Energy & Fuels 2009, 23, 2035–2040
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Fly Ash Properties and Mercury Sorbent Affect Mercury Release from Curing Concrete Danold W. Golightly, Chin-Min Cheng, Linda K. Weavers, Harold W. Walker,* and William E. Wolfe Department of CiVil and EnVironmental Engineering and Geodetic Science, The Ohio State UniVersity, 470 Hitchcock Hall, 2070 Neil AVenue, Columbus, Ohio 43210 ReceiVed October 22, 2008. ReVised Manuscript ReceiVed January 29, 2009
The release of mercury from concrete containing fly ashes from various generator boilers and powdered activated carbon sorbent used to capture mercury was measured in laboratory experiments. Release of gaseous mercury from these concretes was less than 0.31% of the total quantity of mercury present. The observed gaseous emissions of mercury during the curing process demonstrated a dependency on the organic carbon content of the fly ash, with mercury release decreasing with increasing carbon content. Further, lower gaseous emissions of mercury were observed for concretes incorporating ash containing activated carbon sorbent than would be expected based on the observed association with organic carbon, suggesting that the powdered activated carbon more tightly binds the mercury as compared to unburned carbon in the ash. Following the initial 28day curing interval, mercury release diminished with time. In separate leaching experiments, average mercury concentrations leached from fly ash concretes were less than 4.1 ng/L after 18 h and 7 days, demonstrating that less than 0.02% of the mercury was released during leaching.
1. Introduction Of the 72 million tons of fly ash produced in the United States in 2006, 15 million tons, or 21%, was beneficially used in concrete, concrete products, and grout.1 It is known, however, that at least a fraction of the mercury released upon combustion of coal is captured by electrostatic precipitation or fabric filtration in the fly ash fraction. A recent study found that the mercury content of fly ash ranged from 13.0 to 650.6 µg/kg without activated carbon injection (ACI) for mercury control and 37.7 to 1529.6 µg/kg with ACI.2 In order to understand the fate and impact of mercury originating from the combustion of coal, the potential release of mercury upon incorporation of fly ash in concrete and concrete products must be known. Quantifying mercury releases from fly ash concrete is important for defining the ultimate capture of mercury from combustion sources, understanding indoor air quality issues and “green” benefits associated with the use of fly ash concrete in buildings, and predicting mercury releases to air and water upon final disposal of concrete in landfills. Previous studies examined the release of mercury from kilogram masses of concrete incorporating a single class F fly ash and activated carbon sorbent exposed to mercury under laboratory conditions.3-5 For air-cured, portland cement fly ash concretes, less than 0.1% of the total mercury contained in these * To whom correspondence should be addressed. Phone: (614) 292-8263. Fax: (614) 292-3780. E-mail:
[email protected]. (1) American Coal Ash Association. 2006 Coal Combustion Product (CCP) Production and Use SurVey; American Coal Ash Association: Aurora, CO, 2006. (2) U.S. Environmental Protection Agency. Characterization of MercuryEnriched Coal Combustion Residues from Electric Utilities Using Enhanced Sorbents for Mercury Control, EPA/600/R-6/008, USEPA: Washington DC, 2006. (3) Golightly, D. W.; Sun, P.; Cheng, C.-M.; Taerakul, P.; Walker, H. W.; Weavers, L. K.; Golden, D. M. EnViron. Sci. Technol. 2005, 39 (15), 5689–5693.
