Hydrogen sulfide is toxic to humans and most other animals by inhibitingcellular respiration in a manner similar tohydrogen cyanide. When it is inhaled or its salts are ingested in high amounts, damage to organs occurs rapidly with symptoms ranging from breathing difficulties to convulsions and death.[13][14] Despite this, thehuman body produces small amounts of this sulfide and its mineral salts, and uses it as asignalling molecule.[15]
Hydrogen sulfide is often produced from themicrobial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers; this process is commonly known asanaerobic digestion, which is done bysulfate-reducing microorganisms. It also occurs involcanic gases, natural gas deposits, and sometimes in well-drawn water.
In general, hydrogen sulfide acts as areducing agent, as indicated by its ability to reduce sulfur dioxide in theClaus process. Hydrogen sulfide burns in oxygen with a blue flame to formsulfur dioxide (SO2) andwater:
It is slightly soluble in water and acts as aweak acid (pKa = 6.9 in 0.01–0.1 mol/litre solutions at 18 °C), giving thehydrosulfide ionHS−. Hydrogen sulfide and its solutions are colorless. When exposed to air, it slowly oxidizes to form elemental sulfur, which is not soluble in water. Thesulfide anionS2− is not formed in aqueous solution.[16]
H2S and H2O exchange protons rapidly. This behavior is the basis of technologies for the purification of deuterium oxide ("heavy water" orD2O), which exploits the easy distillation of these compounds.[17]
At pressures above 90 GPa (gigapascal – 900,000atmospheres), hydrogen sulfide becomes a metallicconductor of electricity. When cooled below acritical temperature this high-pressure phase exhibitssuperconductivity. The critical temperature increases with pressure, ranging from 23 K at 100 GPa to 150 K at 200 GPa.[18] If hydrogen sulfide is pressurized at higher temperatures, then cooled, the critical temperature reaches 203 K (−70 °C), which was the highest accepted superconducting critical temperature until the discovery ofLanthanum decahydride in 2019. By substituting a small part of sulfur withphosphorus and using even higher pressures, it has been predicted that it may be possible to raise the critical temperature to above 0 °C (273 K) and achieveroom-temperature superconductivity.[19]
Under atmospheric pressure and in the absence of a catalyst, hydrogen sulfide decomposes around 1200 °C into hydrogen and sulfur.[20]
Hydrogen sulfide reacts with metal ions to form metal sulfides, which are insoluble, often dark colored solids. This behavior is the basis of the use of hydrogen sulfide as a reagent in thequalitative inorganic analysis of metal ions. In these analyses, heavy metal (andnonmetal) ions (e.g., Pb(II), Cu(II), Hg(II), As(III)) are precipitated from solution upon exposure toH2S. The components of the resulting solid are then identified by their reactivity.Lead(II) acetate paper is used to detect hydrogen sulfide because it readily converts tolead(II) sulfide, which is black.[21][22]
Hydrogen sulfide is also responsible fortarnishing on various metals includingcopper andsilver; the chemical responsible for blacktoning found on silver coins issilver sulfide (Ag2S), which is produced when the silver on the surface of the coin reacts with atmospheric hydrogen sulfide.[23] Coins that have been subject to toning by hydrogen sulfide and other sulfur-containing compounds may have the toning add to the numismatic value of a coin based on aesthetics, as the toning may producethin-film interference, resulting in the coin taking on an attractive coloration.[24] Coins can also be intentionally treated with hydrogen sulfide to induce toning, though artificial toning can be distinguished from natural toning, and is generally criticised among collectors.[25]
Hydrogen sulfide is most commonly obtained by its separation fromsour gas, which is natural gas with a high content ofH2S. It can also be produced by treatinghydrogen with molten elemental sulfur at about 450 °C. Hydrocarbons can serve as a source of hydrogen in this process.[26]
S + H2 → H2S
The very favorable thermodynamics for the hydrogenation of sulfur implies that the dehydrogenation (orcracking) of hydrogen sulfide would require very high temperatures.[27]
This gas is also produced by heating sulfur with solid organic compounds and by reducing sulfurated organic compounds with hydrogen.It can also be produced by mixing ammonium thiocyanate to concentrated sulfuric acid and adding water to it.
