Zinc Isotope Variability in Three Coal-Fired Power ... - ACS Publications

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Zinc Isotope Variability in Three Coal-Fired Power Plants: A Predictive Model for Determining Isotopic Fractionation during Combustion R. Ochoa Gonzalez* and D. Weiss Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, United Kingdom S Supporting Information *

ABSTRACT: The zinc (Zn) isotope compositions of feed materials and combustion byproducts were investigated in three different coal-fired power plants, and the results were used to develop a generalized model that can account for Zn isotopic fractionation during coal combustion. The isotope signatures in the coal (δ66ZnIRMM) ranged between +0.73 and +1.18‰, values that fall well within those previously determined for peat (+0.6 ±2.0‰). We therefore propose that the speciation of Zn in peat determines the isotope fingerprint in coal. All of the bottom ashes collected in these power plants were isotopically depleted in the heavy isotopes relative to the coals, with δ66ZnIRMM values ranging between +0.26‰ and +0.64‰. This suggests that the heavy isotopes, possibly associated with the organic matter of the coal, may be preferentially released into the vapor phase. The fly ash in all of these power plants was, in contrast, enriched in the heavy isotopes relative to coal. The signatures in the fly ash can be accounted for using a simple unidirectional fractionation model with isotope fractionation factors (αsolid−vapor) ranging between 1.0003 and 1.0007, and we suggest that condensation is the controlling process. The model proposed allows, once the isotope composition of the feed coal is known, the constraining of the Zn signatures in the byproducts. This will now enable the integration of Zn isotopes as a quantitative tool for the source apportionment of this metal from coal combustion in the atmosphere. deviation of an isotope ratio (e.g., 66Zn/64Zn) measured in the sample from that measured in a standard and is typically expressed in per mil, e.g., δ66Zn = ((66Zn/64Zn)sample/ (66Zn/64Zn)standard −1) × 1000. The isotopic composition of Zn expressed as δ66ZnLyon in natural sphalerites (ZnS), and anthropogenic materials produced through low-temperature processes such as electrochemical separation range typically between +0.1 and +0.3‰.10,11 High-temperature processes such as smelting and combustion, in contrast, seem to induce significant isotope fractionation, with lighter Zn isotopes entering the vapor phase and leaving the residual material enriched with the heavier isotopes.11−14 A recent study found large isotope fractionation between Zn ores and emitted dust through the stack in a Pb−Zn refinery.12 Dust collected from the chimney displayed δ66ZnLyon values as low as −0.67‰. The δ66ZnLyon of the largest emitted dust particles (>10 μm) ranged from +0.02 to +0.19‰, in line with the signatures measured in ore dust and slag heaps. The PM10 emitted, however, showed negative δ66ZnLyon values ranging between −0.52 to −0.02‰ that are attributed to evaporation within the refinery.12,15 Zn

1. INTRODUCTION The concentration of Zn in coal ranges typically between 5 and 300 μg g−1.1 When coal is combusted at high temperatures (typically around 1500 °C), most of the Zn evaporates and condenses onto the fly ash particles, most likely as ZnO or ZnSO4.2,3 These are removed in electrostatic precipitators (ESPs). Although ESPs exhibit a good retention for large particles (∼99%), their efficiency is significantly decreased for particles smaller than 0.8 μm remaining in the flue gas.4 Most of the particulate matter (PM) emitted from the top of the stack are in the size range of inhalable PM, i.e., particles with diameters lower than 10 μm (PM10) or 2.5 μm (PM2.5). Significant emissions of PM10 and PM2.5 and high concentrations of Zn have been found in the flue gas that is released into the atmosphere.5,6 This is leading to coal or other hydrocarbon-based fuels combustion being an important source of Zn in the atmosphere, particularly in countries where these fuels are the primary energy sources.7 Analyses of stable isotope ratios of transition elements precise enough to resolve natural and anthropogenic variability in the environment have only been possible since the development of multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) and the associated purification techniques.8,9 Variations in the isotopic composition of Zn are expressed using the δ notation (δ66Zn), which is the © XXXX American Chemical Society