concretes was released over the initial 28-day curing period, suggesting that nearly all of the mercury was retained in the concrete.3,4 Moreover, the mercury fluxes measured from exposed fly ash concrete surfaces were similar to fluxes observed for multiple natural soils.6 Mean rates of mercury release generally increased with increasing fly ash and/or powdered activated carbon (PAC) substitution in the curing concrete. The mercury release further diminished following the initial 28-day curing interval, corresponding to expected mineralogical changes and loss of porosity and loss of water to cement hydration, in the curing concrete. The source and characteristics of fly ash influence the final properties of the cured concrete (e.g., porosity), as well as the associations of mercury with fly ash in the concrete mix. For example, class C fly ash produced from lignite and subbituminous coals contains higher levels of calcium oxide but lower levels of sulfur and chlorine than class F fly ashes derived from anthracite and bituminous coal. Despite the significant use of fly ash in concrete, no prior research has been conducted to examine how the release of mercury from fly ash concretes is affected by the source and properties of fly ash. Differences in the physical and chemical properties of fly ashes from various combustion sources necessitate further study of mercury release beyond the previous work focusing on concretes prepared from a single class F ash. The goal of this research was to investigate the release of mercury during the curing of concretes made from fly ashes (4) Cheng, C.; Golightly, D.; Sun, P.; Taerakul, P.; Walker, H.; Weavers, L.; Wolfe, W. Mercury Emissions During Steam-Curing of Cellular Concretes that Contain Fly Ash and Mercury-Loaded Powdered ActiVated Carbon; Report #1008308; Electric Power Research Institute: Palo Alto, CA, 2005. (5) Golightly, D.; Cheng, C.-M.; Sun, P.; Weavers, L. K.; Walker, H. W.; Taerakul, P.; Wolfe, W. Energy Fuels 2008, 22, 3089–3089. (6) Zehner, R. E.; Gustin, M. S. Estimation of mercury vapor flux from natural substrate in Nevada. EnViron. Sci. Technol. 2002, 36, 4039–4045.
10.1021/ef800918v CCC: $40.75 2009 American Chemical Society Published on Web 03/20/2009
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Table 1. Mercury Concentration in Concrete Ingredientsa ingredient
Hg (µg/kg)
MER0357 fly ash NRT1019 fly ash MER032 fly ash CCS1 fly ash cement coarse aggregate MicroAir100 sand
1370 ( 40 1160 ( 20 350 ( 20 13 ( 2 8.01 7.42 1.52 1.37
a Note MER0357 and NRT1019 ashes contained activated carbon sorbent.
Table 2. Fly Ash LOI and Organic Carbon and Inorganic Carbon Content fly ash
LOI (%)
organic carbon (%)
inorganic carbon (%)
CCS1 NRT1019 MER032 MER0357
0.043 ( 0.005 4.44 ( 0.03 1.59 ( 0.03 1.96 ( 0.05
0.054 3.91 1.40 1.89
0.008 0.018 0.0064 0.0074
derived from lignite and subbituminous coal, with and without PAC injection for mercury capture. Air above curing concretes was sampled by iodated carbon traps to collect gas-phase mercury over a standard 28-day concrete curing interval. Sampling over an additional 28-day interval was carried out for select concrete to better understand the extent of mercury release subsequent to the primary curing period. The aqueous leaching of mercury from fully cured concretes also was examined. 2. Experimental Section 2.1. Concrete Preparation. The ingredients used in concrete were coal fly ash and commercially available type I portland cement, sand, coarse aggregate, and an air-entraining admixture (AEA). Four class C fly ashes originating from power plants that burn either lignite or subbituminous coal were utilized. The four fly ashes are designated CCS1, NRT1019, MER032, and MER0357. CCS1 is a fly ash produced from lignite coal. NRT1019, MER032, and MER0357 are ashes from subbituminous coals. MER032 is a “baseline” fly ash and MER0357 contains DARCO Hg-LH (Norit Americas, Marshall, TX), a brominated PAC from a full-scale mercury capture demonstration. NRT1019 was collected during a mercury capture demonstration using a nonbrominated DARCO powdered activated carbon. The major inorganic element composition of the fly ashes was determined by a microwave-assisted acid digestion procedure based on previous methods7 and is provided in Table S1 of the Supporting Information. All four ashes were determined to be class C ashes on the basis of the sum of aluminum, iron, and silicon oxides.8 The mercury content of the fly ashes and other concrete ingredients were determined by microwave-assisted acid digestion followed by cold vapor atomic fluorescence spectroscopy (CVAFS) by Studio Geochemica (Seattle, WA), and the data are shown in Table 1. Fly ashes also were characterized by combustion for loss on ignition (LOI),9 and organic carbon10 and inorganic carbon10 content, as shown in Table 2. High-purity water (18.2 MΩ cm) was used to (7) United States Environmental Protection Agency. Method 3052 MicrowaVe-Assisted Acid Digestion of Siliceous and Organically Based Matrices; USEPA: Washington, DC, 1996. (8) American Society for Testing and Materials (ASTM). Method C618 Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete; ASTM: West Conshohocken, PA, 2008. (9) American Society for Testing and Materials (ASTM). Method C31104 Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use in Portland Cement Concrete; ASTM: West Conshohocken, PA, 2004. (10) United States Environmental Protection Agency. Method 9060A Total Organic Carbon; USEPA: Washington DC, 2004.
