Sulfuric acid is one of the most produced industrial chemicals in the world, but nature makes it constantly too, in volcanic plumes, hot springs, weathering rock, and even the air above the sea. Almost every source follows the same route: sulfur combines with oxygen, then meets water. Factories largely copy that chemistry, and much of their raw sulfur is pollution captured from fuel and metal processing.
How Volcanoes Make Sulfuric Acid in the Sky
Eruptions release ash and gases, but the ash falls out of the air fairly quickly.1Wikipedia. Stratospheric aerosol injection The invisible gas does the long-term work. Sulfur dioxide (SO2), a common volcanic gas, can rise into the stratosphere, the layer above where weather happens.2Wikipedia. Stratospheric aerosol injection There it turns into sulfuric acid, which condenses into fine droplets that stay suspended in the air.3USGS. Volcanoes Can Affect Climate Those droplets linger long enough to affect climate, which is the biggest way volcanoes change it.4USGS. Volcanoes Can Affect Climate
The droplets cool the planet because they reflect some sunlight back to space. After a large eruption, the surface can stay cooler for months to a few years. Small eruptions that stay low have little lasting effect, since rain washes the acid out.
Water speeds up the reaction. After the 2022 Hunga Tonga eruption, the conversion was unexpectedly fast in wetter parts of the plume.5PubMed Central. Unexpectedly rapid aerosol formation in the Hunga Tonga plume In the wettest part, about 90% of the sulfur had become acid, at a pace of roughly 3 days.6PubMed Central. Unexpectedly rapid aerosol formation in the Hunga Tonga plume In drier parts the same change took about 8 to 24 days.7PubMed Central. Unexpectedly rapid aerosol formation in the Hunga Tonga plume More water vapor produces more of the reactive molecules that drive the change.8PubMed Central. Unexpectedly rapid aerosol formation in the Hunga Tonga plume
Hot Springs, Mudpots, and Acid Rock Drainage
Some hot springs are fed by fluids rich in hydrogen sulfide, the rotten-egg gas.9Wikipedia. Hot spring When that gas meets groundwater and oxygen, it forms sulfuric acid, along with weaker carbonic acid from carbon dioxide.10U.S. National Park Service. Stop 3: Sulphur Springs The water can reach a pH of about 2, on the acidity scale where 7 is neutral and lower is more acidic.11USGS. Where do acid-sulfate hot springs come from and why are they important? In extreme cases the pH drops as low as 0.8, more acidic than stomach acid.12Wikipedia. Hot spring These fluids rapidly break down nearby rock into colorful clay, forming bubbling mudpots.13USGS. Where do acid-sulfate hot springs come from and why are they important?
Yellow crusts around vents are often deposits of elemental sulfur. Acid pools can cause severe burns, which is why boardwalks and warning signs exist at places like Yellowstone.
Acid also forms in ordinary weathering rock. Rocks containing pyrite, an iron sulfide mineral, produce acid when exposed to air or oxygen-rich water.14Colorado Geological Survey. B-54 Natural Acid Rock Drainage Associated with Hydrothermally Altered Terrane in Colorado The runoff becomes acidic, below pH 5, and carries dissolved sulfate and metals.15Colorado Geological Survey. B-54 Natural Acid Rock Drainage Associated with Hydrothermally Altered Terrane in Colorado This happens naturally, but mining exposes far more pyrite, creating acid mine drainage.16Geological Quarterly. Geomicrobiology of Acid Mine Drainage in the weathering zone of pyrite-bearing schists in the Rudawy Janowickie Mountains (Poland) Microbes that feed on sulfur and iron live in these waters and speed the process.17Geological Quarterly. Geomicrobiology of Acid Mine Drainage in the weathering zone of pyrite-bearing schists in the Rudawy Janowickie Mountains (Poland)
In a stream, orange or rust-colored beds often mark iron from pyrite runoff, and such streams may have few fish or insects. Natural drainage occurs in mineral-rich mountains, not only near mines, while old abandoned mines can keep leaking acid for decades.
