Zinc Isotopic Composition of Particulate Matter Generated during the

Nov 3, 2010 - Zn isotopes are fractionated during heating through evaporation (-3, 6) and ... production, tire-tread material has a Zn content of abou...
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Environ. Sci. Technol. 2010, 44, 9219–9224

Zinc Isotopic Composition of Particulate Matter Generated during the Combustion of Coal and Coal + Tire-Derived Fuels ´ ,‡ D A V I D M . B O R R O K , * ,† R E T O G I E R E MINGHUA REN,† AND EDWARD R. LANDA§ Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968, United States; Geowissenschaften, Albert-Ludwigs-Universita¨t, D-79104, Freiburg, Germany; and U.S. Geological Survey, MS 430, 12201 Sunrise Valley Drive, Reston, Virginia 20192, United States

Received July 18, 2010. Revised manuscript received September 30, 2010. Accepted October 13, 2010.

Atmospheric Zn emissions from the burning of coal and tirederived fuel (TDF) for power generation can be considerable. In an effort to lay the foundation for tracking these contributions, we evaluated the Zn isotopes of coal, a mixture of 95 wt % coal + 5 wt % TDF, and the particulate matter (PM) derived from their combustion in a power-generating plant. The average Zn concentrations and δ66Zn were 36 mg/kg and 183 mg/kg and +0.24‰ and +0.13‰ for the coal and coal + TDF, respectively. The δ66Zn of the PM sequestered in the cyclone-type mechanical separator was the lightest measured, -0.48‰ for coal and -0.81‰ for coal+TDF. The δ66Zn of the PM from the electrostatic precipitator showed a slight enrichment in the heavier Zn isotopes relative to the starting material. PM collected from the stack had the heaviest δ66Zn in the system, +0.63‰ and +0.50‰ for the coal and coal + TDF, respectively. Initial fractionation during the generation of a Zn-rich vapor is followed by temperature-dependent fractionation as Zn condenses onto the PM. The isotopic changes of the two fuel types are similar, suggesting that their inherent chemical differences have only a secondary impact on the isotopic fractionation process.

Introduction Atmospheric Zn contamination is derived in part from the burning of hydrocarbon-based fuels such as coal and tires (1, 2). The isotopic ratios of Zn, reported in parts per thousand relative to the Johnson Matthey Catalysts (JMC) Zn metal standard batch 3-0749 L (δ66Zn), have recently been used in an attempt to “fingerprint” and track atmospheric contaminant sources (e.g., refs 3-7). This technique is promising because the δ66Zn of nonbiologic natural materials cluster over a narrow range of ∼0.1‰ to 0.5‰, whereas anthropogenic processes like high-temperature combustion extensively fractionate Zn isotopes (8). Zn isotopes are fractionated during heating through evaporation (3, 6) and * Corresponding author phone: (915) 747-5850; fax: (915) 7475073; e-mail: [email protected]. † Department of Geological Sciences, University of Texas at El Paso. ‡ Geowissenschaften, Albert-Ludwigs-Universita¨t. § U.S. Geological Survey, MS 430. 10.1021/es102439g

