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Assessing the solubility of inorganic compounds from size-segregated coal fly ash aerosol impactor samples
Aerosol Science 33 (2002) 77–90
www.elsevier.com/locate/jaerosci
Assessing the solubility of inorganic compounds from size-segregated coal %y ash aerosol impactor samples Wayne S. Seames 1 , Jamshid Sooroshian, Jost O.L. Wendt ∗ Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ 85721, USA Received 17 November 2000; received in revised form 7 May 2001; accepted 8 May 2001
Abstract An important issue for the coal-1red utility industry is the release of heavy metals contained in %y ash. Before companies can plan emission minimization strategies for these compounds, they must have an accurate means of predicting the forms of occurrence in the waste stream and the solubility of these forms into the surrounding environment. The EPA’s Toxicity Characterization Leaching Protocol (TCLP) Method 1310 was modi1ed to allow the testing of inorganic compounds contained on greased impactor membranes. The modi1ed method was used to explore the solubility of four trace elements from 1ve di9erent types of coal %y ash. For most coals, selenium has limited solubility at pH 5.0 and is not expected to signi1cantly impact groundwater sources. Arsenic, antimony, and cobalt are typically partially soluble at pH 5.0 and a portion of these metals may migrate out of the ash particles. Further, c 2001 Elsevier Science Ltd. All rights reserved. these metals show increased solubility at pH 2.9. Keywords: Leaching; Solubility; Coal; Fly ash; Trace element; Combustion; TCLP
1. Background An important issue facing many industries, including the coal-1red electric utility industry, is the environmental impact resulting from the release of semi-volatile heavy metals and other inorganic compounds contained in solid wastes (Linak & Wendt, 1994). One area of particular interest is the environmental impact of inorganic compounds from submicron particles deposited ∗
Corresponding author. Tel.: +1-520-621-6050; fax: +1-520-621-6048. E-mail address: [email protected] (J.O.L. Wendt). 1 Currently at the University of North Dakota, Department of Chemical Engineering, Grand Forks, ND 58202, USA. c 2001 Elsevier Science Ltd. All rights reserved. 0021-8502/01/$ - see front matter PII: S 0 0 2 1 - 8 5 0 2 ( 0 1 ) 0 0 0 7 1 - 4
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downwind of the emission source. The collection eIciency of the electrostatic precipitators and 1lter bag control equipment often used to remove particles from emission sources is lower for submicron particles than for supermicron particles (Clarke, 1993; Germani & Zoller, 1988; Shendrikar, Ensor, Cowen, & WoInden, 1983). The lower removal eIciency in %ue gas particle collection equipment may increase the deposition of submicron particles in the downwind environment and subsequent migration into the water table (Kauppinen & Pakkanen, 1990; Swaine, 1994). One of the more challenging sources of particle emissions is 1ne %y ash particles generated during pulverized coal combustion. Every coal contains at least some small quantity of every element that naturally occurs in the earth’s crust. Because the chemical composition of these elements is complex and diIcult to de1ne, prediction of environmental emission impact is diIcult. This is particularly true of trace metals. It is diIcult to isolate and quantify the low concentrations of these metals in the complex %y ash inorganic matrix. Before companies can plan or implement emission minimization strategies for these compounds, they must have an accurate means of predicting the forms of occurrence in the waste stream and the solubility of these forms into the surrounding environment. Procedures to identify the forms of occurrence and solubility of inorganic elements from bulk %y ash particle samples are well developed (Cox, Lundquist, Przyjazny, & Schmulbach, 1978; Dusing, Bishop, & Keener, 1992; Dreesen et al., 1977; Finkelman et al., 1990; Karuppiah & Gupta, 1997; Theis & Wirth, 1977; Wadge & Hutton, 1987). The US Environmental Protection Agency has established standard procedures for assessing the toxicity of trace elements from bulk %y ash samples. One of the most important of these procedures is EPA TCLP Method 1310, Toxicity Characterization Leaching Protocol for bulk %y ash samples. This procedure provides a uniform method to compare the tendency of inorganic elements to leach out of %y ash samples into moderate-to-highly acidic aqueous environments. TCLP Method 1310 has some limitations that impact the ability to evaluate submicron particles. In Method 1310, a 1xed amount of ash in the 100 g range is required. However, submicron particles typically account for only 1–5 wt% of the total %y ash particles for most coal combustion %y ash. Therefore, except with very large combustion systems, the time required for particle collection from a single test is substantial. Furthermore, Method 1310 requires loose samples of ash. This requirement eliminates inertial impactors as a collection device. To address these limitations, a leachability protocol was developed to investigate the solubility of the inorganic constituents of size-segregated %y ash particles collected on greased membranes in impactors. The objectives of this program of study were: 1. Develop and demonstrate a technique applicable for examining the leachability of inorganic constituents on submicron-sized %y ash particles contained on greased impactor membranes. 2. Gain insight into the environmental impact of submicron particle deposition in (a) soil downwind of coal-1red combustors and (b) %y ash disposal sites (to compare this impact to that of supermicron particles). 3. Gain insight into how trace elements such as As, Se, and Sb partition to both submicron and supermicron particle surfaces by determining the degree of solubility of %y ash particle compounds containing these trace elements.
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2. Procedure The solubility procedure developed involves the following steps: 1. Place an ash-laden membrane (net weight of ash must be known) in a test tube. 2. Expose the membrane to the 1st solubility %uid for 18–24 h. This step is used to assess the mobility of inorganic elements in mildly acidic solutions such as rainwater. 3. Decant the liquid from the membrane—analyze liquid for trace elements. 4. Expose the membrane to the 2nd solubility %uid for 18–24 h. This step is used to assess the mobility of inorganic elements in strongly acidic solutions such as water migrating through an acidi1ed %y ash disposal pile. 5. Decant the liquid from the membrane—analyze liquid for trace elements. 6. Expose the membrane to the total inorganic digestion %uid—analyze for trace elements. This step is used to quantify the total elemental content of the %y ash sample. The detailed procedure is listed in Fig. 1. This procedure allows an assessment of the solubility of inorganic constituents from size segregated %y-ash samples analogous to EPA’s method TCLP 1310. The method is designed for use with %y ash samples collected using an impactor onto greased polycarbonate membrane substrates. The distribution of ash mass on these membranes is typically 0.1–100 mg. The method can also be used for loose solid samples by the addition of 1ltration or drying steps during %uid transfer. Di9erences from the standard TCLP Method 1310 protocol are summarized in Table 1.
