Mercury and Selenium

foreseeable future. China also is estimated to be the largest producer of mercury emissions (Dastoor and Larocque, 2004). A recent comprehensive study of anthropogenic mercury emissions in China (Streets et al., 2005) yielded a figure of 536 t of mercury for the year 1999 with coal combustion (all types) accounting for 38% of the total. Atmospheric mercury emission is an international problem as the troposphere provides effective global transport of mercury. Although the estimates vary, China produces about three times more mercury per ton of coal burnt than the USA because of the lack of modern pollution technology and limited use of cleaned coal. Knowledge of the mercury content, mode of occurrence, and regional distribution in Chinese coal is vital in order to assess the global atmospheric contribution from Chinese coal combustion.

Under the aegis of the USGS WoCQI Program, we have collected and analyzed 305 Chinese coal samples from mines with the highest production from 25 provinces, municipalities, or autonomous regions, which thus reflects much of the coal currently supplied for power generation and industrial use. The method of determination was routine cold-vapor atomic absorption spectroscopy using two methods of sample dissolution. The arithmetic mean, on an as-determined, whole-coal basis, of 305 samples is 0.15 ppm (1 sigma = 0.14), with a minimum of 0.02 ppm and a maximum of 0.69 ppm. All data averaged to a dry basis is 0.16 ppm. A small percentage (~ 9 %) of the mercury data were below the method detection limit of 0.02 ppm. Duplicate analyses and inter-laboratory comparisons suggest that analytical uncertainties can arise from imperfect sample splitting using a mesh sample size in some samples.

Research continues to assess relationships between mercury content in coal and other measured chemical and physical parameters and topical studies related to mercury mineralization in coals of Guizhou Province.

References

Dastoor, A.P., Larocque, Y., 2004, Global circulation of atmospheric mercury: a modeling study: Atmospheric Environment, v. 38, p. 147-161.

Streets, D.G., Hao, J., Wu, Y., Jiang, J., Chan, M., Tian, H., Feng, X., 2005, Anthropogenic mercury emissions in China, v. 40, p. 7789-7806.

show that pyrite is the primary host of mercury in bituminous coals, whereas the proportion of mercury present in organic parts of coal is generally greater in low-rank (lignite and sub-bituminous) coal. Many Eastern U.S. bituminous coals are “cleaned” prior to use in utility power stations, to reduce sulfur emissions. This coal preparation reduces sulfur content, primarily by removing pyrite from coal. In doing so, a portion of the mercury present may also be removed, as a co-benefit to sulfur reduction. Currently about 35% of the mercury that goes into coal-fired utility power stations in the U.S. is captured by air pollution control devices, whose primary function is to trap particulates from coal combustion, or sulfur emitted from flue gasses. Under the EPA Clean Air Interstate Rule (CAIR), the initial phase of mercury reductions from coal-fired utility power station will primarily result as a co-benefit of reducing other pollutants with existing control technology. Under the second phase of the plan, the EPA Clean Air Mercury Rule (CAMR), greater reductions of mercury emissions are specified, probably requiring new mercury-specific control technology for utilities. By understanding the mode of occurrence of mercury in coal, and the concentration of constituents that affect capture of mercury, such as chlorine, the USGS provides information needed to help predict and control mercury emissions.

Figure 1. Selective leaching results for 15 coal samples (12 from the U.S.). Yellow bars indicate the proportion of mercury leached with nitric acid, thought to be contained in sulfide minerals, such as pyrite. The fraction of mercury not leached (100 percent minus total bar height) represents organic-associated mercury not removed by the leaching process. Arrows indicate minimum values. Figure is from Palmer and others, 1998.

Figure 2. Plot of mercury in pyrite plotted versus arsenic content (log scale) based on reconnaissance laser ablation ICP-MS analysis of 6 U.S. eastern bituminous coal samples. Data points represent individual analysis points ranging in size from 10 to 50 micrometers. Results for pyrite grains analyzed indicate mercury concentrations that are 10’s to 100’s of times higher than mercury concentrations in the whole coal. Samples include an Ohio 5/6/7 blend (Ohio), Illinois #6 (Illinois), Pittsburgh Coal from West Virginia (Pitts), the Warrior Basin of Alabama (AL), the Upper Coalburg Coal from West Virginia (WV), and an eastern Kentucky coal (KY). Data are from Diehl and others, 2005 and Kolker and others, 2002.

References

Diehl, S.F., Goldhaber, M.B., Tuttle, M.L.W., Ruppert, L.F., Hatch, J.R., Koenig, A.E., and Lowers, H.A., 2005, Distribution of arsenic, selenium, and other trace elements in pyrite-filled structures in Appalachian coals of Alabama, Kentucky, and West Virginia: Proceedings of the 22nd International Pittsburgh Coal Conference, Pittsburgh, PA, September, 2005, 24 p., CD-ROM.

Kolker, Allan, Mroczkowski, S. J., Palmer, C. A., Dennen, K. O., Finkelman, R. B., and Bullock, Jr., J. H., 2002, Toxic substances from coal combustion- A comprehensive assessment, Phase II: Element modes of occurrence for the Ohio 5/6/7, Wyodak, and North Dakota coal samples: U.S. Geological Survey, Open File Report 02-224, 79 p.

Palmer, C. A., Mroczkowski, S. J., Finkelman, R. B., and Crowley, S. S., 1998, The use of sequential leaching to quantify the modes of occurrence of elements in coals: Proceedings of the Fifteenth Annual International Pittsburgh Coal Conference, 28 p., CD-ROM.

continuously since November of 2002 and was modified to collect samples for trace metal analysis beginning in November of 2004 (Figure 1).

