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Physical and Chemical Characterization of Fly Ashes from SwissWaste Incineration Plants and Determination of the Ash Fraction inthe Nanometer RangeJelena Buha,*,†,‡ Nicole Mueller,§ Bernd Nowack,§ Andrea Ulrich,† Sabrina Losert,† and Jing Wang*,†,‡

†Analytical Chemistry Laboratory, Empa - Swiss Federal Laboratories for Materials Science and Technology, 8600 Dubendorf, Zurich,Switzerland‡Institute of Environmental Engineering, ETH Zurich, 8093 Zurich, Zurich, Switzerland§Technology and Society Laboratory, Empa - Swiss Federal Laboratories for Materials Science and Technology, 9014 St. Gallen, St.Gallen, Switzerland

*S Supporting Information

ABSTRACT: Waste incineration had been identified as animportant source of ultrafine air pollutants resulting inelaborated treatment systems for exhaust air. Nowadays,these systems are able to remove almost all ultrafine particles.However, the fate of ultrafine particles caught in the filters hasreceived little attention so far. Based on the use of engineerednano-objects (ENO) and their transfer into the waste stream,it can be expected that not only combustion generatednanoparticles are found in fly ashes but that many ENO finallyend up in this matrix. A more detailed characterization of thenanoparticulate fraction of fly ashes is therefore needed.Physical and chemical characterizations were performed for flyashes from five selected waste incineration plants (WIPs) withdifferent input materials such as municipal waste, wood and sewage sludge. The intrinsic densities of the fly ashes were in therange of 2.7−3.2 g/cm3. When the fly ash particle became airborne, the effective density depended on the particle size, increasingfrom 0.7−0.8 g/cm3 for 100−150 nm to 2 g/cm3 for 350−500 nm. The fly ash samples were fractionated at 2 μm, yielding finefractions (<2 μm) and coarse fractions (>2 μm). The size distributions of the fine fractions in the airborne form were furthercharacterized, which allowed calculation of the percentage of the fly ash particles below 100 nm. We found the highest mass-based percentage was about 0.07%; the number percentage in the fine fraction was in the range of 4.8% to 22%. Comparison withmodeling results showed that ENO may constitute a considerable part of the fly ash particles below 100 nm. Chemical analysesshowed that for the municipal waste samples Ca and Al were present in higher concentrations in the coarse fraction; for themixed wood and sludge sample the P concentration was higher in the coarse fraction; for most other samples and elements theywere enriched in the fine fraction. Electron microscopic images of fly ashes showed a wide range of particle sizes, from nanometerrange to micrometer range. Many aggregated particles were observed, demonstrating that ENO, bulk-derived nano-objects andcombustion-generated nano-objects can form aggregates in the incineration process.

1. INTRODUCTION

Nanotechnology has gained growing interest not only inresearch and development but also increasing attention fromregulatory authorities.1,2 Nanoparticle applications are sus-pected to cause consequences to human health and theenvironment as already reported.1,2 Concerns have been raisedon the potential toxicity of such tiny particles as the damagingeffects of exposure to unintentionally produced ultrafineparticles (e.g., by combustion processes) have been proven.3−5

The EU recommendation6 states that “Nanomaterial” means anatural, incidental, or manufactured material containingparticles, in an unbound state or as an aggregate or as anagglomerate and where, for 50% or more of the particles in thenumber size distribution, one or more external dimensions is in

the size range 1 nm-100 nm. “Nano-objects” which defineparticles, plates, or fibers with at least one external dimensionbetween 1 and 100 nm7 are building blocks of nanomaterials.The term “engineered nano-objects” (ENO) restricts the nano-objects to intentionally produced materials. The occupationalrisks of engineered materials that consist of nano-objects suchas nanoparticles, nanofibers, nanotubes, and nanowires, as wellas aggregates and agglomerates of these materials have attractedincreasing attention.8 Release studies9−14 and analyses

Received: October 25, 2013Revised: March 24, 2014Accepted: April 10, 2014Published: April 10, 2014