3. Method validation A special set of experiments was performed to validate the leaching protocol given in Fig. 1. First, size-segregated impactor membrane %y ash samples were collected from a laboratory-scale down%ow pulverized coal combustor. A description of the combustor facilities utilized and the experiments performed is provided in Appendix A. In these experiments, special size-segregated impactor sample sets were collected. For these special sample sets, impactor membranes were cut in half and each half was de1ned gravimetrically. The “A” sets of membranes were subjected to the modi1ed TCLP 1310 protocol described in Fig. 1. The “B” sets were digested directly as per Section 3 of the modi1ed TCLP 1310 protocol. By adding up the mass of arsenic collected for each %uid and for each impactor stage, the mass balance closure of the modi1ed protocol was determined. We also determined the mass of %y ash collected on the “A” and “B” membranes which provides an indication of the error in the gravimetric measurements. The results of both calculations are summarized in Table 2. In Fig. 2 the error in the arsenic mass balance for each impactor membrane is plotted versus the di9erence in the %y ash mass measured on the “A” and “B” membranes. The results in Fig. 2 indicate that error in the arsenic mass balance closure correlates with the di9erences in the %y ash mass determination (within the accuracy of this experimental program) for those impactor
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Fig. 1. Modi1ed TCLP 1310 method for determining the solubility of inorganic elements from %y ash samples contained on greased impactor membranes.
stages having the most signi1cant sources of error. Therefore large errors in the mass balance are primarily due to gravimetric measurement uncertainty rather than due to errors from the leaching protocol or analytical procedures.
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Table 1 Summary of changes for the modi1ed protocol compared to the standard TCLP Method 1310 protocol Modi1ed Protocol
Standard TCLP Protocol
Ash mass: 0.1–100 mg Ash 1xed on greased membranes (free ash samples can also be utilized) Sequential leaching with three solubility %uids
100 g Free ash samples required Leaching with only one solubility %uid
Table 2 Leaching protocol validation: Arsenic closure
Impactor stage
As recovery in all three %uids (ppmw)
As recovery in control sample (ppmw)
Relative di9erence (%)
Ash mass leached membranes (g)
Ash mass control membranes (g)
Relative di9erence (%)
1ca 2c 3c 4c 5 6c 6 7 8 9 10 11
327 507 538 754 648 489 778 535 339 213 102 141
263 371 549 940 419 327 308 494 203 169 95 146
20 27 2 25 35 33 60 8 40 21 7 3
0.000465 0.000415 0.000275 0.00019 0.000165 0.00039 0.000155 0.0003 0.00043 0.00073 0.00116 0.0008
0.000495 0.000575 0.00027 0.000185 0.00026 0.000615 0.00029 0.00031 0.00055 0.000625 0.00117 0.000675
6 39 2 3 58 58 87 3 28 14 1 16
a
c—indicates sample collected with inlet preseparator cyclone attached to impactor.
Fig. 2. Cross correlation of arsenic closure error to membrane gravimetric variability.
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Table 3 Key information for the study of coals Kentucky Wyodak power North Dakota Pittsburgh seam Ohio 5=6=7 blend Elkhorn=hazard river basin sub- Falkirk seam bituminous coal bituminous coal bituminous coal bituminous coal lignite coal Proximate (dry wt%) Fixed Carbon Volatile Matter Moisture
62.23 30.66 1.44
41.31 48.73 2.33
56.46 33.80 2.33
33.31 34.85 25.81
26.59 28.15 35.88
Ultimate (dry wt%) Carbon Hydrogen Nitrogen Sulfur Oxygen Ash
77.74 4.87 1.50 1.66 7.01 7.41
72.60 5.10 1.40 2.60 8.30 10.00
74.87 4.59 1.43 0.82 8.38 7.41
51.19 6.51 0.72 0.32 35.23 6.03
38.57 6.59 0.42 0.63 44.41 9.38
4.51 19.18 22.87
2.24 13.03 39.41
1.84 5.18 34.27
22.74 6.32 17.17
30.25 11.75 20.38
4.1 0.6 0.3 2.5
19 1.4 2.3 5.8
4.0 3.1 1.0 6.2
1.4 1.1 0.4 5.1
8.1 1.5 0.9 6.4
1090 2.2
1050 3.2
1182 2.9
1030 6.6
840 9.4
Major Elements (wt% of ash) CaO Fe2 O3 Al2 O3
Trace Elements (ppmw) Arsenic Selenium Antimony Cobalt Sample Temp (K)1 Residence time (s)2 1 2
Furnace bulk temperature at particulate sampling location. Residence time from burner to particulate sampling location (seconds).
4. Application of the procedure The solubility of selenium, arsenic, antimony, cobalt, calcium, iron, and aluminum in the two TCLP %uids was determined using the protocol described in Fig. 1 for %y ash samples from the combustion of 1ve di9erent pulverized coals. The experimental facilities and experiments used to generate these %y ash samples are described in Appendix A. Information about the coal is provided in Table 3. The relative solubilities of arsenic, selenium, antimony, and cobalt from both submicron and bulk %y ash at pH 5 and 2.9 for all 1ve coals are shown in Fig. 3. Three impactor stages in the submicron region were used to characterize the solubility of the inorganic elements in the submicron fume and three impactor stages in the supermicron region were used to characterize the solubility of the inorganic elements in the bulk %y ash. It should be noted that
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Fig. 3. Solubility of inorganic trace elements from submicron and supermicron %y ash particles collected on impactor membranes during the combustion of 1ve coals using the modi1ed TCLP leaching method. (a) Selenium, (b) Arsenic, (c) Antimony, (d) Cobalt.