prepare all concretes. Quantities of AEA, MicroAir 100 (Degussa, Cleveland, OH), added to each batch of concrete, were based on foam index measurements11 on the cement and fly ash. The formulation used for preparation of each concrete batch is presented in the Supporting Information (Table S2). Fly ash was used to replace 55% of the cement in the concrete mix. The contribution of mercury from each ingredient in the concrete and resulting total mercury concentration of the concrete is also provided in the Supporting Information (Table S3). For each batch of concrete, dry components first were mixed in a 20-L polypropylene (PP) container (Curtec, MO Industries, Whippany, NJ) by tumbling the closed container for 10 min. A weighed quantity of high-purity water (18 MΩ cm) then was poured gradually into the powder mix as stirring occurred. Mixing then was accomplished with a steel helical impeller driven at 450 rpm by a high-torque electric drill. The resulting wet concrete mixture then was sealed inside the PP container. A bead of silicone adhesive sealant (GE RTV-6700 series, Grainger, Lake Forest, IL) provided assurance of a gastight seal at the lid-container interface. This arrangement defined a sampling volume of approximately 4 L (headspace) for air in direct contact with the upper surface of the curing concrete. 2.2. Mercury Emission Measurements. The experimental system used to measure mercury emissions from curing concrete is shown schematically in Figure 1. Experiments were conducted to observe mercury release from (1) an empty sampling container to establish the emission baseline level of mercury (blank) for each experiment, (2) an ordinary portland cement (OPC) concrete that contained no fly ash to establish a mercury emission level for the basic concrete mixture, and (3) concretes for which fly ash replaced 55% of the portland cement. Iodated carbon (IC) traps (Studio Geochimica, Seattle, WA) were affixed at entrance and exit ports of the PP sampling container, and the container then was placed inside an environmental chamber where gas lines from a manifold were connected to the sampling IC traps. IC traps serve as effective collectors of multiple mercury species from the gas phase.12 IC traps at the entrance ports removed background levels of mercury present in ambient air within the environmental chamber. Sampling of the headspace gas above each concrete mass was accomplished by pulling air through a second IC trap, at a rate of 0.285 ( 0.010 L/min for 28 days. All gas lines used to interconnect components of the gas sampling system were silicone tubing (Dow, Midland, MI). Airflows were measured by glass-float rotameters (Cole Parmer, Vernon Hills, IL) with calibrations traceable to NIST. At the conclusion of a sampling interval, IC traps were removed from air sampling lines and each end immediately blocked with a Teflon plug. Then, each sealed tube was enclosed within a labeled zip-lock polyethylene bag and shipped to Studio Geochimica (Seattle, WA) for analysis. The iodated carbon from each trap was digested in a 15 mL mixture of nitric and sulfuric acids (70% HNO3 and 30% H2SO4, by volume). Following digestion, the resulting mixture was diluted to 40 mL with 0.07 N BrCl solution. Mercury measurements were accomplished upon SnCl2 reduction and capture by gold-coated sand, followed by thermal desorption and quantification by a cold vapor atomic fluorescence spectrometer.12 The detection limit of mercury in the IC traps ranged from 0.3 to 0.7 ng per IC trap. For control experiments, background mercury was determined as a “sampling blank” by pulling mercury-free air through an empty sampling container. A sampling blank container accompanied each set of three identical concrete samples through the curing interval. Mercury from the air that passed through the container was collected at the exit port by an IC trap. This sampling blank was subtracted from quantities of mercury collected on individual traps for the associated set of concretes sampled at the same time. Furthermore, the OPC concrete served as a control reference for emissions from concretes that contained fly ash. (11) Kulaots, I.; Hsu, A.; Hurt, R. H.; Suuberg, E. M. Cem. Concr. Res. 2003, 33, 2091–2099. (12) Bloom, N. S.; Prestbo, E. M.; Hall, B.; Von Der Geest, E. J. Water Air Soil Pollut. 1995, 80 (1-4), 1315–1318.
