Environ. Sci. Technol. 1997, 31, 2085-2091
Sunlight-Mediated Emission of Elemental Mercury from Soil Amended with Municipal Sewage Sludge A N T H O N Y C A R P I * ,† A N D STEVEN E. LINDBERG‡ Field of Environmental Toxicology, Boyce Thompson Institute at Cornell University, Tower Road, Ithaca, New York 14853, and Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6038
We studied the fate and atmospheric emission of mercury (Hg) from soil amended with municipal sewage sludge using a Teflon dynamic flux chamber. A sunlight-mediated, in situ reduction of oxidized Hg to volatile elemental mercury (Hg0) resulted in the atmospheric transport of Hg from land-applied sludge. The reduction of oxidized Hg to Hg0 occurred in a shallow layer of surface soil (90% closed, and the soil is naturally shaded from sunlight. The soil is classified as Colbert silty clay loam; it has an organic matter content (OM) of 8.7% (loss on ignition) and a pH of 6.14 (9). The surface soil was not disturbed during flux measurements at Watson Forest, and sludge was applied over litterfall. Nelson Field is a small agricultural field; the soil is classified as Montevallo shaly silt loam; it has an OM of 6.2% and a pH of 5.52 (9). At Nelson Field, the surface grass was first removed and the sludge was incorporated into the surface 2-3 cm of soil to mimic plowed, agricultural fields. The remaining two sludge-amended sites (Upper Watson and Hayfield) are routinely used by the City of Oak Ridge for surface application of stabilized sludge from their wastewater treatment facility. These two long-term application sites have been amended with sludge by surface spray for 5-10 yr. Longterm sludge site Upper Watson is located approximately 0.2 km south of Watson Forest and in the same deciduous forest catchment; the soil type and canopy cover here are similar to that at Watson Forest. Long-term sludge site Hayfield is located on a small, hill top field, approximately 0.66 km north of Barn Field, a background monitoring location (9). The soil at Hayfield is classified as Fullerton cherty silt loam (11); it has an OM of 24% (due to the long-term application of
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TABLE 1. Soil Hg0 Emission at Four Sites Amended with Municipal Sewage Sludge in Oak Ridge, TNa flux range site
mean emission (( 90% CI)
Watson Forest (canopy shade) Upper WatsonL-T (canopy shade) Nelson Field (shade) Nelson Field (sunlight) HayfieldL-T (shade-summer) HayfieldL-T (sunlight-summer) HayfieldL-T (shade-winter)d HayfieldL-T (sunlight-winter)
b22.9
8.9c
( 1.28 ( 0.8 b21.9 ( 4.3 b513 ( 65.3 27.3 ( 16.6 b513 ( 115 32.9 ( 16.0 357 ( 183
min
max
mean background flux (9)
9.40 0.20 9.44 333 9.51 334 25.6 171
42.7 2.66 50.1 711 59.3 647 43.7 566
2.7 2.7 -0.66 to 1.2 12.5 16.8 44.8
a All numbers are in ng m-2 h-1. Means at Watson Forest and Nelson Field do not include measurements taken the day of sludge application (see Figures 1 and 2). L-T designates long-term sludge-amended sites. b Significantly greater than background at p < 0.01. c Measurements May 23 through June 9. Mean for all data ) 16.6 ng m-2 h-1. d Winter shade at Hayfield was due to cloud cover; shade emission measurements are those in which solar radiation < 100 W m-2. At Nelson Field and Hayfield-summer, shading was with an opaque, plastic tarp (mean radiation under tarp ∼60 W m-2).