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especially in complex matrices or the environment arechallenging.15 Studies focusing on end-of-life treatment ofnanomaterials, i.e. waste management,16−18 treatment likeincineration,19,20 deposition on landfills or recycling possibil-ities are still scarce.The modeling of ENO in the environment21−23 revealed a

significant flow of ENO to landfills either via waste and sludgeincineration and the subsequent deposition of bottom and flyash, or via direct dumping of construction waste. Based on thevery high cleaning efficiency of modern filter systems in WIPsthese ENO eventually end up in bottom or fly ash and hence inlandfills.23

ENO constitute not necessarily the major fraction ofnanomaterial entry into landfills. Two other categories ofnanosized objects are bulk-derived nano-objects and combus-tion-generated nano-objects.24 Bulk-derived-NO are defined asthe nanosized fraction of bulk materials (e.g., pigments),25

while combustion-generated-NO are nanosized particlesunintentionally produced in incineration processes.26,27 It hasbeen shown that the size distributions as well as the chemicalcomposition of combustion residues can vary significantlydepending on the input material.28−31 In the past, wasteincineration processes had been identified as an importantsource of ultrafine air pollutants, whereas today the filtersystems in WIPs are able to remove almost all ultrafineparticles.32 A field study has shown that also engineerednanoparticles (with nano-CeO2 as example) are transferredalmost quantitatively into bottom and fly ash and that theconcentration in exhaust air is extremely low.20 In most casesthe exhaust air released from incineration plants into theenvironment easily meets the standards for air quality. Based onthe development in the past 15−20 years, it is not surprisingthat, thus far, research on waste incineration has focused on theemission of ultrafine particles and other pollutants into the air.However, almost no studies are available investigating sizedistribution and particle number concentration of ultrafineparticles caught in the filters.Fly ashes are byproducts of the combustion of pulverized

coal in electric power generating plants and waste incinerationprocesses.33 Some studies concerning the chemical compositionof fly ashes from power plants can be found in the literature. In2003 the chemical composition of different coal fly ashes fromfour thermoelectric power stations in Spain was analyzed.34 Theelemental composition in fly ashes from thermal power plantsin Argentina was also determined.35 In addition, the samegroup also studied particle sizes using a laser based particle sizeanalyzer. Different methods for digestion of fly ashes were alsopreviously tested.36 The general impression from all the resultspublished so far is that the chemical composition differsstrongly depending on the type of material which is burned andalso on the digestion method. According to the aboveobservation, the properties of the fly ashes from incinerationplants may be significantly different from those in power plants.To our best knowledge, the nanoparticulate fraction andchemical composition of fly ashes from incineration plants havenot been systematically studied.The aim of the present study is therefore to characterize the

physical and chemical properties of fly ashes from incinerationplants. We measured the intrinsic density of fly ash samples, inaddition, the particle size and effective density in the airborneform, because part of the fly ash can become airborne duringcleaning for bag-house-filter and electrostatic precipitator,37,38

during collection and dumping, or by wind after entering the

landfill. Airborne particles have the highest mobility, highpossibility for transport among different environment compart-ments,39 and high exposure risks for human.40 Knowing the sizeand density of airborne fly ash particles helps to understandtheir transport properties and exposure risks for the environ-ment and human. A key feature of the present study isdetermination of the fraction smaller than 100 nm (nano-fraction) in the fly ash samples, both in terms of number andmass. We also compared the measured nanofraction with themodeled concentration of engineered nano-TiO2 and nano-ZnO in fly ashes. We fractionated fly ash samples into fine (<2μm) and coarse (>2 μm) particles for elemental analysis. Inview of the higher mobility of fine particles and their strongerassociation with possible health impact,41 such results areneeded in order to fully assess the possible human health risksfrom fly ashes.

2. MATERIALS AND METHODS2.1. Filter Ash Samples. Samples from five different

municipal waste, wood, and sludge incineration plants inSwitzerland were collected. Fly ashes were sampled in onewaste incineration plant (Wa), one combined waste-sludgeincineration plant (WaS), one sludge incineration plant (S),one wood incineration plant (Wo), and one combined wood-sludge incineration plant (WoS). Samples were taken daily overa period of 1 week and then mixed to account for the highheterogeneity of the input materials. The sampling procedurewas differed from plant to plant. In some plants direct samplingfrom the fly ash storage bag/tank was possible. In plants withclosed systems, premixed samples had to be taken from thetruck that picked up the fly ashes. The information on theincineration plants, including input material, plant size, andfiltration system is given Table S1 in the SupportingInformation (SI).

