SSA range of 1.72e2.20 m2 g�1. Thebulk samples showed greater variation, most notably the Chinesesamples, which revealed a dramatic increase in particle sizebetween the PM10 and bulk samples; conversely, the PL2 sampledisplayed little difference between samples.

3.2. Morphology

The FE-SEM analysis revealed a variety of particle morphologies,although the vast majority of particles were spherical. These sphereswere present as solid or hollow spheres (cenospheres); fragments ofspheres were also present. In addition, some spheres were seen tohouse other much smaller sub-micron sized particles, these struc-tures are known as plerospheres. Another common feature observedwas the aggregation of smaller spheres to form largermasses; smallerspheres were also adhering to the surfaces of other larger spheres oragglomerates. The FE-SEM images (Fig. 1aeh) illustrate some of thekey features observed during morphological analysis. The UK1 PM10sample (Fig. 1a) consisted of spheres that were highly uniform inshape and surface texture, their size ranging between 3 mmand 5 mmin diameter (laser diffraction MMD value of 3.50 mm); with theexception of a few larger spheres up to 15 mm. Plerospheres werefrequently observed during analysis (Fig. 1b). The spheres in the UK2sample (Fig.1c) hadnotably rougher surface textures than thoseof theUK1 sample (conditioned CFA). An uncommon chain-like structurewas found in the UK2 PM10 sample, where three adjoining spheres,fairly uniform in size (w2 mm), appeared to have fused together; theirsurface textures varied fromvery rough to quite smooth (Fig. 1d). Theoverallmorphologyof the PL1PM10 samplewas comparable to that ofthe UK1 PM10 sample; however, the spheres possessed a roughersurface texture by comparison. An unusual particle was found in thePL2 sample, its morphology suggests extremely rapid solidificationduring movement-derived deformation (Fig. 1e). FE-SEM images ofthe HF etched bulk samples revealed thatmullite fibres form a three-dimensional mesh cage integral to the particles overall morphology(Fig. 1feh); as such the recrystallized sphere is a pseudomorph ofthe original wholly glass sphere. Fibres were observed to differ inboth length and width, with some spheres possessing thick fibres(Fig. 1f) and others much finer fibres (Fig. 1g). It was also noted thatthe fibres were not restricted to spherical structures, as a number ofirregular shaped particles also exhibited a fibrous scaffold (Fig. 1h).

3.3. Microstructure

The following features highlighted in this section apply to all sixsamples; however, resin-embedded blocks were only made from

the UK and Polish samples. FE-SEM images of the resin-embeddedpolished blocks were taken in back scatter mode (Fig. 2aeh). Thecross-sectional analysis revealed the majority of particles weresmall solid spheres, however both large and small cenosphereswere also observed, as well as a distribution of spheres possessingcomplex internal crystalline structures (Fig. 2a; UK1). The internalstructures observed belong to ferrospheres, although few exhibitsuch complex structures at the surface (Fig. 2b; PL1). These dendriticstructures varied in branch size and complexity (Fig. 2c; UK1). Thevoids and pockets seen in larger spheres were associated withparticle degassing (Fig. 2d; UK2). Ferrospheres were often foundresiding in larger plerospheres, alongwithmany other smaller solidspheres (Fig. 2e; PL2). The microscopy of the HF etched polishedblocks (Fig. 2feh) revealed very clearly that a fibrous componentwas present in nearly all particles, although to a varying degree. Thisfibrousmineral component was found in plerospheres (Fig. 2f; PL1),where fibreswere typically 5 mm in length (Fig. 2g; UK1), alongwithsmaller and larger spheres (Fig. 2h; UK2).