 2010 American Chemical Society

Published on Web 11/03/2010

again as Zn-rich vapors condense to form solids (9). The mechanisms that underpin these processes have not been evaluated experimentally, excepting some initial efforts using a Cu-Zn alloy (10). However, numerous field investigations have empirically demonstrated that lighter Zn isotopes are preferentially incorporated into the vapor phase during waste burning (5), automotive combustion (11), and smelting (3, 4, 6, 12), whereas residual slag and ash material accumulates the heavier Zn isotopes. The δ66Zn of PM in these investigations varied substantially but was as light as -1.0‰ (11). Similarly, Zn condensate has been shown to preferentially incorporate the heavier Zn isotopes and the magnitude of the isotopic fractionation is temperature-dependent (9). In order for Zn isotopes to become an effective tool for studying atmospheric contaminants, more comprehensive investigations that relate Zn isotopic fractionation to industrial processing are necessary. For example, Mattielli and others (3) measured the δ66Zn of particulate matter (PM) at various stages and temperature regimes inside a Zn-Pb refinery, as well as the δ66Zn of atmospheric particles at distances of up to 1.5 km from the plant. Atmospheric PM had a considerably lighter δ66Zn relative to the original ore material. It is unclear, however, whether these results are typical for other high-temperature combustion processes, as the fuel types, temperature regimes, and degrees of sequestration of Zn are process-dependent. In this work, we investigated the δ66Zn of coal, a mixture of ∼95 wt % coal + ∼5 wt % tire-derived fuel (TDF), and the PM derived from their combustion in a power-generating plant. Because ZnO is used as a vulcanizing agent in rubber production, tire-tread material has a Zn content of about 1 wt % (13). Except for a period of approximately stable usage figures for the years 1996-2001 (at about 115 million tires/ yr), TDF usage in the U.S. has grown steadily from 25 million tires in 1990 to 155.1 million tires (2 145 000 tons) in 2005 (14). Hence, TDF may contribute appreciably to global atmospheric emissions of Zn. The δ66Zn of the atmospheric flux from power plants and other industries that burn TDFs (e.g., cement kilns and pulp/paper manufacturing) is currently unknown.

Materials and Methods Fuels. The coal was a high-volatile C bituminous coal from southern Indiana, USA, and the TDF was shredded tire material obtained from Entech, Inc. (Michigan, USA). The sizes, moisture, and ash contents, and bulk chemical compositions of these materials were presented previously (2, 15). The Zn concentrations of the digested fuel separates specific to this investigation are presented in Table 1. Power Plant and Sample Collection. Samples were collected during short-term testing operations at the Walter Wade Utility Plant located at Purdue University, Indiana, USA, during April 2001. The operations and sampling protocols were described previously (15), and are summarized in Figure 1. Incineration of the fuels took place in a Detroit RotoGrate stoker boiler system at ∼1500 °C. Fuel was consumed at a rate of ∼9.0 t/h. During the incineration process PM was sampled from the bottom ash (BA), the mechanical separator (MS), the electrostatic precipitator (EP), and the exhaust stack (Stack). BA samples consisted of PM that dropped through a grate below the boiler where it had cooled to approximately 300 °C prior to collection. The vapor and PM generated during incineration first pass through the cyclone-type MS (280 °C) followed by the EP unit (180 °C) before being emitted through the Stack (180 °C; Figure 1). VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Zn Concentrations and Isotopic Compositions of Fuels, PM, and the IRMM-3702 Zn Standarda sample

TDF fuel coal sample 1 coal sample 2 coal average coal+5%TDF 4/27/01 coal+5% TDF 4/27/01 Ave coal+5%TDF 4/30/01

method Hot plate + Teflon bomb bomb microwave Teflon bomb Teflon bomb Teflon bomb Teflon bomb Teflon bomb

b

coal+5%TDF 4/30/01 Ave coal BA

coal BA average coal MS coal EP coal EP average coal stack filter coal+5%TDF BA coal+5%TDF BA average coal+5%TDF MS coal+5%TDF MS average coal+5%TDF EP coal+5%TDF stack filter coal+5%TDF stack filter ave IRMM-3702 STD

Teflon bomb hot plate hot plate Teflon bomb Teflon bomb hot plate Teflon bomb Teflon bomb fusion hot plate Teflon bomb hot plate Teflon bomb Hot plate + Teflon bomb Teflon bomb Teflon bomb hot plate

Zn recovered

δ66Zn

(mg/kg)

(‰)

13 742 50 22 25 32 259 210 235 133 141 137 46 72 70 63 106 3072 3197 3135 NM 203 232 203 213 4266 4479 4000 4248 60313 NM NM NM NA