2.2. Particle Size Distribution and Density Measure-ments. After the collection, the samples were fractionated at 2μm in a multiplex laboratory Zigzag classifier, model 100 MZR(Alpine corporation, Germany), capable of size classificationbetween 2 and 80 μm based on the centrifugal counter flow, thedensity of the material, and its aerodynamic diameter. The massof all fly ash samples was measured before and after thefractionation. Based on these measurements sample loss as wellas the mass percentage of both fractions was calculated. Thesamples above and below 2 μm are referred to as the coarse andfine fractions, respectively.The fine fractions were aerosolized in a powder dispersion

chamber (model RBG 1000, PALAS, Germany). The generatorincludes a particle reservoir. A piston pushed the particles into arotating brush, and a stream of air is injected to disperse theminto an aerosol. The adjustable parameters are the brushrotation speed, the piston speed, and the air flow rate. Eachexperiment was conducted following the same procedure. Afterchecking the background number concentrations, the materialof interest was placed in the disperser, and with the controlledair flow (compressed air, pressure at 1 bar, which led to 1.25m3/h flow rate), as well as the brush speed (940 rpm) anddispersion speed (50 mm/h), the size distribution from thechamber was measured by the aerodynamic particle sizer (APS,model 3321, TSI, USA) and scanning mobility particle sizer(SMPS, TSI, USA).Due to the high concentrations a diluter was used to control

the flow; the dilution ratio was changed to obtain the mostreliable results and to stay within the detection limits for both

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of the instruments. The best results of the total concentrationof the aerosolized samples were obtained when the dilutionratio was 3:1 in case of SMPS and 10:1 for the APSmeasurements.In the size distribution measurement using SMPS, the

particles are represented by their equivalent electrical mobilitydiameter. SMPS typically provides size distribution curves inthe range below 1 μm. APS provides particle size distributionbased on aerodynamic particle diameter, typically in the rangeof 0.5−20 μm. Since SMPS and APS are based on differentworking principles, the obtained data had to be merged bycalculations that took into account the fundamental physicalprinciples.42 The detailed method for merging the SMPS andAPS data is documented in the SI. The merged size distributionwas used to calculate respective number and mass percentagesof the different size fractions.For the purpose of intrinsic density measurement two steps

were used: a balance (Mettler Toledo AG, Switzerland) tomeasure the mass of a fly ash and a gas displacementpycnometry system (Micromeritics, AccuPyc II 1340) tomeasure the true volume of the solid particles in the powder.Helium is used as the displacement medium because of itsinertness and small atomic number which helps the penetrationinto the small pores of the powder. Finally, the intrinsic densitywas calculated based on the mass and the true volume of thepowder.In order to determine the effective density of aerosolized fly

ash particles, they were first classified according to theirmobility diameter using a differential mobility analyzer (DMA,model 3081, TSI, USA) and then according to their mass tocharge ratio using a Couette centrifugal particle mass analyzer(CPMA, Cambustion, UK).43,44 The DMA-CPMA tandemprovided both the particle size and mass, then the effectivedensity was computed by the mass divided by the effectivevolume, which is the volume of a sphere with the mobilitydiameter. The resultant effective density is a function of theparticle size. An example of the CPMA-scan data is given inFigure S1 in the SI.Our experimental setup allowed us to measure the number-

size distribution directly without assuming the shape of theparticle size distribution, as in the dynamic light scatteringmethod. Our method had a high degree of sizing accuracy andmeasurement repeatability with broad size and concentrationranges. It further allowed determination of the percentage ofparticles below 100 nm, which was one of the main interests ofthis study.2.3. Chemical Analysis. For total elemental analysis of the

fly ashes, acid microwave digestion with subsequent ICP-OES(inductively coupled plasma optical emission spectrometry)analyses were applied. The analyses were carried out on aVarian Vista Pro CCP Simultaneous ICP-OES (Varian AG,Zug, Switzerland) using an external calibration adjusted againstY as internal standard. Detailed description of the analysis canbe found in the SI.2.4. Electron Microscopic Studies of the Fly Ash.