Hg Release from Fly Ash Concrete
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Figure 1. Schematic of the sampling setup.
2.3. Leaching Experiments. Concrete for leaching studies was removed from individual PP containers with conventional tools. Then, pieces from the central portions of the resulting concrete masses were broken loose by a chisel. Smaller pieces of concrete were transferred into two 0.1-mm-thick polyethylene bags for further crushing (inside the bag) with a drilling hammer until all the resulting material passed through a 0.95-cm stainless steel sieve (ASTM E-11). A weighed portion, 100 g, of each pulverized sample was placed into a labeled 2-L FLPE bottle (Nalgene, fluorinated high-density polyethylene, Rochester, NY), and the FLPP-capped (Nalgene fluorinated polypropylene closure, Rochester, NY) bottle then was wrapped in aluminum foil to reduce the possibility of photochemical reduction of any mercury potentially on particle surfaces. Concrete samples prepared as described above were subjected to toxic characteristics leaching procedure (TCLP) and synthetic precipitation leaching procedure (SPLP) tests described in EPA methods 1311 and 1312, respectively. TCLP leaching was carried out with acetic acid, and SPLP leaching was conducted with leaching solution characteristic of eastern U.S. rainwater, as specified in the methodologies. Leachate samples were removed for testing after 18 h and again after 7 days. Leachate samples were filtered (0.7 µm) and ∼200 mL was placed into trace metal-free glass bottles and sent to Studio Geochimica for mercury determinations by CVAFS. Complete recovery of mercury in leachate samples was accomplished by addition of BrCl according to previously published methods.13 A reagent blank was included in the leaching process along with each group of concretes.
3. Results and Discussion 3.1. Mercury Release During 28-Day Curing. During the initial 28 days of curing, total quantities of gaseous mercury collected from air passing over concrete ranged from 149 to 1549 ng or 5.8 to 60 ng/kg of concrete. Release of small quantities of mercury from OPC concrete (concrete without fly ash) occurred during the curing process. Average mercury release from OPC concrete (Table 3) was 5.8 ( 0.8 ng/kg or 0.11% of mercury present from the ingredients mixed into OPC concrete, including cement, sand, coarse aggregate, and water. This value is close to, but slightly higher than, the 0.068% mercury released from OPC concrete (OPC-2003 in Table 3) in a previous study.3 (13) Parker, J. L.; Bloom, N. S. Sci. Total EnViron. 2005, 337 (1-3), 253–263.
Table 3. Mercury Collected on Iodated-Carbon Traps over 28-day Curing Period Hg concentration (µg/kg)
Hg released from concrete
concrete ID
in fly ash
in concrete
ng/kg of concrete
%
OPC CCS1 NRT1019 MER032 MER0357 OPC-2003 FA-2003
s 12.8 ( 1.7 1164 ( 19 354 ( 21 1372 ( 37 s 119
5.2 5.6 89.3 30.4 104.4 4.1 12.6
5.8 ( 0.8 17 ( 3 17 ( 1 72 ( 4 60 ( 5 2.8 ( 0.7 9.5 ( 2.4
0.112 0.307 0.019 0.238 0.058 0.068 0.075
fly ash SO3 (%) 0.9 1.9 1.3 1.3
The release of mercury from concrete containing fly ash varied depending on the source of the fly ash and the presence of activated carbon sorbent (Table 3). Of the different fly ash concretes, concrete made with fly ash CCS1, derived from lignite coal, had the highest percent mercury release (0.307%). Concrete made with subbituminous fly ash MER032 had the second highest percent mercury release (0.238%). Significantly less mercury was released from concrete containing MER0357 (0.058%), fly ash obtained from the same utility as MER032 but containing Darco Hg-LH sorbent. Concrete made with fly ash NRT1019, also a fly ash derived from subbituminous coal with activated carbon sorbent, had the lowest percent mercury release of any of the concretes tested (0.019%). The differences in the percent release of mercury from the concretes may be related to factors similar to those controlling the removal of mercury by ash in combustion flue gases. Mercury capture from flue gas generated from the combustion of lignite or subbituminous coal has been observed to be low, compared to capture following combustion of bituminous coal.14 The lower level of mercury capture in systems burning lignite or subbituminous coals has been attributed to the lower carbon content of these ashes and the lower chlorine levels, which reduce oxidation of elemental mercury.15 Huggins et al.16 have shown by X-ray absorbance spectroscopy that mercury forms a variety of surface species on unburned carbon, and most of (14) Kilgroe, J.; Senior, C. Fundamental Science and Engineering of Mercury Control in Coal-Fired Power Plants. Proceeding, Air Quality IV Conference; Arlington, VA, September 2003; pp2224. (15) Senior, C. L.; Johnson, S. A. Energy Fuels 2005, 19 (3), 859–863.