Hydrogen sulfide can be generated in cells via enzymatic or non-enzymatic pathways. Three enzymes catalyze formation ofH 2S:cystathionine γ-lyase (CSE),cystathionine β-synthetase (CBS), and3-mercaptopyruvate sulfurtransferase (3-MST).[29] CBS and CSE are the main proponents ofH2S biogenesis, which follows the trans-sulfuration pathway.[30] These enzymes have been identified in a breadth of biological cells and tissues, and their activity is induced by a number of disease states.[31] These enzymes are characterized by the transfer of a sulfur atom from methionine to serine to form a cysteine molecule.[30] 3-MST also contributes to hydrogen sulfide production by way of the cysteine catabolic pathway.[31][30] Dietary amino acids, such as methionine and cysteine serve as the primary substrates for the transulfuration pathways and in the production of hydrogen sulfide. Hydrogen sulfide can also be derived from proteins such asferredoxins andRieske proteins.[31]
Sulfate-reducing (resp.sulfur-reducing) bacteria generate usable energy under low-oxygen conditions by using sulfates (resp. elemental sulfur) tooxidize organic compounds or hydrogen; this produces hydrogen sulfide as a waste product.[32]
H2S in the body acts as agaseous signaling molecule with implications for health and in diseases.[29][33][34][35]H2S is involved invasodilation in animals, as well as in increasing seed germination and stress responses in plants.[36]
H2S can signal in three ways: by reacting withreactive oxygen species (ROS) andreactive nitrogen species (RNS),[36] by binding or reduction of metal centers of iron-heme proteins, and bysulfhydration of protein cysteine residues.[35]H2S has been shown to interact with the NO pathway resulting in several different cellular effects, including the inhibition of cGMP phosphodiesterases,[37] as well as the formation of another signal called nitrosothiol.[36] Hydrogen sulfide is also known to increase the levels of glutathione, which acts to reduce or disrupt ROS levels in cells.[36]
The field ofH2S biology has advanced from environmental toxicology to investigate the roles of endogenously producedH2S in physiological conditions and in various pathophysiological states.[38]H2S has been implicated in cancer, Down syndrome, and in vascular disease.[39][40][41][42]
At lower concentrations, it stimulates mitochondrial function via multiple mechanisms including direct electron donation.[43][44] However, at higher concentrations, it inhibits Complex IV of the mitochondrial electron transport chain, which effectively reduces ATP generation and biochemical activity within cells.[36]
Hydrogen sulfide is mainly consumed as a precursor to elemental sulfur. This conversion, called theClaus process, involves partial oxidation to sulfur dioxide. The latter reacts with hydrogen sulfide to give elemental sulfur. The conversion is catalyzed by alumina.[45]
Sodium sulfides are used in thepaper making industry. Specifically, salts ofSH− break bonds between lignin and cellulose components ofpulp in theKraft process.[26]
As indicated above, many metal ions react with hydrogen sulfide to give the corresponding metal sulfides. Oxidic ores are sometimes treated with hydrogen sulfide to give the corresponding metal sulfides which are more readily purified byflotation.Metal parts are sometimespassivated with hydrogen sulfide. Catalysts used inhydrodesulfurization are routinely activated with hydrogen sulfide.[26]
Volcanoes and somehot springs (as well ascold springs) emit someH2S. Hydrogen sulfide can be present naturally in well water, often as a result of the action ofsulfate-reducing bacteria.[47][better source needed] Hydrogen sulfide is produced by the human body in small quantities through bacterial breakdown of proteins containing sulfur in the intestinal tract; it therefore contributes to the characteristic odor of flatulence. It is also produced in the mouth (halitosis).[48]
A portion of globalH2S emissions are due to human activity. By far the largest industrial source ofH2S ispetroleum refineries: Thehydrodesulfurization process liberates sulfur frompetroleum by the action of hydrogen. The resultingH2S is converted to elemental sulfur by partial combustion via theClaus process, which is a major source of elemental sulfur. Other anthropogenic sources of hydrogen sulfide includecoke ovens,paper mills (using the Kraft process), tanneries andsewerage.H2S arises from virtually anywhere where elemental sulfur comes in contact with organic material, especially at high temperatures. Depending on environmental conditions, it is responsible for deterioration of material through the action of some sulfur oxidizing microorganisms. It is calledbiogenic sulfide corrosion.[citation needed]
