Article pubs.acs.org/est
Physical and Chemical Characterization of Fly Ashes from Swiss Waste Incineration Plants and Determination of the Ash Fraction in the Nanometer Range Jelena 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 Dübendorf, 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 an important source of ultrafine air pollutants resulting in elaborated 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 has received little attention so far. Based on the use of engineered nano-objects (ENO) and their transfer into the waste stream, it can be expected that not only combustion generated nanoparticles are found in fly ashes but that many ENO finally end up in this matrix. A more detailed characterization of the nanoparticulate fraction of fly ashes is therefore needed. Physical and chemical characterizations were performed for fly ashes from five selected waste incineration plants (WIPs) with different input materials such as municipal waste, wood and sewage sludge. The intrinsic densities of the fly ashes were in the range of 2.7−3.2 g/cm3. When the fly ash particle became airborne, the effective density depended on the particle size, increasing from 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 fine fractions (2 μm). The size distributions of the fine fractions in the airborne form were further characterized, which allowed calculation of the percentage of the fly ash particles below 100 nm. We found the highest massbased percentage was about 0.07%; the number percentage in the fine fraction was in the range of 4.8% to 22%. Comparison with modeling results showed that ENO may constitute a considerable part of the fly ash particles below 100 nm. Chemical analyses showed that for the municipal waste samples Ca and Al were present in higher concentrations in the coarse fraction; for the mixed wood and sludge sample the P concentration was higher in the coarse fraction; for most other samples and elements they were enriched in the fine fraction. Electron microscopic images of fly ashes showed a wide range of particle sizes, from nanometer range to micrometer range. Many aggregated particles were observed, demonstrating that ENO, bulk-derived nano-objects and combustion-generated nano-objects can form aggregates in the incineration process. the size range 1 nm-100 nm. “Nano-objects” which define particles, plates, or fibers with at least one external dimension between 1 and 100 nm7 are building blocks of nanomaterials. The term “engineered nano-objects” (ENO) restricts the nanoobjects to intentionally produced materials. The occupational risks of engineered materials that consist of nano-objects such as nanoparticles, nanofibers, nanotubes, and nanowires, as well as aggregates and agglomerates of these materials have attracted increasing attention. 8 Release studies9−14 and analyses
1. INTRODUCTION Nanotechnology has gained growing interest not only in research and development but also increasing attention from regulatory authorities.1,2 Nanoparticle applications are suspected to cause consequences to human health and the environment as already reported.1,2 Concerns have been raised on the potential toxicity of such tiny particles as the damaging effects of exposure to unintentionally produced ultrafine particles (e.g., by combustion processes) have been proven.3−5 The EU recommendation6 states that “Nanomaterial” means a natural, incidental, or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in © 2014 American Chemical Society
Received: Revised: Accepted: Published: 4765
October 25, 2013 March 24, 2014 April 10, 2014 April 10, 2014 dx.doi.org/10.1021/es4047582 | Environ. Sci. Technol. 2014, 48, 4765−4773
Environmental Science & Technology
Article
landfill. Airborne particles have the highest mobility, high possibility for transport among different environment compartments,39 and high exposure risks for human.40 Knowing the size and density of airborne fly ash particles helps to understand their transport properties and exposure risks for the environment and human. A key feature of the present study is determination of the fraction smaller than 100 nm (nanofraction) in the fly ash samples, both in terms of number and mass. We also compared the measured nanofraction with the modeled concentration of engineered nano-TiO2 and nanoZnO in fly ashes. We fractionated fly ash samples into fine (2 μm) particles for elemental analysis. In view of the higher mobility of fine particles and their stronger association with possible health impact,41 such results are needed in order to fully assess the possible human health risks from fly ashes.