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Figure 2. Mercury release and mercury content of the different fly ash concretes.
these species interact with chlorine. Thus, differences in carbon and chlorine levels of coals may affect the speciation of mercury in these coal fly ashes. Therefore, the differences in percent release of mercury from these various concretes is potentially attributable to differences in the surface speciation of mercury. Interestingly, a previous study has shown that some bituminous and subbituminous fly ashes act as sinks for atmospheric mercury at room temperature, while certain lignite-derived ashes do not.17 This is qualitatively consistent with our results, which show the greatest percent mercury release from concretes made with lignite-derived fly ash. However, one should note that while concrete made with fly ash derived from lignite coal demonstrated the highest percent release of the different ashes examined, this fly ash had the lowest mercury content and 99.7% of the mercury was not volatilized from the curing concrete. In previous work using a single class F fly ash,3 we found that mercury release increased with increasing mercury concentration in the concrete. Mercury release and mercury content for the different concretes examined in the current work are shown in Figure 2. Data on mercury release and content for an OPC concrete (OPC 2003) and concrete made with a single class F fly ash (Fly Ash 2003) obtained in a previous study are shown for comparison.3 No clear trend was observed between the mercury content of the concrete and the percent mercury release. For example, the highest percent mercury released was for the concrete containing CCS1 fly ash, which had the lowest mercury content of any of the fly ashes tested. This suggests that differences in the interactions of mercury with various components of fly ash are potentially more important than the total mercury content. In our previous work, a single class F fly ash was used, and therefore, the speciation of mercury, or interactions of mercury with various fly ash components, was conserved for each batch. As a result, as the amount of fly ash in the concrete increased, as did the mercury content and the mercury release. For the different fly ashes examined in the current study, total mercury content is not a predictive parameter for estimating mercury release. 3.2. Significance of Fly Ash Organic Carbon Content. The different fly ashes examined in this work varied significantly with respect to LOI and organic carbon content (Table 2). The relationship between mercury released and fly ash organic carbon content for the different concretes is shown in Figure 3. Also shown in Figure 3 is the percent mercury release from (16) Huggins F. E., Yap N., Huffman G. P., Senior C. L. Identification of mercury species in unburned carbon from pulverized coal combustion. Proceeding, Air and Waste Management Association, 92 Annual Meeting, Pittsburgh, PA, June 20-24, 1999; pp 2116-2127. (17) Gustin, M. S.; Ladwig, K. J. Air Waste Manage. Assoc. 2004, 54, 320–330.
