Association of the Sites of HeavyMetals with Nanoscale Carbon in aKentucky Electrostatic PrecipitatorFly AshJ A M E S C . H O W E R , * , †

U S C H I M . G R A H A M , * , † A L A N D O Z I E R , ‡

M I C H A E L T . T S E N G , § A N DR A J E S H A . K H A T R I †

Center for Applied Energy Research, University of Kentucky,2540 Research Park Drive, Lexington, Kentucky 40511,Electron Microscopy Center, University of Kentucky,Lexington, Kentucky 40506, and Department of AnatomicalSciences and Neurobiology, University of Louisville, Louisville,Kentucky 40292

Received May 1, 2008. Revised manuscript received July18, 2008. Accepted August 19, 2008.

A combination of high-resolution transmission electronmicroscopy, scanning transmission electron microscopy, andelectron energy-loss spectroscopy (HRTEM-STEM-EELS)was used to study fly ashes produced from the combustion ofan eastern Kentucky coal at a southeastern-Kentucky wall-fired pulverized coal utility boiler. Fly ash was collected fromindividual hoppers in each row of the electrostatic precipitators(ESP) pollution-control system, with multiple hoppers sampledwithin each of the three rows. Temperatures within theESP array range from about 200 °C at the entry to the firstrow to <150 °C at the exit of the third row. HRTEM-STEM-EELS study demonstrated the presence of nanoscale (10 s nm)C agglomerates with typical soot-like appearance and otherswith graphitic fullerene-like nanocarbon structures. The minutecarbon agglomerates are typically juxtaposed and intergrownwith slightly larger aluminosilicate spheres and often forman ultrathin halo or deposit on the fly ash particles. The STEM-EELS analyses revealed that the nanocarbon agglomerateshost even finer (<3 nm) metal and metal oxide particles. Elementalanalysis indicated an association of Hg with the nanocarbon.Arsenic, Se, Pb, Co, and traces of Ti and Ba are often associatedwith Fe-rich particles within the nanocarbon deposits.

IntroductionHuman health impacts of combustion-derived nanomaterials(CDNs) released to the environment has gained worldwideinterest since such exposure and the incidence of health issuesappears to have increased dramatically within the pastcentury. Evaluation of human exposure and related envi-ronmental effects of nanosized particles from coal combus-tion sources (CDNs typically <100 nm in size) have focusedattention on airborne CDNs released either as coarse, fine,or ultrafine fragmentation mode during combustion (1, 2).

Airborne ultrafine fly ash (FA) particles have provided thebasis for potential evaluations toward nanotoxicology studies(3), and it was previously shown that ultrafine CDNs mighthave adverse toxicological effects unlike the larger aerosolparticles (4). The greater surface areas of ultrafine CDNscompared with larger particles with same chemistries makesthem more environmentally active toward biouptake andassociated health risks (1, 2). Nevertheless, the nature ofprimary ultrafine particles can be significantly different fromsimilarly sized fractions that were the result of continuousparticle fragmentation processes, and it is therefore importantto meticulously evaluate the source of ultrafine aerosolparticles. During the handling and/or other disturbance ofbulk FA, additional aerosols are repeatedly released as canbe observed by the build-up of grayish plumes above themain ash body. The presence of such plumes/aerosolssuggests that bulk FA harbors a vastly greater source ofultrafine material of interest to environmental characteriza-tion. The gray and often dark gray nature of such environ-mental aerosols derived from bituminous and subbituminousfly ashes is indicative of the magnitude of nanosized-carbonaccumulations, yet the quantitative determination of nano-sized carbons and related surface areas vs total fly ash carbonsis complicated since nanocarbons are heavily intergrownwith inorganic ash constituents and complete separation foranalytical purposes cannot be achieved. Relatively highconcentrations of nanocarbons are associated with theultrafine aerosol fractions, and furthermore, these could belinked to the highest toxicology levels (1, 4). From anenvironmental point of view, the continuous release of heavy-metal-host nanocarbons needs further clarification. Thepurpose of this paper is to observe the nature, nanostructure,and spatial relationships of ultrafine CDNs in bulk ash andits continuous release mechanisms which may have furtheradverse environmental impacts. Future studies will focus onidentifying processes that minimize human health impacts.Within coal combustion pollution-control systems, bothelectrostatic precipitators (ESP) and fabric filters (FF orbaghouse), there is a relation between the collection point(ESP or FF row) of the FA and the concentration of traceelements (5-17); with the concentration of volatile elements,such as Pb, As, and Se, generally increasing with a decreasein temperature at the collection point. In addition, coarserFA tends to be preferentially collected in the first row of anESP or FF, with finer, higher surface-area particles passingthrough to the back rows. The combination of decreasingtemperature and increasing surface area of FA particles leadsto enhanced concentrations of volatile trace elements. Inany case, however, the concentration of trace elements in