3.4. Mineralogy

The XRD analysis (Fig. 3aed) revealed the most abundantminerals found in CFA to be quartz, mullite and hematite. The dif-fractogram for the UK1 sample (Fig. 3a) had a clearly visible baselinedistortion due to the amorphous glass, which can cause peaks toappear less prominent; quartz and mullite were the dominantminerals. Upon heating, the diffractogram for the UK1 sampleshowed an increase in mullite peaks, with a decrease in quartzpeaks, and the emergence of a strong hematite peak. The majorpeaks observed during analysis of the UK2 sample were quartz andmullite, with quartz being the most abundant. The number ofidentifiable peaks dropped considerably after heating, though theintensity of the peaks remaining appearednot to have been affected.The PL1 sample had high counts for quartz andmullite (Fig. 3b); theheating program failed to alter the mineral content of the PL1sample. The level of baseline distortionwas high for the PL2 sample,with low counts for quartz and mullite. The PL2 sample showedsome notable changes upon heating. The counts for the quartz andmullite peaks increased slightly, and a number of new hematitepeaks were observed. The CH1 sample displayed peaks for quartz,mullite and hematite, as well as gypsum (Fig. 3c). The CH2 samplealso displayed strong peaks for quartz, mullite and hematite.Overall, the counts for the Chinese CFA crystalline mineral peaksweremuchhigher than those seen for the European CFA; in additionthe baseline was flatter. Along with the CFA samples, the mineralcomposition of PL1 pulverised feed coal was analysed, revealingpeaks for quartz and kaolinite, halite, illite and pyrite (Fig. 3d).

3.5. Geochemical composition

The major elemental compositions of all six samples weredetermined via ICP-MS (Table 2a). In order to assess whetherelements were disproportionally fractionated dependent onparticle size, the elemental compositions of four PM10 sampleswerealso determined. The Chinese samples had the highest Si content,CH1 at 58.49wt% and CH2 at 54.59wt% (PM10; 54.15wt%), consid-erably higher than the UK1 and UK2 samples, whichwere 44.72wt%(PM10; 51.07wt%) and 44.27wt% (PM10; 47.16wt%), respectively. TheAl content of samples was more consistent, ranging between20.55wt%e26.14wt%. The levels of Mg and K were higher in thePolish samples, especially PL2 which had values of 3.31wt% and3.36wt% respectively, compared to CH2, whose levels were thelowest, at 0.82wt% and 0.34wt%. The levels of trace elements variedbetween sub-samples; the UK bulk samples had higher values thanthe PM10 equivalents for every element except Ba and Zr (Table 2b).

Fig. 1. Field Emission-Scanning Electron Microscopy of raw CFA and HF etched CFA. 1a: FE-SEM image of UK1 PM10 sample consisting of spheres highly uniform in shape and size,with the majority around 3 mme5 mm in diameter and smooth in appearance. 1b: An example of a plerosphere found in the UK1 PM10 sample, many smaller sub-micron sizedparticles remain trapped inside of a hollow sphere around 8 mm in diameter. 1c: FE-SEM image of a sphere from the UK2 sample; the sphere possesses a considerably roughersurface texture in comparison to that of the spheres in Fig. 1a. 1d: Three connecting spheres, found in the UK2 PM10 sample, appear to have fused together forming a chain-likestructure. The spheres are fairly uniform in size, around 2 mm, although their surface textures are more varied. 1e: FE-SEM image of a particle found in the PL2 sample, which hashad its shape distorted whilst cooling, presenting an unusual pear-shaped morphology. 1f: A sphere found in the CH2 sample, approximately 15 mm in diameter, exhibiting a fibrousmullite framework. 1g: Image highlighting a large sphere which appears to have opened up following the HF etching procedure. 1h: A large irregular shaped agglomerate displaysthe same fibrous mullite framework seen in Fig. 3b and c.

P. Brown et al. / Environmental Pollution 159 (2011) 3324e3333 3327

TheNi content ranged from52 ppm for CH1 to 541 ppm for CH2. Thesame level of variation was seen for Zn, which was present at83 ppm for CH1 and 702 ppm for PL2. The highest levels of Cr werefound in the CH2 sample at 324ppm,with the lowest levels found inthe CH1 sample at 93 ppm. The PL2 bulk and PM10 samples werevery similar inmajor and trace composition, yet the readings for theCH2 PM10 sample were higher than those seen in the bulk sample

for every trace metal except Cr, Ni and Zr. There appears to be anenrichment of Ca in the PM10 fraction,whichwas noted for 3 of the 4PM10 samples. The levels of Ca in the CH2 PM10 sample were overdouble those seen in the bulk sample. In addition, the SiO2:Al2O3ratio appears to shift in the PM10 fraction, with the exception of CH2PM10, all European samples had increased SiO2 content anddecreased Al2O3 content in the smaller size fraction. Fe content was