In 2011 it was reported that increased concentrations ofH2S were observed in theBakken formation crude, possibly due to oil field practices, and presented challenges such as "health and environmental risks, corrosion of wellbore, added expense with regard to materials handling and pipeline equipment, and additional refinement requirements".[49]
Besides living near gas and oil drilling operations, ordinary citizens can be exposed to hydrogen sulfide by being nearwaste water treatment facilities,landfills and farms with manure storage. Exposure occurs through breathing contaminated air or drinking contaminated water.[50]
Inmunicipal waste landfill sites, the burial oforganic material rapidly leads to the production ofanaerobic digestion within the waste mass and, with the humid atmosphere and relatively high temperature that accompaniesbiodegradation,biogas is produced as soon as the air within the waste mass has been reduced. If there is a source of sulfate bearing material, such as plasterboard or naturalgypsum (calcium sulfate dihydrate), under anaerobic conditionssulfate reducing bacteria converts this to hydrogen sulfide. These bacteria cannot survive in air, but the moist, warm, anaerobic conditions of buried waste that contains a high source of carbon is an excellent environment for the formation of hydrogen sulfide. In inert landfills, paper and glue used in the fabrication of products such asplasterboard can provide a rich source of carbon.[51]
A number of processes have been designed to remove hydrogen sulfide fromdrinking water.[54]
Continuous chlorination
For levels up to 75 mg/Lchlorine is used in the purification process as an oxidizing chemical to react with hydrogen sulfide. This reaction yields insoluble solid sulfur. Usually the chlorine used is in the form ofsodium hypochlorite.[55]
Aeration
For concentrations of hydrogen sulfide less than 2 mg/Laeration is an ideal treatment process. Oxygen is added to water and a reaction between oxygen and hydrogen sulfide react to produce odorless sulfate.[56]
Hydrogen sulfide is commonly found in raw natural gas and biogas. It is typically removed byamine gas treating technologies. In such processes, the hydrogen sulfide is first converted to an ammonium salt, whereas the natural gas is unaffected.[57][58]
RNH2 + H2S ⇌ [RNH3]+ + SH−
The bisulfide anion is subsequently regenerated by heating of the amine sulfide solution. Hydrogen sulfide generated in this process is typically converted to elemental sulfur using theClaus Process.
Process flow diagram of a typical amine treating process used in petroleum refineries, natural gas processing plants and other industrial facilities
Even before hydrogen sulfide was discovered, Italian physicianBernardino Ramazzini hypothesized in his 1713 bookDe Morbis Artificum Diatriba that occupational diseases of sewer-workers and blackening of coins in their clothes may be caused by an unknown invisible volatile acid (moreover, in late 18th century toxic gas emanation fromParis sewers became a problem for the citizens and authorities).[61]
Although very pungent at first (it smells like rotten eggs[62]), it quickly deadens the sense of smell, creating temporaryanosmia,[63] so victims may be unaware of its presence until it is too late. Safe handling procedures are provided by itssafety data sheet (SDS).[64]
Since hydrogen sulfide occurs naturally in the body, the environment, and the gut, enzymes exist to metabolize it. At some threshold level, believed to average around 300–350 ppm, the oxidative enzymes become overwhelmed. Many personal safety gas detectors, such as those used by utility, sewage and petrochemical workers, are set to alarm at as low as 5 to 10 ppm and to go into high alarm at 15 ppm. Metabolism causes oxidation to sulfate, which is harmless.[65] Hence, low levels of hydrogen sulfide may be tolerated indefinitely.[citation needed]
Exposure to lower concentrations can result ineye irritation, a sore throat andcough, nausea, shortness of breath, andfluid in the lungs.[59] These effects are believed to be due to hydrogen sulfide combining withalkali present in moist surface tissues to formsodium sulfide, acaustic.[66] These symptoms usually subside in a few weeks.
