Sulfur is mainly found on Earth as sulfates in rocks or as free sulfur. The largest deposits of sulfur in the United States are in Louisiana and Texas. Sulfur also occurs in combination with several metals such as lead and mercury, as PbS and HgS. Sulfur appears as the yellow aspects of soil in many regions.
Sulfur was mined early in the form of the yellow element and used for gunpowder and fireworks. While bacteria digest plant matter, they emit H2S, hydrogen sulfide, a gas that has the "rotten egg" smell characteristic of swamps and sewage. Sulfur is an essential element of biological molecules in small quantities.
Sulfur and its compounds are important elements of industrial processes. Sulfur dioxide (SO2) is a bleaching agent and is used to bleach wood pulp for paper and fiber for various textiles such as wool, silk, or linen. SO2 is a colorless gas that creates a choking sensation when breathed. It kills molds and bacteria. It is also used to preserve dry fruits, like apples, apricots, and figs, and to clean out vats used for preparing fermented foods such as cheese and wine.
Sulfuric acid, H2SO4, is a very widely used chemical. Over 30 million tonnes of sulfuric acid are produced every year in the U.S. alone. The acid has a very strong affinity for water. It absorbs water and is used in various industrial processes as a dehydrating agent. The acid in the automobile battery is H2SO4. It is used for "pickling" steel, that is, to remove the oxide coating from the steel surface before it is coated with tin or electroplated with zinc.
Sulfur is also a biologically important atom. Although only small amounts of sulfur are necessary for biological systems, disulfide bridges form a critical function in giving biological important molecules specific shapes and properties. (See Biological Systems.)
Sulfur is released into the atmosphere through the burning of fossil fuels --especially high sulfur coal--and is a primary constituent of acid rain. Sulfuric acid (H2SO4) is the primary constituent of acid rain (see Atmospheric System) in about all regions other than California. Sulfur dioxide and carbonyl sulfide (COS) occur in small quantities in the atmosphere; but due to its high reactivity, sulfur is quickly deposited as compound (sulfates) on land and other surfaces.
Figure S1 shows the biogeochemical cycle of sulfur. As in the case of nitrogen, the figure shows the large quantities. Local activities such as coal burning can release large amounts in a small area. Sulfur compounds can also be transported from the higher altitudes from tall "smoke stacks" and contribute to acid rain far from the sources.