Fig. 2. Field Emission-Scanning Electron Microscopy of resin-embedded polished blocks including HF etched blocks and raw material. 2a: A large CFA sphere with a highlycrystalline structure (UK1). 2b: FE-SEM image of a ferrosphere found in the PL1 sample demonstrating a complex surface structure formed from Fe oxides, as confirmed by EDXanalysis. 2c: A close-up image of the dendritic structure seen in Fig. 2a. 2d: CFA sphere with large pockets (formed by degassing) containing numerous small spheres (UK2).2e: A large plerosphere housing a number of smaller spheres, including one with a dendritic crystalline structure (PL2). 2f: A cross-sectional view of fibrous mullite contained withina plerosphere (PL1). 2g: Fibrous mullite meshwork of a sphere approximately 20 mm in diameter (UK1). 2h: Cross-sectional image reveals a fibrous mullite component is present innearly all spheres to a varying extent (UK2).

P. Brown et al. / Environmental Pollution 159 (2011) 3324e33333328

observed to decrease in the respirable fraction, with all four PM10samples presenting lower values than their respective bulk samples.

3.6. Particle-specific composition

The FE-SEM/EDX analysis was performed simultaneously withthe morphological analysis (Fig. 4aef). The data obtained for eachsample generally agreedwith the respective ICP-MS data; themajorelements present were Si, Al, Fe, Ca and Mg. Assumptions based on

elemental ratios suggest that Ca3(PO4)2 and CaSO4 were present inCH1, and NaCl in UK1, as well as CaO in CH2 (Fig. 4a). The levels ofunburned carbon (LOI values) recorded for the UK sampleswere w15% (Table 2a); an example of carbonaceous material (92%carbon) was found in the UK2 bulk sample (Fig. 4b). The dendriticstructures observed in the cross-sectional work (Fig. 2aeh) wereselected for analysis by EDX. The structures were formed from ironoxides (Fig. 4c); these spheres are commonly referred to as ferro-spheres (Sokol et al., 2002). The crystalline mineral fibres identified

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Fig. 3. X-Ray Diffraction Spectra for UK, Polish and Chinese coal fly ash, and one pulverised feed coal sample. Selected spectra of three coal fly ash samples, UK1, PL1 and CH1, andone pulverised feed coal sample (PL1) are shown in Fig. 3aed, respectively. The two UK samples revealed a near identical mineral content, with quartz and mullite the only mineralsto register peaks. The Polish samples also had quartz and mullite as their major crystalline phases. The Chinese spectra revealed the highest levels of quartz and mullite. In addition,haematite and gypsum were found to be present. The overall resolution seen for the CH1 sample is higher, peaks are more pronounced, counts higher, and baselines flatter;indicative of a greater degree of re-crystallisation. The PL1 pulverised feed coal sample displays peaks for quartz, kaolinite, halite, illite and pyrite.

P. Brown et al. / Environmental Pollution 159 (2011) 3324e3333 3329

following the HF etching procedure proved to be an aluminosilicatecomposite (Fig. 4d). Further analysis was performed on exposedcrystals that had been etched, revealing whole particle scaffolds tobe composed of fibrous mullite; this was confirmed using both pin-point analysis and a quadrat setting (Fig. 4e and f).

4. Discussion

4.1. Morphology

The morphology of the CFA showed only marginal differencesbetween samples, mainly in average particle size. However, the

Table 2aFluxy bulk ICP-MS values for UK, Polish and Chinese fly ash, showing the major (2a: wt%)The final column shows the measured values and reference values, in parenthesis, for Jb1aas weight percentage (wt%).

Oxides UK1 UK1 UK2 UK2 PL1 PL2

wt% Cond.FA PM10 ESP PM10 ESP ESP

SiO2 44.72 51.07 44.27 47.16 52.31 43.18Al2O3 24.30 22.11 26.14 24.14 25.31 25.45Fe2O3 6.10 6.06 6.29 5.54 6.96 8.86CaO 1.57 2.66 1.54 2.80 3.63 5.54MgO 1.00 1.04 1.31 1.02 2.43 3.31K2O 2.38 1.67 2.62 1.75 2.78 3.36Na2O 0.80 0.95 0.77 0.97 1.02 1.95TiO2 0.85 0.91 1.02 1.02 1.07 1.23P2O5 0.72 0.51 1.03 0.88 0.43 1.98MnO 0.07 0.12 0.09 0.12 0.11 0.14L.O.I. 17.50 12.91 14.90 14.60 3.94 4.99

