Sulfur

• 1. Introduction

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  Sulfur, the chemical element with the symbol S and atomic number 16, turned up on Mars in July 2024, and nobody was looking for it. The Curiosity rover drove over a rock and crushed it. Inside the broken stone sat crystals of elemental sulfur. The discovery was entirely accidental, and it suits an element with a habit of appearing in unexpected places.

  Sulfur has been known since ancient times, and it is still teaching scientists new things. Where did people first find it, and what did they do with it? Why do two spellings of its name compete, and why did the losing one make a comeback? What made the mines that supplied the industrial world so notorious? And how can a single element belong to an exploding star and to a living cell alike?

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• 2. Ancient Brimstone

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  The Ebers Papyrus describes a sulfur ointment that Egyptian healers used to treat granular eyelids. Around the Mediterranean, the element had other jobs. The Odyssey mentions sulfur as a fumigant in preclassical Greece. Pliny the Elder, in his Natural History, named the island of Melos as its best-known source. He also recorded its use in medicine and in bleaching cloth.

  In China, a natural form called shiliuhuang was known from the 6th century BC and was found at Hanzhong. By the 3rd century, the Chinese had learned to extract sulfur from pyrite. Daoists were fascinated by its flammability and by the way it reacted with certain metals. Its earliest practical uses, however, were in traditional Chinese medicine. In 1044 AD, the Wujing Zongyao set down formulas for Chinese black powder, a mixture of potassium nitrate, charcoal, and sulfur.

  From the eighth century AD, Indian alchemists of the rasaśāstra tradition wrote at length about combining sulfur with mercury. They called it gandhaka, which means the smelly one. European alchemists later gave it a symbol of a triangle atop a cross. They also kept a grey, impure form called black sulfur, or horse brimstone, which was usually the dregs of sulfur purification. It was swallowed as medicine even though it often contained arsenic.

  Sulfur creams were also a traditional treatment for scabies, ringworm, psoriasis, eczema, and acne. The mechanism of action is still unknown, although sulfur slowly oxidizes to sulfurous acid, a mild antibacterial agent. The older English word brimstone means burning stone. The King James Version of 1611 uses it in the Book of Genesis, where fire and brimstone destroy the cities of Sodom and Gomorrah. The modern word sulfur, by contrast, owes its spelling to a long-running mistake.

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• 3. A Name Gone Astray

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  The Latin word sulpur was changed to sulphur by writers who wrongly believed it came from Greek. That false Hellenization inserted a ph where none belonged. Later readers took the ph to represent an f sound, and the spelling sulfur appears in Latin toward the end of the Classical period. The genuine Ancient Greek word was theîon, from the earlier théeion. It survives today in the chemical prefix thio-, and Modern Greek says theío.

  In 12th-century Anglo-French, the word was sulfre. In the 14th century, Middle English restored the mistaken ph in the form sulphre. By the 15th century, both sulfur and sulphur were common in English. The two spellings ran in parallel in Britain until the 19th century, when sulphur became the standard. The United States settled on sulfur, although place names such as White Sulphur Springs keep the ph. Canada uses both forms.

  Then the f returned to Britain. IUPAC adopted the spelling sulfur in 1990, and the Nomenclature Committee of the Royal Society of Chemistry followed in 1992. Oxford Dictionaries notes that in chemistry the f spelling is now standard in British as well as American contexts. It adds that the spelling is increasingly used in general writing too. Whatever the spelling, the substance itself was dug from places that one famous writer could not forget.

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• 4. Mining Sulfur

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  Booker T. Washington wrote that a sulfur mine in Sicily was "about the nearest thing to hell that I expect to see in this life." The island's volcanic regions had once offered sulfur lying on the surface. Workers used the Sicilian process. They stacked deposits in brick kilns built on sloping hillsides, with airspaces between the piles. Some sulfur was pulverized, spread over the stack, and ignited, so that the free sulfur melted and ran down the hill.

  Eventually the surface deposits played out, and miners followed veins underground. Labyrinthine mines came to dot the Sicilian landscape. The work was unmechanized. Pickmen freed the ore from the rock, and mine-boys called carusi carried baskets of it to the surface. The climb often ran through a mile or more of tunnels. Smelting ovens then reduced and extracted the sulfur. Poor nations with volcanoes, such as Indonesia, still mine surface deposits, and problems with working conditions persist.

  By the late 18th century, about 2,000 tonnes of sulfur per year were imported into Marseille, France. It went to make sulfuric acid for the Leblanc process. In 1824, Britain repealed its tariffs on salt, and demand for Sicilian sulfur surged. British control of mining, refining, and transport grew, yet the lucrative export failed to lift Sicily's backward and impoverished economy. The result was the Sulfur Crisis of 1840. King Ferdinand II gave a monopoly of the sulfur industry to a French firm, breaking an earlier trade agreement with Britain from 1816. France eventually negotiated a peaceful solution.

