Environ. Sci. Technol. 2009, 43, 2282–2287
Evaluation of Potential for Mercury Volatilization from Natural and FGD Gypsum Products Using Flux-Chamber Tests S C O T T S . S H O C K , * ,† JESSICA J. NOGGLE,‡ NICHOLAS BLOOM,§ AND LISA J. YOST† Exponent, 15375 SE 30th Place, Suite 250, Bellevue, Washington 98007, Georgia-Pacific, 133 Peachtree Street NE, Atlanta, Georgia 30303, and Studio Geochimica, 4744 University Way NE, Seattle, Washington 98105
Received October 9, 2008. Revised manuscript received February 6, 2009. Accepted February 9, 2009.
Synthetic gypsum produced by flue-gas desulfurization (FGD) in coal-fired power plants (FGD gypsum) is put to productive use in manufacturing wallboard. FGD gypsum wallboard is widely used, accounting for nearly 30% of wallboard sold in the United States. Mercury is captured in flue gas and thus is one of the trace metals present in FGD gypsum; raising questions about the potential for mercury exposure from wallboard. Mercury is also one of the trace metals present in “natural” mined gypsum used to make wall board. Data available in the literature were not adequate to assess whether mercury in wallboard from either FGD or natural gypsum could volatilize into indoor air. In this study, mercury volatilization was evaluated using smallscale (5 L) glass and Teflon flux chambers, with samples collected using both iodated carbon and gold-coated sand traps. Mercury flux measurements made using iodated carbon traps (n ) 6) were below the detection limit of 11.5 ng/m2day for all natural and synthetic gypsum wallboard samples. Mercury flux measurements made using gold-coated sand traps (n ) 6) were 0.92 ( 0.11 ng/m2-day for natural gypsum wallboard and 5.9 ( 2.4 ng/m2-day for synthetic gypsum wallboard. Room air mercury concentrations between 0.028 and 0.28 ng/m3 and between 0.13 and 2.2 ng/m3 were estimated based on the flux-rate data for natural and synthetic gypsum wallboard samples, respectively, and were calculated assuming a 3 m × 4 m × 5 m room, and 10th and 90th percentile air exchange rates of 0.18/hour and 1.26/hour. The resulting concentration estimates are well below the U.S. Environmental Protection Agency (EPA) reference concentration for indoor air elemental mercury of 300 ng/m3 and the Agency for Toxic Substances and Disease Registry minimal risk level (MRL) of 200 ng/m3. Further, these estimates are below background mercury concentrations in indoor air and within or below the range of typical background mercury concentrations in outdoor air.
* Corresponding author email:
[email protected]; phone: 425-519-8722; fax: 425-519-8799. † Exponent, Bellevue, Washington. ‡ Georgia-Pacific. § Studio Geochimica; current address: Columbia Analytical Services, 1317 S. 13th, Kelso, WA 98626. 2282
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 7, 2009
Introduction This work was prompted in part by EPA’s planned Clean Air Interstate Rule and Clean Air Mercury Rule, which would have required the coal-fired power industry to increase the capture of mercury from 70% to 90% by 2018. Although the U.S. Court of Appeals subsequently vacated both rules, before EPA proposes new rulemaking, Congress will likely introduce a single, comprehensive bill that is equally or more stringent than the vacated rules. The prospective legislation and/or rule changes, coupled with ongoing concerns about mercury, raised questions about potential increases in mercury content of coal combustion byproducts (CCBs) such as flue-gas desulfurization (FGD) synthetic gypsum. Mercury concentrations in finished FGD gypsum wallboard have been reported in the range of 0.09-1.4 mg/kg, with concentrations dependent on many factors, including the coal source and the power plant characteristics (1-4). Screening analyses conducted in a companion study indicated that exposure pathways for mercury in current wallboard feedstocks (natural rock and FGD gypsum) are incomplete (5); however, insufficient data existed to evaluate mercury volatilization from wallboard in indoor air environments. Mercury is present in FGD gypsum in oxidized forms (Hg2+), primarily associated with impurities such as iron and aluminum (6). Oxidized forms of mercury cycle between adsorption and desorption at air-solid interfaces (for example, Yin et al. (7)); thus, mercury has the potential to desorb from FGD gypsum and volatilize into air. Mercury flux directly from FGD has been recently studied in the laboratory and under field conditions such as an FGD landfill (8-10). In general, dry FGD acts as a sink or adsorbs Hg2+, but repeated wetting and drying of the material produces a net desorption or mercury release. As with mercury flux from soils, flux from FGD was influenced by temperature, UV light, and moisture. Overall, field measurements have been comparable to natural background mercury flux from soils (9). While mercury flux from raw FGD has been characterized, flux has not been evaluated for products that incorporate FGD such as FGD gypsum wallboard.
Objective The purpose of this study was to assess mercury volatilization from gypsum wallboard in order to evaluate the potential for inhalation exposure in indoor air. A bench-scale method was developed to measure flux from wallboard samples. The measured values of elemental mercury flux were then used to derive conservative (health-protective) estimated ranges of potential mercury concentrations in indoor air, which were compared with risk-based concentrations and background concentrations to further evaluate the significance of the indoor air exposure pathway.