Total 100.00 100.00 100.00 100.00 100.00 100.00

conditioning of the UK1 sample with locally-sourced seawaterresulted in the removal of surface condensates and water solublephases; although EDX analysis revealed these compounds wereoften replaced with NaCl crystals. The difference in surface texturebetween UK1 and UK2 suggests the material has adhered to thesurface as opposed to protruding from the particle body. It isprobable that relative particle surface charge plays a key role indictating the degree of surface coating, as there does not appear tobe a correlation between size and surface texture (Li et al., 2011).Particles will become negatively charged on passing throughthe electrostatic precipitators, the degree to which each particleholds this negative charge is likely dependent on their individual

and trace (2b: ppm) elemental compositions of both the bulk CFA and PM10 samples.; a Japanese basalt and MRG-1, an international rock standard. Major elements given

PL2 CH1 CH2 CH2 Standards (Ref Values)

PM10 ESP ESP PM10 JB1a MRG-1

45.07 58.49 54.59 54.15 52.45 (52.16) 39.17 (39.12)22.54 20.55 25.26 25.15 14.60 (14.51) 8.40 (8.47)7.42 8.28 7.35 3.16 8.97 (9.10) 18.31 (17.94)5.34 3.46 4.93 11.41 9.25 (9.23) 14.65 (14.70)3.31 1.39 0.82 0.73 7.72 (7.75) 13.43 (13.55)2.99 1.06 0.34 0.32 1.45 (1.42) 0.17 (0.18)1.91 0.33 0.11 0.16 2.86 (2.74) 0.75 (0.74)1.21 0.93 1.06 1.19 1.31 (1.30) 3.75 (3.77)1.81 0.24 0.14 0.34 0.25 (0.25) 0.08 (0.08)0.17 0.06 0.06 0.09 0.15 (0.15) 0.18 (0.17)8.23 5.22 5.33 3.30 0.99 (0.78) 1.11 (0.55)

100.00 100.00 100.00 100.00 100.00 100.00

Table 2bFluxy bulk ICP-MS values for UK, Polish and Chinese fly ash, showing the major (2a: wt%) and trace (2b: ppm) elemental compositions of both the bulk CFA and PM10 samples.The final column shows the measured values and reference values, in parenthesis, for Jb1a; a Japanese basalt and MRG-1, an international rock standard. Trace elements givenas parts per million (ppm).

Elements UK1 UK1 UK2 UK2 PL1 PL2 PL2 CH1 CH2 CH2 Standards (Ref Values)

ppm Cond.FA PM10 ESP PM10 ESP ESP PM10 ESP ESP PM10 JB1a MRG-1

Ba 1228 1179 1385 1777 1040 2505 2090 1179 329 587 491 (494) 61 (61)Co 60 18 82 34 35 77 67 19 16 29 38 (41) 92 (87)Cr 99 48 176 67 138 168 166 93 324 101 405 (409) 420 (430)Cu 89 40 202 61 75 179 177 83 73 89 55 (59) 148 (134)Ni 154 56 536 87 77 212 198 52 541 73 125 (142) 207 (193)Sc 32 16 50 20 25 32 31 18 15 32 28 (28) 53 (55)Sr 806 692 1217 1034 398 985 941 1242 579 1546 453 (450) 278 (266)V 218 103 320 143 238 455 465 120 102 203 223 (201) 534 (526)Y 66 62 93 83 49 71 71 38 46 92 23 (24) 13 (14)Zn 105 49 282 117 268 702 648 83 92 198 81 (85) 201 (191)Zr 74 148 70 188 173 81 66 210 234 97 140 (139) 105 (108)

P. Brown et al. / Environmental Pollution 159 (2011) 3324e33333330

composition. Previously, surface charge has been shown to bedependent on composition in the clay mineral montmorillonite forparticles w1 mm (Liu et al., 2008). Slight differences in surfacecharge, either during particle formation or electrostatic precipita-tion, may influence the level of coating for each particle.