sludge) and a pH of 5.1. At Upper Watson, Hg flux was measured over litterfall; at Hayfield, flux was measured over a bare soil plot. To ensure that the plots chosen at the longterm sites had been treated with municipal sewage sludge, the soil was first analyzed for radionuclide contaminants endemic to the sludge matrix. As part of this project, total, methyl, and elemental Hg in sludge and sludge-amended soil were studied in detail, and these results are discussed elsewhere (12). In general, total Hg in sludge averaged 7300 ( 2500 ng g-1 over the course of this study. While this is slightly higher than reported sludge averages of 4000-6000 ng g-1 (4, 5), it is well below the U.S. Environmental Protection Agency’s monthly average, maximum allowable pollutant concentration of 17 000 ng g-1 for Hg in sludge for unrestricted land application (13). Total and methyl Hg concentrations in soil at all sludge-amended sites were significantly elevated over background soil (p < 0.07) (12). Soil Hg was highest at the two long-term sludge sites (Upper Watson and Hayfield) and approached sludge concentrations. Limited lysimeter data for soil water at one sludge-amended site did not show elevated total or methyl Hg during the 10 days after the surface application of sludge. Chamber Method. The primary species of Hg released by soil to the atmosphere is believed to be Hg0 (9). Soil Hg0 flux was measured using the ORNL FEP Teflon dynamic flux chamber (14). The design, use, and testing of this chamber are discussed in detail elsewhere (9, 12, 14). Briefly, triplicate samples of airborne Hg are collected on gold-coated sand traps at the inlet and outlet of the open-bottom chamber after it is sealed to the soil surface. Mercury collected on the gold traps is analyzed by cold-vapor atomic fluorescence spectrophotometry. Sampling and analytical precision of the replicate traps is excellent, with an average relative standard error for our measurements of (2.2% for inlet traps and (1.3% for outlet traps. Flux from the soil surface is calculated from the observed concentration difference between the inlet and outlet traps and the overall flow rate through the flux chamber:
F)
(Co - Ci) ×Q A
where F is the flux of Hg0 in ng m-2 h-1; Co and Ci are the mean Hg0 concentrations in ng m-3 at the outlet and inlet ports, respectively; A is the bottom surface area of the chamber (0.12 m2); and Q is the flushing flow rate through the chamber (0.3 m3 h-1). The contribution from the Teflon chamber surface itself to the Hg signal is measured by sealing the chamber bottom to a clean piece of Teflon and sampling inlet and outlet Hg concentrations over time periods comparable to those used over soil (14). In all cases, this chamber blank was small (mean ) 0.45 ( 0.2 ng m-2 h-1, p ) 0.0001, n ) 25) and much less than field measurements of soil mercury emission (9).
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Generally, flux measurements were approximately 60 min in duration. Several series of measurements over sludgeamended soil were conducted for 24 h. These experiments demonstrate that Hg0 emission closely tracks soil temperature (Figure 3, panels A and B, r 2 ) 0.86 and 0.82, respectively). The two experiments show a markedly similar diel pattern, with emission maxima and minima occurring near the same time of day. The relationship between Hg0 emission and soil temperature at Nelson Field and Hayfield (Figures 4 and 5, respectively) can be further examined using the Arrhenius equation (7, 9, 16). While one assumption of the Arrhenius
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FIGURE 4. Soil Hg emission versus surface soil temperature at Nelson Field.
FIGURE 5. Summer and winter soil Hg emission versus soil temperature at the long-term sludge site Hayfield. When the data from the two seasons are grouped as one set, r 2 ) 0.22. equation is that the reactions proceed through the formation of an activated complex that breaks down into products, the approach can be used to quantify the general dependence of the Hg0 emission process on temperature:
ln (Hg flux) )
-Ea + constant RTs
where Ea is the apparent activation energy for Hg0 emission, R is the gas constant (1.9872 cal K-1 mol-1), and Ts is the soil temperature in K. For the two field sites, these estimates of Ea are in agreement with each other [Nelson Field ) 34.1 kcal mol-1; Hayfield ) 37.2 kcal mol-1 (summer), 37.5 kcal mol-1 (winter)]. The Ea values for these two sites are elevated over those estimated for background soil (8, 9). The elevated Ea values may confirm the localization of the Hg0 emission activity at the soil surface. Because the diel effect on soil temperature decreases with soil depth, the apparent activation energy will be higher if production of the volatile species occurs at depths
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FIGURE 6. Mercury flux from soil at the long-term sludge field site Hayfield. In summer, soil was shaded with an opaque, plastic tarp suspended 1.5 m above the ground; in winter shade is due to cloud cover. Elevated soil Hg0 emission in winter despite low soil temperatures are correlated with high solar radiation.
FIGURE 7. Summer and winter soil Hg emission versus solar radiation at the long-term sludge site Hayfield. closer to the surface than the point of temperature measurement (surface probe at 1 to 2 cm) (17). An infrared surface temperature detector did indicate that, in sunlight, surface soil temperatures were several degrees higher than those recorded with the temperature probe. This discrepancy was not seen on shaded soil. When soil temperature data from the two field sites were corrected for this temperature discrepancy, the resulting Ea values were lower [Nelson Field ) 28.6 kcal mol-1; Hayfield ) 29.6 kcal mol-1 (summer)] and agreed more closely with our own background data (9) and with estimates for mercuriferous volcanic soil (28.0 ( 5.7 kcal mol-1) (16). Data from the two diel experiments at Nelson Field (Figure 3A,B) support a lower estimate for Ea (in shade: Ea ) 22.6 kcal mol-1; in sunlight: Ea ) 26.2 kcal mol-1). Our measured Ea values are greater than the heat of vaporization of Hg0 [14.5 kcal mol-1 at 20 °C (18)] and indicate that soil emission is not accounted for simply by the direct vaporization of existing Hg0. This supports the hypothesis of an in situ reduction discussed earlier. Despite the strong correlation with soil temperature noted above, our data suggest that solar radiation may be the
FIGURE 8. Soil Hg emission versus total solar radiation at sludgeamended site Nelson Field.
