1. What is mercury?
- 1.1 In what forms does mercury exist?
- 1.2 How does mercury exist in the environment?
- 1.3 How can the form of mercury affect living organisms and the environment?
1.1 In what forms does mercury exist?
Mercury occurs naturally in the environment. Sometimes known
as quicksilver, it is a
heavy metal, like lead or
cadmium, that exists in different chemical forms:
- Elemental mercury
or metallic mercury is
the element in its pure, ‘un-combined’ form. It is a shiny,
silver-white metal that is liquid at room temperature, but
is rarely found in this form in nature. If not sealed off,
mercury slowly evaporates into the air, forming a vapour.
The quantity of vapour formed increases as temperatures
rise. Elemental mercury is traditionally used in
thermometers and some electrical switches.
- Inorganic mercury compounds
or mercury salts, more commonly found in nature, include
mercuric sulphide (HgS), mercuric oxide (HgO) and mercuric
chloride (HgCl2). Most of these are white powders
or crystals, except for mercuric sulphide which is red and
turns black after
exposure to light. Some
mercury salts, such as mercury chloride, also form vapour,
but they stay in the air for a shorter time than elemental
mercury because they are more
soluble in water and
- Organic mercury
is formed when mercury combines with carbon and other
elements. Examples of
organic mercury compounds
and methylmercuric chloride. The form most commonly found in
the environment is
1.2 How does mercury exist in the environment?
Several forms of mercury exist naturally in the environment,
the most common being
metallic mercury, mercuric
sulphide, mercuric chloride, and
Natural processes can change the mercury from one form to
another. For instance, chemical reactions in the atmosphere can
transform elemental mercury
Some micro-organisms can produce
from other mercury forms. Methylmercury can
accumulate in living
organisms and reach high levels in fish and marine mammals via a
concentrations increase in
the food chain).
Because mercury is one of the basic chemical elements, of
which all things are made, it cannot be broken down or degraded
into something else. Once released into the
biosphere through natural
events or human activities
(see Question 4),
mercury readily moves and cycles through the environment. Soil,
water bodies and the sediments underneath them are believed to
be the places where mercury comes to rest until it is ultimately
removed from the biosphere again.
1.3 How can the form of mercury affect living organisms and the environment?
Different forms of mercury
affect living organisms and the environment differently.
For exposed living
organisms, the form of mercury affects:
- how available it is to cause effects within the body;
- how it moves around inside the body;
- how toxic it is;
- how it
transformed and leaves the body;
- how it
up) along the food chain.
For the environment, the form of mercury influences how
readily it can move within and between the atmosphere and
oceans, and how far it can travel in the air. For instance,
elemental mercury vapour
can stay in the atmosphere long enough to travel around the
world, whereas other forms of mercury may fall back to earth
relatively close to their source.
Emissions of some forms of mercury into the air (for example
from industry) can be controlled more easily than others.
Inorganic mercury can be
removed from air emissions reasonably well, while
elemental mercury emissions
are more difficult to capture and eliminate.