Imaging was carried out using SEM (Nova NanoSEM 230, FEI,Hillsboro, OR) and TEM (JEM-2200FS, JEOL, Japan).Aerosolized particles were collected on a silicon wafer cube/copper grid covered by an amorphous carbon film (for SEM/TEM, respectively) with a nanometer aerosol sampler (model3089, TSI, USA).

3. RESULTS3.1. Density Measurements. Density is an important

property of fly ashes. It is also necessary for the mergingprocedure of SMPS spectrum to its counterpart APS spectrum(see the SI). The densities of all the samples were above theunit density as presented in Table 1 (details for the density

calculations available in the SI, Table S2). The intrinsicdensities of the Wa, WaS, Wo, and WoS samples were close,distributed in a narrow range from 2.72 to 2.80 g/cm3 (Table1). The intrinsic density of the S sample was considerablyhigher, at 3.23 g/cm3. The result suggested higher concen-trations of heavy elements in the S sample compared to theother samples.The effective densities of the five samples at different

mobility sizes are shown in Figure 1. There was a general

increase of effective density with the mobility diameter for allfly ash samples. When the particle mobility size was 100−150nm, the effective density was about 0.7−0.8 g/cm3; when thesize was 350−500 nm, the effective density was close to orabove 2 g/cm3. All the measured effective densities were lowerthan the intrinsic densities which were in the range of 2.7−3.2g/cm3 (Table 1). The intrinsic density was obtained using thetrue volume of the solid particles, whereas the effective densitywas computed using the volume of an equivalent sphere, forwhich the particle porosity plays a role. Thus, lower values wereobserved for the effective density.

3.2. Size Distribution Measurements and Determi-nation of the Nanofraction. SMPS and APS measurementswere performed for the fine fractionation for all of the fly ashsamples. Size distribution results from SMPS and APS arepresented in Figures 2a and 2b, respectively. The APS results

Table 1. Summary of the Intrinsic and Effective Densities forthe Fly Ash Samplesa

sample name intrinsic density (g/cm3) effective density (g/cm3)

WaS 2.75 2.01Wa 2.78 2.32Wo 2.80 2.08WoS 2.72 1.94S 3.23 1.97

aThe effective densities listed here are for particles with mobilitydiameter of 400 nm.

Figure 1. The effective density of the aerosolized samples at differentmobility diameters.



presented here are already converted to the size distributionwith respect to the mobility diameter using the methoddescribed in the SI. One or two peaks under 650 nm appearedwhen taking SMPS results into account; typically a minor peakat around 100 nm and the main size distribution peak at around400 nm. APS measurements identified the largest particlefraction below 1 μm; the APS results after the conversion didnot provide additional peaks. For all the samples, there werealmost no particles above 2.2 μm because of the fractionation.The combined size distributions for the WaS sample with theSMPS and APS results are shown in Figure S2 in the SI.Smooth transition was obtained not only for the number sizedistribution but also for the surface area and volume sizedistributions.The results from the fractionation experiments, size

distribution measurements, and final calculations regardingthe mass and number percentage for all of the fly ashes arepresented in Table 2. Mass percentage calculations were basedon the mass percentage calculated before and after thefractionation process and the results from merged SMPS andAPS spectra. Number percentage results were based on themerged SMPS and APS data only, because the number ofparticles larger than 2 μm was not determined in thefractionation process. As a result, the number percentagerepresents the portion of the particles smaller than 100 nm inthe fine fraction.

3.3. Elemental Analysis. An overview of the detectedelements in the five fly ash samples are graphically presented inFigures 3a and 3b for the fine and coarse fractions, respectively.

The data are also given in Table S3 in the SI. In addition to theelements presented, Ag, Ni, and Tl were also measured.However, the concentrations were below the detection limit ofthe instrument, thus the results are not shown.The samples from the WoS incineration plant were taken

every day during 1 week. Fly ashes taken on different daysshowed no significant change in the chemical composition.Results for the coarse and fine fractions of the mixed sample arepresented in Table S3. A more detailed table, including thesamples taken on each day of the week, can be found in TableS4 in the SI. For all the other fly ashes only mixed samples wereanalyzed.