Received: May 14, 2015 Revised: September 8, 2015 Accepted: September 30, 2015

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DOI: 10.1021/acs.est.5b02402 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

between 140 and 160 °C. The temperature of the flue gas decreases up to approximately 60 °C in the scrubber of PPC. Concentration and Isotope Ratio Determinations. Analytical work was carried out in a Class 1000 clean laboratory at the MAGIC laboratories. Sample preparation for the trace element and isotopic analysis was performed in Class 10 laminar flow hoods. HNO3, HCl, and HF used were purified by sub-boiling distillation in quartz and Teflon stills. Ultrapure water (≥18.2 MΩ) obtained by Milli-Q systems (Millipore) was used to prepare all solutions. The samples were digested in a microwave (Milestone Ethos) in Teflon vessels using a mixture of concentrated HNO3 and HF (3:1). The coal samples were previously ashed at 500 °C according to the standard ASTM D6357−04. The ash content of the samples was determined by combustion of the organic matter at 815 °C. Digested solutions were evaporated to dryness in PFA vials (Savillex, MN) on a hot plate at 120 °C. Samples were redissolved and refluxed, first in 300 μL of concentrated HNO3 to remove the excess of fluorides and then in 300 μL of 7 M HCl. The solutions were dried down and redissolved in 5 mL of 7 M HCl for subsequent analysis. An aliquot of the solutions was used for concentration analysis by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo iCap 6500 Duo) or ICPMS (Agilent 7700x ICP-MS). The recovery of the trace elements during the digestion was monitored using the certified values for SARM20 (bituminous coal) from Mintek and 1633c (coal fly ash) from NIST. Separation of Zn from the sample matrix was achieved using 1 mL of Bio-Rad AG MP-1 resin (100−200 mesh).16 The Zn fraction was collected in PFA vials, dried down on a hot plate at 120 °C. and treated with 0.3 mL of ∼16 M HNO3 to digest any organic column residue. The solution was dried again and redissolved in 2 mL of 0.1 M HNO3 for isotopic analysis. The possible effect of isotope fractionation during the ion exchange procedure was addressed by achieving the complete recovery of Zn. The recovery of Zn, which was monitored determining Zn contents in the solutions before and after the column passage, varied between 96 and 108%. Isotope ratios of Zn were determined using a multicollector ICP-MS device (Nu Instruments Limited, UK) equipped with a Nu DSN-100 desolvation nebulizer system and a glass nebulizer. The isotopes 62Ni, 63Cu, 64Zn, 65Cu, 66Zn, 67Zn, and 68Zn were measured simultaneously, and the calculated isotope ratios were referenced to IRMM-3702. Zinc isotope ratios in the literature referring to the JMC Lyon Zn standard were recalculated to IRMM 3702, assuming an isotopic offset of δ66ZnIRMM‑JMC ≈ + 0.32‰17 to enable a comparison with our data. All samples were scanned for interfering elements such as Na, Mg, Ba, etc. Isobaric interferences of 64Ni were monitored by measuring the intensity of 62Ni and were negligible for all of the analysis performed in this study. Zinc isotope ratios are reported as δ66Zn IRMM using the δ notation relative to the average value of the bracketing standard analyzed before and after each sample, according to eq 1:

ore tailings in a smelter have been found to be highly fractionated relative to Zn ore (δ66ZnLyon ≈ 0.16‰), reaching δ66ZnLyon values up to +1.49‰.14 Most recently, Borrok and co-workers tested the fractionation of Zn within a coal-fired power plant that employs coal and tire-derived fuels and found fly-ash particles slightly enriched in the heavier Zn isotopes relative to those in the fuels.13 The bottom-ash samples, in contrast, showed enrichments in light and heavy isotopes, which is explained by different condensation or the adsorption of Zn in the plant. Although previous case studies showed strong evidence that smelting and combustion lead to significant Zn isotopic fractionation between input sources or reactants (i.e., coals and ores) and the output products (e.g., fly ash and large particulate matter being isotopically heavy and PM10 being isotopically light relative to the feed material), to date, neither has any study assessed if this fractionation pattern holds in coalfired power plants fed with various fuels nor has one assessed if a generalized model predicting the isotopic composition of the flue gas could be developed.10−15 The latter would not only assist the interpretations of the fractionation mechanisms in calculating additional data but would also enable the prediction of the Zn isotopic composition of the stack emissions and, hence, the monitoring of Zn released into the environment. To this end, the aim of this paper was to investigate the Zn isotopic variability during combustion in three coal-fired power plants fed with different materials and to establish a model that accounts for the observed Zn isotope ratios in the residues and predicts the Zn isotopic composition of the flue gas emissions. We determined first the enrichment and partitioning of Zn among the combustion residues and then the isotopic composition of various feed materials and combustion byproducts, with a special focus on fly ash and bottom ash. We tested possible fractionation mechanisms of Zn isotopes during coal combustion and determined the isotopic variation of the flue gas containing fly ash particles (calculated by Zn mass balances) relative to the feed materials to finally develop a conceptual model that predicts Zn isotopic fractionation in coal-fired power plants.