* Address correspondence to either author. Phone: 859-257-0261(J.C.H); 859-257-0299(U.M.G); E-mail: hower@caer.uky.edu(J.C.H.); graham@caer.uky.edu (U.M.G.).

† Center for Applied Energy Research.‡ Electron Microscopy Center.§ University of Louisville.

FIGURE 1. (a) HRTEM image of nanometer-sized C depositsintergrown with Si-Al glassy fly ash particles (dark spheres);(b) insert showing agglomerated nanocarbons with soot-likeappearance coating fly ash sphere in a porous shell ornanocoating.

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Published on Web 10/15/2008

the FA is strongly related to the concentration of elementsin the feed coal (5, 14, 15, 18-29).

Studies of Hg capture by FA have shown that Hg behaviordeviates from the pattern of other volatile trace ele-ments (22-24, 30-42), as follows (1) a positive correlationbetween Hg and FA C within the same row of an ESP or FF(therefore, at the same T) (7, 8); (2) an increase in Hg capturewith a decrease in flue gas T (14, 15, 26, 39, 40); and (3) arelationship between FA carbon type, BET surface area, andHg capture (41, 42). The latter relationship, particularly withrespect to the petrographically determined C forms, was notas strong as, for example, the correlation between Hg andFA C. Possible explanations include mixed C forms, subtletransitions between C forms, and blinding of the adsorptivesurfaces of the C. All are potentially valid explanations, butthey overlooked the possibility of the contribution of C belowthe optical limit of resolution (at 500× magnification, thepractical limit for valid identification is about 1 µm) ac-counting for the variation in Hg capture.

Shim et al. (43) and Veranth et al. (44) discussed theformation of soot in the combustion of tar-producingbituminous coals, with particular attention to the combustionconditions prevailing in low-NOx combustion. Chen etal. (45-47), Lu et al. (48), and Linak et al. (1) described thepresence of sooty C in the ultrafine FA fraction. Their highresolution transmission electron microscopy studies alsodemonstrated the presence of Si, Al, Ti, and Fe associatedwith the ultrafine C. Differing from this study, they did notidentify trace element associations in the inorganic particles.

In this study, we expand the study of FA C and theirassociation with heavy metals via the investigation of theultrafine fraction of ESP FA. We address (a) the spatial

relationship of ultrafine C with respect to the glassy ashspheres, (b) describe variations in the structural character-istics of the nanocarbons, and (c) analyze the compositionof nanosized metal inclusions.

Materials and MethodsFly ash samples for this study were selected from a setpreviously collected and analyzed (14). The FA’s representsamples from individual ESP hoppers from a southeasternKentucky wall-fired 220 MW utility boiler retrofitted for low-NOx combustion. For their investigation, a single-mine/single-seam high volatile A bituminous coal was burned inorder to study the connection between the coal geochemistryand the FA chemistry. The samples, with about 10% carbon,selected for this investigation were a subset from the secondand third ESP rows, of interest because of relatively high As(102-103 µg/g), Se (102 µg/g), and Hg (1-3 µg/g) contents.

High-resolution transmission electron microscopy (HR-TEM), scanning transmission electron microscopy (STEM),and electron energy-loss spectroscopy (EELS) investigationswere conducted in the University of Kentucky’s AdvancedScience and Technology Commercialization Center (ASTeCC)Electron Microscopy Center. Details of the equipment areprovided in the Supporting Information.