Sulfur From the Ocean and Everyday Air
Most kinds of ocean phytoplankton release a sulfur gas called dimethyl sulfide.18Nature. Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate Marine plankton put more than 20 million tons of sulfur into the air each year.19NOAA CSL. NOAA CSL: 2021 News & Events: Ocean sulfur helps produce new clouds In the air, this gas can turn into sulfuric acid.20NOAA CSL. NOAA CSL: 2021 News & Events: Ocean sulfur helps produce new clouds The acid gives water droplets a place to form, helping make clouds over the sea.21NOAA CSL. NOAA CSL: 2021 News & Events: Ocean sulfur helps produce new clouds
Over land and sea alike, sulfur dioxide is turned into acid mainly by a short-lived, highly reactive molecule in the air.22PubMed Central. Probing the dynamics and bottleneck of the key atmospheric SO2 oxidation reaction by the hydroxyl radical The gas is oxidized in steps and finally combines with water to make sulfuric acid.23PubMed Central. Probing the dynamics and bottleneck of the key atmospheric SO2 oxidation reaction by the hydroxyl radical This can happen in the open air or inside cloud droplets.24Atmospheric Chemistry and Physics. Contribution of the gas-phase reaction between hydroxyl radical and sulfur dioxide to the sulfate aerosol over West Pacific Within a few days the acid dissolves into cloud water and falls in rain.25RADICAL. How atmospheric radicals transform the air
Burning coal and oil adds sulfur dioxide on top of the natural sources, and the same air chemistry turns both into acid. That is why acid rain was worst downwind of power plants.
Sulfuric Acid Beyond Earth
Venus’s clouds are made of liquid sulfuric acid droplets.26Planetary and Space Science. Cloud chemistry on Venus: Sulfuric acid reactions and supercooling in Venus liquid cloud droplets Sunlight breaks apart sulfur dioxide, leading to acid molecules that build the clouds.27ESA. Acid clouds and lightning The acid mixture freezes at such low temperatures that the droplets stay liquid from about 47 to 70 km up.28Planetary and Space Science. Cloud chemistry on Venus: Sulfuric acid reactions and supercooling in Venus liquid cloud droplets On Jupiter’s moon Europa, radiation constantly cycles surface sulfur, and sulfuric acid is about 50 times more common there than other sulfur forms.29PubMed. Sulfuric acid on Europa and the radiolytic sulfur cycle
How Factories Make Sulfuric Acid
Most sulfuric acid today is made by the double contact process.30PubMed Central. Exergy Analysis of Sulfuric Acid Production: A Systematic Framework Using UniSim Design Molten sulfur is burned with dry air, giving hot sulfur dioxide gas near 900°C.31PubMed Central. Exergy Analysis of Sulfuric Acid Production: A Systematic Framework Using UniSim Design The air is dried first because any water would react with the sulfur trioxide formed later and make acid mist in the wrong place. The heat is captured to make steam, which plants often use for power.32PubMed Central. Exergy Analysis of Sulfuric Acid Production: A Systematic Framework Using UniSim Design
The cooled gas then passes over a catalyst, a substance that speeds a reaction without being used up.33PubMed Central. Exergy Analysis of Sulfuric Acid Production: A Systematic Framework Using UniSim Design The catalyst cuts the energy the reaction needs to about one fifth.34Russian Journal of Physical Chemistry A. Modeling the Oxidation of Sulfur Dioxide on a Vanadium Catalyst The resulting sulfur trioxide is absorbed into strong sulfuric acid rather than plain water.35Journal of Cleaner Production. Multi-objective optimisation of a double contact double absorption sulphuric acid plant for cleaner operation Water is then added carefully to keep the acid at the right strength.
The “double” part matters. Older single-step plants converted only 97 to 98% of the sulfur dioxide and released the rest.36Journal of the Air Pollution Control Association. The Double Contact Process For Sulfuric Acid Production Absorbing acid midway, then converting again, raises conversion to 99.5% or more.37Journal of the Air Pollution Control Association. The Double Contact Process For Sulfuric Acid Production The final product reaches about 99.98% purity.38PubMed Central. Exergy Analysis of Sulfuric Acid Production: A Systematic Framework Using UniSim Design Higher yield helps offset the extra equipment cost.39Journal of the Air Pollution Control Association. The Double Contact Process For Sulfuric Acid Production
Where Industry Gets Its Sulfur
Hydrogen sulfide is a byproduct of processing natural gas and refining high-sulfur oil.40US EPA. Sulfur Recovery Refineries and gas plants convert it to pure sulfur in a standard process.41Wikipedia. Claus process Part of the gas is burned so some becomes sulfur dioxide. That sulfur dioxide then reacts with the remaining hydrogen sulfide over a catalyst to give elemental sulfur and water. The sulfur comes out molten, and it is cooled and stored as a solid or shipped hot, ready to feed an acid plant.
