Sulfur

Abstract

Sulfur is a nutrient for plants and animals; hydrogen sulfide is an odorous, toxic material; and sulfur dioxide is an air pollutant responsible for acid rain. Plants primarily use sulfate as a sulfur source, and sulfur containing amino acids in plants are important to animal nutrition. Sulfur compounds undergo oxidations and reductions in the environment. The most famous sulfur oxidizing bacteria are of the genus Thiobacillus that oxidize elemental sulfur, sulfides, and other reduced sulfur compounds releasing sulfuric acid into the environment—acid sulfate soils and acidic mine drainage result from sulfur oxidation. Bacteria of the genus Desulfovibrio use certain sulfur compounds as electron and hydrogen acceptors in respiration allowing them to decompose organic matter in anaerobic environments. Ferrous iron and other metals may react with sulfide in anaerobic sediments to form metallic sulfides, e.g., iron sulfide (iron pyrite). If such sediment later is exposed to oxygen, sulfides will be oxidized resulting in sulfuric acid production. Sulfide in anaerobic zones sometimes diffuses or is mixed into overlaying aerobic water at a rate exceeding the oxidation rate of sulfide—toxicity to aquatic animals can result. Sulfide toxicity is favored by low pH, because un-ionized hydrogen sulfide (H₂S) is the toxic form. Elevated sulfate concentration and the presence of sulfide degrades the quality of drinking water.