Sulfur Cycle

Human activities influence the rates and character of certain aspects of the sulfur cycle in important ways, sometimes causing substantial environmental damages.

Acid rain is a well-known environmental problem. Acid rain is ultimately associated with large emissions of sulfur dioxide to the atmosphere by human sources, such as oil- and coal-fired power plants, metal smelters, and the burning of fuel oil to heat homes. The SO₂ is eventually oxidized in the atmosphere to sulfate, much of which is balanced by hydrogen ions, so the precipitation chemistry is acidic. In addition, the vicinity of large point-sources of SO₂ emission is generally polluted by relatively large concentrations of this gas. If its concentration is large enough, the SO₂ can cause toxicity to plants, which may be killed, resulting in severe ecological damages. In addition, atmospheric SO₂ can be directly deposited to surfaces, especially moist soil, plant, or aquatic surfaces, since SO₂ can readily dissolve in water. When this happens, the SO₂ becomes oxidized to sulfate, generating acidity. This means a direct input of sulfur dioxide is called dry deposition, and is a fundamentally different process from the so-called wet deposition of sulfate and acidity with precipitation.

Acid mine drainage is another severe environmental problem that is commonly associated with coal and metal mining, and sometimes with construction activities such as road building. In all of these cases, physical disturbance results in the exposure of large quantities of mineral sulfides to atmospheric oxygen. This causes the sulfides to be oxidized to sulfate, a process accompanied by the generation of large amounts of acidity. Surface waters exposed to acid mine drainage can become severely acidified, to a pH less than 3, resulting in severe biological damages and environmental degradation.

Sulfur is also an important mineral commodity, with many industrial uses in manufacturing. Sulfur for these purposes is largely obtained by cleaning sour natural gas of its content of H₂S, and from pollution control at some metal smelters.

In a few types of intensively managed agriculture, crops may be well fertilized with nitrogen, phosphorus, and other nutrients, and in such cases there may be a deficiency of sulfate availability. Because sulfate is an important plant nutrient, it may have to be applied in the form of a sulfate-containing fertilizer. In North America, sulfate fertilization is most common in prairie agriculture.