Sulphur (S) is a reactive, multivalent non-metal located in Group 16 (Chalcogens) of the Periodic Table. It is the tenth most abundant element in the universe and plays a twin role in environmental chemistry: it is an essential macronutrient for living organisms and a major constituent of atmospheric pollutants and acid precipitation.
Physical and Chemical Properties of Sulphur
Allotropy of Sulphur
Sulphur exists in several allotropic forms, the most important being crystalline forms:
- Rhombic Sulphur (α-sulphur): The most stable form at room temperature, possessing a bright yellow color.
- Monoclinic Sulphur (β-sulphur): Stable above 95.6°C.
- Structure: Both forms consist of puckered, ring-shaped S8 molecules resembling a crown.
Chemical Valency and Bonding
Sulphur exhibits oxidation states ranging from -2 (in H2S) to +4 (in SO2) and +6 (in SO42-). This variable valency drives the complex redox reactions observed in biogeochemical cycles and atmospheric chemistry.
The Global Sulphur Cycle
The sulphur cycle is primarily sedimentary, but it features a significant gaseous phase, distinguishing it from purely sedimentary cycles like phosphorus.
Weathering and Terrestrial Assimilation
Sulphur is locked up in rocks as pyrite (FeS2), gypsum (CaSO4·2H2O), and Epsom salts (MgSO4·7H2O). Weathering releases sulfate ions (SO42-), which are absorbed by plants and micro-organisms to synthesize sulfur-containing amino acids.
Biological Role of Sulphur
Sulphur is a vital constituent of:
- Amino Acids: Cysteine and Methionine, which are foundational for protein folding via disulphide bonds (-S-S-).
- Vitamins and Enzymes: Thiamine (Vitamin B1), Biotin (Vitamin B7), and Coenzyme A.
- Ferredoxins: Iron-sulphur proteins involved in electron transfer during photosynthesis and nitrogen fixation.
Microbial Transformations
Micro-organisms drive the redox states of the sulphur cycle:
- Sulphate Reducing Bacteria (SRB): Anaerobic bacteria like Desulfovibrio reduce SO42- to toxic Hydrogen Sulphide (H2S) gas in wetlands and marine sediments.
- Sulphur Oxidizing Bacteria: Chemolithotrophic bacteria like Thiobacillus oxidize H2S or elemental sulphur back into sulfates (SO42-).
Atmospheric Sulphur Compounds and Environmental Pollution
Major Sulphur Pollutants
- Sulphur Dioxide (SO2): A colorless gas with a pungent, suffocating odor. It is primarily generated by burning fossil fuels (coal and petroleum) in thermal power plants and oil refineries, as well as via volcanic eruptions.
- Hydrogen Sulphide (H2S): A colorless, highly toxic gas with a characteristic “rotten egg” smell, produced by decaying organic matter in anaerobic environments like swamps, sewers, and industrial effluents.
Mechanism of Acid Rain Formation
Atmospheric SO2 undergoes photo-oxidation and catalytic oxidation by suspended particulate matter (SPM) or ozone to form sulphur trioxide (SO3). This readily reacts with atmospheric water vapor to form sulphuric acid (H2SO4), a chief component of acid rain.
Environmental Impact of Acid Rain
- Soil Leaching: Acid rain lowers soil pH, causing the leaching of essential nutrients like Calcium (Ca) and Magnesium (Mg), while mobilizing toxic Aluminium (Al3+) ions into water bodies.
- Stone Leprosy: The chemical degradation of marble and limestone monuments (composed of CaCO3) by sulphuric acid, converting it into flaky gypsum (CaSO4). This process is notably affecting the Taj Mahal due to emissions from the Mathura Refinery.
Comparison of Sulphur Oxides and Secondary Aerosols
| Pollutant / Compound | Primary Source | Environmental Action | Impact / Health Hazard |
| Sulphur Dioxide (SO2) | Fossil fuel combustion, Volcanic activity, Smelting of sulfide ores. | Primary gaseous pollutant; precursor to secondary pollutants. | Severe respiratory irritation, bronchoconstriction, chlorosis (bleaching of chlorophyll) in plants. |
| Sulphate Aerosols (SO42-) | Atmospheric oxidation of SO2. | Reflects solar radiation back into space; acts as Cloud Condensation Nuclei (CCN). | Albedo enhancement (global cooling effect), contributes to PM2.5 and haze formation. |
| Dimethyl Sulphide (DMS) | Marine phytoplankton (specifically coccolithophores). | Natural biogenic source of atmospheric sulphur; oxidizes to sulfate aerosols. | Regulates global climate via cloud formation (CLAW Hypothesis). |
High-Yield Facts for UPSC Prelims
- Flue-Gas Desulphurization (FGD): A techno-environmental process used in coal-fired power plants to remove SO2 from exhaust flue gases. Slurry of limestone (CaCO3) or lime (CaO) is sprayed into the gas, reacting to form synthetic gypsum, thereby neutralizing the pollutant.
- Sulphur-Heavy Fuels and BS-VI Norms: India transitioned directly from BS-IV to BS-VI emission standards to drastically reduce sulfur content in transportation fuels. BS-VI diesel and petrol contain a maximum of 10 ppm (parts per million) of sulphur, down from 50 ppm in BS-IV.
- The CLAW Hypothesis: A negative feedback loop proposing that warmer oceans trigger marine phytoplankton to release more Dimethyl Sulphide (DMS). The resulting sulphate aerosols increase cloud albedo, reflecting solar radiation and cooling the Earth.
- Vulcanization of Rubber: A chemical process where natural rubber is heated with sulphur (approx. 3-5%) to form cross-links between polymer chains. This enhances the tensile strength, elasticity, and thermal stability of the rubber.
- Anoxygenic Photosynthesis Counterpart: Certain deep-sea hydrothermal vent ecosystems rely on chemolithoautotrophic bacteria that use H2S instead of water as an electron donor to fix carbon, forming the basis of a food web completely independent of sunlight.