especially in complex matrices or the environment are challenging.15 Studies focusing on end-of-life treatment of nanomaterials, i.e. waste management,16−18 treatment like incineration,19,20 deposition on landfills or recycling possibilities are still scarce. The modeling of ENO in the environment21−23 revealed a significant flow of ENO to landfills either via waste and sludge incineration and the subsequent deposition of bottom and fly ash, or via direct dumping of construction waste. Based on the very high cleaning efficiency of modern filter systems in WIPs these ENO eventually end up in bottom or fly ash and hence in landfills.23 ENO constitute not necessarily the major fraction of nanomaterial entry into landfills. Two other categories of nanosized objects are bulk-derived nano-objects and combustion-generated nano-objects.24 Bulk-derived-NO are defined as the nanosized fraction of bulk materials (e.g., pigments),25 while combustion-generated-NO are nanosized particles unintentionally produced in incineration processes.26,27 It has been shown that the size distributions as well as the chemical composition of combustion residues can vary significantly depending on the input material.28−31 In the past, waste incineration processes had been identified as an important source of ultrafine air pollutants, whereas today the filter systems in WIPs are able to remove almost all ultrafine particles.32 A field study has shown that also engineered nanoparticles (with nano-CeO2 as example) are transferred almost quantitatively into bottom and fly ash and that the concentration in exhaust air is extremely low.20 In most cases the exhaust air released from incineration plants into the environment easily meets the standards for air quality. Based on the development in the past 15−20 years, it is not surprising that, thus far, research on waste incineration has focused on the emission of ultrafine particles and other pollutants into the air. However, almost no studies are available investigating size distribution and particle number concentration of ultrafine particles caught in the filters. Fly ashes are byproducts of the combustion of pulverized coal in electric power generating plants and waste incineration processes.33 Some studies concerning the chemical composition of fly ashes from power plants can be found in the literature. In 2003 the chemical composition of different coal fly ashes from four thermoelectric power stations in Spain was analyzed.34 The elemental composition in fly ashes from thermal power plants in Argentina was also determined.35 In addition, the same group also studied particle sizes using a laser based particle size analyzer. Different methods for digestion of fly ashes were also previously tested.36 The general impression from all the results published so far is that the chemical composition differs strongly depending on the type of material which is burned and also on the digestion method. According to the above observation, the properties of the fly ashes from incineration plants may be significantly different from those in power plants. To our best knowledge, the nanoparticulate fraction and chemical composition of fly ashes from incineration plants have not been systematically studied. The aim of the present study is therefore to characterize the physical and chemical properties of fly ashes from incineration plants. We measured the intrinsic density of fly ash samples, in addition, the particle size and effective density in the airborne form, because part of the fly ash can become airborne during cleaning for bag-house-filter and electrostatic precipitator,37,38 during collection and dumping, or by wind after entering the
2. MATERIALS AND METHODS 2.1. Filter Ash Samples. Samples from five different municipal waste, wood, and sludge incineration plants in Switzerland were collected. Fly ashes were sampled in one waste incineration plant (Wa), one combined waste-sludge incineration plant (WaS), one sludge incineration plant (S), one wood incineration plant (Wo), and one combined woodsludge incineration plant (WoS). Samples were taken daily over a period of 1 week and then mixed to account for the high heterogeneity of the input materials. The sampling procedure was differed from plant to plant. In some plants direct sampling from the fly ash storage bag/tank was possible. In plants with closed systems, premixed samples had to be taken from the truck that picked up the fly ashes. The information on the incineration plants, including input material, plant size, and filtration system is given Table S1 in the Supporting Information (SI). 2.2. Particle Size Distribution and Density Measurements. After the collection, the samples were fractionated at 2 μm in a multiplex laboratory Zigzag classifier, model 100 MZR (Alpine corporation, Germany), capable of size classification between 2 and 80 μm based on the centrifugal counter flow, the density of the material, and its aerodynamic diameter. The mass of all fly ash samples was measured before and after the fractionation. Based on these measurements sample loss as well as the mass percentage of both fractions was calculated. The samples above and below 2 μm are referred to as the coarse and fine fractions, respectively. The fine fractions were aerosolized in a powder dispersion chamber (model RBG 1000, PALAS, Germany). The generator includes a particle reservoir. A piston pushed the particles into a rotating brush, and a stream of air is injected to disperse them into an aerosol. The adjustable parameters are the brush rotation speed, the piston speed, and the air flow rate. Each experiment was conducted following the same procedure. After checking the background number concentrations, the material of interest was placed in the disperser, and with the controlled air flow (compressed air, pressure at 1 bar, which led to 1.25 m3/h flow rate), as well as the brush speed (940 rpm) and dispersion speed (50 mm/h), the size distribution from the chamber 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 most reliable results and to stay within the detection limits for both 4766