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Figure 3. Mercury release as a function of fly ash organic carbon content. Note that MER0357 and NRT1019 contain activated carbon sorbent.
concrete containing a class F fly ash tested in a previous study (FA-2003).3 The percent mercury released decreased as the organic carbon content of the fly ash increased. The class C fly ash with the highest organic carbon content tested was the NRT1019 fly ash, and this fly ash demonstrated the lowest percent mercury release. The concrete made with FA-2003 class F fly ash, which had an organic carbon content of 4.1%, demonstrated similarly low mercury release as the NRT1019. Concrete made with the fly ash with the lowest carbon content (CCS1) demonstrated the greatest percent mercury release. There is a conspicuous inverse association between mercury release and organic carbon content for concretes made with fly ashes not containing activated carbon, though only three data points were available for this least-squares regression fit. Concretes made with fly ashes containing activated carbon sorbent (MER0357 and NRT1019) had lower percent mercury release than would be expected given their organic carbon content and the inverse association established above. For example, the mercury release from concrete made with MER0357 (0.058%) is 4 times less than that for MER032 (0.238%). MER032 (1.40% organic carbon) is a baseline fly ash that contains no sorbent. In contrast, MER0357 (1.89% organic carbon) contains Norit Darco Hg-LH carbon sorbent. In instances where powdered carbon sorbents are used to capture mercury in flue gas, a significant fraction of mercury is expected to be sorbed to carbon sites on unburned coal particles and to reactive sites on brominated carbon powders, such as Norit Darco Hg-LH, injected into the flue gas. Our measurements cannot differentiate directly between these potential binding sites. However, the data suggest that for the MER0357 ash the manufactured activated carbon binds the mercury more strongly, resulting in less percent mercury release compared to organic carbon formed during the combustion process. Other differences in the fly ash properties, in addition to organic carbon content, may have influenced the extent of mercury release from the concretes. For example, the levels of sulfur also varied (Table S1, Supporting Information) between the ashes, with the NRT1019 ash having the highest sulfur (1.9% SO3) and CCS1 having the lowest sulfur level (0.9% SO3). Percent mercury emissions from concrete made with the NRT1019 ash were the lowest of the different concretes, while emissions from concrete made from the CCS1 were the highest. Krishnan et al.18 suggested that the activated sites causing Hg(0) adsorption in activated carbon may include oxygenated organic species and functional groups containing inorganic elements, i.e. chlorine (Cl) or sulfur (S). Our data suggest that increasing (18) Krishnan, S. V.; Gullett, B. K.; Jozewicz, W. EnViron. Sci. Technol. 1994, 28, 1506–1512.
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Energy & Fuels, Vol. 23, 2009 2039 Table 4. TCLP Mercury Concentrations and Leachate Final pH Values 18 h OPC CCS1 MER032 MER0357 NRT1019 a
7 day
Hg (ng/L)
pH
Hg (ng/L)
pH
31 ( 20 0.1 ( 1.6 2.4 ( 1.6 1.1 ( 0.6 3.2 ( 1.7
7.6 6.6 6.6 6.5 6.4
9.9 ( 1.6 UDLa 1.7 ( 1.7 4.1 ( 3.9 UDLa
10.6 ( 1.1 6.8 ( 0.2 7.0 ( 0.4 7.2 ( 0.7 7.1 ( 0.6
UDL: under detection limit.
Table 5. SPLP Mercury Concentrations and Leachate Final pH Values Figure 4. Mercury release rate during extended curing.
