Sampling and analysis of mercury and its compounds in the atmosphere
William H. Schroeder Environment Canada Downsview, Ontario, M3H 5T4 Since the late 1960s and early 1970s, growing recognition of the widespread occurrence and toxicological importance of mercury in the environment (Johnels et al:, 1967; Friberg and Vostal, 1972; D'ltri, 1972) has generated a requirement for highly sensitive, selective, and reliable methods for determining this trace contaminant in a diverse array of environmental matrices. Instrumental methods of analysis have almost entirely replaced the cumbersome, classical wet chemical schemes. As a result of the foregoing as well as more recent investigations, both inorganic and organic chemical forms of mercury are now known to be extensively dispersed
throughout the environment. Contributions to the atmospheric mercury burden come from both natural and anthropogenic emission sources (McCarthy et al., 1970; Eshleman et al., 1971; Van Horn, 1975). The behavior and importance of Hg as an environmental contaminant are intimately related to the special physical, chemical, and toxicological features (Tables 1 and 2) of this "heavy metal" belonging to the Group IIB elements in the periodic table. Thus, for example, mercury is the only metal (and the only element besides bromine) that is liquid at ordinary room temperatures. As a consequence, the vapor pressure of mercury in its elemental state is substantial even at ambient temperatures. In nature, mercury can be found in any one of three different oxidation states: elemental (0), mercurous (+1),
TABLE 1
Features of the toxicology of mercury and its compounds Mercury is the only metal (and, besides bromine, the only element in the periodic table) that is a liquid at room temperature. Mercury, in its elemental state and in several of its organometallic forms, is a highly volatile environmental contaminant. Mercury and its compounds (both inorganic and organic types) are widely dispersed in the environment. Organomercuriats, particularly monomethyl mercury, are highly toxic. Inorganic forms of mercury can be converted to highly toxic organic forms by microbial action in the biosphere. Mercury and its compounds have a pronounced tendency to bioaccumulate in the food chain and ultimately in humans.
394A Environ. Sci. Technol., Vol. 16, No. 7, 1982
and mercuric ( + 2 ) . Except for their novelty, insofar as they involve a binuclear cation, the compounds of mercury (I) are typical metallic compounds. All are ionized in aqueous solution; however, most—with the no-
TABLE 2
Physical and chemical properties of Hg° influence its behavior and importance as an environmental contaminanta Melting point Boiling point Density Vapor pressure
—38.9 °C 356.6 °C 13.53 g/cm 3 at 25 °C 0.246 Pa at 25 °C (1.85 X 1(r 3 Torr) Solubility 6.4 X 1(r 5 g/L H20 at 25 °C Ionization potentials 1st 10.4 eV 2nd 18.7 eV 3rd 34.3 eV Electrode potentials Hg 2 2 + + 0.789 V 2e~ 2Hg 2+ Hg + 0.854 V 2e- Hg 2+ 2Hg + 0.920 V 2 e _ Hg 2 2+ Hg 2 2+ -0.115 V Hg2+ 3 Falchuk et al., 1977; Krenkel, 1974; Levason and McAuliffe, 1977.
0013-936X/82/0916-0394A$01.25/0
© 1982 American Chemical Society
Sampling. These collector tubes are filled with silver wool and are used for field sampling of atmospheric mercury.
Monitoring system. This atmospheric mercury monitoring system comprises a sampling module with two silver wool collector tubes in tandem and a detector module with a dual beam/cell UV spectrophotometer.
Field unit. This portable, compact, instrumental phase atmospheric mercury concentrations.
Gold film detector. This portable mercury monitor utilizes a thin gold film detection technique; the detector output is displayed on a strip chart recorder.