  In 1867, elemental sulfur was found in underground deposits in Louisiana and Texas. The Frasch process was developed to extract it. Superheated water was pumped down to melt the sulfur, and compressed air carried the melted product up, 99.5% pure. That method dominated the 20th century, but it has seen no significant use anywhere since 2002.

  Today most sulfur comes from petroleum and natural gas, mainly as hydrogen sulfide. Refiners strip organosulfur compounds from oil by hydrodesulfurization, and the Claus process turns the resulting hydrogen sulfide into elemental sulfur. The high sulfur content of the Athabasca Oil Sands has left stockpiles of the element across Alberta, Canada. The Osiek mine in Poland still carries out commercial mining. World production in 2011 reached 69 million tonnes. China led with 9.6 million, followed by the United States at 8.8 million. Canada and Russia each produced 7.1 million.

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• 5. Shape-Shifting Chemistry

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  No other element forms as many solid allotropes as sulfur, which has more than 30. The familiar one is octasulfur, a ring of eight atoms called cyclo-S8. It is soft and odorless, melts at 115.21 C, and boils at 444.6 C. At 95.2 C, below its melting point, the alpha form slowly changes to the beta polymorph. The ring itself barely changes. Only the way the molecules sit against one another shifts.

  Molten sulfur behaves strangely as it heats. Between melting and boiling, it turns into the gamma form. Its density falls while its viscosity rises, because polymers form. At higher temperatures the viscosity drops again as those polymers break apart. Above 200 C, the liquid turns a dark red.

  Pouring molten sulfur into cold water produces amorphous sulfur, also called plastic sulfur. X-ray crystallography suggests it may be a helix with eight atoms per turn. The long coiled molecules make the brownish mass elastic, with a feel like crude rubber. Over hours to days it reverts to the crystalline form and loses its stretch. Other rings exist too. Removing one atom from the S8 crown gives S7, which is a deeper yellow. Larger rings such as S12 and S18 have been prepared.

  Sulfur reacts with nearly every other element except the noble gases. It even attacks iridium, a notoriously unreactive metal, to yield iridium disulfide. Burning sulfur in oxygen gives sulfur dioxide. Making sulfur trioxide demands a temperature of 400 to 600 °C and a catalyst.

  In 1804, C. F. Bucholz reported colored solutions made by dissolving sulfur in oleum. Their cause stayed a mystery until the late 1960s, when chemists worked out that they contained sulfur polycations. One is deep blue, one is yellow, and one is red. Sulfur can also bind to itself in chains, a talent called catenation. A radical anion formed this way gives lapis lazuli its blue color. The same chains, put to work in rubber, changed the modern world.

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• 6. Stellar Fingerprints

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  About 94.99 percent of natural sulfur is the isotope sulfur-32, and its origin lies inside massive stars. There a silicon nucleus fuses with a helium nucleus. The reaction belongs to the alpha process, which makes highly abundant elements, so sulfur ranks tenth in the universe. Sulfur has 23 known isotopes, but only four are stable. The others are sulfur-33 at 0.75 percent, sulfur-34 at 4.25 percent, and sulfur-36 at 0.01 percent. Novae supply proportionally more sulfur-33 than supernovae do.

  Meteorites carry sulfur too. Ordinary chondrites contain 2.1% on average, and carbonaceous chondrites may hold as much as 6.6%. It usually appears as troilite, iron sulfide. The vivid colors of Jupiter's volcanic moon Io are attributed to molten, solid, and gaseous sulfur.

  On Earth, sulfur is the fifth most common element by mass. Elemental sulfur gathers near hot springs and volcanic regions, especially along the Pacific Ring of Fire, and is mined in Indonesia, Chile, and Japan. The largest documented single crystal measures 22 x 16 x 11 cm. Lakes of molten sulfur up to about 200 m across lie on the sea floor beside submarine volcanoes. They sit at depths where water boils at a higher temperature than sulfur melts. In salt domes along the Gulf of Mexico coast, anaerobic bacteria acting on gypsum make native sulfur.

  The ratio of sulfur-32 to sulfur-34 varies between samples by a surprisingly large amount. Scientists read that ratio, called delta-34S, as a record of a sample's chemical history. It can help estimate the temperature at which ore and water reached equilibrium, and it can reveal the work of sulfate-reducing bacteria. Researchers still debate whether biological activity or later alteration causes the shifts.