DiscussionThe HRTEM-STEM-EELS study demonstrated the presenceof nanoscale (10s nm) C deposits juxtaposed and overgrownwith slightly larger aluminosilicate (Al-Si) glassy spheres(Figure 1). These nanocarbon agglomerates form an ultrathinhalo or shell-like deposit on the coarser inorganic FA. The

FIGURE 2. HRTEM image of fullerene-like nanocarbon with concentric ring-structure. (a) shows the fine soot-like or carbonblack-like nanostructure at the core of the agglomerate with fullerene particles (arrows) attached to the surface; (b) showsindividual fullerene particles at the surface of the nanocarbon particle (see arrows); (c) fullerene-like nanocarbon with multipleshells; (d) fullerene-like nanocarbon with double shells.

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C shells or nanocoatings are porous and consist of ag-glomerated nanometer-sized soot particles with characteristicconcentric-onion ring structures. A majority of Al-Si glassyspheres have a C-based nanocoating or at least some fractionof the surface coated. Since the thickness of the C nanocoatingvaries among different FA spheres and can be irregular onthe surface of individual spheres, such C deposits appear toobstruct the smoothness and, hence, fluidity of the ashparticles. The delicate porous appearance of the C depositson the Si-Al glassy particles suggests that fractions of the Cshells could become liberated during ash transport from theESP, affecting the size of the nanocarbon fraction, causingseparation of the C from the bulk FA, and contributingnanocarbons as an atmospheric aerosol.

Among the nanocarbon agglomerates on Al-Si spheres,HRTEM identified a fraction showing crystalline and sem-icrystalline graphitic structures (Figure 2), confirmed bymeasurements of the characteristic d-spacings betweenindividual graphitic layers (distance ∼ 3.3 Å). The largerspherical agglomerates exhibit structures resembling carbonblack samples at the core with concentric aligned graphenelayers, but are characterized by the additional presence oftethered fullerenes at the outer rim (indicated by arrows inFigure 2a and b). The ultrafine structure of the C shown in

Figure 2 may help shed some light on the origin of thenanocarbon particles, but, in addition to the measurementsof characteristic parameters including interplanar spacingsfrom the particle observed in HRTEM, one has to usetechniques involving the application of computational imageanalysis to single soot particles (49). It is suggested to performsuch an analysis at a later stage. Previously, the micro andnanostructures of sooty C from diesel engines have beenlinked to a structure-determining reactivity toward oxidation(50) Furthermore, the reactivity of the nanocarbons wassuggested to play an important role in health effects triggeredby combustion-derived nanocarbon particles (51).

The current HRTEM investigation revealed abundantfullerene concentration over the nanocarbon agglomeratesurfaces, not surprising as the C was formed in a combustionenvironment. Figure 2b suggests that individual fullerenemolecules appear to range in size as typically observed insamples produced from a fullerene-forming flame (52). Inaddition, some nanocarbons in the current study exhibit afullerene-like structure over larger areas, as shown in Figure2c and d, suggesting that the growth mechanisms of thenanocarbons involved fullerene formation. The typicalmorphology of fullerene-like soot in Figure 2d highlights thechain-like assembly of primary fullerene particles, often

FIGURE 3. HRTEM images of C-rich nanoclusters. Large dark round bodies in a and b and at extreme upper right of c are Si-Alglass fly ash particles. Few-nm dark spots in d are metal grains.

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exhibiting multiple shell structures (Figure 2c) that areinterlinked and result in the appearance of strongly bentgraphene sheets or ribbons and double shells as shown inFigure 2d. The size of the shells may be related to the sizeof the nuclei (53), and the variation in the structure in Figure2b suggests multiple nuclei that grow and allow for theformation of fullerene-like nanocarbon agglomerates (Figure2c). The key parameters required for fullerene synthesis onthe FA C may depend in part on T and concentrations of H2Ovapor and CO2. Nasibulin et al. (54) observed that the latterparameters noticeably affect the amount and size of fullerenesformed under experimental conditions on C supports.

The nanocarbon particles and individual fullerene mol-ecules are below the optical limit of resolution and theirgraphitic nature would not have been resolved via previousSEM investigations. The scale of the particles identified inHRTEM is shown on Figure 3. Very-fine ordered structuresin the C can be observed in Figure 3c and d, magnifying theC-rich nanocluster outlined in Figure 3a and b. This is theC previously unaccounted for in our optical microscopicinvestigations and it can be traced as an ultrathin halo aroundthe majority of Si-Al glass FA spheres. The HRTEM view, aswith optical reflected-light microscopy, is a cross-section;the carbon would actually be a nanocoating on the grain.This is not a quantitative technique, so although we can veri-fy the presence of the C, we cannot determine the amountof the total FA C in this phase. The Si-Al particles (Figure3), while appearing large compared to the nanocarbons,would actually be at the optical limit of resolution. Figure 3ddemonstrates that the fullerene-like C agglomerates host <3nm metal particles, possibly the locus of trace elementsassociated with the ultrafine C.