0.14 0.18 0.20 0.34 0.24 0.12 0.09 0.11 0.18 0.09 0.13 0.27 0.34 0.31 0.31 -0.48 0.31 0.42 0.36 0.63 -0.62 -0.62 -0.57 -0.60 -0.79 -0.85 -0.79 -0.81 0.20 0.46 0.53 0.50 0.27

2σ

n

0.16 0.08 0.11 0.07 0.15 0.06 0.21 0.21 0.01 0.07 0.07 0.13 0.15 0.06 0.21 0.13 0.18 0.14 0.23 0.04 0.11 0.19 0.02 0.22 0.06 0.09 0.13 0.17 0.08 0.12 0.08 0.15 0.07

5 5 5 3 13 3 3 6 3 3 6 5 2 3 10 5 5 7 12 3 3 4 2 9 2 3 3 8 3 3 3 6 24

a

The averaged δ66Zn for fuels and PM used in this investigation are set in bold font. When multiple digestions were considered, the averaged 2σ uncertainty was calculated as the square root of the sum of the squares of the individual 2σ’s for each digestion. NM ) not measured; NA ) not applicable. b The 4/30/01 coal+5%TDF sample (not the 4/27/01 sample) was used for comparisons in this study because the PM samples for coal+5%TDF burning were also collected on 4/30/01.

The MS and EP units are pollution controls that are typical of many fuel-burning plants and they sequester the bulk of the PM (i.e., fly ash) generated. Sample Preparation. Bulk chemical analyses of the coal, TDF, coal+5%TDF, and most of the PM samples have been reported previously (2, 15); however, for this study it was necessary to digest new sample splits and to analyze them for their bulk chemical concentrations prior to isotopic analysis. Because these samples are highly refractory, we evaluated the effectiveness of multiple digestion methodologies and their impact on the measured δ66Zn of the samples. Table SI-1 of the Supporting Information (SI) summarizes the four digestion methods employed. Isotopic Analysis. Sampleswerepreparedforisotopicanalysis usingananion-exchangecolumnchromatographytechnique(16). Columnseparatesandblankswerecheckedforrecoveryandpurity using an Inductively Coupled Plasma-Optical Emission Spectrometer. Zinc isotopes were analyzed at the University of Texas at El Paso using a Nu Instruments multicollector ICP-mass spectrometer. Samples were introduced using a desolvating nebulizersystem(NuDSN100)andthestandard-sample-standard bracketing method was used to correct for mass bias (16). The isotopes 64Zn, 66Zn, 67Zn, and 68Zn were measured simultaneously and were referenced to JMC 3-0749-L. The IRMM-3702 Zn standard (also passed through the column chemistry) was measured during each analytical session to evaluate long-term precision. Zinc isotopic ratios are expressed in standard δ notation relative to the average value of the respective bracketing standards as shown in eq 1 9220

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δ66Zn )

[

(66Zn/64Zn)sample (66Zn/64Zn)JMC ave

]

- 1 × 1000

(1)

The 2σ external precision associated with 24 analyses of the IRMM-3702 standard over 12 analytical sessions spanning more than 3 months was 0.07‰. However, the 2σ uncertainty reported herein is typically larger because it includes the uncertainty associated with multiple sample preparations of the refractory PM (see Table 1). Three isotope plots of the data in relationship to the theoretical equilibrium and kinetic mass-dependent fractionation laws (17) are presented in Figure SI-1 of the SI.

