Thursday, June 21, 2012

Does it Smell like Sulfur in here?


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.
 63% of fossil fuels are sulfur-containing 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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