Environmental Engineering Reference
In-Depth Information
Waterborne
Airborne
Reactive Hg (HgR) is DGHg plus
dissolved Hg(II). Note that in contrast to air,
reactive mercury includes Hg
0 .
Hg 0
Dissolved Gaseous Hg
(DGHg) is the sum of
DMHg and Hg
Hg(II)
Hg(II)
0 .
MMHg
PHg
DMHg
PHg
Hg 0
Total Airborne Mercury (TAM) is
often defined as the sum of three
species, Hg 0 , Hg(II) (reactive gaseous
mercury or RGM ), and PHg .
Total Hg (THg) is the sum
of all waterborne species.
FIGURE 1.1 A simplifi ed diagram of the major chemical forms of mercury found in water and air. See the section on “Chemical Forms of
Mercury in Water and Air” for a description of the forms. DGHg
dissolved gaseous mercury; DMHg
dimethylmercury; HgR
reac-
tive mercury; MMHg
monomethylmercury; PHg
particulate-bound mercury; RGM
reactive gaseous mercury; TAM
total airborne
mercury; THg
total mercury.
point of -39°C and a boiling temperature of 357°C. It also
has a rather high vapor pressure despite having a density
13 times greater than water and slightly greater than lead.
This anomalous behavior is caused by weak inter-atomic
bonding, which is due to the nucleus having a tight hold
on its valence electrons. Hg 0 is also relatively insoluble in
water (49 µg/L or 4.4 ppt at 20°C which is 4 to 6 orders of
magnitude smaller than the solubility of the predominant
Hg compounds) and will readily avoid liquids. So, it is often
described as preferring to be in the gas phase. Mercury can
also be found in two ionic forms, oxidation states
soil and ocean reservoirs, and therefore soil and ocean
emissions, include a component that is natural, plus a
portion resulting from human activities at a prior time.
Research has shown that most natural samples exhibit
measurable mass-dependent isotope fractionation and
some exhibit mass-independent fractionation. Although
this subfi eld of mercury research is in its infancy, it shows
promise in providing new insights into the sources and
history of ambient mercury (Bergquist and Blum, 2007).
1 and
CHEMICAL SPECIES OF MERCURY
Hg 0 is elemental mercury. Hg 0 has an anomalously high
vapor pressure (Brown et al., 2008) for a heavy metal.
It is slightly water-soluble (~50 µg/L at 20°C) (Clever
et al., 1985) and has a high Henry's law coeffi cient
(729 at 20°C) (Schroeder and Munthe, 1998). In the
natural environment, it can exist in the gaseous or
liquid state. Gaseous element mercury (GEM), is the
dominant form in the atmosphere. Most natural waters
are nearly saturated, or are supersaturated with respect
to atmospheric Hg 0 (Fitzgerald et al., 2007).
2, which are more prevalent in water than in the atmo-
sphere (Schroeder and Munthe, 1998).
A number of the mercury compounds that occur in the
environment have not been directly identifi ed. Rather, sev-
eral different fractions of mercury have been defi ned based
on how they are collected (e.g., on a fi lter) and how elemen-
tal mercury can be released from them (e.g., heating the fi lter
to 800°C). These distinctions, called “operationally defi ned
fractions,” are then used in place of specifi c compounds. The
properties of the different fractions (e.g., solubility, volatility,
etc.) are used in modeling the fate of mercury. As instrumen-
tation and techniques have developed, the defi nitions of the
fractions have evolved, and, not surprisingly, there is still
some controversy as to the names and defi nitions of some
of the fractions. A simplifi ed chart of the forms of mercury
that occur in water and air is shown in Figure 1.1. The major
operationally defi ned fractions are indicated with color.
One of the challenges of studying the cycling of mer-
cury is that because it is an element, it is never destroyed,
but it can be recycled through the environment. In specifi c
terms, mercury that is deposited to soils, lakes, wetlands,
or oceans may later be re-released to the atmosphere. So,
Hg(II) or divalent mercury. Inorganic and organic
divalent mercury compounds exist in gaseous,
dissolved, and solid states. Their toxicity, solubility,
vapor pressure, and reactivity vary greatly. Hg(II) is
much more prevalent in waters than in the atmosphere.
Methylated mercury compounds (below) are of
particular interest because of their role in the biologic
cycling of Hg and accordingly make up >95% of the
mercury in fi sh (Chen et al., 2008).
DMHg is dimethylmercury, (CH 3 ) 2 Hg. DMHg is
signifi cantly more toxic than Hg 0 on a milligrams
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