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Oxygen Cycle
Oxygen,
like carbon and hydrogen, is a basic element of life. In addition, in
the form of O3, ozone, it provides protection of life by filtering
out the sun's UV rays as they enter the stratosphere. In addition to constituting
about 20% of the atmosphere, oxygen is ubiquitous. It also occurs in combination
as oxides in the Earth's crust and mantle, and as water in the oceans.
Early in
the evolution of the Earth, oxygen is believed to have been released from
water vapor by UV radiation and accumulated in the atmosphere as the hydrogen
escaped into the earth's gravity. Later, photosynthesis became a source
of oxygen. Oxygen is also released as organic carbon in CHO, and gets
buried in sediments. The role of oxygen in life is describe in the unit
on Biological Systems.
Figure
O1. The Oxygen Cycle
Oxygen
is highly reactive. A colorless, odorless gas at ordinary temperatures,
it turns to a bluish liquid at -183° C. Burning or combustion is essentially
oxidation, or combination with atmospheric oxygen. Figure O1 shows a very
broad overview of oxygen cycling in nature. The environment of oxygen
in numerous reactions make it hard to present a complete picture.
Oxygen
is vital to us in many ways (beside the most obvious--for breathing).
Water can dissolve oxygen and it is this dissolved oxygen that supports
aquatic life. Oxygen is also needed for the decomposition of organic waste.
Wastes from living organisms are "biodegradable" because there
are aerobic bacteria that convert organic waste materials into stable
inorganic materials. If enough oxygen is not available for these bacteria,
for example, because of enormous quantities of wastes in a body of water,
they die and anaerobic bacteria that do not need oxygen take over. These
bacteria change waste material into H2S and other poisonous
and foul-smelling substances. For this reason, the content of biodegradable
substances in waste waters is expressed by a special index called "biological
oxygen demand" (BOD), representing the amount of oxygen needed by
aerobic bacteria to decompose the waste. The result of not meeting the
oxygen demand is described in the section on the water
cycle (needs anchor to exact spot).
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