Sulfur (S)

Sulfur is one of the most widely distributed elements on Earth and is a key to the planet’s ability to support life. It is believed that sulfur is one of the few elements at Earth’s core, and it is possible that sulfur played a role in the core’s formation. Sulfur is one of the key indicators of volcanic activity, surfacing during eruptions in liquid and gaseous forms. Sulfur has a number of useful industrial applications, most notably in agriculture and metallurgy. The element also helps scientists find clues about Earth’s continuing development and its geochemical processes.

Physical and Chemical Properties

Sulfur has been an important element throughout human history. The Greeks attributed it to the leader of the gods, Zeus, in light of his perceived power of lightning. Christians later considered sulfur as the fuel of Hell itself. Sulfur also has been known as brimstone, a Middle English term meaning “burning stone.”

Sulfur (whose chemical symbol on the periodic table is S) is a light, odorless, yellow, and solid element found predominantly in and around areas that experience volcanic activity (including hot springs). It is not soluble in water, but it may be dissolved in other liquids.

Sulfur has a number of allotropes (forms of an element with different physical and chemical properties). The alpha form is more yellow than the paler beta form. The most common allotrope of sulfur is the rhombic form, which is denser and more stable. Monoclinic sulfur, however, is less dense and appears in long needles.

Odorless and tasteless when it is inert, sulfur burns easily, emitting a blue flame and releasing sulfur dioxide gas, a strong and offensive odor. This element is highly reactive, bonding easily with other elements. Samples of pure sulfur are found underground and near volcanoes, but sulfur samples are not readily found above ground outside volcanic areas. However, sulfur is often found in other minerals, such as barium sulfate and iron pyrite (also known as fool’s gold).

Ten known sulfur isotopes exist. (Isotopes are forms of an element that have the same atomic number but different atomic weight.) Four of these isotopes are naturally occurring (S-32, -33, -34, and -36). The other six occur when exposed to radiation, as is the case for S-35 (a sulfur isotope commonly used as a tracer in medicine).

Along with oxygen, selenium, and tellurium, sulfur is considered one of the chalcogenides (or ore formers) on the periodic table. It is a nonmetal, but its atoms strongly retain electrons. Sulfur atoms can form long aggregates, sometimes in a stable, circular chain. Its polysulfide (more than one sulfur atom) ions are easily formed as a result, making bonding with other elements and compounds a relatively simple process.

Uses of Sulfur

Sulfur is one of the most prevalent elements on Earth. In addition to its benefits to the natural environment, sulfur and the compounds it forms have a wide range of applications.

For example, sulfur dioxide is often used in winemaking, as it reduces the amount of color lost during the aging process and has antioxidative and antimicrobial properties that act as a preservative. There is concern, however, that too much sulfur dioxide in the wine barrels could cause illness in consumers. An ongoing effort is trying to establish agreeable standards for the use of sulfur dioxide additives. Nevertheless, this additive remains common in this major industry.

Sulfur also has long been a key ingredient in gunpowder and other explosives and in the vulcanization of rubber, preventing the rubber from melting as the mixture is heated. Furthermore, sulfur is used as an insecticide and in the petroleum refining process. Additionally, it is a key ingredient in fertilizer, providing a basic nutrient to plants and crops.

The compounds in which sulfur is found also have a wide range of uses. For example, sulfuric acid is used in iron and steel production. Sulfur dioxide, meanwhile, is used as a disinfectant and bleaching agent. Magnesium sulfate (more commonly known as Epsom salt) is commonly used as a bath additive and laxative. Calcium sulfate is a common ingredient in alabaster and gypsum, which are used in the creation of such decorative items as vases and other artwork.

Because of its ability to form stable rings that in effect become electron traps (which attract and retain electrons), sulfur is one of the most effective electrical insulators on Earth. Researchers at one point used balls of pure sulfur in the generation of static electricity. One of the most effective high-voltage electrical insulator compounds is sulfur hexafluoride.

Sulfur and sulfuric compounds remain in high demand in light of their many uses. The petroleum industry is one source, as sulfuric acid is a by-product of the refining process. However, because this process only produces a set amount of sulfuric acid, the market becomes inelastic. Businesses are therefore always searching for new sulfur deposits and resources.

The Sulfur Cycle

Many different processes are ever-occurring on Earth to sustain life and environmental balance. One such process is the sulfur cycle. Nearly every form of life on the planet requires sulfur-based compounds. Although it is one of the most abundant elements on Earth, sulfur is not formed on the surface. Rather, it is believed to originate in the planet’s core (many scientists theorize that it played a role in the formation of the core when Earth was still forming). Sulfur is brought to the crust and surface through volcanic activity. Once brought to the surface, it bonds with a wide range of proteins, minerals, and other compounds.

There are two components to the sulfur cycle—atmospheric and terrestrial—working in concert. According to the sulfur cycle framework, this element is released from minerals through the chemical weathering process. The sulfur is released into the atmosphere, where it bonds with oxygen to form sulfates. These sulfates return to Earth’s surface in the form of precipitation, where they are consumed by plants, which in turn are consumed by animals along the food chain. When the animals and plants die and decompose, the sulfur is released back into the air and soil. Sulfur also returns to the oceans and other waterways and moves through marine ecosystems. The sulfur that is not absorbed by this ecosystem settles on the marine floor, where it combines with other sediment.

