Sulphur is one of the most common elements on Earth. It can be found near volcanoes in its pure form as a yellow solid, but it also occurs in large quantities in minerals such as pyrite (iron disulphide; FeS2) and gypsum (calcium sulphate; CaSO4). Finally, dissolved sulphate (SO42-) is present in seawater.

Sulfur reacts readily with oxygen (O2) in powder form to form, for example, sulphur dioxide (SO2), sulphite (SO32-) or sulphate (SO42-), depending on the chemical process used.

When oxygen is absent, sulphur, sulphite, and sulphate are transformed to sulfide (S2-). These various capacities are a function of the atom’s sulfur oxidation state. The number of electrons an element has taken up or given up is called its oxidation state. Sulphur can exist in the oxidation states -2, -1, 0, +4, and +6. It is the most oxidised form in sulphuric acid (H2SO4), which is a common applied chemical feedstock: +6.

H2S, the deadly gas that can escape from manure pits, is precisely the most reduced form of sulphur: -2. In addition to the inorganic sulphur compounds mentioned above, sulphur also occurs in combination with carbon (C) in numerous organic compounds. Thiols, for example, are carbon compounds with a so-called sulphhydryl (SH) group. They are known for their strong odour, which you might recognise from garlic and onions. But thiols aren’t just responsible for stink—they’re also behind the fragrance added to natural gas.

Cysteine contains thiols as well. The amino acid cysteine has a sulfur-containing group bonded to its nitrogen atom. Elemental sulfur at room temperature is a solid, but it immediately transforms into a liquid at 1130C. Sulphuric acid is both a liquid and a gas at room temperature, with sulphur dioxide and hydrogen sulfide being gaseous.

Sulpher is an important element in the biological cycles of our ecosystems. It can be used a source of energy. For example, some bacteria can use sulpher as a food source. This is especially important in environments where sulpher is the only source of energy available. In these conditions, sulpher- oxidizing bacteria play a key role in the cycling of sulpher.

Sulpher is also an important element in the process of corrosion. Corrosion is the gradual degradation of a material. It is caused by the interaction between the material and its environment. The most common type of corrosion is called “rusting.” Rusting is a form of corrosion that affects iron and steel. When iron or steel corrodes, it forms a powdery substance called “rust.” Rusting occurs when the metal reacts with oxygen in the presence of water.

Microbial influenced corrosion (MIC) is a type of corrosion that is caused by microorganisms. In some cases it is also referred to as Biocorrosion. The degradation process is ultimately done by bacteria or Archaea. MIC can occur in any environment where there are microorganisms present. The most common known form of MIC is Chemically driven (CMIC), but there are other types of degradation processes possible.

Relationship of Sulphur to the presence in soil and water

Sulphur depositions harm soil and freshwater ecosystems by acidifying them. They are dangerous to one’s health because they contribute to the sulphur fertilization of plants and hide the effects of greenhouse gases by reflecting some incoming solar radiation back. Because they partially reflect back incoming solar radiation, today’s focus on sulphur has moved from its acid rain impact to its climatic and health impacts, especially due to particles in the form of particulate matter.

Emissions decreased owing to flue gas desulphurisation, especially in power plants, the shift from sulphur-rich coal to natural gas or nuclear energy, and the removal of sulphur from oil refining products.

The amount of sulfur in soil is determined by the type of soil and how it’s used. Additionally, pyrite (FeS2; iron disulphide) has a significant part. The overall sulphur supply is usually in the thousands of kg per hectare. The main source is the deposition from the atmosphere, but biological fixation, decomposition of organic matter and rock weathering also contribute.

Pyrite is sometimes found in coal seams. Pyrite oxidizes readily in air to form sulphur dioxide (SO2), which escapes from the coal seam and contributes to acid rain. Sea clay and peat can contain pyrite naturally. When pyrite is exposed to oxygen, such as during dewatering or desiccation, it breaks down and releases sulphur into the air. Additionally, if pyrite comes into contact with nitrate-rich groundwater, that also stimulates its decomposition.

Most of the sulfur in non-pyrite soils comes from organic matter, so it is closely tied to the total amount of carbon and nitrogen. Organic sulphur compounds are present in soil organisms and decaying organic matter. The variety of organic sulfur compounds is vast, but there are two primary categories: sulphate esters (C-O-S) and sulphur carbon compounds (C-S).

Only a minor percentage of the world’s sulphur stock is in the form of sulfate, although it is by far the most abundant. Most often, it is dissociated in soil moisture or bound to iron and aluminum hydroxides. It can also be present as undissolved calcium sulphate in calcareous soils. The sulphate concentration in the soil moisture is approximately 30 micrograms per litre. Different analytical methods can provide a measure of the amount of available sulphur. These methods, which use water or calcium chloride (CaCl2), indicate how much rapidly-available sulphur there is.

Health & safety consequences of biological activity related to the Sulphur cycle

The most dangerous gas found in transport pipes is hydrogen sulphide. This is found in oil and gas pipes as well as (waste) water pipes). It is poorly soluble in liquids and therefore easily escapes into the air. At very low concentrations (< 5 ppm) it has the familiar smell of rotten eggs. But at higher concentrations (>100 ppm) it is practically odourless because it paralyses the olfactory nerves. Hydrogen sulphide causes rapid health damage. At relatively low concentrations (>50 ppm), lung irritations already occur. 

As concentrations and duration of exposure increase, the risk of unconsciousness, cardiac arrhythmias and respiratory arrest increases. 

Besides hydrogen sulphide, manure contains other gases that can lead to unsafe situations due to toxicity or risk of explosion. Blue acid gas (HCN), for example, is just as toxic as hydrogen sulphide. Ammonia (NH3) is also toxic at very high concentrations, but in practice usually does not play a significant role. Methane (CH4) is especially risky from the point of view of fire safety. Carbon dioxide (CO2) is a harmless gas in itself, but in high concentrations it displaces oxygen (O2), mainly because it sticks to the surface, which can lead to asphyxiation can lead to suffocation.