Differences between the imaging capabilities of HRTEMand STEM are illustrated in the comparison of Figures 1, 2,and 3 with Figure 4. HRTEM (Figures 1, 2, 3, and 4b) providesa superior resolution of the C structure, whereas STEM (Figure

4a and c) emphasizes the difference between the low-atomic-number C (gray shades) and the inorganic entities in the FA,either the Si-Al glassy spherical substrate or the metal-richnanosized inclusions (white areas). The advantage of bothapplications is illustrated side-by-side in Figure 4a (STEM)and 4b (TEM). The STEM image in Figure 4a clearly revealsnanosized metal inclusions contained by nanocarbons (seearrows), whereas the TEM image (Figure 4b) of the sameagglomerate reveals the soot-like nanocarbon structure, butdoes not expose the metal inclusions.

The EELS spectra of one selected (<3 nm) metal inclusionin the nanocarbon agglomerate illustrated by the STEM imagein Figure 4c (see arrow) is represented in Figure 4d, indicatingthat the nano metal particle is Fe-rich. The results are basedon the presence of the FeL3 (715 eV) and FeL2 (720 eV) peaksin the EELS spectra. The significantly larger FeL3 peakcompared with FeL2 in Figure 4d suggests that the Fe-richinclusion may contain both ferric and ferrous iron, suggestingit is a Fe3O4 particle or other spinel.

The STEM images in Figure 5a and b, the latter amagnification of the boxed-in area in 5a, show in light shadesthe presence of heavy elements and in light gray thecarbonaceous host material, whereas the Si-Al glassy spheresappear white. Figure 5 shows two additional high-resolutionSTEM images of C nanoclusters (the boxed-in areas in Figure5a and b are represented by the respective spectra in Figure5c and d). The presence of Hg, arguably at higher concen-trations than in the areas illustrated on Figure S1 (below),can be seen in the corresponding X-ray scans. The volatilityof Hg prevented precise determination of the site, but thepresence of Hg within the fine C, whether or not in the Fe-inclusions, is confirmed by this analysis.

The delicate nature of the C deposited on the Si-Al glasswas confirmed by the observed damage to the structure bythe electron beam. This suggests that normal processes, suchas the transport of the FA in the turbulent flue gas, could

FIGURE 4. STEM image of wispy nanocarbon (A), with included metal grains, on round Al-Si glass fly ash sphere. Matching HRTEMimage is shown on (B). The Al-Si particle would be near the limit of resolution with optical microscopy. Background shows lacy Csupport. (C) is an enlargement of a portion of (A) with EELS indication of Fe in the metal inclusion (D).

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potentially separate the fullerene-like C, with the includedtrace elements, from the Si-Al glass. If this happens in orbefore the electrostatic precipitators (ESP), the relativelycoarser glass will be captured by the ESP (and ESP’s routinelycapture over 99% of the FA (55)) and the finer C could escapeinto the atmosphere if the flue gas is not subject to flue gasdesulfurization (FGD) removal of SO2, a process which shouldalso remove most fine FA particles. At the time of this writing,the power plant in question, as with many older utility boilers,did not have FGD. As FGD is one of the methods shown tobe effective for the removal of oxidized Hg, more FGDinstallations will be constructed over the next decade asutilities comply with U.S. Environmental Protection Agency(EPA) Hg-reduction rules (56, 57). However, while utilitycompliance with the U.S. EPA Clean Air Interstate Rule (56)is underway, the nature of U.S. mercury regulations are influx following the February 8, 2008 vacating of the U.S. EPAClean Air Mercury Rule by the District of Columbia CircuitCourt (57).

AcknowledgmentsThis work was first presented at the 2005 InternationalPittsburgh Coal Conference. We are grateful for the con-structive comments from that presentation, as well as for thecontributions of the editor and two anonymous reviewers ofthis manuscript.

Supporting Information AvailableAnalytical equipment details. This material is available freeof charge via the Internet at http://pubs.acs.org.

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