Mercury concentrations

The high temperatures in the MHD combustion system mean that no complex organic compounds should be present in the combustion products. Gas chromatograph/mass spectrometer analysis of radiant furnace slag and ESP/baghouse composite, down to the part per biUion level, confirms this behef (53). With respect to inorganic priority pollutants, except for mercury, concentrations in MHD-derived fly-ash are expected to be lower than from conventional coal-fired plants. More complete discussion of this topic can be found in References 53 and 63. 

Atomic Absorption Spectroscopy. Mercury, separated from a measured sample, may be passed as vapor iato a closed system between an ultraviolet lamp and a photocell detector or iato the light path of an atomic absorption spectrometer. Ground-state atoms ia the vapor attenuate the light decreasiag the current output of the photocell ia an amount proportional to the concentration of the mercury. The light absorption can be measured at 253.7 nm and compared to estabUshed caUbrated standards (21). A mercury concentration of 0.1 ppb can be measured by atomic absorption. 

Neutron Activation Ana.lysis, A measured sample activated by neutron bombardment emits gamma rays that are used to determine the mercury content by proton-spectmm scanning. Mercury concentrations as low as 0.05 ppb have been determined by this method. 

Colorimetric Method. A finely powdered sample treated with sulfuric acid, hydrobromic acid [10035-10-6] and bromine [7726-95-6] gives a solution that when adjusted to pH 4 may be treated with dithi one [60-10-6] ia / -hexane [110-54-3] to form mercuric dithi2onate [14783-59-6] (20). The resultant amber-colored solution has a color iatensity that can be compared against that of standard solutions to determine the mercury concentration of the sample. Concentrations below 0.02 ppm have been measured by this method. 

Note added in proof. J. C. Strijland and A. J. Nanassy (Physica 24, 935 (1958)) have shown recently that there are no changes of oscillator strength in argon + mercury. Increases in spectral intensity are entirely accounted for by increases in mercury concentration. 

Fig. 15-17 The effect of chloride on adsorption of mercury by hydrous iron oxide at constant total mercury concentration of 3.4 x 10 M. The lines represent the predicted adsorption assuming that Hg-Cl complexes do not sorb at all. (Reprinted with permission from P. V. Avotins, Adsorption and coprecipitation studies of mercury on hydrous iron oxides," 1975, Ph.D. dissertation, Stanford University, Stanford, CA.)...

How will mercury concentrations in air, land, water, and biota respond to changes in mercmy emissions This book proposes a framework for a carefully designed national-scale monitoring program, necessary but currently not in place in the United States, to help answer this question. 

While methylmercury occurs naturally in tlie environment, it is reasonable to expect that methylmercury levels have increased in modem times as a result of increased inorganic mercury concentrations. Whether methylmercmy concentrations have increased to a similar extent as inoiganic mercuiy is not known. It is clear, however, that elevated fish mercuiy concentrations can currently be found in remote lakes, rivers, reservoirs, estuaries, and marine conditions, typically in predators such as sportfish at the top of food webs. As of 2003, 45 states had fish consumption advisories related to mercuiy, and 76% of all fish consumption advisories in the United States were at least partly related to mercury (USEPA 2004a). The number of advisories is increasing with time, although this is due at least partly to more sites being sampled (Wiener et al. 2003). 

Slemr et al. (2003) attempted to reconstruct the global trend of atmospheric mercury concentrations from direct measurements since the late 1970s, and suggested that atmospheric mercury concentrations increased in the late 1970s to a peak in the 1980s, then decreased until the mid-1990s, and have been nearly constant since then. The authors noted, however, that this trend is not consistent with... 

Fish are often the focal point of interest for methylmercury contamination, representing the main exposure pathway for humans and wildlife. Unfortunately, longterm data sets with records of both mercury deposition and fish mercury concentrations over time are limited. In Sweden, Johansson etal. (2001) estimated that... 

Provide guidance regarding additional monitoring needed to help determine whether observed changes in mercury concentrations are related to regulatory controls on mercury emissions. 

Challenges eslabhshing baseline mercury concentrations and temporal trends... 

Challenges isolating the influence of changes in atmospheric emissions on mercury concentrations in the environment... 

Hrabik TR, Watras CJ. 2002. Recent declines in mercury concentration in a freshwater fishery isolating the effects of de-acidification and decreased atmospheric mercury deposition in Little Rock Lake. Sci Total Environ 297 229-237. 

Pollman CD, Porcella DB, Engstrom DR. (In preparation). Assessment of trends in mercury-related data sets and critical assessment of cause and effect for trends in mercury concentrations in Florida biota phase II. 

FIGURE 3.1 Observed mercury concentrations in standardized 50-cm walleye from Clay Lake, Ontario (1970-1983) following reductions in mercury releases from an upstream chlor-aUcali facility. (Source Data from Parks and Hamilton 1987.)... 

Cost concerns (here, High implies there are substantial costs associated with the indicator) Spatio-temporal variation in trophic position can confound and complicate interpretation of trends in mercury concentration in the indicator Low Medium Medium Medium to High Low, if done every 5—10 years Low Medium... 

Kelly CA, Rudd JWM, St. Louis VL, Heyes A. 1995. Is total mercury concentration a good predictor of methylmercury concentration in aquatic systems Water Air Soil Pollut 80 715-724. 

Wang W, Driscoll CT. 1995. Patterns of total mercury concentrations in Onondaga Lake, New York. Environ Sci Technol 29 2261-2266. 

Although the entry of MeHg into the base of the food web and its subsequent transfer in the lowest trophic levels are poorly understood, it is evident that the concentration of MeHg in all trophic levels is strongly correlated with its supply from methylating environments. In fish, for example, much of the modem spatial variation in mercury concentrations (within a given trophic level) can be attributed to variation in factors and processes that affect the microbial production of MeHg and its entry into oxic waters. 

In the United States, the threshold mercury concentration for commercial sale of fish is determined by the Food and Drag Administration, whereas consumption advice for recreational (noncommercial) fish is developed by individual states and tribes. Mercury data collected for development of fish-consumption advisories are typically from analyses of filets (axial muscle tissue, with or without skin) for total mercury, with concentrations expressed on a wet-weight basis. Analysis of filets for total mercury yields a valid estimate of MeHg concentration (Grieb et al. 1990 Bloom 1992), whether the analyzed sample consists of a large filet or a small mass of tissue obtained with a biopsy needle (Cizdziel et al. 2002 Baker et al. 2004). 

In summaiy, periphyton are present in all lakes, easy to sample, and would respond rapidly to changes in the abundance of MeHg. Periphyton are sometimes eaten directly by fish. The diverse, complex, and variable nature of the periphyton community, however, would complicate interpretation of mercury concentrations in periphyton in a monitoring program. 

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