How Is Hydrogen Sulfide Generated?

Infamous for its tell-tale "rotten egg" smell, hydrogen sulfide (H2S) is responsible for the foul odors of natural gas wells, volcanoes and some animal flatulence. From a commercial perspective, hydrogen sulfide is most useful for its role in the production of elemental sulfur (S8). Most commonly, hydrogen sulfide is collected either by separating it from natural gas or harvesting it from an industrial chamber filled with sulfate-reducing bacteria.

Sweetening Natural Gas

Hydrogen sulfide is a common contaminant in the natural gas (e.g., methane, ethane, propane) deposits found within the Earth's crust. If natural gas has a concentration of H2S above 5.7 mg per cubic meter, it is called "sour gas." Although formally known as the Girdler process, removing the H2S from sour gas is commonly referred to as "sweetening." First, the sour gas is bubbled through a solution of monoethanolamine (MEA) and diethanolamine (DEA). These two amine solvents dissolve the H2S vapor, separating it from the gaseous fossil fuel molecules. The H2S-rich solvents are then routed to another chamber and boiled to release the H2S for a collection.

Sulfate-Reducing Bacteria

In nature, the hydrogen sulfide molecule is produced by proteobacteria of the orders Desulfobacterales, Desulfovibrionales and Syntrophobacterales. Under low-oxygen conditions, the organelles of these bacteria will use sulfate (SO4) molecules as oxidization agents for cellular respiration. Using sulfate molecules in this way reduces them to sulfide ions (S2-), which then combine with hydrogen to form hydrogen sulfide. These H2S molecules are released into the environment as cellular metabolic waste.

Laboratory Preparations: Metal Sulfides and Acids

To produce a small amount of hydrogen sulfide for use in the laboratory, chemists will typically mix a few grams of granular iron sulfide or zinc sulfide with a strong acid (e.g., hydrochloric, nitric) and gently heat the mixture inside a sealed glassware apparatus. The generated gas is then bubbled through the water inside a Kipp generator, where it can be stored and released in measured amounts.

Geothermal

Volcanic eruptions are also a notorious source of hydrogen sulfide pollution in the atmosphere. When molten elemental sulfur is heated above 450°C, hydrogen sulfide forms as hydrogen gas or hydrocarbons bubble up through the liquid. Due to the extreme temperatures (and abundance of sulfur) inside the Earth's mantle, this reaction is a probable mechanism for the formation of hydrogen sulfide in magma.

Accidental

Certain common household cleaning products contain a class of chemicals known as organophosphates. If these products are mixed with bleach (also a popular cleaner), a toxic (if not lethal) amount of hydrogen sulfide can be generated. Therefore, manufacturers of organophosphate-based cleaners are required by law to include warnings about accidentally combining the product with bleach-based cleaners.

References

• Creighton.edu: "Hydrogen Sulfide Information"
• DOW.com: "Gas Sweetening"