FIGURE 10. Mercury in ambient air during spring and summer 1995. Watson Forest, Nelson Field, and Barn Field represent background ambient air measurements. Hayfield measurements represent ambient air over a 10-ha sludge-amended field.
TABLE 2. Soil Hg0 Emission at Long-Term Sludge Site HayfieldsA Comparison Between ORNL Dynamic Flux Chamber and Simplified Micrometeorological Method (Modified Bowen Ratio) for Measuring Soil Hg flux (8)a date
start time
Hg0 flux - chamber
Hg0 flux - MBR
Feb 23, 1996 Feb 23, 1996 Feb 23, 1996 Feb 29, 1996 Feb 29, 1996
12:03 13:11 13:43 11:39 12:12
340 ( 114 566 ( 8.7 520 ( 6.9 189 ( 20.9 171 ( 9.8
370 570 570 240 220
a
FIGURE 9. Mercury emission response to sunlight and shading. An opaque tarp was used to shade Nelson Field (A, top) starting at 1104 and was removed at 1145. At Hayfield (B, bottom) the tarp was erected at 1111 and removed at 1154. Flux measurements were short in duration (average time ∼10 min) and separated by only 1-min pauses in an effort to identify rapid changes in soil Hg emission. Because of the short time intervals, individual flux estimates were corrected for residual airborne Hg remaining in the chamber from the previous flux measurement using transient purging equations for a continuous flow system (32). dominant factor affecting soil Hg0 emission. Soil Hg0 emission measured across seasonal cycles at Hayfield was more strongly correlated with solar radiation than with soil temperature (Figure 6). Wintertime Hg0 emission in sunlight at Hayfield
All fluxes are in ng m-2 h-1.
was significantly elevated over shaded, summertime flux despite comparable soil temperatures (Figure 6). In fact, Hg0 emission from Hayfield in sunlight (>513 ng m-2 h-1) was greater than that from a naturally-shaded forest site with a known history of contamination with metallic Hg0 (∼10-200 ng m-2 h-1) (19). The relationship between Hg0 emission and solar radiation at Hayfield was independent of season (r 2 ) 0.85, Figure 7), whereas the relationship with soil temperature differed drastically from summer to winter at Hayfield (Figure 5). Soil Hg0 emission was also highly correlated with solar radiation at Nelson Field (r 2 ) 0.90, Figure 8). In another experiment in which sunlight was physically manipulated at Nelson Field and Hayfield with an opaque tarp suspended ∼1.5 m above the ground, sunlight proved to be more influential than soil temperature on soil Hg0 emission (Figure 9A,B). In this experiment, rapid, continuous measurements of Hg0 emission were made before and after shading the sites with the tarp and again after the tarp was removed. Emission dropped rapidly at both field sites immediately after shading despite only a gradual decline in soil temperature. Mercury emission increased sharply after removing the tarp, again despite a more gradual change in
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TABLE 3. Average Soil Hg0 Emission from Background and Contaminated Areas contamination source none (background soil) planta
nuclear weapons Almade´ n mercury minea gold mill tailingsa municipal sewage sludge chloralkali sludgea a
mean Hg0 flux (ng m-2 h-1)
surface temp (°C)
study area
1-6 8-19 10-200 130-330 37-500 170-700 10 000-150 000
15-20 30-45 5-25 30-35 30-40 10-50 5-30
forest field forest laboratory field field laboratory
ref
9 19 15 30 this work 31
These areas were known to be contaminated with Hg0.
soil temperature. The response to sun and shade was strikingly similar between Nelson Field and Hayfield (Figure 9A,B). Both the magnitude of the emission measurements and the response to shade and sunlight were nearly identical at the two sites. This demonstrates that our hand-amended site (Nelson Field) mimicked the actual long-term, sludgeamended site (Hayfield) closely. The similarities between these two sites support the hypothesis that Hg0 is emitted from a shallow, surface layer of soil (