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the values reported for the percent solubility at pH 2.9 are derived by summing the elemental mass recovered in the pH 5 leachate plus the additional elemental mass recovered in the pH 2.9 leachate divided by the elemental mass recovered in both leachate %uids plus the total dissolution %uid. 4.1. Selenium solubility The Pittsburgh supermicron, Wyodak submicron, Ohio submicron, and North Dakota submicron and supermicron results show very low solubility at both pH conditions. This is consistent with the solubility of selenites such as calcium–selenium complexes. Metal selenite compounds have very low solubilities over a wide pH range (Jones, 1995). For example, calcium selenite is only slightly soluble in neutral aqueous solutions (Coney & Hahn, 1921). The Pittsburgh submicron and Kentucky submicron and supermicron results show a fairly high degree of selenium solubility. This is consistent with the solubility of selenates such as iron– selenium complexes. Metal selenate compounds, such as iron selenate, are very soluble (Jones, 1995). The Ohio supermicron results are most consistent with the solubility of selenides (such as SeO2 ). SeO2 is partially soluble in water (38% is reported in Dean, 1992 and Weast, 1987). There are distinct di9erences between some of the selenium solubility results for submicron particles compared to supermicron particles. For example, the Pittsburgh submicron solubility results are most consistent with iron–selenium complexes whereas the supermicron solubility results are more consistent with calcium–selenium complexes. This suggests that the partitioning mechanism may be di9erent for the submicron and supermicron regimes. A more detailed explanation of these di9erences can be found in Seames (2000). The pH 5.0 leachability data helps assess the potential for selenium contained in submicron and supermicron particles to migrate into the water supply after ground deposition downwind of the combustor. Calcium selenite complexes are not very soluble and are not expected to leach into groundwater. Iron selenate complexes are fairly soluble and are likely to contribute selenium to groundwater due to leaching. SeO2 is partially soluble and is likely to contribute selenium to groundwater due to leaching. The pH 2.8 leachability data helps assess how selenium might leach from an ash disposal pile=land1ll. As water migrates through the ash pile it can become much more acidic than the original water source. Calcium selenite complexes are not very soluble and are not expected to leach out of the %y ash. Iron selenate complexes and SeO2 are fairly soluble and are likely to leach from the %y ash. 4.2. Arsenic solubility The results for all 1ve coals are fairly similar—partial solubility of arsenic in both leaching %uids. Calcium arsenate is slightly soluble in neutral aqueous solutions and is partially soluble in acidic solutions (Weast, 1987; Finkelman, 1994). Iron arsenate has solubility characteristics similar to calcium arsenate—slightly soluble in neutral aqueous solutions and partially soluble in acidic solutions (Dean, 1992). To further complicate the situation, arsenic trioxide (As2 O3 ) is also slightly soluble in neutral aqueous solutions and is partially soluble in acidic solutions (Weast, 1987). Of the species that are postulated as possibly present, only arsenic pentoxide
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(As2 O5 ) has di9erent solubility characteristics. AsO5 is very soluble in both neutral and acidic aqueous solutions (Weast, 1987; Finkelman, 1994). For all of the coals except Ohio, the results are consistent with the solubility of calcium arsenate, iron arsenate, or As2 O3 . Arsenic in the Ohio supermicron %y ash is more soluble than arsenic in the %y ash from the other coals. This is most likely due to the oxidation of some of the As2 O3 predicted to be present (Seames, 2000) on Ohio %y ash surfaces to As2 O5 . There are no appreciable di9erences between the arsenic solubility results for submicron particles compared to supermicron particles. This is not surprising since all the forms of occurrence of arsenic expected to be present have similar solubility characteristics. The pH 5.0 leachability data helps assess the potential for arsenic contained in submicron and supermicron particles to migrate into the water supply after ground deposition downwind of the combustor. In every case examined, arsenic is partially soluble and some arsenic is expected to leach out of ash particles exposed to aqueous environments (e.g. rainfall). If the dominant arsenic partitioning mechanism is physical absorption of As2 O3 (e.g. Ohio coal), oxidation of As2 O3 to As2 O5 may result in increased arsenic leachability. The pH 2.9 leachability data helps assess how arsenic might leach from an ash disposal pile=land1ll. All the forms of occurrence of arsenic are partially soluble and are likely to leach from the %y ash. If the dominant arsenic partitioning mechanism is physical absorption of As2 O3 (e.g. Ohio coal), oxidation of As2 O3 to As2 O5 may result in increased arsenic leachability. 4.3. Antimony and cobalt solubility For four of the coals studied, antimony is fairly soluble at pH 5.0 and very soluble at pH 2.9. These results are consistent with the following compounds: Sb, Sb2 O4 , and Sb2 O5 (Dean, 1992; Coney & Hahn, 1921; Norman, 1998) but are inconsistent with Sb2 O3 . The exception is the Pittsburgh %y ash which has a fairly low solubility at pH 5.0 but is very soluble at pH 2.0. Unfortunately, solubility information on Ca–Sb and Fe–Sb complexes is not readily available, so the results cannot be compared to the solubility characteristics of the expected forms of occurrence. For the Pittsburgh submicron, Ohio supermicron, Wyodak submicron and supermicron, Kentucky submicron, and North Dakota submicron and supermicron samples, cobalt is partially soluble at pH 5.0. For the Pittsburgh supermicron, Ohio submicron, and Kentucky supermicron samples, cobalt has very limited solubility at pH 5.0. At pH 2.9, cobalt is very soluble from the Ohio submicron and supermicron, Kentucky submicron, Wyodak submicron and supermicron, and North Dakota submicron and supermicron samples. For the Pittsburgh submicron and supermicron, and Kentucky supermicron samples, cobalt is partially soluble at pH 2.9. Information on the solubility of Al–Co, Ca–Co, and Fe–Co complexes is not readily available. The pH 5.0 leachability data help assess the potential for antimony and cobalt contained in submicron and supermicron particles to migrate into the water supply after ground deposition downwind of the combustor. In most cases, antimony and cobalt are soluble and compounds containing these elements on %y ash surfaces are expected to leach out of ash particles exposed to aqueous environments (e.g. rainfall).
