Global
Mercury Assessment
CHAPTER
2
Chemistry
2.1
Overview
Mercury occurs
naturally in the environment and exists in a large number of forms. Like
lead or cadmium, mercury is a constituent element of the earth, a heavy
metal. In pure form, it is
known alternatively as “elemental” or “metallic” mercury (also
expressed as Hg(0) or Hg0).
Mercury is rarely found in nature as the pure, liquid metal, but
rather within compounds and inorganic salts.
161.
Elemental
mercury is a shiny, silver-white metal that is a liquid at room
temperature and is traditionally used in thermometers and some electrical
switches. If not enclosed, at room temperature some of the metallic
mercury will evaporate and form mercury vapours. Mercury vapours are
colourless and odourless. The higher the temperature, the more vapours
will be released from liquid metallic mercury. Some people who have
breathed mercury vapours report a metallic taste in their mouths.
162.
Mercury is
mined as mercuric sulfide (cinnabar) ore. Through
history, deposits of mercuric sulphide have been the source ores for
commercial mining of metallic mercury.
The metallic
form is refined from mercuric sulfide ore by heating the ore to
temperatures above 540 ºC. This vaporises the mercury in the ore, and the
vapours are then captured and cooled to form the liquid metal mercury.
163.
Inorganic mercuric
compounds include mercuric sulfide (HgS), mercuric oxide (HgO) and
mercuric chloride (HgCl2).
These
mercury compounds are also called mercury salts. Most inorganic mercury
compounds are white powders or crystals, except for mercuric sulphide,
which is red and turns black after exposure to light.
Some mercury salts
(such as HgCl2) are sufficiently volatile to exist as an
atmospheric gas. However, the
water solubility and chemical reactivity of these inorganic (ionic)
mercury gases lead to much more rapid deposition from the atmosphere than
for elemental mercury. This
results in significantly shorter atmospheric lifetimes for these ionic
(e.g. divalent) mercury gases than for the elemental mercury gas.
164.
When
mercury combines with carbon, the compounds formed are called
"organic" mercury compounds or organomercurials. There is a
potentially large number of organic mercury compounds (such as
dimethylmercury, phenylmercury, ethylmercury and methylmercury); however,
by far the most common organic mercury compound in the environment is
methylmercury. Like the
inorganic mercury compounds, both methylmercury and phenylmercury exist as
"salts" (for example, methylmercuric chloride or phenylmercuric
acetate). When pure, most forms of methylmercury and phenylmercury are
white crystalline solids. Dimethylmercury, however, is a colourless
liquid.
165.
Several
forms of mercury occur naturally in the environment. The most common
natural forms of mercury found in the environment are metallic mercury,
mercuric sulphide, mercuric chloride and methylmercury. Some
micro-organisms and natural processes can change the mercury in the
environment from one form to another.
166.
Elemental
mercury in the atmosphere can undergo transformation into inorganic
mercury forms, providing a significant pathway for deposition of emitted
elemental mercury.
167.
The most
common organic mercury compound that micro-organisms and natural processes
generate from other forms is methylmercury. Methylmercury is of particular
concern because it can build up (bioaccumulate and biomagnify) in many
edible freshwater and saltwater fish and marine mammals to levels that are
many thousands of times greater than levels in the surrounding water.
168.
Methylmercury
can be formed in the environment by microbial metabolism (biotic
processes) and by chemical processes that do not involve living organisms
(abiotic processes). Although, it is generally believed that its formation
in nature is predominantly due to biotic processes.
Significant direct anthropogenic (or human-generated) sources of
methylmercury are currently not known, although historic sources have
existed. Indirectly, however, anthropogenic releases contribute to the
methylmercury levels found in nature because of the transformation of
other forms. Examples of direct release of organic mercury compounds are
the Minamata methylmercury-poisoning event that occurred in the 1950’s
where organic mercury by-products of industrial-scale acetaldehyde
production were discharged in the local bay, and the Iraqi poisoning
events where wheat treated with a seed dressing containing organic mercury
compounds were used for bread. Also, new research has shown that
methylmercury can be released directly from municipal waste landfills
(Lindberg et al., 2001) and
sewage treatment plants (Sommar et
al., 1999), but the general significance of this source is still
uncertain.
Being an element, mercury cannot be
broken down or degraded into harmless substances. Mercury may change
between different states and species in its cycle, but its simplest form
is elemental mercury, which itself is harmful to humans and the
environment. Once mercury has been liberated
from either ores or from fossil fuel and mineral deposits hidden in the
earth’s crust and released into the biosphere, it can be highly mobile,
cycling between the earth’s surface and the atmosphere.
The earth’s surface soils, water bodies and bottom sediments are
thought to be the primary biospheric sinks for mercury.
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Mercury exists in the
following main states under natural conditions
-
As
metallic vapour and liquid/elemental mercury;
-
Bound
in mercury containing minerals (solid);
-
As
ions in solution or bound in ionic compounds (inorganic and
organic salts);
-
As
soluble ion complexes;
-
As
gaseous or dissolved non-ionic organic compounds;
-
Bound
to inorganic or organic particles/matter by ionic,
electrophilic or lipophilic adsorption.