Long-term, low-level exposure may result infatigue, loss of appetite,headaches, irritability, poor memory, anddizziness. Chronic exposure to low levelH2S (around 2ppm) has been implicated in increased miscarriage and reproductive health issues among Russian and Finnish wood pulp workers,[67] but the reports have not (as of 1995) been replicated.
Short-term, high-level exposure can induce immediate collapse, with loss of breathing and a high probability of death. If death does not occur, high exposure to hydrogen sulfide can lead tocortical pseudolaminar necrosis, degeneration of thebasal ganglia andcerebral edema.[59] Although respiratory paralysis may be immediate, it can also be delayed up to 72 hours.[68]
Inhalation ofH2S resulted in about 7 workplace deaths per year in the U.S. (2011–2017 data), second only to carbon monoxide (17 deaths per year) for workplace chemical inhalation deaths.[69]
Exposure limits stipulated by the United States government:[70]
10ppmREL-Ceiling (NIOSH): recommended permissible exposure ceiling (the recommended level that must not be exceeded, except once for 10 min. in an 8-hour shift, if no other measurable exposure occurs)
20 ppmPEL-Ceiling (OSHA): permissible exposure ceiling (the level that must not be exceeded, except once for 10 min. in an 8-hour shift, if no other measurable exposure occurs)
50 ppm PEL-Peak (OSHA): peak permissible exposure (the level that must never be exceeded)
100 ppmIDLH (NIOSH): immediately dangerous to life and health (the level that interferes with the ability to escape)
0.00047 ppm or 0.47 ppb is the odor threshold, the point at which 50% of a human panel can detect the presence of an odor without being able to identify it.[71]
10–20 ppm is the borderline concentration for eye irritation.
Hydrogen sulfide was used by theBritish Army as achemical weapon duringWorld War I. It was not considered to be an ideal war gas, partially due to its flammability and because the distinctive smell could be detected from even a small leak, alerting the enemy to the presence of the gas. It was nevertheless used on two occasions in 1916 when other gases were in short supply.[77]
On September 2, 2005, a sewage line leak in the propeller room of aRoyal Caribbean Cruise Liner docked inLos Angeles resulted in the deaths of 3 crewmen. As a result, all such compartments are now required to have a ventilation system.[78][79]
In September 2008, three workers were killed and two suffered serious injury, including long term brain damage, at a mushroom growing company inLangley,British Columbia. A valve to a pipe that carried chickenmanure,straw andgypsum to the compost fuel for the mushroom growing operation became clogged, and as workers unclogged the valve in a confined space without proper ventilation the hydrogen sulfide that had built up due to anaerobic decomposition of the material was released, poisoning the workers in the surrounding area.[81] An investigator said there could have been more fatalities if the pipe had been fully cleared and/or if the wind had changed directions.[82]
In 2014, levels of hydrogen sulfide as high as 83 ppm were detected at a recently built mall inThailand called Siam Square One at theSiam Square area. Shop tenants at the mall reported health complications such as sinus inflammation, breathing difficulties and eye irritation. After investigation it was determined that the large amount of gas originated from imperfect treatment and disposal of waste water in the building.[83]
In 2014, hydrogen sulfide gas killed workers at the Promenade shopping center in NorthScottsdale, Arizona, USA[84] after climbing into 15 ft deep chamber without wearingpersonal protective gear. "Arriving crews recorded high levels ofhydrogen cyanide and hydrogen sulfide coming out of the sewer."
