Last week in lab, Dario Caro spoke
to us about the greenhouse enhancement effect due to the environmental
pollutants carbon dioxide (CO2), methane (CH4), and Ozone
(O3). But when I was writing my article summary I also learned about
the detrimental effects of sulfur dioxide (SO2) pollution on wall
paintings in the city of Pompeii. The three compounds mentioned above are
generally considered the “bad guys” in discussions about the environment, but I
decided to learn more about sulfur dioxide as an environmental pollutant.
Sulfur dioxide is a poisonous,
pungent-smelling gas that is primarily produced by the burning of
sulfur-containing substances. The greatest sources of sulfur dioxide emissions
are man-made, as 93% of emissions are from the combustion of fossil fuels in
power plants and other industrial settings. Volcanic eruptions also emit sulfur
dioxide, which helps to account for the high SO2 content in the air
around Pompeii. Two major fuel sources that contain sulfur are coal and
petroleum.
The reaction that produces sulfur
dioxide is a simple combustion reaction with oxygen:
S8 +
8 O2 → 8 SO2
Industry’s use of sulfur-containing
fossil fuels for energy releases sulfur dioxide gas into the air, where it is
harmful to living species of all kinds. For humans, sulfur dioxide is a
respiratory irritant. Even short-term SO2 exposure (5 minutes to 24
hours) leads to airway constriction and other asthma-like symptoms. SO2
also forms particulates, which are basically just tiny bits of SO2
and other atmospheric compounds that float around in the air. These particles,
when ingested into the lungs, can cause or worsen respiratory diseases such as
emphysema and bronchitis, as well as heart disease.
Sulfur dioxide is also dangerous to
other living organisms because of the atmospheric formation of sulfuric acid, which
falls to the ground as acid rain. This occurs by an oxidation reaction, often
with nitrogen dioxide (NO2) as a catalyst. In this reaction, sulfur
dioxide gas reacts with a hydroxyl radical and is then oxidized by O2
to form sulfur trioxide, which is rapidly converted to sulfuric acid in the
presence of water to create acid rain:
SO3 (g)
+ H2O (l) → H2SO4 (l)
Acid rain affects both plant and animal
life, including freshwater and soils, which are the foundations of many
ecosystems. Acid rain lowers the pH of surface water, killing fish and
preventing their eggs from hatching. In soil, higher acidity destroys certain
microbes by denaturing their enzymes. The hydronium ions present in acid rain
also drain essential nutrients, such as calcium and magnesium, from the soil,
endangering plant life. By changing the composition of water and soil and
causing the death of certain plants and animals, acid rain can effectively threaten
the livelihood of entire ecosystems.
Luckily, sulfur dioxide emissions
in the U.S. have decreased by 33% since 1983 due to the Acid Rain Program set
up by the EPA (Environmental Protection Agency). Awareness of the dangers of SO2
pollution has led to new technologies to help keep this dangerous substance out
of the environment. One such technique is flue-gas desulfurization, which
removes SO2 generated in power plants by reacting it with calcium
oxide (lime) followed by oxidation, forming CaSO4, also known as
anhydrite. SO2 can also be removed prior to fuel combustion, and
sometimes calcium or magnesium oxides are added to gasoline to prevent SO2
production when the fuel is burned. These techniques have combined to reduce
the effects of sulfur dioxide pollution and acid rain, which is good news both for
us and for our plant and animal friends!
References:
http://en.wikipedia.org/wiki/Acid_rain
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