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The pH 2.9 leachability data helps assess how antimony and cobalt might leach from an ash disposal pile=land1ll. For all of the coals studied, antimony and cobalt are moderately-to-very soluble and are likely to leach from the %y ash. 4.4. Calcium, iron, and aluminum solubility The relative solubilities of calcium, iron, and aluminum are shown in Fig. 4. Calcium is partially soluble at pH 5.0 for the Kentucky and Wyodak %y ash and very soluble at pH 5.0 for the other four coals. Calcium is very soluble at 2.9 for all 1ve coals. Iron is very soluble from the Pittsburgh %y ash at both pH levels but has limited solubility for the other four coals. Aluminum shows a great variety of solubility amongst the various %y ash samples. Most notably, Ohio submicron aluminum is substantially more soluble at pH 2.9 than aluminum in the bulk %y ash samples. It is important to note that the trace element solubilities shown in Fig. 3 do not correlate to any of the major element solubilities shown in Fig. 4. This suggests that the solubility of trace elements from %y ash is due to the physical dissolution of speci1c compounds not due to trace elements being released from the major element matrix structures (e.g. kaolinite, calcium carbonate, iron sulfate, etc.) of the %y ash as the major element compounds dissolve in the leaching %uid. Acknowledgements Financial support was provided by the US Department of Energy through contract DE-AC2295PC95101 under the overall direction of Dr. Constance Senior of Physical Sciences Inc. Technical contributions were also provided by E. Kitchen, University of Arizona. Appendix A. Experimental facilities Experimental work was conducted in a 6 m tall, 0:15 m ID vertical down%ow aerosol furnace. This furnace is designed to simulate the time=temperature histories and complex particle interactions of commercial-scale combustors while still providing a %ow stream that is suIciently well characterized to allow extraction of rates and mechanisms. The furnace, shown schematically in Fig. 5, is described in detail elsewhere (Seames, 2000). A pulverized coal was fed to the furnace using a volumetric screwfeeder. Coal was blown o9 the screw tip and into the furnace using a metered stream of 5 psig, transport air, then mixed with the primary air prior to discharge into the furnace. All of the experiments were run at a naturally evolving temperature pro1le, 20% excess oxygen in the combustion air, and slight vacuum pressure conditions. Fly ash particles were withdrawn from a sampling port located 4:3 m below the burner. Samples collected at this port represent typical conditions late in the post-combustion zone. Sampling conditions for the experiments performed in this study are included in Table 3. The temperature pro1les for the 1ve di9erent coals used in these experiments are shown in Fig. 6.
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Fig. 4. Solubility of (a) Calcium, (b) Iron and (c) Aluminum from submicron and supermicron %y ash particles collected on impactor membranes during the combustion of 1ve coals using the modi1ed TCLP leaching method.
Particle sampling was performed isokinetically using a portable water-cooled, aspirated sampling probe. Dilution nitrogen is fed to the sample tip and a vacuum pump is used to draw the nitrogen plus the furnace sample from the centerline of the furnace. The dilution nitrogen
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Fig. 5. University of Arizona down%ow combustor system.
Fig. 6. Furnace temperature pro1les from the validation experiments using the University of Arizona down%ow combustor system.
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Fig. 7. Simpli1ed schematic of isokinetic particle sampling and berner low pressure impactor collection system.
plus the cooling jacket on the probe serves to freeze subsequent reaction in the probe. It also condenses any species that will condense above atmospheric conditions (Davis, Gale, Wendt, & Linak, 1998). The sample is routed to a Berner-type low-pressure impactor (BLPI; Hillamo, & Kauppinen, 1991). This sampling system, shown in Fig. 7, is described in detail elsewhere (Seames, 2000). Sample sets that most closely duplicated the typical ash mass particle size distribution were selected for elemental analysis. Arsenic, selenium, cobalt, and antimony analyses were performed by graphite furnace atomic absorption spectroscopy (GFAA) while calcium, iron, and aluminum were analyzed by %ame atomic absorption spectroscopy (AAS). References Clarke, L. B. (1993). The fate of trace elements during coal combustion and gasi1cation: An overview. Fuel, 72, 731–736. Coney, A. M., & Hahn, D. A. (1921). A dictionary of chemical solubilities. New York: MacMillan. Cox, J. A., Lundquist, G. L., Przyjazny, A., & Schmulbach, C. D. (1978). Leaching of Boron from Coal Ash. Environmental Science and Technology, 12, 722–723. Davis, S. B., Gale, T. K., Wendt, J. O. L., & Linak, W. P. (1998). Multi-component coagulation and condensation of toxic metals in combustors. Proceedings of the Combustion Institute, 27, 1785–1791. Dean, J. A. (1992). Langes handbook of chemistry. (14th ed.). New York: McGraw-Hill. Dreesen, P. R., Gladney, E. S., Owens, J. W., Perkins, B. L., Wienke, C. L., & Wangen, L. E. (1977). Comparison of levels of trace elements extracted from %y ash and levels found in eSuent waters from a coal-1red power plant. Environmental Science and Technology, 11, 1017–1019. Dusing, D. C., Bishop, P. L., & Keener, T. C. (1992). E9ect of Redox potential on leaching from stabilized=solidi1ed waste materials. Journal of Air Waste Management Association, 42, 56–62. Finkelman, R. B., Palmer, C. A., Krasnow, M. R., Aruscavage, P. J., Sellers, G. A., & Dulong, F. T. (1990). Combustion and leaching behavior of elements in the argonne premium coal samples. Energy and Fuels, 4, 755–766.