2. EXPERIMENTAL SECTION Sample Collection. The sampling campaigns were performed in three pulverized fuel power stations in Spain (thereafter labeled PPA, PPB, and PPC) operating at capacities between 350 and 600 MW. PPA and PPB were fed with bituminous coal, while PPC used a mixture of bituminous coal (82%) and pet coke produced by delayed coking (18%). The samples of the feed blend in PPA (denoted as C1A and C2A), PPB (denoted as C1B and C2B), and PPC (denoted as CC) were obtained by sampling the mills feeding the boilers. Bottom ash samples (denoted as BAA, BAB, and BAC) and a representative sample of fly ash were collected in the mills of the ESPs of each coal-fired power station (denoted as FAA, FAB, and FAC). FA1A and FA1B were taken in the last hoppers of the ESPs of PPA and PPB, where only the fly ash particles of lowest particle size arrive. Due to technical reasons, FA1C was collected in the first hopper of the ESP. PPA and PPB operate without desulfurization systems, while PPC has a scrubber unit. A representative sample of the limestone (LC) and the gypsum (GC) were also taken for analysis in PPC. Combustion temperatures are in the range of 1200−1500 °C in the boiler, while the ESPs of these power plants operate

⎡ (66Zn/64 Zn) ⎤ sample ⎥ × 1000 δ 66 ZnIRMM = ⎢ 66 − 1 ⎢⎣ ( Zn/64 Zn)IRMM average ⎥⎦ (eq 1)

Drifts in instrumental mass bias during the analytical sessions were accounted for using Cu ERM-AE633 as external dopant.9,18 Standards and samples were concentration-matched B


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Environmental Science & Technology within 10%, and the Cu/Zn ratio was adjusted to 1. Accuracy and precision were assessed for each analytical session using repeated measurements of BCR-176 (incineration fly ash) and in-house single-element solutions from Romil and Johnson Matthey19 (details of the reproducibility in the measurements can be found in Figure S1). The δ66ZnIRMM determined over various individual analytical sessions over a year were, for BCR176, −0.17 ± 0.13‰ (n = 45); for Romil, −9.13 ± 0.15‰ (n = 29); and for Johnson Matthey, −0.03 ± 0.11‰ (n = 31), respectively. Values for single-element solutions from Romil and Johnson Matthey are in good agreement with previously published data.19,20 We also provide a new reference value for BCR-176, obtained during ten sessions, that has only been analyzed once previously and showed isotopically heavier Zn (δ66ZnIRMM = 0.33 ± 0.11‰).17 The reproducibility for the analysis of the samples has been quantified using the uncertainty associated with replicates of digestion and three passages through the column, expressed as 2SD.

3. RESULTS AND DISCUSSION Enrichment and Partitioning of Zn among the Combustion Residues. Semivolatile elements such as Zn tend to be enriched in fly ash generated from coal combustion because they condense on the surface of the particles following evaporation processes during combustion. Trace elements undergo enrichment along the stream in coal-fired power plants.21 To elucidate the behavior of Zn in the different combustion utilities studied here, we calculated the relative enrichment factor (RE). The partitioning of Zn among the bottom-ash and fly-ash fractions was evaluated using the RE as defined by Meij:21 ⎛ [Zn] fly ash/bottom ash ⎞⎛ % ash fuel blend ⎞ ⎟ ⎟⎜ RE = ⎜ ⎠ 100 ⎝ [Zn] fuel blend ⎠⎝

Figure 1. (a) Relative enrichment factors (RE) for Zn in the bottomash (BA) and fly-ash samples (FA) from PPA, PPB, and PPC; (b) partitioning of Zn in the BA, FA, and flue gas (FG) after the ESP.