dx.doi.org/10.1021/es4047582 | Environ. Sci. Technol. 2014, 48, 4765−4773
Environmental Science & Technology
Article
3. RESULTS 3.1. Density Measurements. Density is an important property of fly ashes. It is also necessary for the merging procedure of SMPS spectrum to its counterpart APS spectrum (see the SI). The densities of all the samples were above the unit density as presented in Table 1 (details for the density
of the instruments. The best results of the total concentration of the aerosolized samples were obtained when the dilution ratio was 3:1 in case of SMPS and 10:1 for the APS measurements. In the size distribution measurement using SMPS, the particles are represented by their equivalent electrical mobility diameter. SMPS typically provides size distribution curves in the range below 1 μm. APS provides particle size distribution based on aerodynamic particle diameter, typically in the range of 0.5−20 μm. Since SMPS and APS are based on different working principles, the obtained data had to be merged by calculations that took into account the fundamental physical principles.42 The detailed method for merging the SMPS and APS data is documented in the SI. The merged size distribution was used to calculate respective number and mass percentages of the different size fractions. For the purpose of intrinsic density measurement two steps were used: a balance (Mettler Toledo AG, Switzerland) to measure the mass of a fly ash and a gas displacement pycnometry system (Micromeritics, AccuPyc II 1340) to measure the true volume of the solid particles in the powder. Helium is used as the displacement medium because of its inertness and small atomic number which helps the penetration into the small pores of the powder. Finally, the intrinsic density was calculated based on the mass and the true volume of the powder. In order to determine the effective density of aerosolized fly ash particles, they were first classified according to their mobility diameter using a differential mobility analyzer (DMA, model 3081, TSI, USA) and then according to their mass to charge ratio using a Couette centrifugal particle mass analyzer (CPMA, Cambustion, UK).43,44 The DMA-CPMA tandem provided both the particle size and mass, then the effective density was computed by the mass divided by the effective volume, which is the volume of a sphere with the mobility diameter. The resultant effective density is a function of the particle size. An example of the CPMA-scan data is given in Figure S1 in the SI. Our experimental setup allowed us to measure the numbersize distribution directly without assuming the shape of the particle size distribution, as in the dynamic light scattering method. Our method had a high degree of sizing accuracy and measurement repeatability with broad size and concentration ranges. It further allowed determination of the percentage of particles below 100 nm, which was one of the main interests of this 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 a Varian Vista Pro CCP Simultaneous ICP-OES (Varian AG, Zug, Switzerland) using an external calibration adjusted against Y as internal standard. Detailed description of the analysis can be 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 (model 3089, TSI, USA).
Table 1. Summary of the Intrinsic and Effective Densities for the Fly Ash Samplesa sample name
intrinsic density (g/cm3)
effective density (g/cm3)
WaS Wa Wo WoS S
2.75 2.78 2.80 2.72 3.23
2.01 2.32 2.08 1.94 1.97
a
The effective densities listed here are for particles with mobility diameter of 400 nm.
calculations available in the SI, Table S2). The intrinsic densities of the Wa, WaS, Wo, and WoS samples were close, distributed in a narrow range from 2.72 to 2.80 g/cm3 (Table 1). The intrinsic density of the S sample was considerably higher, at 3.23 g/cm3. The result suggested higher concentrations of heavy elements in the S sample compared to the other samples. The effective densities of the five samples at different mobility sizes are shown in Figure 1. There was a general
Figure 1. The effective density of the aerosolized samples at different mobility diameters.
increase of effective density with the mobility diameter for all fly ash samples. When the particle mobility size was 100−150 nm, the effective density was about 0.7−0.8 g/cm3; when the size was 350−500 nm, the effective density was close to or above 2 g/cm3. All the measured effective densities were lower than the intrinsic densities which were in the range of 2.7−3.2 g/cm3 (Table 1). The intrinsic density was obtained using the true volume of the solid particles, whereas the effective density was computed using the volume of an equivalent sphere, for which the particle porosity plays a role. Thus, lower values were observed for the effective density. 3.2. Size Distribution Measurements and Determination of the Nanofraction. SMPS and APS measurements were performed for the fine fractionation for all of the fly ash samples. Size distribution results from SMPS and APS are presented in Figures 2a and 2b, respectively. The APS results 4767
dx.doi.org/10.1021/es4047582 | Environ. Sci. Technol. 2014, 48, 4765−4773
Environmental Science & Technology
Article
Table 2. Summary of Mass and Number (no.) Percentage for All of the Fly Ash Samplesa mass % sample name
CF
FF
FF*
mass % < 100 nm in the FF
WaS Wa Wo WoS S
88 70 56 98 86
7.5 17 32 0.94 8.5
8.5 25 36 0.96 9.9
0.042 0.103 0.127 0.010 0.037
mass %