sulfur content of ash may noticeably decrease the release of mercury from concrete made with this ash. However, the variation in sulfur content (0.9-1.9% SO3) among the different fly ashes was relatively small. Moreover, the sulfur content of MER032 and MER0357 were similar, yet percent mercury release was significantly different, suggesting that activated carbon played a more important role in controlling mercury release. Associations with iron may also impact the release of mercury from the curing concrete. However, the correlation between fly ash iron content (as Fe2O3) and percent mercury released was weak (r2 ) 0.13). The levels of chlorine and bromine in the different fly ashes were not measured, and therefore, the possible effects of these halogens on the mercury release process are unresolved. 3.3. Mercury Release during Longer-Term Curing. For MER032 and MER0357 concretes, air sampling conducted over 4 weeks of curing beyond the initial 4 weeks revealed that mercury release diminished with time, as shown in Figure 4. On the basis of the data in Figure 4, mercury emission is expected to decrease to the 28-day curing time level observed for OPC concrete at curing times of 10.4 weeks for MER032 concrete and 12.3 weeks for MER0357. This slightly greater interval is perhaps a result of the reduced early strength of fly ash concrete19 compared to ordinary portland cement concrete. During curing, there is a loss of porosity for the concrete and loss of water to hydration reactions by components of the cement. Studies on water adsorption by activated carbon report that H2O is adsorbed on the carbon surfaces by means of hydrogen bonding.20 Oxygen complexes on carbon surfaces form primary adsorption centers, while adsorbed H2O molecules then can become secondary adsorption centers as the H2O vapor pressure increases. Chemisorption of Hg(0) is a dominant process for moisture-containing samples.21 Thus, the mercury emitted from other components in the concrete-making process may be adsorbed onto these secondary adsorption centers due to the water-to-binder ratio of 0.459 during concrete curing. As the hydration of cement proceeds, a build-up of a gelmembrane outside the carbon pores has been postulated. Once in the solidified waste form, activated carbon particles will retain most of the adsorbed mercury by forming a barrier outside of the activated carbon particles,22 which accounts for the overall low release rate of mercury in our experiments. 3.4. Mercury in Concrete Leachates. Leaching of mercury from fly ash concretes, either during in-service use or following (19) Babu, K. G.; Rao, G. S. N. Cem. Concr. Res. 1994, 24, 277–284. (20) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. ActiVe Carbon; Dekker: New York, 1988. (21) Li, Y. H.; Lee, C. W.; Gullett, B. K. Carbon 2002, 40 (1), 65–72. (22) Golightly, D.; Walker, H.; Weavers, L.; Wolfe, W. Mercury Leachability from Concretes That Contain Fly Ashes and ActiVated Carbon Sorbents; Report #1014193; Electric Power Research Institute: Palo Alto, CA, 2007.
18 h Hg (ng/L) OPC CCS1 MER032 MER0357 NRT1019
8.4 ( 3.8 1.1 ( 0.3 1.9 ( 0.3 3.2 ( 0.6 0.9 ( 0.2
7 day pH
12.3 ( 0.1 11.9 ( 0.1 11.8 ( 0.1 11.8 ( 0.0 11.8 ( 0.1
Hg (ng/L) 3.6 ( 0.6 0.2 ( 0.3 0.6 ( 0.3 1.4 ( 1.3 0.5 ( 0.7
pH 12.4 ( 0.0 12.0 ( 0.1 11.9 ( 0.1 11.9 ( 0.1 11.8 ( 0.0
demolition and disposal, represents another potential pathway for mercury release. In batch leaching experiments, very low levels of mercury were leached from the fly ash concretes at both 18 h and 7 days, in both the TCLP (Table 4) and SPLP (Table 5) tests. Leaching of other inorganic elements from these concretes is discussed elsewhere.23 Average mercury concentrations leached from fly ash concretes were less than 4.1 ng/L for all samples analyzed, which is significantly below the United States Environmental Protection Agency (USEPA) standard for drinking water of 2 µg/L. Leaching of mercury from fly ash concretes was less than the release for the OPC concretes in both TCLP and SPLP tests, perhaps due to the greater amount of carbon and possible lower permeability of the fly ash concretes. At least 99.98% of the mercury was retained in the fly ash concretes in all the leachate samples analyzed. Comparing the TCLP to the SPLP results in Tables 4 and 5, for most fly ash concretes the leaching of mercury during the two different leaching protocols was similar within the error of the measurements. An exception to this was the MER0357 concrete, which showed greater release of mercury at 18 h in the SPLP test compared to the TCLP test. The NRT1019 fly ash concrete, on the other hand, showed slightly greater leaching of mercury in the TCLP test at 18 h compared to the SPLP leaching over the same time interval. The leachate pH values for the various fly ash concretes were noticeably lower than leachate pH for OPC. For example, the 18-h leachate pH using the TCLP protocol ranged from 6.4 to 6.6 for the fly ash concretes, while a pH of 7.6 was observed for the OPC leachate. Previous studies indicate that the adsorption of mercury on ash24 and activated carbon24 increases as the pH increases. However, the mercury leachate data in Tables 4 and 5 show the opposite trend, namely, that mercury leaching was greater for the higher pH leachate solutions. For example, the 18-h mercury concentration in TCLP leachate was 3.2 ng/L or less for fly ash concretes but 31 ng/L for OPC concrete. A similar trend was observed for the 7-day data for the TCLP test, as well as for the SPLP test. Thus, differences in the leaching fluid and final leachate pH apparently were less important for the fly ash concretes than OPC concrete, perhaps due to the very low levels of mercury released. For the SPLP testing, mercury concentration in the leachate decreased with time for CCS1, MER032, and MER357. (23) Feng, Q.; Lin, Q.; Gong, F.; Sugita, S.; Shoya, M. J. Colloid Interf. Sci. 2004, 278 (1), 1–8. (24) Zhang, J.; Bishop, P. L. J. Hazard. Mater. 2002, 92 (2), 199–212.