Figure 2. (a) Number size distribution from the SMPS for all the flyashes. The size range covered 0.016−0.66 μm; (b) Number sizedistribution from the APS after conversion to the mobility size for allthe fly ashes. The mobility size range covered approximately 0.6−15μm.

Table 2. Summary of Mass and Number (no.) Percentage forAll of the Fly Ash Samplesa

mass %

samplename CF FF FF*

mass % <100 nm inthe FF

mass % <100nm in the WS

no. % <100 nm inthe FF

WaS 88 7.5 8.5 0.042 3.55 × 10−3 11Wa 70 17 25 0.103 2.56 × 10−2 17Wo 56 32 36 0.127 7.12 × 10−2 22WoS 98 0.94 0.96 0.010 8.84 × 10−5 4.8S 86 8.5 9.9 0.037 3.64 × 10−3 13

aResults are based on the mass percentage calculated after thefractionation and the merged SMPS and APS spectra (fine fraction:FF; coarse fraction: CF; whole sample: WS). The FF* value wasestimated taking in consideration the particles smaller than 2 μm in thelost sample, which was deduced from the mass values of the fine andcoarse fractions and the whole sample.

Figure 3. Elemental concentrations of fine fraction (a) and coarsefraction (b) of the fly ash samples in weight percentage as determinedby ICP-OES measurements.



The results from ICP-OES measurements illustrated thatthere were differences in the chemical composition of the fineand coarse fractions. Figure 4a shows the elemental

concentration difference between the two fractions (concen-tration of the coarse fraction−concentration of the finefraction). The same data are used in Figure 4b, but the relativedifference normalized by the concentration in the coarsefraction is shown. These differences depend on the element andthe origin of the input materials. For the municipal wastesamples Ca and Al were present in higher concentrations in thecoarse fraction; for the WoS sample the P concentration washigher in the coarse fraction; for most other samples andelements they were enriched in the fine fraction. This was mostpronounced for Na, Pb, and Zn in the municipal waste ash andfor Na, S, and Zn in the wood ash. The WoS and S samples hadless enrichment in the fine fraction.3.4. Electron Microscopy and EDX Measurements.

Microscopy analyses of the fly ash samples were performed inorder to get information on the primary particle size, particlemorphology, and aggregation status. EDX (energy dispersive X-ray) analysis was performed to obtain information on thespatially resolved chemical composition, and the results werefurther compared to those from ICP-OES.

Analysis was performed for all of the fly ash samples, but onlyresults from three representative samples are presented here.SEM, TEM, and EDX results for Wo (Figure 5a, b, c), S(Figure 5d, e, f), and Wa (Figure 5g, h, i) samples are shown.Agglomerates with hundreds of nanometers to several

micrometers in size were observed in the fly ashes. The SEMimages showed that the smaller particles were agglomerateswith relatively open structures and small primary particles (20to hundreds nm), thus higher porosity and lower effectivedensity; the larger particles had quite compact structures withlarge cores and some small primary particles attached to thesurface, thus lower porosity and higher effective density. This isconsistent with the effective density results shown in Figure 1.In both the Wo and S samples we observe highly

agglomerated particles and some small scattered primaryparticles. The Wo sample is rich in K, Ca, Zn, and Mg. Onthe other hand the sludge sample shows Fe, Si, Ca, and P as themain components present. The last sample shown is the Wasample, for which the particle size variation seems to be morepronounced and the large particles in the SEM image havemore compact structures than the other two samples. In theTEM image of the Wa sample (Figure 5h), spherical particlesclose to the nanometer range were observed. They might beengineered nanoparticles; however, EDX analysis did notprovide a distinctive elemental composition for confirmation.The particles were presumably coated with amorphous C

(dark film-like structure especially in Figure 5h) originatingfrom incineration processes and the EDX analysis demon-strated higher number of detected elements in the Wa samplethan in the other two samples. Cu detected in all of the samplescomes from the TEM copper grid and thus cannot bequantified. When comparing the EDX results with the resultsfrom the ICP-OES measurements we observe that for the Wosample Ca and Zn were detected in high concentrations withboth methods. For the S sample, Ca and P were detectedinstead. EDX analysis pointed out a high number of elementsfor Wa samples as expected due to heterogeneous inputmaterials. This is also confirmed by the ICP-OES measure-ments shown in the previous section. Higher intensity of Feobserved in the S sample may be related to its higher intrinsicdensity than the other samples. Unfortunately Fe was notmeasured in the ICP-OES analysis and the weight percentage ofFe was not available.