In November 2014, a substantial amount of hydrogen sulfide gas shrouded the central, eastern and southeastern parts ofMoscow. Residents living in the area were urged to stay indoors by the emergencies ministry. Although the exact source of the gas was not known, blame had been placed on a Moscow oil refinery.[85]
In June 2016, a mother and her daughter were found dead in their still-running 2006Porsche CayenneSUV against a guardrail onFlorida's Turnpike, initially thought to be victims ofcarbon monoxide poisoning.[86][87] Their deaths remained unexplained as the medical examiner waited for results of toxicology tests on the victims,[88] until urine tests revealed that hydrogen sulfide was the cause of death. A report from the Orange-Osceola Medical Examiner's Office indicated that toxic fumes came from the Porsche'sstarter battery, located under the front passenger seat.[89][90]
In January 2017, three utility workers inKey Largo, Florida, died one by one within seconds of descending into a narrow space beneath amanhole cover to check a section of paved street.[91] In an attempt to save the men, a firefighter who entered the hole without his air tank (because he could not fit through the hole with it) collapsed within seconds and had to be rescued by a colleague.[92] The firefighter was airlifted toJackson Memorial Hospital and later recovered.[93][94] A Monroe County Sheriff officer initially determined that the space contained hydrogen sulfide andmethane gas produced by decomposing vegetation.[95]
On May 24, 2018, two workers were killed, another seriously injured, and 14 others hospitalized by hydrogen sulfide inhalation at aNorske Skog paper mill inAlbury, New South Wales.[96][97] An investigation bySafeWork NSW found that the gas was released from a tank used to holdprocess water. The workers were exposed at the end of a 3-day maintenance period. Hydrogen sulfide had built up in an upstream tank, which had been left stagnant and untreated withbiocide during the maintenance period. These conditions allowed sulfate-reducing bacteria to grow in the upstream tank, as the water contained small quantities ofwood pulp andfiber. The high rate of pumping from this tank into the tank involved in the incident caused hydrogen sulfide gas to escape from various openings around its top when pumping was resumed at the end of the maintenance period. The area above it was sufficiently enclosed for the gas to pool there, despite not being identified as aconfined space by Norske Skog. One of the workers who was killed was exposed while investigating an apparent fluid leak in the tank, while the other who was killed and the worker who was badly injured were attempting to rescue the first after he collapsed on top of it. In a resultingcriminal case, Norske Skog was accused of failing to ensure the health and safety of its workforce at the plant to a reasonably practicable extent. It pleaded guilty, and was fined AU$1,012,500 and ordered to fund the production of an anonymized educational video about the incident.[98][99][96][100]
In October 2019, anOdessa, Texas employee of Aghorn Operating Inc. and his wife were killed due to a water pump failure.Produced water with a high concentration of hydrogen sulfide was released by the pump. The worker died while responding to an automated phone call he had received alerting him to a mechanical failure in the pump, while his wife died after driving to the facility to check on him.[101] ACSB investigation cited lax safety practices at the facility, such as an informallockout-tagout procedure and a nonfunctioning hydrogen sulfide alert system.[102]
The gas, produced by mixing certain household ingredients, was used in asuicide wave in 2008 in Japan.[103] The wave prompted staff at Tokyo'ssuicide prevention center to set up a special hotline during "Golden Week", as they received an increase in calls from people wanting to kill themselves during the annual May holiday.[104]
As of 2010, this phenomenon has occurred in a number of US cities, prompting warnings to those arriving at the site of the suicide.[105][106][107][108][109]
In 2020,H2S ingestion was used as a suicide method by Japanese pro wrestlerHana Kimura.[110]
In 2024, Lucy-Bleu Knight, stepdaughter of famed musicianSlash, also usedH2S ingestion to commit suicide.[111]
A drained pond, showing the layer ofsludge on the bottom; its black colour is due to the presence of metal sulfides, the result of reactions with hydrogen sulfide produced by bacteria
In the absence ofoxygen,sulfur-reducing andsulfate-reducing bacteria derive energy fromoxidizing hydrogen or organic molecules by reducing elemental sulfur or sulfate to hydrogen sulfide. Other bacteria liberate hydrogen sulfide from sulfur-containingamino acids; this gives rise to the odor of rotten eggs and contributes to the odor offlatulence.