Figure 1. View of the VA-08 MDN site showing the recording rain gauge (foreground) and an MDN precipitation collector (background). Precipitation samples are collected through one of two chimneys on the left side of the collector (one for mercury and one for other trace metals) when the moisture sensor activates the lid covering the sample chimneys. Photo by Allan Kolker, USGS.

Data generated from the sampler include weekly mercury and trace element concentration and deposition. These data show variations in mercury and other trace metal wet deposition across the country (Figure 2). Statistical and air mass trajectory modeling are being used to identify potential sources for the mercury and other trace metals, and suggest processes important in controlling wet deposition. Data from the VA-08 site are also being compared to data from the nearby Shenandoah National Park-Big Meadows MDN site (VA-28). The sites are within 31 kilometers of one another but vary in elevation (VA-08 = 183 m; VA-28 = 1,074 m) allowing for investigation on the effect of elevation on deposition.

Figure 2. MDN results for 2004, including the Culpeper VA-08 site. Warmer colors indicate areas of higher wet deposition while cooler colors represent lower yearly deposition. Mercury deposition and concentration maps available from the National Atmopsheric Deposition Program – Mercury Deposition Network.

notable for being quickly deposited from the atmosphere to the surface and are thought to be readily available for conversion to methyl mercury, a highly toxic form of mercury. Particulate mercury (Hg-P) is also quickly deposited and is often found in high concentrations near combustion sources. Although much lower in proportion than Hg^o, the greater reactivity and deposition rates of RGM and Hg-P make them a greater environmental concern. Chemical reactions that occur in the atmosphere can transform mercury between these various species.

Figure 1. USGS Mobile Mercury Laboratory operated by the USGS Wisconsin Water Science Center. Instruments include an atmospheric mercury speciation unit; NO_x, SO_x, O₃, and PM_2.5 analyzers; a meteorological station; and a wet deposition sampler. Photo by James Robinson, USGS.

Recently, USGS researchers from the Energy Resources Program and the Water Mission Area have started characterizing tropospheric Hg^o, RGM, and Hg-P concentrations in southern Alabama, near the Gulf of Mexico. The project relies upon the USGS Mobile mercury laboratory (Figure 1) which measures near real-time concentrations of the three mercury species (Figure 2). Potential sources of mercury to the region include a nearby coal-fired power plant and industrial emissions from nearby cities such as Mobile, Alabama; Pensacola, Florida; and New Orleans, Louisiana. Natural processes are also being investigated that could cause conversion of Hg^o to RGM or Hg-P.

Figure 2. Plot of preliminary RGM, Hg-P, and Hg^o concentration data determined by the atmospheric mercury speciation unit on the USGS Mobile Mercury Laboratory deployed at Weeks Bay, Alabama from 6/6/05 to 6/16/05 (Julian days 157 to 167). The gap in the data is a result a shut down during tropical storm Arlene.

Additional total suspended particulate samplers were used to collect atmospheric particulates on filters that are analyzed for mercury as well as a variety of other constituents (Figure 3). Combining the mercury speciation data with multivariate statistical methods and air mass trajectory modeling, potential sources of mercury and other trace metals can potentially be identified as well as processes that produce RGM and Hg-P.

Figure 3. High-volume (left) and low-volume (right) total suspended particulate samplers deployed at Gulf Shores State Park, Alabama. The high volume sampler collects particles on a large filter located under the triangular lid. The high volume filters were analyzed for mercury and soluble components including Cl and SO₄. The low volume sampler contains two filters housed in filters packs at each end of the tripod crossbar. One filter is analyzed for mercury and the other for other trace metals. Photo by Allan Kolker, USGS.

potential hazards to Gorlovka. An integrated environmental/human health study is underway to define mercury exposure levels, assess possible health effects to exposed individuals, and determine the feasibility of further epidemiologic studies on a larger scale. The study, Feasibility of Assessing Health Risks from Long-term Mercury Exposure in Gorlovka, Ukraine funded by the U.S. Civilian Research and Development Foundation, is part of USGS studies on health effects of inorganic substances in coal.

Figure 1. Defunct mercury processing plant at Nikitovka Mines, Gorlovka, Ukraine, operated from 1968 to 1991. A small portion of this site is currently used to recycle fluorescent lamps and batteries to recover mercury. Photo by Allan Kolker, USGS.

Figure 2. Average mercury content of four Nikitovka coal samples collected by USGS scientists and collaborators at Donetsk National Technical University, compared to mercury in active mines of the region and U.S. averages for in-ground coal, and for the Warrior Basin of northern Alabama. U.S. coal data are from Bragg et al., 1998. Note break in vertical scale between 1 and 10 ppm mercury.

Current work on human mercury exposure in Gorlovka began in August, 2005, following a more general study of mercury-rich coals in the region, funded by the NATO Science Program (Figure 2). In the current study, a group of 30 workers at a mercury recycling facility on the site of the defunct mercury extraction plant was selected for sampling hair, nails, blood, and urine, to assess their mercury exposure. In conjunction with tissue sampling, environmental samples were taken to assess mercury levels and potential exposure near the mercury mines and over a larger portion of Gorlovka (Figure 3). Additional tissue sampling is planned for non-exposed individuals in Gorlovka and in a nearby control municipality. Our work in Ukraine has the potential to be an important case study of human exposure to mercury.

Figure 3. Environmental sampling in Gorlovka by Kathryn Conko (USGS) to assess exposure of residents to mercury. Photo by Allan Kolker, USGS.