to that in this study (1−7%), which is, therefore, in good agreement with the relative volatility of Zn and its enrichment in the fly-ash particles.30 Thermodynamic calculations to predict Zn speciation in the vapor phase in a typical combustion atmosphere31 suggest the dominant condensation of ZnSO4 at the ESPS operating temperatures (Figure S2). These calculations agree with previous thermodynamic models simulating Zn speciation in the solid and vapor phases that identified mainly ZnSO4 at temperatures lower than 500 °C and ZnAl2O4, ZnFe2O4, or Zn2SiO4 occurring at higher temperatures.23 Zn Isotope Variability in Feed Materials and Combustion Residues. The three isotope plots of δ67Zn versus δ66Zn and δ68Zn versus δ66Zn for all of the samples define straight lines with slopes of 1.503 ± 0.009 and 1.965 ± 0.007 and correlation coefficients of 0.9953 and 0.9986, respectively (Figure S3), suggesting mass-dependent fractionation from equilibrium or kinetic processes. Figure 2 shows the isotope variability of the samples collected from PPA, PPB, and PPC. The δ66ZnIRMM of the coal mixtures feeding the boiler of PPA ranges between +1.07 ± 0.17 to +1.03 ± 0.10‰ (Figure 2a). For the bottom ash collected in PPA, δ66ZnIRMM is +0.53 ± 0.17‰. FAA and FA1A are enriched in the heavier isotopes relative to the feed blend, with δ66ZnIRMM values of +1.48 ± 0.14 and +1.58 ± 0.16‰, respectively. The δ66ZnIRMM in the coals collected from PPB are +1.14 ± 0.13 and +1.18 ± 0.14‰ (Figure 2b). These isotopic signatures are close to those of the coals that feed the boiler of PPA. The bottom ash collected in this coal-fired power plant is isotopically lighter (δ66ZnIRMM = +0.84 ± 0.11‰), while the samples of fly ash are isotopically heavier (δ66ZnIRMM = +1.66 ± 0.15 to +1.61 ± 0.11‰) relative to the coal. Figure 2c shows the Zn isotopic composition of the feed blend and the byproducts collected in PPC. The fractionation of Zn between the fuel blend and the bottom ash or fly ash in

(eq 2)

Figure 1a shows the RE of Zn for the byproducts collected in PPA, PPB, and PPC. The RE of the bottom ash collected in PPA, PPB, and PPC (BAA, BAB, and BAC) are ≤0.6. The bottom ash is mostly depleted in Zn as high temperatures are reached in the boilers, and most of the Zn is released into the flue gas. In contrast, Zn shows RE ranging between 0.9 ± 0.6 and 3.4 ± 1.1 for the fly ash collected in PPA, PPB, and PPC. This likely reflects the significant enrichment of this element in the fly ash due to condensation of Zn onto the particles.22,23 To assist the interpretations of Zn isotope variations in the coal combustion plants, we conducted mass balances across the ESPs (Figure 1b).24 The retention of Zn in the bottom ash collected in PPA, PPB, and PPC ranges between 3.4 and 12%, while the proportion of Zn in the fly ash is higher than 87% of the Zn entering the boilers. As a consequence, particles that are not captured in the ESPs and are emitted to the atmosphere by the chimneys are enriched in Zn relative to the fuels. The speciation of Zn in coal fly ash has been little studied to date. Previous studies suggest that Zn occurs predominantly as ZnO associated with amorphous and crystalline nanominerals or as ZnSO4,22 although ZnFe2O4, Zn5(OH)6(CO3)2, Zn2SiO4, and ZnAl2O4 have also been identified.23,25−27 The mass-balance calculations suggest that the fraction of Zn in the flue gas leaving the ESPs is lower than 2%. This might be due to the low concentration of chlorine in the feed materials, the interaction of Zn with other minerals, or the formation of Zn species that would condense onto the particles.28,29 Previous work found a similar amount of Zn in the flue gas C