Materials and Methods Residential paper-faced gypsum wallboard was collected from the manufacturing line for three consecutive days at two facilities: one using natural gypsum rock mined in Nova Scotia as feedstock (n ) 3 boards) and the other using FGD gypsum (n ) 3 boards). The natural gypsum was assessed to provide a comparison for the FGD gypsum because as described above, it also contains unavoidable naturally occurring mercury. Table 1 describes characteristics of the power plant source of FGD gypsum used to produce the wallboard sampled. Mercury concentrations 10.1021/es802872n CCC: $40.75
2009 American Chemical Society
Published on Web 03/09/2009
TABLE 1. Power-Plant Characteristics for Synthetic (FGD) Gypsum coal rank sulfur SCR (NOx control) FGD (SO2 control) sorbent oxidation fines blow down
blended bituminous 2-3% no wet limestone forced, in situ no
in finished FGD gypsum wallboard have been reported in the range of 0.09-1.4 mg/kg, with concentrations dependent on many factors, including the coal source and the power plant characteristics (1-4). In this study, the mean total mercury concentrations in FGD gypsum collected from wallboard samples was 0.14 ( 0.004 mg/ kg. Thus, this study represents an evaluation with a “midrange” mercury concentration in FGD gypsum, rather than a high-end concentration, with respect to the wallboard industry overall. However, the FGD gypsum source used to manufacture the wallboard used in this study was the only source of FGD gypsum used in wallboard manufacturing by the study sponsor at the time the work was conducted. Variability in mercury concentrations was evaluated in a companion study at 15 natural gypsum facilities and 3 synthetic gypsum facilities, in six source material samples from a 12-month period for each facility (5). Samples for use in this study were collected from 3 consecutive days of production, which allowed for assessment of short-term variability. Ultraclean sample-handling techniques (EPA Method 1669) were employed to prevent mercury contamination from other sources in collection and analysis of the finished wallboard samples. For all analyses, mercury was detected using cold vapor atomic fluorescence spectrometry following the method of Bloom and Fitzgerald (11). To assess the representativeness of the wallboard (board) samples analyzed, variability of total mercury concentrations was examined in both types of board (natural and synthetic) at multiple levels: analytical variability; 10-cm scale variability within a board; 100-cm scale variability within a board; and 100-cm scale between boards produced on different days (3 consecutive days). In addition, total mercury concentrations were analyzed within the front and back paper from the wallboard and within the gypsum core of six 1-cm2 samples: three from natural gypsum wallboard (including one replicate sample) and three from synthetic gypsum wallboard (also including one replicate sample). The flux of elemental mercury (Hg0) from wallboard was measured in a small (5 L) glass and Teflon chamber (Figure 1), with air flow at 0.4 L/minute supplied by an air compressor or by bottled air and scrubbed of any ambient mercury by an iodated carbon trap followed by a gold-coated sand trap, and with controlled (room) temperature (22-24 °C) and ambient light. Flux-chamber tests were conducted using 0.0788 m2 × 0.5 in (1.6 cm) thick samples stood vertically in the chamber for full exposure (front and back) to the air flow. Accounting for the sample volume, the air exchange rate in the chamber was approximately 6.5 times per hour. The edges of the samples were taped using masking tape to eliminate exposed edges, representing an installed but unpainted condition. Samples were left unpainted because a paint film could confound assessment of mercury volatilization by acting as a source or sink, or could potentially limit volatilization from the board itself. Hg0 samples were collected from the flux chamber using two different methods: a method with iodated carbon traps, and a more sensitive
method with gold-coated sand traps to achieve lower detection limits. Using the iodated carbon traps and compressed ambient air, flux was measured from three natural gypsum wallboard specimens, three synthetic gypsum wallboard specimens, and one replicate synthetic gypsum sample (n ) 7). Moisture has been reported to affect the release of mercury vapor from coal combustion byproducts (8, 12, 13). The relative humidity of the compressed influent air was not measured in this study. Indoor humidity during the period of the study would likely have been in the range of 25-40%, as calculated using average climatic data on temperature and humidity ranges (14) and a simple webbased indoor relative humidity calculator (15). Relative humidity in the compressed indoor air supplied by the compressor would be lower than the estimated range of indoor values. Using the gold-coated sand traps and bottled air (relative humidity approximately 1%), the flux was measured from three natural gypsum wallboard specimens, three synthetic gypsum wallboard specimens, and one replicate sample for each gypsum type (n ) 8). Laboratory quality control included blanks, duplicates, certified reference materials (National Institute for Standards and Technology), matrix spike recovery, and initial and continuous calibration verification (EPA Method 1631).
Results Wallboard Variability Data. Samples were collected from the same boards used for the flux-chamber analyses to assess the variability of total mercury concentrations in wallboard. Samples were approximately 1 cm2 and included front and back paper and gypsum core. Overall, variability was relatively low in each of the categories evaluated (see Table 2), indicating that the wallboard samples used in the study were representative samples. Mercury concentrations were very low in the natural gypsum wallboard (0.00088 ( 0.00019 mg/kg), and analytical variability was higher than in the synthetic gypsum wallboard (Table 2) where concentrations were higher, but still relatively low (0.15 ( 0.02 mg/kg). Wallboard Component Data. Mean total mercury concentrations in gypsum from the wallboard core (excluding front or back paper) were 0.00095 ( 0.00008 mg/kg in natural gypsum, and 0.14 ( 0.004 mg/kg in synthetic gypsum. As for the results detailed in a companion study (5), these natural and synthetic gypsum mercury concentration ranges were within the available background concentration range for mercury in soil (16) and were well below the risk-based concentration for mercury in residential soils (EPA Region 9 residential soil preliminary remediation goal (17)). The concentrations in this study were lower than the mean total mercury concentrations of 0.021 and 0.38 mg/kg for raw feedstock of natural and synthetic gypsum, respectively, from the companion study (5). These differences may be explained partly by mercury loss during the drying, calcining, and forming processes (1-3). In samples of paper taken from the front and back of sample wallboard, the concentrations of mercury in backing paper were higher than those in facing paper (0.018 ( 0.005 versus 0.0065 ( 0.003 mg/kg, respectively). In the synthetic gypsum samples, mercury mass in paper covering was an insignificant part (