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Finkelman, R. B. (1994). Modes of occurrence of potentially hazardous elements in coal: levels of con1dence. Fuel Processing Technology, 39, 21–34. Germani, M. S., & Zoller, W. H. (1988). Vapor-phase concentrations of As, Se, Br, I, and Hg in the stack of a coal-1red power plant. Environmental Science and Technology, 22, 1079–1085. Hillamo, R. E., & Kauppinen, E. I. (1991). On the Performance of the Berner Low Pressure Impactor. Aerosol Science and Technology, 14, 33–47. Jones, D. R. (1995). In D. J. Swaine, & F. Goodarzi, Environmental aspects of trace elements in coal. Boston: Kluwer Academic (Chapter 12). Kauppinen, E. I., & Pakkanen, T. A. (1990). Coal combustion aerosols: A 1eld study. Environmental Science and Technology, 24(12), 1811–1818. Karuppiah, M., & Gupta, G. (1997). Toxicity of metals in coal combustion ash leachate. Journal of Hazardous Materials, 56, 53–58. Linak, W. P., & Wendt, J. O. L. (1994). Trace metal transformation mechanisms during coal combustion. Fuel Processing Technology, 39, 173–198. Norman, N. C. (1998). Chemistry of Arsenic, Antimony, and Bismuth.. New York: Blackie Academic and Professional. Seames, W. S. (2000). The partitioning of trace elements during pulverized coal combustion. Doctoral Dissertation, University of Arizona. Shendrikar, A. D., Ensor, D. S., Cowen, S. J., & WoInden, G. J. (1983). Size-dependent penetration of trace elements through a utility baghouse. Atmospheric Environment, 17, 1411–1421. Swaine, D. J. (1994). Trace elements in coal and their dispersal during combustion. Fuel Processing Technology, 39, 121–137. Theis, T. L., & Wirth, J. L. (1977). Sorptive Behavior of Trace Metals on Fly Ash in Aqueous Solutions. Environmental Science and Technology, 11, 1096 –1100. Wadge, A., & Hutton, M. (1987). The leachability and chemical speciation of selected trace elements in %y ash from coal combustion and refuse incineration. Environmental Pollution, 48, 85–99. Weast, R. C. (1987). Handbook of chemistry and physics (68th ed.). Cleveland: CRC Press.
www.elsevier.com/locate/jaerosci
Assessing the solubility of inorganic compounds from size-segregated coal %y ash aerosol impactor samples Wayne S. Seames 1 , Jamshid Sooroshian, Jost O.L. Wendt ∗ Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ 85721, USA Received 17 November 2000; received in revised form 7 May 2001; accepted 8 May 2001
Abstract An important issue for the coal-1red utility industry is the release of heavy metals contained in %y ash. Before companies can plan emission minimization strategies for these compounds, they must have an accurate means of predicting the forms of occurrence in the waste stream and the solubility of these forms into the surrounding environment. The EPA’s Toxicity Characterization Leaching Protocol (TCLP) Method 1310 was modi1ed to allow the testing of inorganic compounds contained on greased impactor membranes. The modi1ed method was used to explore the solubility of four trace elements from 1ve di9erent types of coal %y ash. For most coals, selenium has limited solubility at pH 5.0 and is not expected to signi1cantly impact groundwater sources. Arsenic, antimony, and cobalt are typically partially soluble at pH 5.0 and a portion of these metals may migrate out of the ash particles. Further, c 2001 Elsevier Science Ltd. All rights reserved. these metals show increased solubility at pH 2.9. Keywords: Leaching; Solubility; Coal; Fly ash; Trace element; Combustion; TCLP
1. Background An important issue facing many industries, including the coal-1red electric utility industry, is the environmental impact resulting from the release of semi-volatile heavy metals and other inorganic compounds contained in solid wastes (Linak & Wendt, 1994). One area of particular interest is the environmental impact of inorganic compounds from submicron particles deposited ∗
Corresponding author. Tel.: +1-520-621-6050; fax: +1-520-621-6048. E-mail address: [email protected] (J.O.L. Wendt). 1 Currently at the University of North Dakota, Department of Chemical Engineering, Grand Forks, ND 58202, USA. c 2001 Elsevier Science Ltd. All rights reserved. 0021-8502/01/$ - see front matter PII: S 0 0 2 1 - 8 5 0 2 ( 0 1 ) 0 0 0 7 1 - 4
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downwind of the emission source. The collection eIciency of the electrostatic precipitators and 1lter bag control equipment often used to remove particles from emission sources is lower for submicron particles than for supermicron particles (Clarke, 1993; Germani & Zoller, 1988; Shendrikar, Ensor, Cowen, & WoInden, 1983). The lower removal eIciency in %ue gas particle collection equipment may increase the deposition of submicron particles in the downwind environment and subsequent migration into the water table (Kauppinen & Pakkanen, 1990; Swaine, 1994). One of the more challenging sources of particle emissions is 1ne %y ash particles generated during pulverized coal combustion. Every coal contains at least some small quantity of every element that naturally occurs in the earth’s crust. Because the chemical composition of these elements is complex and diIcult to de1ne, prediction of environmental emission impact is diIcult. This is particularly true of trace metals. It is diIcult to isolate and quantify the low concentrations of these metals in the complex %y ash inorganic matrix. Before companies can plan or implement emission minimization strategies for these compounds, they must have an accurate means of predicting the forms of occurrence in the waste stream and the solubility of these forms into the surrounding environment. Procedures to identify the forms of occurrence and solubility of inorganic elements from bulk %y ash particle samples are well developed (Cox, Lundquist, Przyjazny, & Schmulbach, 1978; Dusing, Bishop, & Keener, 1992; Dreesen et al., 1977; Finkelman et al., 1990; Karuppiah & Gupta, 1997; Theis & Wirth, 1977; Wadge & Hutton, 1987). The US Environmental Protection Agency has established standard procedures for assessing the toxicity of trace elements from bulk %y ash samples. One of the most important of these procedures is EPA TCLP Method 1310, Toxicity Characterization Leaching Protocol for bulk %y ash samples. This procedure provides a uniform method to compare the tendency of inorganic elements to leach out of %y ash samples into moderate-to-highly acidic aqueous environments. TCLP Method 1310 has some limitations that impact the ability to evaluate submicron particles. In Method 1310, a 1xed amount of ash in the 100 g range is required. However, submicron particles typically account for only 1–5 wt% of the total %y ash particles for most coal combustion %y ash. Therefore, except with very large combustion systems, the time required for particle collection from a single test is substantial. Furthermore, Method 1310 requires loose samples of ash. This requirement eliminates inertial impactors as a collection device. To address these limitations, a leachability protocol was developed to investigate the solubility of the inorganic constituents of size-segregated %y ash particles collected on greased membranes in impactors. The objectives of this program of study were: 1. Develop and demonstrate a technique applicable for examining the leachability of inorganic constituents on submicron-sized %y ash particles contained on greased impactor membranes. 2. Gain insight into the environmental impact of submicron particle deposition in (a) soil downwind of coal-1red combustors and (b) %y ash disposal sites (to compare this impact to that of supermicron particles). 3. Gain insight into how trace elements such as As, Se, and Sb partition to both submicron and supermicron particle surfaces by determining the degree of solubility of %y ash particle compounds containing these trace elements.