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Significance
of mercury speciation
170.
Different forms mercury exists in (such as elemental mercury vapour,
methylmercury or mercuric chloride) are commonly designated “species”.
As mentioned above, the main groups of mercury species are elemental
mercury, inorganic and organic forms. Speciation
is the term commonly used to represent the distribution of a quantity of
mercury among various species.
171.
Speciation plays an important part in the toxicity and exposure of
mercury to living organisms. Among other things, the species influence:
-
The
physical availability for exposure - if mercury is tightly bound to
in-absorbable material, it cannot be readily taken up (e.g. into the
blood stream of the organism);
-
The
internal transport inside the organism to the tissue on which it has
toxic effects - for example the crossing of the intestinal membrane
or the blood-brain barrier;
-
Its
toxicity (partly due to the above mentioned);
-
Its
accumulation, bio-modification, detoxification in – and excretion
from – the tissues;
-
Its
bio-magnification on its way up the trophic levels of the food chain
(an important feature particularly for methylmercury).
172.
Speciation also influences the transport of mercury within and
between environmental compartments including the atmosphere and oceans,
among others. For example, the speciation is a determining factor for how
far from the source mercury emitted to air is transported. Mercury
adsorbed on particles and ionic mercury compounds will fall on land and
water mainly in the vicinity of the sources (local to regional distances),
while elemental mercury vapour is transported on a hemispherical/global
scale making mercury emissions a global concern. Another example is the
so-called "polar sunrise mercury depletion incidence", where the
transformation of elemental mercury to divalent mercury is influenced by
increased solar activity and the presence of ice crystals, resulting in a
substantial increase in mercury deposition during a three month period
(approximately March to June).
173.
Moreover, speciation is very important for the controllability of
mercury emissions to air. For example, emissions of inorganic mercuric compounds (such
as mercuric chloride) are captured reasonably well by some control devices
(such as wet-scrubbers), while capture of elemental mercury tends to be
low for most emission control devices.
2.2
Mercury species and transformation in the atmosphere
174.
The atmospheric chemistry of mercury involves several interactions:
-
Gas phase
reactions;
-
Aqueous
phase reactions (in cloud and fog droplets and deliquesced aerosol
particles);
-
Partitioning
of elemental and oxidised mercury species between the gas and solid
phases;
-
Partitioning
between the gas and aqueous phases; and also
-
Partitioning
between the solid and aqueous phases in the case of insoluble
particulate matter scavenged by fog or cloud droplets.
175.
The interplay between mercury atmospheric processes and chemistry
is summarised in figure 2.1 below. The atmospheric speciation plays an
important role in the long-range transport of mercury, as well as in
deposition mechanisms. Atmospheric mercury transport is described in
chapter 6.
Figure
2.1
Model of interactions between mercury species in the atmosphere.
(Frontispiece of
the 2001 Special Issue of Atmospheric Environment
(vol. 35, no. 17) dedicated to
mercury research in Europe.) (Pirrone
et al.,
2001a)
176.
Since the first serious attempt at modelling the atmospheric
chemistry and speciation of mercury within the framework of a tropospheric
photochemical box model, which included fog and cloud chemistry as well as
particulate matter (Pleijel and Munthe, 1995), a number of additional
mercury atmospheric reaction parameters have been measured and two major
reviews of atmospheric chemistry have been published (Schroeder and Munthe,
1998; Lin and Pekhonen, 1999).
177.
The determination of the Hg(0)+ OH
(hydroxyl radical) gas phase rate constant (Sommar et al., 2001; Ariya et al.,
2002) and the very recent measurements of Hg+ halide atom rate constants (Ariya
et al., 2002) has shown that the
oxidation of elemental mercury (previously thought to occur mostly in the
aqueous phase, and only slowly in the gas phase as a result of reaction
with O3), in fact occurs relatively rapidly, and estimates of
the atmospheric lifetime of elemental mercury have had to be reduced from
around a year to matter of a few months. The rate of oxidation of
elemental mercury is fundamental to atmospheric mercury chemistry because
the oxidised mercury compounds (such as HgO and HgCl2) produced
are more soluble (and so are readily scavenged by clouds), less volatile
(and therefore more rapidly scavenged by particulates), and have a higher
deposition velocity. Thus oxidation may increase dry and wet deposition
fluxes and also deposition via PM. Oxidised mercury can also be reduced to
elemental mercury in atmospheric droplets, thus limiting the overall rate
of oxidation and deposition. The quantitative description of these
processes is associated with some uncertainty. (Munthe
et al., 1991)
178.
A simplified version of atmospheric mercury chemistry has been used
by Petersen et al. (1998) in a
regional scale dispersion/meteorological model.