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blend analyzed in this work, but they are within the range found in peat.35 Isotopically heavy Zn has been found in fly ash from coal combustion (δ66ZnIRMM = +1.04‰) and tailings collected at a Zn refinery (δ66ZnIRMM = +0.50 to +1.81‰),14 explained by evaporation and condensation occurring during combustion and metallurgical processes. Also, coal fly ash and tire-derived fly ash collected in ESP were enriched in the heavier isotopes.13 We observe significant isotopic variability among the different byproducts collected in PPA, PPB, and PPC. The similarity of the δ66Zn patterns in this study (Figure 2) and in previous work13 suggest that isotopic fractionation during combustion is controlled by the same underlying physicochemical processes. Our results suggest that Zn in different feed materials will fractionate following the same pattern in coalfired power plants as they are controlled by the same chemical and physical processes (i.e., speciation, evaporation, and condensation), but the isotope signatures of the byproducts depend on those of the feed material. This reflects the importance of the isotopic characterization of the feed materials used in each coal-fired power plant for source tracing purposes. Figure 3 illustrates the relationship between the Zn isotopic composition and the concentration of Zn in the coals, bottom ash, and fly ash collected in PPA, PPB, and PPC. The concentrations of Zn in the coals and the blend of coal and pet coke vary between 10 and 53 μg g−1. The concentrations of Zn in the bottom ash (21 to 90 μg g−1) are much lower than those in the fly ash (70 to 282 μg g−1) due to the condensation of this element onto the particles in the ESPs. We observe a correlation of the Zn isotopic composition and the amount of Zn in the different byproducts collected in all the coal-fired power plants (Figure 3). This suggests that the heavier isotopes of Zn are preferentially retained in the fly ash particles. The variability in the isotopic signature of Zn may be driven by the condensation and adsorption of ZnSO4 at temperatures lower than 160 °C in the ESPs or the presence of other Zn minerals in the fly ash (Zn5(OH)6(CO3)2, ZnAl2O4, ZnFe2O4, or Zn2SiO4). Modeling Zn Isotope Fractionation during HighTemperature Combustion. a. Model for Zn Isotope Signatures in the Fly Ash and Flue Gas at the ESPs. Several reports have investigated the isotopic composition of residual gaseous Zn species emitted with the flue gas and suggested that lighter isotopes are predominantly present.12,13,36 To test this, we performed isotopic mass balances across the boilers to estimate δ66Zn at the inlet of the ESP (δ66ZnESP):

Figure 2. δ 66Zn values obtained for feed coal (C), bottom ash (BA), fly ash (FA), limestone (L), and gypsum (G) samples collected in PPA (a), PPB (b), and PPC (c).

PPC follows the same pattern as in PPA and PPB. Also, the gypsum collected in PPC is isotopically enriched in the heavier isotopes in comparison with the solid starting material, i.e., limestone (Figure 2c). This might be due to the preferential adsorption of the heavier isotopes onto the gypsum particles in the slurry, as was previously found with other inorganic mineral surfaces,32 or the coprecitation of ZnSO4. The isotopic ratio of the mixture of coal and pet coke (δ66ZnIRMM = +0.73 ± 0.19‰) is slightly lower than that of the coals that feed PPA and PPB (δ66ZnIRMM = +1.11 ± 0.30‰, n = 4 samples). The speciation of Zn in coals and pet coke is poorly constrained, but it is likely that Zn is present as secondary sulfide phases, associated with organic matter or pyrite in coals, while porphyrins of Zn have been identified in oil shales.33,34 The isotopic signature of the Zn species in coal is determined by diagenetic processes occurring in peat, the precursor of coal. We note that the isotopic signatures of Zn determined in the coals that feed the boilers of PPA, PPB, and PPC fall within the previously determined isotope range for peat collected from three different bogs in Finland (+0.6 − + 2.0‰).35 The isotopically heavy Zn in the peat has been explained by various diagenetic alterations, the uptake of light Zn by plants, and further fractionation in the pore waters via various biogeochemical processes such as adsorption, diffusion, or ion exchange. Previous studies reported δ66ZnIRMM in coal and tirederived fuels of +0.56 and +0.45‰, respectively.13 These values are significantly different from the coals and the coke/coal

fC ·δ 66 Zn C = fBA ·δ 66 ZnBA + f ESP·δ 66 Zn ESP

(eq 3)

where δ66ZnC and δ66ZnBA are the isotopic compositions of Zn in the fuel blend and bottom ash, respectively, and δ66ZnESP is the isotopic signature for the fly ash and flue gas at the inlet of the ESP. f C and f BA are the fractions of Zn in the feed material and bottom ash, respectively, and f ESP is the fraction of Zn in the fly ash and flue gas at the inlet of the ESP. The sum of these fractions is 1: fC = fBA + fESP = 1

(eq 4)

By combining eq 3 and eq 4, the δ66ZnESP is calculated as 66

δ ZnESP = D

δ 66 Zn C − fBA δ 66 ZnBA 1 − fBA

(eq 5) DOI: 10.1021/acs.est.5b02402 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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fuel blends (δ66ZnC) and those of the combustion by-products (δ66ZnBA or δ66ZnFA): Δ66Zn C − BA/FA = δ 66 Zn C − δ 66 ZnBA/FA

(eq 6)