Results and Discussion Metal Enrichment in PM Samples. Elemental enrichment factors were calculated for the PM collected from the BA, MS, EP, and Stack by comparing the metal/Al ratios of the PM samples to those of the fuel sources according to eq 2: Ej )

[

(Me/Al)j (Me/Al)initial

]

(2)

where E is the enrichment factor; (Me/Al) is the metal/Al ratio of the sample of interest; j ) BA, MS, EP, or Stack; and (Me/Al)initial is the metal/Al ratio of the fuel prior to combustion. Aluminum is used for the normalization process because it is one of the most refractory elements in the PM. Figure 2 presents E values for Zn, Pb, and Fe for the combustion of

FIGURE 2. Enrichment factors (see eq 2) for Zn, Pb, and Fe for PM collected from the bottom ash, mechanical separator, electrostatic precipitator, and stack filter for (a) coal and (b) coal+5%TDF. The shaded regions represent depletion. The analytical uncertainty associated with the enrichment factors is (10%.

FIGURE 1. Schematic diagram of the power plant. coal and coal+5%TDF. The latter two metals are plotted for comparative purposes because Pb, like Zn, is known to volatilize during high-temperature combustion (18, 19) in contrast to Fe (20). On average the coal and coal+5%TDF respectively contained 12 mg/kg and 20 mg/kg Pb, 0.8 wt % and 1.2 wt % Fe, and 1.2 wt % and 1.4 wt % Al (15). The metal enrichment trends are similar for the coal (Figure 2a) and coal+5%TDF (Figure 2b) in that Zn and Pb are considerably depleted in the BA and become progressively enriched in the MS and EP. Additional enrichment of Zn and Pb in the Stack PM is only important for the coal+5%TDF. Because the chlorine content of the TDF (2940 mg/kg) is greater than that for coal (215 mg/kg), a greater amount of gaseous Pb- and Zn-chloride species were formed during combustion of the coal+5%TDF (15). This resulted in an increased rate and amount of metal volatilization. Because gaseous Zn- and Pb-chloride species are stable from high temperatures (e.g., 1500 °C) to around 300 °C for Zn and 350 °C for Pb (20), the chlorine added by the TDF may have additionally stabilized these metals in the vapor phase. The enrichment trends (for both fuel types) demonstrate that Zn and Pb were volatilized during combustion but later condensed onto PM as temperature decreased. The fact that Fe is not depleted in the BA and shows no enrichment in the MS, EP, and Stack PM adds strength to this argument. Zn Isotopes in PM. The δ66Zn of the IRMM-3702 standard (+0.27‰ ( 0.07‰; Table 1) is comparable to that reported by Cloquet and others (8) of +0.32‰ ( 0.16‰. Figure 3

