6.1 General.
It is well known that there are large areas of the world where there are
naturally elevated levels of trace metals that have resulted in metal excesses
and chronic poisoning in people / animals that live in the area (Scoullos et al.
2000; UNEP 2002). Man’s use of the earth’s resources has been very
inefficient and wasteful. Through a lack of knowledge, and in some cases a
lack of care, he has contaminated the environment with industrial waste in the
rush to make money and exploit those resources. Environmental pollution is
increased by this attitude, and a combination of man made and natural
mercury releases to the environment has resulted in high levels of
contamination of land, water, air and food.
6.2. Airborne mercury.
Mercury in the air eventually ends up in rivers and lakes where it can be
readily taken up into the food chain. Lacerda (1997) estimated the yearly
global releases of mercury to the environment from gold extraction at up to
460 metric tonnes / year in the late 1980’s and early 1990’s, being equivalent
to about 10 % of total global anthropogenic releases. Mercury is present at
account for the presence of mercury within fossil fuels like coal, oil and gas
(WHO 1990; NTIS 2007). Nearer to New Zealand, Dhindsa et al. (2003)
estimated that 1903 tonnes of mercury was released to the environment at
Gympie in Queensland, Australia during 60 years of gold mining and, of that,
1236 tonnes were released into the air.
Recent estimates, which are highly uncertain, of annual total global mercury
emissions from all sources, natural and anthropogenic, are about 4,400 to
7,500 metric tonnes emitted per year. Fig 15 below provides information
about the worldwide distribution of mercury emissions.
Fig 15.
Source: Presentation by J. Pacyna and J. Munthe at mercury workshop in Brussels, March 29-30, 2004 quoted by USEPA mercury emissions: The
6.3. Mercury as a pollutant.
Mercury is a global pollutant with complex and unusual chemical and physical
properties. The major natural source of mercury is the degassing of the
Earth’s crust, emissions from volcanoes and evaporation from natural bodies
of water. The non–natural causes are man made.
Mercury is mostly present in the atmosphere in a relatively unreactive form as
a gaseous element (WHO 1991). The long atmospheric lifetime of its gaseous
form means the emission, transport and deposition of mercury is a global
issue (Lacerda 1997). Once in the environment, mercury is persistent,it never
goes away and mercury vapour is converted to soluble forms and deposited
by rain onto soil and water. The atmospheric residence time for mercury
vapour is up to 3 years, whereas soluble forms have a residence time of only
a few weeks (WHO 1990). Mercury will bioaccumulate and increase the
concentration in the environment and any biological organism exposed to that
environment over time (Heiserman 1992). Mercury occurs in three valence
states in nature: elemental mercury [metallic, Hg°], monovalent mercury
[mercurous, Hg2+2 ] and divalent mercury [mercuric, Hg2+]. Of the two ionized
states, monovalent and divalent mercury, the latter is more stable and more
The earth’s surface soils, water bodies and bottom sediments are thought to
be the primary biospheric sinks for mercury (UNEP 2002). The adsorption of
mercury to soil is dependent upon the organic content of the particular soil or
sediment (Blume & Brummer 1991).
6.4. Mining.
Gold mining is a source of mercury discharge to the environment as well as
being a source of harm to the health of the handlers. Mercury that is not
inhaled or washed away during the amalgamation process settles into the
surrounding environment, where it is absorbed and processed by a variety of
living organisms, this process transforms elemental mercury into methyl
mercury, one of the most toxic organic compounds and a powerful neurotoxin
and food chain bioaccumulator (UNEP 2005). Approximately 95% of all
mercury used in small-scale gold mining is released into the environment,
constituting a danger on all fronts - economic, environmental and human
health (EPA 1997). It is estimated that over 13 million people work as
artisanal miners [S6.5 p 68] worldwide (Agency 1999) and many of those are
6 .5. Artisanal.
Hentschel et al. (2002) has described artisanal mining as, “small-scale mining
by individuals, groups, families or cooperatives with minimal or no
mechanisation, often in the informal [illegal] sector of the market.” Artisanal
gold mining is a significant source of mercury release into the environment in
the developing world, with at least a quarter of the world's total gold supply
coming from such sources, according to the Basil Action Network (BAN
2007). Small-scale gold miners combine mercury with gold-carrying silt to
form an amalgam to make gold recovery easier. Much of the retorting to
recover gold from the mercury is done in the home using very primitive
equipment and polluting the home. A Chemical & Engineering (2007) report
stated 220 - 250 metric tonnes of mercury are discharged into the atmosphere
from gold mining in China, with Indonesia discharging 100-150 tonnes and a
number of other countries discharging variable amounts between 10 -30
tonnes. [Fig 15 page 65].
It is very difficult to pour mercury without splashing or spilling. Falling drops
break into small droplets, many of which are too small to be seen with the
naked eye. Such small droplets will not agglomerate, so that decontamination
of an area where mercury has been spilt is extremely difficult (Goldwater
6. 6 Food chain.
A very important factor in the impact of mercury to the environment is its
ability to build up in organisms and the food chain. Although all forms of
mercury can accumulate to some degree, methylmercury is absorbed and
accumulates to a greater extent than other forms. Whereas absorption is a
property of the body, bioavailability reflects the nature of the medium or matrix
(ATSDR 1999). Inorganic mercury can also be absorbed, but is generally
taken up at a slower rate and with lower efficiency than is methylmercury
(EPA 1997). Elemental mercury can be converted to methylmercury [CH3Hg]
in aquatic environments by microbial metabolism [biotic processes] such as
by certain bacteria and by chemical processes that do not involve living
organisms (Lindberg et al. 2001; Ullrich et al. 2001)
A typical pattern of biomagnification is shown in figure 16 on page 70.