The average Δ66ZnC is 0.41 ± 0.25‰, and the shifts are similar for the bottom ash and the fly ash in all the samples analyzed in this study (Figure S4). This suggests that the physical and chemical processes affecting the Zn isotopic fractionation in these coal-fired power plants are similar. We suggest that condensation processes are the mechanisms governing the isotopic fractionation of Zn in the ESP, while the speciation may control the initial isotopic signature of the fuels. The Zn that remains in the bottom ash is isotopically lighter than the feed blend because the heavy isotopes, which are also typically associated with the organic matter,37 might be preferentially released into the vapor phase. Isotopic fractionation of Zn in smelters has been successfully modeled using the Rayleigh fractionation model,10,12 which describes unidirectional mass transport processes such as evaporation and condensation from an isotopically uniform reservoir. The major differences between Zn ore refining in a smelter and coal combustion in a power plant are the concentration of Zn in the feed materials and the operating temperatures. Smelters operate between 600 and 700 °C, while coal-fired power stations operate between 1200 and 1500 °C. The concentration of Zn in the feed ores (40−60%) is much higher than those in coals.1,38,39 In this study, the progressive depletion of the light isotope in fly ash as more Zn is removed from the system (Figure 3) is in line with an unidirectional fractionation. The theoretical kinetic fractionation factor, the Rayleigh fractionation model, and the variation of the Zn isotopic compositions in the fly ash are used to test kinetic or equilibrium mechanisms involved in Zn isotope fractionation in coal-fired power plants. The theoretical kinetic fractionation factor (α66−64) is approximated as the square-root of the atomic masses of each isotope.40 In the case of Zn, α66−64 is 1.0155.

Figure 3. Correlation between δ66ZnIRMM values obtained for feed coals (C1A, C2A, C1B, C2B, and CC), bottom ash (BAA, BAB, and BAC), and fly ash (FAA, FA1A, FAB, FA1B, FAC, and FA1C) collected in PPA (a), PPB (b), and PPC (c) and the concentration of Zn. The black arrows indicate the direction of isotope fractionation as a function of the concentration of Zn in the byproducts.

α66 − 64 =

Our mass-balance calculations show that most of the Zn is captured in the fly ash (Figure 1b). 66δZnESP is calculated using the isotope data of the feed materials and the bottom ash analyzed in this study. The isotopic mass balances indicate that δ66Zn in the fly ash and the flue gas is 1.07, 1.20, and 0.76‰ at PPA, PPC, and PPC, respectively. These signatures are not significantly different from the isotopic composition of the starting materials, suggesting that the contribution of the Zn in the bottom ash to the total isotopic mass balance is negligible. These signatures are lighter than those found for the fly ash (Figure 2), suggesting that emissions at the stack might be enriched in the lighter isotopes. However, other processes, such as the leaching of Zn from the particles in the scrubber or the condensation of this element within the plants, may exert a significant effect on the isotopic fingerprint of the stack emissions. b. Modeling Zn Isotope Composition in Combustion Residues. The bottom ash samples are enriched in the lighter Zn isotopes, and the fly ash samples are enriched in the heavier Zn isotopes relative to the feed materials. The magnitude of the fractionation during combustion (Δ66ZnC‑BA/FA) can be estimated by comparison of the isotopic composition of the

M66 M64

(eq 7)

The Rayleigh distillation equation is used to determine the isotope composition of the vapor fraction (δ66Znv):41 ⎡ 1 − f 1/ α ⎤ ⎥ − 1000 δ 66 Zn v = ⎢(δ 66Zn vo + 1000) ⎢⎣ 1 − f ⎥⎦

(eq 8)

where f is the solid residual Zn fraction and δ66Znvo is the initial isotopic composition of the vapor. Various authors reported experimental fractionation factors (α s o l i d − v a p o r = 66 δZnsolid/66δZnvapor) between 1.0002 and 1.0012 using the Rayleigh isotope fractionation model.12−15 Figure 4 shows the corresponding Rayleigh diagrams for coal starting materials with an isotopic composition of 1.05, 1.16, and 0.73‰ for PPA, PPB, and PPC, respectively. Assuming an extraction of 50% ( f FG = 0.5) from the starting phase reflecting the integration of the evaporation process, these models predict isotopic compositions of 0.84, 0.95, and 0.64‰ for the vapor phase in PPA, PPB, and PPC, which is isotopically lighter than the feed materials. The calculated fractionation factors αsolid−vapor are 1.0007 for PPA and PPB and 1.0003 for PPC and, E