FIGURE 3. Average δ66Zn and 2σ uncertainty (including data from multiple digestions of the refractory PM; see Table 1) for coal, coal+5% TDF, and the PM collected from the bottom ash, mechanical separator, electrostatic precipitator, and stack filter. presents the average δ66Zn values for the fuels and PM. Excepting the δ66Zn of the BA, the magnitude of the isotopic changes for the PM generated from the coal and coal+5%TDF were similar (within analytical uncertainty) throughout the power-generation cycle (Figure 3). The δ66Zn of the PM increased as it moved through the various collection points and entered the exhaust stack. The MS samples had the lightest δ66Zn, 0.72‰ and 0.94‰ lighter than the δ66Zn of the coal and coal+5%TDF samples, respectively (Table 1, Figure 3). The EP samples had a δ66Zn that was similar to that of the fuels, only 0.12‰ and 0.07‰ heavier than those of the coal and coal+5%TDF, respectively. The PM samples from the stack had the heaviest δ66Zn, which were 0.39‰ and 0.37‰ heavier than those of the starting coal and coal+5%TDF, respectively. The discrepancy between the δ66Zn of the BA samples for coal and coal+5%TDF is likely a result of adsorption or condensation of a small amount of Zn onto the BA during the combustion of the coal+5%TDF. The coal+5%TDF contains more than four times as much VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Rayleigh distillation behavior for Zn isotopes during evaporation plotted at an example rfuel/vapor of 1.0010. As Zn vapor is formed during combustion, the δ66Zn of the ash and vapor evolve along pathways A and B, respectively. Pathway C is the cumulative δ66Zn of the vapor phase, which is the running sum of the δ66Zn of pathway B (23). Zn and much greater quantities of chlorine than the coal (Table 1). This leads to larger quantities of Zn-rich vapor generation during combustion (see previous discussion of gaseous metal-chlorides). The adsorption of Zn onto BA particles could be driven by the greater amount of Zn-rich vapor, the increased rate of its release, and/or the increase in Zn vapor pressure. Because the BA is severely depleted in Zn relative to the fuel sources (Figure 2 a), small amount of Zn vapor captured with the BA could drastically alter the δ66Zn. The fact that the δ66Zn of the coal+5%TDF BA is similar to that of the MS supports the interpretation that the Zn isotopes of both samples reflect the addition of lighter Zn isotopes from the vapor phase. The similarity of the δ66Zn of the PM derived from both fuels (Figure 3) suggests that the isotopic fractionation process for Zn during high-temperature combustion is not primarily dependent upon the form of Zn in the fuel. However, the form of Zn does impact the degree of volatilization (e.g., ref 21). Zinc in tires is present as a mixture of ZnO, Zn stearate, and ZnS (22), whereas Zn in coal is associated with organic matter (23), and can be found as an impurity in pyrite (FeS2) or as sphalerite (ZnS (24)). Moreover, the δ66Zn of the PM from both fuels is similar despite differences in the postcombustion mineralogy of Zn associated with the PM. For example, gunningite (ZnSO4 · H2O) is a mineral unique to the PM generated from burning coal+5%TDF (25). Because this mineralogy is a reflection of the chemical form of Zn in the vapor phase, it may suggest that vapor composition is not a first-order control on the fractionation of Zn isotopes during evaporation or condensation. Fractionation of Zn Isotopes during High-Temperature Combustion. When Zn-rich material is combusted at high temperatures, the Zn evaporates and the lighter Zn isotopes preferentially enter the vapor phase and the heavier Zn isotopes accumulate in the residual spent material or ash. This fractionation behavior can be illustrated using a Rayleigh distillation model (Figure 4). As the fuel is progressively combusted to produce increasing amounts of Zn vapor, the δ66Zn of the ash and vapor evolve along pathways A and B, respectively. Pathway C is the cumulative δ66Zn of the vapor phase, which is the running sum of the δ66Zn of pathway B. The mathematical relations that describe these pathways (26) have been used previously to describe Zn isotope fractionation behavior for smelting (3, 6). During smelting, ∼98% of the Zn from sulfide ore is volatilized. The Zn vapor is cumulatively recovered (i.e., pathway C in Figure 4) to produce Zn metal with an isotopic composition nearly identical to that of the ore (4, 12). During this process a small fraction of the Zn vapor may escape, which is reflected in the light δ66Zn measured for vapor-based emissions in previous studies (3). Mattielli and others (3) proposed a fractionation 9222