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2. Procedure The solubility procedure developed involves the following steps: 1. Place an ash-laden membrane (net weight of ash must be known) in a test tube. 2. Expose the membrane to the 1st solubility %uid for 18–24 h. This step is used to assess the mobility of inorganic elements in mildly acidic solutions such as rainwater. 3. Decant the liquid from the membrane—analyze liquid for trace elements. 4. Expose the membrane to the 2nd solubility %uid for 18–24 h. This step is used to assess the mobility of inorganic elements in strongly acidic solutions such as water migrating through an acidi1ed %y ash disposal pile. 5. Decant the liquid from the membrane—analyze liquid for trace elements. 6. Expose the membrane to the total inorganic digestion %uid—analyze for trace elements. This step is used to quantify the total elemental content of the %y ash sample. The detailed procedure is listed in Fig. 1. This procedure allows an assessment of the solubility of inorganic constituents from size segregated %y-ash samples analogous to EPA’s method TCLP 1310. The method is designed for use with %y ash samples collected using an impactor onto greased polycarbonate membrane substrates. The distribution of ash mass on these membranes is typically 0.1–100 mg. The method can also be used for loose solid samples by the addition of 1ltration or drying steps during %uid transfer. Di9erences from the standard TCLP Method 1310 protocol are summarized in Table 1.
3. Method validation A special set of experiments was performed to validate the leaching protocol given in Fig. 1. First, size-segregated impactor membrane %y ash samples were collected from a laboratory-scale down%ow pulverized coal combustor. A description of the combustor facilities utilized and the experiments performed is provided in Appendix A. In these experiments, special size-segregated impactor sample sets were collected. For these special sample sets, impactor membranes were cut in half and each half was de1ned gravimetrically. The “A” sets of membranes were subjected to the modi1ed TCLP 1310 protocol described in Fig. 1. The “B” sets were digested directly as per Section 3 of the modi1ed TCLP 1310 protocol. By adding up the mass of arsenic collected for each %uid and for each impactor stage, the mass balance closure of the modi1ed protocol was determined. We also determined the mass of %y ash collected on the “A” and “B” membranes which provides an indication of the error in the gravimetric measurements. The results of both calculations are summarized in Table 2. In Fig. 2 the error in the arsenic mass balance for each impactor membrane is plotted versus the di9erence in the %y ash mass measured on the “A” and “B” membranes. The results in Fig. 2 indicate that error in the arsenic mass balance closure correlates with the di9erences in the %y ash mass determination (within the accuracy of this experimental program) for those impactor
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Fig. 1. Modi1ed TCLP 1310 method for determining the solubility of inorganic elements from %y ash samples contained on greased impactor membranes.
stages having the most signi1cant sources of error. Therefore large errors in the mass balance are primarily due to gravimetric measurement uncertainty rather than due to errors from the leaching protocol or analytical procedures.
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Table 1 Summary of changes for the modi1ed protocol compared to the standard TCLP Method 1310 protocol Modi1ed Protocol
Standard TCLP Protocol
Ash mass: 0.1–100 mg Ash 1xed on greased membranes (free ash samples can also be utilized) Sequential leaching with three solubility %uids
100 g Free ash samples required Leaching with only one solubility %uid
Table 2 Leaching protocol validation: Arsenic closure
Impactor stage
As recovery in all three %uids (ppmw)
As recovery in control sample (ppmw)
Relative di9erence (%)
Ash mass leached membranes (g)
Ash mass control membranes (g)
Relative di9erence (%)
1ca 2c 3c 4c 5 6c 6 7 8 9 10 11
327 507 538 754 648 489 778 535 339 213 102 141
263 371 549 940 419 327 308 494 203 169 95 146
20 27 2 25 35 33 60 8 40 21 7 3
0.000465 0.000415 0.000275 0.00019 0.000165 0.00039 0.000155 0.0003 0.00043 0.00073 0.00116 0.0008
0.000495 0.000575 0.00027 0.000185 0.00026 0.000615 0.00029 0.00031 0.00055 0.000625 0.00117 0.000675
6 39 2 3 58 58 87 3 28 14 1 16
a
c—indicates sample collected with inlet preseparator cyclone attached to impactor.
Fig. 2. Cross correlation of arsenic closure error to membrane gravimetric variability.
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Table 3 Key information for the study of coals Kentucky Wyodak power North Dakota Pittsburgh seam Ohio 5=6=7 blend Elkhorn=hazard river basin sub- Falkirk seam bituminous coal bituminous coal bituminous coal bituminous coal lignite coal Proximate (dry wt%) Fixed Carbon Volatile Matter Moisture
62.23 30.66 1.44
41.31 48.73 2.33
56.46 33.80 2.33
33.31 34.85 25.81
26.59 28.15 35.88
Ultimate (dry wt%) Carbon Hydrogen Nitrogen Sulfur Oxygen Ash
77.74 4.87 1.50 1.66 7.01 7.41
72.60 5.10 1.40 2.60 8.30 10.00
74.87 4.59 1.43 0.82 8.38 7.41
51.19 6.51 0.72 0.32 35.23 6.03
38.57 6.59 0.42 0.63 44.41 9.38
4.51 19.18 22.87
2.24 13.03 39.41
1.84 5.18 34.27
22.74 6.32 17.17
30.25 11.75 20.38
4.1 0.6 0.3 2.5
19 1.4 2.3 5.8
4.0 3.1 1.0 6.2
1.4 1.1 0.4 5.1
8.1 1.5 0.9 6.4
1090 2.2
1050 3.2
1182 2.9
1030 6.6
840 9.4
Major Elements (wt% of ash) CaO Fe2 O3 Al2 O3
Trace Elements (ppmw) Arsenic Selenium Antimony Cobalt Sample Temp (K)1 Residence time (s)2 1 2
Furnace bulk temperature at particulate sampling location. Residence time from burner to particulate sampling location (seconds).