  Before the 2010s, experts thought sulfate reduction could shift the isotopes by up to 46 permil. Experiments since then have shown that sulfate-reducing bacteria can reach 66 permil. In Rocky Mountain lakes, those thought to draw sulfate from the atmosphere show measurably different sulfur-34 values from lakes fed by their watersheds. The rare radioactive isotope sulfur-35, with a half-life of 87.37 days, became a tracer in the Hershey-Chase experiment.

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• 7. Industry's Workhorse

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  A patent granted in 1843 covered heating natural rubber with sulfur, which builds disulfide bridges between the polymer's isoprene units. The process is called vulcanization, after the Roman god of the forge and volcanism. It turned rubber into a major industrial product, especially for automobile tires. Rubber is only one of many jobs the element performs.

  About 85% of elemental sulfur, by a 1989 figure, becomes sulfuric acid. In 2010, the United States produced more sulfuric acid than any other inorganic industrial chemical. Its principal use is extracting phosphate ores to make fertilizer. Oil refining, wastewater processing, and mineral extraction use it too.

  Sulfur reacts with methane to give carbon disulfide, which is used to make cellophane and rayon. Large quantities of sulfites bleach paper and preserve dried fruit. Detergents such as sodium lauryl sulfate are sulfate derivatives. Gypsum, calcium sulfate, is mined at a scale of 100 million tonnes a year for Portland cement and fertilizers. When silver-based photography was widespread, thiosulfates served as fixing agents. In the late 18th century, furniture makers used molten sulfur for decorative inlays. Molten sulfur still sets steel bolts into drilled concrete where shock resistance matters.

  On farms, dusting sulfur is one of the oldest fungicides and pesticides. Growers use it on grapes, strawberries, and many vegetables against powdery mildew and black spot. In organic production it is the most important fungicide. It is the only fungicide used in organic apple orchards against apple scab in colder conditions. Lime sulfur, made from calcium hydroxide and sulfur in water, is a dip that destroys ringworm and mange on pets.

  Sulfur deficiency has become widespread in many European countries, because atmospheric inputs keep falling with the measures taken against acid rain. Farmers therefore add sulfur to fertilizers, most often as gypsum. Medicine relies on the element as well. Sulfur preparations treat acne, and the early sulfa drugs were antibacterial sulfonamides. Penicillins, cephalosporins, and monobactams all contain sulfur.

  The newest chapter is the lithium-sulfur battery, prized for its energy density. Carbonate electrolytes had caused such batteries to fail after a single cycle. In February 2022, researchers at Drexel University built a prototype that lasted 4000 recharge cycles. They also found the first monoclinic gamma sulfur that stayed stable below 95 degrees Celsius.

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• 8. Life, Smell, Poison

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  A 70 kg human body contains about 140 g of sulfur, making it the eighth most abundant element in the body by weight. Most of it sits in two amino acids, cysteine and methionine. Disulfide bonds between cysteine residues give keratin its mechanical strength and insolubility in skin, hair, and feathers. Two vitamins, biotin and thiamine, contain sulfur. So does glutathione, an antioxidant that protects cells against oxidative stress.

  Sulfur compounds are also behind some of the world's most recognizable smells. Organosulfur compounds give odorized natural gas, skunk spray, garlic, and bad breath their odors. Grapefruit mercaptan is the characteristic scent of grapefruit in small concentrations, but it smells like a generic thiol at larger ones. Hydrogen sulfide gives rotting eggs their stench.

  In the 1880s, Sergei Winogradsky studied Beggiatoa, a bacterium living in sulfur-rich surroundings. He found that it oxidized hydrogen sulfide as an energy source and stored sulfur droplets inside its cells. Robert Ballard later found primitive bacteria around deep ocean volcanic vents that oxidize hydrogen sulfide for food. The giant tube worm lives in symbiosis with such bacteria. Other microbes, the sulfate-reducing bacteria, effectively breathe sulfate in place of oxygen. In bacteria, the nitrogenase enzyme uses an iron-molybdenum-sulfur cluster to turn atmospheric nitrogen into ammonia.

  The same chemistry turns dangerous at the wrong concentration. Hydrogen sulfide is about half as toxic as hydrogen cyanide and acts by the same mechanism. Its permissible exposure limit is 20 ppm, and its odor usually warns people away from small amounts. Above 100 to 150 ppm, however, it quickly deadens the sense of smell. A victim may keep breathing more of it without noticing until severe symptoms set in.

  Burning sulfur makes sulfur dioxide, which forms sulfurous acid in water. When coal or petroleum burns in industry and power plants, that gas reacts with atmospheric water and oxygen. The resulting acid rain lowers the pH of soil and freshwater and weathers statues and structures. Sulfur mustard, a potent vesicant, was used as a disabling agent in World War I.

  Studies suggest that many native sulfur deposits on ancient sea bottoms have a biological origin. Scientists still cannot say whether sulfur-oxidizing or sulfate-reducing bacteria made them.

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