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FIGURE 5. Conceptual model for Zn isotopic fractionation during combustion and condensation inside the power plant. The cumulative δ66Zn of the stack emissions is determined by the relative proportions and δ66Zn of Zn condensate, Zn vapor, and fly ash. factor (Rore/vapor ) ((66/64)Znore)/((66/64)Znvapor)) of 1.0004 to 1.0008 for low- and high-temperature smelting processes, respectively. Sivry and others (6) proposed an Rslag/vapor of 1.0002 to 1.0004 based on δ66Zn of slag and estimated vapor recovery. Although our investigation may not be directly comparable to those for smelting because of the inherent differences in materials and processing, we can similarly estimate a minimum fractionation factor for Zn evaporation during fuel burning. The phrase “minimum fractionation factor” reflects the fact that we are neglecting the impact of condensation; at high temperature the fractionation factor for condensation is small (9). At 50% Zn vapor generation along pathway C (Figure 4), Rfuel/vapor values of 1.0007 and 1.0012 are necessary to achieve the δ66Zn of -0.48‰ and -0.81‰ measured for the coal and coal+5%TDF MS samples, respectively. The 50% value was chosen because the isotopic composition of the MS is a reflection of the integration of the entire evaporation process (0 to 100%). This simplistic conceptual model (Figure 4) becomes more complex for power generation because Zn isotopes are additionally fractionated through condensation reactions. The 1500 °C temperature of the Zn vapor evolved during combustion changes rapidly to 280 °C in the MS and to 180 °C in the EP and Stack (Figure 1). These temperature shifts facilitate the condensation of Zn from the vapor phase onto the fly ash particles, enriching them in Zn (Figure 2). Toutain and others (9) investigated Zn isotopic fractionation during condensation of Zn from volcanic fumarolic gases and found that the heavier Zn isotopes are preferentially incorporated into the condensate whereas the lighter isotopes remain in the vapor. They proposed a temperature-dependent fractionation factor (Rcondensate/vapor) for this reaction that changes from 1.0001 during initial condensation at high temperatures (∼800 °C) to 1.0017 at 280 °C and 1.0029 at 180 °C (9). Note that these values are used only to illustrate the paradigm, as it is unclear how differences between natural fumarolic gases and gases generated from fuel burning might impact fractionation factors for Zn condensation. Figure 5 is a conceptual illustration of how the δ66Zn of the condensate becomes enriched in the heavier Zn isotopes and drives the δ66Zn of the remaining Zn in the vapor phase to lighter values. Our data fit this conceptual model in that the lightest δ66Zn is found in the MS where limited condensation occurs at the highest temperatures (∼800 °C to 280 °C). Because the Rcondensate/vapor at high temperatures is probably small, the condensed Zn largely retains the light δ66Zn of the vapor phase that was acquired during evaporation. The δ66Zn of the PM in the EP and Stack, however, is heavier than that of the starting fuel (Figure 5). This is likely attributable to the

fact that the Rcondensate/vapor becomes large at these lower temperatures. The isotopic data in this study plot within error of both the theoretical kinetic and equilibrium mass-dependent fractionation lines (Figure SI-1 of the SI). Hence, without experimental testing, it will likely remain unclear as to what degree evaporation and condensation of Zn are governed by equilibrium or kinetic fractionation reactions. Toutain and others (9) suggest that Zn is fractionated via a temperaturedependent equilibrium mechanism during condensation. The available data fit well an equilibrium model rooted in vibrational spectroscopy theory (27) in that the heavier Zn isotopes are incorporated into the stiffer bonding environment of the condensate and the magnitude of the fractionation decreases with increasing temperature. This model can be used to explain the changes in δ66Zn within the power plant. However, we cannot rule out the possibility that rapid condensation (or direct adsorption of Zn from the vapor phase) could induce kinetic isotopic fractionation. In this case, the patterns in Figure 5 would hold, but the underlying processes controlling the δ66Zn of the PM and vapor would be more complicated. Evaporation reactions involve the opposite behavior in that the lighter Zn isotopes preferentially associate with the less stiff bonds of the gas phase. However, the temperature-dependence of isotopic fractionation is likely complicated by changes in the degree of volatilization (e.g., ref 3). δ66Zn of Stack Emissions. The δ66Zn of the bulk stack emissions can be described by the following equation 3: δ66Znstack ) δ66ZnPM(fPM) + δ66Znvapor(fvapor)