The FE-SEM analysis of the polished block cross-sections(Fig. 2aeh) provided valuable insight into CFA microstructure thatcould not be determined by conventional FE-SEM. The majority ofparticles were composed of aluminosilicate glass, often with gaspockets and iron oxide inclusions (ferrospheres); these findingssuggest that CFA is perhaps not as homogeneous as previouslythought (Goodarzi, 2006). Ferrospheres are known to be derivedfrom the decomposition and oxidation of Fe-bearing mineralspresent in the feed coal, such as pyrite (Fe2S) and clay minerals,during combustion (Fe2þ/ Fe3þ) (Hubbard et al., 1984); pyrite wasidentified in the XRD analysis of the PL1 pulverised feed coalsample (Fig. 4g). This suggests that the Fe from the pyrite wasincorporated into the aluminosilicate matrix and also present asoxidised spherules (ferrospheres).

4.2. Geochemistry

Cho et al. (2009) suggested that enrichment of elements in thesmaller particle size fraction occurs while the incombustiblematerial is molten, the sulphur and metals vaporise and subse-quently nucleate and combine with small droplets of fused inor-ganic material that can be coated with condensed or reacted semi-volatile metalloids. However, our ICP-MS data revealed littledifference between the compositions of the bulk samples and theirPM10 fractions (Tables 2a and 2b), despite notable differences inparticle median mass diameter as seen in the laser diffractionanalysis (Table 1). It is worth noting that the surface compositionsof fugitive CFA re-suspended from power stations, or emitted fromthe stack, may differ from CFA collected directly from an ESP unit,as particles readily acquire new surface compounds duringlong-distance travel, such as polycyclic aromatic hydrocarbons(Eiceman and Vandiver, 1983). The increased Ca content in the CH2PM10 sample is likely to be Ca(OH)2 particle surface materialassociated with the smaller size fraction derived from contami-nating limestone bedrock in the coal that was initially converted toCaO, and then hydrated to Ca(OH)2. This would explain the highlybasic pH of the Chinese CFA, which was determined duringprevious pH elution analysis (data not shown). The analysis foundall six CFA samples and their PM10 fractions to be alkaline, withlittle variation between fractions over a seven day trial period.Surface Ca(OH)2 has been cited previously as a source of CFAalkalinity (Dutta et al., 2009). It is noted that post-combustion

‘conditioning’ of the UK1 sample appeared to reduce CFA alka-linity (pH 11.7 for UK2 reduced to 8.8 for UK1), suggesting surfacematerial or accessible soluble elements dissociated during ‘wetting’of the CFA with local seawater (pH w7.5). Broncheo-alveolar lungfluid is known to have a pH w7.3, thus it is possible the samesurface dissolution chemistry of soluble components would applyin the lung environment. It is also observed that as the Ca contentincreases from bulk to PM10 samples, the Fe content is seen todecrease, especially for CH2. This is likely to be a result of theseparation process causing a decrease in the number of Fe-bearingspheres (ferrospheres) present in the sample. Due to their relativelylarge average particle size (5e50 mm; Xue and Lu, 2008), pro-portionately fewer ferrospheres will reside in the respirable(<10 mm) fraction. The magnetic fraction, including ferrospheres, isbelieved to account for approximately 1.5e1.8wt% of bulk CFA(Hansen et al., 1981; Dai et al., 2010). Interestingly, Kukier et al.(2003) found that when comparing magnetic (haematite, magne-tite) and non-magnetic (quartz, mullite) fractions, more elementsleached out of non-magnetic than magnetic fractions under alka-line conditions created by CFA.

4.3. Mineralogy

Mullite and quartz were the two minerals most frequentlyidentified during XRD analysis. The coexistence of hematite, mulliteand quartz in the Chinese samples could have resulted from ironspherules becoming embedded in the amorphous-silicate matrix(Lu et al., 2009). The counts observed for each sample differedconsiderably indicating that the Chinese samples had a highermineral content; this was also supported by the flatter baselinesseen for these samples, which is indicative of a lower amorphoussilica content (Diamond, 1984). Zhao et al. (2008) reported that CFAparticles becomemore crystalline (quartz) and less amorphouswithdecreasing particle size, however, our ICP-MS data would suggestthere is little difference between bulk and PM10 CFA. The coal usedin the Chinese power stationswas obtained from limestone and claymineral sedimentary sequences (Shao et al., 2003); the limestone(CaCO3) accounts for the high Ca levelsmeasured in the CFA and thekaolinite, mica-illite and smectitic clay minerals provided the Si, Aland O (along with other trace elements) required for the alumino-silicate glass (McCarthy et al., 1987). When gradually heatedkaolinite undergoes several intermediary changes (kaolinite /

metakaolin/ aluminium silicon spinel) before eventually formingmullite, with temperatures exceeding w1050 �C required for thefinal transformation step (Zhao et al., 2010). This processmimics thenatural formation of mullite at the type location at Port Na