unit was used for field measurements
of vapor-
Environ. Sci. Technol., Vol. 16, No. 7, 1982 395A
table exception of the nitrate and the perchlorate—are only sparingly soluble in water at ordinary temperatures. Another common feature of all mercury (1) salts is their tendency to disproportionate. Since mercury (I) does not form covalent bonds with other elements, organometallic dérivâtes containing mercury in a + 1 oxidation state are unknown. Probably the most characteristic property of mercury in its highest oxidation state is its tendency toward the formation of covalent rather than ionic bonds. Exceptions to this rule are the sulfate, the nitrate, and the perchlorate derivatives that are salts, being completely dissociated in aqueous solution and, by inference, in rain droplets or cloud water. Despite the obvious importance of knowing the identity of the individual chemical species of mercury present in the atmosphere, only a few attempts have so far been made to separate a n d / o r identify qualitatively the predominant compounds in ambient air (Campbell et al., 1973; Braman and Johnson, 1974; Soldano et al., 1975; Henriques and Isberg, 1975; Rawlings and Cooper, 1975). Based on the limited amount of information available to date, the principal mercury species found to be present in samples of ambient air are described in Table 3. Vapor-phase mercury Whereas conventional gaseous pollutants such as sulfur dioxide, carbon monoxide, and nitrogen oxides, for example, are usually present in ambient air at concentrations in the partper-million (ppm) or part-per-billion (ppb) range, vapor-phase mercury concentrations encountered in the atmosphere are normally in the partper-trillion (ppt) (v/v) or nanogram per cubic meter (w/v) range. Vaporphase a n d / o r particulate-phase mer-
cury concentrations indicative of three "types of environments" (remote and rural areas, urban areas, and industrial sites) are summarized in Table 4. Because of the low concentrations (trace to ultra-trace levels) of vaporphase mercury found in remote, rural, and even urban settings, continuous real-time monitoring of vapor-phase mercury (or particulate-phase mercury, for that matter) is generally not practicable even with the most sensitive types of detectors currently available. Thus, for most ambient air measurements, in order to collect a sufficient amount of mercury for quantitative analysis, it is necessary to sample over a period of time generally ranging from a fraction of an hour up to 24 hours or more. The actual sampling period required is contingent upon the mercury concentrations encountered and the sensitivity of the analytical method employed. This cumulative (or integrated) sampling technique allows selective preconcentration of the mercury in the air stream being sampled. However, the resulting data quite often lack sufficient time resolution for investigations of temporal a n d / o r spatial trends involving airborne mercury. Sampling—vapor-phase mercury Ambient air sampling for elemental mercury vapor a n d / o r volatile mercury compounds has primarily involved threee techniques for collection and preconcentration: • absorption into a liquid (e.g., aqueous KMnCXt) • collection by a solid sorbent (e.g., charcoal) • amalgamation on a metal surface
(e.g., silver wool). Selected information on these techniques is provided in Table 5. For acceptable collection efficiencies at ambient temperature, with either absorption or adsorption techniques, sampling flow rates must be kept relatively low, typically in the range from several hundred m L / m i n to a few liters per minute ("low-vol" sampling regime). Using a suitable analytical methodology subsequent to the sampling step, the mercury collected is then determined: • directly in the absorbing solution with the aid of a colorimetric reagent • after stripping the mercury (in its elemental form) from the absorbing solution with a stream of inert carrier gas • after solvent extraction or thermal desorption from a solid sorbent or noble metal surface. The collection efficiencies of the various noble metals, solid sorbents, and absorbing solutions described in Table 5 have only been tested with respect to a selected few mercury species (Hg°, HgCl 2 , RHgCl, and R H g R with R being a methyl, ethyl, or phenyl moiety). None of the trapping media appear to have a collection efficiency of 100% even for the few species tested; nevertheless, most, if not all, are highly efficient in trapping at least elemental mercury. In those instances where this parameter is desired, "total vapor-phase mercury" is most easily determined by subjecting the air sample to a pyrolysis step (with or without the use of a catalyst) followed by noble metal amalgamation of the elemental mercury
TABLE 4
Summary of recent data on atmospheric mercury levels for various types of locations Type of location
Concentration range (ng/m 3 )
Mean vaiue
TABLE 3
Principal mercury species reported to be occurring in the atmosphere Elemental mercury vapor Hg° Mercury (II) chloride vapor HgCI2 (and possibly other volatile inorganic compounds) Organomercury compounds Methylmercuric chloride CH3HgCI Dimethyl mercury (CH3)2Hg Particulate mercury—unknown inorganic and/or organic mercury species associated with airborne particulate matter/atmospheric aerosols
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REMOTE & RURAL AREAS Oceanic Particulate Vapor Terrestrial Nonmineralized Particulate Vapor Mineralized Particulate + vapor URBAN AREAS Particulate Vapor INDUSTRIAL8