“Inorganic mercury settles to the bottom sediment where bacteria transform it
to methylmercury through the process of biomethylation. It begins with a
hypothetical water concentration of 1ng/kg [or 1 part per trillion, 1ppt]. After
methylation, the methylmercury [CH3Hg] is readily absorbed and retained by
any organism in the food chain. Each organism eventually bioaccumulates
mercury to a concentration of about 10 times greater than in its food. Hence
The next, protozoa and zooplankton, would accumulate 100 ng/kg and so on
up the food chain and human or other predators (illustrated by a kingfisher)
consume fish with 1 million ng/kg [1 ppm] concentration. The entire process is
referred to as food chain biomagnification” (ATSDR 1999).
6.7. Atmospheric mercury.
The atmosphere is essential to man’s wellbeing and the pollution of it is
directly related to increased industrialisation and growing urban populations
and these are a greater source than natural pollution [e.g. global warming or
volcanic eruption].
6.7.1. Mercury cycle.
The environmental mercury cycle is further complicated because certain
forms of mercury are volatile (Heiserman 1992). Unlike most metal pollutants
whose movement is limited to erosion or leaching pathways, mercury is
readily transported in the atmosphere (Carpi 1997) and has an atmospheric
half-life of approximately one year (Lindqvist & Rodhe 1985). Elemental
mercury is eventually removed from the atmosphere by oxidation to water
soluble species and by dry deposition (Carpi 1997).
There are two main types of reactions in the mercury cycle that convert
mercury through its various forms: oxidation-reduction and methylation-
demethylation. In oxidation-reduction reactions, mercury is either oxidized to a
through the loss of electrons, or mercury is reduced, the reverse of being
oxidized, to a lower valence state (Environment Canada 2007).
The oxidation of Hg0 in the atmosphere is an important mechanism involved in the deposition of mercury on land and water. Elemental mercury Hg0 can volatilize relatively easily and be emitted to the atmosphere, where it may be
transported on wind currents for a year or more and be re-deposited in the
environment for further cycling. In contrast, Hg2+ has an atmospheric residence time of less than two weeks due to its solubility in water, low
volatility and reactive properties. Hence, when Hg0 is converted to Hg2+, it can be rapidly taken up in rain water, snow, or adsorbed onto small particles, and
be subsequently deposited in the environment through "wet" or "dry"
deposition (Environment Canada. 2007).
The partition of mercury is affected by environmental parameters such as pH,
temperature, redox changes, availability of nutrients and complexing agents
(Ullrich et al. 2001). Inorganic mercury has a propensity to bind with mineral
particle and detrital organic matter, whereas methylmercury tends to bind with
Fig 17. Pctorial depiction of the mercury cycle showing the deposition and volatisation of mercury.
Fig 17 is a simplified mercury cycle that shows the transport and fate of
mercury and any contaminated sediments into waterways. It includes overall
methylation reactions and bioaccumulation. The actual cycle is much more
complex.
6.7.2. Anthropogenic pollution.
Anthropogenic effects or processes are derived from human activities, as
opposed to natural effects or processes that occur in the environment without
human influences. The atmospheric total of vapour-phase mercury is
attributed to anthropogenic and natural sources (Nriagu 1989). Important
anthropogenic sources of mercury include the combustion of coal, municipal
solid waste and sewage sludge, mining and smelting of metals, and
production of chloralkali (EPA 1997). Natural sources of atmospheric mercury
include volcanoes, degassing from mercury mineral deposits, emission from
surface waters and natural terrestrial emission. In addition, water and soil are
affected by atmospheric deposition and thus are integral to the continual
global cycling of environmental mercury (WHO 1990).
In fig 18 on page 75 the anthropogenic emissions are roughly divided
between re-emitted emissions from previous human activity, natural
Fig 18. Natural and man made global mercury emissions.
Source: Seigneur et al. 2004. Mason and Sheu 2002 quoted by USEPA mercury emissions: The global context .
6.7.3. Speciation.
Speciation is the term commonly used to represent the distribution of a
quantity of mercury among various species of which the main groups are
elemental mercury, inorganic and organic forms (Carpi 1997). Speciation
organisms especially its biomagnification (Nriagu 1989). Atmospheric
speciation plays an important role in the long-range transport of mercury,
[Fig 16 page 70] as well as in deposition mechanisms (Lindqvist & Rodhe
1985)
The atmospheric chemistry of mercury involves several interactions (Pirrone
et al. 2001):
• Gas and aqueous phase reactions;
• Partitioning of elemental and oxidised mercury species between the
gas and solid phases; the gas and aqueous phases; and also the solid and aqueous phases
The change in speciation of mercury from inorganic to methylated forms is the
first step in the aquatic bioaccumulation process (Pirrone et al. 2001). This
can occur non-enzymically or through microbial action. (Carpi,1997; Pirrone et
al. 2001a). Organic matter affects the level of methyl mercury through
influencing the microbial activity and controlling the partition of Hg between
solid and dissolved phase by serving as complexing agents for Hg2+ and methylmercury (Lambertsson & Nilsson 2006).