As organic matter decays under low-oxygen (orhypoxic) conditions (such as in swamps,eutrophic lakes ordead zones of oceans), sulfate-reducing bacteria will use the sulfates present in the water to oxidize the organic matter, producing hydrogen sulfide as waste. Some of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides, which are not water-soluble. These metal sulfides, such as ferrous sulfide FeS, are often black or brown, leading to the dark color ofsludge.
Several groups of bacteria can use hydrogen sulfide as fuel, oxidizing it to elemental sulfur or to sulfate by using dissolved oxygen, metal oxides (e.g.,iron oxyhydroxides andmanganese oxides), or nitrate as electron acceptors.[113]
The biochemistry of hydrogen sulfide is a key part of the chemistry of theiron-sulfur world. In this model of theorigin of life on Earth, geologically produced hydrogen sulfide is postulated as an electron donor driving the reduction of carbon dioxide.[114]
Hydrogen sulfide is lethal to most animals, but a few highly specialized species (extremophiles) do thrive in habitats that are rich in this compound.[115]
In the deep sea,hydrothermal vents andcold seeps with high levels of hydrogen sulfide are home to a number of extremely specialized lifeforms, ranging from bacteria to fish.[which?][116] Because of the absence of sunlight at these depths, these ecosystems rely onchemosynthesis rather thanphotosynthesis.[117]
A hydrogen sulfide bloom (green) stretching for about 150km along the coast of Namibia. As oxygen-poor water reaches the coast, bacteria in the organic-matter rich sediment produce hydrogen sulfide, which is toxic to fish.
Hydrogen sulfide has been implicated in severalmass extinctions that have occurred in the Earth's past. In particular, a buildup of hydrogen sulfide in the atmosphere may have caused, or at least contributed to, thePermian-Triassic extinction event 252 million years ago.[123][124][125]
Organic residues from these extinction boundaries indicate that the oceans were anoxic (oxygen-depleted) and had species of shallow plankton that metabolizedH2S. The formation ofH2S may have been initiated by massive volcanic eruptions, which emittedcarbon dioxide andmethane into the atmosphere, which warmed the oceans, lowering their capacity to absorb oxygen that would otherwise oxidizeH2S. The increased levels of hydrogen sulfide could have killed oxygen-generating plants as well as depleted the ozone layer, causing further stress. SmallH2S blooms have been detected in modern times in theDead Sea and in theAtlantic Ocean off the coast ofNamibia.[123]
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^Amnesty International and Greenpeace Netherlands:The Toxic Truth - About a company called Trafigura, a ship called the Probo Koala, and the dumping of toxic waste in Côte d'Ivoire. Amnesty International Publications, London 2012,ISBN978-0-86210-478-8. (PDF, 232 pages, 7.6 MbArchived 3 March 2016 at theWayback Machine)
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^Casey, Connor (26 May 2020)."Hana Kimura Cause Of Death Revealed".ComicBook.com.More details about her death have since come to light, as Dave Meltzer provided details about what happened on the night of her death during a recent Wrestling Observer Radio. According to Meltzer, Kimura died after inhalation of hydrogen sulfide. He explained that concerns about her health first popped up when she posted a tweet early Saturday morning indicating that she was going to cause self-harm.
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