Environmental Engineering Reference
In-Depth Information
Air
Sunlight
g
Sunlight
Sunlight
Hg
0
(g)
g(g)
Hg(II)
g()
Hg(II)
Hg(II)
Hg
0
(g)
Sunlight
Sli t
Bacteria
Hg(II)
Hg(II)
Hg
0
(g)
Hg (g)
Hg (g)
Hg(II)
Hg(II)
CH Hg
+
CH
3
Hg
+
Biota
Biota
Phyto -
plankton
g
colloid
Hg
MeHg
colloid
MeHg
colloid
Zoo -
plankton
Z
H
g( )
II
Hg(II)
MeHg
plankton
particle
particle
particle
particle
Water
Land
Land
Fish
Fish
Bacteria
Bacteria
Hg(II)
Hg(II)
CH
3
Hg
+
g
3
HgS (s)
HS()
Sdi
Sediment
t
FIGURE 3.1
A biogeochemical cycle of mercury in the environment, illustrating the common forms of mercury often quantifi ed.
protective role of selenium from Hg intoxication (Yaneda
and Suzuki, 1997). Monovalent Hg, Hg (I), is found only in
dimeric salts such as Hg
2
Cl
2
(calomel), which is sparingly
soluble in water and therefore correspondingly much less
toxic than HgCl
2
(sublimate).
(Horvat, 2005). Monomethylmercury compounds are of the
greatest concern, as these highly toxic compounds are formed
by microorganisms in sediments and bio-accumulated
and biomagnifi ed in aquatic food chains, thus resulting in
exposures of fi sh-eating populations, often at levels exceed-
ing what is regarded as safe. MMHg is also bio-accumulated
and biomagnifi ed in terrestrial food chains. Although it is
less researched and not typically an issue for human con-
sumption, this is still a major concern for wildlife health.
Organo-Mercury Compounds
Organo-Hg compounds consist of diverse chemical structures
in which divalent Hg forms one covalent bond (R-Hg-X) or
two covalent bonds (R-Hg-R) with carbon. In environmen-
tal samples, organo-Hg compounds are, for the most part,
limited to the alkylmercurials monomethylmercury, mono-
ethylmercury and, more rarely, dimethylmercury, as well
as alkoxymercury compounds, and arylmercurials (phen-
ylmercury). Organo-Hg cations (R-Hg
) form salts with
inorganic and organic acids (e.g., chlorides and acetates),
and react readily with biologically important ligands, nota-
bly sulfhydryl groups. Organo-mercurials also pass easily
across biologic membranes, since the halides (e.g., CH
3
HgCl)
and dialkylmercury are lipid-soluble. The major difference
among these various organo-Hg compounds is that the sta-
bility of carbon-mercury bonds in vivo varies considerably.
Thus, alkylmercury compounds are much more resistant to
biodegradation than either arylmercury or alkoxymercury
compounds. The term “methylmercury” is used through-
out this text to represent monomethylmercury (MMHg)
compounds. In many cases, the complete identity of these
compounds is not known except for the MMHg cation,
CH
3
Hg
, which is associated either with a simple anion,
like chloride, or a large charged molecule (e.g., a protein)
Determination of Total Mercury
and Inorganic Mercury Species
There are numerous analytical techniques available for the
analysis of total Hg and the inorganic Hg species in envi-
ronmental samples. A brief summary of the methods avail-
able for analysis of Hg species in environmental samples is
given in Table 3.2. Traditional analytical methods for Hg
detection are largely based upon room temperature, gas
phase (often referred to as cold vapor), and atomic absorp-
tion techniques, but inductively coupled plasma mass spec-
trometry (ICP-MS) can also be used for parts-per-million
to higher parts-per-billion level measurements in solids.
More recently, cold-vapor atomic fl uorescence techniques
have been developed that allow determination at parts-
per-trillion and subparts-per-trillion concentration levels.
The most appropriate specifi c method is dictated by the
detection limit required to produce meaningful data, as
well as the sample size, sample matrix and potential inter-
ferences specifi c to the method.