4. Application of the procedure The solubility of selenium, arsenic, antimony, cobalt, calcium, iron, and aluminum in the two TCLP %uids was determined using the protocol described in Fig. 1 for %y ash samples from the combustion of 1ve di9erent pulverized coals. The experimental facilities and experiments used to generate these %y ash samples are described in Appendix A. Information about the coal is provided in Table 3. The relative solubilities of arsenic, selenium, antimony, and cobalt from both submicron and bulk %y ash at pH 5 and 2.9 for all 1ve coals are shown in Fig. 3. Three impactor stages in the submicron region were used to characterize the solubility of the inorganic elements in the submicron fume and three impactor stages in the supermicron region were used to characterize the solubility of the inorganic elements in the bulk %y ash. It should be noted that
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Fig. 3. Solubility of inorganic trace elements from submicron and supermicron %y ash particles collected on impactor membranes during the combustion of 1ve coals using the modi1ed TCLP leaching method. (a) Selenium, (b) Arsenic, (c) Antimony, (d) Cobalt.
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the values reported for the percent solubility at pH 2.9 are derived by summing the elemental mass recovered in the pH 5 leachate plus the additional elemental mass recovered in the pH 2.9 leachate divided by the elemental mass recovered in both leachate %uids plus the total dissolution %uid. 4.1. Selenium solubility The Pittsburgh supermicron, Wyodak submicron, Ohio submicron, and North Dakota submicron and supermicron results show very low solubility at both pH conditions. This is consistent with the solubility of selenites such as calcium–selenium complexes. Metal selenite compounds have very low solubilities over a wide pH range (Jones, 1995). For example, calcium selenite is only slightly soluble in neutral aqueous solutions (Coney & Hahn, 1921). The Pittsburgh submicron and Kentucky submicron and supermicron results show a fairly high degree of selenium solubility. This is consistent with the solubility of selenates such as iron– selenium complexes. Metal selenate compounds, such as iron selenate, are very soluble (Jones, 1995). The Ohio supermicron results are most consistent with the solubility of selenides (such as SeO2 ). SeO2 is partially soluble in water (38% is reported in Dean, 1992 and Weast, 1987). There are distinct di9erences between some of the selenium solubility results for submicron particles compared to supermicron particles. For example, the Pittsburgh submicron solubility results are most consistent with iron–selenium complexes whereas the supermicron solubility results are more consistent with calcium–selenium complexes. This suggests that the partitioning mechanism may be di9erent for the submicron and supermicron regimes. A more detailed explanation of these di9erences can be found in Seames (2000). The pH 5.0 leachability data helps assess the potential for selenium contained in submicron and supermicron particles to migrate into the water supply after ground deposition downwind of the combustor. Calcium selenite complexes are not very soluble and are not expected to leach into groundwater. Iron selenate complexes are fairly soluble and are likely to contribute selenium to groundwater due to leaching. SeO2 is partially soluble and is likely to contribute selenium to groundwater due to leaching. The pH 2.8 leachability data helps assess how selenium might leach from an ash disposal pile=land1ll. As water migrates through the ash pile it can become much more acidic than the original water source. Calcium selenite complexes are not very soluble and are not expected to leach out of the %y ash. Iron selenate complexes and SeO2 are fairly soluble and are likely to leach from the %y ash. 4.2. Arsenic solubility The results for all 1ve coals are fairly similar—partial solubility of arsenic in both leaching %uids. Calcium arsenate is slightly soluble in neutral aqueous solutions and is partially soluble in acidic solutions (Weast, 1987; Finkelman, 1994). Iron arsenate has solubility characteristics similar to calcium arsenate—slightly soluble in neutral aqueous solutions and partially soluble in acidic solutions (Dean, 1992). To further complicate the situation, arsenic trioxide (As2 O3 ) is also slightly soluble in neutral aqueous solutions and is partially soluble in acidic solutions (Weast, 1987). Of the species that are postulated as possibly present, only arsenic pentoxide
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(As2 O5 ) has di9erent solubility characteristics. AsO5 is very soluble in both neutral and acidic aqueous solutions (Weast, 1987; Finkelman, 1994). For all of the coals except Ohio, the results are consistent with the solubility of calcium arsenate, iron arsenate, or As2 O3 . Arsenic in the Ohio supermicron %y ash is more soluble than arsenic in the %y ash from the other coals. This is most likely due to the oxidation of some of the As2 O3 predicted to be present (Seames, 2000) on Ohio %y ash surfaces to As2 O5 . There are no appreciable di9erences between the arsenic solubility results for submicron particles compared to supermicron particles. This is not surprising since all the forms of occurrence of arsenic expected to be present have similar solubility characteristics. The pH 5.0 leachability data helps assess the potential for arsenic contained in submicron and supermicron particles to migrate into the water supply after ground deposition downwind of the combustor. In every case examined, arsenic is partially soluble and some arsenic is expected to leach out of ash particles exposed to aqueous environments (e.g. rainfall). If the dominant arsenic partitioning mechanism is physical absorption of As2 O3 (e.g. Ohio coal), oxidation of As2 O3 to As2 O5 may result in increased arsenic leachability. The pH 2.9 leachability data helps assess how arsenic might leach from an ash disposal pile=land1ll. All the forms of occurrence of arsenic are partially soluble and are likely to leach from the %y ash. If the dominant arsenic partitioning mechanism is physical absorption of As2 O3 (e.g. Ohio coal), oxidation of As2 O3 to As2 O5 may result in increased arsenic leachability. 4.3. Antimony and cobalt solubility For four of the coals studied, antimony is fairly soluble at pH 5.0 and very soluble at pH 2.9. These results are consistent with the following compounds: Sb, Sb2 O4 , and Sb2 O5 (Dean, 1992; Coney & Hahn, 1921; Norman, 1998) but are inconsistent with Sb2 O3 . The exception is the Pittsburgh %y ash which has a fairly low solubility at pH 5.0 but is very soluble at pH 2.0. Unfortunately, solubility information on Ca–Sb and Fe–Sb complexes is not readily available, so the results cannot be compared to the solubility characteristics of the expected forms of occurrence. For the Pittsburgh submicron, Ohio supermicron, Wyodak submicron and supermicron, Kentucky submicron, and North Dakota submicron and supermicron samples, cobalt is partially soluble at pH 5.0. For the Pittsburgh supermicron, Ohio submicron, and Kentucky supermicron samples, cobalt has very limited solubility at pH 5.0. At pH 2.9, cobalt is very soluble from the Ohio submicron and supermicron, Kentucky submicron, Wyodak submicron and supermicron, and North Dakota submicron and supermicron samples. For the Pittsburgh submicron and supermicron, and Kentucky supermicron samples, cobalt is partially soluble at pH 2.9. Information on the solubility of Al–Co, Ca–Co, and Fe–Co complexes is not readily available. The pH 5.0 leachability data help assess the potential for antimony and cobalt contained in submicron and supermicron particles to migrate into the water supply after ground deposition downwind of the combustor. In most cases, antimony and cobalt are soluble and compounds containing these elements on %y ash surfaces are expected to leach out of ash particles exposed to aqueous environments (e.g. rainfall).