(3)

where δ66Znvapor and fPM are the isotopic signature and fraction of Zn in the stack PM, respectively, and δ66Znvapor and fvapor are the isotopic signature and fraction of Zn in the vapor, respectively. Our measurements of the PM on the stack filter provide data for δ66ZnPM, but not for δ66Znvapor and therefore not forδ66Znstack. We can use the following isotopic massbalance equation to estimate δ66Znstack as well as to illustrate the factors that most impact it: δ66Znstack ) [δ66Znfuel - (δ66ZnBA(fBA) + δ66ZnMS(fMS) + δ66ZnEP(fEP))] fstack (4) Here the δ66Znfuel, δ66ZnBA, δ66ZnMS, and δ66ZnEP represent the Zn isotopic compositions of the fuel and the PM collected in the BA, MS, and EP, respectively. ffuel, fBA, fMS, fEP, and fstack are the fractions of Zn that correspond to each of these individual reservoirs. We assume the fractions sum to 1. On the basis of the ∼10-fold depletions in the Zn/Al ratio in the BA, which accumulates below the boiler after undergoing combustion (Figure 2), we estimate that ∼90% of the Zn was volatilized. As a starting point we choose a δ66ZnBA that is 0.1‰ heavier than that of the fuel source, similar to the BA for coal burning (the BA for coal+5%TDF was likely impacted by interaction with Zn vapor; see above). Since the average δ66Zn for the fuel, MS, and EP are measured (Figure 3), we can calculate the average δ66Znstack given the fraction of Zn in the MS, EP, and Stack reservoirs. Our mass-balance calculation reflects the averaged or composite behavior specific to this system, as the instantaneous behavior of the system is governed by the fraction of Zn evaporated at any given time (Figure 4). Figure 6 illustrates the changes in δ66Znstack for coal burning (Figure 6a) and coal+5%TDF burning (Figure 6b) as a function of the fraction of Zn in the EP relative to the fraction of Zn in the MS (i.e., fEP/fMS) at stack emissions (i.e., fstack) of 0.1, 0.2, 0.4, and 0.6. Note that

FIGURE 6. Changes in δ66Znstack for (a) coal burning and (b) coal+5%TDF burning as a function of the fraction of Zn sequestered in the EP relative to that in the MS (fEP/fMS) when the fraction of stack emissions (i.e., fstack) is 0.1, 0.2, 0.4, and 0.6. the initial δ66Zn of the fuel has been normalized to 0‰ for this illustration. Figure 6 demonstrates that the δ66Znstack is positive relative to the δ66Zn of the fuel at fEP:fMSratios of less than ∼7:1 for coal and ∼10:1 for coal+5%TDF, but becomes negative at higher fEP:fMSratios. The location of these inflection points, where δ66Zn changes from positive to negative, is a reflection of the balance between the lighter Zn isotopes sequestered in the MS and the heavier Zn isotopes sequestered in the EP. The magnitude of the change in δ66Znstack is controlled by the relative proportion of Zn leaving the stack. As fstack increases there is less influence on the δ66Zn of material sequestered in the MS and EP reservoirs and δ66Znstack evolves toward the δ66Zn of the fuel. As the amount of Zn left behind in the used fuel increases (i.e., less efficient volatilization), the inflection point shifts further to the left (to lower fEP:fMS) because the used fuel reservoir contains heavier Zn isotopes. For situations with no pollution control mechanisms, eq 4 simplifies to eq 5: δ66Znstack )

[δ66Znfuel - (δ66ZnBA(fBA))] fstack

(5)