4. DISCUSSIONIn this work different fly ashes of waste incineration plantsreceiving various waste materials were characterized. Fly andbottom ashes of coal and waste incineration have already beenstudied to a great extent. However, when a fine and coarsefraction was investigated in the past, the “fine fraction” wasrepresenting material in the μm-range. The current discussionon the human health and ecotoxicological effects of engineerednanomaterials also demands additional re-evaluations ofmaterials that contain an incidental nanosized fraction such asfly ashes. This information is needed to allow a comparison ofthe possible risks from the incidental nanosized fraction thatwas always present and the additional possible risks fromengineered materials. Our work provides a first evaluation ofthe nanoparticulate fraction of waste incineration residues. Inmany countries all or at least a significant fraction of solid wasteis burned in waste incinerations plants, and modeling hasshown that the flow of engineered nanomaterials through thissystem is one of the most important of all nanomaterial flows.22

Figure 4. (a) The elemental concentration difference between thecoarse and fine fractions. (b) The elemental concentration differencenormalized by the concentration in the coarse fraction.



The mass percentage of the fine fraction of the fly ashesvaried significantly among the different samples. The reasonmay be that the incineration conditions in the furnaces weredifferent and the input materials affected the size of formed flyash particles. In the WoS sample the share of fine fraction wasless than 1%, whereas in the Wo sample the portion of the finefraction was almost one-third. This particularly low masspercentage in the WoS sample might be related to the differentfilter type. Majority of the waste incineration plants in thisstudy used electrostatic precipitators (ESP) (Table S1 in SI),while the WoS plant had bag house filters (fabric filter)installed. Bag house filters lead to the formation of filter cakeson the fabric which gives rise to an increased pressure on thefilter. This pressure favors particle aggregation. Hence, theparticles caught in the filter cake (cleaned by back pulse) aremore likely to be in an aggregated status and in the larger sizefraction.The percentage of the particles below 100 nm was low in the

fly ash. The highest mass-based percentage was about 0.07%,and the lowest was about 0.00009% for the WoS sample. The

number-based percentage was much higher than the mass-based percentage, which was confirmed by comparison of themass and number percentages of particles <100 nm in the finefraction. The number percentage was in the range of 4.8% to22%; these values would be lower if the whole sample wasconsidered. However, even with the number-based evaluationin the fine fraction the fly ashes fall below the 50% threshold fornanomaterials proposed by the EU.6

The particles below 100 nm include bulk-derived nano-objects, combustion-generated nano-objects, and possibly alsoengineered nano-objects (ENO) surviving the incinerationprocess. ENO made of high-boiling-point materials, such asTiO2, could survive the incineration process and remain asnano-objects in the fly ash23 as has been shown for CeO2−nanoparticles.20 Sun et al.45 modeled the concentration ofengineered nanomaterials (ENM) in filter ash and bottom ashfrom WIPs. The predicted nano-TiO2 concentration in filterash is 280 mg/kg, for nano-ZnO 4.1 mg/kg. These materialsmay make up part of the nanosized fraction measured in thiswork. Nano-TiO2, the ENM with the highest concentration,

Figure 5. (a), (d) and (g) SEM images; (b), (e) and (h) TEM images; (c), (f) and (i) EDX spectra of Wo, S and Wa samples, respectively.



could therefore be responsible for up to 0.028 wt % of the filterash, which is within the range of the measured nanofraction.However, the SEM/TEM analyses clearly showed that theparticles were extensively aggregated in the fly ashes. Theseaggregates were not broken up during the aerosolization usedin this study and therefore not measured as particles below 100nm. Further characterization of the nanosized fraction istherefore needed to clarify the potential impact that ENOcurrently have in this matrix. The measurements performed inthis work indicate which amount of nanoparticles can beproduced from the fly ashes by relatively gentle forces that arerepresentative for handling the ashes.Quantitative measurement of the particle size distribution

from microscopic images faces the difficulties of identificationof the individual and overlapping particles, identification of therepresentative size of irregular-shape particles, laborious processof analyzing large amounts of samples, etc. The measurementswe have performed targeted airborne fly ash particles, whichprovided the relevant sizes in the context of collection,handling, and disposal of fly ashes. The dispersion process,based on a brush feeder and compressed air, is comparable tothe processes such as pulse cleaning for filtration systems,dumping in collection trucks, or resuspension by wind afterentering the landfill. These processes may only break looselybound agglomerates but not tightly bound aggregates.46