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The pH 2.9 leachability data helps assess how antimony and cobalt might leach from an ash disposal pile=land1ll. For all of the coals studied, antimony and cobalt are moderately-to-very soluble and are likely to leach from the %y ash. 4.4. Calcium, iron, and aluminum solubility The relative solubilities of calcium, iron, and aluminum are shown in Fig. 4. Calcium is partially soluble at pH 5.0 for the Kentucky and Wyodak %y ash and very soluble at pH 5.0 for the other four coals. Calcium is very soluble at 2.9 for all 1ve coals. Iron is very soluble from the Pittsburgh %y ash at both pH levels but has limited solubility for the other four coals. Aluminum shows a great variety of solubility amongst the various %y ash samples. Most notably, Ohio submicron aluminum is substantially more soluble at pH 2.9 than aluminum in the bulk %y ash samples. It is important to note that the trace element solubilities shown in Fig. 3 do not correlate to any of the major element solubilities shown in Fig. 4. This suggests that the solubility of trace elements from %y ash is due to the physical dissolution of speci1c compounds not due to trace elements being released from the major element matrix structures (e.g. kaolinite, calcium carbonate, iron sulfate, etc.) of the %y ash as the major element compounds dissolve in the leaching %uid. Acknowledgements Financial support was provided by the US Department of Energy through contract DE-AC2295PC95101 under the overall direction of Dr. Constance Senior of Physical Sciences Inc. Technical contributions were also provided by E. Kitchen, University of Arizona. Appendix A. Experimental facilities Experimental work was conducted in a 6 m tall, 0:15 m ID vertical down%ow aerosol furnace. This furnace is designed to simulate the time=temperature histories and complex particle interactions of commercial-scale combustors while still providing a %ow stream that is suIciently well characterized to allow extraction of rates and mechanisms. The furnace, shown schematically in Fig. 5, is described in detail elsewhere (Seames, 2000). A pulverized coal was fed to the furnace using a volumetric screwfeeder. Coal was blown o9 the screw tip and into the furnace using a metered stream of 5 psig, transport air, then mixed with the primary air prior to discharge into the furnace. All of the experiments were run at a naturally evolving temperature pro1le, 20% excess oxygen in the combustion air, and slight vacuum pressure conditions. Fly ash particles were withdrawn from a sampling port located 4:3 m below the burner. Samples collected at this port represent typical conditions late in the post-combustion zone. Sampling conditions for the experiments performed in this study are included in Table 3. The temperature pro1les for the 1ve di9erent coals used in these experiments are shown in Fig. 6.
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Fig. 4. Solubility of (a) Calcium, (b) Iron and (c) Aluminum from submicron and supermicron %y ash particles collected on impactor membranes during the combustion of 1ve coals using the modi1ed TCLP leaching method.
Particle sampling was performed isokinetically using a portable water-cooled, aspirated sampling probe. Dilution nitrogen is fed to the sample tip and a vacuum pump is used to draw the nitrogen plus the furnace sample from the centerline of the furnace. The dilution nitrogen
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Fig. 5. University of Arizona down%ow combustor system.
Fig. 6. Furnace temperature pro1les from the validation experiments using the University of Arizona down%ow combustor system.
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Fig. 7. Simpli1ed schematic of isokinetic particle sampling and berner low pressure impactor collection system.
plus the cooling jacket on the probe serves to freeze subsequent reaction in the probe. It also condenses any species that will condense above atmospheric conditions (Davis, Gale, Wendt, & Linak, 1998). The sample is routed to a Berner-type low-pressure impactor (BLPI; Hillamo, & Kauppinen, 1991). This sampling system, shown in Fig. 7, is described in detail elsewhere (Seames, 2000). Sample sets that most closely duplicated the typical ash mass particle size distribution were selected for elemental analysis. Arsenic, selenium, cobalt, and antimony analyses were performed by graphite furnace atomic absorption spectroscopy (GFAA) while calcium, iron, and aluminum were analyzed by %ame atomic absorption spectroscopy (AAS). References Clarke, L. B. (1993). The fate of trace elements during coal combustion and gasi1cation: An overview. Fuel, 72, 731–736. Coney, A. M., & Hahn, D. A. (1921). A dictionary of chemical solubilities. New York: MacMillan. Cox, J. A., Lundquist, G. L., Przyjazny, A., & Schmulbach, C. D. (1978). Leaching of Boron from Coal Ash. Environmental Science and Technology, 12, 722–723. Davis, S. B., Gale, T. K., Wendt, J. O. L., & Linak, W. P. (1998). Multi-component coagulation and condensation of toxic metals in combustors. Proceedings of the Combustion Institute, 27, 1785–1791. Dean, J. A. (1992). Langes handbook of chemistry. (14th ed.). New York: McGraw-Hill. Dreesen, P. R., Gladney, E. S., Owens, J. W., Perkins, B. L., Wienke, C. L., & Wangen, L. E. (1977). Comparison of levels of trace elements extracted from %y ash and levels found in eSuent waters from a coal-1red power plant. Environmental Science and Technology, 11, 1017–1019. Dusing, D. C., Bishop, P. L., & Keener, T. C. (1992). E9ect of Redox potential on leaching from stabilized=solidi1ed waste materials. Journal of Air Waste Management Association, 42, 56–62. Finkelman, R. B., Palmer, C. A., Krasnow, M. R., Aruscavage, P. J., Sellers, G. A., & Dulong, F. T. (1990). Combustion and leaching behavior of elements in the argonne premium coal samples. Energy and Fuels, 4, 755–766.
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