Although the sulfur cycle is invaluable to the myriad life forms on Earth, it also constitutes a danger when the delicate balance this cycle maintains is altered. For example, when a volcano erupts, it spews great volumes of sulfur high into the atmosphere. The sulfur that does not immediately return to the earth can block the sun’s rays. A large number of volcanic eruptions can eventually lead to climate change, triggering global warming and global cooling.

As humans continue to develop industrial capabilities, the sulfur cycle is again taken out of balance because of the enormous volume of industrial pollution (which includes sulfur) released into the atmosphere. Scientists are concerned that sulfates (a greenhouse gas) can contribute to global warming.

Mining and Sulfide Ores

Sulfur deposits are generally found underground, most commonly in areas near inactive volcanoes and salt domes (protrusions in sedimentary basins in which large salt masses have been pushed outward). Because elemental sulfur melts at relatively low temperatures, the preferred method for mining this element involves the pumping of hot saltwater into a mine and using air pumps to extract the melted sulfur.

In this process the sulfur can be allowed to return to solid form or remain as a liquid, depending on the preferences of those who are purchasing it. This method has been dubbed the Frasch process. Its inventor, Herman Frasch, developed this mining practice in 1891 for the Standard Oil Company, enabling the launch of the Union Sulfur Company in 1892.

Sulfur also is found in ores (solid masses from which certain minerals may be extracted). For example, sulfide ore is a solid deposit of sulfide (a compound of sulfur with a negative charge, allowing it to bond with other minerals and elements) and other metals. Often, mining companies will extract sulfide ore to obtain other metals that are part of the deposit. This “involuntary” sulfur is then refined and sold. The trend of mining involuntary sulfur while pursuing other metals accounts for more than 70 percent of the sulfur produced in the world, which means that the Frasch process, in terms of sedimentary mining, is being replaced.

Sulfur and Climate Change

In addition to its many implications for life on Earth, sulfur plays a significant role in climate change. Indeed, one of the most voluminous gases spewed from a volcanic eruption is sulfur dioxide.

Sulfur dioxide may travel as high as the stratosphere, where it blocks the sun’s warming radiation, triggering a drop in Earth’s temperature by as much as 0.5 degree Celsius (0.9 degree Fahrenheit). Such events occur frequently; they take place nearly once every eighty years. However, in past, such occurrences were more frequent and happened every few to a dozen years. Scientists attribute a number of prehistoric ice ages to sulfur dioxide emissions from high levels of volcanic activity.

Another form of sulfur emission is threatening to create global climate change in the modern era. By the 1960s, the emissions caused by fossil fuel use amounted to the equivalent of a volcanic eruption every 1.7 years. Since that period, however, better awareness of the issue has led to the reduction of fossil fuel use through the increased use of more fuel-efficient vehicles and technologies and by the introduction of alternative energy sources.

Sulfur and its compounds also are involved in climate change as indicators. In a 2010 study of lakes in the Canadian Arctic and in Norway, researchers found evidence of sulfur accumulation that could be caused by an increase in algae development. When normally cold seasons are shortened, the continuing warmth causes an increase in algae growth. This growth kills other vegetation, and the resulting degradation of the plants leads to the release of sulfur. This sulfur is returned to the water, soil, and air as part of the aforementioned sulfur cycle. It is feared that an increase in the pace of this cycle could hasten major climate change. Scientists are carefully watching such trends.

Future Implications

Sulfur and sulfates remain important factors scientists consider when analyzing the ongoing issue of global warming and climate change. For example, later studies point to sulfur in the atmosphere binding in time with soot from industrial pollution. Because sulfur absorbs sunlight, this combination is considered by some scientists to be a possible remedy against global warming trends. This research is at an early stage, although it is clear that sulfur remains on the minds of scientists in pursuit of answers to the issues of global climate changes.

Similarly, sulfur is likely to remain an important natural resource for industry. Sulfuric acid is one of the most-used commodities in the industrial world. Some 265 million metric tons (292 million tons) of this sulfuric compound was consumed annually as of 2022. Experts believe demand for sulfuric acid will continue, especially from makers of fertilizer. In particular, markets are in search of acid that has been derived from natural sulfur deposits rather than that which has been produced from the petroleum refining process (which in comparison with natural deposit sources is a more inelastic source). Sulfur exploration for commercial purposes, therefore, is likely to continue for the foreseeable future.

Principal Terms

allotrope: form of an element with different physical and chemical properties

chalcogenides: nonmetallic elements that are known to form ores with certain metal elements; also called ore formers

monoclinic: a sulfuric type that is less dense than rhombic sulfur and that appears in the form of long needles

rhombic: a more common form of sulfur, with a dense and more stable composition

weathering: a geochemical process whereby rocks and minerals are broken down by water and wind

Bibliography

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