Fig. 4. FE-SEM/EDX analysis of coal fly ash. 4a: EDX-determined composition for the rough surface texture of a particle from the CH1 sample. 4b: Identification and confirmation ofhighly carbonaceous material found in the UK2 sample. 4c: A sphere possessing a dendritic crystalline structure formed from iron oxide, known as a ferrosphere. 4d: Fibrous mulliteidentified during the cross-sectional analysis of the HF etched polished resin-embedded blocks. 4e: Further pin-point analysis of mullite fibres confirming an aluminosilicatecomposition. 4f: EDX spectrum gathered using a quadratt setting reveals the mullite composition to be consistent throughout.

P. Brown et al. / Environmental Pollution 159 (2011) 3324e3333 3331

Cloidheig, Isle of Mull, Scotland, where clay minerals are trappedand transformed between hot volcanic lava flows (Jones et al.,2009).

For all six CFA samples (UK, Poland, and China) in this study, wedid not identify any ’original’ mineral grains, as has been seen inprevious CFA studies. For example, Ribeiro et al. (2011) show imagesof CFA from a deactivated Portuguese power station that are clearlyoriginal quartz grains (their Fig. 3, image A1), which indicated that

the furnace temperatures did not reach 1650 (þ/�75)�C, themeltingpoint of quartz. Furthermore, our XRD of the Polish coal feedstock(PL1) indicates that the clay minerals in the coal (kaolinite and illite)did not survive the heating. There is ambiguity in the scientificliterature and commercial information, but a consensus would havethe melting point of clays around 1750 �C. Therefore, as all ourCFA appears to be derived from glass, all the original minerals musthave melted, and this sets a minimum furnace coal combustion

P. Brown et al. / Environmental Pollution 159 (2011) 3324e33333332

temperature of 1750 �C. In addition to the mineral temperatureinformation, the spherical shape of the CFA is critical in under-standing the mineralogical processes. All the coal minerals weremelted (1750 �Cþ), converted into liquid glass, and formed thespherical shapes. The temperature then decreased resulting in theglass solidifying (900�Ce1100 �C) and retaining that spherical shape,regardless of any other mineralogical changes. At these lowertemperatures, minerals recrystallized (devitrification) from theamorphous non-crystalline glass, and the order in which theyrecrystallise is controlled by their melting temperatures. Thefirst mineral to start forming is mullite which has a meltingtemperature w1760 �C; however the recrystallization occurs ata much lower temperature, 900 �C and less. The mullite forms asfibrous crystals in a three-dimensional framework (pseudomorph)the same overall shape as the original glass sphere (Hulett andWeinberger, 1980; Joshi et al., 1985). In the bulk glass compositionthe ratio of Si:Al is approximately 2:1 (Table 2a), in the mullite theSi:Al ratio is reversed at approximately 1:2 (Fig. 4d, e, f). Therefore, asthe mullite fibres form, the remaining glass matrix becomesprogressively depleted in Al. Once depleted in Al, and at lowertemperatures, thematrixwill then start recrystallising a groundmassof quartz around the mullite fibres. This will continue until thetemperature reaches a sufficiently lower level to stop any furtherrecrystallization. The mullite content of a given CFA sample istherefore highly dependent on the post-melting cooling pathway,and the Al content derived fromminerals in the coal (Dai et al., 2010).