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the boiler, most of the Zn evaporates. The speciation of Zn in fuels likely plays a major role in controlling their isotopic fingerprint and hence explains the range found in the coals analyzed. A small amount of light Zn is found in the bottom ash, while most of the Zn remains in the fly ash and flue gas. As the flue gas containing fly-ash particles is cooled when it enters the ESP at a temperature of about 150 °C, a significant condensation of Zn onto the particles occurs. The calculated αsolid−vapor values are similar, suggesting that fractionation may be temperature-controlled by condensation. The high αsolid−vapor value would explain that δ66Zn is heavier in the particles that are collected in the ESP while the vapor becomes enriched in the lighter isotopes. Implications of Zn Isotopes for Source Tracing. Our findings demonstrate that the isotopic composition of the fuel influences the δ66Zn of the particles that are emitted to the atmosphere, which is likely controlled by the speciation in the fuels. Although coal-fired power plants are designed for rather different operating conditions (fuel input and flue gas rates, type of fuel, etc.), Zn speciation, evaporation, and condensation are the processes governing the Zn isotopic variations within the products of a coal power plant. Our model suggests that fly ash is enriched in the heavier isotopes of Zn, while the flue gas is likely enriched in the lighter ones. A δ66ZnIRMM value of +0.48 ± 0.20‰ (2SD, n = 54) has been proposed for the world average ore-grade sphalerite,10 and this value is similar to that of the δ66ZnIRMM of common metallic Zn products, ranging from +0.42‰ to +0.62‰.11 During smelting, Zinc concentrates are processed to produce Zn metal that has an isotopic composition similar to that of the ore.12,14 This similarity suggests that there is no isotopic fractionation between the Zn in the ore and that in the final product that is used in the manufacture of tires and brakes. However, the 66δZnIRMM of the fly ash analyzed in this study (+1.39 ± 0.40‰) and that predicted for the Zn emitted through the stack (between 0.65 and 0.95‰) are significantly different from that of the metallic Zn products. Because different anthropogenic and natural metal sources have distinctive Zn isotopic fingerprints, we suggest that Zn isotopes are a powerful tool for discriminating between Zn emitted during high-temperature processes such as combustion or smelting and low-temperature processes (i.e., abrasion from tires and brakes or electrochemical refining). In addition, the significant difference between the isotopic composition of Zn in fly-ash and flue-gas emissions would provide valuable information for predicting the behavior of this element in coal-combustion installations.

Figure 4. Rayleigh fractionation models showing δ66Zn vs the fraction of Zn in the flue gas ( f FG). Triangles, circles, and dotted lines represent the solid, vapor, and cumulative gas phases, respectively. Our experimental data for the fly-ash samples are marked with diamonds.

therefore, correspond well with the αsolid−vapor reported in previously published literature.12−15 c. General model for Zn Isotope Fractionation. Combining the experimental data of our work with the Rayleigh distillation model, we developed a conceptual model for predicting the fractionation of Zn in coal-combustion systems (Figure 5). During fuel combustion at temperatures higher than 1200 °C in

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b02402. Figure S1 shows the long-term reproducibility for repeated measurements of 66Zn/64Zn (BCR-176) and 65 Cu/63Cu (ERM-AE633) over the course of this study. Figure S2 illustrates the Zn species in the flue gas predicted by using HSC Chemistry 6.1 software, considering the composition of the flue gas in PPC. Figure S3 shows the Zn isotopic compositions of the samples analyzed in this study: (a) plot of δ67Zn versus δ66Zn and (b) plot of δ68Zn versus δ66Zn. Figure S4 shows the magnitude of Zn fractionation during combustion (Δ66ZnC) estimated by the difference

Figure 5. Conceptual model for predicting Zn isotopic fractionation during coal combustion. F


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between the isotopic composition of the fuel blend (δ66ZnC) and that of bottom ash (δ66ZnBA) or fly ash (δ66ZnFA). (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +44 75946547; e-mail: [email protected]. uk. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.O.G. thanks The European Commission (FP7-PEOPLE2012-IEF) for funding the project ISOTRACE (proposal 329878). The authors thank the Contamination by Metals Group at INCAR-CSIC for providing the samples. The MAGIC group at Imperial College, specially Katharina Kreissig and Barry Coles, are thanked for their invaluable assistance in the laboratories.

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