Here fstack ) 1 - fBA. Because δ66ZnBA is always greater than the δ66Znfuel for evaporation processes (with the exception of systems where the fuel is impacted by back-reaction with Zn vapor, see earlier discussion), all emissions should be lighter than the fuel and the δ66Znstack is dependent only upon the fraction of Zn evaporated and the fractionation factor for the reaction (i.e., Figure 4). Admittedly, this is a simple restatement of a rather an intuitive result, but one that is useful for comparison to the complexities that occur when Zn of differing δ66Zn is sequestered prior to being released to the atmosphere. Our best estimate of a Zn mass balance based on Zn concentrations and historical records of ash accumulation fluxes within the power plant (B. High, personal communication) suggests that the EP:MS ratio for coal and coal+5%TDF burning is about 10:1. The fraction of Zn leaving the plant through stack emissions is less certain, but appears to be smaller (10-30%) for the coal than for the coal+5%TDF VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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(>50%). This may be attributable to increased formation of gaseous metal-chloride complexes during combustion of the coal+5%TDF (15). On the basis of these estimates, the coal fuel likely produces average stack emissions with a δ66Zn that is lighter than that of the coal by 0.05 to 0.5‰., whereas the coal+5%TDF likely produces average stack emissions with a δ66Zn that is nearly the same as that of the fuel. TDF emissions can be substantial on a global scale. For example, if coal+5%TDF fuel were burned as the sole fuel source, Zn emissions from this plant alone would exceed 19 t per year (15). Implications. The conceptual model for Zn isotope fractionation during fuel burning (Figure 4 & 5) and the calculations of δ66Znstack (Figure 6) demonstrate that the δ66Zn of atmospheric PM will depend on the following: (1) the δ66Zn of the fuel; (2) the efficiency of the combustion process; (3) the fraction(s) and δ66Zn of the Zn sequestered in the various pollution control systems; and (4) the fraction of the total Zn leaving the stack. Temperature plays a primary role in controlling the δ66Zn of the sequestered Zn. Hence, the average δ66Zn of atmospheric emissions will depend greatly on the design and efficiency of the plant. For this reason, many individual power plants, smelters, kilns, paper production plants, and waste burning facilities are likely to produce distinctive δ66Zn emissions regardless of the fuels used. Zn isotopes may prove to be useful in this context for “fingerprinting” and tracking industrial Zn emissions, but only in cases where the δ66Zn is substantially different from that of ambient PM.

Acknowledgments We thank Glenda Singleton, Steven Lev, and Dale Simmons for providing helpful feedback on early drafts of the manuscript; Bruce High, Operations Supervisor of the Wade Utility Plant at Purdue University for providing technical information; and Thomas Jauss for drawing Figure 1. The manuscript was additionally improved through the thoughtful comments of 3 anonymous reviewers. The use of firm, trade, or brand names in this paper is for identification purposes only and does not constitute endorsement by the U.S. Government. This is CEEIR contribution #2 (NSF MRI 0820986).

Supporting Information Available A table summarizing the digestion methods (Table SI-1) and a figure that demonstrates the mass-dependency of Zn isotopic fractionation (Figure SI-1). This information is available free of charge via the Internet at http://pubs.acs. org/.

Literature Cited (1) Pacyna, J. M.; Pacyna, E. G. An assessment of global and regional emissions of trace metals to the atmosphere from anthropogenic sources worldwide. Environ. Rev. 2001, 9, 269–298. (2) Giere’, R.; LaFree, S. T.; Carleton, L. E.; Tishmack, J. K. Environmental impact of energy recovery from waste tyres. In: Energy, Waste, And the Environment: A Geochemical Perspective; Giere’, R., Stille, P., Eds.; The Geological Society: London, 2004; Vol. 235, 475-498. (3) Mattielli, N.; Petit, J. C. J.; Deboudt, K.; Flament, P.; Perdrix, E.; Taillez, A.; Rimetz-Planchon, J.; Weis, D. Zn isotope study of atmospheric emissions and dry depositions within a 5 km radius of a Pb-Zn refinery. Atmos. Environ. 2009, 43, 1265–1272. (4) Sonke, J. E.; Sivry, Y.; Viers, J.; Freydier, R.; Dejonghe, L.; Andre, L.; Aggarwal, J. K.; Fontan, F.; Dupre, B. Historical variations in the isotopic composition of atmospheric zinc deposition from a zinc smelter. Chem. Geol. 2008, 252, 145–157. (5) Cloquet, C.; Carignan, J.; Libourel, G. Isotopic composition of Zn and Pb atmospheric depositions in an urban/periurban area of northeastern France. Environ. Sci. Technol. 2006, 40, 6594– 6600.

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