While such a model can provide a reasonable approximation of
mercury transport and deposition, recent developments noted above on the
reaction of elemental mercury with both halides and hydroxyl radicals
indicate that these reactions must be incorporated in order to improve
model accuracy. The hydroxyl
reaction has been included in a Chemical Transport Model (Bergan and
Rohde, 2001), however, the results suggested that perhaps the rate
constant from Sommar et al. (2001) was too high, whereas recently published results from
Ariya et al. (2002) suggest that
it may be too low. Clearly, if atmospheric oxidation processes are faster
than previously thought, then in order for the hemispherical background
concentration to remain as stable as it does, emission (or re-emission) of
Hg(0), most probably from the sea, also occurs at a faster rate than once
supposed.
179.
The tropospheric chemistry of mercury has been much discussed in
the last four or five years since the publication of the results of long
term measurements from the Arctic, (Shroeder et
al., 1998) where contemporaneously with tropospheric ozone depletion
events, seen periodically after polar dawn, the concentration of Hg(0)
diminished to as low as 10-20 percent of its typical value over a period
of three or four days. Since then this phenomenon has been confirmed by
further measurements of the concentration of Hg(0) and also of gas phase
oxidised mercury compounds (Lindberg et
al., 2002a) and mercury associated with particulate matter (Lu et
al., 2001), and mercury depletion has also been seen in Antarctica (Ebinghaus
et al., 2002). The results are
consistent with gas phase oxidation of Hg(0), probably by halogen atoms or
halogen containing radicals (Boudries and Bottenheim, 2000), and
subsequent condensation on to particulates or deposition to the snow pack.
This phenomenon has naturally caused concern due to the possible
toxicological effects of increased mercury input to a fragile ecosystem at
the time in which biological activity is increasing after the long polar
night.
180.
Another region of much interest in terms of the tropospheric
chemistry of mercury is the Marine Boundary Layer (MBL, i.e. the air
directly above the sea surface). Studies performed during European
projects have shown that the concentrations of oxidised mercury are as
high in the Mediterranean area as they are in the more industrial areas of
northern Europe (Pirrone et al., 2001b; Wangberg et al.,
2001, AE special issue). This fact is another example of how the accepted
view of mercury atmospheric chemistry has changed in the last few years.
At one time it was assumed that most if not all gas phase oxidised mercury
was due to direct emission from industrial sources, and that given its
solubility and higher deposition velocity, oxidised mercury would not be
found very far from these sources. Thus, the presence of these compounds
in the open sea of the Mediterranean during anticyclonic conditions when
transport is negligible (Sprovieri et
al., 2002) would not have been expected.
181.
Recent modelling studies of mercury chemistry in the MBL suggest an
important role for sea salt aerosol in mercury cycling (Hedgecock and
Pirrone, 2001; Hedgecock et al.,
2002). The presence of deliquesced sea-salt aerosol in the MBL provides
both a scavenging phase for oxidised mercury compounds resulting from the
gas phase oxidation of Hg(0) and also an almost unlimited supply of
chloride ions with which mercury can form aqueous phase complexes
resulting in high aqueous phase concentrations of Hg(II) in solution (Pirrone
et al., 2000). Interestingly,
many of the abrupt changes in tropospheric photochemistry seen at polar
dawn are repeated on a lesser scale each day in the MBL, as shown by the
recent discovery of sunrise ozone destruction in the MBL, (Nagao et
al., 1999). It is most likely therefore that the same reactions which
result in polar mercury depletion events, occur daily in the MBL, hence
the presence of notable concentrations of oxidised mercury compounds in
the MBL. The diurnal variation of Hg(II) compound concentrations (Sprovieri
et al., 2002; Hedgecock et
al., 2002) shows that oxidation is slower at night and also that
deposition is constantly removing these compounds from the atmosphere, and
thus mercury must be replenished either from the sea or the free
troposphere at more or less the same rate.
182.
Axenfeld et
al. (1991, as quoted by Pirrone et
al., 2001) concluded that as much as 60
percent of the anthropogenic emissions in Europe were in gaseous
elemental form, 30 percent as gaseous
divalent mercury and 10 percent as
elemental mercury on particles.
183.
Most of the emissions from combustion of fuels (an important source
of emissions) occur in the gaseous phase. In the combustion zone, mercury
present in coal and other fossil fuels is
thermally converted into the elemental form. While in the flue
gases, some of it may be oxidised, depending on
the presence of oxidizing constituents such as chlorine. The
oxidised form can be retained in modern flue gas cleaning systems. The
emission generation process for mercury during incineration of wastes is
similar, except that more mercury in the oxidised form is expected from
incinerators, due to the higher content of chlorine in waste matter than
in fossil fuels (AMAP, 1998).
184.
In table 2.1 an overview of the speciation of emissions from a
number of major anthropogenic source types is given. The table was
prepared by Pirrone et al.
(2001).
Table
2.1 Emission profiles (fraction of the total) of mercury from
anthropogenic sources, 1995
(table from Pirrone et
al., 2001).
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