Smelters that process metal ores give off sulfur dioxide-rich gas.42archive.epa.gov. Multi-Media Assessment of the Inorganic Chemicals Industry Making sulfuric acid is the main way to remove those sulfur oxides from smelter gas.43911Metallurgist. How to Control Sulfur Oxide Emissions in Smelting Smelter gas must be filtered of dust and dried before use.44archive.epa.gov. Multi-Media Assessment of the Inorganic Chemicals Industry Some plants instead roast pyrite to release sulfur dioxide, leaving iron oxide behind.45archive.epa.gov. Multi-Media Assessment of the Inorganic Chemicals Industry Supply of sulfuric acid now follows oil, gas, and metal production rather than sulfur mines.
What Sulfuric Acid Does to Land and Water
Acid reaches the ground in rain, snow, and fog.46Wikipedia. Acid rain It also settles out dry, which can account for 20 to 60% of total acid deposits.47Wikipedia. Acid rain Dry deposition means sulfur gases and acidic particles land on leaves, soil, and buildings without any rain. They turn acidic when moisture such as rain, dew, or fog washes them into soil and streams.
Soils with limestone can neutralize some of the acid.48USGS. Acid Rain and Water Where bedrock lacks limestone, as in parts of the Rocky Mountains and eastern US, acid harms the environment.49USGS. Acid Rain and Water It strips nutrients like calcium and magnesium from soil and leaches aluminum into lakes and streams.50Cary Institute of Ecosystem Studies. Guide to acid rain: Causes, effects, and ongoing solutions Fish, frogs, and salamanders suffer, and evergreen trees are damaged.51USGS. Acid Rain and Water Recovery is slow even after emissions drop, because soil nutrients take years to rebuild.
What Sulfuric Acid Is Used For and How Waste Is Handled
Most phosphate fertilizer is made by treating phosphate rock with concentrated sulfuric acid to make phosphoric acid.52UMRP. Beyond Wet-Process Phosphate — UMRP Many common phosphate fertilizers depend on it.53Elessent Clean Technologies. Phosphate Fertilizer Phosphate rock holds its phosphorus in a form plants cannot easily take up, and the acid breaks the rock down so the phosphorus becomes available. The phosphoric acid is then reacted with ammonia. The acid step also yields a calcium sulfate byproduct, which plants must store or dispose of. Car lead-acid batteries contain diluted sulfuric acid, it is used in metal processing and refining, and some drain cleaners contain it concentrated.
Refineries can regenerate used acid by heating it to about 1,000 to 1,200°C.54Veolia North America. How Sulfuric Acid Regeneration Lowers Refinery Costs and Environmental Impact It breaks back into sulfur dioxide and steam, which re-enter the production process.55Digital Refining. Spent sulphuric acid regeneration (SAR) process Acid can instead be neutralized with limestone, producing synthetic gypsum.56Environmental Technology and Innovation. Structurally controlled synthesis of calcium sulphate dihydrate from industrial wastes of spent sulphuric acid and limestone That gypsum can be used in building materials.57Google Patents. The neutralization process of waste sulphuric acid with by product from iron and steel works and the method of manufacturing gypsum with precipitate made the neutralization process
Handling Sulfuric Acid Safely
Concentrated sulfuric acid reacts violently with water, releasing a lot of heat.58SLAC National Accelerator Laboratory. Sulfuric Acid Safe Handling Guideline So always add acid slowly to water while stirring, never water to acid.59CORECHEM Inc.. Safe Handling Guide: Sulfuric Acid A little water hitting concentrated acid flashes to steam and spatters hot acid outward, while adding acid to water lets the larger volume of water absorb the heat.
It also reacts violently with organic materials and bases.60SLAC National Accelerator Laboratory. Sulfuric Acid Safe Handling Guideline It corrodes most metals and can produce flammable hydrogen gas.61SLAC National Accelerator Laboratory. Sulfuric Acid Safe Handling Guideline The acid itself does not burn.62SLAC National Accelerator Laboratory. Sulfuric Acid Safe Handling Guideline
With household acid drain cleaners, wear gloves and eye protection, and never mix products, since they can boil or spatter. Flush skin splashes with lots of running water, and seek medical help for eye contact. Store them in their original containers, out of reach of children.