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However, no statistically significant difference was determined in the leaching of mercury from MER357 and NRT1019 in the SPLP test at the different time intervals. The decrease in mercury at the longer leaching times suggests a fraction of the mercury released during the early stages was readsorbed onto components of the concrete or incorporated into secondary mineral phases formed from other leached elements, as has been suggested for the leaching of municipal solid waste incinerator (MSWI) ash.25 For the MER0357 and NRT1019, the statistically similar concentrations of mercury at the two leaching times were potentially related to the higher carbon contents of these ashes. For the TCLP testing, no statistically significant difference was observed in leaching over 18 h versus 7 days for any of the fly ash concretes. Some differences in leaching between the ashes were observed during the SPLP testing. Comparing the different fly ashes after 18 h of SPLP testing, NRT1019 and CCS1 fly ash concretes showed similar levels of mercury release, MER032 had higher mercury release compared to NRT1019 and CCS1, while MER0357 showed the highest mercury release. After 7 days in the SPLP test, CCS1 had the lowest mercury release, MER032 and NRT1019 had similar and higher mercury release compared to CCS1, and MER0357 again showed the highest mercury release. Previously, we established that gaseous mercury emissions from curing concrete were related to the carbon content of the fly ash, with release decreasing with increasing carbon content. When comparing the leaching of mercury from these concretes, however, no clear relationship between carbon content and mercury release was observed. While mercury release was lowest from the concrete containing NRT1019, which had the highest carbon content, mercury release increased with carbon content for the other fly ash concretes. These data suggest that mercury may be bound more (25) Dijkstra, J. J.; van der Sloot, H. A.; Comans, R. N. J. Appl. Geochem. 2006, 21, 335–351.
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strongly to the NRT1019 ash, resulting in lower release during leaching in the SPLP test, compared to the other ash samples. Little or no statistical difference was observed in the amount of leaching of mercury from the different fly ash concretes during the TCLP testing. 4. Conclusions The release of mercury from fly ash concretes to the gas phase for all samples was less than 0.31% of the total mercury content over the initial curing period and diminished thereafter. Release of mercury to the liquid phase during leaching was less than 0.02% for all fly ash concretes tested and below levels for concrete without fly ash (i.e., OPC). Mercury volatilization did not depend on the initial total mercury content in the concrete, but it did correlate with the organic carbon content of the fly ash. Mercury release to the gas phase decreased with increasing organic carbon. Activated carbon sorbents were more effective in reducing mercury volatilization than unburned carbon from coal. These results demonstrate that nearly all of the mercury initially present in the fly ash concrete was retained during curing and upon exposure to leaching fluids. Acknowledgment. This research was funded by the Electric Power Research Institute (EPRI), Palo Alto, CA. The authors extend thanks to Kenneth Ladwig, EPRI project manager, Thomas Bishop for statistical analysis support, Kevin Jewell for measurements of total carbon and inorganic carbon, Jennifer Parker for Hg analyses, and Jay Hunter for fabrication of our TCLP rotator. Supporting Information Available: Tables summarizing the percent alumina, calcium oxide, ferric oxide, silica, and sulfur trioxide in fly ashes (Table S1); the concrete formulations (all in kg) for individual 20-kg batches (Table S2); and the estimated mercury in 20-kg batches of concrete (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org. EF800918V