The composition of the fly ashes differed strongly dependingon the type of the waste processed. Sludge and woodincineration plants produce only fly ash and no bottom ash,whereas in municipal solid waste incineration the majority ofthe residues are in the bottom ash that was not characterized inthis work. In bag house filters, additives (e.g., Ca(OH)2,NaHCO3, active carbon) may be added for the precipitation ofvolatile acids or gaseous components like dioxins or furans aswell as heavy metals.30 The concentrations of certain elementsdue to added additives may be significantly higher than those infly ash particles. Municipal waste incineration ashes showhigher values for sodium, lead, and especially zinc than thewood and sludge ashes. Ashes from wood and sludgeincineration show high concentrations of calcium andphosphorus. Depending on the elements, they could be eitherenriched in the fine fraction or in the coarse fraction. Pb, Zn,and Na had higher concentrations in the fine fraction, whereasAl and Ca was differently distributed depending on the sample,with the municipal waste ashes having more in the coarsefraction and the sludge sample more in the fine fraction. Thisreflects both the different origins of the raw material as well asthe different incineration processes, e.g. whether both bottomand fly ash is produced or just fly ash as in the wood and sludgeincineration. Overall the results further show that moreelements were enriched in the fine fraction than in the coarsefraction.The distributions of elements between various residues from

incineration have been investigated before, mainly with the aimto analyze distribution between bottom ash and fly ash. Querolet al.47 distinguished elements with volatile behavior that(partially) condense in flue gases and thus enrich in fly ash(e.g., As, B, Bi, Pb, S, Cd), elements that are concentrated inthe slag (e.g., C, Fe and elements with iron oxide affinity suchas Cu, or Mn) and elements that show no fractionationbetween fly ash and slag (e.g., Al, Ca, Na, Mg, Li, Cr, Co, Ni,Zn). According to Querol et al. elements with calcium oxide-sulfate affinities enrich in fly ash. Thipse et al.48 analyzed sizefractions of fly ash samples from a lab scale municipal waste

incinerator and found larger concentrations of Cr, Ni, and Fe incoarse particles (up to 1 mm), while Al and Si were moreconcentrated in the fine fraction (≤75 μm). Pb and Hg showedthe highest concentration in the 150−300 μm fraction. Chen etal.49 observed the copper speciation in a Taiwanese WIP andfound that about 24% of the copper in the fly ash wasnanosized. Dahl et al.50 found more than 90% of the massloadings of heavy metals from wood incineration in the finestfraction of fly ash (<74 μm), whereas in the bottom ash 84−92% were found in the fraction between 0.5 and 2 mm. Zincwas found to be dominant in the finest fly ash fraction with 500mg Zn per kg of fly ash. Whereas these studies show that somedata are available about size fractionated analysis if a “fine”fraction, this fraction is often quite large when looked at in thecontext of the nanomaterial discussion. Nonetheless, the resultsare in general in agreement with our results and show that theenrichment in μm-sized fractions is also reflected whencomparing <2 μm fraction to the larger fraction of the flyashes. However, only a separate analysis of the <0.1 μm fractioncan finally give information about the nanorelevance of thismaterial.

■ ASSOCIATED CONTENT*S Supporting InformationInformation of the incineration plants from which the fly ashsamples were taken (Table S1), example of the data foreffective density measurement (Figure S1), results of theintrinsic and effective density measurement (Table S2), methodfor combination of the data from the SMPS and APS, methodand data for the chemical analyses of fly ash samples (TablesS3, S4). This material is available free of charge via the Internetat http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*Phone: +41 58 765 6136. E-mail: [email protected].*E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Swiss Federal Office for theEnvironment (FOEN).

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