The heating experiments were designed tomimic the conditionsof a “hot-side” ESP unit, which operate between 300 and 450 �Cupstream of the air pre-heater. We were advised that this was thetemperature range by the power station operators. It is possiblethat ash may become trapped on the ESP for long periods beforeremoval, thereby exposing it to conditions capable of thermody-namically altering the mineral content. The results of the heatingexperiments showed some evidence of changes to CFA mineralogy,though mostly concerned with the appearance or increase inhematite peaks, rather than the expected overall increase in crys-talline material. Similar results have been reported previously(Medina et al., 2010). A slight increase in mullite and decrease inquartz was noted for the UK1 sample, a possible thermodynamicalteration of the SiO2-Al2O3 system, as reported previously(Ilic et al., 2003). New hematite peaks were observed in twosamples, UK1 and PL2, suggesting that iron present had been oxi-dised to hematite (a-Fe2O3), possibly associated to the iron-rich rimfound on many CFA particles (�Skvára et al., 2009; Smith and Baer,1983). The decrease in the number of peaks observed for the UK2sample following the heating program is suggestive of slightdifferences in the mineralogy of the two UK samples despite thesamples having both originated from the same power station.

4.4. Fibrous mullite hazard

Whilst the respiratory hazards of quartz dusts have been heavilyresearched (Fubini et al., 2004), there is a paucity of research intothe risks associated with exposure to mullite. When CFA is treatedwith HF acid, the quartz and amorphous glass matrix is rapidlydissolved, whereas mullite is a more refractory material, enabling itto withstand the etching process much longer. Fibre length appearsto be dependent on the size of the spheres, with the majorityapproximately 5 mm in length and 0.5e1 mm in width. Fibrousminerals have been the subject of intensive research in the field ofrespiratory toxicology, most notably asbestos, but also includingman-made mineral fibres such as glass or mineral wools (Dörgeret al., 2001). The most dangerous known mineral is the fibrousnatural zeolite erionite, which is linked to mesothelioma (Doganet al., 2008; Fach et al., 2003). A result of all this intense interest

in fibrous minerals has been the development of the ’fibreparadigm’, where the shape of the mineral is believed to havea direct bearing on disease initiation (Donaldson et al., 2006). It isconceivable that these mullite fibres could prove to be damaging torespiratory health should they accumulate in the tracheo-bronchialand broncho-alveolar regions of the lungs. The determining factorof whether the mullite fibres come into direct contact with cellslining the respiratory tract is the dissolution rate of the amorphoussilica matrix in lung fluid. In addition, we must also consider thepossibility that this dissolution rate could be markedly increased ifthe particle were in an acidic environment, for example, followingphagocytosis by a macrophage (intracellular pHw4.5). Rothenberget al. (1989) recovered CFA from rat lungs at 1, 3, 6 and 12 monthsfollowing a 4 week exposure; particles were characterised usingSEM and EDX. They were able to distinguish between inhaled andcontrol CFA, noting that the number of Si-rich particles decreasedwith time. After 6 months, 1% of particles had been transformed,producing numerous ‘needles’ associated with residues of CFAparticles. These ‘needles’ are almost certainly the mullite fibrespresented in this study. Furthermore, macrophages have beenshown to unsuccessfully attempt phagocytosis of fibres between 5and 10 mm in length, leading to the release of inflammatorymediators (Tomatis et al., 2010). Mullite is a refractory material andone would expect the half-life of the fibres within the lung envi-ronment would be substantial, and this level of biopersistencecould contribute towards a chronic lung condition (Osmond-McLeod et al., 2011).

5. Conclusions

The physicochemistry and potential health effects of CFA aredependent on a number of factors, including feed coal geochem-istry, furnace operating conditions and post-combustion process-ing. Notable geochemical differences between the bulk and PM10samples studied in this paper included a shift in the SiO2-Al2O3ratio towards SiO2 for the UK samples, as well as an increased Caand decreased Fe content in the respirable size fraction of allsamples. XRD analysis revealed the Chinese CFA to be more crys-talline. Feed coal geochemistry and furnace temperature are criticalin dictating the mineralogy and post-furnace recrystallization ofpotentially hazardous minerals from the glass matrix. The fibrousmullite component present in CFA forms a crystalline frameworkthat pseudomorphs the original spherical structure of the CFAparticles. CFA and its constituent mineral mullite should be viewedas a potential respiratory hazard due to the abundance of CFA in ourenvironment, and the insolubility and therefore likely bio-persistence of mullite in the lung environment. The fibres are ofsignificant length to cause chronic respiratory health problemsshould they become deposited throughout the lower respiratorytract.

Acknowledgements

We would like to thank the anonymous coal-burning powerstations for their generous donations of fly ash for this research. Wethank technicians Iain McDonald (ICP-MS), Tony Oldroyd (XRD),and Peter Fisher (FE-SEM).

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